Composite layering in the Isle au Haut Igneous Complex, Maine: evidence for periodic invasion of a mafic magma into an evolving magma reservoir

Marshall Chapman and J.M. Rhodes Department of Geology and Geography, University of Massachusetts, Amherst

Abstract. The Paleozoic Isle au Haut Igneous Complex, Maine, is a composite layered sequence of alternating gabbroic and dioritic units situated between two granitic bodies. Field and textural relationships for a layered sequence of ten alternating gabbroic and dioritic units show that they were liquid or largely liquid contemporaneously. The bases of the gabbros are chilled against the underlying diorite, from which cylindrical pipes of a component of the diorite intrude vertically into the gabbro. Immediately below the chilled gabbro contacts, pillow-like lobate structures, also chilled, have become detached and sunk into the diorite for a half meter or less. These pillows are compositionally identical to the chilled margins of the overlying gabbro and preserve their liquid composition. Chemistry of the gabbros indicates that they are similar to within-plate tholeiites and that they are uniform in average composition throughout the stratigraphic succession. The 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. However, a gravita- tionally stable situation would exist if the diorite were in fact the cumulate floor of a more felsic magma reservoir. Thus, we 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.

Location and generalized geologic map of lithologic units which comprise the Isle au Haut Igneous Complex, taken from Smith et al. (1907) and Luce (1962). Box shows area of our concentrated research.

Geologic map of the composite layering of Gabbro-Diorite units and Granites exposed on Eastern Head peninsula and Eastern Ear island.

Block diagram showing schematic relationship between gabbros E and G and quartz monzodiorite F. This relationship is consistent for all contacts with the exceptions of diorite B overlying gabbro A and diorite D over gabbro C (which shows complex mixing and intermingling). Sample location indicated by numbering.

From: Journal of Volcanology and Geothermal Research, 51 (1992) 41-60 Elsevier Science Publishers B.V., Amsterdam