The geochemistry of differentiation process in granite magma chambers

Abstract

Three major, though interrelated, topics are addressed in this study and these are: 1) the crystal sorting mechanism in compositionally zoned granite plutons; 2) compositional variations in U-, Th- and rare-earth element concentrating accessory minerals; and, 3) the distribution of radioactive heat production in granites. Three examples of concentrically-zoned, I-type, granite plutons from the Sierra Nevada batholith, California, U.S.A., yield a consistent model for crystal fractionation leading to compositional zoning. These plutons include the compositional range 59$ to 75% SiO-p and the rock types: potassium-rich granite-granodiorite (two plutons) and low-potassium, high-sodium tonalite-trondhjemite. Crystallizaton and accumulation of dense phases along the cooler magma chamber sidewalls decreases the density of the magma in the immediate vicinity. This lighter melt-magma then rises buoyantly upward along the sidewall as a boundary layer. Some mixing with the denser bulk magma occurs during the upward boundary layer flow, thereby reducing the overall density of the convecting bulk magma system. This in turn causes progressively lighter magma differentiates to be produced and move buoyantly upward along the sidewall toward the chamber roof. As these light magma differentiates pool at the top of the magma chamber normal or reversed density stratification occurs. Normal density stratification occurs when the lightest magmas continue to be emplaced above the earlier emplaced denser magmas. This normal density stratification process requires that the earlier emplaced denser magmas behave as liquids when the lighter magmas are emplaced later. In reverse density stratification, the earlier emplaced, denser magmas, solidify and behave as solids thus prohibiting the less dense magmas, emplaced later, from rising through to higher levels. The compositions of U-, Th- and REE-rich accessory minerals change dramatically during whole rock fractionation. In sphene, allanite, zircon and apatite, the concentrations of U, Th and REE tend to increase during whole rock fractionation. These accessory mineral compositional trends are not, however, always systematic. In most cases, the composition of the accessory minerals are determined by the paragenetic sequence rather than by the whole rock composition. The distribution of radioactive heat production in granites from the Sierra Nevada batholith, U.S.A. and the Lachlan Fold Belt, Australia is controlled mainly by magmatic processes and also by the granite source rock composition. In general, I-type granites will tend to decrease in radioactive heat production with depth whereas, S-type granites will remain essentially uniform or will increase in radioactive heat production with depth. Therefore, S-type granites will normally have higher surface heat flows than I-type granites, due mainly to the different vertical distributions of heat production in these rocks types. Both I- and S-type granite residual source regions will potentially contain significant amounts of radioactive heat production. However, S-type granite residual source regions potentially contain even higher radioactive heat productions than those of the surface granites. This is due to residual monazite in S-type granites which is related to the chemical weathering of the sedimentary source rocks.