- Email: [email protected]
The Proto Andean Margin of Gondwana
Goiidruaiia Research (Gondzuana Neiusletter Section) V. 2, No.4, p p . 685-686. 01999 International Association for Gondzuana Resmrch, Jnpan.
GNL BOOK REVIEW
The Proto Andean Margin of Gondwana A.B. Roy Department of Geology, Mohanlal Sukhudia University, Udaipiir 313 002, India R.J. Pakhiirst and C.W. Rapela (Eds.) The Proto-Andean
margin of Gondwana. The Geological Society London, Special Piiblication No. 142, 1998, ISBN: 1-86239-021-5, 384p. 17 Chapters, 165figtires, index. List Price: $125/-. Web site: http:// bookshop.geolsoc.org.irk
The early nineteen nineties witnessed the emergence of a new concept in the supercontinent framework and continent-continent relationship. Inspired by this novel idea, the Earth Scientists started looking for signatures of Grenvillian orogeny in far away continental blocks outside North America, presumably with the hope of reconstructing the Mesoproterozoic Rodinia supercontinent framework. At this stage E. H. Moores (1991) advocated the SWEAT (Southwest US- East Antarctica) concept. I. W. D. Dalziel (1991), however, drew attention to the possible North America (NAM)- South America (SAM) interaction via Laurentia-Gondwana collision.In order to solve the problem of opening of the southern Iapetus Ocean, there were suggestions about Taconic continent-con tinent collision and simultaneous evolution of the tectonic belts which are now parts of the South American continent. The unique concept received a big boost with the discovery that the ‘Argentine Precordillera’ (AP) in the south-western part of South America is a n allochthonous bell (a siispcct terrane),
originally detached from Laurentia and accreted to the southwestern margin of the Gondwana. It was but natural that the Precordillera would attract immediate international attention. In the 1995 Penrose Conference, there was an overall consensus amongst the participants about the allochthonous status of the Precordillera of western Argentina. The concensus was destined to have a farreaching palaeogeographic and tectonic implication. In October, 1996, an international symposium was held at Buenos Aires, Argentina, under the joint IGCP projects 345 and 376, where the latest results from various Palaeozoic provinces were discussed. Most of the papers included in the volume under review were presented in the Buenos Aires symposium. Although the region constituting the theme of the volume forms a part of Gondwana supercontinent, yet the territory as well as its geological history appear quite exotic to the distant ’Gondwana people’. The editors anticipating this difficulty, provided the necessary geographical, geological a n d tectonic background of the terrane in the introduction. Further, to help the outside audience understand the Precordillerian scenario, the different terminology have been suitably explained, particularly the names of the orogenic cycles used in South American geology with an indication of their probable equivalents in Europe and North America. The basic issues discussed in different papers in the volume are the timing of transfer of the siispect terrune known as the Argentine Precordillera, and the events and processes involved in its arrival and collision with the Gondwana authochthon. Apart from different perceptions on the time aspect, different works are clearly divided on the tectonics involved in the process of transfer and accretion. Two major hypotheses have been suggested in regard to the tectonics: (1) the terrane hypothesis suggesting that the AP is a rifteddrifted microcontinent, accreted to the western most margin of the Western Gondwana; and (2) the tectonic tracer theory of I.W.D. Dalziel, which suggests an initial continentcontinent collision, followed by rifting, detachment, and drifting of ’the Laurentia sliver’ and final accretion to the Gondwana. The microcontinent model is stoutly
686
advocated by J. L. Benedetto and R. A. Astini in their papers. Benedetto, based on the evidence of strong 'provincialism' exhibited by brachiopods and associated shelly faunas, proposed a mid-Ordovician accretion. Astini advanced stratigraphical evidence in support of the riftingdrifting and collision of the 'Laurentia terrane'. Some confusion seems to prevail on the timing of accretion of AP to the Gondwana. In a paper highlighting slratigraphic evidence, M. Keller, W. Buggisch and 0.Lehnert while confirming that the AP is indeed a Laurentia-derived terrane, disagreed with the views of Benedetto and Astini on the question of timing of accretion. These authors favoured Carboniferous accretion of the Precordillera. P. W. Dickerson and M. Keller suggest severance of AP terrane (identified as the Cuyania block) from Laurentia before the onset of 'Taconic or Ocloyic' orogenesis. An interesting piece of evidence comes from the occurrence of Ordovician ash beds (bearing K-bentonites) in the AP, reported by W.D. Huff, S. M. Bergslvm, D. R. Kolata, C.A. Cingolani and R. A. Astini. These authors identified the Puna-Famatina terrane northwest of the Precordillera as the possible source of the K-bentonite ashes, thus linking it with the Gondwana plate magmatism. A very similar conclusion is reached by H. Bahlburg on the basis of his studies on the Ordovician turbidites. The similarity of the Famatinian orogenic belt in South America (SAM) and the Taconian-Appalachian orogeny in North America (NAM) impressed L. H. Dalla Salda, M. G. Lspez De Luchi, C.A. Cingolani and R.Varela to propose a common NAM-SAM collisional origin for both the orogenies. The authors also suggest that the Occidentalia terrane and the Proterozoic inliers in the western Famatinian belt are equivalent to the Grenville basement. Isotope data have been used by different workers to constrain the timing of diiferent events in the evolution of the Precordillera, from the pre-Grenvillian time to that of the final accretion to the Gondwana. V. A. Ramos, R.D. Dallmeyer and G. I. Vujovich used 4aAr/79Ar plateau ages in constraining a series of ductile deformation events during the early Palaeozoic evolution of the Precordillera. C. W.Rapela, R. J. Pankhrust, C. Casquet, E. Baldo, J. Saavedra, C. Galido and C. M. Fanning determined the sequence of the pre-Silurian metamorphic and magmatic events by using U-Pb dating on abraded zircons, U-Pb SHRIMP analysis, Rb-Sr whole rock and K-Ar determinations. SHRIMP zircon and monazite 2a6Pb/218U and 206Pb/232Th ages helped J. P.Sims, T. R. Ireland, A. Camacho, P. Lyons, P. F. Pieters, R. G. Skirrow, P. G. Stuart-Smith and R. Mirs to define three distinct regional tectonic events in the southern Sierras Pamp eanas. Petrological characterisation of the AP rocks is the theme of three papers. G. I. Vujovich and S . M. Kay mastered evidence of juvenile arc/back arc protoliths with Grenville
crystallisation and metamorphic ages in both the Pie de Palo Complex and the Precordillera basement. The authors have been able to trace out petrological signatures suggesting Mesoproterozoic collision of these terranes along the Laurentian margin. E. J. Lambias, A. M. Sato, A. Ortiz Suarez and C. Prozzi related granitoids of Sierra de San Luis to late Cambrian to early Ordovician Famatinian cycle. According to these workers the deformed tonalitic rocks are a part of an Early Famatinian magmatic arc, emplaced into shallow levels of the crust. J. Saavedra, A. Toselli, J. Rossi, E. Pellitero and F. Durand made a detailed review of the Famatinian magmatism. G. C. Grissom, S.M. DeBari and L.W. Snee documented tectono-thermal history of exhumed deep crustal rocks in the Sierra de Fiambala in northwest Argentina. This is the only paper which attempted detailed structural characterisation of rocks based on geological mapping and geometric studies of structural elements. Analysis of structtual, metamorphic and geochronologicaldata helped the authors in recognising two periods of metamorphism, deformation and magmatism corresponding to the Cambrian Pampean orogeiiy and Ordovician Famatinian orogeny. The authors have also recorded evidence of carboniferous reheating and retrograde metamorphism associated with emplacement of post-tectonic granites. In the concluding paper, R. J. Pankhurst, C. W. Rapela, J. Saavedra, E. Baldo, J. Dahlquist, I. Pascua and C. M. Fanning p u t forward evidence to prove that the Famatinian magmatic arc in the central Pampeans is an Early to Mid Ordovician continental arc on the Gondwana margin. These authors expressed serious reservation about the suggestion made by certain workers that Famatinian arc is totally an ocean closure related phenomenon. The volume presents a comprehensive account of the geology of the terrane which is now proved to be an allochthonous block originally detached from Laurentia continental block, having sailed across the extinct lnpetiis Ocean, and now moored safely between the Andean Cordillera in the west and the Gondwana foreland in the east. The book is bound to have an inspiring influence on the Earth Scientists interested in the tectonics related to the break-up and assembly of supercontinents. The editors deserve to be congratulated for a fairly good editorial job, keeping the lapses to the minimum, and the nice collation of papers which cover diverse fields and ideas. However, we hope that the book will encourage Earth Scientists working in that terrane to find palaeomagnetic evidence in support of this story of continental drift. The followers of global tectonics and processes of continental growth, and also others having a broad interest in the geological evolution of oceans and continents would certainly like to have this volume in their own shelves or in the libraries which they generally visit.
Gondzuana Research, V. 2,No.4, 1999
GNL BOOK REVIEW
The Proto Andean Margin of Gondwana A.B. Roy Department of Geology, Mohanlal Sukhudia University, Udaipiir 313 002, India R.J. Pakhiirst and C.W. Rapela (Eds.) The Proto-Andean
margin of Gondwana. The Geological Society London, Special Piiblication No. 142, 1998, ISBN: 1-86239-021-5, 384p. 17 Chapters, 165figtires, index. List Price: $125/-. Web site: http:// bookshop.geolsoc.org.irk
The early nineteen nineties witnessed the emergence of a new concept in the supercontinent framework and continent-continent relationship. Inspired by this novel idea, the Earth Scientists started looking for signatures of Grenvillian orogeny in far away continental blocks outside North America, presumably with the hope of reconstructing the Mesoproterozoic Rodinia supercontinent framework. At this stage E. H. Moores (1991) advocated the SWEAT (Southwest US- East Antarctica) concept. I. W. D. Dalziel (1991), however, drew attention to the possible North America (NAM)- South America (SAM) interaction via Laurentia-Gondwana collision.In order to solve the problem of opening of the southern Iapetus Ocean, there were suggestions about Taconic continent-con tinent collision and simultaneous evolution of the tectonic belts which are now parts of the South American continent. The unique concept received a big boost with the discovery that the ‘Argentine Precordillera’ (AP) in the south-western part of South America is a n allochthonous bell (a siispcct terrane),
originally detached from Laurentia and accreted to the southwestern margin of the Gondwana. It was but natural that the Precordillera would attract immediate international attention. In the 1995 Penrose Conference, there was an overall consensus amongst the participants about the allochthonous status of the Precordillera of western Argentina. The concensus was destined to have a farreaching palaeogeographic and tectonic implication. In October, 1996, an international symposium was held at Buenos Aires, Argentina, under the joint IGCP projects 345 and 376, where the latest results from various Palaeozoic provinces were discussed. Most of the papers included in the volume under review were presented in the Buenos Aires symposium. Although the region constituting the theme of the volume forms a part of Gondwana supercontinent, yet the territory as well as its geological history appear quite exotic to the distant ’Gondwana people’. The editors anticipating this difficulty, provided the necessary geographical, geological a n d tectonic background of the terrane in the introduction. Further, to help the outside audience understand the Precordillerian scenario, the different terminology have been suitably explained, particularly the names of the orogenic cycles used in South American geology with an indication of their probable equivalents in Europe and North America. The basic issues discussed in different papers in the volume are the timing of transfer of the siispect terrune known as the Argentine Precordillera, and the events and processes involved in its arrival and collision with the Gondwana authochthon. Apart from different perceptions on the time aspect, different works are clearly divided on the tectonics involved in the process of transfer and accretion. Two major hypotheses have been suggested in regard to the tectonics: (1) the terrane hypothesis suggesting that the AP is a rifteddrifted microcontinent, accreted to the western most margin of the Western Gondwana; and (2) the tectonic tracer theory of I.W.D. Dalziel, which suggests an initial continentcontinent collision, followed by rifting, detachment, and drifting of ’the Laurentia sliver’ and final accretion to the Gondwana. The microcontinent model is stoutly
686
advocated by J. L. Benedetto and R. A. Astini in their papers. Benedetto, based on the evidence of strong 'provincialism' exhibited by brachiopods and associated shelly faunas, proposed a mid-Ordovician accretion. Astini advanced stratigraphical evidence in support of the riftingdrifting and collision of the 'Laurentia terrane'. Some confusion seems to prevail on the timing of accretion of AP to the Gondwana. In a paper highlighting slratigraphic evidence, M. Keller, W. Buggisch and 0.Lehnert while confirming that the AP is indeed a Laurentia-derived terrane, disagreed with the views of Benedetto and Astini on the question of timing of accretion. These authors favoured Carboniferous accretion of the Precordillera. P. W. Dickerson and M. Keller suggest severance of AP terrane (identified as the Cuyania block) from Laurentia before the onset of 'Taconic or Ocloyic' orogenesis. An interesting piece of evidence comes from the occurrence of Ordovician ash beds (bearing K-bentonites) in the AP, reported by W.D. Huff, S. M. Bergslvm, D. R. Kolata, C.A. Cingolani and R. A. Astini. These authors identified the Puna-Famatina terrane northwest of the Precordillera as the possible source of the K-bentonite ashes, thus linking it with the Gondwana plate magmatism. A very similar conclusion is reached by H. Bahlburg on the basis of his studies on the Ordovician turbidites. The similarity of the Famatinian orogenic belt in South America (SAM) and the Taconian-Appalachian orogeny in North America (NAM) impressed L. H. Dalla Salda, M. G. Lspez De Luchi, C.A. Cingolani and R.Varela to propose a common NAM-SAM collisional origin for both the orogenies. The authors also suggest that the Occidentalia terrane and the Proterozoic inliers in the western Famatinian belt are equivalent to the Grenville basement. Isotope data have been used by different workers to constrain the timing of diiferent events in the evolution of the Precordillera, from the pre-Grenvillian time to that of the final accretion to the Gondwana. V. A. Ramos, R.D. Dallmeyer and G. I. Vujovich used 4aAr/79Ar plateau ages in constraining a series of ductile deformation events during the early Palaeozoic evolution of the Precordillera. C. W.Rapela, R. J. Pankhrust, C. Casquet, E. Baldo, J. Saavedra, C. Galido and C. M. Fanning determined the sequence of the pre-Silurian metamorphic and magmatic events by using U-Pb dating on abraded zircons, U-Pb SHRIMP analysis, Rb-Sr whole rock and K-Ar determinations. SHRIMP zircon and monazite 2a6Pb/218U and 206Pb/232Th ages helped J. P.Sims, T. R. Ireland, A. Camacho, P. Lyons, P. F. Pieters, R. G. Skirrow, P. G. Stuart-Smith and R. Mirs to define three distinct regional tectonic events in the southern Sierras Pamp eanas. Petrological characterisation of the AP rocks is the theme of three papers. G. I. Vujovich and S . M. Kay mastered evidence of juvenile arc/back arc protoliths with Grenville
crystallisation and metamorphic ages in both the Pie de Palo Complex and the Precordillera basement. The authors have been able to trace out petrological signatures suggesting Mesoproterozoic collision of these terranes along the Laurentian margin. E. J. Lambias, A. M. Sato, A. Ortiz Suarez and C. Prozzi related granitoids of Sierra de San Luis to late Cambrian to early Ordovician Famatinian cycle. According to these workers the deformed tonalitic rocks are a part of an Early Famatinian magmatic arc, emplaced into shallow levels of the crust. J. Saavedra, A. Toselli, J. Rossi, E. Pellitero and F. Durand made a detailed review of the Famatinian magmatism. G. C. Grissom, S.M. DeBari and L.W. Snee documented tectono-thermal history of exhumed deep crustal rocks in the Sierra de Fiambala in northwest Argentina. This is the only paper which attempted detailed structural characterisation of rocks based on geological mapping and geometric studies of structural elements. Analysis of structtual, metamorphic and geochronologicaldata helped the authors in recognising two periods of metamorphism, deformation and magmatism corresponding to the Cambrian Pampean orogeiiy and Ordovician Famatinian orogeny. The authors have also recorded evidence of carboniferous reheating and retrograde metamorphism associated with emplacement of post-tectonic granites. In the concluding paper, R. J. Pankhurst, C. W. Rapela, J. Saavedra, E. Baldo, J. Dahlquist, I. Pascua and C. M. Fanning p u t forward evidence to prove that the Famatinian magmatic arc in the central Pampeans is an Early to Mid Ordovician continental arc on the Gondwana margin. These authors expressed serious reservation about the suggestion made by certain workers that Famatinian arc is totally an ocean closure related phenomenon. The volume presents a comprehensive account of the geology of the terrane which is now proved to be an allochthonous block originally detached from Laurentia continental block, having sailed across the extinct lnpetiis Ocean, and now moored safely between the Andean Cordillera in the west and the Gondwana foreland in the east. The book is bound to have an inspiring influence on the Earth Scientists interested in the tectonics related to the break-up and assembly of supercontinents. The editors deserve to be congratulated for a fairly good editorial job, keeping the lapses to the minimum, and the nice collation of papers which cover diverse fields and ideas. However, we hope that the book will encourage Earth Scientists working in that terrane to find palaeomagnetic evidence in support of this story of continental drift. The followers of global tectonics and processes of continental growth, and also others having a broad interest in the geological evolution of oceans and continents would certainly like to have this volume in their own shelves or in the libraries which they generally visit.
Gondzuana Research, V. 2,No.4, 1999