Unraveling Magmatic and Orogenic Provenance in Modern Sand: The Back-Arc Side of the Apennine Thrust Belt, Italy

Abstract

Modern river and beach sands carried to and deposited along the coasts of the Tyrrhenian Sea are derived from multiple mag- matic-arc and orogenic sources, including ophiolitic sequences and tur- bidites issued from the Ligurian Ocean, carbonate-platform to pelagic sedimentary and metasedimentary successions originally deposited onto the Adria continental margin, and overlying foredeep clastic wedges. ''Undissected arc'' feldspatholithic signatures characterize limited areas (Capraia Island, Tarquinia and Napoli gulfs). Contrasting fea- tures, with respect to Circum-Pacific suites (low to very low P/F ratio), include sanidine as the most abundant detrital feldspar and common leucite and leucite-bearing lithic fragments, reflecting the potassic to ultrapotassic character of Neogene-Quaternary magmatism. Green au- gite, associated with oxyhornblende and either abundant hypersthene (Tuscan magmatic province) or minor olivine and spinel (Roman mag- matic province) is the dominant dense mineral. ''Dissected arc'' ar- kosic signatures reflect unroofing of Miocene monzogranitic stocks in the Tuscan archipelago. Sands from ophiolitic sequences and remnant-ocean sediments (''subduction-complex provenance'') are characterized by common cel- lular serpentinite with few chert grains, basaltic to diabase and gab- broic grains, and by abundant shale to slate lithic grains, respectively. Dense mineral suites are dominated by either diallage or chrome spinel. Sedimentaclastic sands ranging in composition from lithic to quartz- ofelspathic are widespread (''thrust-belt provenance''). Richest in lith- ic grains is detritus from Mesozoic platform (dominant carbonates with abundant dolostone) to pelagic (dominant limestone with abundant chert) sedimentary successions. Metamorphiclastic sands with poly- crystalline quartz, phyllite, and quartz-mica or marble lithic grains from denuded core complexes are found locally (Apuane Alps). Rela- tively quartzose sands with abundant feldspars and garnet-rich dense- mineral assemblages are recycled from foredeep turbidites derived, di- rectly or indirectly, chiefly from the Alps. This signature is foreign to the Apennine accretionary wedge, where continental basement is not structurally involved. We calculate that one-third of the sand grains deposited along Tyrrhenian shores today are ultimately derived, through long-distance multistage transport and repeated recycling, from the Alps.