A new, high precision U–Pb date from the oldest known rocks in southern Britain

Abstract

A new high precision U-Pb zircon age of 710.8 ± 1.5 Ma for granophyric granitic rock from the Stanner Hanter Complex of the Welsh Borderland lies just within error of an older Rb-Sr isochron age. εNd values of −0.3 and −1.2 combined with T DM of 1394 Ma and 1468 Ma indicate that the magma incorporated an older crustal source component. The Nd data highlight differences with western Avalonia, the widely considered Late Neoproterozoic north American counterpart to southern Britain, and point toward a closer similarity with other Peri-Gondwanan terranes that incorporate older, cratonic source material.

Does the Meguma Terrane Extend into SW England?

“…In support of this, inherited zircon grains of Mesoproterozoic age occasionally occur in Avalonian felsic bodies (e.g. Tucker and Pharaoh 1991; Thompson et al 2007;Schofield et al 2010).…”

“…Davies et al 1985;Thorogood 1990). Although older model ages that require a contribution from an ancient crustal source have also been recognized in the Welsh Borderlands and New England (Schofield et al 2010;Thompson et al 2012), the bulk of the Nd isotopic data strongly suggest that the T DM model ages are characteristic of Avalonia. This includes data from complexes that predate the main phase of arc magmatism (630-570 Ma), suggesting that the model ages record a genuine tectonothermal event in the terrane's early history rather than the mixing of juvenile Neoproterozoic-Paleozoic and ancient crustal magmas (cf.…”

Zircon LA-ICPMS geochronology of the Cornubian Batholith, SW England

“…Schofield et al (2010) also regressed one highly discordant zircon age with a Mesoproterozoic upper limit age of 1648 Ma in the Stanner Hanter complex (Wales, UK). Sandeman et al (2000) conducted U-Pb analysis of the Kennack Gneiss (which intrudes the Lizard ophiolite to the east of the Cornubian Batholith) that resulted in a Concordia upper intercept of 2012 ± 13 Ma and a lower intercept of 376.4 ± 1.7 Ma.…”

The Anglo-Brabant Massif: Persistent but enigmatic palaeo-relief at the heart of western Europe

“…The complex was subjected to metamorphism at greenschist and amphibolite facies between 670 and 650 Ma (Strachan et al, 1996). The presence of a Mesoproterozoic zircon inheritance in both Stanner-Hanter and Malvern complexes (Tucker and Pharaoh, 1991;Schofield et al, 2010;Strachan et al, 2007), supported by Sm-Nd model ages, favours an origin on the margin of Gondwana closest to the Amazon Craton and Mexico (Nance and Murphy, 1994;Murphy et al, 2000;Samson et al, 2005;Strachan et al, 2007;Nance et al, 2008) rather than the West African Craton, where such rocks are absent. Detrital zircon suites from early Cambrian quartzites also support this conclusion (Murphy et al, 2004).…”

“…7). Concordant zircons from a granophyric granite component have yielded a Weighted Mean U-Pb age of 710.8 ± 1.5 Ma, interpreted as the crystallisation age of the complex (Schofield et al, 2010). Slightly younger U-Pb ages have been obtained from the Malverns Complex, involved in a transpressional flower structure of Variscan age on the western side of the Worcester Graben (Fig.…”