The continental crust as a source of sand (Southern Alps cross section, northern Italy)

All tectonostratigraphic levels of the continental crust, from carbonate platforms in the Dolomites to granulites in the Ivrea Zone, are exposed along strike in the world-famous Southern Alps cross section. In this extraordinary natural laboratory, we studied the composition of modern stream sands shed by each distinct type of source rock in order to improve on existing models of sediment provenance and specifically to reconstruct the step-by-step changes of detrital signatures ideally produced during unroofing of a continental block. Detritus from progressively deeper crustal levels of the Southern Alps changes systematically from lithic sedimentaclastic (unmetamorphosed cover sequences) to lithoquartzose, quartzolithic, and quartzofeldspathic metamorphiclastic (greenschist facies to amphibolite facies basement units) and finally to feldspathoquartzose and feldspathic composition (granulite facies basement units). Six progressively richer types of heavy mineral assemblages (ZTR-brookite, garnet-chloritoid, garnet-staurolite, blue/green hornblende-kyanite, green/brown hornblende-fibrolitic sillimanite, and garnet-brown hornblende-prismatic sillimanite) faithfully reflect the increasing grade of metamorphic source rocks. Hypersthene-brown hornblende and olivine-enstatite assemblages characterize detritus from lower crustal gabbros and mantle peridotites. In alpine settings undergoing rapid erosion and minor chemical weathering, the mineralogy of detrital sediments faithfully mirrors the mineralogy of their parent rocks. Therefore, our database can be used to integrate direct measurements of physical and chemical properties of exposed bedrock and to reconstruct an ideally complete mineralogy and density profile from the base to the top of the South Alpine crust. This shows a progressive upward increase in quartz and muscovite at the expense of plagioclase and heavy minerals and an effective density stratification from lower crustal (3.10-3.00 g/cm(3)) to middle crustal (similar to 2.85g/cm(3)) and upper crustal levels (2.75-2.70 g/cm(3)). Sharp jumps in average rock density observed across major structural discontinuities (e.g., Cossato-Mergozzo-Brissago Line) mirror the fact that the South Alpine basement is the extensional stack of several slivers that followed distinct metamorphic and exhumation paths during the multistage tectonic evolution linking the Carboniferous Variscan Orogeny with the Jurassic opening of the Alpine Tethys.