Meta-Carbonatites in the Metamorphic Series Below the Semail Ophiolite in the Dibba Zone, Northern Oman Mountains

Abstract

Within the metamorphic series below the Semail Ophiolite, meta-carbonatites occur in a meta-volcanic unit in the Dibba region of the United Arab Emirates (UAE). The meta-carbonatites occur as flows, sills or constituents of meta-tuffites. An intimate association with alkaline basaltic meta-pillow lavas, meta-hyaloclastites, meta-pillow breccias, meta-tuffites, meta-radiolarian cherts and meta-siliceous carbonates can be observed. The succession represents a suite of subaerial and subaquatic volcanic rocks deposited together with deep sea sediments.

The meta-carbonatites are geochemically characterized by high P205 contents and Ce, La and Nd contents between 500 ppm and 15000 ppm. Compared to N-type MORB, these rocks are strongly enriched in LIL-elements. Chondrite normalized REE patterns show a strong enrichment in light REE compared to heavy REE. With the exception of the meta-tuffites, where orthite is the major REE carrying phase, the REE are favouredly enriched in apatite. The initial ⁸⁷Sr/⁸⁶Sr ratios between 0.7037 and 0.7043 confirm a mantle source for the analyzed meta-carbonatites. The Sm-Nd isotope data reflect ∈_Nd(T) values below those commonly observed in magmatic rocks deriving from depleted mantle sources and thus imply a source in a low ∈_Nd subcontinental mantle. A comparative sedimentary carbonate sample yields ∈_Sr—∈_Nd characteristics strongly differing from the meta-carbonatites and, furthermore, indicates the presence of a 1.68 Ga old source region.The meta-volcanics associated with the meta-carbonatites are considered to be the metamorphic equivalents of the mid-Triassic non-metamorphic alkaline volcanic rocks from the Haybi complex and the Umar group. They probably represent magmas of volcanism related to the Triassic rifting of the Neotethys along the passive eastern margins of the Arabian platform. They possibly formed in a transition zone of continental to oceanic crust similar to the situation of the Canary Islands, where carbonatites are encountered at the margin of the African continent.(1980) or the Umar Group (Ziegler and StÖssel,1985). As there seems to be a general agreement with respect to the opening of the Neotethys NE of the Arabian continental block during Triassic times (Stöcklin,1974;Sengör,1985;Descourt et al.,1986 ), it can be inferred that the meta-carbonatites represent magmas related to the volcanism associated with the Triassic rifting. A relationship with the movements of the Paleo-Tethys of SengÖr (op. cit.) or Tethys of Dewey et al. (1983) is ruled out based on the observations by Bèchenec (1987) who clearly assigned the alkaline volcanic rocks to the aforementioned Neotethys. Glennie et al. (1974,1973), Lippard et al. (1986) and BÖchenec (1987) place the origin of the allochthonous sediments NE of the Arabian platform at the passive margin of the Neotethys. According to this model, the meta-carbonatites can be explained as mantle magmas marking the site where the deep reaching Dibba fault zone crosscuts the Triassic SE-NW running rift faults. In order to explain the measured paleocurrent data of the Exotics and the Umar Group, Blendinger (in press) assumes an original position further south and movements along a transform faulting adjacent to an SW directed thrust to bring them into their present position. Using this model, the connection with the Dibba fault zone becomes arbitrary. In both models, however, the meta-carbonatites would have formed along the passive margin of the Arabian continent in a transition zone from continental to oceanic crust in an oceanic island setting similar to the situation of the Canary Islands at the margin of the African continent.