Petrology of Predazzo magmatic complex (Trento, Italy)

Predazzo Magmatic Complex represents the main intrusive boby of the magmatic activity which affected the South Alpine domain during the Triassic age (238-232 My). Field observations and petrological data allow to distinguish four main intrusive sequences characterised by a clear subduction-related signature: a shoshonitic SiO2-saturated series (M1); a shoshonitic SiO2-oversaturated series (M2); a K-alkaline SiO2-undersaturated series (M3) and a Granitic Unit (G). As concerns the subvolcanics products three main series have been distinguished: 1) shoshonitic dikes, characterised by incompatible element distribution analogous to the corresponding M1 and M2 intrusive lithologies; 2) K-basanitic dikes, cutting the granitic and monzonitic M2 intrusions, genetically linked to M3 lithologies; 3) lamprophyric dikes, cutting both the granitic and monzonitic intrusions. Lamprophyres, Middle Triassic in age, are characterised by the presence of ultramafic xenoliths and carbonatite ocelli, as well as by the absence of Nb and Ti negative anomalies. Initial 87Sr/86Sr isotopic ratios of M1 and M2 Units range between 0.70385 and 0.70649 with initial 143Nd/144Nd isotopic ratios between 0.512227 and 0.51216. On the other hand the K-basanitic dikes and M3 Unit rocks show higher initial 143Nd/144Nd isotopic ratios (0.512281-0.512386) for quite similar initial 87Sr/86Sr isotopic ratios (0.70404-0.70533). As expected lamprophyric dikes show a completely different isotopic signature, with higher 143Nd/144Nd (0.512483-0.512595) and lower 87Sr/86Sr (0.70362-0.70429) isotopic ratios. A mass-balance closed system fractionation model was developed for M1, M2 and M3 Units. According to field relationships, petrography, mineral chemistry and isotopic data, model suggests that the Predazzo plutonic complex was predominantly generated by fractional crystallisation processes, with different magma pulses injected in multiple adjacent magma chambers in a short time span. A possible sequence of the magmatic events can be summarised as follows: a first intrusion of M1 Unit (almost contemporaneous to the volcanic products), followed by the emplacement of M2 Unit. The two intrusions were followed by the Granitic Unit, which is in turn cut by both K-basanitic and lamprophyric dikes.