OXYGEN ISOTOPE, TitaniQ, AND CATHODOLUMINESCENCE ANALYSES OF THE ALTA STOCK, UT: PRELIMINARY INSIGHTS INTO PLUTON ASSEMBLY

The Alta Stock in the Wasatch Mountains of Utah provides a unique opportunity to test new geochemical approaches for investigating the assembly history of plutons. Oxygen isotope values of quartz were analyzed and compared with quartz crystallization temperatures using the newly calibrated TitaniQ (titanium-in-quartz) geothermometer. Initial geochemical data suggests a complex assembly history. Oxygen isotope values range from 8.8‰ to 10.0‰, and do not correlate in any obvious way with location within the stock nor do they correlate with rock type (i.e. granodiorite, mafic enclave, aplite, etc.). In addition, several quartz-magnetite pairs yield sub-solidus equilibrium temperatures (~625°C). TitaniQ temperatures are variable, showing both subsolidus (615-630°C) and magmatic temperatures (700-825°C). These temperatures are variable between different rock types at the same sample locations, but over the whole stock average out to be cooler in the border phase (757°C) and hotter in the central phase (787°C). Cathodoluminescence (CL) imaging of some quartz grains used in TitaniQ temperature calculations reveals complex tile-like and mottled domains with only rare core-to-rim zoning. These patterns suggest either synkinematic emplacement of the stock and/or fracturing and annealing during sub-solidus (>600°C) hydrothermal alteration of the quartz. The lack of correlation of δ18O values of quartz with rock type suggests that measured variations in δ18O values reflect primary variations in isotopic composition of magma rather than magma mixing or fractional crystallization. This suggests that both the border and central phases of the stock were assembled by the amalgamation of many small aliquots of magma from slightly different sources. These techniques show promise, although careful CL imaging of quartz must be undertaken to insure meaningful results from TitaniQ geothermometry.