Part Of Gondwana Research Articles

U–Pb ages of detrital zircons from the Basal allochthonous units of NW Iberia: Provenance and paleoposition on the northern margin of Gondwana during the Neoproterozoic and Paleozoic

LA-ICP-MS U–Pb ages of detrital zircons from eight siliciclastic samples from the Basal units of the Variscan allochthonous complexes of NW Iberia are used to establish the maximum depositional age and provenance of two tectonically-stacked metasedimentary sequences deposited on the outermost margin of Gondwana, and subsequently involved in the Rheic Ocean suture. The maximum depositional ages for the two sequences is latest Neoproterozoic and latest Cambrian, respectively. The age spectra are also used to discuss the paleoposition of the NW Iberian basement on the continental margin of Gondwana prior to the opening of the Rheic Ocean, which is tentatively placed in northern Africa, between the West African and Saharan cratons. Based on similarities and differences with age data from the NW Iberian autochthon and other allochthonous terranes involved in the Rheic suture, the relative proportions of Mesoproterozoic zircons in both assemblages are proposed as markers of proximity to the eastern part of the West African craton during late Neoproterozoic and late Cambrian. The geodynamic processes that took place along this part of Gondwana during the late Neoproterozoic, late Cambrian and Early Ordovician are discussed in the light of the LA-ICP-MS results, as well as the sedimentological record, magmatic evolution and plate tectonic setting of NW Iberia. These processes are linked to late Neoproterozoic and Cambro-Ordovician subduction events beneath the northern Gondwanan margin.

Orbital forcing and Mississippian sea level change: time series analysis of marine flooding events in the Visean Windsor Group of eastern Canada and implications for Gondwana glaciation

Abstract Orbital forcing is strongly expressed in spectra of time series delineating transgressive-regressive (TR) cycles in the late Visean Windsor Group in the Shubenacadie Basin of southern Nova Scotia. These spectra compare closely with those of synthetic, variously degraded time series constructed from summed sine waves representing orbital periods for the eccentricity, obliquity and precession-index as predicted for late Visean time. The introduction of significant new macrofaunal elements in the Windsor Group accompanied transgressions separated in time by 2.2 to 2.4 m.y., perhaps reflecting a longer-period orbital influence by the eccentricity or by modulation of the obliquity. Orbital forcing in the late Asbian substage was apparently dominated by the 100 k.y. eccentricity, whereas in the Brigantian, TR cycles responded mainly to forcing by the 400 k.y. eccentricity. High-magnitude sea-level change in late Visean successions elsewhere, and an orbital signature for Windsor Group TR cycles, suggest that orbitally forced waxing and waning of significant continental ice sheets is the most appropriate explanation for late Visean sea-level change. Ice sheets were fully developed by the end of the early Asbian substage. Sea-level high-stands, interpreted as the result of melting of continental ice sheets, represent a very small temporal percentage of the Windsor Group stratigraphic succession, and mark the late Visean as a cold period in Earth’s history. In this part of Gondwana, a late Asbian or even older start to Gondwana glaciation is indicated, contrasting with the Australian record which places the first significant glaciations of Gondwana in the early Namurian.

Taphonomic analysis in lacustrine environments: Two different contexts for Triassic lake paleofloras from Western Gondwana (Argentina)

During the earliest Triassic several rift basins developed along the western margin of Gondwana associated with the pre-breakup of Pangea. They were filled by exclusively non-marine sediments including alluvial, fluvial, and lacustrine deposits. In the Ischigualasto–Villa Unión Basin, the lacustrine-deltaic succession is placed in the Los Rastros Formation and consists of several coarsening-upward cycles of black shale, siltstone, and sandstone. The paleontologic content of the succession includes abundant floral remains (related to the Dicroidium-type flora), invertebrates (conchostracans, insects), and vertebrates (fishes, a temnospondyl amphibian, ichnites). At the Cerro Puntudo area in the Cuyana Basin, the lacustrine succession forms the upper part of the Cerro Puntudo Formation and consists of limestone, stromatolitic limestone, mudstone, sandstone, and tuff. The paleontologic content includes scarce floral remains and rhizoliths; invertebrates are represented exclusively by traces (associated with ichnofacies of Skolithos and Scoyenia), and vertebrates by a fragment of the pelvic girdle of a basal arcosaur. The taphonomic analysis performed in the two Triassic lacustrine successions allows recognition of two different taphonomic histories for the plant remains. The Los Rastros lake preserved both autochthonous (originated in the littoral zone) and allochthonous (originated in the upstream fluvial system) elements. The offshore lacustrine area was dominated by autochthonous well-preserved elements and allochthonous plant debris and wood, which formed time-averaged accumulations. The delta deposits are characterized by allochthonous elements with varied preservational conditions, usually showing evidence of mechanical degradation and accumulation within a short time. Autochthonous and allochthonous material were preserved in the Los Rastros Lake by means of anoxic conditions in the offshore lacustrine area and high sedimentation rates in the delta. In contrast, the Cerro Puntudo Lake preserved only autochthonous elements (originated in the littoral zone), including rhizoliths and foliar material, which formed autochthonous and parautochthonous accumulations at the littoral zone in spite of aerobic conditions. This was the result of tuffaceous material that enhanced preservation. Thus, these very different lacustrine environmental contexts are showing different preservational modes. The fossil assemblages allowed the reconstruction of the original communities from this part of Gondwana. Thus, the Los Rastros lake margins were characterized by shrubs and small trees of Ginkgoales and Corystospermales, and herbaceous members of the Sphenophyta. The sphenophytes were also the dominant floral component along the river margins whereas the Corystospermales, Cycadales, Pteridophyta, and conifers formed the woodland upstream probably related to the floodplains of a trunk fluvial system. The littoral zone of the Cerro Puntudo Lake was dominated mainly by herbaceous sphenophytes and lycopsids. These fossil assemblages characterize the paleoflora associated with lacustrine systems. A fuller understanding of the processes that generate these assemblages is essential for comparisons with other continental paleobotanical records in the Middle Triassic of Gondwana (e.g., Australia, South Africa).

The drift history of Iran from the Ordovician to the Triassic

Abstract New Late Ordovician and Triassic palaeomagnetic data from Iran are presented. These data, in conjunction with data from the literature, provide insights on the drift history of Iran as part of Cimmeria during the Ordovician–Triassic. A robust agreement of palaeomagnetic poles of Iran and West Gondwana is observed for the Late Ordovician–earliest Carboniferous, indicating that Iran was part of Gondwana during that time. Data for the Late Permian–early Early Triassic indicate that Iran resided on subequatorial palaeolatitudes, clearly disengaged from the parental Gondwanan margin in the southern hemisphere. Since the late Early Triassic, Iran has been located in the northern hemisphere close to the Eurasian margin. This northward drift brought Iran to cover much of the Palaeotethys in approximately 35 Ma, at an average plate speed of c . 7–8 cm year −1 , and was in part coeval to the transformation of Pangaea from an Irvingian B to a Wegenerian A-type configuration.

Argentinatachoides, a new genus from Argentina putatively related to the Australian-Tasmanian Tasmanitachoides Erwin, 1972 (Coleoptera: Carabidae: Bembidiini)

ABSTRACT Argentinatachoides balli, new genus, new species, in the tribe Bembidiini is described and illustrated, and its microhabitat noted. Adults of this new species were found in Argentina, east of the Andean Cordillera, in Mendoza and La Rioja Provinces. The subulate apical palpomeres and other characters place this new genus in Bembidiini and the sub-oblique notched anterior tibiae assign it to the subtribe Tachyina. Structural attributes, for example the elytral recurrent groove appearing double and the symmetrical basal lobes of the aedeagal median lobe, indicate that this species is related to the Australian-Tasmanian genus, Tasmanitachoides Erwin, 1972. This discovery amplifies the known range of a particular fauna of Carabidae that was once widely distributed in the western part of Gondwana. Some aspects of the biogeography of the group are discussed.

Records of Precambrian–Early Palaeozoic volcanic and sedimentary processes in the Central European Variscides: A review of SHRIMP zircon data from the Kaczawa succession (Sudetes, SW Poland)

The early, pre-orogenic stages of evolution in the Variscan belt, i.e. rifting processes, opening of sedimentary basins and associated igneous activities, are often obscure because many successions have yielded little or no biostratigraphic data, have a strong metamorphic overprint and are tectonically deformed and dislocated. The increasing application of SHRIMP zircon dating has provided useful constraints on magmatic and metamorphic processes, helped locate probable source areas for detritus within sedimentary successions and facilitated large-scale palaeogeographic correlations. This methodology has recently thrown considerable light on the age and relationships of the previously poorly constrained rock units of the Kaczawa Complex in the Polish West Sudetes. Thus, recent SHRIMP studies in the Kaczawa Mountains have yielded Early Ordovician ages of the initial rift type bimodal volcanic suites at the bottom part of the Kaczawa Succession: c. 503 Ma for metarhyodacites of crustal derivation, and c. 485 Ma for alkaline metatrachytes of mantle signature. These dates provide a firm temporal constraint on the initial rift magmatism interpreted as related to the continental break-up of the northern peripheries of Gondwana. New SHRIMP data from metavolcaniclastic and metasedimentary rocks of the Kaczawa Complex have yielded results that have provided significantly changed interpretations on their age and relationships. For instance, a siliciclastic sequence interpreted as belonging to the lower part of the Kaczawa Complex (the Gackowa Sandstones) and seemingly sourced (using an array of geochemical and mineralogical evidence) from nearby early Ordovician volcanic rocks has, surprisingly, yielded zircon ages not younger than Precambrian and thus this unit has tentatively been reinterpreted as a possible correlative of the Neoproterozoic Lusatian Graywackes. Felsic metavolcaniclastic rocks embedded in the carbonate succession of the Wojcieszów Limestone have yielded SHRIMP zircon ages of c. 497 Ma, thus supporting a late Cambrian/early Ordovician age for this shallow-water limestone sequence (which had been proposed to be late Ordovician/Silurian or younger in age, based on controversial foraminifera evidence, and more recently assigned to the Mid Cambrian, based on newly found archaeocyathid fragments). And, age spectra of zircon populations in the metamudstone sequence of the Radzimowice Slates of thus far enigmatic stratigraphic position indicate that these deposits may have accumulated during early Palaeozoic rifting, with source areas similar to those known from parts of Gondwana (e.g. NW Africa). Overall, the new SHRIMP zircon studies, both in metavolcanic and metasedimentary sequences, have considerably illuminated their affinities, and should provide encouragement for further application of this technique in this region. In general the results provide support and detail for interpretation of this rock complex as reflecting the early stages of development of the volcanic and sedimentary processes operating along North Gondwana during latest Precambrian and early Palaeozoic times.

SHRIMP U-Pb zircon geochronology of the High Himalayan rocks in the Nyalam region, Tibet

FIGURE 1. Detrital zircon age spectra from Nyalam High Himalaya (Sample NY93-1) U-Pb age patterns of individual detrital zircons are a potentially powerful tool for constraining metasedimentary provenance. Each zircon grain has a characteristic age reflecting its genesis, and the overall population of detrital zircons represents the spectrum of primary zircon-bearing source rocks, including detritus derived from a number of sedimentary cycles (Cawood et al. 1999). The complex of sillimanite-, kayaniteand garnet-bearing gneiss, calc-silicates and augen gneiss in Nyalam High Himalaya are intruded by granitoids. The garnet-sillimanite paragneisses have experienced an early Paleozoic metamorphism (Gehrel et al. 2003) and Tertiary metamorphism. The source of metasedimentary rocks of the Nyalam High Himalaya zone has been studied using SHRIMP ion microprobe at Beijing SHRIMP Lab, Chinese Academy of Geological Sciences. SHRIMP U/Pb age data from the metasedimentary rocks of the High Himalayan terranes in South Tibet range from ca.23 Ma to 3221 Ma. The U-Pb data allow grouping of the zircons into five major age components with the exception of Cenozoic ages (Figure 1). (1) Archean grains with a maximium age frequency between 3221 and 2509 Ma, (2) Paleoproterozoic grains ranging from 2453 to 1631 Ma, (3) Mesoproterozoic to early Neoproterozoic grains ranging in age between 1530 Ma and 944 Ma, (4) Neoproterozoic grains ranging between 852 and 540 Ma, and (5) Pan-African ages (543-443 Ma). Potential source regions for the detrital zircons occur within the Gondwana terranes: zircons of Archean age correspond to the age of rock unit formed during major a mgmatic and tectonothermal pulses in the Bhandara Craton or Singhbum craton, zircons with paleo-Mesoproterozoic ages reflect tectonic or magmatic events related to or older than the assembly of India (Catlos et al. 2002). Neoproterozoic zircons may have been derived from Lesser Himalayan rocks or the Indian craton (Myrow et al. 2003). Pan-African zircons are the Pan-African orogen event along the Himalayan orogenic belts. The U-Pb age data suggest the sedimentary sources of the studied gneisses in the northern part of the Indian plate. During Late Precambrian and the Palaeozoic, the Gondwanian India bounded to the north by the Cimmerian Super terranes, was part of Gondwana and was separated from Eurasia by the Paleotethys Ocean. During the periods, the northern part of India was affected by a Pan-African event, Numerous granitic intrusions dated at around 500Ma are attributed to this event. The Pan-African event is marked by an unconformity between Ordovician continental conglometrates and the underlying Cambrian marine sediments (Zhu Tongxin et al. 2003). The PanAfrican event form a wide belt stretching from the Alps over the Arabic peninsula, Africa, India, Australia and down to Antarctica. These evidences suggests the presence of the Pan-African orogenic event in the Himalaya. It is tempting to correlate the early Palaeozoic thermal event with a late extensional stage of the long-lasting Pan-African orogenic events, which ended with the formation of the Gondwana supercontinent. The protolith age of the High Himalayan metamorphic rocks is generally regarded to be Precambrian to early Paleozoic. It seems plausible that the High Himalayan metamorphic rocks represent a minimum depositional rage at ∼500 Ma. Our data show strong similarities to previously published spectra for the Greater Himalayan zone, Lesser Himalayan zone and Tethyan Himalayan zone (Parrish et al. 1996; DeCelles et al. 2000; Myrow et al. 2003). Detrital zircon spectra from the High Himalayan range is as young as 500 Ma, so the High Himalaya and Tethyan Himalaya were deposited contemporaneously.

TRIASSIC-LOWER JURASSIC FORAMINIFERAL INDICES FOR BAHAMAN-TYPE CARBONATE-BANK LIMESTONES, CABLAC MOUNTAIN, EAST TIMOR

Peloidal and oolitic limestones on Cablac Mountain in East Timor contain small calcite-cemented agglutinated and porcelaneous foraminifera that place these limestones in the Triassic or Lower Jurassic, in contrast to the Lower Miocene as previously mapped. An Early Jurassic (Sinemurian-Pliensbachian) age is indicated for some of the limestone by the presence of Meandrovoluta asiagoensis Fugagnoli and Rettori, Everticyclammina praevirguliana Fugagnoli and a palynomorph assemblage. The age of other limestones on the mountain is identified broadly as Late Triassic to Early Jurassic, based on the occurrence of Duotaxis metula Kristan. In basinal facies of the nearby Wai Luli Valley, Gsollbergella spiroloculiformis (Oraveczne Scheffer), palynomorphs, a brachiopod and halobiid bivalves indicate a Late Triassic (Carnian) age for a transported foraminiferal assemblage associated with peloids, ooids and Duotaxis and Siphovalvulina characteristic of carbonate-bank deposits. This occurrence suggests that carbonate banks were developed locally on submerged topographic highs in seas that flooded interior-rift basins in this part of Gondwana and that a complex facies array of deep-water muds, deltaic sands, and carbonate shoals were present in the basins. Taxonomic assessment of Triassic and Early Jurassic species previously placed in Tetrataxis suggests that these are better accommodated in Duotaxis , and some species placed in Trochammina are transferred to Siphovalvulina . The distinction between Duotaxis and Siphovalvulina , as now understood, rests on the shape of adult chambers.

Protolith Ages and Timing of Metasomatism Related to the Formation of Whiteschists at Mautia Hill, Tanzania: Implications for the Assembly of Gondwana

Mautia Hill, situated in the Mozambique Belt of East Africa, is a locality famous for its unique high‐pressure whiteschist mineral assemblages. The rocks are located in a complex regional tectonic setting that is critical to understanding the amalgamation history of Gondwana.The timing of the Pan‐African Gondwana collision in this region is still a topic of considerable debate, especially because the age of the Mautia high‐pressure metamorphism/metasomatism has yet to be established. We have extracted detrital zircons with extensive low‐U overgrowths and dated them using the U‐Pb SHRIMP method. Detrital zircon cores indicate that the sediments came from a region identical to the adjacent Tanzania Craton and adjacent Western Mozambique Belt. One detrital grain has a 207Pb/206Pb age of ca. 3.5 Ga and is the oldest dated zircon in the region. The zircon rims provide 207‐corrected 206Pb/238U and 208Pb/232Th weighted mean ages of $$549\pm 41$$ and $$535\pm 40$$ Ma, respectively, dating the age of collision between the Tanzania Craton and Madagascar‐India during the assembly of Gondwana. This new age indicates that collisional orogenesis in this part of the East African Orogen occurred in a similar time frame as in many other parts of Gondwana, including South America, southern Africa, India, Australia, and Antarctica, but later than a ∼640‐Ma age previously attributed to the collision in this area by some authors.

Early Silurian glaciation along the western margin of Gondwana (Peru, Bolivia and northern Argentina): Palaeogeographic and geodynamic setting

The record of the Ordovician–Silurian transition within the southern Peru–Bolivia Palaeozoic basin (extending across Peru, Bolivia and northern Argentina) is characterized by a diamictite-bearing unit which overlies several different Ordovician units, and underlies mid Silurian shales. We review the lithostratigraphy, sedimentology and biostratigraphy of this unit, revising stratigraphic relationships and several ideas and interpretations previously proposed. Analysis of facies and lithofacies associations allow for a reinterpretation of sedimentary processes and environments involved during its deposition. Turbidite and shale interbeds within the diamictites indicate a deep marine environment, with interbedded mud flows, debris flows, slumps and large displaced slabs providing evidence for sediment instability and resedimentation. Chitinozoan biostratigraphy indicates a Llandovery age, in contrast with previous proposals of a Hirnantian age, and suggests the need for more detailed sedimentologic and biostratigraphic studies, and a reassessment of this diamictite unit in relation to adjacent regions in South America. The resedimented character of the deposit explains some of the Ordovician fauna previously described, which must be considered as recycled from underlying units. In situ fauna needs to be reassessed, as it may be endemic, diachronic and/or indicative of migrations within Gondwana. Glacially-faceted and striated clasts, as well as large granitoid boulders within the resedimented materials, provide evidence for glaciation of the source area, and are interpreted as recycled from former glacigenic deposits. The evidence found in the Central Andes indicates a glaciomarine origin for the diamictites and corroborates glaciation of a western source area prior to the late Llandovery (Telychian). The precise age of glaciation in this part of Gondwana cannot be confirmed do to a lack of true tillites. Tectonic deformation and the resulting relief are respectively identified as the origin for sediment instability and for local glaciation along the active margin of western Gondwana during the Late Ordovician and Early Silurian. The new data imply that the Ordovician–Silurian boundary was not preserved in some areas of the basin, and that an erosional hiatus (disconformity with variable regional significance) is present between different Ordovician units and the diamictite unit. Late Ordovician (?late Caradoc–?early Ashgill) euxinic black shales locally present in the central Altiplano and Eastern Cordillera of Bolivia may represent the youngest Ordovician unit in the region, and more research should be focused on them. Paleogeographic reconstruction of the Early Silurian Peru–Bolivia Basin depicts the extension of the diamictite unit southwards into northern Argentina and western Paraguay, connecting with the Paraná basin through the Asunción Arch. To the north it continued into northern Peru and Ecuador, connecting with the Silurian record in Colombia and Venezuela, and to the northeast into Brazil (Solimões and Amazonas basins). The sedimentary record of glaciation in the Peru–Bolivia basin was due to the development of local ice fields to the west of the basin, related with tectonism and resulting reliefs along the active margin of Gondwana, and probably did not coincide with the development of the North African late Ashgillian ice cap.

Gondwanide continental collision and the origin of Patagonia

A review of the post-Cambrian igneous, structural and metamorphic history of Patagonia, largely revealed by a five-year programme of U–Pb zircon dating (32 samples), geochemical and isotope analysis, results in a new Late Palaeozoic collision model as the probable cause of the Gondwanide fold belts of South America and South Africa. In the northeastern part of the North Patagonian Massif, Cambro-Ordovician metasediments with a Gondwana provenance are intruded by Mid Ordovician granites analogous to those of the Famatinian arc of NW Argentina; this area is interpreted as Gondwana continental crust at least from Devonian times, probably underlain by Neoproterozoic crystalline basement affected by both Pampean and Famatinian events, with a Cambrian rifting episode previously identified in the basement of the Sierra de la Ventana. In the Devonian (following collision of the Argentine Precordillera terrane to the north), the site of magmatism jumped to the western and southwestern margins of the North Patagonian Massif, although as yet the tectonics of this magmatic event are poorly constrained. This was followed by Early Carboniferous I-type granites representing a subduction-related magmatic are and Mid Carboniferous S-type granites representing crustal anatexis. The disposition of these rocks implies that the North Patagonian Massif was in the upper plate, with northeasterly subduction beneath Gondwana prior to the collision of a southern landmass represented by the Deseado Massif and its probable extension in southeastern Patagonia. This ‘Deseado terrane’ may have originally rifted off from a similar position during the Cambrian episode. Intense metamorphism and granite emplacement in the upper plate continued into the Early Permian. Known aspects of Late Palaeozoic sedimentation, metamorphism, and deformation in the Sierra de la Ventana and adjacent Cape Fold Belt of South Africa are encompassed within this model. It is also compatible with modern geophysical and palaeomagnetic data that do not support previous hypotheses of southward-directed subduction and collision along the northern limit of Patagonia. Subsequent Permian break-off of the subducted plate, perhaps with delamination of the lower part of the upper plate, allowed access of heat to the overlying Gondwana margin and resulted in voluminous and widespread silicic plutonism and volcanism throughout Permian and into Triassic times. Thus the new model addresses and attempts to explain three long-standing geological enigmas—the origin of the Gondwanide fold belts, the origin of Patagonia, and the cause of widespread Permian silicic magmatism (Choiyoi province) in southern South America. Differing significantly from previous models, it has new implications for the crustal structure, mineral resources, and plant and animal distribution in this part of Gondwana, since the southern landmass would have had an independent evolution throughout the Early Palaeozoic.

Cold climate in the eastern Australian mid to late Permian may reflect cold upwelling waters

A suite of ice-rafted dropstones and glendonites throughout the Permian succession of eastern Australia indicates the cold climate associated with the late Palaeozoic ice age persisted longest in this part of Gondwana. Paradoxically, these cold climate indicators are preserved in transgressive and highstand facies and formed at mid to high latitudes at a time when palaeofloral and sedimentological data suggest equable onshore environments during the intervening lowstands and temperate conditions at the pole. These apparent inconsistencies suggest that eastern Australia was anomalously cold in the context of post-Sakmarian Gondwanan climates, and the distribution of sedimentary indicators could indicate localized cooling by oceanographic processes. Modern upwelling of cold abyssal waters produces the specific physiochemical conditions necessary for ikaite formation, and is likely to have contributed to the development of glendonites in the eastern Australian Permian system. Such upwelling would have locally lowered surface water temperatures such that seasonal sea or river ice could form, which rafted debris across the marine shelf, and a cold-water fauna could develop. This hypothesis is supported by coupled atmosphere–ocean models for the Permian, which suggest that wind systems may have driven upwelling along this section of the Gondwanan coast. Colder conditions dissipated with the onset of the Hunter–Bowen Contractional Event and the transformation of the basin system from an open marine shelf to a foreland basin. This new hypothesis reconciles the prolonged deposition of cold-climate indicators in the eastern Australian Permian with an apparent post-Sakmarian warming elsewhere in Gondwana.

Episodic growth of the Gondwana supercontinent from hafnium and oxygen isotopes in zircon

It is thought that continental crust existed as early as 150 million years after planetary accretion, but assessing the rates and processes of subsequent crustal growth requires linking the apparently contradictory information from the igneous and sedimentary rock records. For example, the striking global peaks in juvenile igneous activity 2.7, 1.9 and 1.2 Gyr ago imply rapid crustal generation in response to the emplacement of mantle 'super-plumes', rather than by the continuous process of subduction. Yet uncertainties persist over whether these age peaks are artefacts of selective preservation, and over how to reconcile episodic crust formation with the smooth crustal evolution curves inferred from neodymium isotope variations of sedimentary rocks. Detrital zircons encapsulate a more representative record of igneous events than the exposed geology and their hafnium isotope ratios reflect the time since the source of the parental magmas separated from the mantle. These 'model' ages are only meaningful if the host magma lacked a mixed or sedimentary source component, but the latter can be diagnosed by oxygen isotopes, which are strongly fractionated by rock-hydrosphere interactions. Here we report the first study that integrates hafnium and oxygen isotopes, all measured in situ on the same, precisely dated detrital zircon grains. The data reveal that crust generation in part of Gondwana was limited to major pulses at 1.9 and 3.3 Gyr ago, and that the zircons crystallized during repeated reworking of crust formed at these times. The implication is that the mechanisms of crust formation differed from those of crustal differentiation in ancient orogenic belts.

Millerocaulis (Erasmus) ex Tidwell (Osmundales, Filices) de la Formación Carrizal (Triásico Superior) de Marayes, provincia de San Juan, Argentina

Millerocaulis (Erasmus) ex Tidwell (Osmundales, Filices) from the Carrrizal Formation (Upper Triassic) from Marayes, San Juan province, Argentina. Based on newly found specimens, «Marayea lutzi» Herbst 1994, of the Upper Triassic Carrizal Formation, from Marayes (San Juan province , Argentina) is analyzed. Anatomical characters, specially those from the central cylinder, now defines the species as an ectophloicdictyoxylic siphonostele (sensu Miller) and shows that it must be changed to Millerocaulis. Thus, Marayea defined as a siphonostelic genus becomes invalid and an emended diagnosis for the type species is given. Also, from the same locality and age other, rather big specimens, are described. They are not well preserved but the available characters allow to incorporate them as Millerocaulis sp. These two species and others formerly described of this genus, M. herbsti (Archangelsky & de la Sota) Tidwell and M. stipabonettii Herbst, from the Laguna Colorada Formation (Santa Cruz province) and Cepeda Formation (San Juan province) respectively from Argentina along with M. beardmorensis Schopf and Ashicaulis woolfei Rothwell, Taylor & Taylor, from the Fremouw Formation (Antarctica) and the frond Osmunda claytoniites Phipps et. al.,as well as some petioles lacking cellular structure but undoubtely belonging to Millerocaulis, from the Los Rastros Formation (San Juan province), all from the upper Middle to Upper Triassic, show the diversification of the Osmundales as early as this time span, at least in this part of Gondwana

Geology, Climate, and Landscape of the PABITRA Wet-Zone Transect, Viti Levu Island, Fiji

The PABITRA Gateway Transect in Fiji covers most of the eastern part of Viti Levu, the largest island in the archipelago. Viti Levu is located exclusively on the Fiji Plate, a microplate between the giant Pacific Plate and the Indo-Australian Plate that has been moved counterclockwise within the past 42 million yr as a result of their oblique convergence. There is no secure geologic evidence that Viti Levu was ever in contact with part of Gondwana, despite the presence of Gondwana flora. The oldest rock series in the area is the submarine Eocene Wainimala Group, intruded in places by the Colo Plutonics. These are succeeded by the Medrausucu Andesitic Group, the Ba Volcanics, and the Verata Sedimentary Group, a Plio-Pleistocene group of sediments representing deltaic and shallow-water deposition in the southeast of the area. The modern Rewa Delta and associated alluvial flats compose the youngest rocks in the area. The geology of the six study sites within the PABITRA Transect is explained in detail. Being on the windward side of the island, the area's climate is humid tropical, with the lowest temperatures and highest precipitation being associated with the highest elevations. A short account of the area's landscape is given.

South China's Gondwana connection in the Paleozoic: Paleomagnetic evidence *

Abstract The reconstruction in which the South China Block (SCB) lies along the Great India-Australia region of Gondwana margin is tested by using existing Paleozoic paleomagnetic data. Under the proposed reconstruction, all six high quality paleopoles from the SCB between the Early Cambrian and late Early Devonian (∼400 Ma) are in good agreement with coeval paleopoles from Gondwana. Thereafter, the apparent polar wander paths for the SCB and Gondwana diverge markedly. This indicates that the SCB had been part of Gondwana and the connection was intact from at least Early Cambrian until late Early Devonian, but they began to separate by late Middle Devonian (∼375Ma).

Palaeobiogeography of the Ordovician trilobite Prosopiscus, with a new species from western New South Wales

Prosopiscus is particularly important in Ordovician palaeobiogeography because of its wide geographic distribution in Gondwana and peri-Gondwanan regions. It appears to have been confined to low palaeolatitudes, representing a characteristic member of the warm water eastern Gondwanan shelf faunas. Trends in the distribution of the Ordovician genus can be observed due to its long stratigraphic range. Prosopiscus was restricted to, and may have originated in, Australia during the late Early Ordovician (Bendigonian-Chewtonian). By the Middle Ordovician (Darriwilian), Prosopiscus had dispersed to other parts of Gondwana and peri-Gondwana, including the North and South China blocks, Tarim, central Himalayas, and the Argentine Precordillera (South America). Possible explanations for the distribution of Prosopiscus are that: (1) there were no oceanic barriers preventing dispersal of trilobites between different regions of Gondwana, thus permitting uninhibited migration over vast distances; (2) Prosopiscus was not restricted to a specific biofacies; (3) a major eustatic transgression during the early Darriwilian may have facilitated the dispersal of Prosopiscus in allowing further development and expansion of marine environments; and (4) a prolonged planktonic larval stage may have permitted wide dispersal. Prosopiscus lauriei sp. nov. is described from the late Early Ordovician (Bendigonian-Chewtonian) Tabita Formation at Mount Arrowsmith, northwestern New South Wales, Australia. The new species is closely related to P. praecox, from the Nora Formation, Georgina Basin, central Australia, and to P. magicus from northwest China.

Cambroclaves from the Cambrian of Sardinia (Italy) and Germany: constraints for the architecture of western Gondwana and the palaeogeographical and palaeoecological potential of cambroclaves

Cambroclaves represent a group of problematic microfossils previously known from strata close to the Early/Middle Cambrian boundary of only a few palaeogeographic regions (Kazakhstan, China, Australia). Because of their frequent occurrence as disarticulated remains, they have hitherto been assumed to be provincially restricted microfossils of unclear palaeobiological affinity. Discoveries of cambroclaves from the Early to early Middle Cambrian of southern (Sardinia) and central (Germany) Europe point to a much wider distribution during their short stratigraphic range, and imply closer palaeogeographic relations between the European shelf of western Gondwana and the areas from which cambroclaves were previously known. These relations are also supported by the common occurrence of other small shelly fossils. These facts support the existence of a widespread uniform facies belt (shelf) around parts of Gondwana during the Early–Middle Cambrian time interval, and contradict the interpretation of the European depositional areas as isolated basins or as distinctly separate Cambrian terranes. The western Gondwana cambroclaves occur in carbonate successions indicative of special palaeoecological conditions. The specimens are limited to distinct layers formed during transgressive phases that opened inner and partly restricted platform areas to open-marine and more distal (deeper subtidal) environments, possibly accompanied by a transition from a rather arid to more humid climatic conditions. Because of the short stratigraphic window of occurrence and of distinctive facies characteristics, cambroclaves are palaeoecologically and palaeobiogeographically useful, and consequently contribute important evidence for both the reconstruction of the Perigondwana realm and the relations to other palaeocontinents in the Cambrian.

Magnetostratigraphic constraints on the Gondwanan origin of North China: Cambrian/Ordovician boundary results

SUMMARY A significant gap in the middle Palaeozoic apparent polar wander (APW) path precludes polarity definition of the early Palaeozoic palaeopole for North China. This problem can in part be resolved by the intercontinental correlation of magnetic polarity patterns across small time intervals. A magnetostratigraphic study was carried out on upper Cambrian to lower Ordovician sediments near Zhaogezhuang (long. 118.5 ◦ E, lat. 39.7 ◦ N), North China. After stepwise thermal or thermal and alternating field demagnetizations, a characteristic magnetic component with normal and antipodal reversed directions was identified. These data, drawn from 49 samples, yield a north palaeopole at long. 294.6 ◦ E, lat. 32.9 ◦ N (dp = 3.0 ◦ , dm = 5.3 ◦ ). A concordant magnetic polarity pattern around the Cambrian‐Ordovician boundary and lowest Ordovician obtained from different continents favours a Southern Hemisphere origin (∼17 ◦ ) formation site. Using the Cambrian‐Ordovician APW paths between North China and Gondwana, we suggest that the North China block (NCB) was part of Gondwana during the Cambro‐lowest Ordovician, and started breaking away from Gondwana in the lower Ordovician. This finding is contrary to some palaeomagnetic models where the NCB was quite separate from Gondwana in the late Proterozoic, and was attached to the ‘Pacific’ side of Antarctica.

African provenance for the metasediments and metaigneous rocks of the Cyclades, Aegean Sea, Greece

U-Pb geochronology on detrital and inherited zircons from metasediments as well as gneissic and metaigneous basement rocks of the Cyclades, Aegean Sea, Greece, defines several periods of crustal growth prior to the Carboniferous. These ages are consistent with Cycladic rocks being derived from the northern margin of Gondwana. There is a distinct Mesoproterozoic age gap evident in zircons from the 25 samples considered. This is characteristic of the West African craton, and we suggest it will also prove characteristic of the Aegean region. This Mesoproterozoic age gap may be used as a tool in terrane reconstruction to distinguish crustal material derived from different parts of Gondwana.