Space-time variations in British Caledonian Granites: some geophysical correlations

The characteristics of 30 “granites” from the postulated Caledonian (Iapetus Ocean) suture zone of mainland Britain are discussed. Geophysical (gravity, aeromagnetic), geochemical (U/Pb and initial⁸⁷Sr/⁸⁶Sr) and isotopic age data indicate that these British Caledonian intrusions (390–600 Ma) can be divided into two distinct groups temporally, each of which is further subdivided spatially.The temporal division applies throughout the British province and separates a pre-Silurian (group 1) suite of low-volume, low-mobility magmas, which were intruded under compressive conditions, from a Siluro-Devonian (group 2) suite of large volume, mobile magmas intruded under tensional conditions.The spatial subdivisions of groups 1 and 2 are made between intrusions emplaced to either side of the postulated ENE-WSW Iapetus Ocean suture which runs through the Solway Firth. First, the group 1 granites northwest of the suture probably were produced by partial fusion involving Proterozoic continental crust (group 1N) whereas those to the southeast have isotopic characteristics simply indicating a mantle or ocean crustal source (group 1S). Second, the Siluro-Devonian granites, which were all derived largely at the expense of Caledonian mantle, have different aeromagnetic expressions depending on their position in the northwest (2N) or southeast (2S) Caledonides. These aeromagnetic characteristics are probably related to the differences in basement structure recently identified by seismic surveys and they provide further evidence for the former existence and Siluro-Devonian closure of Iapetus. Another significant implication may be that Proterozoic basement is lacking from beneath the region immediately adjacent to and southeast of the suture — southern Scotland and most of England.