RICERCA ITALIANA     

Rlationships between Lower Carboniferous to Permian underplating and volcanism in Southern Alps

Università degli Studi di Trieste

Abstract

The geodynamic interpretation of the Permo-Carboniferous magmatism at the margin of Adria is still widely debated. In the South Alpine, Permo-Carboniferous magmatic activity is well represented by large volumes of volcanic products, mostly acidic and intermediate, in the Dolomites district, and by a large underplated intrusion in the lower crustal section of the Ivrea-Verbano Zone, exposed as a result of alpine collision.
In the perspective that the Ivrea-Verbano Zone constitutes a realistic model for the processes taking place at the basis of the crust beneath the Dolomites between Late Carboniferous and Permian, the integrated study of the two areas offers the opportunity for the interpretation of the geodynamic meaning of the effusive activity at the light of what is actually exposed in the underplating zone.
Recent geochronological studies (U/Pb zircon) indicate that the thermal peak due to the Ivrea-Verbano mafic intrusion occurred at 287±3 Ma, followed by a much slower cooling, recorded by the closure at about 270 Ma of the Sm/Nd isotopic system in mafic rocks.

Principal Investigator

Silvano SINIGOI Università degli Studi di TRIESTE

Research Objectives

The geodynamic significance of the magmatic events which affected the whole Southalpine domain during Late Carboniferous to Late Permian time is still a matter of debate, probably because of the complex geodynamic setting of Adria between the Hercynian orogeny and the opening of the Alpine Tethys. The Permo-Carboniferous magmatic activity in Southern Alps is recorded by both volcanic and intrusive products. The volcanic products are mainly represented by the Atesina Volcanic District in the Dolomites, while the Ivrea-Verbano Zone provides an example of a magmatically underplated section of the lower crust, which was exposed as a consequence of the Alpine collision.
The objective of the proposed research is to explain the magmatic evolution of the Atesina Volcanic District assuming that the lower crust beneath it was affected by processes analogous to those that occurred, at the same time, in the Ivrea-Verbano Zone. In this view, the integrated study of the two areas offers the uncommon possibility of comparing volcanism and related underplating.
The petrological comparison will be based mainly on a large data set, which is already available for both areas, and which requires only minor integration. What is still lacking is the timing of the igneous events. Preliminary data suggest that magmatic underplating in the Ivrea Zone may have occurred over a very long period, and that the lower crust, heated by mafic magma, remained partially molten for at least 15 Ma >>>

First Results

The aim of the project is to interpret the Permo-Carboniferous volcanic activity at the light of the underplating processes that occurred at the same time in the lower crustal section exposed in the Ivrea-Verbano Zone. The results will constrain more precisely the mantle source and the geodynamic significance of the magmatic event.
The working hypothesis is that underplating has been a much longer process than the associated volcanic activity. The 310 Ma age could represent the beginning of the underplating process. A first stage of mafic diking progressively heated the lower crust, inducing anatexis. The thermal peak was reached at about 287 Ma, with the main inflation of the gabbro body, and the coeval, mainly acidic, volcanic activity as the surface expression. Subsequently, temperatures in the lower crust remained above the limit of anatexis at least until 270 Ma, and possibly until around 250 Ma. In this case, the prolonged presence of a partially molten lower crust conditioned the rheology of the crust itself and the chemistry of volcanic products. This framework is consistent with the preliminary results of a thermal model (in progress), but remains only a working hypothesis, while the new data will either provide confirmation or indicate a different scheme.

Timescale

12 months

National and international background

Permo-Carboniferous magmatism in the South Alpine domain is represented by two major occurrences: the mafic underplating in the southern sector of the Ivrea-Verbano Zone (IVZ) of the western Italian Alps, and the acidic-to-intermediate volcanism of the Atesina Volcanic District (AVD) and related upper-crustal plutons (Cima d'Asta, Bressanone-Chiusa, Ivigna and Monte Croce). The geodynamic interpretation of this magmatic phase is still widely debated. Some authors, on the basis of the calc-alkaline affinity of the volcanic products, suggest that they represent a late-orogenic activity (e.g., Di Battistini et al., 1988; Bonin et al., 1993), while others favour a within-plate magmatism in a transtensional geodynamic setting (e.g., Rottura et al., 1998; Dal Piaz et al., 1993).
At the same time, the process of magmatic underplating took place in the lower crust of Adria. The Ivrea-Verbano Zone exposes an "outcrop" example of lower crust underplated in Carboniferous-Permian time, and subsequently exposed in the Italian Western Alps as a result of alpine collision. It is, therefore, possible to interpret the AVD magmatism on the basis of the processes recorded in the underplated lower-crust of the Ivrea-Verbano Zone.
The volcanic district in the Dolomites mainly consists of rhyolitic volcanic products, frequently ignimbrites, with minor andesites. The southern and northwestern margins of this area are characterized by the presence of shallow intrusions, mainly >>>