Small-scale variations and subtle zoning in granitoid plutons: the Liberty Hill pluton, South Carolina, U.S.A.

Abstract

Imperfect flow separation and remixing of crystals and melt produced chemical, mineral and textural variability on the scale of several centimeters to meters in coarse-grained granitoids of the Liberty Hill pluton. This is believed to develop during emplacement because the resulting layering conforms to the geometry of the pluton and is better developed toward the margins. Mineral-specific, equilibrium and fractional crystallization regimes superimposed on the flow separation and remixing produced more complex compositional variations than expected from such granitoid cumulate processes alone. The process is chemically comparable to restite unmixing, but it is difficult to demonstrate that many minerals involved in the flow are restites. Local compositional inhomogeneities also formed by deformation and disaggregation of enclaves. Depending on whether the enclaves are interpreted as xenoliths, chilled mafic magma, or high-melting-point restite clots, they represent contamination by mechanical assimilation, magma mingling and mixing, or restite unmixing. Marginal granitoids, distinguished from the central granitoids by pink feldspar + biotite ± muscovite rather than white feldspar + amphibole + biotite, are more evolved but overlap the compositional range of the central granitoids. Mineralogic and textural similarity and a gradational contact between the two granitoids indicate they are related and that the pluton's subtle reverse zoning was produced by a continuous in situ process. Flow separation and remixing, operating more efficiently and on a larger scale, may have produced the more evolved composition of the marginal granitoids; however, proximity to the contact is more important than rock composition in determining mineralogy. There is slight geochemical evidence for contamination of the marginal granitoids by the wall rocks; compared to central granitoids they have small LREE depletions and HREE enrichments, which more nearly reflects the LREE depletion and HREE enrichment of the wall rocks. Cross-cutting contacts and slightly differing compositional trends indicate a later and distinct evolution for the finer-grained biotite granitoids. They have silica enrichment, incompatible-element enrichment (Rb, U, Th), compatible-element depletion (major elements, Sr, Ba), depleted HREE and, less frequently, LREE contents as compared to the central granitoids. The fine-grained granitoids could be a contemporaneous and unrelated magma or fractionally crystallized from the coarse-grained magma. Compositional, mineralogic and textural differences among the later finer-grained granitoids indicate either multiple magmas, periodic sampling and intrusion of successive residual melt fractions, or flow separation and remixing during emplacement.