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The Xinxing granitic pluton is a large batholith cropping out in southern South China with its long axis parallel to the regional NE–trending faults. We conduct the first comprehensive study of zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U–Pb chronology and trace element chemistry, rock-forming mineral chemistry and whole-rock element and Sr–Nd isotope geochemistry for this pluton. Our new data indicate that the pluton was emplaced during the interval 234–232 Ma, i.e. early Indosinian rather than late Mesozoic as previously considered. The southern pluton consists of syenogranite and monzogranite, which are composed of K-feldspar, oligoclase, quartz and ferri-biotite. The northern pluton consists of monzogranite and granodiorite, which are composed of andesine, K-feldspar, quartz and magnesio-biotite. The southern granites have high SiO2 contents (72.5–79.2 wt%) and high FeOT/(FeOT + MgO) (T refers to total) ratios with low Mg# [atomic Mg/(Mg + FeT)]. They are enriched in rare earth elements and depleted in Sr and Ba with Zr + Y + Ce + Nb >350 ppm and 10,000 × Ga/Al >2.6. Their initial magma temperatures are unusually high (>886 °C). All those features strongly suggest an affinity with A-type granites. The northern granites have lower SiO2 contents (63.4–72.5 wt%) and lower FeOT/(FeOT + MgO) ratios as well as higher Mg# than the southern granites. They are also enriched in rare earth elements and depleted in Sr and Ba, with high Ga/Al ratios and high Zr + Y + Ce + Nb contents. Geochemical data and major element modeling confirm that the southern A-type granites were generated by the moderate degree (~49%) shallow (ca. 15–20 km depth) dehydration melting of the early Paleozoic granitoids triggered by intraplating of basaltic magmas. Subsequent moderate degree (~57%) fractionation of such crustal melts produced the more felsic granites in the southern pluton, whereas subsequent mixing of such crustal melts with about 8 to 48% intraplated mafic magmas generated the less felsic granites in the northern pluton. Interaction of mafic magmas with A-type crustal melts causes the northern granites to lose some diagnostic geochemical features of A-type granites such as high FeOT/(FeOT + MgO) ratios and to be transitional towards I-type granites. We further suggest their origin to have been related to the transtension of the regional NE–trending strike-slip faults coupled with the early Indosinian continental collisions. [ABSTRACT FROM AUTHOR]

The western Tibet Plateau comprises a series of crustal terranes that were successively accreted to the southern margin of Eurasia attending the transition of the Palaeo- to Neo-Tethys. We present the first detailed SHRIMP zircon U–Pb chronology, major and trace element, and Sr–Nd–Hf isotope geochemistry of three Cretaceous plutons (Kalaqigu, Aranbaotai, and Hongqilapu) from this area. SHRIMP zircon U–Pb dating shows that the three plutons were emplaced in the Early Cretaceous. The 114 Ma Kalaqigu pluton is composed of strongly peraluminous high-K calc-alkaline monzogranite and granodiorite. These rocks consist of plagioclase, K-feldspar, quartz, and biotite, and show variable Sr–Nd and Hf (in situzircon) isotopic compositions [87Sr/86Sr (T) = 0.7098–0.7157, ϵNd(T) = –8.1 to –15.6, and ϵHf(T) = –10.2 to –19.8 (mean –14.8)]. Elemental and isotopic data suggest that the granitoids were generated by partial melting of metasedimentary basement in the normal to thickened lower-crust triggered by underplating or injection of hot mantle-derived magmas in a continental arc setting. The 110 Ma Aranbaotai pluton consists of strongly peraluminous high-K calc-alkaline two-mica monzogranites. These rocks are composed of K-feldspar, plagioclase, quartz, biotite, and muscovite, and also show variable Sr–Nd and Hf (in situzircon) isotopic compositions [87Sr/86Sr (T) = 0.7075–0.7144, ϵNd(T) = –9.4 to –13.3, and ϵHf(T) = –5.4 to –16.4 (mean –10.0)]. Elemental and isotopic data suggest that the granites were formed by partial melting of subducted sediments at a depths of <40 km to ∼40–50 km in the collision (overthrust) zone during continental collision. The 102 Ma Hongqilapu pluton consists of metaluminous high-K calc-alkaline granodiorites. These rocks are composed of plagioclase + K-feldspar + quartz + amphibole + biotite ± clinopyroxene, and show uniform Sr–Nd and Hf (in situzircon) isotopic compositions [87Sr/86Sr (T) = 0.7078, ϵNd(T) = –7.6 to –8.0, and ϵHf(T) = –7.2 to –11.2 (mean –9.4)]. Elemental and isotopic data suggest that the granitoids were generated by crystal fractionation of basaltic magmas in a continental arc setting. Our new data suggest that at 114 Ma, the Karakorum terrane was still an active continental arc fuelled by the northword slab subduction of the Mesozoic Tethys; collision of the Kohistan terrane with the Karakorum terrane most likely occurred at 110 Ma, resulting in the closure of the Mesozoic Tethys; since 102 Ma, the southern Karakorum terrane hosted to a new continental arc coupled with the northward slab subduction of the Neo-Tethys. [ABSTRACT FROM PUBLISHER]

Abstract: The western Kunlun orogen, the northwest Tibet Plateau, occupies a key tectonic position at the junction between the Pan-Asia and the Tethyan tectonic domains. It is related to subduction of Proto- and Paleo-Tethys and terrane collision from early Paleozoic to early Mesozoic. This paper presents the first detailed SHRIMP zircon U–Pb chronology, mineral chemistry, major and trace element, and Sr–Nd–Hf isotope geochemistry of three Middle Triassic plutons (Yuqikapa, Muztaga and Taer) and their microgranular enclaves in the western Kunlun orogen. SHRIMP zircon U–Pb dating shows that the Yuqikapa pluton was emplaced in the Anisian (~243Ma) and the Muztaga and Taer plutons that contain microgranular enclaves were emplaced in the Ladinian (234–227Ma). The Yuqikapa pluton is composed of peraluminous high-K calc-alkaline two-mica granites (monzogranite–syenogranite–alkali-feldspar granite), with initial 87Sr/86Sr ratios of 0.7071–0.7077, ε Nd (T) of −4.4 to −6.3 and ε Hf (T) (in-situ zircon) of −3.3. Elemental and isotopic data suggest that they were formed by “wet” anatexis of the Precambrian metasedimentary and metaigneous rocks in the continental collision (overthrust) zone. The Muztaga and Taer plutons consist of metaluminous to weakly peraluminous high-K calc-alkaline granitoids (granodiorite–monzogranite), with initial 87Sr/86Sr ratios of 0.7052−0.7104, ε Nd (T) of −3.6 to −7.3 and ε Hf (T) (in-situ zircon) of −3.1 to −1.0. Elemental and isotopic data suggest that they were formed by partial melting of the Precambrian metasedimentary and metaigneous rocks in the normal lower-crust (<40km) for the Muztaga pluton and in the thickened lower-crust (~40–50km) for the Taer pluton, triggered by underplating of coeval (233–226Ma) mantle-derived enclave-forming magmas. The enclaves are basic (SiO2 48.0–51.6wt.%) to intermediate (SiO2 53.0–54.6wt.%) with high K2O (1.4–4.2wt.%). They generally have lower initial 87Sr/86Sr ratios (0.7034–0.7084) and high initial ε Hf (−1.1 to +1.2) than their respective host granitoids. The enclave magmas are considered to have been derived from the partial melting of subduction-modified mantle in the spinel stability field (<60km) (enclaves in the Muztaga pluton) and the spinel-garnet transition zone (~60–80km) (enclaves in the Taer pluton). Final closure of the Paleo-Tethys occurred in the Middle Triassic Anisian (~243Ma). The Middle Triassic Ladinian (234–227Ma) dates the post-collisional regime caused by Paleo-Tethyan oceanic slab break-off. [Copyright &y& Elsevier]

The origin of microgranitoid enclaves in granitic plutons has long been debated (hybrid magma blobs vs. refractory restites or cognate fragments). This article presents detailed petrography, SHRIMP zircon U–Pb chronology, bulk-rock major and trace element analyses, and Sr–Nd isotope and in situ zircon Hf isotopic geochemistry for microgranitoid enclaves within two Late Triassic granitic plutons in the Qinling orogen. Zircon U–Pb dating shows that the enclaves formed during the Carnian (222.5 ± 2.1 to 220.7 ± 1.9 Ma) coeval with their host granitoids (220.0 ± 2.0 to 218.7 ± 2.4 Ma). Field and petrological observations (e.g. double enclaves, xenocrysts, acicular apatite, and poikilitic K-feldspar or quartz) suggest that the enclaves are globules of a mantle-derived more mafic magma that was injected into and mingled with the host magma. The enclaves are mainly ultrapotassic, distinct from the host granitoids that have high-K calc-alkaline bulk-rock compositions. Although the enclaves have closely similar bulk-rock Sr–Nd isotope [initial 87Sr/86Sr = 0.7046–0.7056, ϵNd (T) = –0.3 to –5.0] and in situ zircon Hf isotope [ϵHf (T) = –1.5 to +2.9] ratios as the granitoids [initial 87Sr/86Sr = 0.7042–0.7059, ϵNd (T) = –0.6 to –6.3, ϵHf (T) = –2.2 to +1.6], chemical relationships including very different bulk-rock compositions at a given SiO2 content lead us to interpret the isotopic similarities as reflecting similar but separate isotopic source rocks. Detailed elemental and isotopic data suggest that the enclaves and the host granitoids were emplaced in a continental arc environment coupled with northward subduction of the Palaeo-Tethyan oceanic crust. Partial melting of subducted sediments triggered by dehydration of the underlying igneous oceanic crust, with melts interacting with the overlying mantle wedge, formed high-K calc-alkaline granitic magmas, whereas partial melting of diapiric phlogopite-pyroxenites, solidified products of the same subducting sediment-derived melts, generated ultrapotassic magmas of the microgranitoid enclaves. Our new data further confirm that in the Late Triassic time the Qinling terrane was an active continental margin rather than a post-collisional regime, giving new insights into the tectonic evolution of this orogen. [ABSTRACT FROM AUTHOR]

Abstract: The Gan-Hang rift, trending at least 450km in a NE–SW direction, is a part of a Mesozoic Basin and Range Province in southeastern China. Detailed SHRIMP zircon U–Pb chronology, major and trace element, and Sr–Nd–Hf isotope data of three granitic plutons and a diabasic dike in the northwest of the Gan-Hang rift, are used to explore the origin of these granites and their relationship to the evolution of the Gan-Hang rift. SHRIMP zircon U–Pb dating shows that the granitic plutons and diabasic dike were emplaced in the Early Cretaceous (122–129 Ma). The Tongshan and Damaoshan plutons, close to the Gan-Hang rift, consist mainly of weakly peraluminous granitic rocks, which show A2 subtype affinity. These granites have initial 87Sr/86Sr ratios of 0.7080–0.7103, εNd (T) values of−1.4 to−5.6 and εHf (T) (in-situ zircon) values of −3.8 to +1.2. Detailed elemental and isotopic data suggest that they were formed by partial melting of granulitized Mesoproterozoic metamorphic basement (including metasedimentary and metaigneous rocks) in the shallow (<30km) crust at a high temperature (~823°C). These pure crustal melts underwent crystal fractionation of biotite, plagioclase and K-feldspar, forming the more felsic end-member of granitic rocks. The Ehu pluton, more distant from the Gan-Hang rift, is composed of two-mica granites, which are strongly peraluminous and are S-type granites. These granites have higher initial 87Sr/86Sr ratios (0.7162−0.7191) and lower εNd (T) (−7.1 to −7.4) and εHf (T) (−6.8; in-situ zircon) values than the Tongshan and Damaoshan granites. Detailed elemental and isotopic data suggest that the Ehu granites were formed by partial melting of Paleoproterozoic metasedimentary basement in the deep (~40–50km) crust at a relatively low temperature (~757°C), with no fractional crystallization and mantle-derived magma mixing in their evolution. The coeval diabasic dike is calc-alkaline and has initial 87Sr/86Sr ratio of 0.7085, εNd (T) value of +0.9 and εHf (T) (in-situ zircon) value of −7.1, suggesting that it was derived from the asthenospheric mantle wedge that had been metasomatised via addition of subducted sediment-derived melt with monazite in the residual phase. The association of Early Cretaceous (122–129Ma) S- and A-type granites in the northwest of the Gan-Hang rift marks the onset of back-arc extension or intra-arc rift. With ongoing extension the crust and lithospheric mantle became progressively thinned. The upwelling of asthenosphere triggered partial melting of both metasedimentary and metaigneous rocks in the more thinned crust close to the Gan-Hang rift, forming the A-type granitic magmas such as Tongshan and Damaoshan, whereas partial melting of metasedimentary rocks in the less thinned crust farther from the Gan-Hang rift formed the S-type granitic magmas such as Ehu. The red sediments with the total thickness more than 10,000m have been successively deposited in the Gan-Hang rift valley since the late Early Cretaceous (~105Ma), suggesting that this region experienced the most back-arc extension. [Copyright &y& Elsevier]

The Late Mesozoic geology of Southeast China is characterized by extensive Jurassic to Cretaceous magmatism consisting predominantly of granites and rhyolites and subordinate mafic rocks, forming a belt of volcanic-intrusive complexes. The Xiangshan volcanic-intrusive complex is located in the NW region of the belt and mainly contains the following lithologies: rhyodacite and rhyodacitic porphyry, porphyritic lava, granite porphyry with mafic microgranular enclaves, quartz monzonitic porphyry, and lamprophyre dyke. Major and trace-element compositions, zircon U-Pb dating, and Sr-Nd-Hf isotopic compositions have been investigated for these rocks. The precise SHRIMP and LA-ICP-MS zircon U-Pb dating shows that the emplacement of various magmatic units at Xiangshan took place within a short time period of less than 2 Myrs. The stratigraphically oldest rhyodacite yielded a zircon U-Pb age of 135 ± 1 Ma and the overlying rhyodacitic porphyry has an age of 135 ± 1 Ma. Three porphyritic lava samples yielded zircon U-Pb ages of 136 ± 1 Ma, 132 ± 1 Ma, and 135 ± 1 Ma, respectively. Two subvolcanic rocks (granite porphyry) yielded zircon U-Pb ages of 137 ± 1 Ma and 137 ± 1 Ma. A quartz monzonitic porphyry dyke, which represented the final stage of magmatism at Xiangshan, also yielded a zircon U-Pb age of 136 ± 1 Ma. All these newly obtained precise U-Pb ages demonstrate that the entire magmatic activity at Xiangshan was rapid and possibly took place at the peak of extensional tectonics in SE China. The geochemical data indicate that all these samples from the volcanic-intrusive complex have an A-type affinity. Sr-Nd-Hf isotopic data suggest that the Xiangshan volcanic-intrusive complex derived mainly from remelting of Paleo-Mesoproterozoic crust without significant additions of mantle-derived magma. However, the quartz monzonitic porphyry, which has zircon Hf model ages older than the whole-rock Nd model ages, and which has ε(T) value higher than the other rocks, may indicate involvement of a subordinate younger mantle-derived magma in its origin. Geochemical data indicate that the various rocks show variable REE patterns and negative anomalies of Ba, Nb, Sr, P, Eu and Ti in the trace element spidergrams, suggesting that these rocks may have undergone advanced fractional crystallization with separation of plagioclase, K-feldspar and accessory minerals such as allanite. We suggest that this Cretaceous volcanic-intrusive complex formed in an extensional environment, and the formation of the Xiangshan mafic microgranular enclaves can be explained by the injection of mafic magma from a deeper seated mantle magma chamber into a hypabyssal felsic magma chamber at the crustal emplacement levels. [ABSTRACT FROM AUTHOR]