PROTRACTED MAGMATIC K-FELDSPAR MEGACRYST GROWTH REVEALED WITH CA-ID-TIMS-TEA U-PB ZIRCON GEOCHRONOLOGY IN THE TUOLUMNE INTRUSIVE COMPLEX, CALIFORNIA

K-feldspar megacrysts (Kfsm) are common in felsic plutons, and occur in volcanic lavas. They are typically euhedral, have crystallographically aligned mineral inclusions that trace mineral growth zones, and show Carlsbad twinning, accumulations and oscillatory Ba zoning, all of which support a magmatic origin for Kfsm. Arguments based on phase equilibria and experimental work have been used to claim that magmatic megacrystic growth is difficult and that metasomatic crystal coarsening is necessary.

We used CA-ID-TIMS-TEA U-Pb zircon geochronology on zircons from the core and rim of two, 7 cm Kfsm and the groundmass of one Kfsm near the margin of the Cathedral Peak unit in the 95-85 Ma Tuolumne intrusive complex. Eight zircon ages from one megacryst core range between 88.8 Ma to 89.2 Ma. Three zircons are older at ~90 Ma. Nine rim zircon ages cluster between 88.3 Ma and 88.5 Ma. Three older grains range 88-99 Ma. Ten groundmass zircon ages range from 88.2 to 88.4 Ma with three older grains <92 Ma. These results yield an age difference of 0.5 myrs between the youngest zircon in the Kfsm core and the Kfsm rim, while the youngest rim zircon age and the youngest groundmass zircon are the same within error. The older zircons are antecrysts and xenocrysts. Trace element compositions of Kfsm core and rim overlap in Zr, Hf, Y, and REE, while groundmass zircons are chemically more evolved.

The zircon ages consistently young from Kfsm core to rim indicating protracted magmatic growth for at least 0.5 myrs. Rim growth ended at the same time as the crystallization of its groundmass. This result is at odds with a textural coarsening model. The zircon age range for the cores is the same as single zircon ages from bulk rock samples of the slightly older porphyritic Half Dome, while the rim ages conform to ages from Cathedral Peak bulk rocks. This observation matches observations of mineral inclusion density differences found in Kfsm from both units and zircon chemistry, suggesting prolonged megacryst growth via crystal recycling in a growing and maturing Half Dome-Cathedral Peak magma mush system. Our results are similar to results from a similar study on a laccolith in Elba Island, however, the 6-7 cm megacrysts there indicate magmatic growth at much faster time scales (24-64 kyrs). This infers that Kfsm growth rates are dependent on pluton crystallization rates.