Deformation and metamorphism of Dalradian rocks and the evolution of the Connemara Cordillera
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Dalradian metasediments in the Cur district of north-east Connemara have had a complex history of deformation and metamorphism. The earliest schistosity present formed during biotite-grade metamorphism and may be mimetic after a compaction fabric. Subsequently, the rocks attained garnet grade and major D 2 folds developed. This deformation involved an initial buckling followed by a coaxial homogeneous flattening and the structures produced were probably initially upright. After D 2 , staurolite, and locally kyanite, grew under moderately high pressure conditions. Continued increase in metamorphic grade was, however, apparently accompanied by regional uplift and erosion, for staurolite breakdown occurred at lower pressures than those required to form kyanite. Furthermore, the highest grade metamorphism was accompanied by a steepening of the thermal gradient, since breakdown of the assemblage staurolite + muscovite + quartz produced andalusite at high structural levels, but sillimanite deeper down. D 3 deformation began after the initial uplift at the peak of metamorphism and produced two major northward-facing nappes thrust over a basal fold-nappe. The nappes root to the south of the Corcogemore Mountains but continued uplift in south Connemara faulted out the root zones as later nappes developed. After cooling, broad open D 4 folds were formed and the Connemara Schists were thrust up and to the south over lower grade rocks. Uplift of the Connemara region may have been complementary to the subsidence of the Mayo Trough to the north, in which case the oldest Ordovician rocks in South Mayo may have been deposited at the same time that the peak of metamorphism was attained in Connemara.Keywords:
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Abstract The preserved array of pressures in the eastern Dalradian indicates that considerable syn‐ to post‐metamorphic differential uplift has occurred. This inferred differential uplift suggests that Buchan sillimanite zone rocks originally lay at higher structural levels than presently adjacent cooler kyanite zone rocks to the west. A number of features are believed to coincide with the western margin of the sillimanite zone. These are a maximum in temperature, sharp thermal features, a high strain zone, and a train of metabasites. These features are explained by invoking syn‐metamorphic movement between the Buchan sillimanite zone and the kyanite zone to its west, involving some horizontal component of movement. It is suggested that the lateral, now eroded, equivalents of the Buchan area once provided part of the required tectonic thickening for other parts of the Dalradian. Areas surrounding the Buchan area suffered tectonic burial followed by metamorphism during uplift relative to the Buchan area.
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Dr B. W. D. Yardley writes: the paper by Ferguson & Al-Ameen (1986) is welcome for the detailed information it provides for the first time on the regional metamorphism in western Connemara. There are, however, a number of problems with the temperature and pressure determination made in the paper and these have important repercussions for both the determination of the thermal history of western Connemara and the comparisons made with the region to the east described by Yardley et al. 1980. Geothermometry. The authors rely primarily on the garnet–biotite cation exchange geothermometer, as did Yardley et al. 1980. Unfortunately, however, they have used a different calibration of the thermometer (due to Hodges & Spear 1982) from that on which the earlier study was based. Yardley et al . (1980) relied primarily on Ferry & Spear (1978). The effect of this is that for the rocks in question the Hodges & Spear (1982) calibration yields temperatures that are consistently higher (by at least 30°) than if the Ferry & Spear (or most other) calibrations had been used. This effect accounts entirely for the temperature differences claimed by Ferguson & Al-Ameen between rocks of similar grade in east and west Connemara. It is hard to decide which of the two calibrations is actually correct, and indeed their discrepancy might best be seen as an indication of the uncertainty that actually attends estimates of metamorphic temperature. Nevertheless, there are several independent lines of evidence which suggest that the temperature estimates of Ferguson &
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During the last 10 m.y., the Nanga Parbat Haramosh Massif in the northwestern Himalaya has been intruded by granitic magmas, has undergone high‐grade metamorphism and anatexis, and has been rapidly uplifted and denuded. As part of an ongoing project to understand the relationship between tectonism and petrologic processes, we have undertaken an isotopic study of the massif to determine the importance of hydrothermal activity during this recent metamorphism. Our studies show that both meteoric and magmatic hydrothermal systems have been active over the last 10 m.y. We suggest that the rapid uplift of the massif created a dual hydrothermal system, consisting of a near‐surface flow system dominated by meteoric water and a flow regime at deeper levels dominated by magmatic/metamorphic volatiles. Meteoric fluids derived from glaciers near the summit of Nanga Parbat were driven deep into the massif along the transpressional faults causing δ 18 O and δD depletions in the gneisses and marked oxygen isotopic disequilibrium between mineral pairs from the fault zones. The discharge of these meteoric fluids occurs in active hot springs that are found along the steep faults that border the massif. At deeper levels within the massif, infiltration of low δ 18 O magmatic fluids caused δ 18 O depletions in the gneisses within the migmatite zone. These low δ 18 O fluids were derived from the young (<4 Ma), relatively low δ 18 O granites (∼8‰c) that are found within the core of the massif. Geochronological evidence in the form of fission track and 40 Ar/ 39 Ar cooling ages and U/Pb ages on accessory minerals from the granites and gneisses provide a constraint on the timing of fluid flow in the surface outcrops we examined. Fluid infiltration in the migmatite zone rocks located along the Tato traverse was coeval with metamorphism, granite emplacement, and rapid denudation, in the interval 0.8–3.3 Ma. Finally, we infer from the presence of active hot springs that significant flow systems continue to be active at depth within the central portion of the Nanga Parbat‐Haramosh Massif.
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Research Article| March 01, 1974 Rb-Sr Whole-Rock Chronology of Caledonian Events in Northeast Scotland R. J. PANKHURST R. J. PANKHURST 1Department of Geology and Mineralogy, University of Oxford, Parks Road, Oxford OX1 3PR, England Search for other works by this author on: GSW Google Scholar Author and Article Information R. J. PANKHURST 1Department of Geology and Mineralogy, University of Oxford, Parks Road, Oxford OX1 3PR, England Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1974) 85 (3): 345–350. https://doi.org/10.1130/0016-7606(1974)85<345:RWCOCE>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation R. J. PANKHURST; Rb-Sr Whole-Rock Chronology of Caledonian Events in Northeast Scotland. GSA Bulletin 1974;; 85 (3): 345–350. doi: https://doi.org/10.1130/0016-7606(1974)85<345:RWCOCE>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Rb-Sr whole-rock isochrons are presented for four late Caledonian granites from northeast Scotland. These isochrons are believed to define a single 460–m.y. B.P. magmatic event, which caused isotopic homogenization of strontium on a whole-rock scale in slate of the Macduff Group, which is part of the Upper Dalradian Series intruded by the granites. The geochronology of the region supports the suggestion that all Caledonian deformation and metamorphism took place during a relatively short episode between the Early Ordovician sedimentation of the Upper Dalradian Series and the Middle to Late Ordovician granitic magmatism.A premetamorphic granite at Portsoy, which intrudes strata of the Middle Dalradian Series, yields a Precambrian isochron age of 669 ± 17 m.y. This is probably a late or postorogenic granite related to a pre-Caledonian metamorphism already recognized within the Moine Series. This Precambrian age cannot be used to support a recent hypothesis in which the Older basic igneous rocks of northeast Scotland are related to a Cambrian(?) subduction zone.All the granitic rocks of northeast Scotland have initial Sr87/Sr86 ratios of 0.714 to 0.717, irrespective of age. This indicates that the granites are derived from a deep, underlying crustal layer, possibly Lewisian (Precambrian) gneiss, and not from the mantle or subducted oceanic lithosphere. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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The Green Beds of the Scottish Dalradian Series are greenschists and amphibolites of metasedimentary origin. Field relations, petrography, and chemical data are used to demonstrate that these rocks, approximately of basaltic composition, were derived by metamorphism of basic tuffs deposited in an area of pelite sedimentation. The Dalradian Epidiprites, a series of orthoamphibolites associated with the Green Beds are less siliceous and less magnesian, in general, than the metasediments. Pelitic material is more abundant in the Green Beds of the Southwest Highlands than to the northeast. Amphibole occurrence and abundance are related to calcium content of the rocks. Fifty-eight metasediments and three Epidiorites were analyzed for thirty elements each, by X-ray fluorescence methods.
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The petrochemistry of the major elements (33 new rock analyses) of the Dalradian Glencolumbkille metadolerite sills, now amphibolite, shows them to have been quartz tholeiites like those of most of Donegal. Their compositions overlap with those of Connemara (Co. Galway) and Knapdale (Scotland), except the latter two also extend into the olivine tholeiite field. The syn-D2 (~470Ma) sheared schistose margins of the Glencolumbkille bodies are garnet amphibolites, unlike the generally garnet-free centres, and have suffered fluid-derived metasomatism and garnet growth. Garnets are postulated to have been nucleated by the activation energy from the shearing, but the main garnet growth and metamorphism was under static post-D2 pre-D3 conditions in certain favourable rock compositions. Garnet growth in the Glencolumbkille and Connemara amphibolites was generally restricted to rocks with low Mg/Fe, low Fe2O3/FeO and high MnO compositions, whether original or metasomatic. The long-standing puzzle as to why amphibolites that were originally dolerite sills in the Dalradian succession are clustered at certain horizons (at Glencolumbkille in and near the 654–635Ma Marinoan glaciation Portaskaig Tillite Formation) is examined. The clustering is suggested to be partly explained by recent structural research showing that the intrusion of widespread lateral sheets of basaltic magma is restricted to closely interbedded competent beds with thin incompetent pelites. The former act as lids, while the latter allow lateral expansion of the original sills. The whole magmatic suite studied intruded sediments deposited before 600Ma. It is thought to be part of the major ~600Ma magmatism and crustal extension that affected the Scottish Highlands and eastern North America as the supercontinent Rodinia split apart and the Iapetus Ocean opened. Of the metabasites, only the Knapdale ones have been U-Pb dated, yielding a magmatic 600Ma age and thus definitely shown to be pre-D1.
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