THERMOBAROMETRIC MODELING OF A FE-TI RICH RESTITE BLOCK IN AMPHIBOLITE-FACIES MÉLANGE

The conditions of metamorphism of garnet-hornblendite blocks within the Santa Catalina amphibolite unit remain ambiguous despite many years of study. The lack of visible plagioclase and modes of garnet almost equal to that of hornblende, 31% and 34% respectively, suggest a different history than typical of orogenic amphibolites. One model of these blocks’ formation is that they are restites of a Fe-Ti rich protolith, possible a ferro-basalt or gabbro, following the formation and extraction of a near 20 mol% melt. Using Equilibrium Assemblage Diagrams (EAD) calculated with Theriak Domino this study quantitatively evaluates this model, and estimates the peak of metamorphism for one such block at temperatures between 700 ˚C and 800 ˚C and pressures above 14 kbar.

The block in this study is composed of garnet, hornblende and clinopyroxene and contains abundant rutile and ilmenite. Trace plagioclase coronas are found between garnet and hornblende. Bulk rock compositions for modeling were determined by XRF, and by image analysis of thin sections for modal estimation coupled with quantitative mineral data. Both methods yield results consistent with published compositions. EADs were calculated with 80% garnet removed from the bulk composition to account for elemental fractionation during prograde garnet growth.

The peak assemblage (grt-cpx-hbl-ilm) is predicted at temperatures directly below the loci of the solidus as one would expect for a restite. Melt near the peak assemblage is roughly 75% albite, 20% silica, and 5% anorthite. Adding 19 mol% of this melt to the sample produces a composition in close agreement with that of Fe-Ti rich gabbros supporting it as a possible protolith. Melt extraction explains both the apparent lack of plagioclase and a notably low Na content of clinopyroxene below what one would expect under eclogitic conditions.

The loci of the melt in reaction on the EAD provides an upper temperature limit of 800 ˚C on the P-T path, while the loci of melt in reaction on an EAD modeled with the addition of the melt provides a lower limit of 700 ˚C. The presence of rutile constrains the pressure to above 14 kbar. Concentric plagioclase and hornblende coronas around garnet, and in contact with clinopyroxene, record stages of stable equilibrium as well as incomplete reactions along a clockwise decompression path.