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Petrogenesis and tectonic implications of Early Cretaceous S- and A-type granites in the northwest of the Gan-Hang rift, SE China [查看] Yao-HuiJiang、PengZhao、QingZhou、Shi-YongLiao、Guo-DongJin

The Gan-Hang rift, trending at least 450 km 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 (b30 km) 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 (~40–50 km) 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 (122–129 Ma) 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 10,000 mhave been successively deposited in the Gan-Hang rift valley since the late Early Cretaceous (~105 Ma), suggesting that this region experienced the most backarc extension.

Geochemical, zircon U–Pb dating and Sr-Nd-Hf isotopic constraints on the age and petrogenesis of an Early Cretaceous volcanic-intrusive complex at Xiangshan,Southeast China [查看] Shui-YuanYang、Shao-YongJiang、Yao-HuiJiang、Kui-DongZhao、Hong-HaiFan

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 volcanicintrusive 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 εNd(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.

Subducting sediment-derived arc granitoids evidence from the Datong pluton and its quenched enclaves in the western Kunlun orogen, northwest China [查看] Shi-YongLiao、Yao-HuiJiangShao-YongJiang、Wan-ZhiYang、QingZhou、Guo-DongJin、PengZhao

Abstract This paper presents detailed SHRIMP zircon U–Pb chronology, mineral chemistry, major and trace element, and Sr–Nd–Hf isotope geochemistry of the Datong pluton and its quenched enclaves from the western Kunlun orogen, northwest China, in an attempt to achieve a better understanding on the origin of diverse arc magmas. The Datong host granitoids are intermediate to acid in composition (SiO2=57.5~73.1 wt.%), and exhibit high-K calc-alkaline to shoshonitic affinities.The quenched enclaves are silica-rich ultrapotassic rocks. Detailed SHRIMP zircon U–Pb dating indicates that the Datong pluton was emplaced in Ordovician time (473.4–447.7 Ma), which places the Datong pluton in an active continental margin setting, rather than a syn-collision setting of Early Silurian age. The Datong host granitoids were derived by partial melting of subducted sediments, with the subsequent melt interacting with the overlying mantle wedge during its ascent. Partial melting of the veined mantle wedge hybridized by sediment-derived melts generated the silicarich ultrapotassic magma, which was injected into the Datong granitoid magma chamber and quenched, resulting in enclaves hosted by granitoids. This contribution provides evidence that arc magmas can be derived directly by partial melting of subducted sediments, which is helpful to further understand the origin of diverse arc magmas.

An island arc origin of plagiogranites at Oytag, western Kunlun orogen, northwest China SHRIMP zircon U-Pb chronology, elemental and Sr –Nd– Hf isotopic geochemistry and Paleozoic tectonic implications [查看] Yao-HuiJiang、Shi-YongLiao、Wan-ZhiYang、Wei-ZhouShen

This paper presents detailed SHRIMP zircon U–Pb chronology, major and trace element, and Sr–Nd–Hf isotope geochemistry of three intrusive plagiogranite bodies formed in an island arc setting within the Oytag ophiolite suite of the Oytag–Kuda Suture Zone. SHRIMP zircon U–Pb dating of two of these intrusions shows that they were emplaced into the Oytag volcanic rocks at 337.5±4.1 Ma and 327.7±4.9 Ma, and that the younger intrusion contains inherited zircons ranging from 435.3 to 480.8 Ma. Both intrusions are composed of low-Al trondhjemite and tonalite. The rocks have low K2O and Sr and high Y contents with low Sr/Y ratios. They have relatively low rare earth element (REE) concentrations and are depleted in light REE (LREE) and relatively enriched in heavy REE (HREE), with marked negative Eu anomalies in the chondrite-normalized REE patterns. They have initial 87Sr/86Sr ratios of 0.7048 to 0.7068 and εNd (T) values of 6.2 to 7.6, which are lower than those of typical MORB. In situ Hf isotopic compositions of zircons with Early Carboniferous age from both intrusions are characterized by positive initial εHf values, ranging from +12.5 to +19.5, similar to typical MORB. The inherited Ordovician zircons in the younger intrusion have initial εHf values of −1.9 to +3.1, similar to zircons from Ordovician granitoids in the region. Detailed elemental and isotopic data suggest that the Oytag plagiogranites were formed in an oceanic island arc setting, by differentiation of arc tholeiitic magmas that formed in response to the intra-oceanic subduction. The inherited zircons are interpreted as the xenocrysts captured from the incipient oceanic crust that contains relicts of discrete fragments of continental crust with Ordovician zircons, through which the plagiogranite was emplaced. The Oytag ophiolite suite might be the remnants of an ancient island arc, the basement of which was the incipient oceanic crust. The Oytag–Kuda Suture Zone is suggested to be a multiple suture zone that represents the remnants of both the Proto-Tethyan and Paleo-Tethyan oceans. The western Kunlun orogen was therefore not a long-lived active continental margin during the early and late Paleozoic times. It most likely underwent at least two orogenic cycles and thus might represent a multiple orogenic belt.

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