Origin of pegmatitic melts from granitic magmas in the formation of the Jiajika lithium deposit in the eastern Tibetan Plateau

Abstract

Granitic pegmatites may be genetically related to granites. However, it is still debatable how pegmatitic melts originated from granitic magmas. For example, a possible mechanism of melt–melt immiscibility for the origin of pegmatitic melt has not been well documented. The Jiajika Li deposit in the eastern Tibetan Plateau is a world-class deposit and is hosted in pegmatites, which occurred as dikes surrounding their source granite (two-mica granite). This paper concerns melt inclusions (MIs) in quartz from the granite–marginal pegmatite (GMP) at the roofward contact of the two-mica granite with the country-rock schist, as the MIs from such pegmatite are considered to represent the original pegmatitic melt and hence are ideal for investigating the origin of pegmatitic melts from granitic magmas. The MIs can be classified into H2O–CH4- and H2O–CO2-bearing types, both containing solid phases, including muscovite, albite, and apatite; the H2O–CO2-bearing MIs are usually associated with the H2O–CO2 fluid inclusions. To prevent leakage or decrepitation of these MIs during heating, external pressures were applied by using a hydrothermal diamond-anvil cell. The compositions of muscovite inside the unheated MIs and glass inside the MIs, which was quenched from a homogenous state, were analyzed using electron microprobe analysis (EMPA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). During heating, the H2O–CH4-bearing MIs were homogenized into an H2O-saturated melt mostly at 750–800 °C, while the H2O–CO2-bearing MIs, when similarly treated, showed H2O–CO2-oversaturated entrapment features. The H2O–CH4-bearing MIs are considered to be trapped from the original pegmatitic melt containing ∼12 wt% H2O with up to ∼1000 ppm Li, ∼1500 ppm Rb, and ∼1800 ppm Cs, at pressures of ∼550–700 MPa. Based on previous MI-homogenization experiments, it has been reported that the microthermometry features and compositions (e.g., H2O contents) of the H2O–CH4-bearing MIs in the GMP quartz are similar to those in an H2O-rich melt, which was immiscible with an H2O-poor melt inside the heated MIs in quartz from the two-mica granite. Accordingly, in this study, we demonstrate that pegmatitic melts could be originated via melt–melt immiscibility of granitic magma, and that the initial exsolution of the pegmatitic melt from granitic magma can occur on a very small scale, as tiny droplets whose sizes may be similar to those of the observed MIs in granite. In addition to the enrichment of Li, B, and P, the enhanced H2O content under low oxygen fugacity condition in the granitic magma, as well as the decrease in pressure following the magma emplacement, could promote the melt–melt immiscibility.