Geochemical and geophysical constraints on the origin and evolution of Caledonian granites

Abstract

Summary Most British Caledonian granites, which were emplaced during the period 600–390 Ma, are characterised by temporal progression towards low K/Rb ratios, low total Sr, low initial 87 Sr/ 86 Sr ratios and high U contents coupled with increasingly large negative Bouguer gravity anomalies and sometimes large magnetic anomalies. It is proposed that some early granites from N Scotland represent low mobility, migmatitic low volume melts resulting from overthrusting which reached a climax in mid-Ordovician times. Early granites from S Britain and late granites from both the north and south provinces have geochemical characteristics of mantle-derived parental magmas capable of metamorphosing the lower crust to a high grade and ascending to become voluminous high level and possibly sub-volcanic intrusions. Late Caledonian granite emplacement ignores the major lateral break in crustal continuity near the Southern Uplands fault that is identified on seismic, geochemical, aeromagnetic and geological evidence. Seemingly, these granites were produced after Silurian continental plate closure. Yet their characteristics have little in common with the modern Alpine-Himalayan plutonic rocks and a source within sub-continental lithosphere under tension is postulated.