U-Th-Pb and Lu-Hf systematics of zircon from TTG's, leucosomes, meta-anorthosites and quartzites of the Limpopo Belt (South Africa): Constraints for the formation, recycling and metamorphism of Palaeoarchaean crust
Zeh A.,Am Hubland |
Gerdes A.,Institute For Geowissenschaften |
Barton Jr. J.,University Of Fort Hare |
Precambrian Research | Year: 2010
Results of more than 400 in situ U-Th-Pb and Lu-Hf zircon isotope analyses combined with cathodoluminescence images and field relationships reveal that the oldest magmatic rocks of the Limpopo Belt (Musina area) formed during the Palaeoarchaean between 3.40 and 3.27 Ga, experienced a first anatexis at 3.24-3.07 Ga (M1) and were overlain by sediments mainly between 3.24 and 3.0 Ga. Subsequently, the infra- and supracrustal rocks together underwent two high-grade metamorphic overprints, a first during the Neoarchaean at 2.65 Ga (M2), and a second at 2.025 Ga (M3). Hafnium isotope data additionally reveal that the Palaeoarchaean magmatic rocks result from substantial recycling of much older crust. This is well reflected by nearly chondritic εHf int obtained from primary magmatic zircon domains of four Sand River TTG-gneiss samples (εHf 3.27-3.28 Ga = -1.3 to -1.8), from leucosomes within the Sand River Gneisses (εHf 3.40 Ga = -2.0; εHf 3.28 Ga = -1.5), and from two meta-anorthosite samples of the Messina layered intrusion (εHf 3.35 Ga = 0.1-1.4). Crustal recycling is also supported by hafnium isotope data of detrital zircon grains from two quartzite samples. These data reflect a general increase of the εHf int between 3.65 and 3.2 Ga from ca. -4.0 to 0.0, indicating that an Eo- to Palaeoarchaean crust was mixed with juvenile mantel material in the hinterland of the Limpopo Belt. The present study also demonstrates that Hf isotope data provide a powerful tool, which makes it possible to distinguish zircon domains formed during distinct magmatic or metamorphic events, from such affected by a pseudomorphic alteration, even if the investigated zircon grain underwent multiple alteration processes. © 2010 Elsevier B.V. All rights reserved.
Martins M.V.A.,State University of Rio de Janeiro |
Martins M.V.A.,University of Aveiro |
Hohenegger J.,Geozentrum |
Frontalini F.,Urbino University |
And 3 more authors.
Palaeogeography, Palaeoclimatology, Palaeoecology | Year: 2016
This work provides a comparison between living (LAs) and dead (DAs) benthic foraminiferal assemblages in 53 sampled sites located in intertidal and subtidal areas of Ria de Aveiro (Portugal).The results of multivariate statistical analysis provide evidence for the main factors that control the distribution of DAs and LAs, which seems to correspond to different gradients of marine/continental influence, conditioned by differences in the hydrodynamics of tidal currents. In addition to these factors, the LAs are clearly influenced by the total organic carbon, biopolymers and pollutants. The main factor that however drives the high level of agreement or the degree of disorder between the DAs and the LAs seems to be the hydrodynamics. The highest level of agreement between the DAs and the LAs is found in sheltered inner lagoon areas, out of the tidal currents strongest influence. This study shows an interesting application of advanced statistical analysis to study the similarity between LAs and DAs aiming to identify the zones or conditions of a coastal system in which the DAs can potentially be a better record of Las. Through similar studies conducted in coastal systems it is possible to select the most suitable sites to collect cores in view of the development of paleoenvironmental studies. © 2015 Elsevier B.V.
Miller C.,University of Innsbruck |
Thoni M.,Geozentrum |
Goessler W.,University of Graz |
Tessadri R.,University of Innsbruck
Lithos | Year: 2011
The northern part of the Karawanken plutonic belt is a gabbro-granite complex located just north of the Periadriatic lineament near the Slovenian-Austrian border. Petrographic and geochemical studies of the Eisenkappel intrusive complex indicate that this multiphase plutonic suite developed by a combination of crystal accumulation, fractional crystallization and assimilation processes, magma mixing and mingling. The mafic rocks are alkaline and have within-plate geochemical characteristics, indicating anorogenic magmatism in an extensional setting and derivation from an enriched mantle source. The mafic melts triggered partial melting of the crust and the formation of granite. The granitic rocks are alkalic, metaluminous and have the high Fe/Fe. +. Mg characteristics of within-plate plutons. Temperature and pressure conditions, derived from amphibole-plagioclase and different amphibole thermobarometers, suggest that the analysed Eisenkappel gabbros crystallized at around 1000 ± 20 °C and 380-470. MPa, whereas the granitic rock crystallized at T ≤ 800 ± 20 °C and ≤ 350. MPa. Mineral-whole rock Sm-Nd analyses of two cumulate gabbros yielded 249 ± 8.4. Ma and 250 ± 26. Ma (εNd: +. 3.6), garnet-whole rock Sm-Nd analyses of two silicic samples yielded well-constrained ages of 238.4 ± 1.9. Ma and 242.1 ± 2.1. Ma (εNd: -2.6). © 2011 Elsevier B.V.
Barrier E.,French National Center for Scientific Research |
De Boisgrollier T.,ISTEP UPMC |
Brunet M.F.,ISTEP UPMC |
Bourillot R.,ENSEGID |
And 6 more authors.
75th European Association of Geoscientists and Engineers Conference and Exhibition 2013 Incorporating SPE EUROPEC 2013: Changing Frontiers | Year: 2013
We propose a model of tectono-stratigraphic evolution of Western Central Asia since the Late Palaeozoic. The evolution during this 300My-long period is mainly controlled by successive accretions of continental blocks and collisions that succeeded in Late Carboniferous-Early Permian, Late Triassic-Early Jurassic, Early Cretaceous and Cenozoic times. These orogenies controlled the deposition of thick clastics sequences in the surrounding platforms and basins. The eastern part of Tien-Shan, emerged since the Late Paleozoic, was the main source of sediments during the Mesozoic. Between the main orogenic periods the sea periodically invaded part of the domain. It is only in Cenozoic, with the beginning of the India/Eurasia collision, that the whole western Central Asia domain was emerged. Copyright © (2012) by the European Association of Geoscientists & Engineers All rights reserved.
Miller C.,University of Innsbruck |
Zanetti A.,CNR Institute of Geosciences and Earth Resources |
Thoni M.,Geozentrum |
Konzett J.,University of Innsbruck |
Lithos | Year: 2012
Anhydrous spinel peridotite xenoliths in Quaternary nepheline-basanite and melilite- or sodalite-bearing lavas of the Wau-en-Namus volcano in S Libya range from lherzolites to harzburgites recording melt extraction in a shallow setting (≤2GPa). Primary clinopyroxenes have distinct trace element characteristics documenting LILE (large ion lithophile element) depletion or enrichment events predating the formation of glass pockets and veins in the xenoliths. These glasses are aluminous and alkali-rich, range in composition from ultrabasic to silicic (43-67wt.% SiO 2) and may contain empty vugs and micro-phenocrysts of olivine, clinopyroxene, spinel, plagioclase, sodalite, apatite that are similar in composition to phenocrysts in the host lavas. Reactions of infiltrating melt and xenolith minerals are documented by diffuse Fe-Ca-rich rims of olivine in contact with glass, and by spongy-textured reaction domains caused by incongruent dissolution of primary pyroxenes and spinel. Some glasses have trace element characteristics similar to that of the host Ne-basanite, suggesting they were derived from the same source during entrainment and transport to the surface. Incompatible element enrichment and Sr-Nd isotopic compositions of the analyzed host lava are similar to HIMU (high μ; μ= 238Pb/ 204Pb)-type magmas, but the Pb isotopic composition is less radiogenic compared to other intra-plate Neogene magmatic rocks from N Africa. © 2011 Elsevier B.V..