Claudio Mazzoli, Sandro Meli & Raffaele Sassi
Dipartimento di Mineralogia e Petrologia, Corso Garibaldi 37, I-35137 Padova
In the basements of the Eastern Alps belonging to the Austroalpine and Southalpine units, several metamorphosed acidic magmatics can be recognised, which have been reported to be Ordovician in age. This pre-Variscan magmatics emplaced partly as volcanic and partly as plutonic rocks. Their metamorphic products are described as "porphyroids" and "orthogneisses" respectively in the literature. Porphyroids outcrop both in the Southalpine and Austroalpine units whereas orthogneisses are found exclusively in the Autroalpine.
The Austroalpine orthogneisses from Oetztal, Pitztal and Casies are considered. They mostly have a granodioritic composition, and show a calcalkaline character (Mazzoli & Sassi, 1992), with minor compositional peculiarities: two sub-groups can be recognized among the Oetztal gneisses on the basis of major, minor and trace elements, including REE; Casies gneisses are less acidic than most of the other orthogneisses. REE patterns indicate a weak and uniform fractionation of LREE and a clear negative Eu anomaly. Initial Sr and Nd isotopic ratios have been calculated on the basis of the age of 434 Ma taken from the literature. They are: (87Sr/86Sr)i = 0.710-0.720; Ndi=-2.86/-6.89. These values indicate an important crustal contribution to the genesis of the parent magma.
The Eastern Southalpine "porphyroids" are mostly acidic, with SiO2>65wt% in 95% of studied rocks, and with a peralouminous character (Meli, 1995). All these metavolcanics are classified as metarhyolites and metarhyodacites on the basis of immobile elements. Variation diagrams show that alkali and, in minor extent, Ca, Sr and Ba underwent mobilization due to a late magmatic or metamorphic alteration. REE patterns show a weak, LREE fractionation and a small negative Eu anomaly. NASC normalization displays flat LREE and fractionated HREE trends. Other peculiar geochemical features of these rocks are the low contents in Nb, Zr, Hf and Ta, their negative correlation vs. silica and the high Ba content.
The Variscan age of the metamorphic event that affected the "porphyroids" is given by a Rb/Sr whole-rock isochron from Val Sarentino samples (345+9 Ma) and by five 39Ar/40Ar age data on muscovite (325-330 Ma).
In conclusion, the Ordovician acidic meta-igneous rocks from the Eastern Alps record relevant crustal contributions to the origin of the related melts. In particular, a dehydration melting process from a fertile metapelitic source may be assumed for the protoliths of the metavolcanics. Furthermore, according to suitable discriminant diagrams, both "porphyroids" and orthogneisses are consistent with a late- to post-orogenic tectonic setting. All data are consistent with the hypothesis (Sassi & Zirpoli, 1979) that the Ordovician volcanisms and plutonisms can be considered as belonging to the same event.