MAGMATIC PLUMBING PROCESSES OF BACK-ARC BASINS: TEXTURAL OBSERVATIONS AND CHEMICAL MODELING OF THE TUDOR GABBRO

Subduction systems are fundamental to our understanding of mass exchange between the crust and mantle. In modern settings, much of our insight of processes active within the mantle wedge and overlying lithosphere comes from the erupted products. The mid and lower lithospheric roots of subduction zones that are preserved in ancient orogenic belts provide us a mechanism to probe subduction-related magma differentiation processes. We have examined the remnants of a Mesoproterzoic arc preserved in the Central Metasedimentary Belt of the Grenville Province in eastern Ontario, with a particular focus on the Tudor Gabbro. Combining bulk rock major and trace element values with Monte Carlo simulation of Raleigh fractionation, we have found the geochemical characteristics of the Tudor Gabbro to be most easily explained through the crystallization of an island arc parental magma. Furthermore, out of thousands of model simulations we have discovered the best agreement for a parent magma is the nearby Jordan Lake basalt complex. While our chemical analysis is consistent with a subduction setting, both the macro and micro textures of this pluton indicate processes active within an oceanic spreading center. Gabbro textures are extremely variable with evidence of crystalline “dikes” and other evidence of interaction between mostly-solidified masses. However, chemical analysis shows these domains to be equilibrated. These physical similarities with core samples of modern mid-ocean rift systems, coupled with the chemical evidence, suggest that the Tudor Gabbro represents the crystal mush near the melt lens in a back-arc system. Based on these findings, we are able to demonstrate some of the magmatic processes and characteristics of this ancient back-arc basin. For example, the modeling allows us to determine the degree of crystallization that had occurred. Furthermore, using comparative models of modern back-arc systems, we are able to determine the relative proximity of the basin to the ancient subduction trench. These new findings are valuable to studies of both modern and ancient systems as back-arc processes have large influences on the chemistry, flow, and temperature of subduction products.