Geochimica dei gabbri del plutone di Bressanone (Sud Tirolo, Italia)

The Bressanone (Brixen) magmatic complex has an age of about 280 Ma. It belongs to a series of intrusions (Ivrea-Verbano, Sondalo, Malenco, Koralpe, External Ligurides) that outcrop through the South alpine and Austro alpine domains which formed during the post Hercynian period in Permian times. The complex outcrops in the North-West of Bressanone. In its northern side, it is delimited by the Insubric line, while in the remaining sides it is confined by the metamorphic basement that in this area is composed by quartz-phyllite. The pluton is mainly constituted by granodioritic peraluminous rocks, bearing aluminum rich minerals such as garnet and aluminous-silicates. In its southern portion there is a small outcrop of gabbros and norites. The gabbros of the Bressanone complex are the object of this thesis. The gabbros can be subdivided in three different groups based on their structure: fine grained, coarse grained and pseudo-porphyric rocks. In all three, the mineralogy is formed by pyroxenes, amphiboles (as result of pseudomorphosis), plagioclase, oxides and quartz. The samples (and also the outcrops) show a weak cumulate structure with alternation of Si-poor silicate minerals (mafic) and Si-rich silicates (plagioclase and quartz). The chemical analyses have been made on samples BR08 and BR10. Clinopyroxenes show a normal zonation with decrease of #Mg and En content from the center to the rim of the crystals. Trace element contents in clinopyroxene, orthopyroxene and plagioclase have been obtained by means of laser-ablation ICP-MS. The REE chondrite normalized pattern of clinopyroxenes is slightly enriched in LREE vs HREE in both samples. Plagioclase show an inverse zonation with An content that increases from the center to the rim of the crystals. The REE normalized pattern shows a marked enrichment of LREE over HREE and a huge positive anomaly in Eu. The equilibrium magma has been calculated considering KD values from the literature. The pattern of the calculated magmas for clynopiroxenes and plagioclase are very similar and show significant negative anomalies in HFSE (Nb, Ta, Zr, and Hf), Sr (LILE) and Pb, and a slight through for Ti. From a general point of view, the chondrite normalized trace element pattern of the computed magma shows higher concentration in high incompatible elements than less incompatible ones. The bulk rock compositions show yielded similar trace elements compositions and patterns for BR08 and BR10, with BR10 being a bit more enriched than BR08. The composition is very similar to the one calculated from mineral trace element data, with negative anomalies in HFSE, Sr, Pb, Ti. Sr-Nd-Pb isotopic compositions of whole rocks (recalculated to the initial value at 275 Ma, show that Bressanone gabbros have enriched compositions. While Sr and Nd isotopic values fall close to the EM I and between EM II domains in the Pb isotopic systems they fall close to EM II. From the evidences of the chemical compositions of major and trace elements in rock and minerals it is possible to affirm that the samples are related to the same magmatic system, and crystallized from the same magma. The crystallization took place at least in two different magmatic chambers at different crustal depth, as suggested by the En composition of clinopyroxene from BR 10, in which the inner part of the crystal has higher En composition than the external portion. This feature might be the consequence of the rise of magma from a deeper chamber to a less deep one. The composition of the gabbros is enriched and this can be the consequence of two main processes: enriched composition due to an enriched peridotite linked to a subduction zone; assimilation-fractional crystallization. In order to constrain the latter hypothesis, an AFC model has been calculated. Since we have no sample from the quartz-phyllite and of “uncontaminated” gabbro, the AFC model has been calculated on similar rocks belonging to the Ivrea-Verbano and Val Sesia zones. The calculations show that with R=0.3 and F=0.68, the AFC is able to reproduce the isotopic composition of BR08 and BR10.