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Abstracto

The behaviour of Feldspar Megacrysts in Granitic Pegmatite Veins (western Oban massif, Nigeria)

Oden, M.I. and Igonor E.E.

Megacrysts of feldspar occur randomly and abundantly in pegmatite veins hosted by gneisses in the Oban massif (SE Nigeria). Earlier work has shown that these megacrysts are k-feldspars and magmatic products, formed at liquidus temperature in the presence of enough liquid that allowed the crystals to flow. Megacryst size, frequency and other physical characteristics were used to unravel their nucleation and crystal growth history. Analyses of over 400 megacrysts present in the pegmatite veins, showed that: (a) larger megacrysts occur closer to the middle of the veins, while smaller ones occur near the vein wall; (b) the smaller grains have larger angle difference between their long axes and the vein wall, while the longer grains have smaller angle difference; and (c) the wider veins contain larger-size but fewer grains, while the narrower veins contain smaller but more grains. It is deduced that the largesize megacrysts, which are also fewer in frequency, are most likely crystals that nucleated and started growing during a small degree of temperature change (during the cooling of the magma melt). Under favourable conditions they grew to large sizes, while the more abundant and smaller-sized megacrysts nucleated and grew during a greater degree of temperature change. Since under such conditions nucleation and growth rates are high, with the crystals competing for solute particles, more abundant but smaller sizes of megacrysts were formed.