In the Santa Catalina Island, CA subduction complex, there are garnetiferous mafic gneiss blocks, from a tholeiitic protolith, that occur in a serpentinite mélange, a unit that is poorly understood and has been little studied in the last twenty-five years. Study of the microstructural relationships and geothermometry of two hand samples of garnet-rich gneiss yield information about processes and conditions for high temperature metamorphism in the Catalina complex and possibly more broadly. Results of the study provide insight into 1) the nature of HT metamorphism in the subduction setting, and 2) the metamorphic history of disparate gneiss blocks within the Catalina serpentinite mélange. Thin section petrography and scanning electron microscope (SEM) analysis were performed on polished thin sections of samples to identify minerals and microstructures. Compositional maps of garnets and quantitative mineral analysis of garnet, garnet inclusions, and matrix phases were acquired using an electron microprobe (EMP). The Zr-in-rutile geothermometer (Watson et al., 2006) was used to calculate peak metamorphic temperatures based upon Zr concentrations (ppm) for rutile inclusions and matrix measured by EMP. One of the garnet gneisses exhibits compositional zoning with Mg increase / Ca decrease toward the rim, and by a pattern of distribution of inclusions. The other sample has unzoned, compositionally homogeneous garnet with an unusual abundance of rutile for a metabasalt. The excess of rutile is a possible indication of metasomatism. Zr-in-rutile thermometry yielded temperatures of 480 to 516 ˚C for the study samples, in contrast to the results of Zack et al. (2004) of 764 to 800 ˚C for clinopyroxene-bearing garnet amphibolite in mélange blocks. The differing Zr-in-rutile results provide a possible indication that blocks in the mélange originated at vastly different depths in the subduction zone, then were brought together during mélange formation. My findings indicate that the two gneiss blocks studied have distinct protoliths and metamorphic histories, suggesting that mélange blocks within the upper tectonic unit of Catalina Island derived from different locations within a subduction zone.