Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

GEOCHEMISTRY AND  K-Ar DATING OF THE TERTIARY UNDERSATURATED VOLCANIC PROVINCE OF JABAL AL HASAWINAH, LIBYA.

 Oun, K. M.*, Hertogen, J.**, Pasteels, P.***, Sanders, I. S.****

*University of Tripoli, Faculty of Science, Department of Geology, P. O. Box 13258, Tripoli, Libya. khaledoun@tripoliuniv.edu.ly

**Katholeike Universiteit Leuven, Afdeling Fysico-Chemische Geologie, Celestijnenlaan 200C, B-3030, Belgium.

***Vrije Universiteit, Eenheid Geochronologie, Pleinlaan 2, 1050 Brussels, Belgium.

****Trinity College, Department of Geology, Dublin 2, Ireland.

ABSTRACT

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 ago.

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 (0.51285-0.51294).