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

Regional-scale Grenvillian-age UHT metamorphism in the Molleonde-Camana Block (basement of the Peruvian Andes).

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The cordierite-orthoamphibole rocks of the variscan dome of gavarnie-gedre-heas : the gedrite of Gèdre (Hautes Pyrénées, France)

The low-P/high-T Variscan metamorphic dome of Gavarnie-Gedre-Heas, Central Pyrenees, harbors a small (1 m(3)) but special cordierite and orthoamphibole-bearing enclave embedded within an anatectic granite. Compared to other cordierite-orthoamphibole occurrences described in the literature, the enclave shows singular characteristics: (1) its mineral composition is unique with cordierite (Crd)+gedrite (Ged) + biotitess (Bt) +staurolite (St) + garnet (Grt) + spinel (Spl) + corundum (Crn) + ilmenite (Ilm) and sulfides (Sulf), but lacking quartz (Qtz), plagioclase (Pl), and aluminosilicate minerals; and (2) its chemical composition, with low SiO2, high Al2O3, K2O, and TiO2 and high Fe2O3T/MgO. The Crd-Oam-bearing rocks also occur as layers in stromatic migmatites that show a mineral composition (Bt+Crd+Ged+Pl+Qtz+Ilm+Grt 6 St) and a chemical composition more akin to other described occurrences. The cordierite-orthoamphibole enclave records a succession of metamorphic reactions: (1) local Crd-Oam growth at the expense of Bt with release of K2O; (2) development of coronas of Crd 6 Ilm 6 Grt around St; (3) isochemical breakdown of St to Crd+Spl+Crn. In the Ged-bearing layers in interlayered migmatites, the formation of the Crd-Oam assemblage involves the consumption of Bt, Pl, and Qtz. Mass balance calculations show that in the enclave most reactions took place in an open system with addition of Si and release of K2O. Pseudosections indicate a clockwise P-T path, with high-temperature conditions (700-730 degrees C) at decreasing pressure (from 5.6 to 3 kbar). This fits the P-T path of other metamorphic domes of the Variscan Central Pyrenees

Rapport final / Granites noirs de l'Uruguay

Partie du texte en anglai

Rapport final / Granites noirs de l'Uruguay

Partie du texte en anglai