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Journal of the Geological Society

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Chemical and Nd isotope constraints on granitoid sources involved in the Caledonian Orogeny in Scotland

¹ Department für Geo- und Umweltwissenschaften, Universität München, Theresienstr. 41, D-80333 München, Germany
² Present address: Institut für Mineralogie, Universität Hannover, Callinstr. 3, D-30167 Hannover, Germany (e-mail: g.steinhoefel{at}mineralogie.uni-hannover.de)
³ Crustal Geodynamics Group, School of Geography and Geosciences, University of St. Andrews, Irving Building, St. Andrews KY16 9AL, UK

Major- and trace-element data and Nd isotope compositions for granitoid samples from the Grampian Highlands in Scotland show a systematic evolution in the composition of their sources in the course of the Caledonian Orogeny. Granitoids of 511–451 Ma, related to the collision of the Midland Valley island arc with the Grampian terrane, show S-type affinity and fractionated REE patterns with minor Eu anomalies and low initial _Nd values of –14.1 to –11.2 suggesting melting of predominantly Dalradian metasediments. Subsequently formed granitoids of 425–406 Ma derived from an assumed Andean plate margin comprise a wide spectrum of rock types including I-type granite–granodiorite, and S-type granitoids, monzonites and alkali granites. The trace-element patterns of these rocks and the range of initial _Nd values of –2.1 to –6.9 are consistent with melting of variably rejuvenated crust as found in continental margin settings. We conclude that the Grampian Highlands were affected by two major crust-modifying events during the Caledonian Orogeny: predominantly recycling of older crust during docking of the Midland Valley arc and addition of juvenile, mantle-derived material to the crust during the convergence of Avalonia with Laurentia.