Impact of varying degrees of seafloor alteration on the metamorphic and rheological evolution of mid-ocean ridge basalts

Peter Lindquist, Department of Earth and Space Sciences, University of Washington
Cailey Condit, Department of Earth and Space Sciences, University of Washington
Deven Loska, Department of Earth and Space Sciences, University of Washington
Sophie Johnson, Department of Earth and Space Sciences, University of Washington
Jason Ott, Department of Earth, Environmental, and Planetary Sciences, Rice University
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Abstract

The Catalina Schist in southern California contains several suites of oceanic crustal rocks that were underplated over a range of pressure and temperature conditions during Cretaceous subduction. Despite similar pressure-temperature paths during prograde subduction metamorphism, metabasalts in the Epidote Amphibolite and Epidote Blueschist Units exhibit strikingly different degrees of deformation, with the former mostly undeformed and the latter hosting strong penetrative deformation and early deformation fabrics preserved by inclusion trails of low-grade metamorphic minerals. Subtle differences in the bulk composition of these metabasalts may have affected their rheology during prograde subduction, explaining the differences in their structural patterns. Using phase equilibrium modeling, we show that bulk compositional differences between the Epidote Amphibolite and Epidote Blueschist Unit metabasalts controls the occurrence of Na-amphiboles (glaucophane and riebeckite) and Ca-amphibole (actinolite) during subduction metamorphism through greenschist facies conditions. The Epidote Blueschist metabasalts, which likely had Na-amphibole present throughout early subduction metamorphism, deformed early in their subduction history at low-grade, seismogenic zone conditions. On the other hand, our models predict that the more Ca-rich Epidote Amphibolite metabasalts would not have stabilized Na-amphibole. Based on the crystal chemistry of Na- vs Ca-amphiboles, Na-amphiboles are expected to deform via dislocation-accommodated crystal plastic deformation more readily than Ca-amphiboles, and we further suggest that difference in mineral and elemental solubilities may affect the relative efficiency of dissolution-precipitation creep between Na- and Ca- amphiboles. The presence of the different amphiboles during early subduction metamorphism is therefore likely to result in distinct rheologies of the two suites of metabasalts. Sr isotope ratios further indicate that the Epidote Amphibolite Unit metabasalts may have experienced greater degrees of seafloor alteration than those in the Epidote Blueschist Unit, suggesting that the type and extent of seafloor alteration of basalts prior to subduction exert and important control on their behavior during subduction.

Session

Large-scale tectonics