| 1. |
- Lewerentz, Alexander, et al.
(författare)
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Baddeleyite formation in zircon by Ca-bearing fluids in silica-saturated systems in nature and experiment : resetting of the U-Pb geochronometer
- 2019
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Ingår i: Contributions to Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0010-7999 .- 1432-0967. ; 174:8
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Tidskriftsartikel (refereegranskat)abstract
- Intergrowths of baddeleyite have been found in zircon grain interiors from amphibolite- and granulite-facies felsic rocks from southwest Greenland. The zircon grains are either close to or in direct contact with quartz. A series of experiments has been conducted using natural, unaltered zircon grains +/- SiO2 in H2O-CaCl2, and H2O-Ca(OH)(2) solutions with varying molar proportions of Ca to Si at 900 degrees C; 1000MPa and 600 degrees C; 400MPa for 4-50days. Experimental results indicate that baddeleyite formed in the reacted zircon if the molar amount of Ca was close to or greater than Si in the system. The baddeleyite primarily takes the form of bead-like trails along the reaction front between the altered and unaltered zircon. Uranium, Th, and Y+REE were detected in both the newly formed baddeleyite and in the altered zircon, while Pb was effectively absent in both phases. Formation of baddeleyite from zircon in the silica-saturated rocks only appears to be possible when Ca saturates the system, such that the Si is tied up as CaSiO3 lowering the silica activity to <1. This highly localized (mu m to nm scale) effect in natural quartz-bearing rocks, where baddeleyite forms in the interiors of zircon grains in contact with quartz, implies that metastability in natural rock-forming systems can occur on a very small scale. Non-incorporation of Pb in the newly formed baddeleyite, or in areas of the zircon altered by fluids, implies that either could be used to date the metasomatic event responsible for their formation.
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| 2. |
- Grenholm, Mikael, et al.
(författare)
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A hypothesis for Proterozoic-Phanerozoic supercontinent cyclicity, with implications for mantle convection, plate tectonics and Earth system evolution
- 2015
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Ingår i: Tectonophysics. - : Elsevier BV. - 0040-1951. ; 662:S1, s. 434-453
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Hoffmann, J. Elis, et al.
(författare)
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The geodynamic evolution of Mesoarchean anorthosite complexes inferred from the Naajat Kuuat Complex, southern West Greenland
- 2012
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 196, s. 149-170
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Tidskriftsartikel (refereegranskat)abstract
- Layered anorthosite complexes are typical components of Archean crustal domains. However, the geodynamic settings in which they were emplaced are still discussed as geological relationships are often ambiguous. Here we report major, trace element and high-precision high-field-strength-element (HFSE) data for the recently discovered well preserved Naajat Kuuat Anorthosite Complex from the inner Ameralik fjord region, southern West Greenland. The dataset is complemented by the first combined Hf-Nd isotope analyses for Archean layered anorthosite complexes and U-Pb zircon geochronology. The data contribute to the small database on Archean layered anorthosite complexes and are used to unravel the origin of these complexes and the tectonic regime involved. Fractional crystallisation of olivine, pyroxene, plagioclase and possibly amphibole controls major and trace element variations in the layered intrusion. There are two groups of amphibolites: (1) a group with primitive mantle normalized trace element patterns are similar to those of MORB-like basalts and (2) typical island-arc tholeiites (IAT), apparently indicating an island-arc setting. Lu-Hf regression lines yield an age of 2985 +/- 59 Ma (MSWD 4) within the error of the Sm-Nd regression age of 2929 110 Ma (MSWD 17). The initial epsilon Hf(2985) for the Naajat Kuuat rocks range from +1.6 to +5.8 and the initial epsilon Nd(2985) range from +0.4 to +3.9, either indicating variably depleted mantle sources or variable degrees of crustal contamination. In contrast to most mafic assemblages, ratios of Nb/Ta are highly variable (7.85 to 18.6), reflecting fractionation and accumulation of amphibole, ilmenite and pyroxene. The MORB-like parental liquids have the highest Nb/Ta of ca. 18, consistent with a mantle source overprinted by melt-like components from subducting oceanic crust with high Nb/Ta. The accumulation of plagioclase forming the anorthosites and the primary fractionation of amphibole as well as the occurrence of high-Al basalts within the Naajat Kuuat complex argue for hydrous parental liquids in support of an island-arc related setting. Zircon U-Pb geochronology from the anorthosite and adjacent tonalites reveal major tonalite intrusion into the complex at ca. 2802 Ma and a second regional event at ca. 2710 Ma, in accord with crustal heating due to micro-continent amalgamation and crustal thickening. Altogether, the geochemical data can be interpreted with a geodynamic model, where anorthosite-complex associated rocks intrude into tectonically thickened island-arc crust. Crustal thickening is possibly triggered by island-arc accretion, leading to the emplacement of TTG bodies that further thickened the crustal pile. Further collision and amalgamation with other proto-crustal assemblages might have led to enhanced crustal magmatism and granulite facies metamorphism.
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| 4. |
- Petersson, Andreas, et al.
(författare)
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Zircon U-Pb, Hf and O isotope constraints on growth versus reworking of continental crust in the subsurface Grenville orogen, Ohio, USA
- 2015
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 265, s. 313-327
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- van Schijndel, Valby, et al.
(författare)
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Evidence for Mesoproterozoic collision, deep burial and rapid exhumation of garbenschiefer in the Namaqua Front, South Africa
- 2020
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Ingår i: Geoscience Frontiers. - : Elsevier BV. - 1674-9871. ; 11:2, s. 511-531
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Tidskriftsartikel (refereegranskat)abstract
- Metamorphic provinces such as the ~1 Ga Grenvillian, ~400 Ma Caledonide and Triassic Qinling Provinces oftencontain rocks with high-pressure assemblages such as eclogites, which formed at mantle depths in subductionzones. These are evidence of the accretion of terranes by subduction of oceans and collision to form large tec-tonostratigraphic provinces. The Mesoproterozoic Namaqua-Natal Province comprises a number of terranesthought to have been assembled by plate-tectonic processes, but they have generally yielded metamorphicpressures below 5 kbar, corresponding to<20 km, crustal depths, lacking evidence for subduction processes. TheKaaien Terrane in the Namaqua Front contains two large garbenschiefer units with the unusual paragenesisgarnet-hornblende-epidote-white mica-plagioclase-ilmenite-quartz. Their protoliths are graywackes influenced byandesitic volcanism during their deposition at ~1870 Ma, in a passive margin of the Rehoboth Province orKaapvaal Craton. Prograde garnet growth dated at 1165�5 Ma culminated in peak metamorphic conditions of645�30�C and 10.4�0.7 kbar, corresponding to 40 km depth. This is attributed to subduction of these rocksbefore collision between the overriding arc-related Areachap Terrane, the Kaaien Terrane and the Kaapvaal-Rehoboth cratonic block during the Namaqua orogeny. Exhumation of the garbenschiefer slabs was followedby rapid cooling, as the 1143�5 Ma argon dates of hornblende and white mica, with closure temperatures ~540�C and ~440�C respectively, are the same within error. This was probably due to tectonic juxtaposition of thegarbenschiefer slab with much cooler rock units. The exhumation was accommodated along the Trooilapspan-Brakbosch Shear Zone due to ongoing transpression. Other components of the Namaqua Front have distinctlydifferent P-T-t paths, exemplified by greenschist metamorphism in the 1300 Ma Wilgenhoutsdrift Group, andmedium-pressure metamorphism in the Areachap Terrane. They were juxtaposed by late-tectonic uplift andtranspressional movements. The ~40 km depth of garbenschiefer peak metamorphism is the deepest yet found inthe Namaqua-Natal Province and strengthens the plate tectonic model of accretion by collision of terranes at theend of a Wilson cycle. The high pressure paragenesis of the garbenschiefer was preserved due to its location in theNamaqua Front, whereas most other parts of the Namaqua-Natal Province were overprinted by 1100–1020 Mathermal events after the collision events.
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| 6. |
- Lindh, Anders, et al.
(författare)
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A model for granite evolution based on non-equilibrium magma separation : evidence from the Gharib and Qattar fluorite-bearing granites, Eastern Desert, Egypt
- 2019
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Ingår i: International Journal of Earth Sciences. - : Springer Science and Business Media LLC. - 1437-3254 .- 1437-3262. ; 108:4, s. 1201-1232
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Tidskriftsartikel (refereegranskat)abstract
- We present 77 new granite whole-rock analyses from the Qattar and Gharib areas, Eastern Desert, Egypt. Both areas include a “normal” granite and either a hypersolvus (Gharib) or an almost plagioclase-free granite (Qattar) enriched in fluorite. According to earlier results, F influences element distribution in granitic melts forming complexes with specific elements as Nb, Ta, Ga, Hf, Th, Zn, Sn, whereas F excludes Ba and Sr. We use principal component analyses to split the granite into chemical groups allowing an unbiased study of the inter-group element distribution. This adds the heavy REEs and Y to the earlier lists of elements with an affinity for F. The light REEs show a decreasing affinity with decreasing atomic mass; fluorine separates Sm from Nd, whereas Zr follows La. Opposite to some, but in accordance with other earlier results, the ratio Nb/Ta is higher in the fluorite-enriched than in the other granite. Weak tetrad effects are present. Zircon in the hypersolvus granite is high in common lead. We suggest F to be instrumental for separating Pb 2+ from Pb 4+ . Two hypotheses may explain the occurrence of the two contrasting granites: they have either different sources, or they are co-magmatic, but the magma was split into two discrete types. We apply the second hypothesis as our working hypothesis. The liquidus has a gentler slope with pressure than the diapir requiring crystallisation to be most important in the lower part of the magma chamber. Our hypothesis suggests that globules of magma, enriched in volatile components, form during crystallisation due to slow diffusion rates in the crystallizing magma. Elements accompanying F are distributed into this magma batch, which has a lowered density and viscosity than the rest of the magma due to its increased contents of volatile components. A mushroom-formed diapir rises, forming the hypersolvus (or almost plagioclase-free) granite. Due to an edge effect, it is concentrated close to the wall of the magma chamber. The size and form of the outcropping granite depend on the intersection of the diapir with the erosion surface. Fluorine only makes it possible to follow the process. The model may be generalised to explain the diversification of non-F enriched granite, since the buoyancy of a magma batch several thousand m 3 in size has a much larger impact on the system than the small negative buoyancy of crystals or small crystal aggregates. A-type granite classified merely from its trace element content may form from separated F-enriched magma batches. This may be the reason for their high frequency in the Eastern Desert.
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| 7. |
- Petersson, Andreas, et al.
(författare)
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Birimian crustal growth in the West African Craton : U-Pb, O and Lu-Hf isotope constraints from detrital zircon in major rivers
- 2018
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Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 479, s. 259-271
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Tidskriftsartikel (refereegranskat)abstract
- Combined U-Pb, O and Lu-Hf isotope analyses of detrital zircon grains from five major rivers in Ghana are used to investigate the growth and evolution of the Birimian terrane of the West African Craton. The majority of the analysed zircon yields supra-chondritic εHf values, suggesting derivation from primarily juvenile host magmas. Zircon grains from the Birim River in southwestern Ghana deviate from the generally juvenile trend, where the sub-chondritic εHf but mantle-like δ18O of the 2.15 Ga population suggest reworking of ancient, but unweathered, meta-igneous crust. Hf-O isotope data from detrital zircons of the remaining rivers are consistent with sequential crust generation between ca. 2.2 and 2.1 Ga. Oxygen isotope data suggest that the contribution from Archaean sedimentary material to felsic magmas was below 30%, and in most cases below 10%. Zircon with strongly positive εHf values and elevated δ18O indicate rapid reworking of juvenile crust within southern Ghana. An increase towards heavier oxygen isotope signatures over time suggests a maturing arc system with rapid recycling of juvenile supracrustal material during the evolution of the Birimian terrane.
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| 8. |
- Petersson, Andreas, et al.
(författare)
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Extensive reworking of Archaean crust within the Birimian terrane in Ghana as revealed by combined zircon U-Pb and Lu-Hf isotopes
- 2018
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Ingår i: Geoscience Frontiers. - : Elsevier BV. - 1674-9871. ; 9:1, s. 173-189
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Tidskriftsartikel (refereegranskat)abstract
- Located in the southeastern parts of the Baoulé Mossi domain of the West African Craton, the Birimian terrane in Ghana is known to consist of predominantly juvenile crust emplaced during the Eburnean orogeny. Here we present novel coupled zircon U-Pb and Lu-Hf isotope data from nine Birimian granitoids intruding different domains from northwestern, western and southeastern Ghana. Sub-chondritic e(open) Hf-values indicating reworking of Archaean crust are recorded in zircon from both northwestern and southeastern Ghana. As a conservative estimate 71% of all analysed zircon grains spanning in age from 2220 Ma to 2130 Ma require contribution from a reworked Archaean source, contradicting the common belief that the Birimian terrane consist of predominantly juvenile crust. A minimum e(open) Hf-value of -10.5 at 2139 Ma suggests a Palaeoarchaean to late Mseoarchaean component as the contributing ancient source. Combined with previously reported heavy zircon δ 18O signatures from Birimian river zircon in Ghana, our new data suggests reworking of Archaean aged sediments during subduction initiated crustal growth in the vicinity of one, or possibly in between two Archaean aged cratons.
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| 9. |
- Petersson, Andreas, et al.
(författare)
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The robustness of the Lu-Hf and Sm-Nd isotopic systems during metamorphism – And case study of the Åker metabasite in southern Sweden
- 2023
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Ingår i: Precambrian Research. - 0301-9268 .- 1872-7433. ; 394
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Tidskriftsartikel (refereegranskat)abstract
- While whole-rock Lu-Hf isotope analysis remains one of the only ways to obtain initial Hf isotope signatures of old mafic rocks, Hf isotope analyses of more robust accessory zircon in intermediate to silicic rocks have largely replaced whole-rock analyses during the last decade. This has led to a discrepancy in the amount of existing data from mafic and felsic lithologies. However, especially in mafic, Si-poor rocks with a metamorphic imprint, Hf isotope data rely on whole-rock analysis since baddeleyite, commonly used for U-Pb age analyses of mafic rocks, is sensitive to alteration and metamorphism. Hence, to accurately evaluate the trace element and isotope signatures of altered mafic rocks, it is important to understand the mechanisms of element mobility during metamorphism. Here, we report whole-rock trace element compositions, Lu-Hf and Sm-Nd isotope data from variably deformed and metamorphosed samples of a mafic intrusion in southern Sweden, the Åker metabasite. These data suggest that trace elements were undisturbed on a whole-rock sample scale during deformation at upper amphibolite facies (at least 1000 MPa and 600 °C) metamorphism under hydrated conditions. Despite redistribution of Zr associated with the breakdown of baddeleyite and other igneous phases, the Åker metabasite has retained its chemical and isotopic integrity since igneous crystallisation at ca. 1565 Ma. This study demonstrates and strengthens the feasibility of whole-rock analyses of (meta-)mafic rocks for determining initial εNd and εHf values, despite deformation and metamorphism under hydrated amphibolite-grade metamorphic conditions. Testing the coherence of the calculated initial Nd and Hf isotope ratios by examining variably deformed and metamorphosed varieties of a rock in a single outcrop, could be used as a model for research on more complex Archean rocks.
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| 10. |
- 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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