Geochemistry and Tectonic Significance of Paleoproterozoic High-temperature and Low-temperature Granites in Southwestern Zhejiang Province, Northeastern Cathaysia Block
|Course||Mineralogy,petrology, Mineral Deposit Geology|
|Keywords||Whole-rock major and trace elements Zircon U-Pb-Hf isotopes Granite Paleoproterozoic orogeny southwestern Zhejiang northeastern Cathaysia Block|
Significant Paleoproterozoic granitic magmatism took place in the southwestern Zhejiang Province, which provides a window to demonstrate the compositions and evolution of early Precambrian basement rocks in the northeastern Cathaysia Block. New whole-rock major and trace element geochemical and LA-ICPMS zircon U-Pb dating and MC-ICPMS zircon Hf-isotope composition analyses were carried out for the Huaqiao, Chimushan, Xiaocu, Wongkeng and Yuyan plutons in southwestern Zhejiang to constrain the formation ages, nature, petrogenesis and tectonic settings of the Paleoproterozoic granites in the northeastern Cathaysia Block.Based on the whole-rock major and trace element analysis for twenty-seven Paleoproterozoic granites, S-, I-and A-type granites can be recognized. A-type granites were found in Huaqiao, Xiaocu and Wongkeng plutons. They show low SiO2(62.5～70.5%), high Fe2O3t(3.82～9.95%), CaO, P2O5, Ba, Ga, Zn, Y and high field strength element (HFSE, e.g. Zr, Hf, Nb and Ta) contents, and high FeOt(MgO+FeOt)(0.78～0.89) and Ga/Al (>3.1) ratios. These A-type granites have rare earth element (REE) ranging from130～501ppm with moderate LREE/HREE fractionation (LREE/HREE=6.76～12.9) and moderate Eu negative anomalies (Eu/Eu*=0.39-0.50). The Huaqiao and Wongkeng A-type granites contain lower SiO2(62.5～63.1%), higher TiO2(1.46～1.62%), P2O5, REE (354～501ppm) contents, and low A/CNK (0.98～1.01) and Rb/Ba (0.12～0.35) ratios, similar to the Tianhou A-type granites. Whereas the Xiaocu A-type granites have relatively high SiO2(66.4～70.5%) contents and A/CNK (1.10～1.38) ratios with low REE contents of130～319ppm, akin to Lizhuang, Jingju and Danzhu A-type granites. S-type granites occur in some Huaqiao, Chimushan, Xiaocu and Wongkeng granites. In contrast with A-type granites, they share high SiO2(65.9～74.1%) ratios and high A/CNK (1.04～1.37) ratios, low Fe2O3t, Ba contens and Al2O3/TiO2ratios. The Xiaocu S-type granites show lower CaO (0.55～0.63%), MgO, Fe2O3t(3.82～9.95%), REE (170～210ppm), low CaO/Al2O3(<0.3) ratios, and high Rb/Sr (3.48～3.67) and LREE/HREE (15.1～17.1) ratios, the characteristics of Xiaji S-type granites, while other S-type granites process the features of Lizhuang S-type granites with high CaO/Al2O3ratios (>0.3). I-type granites were first recognized in the study area, showing large variations in geochemistry with SiO2changing from63.5%to73.5%and Al2O3of12.3～15.5%. They show characteristics of low TiO2, Ga, Zn, HFSE, Al2O3/TiO2ratios and high Na/K ratios. High CaO contents and CaO/Na2O (～0.80) ratios, and low A/CNK (0.94～1.07, a mean of1.01), Rb/Ba (0.17～0.33) and Rb/Sr (0.48～1.14, a mean of0.87) ratios separate these I-type granites from other S-type granites. Xiaocu I-type granites have lower SiO2(63.5～64.6%), higher Al2O3, CaO, Fe2O3t, Rb, Sr, Ba, Sc, V and Cr contents than the Chimushan and Yuyan I-type. A Yuyan I-type granite show highest SiO2(72.5%) content and Sr/Y ratio, lowest TiO2, Fe2O3t, MgO, CaO and REE (120ppm) contents, strongest LREE/HREE fractionation (LREE/HREE=16.5) with highest Eu/Eu*ratio (Eu/Eu*=0.66).According to the high-precision zircon U-Pb dating results for three A-type granites, three S-type and two I-type granites, the Huaqiao, Xiaocu and Chimushan A-type granites were formed at～1859Ma,～1869Ma and～1878Ma, Chimushan, Xiaocu and Wongkeng S-type granites are～1876Ma,～1882Ma and～1884Ma, and Chimushan and Xiaocu I-type granites have formation ages of～1887Ma and～1912Ma. Combined available accurate ages and geochemical data, it can be concluded that Paleoproterozoic I-type and S-type granites were generated synchronously during1912Ma to1875Ma, prior to Paleoproterozoic A-type granites generated at1878-1832Ma, and they suffered early Mesozoic (250-230Ma) and late Mesozoic (110-90Ma) metamorphism.Al2O3/TiO2ratio and Zr saturated thermometry can divide the Paleoproterozoic granites into low-temperature and high-temperature groups. The S-and I-type granites belong to low-temperature group with formation temperature of725～840℃while the A-type granites falls into the high-temperature group (840～920℃). Combined with the types and formation ages of the Paleoproterozoic granites, low-temperature and high-temperature groups can reflect their different tectonic settings and geodynamics. The former was related with early post-orogenic tectonic settings, while the latter was produced in the following post-orogenic settings. These new results indicate a complete late Paleoproterozoic orogeny from syn-orogenic to post-orogenic and final anorogenic (rifting) tectonic settings during a～150Ma span from1912Ma to1766Ma in the northeastern Cathaysia Block, which was once a part of Columbia supercontinent during Paleoproterozoic.The Paleoproterozoic S-and A-type granites share similar zircon Hf-isotope compositions with an average sHf(t) value of ca.-4.19and TC DM of2.78Ga, suggesting that they have similar sources that probably were the Badu Complex. The Xiaocu I-type granites show a large variation of zircon Hf-isotope compositions and probably were originated from felsic meta-igneoud rocks. Some zircons in the Chimushan I-type granites have low176Hf/177Hf ratios and negative εHf(t) values (-2.55to-4.79), similar to those from S-and A-type granites, while some other zircons show high176Hf/177Hf ratios and positive εHf(t) values (+2.60to+7.92). It illustrates that the Chimushan I-type granites was originated from the mixing magma of old meta-sedimentary and juvenile meta-mafic rocks in the Badu Complex. The complex types and compositions of the Paleoproterozoic granites imply the heterogeneous lower crust in the northeastern Cathaysia Block. The discovery of Paleoproterozoic I-type granites, the～-2.8Ga model ages and inherited zircons in this study demonstrates the exact existence of Archean basement rocks under the Cathaysia Block.The Chimushan, Xiaocu and Wongkeng granites, at the east side of the Zhenghe-Dabu fault, show the same petrographic features, geochemistry and formation ages with the Paleoproterozoic granites at the west side of the fault. It is suggested that the Precambrian basement rocks on both west and east sides of the Zhenghe-Dabu fault are similar, and this fault may not be an important boundary fault in the Cathaysia Block.