Petrogenesis of high Ba–Sr plutons from the Northern Highlands Terrane of the British Caledonian Province

Abstract

A variety of high Ba–Sr granites and syenites is intruded into the Caledonian terrane of the Northern Highlands of Scotland. Volumetrically rather small, the plutons display a considerable range of compositions from ultramafic and mafic to granitic and syenitic. Most intrude Proterozoic Moine metasediments, though some are exposed within the Moine Thrust Zone in the west, beyond which lies the Archaean Lewisian foreland. Although it is generally agreed that they are related in some way to the subduction of Iapetus Ocean crust beneath Laurentia, their petrogenesis is much debated, especially the enigmatic relationship between alkaline syenites and calc-alkaline granites. New isotope (Sr–Nd–O) data suggest that both may be derived from mantle sources that include small fractions of subducted pelagic sediment. Juvenile material is volumetrically dominant, although subsequent fractionation involved contamination with local continental crust. Direct derivation from a mafic underplate is unlikely given the absence of pronounced HREE and Y depletion, the major element dissimilarity with adakitic magmas, and the presence of coeval and geochemically-contiguous mantle-derived rocks. The parent magmas were derived from a Caledonian Parental Magma Array (CPMA) that extended from isotopically depleted to significantly enriched compositions, with high δ 18O in the latter. The oxygen isotope values and the high Ba–Sr signature in particular strongly suggest that mantle contamination with subducted pelagic carbonates generated the CPMA, and Nd–Sr isotopes constrain the sediment contribution to less than 10%. Differentiation of the parent magmas proceeded by fractional crystallisation and variable concurrent assimilation of small magma batches, involving different crystallising assemblages and different continental reservoirs. Caledonian syenites evolved by extensive pyroxene-dominated fractionation sometimes involving contamination with Lewisian granulites. On the other hand, the granites fractionated amphibole and plagioclase, during which most were contaminated with local Moine metasediment. The extended time interval over which the syenites were intruded (30 Ma) accords with continuing Iapetus subduction beneath the Laurentian margin, while the late pulse of isotopically-variable appinites and granites at about 425 Ma sits comfortably with Scandian slab breakoff. However, the high Ba–Sr granites studied here represent small-volume assimilation-fractionation products of mantle-derived appinitic magmas, rather than the voluminous melts of lamprophyre-underplated crust also predicted by slab breakoff, which are absent from the Northern Highland Terrane.