242 Crustal Petrology, Part I

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.

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 216AB

Plumbing the depths of the Colorado Plateau crust using mafic xenoliths hosted in shallow intrusions, southeastern Utah, USA.

Paper number: 242-1
Program: Technical Sessions
Time: 8:05 AM – 8:20 AM

Authors

Abstract

Oligocene shallow intrusions exposed on the Colorado Plateau, such as the Henry and La Sal mountains, contain mafic crustal xenoliths that present a unique opportunity to study the crustal evolution of the North American continent. The structural integrity of the Colorado Plateau is poorly understood but could be partly controlled by Mesoproterozoic mafic underplating. To better understand the origins of the crustal xenoliths from the Henry Mountains laccolith complex and their relationship to the host intrusions, we surveyed major and trace elements from amphibole, pyroxene, garnet, plagioclase, and titanite to reconstruct metamorphic and igneous histories. The mafic crustal xenoliths commonly show hydration reaction textures where primary mineral assemblages of pyroxene and garnet were variably reacted to amphibole prior to host entrainment. Xenoliths consist of two primary crustal compositions of garnet-bearing or pyroxene-bearing (cpx ± opx) assemblages. Amphibole is generally calcic where igneous phenocrysts are pargasitic with complex oscillatory zoning and xenolithic crystals are hornblende with diffuse zoning near the rims. Chondrite normalized trace element patterns show some xenolithic amphibole contains positive Eu anomalies and is less enriched in rare earth elements (REE) compared to igneous amphibole, which displays higher concentrations of REEs and negative Eu anomalies. Positive Eu anomalies suggest that xenolithic amphibole grew during metamorphism formed at the expense of Ca- plagioclase and replaced by Na- plagioclase. Preservation of mafic crustal compositions are also supported by distinct titanite and plagioclase chemistry from xenoliths and host. However, some xenolithic amphibole rims overlap with igneous amphibole compositions and some host amphibole is xenocrystic, suggesting some subsequent interaction with the host magma. Preliminary geothermobarometry suggests maximum depth conditions of ~ 0.9 GPa and 920°C (± 30°C) for xenolith amphibole whereas most igneous amphibole grew in a staging zone of ~ 0.5 GPa and 900°C (± 30°C).  Thus, Colorado Plateau xenoliths sample mafic parts of the mid to lower crust that was hydrated prior to xenolith entrainment in the host and that perhaps originated from Mesoproterozoic mafic underplating. The host Oligocene magmas likely originated from a deep, mantle- or lower crust-derived source probably unrelated to the xenoliths. These magmas staged and fractionated in middle crust magma chambers, where minor re-equilibration of xenoliths occurred before emplacement into the upper crust.

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

doi: 10.1130/abs/2025AM-413014

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