Abstract

By heavy-mineral analyses and statistical studies of zircon morphology the author has tried to correlate a series of granitic plutons in the Coast Ranges of central California and the Farallon Islands. Zircon-elongation data, analyzed by reduced major axis and elongation-frequency studies, are interpreted to be indicative of two distinct crystallization histories for quartz diorite. Intrusions forming segments of the separate areas can possibly be correlated. A distinctive elongation trend for zircon in granite represents crystallization under different conditions and suggests that true granites are not simple differentiates of the quartz diorite magma. Zircon in gabbro and in mafic xenoliths in quartz diorite is predominantly anhedral, whereas in granitic rocks it is euhedral. Zircon morphology and mineralogy of other accessory minerals suggest that the dark xenoliths are fragments of mafic igneous rocks, possibly cognate.

Characteristic accessory-mineral assemblages for the major rock types are as follows: apatite-sphene-epidote-clinozoisite-allanite-zircon in quartz diorite, the most abundant rock type, and in closely associated granodiorite and adamellite; garnet-zircon-monazite-xenotime in granite; and brown hornblende-ilmenite-cummingtonite-apatite in gabbro. Heavy minerals from the large quartz diorite bodies show general similarity but wide variations in total and relative amounts. The mineral assemblages indicate similar sources for the quartz diorite, but definite correlations based on accessory mineralogy are uncertain because the suites are not unique compared to accessory minerals of granitic rocks in general.

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