USING MULTI-MINERAL CRYSTAL-SCALE GEOCHEMISTRY TO TRACK THE FORMATION AND MAGMATIC EVOLUTION OF THE TUOLUMNE INTRUSIVE COMPLEX, CALIFORNIA, THROUGH TIME

Composite intrusions are shaped by a multitude of magmatic processes over long durations that cannot be resolved with bulk rock analyses. In contrast, the judicious use of mineral zoning and their compositions can reveal detailed magma histories. We used mineral geochemistry on different minerals from the 95-84.5 Ma Tuolumne intrusive complex (TIC) to investigate its formation and evolution. The TIC is composed of four, partially nested, inward younging units, separated by gradational to sharp contacts: Kuna Crest diorite-granodiorite (KC), equigranular and porphyritic Half Dome granodiorites (eHD, pHD), and Cathedral Peak K-feldspar megacrystic granodiorite-granite (CP).

Hornblende (hbl), K-feldspar (Kfs), plagioclase (plag), and titanite (ttn) were analyzed for major and trace element compositions. While plag is a liquidus phase, hbl and Kfs crystallized at <850°C, and ttn at <760°C, preserving magmatic histories over a range of temperatures. Plag shows eHD- and CP-type magmas mixed to form pHD. PHD closest to eHD contains more eHD-type plag and closest to CP more CP-type plag, revealing a mixing gradient from eHD into CP. Kfs megacrysts in CP contain 0.5 Myr older pHD cores. Small Kfs and hbl indicate at most local mixing, and with ttn, mostly record in-situ crystallized melts of rhyolitic composition that were later mostly lost from the magma mush (bulk rocks are more mafic than predicted by mineral compositions). In contrast, Hbl from the sheeted KC in the KC lobe of the southeastern TIC largely records closed-system crystallization.

In sum, the TIC initiated during incremental emplacement of small, local sheets. KC pulses gradually merged to form magma mush zones in the KC lobe, which underwent rhyolitic (reactive?) melt percolation, transitioning from closed- to open-system behavior. Magma mixing across units began with the eHD emplacement into fractionated KC to form the KC-eHD hybrid. Continued emplacement of eHD involved generally similar magmas and fractional crystallization forming leucogranites. Mixing resumed when CP-type magmas incrementally intruded into fractionated eHD magmas to form pHD. As the volume of CP magma increased, pHD magmas became increasingly more CP-like, merging into CP magma in the interior. The CP underwent fractional crystallization before final crystallization.