A REEVALUATION OF METASEDIMENTARY XENOLITHS IN THE WEST HIGHLAND GRANITIC GNEISS OF INVERNESS-SHIRE

Unusual syn-kinematic amphibole-bearing 'metasedimentary' lithologies within the outcrop of the Neoproterozoic Moine of the northern Highlan ds of Scotland have previously been interpreted as evidence of metasom atism apparently contemporaneous with alkaline igneous activity at the onset of the Ordovician tectonothermal event in this part of the Cale donian nappe belt. Several of these localities coincide with apparent metasedimentary enclaves within components of the Neoproterozoic West Highland Granitic Gneiss. These 'enclaves' are reinterpreted as xenoli th-rich intrusions of quartzose amphibolite which are compositionally unique in terms of known meta-igneous rocks in the Moine, with unusual ly low Al2O3 and elevated trace element contents. Mineral parageneses comprise essentially quartz, hastingsitic amphibole and biotite, with feldspar, sphene, allanite, zircon, apatite and monazite. Xenolith pop ulations include both granitic gneiss and metasedimentary rocks. At th e level of emplacement evidence of hybridization is lacking, with comp ositions appearing to reflect partial unmixing. Emplacement of these u nusual magmas appears structurally to be broadly contemporaneous with the Ordovician (D3) tectonothermal event in the Sgurr Beag Nappe. Thes e intrusions are considered to provide evidence of minor intrusive act ivity coincident with the pre- to syn-D3 alkaline magmatic event repre sented by the deformed pluton of the Glen Dessary Syenite.