242-17 Discriminating between multiple metasomatic events in the Big Maria Mountains, CA

Abstract:

The migration of fluids within the crust plays a significant role in crustal evolution, driving metasomatic reactions that change rheology or induce melting, and transporting and concentrating critical metals. The Big Maria Mountains (BMM), in southeastern California, expose metasomatic calc-silicate rocks with protoliths correlative to the Paleozoic-Mesozoic sedimentary sequence in the Grand Canyon. These supracrustal rocks and associated Jurassic granitoids were deformed and metamorphosed as part of the Late Cretaceous Maria Fold and Thrust Belt, forming a map-scale nappe structure with strata locally attenuated to ~1% of their original thickness. Syn- and post-deformational Late Cretaceous leucogranite dikes occur in a swarm in the western BMM. The calc-silicate rocks have experienced varying degrees of complete to partial wollastonitization and decarbonation (quartz + calcite reacting to wollastonite + CO₂). Previous modeling of these reactions produced calculations of a large (≥17:1) fluid-rock ratio assuming pervasive fluid flow across the BMM from an unresolved source. To resolve metasomatic events and fluid sources in the BMM, we study the petrography, major-element mineral compositions, geochronology, and oxygen isotope records for BMM metasomatic rocks.

The metasomatic rock types include 1) calc-silicate gneisses with layering defined by variable amounts of wollastonite, K-feldspar, diopside, grossular-andradite, quartz, and calcite; 2) epidote-andradite ± diopside ± amphibole skarn, which occurs as lenses, layers, and pods folded and boudinaged within the dominant calc-silicate fabric; and 3) a Late Cretaceous kyanite quartzite. Titanite U-Pb geochronology reveals two populations; Jurassic titanite are found in most outcrops, while Late Cretaceous titanite are generally restricted to outcrops proximal to leucogranite dikes. In the most comprehensive SIMS oxygen isotope survey of calc-silicate phases to date, we observe inter-sample d¹⁸O variabilities in diopside (up to 8‰), wollastonite (up to 7‰), garnet (up to 5‰), feldspar (up to 4.5‰), and titanite (up to 4‰), and intra-sample mineral d¹⁸O variabilities of 1–3‰, suggesting widespread d¹⁸O heterogeneity at the grain-scale and outcrop-scale. Reconstructed bulk-rock d¹⁸O values (~4–11‰) indicate widespread lowering of d¹⁸O relative to sedimentary protoliths, providing evidence for spatially heterogeneous interactions with lower d¹⁸O fluids. Although our oxygen isotope data cannot rule out a Farallon slab fluid source (~3–6‰), the abundance of metasomatic K-feldspar suggests fluids sourced from nearby Jurassic and Cretaceous granitoids (7-8‰) and/or pluton-associated meteoric-hydrothermal systems (≤0‰) over two periods in the BMM. 

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-19768

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