PROVENANCE FROM OPHIOLITES AND OCEANIC ALLOCHTONS: MODERN BEACH AND RIVER SANDS FROM LIGURIA AND THE NORTHERN APENNINES (ITALY)

Abstract

The Northern Apennine arc consists of allochtonous oceanic units (Ligurids) and synorogenic terrigenous wedges, and includes both alpine (Voltri Massif) and apenninic (Bracco Unit) ophiolitic complexes and isolated slabs of oceanic lithosphere (Casanova Complex). Detritus transported by streams in this vast region, comprised between Bologna and Alessandria, and sand of Liguria beaches from Savona to La Spezia can be distinguished - through analysis of framework composition and transparent heavy mineral suites - in four provinces, further subdivided in sub-provinces and variants. The Apennine Province and the Antola Province are carbonaticlastic and characterized by recycling of Ligurid flysches, whereas the Voltri Province and the Bracco Province are dominated by ophioliticlastic detritus. Along the boundaries of major ophiolitic complexes various sub-provinces with mixed provenances characerized by significant ophioliticlastic detritus are recognized (Bormida di Spigno and Polcevera river sands, and Celle-Varazze beach sands for the Voltri Massif; Vara, Lavagna and Ghiaiaro river sands for the Bracco Unit). Ophiolite-derived material dominates the heavy mineral fraction also in the sand transported by major streams draining the Emilia Apennines (Trebbia, Nure, Taro). Framework composition and heavy mineral suites clearly differentiate the serpentineschist- and amphibole-rich (glaucophane, actinolite) detritus derived from alpine ophiolites involved in thick-skinned deformation and high-pressure metamorphism (Voltri Massif), from the serpentinite- and pyroxene-rich (diallage) detritus derived from oceanic lithosphere involved in thin-skinned deformation within the Apennine belt (Bracco Unit; Casanova Complex). In the study of ancient perisutural-basin clastic wedges, these criteria can help reconstructing subduction style and paleogeodynamic evolution of associated suture belts.