Combustion metamorphism of bituminous sediments and the formation of melts of granitic and sedimentary composition

During combustion metamorphism, i.e. the heating of sediments rich in organic matter by spontaneous subsurface combustion, temperatures obtained are frequently high enough to cause partial or total melting of the original rocks. These melts are of particular geochemical interest because at lower temperatures their composition closely simulates that of granitic and at higher temperatures that of common sedimentary rocks. At the California localities, studied here, the parent rocks are mainly bituminous mudstones with smaller amounts of diatomites and phosphorites and lesser dolomites, limestones, shales and cherts. It is estimated that rock melting started below 1000° C. The first partial melts correspond to a melt fraction of about 5% have a constant composition which is controlled by the eutectic of the dominant mudstones, and are undistinguishable in major and trace element composition from common S-type granites. Compared with the original rocks, these melts are enriched in SiO₂, Al₂O₃, K₂O, Na₂O, as well as in Ta, Hf, Zr, Th and REE, and are depleted in all other elements studied. The only difference between these low-temperature melts and granitic ones is their oxygen isotope composition, which is that of the sedimentary parent rocks. These melts intruded the country rocks as sills and dikes and cooled to glassy rocks simulating obsidians. As temperatures rose, the melts changed composition and left the magmatic field. At the highest temperatures (in excess of 1650 C), their composition equals that of the original sediments from which they are derived, except for volatile components such as H₂O and CO₂. These melts formed intrusive bodies, mainly stocks, up to several kilometers across. On cooling, these melts formed fine-grained but holocrystalline rocks, the dominant minerals of which are cristobalite, α- and β-tridymite, calcic plagioclase, cordierite, wollastonite, gehlenite, andradite and apatite. Where the parent rocks included phosphorites, two immiscible melts formed an emulsion in which droplets of apatitic composition are surrounded by a silicate melt, almost devoid of P₂O₅.