SANDRO MELI & URS KLÖTZLI
In the Eastern Southalpine basement, large metavolcanic bodies (i.e. "porphyroids") occur as thick
layers interbedded within phyllites in six different areas, which outcrop along two discontinuous belts
(Sassi and Zirpoli, 1989); their protoliths were represented by prevailing acidic effusive bodies like
ignimbrites and lava flows.
The "porphyroids" are mostly silica-rich (metarhyolites and metarhyodacites) with a marked
peraluminous character. Ca, Sr and Rb behaved as mobile elements due to a late magmatic alteration.
REE patterns show a weak LREE fractionation and a small negative Eu anomaly, while a NASC
normalization shows 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 (Meli, 1994).
The primary (magmatic) age of the volcanics has been deduced on a stratigraphical basis, through a
long distance correlation between the rock sequence in which the Comelico metavolcanics occur and
that described and dated by Flajs and Schönlaub (1976) at the Caradoc-Ashgill boundary in the
Northern Grauwackenzone. According both to microstructural and geochronological data (Sassi &
Zirpoli, 1989, and references therein; Cavazzini et al., 1991) the volcanics were overprinted by a
two-stage Variscan metamorphism.
Different geocrhonological methods have been applied to the metavolcanics in order to define the
chronology of their multistage evolution: Rb/Sr whole rock data from Levico and Val Sarentino
samples, 39Ar/40Ar plateau ages on coarse grained, post-kinematic phengites from the same areas,
and 207Pb*/206Pb* ages on single zircons from Comelico.
The Rb/Sr method has been applied to seven samples for each zone. These data do not yield true
isochrons, as an excess of scatter is observed. However, the Val Sarentino samples define an
isochron assuming a model in which the initial ratio varies according to a normal distribution. In this
way, we obtain an age of 345±9 Ma, with a (87Sr/86Sr)i=0.7125 and an initial scatter of ±0.0011.
The age obtained for the Levico samples agrees with the former within errors, but the larger scatter
prevents a rigorous interpretation.
The 39Ar/40Ar technique has been successfully applied on five phengite separates, which yielded
good plateau ages. For the Val Sarentino area, a very good agreement is observed: 324±1.3 Ma,
325.1±1.9 Ma, 324.7±1.8 Ma. The Levico samples give slightly older ages: 330.4±1.5 Ma and
326.1±1.8 Ma. All these plateaux can be interpreted as crystallisation ages, considering that the
thermal peak of the Variscan metamorphism in the study areas did not exceed the closure
temperature of muscovite.
The single zircon direct evaporation technique described by Kober (1987) has been applied to
crystals extracted from two Comelico specimens. Based upon colour, roundness and elongation,
different population of zircons have been distinguished and classified using typology diagrams (Pupin
1980). Clear, colourless, highly elongated zircons (elongation ratio of 1:8-1:15) have been
interpreted as "true magmatic", while pinkish, slightly rounded and weakly elongated (e.r. of 1:2-1:4)
crystals have been considered as inherited grains. The attempts to date the magmatic crystals were
unsuccesful, due to their very low lead content. Among the inherited zircons, one of them had
enough lead to allow good measurements and reliable 207Pb*/206Pb* ages. A mean value of 668 Ma,
which must be interpreted as a minimum age, testifies to a Cadomian contribution to the crustal
source of the volcanics.
The two metamorphic Variscan stages are well defined by the Rb/Sr and 39Ar/40Ar data, and the
inheritance of older crustal material is supported by the available zircon age. However, radiometric
age constraints on the timing of magma production and emplacement are needed to complete the
geochronological picture of the Southalpine metavolcanics. Future efforts will be focussed towards
single zircon conventional dating technique on the magmatic crystals.
REFERENCES
Cavazzini G.c., Del Moro A., Sassi F.P. & Zirpoli G. (1991): Convegno in memoria di T. Cocozza,
Siena, 21-22 marzo 1991 (Abstr. Vol.).
Flajs G. & Schonlaub H.P. (1976): Verh. Geol. B.A., 257-303, Wien.
Kober B. (1987). Contr. Min. Petr., 96, 63-71.
Meli S. (1994): Ph.D. Thesis, University of Padova, Italy, 298 pp.
Pupin J.P. (1980). Contr. Min. Petr., 73, 207-220.
Sassi F.P. & Zirpoli G. (1989): Rend. Soc. Geol. It., 12, 397-402.