MAGMA MUSH BODIES AND CRYSTAL RECYCLING CONSTRAINED BY ZIRCON DATES FROM CORES AND RIMS OF K-FELDSPAR MEGACRYSTS FROM THE TUOLUMNE INTRUSIVE COMPLEX

Determining the size and longevity of upper crustal magma bodies is important for understanding the evolution of arcs. One way to track magma extent and interconnectivity is to investigate recycling of crystal populations between different plutonic units. Recycling has been suggested for the Tuolumne Intrusive Complex (TIC), Sierra Nevada batholith, CA, based on field observations and presence of multiple feldspar populations established through geochemistry. The 89.7-88.6 Ma porphyritic Half Dome unit (pHD) contains 2-4 cm K-feldspar crystals often with abundant mafic mineral inclusions. The pHD gradually transitions into the 88.6-84.6 Ma Cathedral Peak (CP) unit forming a hybrid with 4-12 cm K-feldspar megacrysts, typical for the CP, and pHD-type euhedral hornblende and biotite. These pHD-CP K-feldspar megacrysts contain inclusion rich cores and inclusion poor rims, also suggesting feldspar recycling from pHD into CP.

To test this hypothesis, zircons from inclusion rich cores and inclusion poor rims of two megacrysts from the pHD-CP transitional zone in the SW TIC were dated with ID-TIMS U-Pb zircon geochronology. Zircons from the cores of the megacrysts span between 89.7 to 90.7 Ma consistent with youngest equigranular HD or oldest pHD ages. Rim ages span 100-200 kyr at ca. 88.4-88.6 Ma, in agreement with oldest CP ages. This is consistent with a model where inclusion rich pHD K-feldspar phenocrysts were entrained into rising CP magmas and grew inclusion poor rims over a duration of several hundred kyr. Older zircons (antecrysts?) are interpreted to be remobilized from the host magma during magma mixing.

The consistent younging of ages from core to rim confirm a magmatic origin for the megacrystic K-feldspars, which grew to their large size due to a prolonged growth history in pHD and CP magmas, and not through metasomatism or Ostwald ripening. Crystal recycling in a ≥100 m broad pHD-CP transition indicates that these units were both magma mush along their interface forming an interconnected magma chamber at one point. The ca. 0.6 Ma age gap between cores and rims is permissive of either “hot storage” with zircon-undersaturated CP magma being intruded some time during the age gap followed by renewed zircon saturation and crystallization at ca. 88.4 Ma, or of cold storage of pHD after ca. 89 Ma and reheating by CP at ca 88.4 Ma.