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 magmatic activity and characterized by arid climate and negligible anthropic modifications, provides an ideal natural laboratory for studies aimed at defining actualistic references for both volcanic and nonvolcanic rifted-margin provenances. Rifted-margin sands are derived in various proportions from volcanic to plutonic rocks emplaced before, during, or after the climax of tectonic extension (volcanic rifted-margin provenance) and from prerift sedimentary successions and underlying crystalline basements progressively unroofed during uplift of rift blocks (rift-shoulder provenance). Volcaniclastic rifted-margin sands are feldspatholithic, as are those shed by Pacific-type magmatic arcs, but are characterized by bimodal (basalt/rhyolite) lithics, abundant granophyre grains, and low plagioclase/total feldspar (P/F) ratios due to supply from synrift hypersolvus alkali granites, representing the upper levels of rift-generated juvenile crust. Augite dominates among dense minerals; detritus from postrift alkali-basalt fields includes olivine and, locally, enstatite and spinel. Sedimentary detritus from undissected rift shoulders consists of recycled quartz and carbonate sedimentary lithics; dense mineral assemblages include largely rounded to subrounded, recycled durable grains, zircon, and rutile being concentrated locally due to their higher density. Arkosic sands from basement rocks exposed on dissected rift shoulders display remarkably consistent compositions, with excess quartz with respect to "ideal arkose"; hornblende-rich assemblages from amphibolite-facies gneiss terranes contrast with epidote-dominated assemblages from greenschist-facies arc terranes. Diagnostic signatures and compositional trends recorded by modern Yemen sands may help in interpreting provenance of ancient rift-related sandstone suites.