ORIGIN OF EXOTIC BLOCKS IN THE CATALINA SCHIST: IMPLICATIONS FOR MOVEMENT ALONG THE SUBDUCTION INTERFACE

Evidence from rocks exhumed from subduction zones has led to a range of ideas about the behavior of materials at the plate interface (the boundary between a subducting slab and the overriding plate). In many exhumed terrains, a fine-grained mélange matrix encloses blocks of markedly different metamorphic grades, which is often interpreted as evidence for substantial tectonic mixing along the subduction interface. The Catalina Schist terrain preserves distinct km-scale, thrust separated, isofacial zones, ranging from lawsonite-albite to amphibolite facies. Block-in-matrix mélange exposures are found at all grades, and the isofacial character of the majority of these exposures suggests that large-scale tectonic mixing ceased during or prior to peak metamorphic conditions. There are, however, several amphibolite-facies blocks located within lower-grade units. These ‘exotic’ blocks preserve peak metamorphic conditions roughly 100-400°C hotter than the surrounding rocks suggesting some differential mobility of blocks. To aid in determining the timing and scale of tectonic mixing in the Catalina Schist, the peak metamorphic conditions for several exotic blocks were determined by coupling zirconium-in-rutile trace element thermometry and quartz-in-garnet thermoba-Raman-try. These P-T conditions were then compared to non-exotic blocks from the amphibolite-facies mélange. Results show that exotic blocks reached peak conditions between 580-700°C and 1.2-1.6 GPa. In comparison, amphibolite-facies mélange blocks record significantly narrower peak temperature and pressure ranges (650-730°C, 1.3-1.5GPa). Although the P-T ranges for the exotic blocks and amphibolite-facies mélange blocks overlap, many exotic blocks record lower temperatures at the same pressure compared to blocks in the amphibolite-facies mélange. These data suggest that the exotic blocks were sourced either from different depths along the subduction interface, or from different stages of subduction. The data raise questions about the timing and scale of differential movement within subduction interfaces, and what mechanism may allow some blocks to be transported over km-scale distances while others are not.