Geochemistry and petrogenesis of the post-collisional high-K calc-alkaline magmatic rocks in Tengchong, SE Tibet

Abstract

Post-collisional high-K calc-alkaline (HKCA) magmatic rocks (8 Ma to present) are extensively distributed in the Tengchong volcanic field (TVF), SE Tibet. Here, an integrated study of whole-rock major-trace elements, Sr-Nd-Pb-O isotopes and zircon U-Pb-Hf-O isotopes, together with mineral texture and geochemistry, are presented for these volcanic suites. The HKCA magmatic rocks exhibit high concentrations of incompatible trace elements with negative Nb-Ta-Ti anomalies and high (87Sr/86Sr)i (0.705–0.710), low (143Nd/144Nd)i (0.5120–0.5126) with variable δ18O (5.7–9.3‰). They may be divided into four groups from early to late stages: Groups I and III are basaltic-trachybasaltic-trachyandesitic assemblages (MgO = 3.3–9.2 wt.%); Groups II and IV mainly consist of trachyandesites and dacites (MgO = 0.6–5.1 wt.%). Incompatible trace element and Sr-Nd isotope compositions of the mafic magmas (MgO = 6.0–9.2 wt.%) from Groups I and III are postulated to reflect fluid enrichment of a mantle source by silicate and carbonatitic metasomatism as a consequence of eastward underthrusting of the Indian continental slab following the India-Asia collision. The petrogenesis of differentiated magmas (Groups II and IV) is interpreted in terms of control by crustal magma chamber AFC processes. The results of P-T estimation and EC-AFC modelling suggest at least two magma storage regions: a deep one (17–28 km) in which the trachyandesitic magmas were produced by AFC of basaltic magmas and a shallow one (9–14 km) fed by trachyandesitic magmas which gave rise to dacitic magmas via AFC. Phenocryst zoning textures also record mafic replenishment into silicic magma chambers.