Salah Al-Khirbash

Precambrian basement character of Yemen and correlations with Saudi Arabia and Somalia

Abstract The Precambrian basement of Yemen occupies a key location in the Pan-African orogen of Gondwana. This paper reviews geological, isotopic and geochronological data and presents new Pb- and Nd-isotope data which help define distinct gneiss terranes within this basement, constraining correlations of these terranes with neighbouring regions of Saudi Arabia and Somalia. Existing whole-rock Pb- and Nd-isotopic data are also summarised. These data should facilitate a more objective assessment of the contribution of the Yemen Precambrian to Cenozoic magmatism associated with the opening of the Red Sea and the Gulf of Aden.

Neoproterozoic crustal growth at the margin of the East Gondwana continent – age and isotopic constraints from the easternmost inliers of Oman

ABSTRACT The Jebel Ja’alan and Qalhat inliers of Oman represent the easternmost exposures in the Arabian peninsula of the Neoproterozoic basement associated with the East African Orogen (EAO) and the assembly of East and West Gondwana. These inliers expose tonalitic gneisses and metasediments intruded by granodiorites and granites of the Ja’alan batholith. Zircons from the gneisses yield U–Pb SIMS ages of ca. 900–880 Ma, which are interpreted as crystallization ages. These represent the oldest magmatic events associated with the closure of the Mozambique Ocean reported to date. Zircon of this age is also the dominant component in the metasediments. The Ja’alan batholith yields ages of ca. 840–825 Ma. Nd isotopes indicate that both the gneisses and the batholith range from juvenile to slightly more evolved, with εNd(t) of +6 to +1.5 interpreted to reflect variable contamination by older, evolved continental material; this is also indicated by >900 Ma detrital zircon from the metasediments. The Nd data also contrast with the uniformly juvenile signature of younger, ca. 840 Ma, rocks of the Marbat region of southern Oman that lie structurally to the west. The Ja’alan and Qalhat inliers thus document eastward increasing age and continental influence, consistent with the progressive development of arc rocks onto the western margin of East Gondwana, although the location and nature of the eastern continental block remain elusive.

Rare earth element mobility during laterization of mafic rocks of the Oman ophiolite

Eleven samples representing a sequence of progressive alteration through a laterite profile of Upper Cretaceous age developed on a layered gabbro from the Oman ophiolite have been analyzed for their rare earth element (REE) contents. The results provide new insights into the fractionation and vertical movement of REEs during the laterization of mafic parent rocks. Compared to the fresh parent rock, from the same section nearly flat chondrite-normalized REE patterns are observed in the saprolite zone, while light REE (LREEs: La-Nd)-enriched patterns characterize the oxide and clay zones. The altered gabbro shows a depletion in middle REEs (MREEs: Sm-Ho) compared to its unaltered counterpart. A negative cerium (Ce) anomaly is observed in the upper part of the altered gabbro, while the lower part appears to be depleted in Ce. The similarity in the LREE-enriched fractionation patterns throughout the alteration profile suggests that the REE fractionation process is independent of the total REE content of the parent rock.

Nature, genesis and industrial properties of the kaolin from Masirah Island, Oman

Abstract Kaolin deposits >10 m thick overlie unconformably a Mesozoic ophiolite sequence at Jabal Humr, Masirah Island, Oman. The clay’s mineralogical and chemical composition, plasticity and moisture content were measured to determine its genesis and suitability for commercial usage. The clayrich raw material contains 76-94% kaolinite and varying amounts of quartz (micro sheets coating kaolinite) and calcite as well as secondary sulfates. The mode of occurrence, an associated shallowmarine iron oolite/pisolite unit, various secondary minerals which can only form in a gossan environment (oxidation zone of a much older sulfide deposit), and minerals such as gypsum that are highly unstable within a laterite, have led to the conclusion that the Jabal Humr kaolinite deposit cannot have the lateritic origin that has been suggested previously. Rather, it must have formed in a coastal marine environment with a subsequent strong geochemical overprint from the underlying gossan environment, after being enveloped by Tertiary carbonates. A high plasticity and its light colour after firing indicate that this material is suitable for industrial use, especially in pottery. Occasional high contents of up to ∼25% extremely fine-grained quartz (sheet-like, <50 nm thick) reduce the need for quartz addition during the processing for ceramic materials; such natural kaolinite-quartz mixtures already produce a suitable blend of materials. The possible occurrence of spalling during or after firing, caused by the sporadic presence of accessory calcite, can be avoided by by further addition of quartz which leads to the formation of calcium silicate.