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

Incremental plutonic growth is a complex process that, dependent on magma pulse size and the frequency of magma intrusion, might result in either freezing into a sheeted complex or in the coalescence of multiple magma batches into larger magma bodies. The differences in behavior have significant implications for how big and dynamic magma bodies get with time 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. The goal is 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. This goal is accomplished using field mapping, and 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 determine 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 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 show evidence of recycling between the two; 4) plagioclase in eHD and CP contain distinct ranges of An-content, Sr and LREE’s; 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, lower in Sr, and higher in Pb, than corresponding whole-rock samples, indicating the magmas have undergone a combination of plagioclase accumulation and melt loss. 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 emplacement and evolution stages, but before groundmass and small phenocryst K-feldspars crystallized. The magma mushes were modified by late-stage “leaking” of rhyolitic melts.