Stable isotope geochemistry of Caledonian granitoids from the British Isles and East Greenland

Abstract

The ‘British Caledonian Granites’ (BCG) comprise a compositionally diverse group of intrusive rocks ranging from diorite to leucogranite, which were emplaced across the British Isles in and adjacent to the Caledonian metamorphic belt during the Early Paleozoic. Although the largest volume of Caledonian magmatic activity in Britain postdates the polyphase deformation and metamorphism of the Grampian orogeny, a small number of plutons were emplaced prior to the main orogenic event and another group are essentially contemporaneous with it. O-isotope ratios for the BCG are extremely variable and essentially overlap the full range of O-isotope variations documented for orogenic granitoids. The margins of two plutonic complexes have been affected by subsolidus hydrothermal exchange with meteoric fluids. For the older (> 440 Ma) BCG plutons δ 18O = 6.6–12.1% and δD = −83 to −45%, whereas for the younger (c. 400) Ma) plutons δ 18O = 7.1–12.4%. and δD = −97 to −44%. There is a general positive correlation between O- and Sr-isotope variations in the post-tectonic granitoids which is absent in their pre-tectonic counterparts. Caledonian plutonic rocks occur in East Greenland in the southern half of the fold belt between 70–75°N. The earliest intrusions are sheets of monzonite and granodiorite emplaced slightly before the climax of Caledonian orogenic activity. Subsequently, a suite of post-tectonic granites was emplaced c. 450–370 Ma. Two of the East Greenland plutons have been affected by post-emplacement meteoric-hydrothermal alteration. The δ 18O range for the unaltered East Greenland plutons is 8.5–13.9‰.