Generation of Early Indosinian enriched mantle-derived granitoid pluton in the Sanjiang Orogen (SW China) in response to closure of the Paleo-Tethys

Abstract

A diverse suite of intermediate to felsic rocks from the Baimaxueshan pluton within the Sanjiang Orogen, SW China, documents the tectonomagmatic history of closure of the eastern Paleo-Tethys and associated arc accretion onto the margin of Asia. The pluton consists mainly of diorite, tonalite and granodiorite, and contains mafic magmatic enclaves (MMEs). SHRIMP U–Pb analyses on zircons from the MMEs and host granitoids yield crystallization ages of 253–248Ma. Considering analytical errors, the age of MMEs and their host granitoids are indistinguishable with a weighted mean of 249±1Ma (N=71, MSWD=1.09), indicating Early Triassic emplacement. They display similar chemical and isotopic characteristics with a medium- to high-K, calc-alkaline, metaluminous I-type character, and similar whole rock Sr–Nd isotopic ratios, zircon εHf (t) (−10±1) and identical zircon δ18O (>8‰ VSMOW). These observations, in combination with variably elevated Mg# (0.53–0.65), MgO content and Cr and Ni concentrations for these rocks suggest an origin from hydrous partial melting of subduction-modified subcontinental lithospheric mantle (SCLM). The MMEs may represent a mantle-derived mafic component that replenished the magma chamber and which survived mixing with the evolved host magma. The isotopically evolved signatures and arc-like pattern of incompatible elements on a primitive-mantle normalized spidergram suggest considerable input of supracrustal materials to the SCLM magma source (20–30% estimated by mixing calculation based on Sr–Nd isotopes). The striking resemblance of the zircon Hf–O isotopic systematics in both MMEs and host rocks also favors a crustal recycling paradigm dominated by source mixing rather than assimilation en route, and suggests that zircon crystallization commenced after the ingestion of supracrustal materials. Fractional crystallization operated during magma ascent and emplacement, and produced the wide spectrum of rock types seen in the pluton. Our geochronological and geochemical results also demonstrate that the Early Triassic Baimaxueshan pluton constitutes a part of the volcano-plutonic arc created by westward-directed Jinshajiang subduction system, and cannot be treated as the northern continuation of the Lincang batholith in the southern Lancangjiang zone.