Extension-shortening cycles at San Carlos Massif (Sierras de Córdoba): a “pull-push” orogen at Eastern Pampean Ranges of Argentina?

MARTINO, ROBERTO DONATO; GUERESCHI, ALINA BEATRIZ

Brasilia

Simposio; Granulites & Granulites 2006; 2006

Universidad de Brasilia

The Sierras de Córdoba, belonging to the Eastern Pampean Ranges of Argentina, has been interpreted tectonically as an active margin at 535 Ma. They are composed by a Type I continental magmatic arc on the east, integrated by calcalkaline granitoids (555-525 Ma) and a metamorphosed accretional prism towards the west. Later a collision of the Pampia Terrane produced migmatization and Tipe S granitoids at ≈ 520 Ma, mainly in the accretional prism. This ensemble was named Eastern Pampean Arc or Pampean Mobile Belt. Detrital materials involved in this Pampean orogen were supplied from western Gondwana sources during Neo- and Paleoproterozoic time. The Pampean orogen would have been developed during Cambrian times along the Pacific margin of Gondwana, coeval with Ross-Delamerian orogenies that developed the Transantartic Mountains. The most high-grade rocks (Central Migmatic Belt, CMB), mainly in granulitic facies conditions, are represented by migmatites and Tipe S granitoids. This belt is oriented NNW, extended by 140 Km in length and 40 Km in width, only interrupted by the 40 Km wide Achala Devonian batholith which major axis is oriented NNE. The CMB has two extremes: the NW San Carlos Massif (SCM, > 1500 Km2) and the SE Yacanto Group, which extends near 1600 Km2. The SCM is predominantly integrated by homogeneous and heterogeneous migmatites composed by Pl – Qtz – Bt – Grt – Crd ± Kfs ± Sil. Often, the remainder of paleosomatic rocks is recognized. This ensemble is cut by Type S granitoids plutons of a few kilometers in diameter composed by Kfs – Pl – Qtz – Bt – Grt ± Al2SiO5 trimorphs. Shortly, the evolution of the SCM could be arranged in: (1) main M2 MP-HT metamorphism (5.5–6.5 Kb, ≈ 700°C), linked to a pre-D2 extension associated with emplacement of basic rocks. S2 is the main metamorphic foliation developed here. (2) Strongly postmetamorphic contractional non-coaxial deformation D2 reworks S2 in complex non-cylindrical F2 mesoscopic folds, and produces the imbrication of the entire SCM with top to the SW movements. Crustal thickening associated with this D2 event would be produced a second migmatic event M´2, mainly affecting high-grade rocks. (3) Isostatic exhumation and decompression of the SCM, associated with slight D3 extensional movements, develops delicate corona textures (M3). Thin S3 shear zones are present. (4) Contractional D4 event represented by megascopic asymmetric F3 folds, verging its S4 axial surfaces to the NE, folds S2 and F2. New coronas are developed related to the M4 cooling. (5) Final contractional D5 event localized deformation in thin S5 shear zones. Phyllosilicates due to hydration M5 are recognized throughout the rocks. Thus, the tectonic evolution of the SCM implies at least two cycles of extension–shortening: pre D2-D2 and D3-D4 respectively, and a hemicycle of D5 with shortening. Such cycles could be interpreted like “pull-push” cycles mirroring the tectonic processes of subduction and collision of Pampia terrane against the Gondwana margin. This cycles would had controlled the evolution not only of San Carlos Massif but probably the entire evolution of the Eastern Pampean Ranges, during the Neoproterozoic? – Early Palaeozoic times.