Crystallization processes in the Rocky Hill granodiorite Pluton, California: An interpretation based on compositional zoning of plagioclase

Abstract

The Rocky Hill Pluton was chosen for study of plagioclase zoning because it has an apparently simple geometry and one-stage emplacement history, documented by Putman and Alfors (1969). The pluton is divided texturally into a hypidiomorphic granular rim fades and porphyritic core facies, both comprising zoned plagioclase phenocrysts averaging 2–4 mm in diameter. Major features of compositional zoning profiles of plagioclase were found to be consistent within a sample, showing resorbed core regions followed by a sharp drop of ca. 10% An to a plateau region, and ending with normal zoning. The width of the plateau region is narrow in samples near the pluton rim but increases systematically toward the pluton core, and the outer part of the plateau develops reverse zoning in the core facies. A review of the effects of pressure, water content of the melt, and temperature on equilibrium and disequilibrium crystallization processes concludes that only variations of temperature and water content during disequilibrium crystallization of plagioclase phenocrysts can produce the observed zoning patterns. Experimental data suggest that the trend toward finer grain size toward the interior of the pluton may be due to increasing water content rather than due to increasing cooling rate. The compositional zoning of plagioclase and textural variations in the pluton can be explained by a model in which the pluton crystallized by migration of a partly crystalline “mush zone” inward from the edge of the plutonic chamber while convection within the residual magma: (1) redistributed water excluded from the mush zone and (2) slowed the cooling rate of the mush zone. The reverse zoning of plagioclase growing in the convecting magma changed to normal zoning after capture in the mush zone because the cooling rate increased and water was removed. Comparison of the results from our study of Rocky Hill with published descriptions of plutons and experimental and theoretical studies of crystallization processes leads to the conclusions that: (1) convection is an active process during the crystallization of many plutons, causing redistribution of mainly water and heat in silicic ones but differentiating other components in intermediate ones; (2) most of the crystallization of granitic rocks may occur in a “mush zone” or static boundary layer between the solid boundary and convecting inner magma body; (3) water content may be an important factor controlling grain size; (4) reverse zoning of plagioclase may be more typical of plagioclase grown under slowly-cooled conditions than “normal” zoning; and (5) variations of plagioclase zoning in plutons can provide useful information on crystallization processes.