CRYSTALLIZATION OF AN INTERNALLY-ZONED GRANITIC PEGMATITE: INSIGHTS FROM FLUID AND MELT INCLUSIONS, EMMONS PEGMATITE, OXFORD COUNTY, MAINE
Primary FIAs are mainly H2O-CO2 averaging 2 to 8.5 NaCleq wt%. MIAs have a simple hydrous granitic composition, but some contain Li-Al-silicates, garnet, and apatite. FIAs in cores of early prismatic tourmaline of EM outer zones are very rare. They are coeval with MIAs, suggesting that crystallization started under fluid-unsaturated conditions. Occasional clusters of primary FIAs in skeletal tourmaline and quartz within QTIs suggest that local fluid exsolution took place at the QTI – magma interface. FIAs in skeletal tourmaline and adjacent quartz have similar compositions and volumetric properties, consistent with coeval formation of the two minerals.
Emplacement pressure is estimated around 250 ± 40 MPa based on isochores of coexisting immiscible CO2-rich and CO2-poor FIAs trapped within drusy quartz in the EM core. In the rapidly cooled wall-zone QTIs, average trapping temperatures estimated for 250 MPa range from 405 °C in central tourmaline to >340 °C in marginal skeletal tourmaline and quartz. In the hotter, slower cooling EM core, trapping temperatures are 570 ± 40°C. These temperatures are hundreds of degrees below the equilibrium crystallization temperature of a granitic magma, in agreement with the model of undercooled magma rapidly crystallizing under disequilibrium conditions.