INTERACTION BETWEEN IGNEOUS, METAMORPHIC, AND STRUCTURAL PROCESSES FROM THE LOWER TO MIDDLE CONTINENTAL CRUST: INSIGHTS FROM TWO ISOBARIC TERRANES

Isobaric terranes preserve evidence of nearly constant-pressure metamorphism, and can be used as 3- and 4-dimensional models for tectonic processes at specific crustal levels. The Athabasca granulite terrane (AGT) in the western Churchill Province and the Adirondack Highlands (AH) of the southern Grenville Province have both been interpreted as isobaric terranes, the AGT at 1.9 Ga and the AH at 1.06 Ga. Individually, they display excellent examples of how magmatism influences subsequent deformation, and how deformation facilitates both the generation and transport of magma through the crust. The AGT contains evidence for extensive Neoarchean plutonism (2.6 Ga) and subsequent tectonism (2.57 Ga) in lower continental crust (ca. 1.0 GPa). Lateral crustal flow was concentrated in young plutons Deformation catalyzed metamorphic reactions and partial melting in peraluminous rocks. The composition of these melts was likely leucogranitic, and may represent syn-collisional type granitoids. The AH preserves evidence of granulite facies metamorphism within ca. 1.15 Ga plutonic rocks. However, the coeval Marcy anorthosite massif shows only local evidence of subsolidus deformation, preserves igneous features, but contains a static, granulite metamorphic overprint. Deformation was focused along the margins of the anorthosite body, and given its large size, this body influenced the structural evolution of a vast region. The ca. 1.06 Ga Lyon Mountain Granite gneiss (LMG) variably cross cuts and contains xenoliths of older lithologies. Magmatic layering mimics the gneissosity of country rock, but large LMG plutons occur in the hinges of extension-related folds. These data suggest that the extensional folds are an efficient means to transport mid-crustal magma, and are also potential sinks for pluton construction. Juvenile plutons are some of the weakest rocks in the lower crust and can act to localize strain, but ensuing deformation may catalyze further partial melting reactions. Deformation aids in magma transport and can focus pluton construction, whereas cooler and relatively strong plutons may partition strain. Thus, isobaric terranes contain evidence of how magmatism and tectonism can interact and how these interactions facilitate the spatial and temporal evolution of the crust as a whole.