La Espuela mylonitic anorthosite (Western Sierras Pampeanas of Argentina): petrography, structure and tectonic hypothesis

MARTINO, ROBERTO D.; GUERESCHI, A.B.; VUJOVICH, G.I; SFRAGULLA, J.A.; OTAMENDI, J.; TIBALDI, A.

Foz de Iguazú, Brasil

Congreso; The Meeting of the Americas 2010; 2010

American Geophysical Union (AGU)

In the Sierras de Maz and del Espinal (Western Sierras Pampeanas, Argentina), two Neoproterozoic massif-type anorthosites were described petrologically and geochemically. Recently we study a third body at the E-W La Espuela creek, on the eastern margin of the Sierra de Maz, totally covered by the unconformably Carboniferous-Permian Paganzo basin. The kilometric-scale La Espuela body is tabular in section and lenticular on the map. It is included in a strongly deformed metamorphic basement composed of quartz-feldspathic gneisses, mylonitized red granites and amphibolites. In the western tract, the rocks are strongly deformed (laminated), while in the central to eastern tracts have an anastomosed pattern with less deformed metric boudins, whose necks are oriented N 134º/60º. In the western tract, fine banded (< 2 mm wide) blastomylonitic anorthosites are composed of white plagioclase-rich bands in a fine grained (< 1 mm) polygonal to elongate granoblastic texture; and light gray bands composed of plagioclase, epidote, muscovite and biotite in a fine-grained (< 0.5 mm) granolepidoblastic texture. In the central to eastern tract, the yellowish white mylonitic anorthosites are composed of sigmoidal porphyroclasts of plagioclase (< 5 mm long) in a very fine-grained (< 0.5 mm) plagioclase matrix. Main accessories are epidote, biotite and muscovite. Minor accessories common to both types of rocks are 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. The mylonitic foliation, oriented N 20º/85-90º E, is defined by the crystallographic and shape preferred orientation of plagioclase and accessories, together with a pervasive stretching lineation, oriented N 170º/45º. Using rotated porphyroclasts, a reverse movement with a sinistral component is deduced. The boudinage would indicate an associated extension parallel to the Y-axis. Laminated to anastomosed nature of the mylonitic foliation show a west-sense deformation gradient. All this fabric features would be probably produced by a convergent oblique shear with the upper plate as the active block. Based on Rodinia palaeogeographical reconstructions, several authors suggested that massif-type anorthosites were part of a large anorthosite province in the late Mesoproterozoic, petrologically and geochemically comparable with the Grenville province of Laurentia. The mylonitization age, roughly coincident with the metamorphism age of these bodies, is 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. In the course of this phase, part of the Cuyania Grenvillian basement (lower plate), would be slivered and accreted against the Pacific margin of Gondwana.