David H. Cornell

Geochronology of the Proterozoic Hartley Basalt formation, South Africa: constraints on the Kheis tectogenesis and the Kaapvaal Craton's earliest Wilson Cycle

Abstract The Hartley Basalt Formation in the Northern Cape of South Africa comprises dominant Fe rich tholeiitic basalt and tuff, with porphyritic andesite found at only one locality. Geological evidence points to a rift-related setting which evolved into a passive continental margin as the early Kaapvaal Craton was split up. This marks a change in the stratigraphical record of the Archaean Kaapvaal Craton, from successively larger Witwatersrand, Ventersdorp and Transvaal intracratonic basins, to localised Waterberg Rifts, following the 2050 Ma Bushveld event. These Waterberg rifts are the earliest feature on the craton which can plausibly be related to a plate tectonic Wilson Cycle. The Hartley Formation now crops out within a linear mountain belt, formed at the end of the Wilson Cycle as rocks of the Kheis Tectonic Province overthrust the Archaean Kaapvaal Province along the Kheis Front. Variable degrees of low grade metamorphism are seen in samples from different localities. A 1928 ± 4 Ma PbPb age by the Kober method on clear magmatic zircons from a porphyry sample is consistent with, and represents a substantial refinement of, the 1984 ± 78 Ma RbSr isochron age for the Hartley basalts. This is the most precise age determination for a rock formed in any of the Waterberg basins. Ancient Pb loss from large zircons after 1906 Ma is thought to represent a maximum age for Kheis metamorphism. A 1750 ± 60 Ma RbSr biotite age on a dolerite postdates the Kheis deformation in the foreland region and is a minimum age for the end of the Kheis Wilson Cycle.

Apatite in early Archean Isua supracrustal rocks, southern West Greenland: its origin, association with graphite and potential as a biomarker

Abstract Rare earth element (REE) abundances in individual apatite crystals in banded iron formations (BIFs), metacherts, metacarbonates and mafic dykes in the Isua supracrustal belt (ISB) have been determined by laser ablation inductively coupled plasma mass spectrometry. The results together with petrographic observations on the distribution of graphite have been used to track the origin of the different compositional types of apatite and to evaluate the potential, proposed in earlier studies, for use of the apatite-graphite association as a biomarker. The chondrite-normalized distribution patterns of apatite in metasedimentary BIFs and metacherts fall into three groups. Relatively flat profiles with distinct positive Eu anomaly are interpreted as characterizing sedimentary (diagenetic) apatite that carry the REE signature of the Archean ocean. Secondary apatite in Isua metasdiments with either middle REE enriched profiles or with light REE depleted profiles is interpreted to have crystallized from percolating carbonate-rich metasomatic fluids or from fluids derived from cross-cutting mafic dykes, respectively. The occurrence together of these different genetic types of apatite with distinct REE signatures within cm-scale samples shows the immobility of REE in preexisting apatite during metamorphic episodes. Apatite crystals in Isua rocks of uncontested chemical sedimentary origin (BIF and metachert samples) do not have graphite inclusions or coatings. Graphite inclusions and coatings on the other hand characterize apatite in secondary metacarbonate rocks. In these rocks graphite is produced by thermal-metamorphic reduction of carbonate ion, derived from dissociation of the metasomatic ferrous carbonate where iron serves as electron donor, oxidizing to form magnetite. In view of the non-sedimentary, metasomatic origin of Isua metacarbonates and the abiogenic source of graphite, the apatite–graphite assemblage can not be considered as a biomarker and does not provide information on early Archean life in the ISB.

Protolith interpretation in metamorphic terranes: a back-arc environment with Besshi-type base metal potential for the Quha Formation, Natal Province, South Africa

Abstract A scheme for protolith interpretation of high-grade gneisses is presented which integrates field, petrographic and geochemical criteria to interpret not only the nature of the original rock types, but also the geological setting in which they formed. The amphibolite to granulite-grade supracrustal gneisses of the ∼ 1.2 Ga Quha Formation from the Mzumbe Terrane, Natal Metamorphic Province, South Africa, represent a metamorphosed sequence of predominantly volcanic and volcaniclastic rocks of K-feldspar-absent basaltic to andesitic compositions, along with penecontemporaneous (greywacke) sedimentary rocks derived from their erosion. Interlayered, finely laminated quartz-garnet-pyrite rocks (coticules) point to localised volcanogenic exhalative activity, which may indicate the existence of Besshi-type base metal sulphide deposits. Two types of nearly concordant amphibolite layers represent metamorphosed dykes of pre- and late-tectonic age. Geochemical parameters including ACF, TAS and REE plots show strong calc-alkaline volcanic-arc signatures, while bulk compositional data can be successfully matched with proposed protolith mineral compositions. UPb dating of selected zircon separates from Quha metavolcanic volcanic rocks suggest an age of crystallisation of ∼ 1200 Ma, indistinguishable from that of the pre-tectonic, arc-related Mzumbe tonalite-trondhjemite with which the Quha Formation is intimately associated. However, none of the Quha rocks are cogenetic with the Mzumbe Suite.

Eclogites in the central part of the Sveconorwegian Eastern Segment of the Baltic Shield: Support for an extensive eclogite terrane

Abstract Mafic boudins found at Viared in the central Sveconorwegian Eastern Segment of the Baltic Shield display both mineralogical and textural features demonstrating that these rocks were once eclogites. The mafic boudins are hornblende-plagioclase amphibolite, but the interiors show evidence of retrogression from eclogite: a grid-like pattern in clinopyroxene grains containing exsolved plagioclase from former omphacite, and garnets commonly surrounded by plagioclase coronas. Geothermobarometry was carried out on two samples and the peak pressure conditions were estimated by re-integrating the original clinopyroxene (Jd23) composition, giving 15.0-16.7 kbar and temperatures of 719 to 811°C. Calculations using the existing retrograde assemblage of clinopyroxene-garnet-plagioclase-quartz give values of 10.5-12.5 kbar and 700-770°C. A 974±3 Ma Re-Os age on molybdenite and 961±26 Ma from titanite represent a minimum age for boudin formation and retrogression. Pre-Sveconorwegian regional migmatization in the granodioritic country rock is represented by well developed CL-dark zircon rims seen in two samples, dated at 1426±18 and 1415±15 Ma. The protolith age of the country rock is 1701±10 Ma from zircon cores. Two related molybdenite samples gave ages at 957±4 and 949±4 Ma respectively, representing either protracted amphibolite facies conditions or a low grade 230-320°C alteration event. The textural and mineralogical features, together with the calculated P-T conditions show that the mafic boudins at Viared were subjected to eclogite facies conditions at ∼50 km depth. The evidence for high-P metamorphism at Viared, together with other known occurrences at Ullared, Skene and Kedum, shows that a significant part of the Eastern Segment was subjected to eclogite conditions.

Igneous and metamorphic geochronologic evolution of granitoids in the central Eastern Segment, southern Sweden

The Eastern Segment abutting the Transscandinavian Igneous Belt (TIB) mostly consists of rocks with overlapping igneous ages. In the Eastern Segment west of Lake Vättern, granitoids of clear TIB affinity exhibit strong deformational fabrics. This article presents U–Pb zircon ages from 21 samples spanning the border zone between these deformed TIB rocks in the east, and more thoroughly reworked rocks in the west. Magmatic ages fall in the range 1710–1660 million years, irrespective of the degree of deformation, confirming the overlapping crystallization ages between deformed TIB rocks and orthogneisses of the Eastern Segment. A common history is further supported by leucocratic rocks of similar ages. Prolonged orogenic (magmatic) activity is suggested by continued growth of zircon at 1.66–1.60 Ga. Six of the weakly gneissic rocks show zircons with cathodoluminescence-dark patches and embayments, possibly partly replacing metamict parts of older magmatic crystals, with 207Pb/206Pb ages dominantly between 1460 and 1400 million years, whereas three of the gneisses have zircon rims with calculated ages of 1440–1430 million years. Leucosome formation took place at 1443 ± 9 and 1437 ± 6 Ma. The minimum age of SE–NW folds was determined by an undeformed 1383 ± 4 million years crosscutting aplitic dike. Sveconorwegian zircon growth was not found in any of the samples from the studied area. To our knowledge, 1.46–1.40 Ga metamorphism affecting the U–Pb zircon system has not previously been reported this far northeast in the Eastern Segment. We suggest that the E–W- to SE–NW-trending deformation fabrics in our field area were produced during the Hallandian–Danopolonian orogeny and escaped later, penetrative Sveconorwegian reworking.

Eclogite-hosting metapelites from the Pohorje Mountains (Eastern Alps): P-T evolution, zircon geochronology and tectonic implications

Phase-equilibrium modelling, geothermobarometry, ion-microprobe dating and mineral chemistry of zircon have been used to constrain the P-T-t evolution of metapelitic kyanite-bearing gneisses from the ultrahigh-pressure (UHP) metamorphic terrane of the Pohorje Mountains in the Eastern Alps. These eclogite-hosting rocks are part of the continental basement of the Austroalpine nappes. Based on calculated phase diagrams in the system Na2O-CaO-K2O-FeO-MgO-MnO-Al2O3-SiO2-H2O (NCKFMMnASH) and conventional geothermobarometry, the garnet-phengite-kyanite-quartz assemblages of gneisses record metamorphic conditions of 2.2-2.7 GPa at 700-800 � C. These are considered as minima because of the potential for a diffusion-related modification and re- equilibration of the garnet and phengite during early stages of decompression. It is therefore most likely that the gneisses experienced the same peak UHP metamorphism at � 3 GPa as associated kyanite eclogites. Decompression and cooling to � 0.5 GPa and 550 � C led to the consumption of garnet and phengite, and the development of matrix consisting of biotite, plagioclase, K-feldspar � sillimanite and staurolite. Textures and phase diagrams suggest a low extent of partial melting during decompression. Cathodoluminescence images as well as zircon chemistry reveal cores encompassed by two types of metamorphic zircon rims. Ion probe U-Pb dating of three zircon cores yielded Permian (286 � 10, 258 � 7 Ma) and Triassic (238 � 7 Ma) concordia ages. The zircon rims are Cretaceous with a mean concordia age of 92.0 � 0.5 Ma and some cores gave a similar age. The Cretaceous zircons all exhibit very low Th/U ratio (,0.02) typical of metamorphic origin. In these zircons, nearly flat HREE patterns, (Lu/Gd)N ¼ 1-4, and only small negative Eu anomalies indicate formation in the presence of garnet and absence of plagioclase, which is corroborated by occurrence of Mg- and Ca-rich garnet inclusions. Therefore, these zircons are interpreted to record the Cretaceous HP/UHP metamorphism. The 92.0 � 0.5 Ma age obtained in this study agrees with that (93-91 Ma) determined earlier in the Pohorje eclogites from U/Pb zircon, Sm-Nd and Lu-Hf garnet-whole-rock dating. This implies that the eclogites and their country rocks were subducted and exhumed together as a coherent piece of continental crust. There is no evidence for a melange-like assemblage of rocks, which followed different P-T-t paths, or several subduction and exhumation cycles as proposed for some other UHP metamorphic terranes.

Emplacement ages and metamorphic overprinting of granitoids in the Sveconorwegian Province in V rmland, Sweden - an ion probe study

North of Lake Vanern in the Sveconorwegian province of Sweden, veined and banded grey to red granitoids characterize the Western Segment in western Varmland. Two gneiss samples from Rackstad, north of Arvika, yielded concordant U-Pb emplacement ages of 1596 - 11 Ma and 1590 - 14 Ma, respectively from zircon cores, while metamorphic zircon rims in one of the samples gave a weighted mean Pb-Pb age of 971 - 8 Ma. Patches of metamorphic zircon overgrowth pre-dating the metamorphic rims indicate that an even older metamorphic event influenced this rock. East of the Mylonite Zone, a N-S-trending Sveconorwegian shear zone, more massive reddish granitoids occupy the segment between this zone and another N-S-trending Sveconorwegian shear zone, the Protogine Zone. A sample from Hansjon, north of Torsby, yielded concordant ages with a weighted mean of 1689 - 12 Ma. One grain has a Pb-Pb age of 1770 - 29 Ma, which is inherited from the source of the granite, and is similar to the age of early magmatism in the Transsc...

Three episodes of crustal development in the Rehoboth Province, Namibia

Abstract The African continental crust was assembled by a series of orogenies over a period of billions of years mainly in Precambrian times. Tracing the build-up history of this stable crust is not always straightforward due to multiphase deformation and regions with poor outcrop. Episodes of metamorphism and magmatism associated with multiple Wilson cycles are recorded in zircons, which found their way into sediments derived from the hinterland. Dating of zircon populations in detrital rocks can hence provide age spectra which reflect the metamorphic and magmatic events of the region. Microbeam dating of detrital zircon is used to characterize the crustal development history of the Rehoboth Province of southern Africa. We investigated a quartzite of the Late Palaeo-Early Mesoproterozoic Billstein Formation, formed in a continental basin, and a quartz-feldspar arenite layer of the late Mesoproterozoic Langberg Formation conglomerates, immature sediments formed within a felsic volcanic system (both close to Rehoboth Town). The combined data indicate three episodes of crustal evolution in the Rehoboth Province. The oldest phase is only documented in the Billstein quartzite by three 2.98–2.7 Ga Archaean zircons. A Palaeoproterozoic phase between 2.2 and 1.9 Ga is older than any known exposures of the Rehoboth Province. The Billstein quartzite shows a main peak at 1.87 Ga, corresponding to the 1863±10 Ma Elim Formation. The Langberg sample reflects magmatism related to the entire Namaqua–Natal Wilson cycle between c. 1.32 and 1.05 Ga. The absence of zircons of that age range in the Billstein quartzite indicates a pre-Namaqua age for the Billstein Formation. Our data shows that there were at least three episodes of crustal development at 2.98–2.7 Ga, 2.05–1.75 and 1.32–1.1 Ga. We have documented the existence of a previously unrecognized 2.98–2.7 Ga Archaean crustal component, which was probably exposed in the Rehoboth Province during the Palaeoproterozoic and thus indicates a much longer geological history for the Rehoboth Province than previously known.

Sm-Nd data for granitoids across the Namaqua sector of the Namaqua-Natal Province, South Africa

Abstract Sm–Nd data for rocks of granitic composition in an east–west traverse across the Namaqua Sector of the Mesoproterozoic c. 1.2 Ga Namaqua–Natal Province of southern Africa provide new evidence about the timing of crustal extraction from the mantle. Recent ion probe zircon dating has shown that, contrary to previous indications that pre-Namaqua basement had been preserved in parts of the Namaqua Sector, the majority of the magmatic rocks were emplaced during the 1.4–1.0 Ga Namaqua tectogenesis and very little U–Pb evidence of older precursors remains. Sm–Nd model ages show that Mesoproterozoic crustal extraction did occur, but was also strong in the Palaeoproterozoic, with local evidence for the existence of Archaean crust. The distribution of model ages generally corresponds with the established terranes, subdivided using lithostratigraphic and structural criteria. The northern part of the Bushmanland Terrane shows early Palaeoproterozoic to Archaean model ages and is clearly different from the southern part, which has Mesoproterozoic model ages. This supports the previously published results of a north–south traverse and justifies the separation of the Garies Terrane from the Bushmanland Terrane, though further subdivisions are not supported. The continuation of the Richtersveld Province east of Pofadder is supported by one sample, and the terrane boundary between the Bushmanland and Kakamas terranes is marked by an abrupt change from Palaeoproterozoic to Mesoproterozoic model ages. Model ages for the Kakamas and Areachap terranes do not distinguish them well. They suggest a Mesoproterozoic to late Palaeoproterozoic origin for both terranes, neither of which has a purely juvenile character. The influence of Palaeoproterozoic crust-forming events is clear in the Kaaien Terrane to the east, possibly reflecting reworking of the Kaapvaal Craton. The Namaqua Sector of the Province thus has a history of crustal extraction and evolution which reaches back locally to the Archaean, with major Palaeoproterozoic and Mesoproterozoic crust-forming events. This differs from the Natal Sector, which has a largely juvenile character related to a Mesoproterozoic Wilson cycle. Attempts to reconstruct the Mesoproterozoic Supercontinent Rodinia will have to take into account the extensive Nd-isotopic evidence for older crustal events in the Namaqua sector.

Emplacement ages of the mid-Proterozoic Kungsbacka Bimodal Suite, SW Sweden

Abstract Ion-microprobe U-Pb geochronological data of zircon grains from three Mesoproterozoic deformed granite plutons in the Western and Median Segments of the Sveconorwegian Province, SW Sweden give intrusion ages of 1336±10 Ma for the Askim Granite, 1311±8 Ma for the Kärra Granite and ≥1304±6 Ma for the Göta Granite. In addition, oscillatory zoned zircon grains in a pegmatite dyke in the Kärra Granite were dated at 1325±8 Ma, i.e. identical within analytical uncertainty with the age of the host granite. The zircon grains from the pegmatite are interpreted as xenocrysts from the Kärra Granite. The plutons are part of a 1.34-1.30 Ga bimodal magmatic suite which we call the Kungsbacka Bimodal Suite, forming a N-S trending linear belt from Kungsbacka in the south to Trollhättan in the north. Other possible members are the 1.32 Ga Ursand Granite, the 1.33 Ga Hästefjorden Granite, the 1.31 Ga Veddige Augen gneiss, the 1.33 Ga Stråvalla Augen gneiss and the 1.33 Ga Chalmers Mafic Intrusion. The suite probably formed in a continental rift environment. This study demonstrates that the Kungsbacka Bimodal Suite, which is restricted to the Western and Median Segments, is younger than the 1.38 Ga Tjärnesjö and Torpa Granites in the Eastern Segment of the Sveconorwegian Province. This difference indicates that the Mylonite Zone may be interpreted as a first order Sveconorwegian tectonic boundary, separating crustal segments with distinct pre-Sveconorwegian histories. Metamorphic overgrowths on zircons from the Kärra pegmatite are dated at 1043±11 Ma and confirm the age of Sveconorwegian metamorphism at 1.04 Ga in the Western Segment. This study also documents the occurrence of post-1.3 Ga penetrative deformation and high-grade metamorphism including partial melting in the Western and Median Segments. It also proves that in some areas it is impossible to distinguish post-Gothian from older units, based only on their degree of deformation.