Strongly peraluminous fractionated S-type granites in the Baoshan Block, SW China: implications for two-stage melting of fertile continental materials following the closure of Bangong-Nujiang Tethys

Abstract

Strongly peraluminous fractionated S-type granites are important indicators of compositional maturity of the continental crust. Here we report the finding of strongly peraluminous fractionated S-type granites from the western Baoshan Block in the southeastern Tibetan–Himalayan orogenic belt. We present LA ICP-MS zircon U–Pb ages from biotite- and two-mica- granites which reveal two distinct periods of magmatism at 121.8 ± 1.1 Ma and 81.9–79.7 Ma. These Cretaceous granites are strongly peraluminous (A/CNK = 1.18–1.31), high-K calc-alkaline, enriched in Rb, Th, K, and Pb, and depleted in Ba, Sr and Ti. They also show high normative- corundum contents of 2.84–4.42 wt%, moderately high P2O5 content (0.22–0.28 wt%) and high differentiation index (DI) of 93.1–96.8, similar to the features of fractionated S-type granites. The low total Fe₂O₃T + MgO + TiO₂ contents (<3.0 wt%) are also consistent with the results from experimental studies on S-type granite melts. The S-type granites from Baoshan show high Al₂O₃/TiO₂ (93.8–182.9), Rb/Ba (6.19–23.5), and Rb/Sr (17.0–30.5) values, and low CaO/Na₂O (0.09–1.19). They also display negative εNd(t) values (−11.7 to −13.6) and εHf(t) ranges (−5.47 to −0.77 for biotite granites and −7.67 to −0.81 for two-mica granites) suggesting evolved and heterogeneous continental crustal sources for the magma. The initial ²⁰⁶Pb/²⁰⁴Pb (17.527 to 19.277), ²⁰⁷Pb/²⁰⁴Pb (15.772 to 15.888), and ²⁰⁸Pb/²⁰⁴Pb (39.237 to 39.498) isotopic ratios are also consistent with lower to upper crustal affinity. Our data, in conjunction with those from experimental studies suggest that the strongly peraluminous S-type granites were derived from partial melting of metasedimentary, pelite-dominated source within continental crustal basement, similar to typical fractionated S-type granites from the Jiangnan Orogen (SE China), Erzgebirge (Central Europe), Eastern Australia and the low-temperature SP granites in the Himalayas. We correlate the Early- to Late- Cretaceous magmatism with two-stage melting of fertile continental materials following the final closure and termination of Bangong-Nujiang Tethys ocean.