Union [U]

 CC:715A  Monday  1630h

Appalachian Connections Through the Americas I

Presiding:  S M Barr, Acadia University; C R van Staal, Natural Resources Canada


The Basement of the Andes: the Gondwana-Laurentia Connections Revisited

* Ramos, V A (andes@gl.fcen.uba.ar), Laboratorio Tectonica Andina Universidad de Buenos Aires, Ciudad Universitaria Pab. 2, Buenos Aires, 1428, Argentina

The research performed in the last decade in the basement of the Andes have shown that the Precambrian and Paleozoic rocks have recorded a series of igneous and metamorphic events through time. These episodes can be grouped in discrete orogenic events, which have different paleogeographic distribution and intensity. The first and most important orogenic event is widely distributed along the margin and correspond to the Sunsas-Grenville orogen. Evidence of metamorphism and associated magmatic rocks are found from Colombia to the southernmost Patagonia. This episode produced the amalgamation of Amazonia, Pampia and Patagonia, among other cratonic blocks, to form Rodinia. The Rodinia break-up leaved several cratonic blocks accreted in the Gondwana side, such as Marañón, Arequipa, and Antofalla, although the generalized extension of this period produced crustal attenuation, rifted basins, and limited oceanic realms during late Proterozoic times. The Brasiliano-Pampean orogeny reamalgamated these blocks against the Gondwana margin. A new episode of break-up produced the dispersal of several Gondwanian blocks, separation along some previous sutures, crustal attenuation and magmatism in Late Cambrian times, until the new amalgamation occurred in Middle Late Ordovician times. These processes led to the Famatinian orogeny when metamorphism and arc magmatism was widely spread along the continental margin, as seen in Chibcha, Marañón, Arequipa and Sierras Pampeanas. Besides the re-accretion of some parautochthonous terranes, new exotic blocks were derived from Laurentia, such as the Cuyania terrane, which finally collided against the Andean proto-margin at ~ 460 Ma to form the Argentine Precordillera and surrounding regions. Late accretion in Early to Middle Devonian times of Chilenia and related terranes formed most of the basement of Central Andes. Final collision between Laurentia and Gondwana in the Late Carboniferous - Early Permian times to form the Alleghanides, left behind some Laurentian pieces like Tahami, Tres Lagunas and Tahuin terranes of Colombia, Ecuador and Perú. This set of rifting episodes and subsequent collisions along the continental margin of western South America were the result of changes of the absolute motion of Gondwana related to global plate reorganizations during Proterozoic to Paleozoic times.


Paleozoic Orogens of Mexico and the Laurentia-Gondwana Connections: an Update

* Ortega-Gutierrez, F (fortega@servidor.unam.mx), Ortega-Gutierrez, Fernando, Ciudad Universitaria, Instituto de Geologia, Universidad Nacional Autonoma de Mexico, Mexico, DF 04510, Mexico

The present position of Mexico in North America and the fixist tectonic models that prevailed prior to the seventies of the past century, have considered the main Paleozoic tectonic systems of Mexico as natural extensions of the orogens that fringed the eastern and southern sides of the Laurentian craton. Well known examples of pre-Mesozoic orogens in Mexico are the Oaxacan, Acatlan, and Chiapas polymetamorphic terranes, which have been correlated respectively with the Grenville and Appalachian-Ouachitan orogens of eastern North America. Nonetheless, several studies conducted during the last decade in these Mexican orogenic belts, have questioned their Laurentian connections, regarding northwestern Gondwana instead as the most plausible place for their birth and further tectonic evolution. This work pretends to approach the problem by briefly integrating the massive amount of new geological information, commonly generated through powerful dating methods such as LA-ICPM-MS on detrital zircon of sedimentary and metasedimentary units in the Paleozoic crustal blocks, which are widely exposed in southern and southeastern Mexico. The Acatlan Complex bears the closest relationships to the Appalachian orogenic system because it shows thermotectonic evidence for opening and closure of the two main oceans involved in building the Appalachian mountains in eastern Laurentia, whereas two other Paleozoic terranes in NW and SE Mexico, until recently rather geologically unknown, may constitute fundamental links between the Americas for the last-stage suturing and consolidation of western Pangea. The buried basement of the Yucatan platform (400,000 squared km) on the other hand, remains as one of the most relevant problems of tectonostratigraphic correlations across the Americas, because basement clasts from the Chicxulub impact ejecta reveal absolute and Nd-model ages that suggest close Gondwanan affinities. Major changes in the comprehension of the Paleozoic orogens in Mexico include the swift of the Acatlan Complex from Iapetus to Rheic scenarios, and the apparent continuation of the Ouachita belt across northern Mexico into south central Sonora, rather than displaced eastwards along the legendary Mojave-Sonora megashear. And yet, poorly known suture-related lithotectonic associations of Paleozoic metamorphic rocks and arc granitoids that underlie the eastern margin of Mexico, have not been explained by existing models dealing with the Appalachian-Mexico-Gondwanan connections.


Neoproteozoic to Cambrian Tectonic Evolution of the Proto-Andean Margin of Gondwana: Implications for the Opening of Iapetus

* van Staal, C R (cvanstaa@nrcan.gc.ca), Geological Survey of Canada, 625 Robson Street, Vancouver, BC V6B 5J3, Canada
Escayola, M (mescayol@NRCan.gc.ca), Conicet, 601 Booth Street, Ottawa, ON K1A 0E8, Canada
Davis, B (bidavis@nrcan.gc.ca), Geological Survey of Canada, 601 Booth Street, Ottawa, ON K1A 0E8, Canada

We present a new tectonic model for the Pampean accretion of the Arequipa-Antofalla-Pampia (AAP) ribbon continent to the Proto-Andean margin of Gondwana represented by the Amazonia and Rio de La Plata cratons, based on our studies of the Puncoviscana Formation in northern and central Argentina. A compilation of existing detrital zircon ages of the Puncoviscana Formation and correlative units along strike in the Pampean orogenic belt to the south combined with our new U-Pb SHRIMP zircon ages of Puncoviscana Formation felsic tuffs and mafic volcaniclastic rocks (c. 531 Ma) suggests this unit mainly represents an Early Cambrian arc- trench gap to foreland basin succession formed during east-directed closure of a late Neoproterozoic marginal basin. The marginal basin, which probably remained relatively narrow, initially had opened behind an east- facing 650-570 Ma island arc (eastern Pampia arc), built upon the rifted, leading edge of the AAP. The 531 Ma felsic tuffs are interpreted to represent the products of a new, short-lived Early Cambrian magmatic arc built upon the now composite Proto-Andean margin, following Late Neoproterozoic, soft-accretion of the eastern Pampia arc and a subduction polarity reversal. Puncoviscana Formation conglomerates previously interpreted as early rift-related deposits are better interpreted as late-orogenic basin fills and/or were deposited after basin closure. Our new U-Pb zircon age of the post-collision Canani tonalite (c. 517 Ma), which intruded into Tilcarian deformed Puncoviscana Formation rocks in the north westernmost part of Argentina in the Puncoviscana type locality, combined with the existing 529-517 Ma zircon ages for post-collision peraluminous granites and tonalites in the Eastern Pampean Ranges to the south indicates that the synorogenic Puncoviscana Formation formed between 540 and 517 Ma, progressively cannibalizing its orogenic hinterland over time. In addition, the Tilcarian and Pampean orogenies represent the same event. We suggest that AAP rifted-off from Laurentia between 700 and 650 Ma, shortly after Amazonia's departure during Rodinia's break-up. We emphasize that it is the departure of AAP, not Amazonia that opened Iapetus in the Late Neoproterozoic. We also suggest that Ganderia in the northern Appalachians, originally formed an extension of the AAP.


New Constraints on Amazonian Versus West African Cratonic Source Components of the Peri-Gondwanan Sedimentary Basins

* Schofield, D I (dis@bgs.ac.uk), British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham, NG12 5GG, United Kingdom
Horstwood, M S (msah@bgs.ac.uk), NERC Isotope Geoscience Laboratories, Kingsley Dunham Centre, Keyworth, Notingham, NG12 5GG, United Kingdom

Unravelling the Palaeogeography of the Peri-Gondwanan terranes during the Early Palaeozoic is in part predicated on the interpretation of detrital zircon analyses from the continental margin sedimentary basins compared to potential source regions. Recent reviews of available detrital zircon data (e.g. Nance et al., 2008) reflect the strong control of the source regions with Gander, Avalonia, Carolina and the cratonic Central American terranes strongly biased toward an Amazonia-Baltica-Laurentia provenance, shown by the relative abundance of Mesoproterozoic zircon populations. In contrast, Meguma, Florida, Cadomia, Iberia and Bohemia show a relative absence of Mesoproterozoic populations and a bias toward palaeoproterozoic populations characteristic of the Eburnean orogeny of the West African Craton. However, understanding of the age compositions of the source areas is often based on sparse, often quite old data. Here, the source regions of the West African Craton are reviewed in light of new U-Pb LA-MC-ICP-MS data from the Reguibat Shield and adjacent Mauritenide orogenic belt, and the implications for the relative dispersal of the peri-Gondwanan terranes considered. Present understanding of the West African source region is based largely on the, historically more economic and better studied, Man Shield of equatorial West Africa. In common with the Man Shield, the Reguibat comprises a western 'Archaean' domain and an eastern Palaeoproterozoic or 'Eburnean' domain. Published U-Pb zircon age data from the western domain show that the Mesoarchaean nuclei in the SW of the shield is relatively abundant in ca. 2.95 and 2.9 Ga magmatic rocks, but preserves a record of earlier ca. 3.5-3.4 Ma magmatism and metamorphism. However, new data from the eastern margin of this domain indicate the presence of previously unrecorded extensive Neoarchaean granite migmatite terrane, making up approximately 25% of the exposed area of the shield, yielding ages between ca. 2.7 and 2.45 Ga. This is juxtaposed against the eastern 'Eburnean' domain along a narrow high strain zone that has yielded metamorphic and syn-tectonic intrusive ages between of c.2.1 Ga. The 'Eburnean' domain itself comprises an extensive region of plutonic rocks, making up approximately 50% of the shield, ranging from ca. 2.1 to 2.0 Ga in age. Following the Palaeoproterozoic orogen, the Reguibat Shield remained relatively stable, with only an episode of rigid extension and emplacement of zircon-poor dykes which has been dated at ca. 1.7 to 1.5 Ga by the Rb-Sr technique. A ca. 1.2-1.0 Ga, Grenvillian age event is also recognised from the Mauritenide Belt, although new zircon ages date the main Neoproterozoic magmatic phase of the orogen at 0.77 to 0.63 Ga. These new data largely support the strongly bimodal detrital zircon record from the adjacent sedimentary basins. However, the newly recognised, extensive, Neoarchaean source area may have implications for the palaeogeography of terranes that have yielded detrital zircon peaks of a similar age. In particular Cadomia- Bohemia, which may have developed immediately outboard of the northern margin of the West African Cratron. Reference: Nance R.D. and nine others, 2008. Neoproterozoic-early Palaeozoic tectonostratigraphy and palaeogeography of the peri-Gondwanan terranes: Amazonian v. West African connections. In The boundaries of the West African Craton, Ennih, W. and Liégeois J-P. (eds), Geological Society, London, Special Publication 297, 345-383.


Supercontinent Reconstruction from Accretionary History at Leading Continental Edges: An Example from the Northern Margin of Gondwana

* Murphy, J (bmurphy@stfx.ca), St. Francis Xavier University, Dept. of Earth Sciences, Antigonish, NS B2G 2W5, Canada
Nance, R (nance@ohio.edu), Ohio University, Dept. of Geological Sciences, Athens, OH 45701, United States
Gutierrez-Alonso, G (gabi@usal.es), Universidad de Salamanca, Departamento de Geología, Salamanca, SAL 33708, Spain
Keppie, J (duncan@servidor.unam.mx), Universidad Autonoma de Mexico, Instituto de Geología, Mexico, MEX o4510, Mexico

Repeated amalgamation and subsequent break-up of continental lithosphere have profoundly affected Earth's evolution since the Archean. Distinctive rift-related stratigraphy and magmatism followed by passive margin development along the trailing edges of dispersing continents have been used to identify such margins in the geologic past. Using the isotopic record of Mesozoic-Cenozoic igneous rocks of western North America as an analogue, we show that the leading edges of dispersing continents have Sm-Nd isotopic characteristics that may be used to identify these margins in the geologic past. The Sm-Nd isotopic signatures of Late Neoproterozoic and Early Paleozoic igneous rocks along the northern Gondwanan margin indicate derivation from 0.7 to 1.1 Ga mantle lithosphere. This lithosphere originated in the Mirovoi Ocean that surrounded Rodinia, accreted to the northern Gondwanan margin by ca. 650 Ma in response to Rodinia breakup, and provided a source for subsequent magmatism. The accretion and subsequent recycling of oceanic mantle lithosphere should be common along the leading edges of dispersing continents following supercontinent breakup. Identification of this phenomenon should therefore provide an additional aid in paleocontinental reconstructions.


U-Pb Ages From Detrital Zircon in Avalonian Sedimentary Rocks: Temporal Changes in Provenance Tied to Terrane Migration?

* Samson, S (sdsamson@syr.edu), Syracuse University, Dept. Earth Sciences, Syracuse, NY 13244, United States
Hamilton, M (mahamilton@geology.utoronto.ca), University of Toronto, Jack Satterly Geochronology Lab, Toronto, ON M5S 3B1, Canada
Barr, S (sandra.barr@acadiau.ca), Acadia University, Depart. Earth and Environmental Science, Wolfville, NS B4P 2R6, Canada
White, C (whitece@gov.ns.ca), Dept. of Natural Resources, P.O. Box 698, Halifax, NS B3J 2T9, Canada
Satkoski, A (amsatkos@syr.edu), Syracuse University, Dept. Earth Sciences, Syracuse, NY 13244, United States

The Avalon microcontinent in the northern Appalachian orogen originated near the margin of the supercontinent Gondwana, but its position along that extensive margin, and its timing of separation, remain disputed. Avalonia is characterized by Neoproterozoic - Cambrian clastic sedimentary sequences. Detrital zircon ages from these sedimentary units may provide constraints on the locations of the terrane prior to its accretion to Laurentia. U-Pb ages have been obtained for detrital zircon from units with depositional ages ranging from c. 630 Ma to c. 505 Ma. The oldest sample, from the Hammondvale Metamorphic Suite (HMS) in southern New Brunswick, contains zircon as young as 630 Ma, providing a maximum depositional age. The dominant Neoproterozoic zircon population has an age of 682 Ma, which likely represents the age of the main sediment source, but the sample also contains a few older Neoproterozoic grains (approaching 800 Ma). Importantly, the HMS sample also contains relatively abundant 1.9 - 1.0 Ga zircon, but no zircon with ages between 2.9 and 1.9 Ga. In contrast, published data from quartzite clasts from a conglomerate thought to be deposited at c. 550 Ma in New Brunswick and Nova Scotia show different detrital zircon age patterns: the percentage of Mesoproterozoic grains is lower than in the HMS, and a population of 2.0 - 1.9 Ga grains is present. Thus the latest Precambrian appears to mark the beginning of an important change in sediment sources to Avalonia. A younger (c. 540 Ma) quartzite (Ratcliffe Brook Formation) reinforces this apparent change in provenance in that Mesoproterozoic zircons are even lower in abundance and the abundance of 2.1 - 1.9 Ga zircon is higher. Additionally, a new c. 800 Ma zircon population is noted. This new age peak may also reflect a fundamental shift in provenance, perhaps as a consequence of migration of the terrane along the Gondwanan margin. Two additional (c. 520 Ma) Cambrian samples also have also been investigated; the vast majority of zircon in the c. 520 Ma Glens Falls Formation is c. 600 Ma, suggesting derivation primarily from local sources. In contrast, the Sgadan Lake Formation, also 520 Ma, also shows a predominant peak at about 600 Ma but also has a secondary age peak at 2.1 - 1.9 Ga, in addition to minor Mesoproterozoic and Archean peaks. The age distribution most closely resembles that found in the quartzite clasts from the 540 Ma conglomerate. The youngest Avalonian sandstone analyzed is sandstone from the c. 505 Ma King Square Formation. The age pattern of detrital zircon for this unit is most similar to the quartzite from the Ratcliffe Brook Formation, except that the 2.1 - 1.9 Ga peak is less, and the 850-800 Ma more, pronounced. A clear temporal shift is seen from Mesoproterozoic dominant and 2.1 - 1.9 absent, towards a minimal Mesoproterozoic record, with a considerably more important 2.1 - 1.9 Ga peak and an increasing importance of 850 - 800 Ma zircon. Various scenarios can be envisioned to explain these results; however, we suggest that the changing ages of detrital zircon are tracking the movement of Avalonia from a South American Gondwanan location towards a West African location prior to its cross-ocean migration towards Laurentia. A similar migration pattern has been suggested for the Carolina terrane of the southern Appalachians, also based on changing age populations of detrital zircon.