PETROGENESIS OF THE UAE CARBONATITES - GEOCHEMICAL AND ISOTOPIC SIGNATURES

Although studies and discoveries of carbonatites have considerably increased in the last few decades, the ambiguous petrogenesis of these rocks has still been an unresolved paradox. The UAE carbonatites are extrusive, uncommonly occurring within the metamorphic sole of the Semail ophiolite at two different locations, Dibba and Hatta zones. Dibba occurrences are relatively more and associated with alkaline metavolcanics. They geochemically display two separate populations. The first one show a relatively significant depletion in the REE and trace elements, with lower CO2 and higher SiO2 (silicocarbonatite). The second one illustrates a significant enrichment in the REE and trace elements, close to, but little above the known average carbonatite composition. The samples of this group include higher CaO, CO2 and FeO and lower SiO2 (Calsiocarbonatite – Ferrocarbonatite).

δ18O and δ13C analysis shows an increase in the isotopic composition of the studied rocks in array from the primary (mantle) igneous carbonatite. However, excluding a few incompatible samples, 2nd group carbonatites exhibit a much higher isotopic enrichment than the 1st group. Moreover, variation trends of REE and trace elements in both groups are analogous, and some reveal similarity with carbonatites from Canary and Cape Verde islands.

These observations draw several important inferences: 1. UAE carbonatites have an oceanic origin and emplacement. 2. They were primarily originated from a silicate (alkaline) melt, and silicocarbonatites might represent a mix cognation between the two melt types. 3. The 2nd group “Calsiocarbonatite – Ferrocarbonatite” was initially produced from the 1st group “silicacarbonatite”. 4. A hot fluid enriched with heavier isotopic composition had affected these carbonatites, especially the 2nd group.

Finally, it is concluded that the UAE carbonatites might have formed in association with alkaline volcanic activities at an oceanic environment prior to the final obduction of the Semail ophiolite, during which they were exposed to metasomatism by a hot hydrothermal fluid.