The geology and geochemistry of the strathy complex of north-east Sutherland, Scotland

AbstractThe Strathy complex is a fault-bounded block of the sub-Moine succession basement characterized by a marked aeromagnetic anomaly. XRF analyses of 94 rocks for 26 elements are used to illustrate the geochemistry of the complex. It comprises siliceous grey gneisses (50 analyses), subordinate amphibolite (13 analyses), hornblende gneiss (13 analyses), and ultramafites (1 analysis), with scapolite-diopside marble and calc-silicate rocks (3 analyses from Strathy, 14 analyses of other calc-silicates). Geochemically the Strathy gneisses are quite distinct from the adjacent Moine lithologies, being unusually low in K, Rb, La, and Ce, and high in SiO2 and Na2O, whilst differences in immobile elements such as Ti and Zr preclude their derivation from a Moine source. The Strathy complex is unlike other high-grade gneiss terrains and apparently acquired its characteristic geochemical features by the removal of an anatectic melt under amphibolite-facies conditions leaving a refractory residue of quartz, plagioclase, Ca-poor amphibole, and garnet ± staurolite in siliceous gneisses. Subsequent influx of water, retrogression to biotite-bearing assemblages, production of sillimanite, and anateclic trondhjemitic melts took place during the maximum metamorphism of the surrounding Moine. The complex is considered to have originated as a dacitic supracrustal sequence with minor sediments and mafic-ultramafic intrusives. REE and Y abundances suggest a Proterozoic age, possibly as part of the Laxfordian cycle.