Petrogenesis of Paleozoic high Ba-Sr granitoids from the Qinling orogen, Central China: Implications to the formation of Archean sanukitoids and TTGs

Modern analogues of Archean granitoids are particularly important, for they offer an opportunity to investigate processes of early crustal formation and tectonic transition. High Ba-Sr granitoids are usually considered as the Phanerozoic equivalents of Archean sanukitoids, however, their petrogenesis is still equivocal. Here, we collected a suite of high Ba-Sr granitoids from the Banshanping pluton in the Qinling orogen, central China, and carried out an elaborated study including mineral chemistry, whole rock elemental and Sr-Nd isotopic compositions, as well as zircon U-Pb, Lu-Hf and O isotopes, to constrain its petrogenesis. Our new secondary ion mass spectroscopy (SIMS) zircon U-Pb geochronology suggests the emplacement age of the Banshanping pluton at 435 +/- 2 Ma (MSWD = 0.37). The rocks have variable SiO2, elevated Mg# (50-53), Cr, and Ni concentrations, as well as strong enrichment of Ba and Sr (Ba + Sr = 1310-3040 ppm), but depletion of high field strength elements (HFSE). Some trace element ratios are positively (e.g., La/Yb, Sr/Y) or negatively (e.g., Dy/Yb) correlated with their SiO2 concentrations, indicating that amphibole fractionation might play a crucial role in their formation. They also exhibit restricted and relatively high initial Sr-87/Sr-86 (0.70437-0.70515) and chondritic to slightly positive epsilon(Nd)(t) values (0.29-1.32), but high zircon eHf(t) (8.0-9.9) and normal mantle zircon delta O-18 (5.45 +/- 0.13 %) values. Isotopic modeling constrains incorporation of about 4% oceanic crust-derived fluid and 0.3% sediment melt into their mantle source. Combined with regional geological evolution, the Banshanping pluton is inferred to originate from a two-stage process: its mantle source was metasomatized by the subducted protoTethyan oceanic slab during the early stage and partial melting of the metasomatites generated the high Ba-Sr granitoids induced by slab roll-back. The high Ba-Sr granitoids geochemical signatures overlap with Archean sanukitoids in many aspects, thus offering essential constraints on their formation. Compared with Archean sanukitoids, our results suggest that the sanukitoids derived from fluid-dominated metasomatic mantle might have the potential to evolve into TTG through amphibole-dominated fractional crystallization.