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

The Alta Stock in the Wasatch Mountains of Utah provides an excellent opportunity to test new geochemical approaches for investigating the assembly history of plutons. Cathodoluminescence (CL) imaging and TitaniQ (Ti-in-quartz) geothermometry of quartz (Qtz) grains reveal several key observations related to Qtz crystallization. Clear core-to-rim zoning in CL is present only in Qtz phenocrysts from the central phase and in one sample of the border phase adjacent to the central phase. TitaniQ temperatures correlate with CL intensity, decreasing from 800-900^oC in bright CL cores down to 685-700^oC in outer (darker CL) portions of the Qtz phenocrysts. Phenocrysts also consistently show a bright CL, high-temperature (700-800^oC) rim; this feature is interpreted as a thermal rejuvenation event. The distribution of thermally rejuvenated rims throughout the central phase suggests either: stock-wide reheating or local reheating events resulting from the emplacement of successive magma increments. Most grains exhibit tile-like and mottled domains, and less often, enigmatic, irregular zoning, in CL. Tile-like domains are defined by irregular, bright CL bands interpreted as cemented fractures. TitaniQ temperatures generally correlate positively with CL brightness within these complex CL patterns, varying from magmatic (700-825^oC) down to subsolidus values (615-630^oC). These patterns suggest multiple periods of quartz growth, resulting from synkinematic emplacement of the stock and/or fracturing and cementing during sub-solidus (>600^oC) hydrothermal alteration of the quartz. Measured δ¹⁸O values of ~2mg samples of whole rock, quartz, feldspar, biotite, hornblende and magnetite indicate extensive exchange and re-equilibration of oxygen isotopes down to sub-solidus temperatures (550-650^oC). While quartz δ¹⁸O values vary significantly (8.8‰ to 10.0‰), up to 0.9‰ of this 1.2‰ variation can be explained by variations in the abundance of quartz and by variations in the extent and temperature of sub-solidus exchange with other minerals, and cannot be ascribed to initial magma heterogeneities. Hence variations in δ¹⁸O values of major minerals do not necessarily record the presence of isotopically distinct magma increments. The geochemical data suggest a complex assembly and cooling history for the Alta stock.