Petrology of Late Jurassic allochthonous ultramafic lamprophyres within the Batain Nappes, Northeastern Oman

Late Jurassic ultramafic lamprophyre (UML) sills and dikes occur as 3 km-long intrusions within the allochthonous Whara Formation of the Batain nappes, eastern Oman. The sills and dikes comprise macrocrystic phlogopite and spinel-bearing aillikite and damtjernite. Aillikite is a light grey, massive fine-grained tuffaceous rock with euhedral laths of mica, while damtjernite is a dark grey, medium- to coarse-grained rock with abundant pelletal lapilli and globular segregationary textures. Both lithologies are composed of calcite, phlogopite, apatite, magnetite, spinel, diopside, and richterite. Orthoclase occurs only within damtjernite. The rocks are strongly silica undersaturated (17.6–33.7 wt.% SiO₂), with low MgO (4.7–10.2 wt. %) and high Al₂O₃ (3.5–8.6 wt.%). The aillikites are distinguished from the damtjernites by their lower SiO₂, Al₂O₃, and Na₂O abundances, and their higher MgO, CaO, and P₂O₅ contents. The rare earth element (REE) patterns of both rock types are similar and show strong light REE (LREE) enrichment. Both are enriched in Ba, Th, U, Nb, and Ta, with normalized concentrations of up to 1000 times those of primitive mantle. Relative depletions are apparent for high REE (HREE), K, Rb, Pb, Sr, P, Zr, and Hf. The rocks have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.70435–0.70646, whereas initial ¹⁴³Nd/¹⁴⁴Nd ratios vary between 0 · 512603 and 0 · 512716 (εNd_i 2.6–3.2). Pb isotopic ratios are more varied among the aillikites and damtjernites: ²⁰⁸Pb/²⁰⁴Pb_i = 38.97–39.39 and ²⁰⁷Pb/²⁰⁴Pb_i = 15.35–15.58, ²⁰⁶Pb/²⁰⁴Pb_i = 18.08–18.96. The abundance of phlogopite, apatite, and rutile and enrichment in LREEs, Ba, Th, U, Nb, and Ta in the Sal UMLs suggest metasomatic enrichment of these rocks following a low degree of partial melting of a depleted source region. Ar–Ar age dating of phlogopite macrocrysts from the aillikites and damtjernites (154 and162 Ma, respectively) correlates with large-scale tectonic events recorded in the proto-Indian Ocean at 140–160 Ma.