A composite C-isotope profile for the Neoproterozoic Dalradian Supergroup of Scotland and Ireland

The Neoproterozoic Dalradian Supergroup is a dominantly siliciclastic metasedimentary succession in the Caledonian orogenic belt of Scotland and Ireland. Despite polyphase deformation and greenschist- to upper amphibolite-facies metamorphism, carbonate units distributed throughout the Dalradian record marked δ13Ccarbonate excursions that can be linked to those associated with key environmental events of Neoproterozoic time. These include: (1) tentative correlation of the Ballachulish Limestone with the c. 800 Ma Bitter Springs anomaly; (2) the presence of the pre-Marinoan Trezona anomaly and 635 Ma marinoan-equivalent cap carbonat sequence in rocks of the middle Easdale Subgroup; (3) the terminal proterozoic (c. 600-551 Ma)Wonoka-Shuram anomaly in the Girlsta Limestone on Shetland. These linkages strengthen previously inferred correlations of the Stralinchy-Reelan formations and the Inishowen-Loch na Cille-MacDuff ice-rafted debris beds to the respectively 635 Ma Marinoan and 582 Ma Gaskiers glaciations, and suggest that the oldest Dalradian glacial unit, the Port Askaig Formation, represents one of the c. 750-690 Ma Sturtian glacial episodes. These δ13C data and resulting correlations provide more robust constraints on the geological evolution of the Dalradian Supergroup than anything hitherto available and enhance its utility in helping refine understanding of Neoprotrozoic Earth history

Accelerating Neoproterozoic research through scientific drilling

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The grandest of them all : the Lomagundi-Jatuli Event and Earth's oxygenation

Funding: K.K., A.L. and T.K. received funding from Estonian Science Agency Project PRG447 and Yu.D., A.R., D.R. and P.M. were supported by the state assignment of the Institute of Geology, Karelian Research Centre of the Russian Academy of Sciences.The Paleoproterozoic Lomagundi–Jatuli Event (LJE) is generally considered the largest, in both amplitude and duration, positive carbonate C-isotope (δ13Ccarb) excursion in Earth history. Conventional thinking is that it represents a global perturbation of the carbon cycle between 2.3–2.1 Ga linked directly with, and in part causing, the postulated rise in atmospheric oxygen during the Great Oxidation Event. In addition to new high-resolution δ13Ccarb measurements from LJE-bearing successions of NW Russia, we compiled 14 943 δ13Ccarb values obtained from marine carbonate rocks 3.0–1.0 Ga in age and from selected Phanerozoic time intervals as a comparator of the LJE. Those data integrated with sedimentology show that, contra to consensus, the δ13Ccarb trend of the LJE is facies (i.e. palaeoenvironment) dependent. Throughout the LJE interval, the C-isotope composition of open and deeper marine settings maintained a mean δ13Ccarb value of +1.5 ± 2.4‰, comparable to those settings for most of Earth history. In contrast, the 13C-rich values that are the hallmark of the LJE are limited largely to nearshore-marine and coastal-evaporitic settings with mean δ13Ccarb values of +6.2 ± 2.0‰ and +8.1 ± 3.8‰, respectively. Our findings confirm that changes in δ13Ccarb are linked directly to facies changes and archive contemporaneous dissolved inorganic carbon pools having variable C-isotopic compositions in laterally adjacent depositional settings. The implications are that the LJE cannot be construed a priori as representative of the global carbon cycle or a planetary-scale disturbance to that cycle, nor as direct evidence for oxygenation of the ocean–atmosphere system. This requires rethinking models relying on those concepts and framing new ideas in the search for understanding the genesis of the grandest of all positive C-isotope excursions, its timing and its hypothesized linkage to oxygenation of the atmosphere.Publisher PDFPeer reviewe

On the Preparation of Some Tertiary Amines Containing the 2-Furfuryl Group. Isomerization of Allyl-aryl( 2-furfuryl)-amines to N-Aryl-4H-5, 7 a-epoxyisoindolines

Six new tertiary 2-furfurylamines of the general formula 2-C 4H 30 · CH2 NRAr, w h ere R represents methyl, ethyl or ally!, and Ar phenyl, p-tolyl or p-methoxyphenyl groups, have been prepared by alkylation of the appropriate secondary aryl-(2-furfuryl)- amines with alkyl or ally! halides. It was found that the oily allyl-aryl-(2-furfuryl)-amines, on standing at room temperature, spontaneously isomerized to crystalline N-aryl-4H-5,7a-epoxyisoindolines, formed by a reversible intramolecular Diels-Alder reaction

Petrogenesis and geochemical halos of the amphibolite facies, Lower Proterozoic, Kerry Road volcanogenic massive sulfide deposit, Loch Maree Group, Gairloch, NW Scotland

The Palaeoproterozoic Kerry Road deposit is one of the oldest examples of volcanogenic massive sulfide (VMS) mineralization. This small VMS deposit (~500,000 tons grading at 1.2% Cu, 3.5% Zn) is hosted in amphibolite facies mafic-siliciclastic units of the c. 2.0 Ga Loch Maree Group, Scotland. Sulfide mineralization consists of pyrite and pyrrhotite with subordinate chalcopyrite and sphalerite, occurring in disseminated, vein and semi-massive to massive textures. The deposit was highly deformed and metamorphosed during the c. 1.8–1.7 Ga Laxfordian Orogeny. Textural relationships of deformed sulfide minerals, related to early Laxfordian deformation (D1/D2), indicate initial high pressure-low temperature (100 MPa, 150 °C) conditions before reaching peak amphibolite facies metamorphism, as evident from pyrrhotite crossing the brittle/ductile transition prior to chalcopyrite. Late Laxfordian deformation (D3/D4) is marked by local retrograde greenschist facies at low pressure and temperature (<1.2 MPa, <200 °C), recorded by late red sphalerite remobilization. δ34S values from all sulfide minerals have a homogeneous mean of 0.8 ± 0.7‰ (n = 21), consistent with interaction of hydrothermal fluids in the host oceanic basalt-island arc setting envisaged for deposition of the Loch Maree Group. Microprobe analyses of amphiboles record evidence of the original alteration halo associated with the Kerry Road deposit, with a systematic Mg- and Si- enrichment from ferrotschermakite (~150 m) to Mg-hornblende (~90 m) to actinolite (0 m) on approach to the VMS deposit. Furthermore, whole rock geochemistry records a progressive enrichment in Si, Cu, Co, and S, and depletion in Al, Ti, V, Cr, Y and Zr with proximity to the VMS system. These elemental trends, together with amphibole geochemistry, are potentially useful exploration vectors to VMS mineralization in the Loch Maree Group, and in similar highly deformed and metamorphosed terranes elsewhere

Sedimentology of the early Neoproterozoic Morar Group in northern Scotland : implications for basin models and tectonic setting

The metasedimentary rocks of the Morar Group in northern Scotland form part of the early Neoproterozoic Moine Supergroup. The upper part of the Group is c. 2-3 km thick and contains two large km-scale facies successions: a coarsening-upwards marine-to-fluvial regression overlain by a fining-upwards fluvial-to-marine transgression. Fluvial facies make up less than a third of the total thickness; shallow-marine lithofacies comprise the remainder. Combining these new findings with previously published data indicates that the Morar Group represents, overall, a transgressive stratigraphic succession c. 6-9km thick, in which there is both an upward and eastward predominance of shallow-marine deposits, and a concomitant loss of fluvial facies. Smaller-scale (100s of m thick) transgressive-regressive cycles are superimposed on this transgressive trend. Collectively, the characteristics of the succession are consistent with deposition in a foreland basin located adjacent to the Grenville orogen, and possibly linked to the peri-Rodinian ocean. Subsidence and progressive deepening of the Morar basin may have, at least in part, been driven by loading of Grenville-orogeny-emplaced thrust sheets, and aided by sediment loading. However, the relative contributions of thrust loading versus plate boundary effects and/or eustatic sea-level rise on basin evolution remain speculative

Fluvial braidplain to shallow marine transition in the early Neoproterozoic Morar Group, Fannich Mountains, northern Scotland

The early Neoproterozoic Morar Group in northern Scotland forms the lower part of the Moine Super-group, deformed and metamorphosed within the Precambrian Knoydartian and Lower Palaeozoic orogenies. It has remained uncertain whether it was deposited in a shallow-marine 'failed rift' basin within Rodinia or a foreland basin on the margin of the Grenville orogen, which is important to determine for tectonic reconstructions. In that context, we assess the sedimentology, depositional environment and tectonic setting of the middle part of the Morar Group in the Fannich Mountains. A ca. 4-6 km thick fining-upward facies succession contains three psammite dominated lithofacies (LF): LE1, at the base, contains amalgamated and multi-storey sets of trough and tabular cross beds, which passes upwards into LF2 consisting of trough and tabular cross-bedded units arranged in coarsening and fining-upward sequences with minor pelitic rocks. The stratigraphically highest lithofacies, LF3, contains finer and more complex coarsening-upward packages of rhythmically interbedded pelite and psammite. Palaeoflow develops from broadly unimodal NW-NE flow in LE1, to weakly bimodal NW-SE flow in LF3. The data indicate that this part of the Morar Group records deposition in a distal fluvial braidplain to tidally influenced shallow-marine setting. All facies and palaeocurrent transitions are gradual and occur systematically over many hundreds of metres vertically; such characteristics are incompatible with a rift-basin setting. We suggest that, instead, deposition occurred in a transition between a foreland basin to the Grenville orogen and a marine basin associated with the Asgard Sea between Baltica and Laurentia. (C) 2010 Natural Environment Research Council. Published by Elsevier B.V. All rights reserved.</p

1.99 Ga mafic magmatism in the Rona terrane of the Lewisian Gneiss Complex in Scotland

The Scourie dyke swarm has long been important to unravelling the geological history of the Lewisian Gneiss Complex (LGC) and the North Atlantic Craton. Recent dating has documented that the majority of those dykes were emplaced between c. 2418–2375 Ma. Here we show that a quartz dolerite dyke in the Rona Terrane of the LGC has a U-Pb zircon age of 1989.08 +4.3/−0.99 Ma. This is the first mafic dyke to be dated from the Rona terrane, the southernmost of those proposed for the terrane model of the LGC. Our new age also overlaps with the c. 1992 Ma age of a previously dated olivine gabbro dyke in the Northern region of the LGC and shows that the LGC contains at least three, and possibly four, temporally discrete episodes of Palaeoproterozoic mafic magmatism: The Scourie dyke swarm ‘sensu stricto’ at c. 2.4 Ga; a suite of younger dykes at c. 1.99 Ga, referred to here as the Strathan dyke swarm; and the c. 1.99–1.90 Ga mafic rocks that are part of the Palaeoproterozoic Loch Maree Group. A c. 2.04 Ga dyke in the Assynt terrain may be part of the younger suite of dykes or perhaps records a temporally separate event. Crucially, our new age data demonstrate that suites of mafic dykes emplaced across the mainland LGC are similar in age and supports the correlation of structural and metamorphic features across that Complex

Detrital zircon geochronology of the Grenville/Llano foreland and basal Sauk Sequence in west Texas, USA

U-Pb detrital zircon ages from Mesoprotero zoic and Cambrian siliciclastic units in west Texas (USA) constrain the depositional setting, provenance, and tectonic history of the region within a late Mesoproterozoic Grenville foreland basin and the early Paleozoic Sauk transgressive sequence. Two key units, the Hazel and Lanoria Formations, have detrital zircon age spectra dominated by detritus derived from the Grenville orogen (the Llano uplift and eroded equivalents), the ca. 1.4 Ga Granite Rhyolite, and the ca. 1.7-1.6 Ga Yavapai/ Mazatzal provinces. These data, combined with sedimentological data, permit interpreting those formations as the proximal and distal deposits, respectively, of a molasse shed into the Grenvillian foreland basin. Detrital zircons as young as ca. 520 Ma show that the Van Horn Formation, previously considered to be Precambrian in age, is no older than middle Cambrian. Further, the overall detrital zircon age spectrum of the Van Horn Formation is similar to that of the overlying Cambro-Ordovician Bliss Formation: both indicate derivation from sources that included the Colorado- Oklahoma aulacogen, Grenville, Granite- Rhyolite, and Yavapai/Mazatzal provinces. The similarities between the depositional history of the Van Horn and Bliss Formations lead us to conclude that the base of the Sauk Sequence in west Texas occurs at the base of the Van Horn Formation. Base-level rise associated with the Sauk transgression affected drainage patterns and sediment deposition along southwestern Laurentia some 20 m.y. earlier than previously assumed