TRACING MANTLE FLUIDS: TRACE ELEMENT SIGNALS IN METASOMATIC GARNET IN QUARTZITES FROM THE CATALINA SCHIST (CALIFORNIA, USA)

Garnet quartzite blocks from the amphibolite mélange of the Catalina Schist subduction complex preserve evidence of massive fluid flow events through extreme δ¹⁸O zoning in garnet (7-15‰). They are found within the metasedimentary mélange along with the more well-studied garnet-hornblende rocks and record similar [Zr] in rutile temperatures of 650-730˚C. Combining new trace element analyses in garnet with existing δ¹⁸O data can provide further insight into the timing of fluid infiltration and composition of these metasomatic fluids in the Catalina subduction zone.

Five garnet quartzite samples with garnets zoned in δ¹⁸O were analyzed by LA-ICPMS to evaluate trace element zoning within the garnets. The rocks are composed of quartz (65-80%), small garnets (40-200 µm, ~10-20%) and minor chlorite, biotite/muscovite, hornblende, sphene, rutile, allanite and monazite. One sample contains large garnets ranging between 1-3mm. Small garnets are almandine-rich (X_alm = 0.55-0.70) and broadly homogenous in cations, with zoning restricted to <5% in Fe-Mg. Large garnets exhibit greater cation zoning. Garnets are strongly zoned in oxygen isotope ratios, with unusually high δ¹⁸O cores (20-25‰, VSMOW) and much lower rims (10-13‰, Cameron et al., 2014, GSA Abstr.).

Garnet cores with high-δ¹⁸O contain sharp peaks in Y (~1000 ppm) and HREE (Dy 300 ppm, Ho 60 ppm, Er 200 ppm, Yb 250 ppm) whereas low δ¹⁸O rims have close to 0 ppm each, contrary to early work on amphibolite garnets from Catalina (Hickmott et al., 1992, Geology). HREE zoning in garnet cores is consistent with Rayleigh fractionation from internal sources. However, low- δ¹⁸O garnet rims were found to be enriched in Cr (~700 ppm) relative to pre-metasomatic cores (~10ppm). There are no other Cr reservoirs in this rock, and the correlation of the decrease in δ¹⁸O with an increase in Cr is a strong indicator of an external Cr source. The presence of Cr suggests the fluid interacted with mantle material before metasomatizing the garnet quartzite. The relatively high (compared to mantle) δ¹⁸O of garnet rims (10-13‰) suggests either the quartzite did not fully equilibrate with a mantle-sourced fluid or the metasomatizing fluid interacted with both mantle and supracrustal sources (e.g. mélange matrix, amphibolite and serpentinite blocks) and developed a hybrid geochemical signature.