A Caradoc Failed-Rift within the Ordovician Marginal Basin of Wales

Campbell, S. D. G.; Howells, M. F.; Smith, Martin J.; Reedman, A. J.

doi:10.1017/s0016756800010190

Cited by 17 publications

(11 citation statements)

References 35 publications

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“…Recent interpretations of the volcanotectonic development of the Welsh Basin on the southeastern margin of Iapetus suggest that late Cambrian to early Ordovician (Tremadoc) island arc volcanism gave way to early Ordovician ensialic back-arc/marginal basin volcanism which developed into a failed rift with extensional volcanism in the Caradoc Kokelaar et al 1984;Campbell et al 1988;Howells, Reedman & Campbell, 1991). In southeast Ireland, Caradoc bimodal (basalt/ rhyolite) volcanism also occurred in an extensional setting (McConnell, Stillman & Hertogen, 1991).…”

Section: Discussionmentioning

confidence: 99%

Distribution and tectonic setting of Ordovician K-bentonites in the United Kingdom

et al. 1993

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Twenty-five Ordovician K-bentonite samples ranging in age from Llanvirn to Ashgill and representing both sides of the Iapetus suture were analysed by INAA and XRF and compared on magmatic and tectonic discrimination diagrams. The proportional distribution of Nb, Y, Ti, Zr, Hf, Ta, Th and REE indicates that source volcanics were erupted in a setting that changed from subduction-related in the Llanvirn/Llandeilo to marginal basin extensional volcanism in the Caradoc/Ashgill. Welsh K-bentonites appear to be derived from volcanoes within the Welsh Basin, but K-bentonites from the Southern Uplands to not have characteristics consistent with a fore-arc setting, further suggesting a source related to ensialic marginal basin extensional volcanism.

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Section: Discussionmentioning

confidence: 99%

Distribution and tectonic setting of Ordovician K-bentonites in the United Kingdom

et al. 1993

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“…Two of the Cambrian metabentonites (BRM 210,613) and the Caradoc metabentonite (BRM 620) plot in the subalkaline, high-K field while BRM 617 plots in the peralkaline field with Zr/Nb > 10, indicating that magmatic evolution and eruption took place in a within plate extensional setting related to subduction (Fig. 7), and it is widely agreed that much of the Caradoc volcanism occurred in a similar setting (Campbell et al 1988). It should be emphasized, however, that Kolata, Frost & Huff (1987) have documented single K-bentonite beds in the Ordovician of eastern North America which extend at least 1300 km from their source volcanoes.…”

Section: A Magmatic Affinities and Volcanotectonic Setting Of The mentioning

confidence: 95%

Cambrian and Ordovician metabentonites and their relevance to the origins of associated mudrocks in the northern sector of the Lower Palaeozoic Welsh marginal basin

Робертс

Merriman²

1990

Geol. Mag.

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Middle Cambrian and Caradoc metabentonites and associated mudrocks are described from the northern part of the Welsh Lower Palaeozoic basin. They occur in areas of slight deformation, distant from thick piles of contemporaneous volcanics. The < 2 /im fraction from anchizonal Middle Cambrian metabentonites consists dominantly of lMd micas (with up to 5% randomly interstratified smectite) with lesser IM and 2M1 muscovite, corrensite, quartz, chlorite and kutnohorite. Interbedded mudrocks are similar but contain, additionally, minor paragonite and regular (6:4) paragonite-muscovite. Late diagenetic grade Caradoc bentonite consists of rectorite, pyrophyllite and small amounts of chlorite. Interbedded mudrocks are similar but contain, additionally, lMd mica with up to 10% smectite interlayers, minor kaolinite and quartz.Metabentonites contain high concentrations of the trace elements Ce, La, Hf, Nb, Ta, Th, Y and Zr in comparison with associated mudrocks. Discriminant diagrams indicate derivation from trachyandesite-dacite-rhyolite magmas in a within-plate setting. Major element variations in Cambrian metabentonites compare closely with those described in Silurian examples whereas the Caradoc metabentonite is closely similar to rhyolitic tuffs in the Caradoc Llewelyn Volcanic Group.Both geochemical and field evidence indicate that whereas Middle Cambrian mudrocks were derived from bentonitic material adulterated variably by a terrigenous input, Lower Cambrian mudrocks were derived from a strongly depleted source such as basic Mona Complex rocks. Geochemical and field evidence suggest Caradoc mudrocks southeast of the Aber-Dinlle fault are largely volcanogenic; and the geological setting of Tremadoc, Arenig and Llanvirn mudrocks demands some volcanogenic input.The Caradoc volcanic succession of Snowdonia is represented by a single bentonite northwest of the Aber-Dinlle fault. The fault system was either a highly effective contemporaneous barrier or, more probably, a site of post-Caradoc transcurrent movement. GEO 127

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“…Magmatism was initially related to subduction (Kokelaar, 1988), but extensional magmatism became predominant toward the end of the Ordovician (Croudace, 1982;Campbell et al, 1988) and magmatic activity had stopped by the end of the Ordovician, ca. 439 Ma.…”

Section: Geologic Setting Of the Welsh Paleozoic Volcanic Rocksmentioning

confidence: 99%

Mineral and isotope features related to the resetting of Rb-Sr whole-rock isotope systems during low-grade metamorphism

Evans¹

1995

Low-Grade Metamorphism of Mafic Rocks

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The interaction between rocks and water during low-grade metamorphism leads to the growth of a predominantly hydrated secondary mineral assemblage and results in the elevation of whole-rock oxygen isotope values. Rb-Sr whole-rock isotope systems are also disturbed during low-grade metamorphism and give reset metamorphic ages. The process of isotope disturbance during low-grade metamorphism is examined by comparing the behavior of the oxygen isotope systems in rocks of similar composition that have undergone differing levels of metamorphic recrystallization.Two intrusions from the Ordovician volcanic province of North Wales represent metamorphically undisturbed systems. They give low oxygen isotope whole-rock values of 5 18 O smow (standard mean ocean water) = 7.3%e ± 0.3%c, their Rb-Sr wholerock systems are undisturbed and they show minimal metamorphic recrystallization. By comparison, other Ordovician igneous rocks have elevated oxygen isotope values enriched between 1 %c and 7%c over undisturbed rocks and give metamorphic Rb-Sr whole-rock regression ages. Interaction with water is shown to be the main influence on the resetting of Rb-Sr systems during low-grade metamorphism. The metamorphic pressure-temperature conditions are not a controlling factor. 157 158 J. A. Evans BT -Biotite. CZ -Clinozoisite, PLL -Pumpellyite Metapelite Kubler Stages Index | I | >0.43° 29 Acid/Intermediate Intrusion Mm 025 " °-43° 20 Basement Gneiss MM <0.25° 29 [H^ Carboniferous @ Sampled unit -see Table A Ordovician age A Reset (Devonian) age ®@A Figure 1. Map (modified after Evans, 1990) showing the regional variations of metamorphic grade in North Wales (after Roberts and Merriman, 1985) and the localities of Ordovician igneous rocks dated by Rb-Sr whole rock (Evans, 1991). Locality numbers of the Garnfor, Nanhoron, and Mynydd Mawr intrusions are in boxes (cf. Table 1).Acid rocks, which are most commonly used for Rb-Sr geochronology, often do not develop a diagnostic secondary metamorphic mineral assemblages during low-grade metamorphism. Assessing the extent of alteration and water-rock interaction in these rocks is therefore often difficult. In this study, mineralogical information, integrated with O isotope data, is used to show the importance of water-rock interaction in disturbing Rb-Sr whole-rock systems in low-grade metamorphism.

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A Caradoc Failed-Rift within the Ordovician Marginal Basin of Wales

Cited by 17 publications

References 35 publications

Distribution and tectonic setting of Ordovician K-bentonites in the United Kingdom

Distribution and tectonic setting of Ordovician K-bentonites in the United Kingdom

Cambrian and Ordovician metabentonites and their relevance to the origins of associated mudrocks in the northern sector of the Lower Palaeozoic Welsh marginal basin

Mineral and isotope features related to the resetting of Rb-Sr whole-rock isotope systems during low-grade metamorphism

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