Mineralogical Characterization of the Plumbago North Pegmatite (Maine, USA): Reading Mineral Textures and Chemistry to Determine the Crystallization Conditions

The Plumbago North pegmatite is an extremely Li-rich body located in the Central Maine Belt (USA). Primary spodumene, which occurs in the inner zones as gigantic-sized crystals, is the main Li-bearing mineral in this body. A detailed petrographic study, combined with the characterization of the mineral chemistry, have made it possible to establish the degree of fractionation of the pegmatite, its crystallization history, and the P-T crystallization conditions. The pegmatite presents an internal zoning, with Li-minerals mainly concentrated in the inner zones, garnet restricted to the outer zones, and mica (muscovite) abundance increasing inward, whereas the K/Rb ratio for micas and K-feldspar decreases from the contacts to the core. According to the mineralogical association and distribution inside the pegmatite, and taking into account the chemical composition of alkali feldspars, muscovite, garnet, phosphates, and Sn-Nb-Ta oxides, the Plumbago North pegmatite presents a high but not extreme degree of fractionation. Internal zoning of the pegmatite and textural evidence indicate that crystallization of the pegmatite-forming melt proceeded from the borders inward, mainly under closed system conditions and not far from equilibrium. The progressive fractionation resulted in a very high concentration of H2O, Li, and P in the residual melt; the high concentration of these fluxes allows the crystallization of very coarse crystals in the core zone of the pegmatite. Taking into account the metamorphic grade of the country rock, and in combination with some petrographic facts (primary character of spodumene and presence of tartan twinning and of plagioclase lamellae in the K-feldspar), the P-T conditions for Plumbago North crystallization are in the ranges 450–550 °C and ≤2.5–3.5 kbar. This study demonstrates that a detailed textural study may give valuable information on the crystallization conditions and history of pegmatitic melts.