A zircon-tourmaline-rutile maturity index and the interdependence of the composition of heavy mineral assemblages with the gross composition and texture of sandstones

Abstract The mineralogical "maturity" of the heavy mineral assemblages of sandstones is quantitatively defined by a proposed zircon-tourmaline-rutile (ZTR) index. The ZTR index is the percentage of the combined zircon, tourmaline, and rutile grains among the transparent, nonmicaceous, detrital heavy minerals. Because of their high mechanical and chemical stability, zircon, tourmaline, and rutile are concentrated with quartz plus chert and metaquartzite rock fragments as sandstones become progressively more quartzose. In most arkoses and graywackes, the average ZTR index is low but varies widely among samples; apparently local lithologic source area variations largely control these relatively unmodified heavy mineral assemblages. In transitional feldspathic and micaceous quartzose sandstones derived by selective sorting and abrasion from feldspathic (arkose) and micaceous (graywacke) detritus, a parallel increase occurs in both the average ZTR index and the percentage of quartz plus chert and metaquartzite rock fragments among the grains, omitting clayey matrix and cements. The ZTR index is over 90% in most ortho-quartzite sandstones. As the ZTR index increases, a concentration occurs of the varieties based on color, inclusions, and form of tourmaline, zircon, and rutile, together with a decrease in the number of species of transparent heavy minerals. Tectonism not only largely controls the gross mineralogy and texture of sandstones but also determines the composition of the heavy mineral assemblages. Intrastratal solution of heavy minerals is not generally volumetrically important.