K2O-poor, aluminous assemblages from the Buchan Dalradian, and the variety of orthoamphibole assemblages in aluminous bulk compositions in the amphibolite facies.

Abstract

Experimentally verified, internal phase relationships of K2O-poor muscovite-free aluminous assemblages, occasionally containing gedrite, with coexisting K2O-rich muscovite-bearing assemblages in Dalradian metasediments of the Buchan area, NE Scotland, demonstrate that transformation of a muscovite-bearing assemblage to a gedrite-bearing assemblage occurs with changing P-T. A Schreinemakers net is constructed for the K2O-poor compositions. Phase assemblages and relations of a variety of orthoamphibole assemblages in aluminous bulk compositions are reviewed with reference to the net. The Buchan rocks are considered to have originated from terrigenous sediments with a component of altered igneous material. Tables of bulk rock and mineral compositions, equilibrium diagrams and petrogenetic grids are included in the discussion of these and other conclusions of the study. -M.S.