284 The Deformation-Metamorphism-Fluid Triplet Governing Plate Boundaries and Orogens

Investigating orogens requires establishing relationships between deformation, fluids, metamorphism, and timing from within and adjacent to faults and shear zones. We seek presentations that combine new and traditional structural, petrochronology, and other microanalytical techniques.

Wednesday, 22 October 2025
1:30 PM - 5:30 PM
HBGCC, 217B

Reconstructing Fluid Histories for Metasomatic Rocks in the Big Maria Mountains and Cargo Muchacho Mountains, Southeastern California, using Oxygen Isotopes and U-Pb Geochronology

Paper number: 284-3
Program: Technical Sessions
Time: 2:15 PM – 2:30 PM

Authors

Abstract

Significant crustal metasomatism is recorded in the kyanite quartzites and calc-silicate rocks of the Cargo Muchacho and Big Maria Mountains. Mineral assemblages of these kyanite quartzite rocks, hosted in Jurassic metavolcanics, and reaction of Paleozoic siliceous limestone in the Big Maria Mountains into metasomatic grossular-diopside-feldspar-titanite pods or layers require relatively large time-integrated fluid fluxes. In the Big Maria Mountains, the minimum fluid-rock ratio was previously estimated to be 17:1 for massive wollastonite in the Supai Formation. The prevailing hypotheses attribute metasomatism to fluids released from devolatilization of either (1) hydrated slab basalt or (2) underplated trench sediments during Late Cretaceous subduction of the shallowly dipping Farallon plate. This study integrates in situ U-Pb monazite and titanite geochronology with in situ oxygen isotope analysis to evaluate the timing and source(s) of metasomatizing fluids.

 

New pressure-temperature constraints from quartz-in-garnet elastic barometry, Ti-in quartz thermometry, and Zr-in-rutile thermometry suggest metamorphism occurred between 400°C and 650°C at crustal pressures >0.3 GPa. Monazite U-Pb dates from the kyanite quartzites range from 61–68 [±1.4] Ma (Cargo Muchacho) to ~72 [±1.4] Ma (Big Maria). Within the Cargo Muchacho kyanite quartzite, quartz (δ18O ~10.5‰), rutile (δ18O ~1‰ to 5‰) and kyanite (δ18O ~10.5‰) do not record a single common equilibrium temperature, suggesting multiple or compositionally evolving fluid interactions. Within the Big Maria kyanite quartzite, quartz (δ18O ~11‰), rutile (δ18O ~2‰), and kyanite (δ18O ~8‰) are largely homogeneous, consistent with expected equilibration temperatures (400°C–650°C). Within the Big Maria calc-silicate rocks, U-Pb titanite dates show mixing between linear arrays that variably intercept concordia in the Jurassic and Late Cretaceous (Campanian). Titanite is interpreted as recording crystallization during intrusion of nearby Jurassic granites followed by fluid-mediated recrystallization during the Late Cretaceous, coeval with models for Laramide flat-slab subduction. Quartz, K-feldspar, plagioclase, diopside, garnet, and titanite vary in δ18O and most minerals are not mutually in equilibrium, but the pattern of mineral-pair fractionations in all samples is similar, indicating heterogeneous fluid isotopic compositions at comparable thermal conditions. These new data suggest metasomatism initiated with low fluid-rock ratio interactions like pervasive flow and/or localized fluid infiltration causing calc-silicate metamorphism, but, over a >7 Myr interval, metasomatism progressed toward higher fluid-rock ratios via channelized flow for the younger kyanite quartzite rocks.

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

doi: 10.1130/abs/2025AM-413863

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