Geochemical contrasts between late Caledonian granitoid plutons of northern, central and southern Scotland

Abstract

ABSTRACTNew major- and trace-element data for granitoid plutons from the Grampian Highlands, the Midland Valley and the Southern Uplands of Scotland are presented and discussed. The study is restricted to ‘late granitoids’ (all younger than 430 Ma); the term ‘granitoid’ is used in a wide sense to encompass all plutonic components of a zoned intrusion of this age, sometimes including diorites and ultrabasic cumulate rocks. The data indicate that as a whole the province is chemically high-K calc-alkalic. Other notable enrichments are in Sr and Ba, and a marked geographical difference in these trace-elements is found between plutons of the SW Grampian Highlands and those of the Southern Highlands, the Midland Valley, and the Southern Uplands. Plutons of the NE Highlands tend to be more geochemically evolved than those further SW and those of the Midland Valley and Southern Uplands.When petrographical and geochemical data are considered, three plutonic suites are recognised: (1) the Cairngorm suite comprising plutons of the NE Highlands, (2) the Argyll suite comprising plutons from the SW Highlands, and (3) the S of Scotland suite comprising plutons from the Southern Highlands, Midland Valley and the Southern Uplands excluding Criffell and the Cairnsmore of Fleet. It is proposed that the more acidic granitoids are dominantly the products of I-type crustal sources, but certain diorites and the more basic members of zoned plutons have a substantial mantle component. The elevated Sr and Ba levels in granitoids of the Argyll suite may reflect the influence of incompatible-element-rich fluids from the mantle in the petrogenesis of this suite. The relatively anhydrous pyroxene-mica diorites of the S of Scotland suite are richer in Ni and Cr and appear to represent mantle-derived melts. The relationships between these data and already published isotopic data are discussed.