Heavy-mineral concentration in modern sands: implications for provenance interpretation

Heavy mineral concentration in sediments depends primarily on the chemistry and tectono-stratigraphic level of rocks eroded within continental-block, arc, or orogenic source terranes. Detritus derived from mantle peridotites and lower crustal gabbros, as well as from high-pressure (oceanic or continental eclogite) and high-temperature (amphibolite, granulite) metamorphic rocks contains one to two orders of magnitude more heavy minerals than detritus derived from upper crustal rocks including granites and sedimentary successions. Concentrated heavy mineral assemblages, however, may result from density-sorting during erosion, transport, or deposition by tractive currents, which can very effectively segregate minerals with even small differences in density within distinct grain-size fractions and sedimentary environments (e.g., fluvial channel versus overbank, beach versus shelf). Conversely, depleted heavy mineral assemblages may result from severe diagenetic dissolution in ancient sandstones, a process that in Alpine and Himalayan foreland basins very extensively affected clastic wedges older than the Pleistocene. Through a series of key modern examples from various geodynamic settings in arid to semi-arid climate, we stress the importance of giving full consideration to heavy mineral concentration while interpreting provenance of terrigenous sediments and sedimentary rocks. The concentration parameters introduced herein (Heavy Mineral Concentration index, Source Rock Density index), coupled with parameters based on heavy mineral species with either contrasting density (% opaque, % ultradense, % ZR) or chemical stability, allow us to reveal and quantify also the effects of hydraulic sorting in the depositional environment and of diagenetic dissolution in ancient terrigenous rocks. We document that in most modern sands, the relative abundance of chemically stable species (e.g., zircon, tourmaline, rutile, apatite, chrome spinel) is considerably less than the experience on ancient sandstones generally induces to believe, and show that the actualistic approach provides crucial insight for a correct interpretation of heavy mineral suites