Petrogenetic significance of Rb-Sr and U-Pb isotopic systems in the 400 Ma old British Isles granitoids and their hosts

Abstract

Summary Rb-Sr isotopic data for 17 Caledonian granitic plutons in Scotland and northern England define low 87 Sr/ 86 Sr i (<0.709) for all granitoids emplaced between ∼435 and 390 Ma with the exception of Moy and Aberdeen (which may be older). There is no apparent correlation of 87 Sr/ 86 Sr i with distance from the proposed Iapetus suture near the Southern Uplands. Zircons from the Moinian and Dalradian rocks and Southern Uplands Ordovician greywacke roughly define U-Pb discordia lines with upper concordia intersections of 1200–1800 Ma and the position along the discordia related to the age of the sedimentary unit. Zircons from the Strontian tonalite and granodiorite and the Loch Doon granite contain little if any Pb isotopic memory, whereas the Strontian central granite contains a marked inherited zircon component. We conclude that, in strong contrast to the c. 550 Ma and c. 460 Ma granitoids, the c. 400 Ma magmas were largely hybrids derived from the mantle and, or, ‘new’ lower crust with varying degrees of incorporation of upper crustal-type material.