TY - JOUR A1 - Ennis, Meg A1 - Meere, Patrick A. A1 - Timmerman, Martin Jan A1 - Sudo, Masafumi T1 - Post-Acadian sediment recycling in the Devonian Old Red Sandstone of Southern Ireland JF - Gondwana research : international geoscience journal ; official journal of the International Association for Gondwana Research N2 - The Upper Devonian Munster Basin of southern Ireland has traditionally been viewed as a post-orogenic molasse deposit that was sourced from the Caledonides of central Ireland and subsequently deformed by the end Carboniferous Variscan orogenic event. The basin fill is composed of super-mature quartz arenite sandstone that clearly represents a second cycle of deposition. The source of this detritus is now recognized as Lower Devonian Dingle Basin red bed sequences to the north. This genetic link is based on the degree of similarity in the detrital mica chemistry in both of these units; micas plot in identical fields and define the same trends. In addition, the two sequences show increased textural and chemical maturity up-sequence and define indistinguishable Ar-40/Ar-39 age ranges for the detrital mica grains. Partial resetting of the Ar ages can be attributed to elevated heat flow in the region caused by Munster Basin extension and subsequent Variscan deformation. The combined evidence from southwest Ireland therefore points to a Caledonian or possibly Taconian primary source area that initially shed detritus into the Lower Devonian Dingle Basin which was subsequently recycled into the Upper Devonian Munster Basin following mid-Devonian Acadian basin inversion. (C) 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. KW - Caledonian KW - Acadian KW - Variscan KW - Old Red Sandstone KW - Sediment recycling Y1 - 2015 U6 - https://doi.org/10.1016/j.gr.2014.10.007 SN - 1342-937X SN - 1878-0571 VL - 28 IS - 4 SP - 1415 EP - 1433 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Rötzler, Jochen A1 - Timmerman, Martin Jan T1 - Geochronological and petrological constraints from the evolution in the Saxon Granulite Massif, Germany, on the Variscan continental collision orogeny JF - Journal of metamorphic geology N2 - Controversy over the plate tectonic affinity and evolution of the Saxon granulites in a two- or multi-plate setting during inter- or intracontinental collision makes the Saxon Granulite Massif a key area for the understanding of the Palaeozoic Variscan orogeny. The massif is a large dome structure in which tectonic slivers of metapelite and metaophiolite units occur along a shear zone separating a diapir-like body of high-Pgranulite below from low-Pmetasedimentary rocks above. Each of the upper structural units records a different metamorphic evolution until its assembly with the exhuming granulite body. New age and petrologic data suggest that the metaophiolites developed from early Cambrian protoliths during high-Pamphibolite facies metamorphism in the mid- to late-Devonian and thermal overprinting by the exhuming hot granulite body in the early Carboniferous. A correlation of new Ar-Ar biotite ages with publishedP-T-tdata for the granulites implies that exhumation and cooling of the granulite body occurred at average rates of similar to 8 mm/year and similar to 80 degrees C/Ma, with a drop in exhumation rate from similar to 20 to similar to 2.5 mm/year and a slight rise in cooling rate between early and late stages of exhumation. A time lag ofc. 2 Ma between cooling through the closure temperatures for argon diffusion in hornblende and biotite indicates a cooling rate of 90 degrees C/Ma when all units had assembled into the massif. A two-plate model of the Variscan orogeny in which the above evolution is related to a short-lived intra-Gondwana subduction zone conflicts with the oceanic affinity of the metaophiolites and the timescale ofc. 50 Ma for the metamorphism. Alternative models focusing on the internal Variscan belt assume distinctly different material paths through the lower or upper crust for strikingly similar granulite massifs. An earlier proposed model of bilateral subduction below the internal Variscan belt may solve this problem. KW - geochronology KW - granulite KW - high-Pmetamorphism KW - metaophiolite KW - Variscan KW - orogeny Y1 - 2020 U6 - https://doi.org/10.1111/jmg.12559 SN - 0263-4929 SN - 1525-1314 VL - 39 IS - 1 SP - 3 EP - 38 PB - Wiley CY - Hoboken ER - TY - JOUR A1 - Scott, James M. A1 - Konrad-Schmolke, Matthias A1 - O'Brien, Patrick J. A1 - Günter, Christina T1 - High-T, low-P formation of rare olivine-bearing symplectites in variscan eclogite JF - Journal of petrology N2 - Extremely rare veinlets and reaction textures composed of symplectites of olivine (similar to Fo(43-55)) + plagioclase +/- spinel +/- ilmenite, associated with more common pyroxene + plagioclase and amphibole + plagioclase varieties, are preserved within eclogites and garnet pyroxenites in the Moldanubian Zone of the Bohemian Massif. Thermodynamic modelling integrated with conventional geothermometry conducted on an eclogite reveals that the symplectite-forming stage occurred at high T (similar to 850 degrees C) and low P (< 6 and > 2 center dot 5 kbar). The development of the different symplectite types reflects reactions that took place in micro-scale domains. The breakdown of high-P garnet controlled the formation of olivine-bearing and amphibole + plagioclase symplectites, whereas breakdown of high-P omphacite led to formation of pyroxene + plagioclase symplectites. In addition, post-eclogite facies but pre-symplectite stage porphyroblastic amphibole and phlogopite were also replaced by olivine-bearing symplectites. Material transfer calculations and thermodynamic modelling indicate that the formation of different symplectite types was linked despite their different bulk compositions. For example, the olivine-bearing symplectites gained Fe +/- Mg, whereas adjacent amphibole + plagioclase and pyroxene + plagioclase symplectites show losses in Fe and Mg; Al, Si and Ca were also variably exchanged. The olivine-bearing symplectites were particularly sensitive to Na despite the small concentration of this element. In eclogites where Na was readily available, the plagioclase composition in the olivine-bearing symplectites shifted from pure anorthite to bytownite, with the less calcic feldspar partitioning Si and inhibiting the formation of orthopyroxene. This regional high-T, low-P granulite-facies symplectite overprint may have been caused by advective heat loss from rapidly exhumed high-T, high-P granulitic bodies (Gfohl Unit) that were emplaced into and over the middle crust (Monotonous and Varied Series) during Carboniferous continent-continent collision. KW - olivine KW - symplectite KW - eclogite KW - thermodynamics KW - Variscan Y1 - 2013 U6 - https://doi.org/10.1093/petrology/egt015 SN - 0022-3530 SN - 1460-2415 VL - 54 IS - 7 SP - 1375 EP - 1398 PB - Oxford Univ. Press CY - Oxford ER -