Age of emplacement and metamorphic overprint of the southalpine porphyroids: a multimethod approach

S. Meli

In the Eastern Southalpine basement, large metavolcanic bodies (i.e., "porphyroids") occur as thick layers interbedded within phyllites (Sassi & Zirpoli, 1989); their protoliths were represented by prevailing acidic effusive bodies like ignimbrites and lava flows. The volcanics underwent a two-stage Variscan metamorphism, as testified both by microstructural and geochronological data (Cavazzini et al., 1991); thermal conditions did not exceed the greenschist facies temperatures in the outcropping rocks (Mazzoli & Sassi, 1988).

The porphyroids are silica-rich (metarhyolites and metarhyodacites), with a marked peraluminous character. Na, K, Ca, Rb, Sr and Ba have been mobilized due to a late to post-magmatic alteration and to metamorphic overprint. REE patterns and other geochemical features point to an anatectic origin for the volcanics, probably derived from a pelitic source (Meli, 1998). Until now, the primary age of the volcanics was inferred on a stratigraphic basis, through a long distance correlation to a sequence in the Northern Grauwackenzone dated by Flajs & Schönlaub (1976) at the Caradoc-Ashgill boundary.

Different dating methods have been applied to the metavolcanics to define their multistage evolution. Rb/Sr whole-rock ages and ³⁹Ar/⁴⁰Ar plateau ages on coarse grained, post-kinematic phengites from Levico and Val Sarentino samples were performed in order to better define the Variscan event; single grain ²⁰⁷Pb*/²⁰⁶Pb* evaporation technique and multigrain U/Pb conventional method have been applied on zircons from Comelico, to trace back to the emplacement age of the volcanics.

The ³⁹Ar/⁴⁰Ar measurements yielded Carboniferous plateau ages. For Val Sarentino samples, a good age agreement is observed between three different phengite separates: 324±1.3 Ma, 325.1±1.9 Ma, 324.7±1.8 Ma. The Levico phengites give slightly older ages: 330.4±1.5 Ma and 326.1±1.8 Ma. All these plateaux can be interpreted as crystallisation ages, as the Variscan thermal peak in the study areas did not exceed the muscovite closure temperature.

The Rb/Sr method has been applied to seven whole-rock samples coming from Val Sarentino and Levico. The data do not define true isochrons, as an excess scatter is present. However, assuming a model in which the initial ratio of the samples had a normally distributed variation, the Val Sarentino porphyroids define an isochron with an age of 345±9 Ma, and (⁸⁷Sr/⁸⁶Sr)_i=0.7125±0.0011. On the contrary, Levico data display an excess scatter which cannot be justified by this model either.

The U/Pb and ²⁰⁷Pb*/²⁰⁶Pb* methods have been applied on zircon crystals belonging to Comelico porphyroids. Based upon morphology, turbidity and colour, three zircon groups have been distinguished in each sample; considering the anatectic nature of the volcanics, they probably reflect different zircon contributions to the overall sample population. Clear, colourless, elongate crystals are thought to be single stage magmatic products, while other typologies represent crystals which exhibit a two-stage growth: inherited xenocrystic core + magmatic overgrowth. Elongate zircons yielded nearly concordant U/Pb ages, whereas other populations were significantly discordant. A discordance produced by mixing between different proportions of small old cores and magmatic overgrowths is likely also for the subconcordant populations, inasmuch as the Variscan thermal regime cannot justify an episodic lead loss. Accordingly, two discordias are defined between discordant and subconcordant populations, with lower intercepts at 477.8 ± 2.6 and 483.4 ± 9.0 Ma, which are identical within the error limits. As only two-point discordias can be drawn, and the data points are close to the lower intercepts, upper intercepts are loosely constrained. Evaporation measurements performed on a core-bearing single grain yielded ages which are interpreted as discordant ages of the inherited domain; Th/U zoning, estimated on the basis of ²⁰⁸Pb*/²⁰⁶Pb* measurements at different evaporation steps, point to a magmatic signature of the cores as well, testifying to a partial reworking of ancient magmatic material by the anatectic melt.

Rb/Sr and ³⁹Ar/⁴⁰Ar data further on confirm the two-stage metamorphic evolution of the Southalpine basement during Variscan orogeny. The 345±9 Ma Rb/Sr age is not useful by itself, as the data points do not fit into a true isochron; however, if compared with previous geochronological work on the porphyroids (Del Moro et al., 1980), its significance is strengthened. The second Variscan stage is better defined at about 325 Ma by high quality plateau ages on post-kinematic micas.

Conventional U/Pb data are the first radiometric ages for the pre-Variscan acidic magmatism of the Eastern Southalpine basement, which now can be referred to the Lower Ordovician: the discordia lower intercepts constrain volcanic activity of the Eastern Southalpine basement within the Arenig. As a consequence, the porphyroids can be correlated with a volcanic event occurred in the Austroalpine domain before the Blasseneck event, for which a Lower Ordovician age is inferred by means of biostratigraphy. The upper intercepts are not related to any geological event, but the isotopic composition of the inherited components suggest a medium-scale heterogeneity in the crustal source of the protoliths.

The Pre-Variscan volcanism can be considered contemporary throughout the Eastern Southalpine basement, as the new isotopic ages are consistent with the biostratigraphy of the Agordo basement outcrops, which have been previously correlated to Comelico on lithostratigraphic grounds only.