SIGNIFICANCE OF THE HORNBLENDE POPULATION DIVERSITY IN THE MARGINS OF THE KUNA CREST LOBE, TUOLUMNE INTRUSIVE COMPLEX, SIERRA NEVADA, CA

Understanding the emplacement and evolution of plutons is essential in understanding igneous crustal growth and arc construction. Initial stages of pluton growth are rarely preserved due to overprinting by younger magmatism. However, such evidence is key as it may give clues to the size and compositions of the earliest magma batches that may subsequently coalesce to form larger magma mush bodies.

Early stages of pluton growth are preserved in the margins of the 95-93 Ma Kuna Crest (KC) lobe of the Tuolumne Intrusive complex in the central Sierra Nevada batholith. The KC margin is composed of cm- to m-scale sheets (sheeted complex) to km-scale, irregular bodies in the lobe interior. Compositions range from granodiorite, tonalite, diorite to gabbro with sharp to gradational contacts and a color index spanning 10% to 32%. KC bulk rock Sr and Nd isotope data indicate an origin from a homogenous, magma source, yet hornblendes vary in size (1-4 mm), shape, and pleochroic color (blue-green to olive-green) indicating the presence of multiple populations.

Bulk rock and hornblende geochemical analysis were conducted to determine the significance of intra-unit variation at the sheeted margin of the KC lobe. KC hornblendes are magnesiohornblende with magmatic temperatures of 730-830 °C (Putirka (2016) thermometer). LA-ICPMS analyses show extensive spread in trace and rare earth element abundances and varying trends within and across samples that are characteristic to mapable units, supporting the results from Barnes et al. (2016) for hornblendes from the lobe interior. Both studies indicate that different hornblende populations grew in local, melt interconnected magma mush undergoing differentiation. The size of the magma batches identified through field observations and characteristic hornblende compositions is significantly smaller at the margin than in the lobe interior. Detailed field mapping of these marginal sheets provides information on the size and shape of these individual magma batches to help understand the mechanics involved in incremental plutonic growth. We conclude that each KC sheet was likely emplaced as a magma increment and grew localized to larger magma mush bodies that underwent homogenization and differentiation while maintaining previous magma histories at the crystal-scale.