Pyroxenite and granulite xenoliths from beneath the Scottish Northern Highlands Terrane: Evidence for lower-crust/upper-mantle relationships

Xenolith suites from Permian host rocks in Orkney and the extreme NE of the Scottish mainland (Duncansby Ness) are described and compared to those fro m elsewhere in the Northern Highlands Terrane. Those from the Tingwall dyke , Orkney, comprise roughly equal proportions of ultramafic rocks (wehrlites , clinopyroxenites, websterites, hornblendites) and mafic to felsic rocks ( gabbroic, noritic and dioritic granulites, with subordinate tonalites and t rondhjemites). Those from Duncansby (45 km to the south) are dominantly oli vine-poor ultramafic rocks (clinopyroxenites, pargasite pyroxenites, biotit e-pyroxenites), together with granulites grading from gabbroic through to t onalites and trondhjemites. Most of the granulites are meta-igneous, compri sing plagioclase and one- or two-pyroxene species with equilibration temper atures of 810-710 degreesC and are regarded as samples of the lower crust. Absence of garnet and olivine, together with the association of relatively sodic plagioclase and aluminous pyroxenes, is consistent with derivation fr om depths corresponding to 5-10 kbar. Positive Eu anomalies in the granulit es imply that most originated as plagioclase-rich cumulates from basaltic m agmas. Scarce peraluminous quartzo-felds-pathic xenoliths, such as a garnet -sillimanite-bearing sample from Duncansby, are regarded as metasedimentary in origin. Pyroxenes (and biotites) in the ultramafic xenoliths tend to ha ve higher mg numbers than those of the granulites, reflecting higher temper atures of formation. Whereas the pyroxene-rich ultramafic rocks may be part ly interleaved with the granulites in the lower crust, it is concluded that they also constitute a zone of substantial thickness at or around Moho lev el, separating the granulites from underlying peridotites, and that they or iginated LIS cumulates cognate to the granulites. They have, however, been variably metasomatised with formation of amphibole. This zone may constitut e a density trap at which melt fractions, rich in K, Fe, Ti and OH and asce nding from the asthenosphere, interact with the ultramafic cumulates modify ing them texturally and modally to produce a complex veined assemblage of c linopyroxene- and pargasite-rich rocks. The metasomatism involved in increa se in LREE, HFSE and LILE contents. Some modal and cryptic metasomatism may also have affected the granulites, accounting for the presence of amphibol e and relatively high LREE/HREE values (La/Lu 38 206). Since closely compar able xenolith assemblages also occur in Mull at the Southwestern extremity of the Northern Highland Terrane, such metasomatised olivine- and orthopyro xene-deficient ultramafic rocks may characterise the shallowest part of the mantle beneath the entire terrane. The strongly bimodal character of the x enolith populations (either ultramafic or mafic grading to felsic) is taken to reflect the sharpness of the petrological Moho in this region.