S. MELI
In the Southalpine basement of the Eastern Alps, the so called "porphyroids" occur as thick levels interbedded within phyllites; they have been interpreted by several authors (Sassi & Zirpoli, 1989, and references therein) as metavolcanics and associated metavolcanoclastic rocks, whose protoliths were represented by prevailing acidic effusive bodies like ignimbrites and lava flows. The primary (magmatic) age assigned to the volcanics on a stratigraphical basis is Upper Ordovician, possibly at the Caradoc-Ashgill boundary, whereas the age of metamorphism, which has developed in two stages according both to structural and geochronological data, is Variscan (350 Ma and 320 Ma approximately).
The Southalpine basement outcrops along three belts oriented SSW-NNE, and the porphyroid level is widespread throughout the whole basement. All the outcrops have been investigated on the field, and among them six representative areas have been chosen for sampling: Val Sarentino, Levico (Val Sugana), Cima d'Arzon, Val di Funes, Agordo-Cereda and Comelico (M. Elmo and M. Cavallino).
The aim of this work is a complete petrographic and geochemical characterization of the porphyroids, and the study of all possible chemical changes occurred after the emplacement of the volcanics, in order to unravel, beyond the present metamorphic situation, the origin and evolution of the magmas which originated the volcanics. Moreover, a geochoronological study has been carried out, in order to yield more detailed data on the age of metamorphism.
PETROGRAPHY AND MINERAL CHEMISTRY
Looking at the present mineralogical association, the first problem is to distinguish between a "metamorphic paragenesis" and a set of pre-metamorphic phases; microstructural and chemical investigations, allowed a nearly complete discrimination. The main textural feature of the porphyroids is a marked bimodality in the grain size of minerals: the larger grains can be interpreted as relics of magmatic phenocrysts, while the surrounding grains, up to 50 times smaller, are derived from a volcanic groundmass through various degrees of recrystallization. The pre-magmatic mineralogical association consists of quartz, K-feldspar, plagioclase, ilmenite, apatite and zircon; the last two phases can be recognized as magmatic with a good level of confidence only in the Comelico area. Sometimes, in the same area, garnet relics are present, and their chemical composition demonstrates a probable xenocristalline origin respect to the host magma. Biotite relics are seldom recognizable, although the occurrence of biotite in the original magmatic paragenesis can be deduced by the development of chlorite + Fe-Ti oxides intergrowths.
The metamorphic paragenesis is consistent with the Variscan zoneography described by Mazzoli & Sassi R. (1988): moving towards NW the metamorphic grade increases, as shown also by the index minerals in the porphyroids. In the Comelico area we find chlorite, while in the Agordo, Cima d'Arzon, Val Sugana and Val di Funes areas also biotite is present; finally, in the westernmost area, Val Sarentino, garnet occurrence marks the onset of the almandine zone. Other metamorphic minerals, common to all porphyroids, are given by quartz, albite, K-feldspar and ilmenite.
The metamorphic hydrous minerals are alogen-poor, probably reflecting low Cl and F contents in the metamorphic fluids; muscovite has a significant phengitic substitution, depending on the bulk rock chemistry.
Garnets from Val Sarentino and Comelico have different compositions: almandine- and spessarrtine-rich in the former zone, almandine- and pyrope-rich in the latter; this chemical feature reflects a xenocrystalline origin of the Comelico garnets.
BULK ROCK CHEMISTRY
The main problem in a geochemical study of metavolcanic rocks is the chance to verify all the possible chemical variations subsequent to the solidification of magmas. For the Southalpine porphyroids, two distinct events could have caused significant chemical variations: a late magmatic deuteresis and the Variscan metamorphism. A suitable rock sampling and several variation diagrams have found out a high mobility for alkali group and, to a lesser extent, for some alkali-earths (Ca, Sr, Ba). In addition, silica has been occasionally mobilized during metamorphism, producing slight silicization. All other elements can be considered nearly immobile (i.e. mobile only at a microscopic scale).
The major elements chemistry of the porphyroids indicates a prevaling acidic character of the volcanics (more than 90% of the samples have a SiO2 content higher than 65%). They are all markedly peraluminous, and belong to the calc-alkaline series (Irvine & Baragar, 1971; Miyashiro, 1974). Owing to the above mentioned chemical alterations, the protolihs have been classified employing the Zr/TiO2 vs Nb/Y diagram (Winchester & Floyd, 1977), based on ratios between immobile elements. On the whole, the porphyroids data points are equally distributed between the rhyolite and the rhyodacite/dacite fields. The variation diagrams involving major elements show good correlations between TiO2, Al2O3, Fe2O3(tot), MgO and P2O5 vs SiO2, while CaO, Na2O and K2O are widely scattered. A particular array shown by the TiO2 - SiO2 and the P2O5 - SiO2 correlations indicates, according to White & Chappell (1977), the possibile formation of anatectic "minimum melts" and "non minimum melts", mixed with variable amounts of restite.
The trace elements geochemistry of the porphyroids is consistent with an anatectic hypothesis. The high Ba and Rb content, the low contents of Zr, Nb, Y, Hf, Ta and Th are in agreement with a crustal anatectic environment: in such a magmatic source micas would yield the former elements to the melt, while some restitic accessory phases, like zircon, monazite, ilmenite and sphene, would retain the latter, causing their low concentrations in the melt.
The REE patterns of selected samples, normalized to the chondrites, show moderate fractionation between LREE and HREE, with a small negative europium anomaly. As we assume an anatectic origin for the volcanics, we have also normalized the REE to an hypothetic crustal source, represented by the NASC (Gromet et al., 1984). The LREE pattern obtained using this normalization, is approximately horizontal for each of the six studied zones, and a small Eu negative anomaly still remains. The HREE patterns, on the contrary, are markedly fractionated for five areas, while the sixth (Val Sarentino) has a complete horizontal pattern. These features can be interpreted in terms of crustal anatexis: LREE behaviour has been controlled mainly by accessory phases which have buffered their partitioning into the melt; Eu anomaly can be caused by restitic K-feldspar, and the marked HREE fractionation probably reflects the crystallization of restitic garnet in the source by incongruent melting; however, the occurrence of garnet in the crustal source is directly observed only in the Comelico zone. In the Val Sarentino area cordierite, which is not able to fractionate HREE, could have replaced garnet in the melting reaction, probably due to lower pressure conditions.
Considering the geochemical informations as a whole, and some of the most suitable petrographic data (e.g. the occurrence of garnet xenocrysts in the Comelico area), we can suggest two possible dehydration reactions, the first for the Val Sarentino zone and the second for the other areas, which are capable of producing significant amounts of melt having the geochemical features of the porphyroids:
biotite + sillimanite + quartz + plagioclase {SIMBOLO 222 \f "Symbol"} cordierite + K-feldspar + melt
biotite + sillimanite + quartz + plagioclase {SIMBOLO 222 \f "Symbol"} garnet + K-feldspar + melt
In order to distinguish among different tectonic scenarios the most suitable one for the geodynamic setting of the protoliths of the porphyroids, some discriminant diagrams for acidic rocks have been used (e.g. Pearce et al., 1984; Whalen et al., 1987). These diagrams indicate that the most likely tettonic setting for this volcanism is orogenic; some further considerations (e.g. the frequency distribution of the silica content, > 65% in 90% of the samples), led us to believe that the hypothesis of a late orogenic setting is reasonable.
GEOCHRONOLOGY
Two different methods have been applied to the porphyroid samples: Rb/Sr and 39Ar/40Ar. Rb/Sr technique has been performed on bulk rocks, while 39Ar/40Ar has been applied on muscovite separates. Val Sarentino and Val Sugana samples have been chosen for measurements; seven bulk rock samples and three muscovite separates for each area have been analized.
Two Rb/Sr sets of data have been performed. Whole rock data obtained for the Val Sugana samples are highly scattered, and do not define any acceptable alignment. Similarly, the Val Sarentino samples did not yield a true isochron. However, using a regression model which implies a slight variation in the initial isotopic ratio of the rocks (York, 1969), i.e. assuming that the metamorphic resetting was not perfect, a metamorphic age of 345 ± 9 Ma is obtained, with a (87Sr/86Sr)i = 0.7125 ± 0.0011. This age probably indicates the first stage of Variscan metamorphism.
The 39Ar/40Ar method has been performed on post-kinematic muscovites using the stepwise heating technique (Dalrymple & Lanphere, 1971). Three concordant plateau ages have been obtained from the Val Sarentino area: 324.7 ± 1.8 Ma, 325.1 ± 1.9 Ma, 324.0 ± 2.3 Ma; two satisfactorily concordant ages, 330.4 ± 1.5 Ma and 326.1 ± 1.8 Ma, have been obtained from the Val Sugana samples. These argon age determinations can be interpreted as crystallization ages of muscovite, and therefore referred to a late stage of the Variscan metamorphic event.
According to these radiometric data, the age of the Variscan metamorphic event in the Southalpine basement of the Eastern Alps is well established, and its two stage development sharply defined.
CONCLUSIONS
The results obtained in this work can be summarized as follows:
1) Southalpine Ordovician porphyroids in the Eastern Alps represent acidic metavolcanics and metavolcanoclastics outcropping throughout the whole Southalpine basement; they formed the so called "Pre-Hercynian Rhyolitic Plateau".
2) The metamorphic mineralogical assemblage of the porphyroids is consistent with the metamorphic zoneography of the Southalpine basement, as described by previous workers.
3) Late magmatic deuteresis significantly mobilized alkalies and, to a lesser extent, alkali-earths; in some samples, a slight silicization also occurred during metamorphism.
4) More than 90% of the porphyroids have silica content higher than 65%, and all of them are strongly peraluminous, belonging to the calc-alkaline series. Their protoliths have been classified as rhyolites and rhyodacites/dacites, using immobile elements ratios.
5) The chemical features of the porphyroids, together with some petrographic observations, indicate a crustal origin of the magmas, through partial melting of a metapelitic source, involving quartz, plagioclase sillimanite and biotite, and leaving K-feldspar and garnet/cordierite in the residue.
6) The tectonic setting suitable for this old volcanism is likely to have been late orogenic, on the basis of the frequency distribution of silica content and according to some discriminant diagrams taken from literature.
7) The radiometric ages obtained indicate a Variscan metamorphic event, which took place in two stages: the older one is well defined by the Rb/Sr whole rock isochron for the Val Sarentino rocks (345 ± 9 Ma); the second is sharply constrained at 325-330 Ma by five concordant 39Ar/40Ar plateau ages.