MINERALOGY, GEOCHEMISTRY, PROVENANCE AND SODIUM METASOMATISM OF TORRIDONIAN RIFT BASIN ELASTIC ROCKS, NW SCOTLAND

Closed-basin deposits of the Steer Group and Diabaig Formation were lo cally derived from Lewisian felsic and mafic rocks in semi-arid to tem perate climates. For open-basin deposits of the Applecross and Aultbea Formations, provenance was Grenville-age rocks and older Proterozoic and Archaean rocks in Laurentia, west of the rift basin. Petrography s hows that detrital Ca-plagioclase, K-feldspar, and matrix clays were a lbitized during diagenesis. Evaluation of geochemical data for Steer a nd Torridonian Group sedimentary rocks indicates widespread sodium met asomatism in these rocks. For the very sodic Steer Group sandstones an d siltstones, containing as much as 6% Na2O, up to 2-3 % Na2O was adde d, during diagenesis, to the detrital sediments. Similar Na-metasomati sm occurred in the Diabaig and Cailleach Head sandstones. The metasoma tic process involved evaporative concentration of salts in closed basi n groundwaters yielding Na-rich brines which reacted with detrital sil icates to develop authigenic albite. Calcium, released from detrital p lagioclase during diagenesis, is resident in authigenic epidote or cal cite, depending on local p(CO2) During metasomatism, potassium mineral s were replaced by albite and the liberated K was incorporated into il lite-mica in shales. Granulite facies provenance is indicated for some of the clastics by K/Rb of 465-1295, much higher than common crustal K/Rb values. In open-basin deposits of the Applecross and Aultbea Form ations, low abundances of Fe, Mg, Co, Cr, and Ni indicate little mafic and/or ultramafic rock in the source area. Petrography indicates that these immature sediments include some recycled sedimentary or metased imentary rocks and were developed in a temperate-humid climate where m oderate weathering removed considerable plagioclase from the source ro cks prior to transport and deposition.