Eclogitized gabbros from the eastern Grenville Province: textures, metamorphic context, and implications

Abstract

Undeformed lenses of olivine -orthopyroxene -gabbro belonging to the 1429 Ma Shabogamo Intrusive Suite (Parautochthonous Belt of the eastern Grenville Province) host the first occurrence of eclogite mineral assemblages reported from the Grenville Province. These rocks preserve igneous textures overprinted by eclogite mineralogy and by textures related to subsequent unloading. High-pressure signatures include omphacite pseudomorphs after augite, plagioclase replacement by garnet + kyanite + corundum, and orthopyroxene-omphacite-garnet coronas between olivine and plagioclase. Transformations related to decompression include replacement of garnet + kyanite + corundum by spinel + plagioclase, growth of plagioclase collars between clinopyroxene and garnet, and de-jadeitization of omphacite. Clinopyroxenes with numerous plagioclase inclusions and garnets surrounded by clinopyroxene-plagioclase symplectites in granoblastic mafic rocks from the same area are also evidence of decompression from high pressures. These rocks record pressure-temperature (P-TI conditions of 1600 MPa at 700-8W°C and steep subsequent P-T paths. It is suggested that the entire parautochthon in this part of the Grenville Province experienced deep burial during the Grenvillian metamorphism. In contrast, the Grenvillian metamorphic signature in the overlying allochthonous sheet is known to be minor, thus precluding any relationship between its emplacement and very high Grenvillian pressures in the parautochthon.Des lentilles non deformkes de gabbro a olivine-orthopyroxkne, appartenant a la suite intrusive de Shabogamo Lgke de 1429 Ma (Ceinture parautochtone de l'est de la province de Grenville), renferment des assemblages minkralogiques du faciks eclogite, decrits ici pour la premikre fois dans la province de Grenville. Ces roches conservent leurs textures ignkes surimprimtes par la minkralogie d'eclogite et par les textures associkes B la dCcompression subskquente. Les signatures des hautes pressions incluent des pseudomorphes d'omphacite aprks augite, le remplacement du plagioclase par grenat + disthene + corindon, et des couronnes d'orthopyroxkne-omphacite-grenat entre l'olivine et le plagioclase. Les transformations dues B la dkcompression incluent le remplacement de grenat + disthkne + corindon par spinelle + plagioclase, la croissance de collerettes de plagioclase entre le clinopyroxkne et le grenat et la de-jadkisation de l'omphacite. Des clinopyroxknes qui renferment de nombreuses inclusions de plagioclase, et des grenats entourks par des symplectites a clinopyroxkne-plagioclase dans les roches mafiques granoblastiques de la mCme region fournissent des indices additionnels d'une decompression partant de hautes pressions. Ces roches rkvklent des conditions de pression-temfirature (P-T) de 1600 MPa 2 700-800°C et des trajets P-T subsequents abrupts. I1 est suggkre que la totalitk du parautochtone dans cette partie de la province de Grenville etait profondkment enfouie durant le mktamorphisme grenvillien. D'autre part, il est reconnu que la sig...

Tectonic Setting and Evolution of the Grenville Orogen: An Assessment of Progress Over the Last 40 Years

“…19), an approach that was quickly abandoned following recognition that it was incompatible with the stacked crustal architecture and ignored the age of the metamorphism. Relict eclogite was first recognized in the western Grenville Province by Davidson (1990) and the first thermobarometric studies of eclogite and associated high-pressure granulite, from a thrust sheet stack in the central Grenville Province, revealed peak P-T around 1800 MPa and 800°C (Indares 1993). This was the first quantitative P-T evidence for the former existence of double thickness crust (≥50-60 km) in the orogen.…”

“…Those who both calibrated thermometers and barometers and tested them in the Grenville Province included Eric Essene and his many co-workers (e.g. Essene 1977, 1980;Essene 1982;Perkins et al 1982;Bohlen et al 1985;Anovitz and Essene 1990;Tuccillo et al 1990), and Indares andMartignole (1984, 1985). Geothermometry on upper amphibolite-and granulite-facies gneisses in the Grenville hinterland revealed temperatures in the range 700 ± 50°C or higher, a not unexpected result although subsequently shown to commonly underestimate peak T due to post-peak resetting (the granulite uncertainty principle; Frost and Chacko 1989).…”

Late Neoproterozoic granulite facies metamorphism in the Menderes Massif, Western Anatolia/Turkey: implication for the assembly of Gondwana

“…However, the origin of such coronitic microstructures between olivine-plagioclase described commonly in basic rocks is still controversial (De Haas et al, 2002 and references therein). This type of corona texture has been documented in high-P terranes (Indares, 1993;Indares & Rivers, 1995;Mørk, 1985;Mørk & Mearns, 1986) as a medium stage of gabbro-eclogite transformation and in medium-to highpressure amphibolite-to granulite-facies rocks (Grant, 1988;McLelland & Whitney, 1977;Whitney & McLelland, 1973). Furthermore, they can be observed even in non-metamorphic gabbros as reactions during magmatic cooling (Joesten, 1986;Turner & Stuewe, 1992).…”

Eclogite-facies shear zones—deep crustal reflectors?

“…Our argument receives further support from the recognition of eclogite-facies overprints on deep crustal rocks in other regions and increased documentation that fluid infiltration and deformation play an important role in the kinetics of the gabbro-eclogite transition (e.g. Blattner, 1976;Compagnoni et al, 1977;Koons et al, 1987;Philippot, 1987;Sanders, 1988;Wilkerson et al, 1988;Biino and Pognante, 1989;Philippot and Kienast, 1989;Lardeaux and Spalla, 1991;Philippot and Selverstone, 1991;Ellis and Maboko, 1992;Selverstone et al, 1992;Indares, 1993).…”