Analysis of detritus from mantle-derived rocks

Abstract

Detrital materials supplied from mantle-derived rocks have peculiar mechanical and chemical characteristics that make them excellent sedimentological and tectonic markers. Chromian spinels are well known and favored petrogenetic indicators because they contain several important cations, including Mg, Fe, Cr, Al, and Fe, as their main components. The Mg/(Mg + Fe) ratio (= Mg#), Cr/(Cr + Al) ratio (= Cr#), and Ti concentration are important parameters that are commonly analyzed to petrologically characterize chromian spinels. The main hosts in the chromian spinels of mantle peridotites (harzburgite and lherzolite) and their serpentinized equivalents are Mg# and Cr#, which are controlled by the equilibrium temperature (degree of subsolidus cooling) and degree of melt extraction, respectively. The chromian spinels in detritus materials are chemically stable during sedimentation processes, and can thus serve as powerful indicators of the tectonic and geologic history of the hinterland. The derivation of detrital chromian spinels, peridotites (serpentinites), chromitites, or volcanics can be partially identified by analyzing their Mg#, Cr#, and Ti concentrations in combination with their textures. The petrologic character of highly sheared serpentinite, such as the matrix of a serpentinite mélange complex, sometimes yields an inconclusive geodynamic history, but this can potentially be inferred from detrital chromian spinels in nearby sediments. Detrital chromian spinels from modern sediments are also useful for obtaining a general view of large peridotite bodies, such as the mantle section of the Oman ophiolite. When we compare the detrital spinels with those from in situ rocks, we should note that the host rocks containing the detritus material have already been eroded. If we analyze the serpentinite sandstones together with their closely associated peridotite bodies, we can possibly obtain information on the petrologic heterogeneity of the upper mantle. Our petrographic investigation of the Circum-Izu Massif Serpentinite Belt, central Japan, provides a good example of such a combined analysis of these peridotite-serpentinite sandstone pairs.