| 1. |
- Chaves, Alexandre de Oliveira, et al.
(författare)
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The 920–900 Ma Bahia-Gangila LIP of the São Francisco and Congo cratons and link with Dashigou-Chulan LIP of North China craton : New insights from U-Pb geochronology and geochemistry
- 2019
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 329, s. 124-137
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Tidskriftsartikel (refereegranskat)abstract
- A widespread mafic sill and dyke unit (the Pedro Lessa suite) adjacent to the southeastern São Francisco (SF) craton (eastern South America) is dated as 940 ± 42 Ma, 912 ± 17 Ma, and 986 ± 64 Ma (based on combined ID-TIMS and LA-ICPMS analyses on baddeleyite grains) on the Santa Maria de Itabira gabbroic intrusion. These ages suggest membership in the previously defined ca. 920–900 Ma Large Igneous Province (LIP) (herein referred to as the Bahia-Gangila LIP) of the reconstructed São Francisco and formerly adjacent Congo craton (Africa). Additional mafic and felsic units throughout the SF craton are also linked (based on a literature review), thus expanding the size of the reconstructed LIP to ca. 1,500,000 km2. Geochemistry reveals high Ti OIB signature for most units and low Ti E-MORB composition for the Gangila flood basalts, marking the initiation of attempted ocean opening. Geochemistry of the Bahia-Gangila LIP is also compared with the ca. 920–900 Ma Dashigou-Chulan LIP of the North China craton, and implications for three possible reconstructions of SF/Congo craton with the North China craton are considered.
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| 2. |
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| 3. |
- Næraa, Tomas, et al.
(författare)
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The ca. 2785–2805 Ma high temperature ilivertalik intrusive complex of southern west Greenland
- 2018
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Ingår i: Geosciences (Switzerland). - : MDPI AG. - 2076-3263. ; 8:9
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Tidskriftsartikel (refereegranskat)abstract
- Ferroan granitoid intrusions are rare in the Archaean rock record, but have played a large role in the evolution of the Proterozoic crust, particular in relation to anorthosite-mangerite-charnockite-granite suites. Here we discuss the petrogenesis of the ca. 2785–2805 Ma ferroan Ilivertalik Intrusive Complex, which has many geochemical similarities to Proterozoic iron rich granitoids. We present major and trace element whole rock chemistry and combined in-situ zircon U-Pb, Hf and O isotope data. The intrusive complex divides into: (i) minor tabular units of mainly diorite-tonalite compositions, which are typically situated along contacts to the host basement and (ii) interior larger, bodies of mainly granite-granodiorite composition. Geochemically these two unites display continuous to semi-continuous trends in Haker-diagrams. Whole rock REE enrichment display increases from Yb to La, from 10–25 to 80–100 times chondrite, respectively. The diorite-tonalite samples are generally more enriched in REE compared to the granite-granodiorite samples. The complex has hafnium isotope compositions from around +1.5 to −2.5 epsilon units and δ18O compositions in the range of 6.3 to 6.6‰. The complex is interpreted to be derived from partial melting in a crustal source region during anomalously high crustal temperatures.
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| 4. |
- Petersson, Andreas, 1980-
(författare)
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Tracing Proterozoic arc mantle Hf isotope depletion of southern Fennoscandia through coupled zircon U–Pb and Lu–Hf isotopes
- 2017
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Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 284–285, s. 122-131
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Tidskriftsartikel (refereegranskat)abstract
- Constraints on the composition of the depleted mantle Sm–Nd and Lu–Hf crust formation ages have a long history of scientific debate. When calculating mantle extraction ages, and constructing crustal growth models, a linear evolution of incompatible trace elements in a depleted mantle since > 4 Ga is routinely used. Mantle depletion however varies regionally and over time and subduction of sediments and oceanic crust renders a mantle-wedge variously enriched relative to a modelled depleted mantle.Here we show that primitive mantle-derived subduction related gabbroic intrusions from southern Fennoscandia have Hf isotope compositions that are enriched relative to a MORB-like linear depleted mantle evolution curve. Extrapolation of primitive Paleoproterozoic gabbro suites enables the construction of a regional mantle evolution curve, providing improved constraints on model ages, crustal residence times and the fraction of juvenile versus reworked continental crust. Convergent margins are assumed to be one of the main sites of continental crust growth, and using an overly depleted mantle source yield model ages that are too old, and hence cumulative crustal growth models show too much crust generation early in the Earth's history. The approach of using the Hf isotope composition of zircon from primitive subduction related gabbroic intrusions as a proxy for mantle Hf isotope composition, piloted in this study, can be applied to other convergent margins.
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| 5. |
- Rey-Solé, Mar, et al.
(författare)
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Siliceous Raw Material Exploitation at Hort de la Boquera Site (Margalef de Monsant, Tarragona, España) : First Results from La-Icp-Ms Analysis
- 2017
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Ingår i: The Exploitation of Raw Materials in Prehistory : Sourcing, Processing and Distribution - Sourcing, Processing and Distribution. - 9781443895972 - 1443895970 ; , s. 234-249
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Konferensbidrag (refereegranskat)abstract
- Research on the sourcing of siliceous raw materials in Prehistory has taken a step forward with the addition of geochemical techniques and elemental analysis to the archaeopetrological method. Counter to the macroscopic techniques, which retain an elevated subjective percentage in their results, geochemical techniques and especially LA-ICP-MS analysis generate objective, quantifiable and usable data, helping us to complete another mineralogical analysis.95% of the siliceous raw material from the Hort de la Boquera archaeological site (Margalef de Montsant, Tarragona, Spain) is a type of flint from an evaporitic origin that petrografically is characterized by a chalcedonic matrix with gypsum lenticules as inclusions.Siliceous Raw Material Exploitation at Hort De La Boquera Site 235Multiple flint outcrops from an evaporitic origin (Eocene-Oligocene) and with many closed characteristics such as the lithic raw material from Hort de la Boquera are located near the site (Cornudella Group-Ulldemolins Complex), making them very suitable to supply the inhabitants of the Hort de la Boquera site.We have two different but complementary goals in this research: on the one hand, to characterize, describe and create a database for all the different types of flint recovered, emphasizing their geochemical marks. On the other hand, we have the aim to compare these results with those obtained from the raw materials recovered at the site, which would provide a range of complementary data to discover the potential catchment areas and help us to understand certain behaviours of the prehistoric communities from the Hort de la Boquera site.
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| 6. |
- Smit, Mattijs, et al.
(författare)
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Formation of Archean continental crust constrained by boron isotopes
- 2019
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Ingår i: Geochemical Perspective Letters. - : European Association of Geochemistry. - 2410-3403. ; , s. 23-26
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Tidskriftsartikel (refereegranskat)abstract
- The continental crust grew and matured compositionally during the Palaeo- to Neoarchean through the addition of juvenile tonalite-trondhjemite-granodiorite (TTG) crust. This change has been linked to the start of global plate tectonics, following the general interpretation that TTGs represent ancient analogues of arc magmas. To test this, we analysed B concentrations and isotope compositions in 3.8-2.8 Ga TTGs from different Archean terranes. The 11B/10B values and B concentrations of the TTGs, and their correlation with Zr/Hf, indicatedifferentiation from a common B-poor mafic source that did not undergo addition of B from seawater or seawater-altered rocks. The TTGs thus do not resemble magmatic rocks from active margins, which clearly reflect such B addition to their source. The B- and 11B-poor nature of TTGs indicates that modern style subduction may not have been a dominant process in the formation of juvenile continental crust before 2.8 Ga.
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| 7. |
- Szilas, Kristoffer, et al.
(författare)
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Highly refractory Archaean peridotite cumulates : Petrology and geochemistry of the Seqi Ultramafic Complex, SW Greenland
- 2018
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Ingår i: Geoscience Frontiers. - : Elsevier BV. - 1674-9871. ; 9:3, s. 689-714
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Tidskriftsartikel (refereegranskat)abstract
- This paper investigates the petrogenesis of the Seqi Ultramafic Complex, which covers a total area of approximately 0.5 km2. The ultramafic rocks are hosted by tonalitic orthogneiss of the ca. 3000 Ma Akia terrane with crosscutting granitoid sheets providing an absolute minimum age of 2978 ± 8 Ma for the Seqi Ultramafic Complex. The Seqi rocks represent a broad range of olivine-dominated plutonic rocks with varying modal amounts of chromite, orthopyroxene and amphibole, i.e. various types of dunite (s.s.), peridotite (s.l.), as well as chromitite. The Seqi Ultramafic Complex is characterised primarily by refractory dunite, with highly forsteritic olivine with core compositions having Mg# ranging from about 91 to 93. The overall high modal contents, as well as the specific compositions, of chromite rule out that these rocks represent a fragment of Earth's mantle. The occurrence of stratiform chromitite bands in peridotite, thin chromite layers in dunite and poikilitic orthopyroxene in peridotite instead supports the interpretation that the Seqi Ultramafic Complex represents the remnant of a fragmented layered complex or a magma conduit, which was subsequently broken up and entrained during the formation of the regional continental crust. Integrating all of the characteristics of the Seqi Ultramafic Complex points to formation of these highly refractory peridotites from an extremely magnesian (Mg# ∼ 80), near-anhydrous magma, as olivine-dominated cumulates with high modal contents of chromite. It is noted that the Seqi cumulates were derived from a mantle source by extreme degrees of partial melting (>40%). This mantle source could potentially represent the precursor for the sub-continental lithospheric mantle (SCLM) in this region, which has previously been shown to be ultra-depleted. The Seqi Ultramafic Complex, as well as similar peridotite bodies in the Fiskefjord region, may thus constitute the earliest cumulates that formed during the large-scale melting event(s), which resulted in the ultra-depleted cratonic keel under the North Atlantic Craton. Hence, a better understanding of such Archaean ultramafic complexes may provide constraints on the geodynamic setting of Earth's first continents and the corresponding SCLM.
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