CRYSTALLIZATION OF AN INTERNALLY-ZONED GRANITIC PEGMATITE: INSIGHTS FROM FLUID AND MELT INCLUSIONS, EMMONS PEGMATITE, OXFORD COUNTY, MAINE

Pegmatites are known for skeletal mineral intergrowths that are thought to form during rapid, undercooled crystallization, as magma intrudes colder rocks in the Earth’s upper crust. The Emmons (EM) pegmatite is internally zoned, including border, wall, intermediate, and miarolitic-core zones. We study quartz and tourmaline across multiple zones in order to constrain the evolution of the pegmatite-forming melt throughout its sequential crystallization from the border to its core. We focus on comb-texture, flaring, chemically-zoned schorl tourmaline from the hanging-wall zone with subhedral-euhedral morphologies at the contact with host rock and skeletal quartz-tourmaline intergrowths (QTIs) towards crystal terminations. We used a freezing-heating microthermometric stage to estimate the compositions and trapping temperatures of fluid inclusion assemblages (FIAs) and Scanning Electron Microscopy to identify microcrystals in exposed crystallized melt inclusions (MIAs).

Primary FIAs are mainly H₂O-CO₂ averaging 2 to 8.5 NaCl_eq 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.