DEVELOPMENT OF IGNEOUS TEXTURES BY MAGMA DYNAMICS

The Deer Isle Pluton, coastal Maine, shows cryptic evidence for the physical processes active during solidification. The granitic intrusion displays a textural progression across a vertical cross section. The monzogranite that forms the lower portion of the chamber, the Flye Point facies, consists of a feldspar supported pseudo-cumulate texture. Abundant plagioclase mantled alkali feldspar indicate pressure, temperature, or compositional changes occurred during crystallization within this portion of the chamber. Abundant mafic enclaves suggest a mixing and/or mingling interaction occurred during the formation of this facies. The central portion of the chamber, the Settlement Quarry facies, consists of heterogeneous textured granite. Schlieren, coarse-feldspar domains, multi-genetic intermediate composition enclave swarms, and aplitic dikes are contained within a matrix of seriate granite. The schlieren and enclave swarms indicate that magmatic flow was an important aspect to the formation of this portion of the magma chamber. Finally, the uppermost portion, the Crotch Island facies, consists of homogeneous, fine-grained granite, which the most chemically evolved granite within the chamber and is relatively-enriched in fluorine. Based upon these observations, a simplified model of how the magma chamber evolved through time can be developed. During initial emplacement, or shortly thereafter, an injection of mafic magma caused a local compositional and/or thermal perturbation which, combined with magma chamber convective flow allowed for the formation of mantled alkali feldspar and the creation and distribution of mafic enclaves. Gravitational or flow induced settling and compaction followed, creating the Flye Point facies. Meanwhile, the volatile-rich uppermost portion of the chamber cooled and crystallized along a solidification front, forming the Crotch Island facies. The interior portion of the chamber, the Settlement Quarry facies, remained hot for a relatively protracted period. This allowed for the development of flow related features due to the extended period of convection. Numerical finite element modeling of heat flow and fluid dynamics of a cooling magma chamber, utilizing a thermally-dependent viscosity, supports the theoretical model.