Central Lhasa Terrane Research Articles

Late Jurassic–Early Cretaceous magmatic activity in the Central Lhasa Terrane: Petrogenesis and implications for the initial subduction of the Slainajap oceanic lithosphere

Models for the magmatic and tectonic evolution of the late Mesozoic Lhasa Terrane remain controversial. This study presents new zircon U-Pb age and Hf isotopic, whole-rock major and trace element, and Sr-Nd isotopic data for silicic igneous rocks within the Shiquanhe and Guchang areas. The Shiquanhe rhyolites yield zircon U-Pb ages of ca. 148 Ma, which are consistent with the 144 Ma age of the Guchang granites. All of these silicic igneous rocks are peraluminous, high-K calc-alkaline, and have negative εHf(t) and εNd(t) values and low (87Sr/86Sr)t values. These characteristics are indicative of derivation from magmas formed by the partial melting of ancient crustal metagraywackes. Combining our new data with the results of previous research suggests that a Late Jurassic–Early Cretaceous (165–135 Ma) magmatic belt is present along the Shiquan River–Nam Tso mélange zone within northern Tibet. These rocks collectively record the magmatic evolution from forearc to continental arc settings during the initial southward subduction of the Slainajap Ocean. Our results provide critical constraints on the tectonic evolution of the Slainajap Ocean and the tectono-magmatic evolution of the Lhasa Terrane.

Tetrad effect of rare earth elements caused by fractional crystallization in high-silica granites: An example from central Tibet

Determining the formation mechanism (fractional crystallization vs. melt-fluid interaction) of the REE tetrad effect in high-silica granites is critical for understanding trace elemental behaviors (e.g., enrichment of rare metals) during magmatic and hydrothermal evolution. This study reports new petrography, zircon U-Pb ages, trace elemental and Hf isotopic, and whole-rock geochemical data of high-silica granites from the Gajin Batholith in the central Lhasa Terrane, Tibet. All samples are dated at ca. 100 Ma and likely formed during Lhasa-Qiangtang collision. These samples are syenogranites and alkali-feldspar granites, belonging to high-silica I-type granites (SiO2 = 74.2–78.2 wt%), having high total-alkalis, low magnesian, marked enrichments in Pb, U, Th, and Rb, and marked depletions in Ti, Eu, P, Sr, and Ba. The petrological and geochemical data indicate moderate (0–20%) fractionation of plagioclase + K-feldspar + biotite (5:3:2). The REE data clearly define regular chondrite-normalized patterns (No-tetrad Group) and pronounced tetrad patterns (Tetrad Group). The REE tread effect in Tetrad Group can be reproduced by fractionation of monazite and allanite as indicated by a Rayleigh fractionation modeling. New modeling results for samples reported elsewhere, along with petrographic observations and decoupling between the REE tetrad effect and fractionation of twin-elements, demonstrate that the REE tetrad effect in high-silica granites is inherited from fractional crystallization rather than formed by melt-fluid interaction as suggested widely.

Post-subdution evolution of the Northern Lhasa Terrane, Tibet: Constraints from geochemical anomalies, chronology and petrogeochemistry

Bangong-Nujiang collisional zone (BNCZ) is an older one in Qinghai-Tibet Plateau and resulted in the famous Bangong-Nujiang metallogenic belt, which plays an important role in evaluating the formation and uplift mechanism of plateau. The northern and central Lhasa Terrane composed the southern part of the BNCZ. Since ore deposits can be used as markers of geodynamic evolution, the authors carried 1∶50000 stream sedimental geochemical exploration in the Xiongmei area in the Northern Lhasa Terrane to manifest the mineralization, and based on this mineralization with geochemical and chronological characteristics of related magmatic rocks to constrain their geodynamics and connection with the evolution of the Lhasa Terrane. The authors find Early Cretaceous magma mainly resulted in Cu, Mo mineralization, Late Cretaceous magma mainly resulted in Cu, Mo, and W mineralization in the studying area. The results suggest a southward subduction, slab rolling back and break-off, and thickened lithosphere delamination successively occurred within the Northern Lhasa Terrane.© 2022 China Geology Editorial Office.

Lower Cretaceous turbidites in the Shiquanhe–Namco Ophiolite Mélange Zone, Asa area, Tibet: Constraints on the evolution of the Meso-Tethys Ocean

Turbidites from the Shiquanhe–Namco Ophiolite Mélange Zone (SNMZ) record critical information about the tectonic affinity of the SNMZ and the evolutionary history of the Meso-Tethys Ocean in Tibet. This paper reports sedimentologic, sandstone petrographic, zircon U-Pb geochronologic, and clastic rocks geochemical data of newly identified turbidites (Asa Formation) in the Asa Ophiolite Mélange. The youngest ages of detrital zircon from the turbiditic sandstone samples, together with ~115 Ma U-Pb concordant age from the tuff intercalation within the Asa Formation indicate an Early Cretaceous age. The sandstone mineral modal composition data show that the main component is quartz grains and the minor components are sedimentary and volcanic fragments, suggesting that the turbidites were mainly derived from a recycled orogen provenance with a minor addition of volcanic arc materials. The detrital U-Pb zircon ages of turbiditic sandstones yield main age populations of 170–120 Ma, 300–220 Ma, 600–500 Ma, 1000–700 Ma, 1900–1500 Ma, and ~2500 Ma, similar to the ages of the Qiangtang Terrane(age peak of 600–500 Ma, 1000–900 Ma, ~1850 Ma and ~2500 Ma) and the accretionary complex in the Bangong–Nujiang Ophiolite Zone (BNMZ) rather than the age of the Central Lhasa Terrane (age peak of ~300 Ma, ~550 Ma and ~1150 Ma). The mineral modal compositions, detrital U-Pb zircon ages, and geochemical data of clastic rocks suggest that the Asa Formation is composed of sediments primarily recycled from the Jurassic accretionary complex within the BNMZ with the secondary addition of intermediate-felsic island arc materials from the South Qiangtang Terrane. Based on our new results and previous studies, we infer that the SNMZ represents a part of the Meso-Tethys Suture Zone, rather than a southward tectonic klippe of the BNMZ or an isolated ophiolitic mélange zone within the Lhasa Terrane. The Meso-Tethys Suture Zone records the continuous evolutionary history of the northward subduction, accretion, arc-Lhasa collision, and Lhasa-Qiangtang collision of the Meso-Tethys Ocean from the Early Jurassic to the Early Cretaceous.

Middle Permian adakitic granite dikes in the Sumdo region, central Lhasa terrane, central Tibet: Implications for the subduction of the Sumdo Paleo-Tethys Ocean

Abstract The Sumdo Paleo-Tethys Ocean was the southernmost part of the Paleo-Tethys Ocean, located on the Qinghai–Tibet Plateau. However, the evolution of the Sumdo Paleo-Tethys Ocean and the origin of eclogites in the Sumdo area of Tibet are still matters of debate. Here we report newly discovered adakitic granite dikes from the Sumdo area of the central Lhasa terrane. The rocks have high SiO2, Al2O3, and Sr contents, high Sr/Y and La/Yb ratios, enrichment in light rare-earth elements (REEs), and depletion in Y, Yb, Th, and heavy REEs. They are characterized by low 87Sr/86Sr (0.703678–0.704427) and high 143Nd/144Nd (0.512825–0.512904) ratios, and positive eNd(t) (+6.41 to +7.38) and eHf(t) (+13.8 to +15.7) values. Zircon U–Pb dating yielded ages of 269.1 ± 4.1 and 266.4 ± 2.0 Ma for the rocks. Our data indicate that the Sumdo adakitic granite dikes were derived from partial melting of subducted oceanic slab, with the magma being influenced by slab-derived fluids and slight interaction with peridotite in the mantle wedge during ascent. By combining our new geochemical data with results of previous studies, we infer a subduction event involving the Sumdo Paleo-Tethys Ocean during the middle Permian, which led to the formation of the Sumdo adakitic granite dikes and the Sumdo, Jilang, and Xindaduo eclogites.

Identification of Forearc Sediments in the Milin-Zedong Region and Their Constraints on Tectonomagmatic Evolution of the Gangdese Arc, Southern Tibet

Abstract The Xigaze forearc sediments revealed the part of the tectonomagmatic history of the Gangdese arc that the bedrocks did not record. However, the sediments’ development is restricted to the region around and west of Xigaze City. Whether the eastern segment of the arc had a corresponding forearc basin is yet to be resolved. In this study, a field-based stratigraphic study, detrital zircon U-Pb geochronology (15 samples), and Hf isotopic analyses (11 of the 15 samples) were carried out on four sections in the Milin-Zedong area, southeast Tibet. The analytical results revealed the existence of three distinct provenances. The lower sequence is characterized by fine-grained sandstone, interbedded mudstone, and some metamorphic rocks (e.g., gneiss and schist). The detrital zircon U-Pb age distribution of this sequence is analogous to those of the Carboniferous-Permian strata and metasediments of the Nyingtri group in the Lhasa terrane. The middle and upper sequences are predominantly composed of medium- to coarse-grained volcaniclastic/quartzose sandstones, which are generally interbedded with mudstone. The detrital zircon U-Pb ages and Hf isotope signatures indicate that the middle sequences are Jurassic to Early Cretaceous in age (~200–100 Ma) and show clear affinity with the Gangdese arc rocks, that is, positive εHft values. In contrast, the upper sequences are characterized by Mesozoic detrital zircons (150–100 Ma) and negative εHft values, indicative of derivation from the central Lhasa terrane. The overall compositions of the detrital zircon U-Pb ages and Hf isotopes of the middle to upper sequences resemble those of the Xigaze forearc sediments, implying that related forearc sediments may have been developed in the eastern part of the Gangdese arc. It is possible that the forearc equivalents were eroded or destroyed during the later orogenesis. Additionally, the detrital zircons from these forearc sediments indicate that this segment of the Gangdese arc experienced more active and continuous magmatism from the Early Jurassic to Early Cretaceous than its bedrock records indicate.

Mesozoic crustal evolution of southern Tibet: Constraints from the early Jurassic igneous rocks in the Central Lhasa terrane

Phanerozoic growth of continental crust on our planet is one of the important research themes in Earth Science. Here, we present the results of a systematic study of newly found and previously reported Mesozoic igneous rocks, including diorite cumulate, granodiorite cumulate, mafic magmatic enclaves (MME) and host granitoids in the central Lhasa terrane, southern Tibet. These igneous rocks give zircon UPb crystallization ages of 199–189 Ma. Based on constituent mineral and bulk-rock compositions, the cumulates are best understood as resulting from amphibole, plagioclase and titanite crystallization from a mafic andesitic magma. The host granitoids also show compositional systematics consistent with amphibole-plagioclase fractional crystallization from andesitic magma. The MMEs share many characteristics with their host granitoids in common, including identical crystallization age, similar mineralogy, mineral chemistry and zircon isotopic compositions, representing earlier cumulate derived from the same magmatic system as their host rocks. The magma parental to the studied Early Jurassic igneous rocks is best explained as resulting from partial melting of hydrated ocean crust together with varying continental material. The increasing zircon εHf(t) values of multiple plutons in the central Lhasa terrane with time during ~215–170 Ma indicate its gradual increase in mantle contribution. We present the Early Mesozoic crustal evolution in the Lhasa terrane.

Geochemistry and petrogenesis of sandstones and their basaltic interlayers of Shexing Formation from Linzhou basin, South Tibet

印度与亚洲大陆的碰撞是青藏高原演化的重要构造事件，碰撞过程被记录在拉萨地块南部的晚白垩世到古新世的沉积-岩浆作用中。林周盆地的晚白垩世设兴组及其之后不整合覆盖的林子宗火山岩，是解析碰撞过程的重要记录。本文对设兴组最高层位的砂岩和玄武岩夹层进行了岩石学、地球化学和年代学研究，探讨了岩石成因和构造意义。设兴组砂岩属于杂砂岩，碎屑物质主要来自中酸性岩浆岩源区；锆石Hf同位素指示设兴组大部分碎屑物质来源于盆地北面的中部拉萨地块，少部分来自盆地南部的冈底斯岩基；砂岩中最年轻的碎屑锆石年龄指示林周盆地设兴组是在98Ma之后接受沉积的。以夹层产出在设兴组顶部的玄武岩和玄武安山岩，富集轻稀土元素、亏损重稀土元素、弱负Eu异常，强烈富集Ba、Th、U、Pb等大离子亲石元素，显著亏损Nb、Ta等高场强元素，属于高钾钙碱性玄武岩系列，与典型安第斯型玄武岩特征吻合。玄武岩和玄武安山岩的锆石均为捕获锆石，其最年轻碎屑锆石年龄限定了设兴组玄武岩的喷发晚于110Ma。综合分析表明，林周盆地晚白垩世时期为夹持在冈底斯岩浆弧与中部拉萨地块之间的弧后盆地，新特提斯洋壳晚白垩世俯冲到冈底斯弧和弧后盆地之下，大约在98~110Ma之后喷发到林周盆地的很少量中基性岩浆构成了设兴组顶部的玄武岩和玄武安山岩夹层，是新特提斯洋俯冲相关的幔源岩浆作用。林周盆地设兴组（晚于98Ma）与上覆的林子宗火山岩（底部约为65Ma）之间呈大约33Myr的构造间断，可能代表了冈底斯弧的碰撞之前的隆升剥蚀过程。;The collision between India and Asia was the most important tectonic event in the evolution of the Tibetan Plateau, which was recorded in the Late Cretaceous to Paleogene sedimentary and magmatic rocks in the southern Lhasa terrane. The Late Cretaceous Shexing Formation strata and later eruption of the Linzizong volcanic succession (ca. 65~45Ma) is the best candidate to show the collisional processes. This study provides new petrological, geochemical and zircon U-Pb geochronological data on the sandstones and basaltic interlayers collected from the highest level of Shexing Formation in the Linzhou basin in southern Tibet. The materials of the Shexing greywacke were from the felsic magmatic source region. Hf isotope implies that the majority of the sediments came from the central Lhasa terrane to the north of the basin, and others from the Gangdese arc batholiths to the south of the basin. The youngest age of the detrital zircons in Shexing sandstone constrains the deposition were later than 98Ma in Linzhou basin. The interlayers close to the top of the Shexing Formation are basalts and basaltic andesites, with enriched light rare earth elements (REE), depleted heavy REE, weak negative Eu anomaly, strongly enriched large ionic lithophile elements (LILEs, such as Ba, Th, U, Pb), and depleted high-field strength elements (HFSEs, such as Nb, Ta). These basalts and basaltic andesites are high potassium calc-alkaline basalt series, with the affinity of the typical Andes-type basalts from continental arc. The detrital zircons from these interlayers define their eruption to be later than 110Ma. Our results suggest that the Linzhou basin is the back-arc tectonic setting, which located between the Gangdese arc and the central Lhasa terrane during the Late Cretaceous. The Neo-Tethys Ocean subducted beneath the Gangdese arc and the back-arc system and triggered the mantle partial melting to produce the eruption of the basalts and basaltic andesites after 98~110Ma. The interval of 33Myr between the upper most Shexing Formation (98Ma) and the Linzizong volcanics (65Ma), may represent the uplift and erosion processes of the Gangdese arc before the collision of India and Asia.

Lithospheric electrical structure in the central Tibetan Plateau and its tectonic significance

The Himalaya-Tibetan Plateau that is situated in the southwest China is an ideal natural field laboratory for continental dynamics. In this study, through the careful processing and analyzing of the magnetotellurics data, the reliable 2-D electrical model was obtained to examine the relationship between the lithospheric electrical structure and dynamics in the central Himalaya-Tibetan Plateau. Six conductors mainly result from the partial melting and aqueous fluids, and three resistors are related to different tectonic evolution in the model. Combined with other observations, we summarize that the upwelling hot material (partial melting), aqueous fluids, subduction erosion and crushed zone contribute to the error between the electrical location and the geographic location of the Indus-Yarlung Zangbo suture. The subducted Indian plate is shown by the different angles and different locations of its frontier in different areas. The electrical structure beneath the Lhasa terrane corresponds well with the geological features of an old, locally reworked crust dominantly in the central Lhasa terrane with two young juvenile crustal blocks surrounding the central terrane, and it also shows the relationship among the dynamics, magmatism and mineralization in the Gangdise metallogenic belt. Meanwhile, we indicate that the subducted polarity of the Bangong-Nujiang Tethyan Ocean lithosphere was double-sided, and the smooth subduction of the Longmu Tso-Shuanghu Tethyan Ocean lithosphere may cut through the Moho surface approximately 20 km. Above all, these results can provide helpful constrains to understand the evolution mechanism of the Himalaya-Tibetan Plateau.

Miocene potassic and adakitic intrusions in eastern central Lhasa terrane, Tibet: Implications for origin and tectonic of postcollisional magmatism

Miocene postcollisional potassic and adakitic rocks are widely distributed in the southern Lhasa terrane and western central Lhasa terrane. However, coeval potassic and adakitic rocks in eastern central Lhasa terrane were rarely recognized, and their origins and formation mechanism remain controversial. In this paper, we provide new geochronological and geochemical data for the Miocene postcollisional potassic and adakitic intrusions exposed in the Qingdu area, eastern central Lhasa terrane, southern Tibet. The Qingdu Miocene intrusions consist of quartz monzonite porphyry and biotite granite with coeval zircon U–Pb ages of 14.1 Ma and 14.0 Ma, respectively. They have high SiO2 (67.98–75.32 wt.%), Al2O3 (14.13–14.78 wt.%), and K2O (4.06–6.18 wt.%) and low MgO (0.25–1.46 wt.%) contents. They are both enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs), with high (La/Yb)N (25.37–42.32) ratios. The biotite granite samples have low Y (7.10–9.96 ppm) and Yb (0.61–0.85 ppm) contents, and high Sr (229–384 ppm) contents and high Sr/Y (30–41) ratios, which show adakitic geochemical characteristics, whereas the quartz monzonite porphyry samples show potassic geochemical characteristics. They display initial (87Sr/86Sr)i ratios of 0.7083–0.7103, εHf(t) values of −6.7 to −0.1, εNd(t) values of −8.96 to −7.44, (208Pb/204Pb)i ratios of 39.028–39.110, (207Pb/204Pb)i ratios of 15.662–15.684, and (206Pb/204Pb)i ratios of 18.541–18.577. These signatures indicate that both the potassic and adakitic intrusions are more likely to originate from partial melting of a thickened lower crust, which were mainly the products of the binary mixing between the juvenile and ancient crust components. Based on the spatial distributions and isotopic features of the postcollisional potassic and adakitic rocks in the Lhasa terrane, we suggest that the differences of the lower crustal composition played a crucial role in causing the geochemical variations of the Miocene postcollisional adakitic and potassic rocks in Lhasa terrane.

The Metamorphic Evolution and Tectonic Significance of the Eclogite in the Sumdo Complex, Tibet, China

AbstractAs the main tectonic component of the Himalayan–Tibetan orogen, the Lhasa terrane has received much attention as it records the entire history of the orogeny. The occurrence of high pressure eclogite in the Sumdo complex in central Lhasa terrane has a significant bearing on the understanding of the Paleo‐Tethys subduction and plate itineration processes in this area. The petrological, geochemical and geochronological data from eclogite and associated blueschist and garnet‐bearing mica schist from Sumdo, Jilang and Bailang area have been briefly review to explore the origin and metamorphic evolution of this suture. Eclogites from the Sumdo complex have experienced low temperature, high pressure to ultrahigh pressure metamorphism, revealing a fast subduction and exhumation process in a typical oceanic subduction zone. The large P‐T span between different eclogites in the literature may be affected by the big error of unappropriated using geothermobarometry and may also because of slices of subducted blocks derived from different depths juxtapose together during exhumation. By summarizing the U‐Pb, Lu‐Hf and Sm‐Nd ages of eclogites, the eclogite facies metamorphism is likely to occur in early Triassic during 245‐225 Ma, but not the previously accepted late Permian at ca. 260 Ma by the reinterpretation of the former geochronological data from literature. The opening of Paleo‐Tethys Ocean between the Lhasa terrane initiate prior to ca. 280 Ma and ultimate closure to integrate the Lhasa terrane was no earlier than 225 Ma and may triggered by the initial subduction of Bangong‐Nujiang Tethys Ocean in the north.

Early Cretaceous sedimentary evolution of the northern Lhasa terrane and the timing of initial Lhasa-Qiangtang collision

Lower Cretaceous strata in the Baingoin basin of the northern Lhasa terrane record initial collision between the Lhasa and Qiangtang blocks, followed by the early uplift of central Tibet. North-south traverses across the Baingoin basin highlight major differences between the Duba Formation in the north and the quasi-coeval Duoni Formation in the south. The Duba Formation documents upward transition from shallow shelf and deltaic environments to coarse-grained siliciclastic fluvial sedimentation. Abundance of detrital zircons yielding Jurassic-Cretaceous ages with εHf(t) values mainly between −2 and +10, occurrence of chert, Cr-spinel, and pyroxene grains, together with southward paleocurrent directions indicate that the Duba Formation was sourced from the southern Qiangtang terrane and Bangong-Nujiang suture zone to the north. The Duoni Formation in the south was deposited in shelfal to fan-delta and fluvial environments. Abundant volcanic clasts, detrital zircons yielding Cretaceous ages with mainly negative εHf(t) values, and northward paleocurrents indicate an active volcanic source located in the central Lhasa terrane to the south, with minor input from the northern Lhasa terrane. Only the northern part of the Baingoin basin was directly controlled by the Lhasa-Qiangtang collision and may thus be considered a peripheral foreland basin, whereas the southern part was mainly influenced by tectonic processes related to the northward subduction of Neotethyan lithosphere, and may thus be comparable to a retroarc foreland basin. But these sedimentary features and the 139–79 Ma Baingoin plutonic intrusion do not fit well with classical foreland-basin models. Zircon chronostratigraphy constrains the final consumption of Bangong-Nujiang oceanic lithosphere and initial collision between the Lhasa and Qiangtang microcontinents to have taken place by 122 Ma, which has major implications for paleotectonic reconstructions of the Tibetan Plateau.

Geochronology, geochemistry and petrogenesis of the Late Jurassic-Early Cretaceous granitoids in Zuozuo, western Central Lhasa Terrane, Tibet

近年来对青藏高原中拉萨地块西部中生代岩浆作用的源区、成因和演化等问题的研究较少。本文针对中拉萨地块西部噶尔县左左乡两个中酸性岩体进行了岩石学、锆石U-Pb年代学、微量元素和锆石Hf同位素，以及主量元素和微量元素地球化学研究。左左乡北侧岩体寄主花岗闪长岩和闪长质包体获得同期（163Ma和160Ma）的年龄，属于晚侏罗世；南侧岩体花岗岩年龄为142~147Ma，属于早白垩世。北侧与南侧的中酸性岩石均属于准铝质-弱过铝质高钾钙碱性I型花岗岩，富集大离子亲石元素和轻稀土元素，亏损高场强元素。晚侏罗世岩体寄主岩具有较为富集的锆石Hf同位素成分（eHf（t）=-16.8~-13.6），可能来源于古老下地壳的部分熔融；包体具有相似的Hf同位素成分（eHf（t）=-15.7~-13.6），结合晚侏罗世寄主岩与包体较高的MgO（4.13%~6.90%）、Cr（146×10-6~370×10-6）和Ni（31×10-6~113×10-6）含量，说明包体代表的中基性岩浆可能源于古老富集地幔的熔融，并且在晚侏罗世寄主花岗闪长岩与闪长质包体所代表的两种岩浆间发生过充分的岩浆混合作用。南侧早白垩世花岗岩具有较负且变化范围大的eHf（t）值（-8.2~-4.8和-3.8~+0.2），指示早白垩世岩浆活动中有更多亏损地幔物质或新生地壳物质的加入，并且亏损地幔物质或新生地壳物质与拉萨地块古老地壳物质熔融形成的酸性岩浆发生了岩浆混合，经历岩浆混合后中酸性岩浆又发生了角闪石、长石的分离结晶，最终形成南侧早白垩世岩体的一系列中酸性岩石。研究区早白垩世亏损地幔物质或新生地壳物质的增多可能与南向俯冲的班公湖-怒江板片的回转有关。

Lithium isotope systematics of the Sumdo Eclogite, Tibet: Tracing fluid/rock interaction of subducted low-T altered oceanic crust

Abstract The Sumdo eclogite, located in the central Lhasa Terrane, Tibet, has received much attention as it has important implications for the tectonic evolution of the Tibet orogenic belt. Aiming to constrain the Lithium (Li) isotope characteristics of rocks in subduction zones, we investigated whole-rock major and trace elements, Li isotopes as well as oxygen (O) and radiogenic (Sr, Nd, and Pb) isotopes on 20 Sumdo eclogite samples. The investigated samples display REE patterns similar to N-MORB, with relatively homogeneous bulk eNd(t=260Ma) values ranging from +4.0 to +7.8. These data suggest that protoliths of the Sumdo eclogite were N-MORB-type oceanic basalts. Yet, initial 87Sr/86Sr and δ18O are heterogeneous and range from 0.703703 to 0.706645 and from +4.9 to +8.9‰, respectively. A positive correlation between Sr and oxygen isotopic compositions suggests that the MORB protoliths had experienced low temperature seafloor alteration prior to subduction. The Li contents and δ7Li values of Sumdo eclogite vary from 3.64 to 29.7 µg/g and −1.1 to +4.0‰, respectively, significantly lower than those of low T altered MORBs (AOC) that usually have high Li contents (up to 33 µg/g) and heavy Li isotopic compositions (δ7Li up to +14‰). Modelling results suggests limited Li isotopic fractionation during dehydration. After eliminating the effect of diffusion, we suggest that a rehydration stage during exhumation may be responsible for the Li isotope systematics of the Sumdo eclogite. The fluids (

Petrogenesis of Late Cretaceous Jiangla'angzong I‐Type Granite in Central Lhasa Terrane, Tibet, China: Constraints from Whole‐Rock Geochemistry, Zircon U‐Pb Geochronology, and Sr‐Nd‐Pb‐Hf Isotopes

AbstractThe Jiangla'angzong granite in the northern part of the Central Lhasa Terrane is composed of syenogranite and adamellite. LA‐ICP‐MS zircon U‐Pb analyses suggest that syenogranite has a weighted mean 206Pb/238U age of 86±1 Ma (mean square weighted deviation=0.37), which is in accordance with the muscovite Ar‐Ar age (85±1 Ma) of Cu‐Au ore‐bearing skarns and the zircon U‐Pb age (84±1 Ma) of adamellite. This suggests that the Jiangla'angzong magmatism and Cu–Au mineralization events took place during the Late Cretaceous. The granite contains hornblende, biotite, and pyroxene, and does not contain Al‐bearing minerals, such as muscovite, cordierite, and garnet. It has high contents of SiO2 (65.10–70.91wt%), K2O (3.44–5.17wt%), and total K2O+Na2O (7.13–8.15wt%), and moderate contents of A12O3 (14.14–16.45wt%) and CaO (2.33–4.11wt%), with a Reitman index (σ43) of 2.18 to 2.33, and A/CNK values of 0.88 to 1.02. The P2O5 contents show a negative correlation with SiO2, whereas Pb contents show a positive correlation with SiO2. Th and Y contents are relatively low and show a negative correlation with the Rb contents. These characteristics suggest that the Jiangla'angzong granite is a high K calc–alkaline metaluminous I–type granite. It is enriched in light rare earth elements (LREE) and large ion lithofile elements (LILE), and depleted in heavy rare earth elements (HREE) and high field strength elements (HFSE), with LREE/HREE ratios of 11.7 to 18.1. The granite has negative Eu anomalies of 0.58 to 0.94 without obvious Ce anomalies (δCe=1.00–1.04). The relatively low initial 87Sr/86Sr ratios of 0.7106 to 0.7179, positive εHf(t) values of 1.0 to 4.1, and two‐stage Hf model ages (TDM2) ranging from 889 Ma to 1082 Ma, These geochemical features indicate that the granite derived from a juvenile crust. The (143Nd/144Nd)t values from the Jiangla'angzong granite range from 0.5121 to 0.5123, its εNd(t) values range from –10.17 to –6.10, its (206Pb /204Pb)t values range from 18.683 to 18.746, its (207Pb /204Pb)t values range from 15.695 to 15.700, and its (208Pb /204Pb)t values range from 39.012 to 39.071. These data indicate that the granite was formed by melting of the upper crust with the addition of some mantle materials. We propose that the Jiangla'angzong granite was formed during the post‐collision extension of the Qiangtang and Lhasa terranes.

Zircon U–Pb dating, geochemistry, and Sr–Nd–Pb–Hf isotopes of the subvolcanic intrusion from Beina Pb–Zn–(Ag) deposit in the southern Lhasa terrane, Tibet: Implications for petrogenesis and mineralization

The Beina Pb–Zn–(Ag) deposit is a newly discovered deposit situated in the southern Lhasa terrane, Tibet. LA‐ICP‐MS U–Pb dating for the ore‐bearing rhyolite porphyry indicates that it was emplaced at 64.6 Ma. The Beina rhyolite porphyry displays high SiO2 (74.31–76.52 wt%), K2O + Na2O (6.99–8.05 wt%), and A/CNK (0.97–1.33), similar to the peraluminous S‐type granites related to Pb–Zn–(Ag) mineralization in the central Lhasa terrane. The Beina rhyolite porphyry is strongly depleted in Nb, Ta, Ti, P, Sr, and Ba, enriched in Rb, K, Th, U, and Pb, enriched in light rare earth elements (LREEs), slightly depleted in heavy rare earth elements (HREEs) (LaN/YbN = 6.52–9.28), and has moderate negative Eu anomalies (δEu = 0.38–0.48), showing an arc affinity. It also shows high CaO/Na2O ratios (0.21–0.73), low Rb/Ba (0.30–0.46), and Rb/Sr ratios (1.17–4.01), indicating that it was mainly derived from partial melting of a clay‐poor, plagioclase‐rich metapsammitic source. The samples have relatively high 87Sr/86Sr(i) (0.7084–0.7092) and Pb isotopes (206Pb/204Pb = 18.61–18.74, 207Pb/204Pb = 15.65–15.68, 208Pb/204Pb = 38.99–39.14) and low εNd(t) (−8.6 to −7.2) and εHf(t) (−8.6 to −4.8) values, yielding ancient two‐stage Nd model ages (1,459–1,569 Ma) and two‐stage Hf model ages (1,433–1,684 Ma), respectively. The Beina rhyolite porphyry was formed at relatively high temperatures (average 817°C) according to zircon saturation thermometer. These results reveal that Beina rhyolite porphyry was generated by partial melting of ancient crustal materials (dominated by metapsammite) heated by mantle‐derived magma during rollback of the Neo‐Tethyan oceanic slab. In addition, our research suggests that both I‐ and S‐type granites related to Pb–Zn mineralization are believed to exist in the Lhasa terrane. Similar to the central Lhasa terrane, the middle segment of the southern Lhasa terrane (83°E–87°E) is also an ancient block, large‐scaled silicic volcanic rocks of the Linzizong Group, and coeval plutons are widely exposed and several Pb–Zn deposits have recently been discovered in the Nuocang–Chazi area. All these facts indicate that there is a great potential for prospecting of intrusions‐related Pb–Zn deposits in the middle segment of the southern Lhasa terrane.

Upper Cretaceous trench deposits of the Neo-Tethyan subduction zone: Jiachala Formation from Yarlung Zangbo suture zone in Tibet, China

The history of convergence between the India and the Asia plates, and of their subsequent collision which triggered the Himalayan orogeny is recorded in the Yarlung Zangbo suture zone. Exposed along the southern side of the suture, turbidites of the the Jiachala Formation fed largely from the Gangdese arc have long been considered as post-collisional foreland-basin deposits based on the reported occurrence of Paleocene-early Eocene dinoflagellate cysts and pollen assemblages. Because magmatic activity in the Gangdese arc continued through the Late Cretaceous and Paleogene, this scenario is incompatible with U-Pb ages of detrital zircons invariably older than the latest Cretaceous. To solve this conundrum, we carried out detailed stratigraphic, sedimentological, paleontological, and provenance analyses in the Gyangze and Sajia areas of southern Tibet, China. The Jiachala Formation consists of submarine fan deposits that lie in fault contact with the Zongzhuo Formation. Sandstone petrography together with U-Pb ages and Hf isotope ratios of detrital zircons indicate provenance from the Gangdese arc and central Lhasa terrane. Well preserved pollen or dinoflagellate cysts microfossils were not found in spite of careful research, and the youngest age obtained from zircon grain was ~ 84 Ma. Based on sedimentary facies, provenance analysis and tectonic position, we suggest that the Jiachala Formation was deposited during the Late Cretaceous (~88–84 Ma) in the trench formed along the southern edge of Asia during subduction of Neo-Tethyan oceanic lithosphere.

Petrogenesis and geodynamic significance of Neoproterozoic (∼925 Ma) high-Fe–Ti gabbros of the RenTso ophiolite, Lhasa Terrane, central Tibet

The Precambrian origin and evolution of the Lhasa Terrane remain unclear. Here, we present new zircon U–Pb isotopic and whole-rock geochemical data for the RenTso gabbros of the central Lhasa Terrane. These data indicate that these gabbros formed during the early Neoproterozoic (∼925 Ma) and have affinities with high-Fe–Ti basaltic rocks, given that they contain low concentrations of SiO2 (39.8–49.0 wt%) and high concentrations of FeOT (11.3–23.5 wt%), TiO2 (2.19–5.70 wt%), and V (333–1204 ppm). These gabbros also have N-MORB-like trace element features such as low (La/Sm)N ratios (0.50–0.57) and the absence of negative Nb–Ta anomalies. Combining these features with the high whole-rock εNd(t) (+5.04 to +7.55) and zircon εHf(t) (+7.8 to +13.0) values for these samples indicate that the RenTso gabbros formed by the partial melting of depleted asthenospheric mantle. The Fe–Ti enrichment in these gabbros reflects conditions of low oxygen fugacity during magma fractionation. These gabbros are spatially associated with troctolite and peridotite units, indicating that together these units form a typical ophiolitic lithological assemblage. These geochemical and field observations suggest that the study area contains a subduction-unrelated-type ophiolite that most likely represents the embryonic crust of the Mozambique Ocean following continental rifting and breakup. The results therefore indicate that the Lhasa Terrane has a Precambrian affinity with the East African orogen and occupied a different position to the Qiangtang Terrane within the Rodinia supercontinent.

Metamorphic evolution and age constraints of the garnet-bearing mica schist from the Xindaduo area of the Sumdo (U)HP metamorphic belt, Tibet

AbstractAs one of the major components of the Himalayan–Tibetan Orogeny, the Lhasa terrane plays a key role in understanding the origin and evolution of this giant orogenic belt and the opening and closure of the Tethys oceans. The eclogite-bearing Sumdo Complex in the central Lhasa terrane was recognized as the main suture of the Palaeo-Tethys Ocean between the north and south Lhasa sub-terranes. Despite the eclogite having been studied for a long time, no attempts have been applied to studying the country rocks, causing confusion in understanding the relationship between the eclogite and the adjacent schist. Petrological investigations and phase equilibrium calculations on the garnet-bearing mica schist of the Sumdo Complex have been performed to constrain its metamorphic evolution. The P–T conditions for three metamorphic stages are constrained as P1 (480–500°C, 2.6–2.7 GPa), P2 (580–600°C, 1.3–1.4 GPa) and R (530°C, 0.9 GPa), which represent the prograde, temperature peak and retrograde stages. Two possible P–T paths were constructed, which experienced isothermal decompression (PT1) or heating with a decompression process (PT2), corresponding to the growth and extinction of garnet porphyroblasts in the matrix. The LA-MC-ICP-MS U–Pb dating method yielded a metamorphic age of 229.7±3.5 Ma, which was interpreted as the age of amphibolite-facies metamorphism at c. 600°C, 1.2–1.4 GPa during the closure of the Palaeo-Tethys Ocean, resulting in the aggregation of the north and south Lhasa sub-terranes. The close relationship between the eclogite and garnet-bearing mica schist, and their similar P–T–t paths indicate an in situ tectonic evolution rather than tectonic juxtaposition during exhumation.

Discovery of Upper Cretaceous Neo-Tethyan trench deposits in south Tibet (Luogangcuo Formation)

Sedimentary basins within the Transhimalayan arc-trench system provide paleotectonic and paleogeographic information on the evolution of the late Mesozoic–early Cenozoic Neo-Tethyan subduction zone along the southern Asian margin. This paper presents detailed stratigraphic, sedimentologic, petrographic, detrital-zircon geochronologic and Hf isotopic data from the Luogangcuo Formation exposed as part of the Xiukang Mélange in south Tibet. The Luogangcuo Formation was deposited (ca. 88–81 Ma) in a trench environment on a deep-sea fan fed from the Lhasa block through a submarine canyon. Dominant chert and subordinate sandstone and limestone clasts in conglomerate beds indicate recycling from the Neo-Tethyan subduction complex during repeated episodes of gravitational failure. The interbedded turbiditic sandstones were sourced directly from the Gangdese magmatic arc and central Lhasa terrane. Detrital volumes from the active Asian margin increased markedly at ca. 88 Ma as a result of uplift of central Lhasa, leading to deltaic progradation, filling of the Xigaze forearc basin, and bypassing of sediments funneled via canyons across the subduction complex to reach the Luogangcuo trench basin at abyssal depths.