Abstract

The Rocky Hill granodiorite stock is chemically a relatively simple, epizonal pluton which is texturally zoned. It intrudes serpentinites and metavolcanic rocks on the western edge of the Sierra Nevada 12½ mi. southeast of Visalia, California. Relationships between the subporphyry core facies of the stock and a series of aplite dikes intruded in the coarse-grained marginal or rim facies of the stock indicate that rupture, attendant upon a build-up of vapor pressure, occurred in the late stages of crystallization. Close similarity of the aplite composition to the isobaric quaternary minimum in the system NaAlSi3O8-KalSi3O8-SiO2-H2O permits an estimate of the water pressure and thereby the load pressure at the time of rupture.

Potassium-argon radiogenic determinations indicate an age of 132 (±4) m.y. for the Rocky Hill stock, which is one of the earlier Sierran plutons. An indicated age of 307 (±30) m.y. has been obtained for a metachert block 4 mi. south of Rocky Hill near Lindsay, California. Metamorphism of this age in pre-Sierran rocks has not previously been recognized in this area.

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