Oxygen isotope, cathodoluminescence, and titanium in quartz geothermometry in the Alta Stock, UT: geochemical insights into pluton assembly and early cooling history

The Alta Stock in the Wasatch Mountains of Utah is a well-studied intrusion and is associated with a large, well-developed contact aureole; thus, the stock provides an excellent opportunity to test new geochemical approaches for investigating the assembly history of plutons. A combination of oxygen isotope analysis, cathodoluminescence imaging, and Ti-in-quartz geothermometry (TitaniQ) was undertaken to document the geochemical signatures of the stock and to examine how these are related to pluton emplacement, growth, and early cooling history. These results suggest a complex assembly and cooling history for the Alta stock. The delta18 O values from whole rock, quartz, feldspar, biotite, magnetite, and hornblende reveal that these minerals have exchanged oxygen isotopes into the subsolidus ( 5 5 0 - 6 5 0 ° C ) . Quartz delta18 0 values range from 8.9%o to 10.0%o, 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.). Most (0.9%o) of this l.l%o variation may be accounted for by variations in abundance of quartz, the temperature at which it equilibrated with other minerals, and in the extent of subsolidus exchange with hydrothermal fluid. Thus clearly identifying magma increments is difficult in the Alta Stock using major mineral delta18 O values.