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The palaeogeographic locations of ancient continental blocks in the Tibetan Plateau, especially of micro-continental blocks during the Neoproterozoic supercontinent cycle, remain enigmatic. To clarify the petrogenesis of Neoproterozoic granitic gneisses and the early evolution of the Amdo micro-continent, we conducted an integrated study of the petrography, geochronology, and geochemistry of biotite monzonitic gneisses from the Precambrian metamorphic basement. The biotite monzonitic gneiss sample yielded a weighted mean zircon U–Pb age of 820 ± 11 Ma, indicating an early Neoproterozoic magmatic event. All gneiss samples show enrichment in light rare-earth elements and depletion in high-field-strength elements in N-MORB-normalized spider diagrams, which suggests that their precursor magma was generated in a subduction-related active continental margin. In addition, these rocks have mostly positive εHf(t) values (−1.0 to 7.0) and corresponding crustal model ages of 1.3 − 1.8 Ga. Zircon Hf isotopes and whole-rock geochemistry indicate that the gneisses were derived from partial melting of juvenile crust material. Recently published age spectra of detrital zircons from different blocks suggest that the Amdo micro-continent had a closer affinity to the Lhasa terrane than to the Qiangtang terrane prior to and during the break-up of the Rodinian supercontinent. However, the gneisses are geochemically distinct from back-arc magmatic rocks in the Lhasa terrane and similar to coeval Andean-type magmatic rocks in blocks that constituted the northwestern margin of Rodinia (including Madagascar, Seychelles, and Malani). This study therefore provides new constraints on the Neoproterozoic palaeogeographic location and evolution of the Amdo micro-continent. [ABSTRACT FROM AUTHOR]

The West Qinling Orogenic Belt (WQOB) is a major portion of the Qinling‐Dabie‐Sulu Orogen and holds essential information for understanding the protracted evolution of the north‐eastern branch of the Paleo‐Tethys in East Asia. In this study, we report our petrological, geochemical, and geochronological study on the five Triassic granitoid plutons of West Qinling with emphasis on the poorly studied Jiaochangba pluton with zircon U–Pb ages of 217.5 ± 1.6 Ma and 215.2 ± 1.2 Ma. The new data and the existing data on the other four plutons support the view that the West Qinling granitoids represent a magmatic response to the continental collision of the Yangtze Block (YB) with the North China Craton (NCC) in the Triassic. Like the other four plutons, the Jiaochangba pluton shows strong light rare earth element (REE) enrichment and weak heavy REE depletion ([La/Sm]N ≈ 7.14 ± 1.89; [Sm/Yb]N ≈ 4.63 ± 1.85) with varying negative Eu anomalies (Eu/Eu* ≈ 0.65 ± 0.20). In the N‐MORB normalized diagram, all the samples show relative enrichment in Rb, Pb, U, and K with negative Nb, Ta, P, and Ti anomalies, resembling those of the model continental crust. The Jiaochangba pluton has relatively lower (87Sr/86Sr)i (0.7062 to 0.7081), higher εNd(t) (−6.91 to −2.09), and εHf(t) (−5.57 to −0.14) than mature continental crust, which are consistent with their source being dominated by lower crust with significant mantle contributions. Mantle‐derived melt, which formed from partial melting of mantle wedge peridotite facilitated by dehydration of the subducted/subducting Mianlue ocean crust, provide the required heat for the crustal melting while also contributing to the compositions of these granitoids. Evolution of such parental magmas in open system crustal magma chambers with continued evolution/replenishment and crustal contamination and assimilation give rise to the observed petrological and geochemical characteristics of these granitoids. [ABSTRACT FROM AUTHOR]

The West Qinling Orogenic Belt (WQOB) is a major portion of the Qinling-Dabie-Sulu Orogen and holds essential information for understanding the prolonged evolution of the northeastern branch of the Paleo-Tethys in East Asia. This study focuses on the petrogenesis of granitoids from Luchuba and Wuchaba plutons in the WQOB. We obtained zircon U-Pb ages of 211 ± 1.4 Ma for the Luchuba pluton and 218.7 ± 1.3 Ma for the Wuchaba pluton, which are the same as the proposed timing of continental collision at ∼220 Ma. We thus interpret the granitoids to represent a magmatic response to the collision between the North China Craton (NCC) and the Yangtze Block (YB). The two plutons are metaluminous to weakly peraluminous I-type granitoids. Samples from the two plutons show strong light rare earth element (REEs) enrichment and weak heavy REE depletion, with varying negative Eu anomalies, which is most consistent with significant plagioclase fractionation although the possible effect of plagioclase as residual phase in the magma source region cannot be ruled out. In primitive mantle normalized multi-element variation diagrams, nearly all the samples show negative Nb, Ta, P and Ti anomalies and relative enrichment in Rb, Pb, U and K. These characteristics resemble those of the average continental crust. The Luchuba pluton has lower (Sr/Sr) (0.7051 to 0.7104), higher ε(t) (−8.11 to −5.73) and ε(t) (−6.70 to −1.65) than mature continental crust ([Sr/Sr] > 0.72, ε(t) < −12). The Wuchaba pluton also has lower (Sr/Sr) (0.7069 to 0.7080), higher ε(t) (−9.86 to −3.34) and ε(t) (−5.69 to 1.58) than mature continental crust. We conclude that the Luchuba and Wuchaba granitoids in the WQOB are best explained as resulting from fractional crystallization with crustal assimilation of parental magmas derived from melting of Mianlue oceanic crust under amphibolite facies conditions during the initial stage of continental collision between the North China Craton and the Yangtze Block. Mafic magmatic enclaves (MMEs) of Wuchaba pluton are earlier cumulates of the same magmatic system. The Mianlue oceanic crust (MORB-like) contributes to the source of the Luchuba and Wuchaba granitoids, pointing to the significance of melting of oceanic crust for continental crust accretion. [ABSTRACT FROM AUTHOR]

Abstract The Laoshan granite is an example of the widespread intraplate granitoids of early Cretaceous age in eastern continental China. The petrogenesis of these granitoids remains in dispute and we choose the Laoshan granite as the representative case to study these granitoids. Zircons from the Laoshan granite give a crystallization age of ~120 Ma, which is consistent with the emplacement age of ~ 126 Ma given by the bulk-rock Rb-Sr isochron. Representative samples from the granite show a large range of major element compositional variation (e.g., SiO 2 /MgO = 64 to 1937), reflecting a varying degree of fractional crystallization of plagioclase, alkali feldspar, amphibole, biotite and accessory phases as observed. The samples are enriched in light rare earth elements, Rb, Th and U, but depleted in Ba and Sr with negative Eu anomalies. The high 87Sr/86Sr (0.7083 to 1.2265) is largely caused by variably high Rb/Sr (~0.31 to 91 with an average of ~13) due to feldspar fractionation. The low ƐNd (t) (−13.8 to −19.5), ε Hf (t) (−14.6 to −24.4), and (206Pb/204Pb) i (16.244 to 17.304) are consistent with significant contributions of the lower continental crust to magmas parental to the Laoshan granite. The origin of the parental magmas is best understood as resulting from anatexis of the lower crust (~20% − 25% partial melting of the mafic granulite) triggered by and mixed with the underplating and intruding basaltic magmas. The basaltic magmas were likely derived from melting of the thinning lithosphere being transformed into the asthenosphere as the result of "basal hydration weakening" with the water ultimately coming from dehydration of the stagnant paleo-Pacific slab in the mantle transition zone. Highlights • The Laoshan granite resulted from fractional crystallization of the parental magma. • Anatexis of the LCC heated by and mixed with basic magma formed the parental magma. • The basal hydration weakening caused the lithosphere thinning with basic magmatism. [ABSTRACT FROM AUTHOR]

Abstract We sampled 22 Mesozoic dykes in eastern continental China and carried out a detailed study on these samples, including K–Ar and zircon U–Pb geochronology, and elemental and Sr-Nd-Pb-Hf isotope geochemistry. Their K–Ar and zircon ages of 130-110 Ma are broadly consistent with the timing of the lithosphere thinning and the emplacement ages of widespread granitoids in the vast region, explicitly pointing to a common cause in space and time. The dykes represent evolved alkaline basaltic melts intruding the Mesozoic granitoids. Their rare earth element (REE) and multi-element patterns differ from the present-day ocean island basalts (OIB), but show strong arc-like signatures (e.g., enrichment in Rb and Pb and depletion in Nb, Ta and Ti). They show high (87Sr/86Sr) i (0.7048 to 0.7103), low ε Nd (t) (−12.3 to −5.7), low ε Hf (t) (−16.5 to −8.0) and low (206Pb/204Pb) i (18.79–18.85). These Mesozoic dykes are best understood as resulting from melting of geochemically enriched sub-continental lithospheric mantle (SCLM), whose geochemical enrichment is consistent with prior metasomatism with the agent being hydrous melt coming from subduction of the Paleo-Pacific plate at ~120 Ma or earlier. Similar to the present-day situation, the paleo-Pacific slab may have also existed stagnant in the mantle transition zone in the Mesozoic. The slab dehydrated and released water in the form of hydrous melt that percolated through and metasomatized the mantle lithosphere, and weakened the base of the lithosphere while producing basaltic melts that evolved to intermediate-felsic compositions of these dykes. The basaltic magmas that underplated and melted the lower crust to generate the widespread Mesozoic granitoids in eastern continental China. Highlights • Our K–Ar and zircon dates indicate that the dykes from the eastern China occurred in the Early Cretaceous (130-110 Ma). • The dykes resulted from partial melting of metasomatized lithospheric mantle. • We interpret this volcanism as indicating the lithosphere thinning in the eastern China. [ABSTRACT FROM AUTHOR]

Abstract Jurassic-Cretaceous granitoids are widespread in eastern continental China and have been considered by many as resulting from paleo-Pacific subduction. However, the actual mechanism of their petrogenesis remains speculative. In order to address this important problem and on the basis of our regional study, we hypothesized that the coastal granitoids may result directly from the paleo-Pacific plate subduction, whereas the widespread granitoids in the continental interiors ultimately result from dehydration of the paleo-Pacific slab stagnated in the mantle transition zone (Niu et al., 2015). Here, we present the very first study testing this hypothesis. We sampled 18 Jurassic-Cretaceous granitoid plutons along a ~ 1300 km long traverse parallel to the inferred paleo-Pacific subduction from the southeast coastline to the Xiaoqinling in the continental interiors and carried out a detailed study on these plutonic samples, including zircon U Pb geochronology, bulk-rock major and trace element compositions and Sr-Nd-Pb-Hf isotopic characteristics. These plutons give varying zircon crystallization ages of ~146 to 100 Ma. They are mostly granitic and minor granodioritic, quartz monzonitic and syenitic in composition, enriched in large ion lithophile elements (LILEs), depleted in high field-strength elements (HFSEs) and have varying negative Sr and Eu anomalies. The plutons in the continental interiors show significant positive correlations of Nd (ε Nd (t) = −25.5 to −10.9) and Hf (ε Hf (t) = −31.5 to −11.3) isotopes with Pb isotopes (206Pb/204Pb (t) = 15.827 to 17.622), with the enriched endmember characterized by low ε Nd (t), ε Hf (t) and 206Pb/204Pb (t). The plutons towards the coastal region have relatively high ε Nd (t) (−9.0 to −5.2), ε Hf (t) (−11.2 to −4.1) and 206Pb/204Pb (t) (18.051 to 18.349). The coastal granitoids are best explained as resulting directly from subduction slab dehydration induced mantle wedge melting and resultant crustal anatexis, whereas the interior granitoids are best interpreted as resulting from mature crustal anataxis caused by basaltic magmatism associated with mantle lithosphere thinning, ultimately triggered by dehydration of paleo-Pacific slab stagnant in the mantle transition zone. Highlights • The interior granitoids have different sources and petrogenesis than coastal granitoids. • The interior granitoids result from basal hydration weakening induced crustal melting. • The coastal granitoids result directly from the paleo-Pacific plate subduction. [ABSTRACT FROM AUTHOR]

The East Kunlun Orogenic Belt (EKOB), an important part of the Greater Tibetan Plateau, is an ideal region for understanding the tectonic evolution of the Anyemaqen Ocean. Here, we present zircon U–Pb ages, bulk-rock major and trace element analyses and Sr–Nd–Hf isotope compositions on representative samples of the syn-collisional Dulan batholith at the eastern end of the EKOB. The zircon U–Pb age data indicate that the bulk of the Dulan batholith was emplaced at 240–235 Ma. The granitoids have high- to medium-K and metaluminous characteristics. They are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) and depleted in some high field strength elements (HFSEs, e.g., Nb and Ta), while having a flat heavy REE (HREEs) pattern. The mafic magmatic enclaves (MMEs) share the same age, mineralogy and indistinguishable Sr–Nd–Hf isotopes with their granitoid hosts except for the higher HREE abundances. We show that the MMEs represent cumulate formed at earlier stages of the same magmatic system. The trace element data (e.g., Nb/Th, Ta/U) and inherited mantle isotopic characteristics of the Dulan batholith are also consistent with an origin via partial melting of the last fragments of underthrusting ocean crust. Simple mass balance calculations using the Sr–Nd–Hf isotopic data show that ~ 85% Paleo-Tethys MORB and ~ 15% mature crustal material (the Proterozoic gneiss of the study area) contribute to the source of the granitoids. The Dulan batholith shows compositional similarities to the bulk continental curst with inherited mantle isotopic signatures. The syn-collisional felsic magmatism must have contributed to the net continental crust growth in the EKOB. We infer that the Kunlun and Qinling orogens may actually be one single orogen offset later by the Wenquan fault system. [ABSTRACT FROM AUTHOR]