PETROLOGY OF AN EXOTIC BLUESCHIST FROM SANTA CATALINA ISLAND

The mélange of Santa Catalina Island has long been a powerful tool in the study of subduction zone metamorphism. The island’s inverted metamorphic gradient [1] presents an opportunity to further understand subduction processes. This study considers the multi-phase subduction [2] of a garnet-blueschist block within the blueschist-facies mélange at the base of the island. Portions of the block contain relict garnet [3] (along with zoned glaucophane, phengite, lawsonite, titanite and quartz). Geochronology studies have determined ages of the block ranging from 150-160Ma [2,4]. Textural, geochemical and pressure/temperature analyses were performed using a combination of backscattered-electron images (BSE), energy dispersive spectroscopy(EDS) maps and Equilibrium Assemblage Diagrams (EAD) generated using Perple_X to invert bulk rock compositions BSE imaging reveals lawsonite and phengite growth between and within fractured garnet domains. EADs predict assemblages consistent with earlier metamorphism in the unit between 400–500˚C at 1.2–1.7 GPa. Analyses excluding garnet in the bulk composition input for EAD calculations to simulate an overprinting metamorphic event are ongoing. The estimated metamorphic conditions for the blueschist unit at 300–400˚C and 0.8–1.1 GPa [5] are lower than the conditions predicted for the garnet-blueschist block, thus peak metamorphism occurred prior its entrainment in the current mélange. Overprinting of glaucophane and lawsonite on pre-existing garnet indicates multiple stages in the block’s metamorphism and, therefore, multiple episodes of subduction. Given these results, it seems likely that the blueschist block underwent an initial higher-grade metamorphism, responsible for garnet formation, resurfaced and underwent a second low-grade metamorphism in a cooler subduction channel producing the conditions preserved in the block that is now exposed.

[1] Platt. (1975), UC Pubs in Geo 112; [2] Grove et al. (2008), GSA Special Paper 436; [3] Adler-Ivanbrook et al. (2018), AGU Abstract T21F-0290; [4] Awalt et al. (2013), GSA Programs with Abstracts 45; [5] Sorenson. (1986). GSA - Memoir 164.