Mineralogy and cryptic layering of the Kunene anorthosite complex of SW Angola and Namibia

The gabbro-anorthosite complex of SW Angola and Namibia (Kunene Complex) is dominated by anorthosite-troctolite cumulates. Other broadly gabbroic rock types are subordinate. An-rich plagioclase (max. An85) and Fo-rich olivine (max. Fo79) are common in the western area of the complex with plagioclase becoming gradually less anorthitic (min. An45) and olivine less forsteritic (min. Fo62) toward the east. This cryptic change is more pronounced in the northern half of the complex where rocks are darker, fresh, and the rhythmic layering is also more conspicuous. Within the white 'massive' anorthosite type, which is largely restricted to the southern half of the intrusion, cryptic layering is less pronounced. Textures indicate that rocks cooled very slowly and the co-existing mineral compositions indicate re-equilibration to usually low temperatures.

Geomorphological conditions of gabbro weathering at Mount Megantic, Quebec

Chemical and mineralogical analyses of a deeply weathered gabbroic rock showed that smectite (nontronite) is the main secondary mineral in association with iron-rich chlorite. The granular disintegration of the rock is mainly induced by the alteration of ferromagnesian minerals. The occurrence of silica-rich minerals is explained by the confining conditions imposed by the depressed topography of the site; along steeper slopes gibbsite is found in thinner saprolites. Weathering, though still proceeding, must have started before the last glaciation in order to explain the thickness of the profile; this is also supported by geomorphological considerations. The succession of erosion and weathering stages during Pleistocene times contributed to the shaping of the gabbroic depression.