EARLY CAMBRIAN OCEANIC PLAGIOGRANITE IN THE SILVRETTA NAPPE, EASTERN ALPS - GEOCHEMICAL, ZIRCON U-PB AND RB-SR DATA FROM GARNET-HORNBLENDE-PLAGIOCLASE GNEISSES

Garnet-hornblende-plagioclase gneisses rich in incompatible elements o ccur in the crystalline basement of the Austro-Alpine Silvretta nappe and are associated with clinopyroxene norites and harzburgite cumulate s. It is proposed here that the gneisses were formerly oceanic plagiog ranites. An E(Nd(530)) value of +5.6 for the gneisses as well as initi al Sr-87/Sr-86 values of 0.7036-0.7037 for the gabbroic rocks and 0.70 26-0.7027 for the ultramafic rocks suggest a mantle source for this ro ck association. The geochemical characteristics of the garnet-hornblen de-plagioclase gneisses indicate that their precursors were derived by fractional crystallization from a basaltic parent magma, by the same process which produced the adjacent clinopyroxene norites and ultramaf ic cumulates as well. The combined U-Pb upper intercept ages of zircon s from two gneiss samples yield an igneous crystallization age of 532/-30 Ma, similar to previously dated (mostly calc-alkaline) orthogneis ses in the same area. High-quality transparent zircons showed the leas t degree of discordance, but contain extremely low U and Pb levels. Th e rock suite, including this plagiogranite, was emplaced within oceani c crust which formed in the latest Precambrian-early Palaeozoic off th e northern continental margin of Gondwana.