CONSTRAINTS ON GARNET-BEARING METASOMATIC RIND GROWTH IN SUBDUCTION MELANGE THROUGH SIMS ANALYSIS OF OXYGEN ISOTOPES

Garnet-hornblende blocks in the amphibolite facies subduction mélange of the Catalina Schist commonly preserve metasomatic rinds. Comparisons of bulk and mineral chemistry have shown that complex processes of mechanical mixing and fluid interaction between block and mélange matrix led to rind growth. New field and petrographic data, along with in situ oxygen isotope analysis of garnet in both rind and block core support and add additional detail to existing models of formation.

The core of the examined block (lithology A) is composed dominantly of hornblende, garnet, phengite, chlorite, and rutile while the rind (B) is made up of concentrically foliated hornblende and tremolite-actinolite and sheet silicates with rare garnets. A newly-sampled, coarser-grained rind zone (C) is found outside of B and appears to be intermediate in mineral composition between A and B.

Preliminary analyses of oxygen isotope ratios in garnet by ion microprobe show that the garnets in A (Alm₅₅Pyp₁₈Grs₂₆Sps₁) are unzoned (δ¹⁸O = 10.1 ± 0.6‰, 2SD, VSMOW) consistent with metamorphism of hydrothermally-altered ocean crust. The garnets in B (Alm₅₆Pyp₂₅Grs₁₆Sps₃) are similarly homogeneous but with higher δ¹⁸O (11.7 ± 0.8‰) and strongly resorbed. The different isotope ratios of garnet in A and B confirm that the Grs-poor garnets in rind B are not relicts of previous garnet growth from the core (A) but grew in different oxygen reservoirs. The garnets in the C rind, however, are zoned with 9.4 ± 0.4‰ cores (Alm₆₁Pyp₁₂Grs₂₃Sps₄) to 11.9 ± 1.2‰ rims (Alm₅₅Pyp₂₇Grs₁₅Sps₃) and also show signs of being resorbed. The overlap of the garnet core isotope ratio and Grs content in C to that of A, suggest that it is relict, predating rind formation. The rim is identical to the garnet from rind B, suggesting renewed garnet growth associated with metasomatism. Additional small euhedral garnets were found in the C and B rinds. Small garnets in C (Alm₅₈Pyp₂₇Grs₁₂Sps₄) are not strongly zoned in δ¹⁸O and match other neoformed rind garnet (11.8 ± 0.6‰). However, small euhedral garnets (Alm₆₀Pyp₂₄Grs₉Sps₇) enclosed in quartz veins in rind B are zoned with 16-20‰ cores and 12-16‰ rims. This high δ¹⁸O is similar to garnets found in quartzite blocks on Catalina, perhaps recording later fluid infiltration and/or mechanical introduction of high δ¹⁸O sediments.