FELDSPAR RECYCLING ACROSS MAGMA MUSH BODIES DURING THE VOLUMINOUS HALF DOME AND CATHEDRAL PEAK STAGES OF THE TUOLUMNE INTRUSIVE COMPLEX, YOSEMITE NATIONAL PARK, CALIFORNIA, USA

Incremental plutonic growth is a complex process that, dependent on magma pulse size and the frequency of magma intrusion, might result in either solidifying into a sheeted complex or coalescence of multiple magma batches into larger magma bodies. The differences in behavior have significant implications for how large and dynamic magma bodies may become and how they are expressed in plutonic complexes. This study investigates the degree of interaction and crystal-melt exchange between different granodiorite and granite units from the southeastern section of the composite Tuolumne intrusive complex (TIC) of the Sierra Nevada batholith. To gain a better understanding of the spatial extent of such material exchange along and away from plutonic contacts at or near the emplacement level, we applied field mapping and determined both petrographic and geochemical characteristics of plagioclase and K-feldspar populations in the equigranular Half Dome (eHD), porphyritic Half Dome (pHD), and Cathedral Peak (CP) granodiorites.

We determined the following characteristics of the units in the southeastern TIC: 1) Contacts between major units predominantly form ~400 m to 3 km thick gradational zones; 2) eHD and neighboring gradational zones with KC and pHD each contain K-feldspar with distinct geochemical signatures and show no evidence of mixing; 3) K-feldspar in a gradational zone between pHD and CP shows evidence of recycling between the two; 4) plagioclase in eHD and CP contain distinct ranges of An-content, Sr and LREE; both populations are observed in pHD; 5) Calculations to determine major oxide (Scruggs and Putirka, 2018) and trace element content of melt in equilibrium with plagioclase indicate a melt that is more silicic and lower in Sr and Rb, than corresponding whole-rock samples, indicating the rocks are probably plagioclase and biotite cumulates. The presence of multiple plagioclase populations in pHD demonstrates that eHD and CP were interconnected magmas and formed pHD as a mixture between the two in an increasingly maturing TIC magmatic system during the eHD-pHD-CP stages of emplacement and evolution, but before groundmass and small phenocryst K-feldspars crystallized.