MINERALOGICAL CHANGES ASSOCIATED DEEP WEATHERING OF GABBRO IN ABERDEENSHIRE

ABSTRACT: The distribution, profile morphology and mineralogy of deeply wea-thered gabbroic rocks in the basic igneous masses of lnsch and Bogancloch, Aberdeen-shire, have been studied. The saprolite is characterized by granular disintegration and spheroidal weathering with a relatively low degree of secondary mineral development. The upper part of the profile often shows evidence of mass movement and incorporation into overlying drift but below this zone the degree of alteration is generally uniform with depth. Feldspar and hornblende are largely unaffected, except by disintegration, but pyroxenes how varying degrees of weathering from the incipient development of amorphous iron oxides to replacement by reticulate networks or complete pseudo-morphs of trioctahedral vermiculite. Biotite weathers to hydrobiotite and vermiculite, giving rise also, under certain conditions, to kaolinite-gibbsite v rmiforms. The corres-pondence between occurrences of deep weathering and coarser textured biotite-bearing rocks can be related to the instability of this mineral. Consideration of the mineralogical changes found and the morphology of the profile of weathering suggest their formation under a warm, humid climatic regime, probably during the mid Tertiary times