Petrology of rifted-margin sand (Red Sea and Gulf of Aden, Yemen)

The Red Sea-Gulf of Aden rift system, displaying a complete record of magma tic activity and characterized by arid climate and negligible anthropic mod ifications, provides an ideal natural laboratory for studies aimed at defin ing actualistic references for both volcanic and nonvolcanic rifted-margin provenances. Rifted-margin sands are derived in various proportions from vo lcanic to plutonic rocks emplaced before, during, or after the climax of te ctonic extension (volcanic rifted-margin provenance) and from prerift sedim entary successions and underlying crystalline basements progressively unroo fed during uplift of rift blocks (rift-shoulder provenance). Volcaniclastic rifted-margin sands are feldspatholithic, as are those shed by Pacific-typ e magmatic arcs, but are characterized by bimodal (basalt/rhyolite) lithics , abundant granophyre grains, and low plagioclase/total feldspar (P/F) rati os due to supply from synrift hypersolvus alkali granites, representing the upper levels of rift-generated juvenile crust. Augite dominates among dens e minerals; detritus from postrift alkali-basalt fields includes olivine an d, locally, enstatite and spinel. Sedimentary detritus from undissected rif t shoulders consists of recycled quartz and carbonate sedimentary lithics; dense mineral assemblages include largely rounded to subrounded, recycled d urable grains, zircon, and rutile being concentrated locally due to their h igher density. Arkosic sands from basement rocks exposed on dissected rift shoulders display remarkably consistent compositions, with excess quartz wi th respect to "ideal arkose"; hornblende-rich assemblages from amphibolite- facies gneiss terranes contrast with epidote-dominated assemblages from gre enschist-facies arc terranes. Diagnostic signatures and compositional trend s recorded by modern Yemen sands may help in interpreting provenance of anc ient rift-related sandstone suites.