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Polat A.,University of Windsor | Fryer B.J.,University of Windsor | Samson I.M.,University of Windsor | Weisener C.,University of Windsor | And 4 more authors.
Precambrian Research | Year: 2012

The Fiskenæsset Complex, SW Greenland, contains the world's best preserved Archean (~2970 Ma) layered anorthosite, leucogabbro, gabbro, and ultramafic association. The complex was emplaced into Archean oceanic crust distal from continental lithosphere and later intruded by tonalites, trondhjemites and granodiorites (TTG) constituting Archean continental crust. The complex and bordering TTG intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions.This study presents new whole-rock major and trace element, petrographic, and SEM (Scanning Electron Microscope)-BSE (backscatter electron) image data for a 45-60 m-thick ultramafic sill and cross-cutting hornblendite veins. The ultramafic sill is composed of ~75% olivine-pyroxene hornblendite, ~15% pyroxene hornblendite, and ~10% hornblende pyroxenite. Despite granulite facies metamorphism and multiple phases of deformation, the Fiskenæsset ultramafic sill shares many petrographic characteristics of unmetamorphosed mafic to ultramafic layered intrusions and Alaskan-type ultramafic complexes. Hornblende appears to have originated as an igneous mineral but underwent extensive recrystallization during granulite facies metamorphism. Formation of orthopyroxene-magnetite symplectitic (vermicular) intergrowths, mainly at the expense of olivine, is attributed to chemical reactions between late stage, residual hydrous melts and olivine.Field, geochemical and petrographic observations suggest that the ultramafic sill did not undergo significant fractional crystallization following its intrusion. High MgO (16-31. wt.%) contents suggest that the sill was emplaced as a crystal mush, rather than as melt only. Petrographic observations and BSE images indicate that olivine, orthopyroxene, and clinopyroxene were the major crystal phases, whereas hornblende represents the main interstitial liquid phase in the crystal mush. The sill exhibits three different REE patterns, corresponding to three cryptic layers. Although the origin of these REE patterns is not fully understood, they may reflect the intrusion of three batches of magma, containing different hornblende-forming melt/crystal ratios. Geochemical data indicate that the cross-cutting hornblendite veins likely originated from evolved, late-stage melts but were not related to the melts of the ultramafic sill.Large negative Nb anomalies (Nb/Nb* = 0.04-0.66) suggest that the Fiskenæsset hornblendites, hornblende pyroxenites, and hornblendite veins were derived from a hydrous sub-arc mantle peridotite. Phanerozoic hornblendites are typically associated with supra-subduction zone ophiolites and magmatic arcs. Accordingly, it is suggested that water was recycled to the source of the Fiskenæsset hornblendites through subduction of altered oceanic crust. Recycling of water to the upper mantle via subduction not only resulted in the generation of hornblendites, but also played an important role in the formation of TTG-dominated Archean continental crust. Therefore, the field, petrographic and geochemical data presented in this study suggest that the origin of the Mesoarchean to Neoarchean terrane in the Fiskenæsset region, SW Greenland, is consistent with subduction zone geodynamic processes, rather than non-uniformitarian, density-driven, vertical, crustal overturn, and diapiric processes. © 2011 Elsevier B.V. Source


Frei R.,Copenhagen University | Frei R.,Nordic Center for Earth Evolution | Polat A.,University of Windsor
Precambrian Research | Year: 2013

This study reports detailed stable Cr, Sm-Nd and Rb-Sr isotope data for a ca. 1.9Ga old subaerial weathering profile at Schreiber Beach, Ontario, Canada, from which detailed major and trace element signatures and δ18O values were previously reported. The weathering profile developed on Neoarchean (∼2.7Ga) pillow basalts and is unconformably overlain by the Paleoproterozoic (∼1.88Ga) Gunflint Chert and basal conglomerates. This stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the Gunflint Formation.The aim was to investigate the behaviour of chromium during ancient weathering processes and to contribute to the question whether or not stable isotope fractionation accompanied the release of chromium from the weathering rocks at times when atmospheric oxygen was supposedly high enough (in a period following the Great Oxygenation Event (GOE) ∼2.45. Ga ago and prior to the deposition of the Gunflint cherts at ∼1.82. Ga). These iron-rich cherts belong to one of the last regionally extensive banded iron formations (BIFs) deposited before the ∼1 billion year-long Mesoproterozoic period during which a drastic change in ocean chemistry prevented deposition of BIFs.The gradual textural, mineralogical, and geochemical changes from unweathered basalts to strongly weathered hematite-bearing basalts with increasing stratigraphic height are associated with shifts in the chromium isotope compositions. The δ53Cr value of unweathered pillow basalt cores (-0.19±0.02‰, 2σ) is within the range of mantle inventory values, whereas weathered brown to green basalts (soils), exhibiting up to 30% lower Cr concentrations compared to unweathered pillow cores, are isotopically lighter (δ53Cr=-0.35±0.11‰). In contrast, red, hematite-rich basalts and hyaloclastites underlying the brown to green basaltic soils, with the highest δ18O enrichment in the profile, are isotopically heavier (δ53Cr=+0.05±0.15‰). Rb-Sr isotope data of weathered basalts define a correlation line with a slope corresponding to an age of 1574±24Ma (MSWD=13) which we interpret to reflect a diagenetic event accompanied by alkaline metasomatism during subsidence/burial of the Gunflint basin. The non-correlation of K enrichment factors with δ53Cr, δ18O and other major and trace elements in the weathered rocks indicates that alkaline metasomatism did not affect the geochemical signatures produced during in situ subaerial weathering. Instead, correlations between δ53Cr and δ18O, and between δ53Cr, Ce/Ce*, U and V, indicate coupled mobilization of the redox sensitive elements during oxidative processes and their partial re-deposition at depth from high δ18O freshwaters. Partial re-precipitation of heavy Cr at depth can be explained on the basis of the weak positive correlation between δ53Cr and Fe enrichment factors; it is seen as a consequence of the reduction of mobile Cr(VI) and co-precipitation as mixed Fe(III)-Cr(III) oxhydroxides during contemporaneous oxidation of the Fe2+-bearing groundwaters at depth, a process similar to the one governing the deposition of most Precambrian BIFs. Redistribution of certain elements during the weathering process, in this case REEs, is furthermore indicated by a correlation line in a Sm-Nd isochron diagram defined by the Schreiber Beach data with a slope corresponding to an age of 1.93±0.19Ga (MSWD=26), an age which is compatible with the direct geological constraints for the timing of the paleosol formation at this locality.An oxidative atmosphere at ∼1.9Ga, as implied by the results from the Schreiber profile, is furthermore supported by positively fractioned Cr isotopes (δ53Cr from +0.06 to +0.39‰) recorded in the iron-rich Gunflint Cherts directly above the palaeo-weathered horizons at Schreiber Beach. These values are interpreted to stem from a positively fractionated shallow seawater chromium composition at ∼1.88Ga, potentially reflecting a continental run-off characterized by positive δ53Cr, and are in accordance with Cr isotope signatures in worldwide BIFs (including the Gunflint Iron Formation).Our results show the potential of Cr isotope studies on ancient paleosols to untangle the presence of oxidative weathering processes, making this isotope system a viable and important tracer for the reconstruction of surface oxygenation in Earth's history. © 2012 Elsevier B.V. Source


Berger A.,University of Bern | Janots E.,ISTerre | Gnos E.,Natural History Museum Geneva | Frei R.,Copenhagen University | And 2 more authors.
Applied Geochemistry | Year: 2014

In this study, rare earth element (REE) distribution has been investigated in a weathering profile from central Madagascar. Combination of bulk rock geochemical data (elements and isotopes) with mineral characterization reveals a remarkable evolution of the REE abundances and REE-minerals in the vertical weathering profile. In the fresh tonalite (bedrock), REE+Y concentrations are typical of granitoids (299-363ppm) and the main REE-minerals are allanite and chevkinite. In the C-horizon (saprolite), primary REE-minerals disappear and REEs are transported via fluid to precipitate rhabdophane group minerals in cracks and pores. The presence of sulfate ligands, produced by sulfide oxidation, may be responsible for the REE speciation, as suggested by the composition of the secondary REE-minerals. Rhabdophane group minerals contain up to 9wt% SO3 and 7wt% CaO, indicating a mixture between rhabdophane sensu stricto, (REE)PO4·H2O, and tristamite, (Ca,U,Fe(III))(PO4,SO4)·2H2O. Due to intense Ca-leaching, rhabdophane disappears and Al-phosphates (alunite-jarosite group) are found in the soil. Cerianite (Ce(IV)O2) also precipitates in the B-horizon of the soil.Mass transfer calculations based on immobile Ti indicate significant REE leaching in A-horizon with preferential leaching of the heavy REE. REEs accumulate partly in the B-horizon. The uniform Nd isotope compositions and the constant proportion of immobile elements do not reveal external input. In the B-horizon, total REE+Y reach 2194ppm with high Ce concentrations (1638ppm; 9*Cebedrock) compared to other REE (3-4*REEbedrock). Tetravalent Ce state is dominant in the B-horizon and requires oxidizing conditions that likely account for the accumulation of redox-sensitive elements in B-horizon (e.g., Mn, Fe, Co). Under oxidizing conditions, cerianite precipitation causes a Ce fractionation from other trivalent REE. In comparison to the ion adsorption clay of southern China, preferential heavy REE enrichment was not observed in the weathering profile.Another remarkable peculiarity of the studied profile is the occurrence of Gd2SO6 grains. The discovery of this new mineral demonstrates that a natural process exists that that can fractionate REE to such an extent to produce a pure gadolinium end-member mineral. An understanding of such a mechanisms is crucial for the REE geochemistry of low temperature alteration processes as well as for the formation of REE ore deposits or industrial processing. © 2013 Elsevier Ltd. Source


Naeraa T.,Lund University | Naeraa T.,Geological Survey of Denmark | Kemp A.I.S.,University of Western Australia | Schersten A.,Lund University | And 3 more authors.
Lithos | Year: 2014

The late Neoarchaean QÔrqut Granite Complex is the youngest large igneous intrusion in the Nuuk region in southern West Greenland, where basement is primarily of Eoarchaean and Mesoarchaean age with a tonalite-trondhjemite-granodiorite (TTG) composition. The QÔrqut granite is generally undeformed and it intruded during a prolonged period, starting at ca. 2730Ma, characterised by crustal reworking, possibly related to syn- or post accretion tectonics or continental collision. We present major and trace element whole rock chemistry and combined U/Pb, Hf and O isotope data from zircon. We obtained a mean zircon U/Pb age of 2547±4Ma (MSWD=0.63). Initial εHf values range from -12 to -18 requiring a long residence time and a rather homogeneous source. Sample averaged zircon δ18O values range from 6.1±0.2‰ to 6.5±0.3/0.7‰ best interpreted with a source region of mainly unweathered mantle derived igneous rocks. Compared to the regional TTG basement, the QGC is characterised by low CaO and Na2O and high K2O, LREE and Rb contents, and a stronger fractionated REE pattern with a negative Eu anomaly. We show that the homogeneous Hf isotope signature of the granite together with its low epsilon value and its pristine oxygen isotope composition are best explained with an Eoarchaean mafic source with a 176Lu/176Hf around 0.015-0.019. Trace element modelling confirms that a mafic source in REE and with an eclogitic residue and with plagioclase as a fractionating phase would generate appropriate melt compositions. Modelling requires residual rutile in the source which constrain the pressures to >ca. 13-18kbar. Zirconium saturation temperatures suggest magma temperatures in the range 750-850°C. The obtained P-T conditions suggest a lower crustal source region in a thickened crustal unit consistent with a post or late continental collisional setting. © 2014 Elsevier B.V. Source


Polat A.,University of Windsor | Frei R.,Copenhagen University | Frei R.,Nordic Center for Earth Evolution | Schersten A.,Lund University | Appel P.W.U.,Geological Survey of Denmark
Chemical Geology | Year: 2010

The Archean Fiskenæsset Complex, SW Greenland, consists of an association of ca. 550-meter-thick layered anorthosite, leucogabbro, gabbro, and ultramafic rocks (peridotite, pyroxenite, dunite, hornblendite). The complex was intruded by tonalite, trondhjemite, and granodiorite (TTG) sheets (now orthogneisses) during thrusting that was followed by several phases of isoclinal folding. The trace element systematics of the Fiskenæsset Complex and associated volcanic rocks are consistent with a supra-subduction zone geodynamic setting.The Fiskenæsset anorthosites, leucogabbros, gabbros and ultramafic rocks collectively yield an Sm-Nd errorchron age of 2973±28Ma (MSWD=33), with an average initial εNd=+3.3±0.7, consistent with a long-term depleted mantle source. Regression of Pb isotope data define an age of 2945±36Ma (MSWD=44); and the regression line intersects the average growth curve at 3036Ma. Slightly lower Pb-Pb errorchron age is interpreted as reflecting partial disturbance of the U-Pb system in gabbros, leucogabbros and ultramafic rocks during intrusion of TTGs.Complex internal structures in zircons from orthogneisses reveal several episodes of zircon growth and recrystallization taking place between ca. 3200 and 2650. Ma. Zircon ages peak at about 3200, 3100, 3000, 2950, 2820, and 2750. Ma. The 3200-3000. Ma zircon cores are interpreted as inherited xenocrysts from older reworked crustal rocks. 2950. Ma is considered as an approximate intrusion age of sampled TTGs. The 2940-2650. Ma ages are attributed to metamorphic overgrowth and recrystallization in response to multiple tectonothermal events that affected the Fiskenæsset region.On the basis of recently published trace element data, and new Nd and Pb isotope and U-Pb zircon age data, a three-stage geodynamic model is proposed to explain the evolution of the Fiskenæsset Complex. Stage 1 represents the formation of depleted shallow mantle source>3000Ma (εNd=+3.3±0.7) for the complex. Stage 2 corresponds to the development of an intra-oceanic island arc between 3000-2950Ma. Stage 3 is characterized by the collision of the island arc with either a passive continental margin or with an older arc between 2950-2940Ma. © 2010 Elsevier B.V. Source

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