Search

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]

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]