The Mönchalpgneiss - Geochemical characteristics and Sm-Nd data of a polymetamorphic S-type granitoid (Silvretta nappe/Switzerland)
Poller, Ulrike; Nägler, Thomas F.; Liebetrau, Volker; Galetti, Giulio
European Journal of Mineralogy Volume 9 Number 2 (1997), p. 411 - 422
published: Jun 26, 1997
manuscript accepted: Oct 23, 1996
manuscript received: Apr 20, 1995
ArtNo. ESP147050902017, Price: 29.00 €
Abstract This paper presents trace element and Sm-Nd isotope data on the Mönchalpgneiss in order to compare the geochemistry of the two polymetamorphic igneous suites that comprise over 30% of the Austroalpine Silvretta nappe. The first are the so-called "Younger Orthogneisses" of the "Fliielagranitic Association" and the second are the "Older Orthogneisses" including the Mönchalpgneisswhich are associated with metagabbros, metadiorites, metatonalites and metagranitoids. U-Pb zircon results from the Mönchalpgneiss are indicative of anatectic processes in late Cambrian to Ordovician times. A volcanic-arc (VA) tectonic environment during intrusion explains the direct association of gabbroic and metagranitoid rocks in the Engadine area. This model is in line with the distribution of major, trace and rare earth elements in these anatectic rocks. However, the significance of the geochemical results remains ambiguous, since the average continental crust and paragneisses of the Silvretta also show VA-type signatures in the respective diagrams. Nd model ages on four Mönchalpgneiss whole-rock samples from the type locality are closely grouped around 1.70 Ga, which is a commonly obtained value for European continental crust. This age is interpreted to be the result of a homogeneous mixture of different crustal components.