LENGTH SCALES OF MAGMA MIXING AND MAGMA CHAMBER DIMENSIONS AS INFERRED FROM ZONING IN FELDSPARS, TUOLUMNE INTRUSIVE COMPLEX, YOSEMITE NATIONAL PARK, CA

The size and longevity of magma chambers in long-lived, composite intrusions are debated. This study uses feldspar populations from the equigranular and porphyritic Half Dome (eHD and pHD) and Cathedral Peak (CP) granodiorites from the SE Tuolumne Intrusive Complex (TIC) in Yosemite National Park to determine the degree of interunit mixing and extent of magma mush bodies during stages of TIC evolution.

Techniques employed included field mapping of contacts, petrography, whole-rock element geochemistry to characterize major and hybrid TIC units, and cathodoluminescence to identify feldspar populations. Selected K-feldspars and plagioclase were analyzed for major and trace elements (e.g., Ba, Sr, Ga, Y, etc.) with EMPA and LA-ICPMS along core-to-rim transects.

Our results include the following: 1) Interunit contacts are primarily gradational and represent hybrids; 2) Samples from the eHD-pHD hybrid are granite, not granodiorite like main eHD and pHD; 3) Rb concentrations in the eHD-pHD hybrid are higher than in the eHD and pHD; 4) the eHD-pHD hybrid zone contains petrographic features indicative of crystal mixing, e.g., biotite inclusions in hornblende; 5) K-feldspars of each unit exhibit distinctive Ba zoning patterns. eHD K-feldspars (≤5 mm) are simple zoned, and pHD and CP K-feldspars (2-6 cm) are oscillatory zoned; 6) Trace element ratios (e.g., Ba/Ga, Rb/Pb) demonstrate that K-feldspars record melt fractionation; 7) K-feldspars from all units contain distinct, inherited cores.

Most K-feldspar cores from the eHD-pHD hybrid show distinct eHD and pHD Ba and Rb values that overlap with the interior and rims of pHD K-feldspar. Higher Rb likely records late-stage fractionated granitic melt from the eHD that mixed with intruding pHD. Increase in the number of Ba zones in K-feldspar from eHD to pHD/CP suggests that the pHD and CP record longer crystallization histories with possible pHD/CP magma recharge. This trace element signature overlap is also seen at the transition between CP and pHD near the pHD transition. These results suggest that K-feldspars from eHD mixed into intruding pHD, and those of pHD mixed into CP. Comparisons of Sr and Eu in plagioclase suggest mixed populations as well. Our results show that two adjacent units, eHD/pHD and pHD/CP, were interconnected magma mushes at different stages of TIC history.