David Stephenson

University of Exeter

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D. J. Fettes

British Geological Survey

B. G. J. Upton

University of Edinburgh

A. Graham Leslie

British Geological Survey

Peter Tanner

Técnicas y Servicios de Ingeniería (Spain)

J. R. Mendum

British Geological Survey

Robie W. Macdonald

Fisheries and Oceans Canada

Richard A. Smith

United States Geological Survey

J. E. Treagus

University of Manchester

M. T. Styles

British Geological Survey

David Gould

British Geological Survey

Cooperative Institutions

British Geological Survey

Met Office

University of Exeter

University of Reading

Institute of Geological Sciences

University of Edinburgh

European Centre for Medium-Range Weather Forecasts

National Institute of Meteorology

Czech Academy of Sciences, Institute of Geology

University of Nottingham

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The South Qôroq Centre nepheline syenites, South Greenland. Petrology, felsic mineralogy and petrogenesis

Grønlands Geologiske Undersøgelse Bulletin (1976)

David Stephenson

The South Qoroq Centre is one of four high-level, major intrusive centres comprising the Igaliko Nepheline Syenite Complex. Three elliptical stocks of foyaite were emplaced in fairly rapid succession by ring fracture and block subsidence, followed by a partial ring· dyke of augite syenite. Inward-dipping microsyenite sheets appear to be associated with the ring-dyke; and four earlier, satellitic stocks occur around the periphery of the centre. Petrographic and mineralogical data show that the intrusions become successively less differentiated with time. Felsic mineral phases (alkali feldspar, nepheline and sodalite) constitute over 80% of most rocks from the centre. Electron-microprobe analyses demonstrate the major role of felsics in the fractionation of the magma and, together with estimates of feldspar structural state from 2V measurements, give indications of the history and conditions of crystallisation. Nepheline compositions fall within a small range of decreasing Si content, but are outside the Morozewicz-Buerger convergence field and are not affected by sub-solidus alkali exchange. Feldspars form a continuous series from Or1Ab67An32 to a K-enriched alkali feldspar Or72Ab28An0. From comparison with other rock suites, this extended feldspar trend seems to be associated with the co-precipitation of nepheline, and increasing peralkalinity of the magma. Major and trace element analyses of the rocks, made by X-ray fluorescence, give variation trends which may be interpreted mainly in terms of fractionation of feldspar and the ferromagnesian phases. In particular, trace element distributions are highly characteristic of fractional crystallisation series, but may not be compatible with fractional melting. Analyses of rocks with co-existing felsic phases compare favourably with phase equilibria in the experimental system Q-ne-ks at 1kb. It is suggested that the centre evolved from an underlying differentiated magma chamber, formed by crystal fractionation and accumulation. Successively lower portions of the chamber were tapped, producing batches of fractionated magma. Later stages of crystallisation were influenced by a build-up in volatiles consisting of H2O, CO2, Cl and F, and post-emplacement differentiation was implemented mainly by outward diffusion of these volatiles together with alkalis under a thermal diffusion gradient. Temperatures of crystallisation deduced from the nepheline geothermometer (Hamilton, 1961) and from phase equilibria in the Ab-Or-H2O system are in reasonable agreement. Assuming a PH2O of about 1 kb, the foyaite feldspars crystallised at about 850°C and augite syenite feldspars slightly higher. Nephelines commenced crystallisation within the range 900-850°C but stabilised at 775-700°C irrespective of rock-type. Physico-chemical conditions during recrystallisation attributable to the later Igdlerfigssalik Centre are inferred from textural, geochemical and mineralogical changes. The recrystallised rocks provide evidence for the behaviour of trace elements during the initial stages of remelting under hydrous conditions in an open system.

Felsic

Nepheline syenite

Nepheline

Alkali feldspar

Fractional crystallization (geology)

Petrogenesis

Trachyte

Carbonatite

Peralkaline rock

Alkali basalt

10.34194/bullggu.v118.6660

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Citations (5)

Geochemical evolution of Dalradian metavolcanic rocks: implications for the break-up of the Rodinia supercontinent

Journal of the Geological Society (2011)

D. J. Fettes Robie W. Macdonald J. Godfrey Fitton David Stephenson Mark Cooper

Abstract: Neoproterozoic basaltic magmatism in the Dalradian Supergroup of Scotland and Ireland was associated with the break-up of the Rodinia supercontinent. Magmas were erupted in rift-related basins along a strike length of at least 700 km and during a time period of c . 80 Ma. New major and trace element analyses of metabasalts from several formations are presented to trace the variations in magma compositions in time and space. The primary magmas resulted from variable degrees of mixing of melts derived from mantle sources similar to those of normal and enriched mid-ocean ridge basalts; some younger lavas also show evidence of contamination with continental crust. In contrast to speculations about magmatism elsewhere in Rodinia, the evidence here suggests that there was no involvement of a mantle plume in basalt generation. For example, the Scottish promontory of Laurentia drifted rapidly southwards through c . 25° over the duration of the magmatism, with no evidence of significant elevation above sea level, as might be expected from involvement of a plume. Generation of the primary magmas might have taken place predominantly through decompression melting in depleted upper mantle containing enriched streaks and blobs. Both the Dalradian lithostratigraphy and the metabasaltic compositions are consistent with extreme lithospheric stretching and possibly rupture during the earliest phase of magmatism, whereas generation of later magmatism appears to have been associated with major fault systems, possibly on a foundering continental margin. Supplementary material: Chemical analyses of Dalradian metavolcanic rocks (major elements recalculated to 100%, anhydrous) are available at www.geolsoc.org.uk/SUP18468 .

Supercontinent

Rodinia

Dalradian

10.1144/0016-76492010-161

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Citations (26)

Architecture of the Oman–UAE ophiolite: evidence for a multi-phase magmatic history

Arabian Journal of Geosciences (2010)

Kathryn Goodenough M. T. Styles David Schofield Robert James Thomas Quentin Crowley

Ultramafic rock

Felsic

Magma chamber

10.1007/s12517-010-0177-3

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Citations (73)

Book Reviews

Journal of Hydraulic Research (2002)

K. Shiono David Stephenson John P. Loveless

Hydraulic Engineering Software VII, proceedings of the 7th International Conference. Series: Water studies 4, Edited by W. R. BLAIN. Published by WIT press. Computational Mechanics Publications, ISBN 1-85312-599-7,1998. Hydraulic engineering, 2nd edition, by J. A. ROBERSON, J. J. CASSIDY and M. H. CHAUDHRY, published by John Wiley, 653 pp, ISBN 0-471-12466-4, 1998 Handbook of coastal engineering, Edited by JOHN B HERBICH, published by McGraw Hill, ISBN 0-07-134402-0, 1999 Coastal Structures 99, vol 1 & 2. Proceedings of the International Conference on Coastal Structures, Santander, June 1999, Edited by INIGO J. LOSADA, published by AA Balkema, ISBN 90 5809 093 0 (vol 1), 90 5809 094 9 (vol 2), 2000

10.1080/00221680209499879

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Basic and ultrabasic volcanic rocks from the Argyll Group (Dalradian) of NE Scotland

Scottish Journal of Geology (2005)

Robie W. Macdonald D. J. Fettes David Stephenson Colin M. Graham

Synopsis The Dalradian Supergroup of the Scottish Highlands is a largely metasedimentary succession of Neoproterozoic to Early Cambrian age, metamorphosed during the Caledonian Orogeny. The rocks were deposited on the Laurentian margin during and following the break-up of Rodinia. This rift setting is evidenced, in the upper half of the succession, by the presence of several volcanic sequences. A significant development of these volcanic rocks occurs in the NE Grampian Highlands. There, the Blackwater Formation, within the Argyll Group of the Huntly area, is dominated by basic and ultrabasic metavolcanic rocks, with intercalated metasedimentary rocks. The rocks were metamorphosed in the amphibolite facies, with the Mg contents of their dominant amphiboles apparently reflecting those of the whole-rock protoliths. The protoliths ranged from ultrabasic picritic types (MgO up to 35 wt%) through high-magnesia basalts to basaltic andesites and andesites. The magmas feeding the volcanism were of tholeiitic affinity, broadly similar to metavolcanic rocks elsewhere in the Dalradian. Higher-Nb and lower-Nb groups can be distinguished. The inferred parental magmas were basalts with MgO c. 10 wt%. The ultrabasic rocks formed by accumulation of olivine and minor Cr-spinel within the parental basalts. Some more evolved rocks show evidence of minor (5–10 wt%) accumulation of Fe–Ti oxides. Primary magmas of the Blackwater metavolcanic rocks were generated from a mantle source, or sources, relatively enriched compared to the MORB source. It is speculated that all the Dalradian metavolcanic rocks represent varying degrees of mixing of magmas from this source and a depleted, MORB-like, mantle source.

Dalradian

Ultramafic rock

10.1144/sjg41020159

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Citations (10)

Mn and Ca enriched olivines from nepheline syenites of the South Qôroq Centre, south Greenland

Lithos (1974)

David Stephenson

Fayalite

Mineral redox buffer

Nepheline

Nepheline syenite

Pigeonite

Peralkaline rock

Fractional crystallization (geology)

10.1016/0024-4937(74)90036-x

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Citations (27)

The geology of the Succoth–Brown Hill mafic–ultramafic intrusive complex, near Huntly, Aberdeeenshire

Scottish Journal of Geology (1996)

Andrew Gunn M. T. Styles K.E. Rollin David Stephenson

Synopsis The Succoth–Brown Hill Intrusion is a deformed mafic-ultramafic body exhibiting multiple phases of recrystallization and alteration as a result of regional metamorphism and shearing deformation associated with the Portsoy Lineament. It comprises dominantly metagabbro within marginal belts of ultramafic rocks, the latter commonly rich in clinopyroxene. The effects of late brittle faulting and attendant hydrothermal alteration are superimposed locally on the sheared lithologies. Primary magmatic features are rarely preserved. The gross structure of the complex, deduced from field mapping, ground magnetic surveys and diamond drilling, has been modelled as a series of sheets steeply dipping to the SE. New gravity observations have revealed a local residual anomaly of about 7 mGal which indicates that the intrusion could extend to a depth of 2.5 km. A comparison of lithological, mineralogical and structural features with those of the nearby Huntly Intrusion reveals many contrasts. It is suggested that the two intrusions are not related, having been derived from magmas of different compositions. A structural separation of the two bodies is indicated by the geophysical data. Furthermore, the much simpler history of alteration and deformation in the Huntly Intrusion suggests that the Succoth–Brown Hill Intrusion may be an earlier body.

Ultramafic rock

10.1144/sjg32010033

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Citations (2)

Phase 1 Intensive Archaeological Survey of a Proposed Cellular Tower Location (Aka Larchwood #160), Section 23, T100N-R45W, Lyon CO, Iowa

David Stephenson Adam Newman

AKA

Section (typography)

Source

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Structural Model of the Basement in the Central Savannah River Area, South Carolina and Georgia

David Stephenson A.L. Stieve

Interpretation of several generations of seismic reflection data and potential field data suggests the presence of several crustal blocks within the basement beneath the Coastal Plain in the Central Savannah River Area (CSRA). The seismic reflection and refraction data include a grid of profiles that capture shallow and deep reflection events and traverse the Savannah River Site and vicinity. Potential field data includes aeromagnetic, ground magnetic surveys, reconnaissance and detailed gravity surveys. Subsurface data from recovered core are used to constrain the model.Interpretation of these data characteristically indicate a southeast dipping basement surface with some minor highs and lows suggesting an erosional pre-Cretaceous unconformity. This surface is interrupted by several basement faults, most of which offset only early Cretaceous sedimentary horizons overlying the erosional surface. The oldest fault is perhaps late Paleozoic because it is truncated at the basement/Coastal Plain interface. This fault is related in timing and mechanism to the underlying Augusta fault. The youngest faults deform Coastal Plain sediments of at least Priabonian age (40-36.6 Ma). One of these young faults is the Pen Branch faults, identified as the southeast dipping master fault for the Triassic Dunbarton basin. All the Cenozoic faults are probably related in time and mechanism to the nearby, well studied Belair fault.The study area thus contains a set of structures evolved from the Alleghanian orogeny through Mesozoic extension to Cenozoic readjustment of the crust. There is a metamorphosed crystalline terrane with several reflector/fault packages, a reactivated Triassic basin, a mafic terrane separating the Dunbarton basin from the large South Georgia basin to the southeast, and an overprint of reverse faults, some reactivated, and some newly formed.

Basement

10.2172/656907

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Citations (3)

Evolution of the Tugtutôq Central Complex, South Greenland: A high-level, rift-axial, late-Gardar centre

Journal of Volcanology and Geothermal Research (1990)

B. G. J. Upton A. R. Martin David Stephenson

Peralkaline rock

10.1016/0377-0273(90)90052-h

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Citations (13)