Research Progresson Petrogenesis of LCT-type Granitic Pegmatite and Lithium Enrichment Mechanism

LCT-type granitic pegmatite is one of the important host rocks of lithium resources in the world. Based on research results of geochronology, geochemistry, inclusion, numerical simulation and petrological experiments on LCT-type pegmatite in the past 40 years, this paper summarizes its the temporal-spatial distribution characteristics, temperature-pressure conditions, magma origin and evolution process and lithium enrichment mechanism, which is aim to provide theoretical reference for future prospecting work. Studys show that the global LCT-type granitic pegmatite was formed at 3040~7Ma, showing a good coupling between the peak of diagenesis and the supercontinent existence. The pressure of pegmatite emplacement is 250~350 MPa, and its liquidus temperature (650~750℃) is related to the abundance of fluxing-element while solidus temperature is about 425℃. Compared with the average composition of continental crust, LCT-type granitic pegmatite is characterized by enrichment of SiO₂, Na₂O, K₂O, Li, Cs, Ta, Nb and depletion of Fe₂O₃, CaO, MgO, TiO₂, Zr, Lower Nb/Ta and Zr/Hf ratios. LCT-type granitic pegmatite-froming magma is derived from granitic magma with high degree crystallization differentiation (>90%), from partial melting of crustal material with low degree (5%~20%), from immiscibility of F-Li-rich granitic magma and from a supercritical fluid(T≈731±21℃). It cooled and consolidated in a short time after emplacement, howere, its evolution process is debatable, including dynamic crystallization and melt-melt immiscibility. Both origin and evolution of pegmatite-forming melt can cause lithium enrichment. The crystallization differentiation origin modle proposes that the supernormal lithium enrichment (Li ≥ 10 000×10^-6) is controlled by the total distribution coefficient (D_Li<0.5), degree of crystallization differentiation (>99%) and initial lithium concentration (>100×10^-6) in deep magma chamber; however, the partial melting origin modle suggests that the lithium abundance in LCT type pegmatite is related to the abundance of lithium in its source component and the proportion of biotite in the residual phase. The immisible origin modle point out that lithium enrichment is affected by the ability of lithium complexes/compounds entering the volatile-rich and silicion-poor melt phase. The enrichment of lithium during the dynamic crystallization evolution of pegmatite-forming magma is related to constitunent zoning-refining process, however, the lithium abundacne is associated with the temperature and water content of volatile-rich melt in the immiscible evolution model.