Petrogenesis of Mesozoic pegmatites in the Dahongliutan Li-mineralized belt (Western Kunlun, NW China)

Abstract

The genesis of pegmatite and the associated Li-mineralization mechanism remain debated. The intimate temporal-spatial relationship between the pegmatites in the Dahongliutan Li-mineralized belt (incl. #509 Daoban and Akshay Li-poor pegmatites, and Akshay Li-rich pegmatite) and the Dahongliutan granite pluton allows us to discuss the origin of pegmatite and the Li-mineralization mechanism by conducting detailed geochemical and geochronological investigations on the two rock units in the Western Kunlun orogen (NW China). The Li-poor pegmatites in the Dahongliutan Li-mineralized belt exhibit similar or much lower Li, Rb, and Cs compared to the adjacent granite, and their εNd(t) values (-8.7–-10.0) are significantly lower than those of the granite (-7.5–-8.1) as well. However, the Akshay Li-rich pegmatite shows extremely high Li, Rb, Nb, Ta, and initial Sr isotopic ratios (0.7223–0.7227, obtained from apatite), in contrast to the Dahongliutan granite (ISr = 0.7108–0.7157). The whole-rock δ7Li values of the Dahongliutan granite, pegmatites, and the wall-rock Bayanharshan Group metapelite are + 0.76–+5.67 ‰, +0.8–+14.4 ‰, and −4.2–+2.8 ‰, respectively. Zircon and cassiterite U-Pb dating yielded 206Pb/238U ages of 216 ± 2 Ma and 195 ± 2 Ma for the #509 Daoban Li-poor pegmatite and Akshay Li-rich pegmatite, respectively. Modeling of trace elements and Li isotopes suggests that the pegmatites in the Dahongliutan Li-mineralized belt did not originate from fractional crystallization of granitic magma. Instead, they may have formed through direct anatexis under amphibolite-facies conditions. Abundant Li-rich claystone, occasionally accompanied by evaporite, may exist in the magma source of the Li-rich pegmatite, which is the key factor controlling pegmatitic Li mineralization.