The Moenchalpgneiss; geochemical characteristics and Sm-Nd data of a polymetamorphic S-type granitoid (Silvretta nappe/Switzerland)

Trace element and Sm-Nd isotope data on the Moenchalpgneiss are presented to provide comparisons with the geochemistry of the two polymetamorphic igneous suites that comprise >><$I> 30% of the Austro-alpine Silvretta nappe: the so-called 'Younger orthogneisses' of the 'Fluelagranitic association' and the 'Older orthogneisses' which include the Moenchalpgneiss and which are associated with metagabbros, metadiorites, metatonalites and metagranitic rocks. U-Pb zircon results from the Moenchalpgneiss are indicative of anatectic processes in the late Cambrian to Ordovician times. A volcanic-arc tectonic environment during intrusion is envisaged to explain the direct association of gabbroic and metagranitic rocks in the Engadine area. Nd model ages for four Moenchalpgneiss samples from the type locality are closely grouped around 1700 m.y., a commonly obtained value for European continental crust; this age is interpreted as the result of a homogeneous mixture of different crustal components.