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The latest Cretaceous magmatic activity in the eastern segment of the Lhasa terrane provides important insights for tracking the magma source and geodynamic setting of the eastern Gangdese batholith, eastward of eastern Himalayan Syntaxis. Detailed petrological, geochemical and geochronological studies of the intrusive rocks (monzodiorites and granodiorites) of the eastern Gangdese batholith are presented with monzodiorites and granodiorites giving zircon U–Pb crystallization dates of 70–66 Ma and 71–66 Ma with εHf(t) values of –4.8 to +6.2 and –1.9 to +5.3, respectively. These rocks are metaluminous to weakly peraluminous I‐type granites showing geochemically arc‐related features of enrichment in LREEs and some LILEs, e.g., Rb, Th, and U, and depletion in HREEs and some HFSEs, e.g., Nb, Ta, and Ti. The rocks are interpreted to be derived from partial melting of mantle material and juvenile crust, respectively, which are proposed to be triggered by Neo‐Tethyan slab rollback during northward subduction, with both experiencing ancient crustal contamination. The studied intrusive rocks formed in a transitional geodynamic setting caused by Neo‐Tethyan oceanic flat subduction to slab rollback beneath the eastern Gangdese belt during the latest Cretaceous. [ABSTRACT FROM AUTHOR]

High‐Sr/Y granitic rocks, characterized by intermediate–high SiO2 contents (> 56 wt.%), high La/Yb ratios (> 20), and low Y (< 18 ppm) and Yb (< 1.9 ppm) contents, have drawn much attention from geologists in the past decades. They were originally named adakites and defined as granitic magma in convergent plate margins formed by partial melting of subducted young (< 25 Ma), hot, and hydrated oceanic slab. Recent studies suggest that these rocks can also be formed in a variety of non‐subduction‐related tectonic settings through different petrogenetic processes including partial melting of thickened lower continental crust, providing important constraints on crustal growth and evolution throughout the Earth's history. The Tibetan plateau, generally considered as an archetype of collisional orogen, was formed by the successive accretion of different tectonic terranes to the southern margin of the Eurasia plates since the Early Paleozoic. Geologically, the Tibetan plateau comprises several W–E trending terranes (the Kunlun, North Qiangtang, South Qiangtang, and Lhasa terranes) and northernmost India (the Himalaya region). These terranes have undergone significant crustal thickening during the continental collision events. The crustal thickness of the Tibetan plateau is asymmetric, being thickest under the Lhasa terrane and thinning more gradually towards the north than to the south. From south to north, the crustal thickness increases from ∼50 km in the Himalaya region, to ∼70–80 km under the Lhasa terrane, and decreases to ∼65 km under the North and South Qiangtang terranes and ∼55 km beneath the Kunlun terrane. The present crustal thickness of the Tibetan plateau has generally been attributed to the Cenozoic collision between the Indian and Eurasian plates and the subsequent intracontinental convergences since ∼55 Ma. However, doubts were raised about this interpretion in the past few years. For example, recent studies indicated the crust of the Lhasa terrane had been significantly thickened during ca. 140–130 Ma in response to the subduction of the Neo‐Tethyan Ocean, although the Cretaceous marine strata across the Lhasa terrane demonstrates that it was thin enough to be within the reach of sea level during at least the early Late Cretacous. Whether the South Qiangtang terrane underwent crustal thickening before Cenozoic or not remains an unanswered question. In this study, new zircon U–Pb geochronologic and Hfisotopic, and whole‐rock geochemical data of the Late Cretaceous high‐Sr/Y granitoids from the eastern South Qiangtang terrane are presented. LA–ICP–MS zircon U–Pb dating produced ages (92.0 ± 1.1 Ma, 88.8 ± 0.7 Ma, and 91.5 ± 0.6 Ma) of formation for the Leiwuqi granitoids of the South Qiangtang terrane, Tibetan Plateau, indicating that they were formed during the Late Cretaceous. The granitoids have low A/CNK ratios (0.94–1.06; <1.10) and P2O5 contents (0.02–0.12 wt.%), geochemically similar to I‐type granitoids. They are characterized by high Sr (350–938 ppm, most > 400 ppm), low Y (5.94–9.67 ppm), with high Sr/Y (36–119, most > 40) and (La/Yb)N ratios (67–394). They exhibit negative zircon ∊Hf (t) values (−9.8 to −1.1) and low MgO (0.32–0.69 wt.%), Cr (2.07–12.17 ppm), and Ni (1.10–5.94 ppm) contents. These features suggest that the granitoids probably were derived from partial melting of thickened lower continental crust (> 50 km). The crust of the South Qiangtang terrane (at least its eastern part) may have been significantly thickened before the Late Cretaceous. [ABSTRACT FROM AUTHOR]

High‐Sr/Y granitic rocks, characterized by intermediate–high SiO2 contents (> 56 wt.%), high La/Yb ratios (> 20), and low Y (< 18 ppm) and Yb (< 1.9 ppm) contents, have drawn much attention from geologists in the past decades. They were originally named adakites and defined as granitic magma in convergent plate margins formed by partial melting of subducted young (< 25 Ma), hot, and hydrated oceanic slab. Recent studies suggest that these rocks can also be formed in a variety of non‐subduction‐related tectonic settings through different petrogenetic processes including partial melting of thickened lower continental crust, providing important constraints on crustal growth and evolution throughout the Earth's history. The Tibetan plateau, generally considered as an archetype of collisional orogen, was formed by the successive accretion of different tectonic terranes to the southern margin of the Eurasia plates since the Early Paleozoic. Geologically, the Tibetan plateau comprises several W–E trending terranes (the Kunlun, North Qiangtang, South Qiangtang, and Lhasa terranes) and northernmost India (the Himalaya region). These terranes have undergone significant crustal thickening during the continental collision events. The crustal thickness of the Tibetan plateau is asymmetric, being thickest under the Lhasa terrane and thinning more gradually towards the north than to the south. From south to north, the crustal thickness increases from ∼50 km in the Himalaya region, to ∼70–80 km under the Lhasa terrane, and decreases to ∼65 km under the North and South Qiangtang terranes and ∼55 km beneath the Kunlun terrane. The present crustal thickness of the Tibetan plateau has generally been attributed to the Cenozoic collision between the Indian and Eurasian plates and the subsequent intracontinental convergences since ∼55 Ma. However, doubts were raised about this interpretion in the past few years. For example, recent studies indicated the crust of the Lhasa terrane had been significantly thickened during ca. 140–130 Ma in response to the subduction of the Neo‐Tethyan Ocean, although the Cretaceous marine strata across the Lhasa terrane demonstrates that it was thin enough to be within the reach of sea level during at least the early Late Cretacous. Whether the South Qiangtang terrane underwent crustal thickening before Cenozoic or not remains an unanswered question. In this study, new zircon U–Pb geochronologic and Hfisotopic, and whole‐rock geochemical data of the Late Cretaceous high‐Sr/Y granitoids from the eastern South Qiangtang terrane are presented. LA–ICP–MS zircon U–Pb dating produced ages (92.0 ± 1.1 Ma, 88.8 ± 0.7 Ma, and 91.5 ± 0.6 Ma) of formation for the Leiwuqi granitoids of the South Qiangtang terrane, Tibetan Plateau, indicating that they were formed during the Late Cretaceous. The granitoids have low A/CNK ratios (0.94–1.06; <1.10) and P2O5 contents (0.02–0.12 wt.%), geochemically similar to I‐type granitoids. They are characterized by high Sr (350–938 ppm, most > 400 ppm), low Y (5.94–9.67 ppm), with high Sr/Y (36–119, most > 40) and (La/Yb)N ratios (67–394). They exhibit negative zircon ∊Hf (t) values (−9.8 to −1.1) and low MgO (0.32–0.69 wt.%), Cr (2.07–12.17 ppm), and Ni (1.10–5.94 ppm) contents. These features suggest that the granitoids probably were derived from partial melting of thickened lower continental crust (> 50 km). The crust of the South Qiangtang terrane (at least its eastern part) may have been significantly thickened before the Late Cretaceous. [ABSTRACT FROM AUTHOR]

The Meso‐Tethys Ocean is generally considered to have opened in the late Early Permian as a result of the Cimmerian continental block drifting away from the Gondwana supercontinent. This ocean is also termed the north branch of the Neo‐Tethys Ocean, and is now represented by the Bangong–Nujiang suture zone in the north‐central Tibetan plateau. However, it is still unknown for the evolutionary history for as such a huge ancient ocean basin. Ophiolites are remnants of oceanic lithosphere and preserve key information in rebuilding the evolutionary history of ancient oceans. In this study, we undertook detailed geological mapping for the Ren Co ophiolite in the middle part of the Bangong–Nujiang suture zone, and a typical Penrose‐type ophiolite sequences was newly documented in the Ren Co area. The Ren Co ophiolite comprises serpentinized peridotite, cumulate rock, gabbro/diabase, sheeted dike, massive and pillow lavas, and minor red chert. These rocks exhibit well‐preserved mantle and crust rock suites, and show close similarities to oceanic lithospheres at modern fast‐spreading ridges. Zircon U–Pb dating for gabbro and plagiogranite yielded ages of 169–147 Ma, which suggest that the Ren Co ophiolites were formed during the Middle to Late Jurassic. Harzburgite in the Ren Co area shows similar features to those of abyssal peridotite indicating the residues of the oceanic mantle. Mafic rocks (basalt, diabase and gabbro) of the Ren Co ophiolite show geochemical features similar to those of typical N‐MORB. Moreover, all samples have positive whole‐rock ∊Nd(t), and zircon ∊Hf(t) and mantle‐like δ18O values. These features suggest that these rocks were derived from a depleted mantle source, thus the Ren Co ophiolite was typical MOR‐type ophiolite. We suggest that the Ren Co ophiolite was formed in a fast‐spreading mid‐ocean‐ridge (MOR) setting, and they most likely represented the ridge segment of the Bangong‐Nujiang Meso‐Tethys Ocean. The Bangong–Nujiang Meso‐Tethys Ocean was a wide paleo‐ocean, and experienced continuous oceanic spreading, subduction, accretion before final Lhasa and South Qiangtang continental assembly. [ABSTRACT FROM AUTHOR]

Discontinuous ophiolite suites along the Yarlung Zangbo suture zone (YZSZ) in southern Tibet representing remnants of the Neo‐Tethyan oceanic lithosphere are considered to be formed between Late Jurassic and Early Cretaceous. Older ophiolite or ophiolitic mélange (e.g. Triassic) to reveal the initial evolution the Neo‐Tethyan ocean within the YZSZ have rarely been documented so far. The western YZSZ extending from the Saga to Ladakh area are composed of the northern ophiolitic sub‐belt, the Zhongba terrane and the southern ophiolitic sub‐belt. In this study, we document structural, petrological and geochronological data of mafic intrusions from the Mayoumu massif in the southern ophiolitic sub‐belt of the western YZSZ. Two lithos‐tectonic sub‐units, the southern ophiolitic complex and the northern ophiolitic mélange, are recognized in the Mayoumu massif in terms of structural pattern and petrology. LA‐ICP‐MS zircon U‐Pb dating of gabbro from the ophiolitic mélange yields an age of 243 ± 1 Ma with zircon ∊Hf(t) values of +7.9 to +13.2. Two samples of diabase samples from the ophiolite yield ages of 131±1 Ma and 124±1 Ma with zircon ∊Hf (t) values ranging from +10.8 to +15.0 and +12.3 to +15.4, respectively. Geochemically, rare earth elements (REEs) patterns show that these mafic intrusions are similar to those of the normal mid‐ocean‐ridge basalt (N‐MORB). Enrichment of fluid‐mobile elements (e.g. Rb and Ba) and depletion of Nb, and Ta suggest that these intrusions were possibly originated from melting of a depleted mantle source influenced by subducted slab. Our data strongly confirms that the Neo‐Tethyan ocean between the Lhasa terrane and Gondwana had been existed since the Middle Triassic at least. The ages of the two diabases from the ophiolitic complex reveal that mafic magma activities within the Neo‐Tethyan ocean could be as a continuing process during 120–130 Ma. Emplacement and preservation of older ophiolitic recorder during evolution of the Neo‐Tethyan Ocean may be closely related to the occurrence of the Zhongba micro‐terrane within the western YZSZ. [ABSTRACT FROM AUTHOR]