New chemical data on the upper Ordovician acidic plutonism in the Austrides of the Eastern Alps.

The Upper Ordovician acidic magmatism is the most important magmatic activity in the Eastern Alps, both as concerns amount of melts and size of the area. It took place both under plutonic and volcanic conditions. New chemical data on this plutonism are presented here. They concern rock samples from Oetztal, Pitztal and Casies Valley (Gsiestal). These new chemical data cover the SiO2 range 67-77 wt%. The data points cluster along a more or less well-defined pattern in several variation diagrams concerning major and trace elements, including REE. These new data are compared with previously published data concerning: (i) similar Austridic orthogneisses; (ii) Austridic sheet-like gneisses, augengneisses and "porphyroids"; (iii) Southalpine "porphyroids". The variation diagrams based on all these data support the following considerations consistently with the conclusion reached by some previous authors: - Upper Ordovician plutonism and volcanism display identica1 chemical features, and can be related to a unique cycle of magma generation; -numerous patches of melts formed in different places in a relatively short time range under identically constrained conditions at expenses of similar parent rocks, so that all these patches of melts could have taken similar geochemical features; - these melts display a calc-alkaline affinity, covering almost continuously the 62-77 SiO2 range without any significant gap; - the lack of cogenetic basic rocks and the insignificant amount of intermediate rocks represent a feature which is meaningful for the genetic interpretation; -crustal anatexis seems to be the most appropriate process for explaining all available data.