Miocene potassic granite–syenite association in western Tibetan Plateau: Implications for shoshonitic and high Ba–Sr granite genesis

Abstract

This paper presents detailed SHRIMP zircon U–Pb chronology, mineral chemistry, major and trace element, and Sr–Nd–Hf isotope geochemistry of two Cenozoic plutons (Karibasheng and Kuzigan) in western Tibetan Plateau, China. The Karibasheng pluton is composed of monzogranite, and the Kuzigan pluton consists of syenogranite, diopside-bearing syenite and diopside syenite. SHRIMP zircon U–Pb dating on monzogranite, syenogranite and diopside syenite shows that they were emplaced in Miocene (12–10Ma). These rocks are metaluminous and show the affinities with shoshonitic rocks, i.e., alkali-rich (K2O+Na2O=8.6–13.4wt.%), high K2O contents (4.7–10.6wt.%) with high K2O/Na2O ratios (1.2–4.6), enrichment in large ion lithophile elements (LILE) (e.g., Ba=2200–9100ppm; Sr=840–3100ppm, most >1100ppm) and rare earth elements (REE), especially light REE (LREE) (La/Yb up to 168). The rocks are depleted in high field strength elements (HFSE) but show high Nb/Ta ratios (13.3–30.7). The granites have lower initial 87Sr/86Sr ratios (0.7091–0.7099) and higher εNd (T) values (−7.1 to −9.5) than the syenites that have initial 87Sr/86Sr ratios of 0.7095–0.7117 and εNd (T) values of −8.6 to −12.2. Their zircon εHf (T) values (−7.2 to −9.7) are coupled with their whole-rock εNd (T) values, and plot along the mantle array. The amphibole–plagioclase thermometer and estimation of apatite saturation temperature indicate that the primary magmas had high temperatures (up to 880°C for the granites and up to 1100°C for the syenites). The source depth for the granites and syenites is <70–100km inferred by the metasomatic volatile phase (amphibole–phlogopite) relation, and thus most probably within the upper part of lithospheric mantle. Detailed elemental and isotopic data and major element modeling suggest that the granites and syenites were derived directly by partial melting of the veined lithospheric mantle that had been hybridized by the continental slab-derived melts. Continental slab break-off was most likely the mechanism associated with the generation of these granites and syenites.