Evolution of pegmatite recorded by zoned garnet from the No. 9 dike in the Jiajika Li polymetallic deposit, eastern Tibetan plateau

Abstract

Granitic pegmatites can be extremely rich in Li, an important metal for modern industries. Being the Asia’s largest hard-rock Li polymetallic deposit, the Jiajika Li deposit is associated with numerous pegmatite dikes in the eastern Tibetan Plateau. The major Li resources in Jiajika are mainly hosted in fine-grained phases of pegmatite dikes. In this study, samples were collected from the typically zoned No. 9 dike, which inwardly consists of fine-grained albite phase, medium-grained albite phase, coarse-grained albite-microline phase, massive microline phase and massive quartz phase. Garnet is one of the common accessory minerals in the fine-grained albite phase. We report the first in-situ LA-ICP-MS U-Pb age of garnet from pegmatite dikes, yielding an age of 216.7 ± 5.9 Ma, which is consistent with the age of the associated granitic pluton. Combined with published isotopic dating results in Jiajika, two age groups can be identified: ∼210 to ∼ 220 Ma and ∼ 190 to ∼ 200 Ma, which are suspected to be resulted from two stages of thermal events, probably separate pulses of pegmatitic melts. The late pulse of melts might be responsible for the weak deformation of garnet grains and Fe-enriched rims along the deformed boundaries. The early pulse of melts derived from the Majingzi-Jiajijiami granitic pluton shows an indistinct time path from granite, to fine-grained phases, to medium- to coarse-grained phases, which resulted in the trace elemental core-rim textures within individual garnet grains. The garnet cores have typically higher trace element concentrations than the rims, and the boundaries between cores and rims are usually sharp and sinuous, which are proposed to record the amalgamation, dissolution and reprecipitation processes. The growth of cores was not in equilibrium with other phases by strong undercooling, and thus inherited the initial Li signature of the incipient pegmatitic melts, which can be up to 373 ppm. Accordingly, it implies that Li and other incompatible element contents of the incipient pegmatitic melts in Jiajiaka could be enriched in a significant amount at the early stage, which may, to some extent, explain why major Li resources are hosted in very-fine to fine-grained phases in Jiajika.