Origins and evolution of two types of Late Triassic granitic magmas in the Caolong-Xiangkariwa area of central-eastern Songpan-Ganze terrane, northern Tibet: Implications for pegmatite lithium mineralization

Abstract

Pegmatite-type lithium (Li) deposits are an important source of the low-carbon energy metal, which is generally considered to be formed by the high degrees of differentiation of strongly peraluminous granitic magma. However, the question of whether the parental rocks of these peraluminous magmas are igneous or sedimentary has been widely debated. Recent studies have identified a world-class pegmatite-type Li deposit belt (up to 2800 km long) closely associated with the Late Triassic granitoids and (meta)sedimentary rocks along the West Kunlun and Hoh Xil-Songpan-Ganze terranes in northern Tibet. This study presents a comprehensive petrological, geochronological, and geochemical study on the two types of Late Triassic granitoids in the Caolong-Xiangkariwa area of the Yushu region of central-eastern Hoh Xil-Songpan-Ganze terrane in northern Tibet: type A includes the Tongtianhe-Zhenqin-Zhaduo-Zhaya-Xiangkariwa (TZX) diorites and granodiorites (218−212 Ma), and type B includes the Caolong two-mica and muscovite granites (213−205 Ma), as well as ore-barren, beryl-bearing, and spodumene-bearing pegmatite dikes (209−196 Ma). The TZX diorites and granodiorites contain variable amounts of amphibolite and biotite. They are metaluminous to slightly peraluminous with variable SiO2 (54.4−71.3 wt%), MgO (0.72−7.26 wt%) contents, and Mg# values (40−70). Some samples with low SiO2 (54.4−58.5 wt%) contents have geochemical features similar to high-Mg andesites: e.g., high MgO contents (5.24−7.26 wt%) and Mg# values (61−70). The TZX diorites and granodiorites exhibit less enriched (86Sr/87Sr)i (0.7080−0.7118) and εNd(t) (−4.8 to −8.0). By contrast, the Caolong two-mica and muscovite granites are characterized by the occurrence of abundant muscovite (plus tourmaline and garnet) and are strongly peraluminous. The Caolong granites and pegmatites are geochemically characterized by high SiO2 contents (68.8−79.8 wt%) and low MgO contents (0.01−0.10 wt%) and Mg# values (4−23). They have εNd(t) values (−8.8 to −11.4; granites: −8.8 to −10.8; pegmatites: −9.2 to −11.4) that are more enriched than the TZX diorites and granodiorites but similar to those of Songpan-Ganze (meta)sedimentary rocks (−7.4 to −12.9), and the Caolong two-mica granites have high zircon δ18O values (11.6−12.1). In addition, decreasing K/Rb ratios correspond to increasing Cs contents in K-feldspar and muscovite from the Caolong granites and pegmatites. Taking into account the Late Triassic granitoids and (meta)sedimentary rocks in the Hoh Xil-Songpan-Ganze terrane, we suggest that the TZX diorites and granodiorites were most probably formed by the partial melting of metasomatized lithospheric mantle that subsequently underwent extensive fractional crystallization. Conversely, the Caolong two-mica and muscovite granites were likely generated purely by the partial melting of (meta)sedimentary rocks rather than via the evolution of dioritic-granodioritic magmas. In summary, there were two kinds of granitic magmas with different sources in the Caolong-Xiangkariwa area. Finally, the Caolong pegmatites were most likely formed by the extreme differentiation of two-mica granitic magmas rather than the dioritic-granodioritic magmas. Therefore, the evolution of magmas derived from (meta)sedimentary rocks were most likely to have formed the Li-rich pegmatites. Moreover, these (meta)sedimentary rocks representing the ultimate Li source must have undergone strong chemical weathering, resulting in significant Li enrichment.