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Schweiz, mineral. petrogr. Mitt. 63, 95-114,1983
Cretaceous metamorphism of metacarbonates in the
Austroalpine Schneeberg complex, Tirol*
by
Georg Hoinkes**
Abstract
In siliceous dolomites of the Schneeberg complex talc and tremolite formed during progressive
Early Alpine metamorphism. Substitution of FeO for MgO, although shifting the equilibrium tem-
peratures only slightly, reduces the stability field of talc and calcite in the T-X
c0
-plane significant-
ly due to acute intersection of equilibrium curves. Equilibrium temperatures of ~560°C are de-
duced for the assemblage Dol-Qua-Tal-Tre-Cal by means of Cal-Dol-thermometry. Significant
impurities in solids besides FeO are sometimes K
2
O, Na
2
O and A1
2
O
3
and lead to phlogopite and
chlorite before tremolite which exhibits a continuous compositional change to pargasitic amphi-
bole. The only occurrence of diopside results from a progressive reaction of pargasitic amphibole
and calcite to diopside and garnet. Breakdown of tremolite to talc is thought to be a retrograde al-
teration during cooling after peak of Early Alpine metamorphism and no hints for a pre-Alpine
crystallisation are seen from the metacarbonates in the Schneeberg complex although Hercynian
mineral phases may be obscured by the Early Alpine event.
Introduction
The Austroalpine Ötztal unit consists of polymetamorphic proterozoic to
early paleozoic basement rocks and a fragmentary metasedimentary cover of
paleozoic to mesozoic age. The only occurrence of paleozoic metasediments is
restricted to the Schneeberg complex which forms complicated narrow syn-
clines to the south of the Ötztal alps (Fig. 1). At least three distinct synclines can
be devided due to their differring lithology and the overall lithology of the
Schneeberg complex is differring from the Ötztal basement rocks
(Schmidegg,
1933).
Metasediments of the Schneeberg complex are characterized by metacarbon¬
ates, metamarls and metapelites in contrast to the quartzofeldspatic basement
where metacarbonates are missing.
*
Dedicated to Dr. Oskar Schmidegg on occasion ofhis 85'
1
' birthday.
**
Institut für Mineralogie und Petrographie der Universität, Universitätsstrasse 4,
A-6020 Innsbruck, Austria.
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