PETROLOGY OF SUBDUCTION-RELATED GARNET QUARTZITES FROM SANTA CATALINA ISLAND, CA

Santa Catalina Island, CA, is famous as a natural laboratory to study subduction processes, but most petrologic work on Catalina has focused on mafic blocks within mélange. In addition to garnet hornblendite, the amphibolite-facies mélange contains blocks of garnet quartzite, sometimes described as metachert, that contain garnets strongly zoned in δ18O, recording fluid metasomatism at peak metamorphism (Page et al., 2019, Geology). Page et al. focused on one block, but this study examines six blocks with differing mineralogies. We used a combination of optical and electron microscopy to characterize these rocks in terms of their mineralogy and bulk chemistry, and we calculated Equilibrium Assemblage Diagrams (EAD) to make P-T estimates. The samples are primarily composed of quartz (85-96%), but they also contain variable amounts of garnet, chlorite, white mica, biotite, anthophyllite, hornblende, rutile, sphene, and apatite. Accessory minerals include monazite, allanite, zircon, and clinozoisite. Texturally, the garnets appear as discrete bands through the quartzite, but individual samples can be quite heterogeneous, sometimes even interlayered with the amphibolite. Temperature estimates for all samples range from 550-650˚C. They are in agreement with estimates by Sorensen and Barton (1987, 640-750˚C) as well as Zr-in-rutile temperatures measured for the same samples (Hartley, 2017, GSA abstract). However, each EAD provides minimal pressure constraints. We are currently exploring QuiG barometry as a way to better refine pressures. Bulk compositions of the samples lie on a linear mixing array between pure SiO2 and a mafic component. Elemental proportions of mafic blocks (Sorensen and Grossman, 1989, Geochimica et Cosmochimica Acta) from the same mélange do not fall on the mixing array, suggesting additional mafic input from a different source, such as volcanic ash deposition prior to subduction. These petrologic constraints combined with unusually high δ18O values (Page et al., 2019) confirm that Catalina quartzites are subducted mixtures of chert and a volcanoclastic component that were subducted contemporaneously and metamorphosed to the same conditions as the basalt that eventually formed garnet amphibolite blocks.