A NEW MODEL FOR THE FORMATION OF THE GEM-BEARING PEGMATITES IN OXFORD, CO., WESTERN MAINE

Be, B, and Li enriched pegmatites in Oxford Co., Maine, are renowned for production of gem tourmaline, beryl, apatite and spodumene. Wise & Brown (2010) suggested the pegmatites formed by fractional crystallization of the Sebago Batholith (SB). The SB was redefined as the Sebago Migmatite Domain (SMD) with a smaller Sebago Pluton (SP). Subsequently the SMD was renamed the Migmatite-Granite Complex (MGC). The migmatites are ~376 Ma (Acadian) and the SP and other small granite bodies are Alleghanian at ~293 Ma (Solar & Tomascak, 2016). U-Pb zircon and apatite dates on Mt. Mica, Emmons and Havey pegmatites (Roda-Robles unpub. dates; Bradley et al. 2016) as well as new Sm/Nd dates (Tanja Knoll & Ralf Schuster, pers comm), give ages from ~250 to 270 Ma, clearly younger than the migmatites and granites in the MGC. As the distance between the pegmatites and the SP is > 30 km, and with a 30-50 Ma age difference, a pegmatite origin by SP fractionation is not feasible. There is no other magmatic activity the same age as the pegmatites. Thus, in the absence of possible parental plutons and the strong chemical similarity of migmatite leucosomes and pegmatites (Simmons et al., 2016), we propose that the pegmatites formed by anatexis of the felsic portions (leucosomes) of the MGC during post-orogenic extension and decompressional melting prior to Pangea rifting.

Our preliminary model for pegmatite formation: Metamorphism and migmatization at ~4 kb, 650°C, ~376 Ma. An uplift and denudation rate of 40 m/Ma over 100 Ma brings the migmatites up from ~13 km to ~9 km with cooling to 450°C with a geothermal gradient (GG) of 50°C/ km. To initiate leucosome melting to generate pegmatite melts, an additional ~200°C is needed. The heat source is proposed to come from initiation of decompression melting associated with Pangaea breakup, introducing higher crustal temperatures prior to rifting. A GG of 70°C/km will heat the leucosomes to ~650˚C, sufficient for melting. The new pegmatite melts ascend to ~8 km, 2.5 kb, conducive for pocket formation, where they crystallize. Upon emplacement, the pegmatite-migmatite country rock thermal contrast of ~50-100°C, may explain comb structure muscovite (Mt Mica) or tourmaline (Emmons) indicating undercooling. Rift related mafic dikes intrude several pegmatites including the ~250 Ma Havey (Webber et al., 2017).