229 Crustal Petrology (Posters)

This session explores various igneous and metamorphic processes that have shaped Earth’s crust. We welcome contributions from the micro- to macroscale, from the deep to shallow crust, and from field- to laboratory- to modeling-based.

Tuesday, 21 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1

Preliminary Results from a Geochemical Analysis of the Basement Rocks of the Mexican Hill Quadrangle, Bighorn Mountains, Wyoming, USA.

Paper number: 229-1
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

The Mexican Hill quadrangle is part of the batholith region of the Bighorn mountains in the Wyoming craton which were exhumed during the Laramide orogeny. Past studies have found an intrusion age of the Bighorns to range over ~200 million years, between ~2.7 Ga and ~2.9 Ga, with multiple igneous pulses of intrusion. This study consists of five different rock types: monzogranite, granodiorite, tonalite, and quartz monzodiorite. There were further discordant mafic dikes scattered around the field area. Petrography shows evidence of myrmekite, sericitization, epidote and chlorite alterations, and antiperthitic texture. Whole rock geochemistry showed a spread of alkalinity with samples in calcic, calc-alkic, and alkali-calcic fields on an alkali-lime index diagram developed by Frost and Frost, 2008.  The felsic intrusives all plotted within the volcanic arc granites field with one sample verging on syn-collisional granite on the Pearce, et al. 1984 tectonic interpretation of granitic rocks diagram. Trace element geochemistry yielded both positive and negative Eu anomalies possibly indicating different pulses of magma that differentiated at varying depths within the lithosphere. The trace element spider diagram showed a depletion in high field-strength elements and a depletion in heavy rare earth elements. The Eu anomalies along with the rest of the whole rock geochemistry will be used with future U-Pb zircon ages to help support the hypothesis that the Mexican Hill is formed from at least two different origins of rock. 

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

doi: 10.1130/abs/2025AM-412618

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