System closure during the combustion metamorphic “Mottled Zone” event, Israel

Abstract

The Hatrurim Formation (or the “Mottled Zone”) is a unique rock complex in Israel and Jordan, which is composed mainly of high-temperature–low-pressure metamorphic minerals, typical of the sanidinite and pyroxene-hornfels facies. These rocks are the metamorphic product of combustion of the bituminous (“oil shales”) Maastrichtian to Paleocene chalk and marls of the Ghareb and Taqiye Formations.Temperatures of 500–700°C were common during the combustion metamorphic process producing the high-grade gehlenite–larnite, calcite–spurrite and anorthite–diopside fels (“dense-olive” rocks). In order to test the degree of system chemical closure the graphical isocon method was applied. We compared the chemical composition of the metamorphic rocks and their protolith equivalents — Ghareb Formation oil shales and Taqiye Formation rocks, on an outcrop scale. With respect to a set of 8 major, minor as well as 21 trace elements, the chemical composition remained similar during metamorphism and yielded almost no element mobility. The main losses were those of organic matter, which was oxidized during combustion, dehydration, upon heating and decarbonation of carbonates. Combustion resulted in an average mass loss of 30% and some volume loss (derived from the mass loss as well as the formation of denser mineral phases). These suggest that the combustion process was essentially isochemical on a large scale. On the other hand, the common presence of veins in some of the rocks implies that the Hatrurim metamorphism was isochemical on a large scale, whereas on a smaller scale open system fractionation characterized some of the elements, which are enriched in the veins.