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 after the cessation of underplating. We expect to better constrain the timing and duration of the underplating and its effects by dating the onset of mafic magma injection, which are represented by mafic/ultramafic dykes, and by dating the cooling of the lower crust below the solidus of the metasedimentary country rocks of underplated intrusions. The age of the volcanism will be better determined to understand the temporal relationship between volcanism and the "magmatic incubation period" in the lower crust.
The main goal of the research is to build an internally consistent petrogenetic framework that could explain the petrological and geochemical features of the two areas in a common geodynamic context. The results are expected to advance comprehension of petrogenetic processes in active volcanic districts where the residence zone of magmatic system in the lower crust can only be investigated through indirect geophysical methods such as InSAR and seismic tomography. <<<

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 granodioritic-to-tonalitic, with minor gabbro-dioritic bodies (Rottura et al., 1998, and ref. therein). Isotopic compositions suggest an anatectic origin for the most acidic magmas, and a hybrid origin for the intermediate terms (Rottura et al., 1998). The magmatic age is estimated at about 270-275 Ma, and up to 301 Ma for the intrusives, but geochronological data are as yet limited (Sassi, 1985, Visonà, p.c.).
The Ivrea-Verbano Zone, in its central and southern portions, exposes one of the most voluminous gabbroic bodies of the Alps (Rivalenti et al., 1975, 1981, 1984; Pin & Sills, 1986). Here, mafic magmas intruded the lower-crustal Kinzigite Formation at depths between 15 and 25 km (Demarchi et al., 1998), reaching a thickness of about 10 km. Detailed mapping of all the lithologies that constitute the gabbroic intrusion in this area revealed a megascopic arcuate structure defined by both foliation and banding, which are mostly concordant, (Quick et al., 1992, 1994, 2003). Structural features are analogous to those observed in ophiolites' gabbros (Nicolas, 1989; Quick et al., 1992). Although it is clear that the geologic setting of the Ivrea-Verbano Zone gabbro intrusion was not an ophiolite, structural analogy suggests that similar emplacement dynamics were involved, at least as far as mechanisms are concerned. The structural observations can be explained altogether assuming that the gabbro body grew from a small magma chamber (km-sized in diameter) continuously fed from below with basaltic magma inputs. Crystallized material at the base and on the sides of the magma chamber was continuously transposed "downward and outward"; as a consequence, this material was smeared out and deformed under conditions varying progressively from synmagmatic to sub-solidus ("gabbro-glacier model", Quick et al., 1992). This growth mechanism explains the features of the "roof" of the gabbro body, which, South of Varallo, displays the features of a stretching fault (Means, 1989; Snoke et al., 1999). The proposed model is consistent with an extensional tectonic regime during the growth of the gabbro body, in agreement with the presence of Permian blastomylonites in the Kinzigite Formation (Brodie & Rutter, 1989, Mulch et al., 2002).
The chemistry of the gabbros reveals that they are contaminated with crustal material by as much as 30% (Voshage et al., 1990; Sinigoi et al., 1991, 1994). Within the gabbro body, granulitic metasediments from the Kinzigite Formation were detached in layers to form the "septa", stretched to an extreme aspect ratio, and transposed parallel to the foliation. Field, density and geochemical data have proved that these septa were incorporated within the gabbro when they were detached from the roof of the intrusion after the extraction and upward migration of anatectic melts which left a crustal restite layer denser than the mafic melt (Sinigoi et al., 1995). The crustal septa that were incorporated continued melting, releasing a depleted crustal component. This process implies a fractional melting of the crust with the production of granitic magmas in two phases. In the first phase, granitic magmas were produced by partial melting of a fertile crust and migrated upward toward shallower crustal levels. In the second phase, charnokitic magmas were derived from advanced melting of the septa; geochemical evidence demonstrates that these melts were the primary contaminant of the gabbros (Sinigoi et al., 1996). Thermal modelling of this process suggests that it can last several tens of millions of years (Annen and Sparks, 2002; Peressini et al., in prep.).
At high crustal levels, melts generated by this process process are represented by the Graniti dei Laghi, intruded in Permian time within the adjacent Strona-Ceneri Zone (Pinarelli et al., 1988), and by the Permian rhyolites cropping out SE of the Cremosina Line. For the latter, though, petrological and geochemical data are extremely scarce: the available age data (Rb/Sr WR isochrones, Hunziker and Zingg, 1980) give ages roughly coeval with both the Graniti dei Laghi (Pinarelli et al., 1988), the Lugano volcanics (Stille and Buletti, 1987), and the AVD (Rottura et al., 1998, and ref. therein; Visonà, p.c.).
An extensive bibliography exists on the age of intrusion of the Mafic Complex in the IVZ (Pin, 1986; Voshage et al., 1990; Vavra et al., 1996, 1999; Henk et al., 1997; Mayer et al., 2000), which indicates a Permian age. SHRIMP data on zircons, recently produced by the Trieste Reasearch Unit with the COFIN 2002 contribution, have shown that:
1) Zircons extracted from gabbros collected at different depths in the complex crystallized at 287±3 Ma
2) Gabbro samples collected in the neighbourhood of metasedimentary septa in the Complex (Paragneiss Bearing Belt, Sinigoi et al., 1996) contain a high number of inherited zircons with ages of about 310 Ma, which testify to an older anatectic event in the lower crust.
3) Sm/Nd mineral isochrones, which give ages around 270 Ma for the gabbros and around 250 Ma for garnet-bearing gabbros, are viewed as cooling ages (Voshage et al., 1987; Mayer et al., 2000). Considering that the closure temperature for Sm/Nd in grt-free gabbroic rocks is likely higher than or close to 750°, it seems reasonable that the crustal rocks associated to the Mafic Complex remained partially molten for about 15 Ma. <<<