Geochemistry and petrology of the rocky hill stock, tulare county, california

The Rocky Hill granodiorite stock is a relatively homogeneous, epizonal pluton which is texturally zoned into a medium-grained, granitic-textured rimfacies and a fine- to medium-grained, subporphyritic core-facies. It is about 1.5 square miles in area and intrudes metamorphosed ultramafic and metavolcanic rocks on the western edge of the Sierra Nevada 12.5 miles southeast of Visalia, California. The exposed contacts of the stock are sharp and steeply dipping. Apophyses and dikes into the wall rocks are present, but few. Although outcrops of the granodiorite appear to be massive, a weak, steeply plunging lineation is generally present. In addition, a northwest-trending planar grain fracture, which is most easily observed on weathered surfaces, parallels the most prominent set of joints. Aplite dikes of various sizes, up to a maximum of 10 feet thick and 4000 feet long, occur throughout the stock; small irregular aplitic segregations are common in rocks of the core-facies. Sparse ellipsoidal inclusions up to 3 feet long are present in most outcrops of the rim-facies granodiorite but are exceedingly rare in the core-facies granodiorite. The effects of protoclastic deformation are found throughout the stock but are most intensive in the rim-facies granodiorite. Limited deuteric alteration has affected rocks of both facies, but the zone of most intense chloritization as well as areas with higher sulfide contents are within or close to the core-facies. Rock samples from 151 localities were collected on an equal-interval (375 feet), orthogonal grid over the entire stock and were used with additional samples to determine the within-pluton (regional) and within-grid variation in trace-element content of the ferromagnesian minerals as well as the variation in petrographic properties. Modal analyses show that rim-facies rocks are characterized by higher hornblende, slightly higher potassium feldspar, and slightly lower plagioclase and quartz contents than the core-facies rocks. Within-hand-specimen variation constitutes a large portion of the total modal variation, especially in the rim-facies rocks, and distribution patterns of the modal minerals could not be effectively determined (except for magnetite) by the sampling design used in this study. Chemical analyses of composite samples of rim- and core-facies granodiorite are similar, but the small differences are consistent with modal differences between rock-facies. Spectrochemical analyses of ferromagnesian mineral separates contain significant regional variation trends, specifically a progressive decrease inward from the walls of the stock in the contents of Be, Cr, V, and Ni, and an erratic increase in Cu and Zn. In preparing regional patterns for Cr, Cu, Ni, V, and Zn, it was necessary to correct the ferromagnesian data for the influence of the modal distribution of co-existing magnetite. Field and laboratory evidence indicate that the Rocky Hill stock was emplaced as a single, nearly vertical intrusion of relatively fluid granodiorite magma (less than 20 percent crystals) which crystallized progressively inward from the walls. When approximately 75 percent of the stock (at the present level of exposure) consisted of solid or nearly solid granodiorite (rimfacies rocks), saturation in volatiles occurred, and isothermal pressure quenching ensued to produce the matrix of the core-facies granodiorite. Portions of the solid rim and newly crystallized core ruptured, and some of the still-fluid, residual magma in the core area was intruded into the fractures as aplite dikes. Temperature of the magma at the time of initial intrusion, as estimated from biotite composition and petrographic evidence, was probably near 850 degrees C; at the time of final consolidation it was probably less than 700 degrees C. Petrographic and field evidence suggests that, at the time of aplite intrusion, water pressure equaled load pressure. A comparison of the composition of the aplites with the position of the quaternary isobaric liquidus minimum in the system NaAlSi (sub 3) O (sub 8) -KAlSi (sub 3) O (sub 8) -SiO (sub 2) -H (sub 2) O indicates that a water pressure of 1.5 to 2 kb existed when the aplites were emplaced. An estimated depth of intrusion of the Rocky Hill stock is 5.5 to 7 km. A model for the intrusion which is a vertical cylinder 7,000 feet in diameter with a depth exceeding 16,000 feet is consistent with a gravity anomaly over the stock.