Mafic dykes as monitors of hp granulite facies metamorphism in the Grenville front tectonic zone (western Quebec)

International audienceMetamorphosed mafic dykes and pods in the Grenville Front Tectonic Zone (GFTZ) are correlated, on the bases of map patterns and geochemical trends, with Paleo- and Mesoproterozoic dykes that cross-cut Archean rocks of the foreland, NW of the Grenville Front. The GFTZ consists of a SE-dipping 30 km thick slab of quartzo-feldspathic granulites and biotite–garnet (±opx) gneisses overriding, along the McLaurin thrust, a 4 km thick zone of muscovite–sillimanite micaschists that abuts on the Grenville Front. The granulite slab in turn dips under amphibolite-grade migmatites of the Réservoir Dozois terrane along the Dorval detachment. According to published U–Pb and new U–Th–Pb ages on monazite, the granulites, the gneisses and the schists of the GFTZ were metamorphosed at ca. 2.7–2.8 Ga and reworked at ca. 1.0 Ga (Grenvillian). Proterozoic mafic dykes and pods cross-cutting the GFTZ are thus monocyclic, i.e. they went through the Grenvillian metamorphic event only. Several of these metamorphosed mafic dykes and pods have preserved evidence of emplacement in a brittle regime such as intersection of the gneissic foliation, chilled margins, magmatic layering, this in spite of a strong metamorphic overprint attested by coronitic fabrics. In particular, metamorphosed mafic dykes of the granulite slab, contain large pyroxenes rimmed by garnet coronas with minute quartz inclusions, suggesting the reaction pl + opx = grt + qtz. These garnet coronas, in turn, are locally replaced by symplectites of opx, hbl, pl and qtz, probably formed through back-reactions grt + cpx + qtz = pl + opx and grt + cpx = hbl + qtz. Thermobarometric calculations using the above assemblages indicate peak equilibration at about 1.2–1.5 GPa at temperatures of about 800 °C followed by a quasi-isothermal decompression down to 0.9 GPa at temperatures around 700 °C. Proterozoic mafic dykes thus record a Grenvillian HP granulite metamorphism resulting from burial of upper crustal levels down to sub-Moho depths with concomitant heating. This event was followed by rapid exhumation of the GFTZ zone probably controlled by normal displacement along the Dorval detachment

U-Pb SHRIMP zircon dating of Grenvillian metamorphism in Western Sierras Pampeanas (Argentina) : correlation with the Arequipa-Antofalla craton and constraints on the extent of the Precordillera Terrane

The Sierras Pampeanas of Argentina, the largest outcrop of pre-Andean crystalline basement in southern South America, resulted from plate interactions along the proto-Andean margin of Gondwana, from as early as Mesoproterozoic to Late Paleozoic times (e.g., Ramos, 2004, and references therein). Two discrete Paleozoic orogenic belts have been recognized: the Early Cambrian Pampean belt in the eastern sierras, and the Ordovician Famatinian belt, which partially overprints it to the west (e.g., Rapela et al., 1998). In the Western Sierras Pampeanas, Mesoproterozoic igneous rocks (ca. 1.0–1.2 Ga) have been recognized in the Sierra de Pie de Palo (Fig. 1) (McDonough et al., 1993 M.R. McDonough, V.A. Ramos, C.E. Isachsen, S.A. Bowring and G.I. Vujovich, Edades preliminares de circones del basamento de la Sierra de Pie de Palo, Sierras Pampeanas occidentales de San Juán: sus implicancias para el supercontinente proterozoico de Rodinia, 12° Cong. Geol. Argentino, Actas vol. 3 (1993), pp. 340–342.McDonough et al., 1993, Pankhurst and Rapela, 1998 and Vujovich et al., 2004) that are time-coincident with the Grenvillian orogeny of eastern and northeastern North America (e.g., Rivers, 1997 and Corrievau and van Breemen, 2000). These Grenvillian-age rocks have been considered to be the easternmost exposure of basement to the Precordillera Terrane, a supposed Laurentian continental block accreted to Gondwana during the Famatinian orogeny (Thomas and Astini, 2003, and references therein). However, the boundaries of this Grenvillian belt are still poorly defined, and its alleged allochthoneity has been challenged (Galindo et al., 2004). Moreover, most of the Grenvillian ages so far determined relate to igneous protoliths, and there is no conclusive evidence for a Grenvillian orogenic belt, other than inferred from petrographic evidence alone (Casquet et al., 2001). We provide here the first evidence, based on U–Pb SHRIMP zircon dating at Sierra de Maz, for a Grenville-age granulite facies metamorphism, leading to the conclusion that a continuous mobile belt existed throughout the proto-Andean margin of Gondwana in Grenvillian times

The formation of garnet in olivine-bearing metagabbros from the Adirondacks

A regional study of olivine-bearing metagabbros in the Adirondacks has permitted testing of the P(pressure)-T(temperature)-X(composition) dependence of garnet-forming reactions as well as providing additional regional metamorphic pressure data. Six phases, olivine, orthopyroxene, clinopyroxene, garnet, plagioclase and spinel, which can be related by the reactions: orthopyroxene+clinopyroxene+spinel +anorthite=garnet, and forsterite+anorthite=garnet occur together both in coronal and in equant textures indicative of equilibrium. Compositions of the respective minerals are typically Fo 25–72 , En 44–75 , En 30–44 Fs 9–23 Wo 47–49 , Pp 13–42 Alm 39–63 Gr 16–20 , An 29–49 and Sp 16–58 . When they occur in the same rock, equant and coronal garnets are homogeneous and compositionally identical suggesting that chemical equilibrium may have been attained despite coronal textures. Extrapolating reactions in the simple CMAS system to granulite temperatures and making thermodynamic corrections for solid solutions gives equilibration pressures (using the thermometry of Bohlen et al. 1980b) ranging from about 6.5 kb in the Lowlands and southern Adirondacks to 7.0–8.0 kb in the Highlands for the assemblage olivine-plagioclase-garnet. These results are consistent with inferred peak metamorphic conditions in the Adirondacks (Valley and Bohlen 1979; Bohlen and Boettcher 1981). Thus the isobaric retrograde path suggested by Whitney and McLelland (1973) and Whitney (1978) for the formation of coronal garnet in olivine metagabbros may not be required. Application of the same equilibria gives >8.7 kb for South Harris, Scotland and 0.9 kb for the Nain Complex. Disagreement of the latter value with orthopyroxeneolivine-quartz barometry (Bohlen and Boettcher 1981) suggests that the use of iron-rich rocks (olivines ≧Fa 50 ) results in errors in calculated pressures.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47335/1/410_2004_Article_BF00371301.pd

Anorthosite-Farsundite Complexes in the Southern Part of the Grenville Province

Three major anorthosite-farsundile complexes in the Grenville province underlie about 50 per cent of a northeasterly area, about 200 km long and 60 km wide, situated north of Montreal and lying parallel to the St-Lawrence River. The plutonic complexes are surrounded by a highly varied, layered assemblage of granulite-facies country rocks, some of which, at least, are of sedimentary origin ("Grenville Supergroup"). The plutonites were emplaced diapirically and came to rest near the interface of the pre-Grenville basement and Grenville metasediments. Diapiric rising, or lateral spreading of parts of the complexes induced large scale deformation in the country rock. Granulite facies metamorphism took place before and during diapirism and lasted until about 1100 Ma ago. Retrograde metamorphism due to cooling at high pressure followed diapirism. The emplacement of the crystallising plutonites is thought to be the fundamental cause of the evolution of this area in terms of deformation and metamorphism. Résumé Trois grand complexes anorthosito-farsunditiques occupent, dans le sud de la province de Grenville, environ la moitié d'une bande de 200 km de long par 60 km de large, parallèle au Saint-Laurent, et située au nord de Montréal. Entre les complexes plutoniques se trouvent divers types de roches de faciès granulite dont certaines sont manifestement d'origine sédimentaire ("Supergroupe de Grenville"). Les plutonites sont montées par diapirisme jusqu'à la limite entre le socle pre-Grenvillien et sa couverture métasédimentaire. La montée diapirique et localement l'étalement latéral de ces complexes ont causé des déformations majeures dans les roches encaissantes. Le métamorphisme de faciès granulite a eu lieu avant et pendant le diapirisme et s'est arrêté il y a 1100 Ma. Un métamorphisme rétrograde correspondant à un refroidissement à haute pression a suivi le diapirisme. L'emplacement de masses plutonites en voie de consolidation est considéré comme la cause fondamentale de l'évolution de cette région en termes de déformation et de métamorphisme