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The far limit of a plate subduction process and its related far‐field dynamic process are fundamental topics for plate tectonics. The Great Khingan Range (GKAR) in the western flank of NE China is currently under the far‐field influence of the Pacific plate subduction. Benefiting from the newly deployed seismic arrays in the northern GKAR, we take full advantage of seismic data from both temporary and permanent networks and employ an improved scheme of the H‐κ stacking method by introducing surface wave dispersion to obtain the integrated maps of Moho depth and crustal bulk Vp/Vs ratio in this region. Our results show that the Moho depth has a giant step near the North–South Gravity Lineament (NSGL). Meanwhile, the distribution of bulk Vp/Vs ratio presents a north–south difference roughly by 50°N, where the south subregion consists of a collage of high and low Vp/Vs ratio; by contrast, the north subregion is characterized by unified high values. The east‐west structural discrepancies across the NSGL from the Earth's surface to the mantle transition zone indicate the difference in the strength of modifications between the near and far‐fields from the Pacific plate subduction. The complicated distribution of the Vp/Vs ratio in the south subregion supports that secondary small‐scale upwellings underneath the Cenozoic volcanic groups dominate the tectonic reworking in this area. The lack of Cenozoic volcanism and a more straightforward distribution of the Vp/Vs ratio in the north subregion might allude to a tectonically inactive part of NE China. Plain Language Summary: We use an improved H‐κ stacking method to construct an integrated map of Moho depth and crustal bulk Vp/Vs ratio across the Great Khingan Range (GKAR). Our results show a significant step in the Moho depth near the North–South Gravity Lineament, which probably marks the boundary between near‐ and far‐field locations from the subduction. We also identify a prominent discrepancy in the crustal bulk Vp/Vs ratio between the south and north subregions of the GKAR. The south subregion is characterized by a collage of high and low Vp/Vs ratio, while the north subregion has a much more straightforward pattern. The localized high Vp/Vs ratio in the south subregion coincides with the Cenozoic volcanism, which tells us the far‐filed effects of the subduction are dominated by a secondary small‐scale mantle upwelling. In contrast, the unified high Vp/Vs ratio and no sign of Cenozoic magmatism in the south subregion may indicate a tectonically inactive region in NE China. The unique mechanism of the far‐field effects and the particular location of the north subregion of the GKAR can explain how the north–south difference in crustal property is formed. Key Points: A new integrated map of Moho depth and bulk Vp/Vs ratio across the Great Khingan Range (GKAR) is built with an improved H‐κ stacking methodThe far‐filed effects of the stagnant Pacific slab on the GKAR are most likely achieved by a small‐scale secondary mantle upwellingThe north subregion of the GKAR is tectonically inactive, which may have retained its crustal state since the Paleozoic Era [ABSTRACT FROM AUTHOR]

Neoproterozoic island arc assemblage of the Arabian–Nubian Shield (ANS) in the Eastern Desert (ED) of Egypt comprises juvenile suites of metavolcanics (MV), large amounts of meta-sedimentary rocks (MS), and voluminous metagabbros-diorites (MGD) and syn-tectonic intrusions of older granitoids (OG). We report here the updates of these four rock units in terms of classification, distribution, chemical characteristics, geodynamic evolution, metamorphism, and ages. In addition, we discuss these integrated data to elucidate a reasonable and reliable model for crustal evolution in the ANS. The main features of these rock units indicate their relation to each other and the geodynamic environment dominated by early immature oceanic island arcs to primitive continental arcs. Integrated information of the island arc metavolcanic and plutonic rocks (gabbros, diorites, tonalites, and granodiorites) furnish evidence of the genetic relationships. These include proximity and a coeval nature in the field; all protolith magmas are subalkaline in nature following calc-alkaline series with minor tholeiitic affinities; common geochemical signature of the arc rocks and subduction-related magmatism; their similar enrichment in LREEs; and similar major element compositions with mafic melts derived from metasomatized mantle wedge. The volcano-sedimentary and the OG rocks underwent multiphase deformation events whereas the MGD complexes deformed slightly. Based on the magmatic, sedimentological, and metamorphic evolutions constrained by geochronological data as well as the progressive evolutionary trend from extensional to compressional regimes, a possible gradual decrease in the subducted slab dip angle is the most influential in any geodynamic model for arc assemblage in the ED of Egypt. [ABSTRACT FROM AUTHOR]

In the Dom Feliciano Belt, Brazil, the Pinheiro Machado Complex (PMC) includes diorites, tonalites, granodiorites, syenogranites and granites, whose evolution is related to several magmatic pulses and complex petrogenetic processes. Two magmatic stages were identified (early and late), resulting in different rock subgroups. The geochemical data showed that the early magmatism was chemically affected by partial melting. Geochemical modelling results suggest that fractional crystallization processes with assimilation of around 40% from the crustal basement and the decoupling of assimilated magma are crucial for the PMC rocks' genesis. Geochemical data also show that during the early magmatism, the subsequent process of early diorite anatexis developed by heating and continuous activity of the underlying magma chamber possibly occurred at a melting rate of 5%–10%. The hybrid rocks have contributions from the mixing process related to early terms, showing geochemical correlations in the major element curves, for the early diorite and syenogranitic melt members, at 60%–50% and 50%–40%, respectively. Slab failure tectonic context is related to the multi‐intrusive events dynamics recorded in the studied rocks. Recharge and melting events of the recently formed crust due to the constant heating of new pulses of deep slab melting would explain the magmatic interactions observed in the Complex. The results demonstrate that the studied rocks crystallized in an open system, including mixing processes to form hybrid rocks, physical disaggregation and assimilation of early intrusions, truncation, dragging and erosion of early mushes by younger pulses. [ABSTRACT FROM AUTHOR]

It was recognized that two magmatic belts in the Lhasa‐Tengchong terrane formed due to the Mesozoic‐Cenozoic Tethyan evolution. Still, their spatiotemporal variations of magmatic flare‐ups/lulls are rarely discussed. Here we use the new U‐Pb and Lu‐Hf isotopic data of captured zircons and a comprehensive data set to show that the flare‐up of northern magmatic belt has peak ages of 110 Ma in central and northern Lhasa and 120 Ma in eastern Tengchong, possibly related to the tectonic transition from Meso‐ and Neo‐Tethyan double subduction to Neo‐Tethyan single subduction. For the southern magmatic belt, the flare‐ups at 100–85 Ma and 65–45 Ma in eastern southern Lhasa indicate obvious juvenile crustal growth, while flare‐ups at 75–45 Ma in western southern Lhasa and Tengchong record ancient crustal reworking. Such flare‐up variations in the southern magmatic belt possibly resulted from asynchronous changes in the Neo‐Tethyan slab dip. Plain Language Summary: The Tethyan slab subduction and following collision of India and Asia produced two magmatic belts in the Lhasa‐Tengchong terrane. How did the magmatic flare‐ups or high‐flux episodes and sources spatiotemporally vary? We combine the new U‐Pb and Lu‐Hf isotopic data of captured zircons (xenocrystic zircons caught by volcanics from country rocks) and a comprehensive data set to show that the peak ages of early Cretaceous flare‐up of the northern magmatic belt are ca.10 Ma younger in central and northern Lhasa than in eastern Tengchong, possibly related to the diachronous closure of the Meso‐Tethys and flat subduction of the Neo‐Tethys. The southern magmatic belt in Tengchong and western southern Lhasa exhibit 75–45 Ma flare‐ups featured by the reworking of ancient crust, different from the eastern southern Lhasa with the flare‐ups at 100–85 Ma and 65–45 Ma which are dominated by the juvenile crustal growth. Such flare‐up variations may be related to changes of the Neo‐Tethyan slab dip. Key Points: The Neo‐Tethyan arc has asynchronous magmatic flare‐ups and diverse magmatic sourcesThe spatiotemporal changes of slab dip triggered the asynchronous flare‐ups of the Neo‐Tethyan arc [ABSTRACT FROM AUTHOR]

The distribution of Oligo--Miocene magmatic rocks from southern Tibet in space and time yields critical information on the geometry and deformation of the subducted Indian lithosphere which impacts on plateau growth following the India and Eurasia collision. A growing body of geophysical evidence has shown that the subducted Indian lithosphere beneath the Tibetan Plateau has been torn apart. However, the spatiotemporal distribution and cause of the tearing remain enigmatic. Timing of the post-collisional magmatic rocks in southern Tibet exhibits four patterns of decreasing ages; magmatism began earlier in the west and east Himalayan syntaxis and evolved to two age undulations in the central southern Tibet. Seismic images show that regions of slab window (both 90°E and 84°E) and flattened subducted lithosphere (both 86°E and 81°E) are present at depth of 135 km. Correspondingly, increasing mineral crystallization temperatures (absolute value of 50 °C) were recorded in the Oligo--Miocene ultrapotassic-potassic rocks at 90°E and 84°E, while opposing trends were shown by coeval ultrapotassic-potassic rocks at 86°E and 81°E. Besides, the melting depth of the Oligo--Miocene ultrapotassic-potassic primitive melts decreases from nearly 100 km to 70 km between 81°E and 90°E, probably indicating progressive rising of the lithosphere-asthenosphere boundary. Such variations were possibly the results of the focused flow and upwelling of asthenosphere, which advanced rapidly but diachronously through weakened and torn sectors within the overlying Indian slab. The upwellings probably induced diachronously upward bending of the residual Indian slab and its flattening, which accelerated the tearing of the Indian lithosphere during continental subduction. [ABSTRACT FROM AUTHOR]

The late Paleozoic tectonic evolution of the Lhasa terrane and opening of the Sumdo Paleo-Tethys Ocean remain controversial due to limited data and the ambiguous tectonic significance of Paleozoic magmatism in the southern Lhasa subterrane. Here, we explore the petrogenesis of meta-basalt from the Tangjia area and meta-gabbro from the Sumdo area to provide new constraints on the evolution of the Lhasa terrane. The geochemical characteristics of meta-gabbro are similar to those of oceanic island basalt and have positive whole-rock εNd(t) values (+0.08 to +1.03) and zircon εHf(t) values (+3.4 to +4.9), which suggests that they originated from asthenospheric mantle with minor contributions from lithospheric mantle in an intracontinental rift setting. The meta-basalt yields positive whole-rock εNd(t) values (+6.47 to +6.60) and zircon εHf(t) values (+14.2 to +15.2) and has enriched mid-oceanic-ridge--like geochemical characteristics; we suggest that the meta-basalt was sourced from the depleted mantle and interacted with enriched components in an initial oceanic basin setting. Zircon U--Pb geochronology of meta-basalt and metagabbro yielded crystallization ages of ca. 338 Ma and ca. 345 Ma, respectively. We suggest that the Sumdo Paleo-Tethys Ocean opened in the early Carboniferous. Negative buoyancy of Paleo-Tethys oceanic slab during subduction toward Eurasia drove the opening of the Sumdo Paleo-Tethys Ocean. Given that the Sumdo Paleo-Tethys Ocean was a limited oceanic basin, the Lhasa terrane was not completely separated from the northern margin of Gondwana before the Permian. [ABSTRACT FROM AUTHOR]

Arc magmatism along convergent plate boundaries has been a major contributor to continental crustal growth. During the late Paleoproterozoic--Mesoproterozoic assembly of the supercontinent Nuna, following arc-backarc-continent collisions, the Makkovik Orogen, along the southern margin of the Archean North Atlantic Craton, experienced a pulse of continental arc magmatism at ca. 1800 Ma. Lithogeochemistry indicates compositions typical of ferroan, alkali to alkali-calcic, A-type granites. The granitic rocks have εNd(t) values varying from -5.3 to +2.0 and model ages [T(DM)] from 2750 to 1880 Ma. The very high εNd(t) and young model ages imply a substantial juvenile mantle component in their formation. Oxygen fugacity (ƒO2) ratios, estimated by the proxy ratio of FeOT/(FeOT + MgO), range from 0.58 to 0.99, indicating the parental magmas formed in both reducing and oxidizing conditions. The granites also preserve an alkali-calcic alteration that together with inferred ƒO2 makes them more prospective for a variety of intrusion-related styles of mineralization including iron-oxide-copper-gold and uranium. Based on the petrology, lithogeochemistry, and isotopic compositions, the ca. 1800 Ma magmatic event is interpreted to have been developed post-collisionally following the docking of the Cape Harrison Arc/micro-continent with the North Atlantic Craton during the assembly of the supercontinent Nuna. This collision was followed by slab rollback (possibly breakoff), extensional collapse, and mantle upwelling resulting in the generation of the abundant ca. 1800 Ma felsic magmatism. The timing of the ca. 1818-1799 Ma plutonism in the Makkovik Orogen is coeval with significant felsic plutonism preserved in the Julianehåb Igneous Complex (Ketilidian Orogen) of southern Greenland. [ABSTRACT FROM AUTHOR]

Differentiating between advancing and retreating subduction zones and delineating the timing and processes for switching between these end-member types are critical factors in understanding orogenic evolution. In this study, we used temporal composition variations and spatiotemporal distribution of igneous rocks to constrain the types and directions of the subduction zone beneath the Lhasa terrane in southern Tibet during the mid-Mesozoic. Comprehensive geochronological and geochemical data from mid- Mesozoic magmatic rocks in five areas of the central Lhasa terrane document two compositionally distinct magmatic suites: older (ca. 172-150 Ma) high-Mg diorite-granodiorite and younger (ca. 150-130 Ma) low-Mg granodiorite-syenogranite. The high-Mg rocks with enriched isotopes were likely generated by the mixing of Lhasa terrane basement- derived magmas with mantle-derived magmas that incorporated extensive contributions from subducted sediments. The low- Mg rocks with relatively depleted isotopes are interpreted as derived from a mixed lowercrust source (ancient basement and juvenile crust) with minor mantle contributions followed by crystal fractionation. Both high-Mg and low-Mg rocks show arc signatures and formed the 1200 km Shiquan-Zhegu-Menba arc. A combination of the Mesozoic geological and geochemical data suggests that both the Neo-Tethyan and Bangong Tethyan slabs were likely subducting beneath the Lhasa terrane, creating two opposing subduction zones. The change from high-Mg to low- Mg magmatism within the Shiquan-Zhegu- Menba arc likely records a tectonic switch from advancing to retreating subduction of the south-dipping Bangong Tethyan slab. Our results provide a good example for identification of subduction polarities and types of ancient orogenic belts through spatiotemporal distributions and changes in geochemical compositions of igneous rocks. [ABSTRACT FROM AUTHOR]

The contributions of deeply subducted slabs to post-collisional magmatism are poorly constrained. Here, we present a study of the post-collisional, Early Permian (ca. 279 Ma), Hamisite basalts from the Western Tianshan NW China to trace the geochemical fingerprints of deeply subducted slab. The Hamisite basalts have arc-like trace element patterns but high Nb/Ta ratios (18.4-23.0). They have slightly enriched Sr--Nd isotopic compositions (87Sr/86Sr = 0.7048-0.7052; εNd[t] = 2.02--3.85), which indicates that the basalts were derived from an enriched mantle source. Extremely low δ7Li values (-5.76‰ to 0.20‰) imply the involvement of a deeply subducted slab (i.e., eclogites) in the source. Correlations among Nb/Ta and (Dy/Yb)N ratios, and rare earth element contents, indicate that the deeply subducted slab contained residual rutile and garnet but no epidote. Given that melting of eclogites requires an anomalously hot mantle source, we propose that mantle upwelling during post-collisional extension triggered melting of the deeply subducted slab. Our results indicate that melting of a deeply subducted slab can generate a metasomatized mantle source with high Nb/Ta ratios, which could be a hidden, high-Nb/Ta reservoir on Earth. Low δ7Li values and high Nb/Ta ratios could be a fingerprint of recycling of a deeply subducted slab. [ABSTRACT FROM AUTHOR]

Arc magmatism can provide crucial information about crustal recycling and growth in subduction zones. However, the crustal material transfer process and mechanism from subducting slab to overlying mantle wedge are controversial. Here, we present detailed petrological, geochronological, geochemical, and Sr-Nd-Hf isotopic data, and previously published data for two episodes of subduction-related magmatism in the Northern Yili Block, NW China, to study these issues in relation to the recycling of continental crust and the growth of two separate magmatic episodes at 400-350 Ma and 350-300 Ma, respectively. The Nd-Sr isotope modeling results, low Nd/Sr, and variable Hf/Nd ratios demonstrate that the arc magmatism of these two episodes could be derived from two distinct mélange sources that formed at the slab-mantle interface during subduction of the Junggar oceanic plate. In the first episode, magmatic rocks exhibit high Th/Yb and (La/Sm)N ratios and the decoupling of Nd-Hf isotopes, which leads to an interpretation that the primary magmas were derived from sources mainly mixed by sediments and mantle-wedge peridotites. In contrast, the magmas of the second episode exhibit high Ba/La and Ba/Th ratios and the coupling of Nd-Hf isotopes, which implies that these magmas were likely produced from mélange diapirs dominated by the mixing of midoceanic-ridge basalts and mantle peridotites. The episodes of Nd-Hf isotopic decoupling and coupling coincided with crustal material recycling and crustal growth in subduction zones, respectively. Considering that the Northern Yili Block at 400-350 Ma was characterized mainly by enriched Nd-Hf isotopes of magmatic rocks and no development of the forearc accretionary complex, while the Northern Yili Block at 350-300 Ma featured high εHf(t)-εNd(t) values of magmatic rocks, occurrences of extension-related magmatism, and distribution of the forearc accretionary complex and immature back-arc basin, we propose that these two different mélange sources probably developed in response to subduction transition. That is, one resulted from advancing and the other was relevant to retreating regime. The partial melting of these mélanges in the mantle wedge generated intermediate felsic magmas, which might have acted as a leading mechanism for crustal recycling and growth of the accretionary orogenic belt. [ABSTRACT FROM AUTHOR]

Subduction zones may be characterised by deep-seated tectonic structures whose effects propagate to the upper plate through faulting and magmatism. The overall geodynamic framework, as well as the roots of the many active faults affecting such regions, can be investigated by the study of the upper mantle anisotropic patterns, through the analysis of core-transiting teleseismic phases. Here, we discuss the results of XKS waves splitting observed in the central Mediterranean, particularly in southern Italy, which is characterised by the Adriatic-Ionian subduction system. Azimuths of polarisation of the fast wave (fast directions) were found to be generally trench-parallel, as an effect of the subducting slab, albeit a change to a perpendicular direction, in central Italy and Sicily, suggests discontinuities in the structure of the slab itself. However, while in central Italy a gradual rotation of fast directions points to a toroidal upper mantle flow through a tear in the Apenninic slab, in central-eastern Sicily, the splitting parameters show an abrupt change that matches well with the main crustal tectonic structures. There, the rapid trench migration, taking place at the transition between the subduction and continental collision domains, produced a rather complex Subduction Transform Edge Propagator fault system. The sharp variation in the pattern of the upper mantle anisotropy marks the main element of such a fault system and suggests its primary role in the segmentation process of the collisional margin. Our findings further show that the study of seismic anisotropy may be fundamental in investigating whether tectonic structures only involve the crust or extend down to the upper mantle. [ABSTRACT FROM AUTHOR]

The West Junggar terrane (WJT) is known to tectonically correlate with the Central and East Kazakhstan terrane; however, the Middle Palaeozoic magmatism has rarely been reported in the southern WJT, leading to significant uncertainties on the tectonic evolution and correlation of the southwestern Central Asian Orogenic Belt (CAOB). Here, we present the adakitic quartz diorites with zircon U-Pb ages of 390–394 Ma in the Alashankou region to demonstrate the presence of Early-Middle Devonian magmatism in the southern WJT for the first time. They are characterized by low Yb (1.00–1.41 ppm) and Y (8.9–14.4 ppm) but high Sr (301.5–475.9 ppm), Sr/Y (23–41), and (La/Yb)N (8.1–11) values, typical of modern adakites. As suggested by the relatively high SiO2 (64.96–67.71 wt.%), moderate K2O (1.53–3.19 wt.%), but low MgO (1.56–2.31 wt.%), TiO2 (0.28–0.42 wt.%), Cr (≤13.7 ppm), Co (2.7–9.9 ppm), Ni (8.0–18.7 ppm), and Th (3.7–4.8 ppm) concentrations as well as the high positive ɛHf(t) values (+5.6 to +10.3) and low Nb/Ta ratios, the adakitic quartz diorites were probably generated by the partial melting of a slightly thickened lower crust, which consisted mainly of the Ediacaran-Middle Palaeozoic intra-oceanic arc rocks in the southern WJT. Combined with regional data, our new results support that the southern WJT probably evolved as the eastern extension of the Kazakhstan Early-Middle Devonian outer volcanic belt during the Middle Palaeozoic. The Early-Middle Devonian tectonic extension and resultant lower crust melting may thus be related to the slab retreat of the Junggar-Balkhash Ocean. [ABSTRACT FROM AUTHOR]

The most intense arc magmatism in the Gangdese arc occurred during the Palaeocene–Eocene, which is widespread across the entire southern Lhasa subterrane with Palaeocene–Eocene I‐type intrusive rocks and Linzizong volcanic successions. What drives the voluminous magmatism remains enigmatic. New geochronological and geochemical data on Late Cretaceous‐Palaeocene magmatic activity from Bumeicun, southern Tibet enables recognition of this distinct magmatic flare‐up event that we ascribe to Neo‐Tethyan Oceanic slab rollback. Bumeicun, located in the southern margin of Gangdese magmatic arc, exposes volcanic and plutonic rocks with close spatial–temporal links, and a wide range of silica content (53.0–74.7 wt.%). Zircon U–Pb dating result shows that the rocks form at 65.24 ± 0.77–67.65 ± 0.31 Ma. The Bumeicun igneous rocks are characterized by relative enrichment of large‐ion lithophile elements, depletion of high field‐strength elements (HFSEs) and enrichment of light rare earth elements, showing typical arc features. These rocks also show positive zircon εHf(t) value (+8.70 to+19.68), which are ascribed to the partial melting of arc mantle wedges. Followed by a succession of melt extraction and fractional crystallization, the coeval intermediate‐felsic volcanic and plutonic rocks in Bumeicun are formed. Previous researches note that Gangdese magmatic arc had a period of magmatic quiescence in the Late Cretaceous, and a subsequent burst of magmatism occurred in the P‐E. Our proposal of slab rollback model is consistent with the distinctly high zircon saturation temperatures (Tzr: 730–827°C) of Bumeicun magmatism. In addition, the slab rollback led to the Gangdese arc magmatism migration from north to south from the Cretaceous to the Eocene. [ABSTRACT FROM AUTHOR]

Passive seismic studies have suggested that slab tearing occurs beneath North Sumatra, albeit the slab morphology is somewhat poorly constrained. The lack of resolution on the slab means that the presence of a tear has not been validated, and that the main factors responsible for slab tearing are hard to discern. We use a large seismic data set to determine a new 3‐D high‐resolution tomographic model of the crust and upper mantle beneath North Sumatra, which reveals a well‐defined low‐velocity channel that cuts across the subducting slab through a cavity. This feature cannot be reconciled with a tear in the slab, as previously suggested. Our relocated earthquakes exhibit a protruding cluster of seismicity along the subducted Investigator Fracture Zone, indicating that the high reliefs of the Investigator Fracture Zone possibly enhance the coupling between the subducted slab and the overriding plate. Shear stress resulting from the differential coupling in the slab may have created an area of mechanical weakness in correspondence of the fossil‐spreading ridges, which eventually led to the formation of a slab window. This window allows the exchange of mantle material from the subslab region up to the Toba volcano. We propose that the previously damaged parts (e.g., fracture zones and spreading centers) of the oceanic plate could help promote the growth of a slab window. Plain Language Summary: Seafloor fabrics are damaged traces of the oceanic plate, which may break again during subduction, leading to tearing, slab window or full slab detachment. Slab breaking is a pivotal process in the tectonic evolution of the lithosphere and underlying mantle, and can lead to complexities in slab deformation and mantle dynamics, new intraplate magmatism, and rapid uplift. Slab tearing in the subducted Indo‐Australian oceanic lithosphere beneath North Sumatra has been suggested by studies of seismicity and 3‐D seismic tomography. However, the crucial contributing factor to the initiation of slab breaking is a matter of debate. Our high‐resolution tomographic results reveal the fine crustal and upper‐mantle structure under North Sumatra, showing a very low P‐wave velocity hole cutting through the subducting slab. According to the location of seafloor fabric structures (e.g., fracture zones and fossil ridges in the Wharton basin) and the corresponding low‐velocity hole in our tomographic images, we suggest that the subducted seafloor fabrics are responsible for the slab window of the subducting Indo‐Australian plate beneath North Sumatra. We propose that the weak compartments of fossil‐spreading ridge and the obstructed Investigator Fracture Zone are keys for the onset of slab breaking. Key Points: A 3‐D P‐wave tomography model beneath North Sumatra is obtained using local and teleseismic data recorded by a dense seismic networkA low‐velocity channel is revealed in the subducting slab beneath the Toba calderaSlab window beneath Toba may be related to pre‐existing seafloor fabric structures in the oblique subduction system [ABSTRACT FROM AUTHOR]

There is considerable debate regarding the petrogenesis of K-rich magma, which are believed to occur in subduction settings. The districts of the potassic rocks in the rear-arc of the Cenozoic Iran have not been thoroughly investigated. In this study, we investigate Late Eocene K-rich rocks of the Roshtkhar area of NE Iran. We employ whole-rock geochemistry, Sr-Nd-Hf-isotopes and zircon U-Pb age data of intrusive rocks to identify the mantle signature, evaluate the timing of emplacement, and determine the geodynamic setting. The Roshtkhar intrusive rocks (RIR) encompass a narrow compositional spectrum from syenite to monzonite, belonging to K-rich series, and are enriched in light rare earth elements and large ion lithophile elements such as Cs, Rb, Ba, Th, and U, with positive K, Pb, and Sr anomalies. U-Pb LA-ICP-MS dating of zircon from two samples yielded ages of ca. 38 Ma. Epsilon Hf values for investigated Eocene zircons display a variable range from +0.3 to +6.5. The RIR show relatively homogeneous initial epsilon Nd values ranging from +0.64 to −1.31 and modest variation in initial 87Sr/86Sr from 0.7048 to 0.7052. The data show mantle contribution with small involvement of Cadomian upper crustal material of up to 12% and an approximately 3% contribution fluid derived from subducted sediment into the mantle source. The lack of rocks that are purely derived from the lower crust in the study area further argues against the partial melting of lower crust that delaminated into the mantle. We suggest slab retreat, extension, and mantle upwelling as the main mechanisms for the generation of the primitive melts of RIR. Accordingly, decompression melting of metasomatized lithospheric mantle (e.g. phlogopite, apatite, and/or pargasite-bearing lherzolite) would be a plausible mechanism for the forming of K-rich basic melt in the mantle source. [ABSTRACT FROM AUTHOR]

The process of slab tearing profoundly affects interchanges of material and energy between the lithosphere and asthenosphere and has been invoked as a trigger of magmatism in modern and ancient subduction‐collision systems. Horizontal slab tears may occur after continental collision and subsequent slab break‐off, whereas vertical slab tears may occur in response to along‐strike variations in rollback velocity, thereby ripping the subducting lithosphere. Previous studies have documented tearing of subducting slabs in modern convergent settings using geophysical methods, but in ancient collisional belts, where these methods cannot be applied, detection of slab tearing is more difficult, particularly if the oceanic plate was fully subducted and the geological evidence for subduction is limited. Here, we employ multiproxy element‐isotope imaging, as well as zircon petrochronology and whole‐rock geochemistry, to assess evidence of vertical tearing of the Mianlue slab in the Triassic Qinling Orogen in central China. Our results show evidence of anomalous, inferred tear‐related magmatism east of 107°E, at 225–210 Ma. Vertical tearing of the northward subducting Mianlue slab is recorded by geochemically anomalous igneous rocks, and the recognition of N–S to NE–SW trending boundaries defined by Sr‐Nd‐Hf isotopes, elemental ratios (Nb/Ta, and Ca/Al), Mg#, and reduced crustal thickness between 107° and 108°E. The inferred vertical tear may have been driven by slab rollback of the Mianlue slab at 225–210 Ma. The results show that multiproxy element‐isotope imaging (e.g., of εNd(t), εHf(t), (87Sr/86Sr)i, Nb/Ta, Mg#, and Ca/Al) can be used to reconstruct the evolution of fossil subduction systems. Plain Language Summary: Subduction zones are natural factories for the circulation of material in the Earth as they allow tectonic plates to descend into the mantle. Pieces of subducting plates, normally referred to as slabs, can be torn and segmented, but reconstructing the geometry of such tears in ancient convergent plate boundary settings is difficult. Based on the observation that slab tearing can trigger anomalous magmatism in convergent plate margins, we present a multiproxy geochemical and isotopic imaging method that enables us to understand the process of slab tearing in the Qinling Orogen (China) during the Triassic. We found that igneous rocks from the Qinling Orogen, east of 107°E, are geochemically anomalous (relative to typical arc rocks). These rocks are dated 225–210 Ma. By imaging various geochemical characteristics, our results demonstrate that the subducting Mianlue slab was likely torn at longitude 107°–108°E, thereby producing geochemically and isotopically anomalous mafic and felsic magmatism. Key Points: The geometry of slab tearing in an ancient orogen is constrained by a multiproxy geochemical approachAnomalous arc magmatism dated at 225–210 Ma is identified in the Qinling Orogen east of 107°EThe Mianlue slab underwent southward vertical slab tearing at 107°−108°E in the Late Triassic [ABSTRACT FROM AUTHOR]

Changes in magmatism and sedimentation along the late Neoproterozoic-early Paleozoic Ross orogenic belt in Antarctica have been linked to the cessation of convergence along the Mozambique belt during the assembly of East-West Gondwana. However, these interpretations are non-unique and are based, in part, on limited thermochronological data sets spread out along large sectors of the East Antarctic margin. We report new 40Ar/39Ar hornblende, muscovite, and biotite age data for plutonic (n = 13) and metasedimentary (n = 3) samples from the Shackleton–Liv Glacier sector of the Queen Maud Mountains in Antarctica. Cumulative 40Ar/39Ar age data show polymodal age peaks (510 Ma, 491 Ma, 475 Ma) that lag peaks in U-Pb igneous crystallization ages, suggesting igneous and metamorphic cooling following magmatism within the region. The 40Ar/39Ar ages are similar to ages in other sectors of the Ross orogen, but younger than detrital mineral 40Ar/39Ar cooling ages indicative of older magmatism and cooling of unexposed inboard areas along the margin. Detrital zircon trace element abundances suggest that the widespread onset of magmatism in outboard localities of the orogen correlates with a ~560–530 Ma decrease in crustal thickness. The timing of crustal thinning recorded by zircon in magmas overlaps with other evidence for the timing of crustal extension, suggesting that the regional onset of magmatism with subsequent igneous and metamorphic cooling probably reflects slab rollback that coincided with possible global plate motion changes induced during the final assembly of Gondwana. [ABSTRACT FROM AUTHOR]

A discrete belt of Albian-Cenomanian granitoid plutons occurs in the Lhasa and Qiangtang terranes in the Central (CTP) and Eastern Tibetan Plateau (ETP) and represents a major magmatic pulse in the plateau's crustal evolution during the Cretaceous. The geochemistry, petrogenesis, and magmatic development of these granitoids are different from those of magmatic arc granitoids along the southern edges of the Lhasa and Qiangtang terranes, indicating different heat and melt sources and tectonic setting of their formation. We present here new mineral, wholerock and isotope geochemistry, and zircon U-Pb age data from the Xiasai pluton in the ETP and discuss its geochemical-petrological characteristics and magmatic development in comparison to the other Cretaceous plutons in the ETP and CTP, and within the tectonic framework of the Mesotethyan geology of Tibet. Zircons from the Xiasai and other plutons in the ETP have yielded U-Pb ages ranging from 106 Ma to 93 Ma in comparison to 115 Ma and 100.3 Ma zircons from the Southern Qiangtang Terrane (SQT) and 113.4 Ma and 109 Ma zircons from the Northern Lhasa Terrane (NLT) farther west. The Cretaceous granitoids in the ETP and CTP range in composition from granite, K-feldspar granite to monzogranite and biotite monzogranite, representing highly fractionated I-type granites with relatively high SiO2 and K2O contents, variable (Na2O + K2O)/CaO and FeOT/MgO ratios, and (Zr + Nb + Ce + Y) abundances. They display significant negative Eu anomalies (Eu/*Eu) = 0.04-0.12) and strong depletions in Sr and Ba, and are strongly enriched in large ion lithophile elements but depleted in high field strength elements. Their eHf(t) values correspond to wide ranging Hf isotope crustal model ages (TDM C) of 0.3-1.9 Ga, and their Sr-Nd isotopic signatures show elevated (87Sr/86Sr)i ratios (0.7034-0.7105) and negative eNd(t) values of -8.8 to -4.9. These high whole-rock (87Sr/86Sr)i ratios and relatively high Th/Nb and Th/Yb ratios indicate incorporation of melts derived from partial melting of subducted sediments into the melt evolution of these granitoids that involved partial melting of the subduction-metasomatized lithospheric mantle and the mafic- to intermediate-composition continental crust. The extant zircon crystallization ages of the granitoid intrusions in the CTP and ETP show eastward younging of their emplacement from 115 Ma to 93 Ma, suggesting an apparent eastward migration of the heat source through time. A diachronous collision of the NLT with the SQT during 145-120 Ma and the subsequent slab breakoff induced, eastward propagated slab tear and asthenospheric upwelling produced the hybrid melts of the Albian-Cenomanian granitoids and their emplacement in a discrete, narrow magmatic belt in the CTP and ETP. [ABSTRACT FROM AUTHOR]

Cenozoic magmatism is present across the Iranian Plateau but, despite playing an important role in the geodynamic evolution of the young plateau, remains poorly understood. In this study, the crustal thermal structure beneath the Iranian Plateau is investigated by high‐resolution Lg‐wave attenuation tomography using a newly compiled data set, which consists of records from three recent temporary seismic arrays (for a total of 197 stations) and previous regional networks. Strong Lg attenuation is mainly observed beneath the Zagros orogenic belt, Urumieh‐Dokhtar magmatic arc (UDMA), Makran magmatic arc, Lut magmatic zone, and Alborz reararc magmatic belt, which covers not only tectonic boundary belts but also the active internal zones of the Iranian Plateau. Combining deep seismological observations and geochemical data, the strong crustal attenuation characteristics beneath the northwestern and southeastern UDMA suggest that Miocene–Quaternary magmatism was activated by slab detachment‐induced upwelling after the cessation of oceanic subduction. Western Makran is characterized by strong Lg attenuation with a NE trend. The age of the Paleogene–Quaternary Makran volcanism decreases toward the NE and is likely related to the change in subduction dip. Diffuse magmatism and strong crustal attenuation are present in the northern Lut Block rather than the entire Lut–Afghan collision zone, which is likely associated with the accumulation of mantle magmas driven by the potential northward asthenospheric flow. In northern Iran, a strong attenuation anomaly spans the entire E‒W Alborz magma belt, suggesting that partial crustal melting and mantle thermal sources may exist in this region. Plain Language Summary: The Iranian Plateau is located in the Middle East and is a part of the Alpine‐Himalayan orogenic system. Intensive Cenozoic volcanic activity is characteristic of the Iranian Plateau, but the origination, storage, and transport of underground magma are poorly understood. Seismic attenuation is sensitive to high temperatures and partial melting and can be an indicator of the distribution of heat sources. In this study, we investigate the attenuation features of the crust with a new data set to constrain the crustal thermal structure beneath the Iranian Plateau by using a specific type of seismic wave (Lg) that travels within the crust. Hot crustal sources were observed beneath the Zagros orogenic belt and the four magmatic zones. Therefore, the origination of magmatism can be inferred based on the crustal attenuation distribution. Typically, the younger magmatism in the northwestern and southeastern Urumieh‐Dokhtar magmatic arc was likely activated by deeper mantle upwelling, which may be related to plate kinematics at depth (slab detachment or break‐off). On the southeastern edge of Iran, the Makran region is characterized by strong attenuation with a NE trend, where the volcanism also gets younger toward the NE. It is likely related to the dip change in oceanic plate subduction. Key Points: A high‐resolution Lg attenuation model is developed beneath the Iranian PlateauStrong Lg attenuation characterizes tectonically active areas or recent magmatic zonesCrustal thermal structure revealed by Lg attenuation links Cenozoic magmatism in Iran with sub‐lithospheric mantle processes [ABSTRACT FROM AUTHOR]

New high-precision trace-element analyses of magmatic olivines point to a pyroxenitedominated source for recent alkali basalts erupted above slab windows formed along the Antarctic Peninsula. Melting occurred at ambient mantle temperature, and basalts have geochemical compositions that are indistinguishable from ocean-island basalts (OIBs). We propose that the pyroxenite component originally resided in the upper mantle beneath the subducted slab; formation of a slab window allowed limited decompression and the generation of melts of garnet-pyroxenite, but little or no melting of mantle peridotite. The pyroxenite component in the mantle formed ca. 550 Ma, an age that does not require long-term recycling of subducting slabs to the core-mantle boundary. Enriched mid-ocean ridge basalt (E-MORB) from the adjacent extinct Phoenix Ridge owes its enriched trace-element compositions to mixing between small melt fractions of pyroxenite and peridotite during a period of decreased spreading rate prior to the death of the ridge ca. 3.3 Ma. It is likely that the variable trace-element enrichment seen in East Pacific Rise E-MORB distal from hotspots results from the same process of interactions between small-melt-fraction (<∼5%) melts of pyroxenite and peridotite. [ABSTRACT FROM AUTHOR]

Distinguishing the initiation of actual collision from flat‐slab subduction of oceanic buoyant highs along convergent margins is elusive because both can lead to inboard deformation and disrupt magmatic arcs. Volcanoes with nascent tear magmatic signatures provide a means to document both the occurrence and timing of actual oceanic buoyant high collision. There is a ~40‐year debate on when the true collision of the Yakutat plateau began in Alaska. Three newly identified ca. 1 Ma volcanoes with a north‐to‐south trench perpendicular orientation, nascent tear geochemical signatures, overlaying an imaged Yakutat slab tear, provide constraints on the timing of Yakutat collision and slab tearing. The ca. 1 Ma slab tear is coincident with Yakutat slab segmentation, northern continental Aleutian Arc rejuvenation, cessation of Wrangell Arc magmatism, increased collisional zone exhumation and eastern Yakutat trench abandonment. The documentation of nascent slab tear volcanoes may help resolve similar debates in other convergent margin settings. [ABSTRACT FROM AUTHOR]

The origin and formation of the continental collision-related magmas remain elusive. The volcanic rocks erupted during India-Asia continental collision offer an ideal opportunity to explore their genesis and geodynamic process. Here, we report new zircon U-Pb dating results and Hf isotope, whole-rock element and Sr-Nd isotope data of volcanic rocks in Linzhou Basin, central Lhasa Terrane, southern Tibet. These volcanic rocks are mainly comprised of andesites and belong to Dianzhong Formation. The timing of their formation is ca. 63–66 Ma, coeval with the India-Asia continental initial collision in the central part of southern Tibet. All these rocks show an arc-like geochemical affinity and they have more depleted Sr-Nd-Hf isotopic compositions (87Sr/86Sri = 0.705006–0.705963, εNd(t) = −1.78 to 3.52, zircon εHf(t) = 1.2–7.0) than the Cretaceous pre-collisional andesites and Eocene ancient lithospheric mantle-derived melts beneath the central Lhasa Terrane. The correlation between Mg# and CaO, TiO2, Al2O3, Sr/Y, 87Sr/86Sr, εNd(t) suggests that they were likely hybrid production between an isotopically depleted end-member and the enriched lithospheric mantle (ELM). The oceanic crust-derived melts would be the best candidate for the former end-member according to their depleted Sr-Nd isotopic compositions. The andesitic rocks of Dianzhong Formation in the central Lhasa Terrane were most likely stemmed from partial melts of altered Neo-Tethyan crust and then mixed with the ELM-derived melts. Given the Cretaceous-Tertiary upper crustal shortening, back-arc extension, and voluminous volcanism in the Lhasa Terrane, along with abrupt increasing in the magma temperature and the convergence rate between India and Asia during Palaeocene, the occurrence of the Dianzhong Formation volcanic rocks can be well explained by the Neo-Tethyan slab roll-back. The mixing between the oceanic crust- and the continental lithospheric mantle-derived melts induced by the oceanic slab roll-back would be a complimentary scenario for the formation of the syn-collisional magmatism in collisional orogeny belts. [ABSTRACT FROM AUTHOR]

Molybdenum isotope is a diagnostic tracer for crustal and mantle components in arc magmatism. However, the mechanism of Mo isotopic variation in arc magmas is still debated, e.g., input of different subduction components into the mantle wedge versus isotopic fractionation during dehydration of subducted slab. Here we present whole-rock Mo-Sr-Nd-Hf-Pb isotopic data for the Continental Arc Basalt (CAB) and Back Arc Basalt (BAB) from Java, Indonesia, to investigate the role of slab inputs in Mo isotopic variation of the Sunda arc magmatism. The CAB samples have variable K 2 O contents (0.44–2.49 wt%) and are mainly classified as calc-alkaline series, while the BAB samples are shoshonitic with markedly high K 2 O contents (2.12–6.90 wt%) relative to the CABs. The Java CABs and BABs have similar Mo isotopic compositions (δ98/95Mo = −0.65 to −0.07‰ and −0.66 to −0.07‰, respectively, relative to NIST SRM3134), suggesting that such a significant Mo isotopic variation should not be caused solely by the isotopic fractionation during the subduction. Instead, δ98/95Mo values of the Java basalts positively correlate with Pb isotopic ratios. This implies that the Mo isotopic variations in the Java arc rocks should result from the metasomatism in the mantle wedge by hybrid agents, including varying proportions of melts from subducted sediments (with heavy Mo isotope) and melts from the subducted altered upper oceanic crust (SAOC) (with light Mo isotope). The light Mo isotope of the Java arc rocks, compared with the Mariana arc basalts, suggests that melts from the SAOC have much lighter Mo isotopic compositions than the components from the lower oceanic crust. Thus, Mo isotope has great potential to distinguish the components from the subducted upper and lower oceanic crust. The Java CABs show along-arc variations in Mo-Sr-Nd-Hf-Pb isotopes, which is related closely with the thermal status of the subducted slab. Upwelling of the asthenosphere due to the slab tearing beneath the Java arc might have enhanced the partial melting of subducted sediments nearby the slab window. The complicated subduction system in the Sunda arc has strongly controlled the geochemical composition of arc magmas, which changes with input of different subduction components into the mantle wedge along arc. [ABSTRACT FROM AUTHOR]

This study aims to review the Jurassic to Cretaceous tectonic setting of the Sanandaj-Sirjan Zone (SaSZ), the inner part of the Zagros orogen. For many years, the SaSZ was interpreted as a part of the Cimmerian microcontinent carrying a Jurassic magmatic arc to the Neo-Tethyan Ocean. Recently, propagating continental rift and mantle plume models have also been considered for this magmatism without taking into consideration the presence of coeval high-pressure metamorphic rocks and S-type granites predating I- and A-type granites. In the middle part of the SaSZ, the association of high-pressure metamorphic basement rocks and S-type granites suggests a collisional tectonic setting during the Early Jurassic, followed by partial melting by decompression and exhumation. These phenomena are considered to reflect compressive forces from the collision of the Cimmerian microcontinent with the Turan plate contemporaneous with the opening of the Neo-Tethys Ocean. The heterogeneity between the rheologically stiff and heavy Upper Neoproterozoic to Cambrian basement with arc-related mafic magmatic rocks and weak Permian-Mesozoic cover rocks of the SaSZ controlled the downward motion of the former. We propose that Early Jurassic intracontinental subduction predates subduction of the Neo-Tethys oceanic lithosphere underneath the SaSZ, and the formation of Jurassic magmatism resulted from plate collision and break-off of the subducted slab of the Paleo-Tethyan Ocean and of its associated former passive margin. The Neo-Tethys subduction and formation of the active continental margin started only during Cretaceous times. The lower mantle Mesopotamia anomaly detected by mantle tomography could represent the subducted Paleo-Tethyan slab. [ABSTRACT FROM AUTHOR]

[Display omitted] • 2.07–2.03 Ga bimodal rocks characterize a late-to-post collisional magmatic event. • Basic rocks and C-type adakites representing mantle addition and crustal reworking. • Alkaline rock associated with upwelling of hot asthenosphere during slab break-off. • Lower crustal reworking of overthickened orogen-related Piedade crust. Understanding of ancient accretionary orogenic systems through the correlation between pre-collisional assemblages described in different tectonostratigraphic terranes is not a straightforward task. The investigation of late-to-post collisional rocks, associated with the end-stage of convergent margin interactions, can be a reliable alternative due to a higher preservation bias. A typical rock assemblage in such settings is represented by C-type adakites chronologically associated with alkaline rocks and basic rocks with intraplate and MORB-like signatures. The Piedade microcontinent, focus of the investigation, represents an important Archean crustal fragment enveloped by magmatic arc terranes, located in the southern segment of the Siderian to Orosirian Minas-Bahia orogenic system. In this work we present new petrographic, geochemical and isotopic (whole-rock Sm-Nd, and zircon U-Pb, and Lu-Hf) data for the bimodal rock association presently cropping out as metabasic and metagranitoid rocks intrusive within the Archean Piedade microcontinent. The timespan of ca. 2.07 Ga to 2.03 Ga represents the reworking of lower crustal lithologies plus the addition of mantle derived magmas during the late stages of the orogenic development. A comparison with similar assemblages of other orogenic systems from different ages demonstrates that the rock association of MORB- and intraplate mafic magmas with C-type adakites and alkaline granitoids is common throughout most of Earth eras, and records an event of lithospheric delamination, ie. slab break-off, followed by asthenospheric rise and partial melting of lithospheric mantle and lower crustal rocks. [ABSTRACT FROM AUTHOR]

The origin of the Chon Aike silicic large igneous province (SLIP) is a matter of intense debate, with contrasting hypotheses that range from intraplate settings linked to mantle plume impingement to subduction-related protracted arc magmatism in an active margin. In this study, we propose a new model for the origin of this SLIP based on a multidisciplinary dataset from Patagonia, a comprehensive literature review of southwestern Gondwana, and the results of 2-D thermochemical modeling. We demonstrate that the partial melting of subducted rocks during a massive slab break-off and the subsequent piecemeal sinking of a previously flattened oceanic lithosphere beneath southwestern Gondwana best reconciles most of the data from this magmatic province. Geophysical, geochronological, geochemical, and isotopic data from Chon Aike SLIP, combined with the understanding of the tectonic regime, ore deposits, Jurassic geological events in southwestern Gondwana and numerical modeling results, support an origin primarily linked to the partial melting of partially eclogitized metabasaltic and metasedimentary rocks, enhanced by a warm ambient mantle associated with supercontinent thermal insulation and the thermal effects of the Karoo mantle plume impingement. The demise of the flat slab through large-scale slab break-off would have led to the partial melting of a mixture largely composed of these extensively underplated components and mantle batches. These melts would have variably interacted with mantle and continental rocks and melts, resulting in the formation of most of the Chon Aike SLIP. The re-establishment of the magmatic arc after the initial stages of slab break-off seems to have only affected the northwestern part of this SLIP. [ABSTRACT FROM AUTHOR]

Carboniferous-Triassic magmatism in northern Qiangtang, central Tibet, China, played a key role in the evolution of the Tibetan Plateau yet remains a subject of intense debate. New geochronological and geochemical data from adakitic, Nb-enriched, and normal arc magmatic rocks, integrated with results from previous studies, enable us to determine the Carboniferous-Triassic (312-205 Ma), arc-related, plutonic-volcanic rocks in northern Qiangtang. Spatial-temporal relationships reveal three periods of younging including southward (312-252 Ma), rapid northward (249-237 Ma), and normal northward (234-205 Ma) migrations that correspond to distinct slab geodynamic processes including continentward slab shallowing, rapid trenchward slab rollback, and normal trenchward rollback of the Jinsha Paleotethys rather than the Longmuco-Shuanghu Paleotethys, respectively. Moreover, varying degrees of coexistence of adakites/High-Mg andesites (HMAs)/Nb-enriched basalt-andesites (NEBs) and intraplate basalts in the above-mentioned stages is consistent with the magmatic effects of slab window triggered by ridge subduction, which probably started since the Late Carboniferous and continued into the Late Triassic. The Carboniferous-Triassic multiple magmatic migrations and ridge-subduction scenarios provide new insight into the geodynamic processes of the Jinsha Paleotethys and the growth mechanism of the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

We integrated new and existing bedrock and detrital zircon dates from the Zealandia Cordillera to explore the tempo of Phanerozoic arc magmatism along the paleo-Pacific margin of southeast Gondwana. We found that episodic magmatism was dominated by two high-magma-addition-rate (MAR) events spaced ~250 m.y. apart in the Devonian (370-368 Ma) and the Early Cretaceous (129-105 Ma). The intervening interval between high-MAR events was characterized by prolonged, low-MAR activity in a geographically stable location for more than 100 m.y. We found that the two high-MAR events in Zealandia have distinct chemistries (S-type for the Devonian and I-type for the Cretaceous) and are unlikely to have been related by a repeating, cyclical process. Like other well-studied arc systems worldwide, the Zealandia Cordillera high-MAR events were associated with upper-plate deformation; however, the magmatic events were triggered by enhanced asthenospheric mantle melting in two distinct arc-tectonic settings--a retreating slab and an advancing slab, respectively. Our results demonstrate that dynamic changes in the subducting slab were primary controls in triggering mantle flare-up events in the Phanerozoic Zealandia Cordillera. [ABSTRACT FROM AUTHOR]

The Early Cretaceous represents a peak period of the North China Craton (NCC) destruction. A comprehensive analysis of crustal deformation during this period can reveal processes and dynamics of the destruction. The peak destruction of the NCC was associated with intense extension whose representative deformation products are metamorphic core complexes (MCCs), extensional domes and rifted basins. These MCCs occurred along both northern and southern margins of the NCC, and resulted from synchronous extension and magmatism, showing difference from the typical orogen-type MCCs in many aspects. The MCCs of the Early Cretaceous were replaced by extensional domes under relatively weak extension and uplift. In contrast to a major depression-type basin of the Early Cretaceous in the western NCC, rifted basins of the same age in the eastern NCC appeared as medium- to small-scale ones extensively. In the eastern NCC, the rifted basins north of the Bohai Bay are characterized by a feature similar to an active rift whereas those south of the Bohai present similarity to a passive rift. Various sorts of extensional structures developed during the peak destruction indicate a stable stress state of NE-SE extension over the entire central to eastern NCC, consistent with the plate margin-driven stress field. Spatial distribution of the extensional structures presents an 1800 km wide back-arc extension region in the central to eastern NCC, consistent with the Paleo-Pacific slab rollback model following flat subduction. Temporal-spatial variation of initial extension and volcanic activity during the peak period also supports the rollback model right after the flat oceanic slab. The crustal deformation evolution demonstrates that the peak destruction of the NCC took place after the B-episode compression of the Yanshan Movement of the earliest Early Cretaceous and terminated with onset of the C-episode compression of the earliest Late Cretaceous. [ABSTRACT FROM AUTHOR]

The Lhasa Terrane in southern Tibet is the leading edge of the Tibet-Himalaya Orogen and represents a fragmentary record of terminal oceanic subduction. Thus, it is an ideal region for studying magmatism and geodynamic processes that occurred during the transition from oceanic subduction to continental collision and/or oceanic slab breakoff. Here we examine a suite of early Cenozoic mafic rocks (ca. 57 Ma) within the central part of Lhasa Terrane, southern Tibet, which erupted during a transitional phase between the onset of India-Asia continental collision and Neo-Tethyan slab breakoff. These rocks display a geochemical affinity with magmas produced by fluid-fluxed melting of the mantle wedge within a subduction zone environment. The whole-rock element and Sr-Nd isotope compositions of these mafic rocks are similar to those of Cretaceous subduction-related magmatism in southern Tibet, demonstrating the sustained influence of the Neo-Tethys Ocean slab on the mantle wedge during the onset of the collision of India and Asia. The results of our geochemical forward modeling constrain the conditions of melt generation at depths of 1.3-1.5 GPa with significant fluid additions from the Neo-Tethyan slab. These results provide the first petrological and geochemical evidence that slab flux-related magmatism continued despite the commencement of continental collision. While existing studies have suggested that magmas were derived from melting of the Neo-Tethyan slab during this period, our new results suggest that additional magma generation mechanisms were active during this transitional phase. [ABSTRACT FROM AUTHOR]

Significant uncertainty remains regarding the exact timing and nature of subduction events during the closure of the Tethyan seas in what is now NW Iran. This study thus presents new geochemical compositions and U–Pb ages for a suite of volcanic rocks emplaced during Cenozoic volcanism in the west Alborz Magmatic Assemblage, which is commonly regarded as the back-arc of the Neotethyan magmatism in Central Iran. The subalkali basalts and andesites are dated to 57 ± 1.2 Ma, and are likely derived from a supra-subduction mantle wedge. Later, trachytic A-type rocks erupted from ∼42 to 25 Ma during an anorogenic (extensional) stage triggered by slab retreat and associated asthenospheric mantle influx. A-type melts were at least partly concurrent with lithospheric mantle magmatism implied by eruption of subalkali basalts–andesites around 26–24 Ma. Next, Amp-Bt trachybasaltic volcanism with high-Nb basaltic affinity at ∼19 Ma likely records slab deepening and slab partial melting, which reacted with the mantle wedge to produce the source material for the high-Nb basalts. Sr–Nd isotopic ratios for SE Ahar mafic as well as A-type rocks imply rather enriched mantle source(s). Some crustal contamination is implied by the presence of inherited zircons dominated by those derived from Neoproterozoic–Cambrian basement rocks and Carboniferous magmatism. Rhyolitic rocks with adakitic affinity probably mark the final volcanism in the study area. The adakitic rocks show crustal signatures such as high K and Th, probably formed as a consequence of higher temperature gradients, at crustal levels, imposed by both slab and mantle partial melts. [ABSTRACT FROM AUTHOR]

Late Cretaceous–Eocene granites are widespread in the Tengchong terrane and related to subduction and closure of the Neo–Tethyan Ocean. However, the reason of their compositional and spatial variations is still under explored. This study reports zircon U–Pb geochronological, whole–rock and Nd isotopic data for Guyong, Houqiao and Menglong granites in the Tengchong terrane. Our data show that the three plutons crystallized at 70.4 ± 0.3Ma, 64.4 ± 0.3Ma and 47.6 ± 0.2Ma. Guyong monzogranites show an I–type affinity and may have experienced fractional crystallization of plagioclase, biotite and sphene. Negative εNd(t) values (−9.19 to −11.01) with two-stage model ages (T2DM = 1.59–1.78 Ga) indicate that Guyong monzogranites are derived from a predominantly Paleoproterozoic crustal source with little involvement of mantle–derived materials in an extensional setting. Lower εNd(t) (−9.33 to −11.63) and negative correlations of HREE, LREE, Th, and Y vs. SiO2 indicate that Houqiao plutons are S–type granites derived from a pure crustal source during crustal thickening. Low Zr/Hf and Nb/Ta ratios, zircon saturation temperature (TZr = 750°C–786°C) and negative δEu (0.11–0.40) of Menglong biotite granites indicate that they have experienced fractional crystallization of plagioclase, biotite and sphene. Relatively high εNd(t) (−6.81 to −7.07) values indicate that they are products of felsic magma mixed with mafic magma, which is also supported by numerous coeval mafic–intermediate rocks (Eocene) such as metagabbros, diorites and MMEs (mafic microgranular enclaves) in the western Tengchong terrane. A process of slab rollback and slab breakoff may have triggered the late Cretaceous–Eocene magmatism and the compositional variation of associated granitoids in the Tengchong terrane. [ABSTRACT FROM AUTHOR]

It is widely believed that the melting of mantle wedge in a subduction zone is triggered by the influx of fluids. However, experimental petrology studies have yielded conflicting results regarding temperatures for the wet solidus of mantle metasomatite, either below or above that of mantle wedge. In the latter case, the influx of fluids cannot trigger mantle melting, particularly in a cold subduction zone. Late Paleozoic magmatic rocks in the Western Yunnan Tethyan belt have been interpreted as being products of the first-stage subduction-related magmatism in the cold subduction zone and are thought to carry crucial information about mantle–wedge melting. Here, we report zircon ages and whole-rock geochemical data for these magmatic rocks to identify both the origin of the magmatism and the mechanism of mantle melting. Rock samples collected from the Nanlianshan complex and the Daxinshan Formation yielded zircon ages ranging from 303 Ma to 297 Ma. The samples can be classified into two groups based on geochemical features. Group I magmatic rocks have higher Ba/Th and lower Th/Nb ratios and depletion in Nd Hf isotopes [ε Nd (t) 2.9–5.0; ε Hf (t) 5.7–14.6], whereas Group II rocks show relatively lower Ba/Th and higher Th/Nb ratios and relative enrichment in Nd Hf isotopes [ε Nd (t) 0.7–4.0; ε Hf (t) 4.1–10.7]. The two groups of rocks may have originated from different mantle domains that were metasomatized by slab-derived aqueous fluids and by sediment-derived melts, respectively. We hypothesize that slab rollback during closure of the Paleo-Tethyan Ocean prompted mixing of different mantle domains; subsequently, the multiple mantle was heated by the laterally flowing asthenosphere, and partial melting occurred. • Two groups of Carboniferous magmatic rocks can be recognized in Nanlianshan, Tethyan Yunnan. • Slab-derived fluid and sediment-derived melt successively modified the mantle wedge. • Rollback of slab triggered melting of different mantle domains without extra fluid influx. [ABSTRACT FROM AUTHOR]

Early Cretaceous magmatic rocks of the Tengchong and Baoshan blocks provide insight into the late Mesozoic evolution of southeastern Tibet—Yunnan. We present LA-ICP-MS U-Pb zircon age, zircon ε Hf (t) isotope, and whole-rock geochemical and Sr-Nd isotopic data for dioritic porphyrite and rhyolite, outcropping in the Eastern Tengchong Belt and the Gaoligong Belt, respectively. The dioritic porphyrite has a 123.0 ± 1.3 Ma (2σ) age, negative whole-rock ℇNd (t) of −1.99 to −2.16, initial 87Sr/86Sr of 0.70690–0.70734, zircon ℇHf (t) of −1.42 to 4.11, and adakitic geochemical features, suggesting an origin from mixing of mantle- and crust-derived magmas; the rhyolite has a 122.2 ± 1.0 Ma age, low whole-rock ℇNd (t) of −9.69 to −8.83, initial 87Sr/86Sr of 0.71104–0.71246, and zircon ℇHf (t) of −8.55 to −2.59, suggesting crustal derivation. Our data—along with the published data—suggest that the Early Cretaceous rocks (140–109 Ma) comprise two magmatic phases with a geochemical and isotopic transition at ∼123 Ma. The 140–123 Ma phase—mainly outcropping in the Gaoligong Belt—is mainly peraluminous, highly silicic, and has negative zircon ε Hf (t) values, suggesting derivation from remelting of ancient crust in a postcollisional setting; the 123–109 Ma phase is mainly metaluminous, restricted to the Eastern Tengchong Belt, and formed in a continental-arc setting with mantle melt involved. We propose that the ∼123 Ma geochemical and isotopic change marks the transition from contraction due to the Tengchong-Baoshan collision to extension resulting from slab rollback of the Neo-Tethyan lithospheric slab. • The first report on the Early Cretaceous adakite-like rock in the Tengchong block. • The Early Cretaceous rocks have a geochemical and isotopic transition at ∼123 Ma. • The ∼123 Ma marks the transition from the collision to slab rollback of the Tethys. [ABSTRACT FROM AUTHOR]

Ar-Ar ages, and petrographical and geochemical characteristics of pyroclastics and an overlying lava from Teshima Island, southwest Japan are presented. Although previous geological and age data suggested Teshima pyroclastics were products of magmatism > 3 my prior to lava flows of Setouchi volcanic rocks generated in association with southward migration of the southwest Japan arc sliver during opening of the Sea of Japan backarc basin at 15 Ma, the present results led to the conclusion that a sequence of Setouchi volcanism, induced by slab melting and subsequent melt-mantle reactions, produced both pyroclastics and lava at 14.6-14.8 Ma. This age is oldest among those reported so far and may represent the timing of onset of characteristic Setouchi magmatism immediately posterior to and hence as a result of the mega-tectonic event including rotation of the southwest Japan arc sliver. [ABSTRACT FROM AUTHOR]

The transition of the geochemical signature in mafic rocks along the eastern Bangong-Nujiang suture in Tibet contains important information about geodynamic processes in the upper mantle. This study recognized two episodes of Early Jurassic gabbros from the Kaqiong terrane, a microblock within the Bangong-Nujiang suture zone. Early gabbros (ca. 197-191 Ma) appear as lenses in the basement complex and were overprinted by amphibolite/granulite-facies metamorphism at ca. 180 Ma. Later undeformed hornblende gabbros (ca. 177-175 Ma) occur as dikes intruding into the basement complex. The early metagabbros are characterized by arc-like geochemical features and enriched Nd-Hf isotopic compositions (whole rock eNd(t) = -0.7 to +0.3; zircon eHf(t) = -5.7 to -2.2), which suggests formation by partial melting of an enriched lithospheric mantle source. In contrast, the later hornblende gabbros have depleted Nd-Hf isotopic compositions (whole rock eNd(t) = +6.1 to +7.1; zircon e Hf(t) = +10.7 to +16.8) and normal mid-oceanic-ridge basalt (N-MORB)-type rare earth element (REE) features. They also show variable enrichments of fluid mobile elements (e.g., Rb, U, Pb), indicative of the input of slab-derived fluids in their mantle source. Thus, the hornblende gabbros were most likely originated from the asthenospheric mantle metasomatized by subducted oceanic slab-derived fluids. The transition in geochemical and isotopic compositions of these mantle-derived magmas reveals a longlasting lithosphere extension and thinning along the southern margin of the Qiangtang terrane in the Early Jurassic. Combined with geological observations, we propose that this transition has resulted from the southward rollback of the subducting Bangong-Nujiang Tethyan oceanic slab. The slab rollback could have initiated the overriding plate extension and the asthenosphere upwelling. Wider implications of this study are that an onset of slab rollback could be an important trigger for the transition of magmatic geochemistry in subduction zones. [ABSTRACT FROM AUTHOR]

Southern Tibet is a key region for investigating the magmatism that occurredduring the collision between India and Asia. The break‐off of the Neo‐Tethyan slab, after the onset of continental collision at ca. 65–60 Ma, affected the lithospheric thermal regime and led to a series of magmatic and tectonic events. Many magmatic rocks generated by slab break‐off have been reported in southern Tibet; however, which of these record the initial slab break‐off (i.e., slab tearing) is still an open question. The Lianxiang gabbros, which intrude an ophiolite in central‐southern Tibet, provide an ideal opportunity to investigate the footprint of the initial slab break‐off. Zircon U–Pb ages show that the Lianxiang gabbros crystallized at 57–53 Ma, coeval with other mafic dikes in southern Tibet. The Lianxiang gabbros are characterized by high Na2O (2.13–5.42 wt%), Al2O3 (13.86–17.13 wt%), and V (188–346 ppm) contents and moderate Mg# (40.8–65.9). The gabbrosare slightly depleted in light rare earth elements [LREE; (La/Yb)N = 0.45–0.76], similar to normal mid‐ocean ridge basalt (N‐MORB). They also have low 87Sr/86Sri ratios (0.703538–0.705698) and high εNd(t) values (9.49–10.68), indicating that they were derived mainly from a shallow asthenospheric mantle source. Combining our results with data from other magmatism at ~57 Ma in the southern Lhasa Terrane, we propose that the N‐MORB‐type rocks reflect the tearing of the Neo‐Tethyan slab. The process of slab break‐off differed between western and eastern southern Tibet, possibly as a result of different lithospheric structures and compositions. [ABSTRACT FROM AUTHOR]

Our field-based geochemical studies of the Triassic, Jurassic and Cretaceous granitoids on Hainan Island indicate that their magmas had different geochemical affinities, changing from alkaline in the Triassic through ocean island basalt (OIB) in the Jurassic, to calc-alkaline in the Cretaceous. We show that these changes in the geochemical affinities of the Mesozoic granitoids on Hainan and in SE China reflect different melt sources and melt evolution patterns through time. Our new geodynamic model suggests that: (1) Triassic geology was controlled by flat-slab subduction of the palaeo-Pacific plate beneath SE China. This slab dynamics resulted in strong coupling between the lower and upper plates, causing push-over tectonics and contractional deformation in SE China. Flat subduction-induced edge flow and aesthenospheric uprising led to the production of high-K granites, syenites and mafic rocks. (2) Slab foundering, accelerated subduction rates and subduction hinge retreat in the Early Jurassic caused rapid rollback of the downgoing slab. Strong decoupling of the upper and lower plates resulted in pull-away tectonics, producing extensional deformation in SE China. Decompression melting of the upwelling aesthenosphere produced OIB-type melts, which interacted with the subcontinental lithospheric mantle (SCLM) to form A- and I-type granitoids. (3) Segmentation of the palaeo-Pacific plate in the Early Cretaceous resulted in steeply dipping slabs and their faster rollback, facilitating lithospheric-scale extension and oceanward migration of calc-alkaline magmatism. This extensional deformation played a significant role in the formation of metamorphic core complexes, widespread crustal melting and development of a Basin and Range-type tectonics and landscape evolution in SE China. [ABSTRACT FROM AUTHOR]

The Neoproterozoic magmatism in South China may have been related to assembly and breakup of the supercontinent Rodinia. The ca. 815-Ma mafic intrusions in the Dahongshan region at the northern Yangtze Block are composed of fine- to medium-grained gabbro and diabase, which show variable major and trace element compositions with (La/Yb) N ratios of 0.53–3.43. The mafic rocks also have large ranges of initial 87Sr/86Sr ratios (0.70318–0.70810), ƐNd values (−2.8 to +4.3) and Pb isotopes (206Pb/204Pb = 17.57–18.46, 207Pb/204Pb = 15.43–15.56, 208Pb/204Pb = 37.45–38.12). These lines of evidence suggest that the Dahongshan mafic rocks underwent significant crustal contamination during the magma emplacement. AFC calculations reveal that 10–20% crustal materials were involved in their petrogenesis. Two primary samples from the Dahongshan intrusions display low U/Th (0.36–0.37), Th/Zr (0.001–0.002), and Nb/Y (0.02–0.03) ratios, and show DMM-like ƐNd and Pb isotopes, indicating that their primary melts were derived from an asthenosphere mantle which had not been modified by slab-derived materials. In the northern margin of the Yangtze Block, the Neoproterozoic magmatism changed from arc-affinity to MORB- and OIB-like characters at around 815 Ma. Such transition is proposed to have resulted from slab tearing and detachment that induced upwelling of the asthenosphere mantle. • The ca. 815 Ma mafic rocks were derived from an asthenosphere mantle. • The rocks experienced strong crustal contamination. • Slab detachment resulted in tectonic change in the northern Yangtze Block. [ABSTRACT FROM AUTHOR]

Postcollisional magmatism in the eastern Sakarya zone was recorded by voluminous basic volcanism and repeated plutonism during the early Cenozoic. The temporal and geochemical evolution of these magmatic rocks is important for understanding the possible geodynamic history of the Sakarya zone. Here, we investigated three representative plutons lying between the towns of Çamlıhemşin (Rize) and İspir (Erzurum), Turkey. These are largely composed of medium-K gabbroic diorites (Marselavat Pluton), shoshonitic monzonites (Güllübağ Pluton), and high-K granites (Ayder Pluton). We present whole-rock geochemistry, 40Ar/39Ar geochronology, and Sr, Nd, and Pb isotope analyses from the plutons to constrain the timing of variations in magmatism and source characteristics, and we provide a new approach to the proposed geodynamic models, which are still heavily debated. The 40Ar/39Ar geochronology reveals a cooling sequence from ca. 45 Ma for the Marselavat Pluton through ca. 41 Ma for the Güllübağ Pluton to ca. 40 Ma for the Ayder Pluton. Whole-rock geochemistry and Sr, Nd, Pb isotopes suggest that crustal contamination was not an important factor affecting magma compositions. Although there was no arc-related tectonic setting in the region during the middle Eocene, the Marselavat Pluton shows some subduction affinities, such as moderately negative Nb and Ta anomalies, and slightly positive Pb anomalies. These signatures were possibly inherited from a depleted mantle source that was modified by hydrous fluids released from the oceanic slab during Late Cretaceous subduction. Geochemical traces of the earlier subduction become uncertain in the Güllübağ samples. They display ocean-island basalt-like multi-element profiles and Nb/Ta, Ce/Pb, and La/Ba ratios. All these point to a mantle source in which earlier subduction signatures were hybridized by the addition of asthenospheric melts. Melting of calc-alkaline crustal material, probably emplaced during the first phase of middle Eocene magmatism (Marselavat), led to the formation of granitic plutonism (Ayder Pluton). Our data in conjunction with early Eocene adakite-like rocks show that melt generation, as in the given sequence, was most probably triggered by breakoff of the northern Neotethyan oceanic slab, ~13 m.y. after the early Maastrichtian collision between the Sakarya zone and Anatolide-Tauride block, and continued until the end of the middle Eocene. A shallow-marine transgression occurred contemporaneously with the middle Eocene magmatism throughout the Sakarya zone. An extension in this magnitude seems unlikely to be the result of orogenic collapse processes only. The main cause of this extension was most probably related to the northward subduction of the southern Neotethys Ocean beneath the Anatolide-Tauride block. The result is a volumetrically larger amount of middle Eocene magmatism than that expected in response to slab breakoff. [ABSTRACT FROM AUTHOR]

In this paper, we present zircon U Pb dating, elemental geochemistry, and Sr–Nd–Hf isotopic compositions for Late Carboniferous intrusive rocks in the northern Great Xing'an Range, northeast China. We decipher the mechanisms which triggered lithospheric thinning under microblock-microblock collision and provide a new perspective on the Late Paleozoic evolution of the eastern Central Asian Orogenic Belt. Zircon U Pb dating indicates that Jifeng gabbroic diorites and Xinlin granites were formed during a short time interval at 322–321 Ma, whereas the Xinlin gabbros were crystallized at 300 ± 2 Ma. The Jifeng gabbroic diorites exhibit typical arc characteristics, depleted in HFSEs and enriched in Pb and have positive ε Nd (t) (+1.88 to +2.00) and ε Hf (t) (+6.6 to +10.7) values, suggesting they originated from the partial melting of depleted lithospheric mantle modified by slab fluids. The Xinlin gabbros belong to alkaline series and are characterized by the enrichment in HFSEs with positive Nb and Ta anomalies and depleted Nd Hf isotopic compositions (zircon ε Hf (t) = +12.5 to +18.9, whole rock ε Nd (t) = +8.0 to +8.1). High Nb concentrations and low La/Nb (0.81–0.83) and La/Ta (12.50–14.61) ratios of the Xinlin gabbros indicate an origin from a depleted asthenospheric mantle. The Xinlin granites have high-K calc-alkaline compositions, belonging to I-type granites. These granites have positive ε Hf (t) values (+0.2 to +15.1) and juvenile two-stage model ages (1314–506 Ma) with low Sr and high Yb contents, indicating they were formed by the partial melting of a juvenile lower crust under low-pressure conditions. These data, together with the temporal and spatial distribution and the geochemistry characteristics of the Carboniferous igneous rocks in the Great Xing'an Range, suggest that the amalgamation between the Erguna–Xing'an and Songnen blocks occurred before the Late Carboniferous. The occurrence of asthenospheric mantle-derived mafic rocks indicates localized thinning process of lithosphere. Late Carboniferous magmatism on the Erguna–Xing'an Block attributed to dripping or piece removal of the lithosphere due to asthenospheric fluctuation triggered by slab break-off. • Lithospheric and asthenospheric sources of mafic rocks represent the different stage of post-collisional magmatism. • The amalgamation of the Xing'an and Songnen blocks occurred before late Early Carboniferous. • Lithospheric dripping triggered by slab break-off generated the planar Late Carboniferous post-collisional magmatism. [ABSTRACT FROM AUTHOR]

The thermal structure of subduction zones controls many important processes such as metamorphic devolatilization, arc magmatism, and seismicity. However, the thermal state of subducted oceanic slab predicted by subduction zone thermal models down to ∼80 km depth is systematically cooler than inferred from exhumed metamorphic slab rocks, raising questions about the current understanding of subduction dynamics portrayed by these models. Upon synthesizing previous petrological studies and estimating slab ages of ancient subduction zones from metamorphic terranes, we think that exhumation of subducted oceanic metamorphic rocks is extremely rare and likely requires unusual processes that are not an integral component of normal subduction dynamics. We construct simple numerical scenarios to illustrate how the thermal regime under some unusual conditions at the beginning and ending stages of a subduction margin may deviate from the normal subduction process. At the beginning stage of subduction, a shallow maximum decoupling depth (MDD) between the slab and mantle wedge enables viscous mantle wedge flow to reach a shallow depth, bringing heat to make slab rocks warmer than in a mature subduction zone. At the ending stage of subduction, if subduction cessation is in the form of slab stalling and if the stalled slab is not immediately exhumed, the slab rocks can be warmed up by the heat from the warm mantle wedge and underlying asthenosphere. In either situation, the slab rocks can be much warmer than normal before they are exhumed by other dynamic processes. Observed exhumed rocks are not expected to represent the normal subduction process and should not be directly compared with thermal models that are designed for the normal process. To extract important information about general geodynamic processes from these rocks, we must first understand the processes these rocks went through. • Exhumed metamorphic oceanic rocks are derived from very young to very old slabs. • Exhumed slab rocks likely represent conditions not typical of normal subduction. • The rocks may not be directly comparable with models designed for normal subduction. • Shallow mantle wedge flow can make slab rocks hot at the beginning of subduction. • Warming of stalled slab can make slab rocks hot at the ending of subduction. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Gaga diorite with adakitic features was generated from Neo-Tethyan oceanic slab. • Mafic enclave was formed by mixing of melts from lithosphere mantle and oceanic slab. • The genetic transition of Eocene granitoids implies the process of slab breakoff. The India-Asia collision holds significant implications for understanding continental collision dynamics. It is essentially controlled by various dynamic processes, including oceanic slab subduction, slab rollback, and slab breakoff, all of which are accompanied by extensive magmatic activities in convergent margins. However, the magmatic consequences of the Neo-Tethyan slab breakoff during the India-Asian collision remain ambiguous. In the middle segment of the southern Lhasa terrane, two episodes of magmatism (∼55 Ma and ∼ 50 Ma) have been preserved. In this study, we present whole-rock major and trace elements, zircon U-Pb ages, and whole-rock Sr-Nd-Hf isotopes of the ∼ 55 Ma Gaga pluton and conduct a comprehensive comparison with the two episodes of magmatic mixing events. The Gaga host diorites were formed by the partial melting of the Neo-Tethyan oceanic slab and underwent amphibole fractional crystallization. The Gaga gabbroic enclaves, which are associated with magma mixing, originated from lithosphere mantle metasomatized by subduction-related fluids. Notably, the Gaga pluton exhibits more prominent arc-type characteristics of Nb-Ta and Zr-Hf negative anomalies than the ∼ 50 Ma intrusive rocks in adjacent areas. The genetic transition of Eocene magmatic rocks indicates that the ∼ 55 Ma Gaga pluton formed during the initial stage of slab breakoff, and the ∼ 50 Ma magmatic pluton was produced during the late stage of slab breakoff. [ABSTRACT FROM AUTHOR]

• Northward migration of arc magmatism and formation of ca. 802–704 Ma and ca. 759–685 Ma magmatic suites. • The two magmatic suites show contrasting geochemical and Sr-Nd-Hf isotopic features. • Magmatism associated with slab rollback during Neoproterozoic oceanic subduction. The South Qinling Belt was located in the northern part of the Yangtze Block during the Neoproterozoic. Here we investigate two major magmatic intrusions in the northern and southern part of the belt, the Douling plutons (DLP) and the Fenghuangshan plutons (FHSP), to gain insights into the Neoproterozoic tectonic processes along the northern segment of the Yangtze Block. Zircon grains in the DLP yield weighted mean 206Pb/238U ages of 750–709 Ma, whereas those from the FHSP show ages of 749–728 Ma. The new results, together with the published data, reveal that these plutons were emplaced at ca. 759–685 Ma and ca. 802–704 Ma, respectively. Geochemically, the intermediate-felsic rocks from the DLP and FHSP show enrichment of light rare earth elements (LREE) and large-ion lithophile elements (LILE) (Rb, Ba and K), depletion of high field strength elements (HFSE) (Nb, Ta, P and Ti), and low Sr/Y and (La/Yb) N values, which are typical features of arc magmatic rocks. The gabbros from the FHSP have high Mg#, and display LREE and LILE (Rb, Ba, K, and Sr) enrichment and depletion of HFSE (Th, U, Nb, Ta, and Ti), suggesting that they were probably formed in a mantle wedge setting. Both DLP and FHSP show arc-related geochemistry modified by slab-derived fluids, and were formed at relatively high-temperature and low-pressure conditions. However, they display distinct Sr-Nd-Hf isotopic compositions. Zircon grains of two samples from the DLP respectively show mean ε Hf (t) values of −2.6 ± 0.3 and 0.9 ± 0.3, whereas zircon grains of four samples from the FHSP show mean ε Hf (t) values ranging from 7.5 ± 0.5 to 8.7 ± 0.4. The DLP shows dominantly negative ε Nd (t) values and inconsistent initial 87Sr/86Sr ratios, whereas FHSP shows relatively constant and positive ε Nd (t) values (3.69–3.74) but variable initial 87Sr/86Sr ratios. The Sr-Nd-Hf isotopic and geochemical features suggest that the enriched mantle and crustal materials were involved in the magma evolution of the DLP, whereas the FHSP is characterized by significant juvenile components with a rising asthenospheric mantle source playing a key role. We propose a subduction-related setting for these intrusions, and envisage slab rollback during the Neoproterozoic. [ABSTRACT FROM AUTHOR]

Abstract Widespread Cenozoic basalts, erupted at ca. 24–0.16 Ma in central Inner Mongolia, eastern China, are significant for understanding the potential links between deep subduction of oceanic slabs and the genesis of continental intra-plate magmatism. Here we examine the whole-rock geochemical and isotopic data, as well as olivine compositions, for Cenozoic alkali and tholeiitic basalts from the Abaga, Huitengliang, and Chifeng areas in central Inner Mongolia. The results suggest that fractional crystallization of olivine + clinopyroxene (in the alkali basalts) and olivine (in the tholeiites), played a key role in their magmatic evolution, without significant crustal contamination or post-magmatic alteration. Geochemical and petrogenetic analyses show that pyroxene and garnet were dominant phases in the residual mineral assemblage for both alkali basalts and tholeiites, implying a pyroxenite source for these basalts. We propose that a pyroxenite-dominated source may be the product of reaction between recycled oceanic slab-derived melts and surrounding mantle peridotites. The spatial and geochemical variations of the Cenozoic basalts in central Inner Mongolia, together with recent geophysical and geological observations in eastern China, imply that the origin and evolution of large-scale Cenozoic continental intra-plate magmatism can be related to subduction of the Paleo-Pacific and Paleo-Asian oceanic slabs. Incorporation of recycled oceanic materials from these two subducted slabs thus influenced the mantle source lithologies and deep-Earth geodynamic processes beneath eastern China. Graphical abstract Unlabelled Image Highlights • A pyroxenite source is inferred for the Abaga, Huitengliang and Chifeng basalts • Recycled altered oceanic crust contributed to the generation of the basalts • Geochemical variation relates to recycled Paleo-Asian and -Pacific oceanic slabs [ABSTRACT FROM AUTHOR]

Periadriatic Alpine magmatism has long been attributed to slab breakoff after Adria–Europe continental collision, but this interpretation is challenged by geophysical data suggesting the existence of a continuous slab. Here, we shed light on this issue based on a comprehensive dataset of zircon U–Pb ages and Hf isotopic compositions from the main western Periadriatic intrusives (from Traversella to Adamello). Our zircon U–Pb data provide the first evidence of Eocene magmatism in the Western Alps (42–41 Ma in Traversella), and demonstrate that magmatism started synchronously in different segments of the Alpine belt, when subduction was still active. Zircon U–Pb ages define younging trends perpendicular to the strike of the European slab, suggesting a progressive Eocene–Oligocene slab steepening. We propose that slab steepening enhanced the corner flow. This process was more effective near the torn edge of the European slab, and triggered Periadriatic magmatism in the absence of slab breakoff. [ABSTRACT FROM AUTHOR]

In the standard paradigm, continental crust is formed mainly by arc magmatism, but because the compositions of magma rising from the mantle are basaltic and continental crust is estimated to contain about 60% SiO 2 and much less MgO than basalt, the two do not match. To resolve this paradox, most researchers argue that large amounts of magmatic fractionation produce residual cumulates at the base of the crust, which because arcs are inferred to have magmatically thickened crust, form eclogites that ultimately founder and sink into the mantle. Not only are there problems with the contrasting bulk compositions, but the standard model also fails because prior to collision most modern arcs do not have thick crust, as documented by their eruption close to sea level, and in cases of ancient arc sequences, their intercalation with marine sedimentary rocks. Our study of Cretaceous batholiths in the North American Cordillera resolves the crustal composition paradox because we find that most are not arc-derived as commonly believed; but instead formed during the waning stages of collision and consequent slab failure. Because the batholiths typically have silica contents >60% and are derived directly from the mantle, we argue that they are the missing link in the formation of continental crust. Slab failure magmas worldwide are compositionally similar to tonalite-trondhjemite-granodiorite suites as old as 3.8 Ga, which points to their collective formation by slab failure and long-lived plate tectonics. Our model also provides (1) an alternative solution to interpret compiled detrital zircon arrays, because episodic peaks that coincide with periods of supercontinent amalgamation are easily interpreted to represent collisions with formation of new crust by slab failure; and (2) that models of early whole-earth differentiation are more reasonable than those invoking progressive growth of continental crust. [ABSTRACT FROM AUTHOR]

Marginal parts of a plate and subducting slab can play important roles in geodynamics. This is because in areas where a plate interacts with other plates or with the mantle thermal, geochemical, and mechanical interactions are expected. The Philippine Sea (PHS) slab that subducts beneath the Japan arcs has such an edge. To examine the relationship between arc magmatism and the slab edge in the transition zone from Northeast Japan to Central Japan, we investigated isotopic systematics of the regional volcanic rocks, incorporating data from literature and new data for five isotopic ratios of Sr, Nd, and Pb. The new data included major element compositions of 22 samples from the back-arc area, and 5 isotopic ratios for 6 samples selected from Pleistocene to early Quaternary epochs. Consequently, several findings were determined based on the spatial variation of the isotopic ratios and the estimated amount of slab-derived fluid: (1) the amount of fluid derived from the two subducting slabs (i.e., the Pacific slab and the PHS slab) decreases northward from a significantly high value (~ 5 wt% fluid added to the source mantle), away from the seismically determined edge of the PHS slab; (2) the proportion of the PHS component in the total slab-derived fluid also decays northward; and (3) the PHS component spreads to the north beyond the seismically determined edge of the PHS slab. These observations strongly suggest that the existence of an aseismic PHS slab beneath southernmost parts of Northeast Japan delivers the PHS component to the arc magmatism. As was indicated by previous geodynamical studies, subduction of the PHS and PAC slabs may generate suction force towards the corner of mantle wedge, which might account for the large amount of fluid near the seismically determined slab edge as described in (1) above. [ABSTRACT FROM AUTHOR]

The Central Asian Orogenic Belt (CAOB), characterized by its significant Phanerozoic accretion, develops tremendous post-Carboniferous granites in its southern part and their origins are still unclear, hindering our understanding on the CAOB late-staged evolution, especially its crust growth history. Herein, we presented zircon U-Pb-Hf-O and whole-rock geochemical and Sr Nd isotopic data for the granitic rocks of early Permian (minor late Carboniferous) from the Khan-Bogd area in southern Mongolia. These early Permian granitic rocks formed at ∼280–290 Ma, together with the coeval previously-studied A-type granites there, are mainly high temperatures (∼800–960 °C) granitic rocks and have contributions of deep substances such as relicts of oceanic slab, lithospheric or asthenospheric mantles, as suggested by their zircon Hf O, and whole-rock Sr Nd isotopes. Similar cases occurred commonly in other areas of southern CAOB and we infer that this is related to a significant shift of the early Permian non-subduction-related magmatism from the earlier arc magmatism. Combined with the accompanying significant contraction of the Paleo- Asian Ocean (PAO), we concur the occurrence of the closure of multiple secondary oceans of the PAO and later accompanying slab-breakoff occurred commonly after the Carboniferous in the southern CAOB. Thus, we propose that the relicts of oceanic slabs, as an important magma origin of these post-Carboniferous granites, were reasonably caused by the commonly-occurred slab breakoff and therefore, it is of great importance for the vertical accretion of the post-Carboniferous evolution of the CAOB, rather than mantle plume and other models. [Display omitted] • The studied early Permian granites have contributions of relicts of oceanic slab. • The relicts of oceanic slab probably resulted from oceanic slab breakoff. • The slab breakoff is an important vertical accretion mechanism there. [ABSTRACT FROM AUTHOR]

While chemical variability in volcanic arc lavas erupted perpendicular to the strike of subduction has been observed and studied for many years, how these variations may reflect slab dehydration or melting processes is still actively debated. Here we report new data for cross-arc geochemical variations in Quaternary volcanic rocks from the Kurile arc. Correlations among multiple isotopic tracers (B–Sr–Nd–Hf) and key elemental ratios (B/Nb, Ba/Nb, Th/Nb and Hf/Nd) show that these arc lavas reflect the influence of three discernible subduction components. Shallow slab-sourced low-temperature hydrous fluids (high B/Nb, Ba/Nb, and δ11B) and deeper high-temperature hydrous melts (moderate B/Nb, Ba/Nb and δ11B, and low Hf/Nd) show characteristics similar to those seen in Izu-Bonin-Mariana arc lavas and likely reflect similar slab-derived origins. In addition to these components, Kurile volcanic front samples document a component with high Th/Nb and depleted mantle-like Hf/Nd. This component may reflect Kurile forearc mantle that had been metasomatized by shallow slab-derived melts associated with the subduction of the Izanagi-Pacific ridge in the Eocene. We propose a new model to interpret the across-arc geochemical variations in the Kurile arc lavas. Kurile forearc mantle, with high Th/Nb due to Eocene ridge subduction, was added to the subduction channel via subduction erosion by the downgoing Pacific plate. This subduction interface reservoir was subsequently metasomatized by deep-sourced hydrous fluids and melts from the slab beneath the volcanic front. The gradual prograde metamorphic modification of these multiply-metasomatized, subducted materials provides the flux that triggered Kurile volcanic front magmatism. • Kurile arc lavas contain three subduction components. • Forearc mantle was metasomatized by shallow slab melt during Eocene ridge subduction. • Forearc mantle is eroded by recent subduction and fluxed by hydrous fluid/melt. • Decomposition of subducted forearc mantle triggered the volcanic front volcanism. [ABSTRACT FROM AUTHOR]