Physics
Scientific paper
Oct 1984
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1984pepi...35..199b&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 35, Issue 1-3, p. 199-211.
Physics
4
Scientific paper
The Nigerian younger granites, as well as the petrologically similar but chronologically older northern centres in Aïr and south Niger, constitute the best example of epidosically migrating mid-plate magmatism. The Nigerian anorogenic ring complexes provide a classic example of alkaline granitoid evolution. Each anorogenic centre represents the exposed roots of an eroded volcano. Volcanic assemblages preserved by caldera collapse at some centres confirm that early erupted liquids evolved from olivine basalt through hawaiite, mugearite and trachyte. Although there are volumetrically abundant rhyolitic ignimbrites, there are no volcanic equivalents of biotite granite. However, at the subvolcanic level biotite granites are extensively mineralized with rich deposits of columbite, sphalerite, cassiterite and wolframite. Related peralkaline granites contain enriched concentrations of ore elements but apart from pyrochlore, thorium-rich monazite, and topaz, most potential ores are contained in alkali amphiboles or alkali pyroxene. Fluid interaction in the granite cupolas has caused subsolidus mineral growth and modified original element abundances. In some instances fluid reaction has been so intense that original granite has been locally converted to albite, microclinite, and greisen. These reactions can be monitored by the new mineral assemblages, the major element chemistry, trace elements (e.g., the rare earths) and isotopically by the initial strontium isotopic ratios.
Bowden P.
Kinnaird J. A.
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