GEOCHEMISTRY AND K-Ar DATING OF THE TERTIARY UNDERSATURATED VOLCANIC
PROVINCE OF JABAL AL HASAWINAH, LIBYA.
Hertogen, J.**, Pasteels, P.***, Sanders, I. S.****
*University of Tripoli,
Faculty of Science, Department of Geology, P. O. Box 13258, Tripoli,
Universiteit Leuven, Afdeling Fysico-Chemische Geologie, Celestijnenlaan
200C, B-3030, Belgium.
Eenheid Geochronologie, Pleinlaan 2, 1050 Brussels, Belgium.
Department of Geology, Dublin 2, Ireland.
Jabal al Hasawinah (JAH) is one of five main Tertiary
volcanic provinces in Libya. It exposes a series of alkaline lavas
composed of an undersaturated basanite-phonolite lineage accompanied by
transitional alkali (olivine) basalt and occurrences of gabbro and
syenite subvolcanic intrusions.
The lavas are highly enriched in incompatible
elements, typical of within-plate magmatism. The basanites and olivine
basalts differ in their mineralogical, chemical, isotopic and REE
characteristics indicating derivation by two different degrees of
partial melting, possibly at different mantle levels.
Quantitative modelling of major and trace elements
and REE variations demonstrate that, within the undersaturated lineage,
the basanites differentiation to mugearites through hawaiites is
controlled fractional crystallization of the phenocryst phases olivine,
clinopyroxene and magnetite. Similarly, the mugearites differentiate to
phonolites. Fractionation within the phonolites is dominated by alkali
feldspar, clinopyroxene and sphene. The nepheline syenites exhibit
mineralogical, chemical and REE patterns similar to the phonolites while
the olivine gabbro show characteristics similar to the basanites.
K-Ar dating of the basanites and olivine basalts
suggests eruption during the Oligocene time ca. 30-29 Ma ago. The
phonolites were erupted over a relatively longer period, starting during
the Oligocene and terminating during the Miocene ca. 30-20 Ma ago. A
minor rejuvenation phase of small basanitic dikes was erupted ca. 17 Ma
Limited Sr and Nd isotope data indicate that the
volcanics were derived from a depleted upper mantle source with no
significant crustal contamination; 87Sr/86Sr
(0.7033-0.70389) and 143Nd/144Nd