A model involving amphibolite lower crust melting and subsequent melt extraction for leucogranite generation

Abstract

Abstract In the southern Tibetan Plateau, leucogranites are dominantly distributed in the Himalayan orogenic belt with minor occurrences in the southern Lhasa subterrane. In this paper, we report the first Miocene Anglonggangri leucogranites in the northern Lhasa subterrane. This finding provides important constraints on both leucogranite petrogenesis and the tectono-magmatic evolution of the Lhasa terrane. The Anglonggangri leucogranites include biotite-muscovite granite and slightly younger garnet-muscovite granite and pegmatite. Zircon U-Pb and muscovite 40Ar-39Ar dating of these leucogranites yields Miocene ages of 11.1–10.2 Ma. The biotite-muscovite and garnet-muscovite granites are characterized by high SiO2 (72.3–74.4 wt.%) and Al2O3 contents (14.4–15.4 wt.%) and are peraluminous. The biotite-muscovite granite displays geochemical signatures with high Sr/Y (29.2–81.0) and (La/Yb)N (37.5–98.9) ratios, low Y (4.30–7.22 ppm) and Yb contents (0.26–0.47 ppm), low to moderate initial (87Sr/86Sr)i ratios (0.7085–0.7192), and moderate εNd(t) values (−10.17 to −6.94). Furthermore, they also exhibit radiogenic Pb isotope and variable zircon εHf(t) values (−9.6 to +4.4) with Proterozoic Nd (1.1–1.4 Ga) and Hf model ages (0.8–1.7 Ga). By comparison, the garnet-muscovite granite has lower CaO, MgO, TiO2, and total FeO contents and is enriched in Rb (380–466 ppm) and depleted in Sr (24.1–38.5 ppm) and Ba (30.7–58.6 ppm) and further characterized by a significant rare earth element (REE) tetrad effect and non-charge and radius-controlled (CHARAC) trace element behaviors. The garnet-muscovite granite shows a negative Eu anomaly and positive correlations among Sr and Eu, Sr and Ba, and Th and light rare earth elements (LREEs). Pegmatite comprising Nb-Ta oxides and cassiterite occurs in the garnet-muscovite granite. Geochronological and geochemical characteristics of the Anglonggangri leucogranites indicate that the magma of the biotite-muscovite granite was derived from partial melting of amphibolite lower crust contaminated with Proterozoic-Archean upper crustal materials. The garnet-muscovite granite was generated through melt extraction from the biotite-muscovite granite crystal mush. These results confirm that partial melting of the amphibolite lower crust not only occurred in the southern and central Lhasa subterranes but also in the northern Lhasa subterrane.