Abstract

The north half of the 15-minute Mt. Abbot quadrangle lies across the crest of the Sierra Nevada. The Cretaceous granitic rocks that underlie most of the area form eight large discordant plutons that range from quartz diorite to alaskite; the largest pluton is coarse prophyritic quartz monzonite. Pre-batholithic metasedimentary and metavolcanic rocks and metagabbro and metadiorite are minor.

The porphyritic quartz monzonite sent gently dipping dikes as much as 200 feet thick into its walls. These dikes in places make up more than half the height of exposed contact zones. A broad border zone of protoclastic flaser gneiss was formed in part of the pluton and in the adjacent wall rocks and dikes.

A contact between granodiorite and calcareous metasedimentary rocks swarms with dark inclusions. Their origin, puzzling in most places, is here clearly due to progressive hybridization of calc-silicate xenoliths; the changes take place within a few tens of yards of the contact. The xenoliths were amphibolitized concentrically and reconstituted to typical dark-inclusion texture and mineralogy and were drawn out from blocky xenoliths into spindle shapes.

Elsewhere, large irregular xenoliths of metadiorite were assimilated by alaskite. Within the resulting complex is a thick series of alaskite flow layers crowded with xenocrysts showing graded “bedding”.

All the granitic plutons were intrusive; assimilation of mafic wall rocks by felsic magmas may have caused much of the diversity.

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