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To constrain the closure mechanism and time of the Paleo-Asian Ocean, we report new geochronological and geochemical data for Triassic granites along a NW-SE corridor from Eastern Tianshan to Beishan, NW China. Seven granites have U-Pb ages that young southwards from 245 Ma to 234 Ma in the Kanguer accretionary complex, to 237 Ma to 234 Ma in the eastern Central Tianshan block, to 229 Ma to 223 Ma in the Liuyuan accretionary complex. Granites in the Kanguer accretionary complex formed by fractional crystallization and are peraluminous, high-K, calc-alkaline, and crust-derived. They have very low MgO (Mg# = 6-9), Cr, and Ni contents, and their high eNd(t) (+3.40) and eHf(t) (+4.49 to +11.91) isotopes indicate that the Dananhu arc crust was juvenile. The Huaniushan pluton in the Liuyuan accretionary complex displays the geochemical signatures of both A1- and A2-type granites (Y/Nb = 0.32-3.39). All other granites in the Central Tianshan block and Liuyuan accretionary complex are aluminous A2-types with high K2O + Na2O, Al, rare earth elements (REE), Zr + Nb + Y, Ga, Fe/Mg, and Y/Nb and remarkable depletions of Eu, Ba, Nb, Ta, Sr, P, and Ti. They have a broad range of MgO (Mg# = 9-59), Cr, and Ni contents, Isr (0.70741-0.70945) values, negative eNd (t) (-2.98 to -1.14), and low to moderate eHf(t) (-1.22 to +7.78), which suggests a mixture of mantle and crustal components. These 245-223 Ma granitoids show marked Nb-Ta depletions that point to a subduction origin. Notable enrichments in Nd-Hf isotopes of Late Triassic granites are likely an indication of collision. Integration with previous data enables us to conclude that the delamination of an oceanic slab and mantle upwelling induced partial melting of thickened arc crust during a tectonic transition from a multiple supra-subduction margin to a collisional setting in the Late Triassic. [ABSTRACT FROM AUTHOR]

A correction is presented to the article "Mineralization and genesis of the orogenic gold system in the Kalamaili area, East Junggar, Xinjiang, northwestern China" which appeared in the last issue of the periodical.

The Bikou Terrane, located at the conjunction of the Longmenshan fold-thrust belt and the west Qinling orogenic belt in centeral China, was involved in the Late Triassic collision between the South China and North China blocks. The Bikou Terrane has preserved crucial information on structural geometry and kinematics of Triassic tectonics, and is therefore of great importance for reconstructing the Paleo-Tethyan evolutionary history. However, multi-phase tectonic events of the Bikou Terrane are unsettled. This work presents detailed structural analysis based on both the field and laboratory works, which reveals three phases of deformation events in Bikou and its adjacent areas, including top-to-the-SW shearing related to SW-ward thrusting (DI) mainly to the north of the Bikou Terrane, top-to-the-NNW shearing related to NNW-ward thrusting (DII) in the Bikou Terrane, and strike-slip faulting (DIII) locally developed in the northern Bikou Terrane. Anisotropy of magnetic susceptibility (AMS) study and related structural analysis not only support the multiphase deformation but also reveal a gradual transition from the DII-related magnetic fabrics to the DIII-related magnetic fabrics in the Bikou Terrane. Integrating published geochronological data, it is constrained that DI occurred at ca. 237-225 Ma, DII occurred at ca. 224-219 Ma, and DIII possibly occurred during the Early Cretaceous. Based on regional tectonics, the DI event corresponds to the collision between the South Qinling block and the Bikou Terrane, and the DII event reflects the intracontinental amalgamation between the Bikou Terrane and the Yangtze block, which indicates a Late Triassic successive amalgamation from the North China block to the South China block. Intracontinental adjustment represented by the strike-slip (DIII event) occurred after the final amalgamation between the North China and South China blocks. By applying AMS on deciphering structural geometry and multiphase deformation, our study suggests that AMS is a useful tool for structural analysis. [ABSTRACT FROM AUTHOR]

To understand the origin of curved subduction zones has been one of the major challenges in plate tectonics. The Mongol-Okhotsk Orogen in Central Asia is characterized by the development of a U-shaped oroclinal structure that was accompanied by the continuous subduction of the Mongol-Okhotsk oceanic plate. Therefore, it provides a natural laboratory to understand why and how a subduction system became tightly curved. In this study, we provide the first structural observation around the hinge of the Mongolian Orocline (the Zag zone in Central Mongolia), with an aim to constrain the oroclinal geometry and to link hinge zone structures with the origin of the orocline. Our results show that rocks in the Zag zone are characterized by the occurrence of a penetrative foliation that is commonly subparallel to bedding. Both bedding and dominant fabric in the Zag zone are steeply dipping, and their strike orientations in a map view follow a simple curve around the hinge of the Mongolian Orocline, thus providing the first structural constraint for 3D geometry of the orocline. A secondary penetrative fabric parallel to the axial plane of the orocline was not observed, indicating a low degree of orogen-parallel shortening during oroclinal bending. Combining with available geological and geophysical data, we conclude that the Mongolian Orocline was developed in a period of Permian to Jurassic, and its origin was linked to the subduction of the Mongol-Okhotsk oceanic slab. We consider that the low-strain oroclinal bending likely resulted from the along-strike variation in trench retreat, which was either triggered by the negative buoyancy of the Mongol-Okhotsk oceanic slab, or driven by the relative rotation of the Siberian and North China cratons. Our results shed a light on 3D geometry and geodynamic mechanisms of large-scale oroclinal bending in an accretionary orogen. [ABSTRACT FROM AUTHOR]

Post-collisional mountain-root collapse and subsequent massive partial melting occurred in the high-temperature (HT) ultrahigh- pressure (UHP) metamorphic terrane of the North Dabie complex zone (NDZ), central China. The NDZ was deeply subducted in the Triassic, producing widespread migmatites and various magmatic intrusions in the Cretaceous. Post-collisional metadiorites with distinctive large K-feldspar augen porphyroblasts, locally reported but rarely exposed in the NDZ, underwent a complex evolutional history. In this contribution, integrated studies including field investigation, petrographic observation and mineral analysis, zircon U-Pb geochronological and Hf isotopic analyses, and whole-rock elemental and Sr-Nd-Pb isotopic analyses of the metadiorites were carried out. Our results provide new constraints on the mountain-root collapse in the Dabie orogen. The metadiorites are enriched in large ion lithophile elements and light rare earth elements, whereas they are depleted in high field strength elements and heavy rare earth elements with significant Ba positive anomalies, a composition consistent with the lower continental crust. All the studied samples have moderately enriched initial 87Sr/86Sr ratios (0.707582-0.708099), low εNd(t) values (-15.3 to -20.4), and low initial Pb isotopic ratios (16.0978-16.8452, 15.3167-15.4544, and 37.1778-37.8397 for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, respectively). However, they have highly negative εHf(t) values and Paleoproterozoic two-stage Hf model ages, which are only partially consistent with data from the associated UHP metamorphic rocks. Such features suggest the metadiorites resulted from a magma produced by mixing of Triassic UHP mafic lithologies and minor amounts of mantle-derived materials. Zircon morphological analysis and U-Pb sensitive high-resolution ion microprobe dating combined with conventional thermobarometry indicate that these upwelling melts crystallized at pressure- temperature (P-T) conditions of 5.4-5.7 kbar and 750-768 °C at ca. 130 Ma and subsequently suffered HT metamorphism at ca. 125 Ma. We conclude that the metadiorites' precursors were derived from partial melting of the Triassic subducted Neoproterozoic mafic lower-crustal rocks, with addition of minor amounts of mantle-derived materials in the Early Cretaceous, in response to mountain-root collapse of the orogen. Based on petrographic textures and mineral compositions, it is moreover inferred that formation of the distinctive K-feldspar porphyroblasts is likely related to a two-stage process, i.e., crystallization derived from biotite breakdown after the formation of the metadiorite at T = 640-703 °C and P < 4.5 kbar and coarsening related to shear deformation. [ABSTRACT FROM AUTHOR]

The Zagros orogen, formed by the collision of the Arabian and Eurasian continental margins, represents one of the largest and richest oil and gas provinces in the world. The Zagros fold-thrust belt records collision and convergence along the Neotethys suture zone. By coupling field observations, sandstone modal analysis, U-Pb zircon dating, and Hf isotopic data from the Upper Cretaceous to Pliocene sedimentary succession of the Neyriz region, this paper documents several major provenance changes that allow us to propose a refined scenario for the Zagros orogeny. An ophiolitic complex dated by detrital-zircon U-Pb geochronology as ca. 95 Ma provided detritus to Upper Cretaceous-Paleocene strata deposited along the northeastern margin of the Arabian lower plate (ophiolite provenance). Yet, on the southwestern margin of the Eurasian upper plate, upper Paleocene-lower Eocene strata indicate provenance from Mesozoic magmatic rocks yielding zircons dated as ca. 240 Ma and 170 Ma as well as the recycling of clastic rocks. Since the early Miocene, the sedimentary basin located on the Arabian plate received both ophiolitic detritus and magmatic-arc, recycled clastic, and axial-belt metamorphic detritus from Eurasia. U-Pb ages of detrital zircons reflect polyphase magmatism at 170 Ma, 95 Ma, and 40 Ma on the Eurasian active margin. Our results indicate that progressive accretion, uplift, and exhumation of the Zagros orogen was well under way by the beginning of the Miocene in the Neyriz region. Literature data from adjacent regions suggest that the Arabia/Eurasia collision may have occurred diachronously and later in the Kermanshah and Lurestan areas to the north. [ABSTRACT FROM AUTHOR]

Accretionary orogens function as major sites for the generation of continental crust, but the growth model of continental crust remains poorly constrained. The Central Asian Orogenic Belt, as one of the most important Phanerozoic accretionary orogens on Earth, has been the focus of debates regarding the proportion of juvenile crust present. Using published geochemical and zircon Hf-O isotopic data sets for three belts in the Eastern Tianshan terrane of the southern Central Asian Orogenic Belt, we first explore the variations in crustal thickness and isotopic composition in response to tectono-magmatic activity over time. Steady progression to radiogenic zircon Hf isotopic signatures associated with syn-collisional crustal thickening indicates enhanced input of mantle-derived material, which greatly contributes to the growth of the continental crust. Using the surface areas and relative increases in crustal thickness as the proxies for magma volumes, in conjunction with the calculated mantle fraction of the mixing flux, we then are able to determine that a volume of ~14-22% of juvenile crust formed in the southern Central Asian Orogenic Belt during the Phanerozoic. This study highlights the validity of using crustal thickness and zircon isotopic signatures of magmatic rocks to quantify the volume of juvenile crust in complex accretionary orogens. With reference to the crustal growth pattern in other accretionary orogens and the Nd-Hf isotopic record at the global scale, our work reconciles the rapid crustal growth in the accretionary orogens with its episodic generation pattern in the formation of global continental crust. [ABSTRACT FROM AUTHOR]

Stratigraphic and thermochronologic data are used to study the processes that shaped the topography of the central Apennines of Italy. These are part of a major, active mountain belt in the center of the Mediterranean area, where several subduction zones control a complex topography. The Apennines were shaped by contraction at the front of the accretionary wedge overlying the subducting Adria microplate followed by extension at the wedge rear in response to eastward slab rollback. In the central Apennines, intermontane extensional basins on the western flank rise eastward toward the summit. We contribute with new data consisting of 28 (U-Th-Sm)/ He and 10 fission track ages on apatites to resolve a complex pattern of thermal histories in time and space, which we interpret as reflecting the transitional state of the orogen, undergoing a two-phase evolution related to initial slab retreat, followed by slab detachment. Along the Tyrrhenian coast, we document cooling from depths ≥3-4 km occurring between 8 and 5 Ma and related to the opening of marine extensional basins. Post--5 Ma, a broader region of the central Apennines exhibits cooling from variable depths, between <2 km in most areas and ≥3-4 km in the northeast, and with different onset times: at ca. 4 Ma in the west, at ca. 2.5 Ma in the center and northeast, and at ca. 1 Ma in the southeast. Between 5 and 2.5 Ma, exhumation is associated with modest topographic growth during the late stages of thrusting. Since 2.5 Ma, exhumation has concurred with the opening of intermontane basins in the west and in the east, with regional topographic growth and erosion, that we interpret to be associated with the locally detaching slab. [ABSTRACT FROM AUTHOR]

Triassic strata in the Greater Barents Sea Basin are important records of geodynamic activity in the surrounding catchments and sediment transport in the Arctic basins. This study is the first attempt to investigate the evolution of these source areas through time. Our analysis of sediment budgets from subsurface data in the Greater Barents Sea Basin and application of the BQART approach to estimate catchment properties shows that (1) during the Lower Triassic, sediment supply was at its peak in the basin and comparable to that of the biggest modern-day river systems, which are supplied by tectonically active orogens; (2) the Middle Triassic sediment load was significantly lower but still comparable to that of the top 10 largest modern rivers; (3) during the Upper Triassic, sediment load increased again in the Carnian; and (4) there is a large mismatch (70%) between the modeled and estimated sediment load of the Carnian. These results are consistent with the Triassic Greater Barents Sea Basin succession being deposited under the influence of the largest volcanic event ever at the Permian- Triassic boundary (Siberian Traps) and concurrent with the climatic changes of the Carnian Pluvial Event and the final stages of the Northern Ural orogeny. They also provide a better understanding of geodynamic impacts on sedimentary systems and improve our knowledge of continental-scale sediment transport. Finally, the study demonstrates bypass of sediment from the Ural Mountains and West Siberia into the adjacent Arctic Sverdrup, Chukotka, and Alaska Basins in Late Carnian and Late Norian time. [ABSTRACT FROM AUTHOR]

The deformation pattern and slip partitioning related to oblique underthrusting of the Tarim Basin in the eastern Tian Shan orogenic belt are not well understood because interior deformation images are lacking. The Baoertu fault is an E-W--striking, ~350-km-long reactivated basement structure within the eastern Tian Shan. In this study, we quantify its late Quaternary activity based on interpretations of detailed highresolution remote sensing images and field investigations. Three field observation sites along an ~80-km-long fault segment indicate that the Baoertu fault is characterized by sinistral thrust faulting. Based on surveying of the displaced geomorphic surfaces with an unmanned drone and dating of the late Quaternary sediments using radiocarbon and optically stimulated luminescence (OSL) methods, we estimate a late Quaternary leftlateral, strike-slip rate of 1.87 ± 0.29 mm/yr and a N--S shortening rate of 0.26 ± 0.04 mm/ yr for this fault. The lithospheric Baoertu fault acts as a decoupling zone and accommodates the left-lateral shearing caused by the oblique underthrusting of the Tarim Basin. In the eastern Tian Shan orogenic belt, the oblique convergence is partitioned into thrust faulting across the entire range and sinistral slip faulting on the high-dip basement structure within the orogen. This active faulting pattern in the eastern Tian Shan of sinistral shearing in the center and thrust faulting on both sides can be viewed as giant, crustal-scale positive flower structures. [ABSTRACT FROM AUTHOR]

The India-Asia collision is the most spectacular, recent, and still active tectonic event of the Earth's history, leading to the uplift of the Himalayan-Tibetan orogen, which has been explained through several hypothetical models. Still, controversy remains, such as how and when it occurred. Here we report a paleomagnetic study of Cretaceous-Tertiary marine sediments from the Tethyan Himalaya (TH) in the Hazara area, north Pakistan, which aims to constrain timing for the onset of the India-Asia collision and to confirm the validity of already proposed models, particularly in western Himalaya's perspective. Our results suggest that the TH was located at a paleolatitude of 8.5°S ± 3.8° and 13.1°N ± 3.8° during the interval of ca. 84-79 Ma and 59-56 Ma, respectively. A comparison between paleopoles obtained from the current study and coeval ones of the India Plate indicates that the TH rifted from Greater India before the Late Cretaceous, generating the Tethys Himalaya Basin (THB). Our findings support a model for a multi-stage collision involving at least two major subduction systems. A collision of the TH with the Trans-Tethyan subduction system (TTSS) began first in Late Cretaceous-Early Paleocene times (ca. 65 Ma), followed by a later collision with Asia at 55-52 Ma. The onset of the collision between the TH (plus TTSS) and Asia could not have occurred earlier than 59-56 Ma in the western Himalaya. Subsequently, the India craton collided with the TH, resulting in the diachronous closure of the THB between ca. 50 and ca. 40 Ma from west to east. These findings are consistent with geological and geochemical evidence and have a broad implication for plate reconfigurations, global climate, and biodiversity of collisional processes. [ABSTRACT FROM AUTHOR]

In the southern Tibetan Plateau, leucogranites are dominantly distributed in the Himalayan orogenic belt with minor occurrences in the southern Lhasa subterrane. In this paper, we report the first Miocene Anglonggangri leucogranites in the northern Lhasa subterrane. This finding provides important constraints on both leucogranite petrogenesis and the tectono-magmatic evolution of the Lhasa terrane. The Anglonggangri leucogranites include biotite-muscovite granite and slightly younger garnet-muscovite granite and pegmatite. Zircon U-Pb and muscovite 40Ar-39Ar dating of these leucogranites yields Miocene ages of 11.1-10.2 Ma. The biotite-muscovite and garnet-muscovite granites are characterized by high SiO2 (72.3-74.4 wt.%) and Al2O3 contents (14.4-15.4 wt.%) and are peraluminous. The biotite-muscovite granite displays geochemical signatures with high Sr/Y (29.2-81.0) and (La/Yb)N (37.5-98.9) ratios, low Y (4.30-7.22 ppm) and Yb contents (0.26-0.47 ppm), low to moderate initial (87Sr/86Sr)i ratios (0.7085-0.7192), and moderate εNd(t) values (-10.17 to -6.94). Furthermore, they also exhibit radiogenic Pb isotope and variable zircon εHf(t) values (-9.6 to +4.4) with Proterozoic Nd (1.1-1.4 Ga) and Hf model ages (0.8-1.7 Ga). By comparison, the garnet-muscovite granite has lower CaO, MgO, TiO2, and total FeO contents and is enriched in Rb (380-466 ppm) and depleted in Sr (24.1-38.5 ppm) and Ba (30.7-58.6 ppm) and further characterized by a significant rare earth element (REE) tetrad effect and non-charge and radius-controlled (CHARAC) trace element behaviors. The garnet-muscovite granite shows a negative Eu anomaly and positive correlations among Sr and Eu, Sr and Ba, and Th and light rare earth elements (LREEs). Pegmatite comprising Nb-Ta oxides and cassiterite occurs in the garnet-muscovite granite. Geochronological and geochemical characteristics of the Anglonggangri leucogranites indicate that the magma of the biotite-muscovite granite was derived from partial melting of amphibolite lower crust contaminated with Proterozoic-Archean upper crustal materials. The garnet-muscovite granite was generated through melt extraction from the biotite-muscovite granite crystal mush. These results confirm that partial melting of the amphibolite lower crust not only occurred in the southern and central Lhasa subterranes but also in the northern Lhasa subterrane. [ABSTRACT FROM AUTHOR]

Two successive and parallel magmatic arcs within the southern Alxa provide an ideal area to examine the influence of tectonic switching on temporal and spatial distribution of magmatism within accretionary orogens. This study presents new geochronological and geochemical data for Yingen and Quagu plutons from the southern Alxa, located in the southern Central Asian Orogenic Belt. Late Permian Yingen granitic dikes (ca. 252 Ma) have depleted whole-rock Nd isotopic compositions, high Sr, low Y and Yb, and high Sr/Y ratios, all of which indicate they were generated by the partial melting of subducted young/hot oceanic slab. The Middle Permian (271 Ma) Yingen hosting granites contain elevated contents of Nb and Zr, and have high 10,000 x Ga/Al ratios, suggesting that they resulted from mixing between Neoproterozoic crust-derived felsic magmas and depleted mantle-derived mafic magmas. The Quagu pluton yields ca. 271-262 Ma zircon U-Pb ages and has an adakitic high-Mg diorite-like geochemical composition, suggesting that it originated from interaction between slab-derived melts and overlying peridotite material. Collectively, these data record the subduction of the Enger Us oceanic slab beneath Mesoproterozoic-Neoproterozoic sialic crust, generating a Japan-type arc within the southern Alxa during Middle-Late Permian. Temporal-spatial variations of zircon Hf isotope for plutons suggest tectonic switching from advancing to retreating subduction during Carboniferous-Early Triassic within the southern Alxa. An advancing subduction resulted from the subduction of the Paleo-Asian Ocean, and a retreating subduction was related to plate boundary reorganization during the assembly of Pangea. [ABSTRACT FROM AUTHOR]

Knowledge of the nature of the earliest metamorphism experienced by collisional orogenic belts is essential for reconstruction of tectonic processes that build high mountain chains and their environmental consequences. Understanding the metamorphic nature of Eohimalayan-phase orogeny of the Himalayan orogen, one of the typical examples of orogenic belts worldwide, could provide some important constraints to test different tectonic models (shallow continental subduction vs. slab breakoff) for the early phases of the development of large-scale orogenic belts. As exhumed middle- to lower-crustal rocks in the Kangmar gneiss dome, the garnet amphibolites with a protolith age of 176.4 ± 3.6 Ma experienced a phase of metamorphism at 47.2 ± 1.8 Ma with an increase in pressure as well as temperature from 3-5 kbar and 550-600 °C to over ~11 kbar and 650 °C. This suggests that the middle- to lower-crustal rocks experienced heating at least by ~50 °C while they underwent compression and thickening. Heat-flow estimation further demonstrates that the self-produced heat was high enough to achieve the observed pressure-temperature conditions recorded by the garnet amphibolite. Therefore, an additional heat supply is not required during early Eocene metamorphism. A breakoff of the leading part of the subducting Indian continental slab, if it occurred, should be younger than ca. 47 Ma. [ABSTRACT FROM AUTHOR]

Knowledge of the subduction to postcollision tectonic transition in response to oceanic closure is crucial for tracking the final stage of orogenic evolution. Here, we report new geochronology, geochemistry, and isotopic data for Carboniferous magmatism in East Junggar (NW China), southwestern Central Asian orogenic belt, which may record such processes following the closure of the Kalamaili Ocean (a branch of the Paleo-Asian Ocean). The early Carboniferous calc-alkaline volcanic rocks (dominated by basalt and basaltic andesite) yielded zircon U-Pb ages of ca. 340-330 Ma and are characterized by arc-like trace-element patterns showing enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs; e.g., Pb) but depletion of high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). Combined with their variable Ba/Nb (9.80-454) and low Nb/La (0.21-0.54) and Sm/Yb (1.77-3.08) ratios as well as depleted mantle-like Sr-Nd-Pb-Hf (whole-rock 87Sr/86Sri = 0.7037-0.7040; eNd[t] = +3.5 to +5.9; 206Pb/204Pbi = 17.728-17.996; zircon eHf[t] = +11.8 to +18.8) isotopic values, we suggest that they were produced by melting of a lithospheric mantle wedge fluxed by slab-derived fluids under spinel-facies conditions. With whole-rock 40Ar/39Ar dating of ca. 320 Ma, the late Carboniferous mafic dikes have geochemical features and Sr-Nd-Pb (87Sr/86Sri = 0.7039-0.7041; eNd[t] = +6.6 to +6.8; 206Pb/204Pbi = 17.905-17.933) isotopic compositions similar to those of the early Carboniferous volcanics, but they show less pronounced Pb anomalies and negative Nb and Ta anomalies. They are interpreted to have formed by partial melting of a spinel-bearing lithospheric mantle metasomatized by limited influx of subduction-related fluids. The late Carboniferous felsic volcanic rocks (dacite and rhyolite) yielded zircon U-Pb ages of ca. 305 Ma and are geochemically equivalent to those of A2-type granites in East Junggar. They have juvenile isotopic compositions (eNd[t] = +4.5 to +6.8; eHf[t] = +13.3 to +18.7) and relatively young Nd and Hf model ages that roughly coincide with the ages of the ophiolites in the area, suggesting that they could have originated from melting of a juvenile basaltic lower crust. Whole-rock geochemistry, assimilation-fractional crystallization (AFC), and isotopic mixing modeling argue for insignificant crustal contamination for the Carboniferous magmatism. We suggest that the early Carboniferous lavas erupted in an island-arc setting related to the northward subduction of the Kalamaili oceanic crust, whereas the late Carboniferous magmatism formed in a postcollisional extensional regime in response to slab breakoff or lithospheric delamination. Combined with regional geological information, we propose that a rapid tectonic transition from oceanic subduction to postcollisional extension may have occurred in East Junggar during the Carboniferous, marking the final closure of the Kalamaili Ocean, which most likely took place ca. 330-320 Ma. This study provides overall geochronological and petrogeochemical evidence to better constrain the amalgamation of the southwestern Central Asian orogenic belt and may be of great importance for understanding the final stage of orogenic evolution elsewhere. [ABSTRACT FROM AUTHOR]

Updating magmatic profile in crucial constituent terranes across the Central Asian Orogenic Belt presents a key to chronicling the timeline of prolonged accretionary processes and termination of the Paleo-Asian Ocean in the northern China-southern Mongolia tract. Here we performed a systematic geochronological and geochemical study on a spectrum of Paleozoic intrusions from the Erenhot region in central Inner Mongolia, North China, within the hinterland of the tract, with four distinct magmatic episodes unraveled. Combining these episodes with the previously documented events from contiguous regions defines two major tectono-magmatic cycles. The early Paleozoic cycle (500-450 Ma) evolved from initial fluid-fluxed tholeiitic and calc-alkaline granitoids to melt-fertilized mafic-intermediate magmatism. It appears to experience the initiation and maturation of a Western Pacifictype intra-oceanic arc system that culminated in ridge subduction. The late Paleozoic cycle expanded in magmatic expression from sporadic Late Devonian (373-365 Ma) calcalkaline intermediate-felsic pulses through Early-Middle Carboniferous (356-320 Ma) medium to high-K calc-alkaline flare-up to a Late Carboniferous-Early Permian (310-277 Ma) province of diverse lithologies. These magmatic episodes seem to encompass a complete active continental arc-back-arc system that spanned from resuming oceanic plate subduction through slab rollback and backarc rifting to ridge-trench collision and backarc basin closure. Featuring a Rodiniaaged terrane affinity and a representative Paleozoic magmatic profile, the Erenhot region provides an optimal site for correlating the evolution of mosaic terranes in southern Mongolia and northern China, and for evaluating the coupled evolution of shifting tectonic regimes and plural crustal generation mechanisms within a retreating accretionary orogen. [ABSTRACT FROM AUTHOR]

Granitoids at convergent plate boundaries can be produced either by partial melting of crustal rocks (either continental or oceanic) or by fractional crystallization of mantlederived mafic magmas. Whereas granitoid formation through partial melting of the continental crust results in reworking of the pre-existing continental crust, granitoid formation through either partial melting of the oceanic crust or fractional crystallization of the mafic magmas leads to growth of the continental crust. This category is primarily based on the radiogenic Nd isotope compositions of crustal rocks; positive eNd(t) values indicate juvenile crust whereas negative eNd(t) values indicate ancient crust. Positive eNd(t) values are common for syn-collisional granitoids in southern Tibet, which leads to the hypothesis that continental collision zones are important sites for the net growth of continental crust. This hypothesis is examined through an integrated study of in situ zircon U-Pb ages and Hf isotopes, whole-rock major trace elements, and Sr-Nd-Hf isotopes as well as mineral O isotopes for felsic igneous rocks of Eocene ages from the Gangdese orogen in southern Tibet. The results show that these rocks can be divided into two groups according to their emplacement ages and geochemical features. The first group is less granitic with lower SiO2 contents of 59.82-64.41 wt%, and it was emplaced at 50-48 Ma in the early Eocene. The second group is more granitic with higher SiO2 contents of 63.93-68.81 wt%, and it was emplaced at 42 Ma in the late Eocene. The early Eocene granitoids exhibit relatively depleted wholerock Sr-Nd-Hf isotope compositions with low (87Sr/86Sr)i ratios of 0.7044-0.7048, positive eNd(t) values of 0.6-3.9, eHf(t) values of 6.5-10.5, zircon eHf(t) values of 1.6-12.1, and zircon d18O values of 5.28-6.26?. These isotopic characteristics are quite similar to those of Late Cretaceous mafic arc igneous rocks in the Gangdese orogen, which indicates their derivation from partial melting of the juvenile mafic arc crust. In comparison, the late Eocene granitoids have relatively lower MgO, Fe2O3, Al2O3, and heavy rare earth element (HREE) contents but higher K2O, Rb, Sr, Th, U, Pb contents, Sr/Y, and (La/Yb)N ratios. They also exhibit more enriched wholerock Sr-Nd-Hf isotope compositions with high (87Sr/86Sr)i ratios of 0.7070-0.7085, negative eNd(t) values of -5.2 to -3.9 and neutral eHf(t) values of 0.9-2.3, and relatively lower zircon eHf(t) values of -2.8-8.0 and slightly higher zircon d18O values of 6.25-6.68?. An integrated interpretation of these geochemical features is that both the juvenile arc crust and the ancient continental crust partially melted to produce the late Eocene granitoids. In this regard, the compositional evolution of syn-collisional granitoids from the early to late Eocene indicates a temporal change of their magma sources from the complete juvenile arc crust to a mixture of the juvenile and ancient crust. In either case, the syn-collisional granitoids in the Gangdese orogen are the reworking products of the pre-existing continental crust. Therefore, they do not contribute to crustal growth in the continental collision zone. [ABSTRACT FROM AUTHOR]

Widespread accumulation of thick carbonates is not typical of orogenic settings. During the mid-Cretaceous, near the Bangong suture in the northern Lhasa terrane, the shallow-marine carbonates of the Langshan Formation, reaching a thickness up to ~1 km, accumulated in an epicontinental seaway over a modern area of 132 x 10³ km², about half of the Arabian/Persian Gulf. The origin of basin-wide carbonate deposits located close to a newly formed orogenic belt is not well understood, partly because of the scarcity of paleogeographic studies on the evolution of the northern Lhasa. Based on a detailed sedimentological and stratigraphic investigation, three stages in the mid-Cretaceous paleogeographic evolution of northern Lhasa were defined: (1) remnant clastic sea with deposition of Duoni/Duba formations (Early to early Late Aptian, ca. 125-116 Ma); (2) expanding carbonate seaway of Langshan Formation (latest Aptian--earliest Cenomanian, ca. 116-99 Ma); and (3) closure of the carbonate seaway represented by the Daxiong/Jingzhushan formations (Early Cenomanian to Turonian, ca. 99-92 Ma). Combined with data on tectonic subsidence and eustatic curves, we emphasized the largely eustatic control on the paleogeographic evolution of the northern Lhasa during the latest Aptian--earliest Cenomanian when the Langshan carbonates accumulated, modulated by long-term slow tectonic subsidence and high carbonate productivity. [ABSTRACT FROM AUTHOR]

High-grade metamorphic rocks and crustal melts provide crucial evidence for growth and differentiation of the continental crust, and are widespread in collisional orogens. However, their importance in the evolution of continental arcs remains poorly understood. Metamorphism and related anatexis in the preserved continental margin of the Neo-Tethys ocean serves as a key natural laboratory to investigate this process. Along the Neo-Tethyan arc margin, the Gaoligong shear zone, Yunnan region of China, is an important locality for linking Lhasa in the north with Sibumasu and Burma in the south. Here, Late Cretaceous granulite-facies metamorphism and crustal anatexis have been identified for the first time in the Gaoligong area. Zircon and monazite U-Pb dating indicates that S-type granites formed at 87-73 Ma, granites and buried pelitic sediments were simultaneously metamorphosed at 75-70 Ma during Neo-Tethyan subduction, and all lithologies were overprinted by a younger 40-30 Ma magmatic and strike-slip event related to India-Asia collision. Phase equilibria modeling of high-grade anatectic gneiss in the MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 system indicates peak pressure-temperature (P-T) conditions of 780-800 °C and 6.5-7.5 kbar and defines a cooling and decompressional P-T path for the metapelites. This demonstrates that sediments within the Neo-Tethyan active continental arc were buried to >20 km depth at 75-70 Ma. In combination with the metamorphic record of the Lhasa, Burma, and Sibumasu blocks, an extensive Late Cretaceous metamorphic belt must have formed along the Neo-Tethyan subduction zone. This spatially correlates with coeval gabbro-diorite suites exposed in the Gangdese, Sibumasu and Burma terranes that were triggered by thinning of the lithospheric mantle. This prolonged Late Cretaceous mantle-derived magmatism and lithospheric thinning may have provided a regional-scale heat source for high-grade metamorphism and crustal anatexis along the active continental margin of the Neo-Tethys ocean. [ABSTRACT FROM AUTHOR]

The paleotopographic history of the North American Cordilleran orogen holds the key to understanding mechanisms of orogenesis and subsequent orogenic collapse. It has been suggested that the orogenic front in western Montana (USA) and Alberta (Canada) was more than 4 km high during Late Cretaceous-early Eocene contractional deformation and during the initial phase of extension in the middle Eocene; however, the late Eocene-Oligocene topographic evolution during continued extensional collapse remains poorly constrained. Here we extend the paleotopographic record in the Kishenehn Basin in northwestern Montana and southeastern British Columbia (Canada) to the late Oligocene by studying δ18O values of fossil mollusks and cement and paleosol carbonates. The molluscan taxa changed from three sympatric groups with preferred habitats ranging from tropical wet, semi-arid subtropical, and temperate during the middle and late Eocene, to mainly a single group associated with temperate environment during the Oligocene, reflecting a decline in molluscan biodiversity induced by climate cooling across the Eocene-Oligocene transition. Reconstructed δ18O values of alpine snowmelt and basinal precipitation decreased by 1.4‰ and 3.8‰, respectively, from the middle to late Eocene, reflecting climate cooling and ~1 km surface uplift of the basin floor. The reconstructed alpine snowmelt δ18O values then increased by 2.9‰ in the Oligocene suggesting a ~0.5 km drop in elevation of the orogenic front. Collectively, the results of our new and previously published δ18O data chronicle the paleotopographic response to the change from flat-slab subduction to slab rollback over a 45 m.y. period. These data suggest that the orogenic front was characterized by high elevation (>4 km) in the ancestral Lewis-Clark-Livingston ranges during latest Cretaceous-early Eocene (ca. 75-52 Ma) contraction. The initial phase of extension related to the Kishenehn Basin created a lowland basin with a surface elevation of only ~1.5 km during the early middle Eocene (ca. 46-44 Ma) whereas the ranges remained >4 km high. The high range elevations were sustained for at least 12 m.y. in the middle to late Eocene concurrent with extension, while the basin floor elevation was uplifted to ~2.5 km by the latest Eocene (ca. 36-34 Ma). Basin aggradation can explain at most half of the 1 km basin floor uplift. The remaining amount (at least 0.5 km) and sustained high range elevation suggest that range denudation and crustal extension was compensated by the isostatic and thermal effects of slab rollback and/or passage of a slab window and infusion of hot asthenosphere beneath the continent. The range elevation in the orogenic front decreased ~0.5 km by the late Oligocene (ca. 28 Ma), associated with a decrease in rock uplift rate associated with extension. A post-Oligocene elevation drop of ~1 km resulted in both the ranges and basin floor reaching modern topography in the Kishenehn Basin drainage, likely due to the regional effect of Neogene Basin and Range extension. This study, along with the previous investigation of the Kishenehn Basin by Fan et al. (2017), are the first studies that systematically investigate paleorelief of the orogenic belt by reconstructing paleoelevations of the mountains and the basin at the same time. The results highlight that the Cordilleran orogenic front of northern Montana and southern British Columbia sustained its high elevation edifice for at least 12 m.y. after the start of extension. We suggest that initial crustal extension did not result in orogenic demise because of concurrent thermal and isostatic uplift. [ABSTRACT FROM AUTHOR]

In cordilleran-type orogens, subduction geometry exerts a fundamental control on the tectonic behavior of the overriding plate. An integrated low-temperature, large thermochronological data set is used in this study to investigate the burial and exhumation history of the overriding plate in northern Patagonia (40°-45°S). Thermal inverse modeling allowed us to establish that a ~2.5-4-km-thick section originally overlaid the Jurassic-Lower Cretaceous successions deposited in half-graben systems that are presently exposed in the foreland. Removal of the sedimentary cover started in the late Early Cretaceous. This was coeval with an increase of the convergence rate and a switch to a westward absolute motion of the South American Plate that was accompanied by shallowing of the subducting slab. Unroofing was probably further enhanced by Late Cretaceous to early Paleogene opening of a slab window beneath the overriding plate. Following a tectonically quiescent period, renewed exhumation occurred in the orogen during relatively fast Neogene plate convergence. However, even the highly sensitive apatite (U-Th)/He thermochronometer does not record any coeval cooling in the foreland. The comparison between Late Cretaceous and Neogene exhumation patterns provides clear evidence of the fundamental role played by inter-plate coupling associated with shallow slab configurations in controlling plate-scale deformation. Our results, besides highlighting for the first time how the whole northern Patagonia foreland was affected by an exhumation of several kilometers since the Late Cretaceous, provide unrivalled evidence of the link between deep geodynamic processes affecting the slab and the modes and timing of unroofing of different sectors of the overriding plate. [ABSTRACT FROM AUTHOR]

Flow of partially molten crust is a key contributor to mass and heat redistribution within orogenic systems, however, this process has not yet been fully understood in accretionary orogens. This issue is addressed in a Devonian migmatite-granite complex from the Chinese Altai through structural, petrological, and geochronological investigations presented in this study. The migmatitegranite complex records a gradual evolution from metatexite, diatexite to granite and preserves a record of two main Devonian phases of deformation designated D1 and D2. The D1 phase was subdivided into an early crustal thickening episode (D1B) and a later extensional episode (D1M) followed by D2 upright folding. The D1M episode is associated with anatexis in the deep crust. Vertical shortening, associated with D1M, gave rise to the segregation of melt and formation of a sub-horizontal layering of stromatic metatexite. This fabric was reworked by the D2 deformation associated with the migration of anatectic magma in the cores of F2 antiforms. Geochronological investigations combined with petro-structural analysis reveal that: (1) D1M partial melting started probably at 420-410 Ma and formed subhorizontal stromatic metatexites at ~30 km depth; (2) The anatectic magma accumulated and migrated when a drainage network developed, as attested by the pervasive formation of massive diatexite migmatites, at 410-400 Ma; (3) Soon after, massive flow of the partially molten crust from orogenic lower to orogenic upper crustal levels, assisted by the interplay between D2 upright folding and magma diapirism, led to migmatite-granite emplacement in the cores of regional F2 antiforms that lasted until at least 390 Ma; (4) a terminal stage was manifested by the emplacement of 370-360 Ma granite dykes into the surrounding metamorphic envelope. We propose that Devonian anatexis assisted by deformation governed first the horizontal and then the vertical flow of partially molten orogenic lower crust, which drove crustal flow, mass redistribution, and crustal differentiation in the accretionary system of the Chinese Altai. [ABSTRACT FROM AUTHOR]

Accompanying Gondwana assembly, widespread but diachronous Ediacaran--early Paleozoic magmatism of uncertain origin occurred along the supercontinent's proto-Tethyan margin. We report new geochemical, isotopic, and geochronological data for Cambrian magmatic rocks (ca. 500 Ma) from the Gondwana-derived North Lhasa terrane, located in the present-day central Tibetan Plateau. The magmatic rocks are composed of basalts, gabbros, quartz monzonites, granitoids (with mafic microgranular enclaves), and rhyolites. Nd-Hf isotopic and whole-rock geochemical data indicate that these rocks were probably generated by mixing of mantle-derived mafic and crust-derived felsic melts. The mantle end-member volumes of mafic, intermediate, and felsic rocks are ~75%-100%, 50%-60%, and 0-30%, respectively. Integration of our new data with previous studies suggests that the North Lhasa terrane experienced long-term magmatism through the Ediacaran to Ordovician (ca. 572-483 Ma), with a magmatic flare-up at ca. 500 Ma. This magmatism, in combination with other Ediacaran--early Paleozoic magmatism along the proto-Tethyan margin, was related to an Andean-type arc, with the magmatic flare-up event related to detachment of the oceanic slab following collisional accretion of Asian microcontinental fragments to northern Gondwana. Diachroneity of the proto-Tethyan arc system along the northern Gondwanan margin (ca. 581-531 Ma along the Arabian margin and ca. 512--429 Ma along the Indian-Australian margin) may have been linked to orogenesis within Gondwana. The North Lhasa terrane was probably involved in both Arabian and Indian-Australian proto-Tethyan Andean-type orogens, based on its paleogeographic location at the northern end of the East African orogen. [ABSTRACT FROM AUTHOR]

Neoproterozoic to Paleozoic sedimentation shows systematic temporal-spatial variations within South China, which must be considered in reconstructing geological evolution of South China in response to global plate reorganization from the breakup of Rodinia to the assembly of Gondwana. We use >1000 new U-Pb and Hf isotopic data for detrital zircons from Neoproterozoic--Cambrian strata across the western (i.e., Longmenshan) and eastern (i.e., Wuyishan) margins of South China, coupled with compiled stratigraphic and magmatic information, to constrain change in provenance through time. First-order conclusions are as follows: (1) detrital zircons from the Neoproterozoic strata of the two margins were mainly sourced from the Panxi-Hannan arc and the Jiangnan orogen, signaling a rough self-sufficient sedimentary system; (2) newly identified Cambrian molasse-like sediments in the western margin, in which abundant detrital zircons are 550-500 Ma old with positive εHf(t) values, were mainly derived from the 580-500 Ma Cadomian arc belt along the Iran-Turkey margin; and (3) the Cambrian sediments in the eastern margin document more increased contributions from the Grenvillian-age provinces most possibly in Australia. Such spatial-temporal provenance variations signal the northward drifting of South China, from a position connecting with Iran-Turkey and northern India to that approaching Australia during the late Neoproterozoic--Cambrian period. We highlight that the activity of oblique oceanic-continental convergence accreted Asian terranes onto the northern margin of Gondwana, hence contributing to the ultimate Gondwana architecture under global plate reorganization. [ABSTRACT FROM AUTHOR]

Amphibolite retrograded from high-pressure (HP) mafic granulite can provide valuable insight into exhumation of deeply buried crust in orogenic belts. In the north Hengshan terrane of the North China Craton, amphibolite occurs as rims of HP mafic granulite block or as smaller homogeneous boudins representing retrograde products of the granulite. Three amphibolite samples were selected. The rocks are mainly composed of hornblende + plagioclase + quartz + biotite + ilmenite with or without garnet pseudomorph consisting of plagioclase + hornblende + ilmenite and symplectite of hornblende + plagioclase ± clinopyroxene. The pseudomorph-, symplectite-bearing sample experienced a post-peak isothermal decompression at >800 °C that was accompanied by breakdown of garnet and clinopyroxene. Isopleths of the maximum An in plagioclase and Ti in hornblende were used to constrain the Tmax stage of ca. 6 kbar/825--850 °C, which was followed by cooling and post-cooling decompression. For the sample showing an "equilibrated" mineral assemblage, a medium-temperature decompression from >6.8 kbar/685 °C to 3.6-4.8 kbar/640-660 °C was inferred. P-T evolution of the north Hengshan terrane is characterized by two discrete (post-peak and post-cooling) decompression processes. Zircon U-Pb dating of amphibolite yields a metamorphic age of 1868 ± 15 Ma, which is interpreted to record the timing of late amphibolization. Synthesized metamorphic P-T-t data in the Hengshan-Wutai area indicate a complicated tectonic evolution that includes an older collisional orogeny at ca. 1.95 Ga and a younger metamorphism at ca. 1.85 Ga. The post-cooling decompression path of the amphibolite may reflect final exhumation of the north Hengshan terrane through the late deformation/metamorphism of the Zhujiafang shear zone. [ABSTRACT FROM AUTHOR]

In accretionary convergent margins, the subduction interface is formed by a lower plate décollement above which sediments are scraped off and incorporated into the accretionary wedge. During subduction, the basal décollement is typically located within or at the base of the sedimentary pile. However, the transition to collision implies the accretion of the lower plate continental crust and deformation of its inherited rifted margin architecture. During this stage, the basal décollement may remain confined to shallow structural levels as during subduction or re-localize into the lower plate middle-lower crust. Modes and timing of such re-localization are still poorly understood. We present cases from the Zagros, Apennines, Oman, and Taiwan belts, all of which involve a former rifted margin and point to a marked influence of inherited rift-related structures on the décollement re-localization. A deep décollement level occurs in the outer sectors of all of these belts, i.e., in the zone involving the proximal domain of pre-orogenic rift systems. Older--and shallower--décollement levels are preserved in the upper and inner zones of the tectonic pile, which include the base of the sedimentary cover of the distal portions of the former rifted margins. We propose that thinning of the ductile middle crust in the necking domains during rifting, and its complete removal in the hyperextended domains, hampered the development of deep-seated décollements during the inception of shortening. Progressive orogenic involvement of the proximal rift domains, where the ductile middle crust was preserved upon rifting, favors its reactivation as a décollement in the frontal portion of the thrust system. Such décollement eventually links to the main subduction interface, favoring underplating and the upward motion of internal metamorphic units, leading to their final emplacement onto the previously developed tectonic stack. [ABSTRACT FROM AUTHOR]

The comprehensive investigation of orogenic-related litho-structural assemblages, metamorphism, and geochronology in early Precambrian orogens can help us better understand the features of plate tectonics in early Earth. The Paleoproterozoic Aketashitage orogenic belt is located at a key position in northwestern China and connects the North China craton, Tarim craton, Altaids orogen, and Tethys orogen. Garnet-bearing mafic and paragneissic granulite occur as interlayers or blocks preserved within paragneissic matrix, and two to three generations of metamorphic mineral assemblages were identified. Geothermobarometry and pseudosection modeling yielded clockwise metamorphic P-T paths passing from 7.5-8.6 kbar/575-715 °C (M1) through 7.4-12.2 kbar/715-895 °C (M2) and finally to 5.2-7.3 kbar/710-800 °C (M3) for the mafic and paragneissic granulite as well as amphibolite, which is indicative of metamorphic features of subduction/collision zones. Peak metamorphic P-T conditions of all the samples lie in the medium P/T facies series, suggesting that the thermal gradient (~20-31 °C/km) of this Paleoproterozoic orogenic belt was obviously higher than most of the Phanerozoic subduction zones. Secondary ion mass spectrometry (SIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb dating of zircon and monazite yielded metamorphic ages of ca. 1.98-1.96 Ga in the eastern part of the orogen, ca. 1.86-1.85 Ga in the western part, and a maximum depositional age of ca. 2.06 Ga for paragneiss. Compared with previous studies, the Aketashitage orogen is composed of unordered juxtaposition of diverse, subduction-related tectono-metamorphic blocks with different protoliths, metamorphic grades, and ages preserved within the paragneissic matrix deposited in the Paleoproterozoic, which is highly similar to Phanerozoic mélange. A Paleoproterozoic subduction-metamorphic-exhumation-accretionary process was deciphered, similar to that found in accretionary/orogenic wedge in Phanerozoic orogens. The juxtaposition of diverse, subduction-related tectonic blocks with contrasting ages and metamorphic features can serve as a marker of early Precambrian orogens and plate tectonics. [ABSTRACT FROM AUTHOR]

Extensive angular unconformities that occur across the Tianshan Orogen provide insight into the Paleozoic tectonic evolution of the southwestern Central Asian Orogenic Belt. This study presents new geochronological and geochemical data to unravel the age and origin of a pronounced angular unconformity in the Baluntai domain of the Chinese Western Tianshan. The angular unconformity separates a ductilely deformed metamorphic complex from overlying unmetamorphosed sequences. Detrital zircon age spectra suggest that the sedimentary samples received detritus from both the Precambrian basement and contemporaneous igneous rocks in the Central Tianshan and Yili blocks. The youngest detrital and metamorphic zircons in metasediments from the metamorphic complex reveal that their protoliths were deposited at ca. 419 Ma, and subsequently overprinted by metamorphism and ductile deformation at ca. 409 Ma. Along with ca. 532-407 Ma metavolcanics and foliated intrusions, the metamorphic complex constitutes a diachronous stratigraphic unit deposited during early Cambrian to late Silurian times, rather than a Precambrian basement as previously thought. The youngest detrital zircon age data from siliciclastic samples immediately above the angular unconformity constrain their deposition to ca. 407-397 Ma. In conjunction with a ca. 406 Ma rhyolitic tuff in the lower terrestrial siliciclastic unit and an early Carboniferous paleontological age of carbonates in the upper shallow-marine unit, it is suggested that the overlying unmetamorphosed sequences were accumulated during the early Devonian to the early Carboniferous. Thus, the angular unconformity in the Baluntai domain was formed between ca. 409 and 407 Ma. New and already published geochemical data reveal that the early Paleozoic (ca. 530-410 Ma) and late Paleozoic (ca. 410-320 Ma) igneous rocks are mainly medium- to high-K calc-alkaline I-type granitoids, which exhibit typical subduction-related geochemical signatures despite contrasting structural features. These geochronological and geochemical results are in accordance with those of similar early Devonian angular unconformities and Paleozoic igneous rocks in the southern Yili Block. The early Paleozoic igneous rocks display highly scattered zircon εHf(t) values, while the late Paleozoic rocks show a progressively increasing trend toward more positive values, attributed to slab rollback of the northward subducting South Tianshan Ocean under the Yili and Central Tianshan blocks. Thus, we propose that the formation of the angular unconformity was caused by tectonic conversion from contraction to extension due to slab rollback during the early Devonian. This study highlights the significance of geological and geochronological investigations of angular unconformities in the context of associated episodic magmatism and slab behavior in order to unravel distinct tectonic processes in the long-lived accretionary evolution of the Chinese Western Tianshan. [ABSTRACT FROM AUTHOR]

The integration of biostratigraphy, strontium isotope stratigraphy, and magnetostratigraphy allowed for the precise dating of the >3.0kmthick marine to nonmarine foreland sedimentary succession within the Dowlatabad growth syncline along the Frontal Fars arc in the Zagros Fold Belt that extends from eastern Turkey to southern Iran. This area was the missing link to complete the dating of syntectonic deposits in the Fars arc and quantify the migration of sedimentary belts as well as the propagation of folding across the entire Mesopotamian foreland basin. Both are essential for defining the interplay of basin evolution and sequence of folding. Deposition of the foreland marine marls in the Mishan Formation started at ca. 11.5 Ma. The transition to a nonmarine basin infill occurred at 4.9 Ma by the progradation of thick fluvial deposits of the Aghajari Formation with a fast accumulation rate of 63 cm/k.y. The beginning of growth strata deposition and thus the onset of folding in the Dowlatabad syncline is dated at 4.65 Ma. The first appearance of carbonate conglomerates sourced from the Guri limestone at 2.8 Ma marked the progressive dismantling of the nearby growing anticlines. The tectonic deformation in the front of the Fars arc was active for at least 2.85 m.y. and ceased at 1.8 Ma before the deposition of the discordant and slightly folded Bakhtyari conglomerates characterized by a clast composition derived from the Zagros hinterland. The compilation of magnetostratigraphic ages reveals that both the migration of the AghajariBakhtyari sedimentary belts and the propagation of the folding front was insequence toward the foreland at a rate close to 20 mm/yr in the Fars arc and 15 mm/ yr in the Lurestan arc, in the last 20 m.y. These high rates of folding propagation are about one order of magnitude larger than age equivalent shortening rates (~4 mm/yr in Fars arc and ~2 mm/yr in Lurestan arc) and thus imply an efficient detachment level at the base of the deformed Arabian sedimentary cover. Numerical experiments on both the cover and basement sequences are designed to test the influence of inherited basement structures on the deformation propagation within the cover sequence, providing clues on the partly coeval insequence deformation of the Zagros Simply Folded Belt and the local outofsequence Mountain Frontal Fault system as illustrated by regional and local geology. [ABSTRACT FROM AUTHOR]

In worldwide orogenic belts, crustal-scale ductile shear zones are important tectonic channels along which the orogenic root (i.e., high-grade metamorphic lower-crustal rocks) commonly experienced a relatively quick exhumation or uplift process. However, their tectonic nature and geodynamic processes are poorly constrained. In the Trans-- North China orogen, the crustal-scale Zhujiafang ductile shear zone represents a major tectonic boundary separating the upper and lower crusts of the orogen. Its tectonic nature, structural features, and timing provide vital information into understanding this issue. Detailed field observations showed that the Zhujiafang ductile shear zone experienced polyphase deformation. Variable macro- and microscopic kinematic indicators are extensively preserved in the highly sheared tonalite- trondhjemite-granodiorite (TTG) and supracrustal rock assemblages and indicate an obvious dextral strike-slip and dip-slip sense of shear. Electron backscattered diffraction (EBSD) was utilized to further determine the crystallographic preferred orientation (CPO) of typical rock-forming minerals, including hornblende, quartz, and feldspar. EBSD results indicate that the hornblendes are characterized by (100) <001> and (110) <001> slip systems, whereas quartz grains are dominated by prism and prism slip systems, suggesting an approximate shear condition of 650-700 °C. This result is consistent with traditional thermobarometry pressure- temperature calculations implemented on the same mineral assemblages. Combined with previously reported metamorphic data in the Trans--North China orogen, we suggest that the Zhujiafang supracrustal rocks were initially buried down to ~30 km depth, where high differential stress triggered the largescale ductile shear between the upper and lower crusts. The high-grade lower-crustal rocks were consequently exhumed upwards along the shear zone, synchronous with extensive isothermal decompression metamorphism. The timing of peak collision-related crustal thickening was further constrained by the ca. 1930 Ma metamorphic zircon ages, whereas a subsequent exhumation event was manifested by ca. 1860 Ma syntectonic granitic veins and the available Ar-Ar ages of the region. The Zhujiafang ductile shear zone thus essentially record an integrated geodynamic process of initial collision, crustal thickening, and exhumation involved in formation of the Trans--North China orogen at 1.9--1.8 Ga. [ABSTRACT FROM AUTHOR]

Subduction of Indian continental lithosphere during the Asia-India collision played an important role in the formation and evolution of the Himalaya-Tibetan orogen. However, the geometry of early Indian continental subduction remains debated. Given that the Indian continent is characterized by enriched isotope ratios (87Sr/86Sr > 0.730, εNd(t) < -10), relative to those in subducted oceanic materials (87Sr/86Sr < 0.704, εNd(t) ≈ +8), changes in the composition of magmatic rocks with time, in particular their radiogenic isotope ratios, is used to constrain the timing and nature of continental subduction. This study reports the field relations, zircon U-Pb ages and geochemical composition of a syn-collisional batholith that crosscuts the central Indus-Yarlung Zangbu suture in the Saga area of southern Tibet. Zircon U/Pb ages for the batholith mainly range from 50 to 46 Ma. Samples from the Lopu Range batholith have enriched zircon Hf (εHf(t) = -0.4 to -8.6) and whole rock 87Sr/86Sri = 0.7094-0.7121 and εNd(t) = -7.3 to -9.8, suggesting that they were derived from a mixture of juvenile Gangdese and isotopically enriched Indian crustal materials. This result indicates that subduction of Indian crustal rocks occurred before 50 Ma in the central Himalaya. The geochemical composition and distribution of high volume ca. 51 Ma magmatism in the Gangdese belt, combined with thermal models of the subduction zone, suggests a steepening of the subducted Indian continental lithosphere occurred between the onset of India-Asia collision (59 Ma) and 46 Ma in the central-eastern Himalaya. [ABSTRACT FROM AUTHOR]

The Conception and St. John's Groups of southeastern Newfoundland contain some of the oldest known fossils of the Ediacaran macrobiota. The Mistaken Point Ecological Reserve UNESCO World Heritage Site is an internationally recognized locality for such fossils and hosts early evidence for both total group metazoan body fossils and metazoan-style locomotion. The Mistaken Point Ecological Reserve sedimentary succession includes ~1500 m of fossil-bearing strata containing numerous dateable volcanogenic horizons, and therefore offers a crucial window into the rise and diversification of early animals. Here we present six stratigraphically coherent radioisotopic ages derived from zircons from volcanic tuffites of the Conception and St. John's Groups at Mistaken Point Ecological Reserve. The oldest architecturally complex macrofossils, from the upper Drook Formation, have an age of 574.17 ± 0.66 Ma (including tracer calibration and decay constant uncertainties). The youngest rangeomorph fossils from Mistaken Point Ecological Reserve, in the Fermeuse Formation, have a maximum age of 564.13 ± 0.65 Ma. Fossils of the famous "E" Surface are confirmed to be 565.00 ± 0.64 Ma, while exceptionally preserved specimens on the "Brasier" Surface in the Briscal Formation are dated at 567.63 ± 0.66 Ma. We use our new ages to construct an age-depth model for the sedimentary succession, constrain sedimentary accumulation rates, and convert stratigraphic fossil ranges into the time domain to facilitate integration with time-calibrated data from other successions. Combining this age model with compiled stratigraphic ranges for all named macrofossils within the Mistaken Point Ecological Reserve succession, spanning 76 discrete fossil-bearing horizons, enables recognition and interrogation of potential evolutionary signals. Peak taxonomic diversity is recognized within the Mistaken Point and Trepassey Formations, and uniterminal rangeomorphs with undisplayed branching architecture appear several million years before multiterminal, displayed forms. Together, our combined stratigraphic, paleontological, and geochronological approach offers a holistic, time-calibrated record of evolution during the mid-late Ediacaran Period and a framework within which to consider other geochemical, environmental, and evolutionary data sets. [ABSTRACT FROM AUTHOR]

As a main part of the North China craton collage system, the tectonic origin of the Bainaimiao arc terrane is still hotly debated, especially its relationship with the North China craton. Thus, we report on a field-based petrological and zircon U-Pb-Hf isotopic study of (meta-)sedimentary and volcanic rocks from the Bainaimiao arc terrane and northern margin of the North China craton in the Damao region. The lower and middle successions of the Bayan Obo Group from the northern North China craton, including the Dulahala, Jianshan, Halahuogete, and Bilute Formations, were deposited ca. 1.81-1.35 Ga and show age peaks at 1.85, 1.90, 2.0, and 2.5 Ga. This age pattern is in accordance with the coeval and extensively outcropped metamorphosed basement rocks of the northern North China craton. The upper succession, including the Bayinbaolage and Hujiertu Formations, deposited ca. 1.19-0.92 Ga, shows age peaks at ca. 1.35, 1.57, and 1.69 Ga, and sediments were derived from coeval rift-related magmatism characterized by a significant increase in positive εHf(t) values in detrital zircons. Thus, the Bayan Obo Group can be subdivided into North China craton basement-derived sediments and Mesoproterozoic to Neoproterozoic rift-derived sediments, and the change of the provenance was probably caused by the ca. 1.35-1.30 Ga rifting event related to the breakup of the Nuna supercontinent. In the Bainaimiao arc terrane, zircon U-Pb geochronological results of (meta-)sedimentary rocks indicate they were formed in the Neoproterozoic and Paleozoic and have mainly late Paleoproterozoic to Neoproterozoic ages (ca. 0.6-0.9, 1.1-1.2, 1.4-1.7, and 1.8-2.0 Ga) with peaks at ca. 0.86, 0.96, 1.15, 1.47, 1.66, 1.75, 1.80-1.87, and 1.94 Ga. The abundant Neoproterozoic ages for the Bainaimiao arc terrane detrital zircons are distinct from the scarcity of these ages in the northern North China craton, southern Siberia (age peaks at ca. 1.87, 1.92, and 2.0 Ga), and Mongolia (age peak at ca. 0.62, 0.83, 0.93, 1.84, and 2.0 Ga), but they are similar to the features of the Tarim craton (age peaks at ca. 0.62, 0.85, 1.15, 1.47, 1.66, 1.75-1.80, 1.85, and 1.94 Ga). Together with the recognition of a possible preexisting Proterozoic basement (ca. 0.7-2.0 Ga) in the Bainaimiao arc terrane, as evidenced by the Hf model ages of continental arc magmatism in the Bainaimiao arc terrane, this late Paleoproterozoic to Neoproterozoic detritus probably resulted from local recycled sedimentation, and the preexisting Bainaimiao arc terrane basement rifted away from a continent with Tarim craton affinity. The rifting probably occurred between ca. 0.6 and 0.52 Ga, because the reworking of the Precambrian rocks in the North China collage system occurred between ca. 520 and 230 Ma. [ABSTRACT FROM AUTHOR]

The Himalayan Orogen is a typical continental collision orogenic belt that formed during India-Asia collision. The orogeny involved a transition in tectonic regime, which led to E-W-trending extension during the middle Miocene. At the same time, widespread post-collisional adakitic magmatism occurred in southern Tibet, particularly in the Lhasa Terrane, and these rocks provide a valuable record of the collisional geodynamic processes within the lithosphere. Few studies have focused on the middle Miocene adakitic rocks of the Himalayan Orogen, so further research is required to constrain their origin and geodynamics. This study presents new geochronological and geochemical data, including the whole rock Sr-Nd and zircon Hf isotopic compositions from the Mayum pluton in the Mayum-Gong Tso area, Northern Himalaya. Zircon U-Pb ages show that the Mayum granodiorite porphyries crystallized at 16.67 ± 0.14 Ma (mean square weighted deviation [MSWD] = 0.82, n = 29) and 16.68 ± 0.16 Ma (MSWD = 1.13, n = 28), indicating that they formed during the middle Miocene. The major and trace element characteristics are as follows: SiO2 = 65.79-67.31 wt%, Al2O3 = 15.28-16.00 wt%, MgO = 1.77-1.89 wt%, Y = 12.0-13.5 ppm, Yb = 0.11-0.99 ppm, Sr = 719-822 ppm, (La/Yb)N = 21.89-27.02, Sr/Y = 56.1-65.5, and the rocks have weak negative Eu anomalies (Eu/Eu* = 0.76-0.85), indicating that they are adakitic rocks and show high Sr-Ba granite affinity. The Mayum granodiorite porphyries have high K2O contents (3.42-3.65 wt%), Na2O (3.76-4.04 wt%), and K2O/Na2O ratios of 0.91-0.95, indicating a high-K calc-alkaline affinity. All samples are enriched in large-ion lithophile elements and depleted in high field strength elements. The initial 87Sr/86Sr ratios are 0.709262-0.709327 and εNd(t) values are -6.36 to -7.07, which correspond to two-stage Nd model ages [TDM2(Nd)] of 1405-1348 Ma. In situ zircon Lu-Hf isotopic compositions are variable, with 176Hf/177Hf ratios of 0.2823845-0.282824, εHf(t) values of -13.37 to +2.17, and two-stage Hf model ages [TDM2(Hf)] of 1704-841 Ma. These geochemical and Sr-Nd-Hf isotopic characteristics indicate that the Mayum granodiorite porphyries may be derived from partial melting of the subducted Indian thickened ancient mafic lower crust (~92%) mixed with depleted mid-ocean ridge basalt mantle material input (~8%) slightly. Crust-mantle interaction was induced by asthenospheric upwelling followed by the underplating of the Himalayan lower crust beneath Southern Tibet during the middle Miocene in response to significant changes in the geodynamics of the India-Asia collisional orogen. These deep geodynamic processes reflect the break-off or rollback of the subducted Indian continental slab, which caused asthenospheric upwelling, the input of juvenile heat, and the addition of depleted mantle material. This study indicates that the middle Miocene Himalayan adakitic rocks, which include the Miocene Yardoi two-mica granite, Gyangzê granite porphyry dike, Bendui two-mica granite, Langkazi enclave, Kuday dacitic dike, Lasa pluton, and Mayum pluton, form a belt of adakitic rocks in the Northern Himalaya to the south of the Indus-Tsangpo Suture Zone. These adakitic rocks have similar temporal and spatial distributions, geochemical features, and Sr-Nd isotopic compositions, indicating that their petrogenesis and geodynamic settings were similar. At that time there was widespread initiation of N-S-trending rifting, exhumation of central Himalayan eclogites, and large changes in the compositions of Himalayan leucogranites in the Himalayan Orogen. These can be attributed to significant changes in the characteristics of the subducted Indian continental lithosphere during the middle Miocene. The middle Miocene Himalayan adakitic rocks provide valuable insights into this transition in the tectonic regime and deep geodynamic processes. [ABSTRACT FROM AUTHOR]

The nature of source rocks and the pressure-temperature-hydration (P-T-H2O) condition are the two main factors that control the geochemical properties of granites. Therefore, the evolution of P-T-H2O conditions can be used to deduce the tectonic setting of granites. In this paper, we report on three Paleozoic granite plutons along the Shangdan suture that revealed increasing melting temperature and decreasing pressure from 437 to 403 Ma, suggesting a crustal thinning process. The Tieyupu granodiorites (437 ± 4 Ma) display Na-rich adakite affinity, i.e., SiO2 = 69.1-70.1 wt%, Na2O/K2O = 1.9-2.26, positive zircon εHf(t) values (+4.29 to +12.04), and high Sr/Y (137-160) and Y/Yb (9.89-10.25) ratios, implying a garnet-rich residue in their source. In combination with moderate zircon saturation temperatures (814-822 °C), we infer that the Tieyupu granodiorites were formed by melting of Neoproterozoic metabasites under high-pressure (>1.5 GPa) and moderate-temperature (HP-MT) conditions. The Liangchahe granodiorites (415 ± 8 Ma) also display Na-rich adakite affinity, i.e., higher Na2O/K2O (2.16-3.11) and lower Sr/Y (77-88) ratios, and higher zircon saturation temperatures (854-874 °C), and they are interpreted to have been derived from melting of metabasites under moderate-pressure (>1.0 GPa) and high-temperature (MP-HT) conditions. Their variable zircon εHf(t) values (-14.97 to +9.80) and the existence of zircon xenocrysts suggest that the primitive adakitic melts were assimilated by evolved crustal components. The Yaogou monzogranites (403 ± 4 Ma) have the highest K2O/Na2O (0.81-1.00) ratios and total rare earth element (ΣREE; 105-191 ppm) contents, lowest Sr/Y (14-43) ratios, positive zircon εHf(t) values (+6.79 to +12.22), and highest zircon saturation temperatures (891-973 °C), showing they were formed by high-temperature melting of intermediate rocks under low-pressure conditions (<1.0 GPa). The evolution of P-T conditions revealed by these three granites suggests that crustal growth and differentiation were related to gradual extensional and melting of mafic protoliths in the orogenic belt. [ABSTRACT FROM AUTHOR]

The evolution of the northern margin of Gondwana, especially to the north of India and Australia, remains enigmatic. Much controversy concerns when and where the Tarim craton was amalgamated with northern Gondwana due to final closure of the North and South Altyn Oceans (two branches of the Proto-Tethys Ocean between southeastern Tarim and northern Gondwana). This study addressed these issues through systematic field-based zircon U-Pb dating and Hf-isotope analyses of early Paleozoic sedimentary rocks in the Altyn Tagh orogen. New dating results reveal depositional ages from ca. 494 to 426 Ma. Provenance tracing indicates the ca. 494-477 Ma samples were dominantly sourced from local Altyn Tagh areas to the south of the North Altyn Ocean, whereas the ca. 465-449 Ma samples are characterized by a significant increase in ca. 2.7-2.4 Ga, 2.0-1.7 Ga, and 840-780 Ma detrital zircons, indicating an augmented supply of detritus from the Tarim craton to the north of the North Altyn Ocean. This change indicates a major provenance shift from a single to multiple source regions between ca. 477 and 465 Ma, marking the timing of the final closure of the North Altyn Ocean. Zircon U-Pb and Hf-isotopic data from the ca. 444-426 Ma samples resemble those from the ca. 465-449 Ma samples, suggesting local sediment recycling related to a postcollisional regime. Considering the South Altyn Ocean and other branches of the Proto-Tethys Ocean, we infer that the entire Proto-Tethys Ocean might have been progressively closed at ca. 500-420 Ma, leading to the amalgamation of most East Asian blocks with northern Gondwana. Detrital zircon U-Pb and Hf-isotope comparisons indicate that Tarim shared a North Indian affinity with many East Asian terranes (such as North Qilian, North Qinling, South China, Indochina, South Qiangtang, etc.), rather than with Arabia-Iran or other terranes (e.g., Lhasa and Sibumasu) that were adjacent to western Australia along the northern margin of Gondwana. [ABSTRACT FROM AUTHOR]

A correction is presented to the article "Mineralization and genesis of the orogenic gold system in the Kalamaili area, East Junggar, Xinjiang, northwestern China" which appeared in the last issue of the periodical.

The Central Asian Orogenic Belt has long been considered the largest Phanerozoic accretionary orogen in the world; it developed through the subduction and final closure of the Paleo-Asian Ocean. However, the architecture and duration of the accretionary orogenesis of the Central Asian Orogenic Belt are still controversial despite decades of investigation. In this study, we present field, compositional, and stratigraphically controlled detrital zircon geochronological data for late Paleozoic sedimentary rocks from the Alxa Tectonic Belt to constrain their provenance, tectonic setting, and the overall tectonic configuration of the southern Central Asian Orogenic Belt. A Devonian sample yields a unimodal age peak (ca. 424 Ma) and broad late Mesoproterozoic ages. A Carboniferous sample has Early Silurian (ca. 438 Ma) and Late Devonian (ca. 382 Ma) peaks along with Neoproterozoic to Archean ages. The Permian samples are dominated by Ordovician-Devonian and Carboniferous-Permian ages. They yield maximum depositional ages ranging from ca. 291 Ma to 248 Ma and contain abundant zircon ages that are close to their depositional ages. These data reveal Ordovician-Silurian and Carboniferous-Permian magmatic flare-ups separated by a Devonian magmatic lull in the southern Central Asian Orogenic Belt. The arc terranes in southern Mongolia, central Beishan, and northern Alxa provided major detritus for the late Paleozoic sediments. An abrupt shift of zircon eHf(t) values at ca. 400 Ma reveals significant late Paleozoic crustal growth and excludes southern Alxa as a source. Oceanic basins prevented detritus from southern Alxa from reaching northern Alxa during Permian-Early Triassic time. A geological and provenance comparison of Permian basins in the southern Central Asian Orogenic Belt reveals the existence of two separate forearcs ascribed to bipolar subduction of the Paleo-Asian Ocean. Combined with recent paleomagnetic data, this leads us to advocate for an archipelago-style accretionary process induced by subduction retreat for the late Paleozoic tectonic evolution of the southern Central Asian Orogenic Belt, which continued into Late Permian-Early Triassic. [ABSTRACT FROM AUTHOR]

The Olympic-Wallowa lineament (OWL) is an alignment of geologic structures extending nearly 650 km across the Pacific Northwest (PNW) and has been a controversial feature since it was first described nearly 80 years ago. It extends from the Olympic Peninsula-Puget Sound area of western Washington to the Wallowa Mountains of northeast Oregon, crossing through the Columbia Basin and forming the southern boundary of the Pasco and Walla Walla Basins. Within the Columbia Basin, the OWL is a wide zone that aligns with >250 km of folds and faults that are part of the Yakima fold and thrust belt. Although the OWL is recognized to be active with deformation continuing from the Miocene to Recent, there are two competing end-member interpretations on the style of deformation: that of a major through-going, dextral strike-slip or oblique-slip fault and that of a series of thrusted anticlines. We focus on that portion of the OWL in the central Columbia Basin that extends from the Walla Walla River in northeast Oregon to Rattlesnake Mountain in central Washington. East of Wallula Gap, the OWL is expressed as the Wallula fault zone along the north flank of the Horse Heaven Hills anticline and the south fork of the Walla Walla River, while west of Wallula Gap, there are three structural alignments: the Rattlesnake-Wallula structural alignment, the Horse Heaven Hills-Badlands structural alignment, and the Horn Rapids-Badger Coulee structural alignment. Except for the Horse Heaven Hills anticline, the structural trends west of Wallula Gap mainly consist of a series of doubly plunging, northeast-verging anticlines. We demonstrate that the OWL consists of northverging, thrusted anticlines whose orientation and structure are controlled by a basement fault without any evidence of strike-slip or significant oblique-slip fault movement since at least the middle Miocene. The difference in structural deformation of the portion of the OWL to the east of Wallula Gap versus that to the west appears to be controlled by the thickness of pre-basalt sediment within the central Columbia Basin and structures therein. [ABSTRACT FROM AUTHOR]

In this study, we use published geologic maps and cross-sections to construct a threedimensional geologic model of major shear zones that make up the Himalayan orogenic wedge. The model incorporates microseismicity, megathrust coupling, and various derivatives of the topography to address several questions regarding observed crustal strain patterns and how they are expressed in the landscape. These questions include: (1) How does vertical thickening vary along strike of the orogen? (2) What is the role of oblique convergence in contributing to along-strike thickness variations and the style of deformation? (3) How do variations in the coupling along the megathrust affect the overlying structural style? (4) Do lateral ramps exist along the megathrust? (5) What structural styles underlie and are possibly responsible for the generation of high-elevation, lowrelief landscapes? Our model shows that the orogenic core of the western and central Himalaya displays significant along-strike variation in its thickness, from ~25-26 km in the western Himalaya to ~34-42 km in the central Himalaya. The thickness of the orogenic core changes abruptly across the western bounding shear zone of the Gurla Mandhata metamorphic core complex, demonstrating a change in the style of strain there. Pressure-temperature-time results indicate that the thickness of the orogenic core at 37 Ma is 17 km. Assuming this is constant along strike from 81°E to 85°E indicates that, the western and central Nepal Himalaya have been thickened by 0.5 and 1-1.5 times, respectively. West of Gurla Mandhata the orogenic core is significantly thinner and underlies a large 11,000 km2 Neogene basin (Zhada). A broad, thick orogenic core associated with thrust duplexing is collocated with an 8500 km2 high-elevation, low-relief surface in the Mugu-Dolpa region of west Nepal. We propose that these results can be explained by oblique convergence along a megathrust with an along-strike and down-dip heterogeneous coupling pattern influenced by frontal and oblique ramps along the megathrust. [ABSTRACT FROM AUTHOR]

The Central Asian Orogenic Belt, as the largest accretionary orogen on Earth, is an ideal candidate to study the geodynamics of convergent plate boundaries through a prolonged period. The evolution of this orogen has been explained by different tectonic models, which incorporated one, or a combination, of the following mechanisms: lateral stacking of arc systems along major shear zones, arc amalgamation, oroclinal bending, and trench migration. Here we elucidate major mechanisms responsible for the tectonic evolution of the Central Asian Orogenic Belt, focusing on the Chinese Tianshan Orogen in the southern Central Asian Orogenic Belt. Structural observations from the ~50-km-long Gangou section show evidence of polyphase deformation. The earliest episode of orogenparallel sinistral shearing, constrained to the Early Devonian (ca. 399 Ma) by syn-deformational intrusions, was possibly controlled by oblique subduction. This was followed by an episode of ~NE-SW contractional deformation, dated at ca. 356 Ma (40Ar/39Ar age of syn-deformational hornblende), and likely associated with an episode of trench advance. The third stages of deformation during the latest Carboniferous and Permian involved ~NE-SW contraction, orogen-parallel extension, and dextral transpression. Our new geochronological data constrain the timing of orogen-parallel extension to ca. 303-293 Ma, and confirm that dextral activation along shear zones occurred during the Permian. The results highlight the role of trench migration, oblique tectonics, and syn-collisional orogen-parallel extension in the build-up of the Central Asian Orogenic Belt, and contribute to the pre-collisional reconstruction of this orogenic system. [ABSTRACT FROM AUTHOR]

Much of the global picture of crustal evolution has been constructed using zircon. While this has revealed a rich and complex history, this view is necessarily incomplete because of the lithology-specific affinity of zircon and the high temperatures needed to reset the U-Pb and Lu-Hf systems inherent within it. Here we use a five mineral, multi-isotope system approach to compare the record of crustal evolution recorded by zircon versus the picture provided by monazite, titanite, apatite, and rutile from the Yong-Ding and Luan rivers, northern China. These other minerals sample more diverse lithologies and temperature-pressure conditions that reflect additional tectonothermal events to those recorded solely by zircon. Zircon from both studied rivers predominantly reflects magmatic features, yielding age peaks at 2.6-2.3, 2.0-1.8, and 0.38-0.13 Ga, corresponding to the major magmatic events in their catchments. However, the detrital zircon record from both catchments fails to record and detail several important tectonothermal events. Specifically, the detrital monazite U-Pb ages cluster into two Paleoproterozoic peaks of ca. 1.95 and 1.85 Ga, while detrital apatite and rutile ages document unimodal and protracted U-Pb age peaks at 1.9-1.6 Ga. The different U-Pb closure temperatures of monazite, apatite, and rutile likely record two metamorphic events and the subsequent cooling history--key details that are absent from or obscured in the zircon record. The Phanerozoic mineral U-Th-Pb ages correspond to multiple magmatic events between 0.40 and 0.24 Ga and subsequent 0.24- 0.20 Ga metamorphism. The 0.60-0.25 Ga rutile U-Pb ages along with 0.33-0.24 Ga U-Pb ages in some zircon grains with radiogenic Hf isotope compositions from the Luan River do not match the geological records in the North China Craton, but instead reflect the protracted subduction-accretionary history of the Central Asian Orogenic Belt. In addition to their U-Th-Pb ages, Nd model ages of monazite, titanite, and apatite, plus zircon Hf model ages provide additional constraints on regional crustal evolution. The Nd model age information is blurred by the fact that the relationship between the Sm/Nd of these minerals and their former host rocks is not precisely known. Taken at face value, the monazite Nd model ages have two Neoarchean peaks at 2.9-2.7 and ca. 2.5 Ga, that may correspond to two crustal growth episodes, while the titanite Nd model ages with predominant peaks at 2.2-1.8 and 1.5-1.3 Ga broadly correspond with those derived from the whole-rock analyses of the wide spread Phanerozoic granitoids, and hence record extensive crustal reworking. In contrast, the zircon Hf model ages are strongly skewed to a 2.9-2.7 Ga period and fail to record the post-Archean evolution of this region. These data highlight the power of integrating the U-Th-Pb age and Lu-Hf/Sm-Nd isotope compositions of multiple detrital minerals, with a broad range in geochemical behavior and closure temperatures, to gain a more complete understanding of tectonothermal history and crustal evolution than zircon alone. [ABSTRACT FROM AUTHOR]

A clear insight of the Indo-Burma amalgamation is required for an understanding of the complex processes of the Indo-Asian collision. Where and when the northeastern part of the Indian subcontinent amalgamated with the Burma plate has not yet been well defined. In order to better understand the tectonic affiliations and evolution of the Indo- Burma Ranges (IBR), we report new detrital zircon U-Pb and Hf isotope data of nine sandstone/siltstone samples (late Miocene- Pliocene) from the Chittagong-Tripura Fold Belt, Bangladesh, which is in the Neogene belt of the IBR. Our results show that the zircon ages of all nine samples have three peaks at <200 Ma, ca. 500 Ma, and 800-1000 Ma. The percentage of young (<200 Ma) zircons increase from the Boka Bil Formation (6.9% to 15.2%) to the Tipam Formation (20.3% to 27.6%). The εHf(t) of most young zircons from the Boka Bil Formation suggest derivation from the Gangdese arc, whereas those from the Tipam Formation show an affinity with the Burma magmatic arc. Therefore, we suggest that the increase of young zircons in the Tipam Formation came from the Burma plate. Comparing provenances of the Paleogene and Neogene belts of the IBR, we suggest that they belong to the Burma plate and Indian subcontinent, respectively, and that the Kaladan Fault, separating these two belts, is the suture between the Indian and Burma terranes. The time of amalgamation is constrained by the ca. 2.5-3.9 Ma boundary between the Boka Bil and Tipam formations, and the eastern Indian promontory likely collided with the Burma plate ca. 11-12 Ma. [ABSTRACT FROM AUTHOR]

Identification of subduction to post-collisional tectonic transitions is critical to the study of orogenic belts. To characterize such a transition in the Tianshan Orogenic Belt, a systematic study was conducted on the late Carboniferous (305-301 Ma) Hongshankou dolerite and Dikan'er basalt of Eastern Tianshan. The Hongshankou dolerites have relatively high Ti and Nb contents, akin to Nb-enriched arc basalts. Based on the Nb/La ratios, these dolerites can be divided into low- Nb/La (0.35-0.40) and high-Nb/La (0.67-1.4) groups, which were likely derived respectively from slab melt-metasomatized mantle wedge and a mixed mantle source involving depleted super-slab and enriched sub-slab asthenospheric components. Like the low-Nb/La dolerites, the Dikan'er basalts possess low Nb/La (0.42-0.46) ratios, suggesting a mantle source previously modified by slab components. In addition, the Dikan'er basalts have variable Nb contents and can be grouped into normal arc basalts and Nb-rich basalts that can be attributed to a common mantle source with different degrees of mantle melting, as demonstrated by the positive correlations of La/Sm with La and Nb. By integrating available data, two late Carboniferous belts of Nb-enriched mafic magmatism are recognized in the Eastern Tianshan, with one in the Yamansu arc (336-301 Ma) and the other in the Bogda Mountains (305-301 Ma). The former is characterized by low Nb/La (<0.6) ratios, reflecting derivation from mantle metasomatized by slab-derived melt during a subduction process; the latter exhibits an abrupt Nb/La increase from 0.6 to 1.4, indicating significant input of sub-slab asthenospheric mantle that was probably induced by slab break off. Accordingly, we propose that the tectonic transition from subduction to post-collision in the Eastern Tianshan occurred in the latest Carboniferous (305-301 Ma) and was marked by the abrupt input of deep and enriched asthenospheric mantle. [ABSTRACT FROM AUTHOR]

Late Paleozoic large-scale transcurrent tectonics and synkinematic intrusions are prominent features in the Eastern Tianshan segment of the southwestern Central Asian Orogenic Belt. However, the spatial and temporal relationship between synkinematic intrusions and crustal-scale shear zones remains unclear. Here we report petrology, geochemistry, and geochronology of the Qiziltag pluton associated with the Kanggur- Huangshan Shear Zone (KHSZ) with a view to characterize the spatial and temporal relationship between synkinematic intrusions and large-scale transcurrent shearing. Field relations and zircon U-Pb ages indicate that the Qiziltag pluton was formed through two stages of magmatism, with earlier stage granitoids (gneissic biotite granite: 288.9 ± 1.9 Ma, biotite monzogranite: 291.5 ± 1.7 Ma, Kfeldspar granite: 287.9 ± 3.1 Ma), and later stage bimodal intrusions (biotite quartz monzonite: 278.5 ± 1.8 Ma, gabbro: 278.1 ± 2.3 Ma). The earlier stage granitoids are high- K calc-alkaline, enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs; e.g., Rb, Th, and U), and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). Combined with their depleted isotopic compositions (Nd(t) = +6.29 to +7.48) and juvenile model ages (TDM2 = 450–610 Ma), we infer that the granitoids were derived from juvenile lower crust in a post-collisional tectonic transition (from compression to extension). The structural and temporal features indicate that the earlier stage (ca. 290 Ma) granitoids formed prior to the regional large-scale dextral strike slip. The later stage bimodal intrusions are dominated by biotite quartz monzonite as the felsic member and gabbro as the mafic component. The biotite quartz monzonite is high-K calc-alkaline with enriched LREEs and LILEs (e.g., Rb, Th, and U), and depleted HFSEs (e.g., Nb, Ta, and Ti), whereas the gabbro is subalkalic with depleted LREEs and HFSEs (e.g., Nb and Ta), resembling normal mid-ocean ridge basalt features. The bimodal intrusions show similar isotopic compositions (Nd(t) = +6.41 to +6.72 and Hf(t) = +9.55 to + 13.85 for biotite quartz monzonite; Nd(t) = +9.13 to +9.69 and Hf(t) = +4.80 to +14.07 for gabbro). These features suggest that the later stage (ca. 280 Ma) bimodal intrusions were derived from partial melting of depleted mantle and anatectic melting of lower crust materials induced by synchronous underplating of basaltic magma in a post-collisional extension. The structural features of the bimodal intrusions indicate that the later stage (ca. 280 Ma) magmatism was coeval with the development of the KHSZ. In conjunction with spatial and temporal evolution of magmatism and sedimentary records of Eastern Tianshan, we infer that transition between the northward closure of the North Tianshan Ocean and subsequent collision between the Central Tianshan Massif and the Qoltag Arc belt occurred at ca. 300 Ma. [ABSTRACT FROM AUTHOR]

New geochemical and geochronological data are used to characterize the geodynamic setting of metasediments, felsic orthogneisses, and eclogite and amphibolite lenses forming the Beishan complex, NW China, at the southern part of the Central Asian Orogenic Belt. The metasediments correspond compositionally to immature greywackes receiving detritus from a heterogeneous source involving a magmatic arc and a Precambrian continental crust. Metagranitoids, represented by felsic orthogneisses, show both composition of greywacke-derived granitic melt with incompatible trace element patterns similar to the host metasediments. The eclogite lenses are characterized by high Nb contents (5.34-27.3 ppm), high (Nb/La)N (>1), and low Zr/Nb ratios (<4.5), which together with variable and negative whole-rock Nd(t) (-4.3 to -10.3) and zircon Hf(t) (-5.0 to + 2.3) values indicate an origin of enriched mantle source as commonly manifested by back-arc basalts at stretched continental margins. Combined with monazite rare earth element analysis, the in situ monazite U-Pb dating of metagraywacke (880.7 ± 7.9) suggests garnet growth during a high-temperature (HT) metamorphic event. Together with U-Pb dating of zircon metamorphic rims in amphibolite (910.9 ± 3.0 Ma), this indicates that the whole crustal edifice underwent a Grenvillian-age metamorphic event. The protolith ages of the eclogite (889.3 ± 4.8 Ma) and orthogneiss (867.5 ± 1.9 Ma) suggest that basalt underplating and sediment melting were nearly coeval with this HT metamorphism. Altogether, the new data allow placing the Beishan Orogen into a Grenvillean geodynamic scenario where: (1) The late Mesoproterozoic to early Neoproterozoic was marked by deposition of the greywacke sequence coeval with formation of an early arc. (2) Subsequently, an asthenospheric upwelling generated basaltic magma underneath the thinned subcontinental mantle lithosphere that was responsible for HT metamorphism, melting of the backarc basin greywackes and intrusion of granitic magmas. These events correspond to a Peri-Rodinian supra-subduction system that differs substantially from the Neoproterozoic ophiolite sequences described in the Mongolian part of the Central Asian Orogenic Belt, thus indicating important lateral variability of supra-subduction processes along the Rodinian margin. [ABSTRACT FROM AUTHOR]

Although continental crust is characterized by the widespread occurrence of granitoids, the causal relationship between continental crust growth and granitic magmatism still remains enigmatic. While fractional crystallization of basaltic magmas (with or without crustal contamination) and partial melting of mafic lower crust are two feasible mechanisms for the production of granitoids in continental arc regions, the problem has been encountered in discriminating between the two mechanisms by whole-rock geochemistry. This can be resolved by an integrated study of zircon U-Pb ages and Hf-O isotopes together with whole-rock major-trace elements and Sr-Nd-Pb isotopes, which is illustrated for Mesozoic granitoids from the Gangdese orogen in southern Tibet. The results provide geochemical evidence for prompt reworking of the juvenile mafic arc crust in the newly accreted continental margin. The target granitoids exhibit high contents of SiO2 (65.76-70.75 wt%) and Na2O + K2O (6.38-8.15 wt%) but low contents of MgO (0.19-0.98 wt%), Fe2O3 (0.88-3.13 wt%), CaO (2.00-3.82 wt%), Ni (<5.8 ppm), and Cr (⩽10 ppm). They are enriched in large ion lithophile elements, Pb, and light rare earth elements but depleted in high field strength elements. The granitoids are relatively depleted in whole-rock Sr-Nd isotope compositions with low (87Sr/86Sr)i ratios of 0.7043-0.7048 and positive Nd(t) values of 0.5-2.6, and have relatively low 207Pb/204Pb and 208Pb/204Pb ratios at given 206Pb/204Pb ratios. Laser ablation-inductively coupled plasma-mass spectrometry and secondary ion mass spectrometry U-Pb dating on synmagmatic zircons yield ages of 77 ± 2-81 ± 1 Ma in the Late Cretaceous for their emplacement. Relict zircons have two groups of U-Pb ages in the late Mesozoic and the late Paleozoic, respectively. The whole-rock Sr-Nd isotopes in the granitoids are quite similar to those of Late Cretaceous mafic rocks in the Gangdese batholith. In addition, both synmagmatic zircons and relict zircons with Late Cretaceous U-Pb ages exhibit almost the same Hf-O isotope compositions to those of the slightly earlier mafic rocks. All these observations indicate that the granitoids were mainly derived from partial melting of the juvenile mafic arc crust. Therefore, reworking of the juvenile mafic arc crust is the mechanism for the origin of isotopically depleted granitoids in southern Tibet. It is this process that leads to differentiation of the juvenile mafic arc crust toward the felsic lithology in the continental arc. In this regard, the granitoids with depleted radiogenic isotope compositions do not necessarily contribute to the crustal growth at convergent plate boundaries. [ABSTRACT FROM AUTHOR]

New metamorphic pressure-temperature (P-T) paths and Lu-Hf garnet ages reveal a temporal correlation between Middle to Late Jurassic retroarc underthrusting and arc magmatism in southwestern North America. P-T paths were determined for 12 garnet porphyroblasts from six samples from the Chloride Cliff area of the Funeral Mountains in southeastern California. The composite path shows a pressure increase from 4.2 to 6.5 kbar as temperature increased from 550 to 575 °C, followed by a pressure decrease to 5.1 kbar during a further increase in temperature to 590 °C. Lu-Hf garnet ages from a pelitic schist (167.3 ± 0.7 Ma) and a garnet amphibolite (165.1 ± 9.2 Ma) place these P-T paths in the Middle Jurassic. We interpret the near-isothermal pressure increase portion of the P-T path to have developed during thrust-related burial, similar to lower grade rocks at Indian Pass, 8 km to the southeast, where garnet P-T paths show a pressure increase dated by the Lu-Hf method at 158.2 ± 2.6 Ma. We interpret the pressure decrease portion of the composite P-T path from the Chloride Cliff area to reflect exhumation contemporaneous with cooling in the Indian Pass area documented from muscovite 40Ar/39Ar step-heating ages of 152.6 ± 1.4 and 146 ± 1.1 Ma. The conditions and timing of metamorphism determined for the Indian Pass and Chloride Cliff areas, and isogradic surfaces that cut across stratigraphy, support the interpretation that the strata were dipping moderately NW during metamorphism, parallel to the thrust ramp that buried the rocks. Burial likely resulted from top-SE motion along the Funeral thrust, which was later reactivated as a low-angle normal fault with opposite motion to become the currently exposed Boundary Canyon detachment that was responsible for Miocene and possibly older exhumation. The part of the burial history captured by garnet growth occurred ~6 m.y. before the 161 Ma peak of high-flux magmatism in the arc. Burial was contemporaneous with metamorphic ages from the western Sierra Nevada metamorphic belt, with the possible timing of accretion of arc terranes in northern California, and with the initiation of Franciscan subduction. Burial ages are also similar in timing with generally E-W crustal shortening in the retroarc that produced the East Sierra thrust system, the Luning-Fencemaker fold and thrust belt, the possible early history of the Central Nevada thrust belt, and the western thrusts of the southern Sevier belt. The timing of tectonic burial documented in this study and of high-flux magmatism in the arc supports the interpretation that the development of a coherent arc-trench system in the Early Jurassic resulted in the underthrusting of melt-fertile material beneath the arc along west- to northwest-dipping faults such as the Funeral thrust in the Jurassic, which penetrated the basement to the west as well as the roots of the magmatic arc, leading to increased magmatism. [ABSTRACT FROM AUTHOR]

Continental strike-slip shear zones that may bear important information about the evolution of convergent tectonics often occur to accommodate plate convergence. When and how shearing along the shear zones responds to plate interactions, however, are often debated. In this study, we investigated the Oligocene-Miocene leucocratic dikes from the Ailao Shan-Red River shear zone, which was active during India-Eurasia plate convergence, to constrain the timing and mechanism of ductile shearing along the shear zone. The dikes are structurally grouped into pre-, syn-, and postkinematic types with respect to ductile shearing. Prekinematic dikes from ca. 41 to 30 Ma have low whole-rock 87Sr/86Sr(i) values (0.707-0.710), generally high eNd(t) values (-3.31~-7.98), and variable eHf(t) values (-7.9~+5.7). Their magma sources involved high thermal perturbation inducing partial melting of the lower crust, and contributions from the mantle that were possibly related to extensional collapse of the orogenic belt prior to tectonic extrusion of the Sundaland block. Syn- and postkinematic dikes from ca. 28 to 20 Ma dominantly have high whole-rock 87Sr/86Sr(i) (0.707-0.725) and low eNd(t) (-5.83 to -9.76) values, and either negative or positive zircon eHf(t) values (broadly in the range of -12 to + 7.6) for coeval but separate crustal magma sources. The results imply that major shearing accompanying retrograde metamorphism along the Ailao Shan-Red River shear zone was localized to crustal level. A synthesis of regional structural data suggests that Oligocene-Miocene shearing along the Ailao Shan-Red River shear zone and lateral tectonic extrusion of the Sundaland block proceeded in response to progressive India- Eurasia plate convergence. Distributed and inhomogeneous middle- to lower-crustal flow along the boundaries of and within the Sundaland block occurred during the tectonic extrusion. [ABSTRACT FROM AUTHOR]

Vitrinite reflectance analysis and the mean diameter of metamorphosed quartz, suggest that the eastern Ouachita region, Arkansas, USA reached the lower greenschist facies, regardless of burial depth. An increase in "crystallinity" may explain that the maturation is from the exhumation of the rocks from a greater depth. Shale and sandstone samples collected from outcrops along the crest of the Benton Uplift exhibit illite and chlorite crystallinities that yield higher indices compared to younger rocks adjacent to this crustalscale anticlinal structure. Illite crystallinity d(001) of air-dried, fine (<0.2 µm) authigenic clays (Δ°2ϴ (CuKa) = 0.417° to 0.875°) yield a range of conditions from diagenetic temperature to lower anchizone temperatures up to ~200 °C. Air-dried, fine authigenic chlorite clays (Δ°2ϴ (CuKa) = 0.259° to 1.570°) yield a wide range of diagenetic to metamorphic conditions that span the diagenetic zone through epizone, which indicate a maximum temperature slightly above ~300 °C. These results are in contrast to prior thermal maturation data. No additional heat sources, such as from pluton emplacement, were identified by major areas of anomalous high maturation based on crystallinity. Regionally, the illite and chlorite crystallinity increases toward the central axis of the Benton Uplift. Exhumation of the rocks from greater depth is all that is required to explain the illite and chlorite crystallinity data. [ABSTRACT FROM AUTHOR]