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Eocene igneous rocks along the Gangdese belt in southern Tibetan Plateau are important for understanding the India–Eurasia collision and Tibetan Plateau uplift. These magmatic rocks are widely considered to be related to roll-back or break-off of the Neotethyan slab during northward subduction. However, Eocene rhyolites in the northwestern Gangdese belt (SW Qiangtang margin) do not fit either the rollback or breakoff models. This paper investigates these Early Eocene lavas and compares them with contemporaneous igneous rocks along the Gangdese belt via detailed field observations, petrology, zircon geochronology, zircon Lu–Hf isotopes and whole-rock geochemistry. These felsic lavas form large outcrops in western Tibet and have two zircon U–Pb ages of 54.71 ± 0.14 Ma and 54.74 ± 0.27 Ma. They have high SiO2 and alkali contents, FeO*/MgO, and Ga/Al ratios as well as strongly negative Eu anomalies and slightly positive zircon εHf(t) values (+2.5 to +4.4). Both Ti-in-zircon and Zr saturation thermometers confirm high magmatic temperatures (~900°C). They are ferroan, A-type rhyolites. Furthermore, the distinctive low εHf(t) values imply that these rhyolites are likely partial melting products of the southwestern Qiangtang Terrane. The lithosphere in this region undergone long-term shortening and thickening since 80–95 Ma. Pre-thickened lithosphere may be a prerequisite for the Early Eocene delamination, and the simultaneous inducement is a far-field tectonic effect from the India–Eurasia collision. We suggest that: (1) India collided with Eurasia before 55 Ma and triggered delamination to form the Shiquanhe A-type rhyolites on the southwestern Qiangtang margin thereafter; and (2) tectonic stresses related to India–Eurasia collision during the Eocene were transmitted efficiently over large distances. [ABSTRACT FROM AUTHOR]

[Display omitted] • Lilong ultramafic igneous rocks are Late Cretaceous sulfide-bearing cumulates. • Mantle sources of Lilong ultramafic igneous rocks had been metasomatized by terrigenous sediment- and altered oceanic crust-derived fluids. • Deeply subducted oceanic crust plays an important role in the formation of sulfide-bearing juvenile mafic lower crust of magmatic arc. Post-collisional porphyry copper deposit (PCD) is a new genetic type of PCDs discovered in the Himalayan–Tibetan orogenic belt. It is believed that the formation of ore-forming magmas results from the re-melting of a sulfide-bearing, thickened, juvenile mafic lower crust in a magmatic arc associated with the oceanic plate subduction. However, the material contribution of subduction processes to the formation of this particular arc magma remains unclear. In this paper, we attempted to present a study that includes petrology, mineral geochemistry, whole rock Sr–Nd–Li isotopes, of Late Cretaceous Lilong ultramafic igneous rocks, south Lhasa sub-terrane, Tibet. The target ultramafic igneous rocks are sulfide-bearing cumulates. They show relative depletion in whole rock Nd isotopic compositions, slightly elevated (87Sr/86Sr) i ratios (0.7043–0.7059), and variable δ7Li values (2.8–9.0‰). Simulation results showed that trace element of melts in equilibrium with cumulus hornblende grains exhibit typical arc-like trace element distribution patterns, indicating that these ultramafic igneous rocks crystallized from arc magma. Additionally, the modeled melts showed enrichment in melt-mobile elements such as Ba, Pb, Sr, Th, and light rare earth elements (LREE), and higher Th/Yb ratios, as compared with normal mid-ocean ridge basalt (MORB). These geochemical properties can be explained by the metasomatism of depleted MORB mantle peridotites by hydrous melts derived from subducted seafloor sediments. Whole rock Sr isotopes show correlations with Ba/Nb and Th/La ratios for the modeled melts, showing that fluids from the basaltic igneous crust, in addition to hydrous melts from the seafloor sediments, contribute to their mantle source. In addition, the heterogeneity of whole rock δ7Li values for ultramafic igneous rocks also indicates that their mantle sources have been metasomatized by terrigenous sediment- and altered oceanic crust-derived fluids. Depleted MORB mantle peridotites react with the mixed fluids at sub-arc depths to produce ultramafic metasomatites, and model calculations for trace element contents and Sr–Nd–Li isotopic compositions further validate that the geochemical compositions of the Lilong ultramafic rocks can be clarified by this process. The results show that the deeply subducted oceanic crust may have played an important role in the formation of sulfide-bearing juvenile mafic lower crust of this magmatic arc. [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]

(1) Background: The widely developed thrust structures in the Jingzhushan Formation of late Cretaceous in Nyima Basin are related to the collision which leads to orogeny between the Lhasa and Qiangtang blocks. Clarifying the tectonic properties of the Nyima Basin is of great significance to characterize the evolution of the collision between the Lhasa block and the Qiangtang block. (2) Methods: Combined with mineralogy and petrology and U-Pb zircon dating of extrusive rocks, a detailed magnetic fabric test and rock magnetic experiment were carried out on the red sandstones of the Late Cretaceous Jingzhushan Formation in Nyima Basin. (3) Results: The sedimentary magnetic fabrics developed at the three sampling locations in the southern part of the basin are related to paleocurrents. The strained magnetic fabrics developed at one sampling site in the north, including pencil-like fabrics and tensile linear fabrics, are related to SE-NW tectonic stress. The average magnetic susceptibility value of the strained fabric with sandstone is lower than that of the sedimentary fabric, and its size is controlled by mica paramagnetic minerals. Ferromagnetic minerals are magnetite or hematite. In addition, the age of sandstone in the Jingzhushan Formation from the Nyima Basin matches the age of the eruptive rock in the Jingzhushan Formation from Coqen Basin, which is 92 Ma. (4) Conclusions: The strained fabric is developed near the thrust fault, which can explain the widely developed thrust structures in the basin. The Nyima Basin of the Jingzhushan Formation in the Late Cretaceous was an ocean-continent transitional foreland basin developed under the background of arc-arc "soft collision". [ABSTRACT FROM AUTHOR]

The timing of the Lhasa–Qiangtang collision following the closure of the Bangong–Nujiang Tethys Ocean has not been well constrained. An integrated study of whole-rock geochemistry and zircon U–Pb–Hf isotopes was carried out for Early Cretaceous quartz diorite-porphyrites and granites from the Yilashan and Amdo areas, northern Tibet. LA–ICP–MS zircon U–Pb dating reveal that the Yilashan and Amdo granitoids were emplaced at ~121–110 Ma. These granitic rocks display selective enrichment of light rare earth elements, large ion lithophile elements (e.g., Rb, U) and Th, but depletion of Sr and high field strength elements (e.g., Nb, Ta, Ti) compared to its neighboring elements. These new data, combined with regional geological setting, show that these igneous rocks were formed under a geodynamic setting of the Lhasa and Qiangtang (–Amdo) collision with oceanic slab breakoff and asthenospheric upwelling. The BNTO had been closed at ~121–110 Ma in the study area. Yilashan-Amdo granitoids roughly yield high (87Sr/86Sr)i ratios and obvious negative εNd(t) and zircon εHf(t) values along with old Nd TDM and zircon Hf TDM2 ages. Together with their variable U–Pb ages, these features indicate a Precambrian "hidden" crustal source beneath the northern Lhasa and Amdo terranes. The YLSS S-type granophyres were derived from partial melting of Paleoproterozoic lower crustal metagraywackes, whereas the YLSZ quartz diorite–porphyrites and the Amdo I- and A-type granites were mainly derived from partial melting of Paleo–Mesoproterozoic lower crustal mafic rocks with a certain amount of addition of mantle-derived melts. Minor amounts of the materials originated from the Amdo orthogneisses may also be involved in the formation of the YLSZ quartz diorite–porphyrites and the Amdo I-type granites. In addition, the Yilashan ophiolite was intruded by the ~112–108 Ma granophyric and quartz diorite–porphyritic intrusions before its final emplacement into the surrounding strata. [ABSTRACT FROM AUTHOR]

Mafic igneous rocks are volumetrically limited in collisional orogens but are significant for revealing specific geological processes. The mafic intrusive rocks in the eastern Lhasa batholiths along the Parlung fault (the easternmost branch of the Jiali fault zone), southern Tibet, are commonly regarded as the easternmost mafic components of an inferred Shiquanhe–Nam Tso–Jiali–Parlung suture zone, which is assumed to mark a back-arc oceanic basin related to the southward subduction of the Bangong-Nujiang Meso-Tethyan Ocean. However, the petrogenesis and tectonic setting of the mafic intrusive rocks have been poorly constrained. In this paper, we present petrological, geochemical and geochronological data for this mafic suite and evaluate the tectonic implications. The mafic intrusive rocks can be approximately divided into pyroxene-amphibole cumulate and medium-grained and pegmatitic hornblende gabbros that are characterized by the occurrence of very calcic plagioclase (An = 80–92) and abundant amphibole crystals, indicating an H 2 O-enriched magmatic signature. Their whole-rock compositions, which include negative Nb and Ta anomalies, also suggest a subduction-modified setting. Zircon Lu Hf isotope data show that their magmas were formed by magmatic mixing between melts derived from enriched subcontinental lithospheric mantle and liquids derived from upwelling asthenosphere. Zircon U Pb dating results reveal crystallization ages of ~114–110 Ma for this mafic suite, corresponding to the late stage of regional granitic magmatism (~135–110 Ma) that was related to the Lhasa-Qiangtang collision after the closure of the Bangong-Nujiang Meso-Tethyan Ocean. Given that coeval mafic and felsic rocks are distinctly controlled by the Parlung fault, we suggest that asthenospheric upwelling due to the break-off of the subducted Bangong-Nujiang oceanic slab not only induced partial melting of the enriched subcontinental lithospheric mantle but also resulted in regional rifting/extension and the initial development of the eastern Jiali (Parlung) fault. • Early Cretaceous mafic magmatism along the eastern Jiali fault is demonstrated. • The mafic suite marks the initial development of the eastern Jiali fault. • The active Jiali fault zone might exploit and connect the pre-existing faults. [ABSTRACT FROM AUTHOR]

Whole-rock geochemical data and compositional balance analysis (CoBA), combined with a magnetic susceptibility model obtained from a high-precision magnetic survey, are useful exploration tools in the Jiama porphyry–skarn deposit, southern Tibet, China. In this paper, three innovative aspects of the model are described that have assisted exploration of the Jiama deposit: an improved geological understanding of a thrust–nappe structure, hydrothermal fluid pathways, and magma mixing; data- and knowledge-driven compositional balance analysis of whole-rock geochemistry; and a high-precision magnetic survey of the deposit. The local strata provided the materials and natural trap for skarn formation, and a site for fluid circulation and porphyry ore precipitation. An interlayer detachment zone, gliding nappe structure, and cylindrical fissure system provided channels for fluid migration, and the Niumatang Anticline provided the accommodation space for the porphyry system. Miocene magma mixing and crustal contamination led to the development of complex mineralization in the Jiama area. The geochemical signatures obtained from CoBA, and the spatial distribution of the strata, structures, igneous rocks, alteration, mineralization, and degree of erosion enable integration of the geochemical and geological data. The "demagnetization skylight" identified by magnetic anomalies and a magnetic susceptibility model obtained with UBCmag3D software revealed the location of a concealed porphyry system. A magnetic susceptibility of 0.014–0.016 was found to identify the location of an interlayer skarn. Our exploration indicators of the Jiama porphyry–skarn deposit can be used to guide ore exploration throughout the region. • Meaningful geochemical associations are gained from data- and knowledge- driven CoBA. • Using geochemical associations to infer geology, alteration, mineralization and erosional degree of Jiama deposit. • Geophysics contributes to positioning porphyry complex and interlayer skarn. • A comprehensive exploration model involves multi-source information is proposed. [ABSTRACT FROM AUTHOR]

Whether there is a relationship between widespread Cenozoic igneous rocks and the activated Ailao Shan–Red River shear zone (ASRR) in western Yunnan, SE Tibet, remains highly controversial. New geochronological, and geochemical data derived from Cenozoic igneous rocks both within and far from the ASRR offers answers. Although these coeval felsic rocks (36.9–33.5 Ma) are enriched in large‐ion lithophile elements relative to high field strength elements, they also display some special geochemical features, such as high Y and Yb values of syenitic porphyries, which contrast with the high Sr and low Y contents of monzoporphyries and granitic porphyries. The syenitic porphyries exhibit geochemical characteristics similar to those of coeval mafic magmas, e.g., high K2O, Y, and Yb contents, indicating that the former were probably generated by fractional crystallization of the latter. However, the high Sr/Y (56.1–109) monzoporphyries and granitic porphyries, combined with their low MgO, and Cr (or Ni) contents, suggest that they were mainly derived from partial melting of a thickened mafic lower crust. Given that these Cenozoic magmatic events exhibiting planar distributions occurred outside of the activation of the ASRR (31–20 Ma), then they were not controlled by the latter. Instead, they were tied to convective removal of a thickened lower lithospheric mantle in western Yunnan during the Paleogene. [ABSTRACT FROM AUTHOR]

Subduction of oceanic lithosphere can cause crustal growth and destruction, but whether this balancing act is common in a single fossil convergent system has been unclear. Here we report geochronologic‐geochemical‐isotopic data on Mesozoic igneous rocks in the central‐eastern Gangdese arc. Our results reveal: (a) A coupling of magmatic arc migration toward the continent interior with a marked incompatible enrichment of sub‐arc mantle magma sources; (b) numerous Late Cretaceous adakites were generated via combined partial melting of both eroded forearc crust debris and subducted oceanic crust; (c) the truncation of forearc crust and extremely short arc‐suture distance. Our work demonstrates that although there was significant vertical growth of juvenile crust throughout the Gangdese arc during the Late Mesozoic, a large amount of continental crust was returned back into the mantle through lateral subduction erosion at the same time. This study provides important insights into long‐term material recycling on Earth. Plain Language Summary: Arc magmatism plays a significant role in the formation of new crust, tectonic erosion has also been suggested to represent a major process associated with the destruction of continental crust. Crust generation and destruction triggered by subduction sustain the volume of the continental crust in some modern arcs, but subduction erosion is difficult to identify in fossil convergent systems and whether or not this balancing act is a common process over long geological period has been unclear. Here we report geochronologic‐geochemical‐isotopic data on Mesozoic igneous rocks in the central‐eastern Gangdese arc, which is widely considered to have been a very important area for Phanerozoic crustal growth. We consider magmatism along with integrated tectonic and sedimentological evidence to constrain the subduction erosion history of the Gangdese arc. We propose that the southern part of Eurasia was a typical erosive plate margin during the Late Mesozoic comparable to modern Andean arc. Although there was significant vertical growth of juvenile crust throughout the Gangdese arc during the Late Mesozoic, a large amount of continental crust was returned back into the mantle through lateral erosion at the same time. This study provides important insights into long‐term material recycling and crustal evolution on Earth. Key Points: The Late Mesozoic Gangdese arc suffered extensive tectonic erosion triggered by the subduction of Neotethyan oceanic slabSignificant vertical growth and lateral erosion of continental crust occurred approximately contemporaneously in the Late Mesozoic GangdeseCrustal generation and destruction triggered by subduction are common and long‐term processes that sustain the volume of continental crust [ABSTRACT FROM AUTHOR]

The character of arcs varies over time with significant temporal fluctuations in the quantity and spatiotemporal patterns of magmatism. However, the driving mechanisms for this episodic behavior of arcs need more constraints. This paper analyzed the published data along with our new zircon U-Pb dating and Hf isotopic and whole-rock geochemical data of plutonic rocks in the Gangdese belt in southern Tibet to explore the features, potential drivers, and tectonic implications of episodic arc activity in the Gangdese arc. A comprehensive compilation of U-Pb ages and Lu-Hf isotopic analyses of zircon grains from igneous rocks in the Gangdese belt, sedimentary rocks in trench fill sequences, forearc basins and foreland basins, and sands from modern river reveals that: 1) Gangdese arc activity was episodic during Late Cretaceous to Middle Eocene, displaying two magmatic flare-ups (ca. 100–80 and 65–45 Ma) and one magmatic lull (ca. 80–65 Ma), and 2) both flare-up magmas show relatively positive ε Hf (t) values (+5 ~ +15) indicative of juvenile sources suggesting these magmas are dominated by contributions from the depleted mantle. In contrast, the magmatic lull between these two magmatic flare-ups could be caused by flat subduction of the Neotethyan slab beneath the southern margin of the Lhasa terrane. These flare-ups likely contributed greatly to the crustal thickening of the Gangdese belt. Constraints from paleo-elevation and geochemical proxies for crustal thickness showed that the ~100–80 Ma flare-up was accompanied by the formation of a thick arc root while the ~65–45 Ma flare-up likely developed in a thinner crust without an arc root. [Display omitted] • Gangdese arc activity was episodic, displaying magmatic flare-ups (~100–80 and ~65–45 Ma). • Flare-up magmatism contributed to crustal thickening. • Gangdese batholith had an arc root at ~90 Ma. [ABSTRACT FROM AUTHOR]

[Display omitted] • The ~91 Ma Chemen intrusion represents a magmatic product of the Late Cretaceous Gangdese magmatic 'flare-up' event. • The Chemen intrusion was derived from partial melting of Neo-Tethys oceanic crust during ridge subduction. • Compositions of zircon and apatite indicate that the Chemen intrusion has a high mineralization potential. • The mineralization potentials of some other Late Cretaceous intrusions may be limited due to low magma oxygen fugacities. Large-scale Late Cretaceous magmatic intrusions have been recognized in the southern Gangdese magmatic belt, although their genesis and mineralization potential remain controversial. In this paper, we present new zircon U-Pb ages, whole-rock compositions, and major- and trace element compositions of zircon and apatite from the Chemen granodioritic intrusion. When integrated with compiled literature data for other Late Cretaceous igneous rocks in the southern Gangdese belt, our new dataset provides new insight into their genesis and mineralization potential. The Chemen intrusion obtains a zircon U–Pb age of ~91 Ma, similar to the age of Late Cretaceous magmatic intrusions across the entire southern Gangdese belt. Both the Chemen and other Late Cretaceous intrusions have geochemical characteristics that resemble those of adakites. When considered alongside the coeval formation of mafic intrusive rocks, high-temperature adakitic charnockites, and thickening of continental crust in the region, ridge subduction is considered most likely to be responsible for this 'flare-up' event. The Ce4+/Ce3+ ratios in zircon and SO 3 contents of apatite from the Chemen granodiorites both indicate a high oxygen fugacity during crystallization. Further, trace elements in Chemen apatite indicate that the Chemen intrusion has a great mineralization potential; however, no mineralization has been found in some of the Late Cretaceous intrusions in the southern Gangdese belt. Compiled data show that these barren Late Cretaceous intrusions generally formed from magmas with a low oxygen fugacity. Thus, we speculate that the low oxygen fugacity of some Late Cretaceous intrusions may limit the extent to which mineralization may occur. [ABSTRACT FROM AUTHOR]

Successive indentations of Eurasia by India have led to the Tibet-Himalaya E–W orthogonal collision belt and the SE Tibetan Plateau N–S oblique collision belt along the frontal and eastern edges of the indenter, respectively. The belts exhibit distinctive lithospheric structures and tectonic evolutions. A comprehensive compilation of available geological and geophysical data reveals two sudden tectonic transitions in the early Eocene and the earliest Miocene, respectively, of the tectonic evolution of the orthogonal belt. Synthesizing geological and geochronological data helps us to suggest a NEE–SWW trending, ~450 km-long, ~250 km-wide magmatic zone in SE Tibet, which separates the oblique collision belt (eastern and SE Tibet) into three segments of distinctive seismic structures including the mantle and crust anisotropies. The newly identified Yongping basin is located in the central part of the magmatic zone. Geochronological and thermochronological data demonstrate that (1) this basin and the magmatic zone started to form at ~48 Ma likely due to NNW–SSE lithosphere stretching according to the spatial coincidence of the concentrated mantle-sourced igneous rocks on the surface with the seismic anomalies at depth; and (2) its fills was shortened in the E–W direction since ~23 Ma. These two dates correspond to the onset of the first and second tectonic transitions of the orthogonal collision belt. As such, both the orthogonal and oblique belts share a single time framework of their tectonic evolution. By synthesizing geological and geophysical data of both collision belts, the indenting process can be divided into three stages separated by two tectonic transitions. Continent–continent collision as a piston took place exclusively during the second stage. During the other two stages, the India lithosphere underthrust beneath Eurasia. [ABSTRACT FROM AUTHOR]

Dong, J.-H.; Zou, H.; Liu, H.; Li, Y., and Jiang, X.-W., 2019. The Mesozoic closure of the Paleo-Tethys Ocean: Constraints from zircon u–pb ages, hf and Nd isotopes of the granitic plutons in the Hoh-Xil-Songpan-Ganzi. In: Gong, D.; Zhu, H., and Liu, R. (eds.), Selected Topics in Coastal Research: Engineering, Industry, Economy, and Sustainable Development. Journal of Coastal Research, Special Issue No. 94, pp. 101–106. Coconut Creek (Florida), ISSN 0749-0208. The Hoh-Xil-Songpan-Ganzi (HXSG) complex is a large triangular fold located in the eastern part of the Tibet Plateau, which recorded of the Closure of Paleo-Tethys Ocean and recorded the tectonic activities of convergence between the three blocks of the Yangtze, North China and Qiangtang during early Mesozoic. This contribution combines geological and Hf-Nd isotopic data for igneous rocks from the HXSG complex, together with the salient features to gain insights into the crustal architecture and tectonic evolution. Two period tectonic-magmatism activities of sys-collision magmatism (ca. 243-227 Ma) and post-collision magmatism (ca. 219-185 Ma) have been recognized in the HXSG complex during Early Mesozoic. The large-scale intermediate-felsic magmatism include high Ba-Sr granite, adakite granitoids, I-type and A-type granitoids, strong aluminous leucogranites. The depth of the source region is different from the mantle, the crust and mantle transition zone, the lower crust and the upper middle crust. As for the closure of the Paleo-Tethys Ocean, the subduction of the Paleo-Tethys oceanic lithosphere leads to the convergence of the North China Block, the Yangtze Block and the Qiangtang Block, the HXSG complex shortened and thickened, the thickened lithosphere was delaminated, then the asthenosphere further upwelled to melting the thicken lower crust, the HXSG complex entered the intraplate extensional tectonic setting, the magmatism involved the partial melting of the lithospheric mantle and even the asthenosphere mantle. [ABSTRACT FROM AUTHOR]