Abstract
Gharyan basaltic subvolcanic bodies, in the north-westernmost Libya, are characterized by extreme irregularity, having short length, numerous braches and abruptly change in thickness with a wide GPS coordinates. Commonly occurring as dykes, a few of them can be classified as small stocks, and others are sills. They have strike N 70-80˚ W. They are nepheline-normative alkaline basalts which are comprised of four distinct suites, namely, olivine basalt, basalt, trachy-basalt and trachy-andesite. The trachy- basalt and trachy-andesite are the end differentiation products of alkali-basalt series. The magmatism that gave rise the Miocene- Pleistocene basaltic dykes of Gharyan district represent intraplate alkaline volcanics generated by a young mantle plume but was contaminated by little crustal materials or were products of fractional crystallization of mantle-derived magma. Such magma is thought to be similar to the ocean island basalt formed in within- continental plate setting. Based on the field observation and K-Ar dating (6) subdivided the volcanic activity along eastern scarp of Jabal Nafusah (Gharyan district) into an old lava series, young lava series, volcanic cones, phonolite dome-shaped intrusions (47.0±1.2-37.7 Ma) and pyroclastic rocks. The young lava series (6.0±1.3-2.3±0.3 Ma) is mostly less common and restricted to the periphery of the old lava series or sometimes occupies ancient burial wadis. Late volcanic centers (7.7±1.5-1.3±0.5 Ma) usually form very small eruptive products which erupted from central volcanoes and dykes. Buserwil and Wadsworth (6) reported that both old and young lava series are composed of basaltic andesites. The late volcanic centers are generally basanitic suite with minor related differentiates, while the domes are all phonolitic suite. The present study intends to outline and compare the petrological and geochemical characteristics of the sheet-subvolcanic intrusions (dykes and sills), and briefly discusses some petrogenetic implications of the results obtained.
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