A hypothesis for Proterozoic-Phanerozoic supercontinent cyclicity, with implications for mantle convection, plate tectonics and Earth system evolution

We present a conceptual model for supercontinent cycles in the Proterozoic-Phanerozoic Eons. It is based on the repetitive behavior of C and Sr isotopes in marine carbonates and U–Pb ages and εHf of detrital zircons seen during the Neoproterozoic-Paleozoic and Paleoproterozoic Eras, respectively. These records are considered to reflect secular changes in global tectonics, and it is hypothesized that the repetitive pattern is caused by the same type of changes in global tectonics. The fundamental premise of this paper is that such repetitive changes should also be recorded in orogenic belts worldwide. This carries the implication that Neoproterozoic-Paleozoic orogenic belts should have Paleoproterozoic equivalents. It is proposed that this is the case for the East African, Uralides and Ouachita–Alleghanian orogens, which have Paleoproterozoic analogs in the West African–Amazon, Laurentian and East European cratons, respectively. The Neoproterozoic-Paleozoic orogenic belts are not isolated features but occur in a specific global context, which correspond to the relatively well-constrained Neoproterozoic break-up of Rodinia, and the subsequent Late Paleozoic assembly of Pangea. The existence of Paleoproterozoic equivalents to Neoproterozoic-Paleozoic orogens requires that the same cycle defined the Paleoproterozoic. We therefore hypothesize that there were Paleoproterozoic supercontinents equivalent to Rodinia and Pangea, and that Proterozoic-Phanerozoic supercontinents are comprised of two basic types of configurations, equivalent to Rodinia (R-type) and Pangea (P-type). The Paleoproterozoic equivalent of Rodinia is likely the first supercontinent to have formed, and Proterozoic-Phanerozoic supercontinent cycles are therefore defined by R- to R-type cycles, each lasting approximately 1.5 Gyr. We use this cyclic pattern as a framework to develop a conceptual model that predicts the configuration and cycles of Proterozoic-Phanerozoic supercontinents, and their relation to mantle convection and Earth system evolution

Zircon U-Pb-Hf evidence for subduction related crustal growth and reworking of Archaean crust within the Palaeoproterozoic Birimian terrane, West African Craton, SE Ghana

Zircon Lu-Hf isotopic data from granites of southern and northwestern Ghana have been used to investigate the contribution of reworked Archaean bedrock to the Birimian crust of Ghana, West African Craton. Zircon from seven localities in southern Ghana and one locality in western Ghana were analysed. Combined U-Pb and Lu-Hf isotope data suggest juvenile crustal addition between 2.3–2.1 Ga, with a short period of reworking of Archaean crust. Until now, evidence for reworking of Archaean basement during Birimian magmatism in Ghana has hinged on whole-rock Nd model-ages of the Winneba pluton, and sparse inherited zircon grains from mainly northwestern Ghana. Our data suggest that reworking of Archaean crust is greater than previously inferred, but was limited to between ∼2.14–2.13 Ga. This period of reworking of older crustal components was preceded and succeeded by juvenile crustal addition. Coupled isotopic data suggest an eastward, mainly retreating arc system with a shorter pulse of accretion between ∼2.18–2.13 Ga and a rapid return to slab retreat during the growth of the Birimian terrane. The accretionary phase initiated melting of sub-continental lithospheric mantle and the overlying Archaean crust, generating magma with sub-chondritic Hf signatures. Subsequent slab retreat led to trench-ward movement of the magmatic activity and the mixture of juvenile and Archaean crust was replaced by uncontaminated juvenile magma. The 2.23 Ga age of the West Accra granodiorite (PK105) demonstrates the emplacement of felsic rocks during the Eoeburnean and pre-dates the suggested plume related rocks, contradicting suggested plume initiated subduction

Zircon U-Pb, Hf and O isotope constraints on growth versus reworking of continental crust in the subsurface Grenville orogen, Ohio, USA

Combined U-Pb, O and Hf isotope data in zircon allows discrimination between juvenile and reworked crust, and is therefore a useful tool for understanding formation and evolution of the continental crust. The crustal evolution of basement rocks in central North America (Laurentia) is poorly constrained, as it is almost entirely overlain by Palaeozoic cover. In order to improve our understanding of the evolution of this region we present U-Pb, O and Hf isotope data from zircon in drill-core samples from the subsurface basement of Ohio. The Hf isotope data suggests juvenile crust formation at similar to 1650 Ma followed by continued reworking of a single reservoir. This similar to 1650 Ma reservoir was tapped at similar to 1450 Ma during the formation of the Granite-Rhyolite Province and subsequently reworked again during the Grenvillian orogeny. The similar to 1650 Ma crust formation model age for the suite of samples along with the presence of similar to 1650 Ma magmatic rocks suggests an eastward extension of the Mazatzal Province (or Mazatzal-like crust) and makes it a possible protolith to the subsurface basement of Ohio and surrounding Mesoproterozoic (i.e. Grenville-age) rocks. The eastward extension of this similar to 1650 Ma crustal reservoir into Ohio requires a revision of the crustal boundary defined by Nd isotopic data to be located further east, now overlapping with the Grenville front magnetic lineament in Ohio. In fact, the easternmost sample in this study is derived from a more depleted reservoir. This limits the extent of >1.5 Ga basement in subsurface Ohio and constrains the location of the crustal boundary. Further, syn-orogenic magmatism at similar to 1050 Ma suggests a potential extrapolation of the Interior Magmatic Belt into Ohio. Oxygen isotopic data in zircon suggests that during Grenvillian metamorphism, zircon recrystallisation occurred in the presence of heavy delta O-18 fluids resulting in zircon with elevated delta O-18 values. (C) 2015 Elsevier B.V. All rights reserved

Remnants of Mesoarchaean oceanic crust in the Tartoq Group, South-West Greenland

The Tartoq Group is located in the Sermiligaarsuk fjord region in South-West Greenland in an area of approximately 20 × 50 km (Fig. 1). The Tartoq Group consists of several discrete, fault-bound blocks of metavolcanic rocks, surrounded by Archaean tonalite-trondhjemite-granodioritetype (TTG) gneisses. A zircon age of 2996.3 ± 5.9 Ma of a TTG intrusion provides a minimum age for the formation of the Tartoq Group (Fig. 2). The metavolcanic rocks probably show the lowest degree of metamorphism found anywhere in the Archaean craton of Greenland. Here we present a new model for the origin of the metavolcanic rocks of the Tartoq Group based on geochemical, metamorphic and structural data. The samples used for this study were collected by the Geological Survey of Denmark and Greenland (GEUS) in 2009 and 2010. The study is part of a joint project between the Greenland Bureau of Minerals and Petroleum and GEUS on the mineral potential of south-western Greenland

Preliminary Archaeopetrological Study of the Lithic Industry From the l’Hort de la Boquera Rock Shelter (Margalef de Montsant, Tarragona, Spain): Applying Mineralogical and Geochemical Techniques

The l’Hort de la Boquera site is located in the northeastern part of Iberia and its stone tool assemblage includes up to 25,000 flint artefacts. This is the first approach to the analysis of the raw material through an archaeopetrological study. Results were obtained by use of mineralogi¬cal techniques: macroscopic and petrographic analysis, Scanning Electronic Microscopy (SEM), Micro-Raman and X-Ray diffraction (XRD); additionally, Laser Ablation Inductively Coupled Plasma Mass Spectrometry was applied. It has been possible to discriminate at least four flint categories, the ‘Evaporitic flint type’ (with two local subvarieties – ‘Common evaporitic’ and ‘Garnet’ varieties) that comes from local outcrops of the Ulldemolins Complex, and two flint types that had their origin further afield: the ‘Charophyta flint type’ (coming from the Torrente de Cinca Unit) and the ‘Dark flint type’ (from the La Serra Llarga Formation).These results make this study the most comprehensive analysis of raw materials that has been carried out in the area so fa

New zircon ages from the Tasiusarsuaq terrane, southern West Greenland

In the last three field seasons the Geological Survey of Denmark and Greenland (GEUS) has undertaken mapping in the south-eastern part of the Nuuk region in southern West Greenland, and here we present new zircon ages that help constrain the northern boundary of the Tasiusarsuaq terrane. The Archaean geology of the Nuuk region is commonly interpreted as a tectonic collage assembled through lateral accretion and collision of oceanic and continental slivers and blocks (e.g. Friend & Nutman 2005). Popular jargon describes these as terranes, bounded by faults or mylonite zones and characterised by rocks of contrasting origin on either side of their tectonic boundaries (Coney et al. 1980). The Isukasia and Færingehavn terranes (Figs 1, 2) are the oldest terranes at ≥3.75 Ga, and extend from the outer part of Godthåbsfjord in the south-west to the margin of the Inland Ice in the north-east, but they might not have a common geological history (Friend & Nutman 2005). The Tre Brødre terrane is mainly represented by the Ikkatoq gneiss and occurs in close spatial relationship with the Færingehavn terrane, and also as a pronounced thrust unit along the Qarliit Nunaat thrust between the Færingehavn and Tasiusarsuaq terranes (Fig. 1; Nutman et al. 1989). The terrane boundaries in the inner fjord region near the Inland Ice margin are less well constrained; the Tre Brødre terrane extends into the region from the south-west, the Kapisilik terrane is defined from the northern and eastern part and borders the Tasiusarsuaq terrane to the south and possibly to the east. The terrane accretion is believed to have taken place in two events. The first terrane accretion is defined from the northern part of the region, and possibly involves the Isukasia, Kapisilik and Akia terranes. The thermal event stitching these terranes is dated to c. 2.99–2.95 Ga (Fig. 2; Hanmer et al. 2002; Friend & Nutman 2005). The second accretion phase of the major continental blocks is believed to have occurred at around 2.725–2.71 Ga. This second event is well described, and includes anatexis and emplacement of continental crust-derived granites, which are associated with contemporaneous metamorphism (Friend et al. 1996). Figure 2 outlines regional plutonic, metamorphic and supracrustal events. Individual terranes were formed during relatively short time periods with active geological processes of creation and recycling of continental crust, and most of the terranes follow a similar pattern of development. The first plutonic events consisted of primitive magmas and produced tonalite–trondhjemite–granodiorite (TTG) and dioritic gneisses. Younger, more evolved granitic magmas were often intruded simultaneously with high-grade metamorphism. This development may reflect a stabilisation of the individual terranes