Petrogenesis of the Lijiagou spodumene pegmatites in Songpan-Garze Fold Belt, West Sichuan, China: Evidence from geochemistry, zircon, cassiterite and coltan U-Pb geochronology and Hf isotopic compositions

Abstract

Spodumene and other pegmatites in the Lijiagou area are located in the central Songpan-Garze Fold Belt (SGFB), West Sichuan. These pegmatites are hosted within the giant Songpan-Garze polymetallic belt. A lack of precise geochronological constraints on the magmatic crystallization of the pegmatite suite and nearby plutonic granitoids has precluded definitive statements on genetic links between spodumene ore formation and the magmatic suite. Systematic zircon, cassiterite and coltan U-Pb dating, Hf isotope and whole-rock geochemical analysis were undertaken. Two-mica granite and muscovite albite granite have S-type granite affinities and chemical compositions suggesting a post-orogenic setting, and the albite spodumene pegmatites belong to the LCT (Li-Cs-Ta) group of pegmatites. Zircon LA-MC-ICP-MS U-Pb dating of the two-mica granite, muscovite albite granite, albite pegmatite and albite spodumene pegmatite give crystallization ages of 219.2 ± 2.3 Ma (MSWD = 0.55), 217 ± 2.8 Ma (MSWD = 0.47), 202.8 ± 4.9 Ma (MSWD = 3.9) and 200.1 ± 4.6 Ma (MSWD = 3.1), respectively. Zircons from the same units yield εHf(t) values of −39.17 to 6.81, −22.73 to −2.83, −11.17 to 8.14, and −4.92 to −2.4, respectively, consistent with mixed crustal sources for the pegmatite and granite magmas. Cassiterite from albite spodumene pegmatite yields a concordia intercept age of 211.4 ± 3.3 Ma (MSWD = 2.9), while coltan yields a weighted mean age of 211.1 ± 1.0 Ma (MSWD = 0.97). The U-Pb zircon ages of the two-mica granite and muscovite albite granite are interpreted as ages of magmatic crystallization. Metamictization of zircon in both albite and albite spodumene pegmatites makes their U-Pb zircon ages liable to inaccuracy due to Pb loss. The coltan U-Pb age is regarded as an accurate measure of the magmatic crystallization age of the albite spodumene pegmatite. Given the differences in magmatic ages and εHf(t) ranges between albite spodumene pegmatite and both the two-mica granite and the muscovite albite granite, the albite spodumene pegmatites probably represent anatectic melts and not fractionation products of either of the granitic magmas. U-Pb coltan and U-Pb cassiterite dating are more likely to provide accurate crystallization ages of albite spodumene pegmatites than U-Pb zircon ages.