282 Crustal Petrology, Part II

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
1:30 PM - 5:30 PM
HBGCC, 216AB

Deep Crustal Metamorphism Revealed by Ultra-high-temperature Mineralogical Features in Southern Sierra Nevada, California

Paper number: 282-8
Program: Technical Sessions
Time: 3:30 PM – 3:45 PM

Authors

Abstract

The southern Sierra Nevada Batholith (SNB) in California exposes the lower crust of a long-lived massive Cretaceous magmatic arc. This intrusive complex reveals a tilted crustal profile, as indicated by previous barometry: shallow levels are exposed in the northern Sierra (~2 kbar equivalent depth), getting progressively deeper (>8 kbar equivalent depth) toward the south near the Tehachapi Mountains. Yet despite this well-documented pressure gradient across the exposed plutonic rocks, pressure–temperature (P–T) conditions of the widespread metamorphosed wall rocks (e.g., calcsilicate roof pendants) remain poorly constrained. These rocks may have experienced conditions of tectono-thermal evolution that differ from the surrounding intrusions. Thus, constraining their metamorphic history complements the existing high-density petrological, structural, geochemical and geochronological datasets across the SNB.

Here, we focus on a garnet-bearing quartzite and a granulite from the southern SNB, which preserve features of ultra-high-temperature (UHT) metamorphism, including: fibrous sillimanite, garnet-hosted mineral lamellae (i.e., rutile, ilmenite, apatite), and the uncommon antiperthitic intergrowth of orthoclase blebs in anorthite host. To our knowledge, the garnet-hosted lamellae are not previously reported in the Sierra. We applied quartz-in-garnet barometry, zirconium-in-rutile and feldspar ternary thermometry, and thermodynamic modeling to reconstruct the thermo-tectonic history of the southern SNB. Our results define a counter-clockwise P–T path marked by peak granulite-facies conditions (around ~900 °C and ~16 kbar), followed by retrograde re-equilibration (to 600 °C at ~4.5 kbar) during exhumation of tens of kilometers. The unusually high peak pressures recorded by the wall rocks suggest deep burial during arc construction and may accordingly reflect significant vertical transport within the arc framework. This study integrates newly documented petrological features in the southern SNB, thermobarometry, and phase equilibria modeling to provide a metamorphic perspective on the Sierran arc, offering insights into the burial and exhumation history of arc wall rocks.

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

doi: 10.1130/abs/2025AM-412688

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