The anorthositic tectonites of the Sierra de Maz (Western Sierras Pampeanas, Argentina): petrography, kinematics and its implication in the amalgamation of Cuyania and Western Gondwana

MARTINO, ROBERTO DONATO; GUERESCHI, ALINA BEATRIZ; VUJOVICH, GRACIELA I.; SFRAGULLA, JORGE ALBERTO; OTAMENDI, JUAN E.; TIBALDI, ALINA M.; CRISTOFOLINI, EBER

Mar del Plata

Congreso; GeoSur 2010: International Geological Congress on the Southern Hemisphere; 2010

Istituto Nazionale di Oceanografia e di Geofisica Sperimentale

In the Sierra de Maz, two massif-type anorthositic bodies are recognized: the Potrerillos body (12 x 1.2 km) and the new one La Espuela body (1 x 0.4 km), which is covered by the unconformable Paganzo basin. Both bodies have a lenticular shape on the map and a tabular shape in section. They are strongly deformed and shared the same foliation than the associated layered metamorphic rocks, composed of garnet gneisses, granites, amphibolites, quartzites, calcsilicate gneisses and pink marbles. In the Potrerillos body, the dominant rocks are fine-grained (< 2 mm) white foliated garnetiferous anorthosites. Main accessories are garnet, hornblende, biotite, muscovite, epidote, apatite, titanite and opaque minerals. Coarse-grained (< 4 mm) rocks also appear. In the La Espuela body, the rocks are mylonitic anorthosites composed of sigmoidal porphyroclasts of plagioclase (< 5 mm long) in a very fine-grained (< 0.5 mm) plagioclase matrix. Main accessories are epidote, biotite, muscovite, titanite, rutile, opaque minerals, apatite and hornblende. Mineral chemistry shows a plagioclase composition variation during the deformation, with a reverse zonation. Porphyroclasts varies from An40 (core) to An50 (rim), while in the matrix varies between An50 and An65 respectively. In the Potrerillos body, the main foliation is oriented N 340º-350º/65-90º E and the stretching lineation N 140º/50º. In the La Espuela body, the main foliation is oriented N 20º/85-90º E and the stretching lineation N 170º/45º. Regionally, both bodies appear with a bent pattern, convex toward the east, and internally the deformation gradient increases toward the western margin. The foliation is mylonitic and defined by the crystallographic and shape preferred orientation of the plagioclase and accessories. Using rotated porphyroclasts, a reverse movement with a sinistral component is deduced. In both bodies, two preferred necks directions of boudinage, N 20º/40-50º and N 110º/70-80º, form a “chocolate tablet” pattern. The foliation shows an anastomosed pattern in the Y-Z section, which changes to planar in the X-Z section. These features, also recognized in the associated metamorphic rocks, would imply a strong extension along the X-axis, but also a minor stretching along the Y-axis. All this fabric features would be probably produced by a sinistral convergent oblique shear (transpression), indicating flattening associated with the shear movement, with the upper plate as the active block. Several authors, based on petrology, geochemistry and Rodinia palaeogeographical reconstructions, suggested that the anorthosites were part of the late Mesoproterozoic Grenville province of Laurentia. The deformation and metamorphism of the anorthositic bodies were probably related with the emplacement of the Cuyania terrane against the Pacific margin of Gondwana during the Ordovician-Silurian Ocloyic phase of the Famatinian Orogeny. During this phase, part of the lower plate, represented by the Cuyania Grenvillian basement carrying the anorthositic tectonites, would have been slivered and accreted against the Pacific margin of Gondwana.