Geochemische Untersuchungen zur Petrographie des Cima d'Asta-Plutons

The chemical composition of the various rocks of the Cima d'Asta Intrusion were investigated. On the basis of 29 analyses it is concluded that the granodioritic rocks of the central area of the intrusion belong to the trondhjemitic, leuco-granodioritic and granodioritic magma types, whereas the quartzdioritic rocks of the southwestern border belong to melaquartzdioritic to quartzdioritic magma types. Xenoliths occuring in the top region of the Cima d'Asta massive are chemically and mineralogically identical with the quartzdioritic rocks of the southwestern border.17 biotites separated from various rocks of this area were analysed.On the basis of their octahedral occupancies the biotites of the quartzdioritic rocks and those of the quartzdioritic xenoliths are both akin to the phlogopite-annite series. On the other hand the biotites of the granodioritic rocks are more closely related to the eastonite-siderophyllite series. A notable feature of the phlogopitic biotites is their high content in calcium.The beryllium contents of 82 rocks and of the biotites separated from these rocks substantiate the chemical and mineralogical identity of the quartzdioritic xenoliths with the quartzdioritic border rocks.Because of this identity an ultrametamorphic and possibly hybrid origin of the Cima d'Asta rocks is considered.

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Die Verfasser danken Herrn Prof. Dr. W. v. Engelhardt und Herrn Prof. Dr. F. Karl für wertvolle Hinweise und Diskussionen. Die chemischen Untersuchungen wurden am Mineralogisch-Petrographischen Institut der Universität Tübingen durchgeführt.