THE FORMATION AND EVOLUTION OF THE TUOLUMNE INTRUSIVE COMPLEX FROM THE PERSPECTIVE OF ITS MINERALS

Composite intrusions experience many magmatic processes. What bulk-rocks preserve is, at best, a partial history. In contrast, mineral zoning has the capacity to record melt compositions and retain a long-term magmatic memory, which can be used to investigate the formation and evolution of long-lived magma systems, such as the 95-84.5 Ma Tuolumne intrusive complex (TIC). The TIC youngs and becomes more felsic (diorite to granite) across four major units (KC, eHD, pHD, CP) towards its interior and the NE, separated by gradational to sharp contacts.

Hornblende (hbl), K-feldspar (Kfs), plagioclase (plag), and titanite (ttn) were analyzed for major and trace element compositions and zoning. While plagioclase is a liquidus phase, hbl and Kfs crystallized at <850°C, and ttn at <760°C, together preserving a complete magmatic history.

Plag shows eHD and CP 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 to CP. Kfs megacrysts in CP contain pHD cores. Small Kfs and hbl indicate minor mixing, and with tnt, mostly record similar chemistry of in-situ crystallizing melts of rhyolitic composition that were lost from the crystal mush, leaving cumulates behind. In contrast, Hbl from the sheeted KC at the SE TIC margin records closed-system crystallization with no melt-loss.

In sum, the TIC initiated during incremental emplacement of small, in-situ crystallizing sheets. KC pulses merged to form thick crystal mushes in the KC lobe, which underwent rhyolitic melt percolation, transitioning from closed- to open-system behavior. Magma and crystal mixing across units began with the eHD emplacement into fractionated KC to form a KC-eHD hybrid. The eHD continued emplacement of generally similar magmas with preservation of local chemical signatures and leucogranite zones. Mixing resumed when CP-type magmas incrementally intruded into fractionating eHD magmas to form pHD. As CP magma emplacement increased in volume, pHD magmas became increasingly more CP-like and merged into CP magmas in the interior.

This study shows that pluton construction can be tracked through judicious use of mineral compositions, and combined with internal and external structural observations (Paterson abstract), can provide a powerful tool for revealing magmatic histories.