USING PLAGIOCLASE GEOCHEMISTRY TO EXAMINE THE DEGREE OF MAGMA MIXING BETWEEN THE KUNA CREST SHEETED COMPLEX AND LOBE MAGMAS DURING THE INITIATION OF MAGMATISM IN THE TUOLUMNE INTRUSIVE COMPLEX, CALIFORNIA

Magmatic mobilization and ascent from the upper mantle and lower crust through the arc crust and the degree to which these magmas are interconnected and mix with one another at different scales is an essential question of igneous petrology. Previous studies of large compositionally zoned batholiths, such as the ~95 - 85 Ma, 1,100 km² Tuolumne Intrusive Complex (TIC), Sierra Nevada, California, demonstrate that batholiths are formed by incrementally emplaced batches of magma. These magmas undergo dynamic evolutionary mixing processes in magma mush systems as they progress through the crustal column that are recorded in Plagioclase (CaAl₂Si₂O₈-NaAlSi₃O₈), a liquidus phase. This study seeks to understand the extent and nature of these mixing processes at the initiation of the Tuolumne Intrusive Complex, which is represented by the mafic, isotopically more mantle-like, 80 km² 94.91 ± 0.53 Ma to 92.75 ± 0.11 Ma Kuna Crest lobe and related sheeted complexes located in the Tuolumne Intrusive Complex.

We present thin section size cathodoluminescence images to highlight zoning in plagioclase populations, petrography, and electron probe microanalysis and LA-ICP-MS element geochemistry to conclude the following: (1) All plagioclase grains assessed have Anorthite contents ranging from An₃₇ to An₈₈. (2) Overall, plagioclase grains collected from the Kuna Crest lobe show trace element ranges of Sr at 500 ppm – 1250 ppm commensurate with plagioclase grains collected from Gaylor Ridge, while plagioclase grains collected from the Marie Lakes sheeted zone have typically higher Sr signals at 900-1500 ppm. (4) Plagioclase grains analyzed from the Kuna Crest lobe, Gaylor Ridge and Marie Lakes sheeted zones display heterogenous intra-unit profiles in Sr and Ba (ppm) versus Anorthite space, which show at least 2 distinct populations of plagioclase in each of the lithologic subdomains and are different from one another. (5) Plots showing the evolution of Sr and Pb across plagioclase grains from Gaylor Ridge show at least two different populations based on average abundances of Pb and Sr.

We interpret these data as evidence that the embryonic construction of the TIC occurred with magmas that already underwent magma mixing and were sourced from different magma bodies stored in a complex magma mush network in the crust prior to emplacement.