Petrogenesis of Triassic granites from the Nanling Range in South China: Implications for geochemical diversity in granites

Abstract

Abstract A combined study of whole-rock major-trace elements and Sr–Nd isotopes, zircon U–Pb ages, Hf and O isotopes as well as biotite geochemistry was carried out for Triassic granite intrusions from the Nanling Range in South China. The results provide insights into the effects of source composition and melting conditions on the geochemical diversity of granites. The granites of interest are peraluminous, and contain primary muscovite and tourmaline. They are characterized by high zircon δ18O values of > 9.0‰, high initial 87Sr/86Sr values of ~ 0.7200, and homogeneous eNd(t) values of − 11.3 to − 9.8, as well as variable zircon eHf(t) values of − 12.2 to − 5.8. Biotite geochemistry is similar to that of common peraluminous granites. An integrated interpretation of these petrological, mineralogical and geochemical data indicates that these granites were derived from partial melting of metasedimentary rocks under variable physicochemical conditions. The differences in whole-rock and biotite geochemistry between the intrusions are ascribed to the variable effects of source heterogeneity and melting temperature. The Luxi intrusion exhibits higher contents of MgO, FeOT, TiO2 and CaO than common melts derived from metasedimentary rocks, tight variations in major-trace elements and homogeneous Sr–Nd isotopic compositions, and homogeneous biotite composition with high Mg# [= Mg / (Mg + Fe) in molar] and lower whole-rock A/CNK values [= Al2O3 / (CaO + Na2O + K2O) in molar]. These can be explained by originating from a relatively mafic metasedimentary source. On the other hand, the geochemical diversity of granites can be caused by the difference in melting temperature in addition to the source heterogeneity. This is suggested by the Xiazhuang and Fucheng intrusions which exhibit similar range of SiO2. Nevertheless, the Fucheng intrusion is ferroan, and high in TiO2, (Na2O + K2O)/CaO, TiO2/MgO, Ga/Al and Zr + Nb + Ce + Y, but low in CaO, MgO and Mg#. Most of its major elements and their ratios are similar to those of A-type granites that were generated from F-rich rocks at high temperatures, consistent with the enrichment of Fe and F in its biotite. Thus, partial melting of a metasedimentary restite at reduced conditions after previous melt extraction is suggested to produce the geochemical feature of A-type granites in the Fucheng intrusion. In contrast, partial melting of fertile metasedimentary rocks would produce normal S-type granites like the Xiazhuang intrusion, with its geochemical variations ascrible to source heterogeneity and differential melting. Therefore, the geochemical composition of granites is primarily dictated by the composition of source rocks in addition to the physicochemical conditions of partial melting.