HR: 1400h
AN: V33B-02    [Abstracts]
TI: Late Jurassic ultramaphic lamprophyres with kimberlitic affinity in the allochthonous Batain nappes of Eastern Oman
AU: * Nasir, S J
EM: sobhi@squ.edu.om
AF: Sultan Qaboos University, Department of Earth Sciences College of Science, P.O.Box 36, Al-Khod, Mus 123, Oman
AB: Carbonatite, alkaline volcanic rocks and ultramafic lamprophyres with kimberlitic affinities have been recently discovered in the allochthonous Batain nappes of Eastern Oman. The main bodies of the ultramafic lamprophyres occur in a diatreme at the coast of the Asseelah village, northeastern Oman. The second major outcrops of ultramafic lamprophyres occur as several 6 km long dykes at the Bomethra area. The diatreme consists of heterogeneous deposits dominated by 'diatreme facies' volcaniclastic rocks. These include accretionary and armoured carbonate lapilli, and carbonate-dominated tuffs, all of which intrude late Jurassic to early Cretaceous cherts and shales of the Wahra Formation. The Asseelah ultramafic rocks may be classified as either aillikite and/or carbonatite with kimberlitic affinity. Garnet (G0), chromite, phlogopite, ilmenite, zircon, apatite, rutile, corundum, and sillimanite have been recovered from heavy mineral concentrates. Zircon grains extracted from the diatreme rocks have a mean age of 137 + 1 Ma (95 % confidence, MSWD = 0.49). The trace element patterns of the zircon gains are typical of kimberlitic to carbonatitic rocks and their Hf isotope ratio (176Hf/177Hf = 0.28286 + 1, å Hf = 6.2) is typical of kimberlitic zircons of early Cretaceous. The lamprophyric dyke swarms of the Bomethra area comprise macrocrystic, spinel and phlogopite bearing hybabyssall facies calcite aillikites/damtjernites with pelletal lapilli and globular segregationary textures. The main dyke extends in length up to 6 km and ranges in width between 1 and 30 meters with two main blows 300-500 m in width. The petrography, mineralogy, trace element and isotopic composition of the dyke rocks are comparable to aillikites and damtjernites with kimberlitic affinity. Kimberlite indicator minerals include chromite, Cr-diopside, G4 garnet, and picroilmenite. The Asseelah and the Bomethra ultramafic rocks are enriched in light REE and have a high modal proportion of Ti-Al rich phlogopite, suggesting that they were derived from a source region which has experienced melt-depletion followed by metasomatic enrichment. This enrichment of the source region could be a consequence of the upward percolation of an alkaline melts that penetrated the base of the subcontinental lithosphere during the break-up of Gondwanaland. There is no obvious age difference (137 Ma; U-Pb zircon dating of Asseelah rocks and 150- 162 Ma; Ar-Ar age dating of Bomethra rocks) between these various rocks, so the initial magmas were formed around the same time. These ages correlate with large-scale tectonic events recorded in the early Indian Ocean at 140-160 Ma. The magmatism is probably a distal effect of the breakup of Gondwana, during and/or after the rift-to-drift transition that led to the opening of the Indian Ocean. The magmas in the Batain area are petrogenetically related and appear to have originated in a single event, possibly triggered by the arrival of the hot material beneath the Batain lithosphere that had been recently metasomatised and is related spatially and compositionally to mantle upwelling associated with the rifting.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1038 Mantle processes (3621)
DE: 1039 Alteration and weathering processes (3617)
DE: 3625 Petrography, microstructures, and textures
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2009 Joint Assembly