ALBERT

Beschreibung: A zircon LA-ICP-MS U-Pb concordia age of 554.5 ± 5.4 Ma (2) from a felsic tuff of the Bloubergstrand Member (Tygerberg Formation) in the Pan-African Saldania Belt of the Western Cape provides the first direct age constraint on the deposition of the low-grade metamorphic volcanosedimentary sequence, collectively referred to as the Malmesbury Group. This age is identical within uncertainty to previously documented U-Pb ages of detrital zircons in the metaturbiditic Tygerberg Formation and constrains deposition of the Malmesbury Group to the latest Neoproterozoic. The age of the sub-aerial volcanism is, within error, identical to the emplacement of the earliest granites of the Cape Granite Suite that cross-cut regional metamorphic fabrics and structures of the Malmesbury Group. This suggests sedimentation and volcanism along an active convergent margin and is consistent with the findings of regional studies that postulate sedimentation and subsequent deformation of the Malmesbury Group in a fore-arc region. Concordant spot ages obtained from inherited xenocrystic cores of zircons in the Bloubergstrand Member define four main age groups at ca. 1200 to 1020 Ma, ca. 970 to 950 Ma, ca. 750 to 720 Ma and ca. 660 Ma. These ages can be correlated with earlier magmatic and/or metamorphic events in terrains to the north of the Saldania belt and point to a trench-parallel sediment supply from north to south and during the diachronous closure of the Adamastor ocean in the late Neoproterozoic.

Beschreibung: The Barberton Granite-Greenstone Belt (BGGB) of South Africa is an exceptionally well preserved Meso-Paleoarchean metamorphic supracrustal belt, one of only a few in the world. Studies of metamorphism in the BGGB have considerable potential to advance our understanding of tectonic processes in the Archean crust. Two current hypotheses persist to explain the origin of amphibolite-facies metamorphism in the southern BGGB. The first interprets these rocks to be the consequence of accretionary tectonics, while the second proposes a "dome-and-keel" vertical tectonic process driven by sinking of greenstone layers and the doming of the underlying granitoid crust. In this study, metamorphic pressure-temperature ( P-T ) analysis has been combined with garnet Lu-Hf and monazite U-Pb geochronology to directly date the amphibolite-facies metamorphism within the Stolzburg terrane of the BGGB. A garnet-biotite-chlorite–bearing sample yields a Lu-Hf garnet age of 3233 ± 17 Ma and a garnet-staurolite-kyanite–bearing sample produces a U-Pb monazite age of 3191 ± 9 Ma, whereas an andalusite-kyanite–bearing sample produces a U-Pb monazite age of 3436 ± 18 Ma. Phase diagrams and garnet compositional modeling produce a clockwise P-T evolution, with rocks reaching peak P-T conditions of 8.5 kbar and 640 °C for the ca. 3200 Ma event and minimum peak P-T conditions of ~4.5 kbar and 550 °C for the ca. 3435 Ma event. The duration of metamorphism for the ca. 3200 Ma event is estimated to be ~50–20 m.y. based on differences in age between U-Pb and Lu-Hf systems and durations needed to fit models of diffusionally modified garnet chemical zoning. Similarly shaped P-T paths over the Stolzburg terrane indicate that the metamorphism occurred in response to crustal thickening due to an accretionary tectonic process. Thus, the Stolzburg terrane constitutes an orogenic core, exhumed along the Komati fault.

Beschreibung: Detrital zircon dating has proven to be an effective way to constrain ages of submerged basement terranes on the margins of the northern Rockall Basin, a region where direct evidence of crustal affinities is scarce or absent. Zircons have been dated from sandstones of Paleocene–Oligocene age known to have been derived from the east (Hebridean Platform) and west (Rockall and George Bligh highs). The results show that the Hebridean Platform is a westward extension of the Lewisian Complex, with Archaean and Palaeoproterozoic ages that can be directly correlated with events identified in the Outer Hebrides and NW Scotland. The detrital zircons derived from the Hebridean Platform also provide evidence for a Mesoproterozoic thermal event and two phases of intrusions in the Palaeozoic. The Rockall High consists of a Palaeoproterozoic terrane dated as c. 1760–1800 Ma, similar to ages previously determined from both basement samples and detrital sediment. The data also provide evidence for the subsequent intrusion of alkaline igneous rocks in the Paleocene–Eocene. The George Bligh High represents an Archaean terrane heavily affected by Palaeoproterozoic tectonothermal events, and was also the site of intrusion of alkaline igneous rocks during Paleocene time.

Beschreibung: 〈p〉The development of laser ablation techniques using inductively coupled plasma mass spectrometry has enabled the routine and fast acquisition of 〈i〉in situ〈/i〉 U–Pb and Pb–Pb isotope ratio data from single detrital grains or parts of grains. Detrital zircon dating is a technique that is increasingly applied to sedimentary provenance studies. However, sand routing information using zircon analysis alone may be obscured by repeated sedimentary reworking cycles and mineral fertility variations. These biases are illustrated by two clear case studies from the Triassic–Jurassic of the Barents Shelf where the use of U–Pb geochronology on apatite and rutile and Pb–Pb isotopic data from K-feldspar is highly beneficial for provenance interpretations. In the first case study, U–Pb apatite ages from the (Induan – Norian) Havert, Kobbe and Snadd formations indicate an evolving provenance and identify possible episodes of storage within foreland basins prior to delivery onto the Barents Shelf. In the second case study, U–Pb rutile and Pb isotopic analyses of K-feldspar from the Norian–Pliensbachian Realgrunnen Subgroup provide a clear distinction between north Norwegian Caledonides and Fennoscandian Shield sources and suggest that a similar approach may be used to test competing models for sand dispersal for this Subgroup in regions farther north than this study.〈/p〉 〈p〉〈b〉Supplementary material:〈/b〉 U–Pb and trace element data for detrital apatite and detrital rutile, Pb isotopic data for detrital K-feldspar, U–Pb data for detrital zircon and a detailed analytical method is available at 〈a href="https://doi.org/10.6084/m9.figshare.c.4363838"〉https://doi.org/10.6084/m9.figshare.c.4363838〈/a〉〈/p〉

Beschreibung: Ion microprobe dating in Wedel Jarlsberg Land, southwestern Spitsbergen, provides new evidence of early Neoproterozoic ( c . 950 Ma) meta-igneous rocks, the Berzeliuseggene Igneous Suite, and late Neoproterozoic ( c . 640 Ma) amphibolite-facies metamorphism. The older ages are similar to those obtained previously in northwestern Spitsbergen and Nordaustlandet where they are related to the Tonian age Nordaustlandet Orogeny. The younger ages complement those obtained recently from elsewhere in Wedel Jarlsberg Land of Torellian deformation and metamorphism at 640 Ma. The Berzeliuseggene Igneous Suite occurs in gently N-dipping, top-to-the-S-directed thrust sheets on the eastern and western sides of Antoniabreen where it is tectonically intercalated with younger Neoproterozoic sedimentary formations, suggesting that it provided a lower Tonian basement on which upper Tonian to Cryogenian sediments (Deilegga Group) were deposited. They were deformed together during the Torellian Orogeny, prior to deposition of Ediacaran successions (Sofiebogen Group) and overlying Cambro-Ordovician shelf carbonates, and subsequent Caledonian and Cenozoic deformation. The regional importance of the late Neoproterozoic Torellian Orogeny in Svalbard's Southwestern Province and its correlation in time with the Timanian Orogeny in the northern Urals as well as tectonostratigraphic similarities between the Timanides and Pearya (northwestern Ellesmere Island) favour connection of these terranes prior to the opening of the Iapetus Ocean and Caledonian Orogeny.

Beschreibung: In central parts of the Scandinavian Caledonides, detrital zircon signatures provide evidence of the change in character of the Baltoscandian crystalline basement, from the characteristic Late Palaeoproterozoic granites of the Transscandinavian Igneous Belt (TIB, c. 1650–1850 Ma) in the foreland Autochthon to the typical, mainly Mesoproterozoic-age profile ( c. 950–1700 Ma) of the Sveconorwegian Orogen of southwestern Scandinavia in the hinterland. Late Ediacaran to Early Cambrian shallow-marine Vemdal quartzites of the Jämtlandian Nappes (Lower Allochthon) provide strong bimodal signatures with TIB (1700–1800 Ma) and Sveconorwegian, sensu stricto (900–1150 Ma) ages dominant. Mid-Ordovician turbidites (Norråker Formation) of the Lower Allochthon in Sweden, sourced from the west, have unimodal signatures dominated by Sveconorwegian ages with peaks at 1000–1100 Ma, but with subordinate components of older Mesoproterozoic zircons (1200–1650 Ma). Latest Ordovician shallow-marine quartzites also yield bimodal signatures, but are more dispersed than in the Vemdal quartzites. In the greenschist facies lower parts of the Middle Allochthon, the Fuda (Offerdal Nappe) and Särv Nappe signatures are either unimodal or bimodal (950–1100 and/or 1700–1850 Ma), with variable dominance of the younger or older group, and subordinate other Mesoproterozoic components. In the overlying, amphibolite to eclogite facies lower part of the Seve Nappe Complex, where the metasediments are dominated by feldspathic quartzites, calcsilicate-rich psammites and marbles, most units have bimodal signatures similar to the Särv Nappes, but more dispersed; one has a unimodal signature very similar to the Ordovician turbidites of the Jämtlandian Nappes. In the overlying Upper Allochthon, Lower Köli (Baltica-proximal, Virisen Terrane), Late Ordovician quartzites provide unimodal signatures dominated by Sveconorwegian ages ( sensu stricto ). Further north in the Scandes, previously published zircon signatures in quartzites of the Lower Allochthon are similar to the Vemdal quartzites in Jämtland. Data from the Kalak Nappes at 70°N are in no way exotic to the Sveconorwegian Baltoscandian margin. They do show a Timanian influence (ages of c. 560–610 Ma), as would be expected from the palinspastic reconstructions of the nappes. Thus the detrital zircon signatures reported here and published elsewhere provide supporting evidence for a continuation northwards of the Sveconorwegian Orogen in the Neoproterozoic, from type areas in the south, along the Baltoscandian margin of Baltica into the high Arctic. Supplementary material: LA-ICP-MS U–Pb analyses are available at http://www.geolsoc.org.uk/SUP18699

Beschreibung: The Journal of Organic Chemistry DOI: 10.1021/acs.joc.7b02457