IMPLICATIONS OF THE TRANSITION OF CORONITIC METAGABBRO TO GARNETIFEROUS AMPHIBOLITE IN THE ADIRONDACK HIGHLANDS

Coronitic metagabbros (CMGs) are the most primitive rocks in the Adirondack Highlands and were derived from an enriched asthenospheric source tapped near the end of the Shawinigan Orogeny (ca. 1150 Ma). They intruded the AMCG plutonic suite and surrounding country rocks as small (<10 km), satellite bodies and dikes. Many are also found as relict cores enveloped within elongate, deformed belts of garnetiferous amphibolite (GA). The transition from CMG to GA is exposed in a 0.5 km long road cut south of Tahawus along Rt. 28N. Coronitic textures and associated mineralogy (Ol+Opx+Cpx+Pl+Spl+Grt) are overprinted by growth of Grt, Hnb, Bt, and more calcic Pl with associated, but variable, foliation and porphyroblast development. Coronitic textures in CMGs include relict Ol surrounded by Cpx rims, then surrounded by Opx with symplectic Spl-clouded Pl. In contrast, GAs contain coronas of Bt and variable amounts of Hnb (5-30%) around Fe-Ti oxides, much less Cpx, minor Opx, and locally, Gnt porphyroblasts up to 10 mm. Whole rock geochemistry indicates, despite large differences in modal mineralogy and fabric development, the samples analyzed have a slight variation in composition, are identical to CMGs, and have LOIs (<0.5%). Grt rims in CMGs typically contain a higher Mg/Mg+Fe, Ca, and lower Al than Grt crystals in their deformed equivalents. Plagioclase in coronitic protoliths are typically spinel clouded and have a limited X_An range with an average of An₃₃, ~10 molar percent lower than the deformed equivalents. Preliminary thermobarometric estimates suggest that coronitic textures developed at notably higher P-T conditions than GA formation. This indicates closed system chemical behavior during the transition and suggests differences in strain in concert with changes in P and T conditions, rather than fluid influx, account for the observed fabrics and mineralogical transformations. A scenario involving intrusion of CMGs into deep orogenic crust, crystallization and equilibration with metamorphic conditions, and deformation during post-peak metamorphic conditions can explain the observed field relations. Such a scenario requires neither fluid infiltration nor the effects of the ca. 1050 Ma Ottawan event.