METASOMATISM AND POLYMETAMORPHISM IN A PANOCHE PASS GARNET HORNBLENDITE

High-grade blocks from the Franciscan Complex preserve complicated fluid histories of exhumation conditions in the subduction channel. A garnet hornblendite block from the Panoche Pass (Hermes, 1973), characterized by oxygen isotope disequilibrium between garnet core, garnet rim/matrix amphibole, and matrix zircon, contains textures and mineral assemblages that can be tied to distinct stages of metasomatism and polymetamorphism with existing geochronology.

The amphibolitic block is composed of hornblende (51%), garnet (29%), ilmenite (9%), apatite (8%), and rutile (3%) with trace allanite and zircon. This assemblage is cut by pumpellyite veins, ~50 µm blue amphibole rims tracing hornblende grain margins, and partially chloritized garnet. Detailed BSE petrography reveals high concentrations of LILE-rich minerals as part of the late assemblage. Phengite is found within chlorite rims on garnet, whereas hornblende grain boundaries are decorated by potassio-katophorite and thin (~2 µm) barite veneers. The intensity of blueschist-facies overprinting increases with proximity to veins and fractures suggesting a metasomatic origin for the K- and Ba-rich minerals. Published phengite geochronology of 145 Ma provides an absolute age for LILE-rich, high-pressure fluid infiltration. Zircon of the same age also occurs in this block correlating LILE enrichment with zircon δ¹⁸O values. Oxygen isotope disequilibrium found between zircon and matrix hornblende further suggests that LILE were introduced by an external fluid.

Thermobarometric modeling of the block is hampered by the high variance mineral assemblage and evidence of multiple metasomatic events. Experimentation with varying estimates of equilibrium assemblage compositions using averaged EDS broad beam scans and spectral classification of plain and polarized light images make thermodynamic modeling possible at distinct stages of fluid infiltration. Similar spectral analysis of garnet cation zoning in BSE images, as a proxy for oxygen isotope zoning, was used to fractionate ¹⁸O-rich garnet cores from the remainder of the sample. Modeling results from the corrected mineral assemblage provide new evidence of a second metamorphic event following metasomatism at low-grade conditions.