Mantle xenolith-bearing Maastrichtian to Tertiary alkaline magmatism in Oman



[1] Mantle xenolith-bearing alkali basalts, basanites and tephrites with 1.5–2 K2O and 4–5 wt% Na2O occur as small (<100 m) plugs and dikes in the Batain and Haushi-Huqf areas, and WNW of Muscat. Their black color and the common presence of peridotite xenoliths allow a separation from older alkaline rocks and from the ophiolitic extrusive rocks. Two main dike directions, approximately E-W and N-S oriented, are observed. Intrusions seem to occur where these two fault systems intersect. The basalts crosscut the Upper Maastrichtian siliciclastic rocks of the Fayah Formation and the nappe stack of the Eastern Ophiolite Belt of the Batain area. They have not been observed intruding the Tertiary shallow marine carbonates although K-Ar whole rock dating on these lavas yielded Late Eocene 37 ± 1 to 44 ± 1 Ma ages. The rocks are aphyric and fine-grained with microgranular, and less common microtrachytic texture. They contain magmatic olivine (Fo80–82), nepheline (Ne82–86Ks14–18), clinopyroxene (Di46En25Wo28) with 2.5 wt% Al2O3, accessory phlogopite (XMg0.6, with 5.5 wt% TiO2), in a microcrystalline or glassy matrix. Plagioclase is only observed in few samples. Locally occurring mm to cm-sized immiscible leucocratic melt droplets consist of potassium feldspar (An1–4Ab37–39Or60–70), plagioclase (An20–22Ab65Or13–15), nepheline (Ne85Ks15), phlogopite (XMg0.8, with ∼6–7 wt% TiO2), and titanomagnetite with ∼8 wt% TiO2. More than 95% of the xenoliths are <5cm-sized weakly to clearly foliated spinel peridotites of mantle origin. Sedimentary xenoliths (hornfelses) are rare and lower crustal xenoliths were not found. The peridotite xenoliths consist of olivine (Fo90–92), enstatite (En89Fs09Wo02) with 3.1–3.3 wt% Al2O3, diopside (Di45En26Wo29) with 3–6 wt% Al2O3, and Cr-spinel with ∼60 mol% spinel, 20% hercynite, and 15–16% magnesiochromite. The age, chemical composition, and structural position suggest a relation with local tectonic movements associated with plate reorganization in the Owens Basin region prior to the Red Sea opening.