Abstract Pyrometamorphism of coal measures that overlie underground burnt-coal seams in the southern area of the Rotowaro coalfield, New Zealand, has produced porcellanites that enclose lenses of iron-rich magnetite and hematite-bearing slag-like rocks, which show various degrees of oxidation. Paralavas that are associated with the iron-rich lenses form stalactites around fissures and gas-escape vents and intrude the porcellanites. The slags are unusually rich in iron and contain magnetite, hematite, hercynite, titaniferous magnetite, and minor fayalite and silicate glass. Iron oxides in these rocks exhibit a variety of textures and morphologies, including dendritic, quench, exsolution, and oxidation. The paralavas contain abundant glass, which encloses feathery crystals of fayalite and orthoferrosilite, plagioclase, tridymite, cristobalite, and minor magnetite. Phase-equilibria data indicate that the paralavas and slags were formed at temperatures in the range of 1000–1600 °C. Composition plots of local coal measure sediments, a siderite nodule, and various porcellanites, slags, and paralavas with respect indicate that the porcellanites are iron-rich when compared to SiO 2 , Fe 2 O 3 , and Al 2 O 3 with the unmetamorphosed coal measures. The slags, irrespective of their oxidation state, plot on the iron-enrichment trend shown by the porcellanites. Although the siderite nodule lies within the iron-enrichment trend, most of the slags are more iron-rich than the siderite nodule. The paralavas diverge from the iron-enrichment trend, suggesting that they formed by partial melting of the porcellanites. The Rotowaro samples represent some of the most iron-rich natural slags collected from a combustion-metamorphic environment to date.

Abstract Unusual dike-like bodies and lenses of paralava, up to 1–5 m long and 10 cm thick, are found in the “olive” unit of the combustion metamorphic complex in the Hatrurim Basin, Israel. High-temperature rocks of the “olive” unit are composed of anorthite and clinopyroxene (diopside-hedenbergite-esseneite) and are connected with pipe-like explosion structures. The paralavas are cryptocrystalline rocks that exhibit vesicular and fluidal textures. The main mineral assemblage in these rocks, identified by electron-microprobe analysis and X-ray diffraction, consists of basic plagioclase + Fe-Ti oxides + clinopyroxene + K-feldspar + tridymite ± apatite. The silica content of the paralavas is similar to that of basalt, whereas the high calcium content suggests similarity to anorthosite. The occurrence of glass in the paralava is evidence for melting. The glasses are compositionally similar to rhyolites and more acidic melts. Melting temperatures were at least 1100 °C. The presence of pipe-like explosion structures, the occurrence of melted rocks, and various geological relationships throughout the Hatrurim Basin provide evidence for a new hypothesis about the genesis of this pyrometamorphic complex. The combustion metamorphic rocks in the Hatrurim Basin in Israel formed as a result of repeated ignition of hydrocarbon gases. The setting we envisage has many geological features typical of mud-volcano provinces. The occurrence of paralavas is restricted to the areas of gas ignition.