Petrology of Predazzo Magmatic Complex (Trento, Italy).

An extensive study of the Triassic volcano-plutonic complex of Predazzo has been carried out. As it is known, Predazzo intrusion represent the main plutonic body of the magmatic activity which affect the South Alpine domain during the Triassic age. It is composed by a ring-shaped small intrusive bodies emplaced between 238 and 232 My (Laurenzi et al., 1997), partly coeval with the volcanic products which overlye the complex. Petrographyc and geochemical features, together with age relationships allow to depict a rather complicated tectono-magmatic evolution, resulting in several small magma pulses with different magmatic affinity injected in a short time gap. According to Visonà (1997) four main sequences were distinguished on the basis of silica saturation degree, mineral phases and compositions. They are: - a shoshonitic silica saturated series (M1), - a shoshonitic silica oversaturated series (M2), - a potassic-alkaline series (M3) and - a probably calc-alkaline series made up only by the granites (G). M1 represents the main part of the outer ring of the complex and is mainly constituted by monzo-gabbros and monzonites, with subordinate gabbros and pyroxenites and syenites. Diopsidic to augitic clinopiroxene is the most common phase, followed by olivine, Mg-horneblende and minor ortopiroxene, while plagioclase is the most abundant sialic phase, followed by K-feldspar and scarse quartz. It represents the most common series in the Dolomitic area as both intrusives (M.ti Monzoni, Cima Pape) and volcanics. M2 is mainly represented by monzo-diorites, with minor quartz-monzonites and quartz-syenites. Clinopiroxene and amphibole, with similar composition to those of M1, are common; quartz is more abundant in monzo-diorites, monzonites and syenites, coherently with he marked silica-oversaturation of the series. M3 outcrops exclusively in the eastern part of the complex (Val di Viezzena). It is characterized by abundance of differentiated products (syenites), with subordinate monzo-gabbros and monzonites, all bearing nepheline. In this series clinopiroxene is salitic to aegirinaugitic in composition and amphibole varies from Mg-hastingsite to Fe-pargasite. The presence of titanite, apatite and garnet (rare) is peculiar. Calc-alkaline granites constitute the core of the complex. An “I Type” nature was proposed by Visonà, (1997) for these rocks, considered differentiated from calc-alkaline basic magmas; the presence of muscovite is related to secondary deuteric processes overimposed on primary biotite crystals. Differentiation processes were modelled using a mass-balance calculations. Results indicate that crystallization played a dominant role in the differentiation of the three series. On the basis of field relationship between the various lithotypes the temporal succession of the intrusion may be put forward: M1 represent the oldest intrusion in the western and north-western part of the complex. It was followed by M2 in the eastern and south-eastern part. A gradational contact between lavas, sills and the M1 and M2 intrusives suggests concomitant, or slightly older, volcanic episodes. The youngest intrusive event is represent by the granitic rocks, which are clearly intruded in M1 and M2 as well as in the volcanics. Granites, in turn, are cut by several K-lamprophyric dykes, which might be analogous to the M3 parental magmas. The timing of M3 intrusion remain still uncertain, since it cuts M1 and M2, but no relationship were found with the granites.