Resetting of Rb-Sr Whole-Rock Ages During Acadian Low-Grade Metamorphism in North Wales
Journal of rhe Geological Society, London, Vol. 148, 1991, pp. 703-710, 3 figs, 2 tables. Printed in Northern Ireland Resetting of Rb-Sr whole-rock ages during Acadian low-grade metamorphism in North Wales J. A. EVANS NERC Isotope Geosciences Laboratory, Keyworth NG12 5GG, UK Abstract: Ordovicianvolcanic rocks andassociated sub-volcanic intrusions in NorthWales give Devonian, metamorphic Rb-Sr whole-rock ages. Isotopic resetting isnot confined to acid volcanic rocks. Three intrusions preserve Ordovician ages. The closed system resettingis not directly related to metamorphic grade, but appears to correlate with the extent to which the secondary metamorphic mineralassemblage is developed.Rocks giving Devonianages have a well developedsecondary mineralogy whereas those retaining Ordovician ages are less extensively altered. The metamorphic ages combine to give a Lower Devonian mean age and weighted 2a error of 399 f 9 Ma. Metamorph- ism is therefore synchronous with Acadian deformation in the North Wales area. The Caradoc Series in North Wales consists of sequences of pre-tectonic,having cleaved, hornfelsed margins (Sargent varied volcanic rocks interbedded with fossiliferous sedim- 1924, Tremlett1964; Bromley 1969), and are therefore entary rocks, the latter enabling stratigraphic controls to be pre-Devonian. placedupon the volcanicepisodes. The stratigraphic Severallines of evidencesupport the intrusive rocks divisions of the Caradoc are described by Reedman et al. being of Caradocage. They have geochemical signatures (1985) who documented two major volcanic groups within indicative of formationinan extensional, within-plate theCaradoc Series; the earliest volcanic rocks are the environmentconsidered typical of thetectonic regime Llewelyn Volcanic Group which is separated from the later present during the Caradoc (Croudace 1982; Campbellet al.
[Show full text]