Chemical and Isotopic Systematics of the Caledonian Intrusions of Scotland and Northern England: A Guide to Magma Source Region and Magma-Crust Interaction

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 (δ <sup>18</sup>O ≈ 8 to 14%<sub>o</sub>, <sup>87</sup>Sr/<sup>86</sup>Sr > 0.710, <sup>206</sup>Pb/<sup>204</sup>Pb ≈ 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 (δ ¹⁸O ≈ 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 (δ ¹⁸O ≈ 11 to 14%_o, ⁸⁷Sr/⁸⁶Sr ≈ 0.705-0.711, 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 (δ ¹⁸O ≈ 8 to 10%_o, ⁸⁷Sr/⁸⁶Sr ≈ 0.705-0.707, ²⁰⁶Pb/²⁰⁴Pb ≈ 16.5-17.0) and (iv) Middle to Late Proterozoic metasedimentary upper crust (δ ¹⁸O ≈ 8 to 14%_o, 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.