Origin of peraluminous A-type granites from appropriate sources at moderate to low pressures and high temperatures

摘要

Four Triassic plutons from South China exhibit compositional features of S-type granites (high A/CNK values, enriched Sr-Nd isotopic compositions and high zircon delta O-18 values), but also of A-type granites (alkali-rich and ferroan compositions, high Zr + Nb + Ce + Y contents and high Ga/Al ratios). The S-type-like features suggest a metasedimentary source. However, the exact anatectic processes are poorly known. Here, phase equilibrium modelling on the melting of five representative metasedimentary rocks with contrasting compositions at 0.4-1.2 GPa and FMQ - 1 to FMQ + 1 indicate that the most likely source should have a composition of Mg#<52, A/CNK>1.14, K2O/Na2O<1.90, and K2O + Na2O>5. Two Neoproterozoic volcanic-sedimentary rocks adjacent to the PAGs have compositions that match these requirements, and modelled melts at P <= 0.8 GPa and T >= 850 degrees C match the main compositional variables of the plutons. This study highlights that source composition and anatectic P - T conditions are the main factors controlling the generation of peraluminous A-type granites. (C) 2019 Elsevier B.V. All rights reserved.