PETROLOGY AND TRACE ELEMENT THERMOMETRY OF GARNET-QUARTZITE FROM THE CATALINA SCHIST

The Catalina Schist has long served as a laboratory for the the study of fluid flow in subduction zones. Garnets in quartzite blocks from the amphibolite unit of the Catalina mélange preserve extreme oxygen isotope zoning recording evidence of high fluid flow within the region: here we present the first temperature estimates and petrologic descriptions of these unusual rocks.

Garnet quartzite from Catalina occurs primarily as meter-scale blocks in the amphibolite facies mélange, but also as 10-100m scale slabs in lower-grade levels of the subduction complex. The rocks consist of quartz (65-80%), fine-grained (40-200µm) garnet (~10-20%) and variable and minor chlorite, biotite, muscovite, hornblende, sphene, rutile, allanite and monazite. Rutile and zircon are commonly found as inclusions within garnet grains. The samples vary in texture, from fine-scale (<1mm) near uniform parallel garnet layering to unfoliated or clumped distributions of garnet.

Trace element analyses by LA-ICPMS were performed on rutile grains (100-300µm) from five samples of quartzite. Generally, trace element concentrations are consistent among rutile from the same samples. Larger grains are zoned in Ta, with cores (~193 ppm) more concentrated than rims (~88 ppm). Grains are also typically zoned in Zr (141-571 ppm) and Nb (147-3095 ppm), with rims having greater concentrations than cores. Rutile from two samples collected from lower-grade coherent slabs (average [Zr] = 155 ppm) are less zoned and have less Zr than rutile from tectonic blocks (438-440 ppm) Grains from all samples are generally homogeneous in V, and Cr concentrations, and are broadly the same within individual samples.

Application of the Zr-in rutile thermometer (with estimated P of 15 kbar) yields temperatures in tectonic blocks of 650-730˚C. These temperatures are consistent with thermometry from the more common mafic and migmatitic blocks in the same mélange. Samples from coherent slabs at lower structural levels in the subduction complex preserve lower temperatures (610-643˚C), consistent with their later formation in a cooler trench.