Petrogenesis and geochemical diversity of Late Mesoproterozoic S-type granites in the western Yangtze Block, South China: Co-entrainment of peritectic selective phases and accessory minerals

摘要

Deciphering the geochemical diversity of S-type granites is crucial for obtaining more profound insight into their petrogenesis. We therefore undertook an integrated study of whole-rock geochemistry, Sr-Nd isotopes, and zircon U-Pb-Hf isotopes for newly recognized Late Mesoproterozoic S-type granites, including two-mica-, biotite-, garnet-bearing two-mica granites, from the western Yangtze Block, South China. The crystallization ages of these granites are ca. 1040 Ma (weighted mean Pb-206/U-238 age = 1036.4 +/- 4.5 Ma to 1042.2 +/- 1.1 Ma). They are peraluminous to strongly peraluminous (A/CNK = molar ratio of Al2O3/(CaO + Na2O + K2O) = 1.02-1.67), high-K talc-alkaline rocks, and display high concentrations of normative-corundum (0.54-7.04 wt %) as well as positive correlations of A/CNK and FeOT + MgO values, which are characteristics of S-type granites. These S-type granites are characterized by enriched in Rb, Th, K, and Pb, depleted in Ba, Sr, Ti, and Eu, with negative whole-rock epsilon Nd(t) (-03 to -6.8) and predominantly negative zircon epsilon Hf(t) values (-8.09 to +5.70), indicating the affinity of middle-upper crustal trends and a heterogeneous metasedimentary source. Compared with the geochemical diversity of S-type granites around the world, the variably negative epsilon Nd(t) values as well as positive and negative epsilon Hf(t) values of our S-type granites may be caused by source heterogeneity and disequilibrium melting processes. More importantly, similar to typical more mafic S-type granites from the Cape Granite Suite (South Africa) and north Queensland (Australia), the high and variable FeOT + MgO contents (2.21-6.64 wt%) are significantly attributed to coupled co-entrainment of peritectic and accessory minerals (e.g., garnet, ilmenite, zircon, and monazite), evidenced by positive relationships between FeOT + MgO and TiO2, CaO, Zr, Th, Hf, Y, Yb, light rare earth elements (LREEs). In conjunction with the existence of coeval rift-related A-type igneous rocks, depleted asthenosphere-derived mafic rocks, marine sedimentary sequences, and subsequent extensive Neoproterozoic arc magmatism along the western Yangtze Block, we infer that the ca. 1040 Ma S-type granites studied here were produced in a continental rifting basin that was transformed into a compression setting during the Neoproterozoic. (C) 2019 Published by Elsevier B.V.