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Geochemistry of coesite-bearing "pyrope quartzite" and related rocks from the Dora-Maira Massif, Western Alps

Hans-Peter Schertl^* and Werner Schreyer

Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany

^* Corresponding author, e-mail: hans-peter.schertl{at}rub.de

A complete set of geochemical data for "pyrope quartzite", various phengite-schist and jadeite-bearing rock intercalations, as well as for enclosing orthogneiss from different localities within the southern Dora-Maira Massif is presented, in order to establish the nature of the different protoliths and their primary relationships. For comparison, Mg-rich rocks from the Eastern Alps occurring along shear zones within granitic gneiss, as well as metaevaporitic rocks from Sar e Sang, Afghanistan, were also studied. Typically, the "pyrope quartzite" and its retrograde derivative are enriched in Mg and strongly depleted in Na, Ca, Fe, Cu, P, Rb, Ba, and Sr with respect to the enclosing orthogneiss of granitic bulk composition. This orthogneiss is usually peraluminous, has normative corundum, and may be best related to S-type granite protoliths. Trace-element contents of the different phengite schists included in the "pyrope quartzite" confirm their close overall kinship to the enclosing orthogneiss. Jadeite-bearing rocks, which occur as conformable layers and boudins within "pyrope quartzite", in general differ by having lower contents in K, Mg, Rb, and higher contents in Na, Fe, Ca, Mn, P, and Zn. The present study indicates that the "pyrope quartzite" has a metasomatic origin, whereas an evaporitic nature of the protolith can be ruled out. Discrimination plots demonstrate that the "pyrope quartzite" can clearly be distinguished chemically from metasedimentary whiteschist derived from former evaporite environments. The field relations of trails of "pyrope quartzite", now forming lensoid inclusions within an orthogneiss (former granite), would also make a metasedimentary origin rather unlikely. On the other hand, consistent genetic relationships between "pyrope quartzite" and its enclosing orthogneiss were obtained that virtually match those of Mg-metasomatic leucophyllites from the Eastern Alps occurring along shear zones within granitic gneisses. Similar Mg-rich rocks within the Tauern Window such as within the Monte Rosa and Gran Paradiso Massifs of the Western Alps indicate that local processes of Mg metasomatism occurred throughout the entire range of the Alps.

Key-words: geochemistry, Dora-Maira Massif, "pyrope quartzite", UHP, metasomatism, whiteschist.

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