The Continental Crust as a Source of Sand (Southern Alps Cross Section, Northern Italy)

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Geology. A B S T R A C TAll 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 (∼2.85 g/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.

From static to dynamic provenance analysis—Sedimentary petrology upgraded

“…Rich information on parent rocks is revealed directly by the detailed study of rock fragments, which provide us with a firm basis for provenance diagnosis. In particular, the rigorous definition and systematic determination of metamorphic rank for each counted metapelite, metapsammite and metabasite rock fragment allows us to estimate the average metamorphic grade of parent rocks (MI index of , and hence the crustal level reached by erosion in source areas (Garzanti et al, 2006(Garzanti et al, , 2010b. Although the recent tendency is to rely more and more on sophisticated instruments for the collection of numerical datasheets, the use of valuable data-acquisition machinery does not automatically lead to valuable science, and basic examination of thin sections under the microscope remains very necessary.…”

“…The Metamorphic Index MI increases steadily when and where erosion cuts deeper into the crustal multilayer (Garzanti et al, 2006).…”

“…Not only arc crust (Dickinson and Suczek, 1979) or continental crust (Garzanti et al, 2006), but even ophiolitic allochthons including oceanic crust and underlying mantle can be envisaged as a progressively eroding multilayer source of sediment (Ophiolite provenance; Garzanti et al, 2002b).…”

“…The concentration (and not just the spectrum) of heavy minerals in a sedimentary deposit depends on the composition of parent rocks, and increases by more than an order of magnitude during progressive unroofing of denser rocks found at deeper-seated crustal levels (Garzanti et al, 2006). Equally drastic modifications of their concentration (as well as of their spectrum) may occur by selective entrainment of low-density grains in the depositional environment (Garzanti et al, 2010a), or by selective leaching of unstable species during diagenesis (Gazzi, 1965;Andò et al, 2012).…”

Bronze Age paleohydrography of the southern Venetian Plain

“…The Po sands are distinguishable due to the larger amount of quartz grains and specific heavy mineral content, among which glaucophane is regarded as diagnostic (Gazzi et al, 1973). In contrast, Adige sands are characterized by large amounts of dolostone and acidic volcanic rock fragments (Gazzi et al, 1973;Garzanti, Andò, & Vezzoli, 2006). Quantitative compositional point counting of the sand samples was conducted following the standard Gazzi-Dickinson technique (Gazzi, 1966;Dickinson, 1970;Zuffa, 1980), which reduces the influence of the grain size on composition.…”