MULTI-SCALE MAGMATIC AND SUB-SOLIDUS PROCESSES OPERATING IN A BIMODAL SHALLOW CRUSTAL MAGMA RESERVOIR

The chemical and isotopic characteristics in a solidified pluton represent the integration of various magmatic and sub-solidus processes that operate at different scales during pluton construction, crystallization, and cooling. Disentangling these processes and understanding where chemical and isotopic signatures were acquired (i.e., lower crust, ascent, emplacement level), requires the combination of multiple tools tracing processes at different time- and length-scales.

We combine whole rock oxygen and Sr-Nd isotopes, zircon oxygen isotopes and trace elements, and the composition of rock-forming minerals to evaluate the differentiation of the Guadalupe Igneous Complex (GIC), a Jurassic tilted pluton, exposed in the Western Foothills of the Sierran Nevada, California. High-precision U-Pb ID-TIMS zircon geochronology shows that the GIC was constructed in ~300 kyr between 148-149 Ma. Felsic magmas crystallized as cm- to m-sized segregations in gabbros in the lower part of the complex and as granites and granophyres structurally above the gabbros. Dominantly mafic and felsic units are separated by a central mingling zone.

Ranges in pluton-wide d¹⁸O_(WR), d¹⁸O_(zircon), and Sr-Nd isotopic values are too large to be explained by in-situ, closed-system differentiation, as indicated by earlier studies, instead requiring open-system behavior during pluton construction. Fractionation in a deeper-seated magma reservoir and assimilation of high temperature hydrothermally altered crust likely during ascent is required to explain the range in whole rock chemistry. However, in-situ differentiation processes operate on a smaller scale (outcrop to 10s of meters) to further add to the observed chemical and isotopic diversity through (a) percolation and segregation of chemically and isotopically diverse interstitial melt from a heterogeneous gabbro mush; (b) localized closed-system fractionation and crystal accumulation; and (c) sub-solidus, high-temperature hydrothermal alteration at the top of the pluton.

This study shows that zircon petrochronology in combination with whole rock and mineral chemistry is a powerful tool to disentangling pluton-wide versus local differentiation processes in the GIC.