New evidence for the relative age of the Ercall Granophyre and its bearing on the Precambrian‐Cambrian boundary in southern Britain

Abstract

Recent extension of the Ercall Quarry, Shropshire, has revealed an unconformable contact between the Ercall Granophyre and overlying Lower Cambrian sediments. Five metres of basal conglomerate with rare granophyre clasts overlie a weathered granophyre surface and precludes the possibility that the granophyre is intrusive into the quartzite. The age of the Cambrian quartzite is reviewed and is considered to be most probably late Tommotian. The granophyre is therefore pre-late Tommotian, but has been radiometrically dated as 533 ? 13 Ma (Rb-Sr WR, Patchett ef al. 1980) and 531 t 5 Ma (U-Pb zircon, Compston ef al. in Cowie and Johnson 1985). If these independent dating methods are recording the age of granophyre intrusion, then the absolute ages of both the Comley Series Lower Cambrian faunas, and therefore the Precambrian-Cambrian boundary, are younger than is generally recognized.

Avalon: Insular continent by the latest Precambrian

“…Avalonian regions in England (e.g., Brasier et al, 1978;Cope and Gibbons, 1987), Cape Breton Island (Hutchinson, 1952;Landing, 1991b), northern mainland Nova Scotia (Landing and Murphy, 1991), and eastern Massachusetts and Rhode Island (Webster et al, 1986;Landing, 1988) also show angular unconformities and nonconformities at the base of the cover sequence. The tectonic regime associated with accumulation of the cover and its minor volcanics may have been similar to the Basin and Range setting proposed by Krogh et al (1988).…”

Sedimentology, structural evolution and tectonic setting of the late Precambrian Longmyndian Supergroup of the Welsh Borderland, UK

“…The granophyre is assigned to the Uriconian Volcanic Complex and has yielded an age-date of 533 -+ 13 Ma (Patchett et al 1980). The Wrekin Quartzite is likely to be of late Tommotian age (Cope & Gibbons 1987). As discussed by Cope & Gibbons this suggests that, in this case, the base of the Cambrian could be in the region of 531 Ma to 541 Ma, which is younger than generally accepted.…”

“…However, this is difficult to reconcile with the radiometric age-dates of c. 520 Ma to 535 Ma. Cope & Gibbons (1987) demonstrated that the Wrekin Quartzite rests unconformably on the Ercall Granophyre which crops out in the vicinity of the Wrekin (Fig. 1).…”

Suspect terrane definition in Anglesey, North Wales

“…Present data, therefore, indicate that the assembly of Monian terranes took place around 550 to 540 Ma. Given the present uncertainty regarding the absolute age of the base of the Lower Cambrian in Britain (Cope and Gibbons, 1987), this places the Monian terrane docking event as latest Precambrian to Early Cambrian.…”