Petrogenesis of Ordovician igneous rocks in the southern part of the Welsh Basin

Bevins, R. E.; Lees, G. J.; Roacht, R. A.

doi:10.1017/s0016756800021786

Cited by 10 publications

(6 citation statements)

References 27 publications

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“…The North Pembrokeshire region of SW Wales is dominated by a sedimentary and volcanic succession of early Ordovician age which was deformed and metamorphosed during the Caledonian Orogeny. The sedimentary rocks are chiefly mudstones with some siltstones and rarer fine-grained sandstones, while the volcanics comprise bimodal basalt-rhyolite lavas, pyroclastics, and volcaniclastics, associated with highlevel intrusions (Bevins & Roach 1979;Kokelaar et al 1984;Bevins et al 1991Bevins et al , 1992. The coeval relationship between the lavas and intrusions in this area has been demonstrated previously by and Bevins et al (1989).…”

Section: Geological Settingsupporting

confidence: 60%

Petrogenesis of the St David's Head Layered Intrusion, Wales: a complex history of multiple magma injection andin situcrystallisation

Bevins

Lees

Roach

et al. 1994

Transactions of the Royal Society of Edinburgh: Earth Sciences

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The St David's Head Intrusion, exposed in North Pembrokeshire, is a tholeiitic sill up to 570 m thick, comprising principally mafic gabbros, which are in part layered. Layering varies from the centimetre to the metre scale. A range of gabbroic compositions is present, defining seven major petrological types, which are cut by thin silicic (aplitic) veins.Log–log plots of incompatible elements from the various lithological units indicate that all of the rocks in the intrusion are petrogenetically linked, although a variety of processes has been operative. Roach (1969) considered the quartz gabbros and dolerites, which form an envelope around the other units, to relate most closely to the parental composition. However, the least evolved compositions in the intrusion are from the xenolithic laminated olivine gabbros, although these mafic compositions are due primarily to the presence of abundant, mafic, cognate xenoliths. These xenoliths are thought to relate to an earlier episode of crystal accumulation in a high-level magma chamber. The various laminated gabbros reflect crystal accumulation in situ after magma emplacement, leading in certain layers to extreme enrichments in Fe, Ti, and V, related to high modal proportions of cumulus ilmenite. Further in situ crystallisation led to differentiation of the residual liquid, producing more silicic gabbros with well-developed granophyric textures, the granophyre reflecting the silicic residuum. Extreme differentiation, possibly combined with expulsion of silicic residual liquid during crystal accumulation and compaction, resulted in the cross-cutting aplite veins.Three different types of layering are present in the intrusion. Firstly, preferred orientation of tabular minerals in the laminated gabbro units is thought to result from discrete sedimentation episodes from a convecting magma chamber. Secondly, macrorhythmic modal layering up to 1 m thick consists of an alternation of relatively ilmenite-rich and ilmenite-poor layers in the laminated gabbro units, although the reason for the modal variation is not certain. Thirdly, a centimetre-scale felsic–mafic microrhythmic layering is present in the envelope quartz gabbros and dolerites, which is similar to the inch-scale layering in the Stillwater Igneous Complex. This layering is thought to relate to metasomatic reaction in the gabbro in the presence of water at a late magmatic stage. Overall, these various lithological units themselves define a large-scale layering in the intrusion.Combined, the petrological and geochemical data suggest that the St David's Head Intrusion was not emplaced in a single event. Rather, a series of magma pulses, of contrasting compositions but petrogenetically linked, was intruded. Some of the chemical variations now seen existed prior to emplacement, indicating the former presence of high-level crustal magma chambers, while other variations developed as a result of in situ crystallisation processes and related chemical differentiation.

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Section: Geological Settingsupporting

confidence: 60%

Petrogenesis of the St David's Head Layered Intrusion, Wales: a complex history of multiple magma injection andin situcrystallisation

Bevins

Lees

Roach

et al. 1994

Transactions of the Royal Society of Edinburgh: Earth Sciences

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“…Sample locations for this study are indicated and the 465 Ma age is a U-Pb zircon age from the Llanrian Volcanic bed within the section (Tucker et al 1990). (Fig 1;Bevins et al 1984;Bevins et al 1992). This center was active from Arenig to Llanvirn times judging from outcrop relationships with associated shales (Fig.…”

Section: Caradoc Shales Southwest Wales Ukmentioning

confidence: 96%

“…They were deposited in a back-arc setting dominated by volcanic deposits (Bevins and Roach 1979;Bevins 1982;Bevins et al 1984;Bevins et al 1992;Kokelaar et al 1984;Woodcock 1984;Kokelaar 1988;Traynor 1988). Chronostratigraphy in the area is based on graptolite zones (Jenkins 1979;Hughes et al 1982).…”

Section: Caradoc Shales Southwest Wales Ukmentioning

confidence: 99%

Mineralogic controls on REE mobility during black-shale diagenesis

Lev¹,

McLennan²,

Hanson³

1999

Journal of Sedimentary Research

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The Llandeilo-Caradoc black shales of southwest Wales, U.K. record a paragenetic sequence that requires redistribution of rare earth elements (REE) on a mineralogic scale during diagenesis. The main evidence for this redistribution is formation and replacement of REE-rich apatite and monazite and variations in whole-rock REE patterns. The processes that produce these mineralogic changes create a diagenetic overprint on the Llandeilo-Caradoc black shales that obscures geochemical information related to both provenance and paleoenvironmental conditions of deposition. Periods of open-system conditions existed during the diagenetic alteration of the Llandeilo-Caradoc black shales, which allow an evaluation of the extent and timing of the REE redistribution inferred from petrography. The observed variations in La/Sm ratios in the whole-rock REE patterns are strong evidence that REE were redistributed on a scale greater than an individual hand sample. This change in the whole-rock REE pattern altered the Cerium anomaly by as much as 20% and produced a range in the Sm/Nd ratio from 0.14 to 0.20. This change in the whole-rock Sm/Nd ratio indicates a disturbance in the Sm-Nd isotopic system at about 460 Ma that can be directly linked to the formation and dissolution of diagenetic apatite and monazite during diagenesis. One effect of this disturbance is to alter the range in Nd model age for the Llandeilo-Caradoc black shales from 1.6 to 1.8 Ga to about 1.4 to 2.25 Ga, thus complicating any interpretation of provenance. The ability to link petrographic observations from a black shale to measurable wholerock geochemical changes provides us with one tool to evaluate the presence of a diagenetic overprint on trace elements commonly used in paleoenvironmental reconstruction.

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“…Allen 1982;Bevins et al 1992) age are common in southwest Wales, but the Skomer Volcanic Group, which includes ignimbrite and mugearite (Thorpe et al 1989), is the only significant Silurian occurrence. Efforts to match Cosheston clasts with these complexes encounter two problems.…”

Section: Provenance Synthesismentioning

confidence: 99%

Enigma variations: the stratigraphy, provenance, palaeoseismicity and depositional history of the Lower Old Red Sandstone Cosheston Group, south Pembrokeshire, Wales

Thomas¹,

Williams

Morrissey

et al. 2006

Geol. J.

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Petrogenesis of Ordovician igneous rocks in the southern part of the Welsh Basin

Cited by 10 publications

References 27 publications

Petrogenesis of the St David's Head Layered Intrusion, Wales: a complex history of multiple magma injection andin situcrystallisation

Petrogenesis of the St David's Head Layered Intrusion, Wales: a complex history of multiple magma injection andin situcrystallisation

Mineralogic controls on REE mobility during black-shale diagenesis

Enigma variations: the stratigraphy, provenance, palaeoseismicity and depositional history of the Lower Old Red Sandstone Cosheston Group, south Pembrokeshire, Wales

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