MODELS FOR METAMORPHIC PORPHYROBLAST GROWTH AND RESORPTION: EXAMPLES FROM GARNET AMPHIBOLITE

Metamorphic minerals such as garnet preserve compositional zoning and textures that provide important information about metamorphic and tectonic processes. Compositional zoning from garnet core to rim can be modeled to construct P-T paths. However, diffusion, resorption, recrystallization, and hiatus in growth/multiple growth events can produce incomplete or unusable garnet zoning. Inclusion rich garnet cores or rims with inclusion free garnet are widespread even in high T metamorphic rocks providing a means to construct P-T paths if zoning and inclusion compositions are preserved. Abrupt textural discontinuities are attributed to several factors, most notably 2 stage garnet growth. Two stage garnets may result from multiple tectonic events or mineralogical reactions producing/consuming garnet along a single P-T path.

Amphibolites from the Salmon River Suture Zone, containing garnet with compositional breaks and abrupt changes in inclusion density, are used to evaluate metamorphic reactions that could cause the growth, absorption/hiatus, and regrowth of garnet. Forward thermodynamic models (pseudosections), are used to predict garnet compositional isopleths (Ca, Mn, Mg, and Fe) and garnet modes. The combination of predictions with thermobarometry and observed mineral compositions allow construction of P-T paths and interpretation of garnet zoning. Preliminary results for staurolite-kyanite-garnet-2 amphibole-amphibolite, containing garnet with a sharp break between an inclusion-free core and inclusion-rich rim, shows that the growth and breakdown of staurolite can cause garnet resorption followed by growth.

Prior work interprets 2 stage garnet as a result of distinct regional metamorphic events (Getty and Selverstone, 1993). However, McKay (2010) using garnet core and rim Sm-Nd ages, inferred garnet growth to result from a single prolonged metamorphic event lasting 10-20 Ma. In this case, textures and compositional zoning in garnet could have resulted from a long term conductive heating event such as thrust faulting, allowing better understanding of tectonic processes during terrane accretion.