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

Abstract

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

Identification and long-range correlation of bentonites in Turonian–Coniacian (Upper Cretaceous) chalks of northwest Europe

“…Winchester & Floyd, 1977;Pearce et al 1984;Leat et al 1986), and their use has been expanded to the study of bentonites (e.g. Roberts & Merriman, 1990;Huff et al 1993;Spears & Lyons, 1995). A plot of Zr/TiO 2 against Nb/Y indicates that samples are acidic, with a rhyodacite-rhyolite-comendite/pantellerite composition (Fig.…”

Ordovician K-bentonites of eastern North America

“…Both primary phenocryst and whole rock chemical analyses have provided much useful information concerning original magmatic composition and have further contributed to interpretations of the tectonic settings of source volcanoes (Delano et al, 1990;Roberts and Merriman, 1990;Huff et al, 1993). Detailed studies of immobile large-ion lithophile (LIL) elements and rare-earth elements (REE) indicate most source volcanoes were associated with collision-margin tectonism and that the parental magmas were silicic in nature.…”

“…Chondrite-normalized REE plots of Deicke and Millbrig samples (Fig. 70) show typical subalkaline and peralkaline patterns (Roberts and Merriman, 1990), and both show some evidence of light rare earth element (LREE) enrichment characteristic of highly evolved calc-alkaline magmas. The Millbrig patterns show somewhat less LREE enrichment and may represent a slightly less fractionated magma.…”

Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

“…By contrast, C/S is common in altered intermediate to mafic igneous rocks, reflecting the high concentration of Mg in those rocks relative to the high At content of most pelites. The presence of C/S in clastic rocks implies that original detrital material had a significant intermediate to mafic volcanogenic component (Helmold and van de Kamp 1984;Chang et al 1986;Roberts and Merriman 1990). Because the composition of volcanic material may vary in a predictable way in response to tectonic setting, the occurrence of pelitic rocks with abundant trioctahedral phyllosilicates may be diagnostic of that origin.…”