Cuyania: an exotic block to Gondwana - introduction

VUJOVICH, G.I., FERNANDES, L.A.D., RAMOS, V.A.

GONDWANA RESEARCH

St. Joseph´s Press

Lugar: Trivandrum; Año: 2004 vol. 7 p. 1005 - 1007

1342-937X

Accretion tectonics has made fundamental contributions to our understanding of mountain-building processes responsible for the growth and evolution of the continental crust. Derived from the major paradigm of Earth Sciences - Plate Tectonics – this branch of orogenic studies, whose bases were launched less than twenty years ago, has witnessed a fast development more recently, especially after it was realized that convergence between lithospheric plates is mostly oblique, resulting in large displacements parallel to plate boundaries. Nowadays it is well established that amalgamation of terranes by accretion tectonics is the most efficient process of continental growth taking place in orogenic belts. In the case of the Andes, the classical example of an Andean-type of belt, amalgamation of terranes by tectonic accretion is thought to be responsible for about forty percent of the continental growth of its basement since the end of the Neoproterozoic. A succession of proto-Andean margins has been recognized, from the present foreland basement to the continental margin. A series of active continental margins, with ancient magmatic arcs, and deformation belts developed along the protomargin of West Gondwana during the Paleozoic. Three large terranes, Pampia, Cuyania, and Chilenia, accreted to West Gondwana during Early Cambrian, Middle Ordovician, and Late Devonian times, and are preserved in the basement of the southern Central Andes. One of the most conspicuous terranes accreted to this margin is Cuyania, a basement block derived from Laurentia that was transferred to Gondwana during early Paleozoic times. In recent years a voluminous amount of work has been performed to characterize this basement, the sedimentary cover and its fauna, the active magmatic arc developed along the Gondwana margin, and the complex stages in the deformation that resulted in the docking and final amalgamation of Cuyania against the supercontinent of Gondwana. The Laurentia derivation was confirmed by paleomagnetic data, paleoclimatic evidence, faunal affinities, K-bentonites ages and provenance studies. Fundamental to these analyses were the age of detrital zircons in the cover, isotopic and geochemical studies on the basement, and the age of the deformation established in the lower and upper plates of the collision zone. To promote discussion on mechanisms of accretion tectonics, an international symposium on the “Accretion of Cuyania Microcontinent to the margin of Gondwana” was held in Porto Alegre between 13 and 14 of March 2003. This meeting, sponsored by Programa Sul-Americano (PROSUL) of the Conselho Nacional de Pesquisas (CNPq) of the Brazilian Government, set out to foster scientific exchange within South America. It brought together 30 geoscientists from Argentina, Brazil, Chile and Uruguay, who presented 18 contributions on the geology of the Cuyania composite terrane and related regions. The present special issue of Gondwana Research stems from this meeting and contains13 papers dealing with this subject (Fig. 1). A review of the state-of-art and the currently accepted concepts about the origin and evolution of the Cuyania composite terrane based on geological and paleontological data was presented by V.A. Ramos and J.L. Benedetto, respectively. They concluded that there is not enough evidence on which to question the Laurentian origin of Cuyania or its well-established history of drifting and final amalgamation to the margin of Gondwana, despite the fact that the northern boundary of this terrane is still to be determined precisely. The following three contributions deal with the nature and evolution of the crystalline basement of Cuyania, presenting new data on ages of crustal addition and areas of provenance, on physical conditions of high-T tectono-metamorphic episodes that affected these rocks, as well as on structural analysis and geological interpretation of geophysical information. G. Vujovich and co-workers, based on U-Pb ages in zircons, confirm the Middle Proterozoic age of the suprasubduction complex of the Pie de Palo basement and the Famatinian ages for the main metamorphic episode imprinted on them. Early Proterozoic Sm-Nd model ages obtained from northern Sierras Pampeanas at Sierra de Maz and Las Ramaditas are interpreted by C. Porcher and co-workers as evidence of a gondwanic signature for these rocks, suggesting that on these grounds the suture zone would be located between the Sierras de Umango and Maz, whereas on geophysical grounds the suture would seem to be located to the east of sierras de Maz-Espinal, coinciding with a deep-seated magnetic and gravimetric discontinuity. A. Sato and co-workers present U-Pb data confirming the Middle Proterozoic ages and discuss the long-lasting evolution of the Las Matras pluton in the San Rafael Block, further south in the Cuyania terrane. The next four papers discuss the geophysical characteristics of Cuyania. A. Rapalini and C. Cingolani present paleomagnetic data from the San Rafael Block and discuss three possible alternatives for the paleogeographic and tectonic setting of Cuyania during the Late Ordovician. C.J. Chernicoff and E. Zapettini define the boundaries between the Cuyania, Chilenia, Pampia and Patagonia terranes over La Pampa province in central Argentina, mostly based on aeromagnetic data. A. Introcaso and co-workers and F. Ruiz present gravimetric and magnetometric models for the northeastern portion of the Cuyania, Pampia and Famatina terranes, discussing the characteristics of these units and the location of their boundaries from the geophysical viewpoint. Out of the last four papers, three deal with the geology of the adjacent Pampia terrane, to the east, and one with the Chilenia terrane, to the west. The paper by J. Otamendi and co-workers examines the thermal and tectonic evolution of rocks from Sierra de Comechingones, interpreted in terms of continuous convergence along the margin of Gondwana from Upper Proterozoic to Devonian times. The contribution of P. González and others presents structural, metamorphic, and isotopic data from rocks of the Nogolí Complex, Sierra de San Luis, where the authors present evidence that the Famatinian orogeny has overprinted an old metamorphic basement. Ages and metamorphic conditions of the main events recorded by these rocks are presented and the most conspicuous Early- to mid-Ordovician tectono-metamorphic episode recognized by these authors is ascribed to collision of the Cuyania terrane. A. Steenken and co-workers deal with the provenance and cooling history of the rocks from Sierra de San Luis based on isotopic data. They propose an Amazonian origin for the Pampia terrane and question the correlation of the phyllitic belts of Sierra de San Luis with the Puncoviscana Formation, discussing the cooling history of these rocks on the basis of K-Ar muscovite ages. The last paper of this volume, by V. López and D. Gregory, deals with the provenance and evolution of the Guarguaráz Complex in the Chilenia terrane. Based on geochemical studies, they recognize the continental derivation of metasediments and MORB signature of interbedded volcanics that together with the tectonic and metamorphic history of evolution of these rocks lead the authors to propose that Chilenia is not an allochthonous terrane but represents the accretionary prism of the Famatinian arc.