EXTRUSIVE CARBONATITES FROM THE UYAYNAH AREA, UNITED-ARAB-EMIRATES

Carbonatites occur in the Uyaynah area, United Arab Emirates, within a tectonic window of oceanic metasediments beneath the Semail Ophiolite Complex at the northern end of the Oman Mountains. The carbonatites form conformable layers, pods, and lenses, up to 10 m thick, associated with deep-sea sediments that include radiolarian cherts and pillow lavas. Although disrupted during deformation, the carbonatites are internally undeformed and contain abundant lapilli, indicating an extrusive, subaerial origin, probably on a small volcanic island. The lapilli comprise calcite, abundant Sr-bearing apatite and iron oxides in a calcite matrix. Alkali amphiboles, biotite, chlorite, and allanite were produced as a result of metamorphism but otherwise the primary igneous carbonatite mineralogy remains. One distinct group of rocks also contains primary calcic amphibole and biotite, Niobian titanite, strontian baryte, and albite are also present, and rare earth elements are held in abundant allanite-(Ce) and rare monazite-(Ce). Analyses of 26 rocks define two groups which are distinguishable in the field. One group consists of silico-carbonatites containing primary calcic amphibole, biotite. and abundant allanite, and is chemically similar to average intrusive ferrocarbonatite. The other group corresponds chemically to average intrusive calciocarbonatite. Unlike other extrusive carbonatites described in the literature, the Uyaynah carbonatites have higher incompatible element abundances than average intrusive carbonatites and are notably rich in rare earth elements (up to 2-6 wt.%) and P2O5 (average > 7 wt.%). The two carbonatite groups are considered to be related magmatically, one to the other, and the presence of rare ferroan chromian spinel is interpreted as evidence for a direct mantle origin.