THE PYROCLASTIC ROCKS AND ASSOCIATED MAFIC DYKES AND PLUGS, NW LIBYA

Abstract

Repeated highly explosive and effusive processes in north of Ghāryān Town and Qumātah area, NW Tripolitania blasted through Mesozoic country rocks, generated numerous small sized and individual isolated exposures of areally limited, explosively fragmented aggregates. They are represented by near-surface, vent-filling, fragmentary rocks identifiable along roadcuts, but most commonly seen along wādi channels. They are comprised of a mixture of tuff, lapilli tuff, comminuted sediments and minor igneous and sedimentary rock fragments.  A period of hydromagmatism, resulting from the interaction of ascending magma with externary-derived water at shallow depth, appears to characterize the early stage of mafic volcanism. When no water interacted with the erupting magma, low viscosity alkalic basaltic magma was emplaced after short hiatuses, forming multiple discordant narrow dykes or plugs of mafic rocks. The dykes appear to be parallel sided and their trend in most cases, lie parallel to the major structure of their host rock. In the Wādi Qābel and Kabdet al Jamel pyroclasts, there is evidence that the dykes were emplaced by injection associated with dilation of the dyke walls. It is possible that the localized nature of the pyroclastic deposits, and their associated mafic dykes, were controlled by regional tectonic stresses. Coarse-grained lherzolite nodules and clinopyroxene megacrysts of presumed upper mantle source origin are widely distributed and locally abundant in some vents and their associated mafic plugs. The presence of accidental blocks and foreign fragments within the volcaniclastic rocks of Wādi Qābel and Kabdet al Jamel, which can be recognized as coming from shallow levels of the walls of the conduit (Al Aziziyah and Abu Shaybah sediments) suggests shallow depth of fragmentation of the melt within/or below the stratigraphic position of the two formations. The dykes appear to have been emplaced along shear planes, the distribution and general similarity of the vents through the area has led to them being regarded as belonging to the same magmatic episode.