Textures, metamorphic reactions and thermobarometry of eclogitized metagabbros: a Proterozoic example
Indares, Aphrodite; Rivers, Toby
published: Feb 8, 1995
manuscript accepted: May 5, 1994
manuscript received: Oct 26, 1993
ArtNo. ESP147050701004, Price: 29.00 €
Abstract The Parautochthonous Belt of the eastern Grenville Province (Canadian Shield) consists of a medium to high pressure (HP) fold-thrust belt (Gagnon terrane) overlain by a thick thrust slice (Molson Lake terrane) with a HP metamorphic signature, both of which were penetratively metamorphosed during the Grenvillian Orogeny about 1000 Ma. Evidence of HP Grenvillian metamorphism is best preserved in the cores of the 1429 Ma Shabogamo gabbros, which may be subdivided into two compositional groups, 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 around olivine and orthopyroxene, and Na-clinopyroxene pseudomorphs after intercumulus augite. Group 1 is also characterized by garnet and/or Na-clinopyroxene (Jd up to 54 %) pseudomorphs after plagioclase, whereas group 2 is characterized by relict plagioclase with garnet-corundum-kyanite inclusions. Using coronitic metagabbros from group 2, estimates of peak P-T conditions have been achieved, despite complex textures and the absence of quartz, by careful selection of small domains for which local equilibrium may be assumed and thermobarometric relationships written. Maximum P-T conditions, calculated by Grt-Cpx thermometry and Grt-Pl-Ky-Crn barometry, show a regional gradient from 700-750°C/l 200-1400 MPa in the vicinity of the "eclogite in" isograd to 800-850°C/1600- 1800 MPa some 30 km farther south. Decompression reactions took place at near-peak temperatures, implying exhumation along steep P-T paths that are compatible with those estimated from the retrogressed country rocks. This metamorphic evolution is indicative of the depth of crustal delamination of Molson Lake terrane, and is compatible with its rapid tectonic uplift over the foreland, thus providing the driving force for the formation of the fold-thrust belt in Gagnon terrane.