SIGNIFICANCE OF CRYSTAL MIXING IN THE TUOLUMNE INTRUSIVE COMPLEX

Minerals are great recorders of magma environments during growth. Plutonic studies are increasingly reinforcing results from the volcanic realm that multiple populations of minerals are more the rule than exception. This suggests that magma mixing, and specifically crystal mixing, is an important process in magma mushes across the arc’s crust. It also implies that antecrystic minerals with different histories and increasing abundance blur compositional whole rock signatures and produce scatter in geochemical data.

High precision U/Pb geochronology on zircon from across major units of the 95-84 Ma Tuolumne Intrusive Complex (TIC) has been central in showing the increased abundance and scatter in age of antecrystic zircons towards the younger TIC interior. Mineral scale geochemistry on major rock-forming minerals shows similar results. This is consistent with an inward increase of magmatic erosional features, hybridization along contacts, minerals displaying variable zoning at thin section scale and increased scatter and overlap in whole rock element and isotope geochemistry.

Zoned K-feldspar megacrysts are a prominent mineral in the two younger TIC units used in the study to further test this hypothesis. The porphyritic Half Dome granodiorite (pHD) contains 1-3 cm K-feldspar phenocrysts and the younger Cathedral Peak (CP) granodiorite has megacrysts up to 15 cm in length. Petrography results show that pHD K-feldspar are generally inclusion-rich, while 1-4 cm CP K-feldspars are rather inclusion-poor. CP K-feldspar megacrysts >4-5 cm, however, tend to display inclusion-rich cores and inclusion-poor rims, hinting at that larger megacrysts may have started their growth in pHD granodiorite and continued to grow in CP magmas. Trace element and Sr- and Pb-isotope data from growth zones of pHD and CP K-feldspar phenocrysts indicate complex trace element zoning and overall isotopic overlap with some degree of scatter in the data like it is the case with whole rock values. When comparing K-feldspar isotope data to whole rock values, K-feldspar zones are less radiogenic, implying that whole rock samples must contain minerals with more radiogenic isotope values. We conclude that K-feldspar phenocryst compositions reinforce the importance of crystal recycling in producing compositional heterogeneity in the TIC.