Petrogenesis of a silicic magma chamber periodically invaded by basaltic magma: The Isle au Haut Igneous Complex, Maine

Marshall Chapman, University of Massachusetts Amherst

Abstract

The plutonic rocks of the Paleozoic Isle au Haut Igneous Complex, Maine (424 $\pm$ 1 Ma.), are a trimodal assemblage of granites, diorites, and gabbros. Exposed from the top to the bottom of the pluton, the complex consists of an enclave-bearing granite, a composite layered sequence of alternating gabbroic and dioritic units, a thick and massive gabbroic unit, and a base of an enclave-void granite. The composite layer shows a sequence of ten alternating gabbroic and dioritic units that were liquid or largely liquid contemporaneously. Complex hybridization has taken place between the lower massive gabbroic unit and the underlying granite, while both were liquid or largely liquid contemporaneously. The gabbros are compositionally similar to within-plate tholeiites and they are uniform in average composition throughout the stratigraphic succession. The interlayered dioritic units, on the other hand, change progressively from mafic quartz diorite to quartz monzodiorite with stratigraphic height. A comparison of their respective densities indicates a gravitationally unstable situation if both the gabbroic and dioritic units were simultaneously liquid. I contend that the composite layered sequence represents the successive invasion, or replenishment, of an evolving dioritic chamber by a compositionally uniform gabbroic magma that intruded sill-like between the cumulate floor of the chamber and the overlying melt. Petrography of the individual gabbroic units shows varying degrees of disequilibrium. These include crystals with heterogeneous dissolution textures and mineral assemblages. The hybrid compositions are typically skewed to one side or the other of a linear bulk mixing line between the two proposed end-members. Fractional Crystallization and Assimilation-Fractional Crystallization models are inconsistent with the observed geology, because $>$60-80% crystallization of the gabbroic melt would be required to account for the most silica-rich of the hybrid compositions. Moreover, in element-element comparisons, the hybrids tend to plot away from a linear bulk mixing line in the direction of the experimentally determined, slower diffusing element. The hybrid compositions, on the other hand, can be bounded, on element-element plots, between paths invoking simple diffusion in response to a compositional gradient. Selective contamination and effects of differing rates of diffusion between elements appear to progress in directions consistent with the hybrid samples. Compositionally, the layered dioritic units and the hybrid samples overlap in almost every major, minor, trace, and rare earth element analyses. The composition of the hybrid matrix and the dioritic units does not conform to processes which I examined for the hybrid rocks. Instead, I propose that the compositions of the large-scale dioritic layers are largely controlled by similar diffusion processes which effected the small-scale hybrid compositions. Selective contamination, effects of differing rates of diffusion between elements, and convective transport of those elements at the granite-gabbro contact appear to progress in directions consistent with the hybrid and diorite compositions. Relative diffusivity controlled the composition of a hybrid liquid which formed at both the top and base of the contact of the massive gabbroic unit overlying the granitic unit and collected to produce the composite layers sandwiched between the massive gabbro and the uppermost granite. (Abstract shortened by UMI.)

Subject Area

Geology|Geochemistry|Mineralogy

Recommended Citation

Chapman, Marshall, "Petrogenesis of a silicic magma chamber periodically invaded by basaltic magma: The Isle au Haut Igneous Complex, Maine" (1996). Doctoral Dissertations Available from Proquest. AAI9638940.
https://scholarworks.umass.edu/dissertations/AAI9638940

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