5 research outputs found
Sedimentology, sandstone provenance and palaeodrainage on the eastern Rockall Basin margin : evidence from the Pb isotopic composition of detrital K-feldspar
The Rockall Basin, west of Ireland, is a frontier area for hydrocarbon exploration but currently the age and location of sand fairways through the basin are poorly known. A recently developed provenance approach based on in-situ Pb isotopic analysis of single K-feldspar grains by laser ablation multi-collector inductively-coupled mass spectrometry (LA-MC-ICPMS) offers advantages over other provenance techniques, particularly when applied to regional palaeodrainage issues. K-feldspar is a relatively common, usually first-cycle framework mineral in sandstones and its origin is typically linked to that of the quartz grains in arkosic and sub-arkosic rocks. Consequently, in contrast to other techniques, the Pb-in-K-feldspar tool characterises a significant proportion of the framework grains. New Pb isotopic data from K-feldspars in putative Permo-Triassic and Middle Jurassic sandstones in Well 12/2-1z (the Dooish gas condensate discovery) on the eastern margin of the Irish Rockall Basin are reported. These data suggest that three isotopically distinct basement sources supplied the bulk of the K-feldspar in the reservoir sandstones and that the relative contribution of these sources varied through time. Archaean and early Proterozoic rocks (including elements of the Lewisian Complex and its offshore equivalents), to the immediate east, north-east and north of the eastern Rockall Margin, are the likely sources. More distal sourcelands to the north-west cannot be ruled out but there was no significant input from southern sources, such as the Irish Massif. These data, together with previously published regional Pb isotopic data, highlight the important role played by old, near and far-field Archaean â Proterozoic basement highs in contributing sediment to NE Atlantic margin basins. The Irish Massif appears to have acted as a significant, but inert, drainage divide from the Permo-Triassic to the Late Jurassic and hence younger, Avalonian and Variscan, sand sources appear to have been less important on the Irish Atlantic Margin.Science Foundation IrelandGriffiths Geoscience Awar
GĂ©ochronologie et gĂ©ochimie du zircon de roches volcaniques et plutoniques granitiques du DĂ©vonien prĂ©coce Ă moyen provenant de la suite ignĂ©e Cashes Ledge, dans le centre du golfe du Maine, Ătats-Unis
New zircon UâPb, trace element, and LuâHf laser ablation inductively coupled mass spectrometry (LA-ICP-MS) data are presented for five Early to Middle Devonian granitic and felsic volcanic rocks from the Cashes Ledge igneous suite, central Gulf of Maine, USA. These samples were previously analyzed by UâPb LA-ICP-MS and whole-rock geochemical methods and the new data generally corroborate the earlier results. Coarse-grained alkali-feldspar granite from northwest of the Fundy magnetic anomaly, the interpreted fault boundary in the offshore between Gondwanan microcontinents Ganderia to the northwest and Avalonia to the southeast, yielded a crystallization age of 414 ± 2 Ma. Southeast of the inferred fault, crystallization ages are 385 ± 3 Ma and 386 ± 3 Ma for two crystal tuff samples near the fault, 403 ± 3 Ma for an alkali-feldspar granite ~50 km southeast of the fault, and 399 ± 5 Ma for syenogranite ~25 km southeast of the fault, which also yielded inherited grains at ~1.3 Ga and between 613 ± 15 Ma and 558 ± 9 Ma. LuâHf LA-ICP-MS data for zircon retaining igneous crystallization ages have ΔHf(t) between 2.9 and 13.1 and model ages based on felsic sources between 0.52 and 1.04 Ga, reflecting a mix of late Mesoproterozoic (Avalonian?) basement and primitive melt, possibly in an extensional setting. Zircon Nb/Hf ratios generally greater than 0.001 indicate a predominately within-plate/anorogenic/rift setting, consistent with their whole-rock chemistry. U/Yb-Nb/Yb and U/Yb-Hf tectonic setting discrimination diagrams show ocean island to continental arc signatures, with a stronger continental arc signature for the syenogranite. Most zircon grains have Eu/Eu* values less than 0.1, indicating a crustal thickness of ~30 km or less at the time of their crystallization.De nouvelles donnĂ©es de datation UâPb sur zircon, dâanalyse dâĂ©lĂ©ments traces et de spectromĂ©trie de masse Ă plasma inductif et ablation par laser (LA-ICP-MS) Ă des fins de datation par le LuâHf sont prĂ©sentĂ©es par rapport Ă cinq roches granitiques et volcanofelsiques du DĂ©vonien prĂ©coce Ă moyen provenant de la suite ignĂ©e Cashes Ledge, dans le centre du golfe du Maine, aux Ătats-Unis. Les Ă©chantillons en question avaient prĂ©cĂ©demment Ă©tĂ© analysĂ©s par les mĂ©thodes de lâĂ©tude gĂ©ochimique sur roche totale et LA-ICP-MS de datation UâPb, et les nouvelles donnĂ©es corroborent de façon gĂ©nĂ©rale les rĂ©sultats antĂ©rieurs. Le granite alcalo-feldspathique Ă gros grains de lâanomalie magnĂ©tique du nord-ouest de Fundy, limite interprĂ©tĂ©e de la faille au large entre les microcontinents gondwaniens de Ganderia au nord-ouest et dâAvalonia au sud-est, a accusĂ© un Ăąge de cristallisation de 414 ± 2 Ma. Au sud-est de la faille infĂ©rĂ©e, les Ăąges de la cristallisation sont de 385 ± 3 Ma et 386 ± 3 Ma dans le cas de deux Ă©chantillons de tuf cristallin prĂšs de la faille, 403 ± 3 Ma dans le cas dâun granite alcalo-feldspathique Ă une cinquantaine de kilomĂštres au sud-est de la faille, et 399 ± 5 Ma dans le cas du syĂ©nogranite Ă environ 25 km au sud-est de la faille, qui a aussi livrĂ© des grains hĂ©ritĂ©s dâenviron 1,3 Ga et dâentre 613 ± 15 Ma et 558 ± 9 Ma. Les donnĂ©es LA-ICP-MS de datation par le LuâHf du zircon rĂ©vĂ©lant les Ăąges de la cristallisation ignĂ©e situent les Ăąges entre 2,9 et 13,1 sâinspirant de sources felsiques entre 0,52 et 1,04 Ga, ce qui tĂ©moigne dâun mĂ©lange dâune fusion primitive et dâune fusion du socle tardive mĂ©soprotĂ©rozoĂŻque (avalonienne?), possiblement dans un milieu dâextension. Les rapports Nb/Hf sur zircon gĂ©nĂ©ralement supĂ©rieurs Ă 0,001 signalent un milieu intraplaque/anorogĂ©nique/de rift correspondant Ă la composition chimique de sa roche totale. Des schĂ©mas de discrimination des milieux tectoniques U/Yb-Nb/Yb et U/Yb-Hf affichent des signatures dâĂźles ocĂ©aniques Ă et des arcs continentaux dans le cas du syĂ©nogranite. La majoritĂ© des grains de zircon prĂ©sentent des concentrations dâEu/Eu* de moins de 0,1 tĂ©moignant dâune Ă©paisseur de la croĂ»te dâune trentaine de kilomĂštres ou moins au moment de leur cristallisation
Geochronology and geochemistry of zircon from Early to Middle Devonian granitic and felsic volcanic rocks from the Cashes Ledge igneous suite, central Gulf of Maine, USA
New zircon UâPb, trace element, and LuâHf laser ablation inductively coupled mass spectrometry (LA-ICP-MS) data are presented for five Early to Middle Devonian granitic and felsic volcanic rocks from the Cashes Ledge igneous suite, central Gulf of Maine, USA. These samples were previously analyzed by UâPb LA-ICP-MS and whole-rock geochemical methods and the new data generally corroborate the earlier results. Coarse-grained alkali-feldspar granite from northwest of the Fundy magnetic anomaly, the interpreted fault boundary in the offshore between Gondwanan microcontinents Ganderia to the northwest and Avalonia to the southeast, yielded a crystallization age of 414 ± 2 Ma. Southeast of the inferred fault, crystallization ages are 385 ± 3 Ma and 386 ± 3 Ma for two crystal tuff samples near the fault, 403 ± 3 Ma for an alkali-feldspar granite ~50 km southeast of the fault, and 399 ± 5 Ma for syenogranite ~25 km southeast of the fault, which also yielded inherited grains at ~1.3 Ga and between 613 ± 15 Ma and 558 ± 9 Ma. LuâHf LA-ICP-MS data for zircon retaining igneous crystallization ages have ΔHf(t) between 2.9 and 13.1 and model ages based on felsic sources between 0.52 and 1.04 Ga, reflecting a mix of late Mesoproterozoic (Avalonian?) basement and primitive melt, possibly in an extensional setting. Zircon Nb/Hf ratios generally greater than 0.001 indicate a predominately within-plate/anorogenic/rift setting, consistent with their whole-rock chemistry. U/Yb-Nb/Yb and U/Yb-Hf tectonic setting discrimination diagrams show ocean island to continental arc signatures, with a stronger continental arc signature for the syenogranite. Most zircon grains have Eu/Eu* values less than 0.1, indicating a crustal thickness of ~30 km or less at the time of their crystallization.De nouvelles donnĂ©es de datation UâPb sur zircon, dâanalyse dâĂ©lĂ©ments traces et de spectromĂ©trie de masse Ă plasma inductif et ablation par laser (LA-ICP-MS) Ă des fins de datation par le LuâHf sont prĂ©sentĂ©es par rapport Ă cinq roches granitiques et volcanofelsiques du DĂ©vonien prĂ©coce Ă moyen provenant de la suite ignĂ©e Cashes Ledge, dans le centre du golfe du Maine, aux Ătats-Unis. Les Ă©chantillons en question avaient prĂ©cĂ©demment Ă©tĂ© analysĂ©s par les mĂ©thodes de lâĂ©tude gĂ©ochimique sur roche totale et LA-ICP-MS de datation UâPb, et les nouvelles donnĂ©es corroborent de façon gĂ©nĂ©rale les rĂ©sultats antĂ©rieurs. Le granite alcalo-feldspathique Ă gros grains de lâanomalie magnĂ©tique du nord-ouest de Fundy, limite interprĂ©tĂ©e de la faille au large entre les microcontinents gondwaniens de Ganderia au nord-ouest et dâAvalonia au sud-est, a accusĂ© un Ăąge de cristallisation de 414 ± 2 Ma. Au sud-est de la faille infĂ©rĂ©e, les Ăąges de la cristallisation sont de 385 ± 3 Ma et 386 ± 3 Ma dans le cas de deux Ă©chantillons de tuf cristallin prĂšs de la faille, 403 ± 3 Ma dans le cas dâun granite alcalo-feldspathique Ă une cinquantaine de kilomĂštres au sud-est de la faille, et 399 ± 5 Ma dans le cas du syĂ©nogranite Ă environ 25 km au sud-est de la faille, qui a aussi livrĂ© des grains hĂ©ritĂ©s dâenviron 1,3 Ga et dâentre 613 ± 15 Ma et 558 ± 9 Ma. Les donnĂ©es LA-ICP-MS de datation par le LuâHf du zircon rĂ©vĂ©lant les Ăąges de la cristallisation ignĂ©e situent les Ăąges entre 2,9 et 13,1 sâinspirant de sources felsiques entre 0,52 et 1,04 Ga, ce qui tĂ©moigne dâun mĂ©lange dâune fusion primitive et dâune fusion du socle tardive mĂ©soprotĂ©rozoĂŻque (avalonienne?), possiblement dans un milieu dâextension. Les rapports Nb/Hf sur zircon gĂ©nĂ©ralement supĂ©rieurs Ă 0,001 signalent un milieu intraplaque/anorogĂ©nique/de rift correspondant Ă la composition chimique de sa roche totale. Des schĂ©mas de discrimination des milieux tectoniques U/Yb-Nb/Yb et U/Yb-Hf affichent des signatures dâĂźles ocĂ©aniques Ă et des arcs continentaux dans le cas du syĂ©nogranite. La majoritĂ© des grains de zircon prĂ©sentent des concentrations dâEu/Eu* de moins de 0,1 tĂ©moignant dâune Ă©paisseur de la croĂ»te dâune trentaine de kilomĂštres ou moins au moment de leur cristallisation
Palaeodrainage in the NW European Triassic
Pb isotopic data from K-feldspars in Middle Triassic (Anisian) sandstones in the Wessex Basin, onshore southwest UK, and the East Irish Sea Basin, some to the north, show that the same grain populations are present. This indicates that the drainage system (the âBudleighensisâ River) feeding these basins originated from the same source/s, most probably the remnant Variscan Uplands to the south. Fluvial and aeolian sandstones have the same provenance, suggesting that if water- and wind-driven sands were originally derived from different sources, this has been obscured through reworking prior to final deposition. Significant recycling of feldspar from arkosic sandstones in earlier sedimentary basins can be ruled out. The provenance data agree with previous depositional models, indicating transport distances in excess of , with a drainage pattern that linked separate basins. This supports the idea that the regional fluvial system was driven by topography and episodic flooding events of sufficient magnitude to overcome evaporation and infiltration over hundredâs of kilometres. Importantly, this drainage system appears to have been isolated and independent from those operating contemporaneously to the northwest of the Irish and Scottish massifs, where the remnant Variscan Uplands apparently exerted no influence on drainage or sand supply.Science Foundation IrelandGriffiths Geoscience Awar