UNDERSTANDING THE DYNAMICS OF MAFIC-SILICIC MAGMA INTERACTIONS IN THE ISLE AU HAUT IGNEOUS COMPLEX, MAINE

Magmatism associated with subduction zones is a major contributor to the growth of continental crust and there is increasing evidence for its bimodal character or mafic-silicic magmatic interaction. This evidence is in the form of contemporaneous silicic and mafic volcanism (where the details of the physical interactions are obscured by the volcanic process) and granitoids made up of inter-mingled silicic-mafic magmas (where the details of the physical and chemical interactions are often preserved). The Isle au Haut Igneous Complex, Maine, emplaced during the Acadian orogeny, has uniquely captured the intimate physical and chemical interactions between contemporaneously emplaced mafic and silicic magmas in a 600 m thick sequence of 11 alternating gabbro and diorite layers (typically 15 – 40 m thick). Purely on the basis of density contrasts (2.65 g/cm³ gabbro vs. 2.55 g/cm³ diorite), the entire system should have undergone wholesale instability and mixing; it is instead arrested in a grossly unstable state of interaction while molten indicated by systematic and widespread dioritic intrusions into gabbro and vice-versa. The differences between juxtaposed layers of diorite and gabbro record variations in cooling, solidification, compaction, and gravitational instability rise times. The timing and rates of each of these processes can be closely bracketed by studying their physics and analyzing the contrasting physical properties of the magmas (density, viscosity, temperature). Chilled margins along the lower contacts of the gabbros and structural integrity of the diorite layers indicate that near liquidus gabbroic magma invaded partly crystalline, cooler diorite. Mineral assemblages, chemical analyses, and phase equilibria calculations indicate initial temperatures during emplacement of ~1180 ^oC (gabbro) and ~1000 ^oC (diorite). Conductive thermal models yield solidification timescales of 15 – 60 years for individual gabbro layers (supported by spatial variation in crystal sizes) and about a thousand years for the entire complex. A thorough study of field relationships combined with detailed geophysical and geochemical analysis is used to construct a comprehensive quantitative model of mafic-silicic magma interaction, which helps to refine our understanding of subduction zone magmatism.