METAGRANITOIDS AND ASSOCIATED METASEDIMENTS AS INDICATORS FOR THE PRE-ALPINE MAGMATIC AND METAMORPHIC EVOLUTION OF THE WESTERN AUSTROALPINEOTZTAL BASEMENT (KAUNERTAL, TIROL)

Metagranitoids associated with metasediments are common in the Otztal crystalline basement which is part of the Austroalpine tectonic unit o f the Eastern Alps. The two main orthogneiss types. muscovite-and biot ite-bearing metagranites (type 1) and mainly biotite-bearing metagrano diorites and metatonalites (type 2), were investigated together with t he associated metasediments in the Kaunertal-Langtaufers area using ge ochemical and geochronological methods. A Sm-Nd errorchron, derived us ing data for ten type 1 metagranites, result in an '' age '' of 494 +/ - 73 Ma. An apatite whole-rock two-point isochron for one of these sam ples yields an age of 455 +/- 8 Ma. Rubidium-Sr whole-rock ages are hi ghly variable, and range from 470 Ma to 309 Ma. The broad scatter is p robably caused by partial resetting of this isotope system during Cale donian and/or Variscan metamorphic events. Rubidium-Sr white mica ages also show a very large range. In few cases, near-Caledonian ages in m uscovite cores have been preserved. whereas the majority of the micas has been reset, or recrystallized, during the Variscan tectonometamorp hic evolution. Three Sm-Nd garnet whole-rock ages from micaschists ran ge from 343 +/- 1 to 331 +/- 1 Ma and indicate a medium to high grade metamorphism of the metasedimentary host rocks of the orthogneisses at that time. The mean epsilon(Nd)(490) values of the micaschists at the time of supposed magmatic crystallization of the metagranitoids (490 Ma) are -9.1 +/- 1.1 and are significantly lower than those of the ort hogneisses. For the two orthogneiss types 1 and 2 the mean epsilon(Nd) values are -4.8 +/- 0.5 and -7.6 +/- 0.5, respectively. The negative epsilon(Nd) values are compatible with a crustal source of the metagra nitoids, but at least some mantle contribution is indicated for the ty pe 1 orthogneisses by the shift to less negative values compared to th e metasediments. Different degrees of fractionation for the two orthog neiss types are evident from major, minor and trace element correlatio n with CaO and from chondrite normalized REE patterns. Metagranites (t ype 1) are highly differentiated due to fractionation of plagioclase, K-feldspar and an accessory phase compatible for REE, whereas at least K-feldspar fractionation is absent in metagranodiorites and tonalites (type 2). The magmatic protoliths of the orthogneisses from the Austr oalpine continental crust of the Otztal basement were formed during an episode of intense crustal melting, possibly in the early Ordovician which is consistent with data on the metamorphic evolution in other Va riscan and Alpine terranes of central and western Europe.