Late Triassic lithium pegmatites in Northwest China: A response to continental collision during Paleo-Tethys Ocean closure

Abstract

Spodumene-bearing pegmatite dyke swarms have recently been discovered in the Chaqiabeishan area, at the northeastern margin of the Qinghai-Tibet Plateau in northwest China. In order to elucidate the connection between the Chaqiabeishan lithium pegmatites and Triassic magmatism and mineralization within the West Kunlun – Songpan -Garzê rare-metal belt, this study presents new columbite-group mineral and monazite U–Pb dating results, mineral chemistry, monazite in situ Nd isotope analyses and gamma-ray spectrometric measurements. Magmatism and mineralization at Chaqiabeishan mainly occurred ca. 216 Ma, and the mineralized pegmatites (Mn-rich columbite-group minerals, highly evolved monazite compositions and high effective uranium contents in the gamma-ray survey) were generated from the high degree of the fractional crystallization in a volatile-rich granitic magma. The source material of the Chaqiabeishan lithium pegmatites (ε_Nd(t) values from −14.4 to −12.8) was more enriched than that of other pegmatite-type lithium deposits (Jiajika and Bailongshan) in the West Kunlun – Songpan – Garzê rare-metal belt and is ascribed to melting of ancient crustal materials in the basement. These late-Triassic mineralizing events were closely related to the collision-related tectonic setting at the northeastern margin of the Tibetan-Qinghai Plateau, a product of Paleo-Tethys Ocean closure.