TRACKING STAGES OF MAGMATIC EVOLUTION BY COMPARISON OF TRACE ELEMENTS AND ISOTOPES IN DIFFERENT MINERAL POPULATIONS IN THE TUOLUMNE INTRUSIVE COMPLEX

Micro-analysis of igneous minerals, spearheaded by J. Davidson >30 years ago, has proven to be powerful in unraveling complex crystal histories of different stages of magmatism. Mineral analysis is especially important in evolved, long-lived plutonic rocks, where magmas crystallized over prolonged durations, a range of temperatures, and different hypersolidus saturation intervals, resulting in complex geochemical records typically hidden in bulk-rock analysis.

To evaluate magmatic histories and the extent of magma mushes in the composite, 95-85 Ma Tuolumne intrusive complex (TIC), trace elements in plagioclase, K-feldspar, amphibole and titanite from the oldest, marginal Kuna Crest granodiorite-diorite (KC), the interior equigranular and porphyritic Half Dome granodiorites (eHD, pHD), and the youngest, innermost Cathedral Peak granodiorite (CP) were analyzed using LA-ICP-MS. Sr isotopes from K-feldspar megacrysts were determined in the pHD and CP.

Plagioclase (with apatite inclusions) range from An₆₅-An₂₇, and locally contain antecrystic calcic cores. Oscillatory zoning is defined by collinear trends of Sr, Ga, and Eu. K-feldspar (with plagioclase, +/- amphibole inclusions) ranges from Or_88-97 and shows large, oscillatory Ba variations (≤3 wt%), and collinear Ga, Y, Eu, and Zn behavior. Plagioclase indicates at least two mixed populations in all units based on element concentrations and patterns. K-feldspar shows one population in KC/eHD, and one shared and two distinct populations in pHD and CP (Oppenheim et al., this meeting), while they overlap in Sr_i. In contrast, amphibole and titanite trace element compositions are distinct within each TIC unit; these phases primarily record fractional crystallization from rhyolitic melts (Werts et al., this meeting). Mixed amphibole populations are only identified in KC/eHD hybrids. Putirka’s (2016) hornblende thermometer indicates crystallization of TIC amphiboles at <830°C. Titanite temperature is estimated at <730°C.

We conclude that while feldspars grow early enough to record identifiable intra- and inter-unit mixing at the emplacement level in increasingly broad interacting magma mush bodies towards the TIC interior, the amphibole and titanite crystallized later, largely recording fractional crystallization and local mixing in hybrids.