Subduction records of the Proto-Tethys Ocean in SW Yunnan: Supported by geochronology, geochemistry and Hf-Nd isotopes of the Ordovician granites in the Baoshan Block

The Southwest China was tonically situated in the northern margin of Gondwana continent during the Early Paleozoic, thus the Early Paleozoic magmatic records in this area were closely related to geodynamic evolution of the Proto-Tethys Ocean. In this paper, we present petrography, whole-rock major- and trace-element, LA-ICP-MS zircon U-Pb ages, zircon in situ Hf and apatite Nd isotopic compositions for the Early Paleozoic granites in the Pingda area of Baoshan Block in Southwest Yunnan to decipher their petrogenesis, magma source, and geodynamic mechanism. The LA-ICP-MS U-Pb isotopic analyses of zircons from the granites of Pingda pluton in Baoshan Block show that they were formed during 465.9 similar to 454.9Ma, representing the product of Middle-Late Ordovician magmatic activities. The Pingda pluton is dominantly composed of monzogranite, with main mineral compositions of plagioclase, quartz and alkaline feldspar (perthite and orthoclase), and minor biotite and muscovite. Geochemical analyses show that these monzogranite samples in the Pingda region have high SiO2 (72.16%similar to 76.87%) and Al2O3 (12.50%similar to 14.36%) contents, low MgO (0.12%similar to 0.64%), Mg-# values (20.9 similar to 35.9), Cr and Ni contents, as well as high Rb/Sr and Rb/Ba ratios. Together with their relatively high A/CNK values (1.06 similar to 1.45), these monzogranites belong to the weakly peraluminous to strongly peraluminous series, indicating a S-type granite affinity. In addition, the monzogranite samples of Pingda pluton have low Al2O3/TiO2 ratios and varying CaO/Na2O, Rb/Sr and Rb/Ba ratios, indicating that the studied granites represent partial melting products of heterogeneous sources with variable proportions of pelite and psammite rocks. Furthermore, these monzogranite samples have variable zircon epsilon(Hf)(t) values (-11.10 similar to-4.05), apatite epsilon(Nd)(t) values (-8.76 similar to-6.96), and heterogeneous two-stage model ages (2140 similar to 1704Ma), indicating that their magma source was dominant by Paleoproterozoic pelite and psammite rocks. Combined with widely distributed Early Paleozoic magmatic rocks and contemporaneous ophiolitic melange records in this area, we argue that the Early Paleozoic S-type granites in the Baoshan Block were originated from partial melting of crustal sedimentary rocks, indicating an active continental margin setting related to westward subduction of the Proto-Tethys Ocean.