MICROANALYSIS OF K-FELDSPAR MEGACRYSTS AT THE EASTERN CATHEDRAL PEAK GRANODIORITE-HOST ROCK BOUNDARY, SADDLEBAG LAKE PENDANT, SIERRA NEVADA, CA

A fervent debate has been rekindled concerning the origin of K-feldspar megacrysts in granites. Some authors favor subsolidus growth of megacrysts by textural coarsening during which thermal cycling may preferentially dissolve smaller crystals to allow larger crystals to coarsen. This explanation contradicts textural and structural observations from outcrop to micro scale. Alternatively, the megacrysts grew to large sizes while always surrounded by melt and are thus magmatic in origin. Repeated replenishment by new magma may frequently change the material supply for crystal growth producing a particularly favorable environment for continued growth of K-feldspar.

To test these hypotheses, three samples were collected from the same location in the 88-86 Ma Cathedral Peak granodiorite (CP) in the Tuolumne Batholith, CA at the eastern host rock boundary with metamorphic rocks in the Saddlebag Lake pendant. One sample was collected from the main CP body, a K-feldspar megacrystic dike, and an aplite dike. Both dikes show clear intrusive relationships with the metamorphic host rock. Thin sections of megacrystic K-feldspar and surrounding fine grained matrix of the granodiorite and aplite were analyzed by creation of X-ray element distribution maps and BSE images using a JEOL JXA-8200 electron microprobe to measure relative abundances of Al, Ca, Fe, Ba, K, Mg, Na, Si, Sr, Ti, and Zr.

K-feldspar megacrysts show extensive sawtooth oscillatory zoning of Ba in samples from the main pluton and porphyritic dike, whereas K-feldspars in the aplite show simple to no zoning. The matrix minerals and abundant mineral inclusions aligned along growth zones in K-feldspars show either simple normal or no zoning. Since a) K-feldspar megacrysts contain simple zoned magmatic inclusions, b) element distributions are consistent with magmatic processes and c) both complexly and simply zoned K-feldspars occur together at cm to outcrop scale with complexity in zoning pattern correlated with crystal size, we interpret the megacrysts to be magmatic and related to preferential mineral growth during chamber replenishment. Textural coarsening requires the same subsolidus thermal conditions for all three spatially related intrusive bodies that contain similar amounts of K-feldspar to coarsen the K-feldspar crystals to the same large size.