244 Subduction Zone Processes: Insights from Geology, Geochemistry, and Petrochronology

We invite contributions that explore the geologic processes of subduction zones from new field observations, analytical methodologies, and numerical modeling of metamorphic rocks along the subduction interface or try to integrate or reconcile the rocks record with geophysical observations.

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 217C

Nitrogen Sources and Mineral Hosts within Shallow Sedimentary Mélange Matrix: Insights from San Simeon, California

Paper number: 244-11
Program: Technical Sessions
Time: 10:50 AM – 11:05 AM

Authors

Abstract

Quantifying the nitrogen (N) cycle within subduction zones relies on the characterization of representative subduction inputs (i.e., sediments, oceanic crust and mantle) and outputs (i.e., exhumed metamorphic equivalents and arc volcano emissions). However, N systematics along the plate interface remain under-characterized due to limited mélange N data. In this study, we report modal mineralogy, bulk geochemistry, and N concentrations and isotopic compositions of the Franciscan Complex (Coast Ranges, Central Belt) sedimentary mélange matrix from San Simeon, California, and assess N sources and mineral hosts. Eight San Simeon mélange matrix samples contain quartz + albite + chlorite ± muscovite ± calcite and have bulk-rock N concentrations of 225-569 ppm and δ15Nair values of +0.4 to +1.3‰. These concentrations and isotope compositions are similar to those of other Coast Ranges metagraywackes (30 to 891 ppm and +0.3 to +3.4‰, respectively), suggesting that N redistribution was limited and the isotopic composition did not change significantly during mélange formation. The range of N concentrations in the samples can be accounted for by variations in modal mineralogy. Nitrogen concentrations positively correlate with albite and muscovite abundances. Both N concentration and muscovite and albite abundances positively correlate with bulk-rock K2O, Na2O, Rb, and Cs, further supporting the interpretation that muscovite and albite are N hosts. Samples with high abundances of chlorite + quartz + calcite have low N concentrations but elevated Cr and Ni concentrations, implying that these minerals hosting fluid-immobile elements dilute N whole-rock concentrations. These trends can be explained by mechanical mixing between or sedimentation of detritus sourced from materials of varying compositions. This interpretation limits the necessity of fluids having modulated the N composition of San Simeon’s sedimentary mélange matrix. Formed under P-T conditions similar those experienced by San Simeon mélange, the sedimentary mélange of the lawsonite-albite unit of the Catalina Schist, CA, has higher N concentrations (860 to 1190 ppm) and higher δ15N (+1.6 to +2.0‰). Like the San Simeon mélange, the Catalina Schist sedimentary mélange hosts N largely in white micas (phengite and fuchsite, a Cr-rich white mica; Bebout, 1997; EPSL). Taken together, these data demonstrate that the behavior of N along subduction interfaces reflects lithological variability and varying combinations of mechanical mixing and fluid-rock interaction tied to complex deformation.

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

doi: 10.1130/abs/2025AM-413496

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