10-12 Tectonically Driven Differentiation at the Emplacement Level of the Alta stock, Utah

Session: How are Plutons Made? Physical and Chemical Records of Pluton Construction and Evolution

Presenting Author:

Emmaline Saunders

Authors:

Saunders, Emmaline M.¹, Bartley, John M.², Stearns, Michael A.³, Bowman, John R.⁴, Beno, Carl J.⁵, Didericksen, Bradley D.⁶

(1) Utah Valley University, Orem, UT, USA, (2) University of Utah, Dept Geology & Geophysics, Salt Lake City, UT, USA, (3) Utah Valley University, Orem, UT, USA, (4) Univ of Utah, Dept Geology & Geophysics, Salt Lake City, UT, USA, (5) United States Geological Survey, Tucson, AZ, USA, (6) N/A, Draper, Utah, USA,

Abstract:

The Alta stock (AS), UT, is part of the Wasatch Intrusive Belt and is well known for its surrounding metamorphic aureole. Wilson (1961) defined two textural units of the AS: an equigranular, more mafic border and porphyritic, more felsic central phases. At the outcrop scales, both phases are texturally and compositionally diverse, host compositionally diverse enclaves, and contain mutually-crosscutting granodiorite and aplite dikes. Contacts between the phases are generally gradational and locally sharp, suggesting complex interaction between magma increments and textural modification during and after emplacement. Zircon U-Pb dates from the AS range from ~35–31 Ma and young inward, suggesting the border phase was emplaced before 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. Plg in equigranular samples form an interlocking framework with abundant Plg-Plg grain boundaries. Crystal-plastic deformation (e.g., crystallographic misorientation 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. Plg crystals in the porphyritic samples most often occur as solitary phenocrysts surrounded by a matrix of fine-grained Qtz+Or, with fewer Plg-Plg grain boundaries. Qtz phenocrysts in porphyritic samples are commonly rimmed by microcrystalline Qtz+Or and have high-Ti rims, consistent with rapid precipitation by a melt/fluid, recording infiltration or transient pressure changes. AS and aplite whole rock concentrations form a linear trend on Rb-Th plot and complementary groups on Rb-(Ba, Sr, Nd) plots. Correlated and spatially coherent patterns of textural, lithologic, chemical, and microstructural variation and zircon U-Pb petrochronology are best explained by tectonically-driven separation of aplitic fluids from the partially molten border phase into the growing porphyritic central phase. This record of differentiation within the AS suggests that the growing intrusion accumulated magmatic rock at tectonic rates (~0.5 mm a^-1) consistent with the protracted (~4 Myr) timescale of emplacement and provides insights into the interplay between tectonic forces and petrologic processes within growing intrusive bodies.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10852

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.