Lower crustal assimilation revealed by sulfur isotope systematics of the Bear Valley Intrusive Suite, southern Sierra Nevada Batholith, California, USA

Abstract

The origin of the wide range of sulfur isotope compositions (i.e., δ³⁴S) measured in arc rocks remains debated. While the observed δ³⁴S variability has been attributed to slab-related fluids that flux the sub-arc mantle, others have argued that it primarily reflects crustal-derived processes by some combination of magmatic differentiation, country rock assimilation, and/or degassing. Here, we present new whole rock sulfur isotopes for the Late Cretaceous Bear Valley Intrusive Suite (BVIS) that represents a continuous arc crustal section in the southern Sierra Nevada Batholith, exposing lower crustal mafic cumulates and cogenetic mid-upper crustal tonalites. Our data reveal a range of δ³⁴S-depleted values (–1.2 to − 5.1‰) for the BVIS with overlapping δ³⁴S between mafic cumulates and tonalites. Complementary δ³⁴S measurements of structurally concordant metasedimentary pendants indicate δ³⁴S-depleted values (–11.5 to − 5.2‰) for deep metasedimentary rocks compared to δ³⁴S-enriched values (+ 1.6 to + 6.4‰) for shallower ones. Quantitative mixing models suggest that assimilation of crustal-derived sulfur from metasedimentary rocks in the lower crust can account for the δ³⁴S-depleted values in the BVIS, whereas assimilation of shallower ones is unlikely. Sulfur degassing modelling indicates that the range of δ³⁴S-depleted values observed within mid-upper crustal tonalites can be reproduced by degassing  ~60–80% of the initial melt sulfur at fO₂ ≤ FMQ + 1 with initial H₂O content of 10–12 wt%. Finally, the identical ranges of δ³⁴S values within the tonalites and mafic cumulates argue for limited sulfur isotope fractionation related to magmatic sulfide immiscibility. Although assimilation, magma degassing and sulfide immiscibility are not mutually exclusive during crustal magmatic processes, field, thermal and geochemical evidence favor lower crustal-derived sulfur assimilation as the primary mechanism to explain the range of δ³⁴S- depleted values within the mafic cumulates, which are ultimately inherited by the derivative tonalitic melts. Overall, this study emphasizes that deep crustal magmatic processes can severely influence the early δ³⁴S evolution of arc magmas.