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Although it has been argued that sediment recycling plays an important role in the differentiation of the continental crust, boron (B) isotopic data does not support a direct input of subducted sediments into arc magmas. This raises questions about the viability of sediment recycling as a process in the differentiation of continental crust. Here, we report B isotopic data from Miocene–Quaternary lavas derived from different sources in the northern margin of Tibet. These lavas have high B contents and negative δ11B values close to those of continental sediments. Strongly peraluminous rhyolites have the highest B (93 to 1559 ppm) contents with negative δ11B (−9.7 to −17.9) values. Adakitic dacites and trachyandesites exhibit the lowest B (18 to 29 ppm) contents with markedly negative δ11B (−12.0 to −35.7) values whereas olivine leucitites have B (37.2 to 59.3 ppm) contents with negative δ11B (−8.3 to −15.6) values. These lavas also have enriched Hf-Pb-Nd isotopic compositions similar to those of sediments. This data, combined with numerical modelling and geophysical and tectonic data for Cenozoic continental subduction in the northern margin of Tibet, indicates that: (1) the strongly peraluminous rhyolites were generated by partial melting of mica-bearing continental sedimentary rocks subducted to the depth of mid-to-lower crust; (2) adakitic lavas were derived by partial melting of sediment-bearing thickened lower crust underwent dehydration and eclogites-facies metamorphism; and (3) olivine leucitites were generated by partial melting of enriched mantle metasomatized by sediment-bearing eclogite-facies crust-derived melts. Thus, at continental convergent margins, continental subduction is an important mechanism for sediment recycling and the evolution of continental crust. • B-Hf-Pb-Nd isotopes show that subducted continental sediments entered the mid-to-lower crust and mantle of northern Tibet. • Subducted sediments in mid-to-lower crust and mantle were recycled to the continental crust via magmatism. • Continental subduction is an important mechanism for sediment recycling and the evolution of continental crust. [ABSTRACT FROM AUTHOR]

Geophysically identified low-velocity zones (LVZs) are widespread within the Tibetan lower–middle crust, but their nature and role in models of the development of the Tibetan Plateau remain controversial. The debate stems mainly from whether the Tibetan crust was hydrous and has therefore undergone substantial melting. Here, we identify hydrous crustal xenoliths incorporated into 28 Ma syenite porphyries from central–northern Tibet. These xenoliths indicate the former existence of a cold (T = 680–790 °C) and water-rich (1.10–1.50 wt%) crustal end-member at a depth of 14–40 km in the Tibetan lower–middle crust. Our new petrological evidence indicates the LVZs are partially molten layers that have existed within the Tibetan crust since at least 28 Ma. High- to ultrahigh-temperature metamorphism since the Miocene is inferred to have triggered widespread melting of the former hydrous crust, which would have promoted the development of LVZs and resulted in a flat Tibet. • A former cold and water-rich crustal end-member has been identified in the central-northern Tibetan lower–middle crust. • Low-velocity zones are partially molten layers that have existed within the Tibetan crust since at least 28 Ma. • Crustal melting since the Miocene generated more intense low-velocity zones and crustal flow, creating a flat Tibet. [ABSTRACT FROM AUTHOR]

High-pressure (HP) granulites are commonly regarded as indicators of plate convergence and collision following the subduction of oceanic or continental crust. In this study we report the discovery of Silurian HP basic granulites from Central Qiangtang on the Tibetan Plateau. Detailed petrology and geochronology reveal a three-stage metamorphic history based on inclusions, reaction textures, and garnet zoning patterns. Peak metamorphism at 830–860 °C and 1.15–1.45 GPa (M 1 ) is defined by high-Ca garnet cores, high-Al clinopyroxene, and high-Na plagioclase. Symplectites or coronas of orthopyroxene + plagioclase ± magnetite around garnet porphyroblasts indicate garnet breakdown reactions at ca. 810–830 °C and 0.65–0.85 GPa (M 2 ). Kelyphites of amphibole + plagioclase around garnet formed during the cooling process at about 590–650 °C and 0.62–0.82 GPa (M 3 ). These results help define a sequential P–T path containing near-isothermal decompression (ITD) and near-isobaric cooling (IBC) stages. Identification of mineral inclusion assemblages in zircons dated by U–Pb SHRIMP and LA–ICP–MS reveals peak HP metamorphism at ca. 427–422 Ma, subsequent near-isothermal decompression with associated retrograde reactions at ca. 392–389 Ma, and continued cooling at ca. 360 Ma. The P–T–t path of HP basic granulites reflects collision followed by extensional exhumation during early Paleozoic orogenesis. The present results indicate the occurrence of a collisional event along the northern margin of Indo-Australian Gondwana during the Silurian. Renewed Gondwana-directed subduction and subsequent collision probably led to the opening of the Paleo-Tethys Ocean. [ABSTRACT FROM AUTHOR]

High-MgO (>9 wt %) basaltic rocks can be primary magmas and are used to constrain the geochemistry and temperature of the mantle. However, high MgO contents can also result from mixing between evolved melts and antecrysts or xenocrysts, and thus, the whole-rock composition might not represent the solidified equivalents of primary magma. Whether such mixing with crystals can result in erroneous interpretations of mantle processes remains unclear. This study presents a petrological and geochemical investigation of the post-collision high-MgO (>9 wt %) Lugu volcanic rocks in the southern Qiangtang terrane, central Tibet. The Lugu volcanic rocks comprise porphyritic and intersertal alkali basalts. Zircon U–Pb ages and 40Ar/39Ar dating suggest that the two types of alkali basalts were erupted at c. 29 Ma. Based on detailed petrographic observations and geochemical analysis, the porphyritic alkali basalts may represent near-primary melts, which are characterised by low SiO2 contents (40.9–45.1 wt %), high CaO/Al2O3 ratios (1.1–1.5), and arc-like trace element patterns. We suggest these basalts were derived by partial melting of enriched garnet peridotite (>3 GPa) in the presence of H2O and CO2. These geochemical features are different from those of the c. 30-Ma (ultra)-potassic rocks in the Qiangtang terrane, indicating that a heterogeneous lithospheric mantle existed beneath the Qiangtang terrane during the Oligocene. In contrast, although the intersertal alkali basalts have high MgO contents (>9 wt %), evidence from mineral chemistry indicates that the whole-rock compositions of the intersertal alkali basalts represent mixtures of evolved residual melts and cumulate crystals. They were the product of polybaric fractional crystallisation and the subsequent mixing of crystals and residual melts in a magmatic plumbing system. Furthermore, when intersertal alkali basalts are assumed to be primary melts, they would have been derived by partial melting of shallow (~2.5 GPa) CO2-poor pyroxenite or peridotite. These conditions are different from interpretations of the nature of the mantle source and melting conditions for porphyritic alkali basalts. Our results highlight that the interpretation of petrogenetic processes should be preceded by detailed mineralogical investigations. [ABSTRACT FROM AUTHOR]

In recent studies of crustal growth using global zircon Hf–O isotopic datasets, high-δ18O zircons are typically attributed to intra-crustal reworking during which very little juvenile mantle-derived magmas were added to the crust. Although arc magmas may originate from a high-δ18O mantle wedge, it has been difficult to decipher the contribution of high-δ18O mantle to zircon-saturated felsic magma due to superimposed intra-crustal processes. We address this issue by combining the data from high-δ18O zircon-bearing ultramafic cumulates and coeval lavas from a Cretaceous magmatic arc in southern Tibet. The cumulates mainly consist of different proportions of cumulus olivine and intercumulus amphibole. Amphibole analyses show a transition from increasing to decreasing Zr with increasing SiO2 (50–74 wt.%) contents in the intercumulus melts, indicating zircon saturation in late-stage interstitial melts. The εNd(t) values (2.4 ± 1.4) of the apatite grains crystallized before and after zircon remain almost constant. Interstitial zircons have δ18O (6.1–7.2‰) values similar to the earliest crystallized olivine (δ18O = 6.3–7.1‰) in the cumulates. The coeval lavas may represent the intercumulus melts extracted from amphibole-rich cumulates at different depths. Both the lavas and cumulates were ultimately derived from high-δ18O arc mantle modified by small amounts (<12%) of subducted sediments, and crystallized zircon during intra-crustal magma evolution without involving crustal contamination or melting. These high-δ18O zircons therefore are not products of crustal reworking, but record crustal growth during their crystallization (110 ± 2 Ma). Our study shows that the combination of zircon and olivine oxygen isotopes for ultramafic to felsic rocks is more effective than zircon data alone in evaluating the role of crustal growth vs. reworking in an arc system. The implication is that global zircon-based crustal evolution models that attribute all high-δ18O zircons to crustal reworking may conceal recent crustal growth. [ABSTRACT FROM AUTHOR]

Recognizing the early-developed intra-oceanic arc is important in revealing the early evolution of East Paleo-Tethys Ocean. In this study, new SIMS zircon U-Pb dating, O-Hf isotopes, and whole-rock geochemical data are reported for the newly-discovered Late Devonian-Early Carboniferous arc in Qiangtang, central Tibet. New dating results reveal that the eastern Riwanchaka volcanic rocks were formed at ~370–365 Ma and were intruded by the ~360 Ma Gangma Co alkali feldspar granites. The volcanic rocks consist of basalts, andesites, dacites, and rhyodacites, whose geochemistry is similar to that typical of subduction-related volcanism. The basalts and andesites were generated by partial melting of the fluid and sediment-melt metasomatized mantle, respectively. The rhyodacites and dacites were probably derived from the fractional crystallization of andesites and from partial melting of the juvenile underplated mafic rocks, respectively. The Gangma Co alkali feldspar granites are A-type granites, and were possibly derived by partial melting of juvenile underplated mafic rocks in a post-collisional setting. The ~370–365 Ma volcanic arc was characterized by basalts with oceanic arc-like Ce/Yb ratios and by rhyodacites with mantle-like or slightly higher zircon δ 18 O values, and it was associated with the contemporary ophiolites. Thus, we propose that it is the earliest intra-oceanic arc in the East Paleo-Tethys Ocean, and was accreted to the Northern Qiangtang Terrane during ~365–360 Ma. The arc -continent collision on the north side of the East Paleo-Tethys Ocean probably induced the contemporary magmatism in the Southern Lhasa Terrane, which was located on the south side of the East Paleo-Tethys Ocean. [ABSTRACT FROM AUTHOR]

The formation of post-collisional mantle-derived rocks in the Tibetan Plateau has been linked to the deep geodynamic processes that cause surface uplift. Co-existing silica-oversaturated to silica-undersaturated mantle-derived rocks have been identified in the northern Qiangtang Terrane (NQT). However, the origins of silica-undersaturated magmas are controversial, and the mechanisms responsible for variable silica activity in the mantle-derived rocks are unclear. Here, we present 40Ar/39Ar chronology, mineral chemistry, and whole-rock geochemical data for the Bamaoqiongzong (BMQZ) volcanic rocks of the NQT. The BMQZ volcanic rocks consist of olivine leucitites, trachybasalts, and phonolites and were erupted at ca. 29 Ma. All samples are unsaturated in silica and characterized by enrichment in light rare earth elements and large-ion lithophile elements, depletion in high-field-strength elements, and the presence of negative Nb–Ta–Ti anomalies and positive Pb anomalies. All samples show limited variation in (87Sr/86Sr)i (0.7079–0.7085) and εNd(t) values (−6.9 to −5.3). The geochemical compositions of the BMQZ volcanic rocks indicate that they were produced by partial melting of carbonated phlogopite–lherzolite within the lithospheric mantle. The formation of the olivine leucitites-trachybasalts-phonolites suite was controlled by fractional crystallization and magma mixing in a magmatic plumbing system. This plumbing system included several independent reservoirs and conduits within the crust. The enriched mantle sources of the BMQZ volcanic rocks were formed by the addition of carbonate-rich melts released from the southward-subducted Songpan–Ganzi Terrane after the Late Cretaceous. Our new results, together with published data, reveal systematic variations in geochemical compositions between silica-undersaturated and silica-oversaturated rocks in the NQT, which are ascribed to variations in the nature of the subducted continental materials added during intracontinental subduction. Carbonate-rich melts that were formed by the breakdown of carbonate minerals helped to generate the mantle sources of silica-undersaturated rocks, whereas silicate melts produced by the partial melting of sediment diapirs contributed to the generation of the silica-oversaturated rocks. On the basis of published numerical modelling of continental subduction and crustal deformation records in the NQT, we suggest that intracontinental subduction and lithospheric thinning together contributed to the generation of post-collisional mantle-derived rocks in the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Intra-oceanic subduction zones are major sites of crust–mantle material exchange and crustal growth; however, the processes responsible for the magmatic evolution and transformation of nascent arcs into mature oceanic arcs with thicker crust are debated. We present mineral chemistry, zircon U Pb age and Hf isotopic data, and whole-rock chemical and Sr–Nd–Hf–Pb isotopic data for Cenozoic volcanic rocks on Lombok in the East Sunda Arc, Indonesia, to investigate their petrogenesis and arc maturation. SIMS zircon U Pb analyses of lavas from southern Lombok yield an early Oligocene age of ∼30 Ma. The lavas can be divided into three groups based on their ages and geochemical characteristics: group I (late Eocene) and II (∼ 30 Ma) lavas from southern Lombok, and group III lavas (∼ 3–0 Ma) from northern Lombok. The lavas are characterized by a progressive enrichment in K 2 O and incompatible element contents, and Nd Hf isotopic compositions with time. Group I lavas are tholeiitic basalts with low K 2 O contents (0.15–0.17 wt%) and Indian-MORB like rare earth element (REE) patterns and Nd Hf isotopic compositions (ε Nd = +7.4; ε Hf = +14.5), enrichment in large-ion lithophile elements (e.g., Rb, U, and K), and typical forearc basalt (FAB) geochemical characteristics. Group II lavas are low-K basaltic andesites to dacites that yield flat chondrite-normalized REE patterns [(La/Sm) N = 0.9–1.4], uniform initial 87Sr/86Sr ratios (0.7038–0.7042), and positive ε Nd (t) (+5.9 to +6.8) and ε Hf (t) (+13.9 to +14.4) values. Group III lavas are typically medium- to high-K calc-alkaline basalt to andesite and are enriched in light REEs [(La/Sm) N = 1.6–3.3] with uniform initial 87Sr/86Sr ratios (0.7038–0.7042) and slightly low ε Nd (t) (+3.9 to +5.4) and ε Hf (t) (+10.5 to +12.5) values. Progressive enrichment in incompatible element contents and Nd Hf isotopic compositions over time, from FAB to high-K calc-alkaline lavas, reflect the maturation of the East Sunda Arc. Sr–Nd–Pb–Hf isotopic mixing models, along with trace element ratios (e.g., Ba/Th, La/Yb, Th/Yb), suggest that increasing amounts of sediments were incorporated into the mantle sources of the magmas from group I (∼0%) to group II (<0.5%) and group III (0.5%–1.0%) lavas. Partial melting models show that the formation of group I and II lavas can be reproduced by high-degree (∼10%) partial melting of spinel peridotite, whereas the group III lavas were formed by low-degree (∼5%) partial melting of spinel peridotite. We propose that the evolution of Lombok arc magmas was controlled mainly by the fluxes of subducted slab material, along with decreasing degrees of partial melting as a result of crustal thickening over time. • Three distinct magmatic periods identified on Lombok. • Three periods of magmas indicate maturation of the island arc. • Arc maturation driven by subducted material fluxes and crustal thickening. [ABSTRACT FROM AUTHOR]

Numerous studies have focused on the effects of subduction zones on the development of back-arc basins. However, few have considered whether or how back-arc extension affects subduction zones. Back-arc spreading is thought to have resulted in reduced arc magma production in many Cenozoic subduction systems, but there are no documented pre-Cenozoic examples of this process. This paper reports new geochronological, mineralogical, geochemical, whole-rock Nd, and zircon Hf O isotope data for Carboniferous mafic dikes (diabase and gabbro) from the Tuotuo River area, Northern Qiangtang terrane (NQT), central Tibet. The dikes were intruded during the middle Carboniferous (330–325 Ma). The dikes are typical arc-like basalts with enrichments of large-ion lithophile elements and depletions of high-field-strength elements. Depleted mantle-like Nd − Hf − O isotopes indicate the mafic rocks were formed by partial melting of metasomatized asthenospheric mantle. Clinopyroxene compositions follow the trends of both back-arc basin basalts (BABBs) and rift cumulates, and the whole-rock compositions are also similar to BABBs. Based on the tectonic evolution of the Paleo-Tethys Ocean in the Tibet segment, we propose that this Carboniferous mafic magmatism was caused by back-arc extension during a period of magmatic quiescence of the main Paleo-Tethyan subduction system. This back-arc extension may have led to the opening of the Xijir Ulan Ocean. • The 330–325 Ma mafic dikes in Northern Qiangtang occurred in back-arc setting. • Back-arc extension coeval with Paleo-Tethys arc magmatic quiescence. • Slab-roll back or mid-ocean ridge subduction caused back-arc extension. • This back-arc extension may represent initial opening of Xijir Ulan Ocean. [ABSTRACT FROM AUTHOR]

The Mo stable isotope system has been used to trace material recycling during subduction-related processes, but the behavior of Mo isotopes during magmatic evolution (e.g., crystal–melt fractionation and melt–fluid interaction) remains contentious, especially in high-SiO 2 granites. This study addresses the issue of Mo isotope variation in high-SiO 2 granites by measuring bulk-rock and mineral Mo isotopes of biotite granites (BGs) and garnet-bearing two-mica granites (GBGs) from the well-characterized Zhengga granite pluton (southern Tibet, China). The GBGs have similar Sr–Nd–O isotope compositions to those of the BGs but show higher SiO 2 and lower TiO 2 , MgO, total Fe 2 O 3 , and CaO contents, and represent the products of advanced fractionation of the BG magmas. The BGs have lower Mo contents (0.02–0.07 ppm) and higher δ98/95Mo values (−0.54‰ to 0.22‰) compared with the GBGs (0.029–2.121 ppm and −0.97‰ to −0.41‰, respectively). Analysis of major silicate minerals suggests that substantial segregation of biotite and feldspar with high δ98/95Mo values of 0.00‰ to 0.38‰ and −1.06‰ to 0.57‰ (most within −0.58‰ to 0.13‰) could have driven the GBGs and the late-stage crystalline phase of garnet (−1.22‰ to −0.98‰) towards very low δ98/95Mo values. However, the trend of decreasing δ98/95Mo with indices of magma differentiation is not linear: one group of GBGs show increasing Mo contents and decreasing δ98/95Mo values with decreasing Y, Ho, and Dy contents; while the other group display increasing Mo contents and slightly decreasing δ98/95Mo values with respect to the increasing contents of Y, Ho, and Dy. These two contrasting behaviors can be ascribed to further crystal fractionation and melt–fluid interaction in a closed magmatic–hydrothermal system. This is also evidenced by the formation of two types of garnets with different contents of Mo and rare earth elements in these two groups of GBGs. Closed-system fluid saturation is inferred to have driven the silicate melt to be enriched in 98Mo, which limited the decrease in melt δ98/95Mo caused by crystal fractionation. These observations are supported by quantitative geochemical modeling. We conclude that both fractional crystallization and melt–fluid interaction control Mo isotope fractionation in high-SiO 2 granites and that Mo isotopes are useful for tracing the evolution of high-SiO 2 igneous rocks. [ABSTRACT FROM AUTHOR]

Identifying basement rocks is critical to understanding the continent formation and evolution during orogenesis. Despite two decades of research, the age and composition of the basement rocks of the Southern Qiangtang Terrane/Microcontinent (SQT), Central Tibet are still unclear. In this study, zircon U–Pb ages and Hf–O isotopes, as well as whole-rock Nd isotopes are presented for the oldest known igneous rocks in the SQT. Precise SIMS zircon U–Pb dating of these rocks reveals that these early Paleozoic granites were crystallized at ca. 480‐465 Ma. They exhibit a wide range of isotopic values, with unradiogenic whole rock ε Nd (t) and zircon ε Hf (t) values ranging between −8.3~−7.7 and −8.2~−2.3, respectively, and by high δ18O zircon values of +8.1~+11.4‰. These results reveal that the early Paleozoic granites were crystallized from magmas that were almost entirely sediment-derived. When viewed in concert with regional data and constraints on the tectonic evolution of northern Gondwana, the sources for the early Paleozoic granites are herein proposed to have been deposited in an extensional basin, implying that these early Paleozoic granites were generated in an extensional setting within an accretionary orogen. Thus, the early Paleozoic granites are considered to represent the basement of the SQT. After a long period of attachment to northern Gondwana, the SQT became a small microcontinent when it was rifted and detached from northern Gondwana during Early Permian. After this time, it drifted northwards to Asia, and collided with the Northern Qiangtang Terrane to form a unified Qiangtang terrane in the Late Triassic. Thus, the SQT records a complete Wilson cycle from construction, rifting and departure, drifting to collision. • Pure sedimentary-derived early Paleozoic granites reported in Southern Qiangtang. • The basement of the SQT formed during early Paleozoic. • The SQT recorded a whole Wilson cycle from construction, rifting to collision. [ABSTRACT FROM AUTHOR]

Abstract Low-δ18O magmas (δ18O zircon < 4.7‰) are rare but significant compared to the more abundant high-δ18O magmas in diverse tectonic settings. Here, we report mantle-like δ18O lavas and low-δ18O granites from the Carboniferous Riwanchaka intra-oceanic arc of the Qiangtang Terrane, central Tibet. SIMS (secondary ion mass spectroscopy) zircon U Pb dating indicates that they were formed simultaneously at ~350 Ma, slightly younger than the Early Carboniferous ophiolites (~355 Ma). The forearc ophiolites and volcanic rock assemblages constitute an intra-oceanic subduction system, evidenced by their similar Sr Nd isotopes. SIMS zircon O isotope analyses indicate that the felsic rocks have δ18O zircon values similar to or lower than that of the mantle. The dacite and trondhjemite samples have homogeneous δ18O zircon values of 5.1 ± 0.4‰ (2SD) and 4.6 ± 0.5‰ (2SD), respectively. The tonalite samples exhibit a large range of δ18O zircon values from 2.7 to 5.0‰, most of which are lower than the mantle-like value of 5.3 ± 0.6‰ (2SD). We suggest that the dacites and trondhjemites were produced by partial melting of unaltered lower crust, whereas the low-δ18O tonalites originated by melting of hydrothermally altered lower crust, and both were generated in the early stage of this intra-oceanic arc. This process resulted in a weak middle arc crust of intermediate composition. By allowing decoupling of upper and lower arc crust, this plays an important role in the accretion of intra-oceanic arcs to continents and ultimately in the crustal growth. Highlights • Partial melting of lower arc crust generates felsic rocks in an intra-oceanic arc • Low-δ18O magmas generated in the early stage of this intra-oceanic arc • Arc crustal partial melting plays an important role in intra-oceanic arc accretion [ABSTRACT FROM AUTHOR]

Abstract: Many studies provide compelling evidence for a fossil mantle plume beneath the Late Permian Emeishan large igneous province located near the southeastern margin of the central Tibetan Plateau, a region associated with the Paleo‐Tethyan subduction system. However, little is known about whether direct plume‐subduction interaction occurred during the Late Permian. Here we report geochronological, mineralogical, geochemical, and Sr‐Nd‐Hf‐O isotopic data for the Late Permian (~259–256 Ma) bimodal volcanic rocks (BVRs) in the northern Qiangtang Terrane (NQT), central Tibet. These BVRs consist mainly of basalts, rhyolites, and rhyolitic tuffs. The rhyolites with A‐type affinity were generated by partial melting of newly underplated basalts. Geochemical and isotopic data suggest that two components (ocean island basalt‐type mantle and subducted sediment‐derived fluid) were involved in the generation of the basalts in an extensional back‐arc region. Considering the Permian tectonic evolution of the NQT, we propose that this enriched ocean island basalt‐type mantle was distinct from the Paleo‐Tethyan depleted upper mantle. It most likely originated from Emeishan‐plume material that flowed westward from the South China Block to the nearby NQT, driven by slab rollback‐induced counterflow. The presence of high‐temperature A‐type rhyolites in bimodal back‐arc magmatism also indicates a special subduction setting influenced by a nearby plume. Thus, we propose that the Late Permian BVRs were the result of interaction between the Emeishan plume and the Paleo‐Tethyan subduction system. [ABSTRACT FROM AUTHOR]

Highly evolved granites are geochemically similar to the mature continental crust, and are sometimes associated with W-Sn-Nb-Ta and rare-metal mineralizations. These rocks have important implications for tracing the compositional evolution of continental crust and magma metallogenesis. They were commonly considered to have been produced by partial melting of crustal materials or by high-degree fractional crystallization. In this study, we report on high SiO 2 granite porphyries in the Xiabie Co area of southern Qiangtang Terrane, central Tibet, which provide new insights into the petrogenesis of the highly evolved granites. Zircon dating results indicate that they were generated in the late Early Cretaceous (107 ± 1 Ma). They consist mainly of orthoclase, albite, and quartz, with accessory minerals including zircon, fluorite, and Fe-Ti oxides. Geochemically, they have high SiO 2 but low MgO, Fe 2 O 3 T , CaO, and P 2 O 5 , and are enriched in Rb but depleted in Ba, Sr, Eu, and Ti, similar to highly evolved granitic rocks. They are characterized by distinct rare earth element (REE) tetrad effects and negative Ce anomalies. They also have slightly higher ε Nd (t) values (−0.99 to −0.36) than those of the adjacent early granites (−2.05 to −3.01), and relatively high and variable zircon ε Hf (t) values ranging from −0.3 to +5.4. Compared to typical highly evolved granites and petrological experiment data, we suggest that the Xiabie Co granite porphyries represent highly evolved melts produced by fractional crystallization. The interaction between fractionated melts and F-rich aqueous fluids played an important role in the genesis of their REE tetrads effect. Involvement of meta-sedimentary rocks that went through surficial chemical weathering into their source region is the predominant reason for the generation of negative Ce anomalies. This indicates that, in addition to fractional crystallization, the melt-fluid interactions and partial melting of source materials also play important roles in the generation of highly evolved granites. [ABSTRACT FROM AUTHOR]

An integrated petrologic, geochronologic, major and trace element geochemical, and Sr-Nd-Hf isotopic study of Late Triassic (~ 215 Ma) diorites from the Hoh Xil area, northern Tibet, provides new constraints on the genesis of intermediate magmas and insights into the origin of the high-Mg# andesitic signature of continental crust. These dioritic rocks are characterized by high MgO contents (3.3–5.0 wt%) and Mg# values (50–57) comparable to the estimates for the bulk continental crust at the same level of SiO 2 contents (61.1–64.5 wt%). They also display continental crust-like trace element distribution patterns and uniformly enriched isotope compositions ([ 87 Sr/ 86 Sr] i = 0.7081 to 0.7094, ɛNd[t] = − 8.0 to − 6.9, and ɛHf[t] zircon = − 10.1 to − 5.0). Combining our results with published data from crystallization experiments, we propose that they were probably produced by fractional crystallization from a primitive andesite parent, rather than a primitive basalt parent. This parental magma may be geochemically similar to the roughly contemporaneous primitive andesites in the adjacent Malanshan area of northern Tibet. Our compilation of modern arc lavas shows that progressive fractional crystallization of primitive andesites is also required to reproduce the Mg# versus SiO 2 array for natural arc magmas, in addition to differentiation of mantle-derived primitive basaltic magmas and/or mixing of basaltic with felsic magmas. Therefore, we emphasize that fractional crystallization of primitive andesitic magmas is potentially a frequent occurrence in arc crust and hence may play an important role in producing the high-Mg# signature of intermediate magmas comprising the continental crust. [ABSTRACT FROM AUTHOR]

The Pianshishan eclogites discovered in central Qiangtang are a key piece of evidence constraining the main suture of the Paleo-Tethys Ocean in Tibet. Based on the eclogite-facies metamorphic inclusions found in them, a previous study interpreted the zircon grains as metamorphic in origin, although their catholuminescence images were similar to zircons of igneous origin. Distinguishing metamorphic from igneous zircons can be difficult, however, especially in high-pressure metamorphic rocks. In this study, we report new SIMS zircon U-Pb dates along with zircon O isotope and trace element analyses, to constrain the origin of these zircons and the interpretation of zircon dates. Twenty-five of 26 zircons analyzed from three samples yielded zircon U-Pb dates ranging from 239 to 236 Ma with an average of 238 ± 1 Ma. The zircon rare earth element (REE) patterns are characterized by positive Ce and negative Eu anomalies, strongly indicating an igneous origin. This interpretation is also supported by the high Ti-in-zircon temperatures (> 700 °C) and depleted mantle-like zircon O isotope ratios (5.15 ± 0.57‰, 2SD). Thus, we conclude that the protoliths of the Pianshishan eclogites were generated at ~ 238 Ma. Their geochemistry is similar to the oceanic island basalts (OIB), and they were probably generated by small extents partial melting of enriched oceanic asthenospheric mantle within the garnet stability field and emplaced or erupted as a seamount in the Paleo-Tethys Ocean. We infer the age of eclogite-facies metamorphism to be ~ 233 Ma based on previously reported Lu-Hf isochron ages, and that this metamorphism marks the collision between the Southern and Northern Qiangtang terranes. Thus, metamorphism of the eclogites occurred within 5 My of protolith generation and exhumation occurred only 13 to 19 My later, based on the 40 Ar/ 39 Ar dates of 220–214 Ma. The time between protolith generation and exhumation was thus < 24 My. This rare rapid evolution may be due to the protoliths having formed as a “petit-spot” seamount in response to outer rise flexure as the Paleo-Tethys Ocean entered the Longmu Co-Shuanghu subduction zone. [ABSTRACT FROM AUTHOR]

Partial melting of granulites generates melts, which causes the modification of continental crust. However, the detailed mechanisms of this partial melting and related magmatic processes are debated. In this study, we investigated how crustal melting of granulites generated late Miocene (ca. 6.0 Ma) silicic volcanic rocks in the Chibuzhangcuo area of the Qiangtang Block, Tibet. Various peritectic textures (e.g., quartz inclusions in clinopyroxene and fluorhydroxyl phlogopite) suggest that the pyroxenes were derived from a residual granulite phase and peritectic reactions, and were then entrained into the partial melts. The F-rich mica, granulite mineral assemblages, and high titanomagnetite–ilmenite (811–975 °C) and zircon saturation (836–867 °C) temperatures indicate the silicic magmas were high-temperature and water-poor. The geochemical and isotopic compositions of the silicic volcanic rocks are similar to those of middle–lower crustal intermediate–mafic granulite xenoliths in Cenozoic volcanic rocks in the Qiangtang Block. We suggest that these volcanic rocks were generated by partial melting of hydrous mineral-bearing granulites, triggered by an underlying thermal anomaly associated with the high crustal heat-flow of the central Tibetan Plateau. This work indicates that the occurrence of restitic hydrous mineral and perturbation of thermal anomaly are the primary conditions to cause the partial melting of granulite. As the melting went on, the residual granulite phase and peritectic minerals were likely to be preferentially entrained into the melt due to the erosion effect caused by the continued melt flow at the sites of melting. • Late Miocene silicic volcanic rocks erupted in central Tibet have been investigated. • These rocks are similar to granulites in textures and carry peritectic phases. • The studied rocks recorded the crustal anatexis of granulite in central Tibet. • Partial melting of the granulites was caused by an underlying thermal anomaly. • A high crustal heat-flow of the central Tibet was established since the late Miocene. [ABSTRACT FROM AUTHOR]

[Display omitted] • Eocene Maliaoshan porphyritic granites evolved from coeval metaluminous magmas. • Fractionation of monazite, apatite and allanite can cause an increase in Eu anomalies. • The initial southward subduction of the Songpan–Garzê Block was at 55 ± 5 Ma. • The Eocene magmatic flare-up was caused by the break-off of the Songpan–Garzê Block. The Eocene magmatic flare-up in the Northern Qiangtiang Block is a key event that was genetically related to the uplift of the Tibetan Plateau. This uplift has been considered to have an important impact on the global cooling and oceanic isotopic compositions. Here, we report zircon ages and bulk rock elemental and isotopic compositions for high-silica porphyritic granites in the Maliaoshan area of the Northern Qiangtang Block. Zircon U–Pb dating shows that the porphyritic granites were generated at ca. 41.02 Ma. The porphyritic granites are characterized by uniformly high SiO 2 contents (74.3–75.0 wt%), high contents of total alkalis, metaluminous compositions, and low contents of TiO 2 , Fe 2 O 3 T, MgO, CaO, and P 2 O 5. Moreover, the Maliaoshan porphyritic granites have Nd–Hf isotopic compositions similar to those of coeval metaluminous magmatic rocks in the Northern Qiangtang Block. These data together with geochemical modeling results suggest that the Maliaoshan porphyritic granites were formed by the fractional crystallization of 43.06% major minerals and 0.76% accessory minerals from the assumed starting composition (average Eocene metaluminous igneous rock). Based on our new data and Eocene magmatic flare-up event in the Northern Qiangtiang Block, we propose a new geodynamic model to explain such a magmatic flare-up event. The stress produced by the collision of India and Eurasia gradually transferred northward, prompting the Songpan–Garzê Block to subduct southward beneath the Northern Qiangtang Block at ca. 55 Ma, accompanied by minor magmatic activity. Subsequently, the break-off of the Songpan–Garzê Block at ca. 46 Ma resulted in the Eocene magmatic flare-up. [ABSTRACT FROM AUTHOR]

As one of the most important parts of the eastern Tethys, the evolution of the Bangong–Nujiang Tethyan Ocean (BNTO) has important geological implications for paleogeographic reconstructions of the Tethyan Realm. However, the closure time of the BNTO remains highly debated. Here, we report geochronological, petrological and geochemical data from three newly identified undeformed stitching plutons that intrude a north–south-oriented section of ophiolitic mélanges in the middle–eastern parts of the Bangong–Nujiang suture zone (BNSZ), central Tibet. The stitching plutons comprise the Chaqu diorite–granite association, the Jiang Co granite, and the Konglong diorite porphyry. Secondary ion mass spectroscopy (SMIS) zircon U Pb dating suggests that these three plutons were emplaced at ca. 116–112 Ma. The stitching plutons show heterogeneous Sr–Nd–Hf–O isotopic compositions. The Chaqu diorites have initial 87Sr/86Sr ratios varying from 0.7062 to 0.7064 and ε Nd (t) values from −2.0 to −1.6, and slightly high zircon δ18O (6.2–6.8‰) and positive zircon ε Hf (t) (+2.6 to +7.4) values. The Chaqu granites show high initial 87Sr/86Sr (0.7064–0.7065) ratios and low ε Nd (t) (−2.3 to −2.1) values, and elevated zircon δ18O (6.2–7.3‰) values and variable zircon ε Hf (t) (−1.7 to +4.9) values, similar to those of the associated Chaqu diorites. The Jiang Co granites exhibit higher initial 87Sr/86Sr (0.7085–0.7094) ratios and negative ε Nd (t) (−8.33 to −8.27) values, and variable zircon δ18O (4.8‰ to 6.0‰) and ε Hf (t) (−6.9 to −2.0) values. The Konglong diorite porphyries have initial 87Sr/86Sr isotopic ratios ranging from 0.7069 to 0.7075 and negative ε Nd (t) values ranging from −4.2 to −4.1, and high zircon δ18O (9.0‰ to 9.8‰) and variable zircon ε Hf (t) (−7.3 to +0.7) values. Geochemical evidence indicates that they were derived from different magma sources that included varying proportions of mantle and crustal components. The Chaqu diorites were derived by partial melting of an ancient metasomatized mantle source, and the Chaqu granites were formed by fractional crystallization of the Chaqu dioritic magmas. The Jiang Co granites were generated by partial melting of low δ18O high-temperature hydrothermally altered ancient continental crust. The Konglong diorite porphyries were likely generated by partial melting of high-δ18O enriched lithospheric mantle. In combination with data from adjacent ophiolites, magmatic rocks, and Cretaceous sedimentary rocks, we suggest that the stitching plutons were generated in a post-collisional setting induced by upwelling of asthenospheric mantle due to slab breakoff beneath the BNSZ. Our data provide an important new upper age limit for the time of collision between the Lhasa and Qiangtang terranes, i.e., the latest closure time for the BNTO. • Stitching plutons provide upper age limits on the timing of closure of the BNTO. • Contemporaneous plutons show heterogeneous Sr–Nd–Hf–O isotopic compositions. • Stitching plutons were formed by slab breakoff in a post-collisional setting. [ABSTRACT FROM AUTHOR]

• Two igneous pulses (ca. 290 Ma and 285 Ma) recognized for the Qiangtang-Panjal LIP. • The LIP was generated by a mantle plume and emplacement guided by slab pull forces. • The Neo-Tethys Ocean was initially opened at ∼285 Ma phase. Northward drift of the Cimmerian microcontinents and opening of the Neo-Tethys Ocean in Early Permian are prominent events in northern Gondwana's protracted breakup history. However, the geodynamic setting responsible for these events is controversial. The dispersal of the eastern Cimmerian microcontinents was accompanied by the emplacement of Qiangtang-Panjal large igneous province (QP-LIP). Previously published zircon U-Pb data imply the QP-LIP was emplaced over a long interval (320-280 Ma) but are inconsistent with the short eruption interval constrained by biostratigraphic studies. New secondary ion mass spectrometry (SIMS) zircon U-Pb ages from 15 mafic dikes and gabbroic intrusions from the Southern Qiangtang terrane as well as age data from other parts of the QP-LIP show a much shorter duration (∼ 290 − 285 Ma) for this LIP. The QP-LIP was characterized by an initial ∼290 Ma alkalic and tholeiitic phases and a subsequent ∼285 Ma tholeiitic phase, in agreement with regional biostratigraphic data constraining the timing of departure of the eastern Cimmerian microcontinents from Gondwana at ∼285 Ma and the opening of the Neo-Tethys Ocean. The QP-LIP was most likely the consequence of the arrival in the upper mantle of a mantle plume beneath the northern Gondwanan margin. This event, in combination with slab pull associated with subduction beneath Laurasia, may have caused extension and rifting of east Cimmerian continental fragments from the northern margin of Gondwana. This study may elucidate the mechanism responsible for repeated episodes of one-way northward drifting terranes from northern Gondwana that commenced in Paleozoic. [ABSTRACT FROM AUTHOR]

Multiple petrogenesis models have been proposed for granitic rocks and the mafic microgranular enclaves (MMEs) hosted in them. The mantle and crust-derived magma mixing/mingling process was widely suggested for the formation of MMEs or the host granitoids. However, whether the mixing between magmas derived from different crustal sources may also form microgranular enclaves (MEs, not restricted to mafic) or the host granites remain unclear. Here, we report petrological, geochemical, and zircon in situ Hf-O isotopic data for MEs and their host granites in the northern Nyima area, southern Qiangtang terrane (SQT), central Tibet. The host granitic rocks consist of amphibole-bearing biotite granites and syenogranite, minor granodiorite and aplite veins, and MEs comprising diorite and granodiorite. Host rocks and MEs are contemporaneous and were generated in the Early Cretaceous (∼120 Ma). Host granitoid samples are high-K calc-alkaline rocks and have high SiO 2 (66–78 wt%) and low MgO (<1.14 wt%). They have enriched whole-rock εNd(t) (−1.82 to −3.06), and variable zircon εHf(t) (+0.64 to +5.12) and δ18O (7.35 to 9.61‰) values. The host granodiorites contain some mafic mineral-rich clots and lower silica contents than the biotite granites. They exhibit distinctive geochemical patterns (such as high K 2 O > 5 wt%, high Zr >500 ppm with Zr/Hf ratios >45, and high Ba >400 ppm), implying biotite and zircon concentrations. Combining this information with embayed feldspars, curved biotites, and closely spaced feldspar grains, we suggest that liquid loss resulted from crystal compaction or filter pressing. Both the fine-grained syenogranites and aplite veins exhibit high SiO 2 and Rb/Sr ratios in addition to low Zr/Hf ratios and Zr and Ba concentrations with remarkably negative Eu anomalies, indicating that they are highly differentiated melts extracted from felsic crystal mush. Such a crystal mush is possibly represented by the biotite granites. Gradual contact relationships between variable granitic phases indicate that melt extraction and crystal concentration were coeval. Residual melt extraction from the crystal mush was facilitated by mush remobilization resulting from magma replenishment. We suggest that for the host rocks, the biotite granites were mainly derived by partial melting of ancient crust, and the granodiorites most likely resulted from the magma mixing/mingling that might be accompanied with the loss of melts, the fine-grained syenogranites and aplites represent highly evolved residual melts. MEs are geochemically characterized by intermediate to felsic compositions (SiO 2 = 56.3–67.8 wt%), with low MgO (<4 wt%) and Mg# (< 53) and low mantle compatible trace element (Cr and Ni) contents. They exhibit enriched whole-rock Nd (εNd(t) = −0.36 to −2.71), depleted zircon εHf(t) (+0.51 to +5.68) and high δ18O values (7.45 to 9.57‰), except for one sample with comparable whole-rock εNd(t) (−1.32), relatively high zircon εHf(t) (+5.05 to +7.11) and δ18O (6.09 to 7.10‰) values. MEs exhibit evidence for quenched magma crystallization, chemical exchange, mineral disequilibrium and mass transfer, indicating inefficient magma mixing/mingling. Mafic minerals from MEs were in equilibrium with evolved melts rather than mantle-derived primitive melts. Given the substantial absence of contemporaneous basaltic rocks from the SQT and the relatively high SiO 2 and low MgO contents and crust-like zircon stable oxygen isotope of MEs, we propose that they most likely resulted from the mixing/mingling of diverse crust-derived magmas. Therefore, our study provides important evidence for crust-derived magma mixing and melt extraction from mush. Unlabelled Image • Granitic host rocks and associated enclaves are coeval and were emplaced at ∼120 Ma. • The biotite granites were mainly derived by partial melting of ancient crust. • Evolved melt extraction from felsic mush was facilitated by hotter magma replenishment. • The enclaves resulted from the mixing/mingling of diverse crust-derived magmas. [ABSTRACT FROM AUTHOR]