75-19 Comparative Study of Depth-Dependent Magmatic Evolution in a Late Cretaceous Arc, Southern California

Session: Mineralogy, Geochemistry, Petrology, and Volcanology Student Session (Posters)

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This study investigates how magmatic processes vary with depth in a Late Cretaceous (82–74 Ma) continental magmatic arc system. We focused on three locations in southern California that represent different arc crustal levels: deeper mid-crustal intrusive rocks and the shallower Porcupine Wash pluton from Joshua Tree National Park, and upper-crustal rocks of the Big Bear Lake Intrusive Suite. To reconstruct the magmatic history at each site, we analyzed mineral textures in hand samples and thin sections, along with whole-rock geochemistry.

All three rock suites contain intermediate to felsic rocks and show evidence of crystallization influenced by crystal fractionation. However, differences in mineralogy, textures, and geochemical trends reflect variations in volume and crystallization depth. The Blue Granodiorite samples, from deeper levels, are coarse-grained and include hornblende, biotite, and magnetite, pointing to slower cooling and extensive fractionation. Porcupine Wash pluton samples from Joshua Tree are finer-grained and contain fewer mafic minerals, suggesting they crystallized at shallower mid- to upper-crustal depths. Big Bear Lake samples, from the shallowest crust, are the finest-grained and have the lowest abundance of early-forming mafic minerals, indicating faster cooling and greater fractionation in the upper crust.

Despite these textural differences, the geochemistry across all three locations is consistent. Shared major and trace element patterns suggest that the magmas likely came from a common source. This supports the idea of a Late Cretaceous vertically connected arc system, where magma rose through the crust and progressively evolved at a range of depths. The variations we see in texture and mineralogy are best explained by depth and cooling rates, while the consistent geochemistry points to a shared magmatic origin.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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