Metamorphic grade of source rocks revealed by chemical fingerprints of detrital amphibole and garnet
Published:January 01, 2014
Sergio Andò, Andrew Morton, Eduardo Garzanti, 2014. "Metamorphic grade of source rocks revealed by chemical fingerprints of detrital amphibole and garnet", Sediment Provenance Studies in Hydrocarbon Exploration and Production, R. A. Scott, H. R. Smyth, A. C. Morton, N. Richardson
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Amphibole and garnet are among the most widespread heavy minerals in orogenic sediments. Their chemical composition and optical properties vary markedly and systematically with temperature and pressure conditions during growth, and thus provide important information on the metamorphic evolution of source areas that is crucial in palaeotectonic and palaeogeodynamic reconstructions. This study investigates the chemical composition of detrital amphiboles and garnets derived from parent rocks of progressively increasing metamorphic grade through a well-studied composite section across the Central and Southern Alps, including the granulite-facies core of the Late Palaeozoic orogen exposed in the Ivrea–Verbano Zone and the amphibolite-facies core of the Cenozoic orogen exposed in the Lepontine Dome. We specifically focus on metamorphic grade because it represents the best proxy for tectono-stratigraphic crustal level, and hence degree of unroofing of source areas. In river sands collected between metamorphic isograds corresponding to crystallization temperatures ranging from c. 500 °C to c. 850 °C, TiO2 gradually increases in detrital amphibole while its colour progressively changes from blue-green in the lower amphibolite-facies where actinolite, hornblende and tschermakite are most abundant, to brown in the granulite facies where pargasite is dominant. Detrital garnets display moderate gradual changes across the amphibolite-facies Lepontine Dome, where low-Mg ‘type B’ garnets predominate. Almandine-spessartine is spatially associated with abundance of pegmatites while entering the zone of anatexis (Southern Steep Belt), where grossular or grossular-andradite-spessartine are occasionally found. A sharp change occurs while reaching granulite-facies in the Ivrea–Verbano Zone, where high-Mn garnets disappear and ‘type A’ almandine-pyrope (from ‘stronalite’ metasediments) and ‘type C’ almandine-pyrope-grossular (from metagabbros of the Mafic Complex) predominate. Also redefined in this article are a series of numerical indices based on amphibole colour and relative abundances of diverse key minerals (chloritoid, staurolite, andalusite, kyanite, fibrolitic and prismatic sillimanite), useful to accurately assess the average metamorphic grade of meta-igneous and metasedimentary source rocks.
Chemical composition of detrital amphiboles and garnets and full information on the location and mineralogical composition of studied samples is available at http://www.geolsoc.org.uk/SUP18618.
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Sediment Provenance Studies in Hydrocarbon Exploration and Production
Sediment provenance studies concern the origin, composition, transportation and deposition of detritus and therefore are an important part of understanding the links between basinal sedimentation, and hinterland tectonics and unroofing. Such studies can add value at many stages of hydrocarbon exploitation, from identifying regional-scale crustal affinities and sediment dispersal patterns during the earliest stages of exploration, to detailed correlation in producing reservoirs and understanding the impact of mineralogy on reservoir diagenesis.
The volume showcases the wide variety of techniques available, using examples and applications from all aspects of sediment provenance research. The papers are organized into four sets around the following themes:
Overview: applications of provenance information in hydrocarbon reservoir sandstones
Provenance, diagenesis and reservoir quality
Provenance studies linking sediment to source
Looking forward: development of techniques and data handling
This book is dedicated to the memory of Maria Mange and Robert A. Scott.