The Pegmatite Veins of Western Oban Massif: Tectonic and Lithological Controls on Physical Properties

摘要

In this study, the pegmatite vein system in western Oban Massif, southeastern Nigeria, is presented against a background of the salient geological structures of the area. It is the ultimate interest of this author to determine the relative contributions of magmatic fluid pressure and prevailing structural control in the emplacement of these igneous bodies, the source of the magmatic fluids as well as any indication of the pressure of the fluid. Is there any simple relationship between present pegmatite dimensions and the paleo-pressures of the fluids or the stresses that existed at the time of emplacement? The granitic pegmatites of Uyanga- Akwa Ibami area in western Oban Massif have drawn reasonable attention since the beginning of the twentieth century. They are genetically related to Uwet granodiorite, a syn-tectonic pan-African granitoid with a long cooling history. The strongest pegmatite vein orientation is in the NNW-SSE, followed by ENE-WSW and E-W sets. Less prominent sets trend in the NNE-SSW and NW-SE directions. These veins are preferentially emplaced mainly in pre-existing discontinuities like fractures, joints, faults and foliations. Finite pegmatite lengths up to 20 m and the infinite variety over 40 m have been measured in this area, although the most frequently occurring lengths are between 2-3 m. Vein thickness generally varies from 0.3 m to 8.4 m, with the most frequently occurring value lying between 1.0 and 1.5 m. The most frequently occurring vein thickness is-1.75 m in schist and 1.25 m in granodiorite host rock. Pegmatite veins tend to dilate as they elongate and a relationship of the form Y = MX exists between their lengths and widths. The material constant M varies from 2.236 for schist to 5.159 for granodiorite. This is interpreted as lithological control on vein dimensions. The presence of veins in tensile (mode I) fractures at the schist level only and a more ubiquitous distribution in shear fractures mainly, at the granodiorite level, is a strong indication of interaction between long range tectonic stresses and magmatic fluid pressures.