Basic intrusions in the Ordovician of North Wales—geochemical data and tectonic setting

Floyd, P. A.; Lees, G. J.; Roach, Robert Allen

doi:10.1016/s0016-7878(76)80016-8

Cited by 9 publications

(6 citation statements)

References 29 publications

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“…Discriminant plots (Fig. 6) of basic rocks cluster about the junction of the fields of ocean-floor, calc-alkaline and within-plate basalts (see also Floyd et al 1976). The derivation of the acid rocks, including the numerous high-level granitoid intrusions, is uncertain• As in southern Snowdonia their large volume relative to basic rocks, and geophysical considerations, tend to favour an origin by crustal fusion.…”

Section: Central and Northern Snowdoniamentioning

confidence: 98%

“…The least fractionated basic rocks in thehigh-level layered Rhiw Intrusion (Fig. 1) Floyd et al 1976;Hughes 1977;Lowman 1977;Dunkley 1978;Furnes 1978;Bevins 1979;Croudace 1982;J. Esson & W. J. Wadsworth (unpubl.…”

Section: Ll~nmentioning

confidence: 99%

“…6), the basic rocks of the complex show strong affinities with ocean-floor basalts, although they are slightly richer in Ti than these and the Aran rocks. Floyd et al 1976;Hughes 1977;Lowman 1977;Dunkley 1978;Furnes 1978 Roach (unpubl.). )…”

Section: Ll~nmentioning

confidence: 99%

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The Ordovician marginal basin of Wales

Kokelaar¹,

Howells²,

Bevins³

et al. 1984

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The Lower Palaeozoic Welsh Basin was founded on immature continental crust. During late Precambrian-early Cambrian times, volcanism and sedimentation were influenced by NE-SW-trending faults which defined the NW and SE margins of the basin. During the Cambrian, marine sediments infilled a graben and at the end of the Tremadoc widespread tectonism was associated with an island-arc volcanic episode. In the Ordovician this subduction-related activity was succeeded by mainly tholeiitic volcanism related to back-arc extension, with the locus of arc volcanism sited further N, in the Lake District--Leinster Zone of the Caledonides. In Wales, the Ordovician volcanic activity shifted in time and space. In S Wales volcanism persisted from the middle Arenig through the Llanvirn. In N Wales the volcanism can be broadly divided into dominantly pre-Caradoc activity in southern Snowdonia and an intra-Caradoc episode in central and northern Snowdonia. In eastern Wales, including the Welsh Borderland, and in Ll~,n, both episodes are represented. In all areas faults greatly influenced both volcanism and sedimentation. Intrusive activity was dominated by high-level emplacement of sills. Granite (s.l.) stocks are restricted to central and northern Snowdonia and Ll~n and many were coeval with extrusive volcanism.Volcanism in the basin was essentially bimodal with voluminous eruptions of tholeiitic basalts with ocean-floor affinities, and of rhyolites. Minor volumes of andesite to rhyodacite resulted from low-pressure fractional crystallization of the tholeiitic basalts. Available evidence suggests that the rhyolites resulted mainly from crustal fusion, although in some instances evolution by crystal fractionation from intermediate magma has been proposed. Calc-alkaline assemblages are petrographically distinct, of minor occurence and, contrary to previous conclusions, are relatively insignificant in the characterization of the tectonic environment of the basin.Throughout the basin, volcanism was generally succeeded by deposition of black muds and then turbidite-dominated sequences.

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Section: Central and Northern Snowdoniamentioning

confidence: 98%

Section: Ll~nmentioning

confidence: 99%

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The Ordovician marginal basin of Wales

Kokelaar¹,

Howells²,

Bevins³

et al. 1984

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“…In Gwynedd itself Hawkins (1970) recorded prehnite from the hornblende-picrite and hornblende-gabbro sills of Rhiw. Floyd, Lees & Roach (1976) recorded the presence of extensive prehnite in dolerite sills of Ordovician age from the Conwy area and prehnite and unspecified zeolites in associated basic pyroclastic rocks. The occurrence of zeolites and prehnite was ascribed to either pre-Caledonian burial metamorphism or to Caledonian regional metamorphism.…”

Section: Introductionmentioning

confidence: 99%

Low grade and very low grade regional metabasic Ordovician rocks of Llŷn and Snowdonia, Gwynedd, North Wales

Робертс

1981

Geol. Mag.

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SummaryMetabasic assemblages belong to the prehnite-pumpellyite and greenschist facies. The isograds pumpellyite-in, pumpellyite-out/clinozoisite-in and biotite-in have been mapped, and divide the area into four metamorphic zones. Field and petrographic evidence indicate that the metamorphic crystallization was largely syn-and immediately post-tectonic, viz. end-Silurian to Devonian. The use of published Schreinemakers' bundles indicates that most of the prehnite-pumpellyite-bearing assemblages formed at high grades within the facies and that the greenschist facies rocks crystallized in the higher pressure field. As the amount of calcite increases so diagnostic Ca–Al silicates are reduced and eventually eliminated from assemblages. Contrary to recent claims, the evidence indicates that the Gwna Group and the Gwyddel Felsitic Beds of Llŷn were metamorphosed prior to the Arenig.

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“…According to Floyd et al (1976) the dolerite sills of northeast Snowdonia are little differentiated, high-alumina basalts of continental calcalkaline variety (Andean type), though they also include 'primitive' types akin to ocean-floor basalt. This reading accords with results by Fitton & Hughes (1970) which indicate the whole North Wales late Caradoc suite, including the potassic and sodic ignimbrites and related potassic acid intrusions to fall mainly within the calc-alkaline field.…”

Section: The Welsh Basin Zonementioning

confidence: 99%

Caledonide volcanism in Britain and Ireland

Stillman¹,

Francis²

1979

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Summary This paper reviews the state of knowledge of the volcanic activity which accompanied the development of the Caledonide orogen in Britain and Ireland. Unlike the relative abundance of stratigraphical data, geochemical information is scarce in earlier accounts, but in the last decade or so there has been a surge of interest coincident with the introduction of rapid analytical techniques and with the development of methods for distinguishing primary petrographical and chemical characteristics from those of subsequent alteration. Many of the Caledonide volcanic rocks were erupted into or beneath water, interbedded with sediments and then affected by orogenic deformation and metamorphism, yet this has apparently had less effect than might be expected on the gross geochemistry of the volcanic rocks. In the paratectonic Caledonides, submarine weathering and low-grade hydrous metamorphism have been the principal causes of element mobility, and the effects produced are, in many cases, remarkably similar to those which affect present-day eruptives in oceanic environments.

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Basic intrusions in the Ordovician of North Wales—geochemical data and tectonic setting

Cited by 9 publications

References 29 publications

The Ordovician marginal basin of Wales

The Ordovician marginal basin of Wales

Low grade and very low grade regional metabasic Ordovician rocks of Llŷn and Snowdonia, Gwynedd, North Wales

Caledonide volcanism in Britain and Ireland

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