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The Jiangnan Orogen is located at a key tectonic position along the junction between the Yangtze and Cathaysia blocks. We obtained detailed major and trace elements, whole-rock Nd + zircon Hf isotope data, and U–Pb age data from several Mesozoic granites, including the Fuling (FL), Taiping–Huangshan (TH), Lingshan (LS), Sanqingshan (SQS), and Baijuhuajian intrusions in order to investigate their sources and petrogeneses related to extension in South China. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of zircon from the FL, TH, SQS, and LS bodies yield Early Cretaceous ages of 124–135 Ma. These plutons are alkali-feldspar granites to syenogranites–monzogranites, and show A-type affinities. They have high K2O and total alkali contents, and are enriched in rare earth elements (except for Eu), Zr, and other high-field-strength elements as well as high Ga/Al ratios, and are depleted in Ba and Sr. These granites are metaluminous to weakly peraluminous (ACNK from 0.81 to 1.27). The whole-rockϵNd(T) values of −5.34 to −0.96 are coupled with zirconϵHf(T) values (from −5.3 to +4.24), and all samples plot along the mantle array. Field observations, geochronology, geochemistry, Nd isotopic, and zircon Hf isotopic compositions suggest that they formed by the partial melting of Mesoproterozoic metamorphic basement, with input from juvenile, mantle-derived materials in the shallow (<30 km) crust at high temperatures (756–965°C). These melts underwent crystal fractionation of biotite, plagioclase, and K-feldspar. The upwelling of asthenosphere triggered partial melting of the metamorphic protolith in a back-arc or intra-arc rift setting, reflecting rollback of the Pacific plate. Our research adds new geochronologic constraints on Cretaceous (135–120 Ma) A-type granites from the NE sector of the Jiangnan Orogen. Combined with previous research, we suggest that three main episodes of late Mesozoic extensional tectonism took place in South China: (1) 190–170 Ma (mainly inland), (2) 165–120 Ma (including 165–150 Ma in SE Shi-Hang, 135–120 Ma in NE Shi-Hang, and ∼125 Ma in the Lower Yangtze River Belt), and (3) 100–90 Ma (coastal area), showing an oceanwards younging trend due to the subduction of the Palaeo-Pacific plate. [ABSTRACT FROM AUTHOR]

The Dexing ore deposit, Jiangxi Province, is the largest porphyry copper deposit in China. Controversies exist regarding the ore-forming source of this deposit. We have conducted Pb isotope analyses of pyrites from the Tongchang and Fujiawu mines. Our results document consistent Pb isotopes from these two orebodies, with 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 17.954–18.320, 15.407–15.517, and 37.888–38.153, respectively. These Pb isotope ratios are consistent with those of ore-bearing adakitic porphyries but distinctly different from those of the Neoproterozoic metamorphic wall rocks, which indicates that the metals were derived from the porphyries. Based on previous S and Os isotopic data and comparisons with more than 20 Mo-bearing deposits worldwide, we further attribute the narrow range of δ34S values of sulphide minerals and high Re–187Os concentrations of associated molybdenites to a mantle origin. This large-scale copper deposit was evidently emplaced in a continental arc setting attending westward subduction of the palaeo-Pacific plate. Partial melting of the downgoing oceanic slab generated the adakitic magmas. The associated metals were extracted from the lithospheric mantle by these magmas during ascent through the mantle wedge. [ABSTRACT FROM AUTHOR]

The Dexing porphyry copper deposit, part of the circum-Pacific porphyry copper ore belt, is the largest porphyry copper deposit in China. We present new LA–ICP–MS zircon U–Pb and molybdenite Re–Os dating, bulk-rock elemental and Sr–Nd–Pb isotopic as well as in situ zircon Hf isotopic geochemistry for these ore-bearing porphyries, in an attempt to better constrain their petrogenesis. LA–ICP–MS zircon U–Pb dating shows that the Dexing porphyries were emplaced in the early Middle Jurassic (∼171 Ma); molybdenite Re–Os dating indicates that the associated Cu–Mo mineralization was contemporaneous (∼171 Ma) with the igneous intrusion. The rocks are mainly high-K calc-alkaline and show adakitic affinities, including high Sr and low Y and Yb contents, high Sr/Y and La/Yb ratios, and high Mg# (higher than pure crustal melts). These porphyries have initial 87Sr/86Sr ratios of 0.7044−0.7047, ϵNd(T) values of –1.5 to +0.6, and ϵHf(T) (in situ zircon) values of +2.6 to +4.6. They show unusually radiogenic Pb isotopic compositions with initial 206Pb/204Pb ratios up to 18.41 and 207Pb/204Pb up to 15.61. These isotopic compositions are distinctly different from either Pacific MORB or Yangtze lower crust but are similar to the subducting sediments in the western Pacific trenches. Detailed elemental and isotopic data suggest that the Dexing porphyries were emplaced in a continental arc setting coupled with westward subduction of the palaeo-Pacific plate. Partial melting involved the subducted slab (mainly the overlying sediments), with generated melts interacting with the lithospheric mantle wedge, thereby forming the investigated high-K calc-alkaline porphyry magmas. [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]

Early Cretaceous felsic intrusions and volcanic rocks are widespread in the Gan-Hang Belt, SE China. In this study, we report a distinctive high-Zr rhyolite (802–1145ppm Zr) from the Dazhou uranium district in the eastern Gan-Hang Belt. SHRIMP zircon U–Pb dating shows that the Dazhou rhyolite erupted at 127.3±1.7Ma. Geochemical data indicate A-type characteristics for the Dazhou rhyolite, such as high contents of alkali elements, high and high Ga/Al ratios, enrichment in some LILEs, HFSEs, and REEs, and depletion in Sr, Ba, P, and Ti. Compared to other A-type granitoids reported previously in the same belt, the Dazhou high-Zr rhyolite shows only a slight enrichment in other HFSE elements and LREE, but has similar whole-rock Nd and zircon Hf isotopic compositions to those A-type granitoids with relatively lower Zr concentrations, indicating that all these A-type rocks may have similar magma sources. Hence, the difference of Zr contents in these rocks is not due to their different source rocks. We found that zircon grains in the Dazhou high-Zr rhyolite are generally inheritance-poor, and the calculated zircon saturation temperatures are extremely high at ∼1000°C. It is therefore suggested that the distinctive high-Zr characteristic of the Dazhou rhyolite was a result of high-temperature suppression of zircon crystallization by ambient mafic magmas correlative with the upwelling of the asthenospheric mantle. The fundamental difference in magma temperature and magmatic evolution mechanism between the Dazhou high-Zr rhyolite and other A-type granitoid suites in the eastern Gan-Hang Belt brought about their different Zr concentrations. [ABSTRACT FROM AUTHOR]

Abstract: The widespread occurrence of Mesozoic granites in the Gan-Hang Belt in Southeast China is associated with similarly widespread, economically important mineralization, but the precise timing, origin, and plate tectonic significance of these granites are not well understood. We have studied two of these (Early Cretaceous) granite bodies, the Yangmeiwan granite and the Daqiaowu granitic porphyry, from the northeastern part of the Gan-Hang Belt in western Zhejiang Province by zircon U-Pb geochronology, major and trace element analyses, and Nd–Hf isotopic analyses. LA-ICP-MS and SHRIMP U–Pb dating of zircon grains from these two granites yield ages of 133–136Ma, representing a Early Cretaceous magmatic event. These granitic rocks are metaluminous to weakly peraluminous and have a pronounced A-type geochemical signature with high Na2O+K2O, Fe2O3*/MgO and Ga/Al ratios. They show low CaO, MgO and TiO2 contents, enrichment in some LILEs (such as Rb and Th) and HFSEs (such as Zr, Y), depletion in Sr, Ba, P, Eu and Ti. They also show A2 subtype affinity and were probably formed at a high temperature (~810°C for Yangmeiwan granite and ~850°C for Daqiaowu granitic porphyry). These A-type granitic rocks show bulk rock εNd(t) values in the range of −6.5 to −3.6 and zircon εHf(t) values from −7.8 to −0.9, with Mesoproterozoic TDM c ages for both Nd and Hf isotopes. Geochemical and isotopic data suggest that these A-type granitic rocks were generated largely by partial melting of granulitized Mesoproterozoic metamorphic basement rocks (including parametamorphic and orthometamorphic rocks), with a possible input of mantle-derived materials and followed by subsequent fractional crystallization. Our new data together with previous published data suggest that the Cretaceous A-type granitic rocks with ages between 137Ma and 122Ma occurred along the Gan-Hang Belt, indicating an important Mesozoic extensional event in Southeast China. This event represents either a back-arc extension or an intra-arc rift due to the roll-back of the paleo-Pacific plate, and it started as early as ~137Ma in Xiangshan and Xinlu basins. It is earlier than ~130Ma as previously suggested in the studied region and later than that along the south part of the Shi-Hang Zone. With ongoing extension during slab roll-back of paleo-Pacific plate, the crust and lithospheric mantle became progressively thinned. The upwelling of asthenosphere would have triggered crustal rocks to partially melt, generating granitic magmas. Our new geochemical data also suggest that mantle-crust interaction in the formation of the A-type granitic magma along the Gan-Hang Belt gradually intensified from early to late stages between ca. 137 and ca. 122Ma. [Copyright &y& Elsevier]