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

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 epsilon(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 epsilon(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.