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 theSemail 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 includeradiolarian cherts and pillow lavas. Although disrupted duringdeformation, the carbonatites are internally undeformed andcontain abundant lapilli, indicating an extrusive, subaerialorigin, probably on a small volcanic island. The lapilli comprisecalcite, abundant Sr-bearing apatite and iron oxides in a calcitematrix. Alkali amphiboles, biotite, chlorite, and allanite wereproduced as a result of metamorphism but otherwise the primaryigneous carbonatite mineralogy remains. One distinct group ofrocks also contains primary calcic amphibole and biotite. Niobiantitanite, strontian baryte, and albite are also present, andrare earth elements are held in abundant allanite-(Ce) and raremonazite-(Ce). Analyses of 26 rocks define two groups whichare distinguishable in the field. One group consists of silico-carbonatitescontaining primary calcic amphibole, biotite, and abundant allanite,and is chemically similar to average intrusive ferrocarbonatite.The other group corresponds chemically to average intrusivecalciocarbonatite. Unlike other extrusive carbonatites describedin the literature, the Uyaynah carbonatites have higher incompatibleelement abundances than average intrusive carbonatites and arenotably rich in rare earth elements (up to 2?6 wt.%) and P2O5(average > 7 wt.%). The two carbonatite groups are consideredto be related magmatically one to the other, and the presenceof rare ferroan chromian spinel is interpreted as evidence fora direct mantle origin.