Showing papers on "Gondwana published in 1998"

Continuation of the Mozambique Belt Into East Antarctica: Grenville‐Age Metamorphism and Polyphase Pan‐African High‐Grade Events in Central Dronning Maud Land

Abstract: The about 500 km long coastal stretch of central Dronning Maud Land (DML), East Antarctica, is critical for understanding both Gondwana and Rodinia assembly. In common Gondwana reconstructions central DML lies at the potential southern extension of the Mozambique Belt. We report the first extensive geochronological study of magmatic and metamorphic rocks from the area. These new U‐Pb SHRIMP zircon and Sm‐Nd‐data of rocks sampled during the German international GeoMaud 1995/96 expedition indicate that the oldest rocks in central DML are Mesoproterozoic in age. The crystallization ages of metavolcanic rocks were determined at c. 1130 Ma. Syn‐tectonic granite sheets and plutons give ages of c. 1080 Ma, contemporaneous with metamorphic zircon growth at granulite facies conditions. An anorthosite intrusion and a charnockite are dated at c. 600 Ma. Subsequent metamorphism is recorded for at least two different episodes at c. 570–550 Ma and between 530 to 515 Ma. The latter metamorphic event reached granulite fa...

The Pampean Orogeny of the southern proto-Andes: Cambrian continental collision in the Sierras de Cordoba

Abstract: A detailed study of the pre-Silurian geology of the Sierras de Cordoba, Eastern Sierras Pampeanas, is used to define the sequence of magmatic and metamorphic events during the Pampean orogeny. This primarily involved Early to Mid-Cambrian subduction and terrane collision at the western margin of Gondwana during the amalgamation of the super-continent. Evidence for this is based principally on new information concerning (a) regional mapping and field relations, (b) analysis of the structures, deformational history and meta-morphic evolution and (c) geochronology and geochemistry of the igneous and metamorphic rocks. The main events recognized are (1) Late Proterozoic break-up of Rodinia (Nd model ages of 1500 ± 200 Ma, inherited zircons 800–1400 Ma), (2) development of an Early Cam-brian passive margin sequence (Puncoviscana Formation and equivalents), (3) emplacement of metaluminous calc-alkaline granitoids (G1a, dated at 530 ± 3 Ma) as a result of NE-directed subduction, (4) crustal thickening, ophiolite obduction, compression and high-grade metamorphism (M2: 8.6±0.8 kbar, 810 ± 50°C, c.525 Ma) related to collision, and culmina-ting in (5) isothermal uplift and widespread low-P anatexis (M3, 4.0±0.5 kbar, 715 ± 15°C, c.520 Ma). The last event is responsible for the linked generation of highly peraluminous granites (G1b) and cordieritites. Subsequent emplacement into the accreted terrane of Ordovician trondhjemite-tonalites (500-470 Ma) and dextral wrench shear are interpreted as inner cordilleran counterparts of the Famatinian arc, which developed to the west along the newly-formed proto-Andean margin.

Predatory Dinosaur Remains from Madagascar: Implications for the Cretaceous Biogeography of Gondwana

Abstract: Recent discoveries of fossil vertebrates from the Late Cretaceous of Madagascar include several specimens of a large theropod dinosaur. One specimen includes a nearly complete and exquisitely preserved skull with thickened pneumatic nasals, a median frontal horn, and a dorsal projection on the parietals. The new materials are assigned to the enigmatic theropod group Abelisauridae on the basis of a number of unique features. Fossil remains attributable to abelisaurids are restricted to three Gondwanan landmasses: South America, Madagascar, and the Indian subcontinent. This distribution is consistent with a revised paleogeographic reconstruction that posits prolonged links between these landmasses (via Antarctica), perhaps until late in the Late Cretaceous.

Development of the early Paleozoic Pacific margin of Gondwana from detrital-zircon ages across the Delamerian orogen

Abstract: Detrital-zircon age spectra have been determined for sedimentary rocks from the Delamerian orogen, southern Australia. In Neoproterozoic sedimentary rocks, patterns progressively change from Mesoproterozoic- to Neoproterozoic-dominated detritus and there are few zircons that are close to the depositional age. The base of the Cambrian Kanmantoo Group marks an abrupt change in provenance to detrital patterns dominated by Ross and Delamerian (600–500 Ma) and Grenvillean ages (1200–1000 Ma). These patterns are strikingly similar to those obtained from Lachlan fold belt sedimentary rocks, indicating that the sedimentation recorded in the Kanmantoo Group marks a change from deposition of sediments derived from the Australian cratons to those representative of the early Paleozoic Gondwana mudpile. If sedimentary rocks with zircon-provenance characteristics such as those of the Kanmantoo rocks extend under elements of the Lachlan fold belt, they would provide suitable protoliths for the S-type granites of southeastern Australia.

Tracing the Indian Ocean Mantle Domain Through Time: Isotopic Results from Old West Indian, East Tethyan, and South Pacific Seafloor

Abstract: The isotopic difference between modern Indian Ocean and Pacific or North Atlantic Ocean ridge mantle (e.g. variably lower 206Pb/204Pb for a given eNd and 208Pb/204Pb) could reflect processes that occurred within a few tens of millions of years preceding the initial breakup of Gondwana. Alternatively, the Indian Ocean isotopic signature could be a much more ancient upper-mantle feature inherited from the asthenosphere of the eastern Tethyan Ocean, which formerly occupied much of the present Indian Ocean region. Age-corrected Nd, Pb, and Sr isotopic data for 46–150 Ma seafloor lavas from sites in the western Indian Ocean and ocean-ridge-type Tethyan ophiolites (Masirah, Yarlung–Zangpo) reveal the presence of both Indian-Ocean-type compositions and essentially Pacific–North Atlantic-type signatures. In comparison, Jurassic South Pacific ridge basalts from Alexander Island, Antarctica, possess normal Pacific–North Atlantic-type isotopic ratios. Despite the very sparse sampling of old seafloor, the age-corrected eNd(t) values of the old Indian Ocean basalts cover a greater range than seen for the much more thoroughly sampled present-day spreading axes and islands within the Indian Ocean (e.g. 18 eNd units for basalts in the 60–80 Ma range vs 15 eNd units for 0–10 Ma ones). The implications of these results are that the upper mantle in the Indian Ocean region is becoming increasingly well mixed through time, and that the Indian Ocean mantle domain may not greatly pre-date the age of earliest spreading in the Indian Ocean.

U-Pb, Th-Pb and Ar-Ar geochronology from the southern Sierras Pampeanas, Argentina: implications for the Palaeozoic tectonic evolution of the western Gondwana margin

Abstract: Abstract New SHRIMP zircon and monazite 206Pb/238U and 208Pb/232Th ages on structurally controlled units and 40Ar-39Ar step-heating ages from shear fabrics, define three distinct regional tectonic events in the southern Sierras Pampeanas. The first, the Pampean orogeny, involved closure of a late Neoproterozoic basin on the western margin of Gondwana. New rims on detrital zircons and concurrent monazite growth suggest that the metamorphic peak was attained by c. 530 Ma. The second event, the Famatinian orogeny, marks the initiation of eastward-dipping subduction on the western Gondwana margin, and may represent a continuation of the earlier Pampean event. Metasedimentary rocks from the Sierras de San Luis have zircons with a predominantly Early Cambrian detrital age, indicating a Pampean source. The metamorphic peak in these rocks was contemporaneous with the emplacement of felsic, mafic and ultramafic rocks at c. 480 Ma in a collisional setting. Monazite ages and limited new zircon growth in the metasedimentary rocks suggest that the Famatinian orogeny had ceased by about 450 Ma. This correlates well with a 450–460 Ma Ar-Ar age for late shearing in the southern sierras of La Rioja province. The third tectonic event, the Achalian orogeny, involved W-directed compression and emplacement of multiple, voluminous, granite intrusions. Deformation during this event was partitioned between discrete shear-zones and regions of open to tight folding. The shear zones alternate between W-directed thrusts and NNW-trending, sinistral shear-zones. Ar-Ar data from the low-grade shear fabrics indicate that transpressional deformation continued through most of the Devonian.

Early evolution of the Proto-Andean margin of South America

Abstract: From a detailed study of a 500 km transect in the Sierras Pampeanas, central-west Argentina, two pre-Silurian tectono-magmatic episodes are recognized and defined, each culminating in micro-continental collisions against the proto-Andean margin of Gondwana. The Pampean orogeny started in Early Cambrian time with short-lived subduction, indicated by ca. 535 Ma calc-alkaline granitoids. Following Pampean terrane collision, burial to granulite facies conditions (ca. 9 kbar) generated widespread migmatites and ca. 520 Ma highly peraluminous granites in the Eastern Sierras Pampeanas. After brief quiescence, a second major episode, the Famatinian orogeny, started with subduction ca. 490 Ma, forming a wide continental arc and ensialic backarc basin. This heralded the approach of Laurentia to Gondwana, during which the Precordillera terrane separated from the southern Appalachian region, finally colliding with Gondwana in Silurian–Devonian time.

Duration of the Early Cambrian: U-Pb ages of volcanic ashes from Avalon and Gondwana

Abstract: Volcanic zircons from three ashes give a U-Pb date of 511 ± 1 Ma on trilobite-bearing, upper Lower Cambrian (upper Branchian Series) strata of southern New Brunswick that correlate into the Siberia...

Orogenias paleozóicas no domínio sul-ocidental do gondwana e os ciclos de subsidência da bacia do paraná

Abstract: The geological development of the Parana Basin was influenced by the geodynamics of southwestern Gondwana, a domain continuously affected during almost all the Phanerozoic eon by compressional stresses derived from a persistently active convergent motion between the continental block and the oceanic lithosphere of Panthalassa. The Parana Basin was supported by a cratonic basement since its inception, but had in its neighbourhood evolving collisional belts fringed by foreland basins. Some areas were selected representing the history of subsidence along the foreland domain of southwestern Gondwana during Paleozoic times. The subsidence analysis showed that such foreland basins experienced cycles of accelerated subsidence that coincide in time with the major orogenic phases, that were related to the docking of terranes along the margin of the continent. The computation of average subsidence rates revealed the main subsidence cycles for the region as a whole. Subsidence and sediment accumulation in the Parana Basin started during Middle to Late Ordovician times when the Precordillera terrane collided against Gondwana and produced the different contractional phases of the Ocloyic Orogeny. The intraplate response to the compressional stresses related to this orogenic cycle was transtensional reactivation of weakness zones, providing the initial subsidence for the Parana Basin. Repeatedly during the geologic history of the Parana Basin orogenic cycles left their signature as periods of accelerated subsidence. Subsidence plots revealed that Early Devonian times, when the stresses generated by the Precordilleran Orogeny affected Gondwana, and Late Permian times, under the yoke of the Sanrafaelic Orogeny, were periods when intracratonic subsidence rates increased remarkably. An integrated analysis of the sedimentary record of the Parana Basin, considering eustatic variations of the sea level and subsidence cycles of southwestern Gondwana led to the conclusion that the stratigraphic cyclicity observed in the Parand Basin was ultimately influenced by its subsidence history. The presumed global correlation peaks shown in Vail's curve, of Late Silurian, Early Carboniferous and Early Permian ages, are not present in the Parana Basin. Instead, local maximum flooding levels developed in each one of the second order transgressive cycles, during Early Silurian, Early Devonian and Late Permian times, defining the particular subsidence history of this interior basin as an intraplate response to geodynamic processes affecting southwestern Gondwana margin.

Late Permian to Late Triassic palaeomagnetic data from Iran: constraints on the migration of the Iranian block through the Tethyan Ocean and initial destruction of Pangaea

Abstract: SUMMARY A palaeomagnetic study of Late Permian to early Jurassic rocks from the Alborz and Sanandaj‐Sirjan zones in Iran and a compilation of selected palaeopoles from the Carboniferous to the present provide an updated history of the motion of the Iranian block within the Tethys Ocean. The Iran assemblage, part of Gondwana during the Palaeozoic, rifted away by the end of the Permian. We ascertain the southernhemisphere palaeoposition of Iran at that time using magnetostratigraphy and show that it was situated close to Arabia, near to its relative position today. A northward transit of this block during the Triassic is shown, with an estimated expansion rate of the Neotethyan ridge of 100‐140 km Myr’1. The northward convergence with respect to Eurasia ended during the Ladinian (Middle Triassic), and is marked by a collision in the northern hemisphere with the Turan platform, which was the southern margin of the Eurasian continent at that time. No north‐south component of shortening is evidenced north of Iran afterwards. An analysis of the declinations from the Late Permian to the present shows diVerent, large rotations, emphasizing the important tectonic phases suVered since the Triassic. Finally, we propose palaeomagnetic reconstructions of the Tethys area during the Late Permian and the Late Triassic, showing that the Palaeotethys Ocean was narrower than previously thought, and did not widen its gate to the Panthalassa before the Triassic period.

The proto-Andean margin of Gondwana: an introduction

Abstract: A basic background is presented for the discussion of the Early Palaeozoic geology of western Argentina covered by this book. This includes the definition and terminology of orogenic cycles on this part of the Gondwana margin, represented by the Eastern Sierras Pampeanas. The Pampean orogeny (Early Cambrian) relates to an intense but short-lived period of terrane collision predating the rifting of the Precordillera terrane from Laurentia. The Famatinian cycle is predominantly represented by intense subductionrelated magmatism of Early-Middle Ordovician age, developed on the continental margin of Gondwana during the rifting and drifting of the Precordillera terrane. The Grenvillian basement of the latter is further exemplified by a new Rb-Sr whole-rock isochron age of 1021 ± 12 Ma for orthogneisses from the Western Sierras Pampeanas. A mid-Ordovician granite in this area (dated at 481 ± 6 Ma by U-Pb ion microprobe data) may be related to rifting while the Precordillera terrane was still attached to Laurentia. A divergence of opinion is pointed out between some authors in this book who favour mid-Ordovician collision of the Precordillera with Gondwana, and others who place it much latter, in Silurian or Devonian times.

Geochronology and geochemistry of pre-Jurassic superterranes in Marie Byrd Land, Antarctica

Abstract: Marie Byrd Land, Antarctica, is a major part of the proto-Pacific supercontinental margin. On the basis of new geochronological and geochemical data relating to its pre-Jurassic evolution, Marie Byrd Land is subdivided into western or interior (“Ross”) and eastern or exterior (“Amundsen”) provinces, equivalent to two superterranes in New Zealand. The Ross province is characterized by Cambrian? metagraywackes and I-type orthogneiss dated at 505±5 Ma by U-Pb SHRIMP (Sensitive High Resolution Ion Microprobe). Its magmatic record consists of Devonian-Carboniferous (375±5 Ma and circa 339±6 Ma), predominantly I-type granitoids, and further minor granitic magmatism in Permo-Triassic times. This Paleozoic history is comparable to that of the Gondwana margin in northern Victoria Land, western New Zealand, and SE Australia. The Amundsen province has no observed Paleozoic graywacke succession; evidence from Rb-Sr and U-Pb SHRIMP dating supports calc-alkaline granitoid events in Ordovician/Silurian (450–420 Ma) and Permian (276±2 Ma) times. The latter may be the previously unknown source of Permian volcanic detritus in the Ellsworth and Transantarctic mountains. The Amundsen province is considered to be the equivalent of the Median Tectonic Zone of New Zealand, and arc magmatism of comparable ages is found in the Antarctic Peninsula and Thurston Island. The underlying lithosphere of the two provinces may be distinguished by Nd isotope data; granitoids and metasedimentary rocks of the Ross province have Meso-Proterozoic Nd model ages, generally 1300–1500 Ma, compared to 1000–1300 Ma for the Amundsen province. On the basis of published palaeomagnetic data, the two provinces amalgamated to form Marie Byrd Land in mid-Cretaceous times, only shortly before rifting of the New Zealand continental block away from Antarctica.

Late glacial and postglacial environmental changes : Quaternary, Carboniferous-Permian, and Proterozoic

Abstract: Part I The quarternary - the last glaciation and deglaciation: northern hemisphere (Laurentide) deglaciation - processes and responses of ice sheet/ocean interactions the last Scandinavian ice sheet and its down-wasting environment and climate of Sartan maximum and the late glacial in Siberia termination of the pleistocene and holocene changes in South America and other glaciated parts of the southern hemisphere. Part II The quarternary - macro-processes: sediment deformation beneath the Laurentide ice sheet megafloods and glaciation. Part III The permo-carboniferous - a paleographic reconstruction: Permian world topography and climate. Part IV The permo-carboniferous - the distribution and effect of the major glaciers of Gondwana glacial-postglacial transition in the late Paleozoic basins of southern South America a review of the permo-carboniferous glaciation in Africa transition from freezing to subtropical climates in the permo-carboniferous of Afro-Arabia and India Permian postglacial environments of the Australian Plate upper paleozoic glacial and postglacial deposits, central Transantarctic mountains, Antarctica some problems of the Permian (Asselian) glaciation and the subsequent climate in the Permian. Part V An older, precambrian glaciation - a comparative analysis: tectonic and glacio-eustatic controls on postglacial stratigraphy - proterozoic examples. Part IV quaternary and permo-carboniferous terrestrial weathering and organic deposits interpreting glacial climate from detrital minerals in sediments paleosols of the northern part of North America - their features and significance as indicators of past climates cold temperate peats and coals - their sedimentology and composition recent cold climate peats of central Canada and permo-carboniferous coals of Gondwana.

Taconian orogeny in the New England Appalachians: Collision between Laurentia and the Shelburne Falls arc

Abstract: Tectonic models of the Ordovician Taconian orogeny in western New England usually invoke a collision between the Laurentian margin and a magmatic arc identified as the Bronson Hill arc. However, in central Massachusetts and southern New Hampshire, rocks in the Bronson Hill arc are 454 to 442 Ma and therefore younger than much of the Taconian deformation and metamorphism in western New England and eastern New York, which began by 470 Ma. U-Pb and single-grain evaporation zircon ages combined with geochemical analyses reveal the presence of an older magmatic arc, the Shelburne Falls arc, that formed west of the Bronson Hill arc at 485 to 470 Ma. The Shelburne Falls arc formed above an east-dipping subduction zone by the Early Ordovician. The Taconian orogeny was the result of the collision between Laurentia and the Shelburne Falls arc beginning ca. 475 to 470 Ma. The younger Bronson Hill arc formed above a west-dipping subduction zone that developed along the eastern edge of the newly accreted terrane during the final stages of and subsequent to the Taconian orogeny. The Taconian orogeny ended when plate convergence between Laurentia and Iapetus was accommodated by the newly developed west-dipping subduction zone instead of by crustal shortening in the Taconian thrust belt. The tectonic history of the New England Appalachians is inconsistent with a Middle Ordovician collision between Laurentia and the proto-Andean margin of Gondwana.

A Tight fit-Early Mesozoic Gondwana, a Plate Reconstruction Perspective

Abstract: Gondwana, with East Antarctica as its center, began to break up during Late Triassic to Early Jurassic time. Use of the satellite derived gravity map to approximate the ocean-continent boundary allows us to generate a much tighter fit for the reconstructed supercontinent then previously attempted. Major mantle plumes such as the Karoo-Ferrar Plume that first split Gondwana at about 182 Ma, the Parana-Etendeka plume at 132 Ma that split South America and Africa, the Marion plume at 88 Ma that split Madagascar and India and finally the Reunion hotspot that split the Mascarene Plateau from India at 64 Ma, were all critical events in the break-up of Gondwana. Our tight-fit produces overlap between cratonic East Antarctica and the Limpopo Plain of Mozambique but there is no evidence that the crustal material underlying the Limpopo Plain pre-dates the break-up of Gondwana. Likewise Madagascar has been recon­ structed so that it substantially overlies coastal East Africa in the vicinity of the Anza Trough, an early Jurassic rift in Kenya. The western margin of the island of Madagascar may in fact be crustal material that is younger than the break-up. It may have been produced as a result of the Karoo mantle plume or some may have been the result of the Marion hotspot. Between South America and Africa there are three significant overlaps. Two of them are deltaic, and the third is the Abrolhos and Royal Charlotte banks which post-date Gondwanide breakup by 80 to 100 million years. key words: Gondwana break-up, Mesozoic Paleoreconstructions, ocean-continent boundaries, Karoo-Ferrar Mantle Plume, overlap

Palaeozoic palaeogeography: A North Atlantic viewpoint

Abstract: Palaeozoic palaeogeography, highlighting the North Atlantic Caledonian evolution and the destruction of the Iapetus Ocean and the Tornquist Sea, is recapitulated with reconstruction maps from Early Ordovician to Mid-Devonian times. In the Early Ordovician (Trem-adoc-Arenig), Laurentia, Siberia, and the North China Block were positioned in equatorial latitudes, Baltica was located at intermediate southerly latitudes, whilst Avalonia and the European Massifs were located together with the North African part of Gondwana in high southerly latitudes. During the Ordovician, Baltica drifted northwards and approached Siberia while undergoing counter-clockwise rotations. Aval-onia rifted away from Gondwana during Arenig-Llanvirn time, and the Tornquist Sea, separating Avalonia and Baltica, narrowed gradually during the Ordovician followed by Late Ordovician ‘soft docking’ of Eastern Avalonia and Baltica prior to their joint collision with Lau-rentia. The main collisional event between Baltica and Laurenti...

The Palaeozoic evolution in the Alps: from Gondwana to Pangea

Abstract: More than 50% of the Alps expose fragments of Palaeozoic basement which were assembled during the Alpine orogeny. Although the tectonic and metamorphic history of the basement units can be compared to that of the Variscan crust in the Alpine foreland, most of the basement pieces of the Alps do not represent the direct southern continuation of Variscan structural elements evident in the Massif Central, the Vosges–Black Forest or the Bohemian massif. The basement units of the Alps all originated at the Gondwana margin. They were derived from a Precambrian volcanic arc suture fringing the northern margin of Gondwana, from which they were rifted during the Cambrian–Ordovician and Silurian. A short-lived Ordovician orogenic event interrupted the general rifting tendency at the Gondwana active margin. After the Ordovician, the different blocks drifted from the Gondwana margin to their Pangea position, colliding either parallel to Armorica with Laurussia or with originally peri-Gondwanan blocks assembled presently in Armorica. From the Devonian onwards, many basement subunits underwent the complex evolution of apparently oblique collision and nappe stacking. Docking started in the External massifs, the Penninic and Lower and middle Austroalpine units in approximately Devonian/early Carboniferous times, followed by the Upper Austroalpine and the South Alpine domains, in the Visean and the Namurian times, respectively. Wrenching is probably the best mechanism to explain all syn and post-collisional phenomena since the Visean followed by post-orogenic collapse and extension. It explains the occurrence of strike-slip faults at different crustal levels, the formation of sedimentary troughs as well as the extrusion and intrusion of crustal and mantle-derived magmas, and allows for contemporaneous rapid uplift of lower crustal levels and their erosion. From the Stephanian onwards, all regions were deeply eroded by large river systems.

Geochemistry, single zircon ages and Sm–Nd systematics of granitoid rocks from theGóry Sowie (Owl Mts), Polish West Sudetes: evidence for earlyarc-related plutonism

Abstract: Granitoid gneisses as well as their migmatitic and anatectic derivates were investigated from theGory Sowie (Owl Mts) Massif of SW Poland in the central West Sudetes. The gneisses and migmatites aretectonically interlayered with paragneisses, and experienced several consecutive tectono-metamorphicevents. Geochemically, the granitoid gneisses are calc-alkaline and similar to orogenic granite suites, whichtherefore lends support to a subduction-related origin. Single zircon Pb–Pb evaporation ages suggest that the gneiss precursors were emplaced between 473 and488 Ma, and most samples analysed contain zircon xenocrysts with minimum ages between 1124 and2620 Ma. An early phase of high-grade metamorphism is documented by an age of c. 440 Ma for ananatectically derived granite. Two late Variscan granites have an age of c. 333 Ma. Nd isotope systematicssuggest variable involvement of crustal material in the generation of the granitoids. We speculate that the Gory Sowie Massif was part of Avalonia in early Palaeozoic times, derived fromthe northwestern margin of Gondwana, probably northern South America. Granitoid emplacement in theOrdovician took place along a Japan-type active continental margin while Avalonia drifted towardsBaltica and the Tornquist Ocean was consumed. Collision with Baltica occurred in mid-Silurian times. Our data are compatible with an active magmatic arc extending from southern England to thesoutheastern Sudetes in Cambrian and Ordovician times.

Ordovician convergent-margin volcanism and tectonism in the Lachlan sector of east Gondwana

Abstract: The enigmatic early Paleozoic tectonic setting of the Lachlan orogen in Australia has obscured reconstructions of east Gondwana. Recognition of an intraoceanic mafic island arc in this paper establishes the presence of a convergent plate boundary in the Tasmanides from the Early Ordovician to the Early Silurian. This in turn facilitates the updating of supercontinent correlations.

Sequences and stratigraphtc hierarchy of the Paraná basin (Ordovician to Cretaceous), southern Brazil

Abstract: The Parana Basin, a vast sedimentation area during Paleozoic and Mesozoic times, holds a stratigraphic record ranging in age from late Ordovician to late Cretaceous and comprising six supersequences or unconformity-bounded units (Milani, 1997): Rio Ivai (Ordovician-Silurian), Parana (Devonian), Gondwana I (Carboniferous-early Triassic), Gondwana II (middle-late triassic), Gondwana III (late Jurassic-early Cretaceous), and Bauru (late Cretaceous). Three of them coincide with major Paleozoic transgressive-regressive cycles, and the others are Mesozoic continental sedimentary packages with associated igneous rocks. These supersequences are the remnant record of successive phases of sediment accumulation alternating with times of erosion. The evolution of each supersequence was constrained by a particular tectonic and climatic setting. The Rio Ivai supersequence is closely associated with basin inception and its geometry suggests that deposition was to some extent controlled by normal faulting. The Parana supersequence deposited during a time of widespread marine flooding over the cratonic area of southwester Gondwana. From the deposition of the Gondwana I supersequence onward tru intracration conditions were established. Sharing Gondwana's dessication trend the Parana Basin sedimentation history culminated with extensive desertic conditions during the Jurassic. The Lower Cretaceous Serra Geral continental flood basalts are related to the initial moments of South Atlantic rifting and the upper Cretaceous Bauru continental cover ended the history of the basin. The hydrocarbon potential of the Parana Basin is related to two well defined source beds: the Devonian shales (Ponta Grossa Formation) and the upper Permian bituminous shales and limestones (Irati Formation). Sandy reservoirs can be found in the lower Devonian Furnas Formation, in the upper Carboniferous/lower Permian Iatarare Group and in the lower Permian Rio Bonito Formation. The role of intrusive bodies in the maturation of source rocks and in the trapping of hydrocarbons seems to be crucial and deserves more investigation. (author)

Pb, Nd, and Sr Isotope Mapping of Grenville‐Age Crustal Provinces in Rodinia

Abstract: New Pb, Nd, and Sr isotope data are presented for geochemically similar, ∼1.1–1.2 Ga, granitoids and tonalitic‐granitic orthogneisses from Antarctica, southern Africa, and the Falkland Islands and adjoining plateau, areas originally within the supercontinents of Rodinia and Gondwana. These data support contentions for the presence of a Mesoproterozoic (∼1.2 Ga) destructive plate margin running from Namaqua‐Natal (southern Africa), through the displaced microplates of the Falkland Plateau and Falkland Islands, the Haag Nunatak crustal block (West Antarctica) and into western Dronning Maud Land (East Antarctica). The bulk of these granitoids represent juvenile Mesoproterozoic additions to the crust, except for in parts of East Antarctica (i.e., the Sverdrupfjella) where older (Paleoproterozoic or Archean) crust was involved in granitoid generation. Our isotope data permit plate reconstructions in which southern Africa, East Antarctica, and the Falkland Islands and plateau were adjacent within Rodinia.

Early Palaeozoic brachiopods and associated shelly faunas from western Gondwana: their bearing on the geodynamic history of the pre-Andean margin

Abstract: Abstract The strong provincialism exhibited by early Ordovician to Devonian brachiopod faunas provides an independent tool for testing palaeogeographical hypotheses. Patterns of biogeographical affinities of early Palaeozoic brachiopods from the Argentine Precordillera, the Sierra de Famatina and the Central Andean basin (NW Argentina, Bolivia, southern Peru) suggest that the pre-Andean margin was linked to northern Iapetus Ocean history. The low-grade Vendian-Early Cambrian Puncoviscana Formation and the broadly coeval rocks from the Carolina and Gander terranes may represent sedimentation in a narrow basin developed between Laurentia and Gondwana during rifting. Palaeontological evidence suggests that the Precordillera developed on the passive margin of Laurentia but moved away as an independent plate during the Ordovician; the hypothesis of the Precordillera as a Laurentian plateau does not explain faunal differences between their Late Arenig-Llanvirn brachiopods. Ordovician subduction beneath Gondwana resulted in formation of the Famatina-Puna-Avalonia volcanic-arc system. Affinities between Celtic brachiopods of this age in volcaniclastic rocks from South America (Famatina) and eastern North America (Maine, Gander, Central Newfoundland) suggest some geographical continuity between them, consistent with palaeomagnetic evidence. It is proposed that accretion of allochthonous terranes to eastern Laurentia was related to collision with the northwestern corner of South America in late Ordovician time, an idea supported by the affinities of Silurian and Devonian brachiopods from Venezuela and Colombia.

Sedimentology, geochronology and provenance of the Proterozoic Itremo Group, central Madagascar, and implications for pre-Gondwana palaeogeography

Abstract: Proterozoic metasediments of the Itremo Group in central Madagascar probably represent a passive margin sequence predating Gondwana assembly. The quartzites are well-sorted quartz arenites that contain flat lamination, wave ripples, current ripple cross-lamination, and dune cross bedding. The carbonate rocks preserve abundant stromatolites and algal laminates. A continental source is indicated by mudrock major and trace element chemistry. The combination of lithologic association, sediment architecture and mudrock chemistry indicates that the sequence was deposited on a continental shelf or platform. SHRIMP data from detrital zircons indicate that the source area included early Proterozoic and late Archean rocks with ages between 1.85 and 2.69 Ga, and that the depositional age of the Itremo Group must be less than 1855 ± 11 Ma. The sequence has been deformed into a series of large-scale folds separated by ductile shear zones. SHRIMP data indicate both massive lead loss from detrital zircons and new zircon growth in the metasediments at 833 ± 112 Ma, which we interpret as the age of metamorphism of the sequence. Comparison of detrital grain ages with basement ages in East Africa and in India indicates that the source area for the Itremo Group probably lay on the present African mainland.