TEXTURES, METAMORPHIC REACTIONS AND THERMOBAROMETRY OF ECLOGITIZED METAGABBROS - A PROTEROZOIC EXAMPLE

The Parautochthonous Belt of the eastern Grenville Province (Canadian Shield) consists of a medium to high pressure (HP) fold-thrust belt (G agnon terrane) overlain by a thick thrust slice (Molson Lake terrane) with a HP metamorphic signature, both of which were penetratively meta morphosed during the Grenvillian Orogeny about 1000 Ma. Evidence of HP Grenvillian metamorphism is best preserved in the cores of the 1429 M a Shabogamo gabbros, which may be subdivided into two compositional gr oups, i.e. low Al/high [Fe + Mg] (group 1) and high Al/low [Fe + Mg] ( group 2). Common to both groups are garnet-Na-clinopyroxene coronas ar ound olivine and orthopyroxene, and Na-clinopyroxene pseudomorphs afte r intercumulus augite. Group 1 is also characterized by garnet and/or Na-clinopyroxene (Jd up to 54%) pseudomorphs after plagioclase, wherea s group 2 is characterized by relict plagioclase with garnet-corundum- kyanite inclusions. Using coronitic metagabbros from group 2, estimate s of peak P-T conditions have been achieved, despite complex textures and the absence of quartz, by careful selection of small domains for w hich local equilibrium may be assumed and thermobarometric relationshi ps written. Maximum P-T conditions, calculated by Grt-Cpx thermometry and Grt-Pl-Ky-Cm barometry, show a regional gradient from 700-750 degr ees C/1200-1400 MPa in the vicinity of the ''eclogite in'' isograd to 800-850 degrees C/1600-1800 MPa some 30 km farther south. Decompressio n reactions took place at near-peak temperatures, implying exhumation along steep P-T paths that are compatible with those estimated from th e retrogressed country rocks. This metamorphic evolution is indicative of the depth of crustal delamination of Molson Lake terrane, and is c ompatible with its rapid tectonic uplift over the foreland, thus provi ding the driving force for the formation of the fold-thrust belt in Ga gnon terrane.