A neoproterozoic age for the chromitite and gabbro of the Tapo Ultramafic Massif, Eastern Cordillera, Central Peru, and its tectonic implications

The ultramafic-mafic rocks of the Tapo Complex are exposed in the Eastern Cordillera of the Central Peruvian Andes. This complex is composed of serpentinised peridotites and metabasites with some podiform chromitite lenses and chromite disseminations and overlies the sandstones, conglomerates, and tuffs of the Carboniferous Ambo Group. The metagabbros and amphibolites showa tholeiitic affiliation and a flat REE spider diagram, with a slight LREE depletion and a positive Eu anomaly suggesting magmatic accumulation of plagioclase, in an ocean ridge or ocean island environment. Sm-Nd isotopic analyses were performed on chromite as well as on whole rock from the gabbro. All samples yielded an Sm-Nd isochrone age of718 ± 47 Ma with an initial 143Nd/l44Nd of0.51213 ± 0.00005. The Nd (718 Ma) values calculated for both chromite and gabbro are in close agreement, around 8.0, implying that they were formed at the same time from the same mantelic magma source. Furthermore a K-Ar age on amphibole of 448 ± 26 Ma was obtained, interpreted as the cooling age of a younger orogenic event. These rocks represent slices of oceanic crust (from a dismembered ophiolitic complex), metamorphosed and later overthrust on upper Palaeozoic continental formations

Assessment of CO2 storage prospect based on physical properties of Rio Bonito Formation rock units

The physical properties of the rock units associated with the Rio Bonito Formation are presented in this study with the focus on modelling reservoir quality based on petrophysics-derived parameters to evaluate CO2 storage potentials. It involves the modelling of the reservoir depths, thicknesses, flow zone indicators (FZI), and effective permeability (Keff) and presenting the CO2 storage efficiency factors peculiar to the rock units of the study location. Research results presented by this study for the stated objectives are not quite common in the region. Keff values range from 200 mD to higher than 2000 mD, and FZI values are generally above 1.0 μm and up to 13.0 μm within the portions covered by the drilled wells. The sandstone units recorded are up to 20 m thick in some cases. The Keff and FZI models indicate the sandstone reservoirs as permeable units to support the injection and circulation of CO2 within the potential reservoir units of the Rio Bonio Formation across São Paulo State. Apart from some points in the southeastern part of the study location, where the Rio Bonito are delineated at depths less than 800 m (minimum CO2 storage depth based on best practices), other portions are deeper, ranging from 950 m to 3500 m. Thin-bedded layers will affect the integrity of the rocks as CO2 storage tanks or reservoir seals/traps/overburden within the region. Sandstone bed thicknesses are up to 20 m in some cases. However, hybrid CO2 reservoir units are feasible, especially in portions where thin siltstone layers are sandwiched between sandstone units to provide considerable thicknesses based on CO2 storage standards. The current study shows that useable areas considering reservoir thickness, depth, and other physical qualities will significantly control the CO2 storage efficiency of the study location. Further studies featuring a detailed geophysical exploration of the site to confirm the availability and saturations of pre-existing fluid (hydrocarbon and water) are encouraged to boost CO2 storage in the region. The related research-based results, as mentioned above, may be combined with the results of this research to determine the area's potentials for CO2 storage or hydrocarbon production with CO2 storage options

PETROGENESIS OF THE EARLY CRETACEOUS VALLE CICO IGNEOUS COMPLEX (SE URUGUAY): RELATIONSHIPS WITH PARANÀ-ETENDEKA MAGMATISM

The early Cretaceous (~130 Ma) igneous complex of Valle Chico (SE Uruguay) is made up of felsic plutonic and subordinate volcanic rocks and dykes cropping out over an area of about 250 km2. This complex is strictly linked with the formation of the Parana´–Etendeka Igneous Province and the first stages of the South Atlantic Ocean rifting. The plutonic rocks range from quartz-monzonite to syenite, quartz-syenite and granite. The volcanic rocks and the dykes range from quartz-latite to trachyte and rhyolite; no substantial differences in term of chemical composition have been found between plutonic and volcanic rocks. Only a sample of basaltic composition (with tholeiitic affinity) has been sampled associated with the felsic rocks. The Agpaitic Index of the Valle Chico felsic rocks range from 0.72 to 1.34, with the peralkaline terms confined in the most evolved samples (SiO2N65 wt.%). Initial 87Sr/86Sr(130) of the felsic rocks range from 0.7046 to 0.7201, but the range of 87Sr/86Sr of low-Rb/Sr samples cluster at 0.7083; 143Nd/144Nd(130) ratios range from 0.5121 (syenite) to 0.5117 (granite). The tholeiitic basalt show more depleted isotopic compositions (87Sr/86Sr(130)=0.7061; 143Nd/144Nd(130)=0.5122), and plots in the field of other early Cretaceous low-Ti basaltic rocks of SE Uruguay. The radiogenic Sr and unradiogenic Nd of the Valle Chico felsic rocks require involvement of lower crustal material in their genesis either as melt contaminant or as protolith (crustal anatexis). In particular, most of the Valle Chico (VC) felsic rocks define a near-vertical array in Sr–Nd isotopic spaces, pointing toward classical EMI-type composition; this feature is considered to reflect a lower crust involvement as observed for other mafic and felsic rocks of the Parana´–Etendeka Igneous Province. Decompression melting of the lower crust related to Gondwana continental rifting before the opening of the South Atlantic Ocean or the presence of thermal anomalies related to the Tristan plume may have induced the lower crust to partially melt. Alternative hypothesis considers contamination of upper mantle by a mafic/ultramafic keel composed of lower crust and uppermost mantle after delamination and detachment processes. This interaction may have occurred after the continent–continent collision during the last stages of the Panafrican Orogeny. This blower crustQ model does not exclude active involvement of upper crust as contaminant, necessary to explain the strongly radiogenic 87Sr/86Sr(130) isotopic composition of some VC SiO2-rich rocks. Mineralogical (sporadic presence of pigeonite, Ca–Na and Na clinopyroxene, calcic- and calco-sodic amphibole) and geochemical evidences (major and trace element as well as Sr–Nd isotopic similarities with the felsic early Cretaceous volcanic rocks of the Arequita Formation in SE Uruguay) allow us to propose for the VC rocks a transitional rock series (the most abundant rock types are of syenitic/trachytic composition) preferentially evolving towards SiO2-oversaturated compositions (granite/rhyolite) also with a strong upper crustal contribution as melt contaminant. This conclusion is in contrast with previous studies according which the VC complex had clear alkaline affinity. Many similarities between VC and the coeval Paresis granitoids (Etendeka, Namibia) are evidenced in this paper. The genetic similarities between VC and the rhyolites (s.l.) of SE Uruguay may find counterparts with the genetic link existing between the early Cretaceous tholeiitic-alkaline Messum complex and the quartz latites (s.l.) of the Awahab Formation (Etendeka region, Namibia)

Zircon provenance in meta-sandstones of the São Roque Domain : Implications for the Proterozoic evolution of the Ribeira Belt, SE Brazil

Detrital zircons from quartzites of the São Roque Domain were subject of studies of crystal growth and stratigraphy, using in situ LA-ICPMS trace element geochemistry and U–Pb dating to deduce their provenance and contribute to understand the tectonic evolution of the Proterozoic metasedimentary successions of southeast Brazil. Trace element chemistry of most detrital cores indicates derivation from plagioclase-rich felsic rocks (e.g., tonalites to granodiorites); however, a small proportion may derive from gabbroic sources, as indicated by a combination of features (strong positive Ce anomaly, high LuN/SmN, low U/Yb; concave-down shape of the intermediate REE patterns). Thin metamorphic overgrowths are chemically distinct (e.g., enriched in several trace elements, especially the LREE), and were dated at 584 ± 47 Ma, reflecting the Ediacaran metamorphism and granite magmatism that affected the São Roque Domain. The detrital zircon U–Pb ages for five quartzite samples from different stratigraphic positions within the São Roque Domain are restricted to the Paleoproterozoic and Archean (∼1730–3440 Ma). A striking feature of the dataset is a main peak at ∼2.2 Ga which is characteristic of all samples; the age and abundance of Archean peaks allow distinguish two groups of samples: Jaraguá and Japi, with 41–43% Archean zircons and a peak at 2.6–2.7 Ga; and Voturuna, Pirucaia and Serra da Viúva, with 16–21% Archean zircons and a peak close to the Archean-Paleoproterozoic boundary (2.4–2.5 Ga). A limited contribution of late Paleoproterozoic (Statherian) sources places an upper limit for the deposition of the sedimentary successions that is consistent with the estimated age of the metaconglomerate-metarkose rift sequence of the São Roque Group (Boturuna Formation), based on U–Pb dating of interlayered meta-trachydacites (1.75 Ga). Our data demonstrate that the different sedimentary successions of the São Roque Domain have an essentially continental (cratonic) provenance which is very similar to those of other sequences in the Ribeira Belt (Açungui and possibly also Embu Domain). The ∼2.2 Ga age of the volumetrically most important sources, also indicated by the age of granite pebbles in metaconglomerates, is a diagnostic signature, consistent with the age of regional basement (e.g., orthogneisses from few basement nuclei in the Açungui Domain). Similarities in provenance and tectonic evolution of the metasedimentary sequences are consistent with the interpretation that the Ribeira Belt is part of the reworked border of the São Francisco Paleoplate, displaced southwestward as a result of significant dextral orogen-parallel tectonics during the Neoproterozoic collision, and may imply that a suture zone between this paleoplate and another stable continental area to the SW lies below the Phanerozoic sediments of the Paraná Basin

Provenance and tectonic setting of Proterozoic metasedimentary sequences of the São Roque Domain, Ribeira Fold Belt, Brazil: a combination of whole-rock geochemistry, Sm-Nd isotopic systematics and detrital zircon U-Pb geochronology

The Proterozoic São Roque Group (Ribeira Fold Belt, southeast Brazil) is a metasedimentary sequence deposited in a marine environment consisting of proximal metasandstones and meta-felspathic wackes with some volcanic layers (Boturuna Formation) and more distal metawackes and metamudstones (Piragibu Formation). A combination of zircon provenance studies in metasandstones (textural and trace-element analysis and U-Pb geochronology) and whole-rock major and trace-element geochemistry and Sm-Nd isotopic systematics in metamudstones was used to understand the provenance and tectonic significance of this sequence, and their implications to the evolution of the Precambrian crust in the region. Whole-rock geochemistry indicates that the sources are largely granitic (as indicated for instance by the LREErich moderately fractionated REE patterns and subtle negative Eu anomalies) with some mafic contribution (responsible for higher contents of Fe2O3, MgO, V, and Cr) and were subject to moderate weathering (CIA - 60 to 82). The trace element signatures in detrital zircons indicate that most of them are derived from plagioclase-rich felsic rocks, as indicated by strong positive Ce anomalies, high (Lu/Sm)N ratios, low U/Yb, and a concave-down shape of the intermediate REE in chondrite-normalized plots. A significant proportion of the zircon crystals show rounded cores with growth zoning truncated and overgrown by a thin rim that has dark color in cathodoluminescence images. These overgrowths are chemically distinct, being enriched in trace elements, especially the LREE, and were dated at 584±47 Ma, reflecting the regional Neoproterozoic metamorphism. Sm-Nd isotope data for Piragibu Formation metamudstones show four main groups of Nd TDM ages at ca. 1.9 Ga, 2.1 Ga, 2.4 Ga and 3.0 Ga. The younger ages define an upper limit for the deposition of the unit, and reflect greater contributions from sources younger than the >2.1 Ga basement. The oldest Nd TDM age (3.0 Ga) is similar to those of 2.2 Ga granitic clasts from the Boturuna Formation metaconglomerates. The age spectra of detrital zircons from five samples of quartzite from the São Roque Domain show a wide range (1.7-3.4 Ga). The youngest detrital crystals, although subordinate, place a minimum limit for the depositional age of the São Roque metasedimentary sequences at ∼1.8-1.7 Ga, which is consistent with U-Pb dating of interlayered metavolcanic rocks. The coincident age peaks of Nd TDM and U-Pb detrital zircons at 2.1-2.2 Ga and 2.4-2.5 Ga, combined with the presence of a small amount of zircons derived from mafic (gabbroid) sources with the same ages, are suggestive that these were major periods of crustal growth in the sources involving not only crust recycling but also juvenile additions. A similar geochemical behavior, consistent with sediments deposited in a passive margin basin for both Formations (Piragibu and Boturuna), suggests that these were coeval, probably with lateral correlation. However, the mudstones (Piragibu Formation) have Ce-depleted REE patterns compatible with clays from open sea, suggesting a more complex paleo-environment, probably with interaction between passive margin basin and volcanogenic sources related to oceanic island arc