High magnesian schist, granitic gneiss, amphibolite and monzogneiss in the eastern Ama Drime Massif in South Tibet (China): A rifted Paleoproterozoic arc fringed the western Columbia supercontinent?

Abstract

Understanding the Paleoproterozoic tectonic setting of the north Indian Craton—as either continental rifting or continent arc—is vital for locating the Indian Craton in the Columbia supercontinent. Herein, we present new petrological, geochemical, Nd–Hf isotopic, and geochronological data of high magnesian schist, granitic gneiss, amphibolite and biotite monzogneiss in the south-eastern margin of the Ama Drime Massif of South Tibet (China). The high magnesian schist contains 20.4 %–31.3 % phlogopite, 12.2 %–17.8 % clinochlorite, 10.5 %–13.4 % cordierite and 5.5 %–5.6 % loss on ignition, implying its protolith of Mg-rich clays and later alteration in the fault zone. Its medium-silicic composition and concentrative magmatic zircon age of ca. 1859 Ma indicates the origin of the clays from locally deposited mafic rocks. Moreover, its negative values of ƐNd(t) (−3.0 to −2.6) and zircon ƐHf(t) (−8.6 to +2.4) correspond to the metavolcanic rock in the same outcrop as well. Granitic gneiss, amphibolite and biotite monzogneiss exhibit their protolith of crystal ages between 1870 and 1849 Ma with characteristics of negative anomalies of Nb, Ta, and Ti in subduction zone setting. They have similar negative values of ƐNd(t) (−3.0 to −1.3) and zircon ƐHf(t) (−8.6 to +4.0), indicating various degrees of contamination in the continental crust. The geochemical characteristics of the granitic gneiss and amphibolite signify the generation of their protolith via various degrees (1.5–2.5 %) of partial melting of spinel lherzolite inside the shallow mantle. As the high alkalinity of biotite monzogneiss implies a greater contribution of water from the subduction plate, and the trace elements of zircons manifest their affinity of continent arc toward island arc, we interpret the origin of the protolith of these rocks in a setting of continental arc-related rifting at the north of the Indian Craton. The finding of this arc reinforces that the reconstruction of the western Columbia supercontinent was fringed with an active margin.