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The Form and Structural Features of Aplite and Pegmatite Dikes and Veins in the Osi Area of the Northern Provinces of Nigeria and the Criteria That Indicate a Nondilational Mode of Emplacement

B. C. King
The Journal of Geology
Vol. 56, No. 5 (Sep., 1948), pp. 459-475
Stable URL: http://www.jstor.org/stable/30071078
Page Count: 17
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
The Form and Structural Features of Aplite and Pegmatite Dikes and Veins in the Osi Area of the Northern Provinces of Nigeria and the Criteria That Indicate a Nondilational Mode of Emplacement
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Abstract

Numerous dikes and veins of aplite and pegmatite in the Osi area of Nigeria were studied with a view to ascertaining the mechanism of emplacement. Offsetting of the invaded rocks of a magnitude and direction appropriate to dilation is so rare as to appear to be quite fortuitous. Where offsetting is not entirely absent, it is commonly of a different magnitude and frequently in the opposite direction from that to be expected on dilation of the invaded rocks. In such instances emplacement has evidently occurred along planes of shearing formed under compressional stress. Nondilational emplacement is also clearly indicated in the case of the irregular bodies and ramifying networks of veins. The criteria advanced are similarly of significant application in the mode of emplacement of "ptygmatic folds." Processes of metasomatic replacement, conveniently termed "aplitization" and "pegmatization," are believed to have been those responsible for the development of the bodies described, while internal evidence for the operation of such processes is sometimes provided by the textural features of the dikes themselves. The broader petrogenetic implications emphasize the need for a closer scrutiny of the criteria that are commonly accepted as indicative of intrusive relations.

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