Geochemistry and tectonic setting of Lewisian clastic metasediments from the Early Proterozoic Loch Maree Group of Gairloch, NW Scotland

Abstract

Abstract The early Proterozoic (∼2.0 Ga) Loch Maree Group (LMG) of the Lewisian complex consists of a supracrustal sequence of metasedimentary schists and associated tholeiitic metavolcanic amphibolites within the Archaean basement. The metasediments comprise three main mapped units: the Flowerdale schists, Kerrysdale schists and Charlestown schists, together with numerous narrower bands. The dominant clastic metasediments within the schist units (semipelites and psammites) are chemically distinguished as a series of greywackes and lithic sandstones with variable LIL and transition element abundances, fractionated REE patterns and overall compositions similar to PAAS. Three chemical groups are recognized, whose range of composition largely reflects the mixing between two main source components, Lewisian gneissose basement with an intermediate-acid continental crust composition and local rift-related amphibolites with a primitive tholeiitic composition, which also incorporates components not preserved within the LMG. The geochemistry of the clastic metasediments confirms a previous tectonic model for the LMG as an intracontinental extensional rift receiving clastic detritus from the adjacent uplifted basement, as well as from internal volcanogenic sources.