Combustion metamorphism in the Nabi Musa dome: new implications for a mud volcanic origin of the Mottled Zone, Dead Sea area

Numerous high-temperature combustion metamorphic foci within brecciated mainly calcareous sedimentary rocks in the Dead Sea area (the so-called 'Mottled Zone' complexes) have been interpreted as resulting from in situ oxidation and ignition of dispersed organic matter in bituminous chalks. Geological, chemical and petrological data for the Nabi Musa dome, one of 15 Mottled Zone complexes data presented in this paper, suggest an alternative interpretation relating the Mottled Zone complexes to Pliocene-Pleistocene mud volcanism and the associated methane combustion. The geochemistry and mineralogy of sedimentary, combustion metamorphic, localized hydrothermally, altered rocks, and ignition foci marked by ultrahigh temperature (up to 1500 °C) pseudowollastonite-rankinite-nagelschmidtite-bearing paralava, indicate that Nabi Musa is a fossil mud volcano, comprising a large diatreme edifice with brecciated sedimentary rocks in its main feeder. The mud volcanic event mobilized sediments of underlying Cretaceous strata from depths of at least 0.8 km, and the eruption was driven by hydrocarbon gases (predominantly methane), with gas flaming causing local combustion metamorphism. Besides ultrahigh temperature combustion metamorphic processes, ejected sedimentary rocks were subsequently altered by low-temperature hydrothermal fluids from various sources, which produced specific rock compositions with local enrichments in Mg, Na, Cl and B. Later, carbonation almost completely replaced the original smectite-bearing parent mud and preserved the edifice from erosion. The proposed mud-volcanic origin of the Mottled Zone complexes may have implications for gas prospecting in the Levantine basin.