A (PLAGIOCLASE) FRAMEWORK FOR GRANITE DIFFERENTIATION IN THE ALTA STOCK, UTAH

The Alta stock (AS), UT, is part of the Wasatch Intrusive Belt and is best known for its surrounding metamorphic aureole. Wilson (1961) defined two textural units of the AS: an equigranular, more mafic border phase and a porphyritic, more felsic central phase. Contacts between the phases are most commonly gradational. Zircon and titanite U-Pb dates from the AS range from ~35–31 Ma and young inward, suggesting the border phase was emplaced prior to the central phase. Plagioclase (Plg) and quartz and orthoclase (Qtz+Or) mineral modes from 40 AS samples are negatively correlated (R2 = 0.86) and range between two end members: an equigranular group with high Plg/low Qtz+Or and aplite with low Plg/high Qtz+Or. Porphyritic samples lie between these end members implying physical mixing processes. Plg in equigranular samples form an interlocking framework with abundant Plg-Plg grain boundaries. Crystal-plastic deformation (e.g., crystallographic misorientation, undulose extinction, fractured and healed, and mechanically twinned Plg) of early-crystallizing minerals is more common in the equigranular samples. Deformation of the interlocking Plg crystals in the border phase indicates the crystals formed a rigid framework capable of transmitting stresses and underwent a volume reduction during melt extraction. Plg crystals in the porphyritic samples are most often solitary phenocrysts surrounded by a matrix of fine-grained Qtz+Or, with fewer Plg-Plg grain boundaries. AS and aplite whole rock data form complementary groups on Zr-Rb and Ba-Rb discrimination plots. Correlated patterns of textural, lithologic, chemical, and microstructural variation and zircon and titanite U-Pb petrochronology are best explained by tectonically-driven separation of aplitic melt from the partially molten border phase into the growing porphyritic central phase. The steep wall-rock contacts and textural variation suggest protracted, tectonic dilation (~0.5 mm a-1) of a dike-like conduit that likely both transmitted magma to the surface and accumulated magmatic rock. These relationships provide insights into shallow crustal magma differentiation at the emplacement level and highlight the interplay between tectonic forces and petrologic processes within growing intrusive bodies.