Chemical and isotopic systematics of the Caledonian intrusions of Scotland and Northern England: a guide to magma source region and magma-crust interaction

Abstract

Abstract Chemical and O-, Sr-, Nd-, and Pb-isotope relations for the British Caledonian granitoids exhibit systematic variations that are attributed to derivation from both mantle and crustal sources. The ‘older’ (more than ca. 470 Ma) pre- and syn-tectonic granites were the product of local anatectic melting of Late Proterozoic metasedimentary upper crust (818O æ 8 to 14%0, 87S r/86Sr > 0.710, 206Pb /204Pb « 18.1-19.2) during the peak thermal conditions of the Grampian Orogeny. The ‘younger’ (less than ca. 440 Ma) post-tectonic granitoids have a complex origin which, in individual cases, involved at least four different source regions: (i) the upper mantle or subducted oceanic crust (818O « 5.7 to 7.0%o, 87Sr/86Sr « 0.7035-0.7040, 206Pb/204Pb = 17.9 to 18.1) and (ii) Lower Palaeozoic geosynclinal sedimentary upper crust (818O « 11 to 14%0, 87Sr/86Sr « 705-0.711, 206Pb/204Pb « 18.4) within the paratectonic Caledonides in the Scottish Midland Valley and Southern Uplands and in Northern England or (iii) Middle Proterozoic (?) mafic to intermediate granulitic lower crust (818O « 8 to 10%0, 87Sr/86Sr « 0.705-0.707, 206Pb /204Pb « 16.5-17.0) and (iv) Middle to Late Proterozoic metasedimentary upper crust (8180 « 8 to 14%0, 87Sr/86Sr > 0.710, 206Pb/207Pb » 18.1-19.2) in the Scottish Highlands. Mantle-derived magmas or their direct derivatives were likely involved in the development of all of the ‘younger’ granitoids, either as end-member components or as the source for a substantial part of the heat required for crustal melting and assimilation. Although the Lower Palaeozoic was a time during which a large amount of igneous material was introduced into the upper crust in Britain, it was not a major crust-forming period because the Caledonian granitoids are dominated by recycled continental crust.