Structural, tectonic and geodynamic significance of Middle Triassic igneous activity in the Dolomites, NE Italy

At the Middle-Late Triassic boundary (~244-235 Ma) a diffuse igneous activity developed in Southern Alps, a wide area where the recently agglutinated Pangea super-continent had begun to disrupt. Sparse remnants of these products now crop out in the Brescian pre-Alps, the Vicentinian Alps (Recoaro-Schio-Posina), Non Valley, Dolomites and Tarvisian Alps. The igneous activity is concentred mainly in the Dolomites mostly as lava flow and pyroclastic successions, with rare intrusive bodies cropping out at Predazzo, Monzoni, and Cima Pape. The Triassic igneous rocks show a wide compositional spectrum with compositions ranging from metaluminous (basalt/gabbros) to peraluminous (rhyolites/leucogranites) and including also cumulitic lithologies (cumulitic gabbros and clinopyroxenites). The study of Triassic magmatism in the Southern Alps is the special interest due to contrasting models proposed to define the geodynamic and tectonic setting. The petrographic, mineral chemistry and geochemical investigations have been performed for Triassic igneous rocks from different localities in the Dolomites and Vicentinian Alps to understand the origin of this magmatism and the main magmatic processes that controlled the evolution of these rocks. We investigated the magnetic and magmatic fabric of Predazzo and Monzoni plutons by means of anisotropy of magnetic susceptibility (AMS) and microstructural data coupled with a field geological study. The AMS technique aimed at revealing the internal fabric of Predazzo and Monzoni plutons and defines the magnetic foliation and magnetic lineation (magmatic flow). Also, this technique is used to study magma migration and emplacement in the upper crust and define the model of emplacement of igneous bodies. Furthermore, the AMS method is aimed to constraining and define the relationship between magma emplacement and Middle Triassic tectonics. Finally, the integration between structural and petrological analyses allowed to constrain the geodynamic and tectonic setting of Middle Triassic magmatism in the Dolomites and Southern Alps. The major findings and contribution of this study are the following: 1) The Triassic magmatism in the Southern Alps has mainly shoshonitic affinity followed by high-K calcalkaline. 2) The studied rocks show a sharp correlation (SiO2, Na2O+K2O) and negative correlation (TiO2, Fe2O3tot, MnO, MgO, CaO, CaO/Al2O3) with differentiation index (D.I.). 3) The Dolomites lavas exhibit enrichment of large ion lithophile elements (LILE) relative to high field strength elements (HFSE) and shows high Ba/Nb (32.8-93.9), low Ce/Pb (1.8-10.4) and low U/Nb (0.11-0.33) coupled with low HFSE (e.g., Nb = 6-12 ppm) with low Nb/Nb* [(Nb/0.46)/((O980/0.023)*(U980/0.017))^0.5] ratio (0.1-0.35), resembling the typical characteristics of magmas generated by subduction-modified mantle sources. 4) The fractional crystallization of Mg-bearing phases such as olivine and clinopyroxene with only a minor role of plagioclase controlled Dolomites volcanic rocks evolution. 5) The 87Sr/86Sr for the Dolomites volcanic rocks ranges from 0.70432 to 0.70577 and do not show any correlation with D.I., with SiO2 or MgO which indicates that the mantle sources, only little modified, if any, by interaction with the local crust. 6) The 143Nd/144Nd(i) ranges from 0.51227 to 0.51237, with Nd(i) values slightly lower to slightly higher than ChUR(230 Ma) (from -1.4 to +0.5). The Pb isotopic compositions are homogeneous, with 206Pb/204Pb clustered around 18.26 and 18.41, 207Pb/204Pb from 15.62 to 15.67 and 208Pb/204Pb from 38.36 to 38.89. 7) The isotopic variation of the Southern Alps Triassic rocks is related to mantle source heterogeneities rather than interaction with upper crustal lithologies. 8) The mildly to strongly evolved rocks are genetically linked with the least differentiated terms by closed system fractional crystallization of gabbroic to monzonitic assemblages. 9) According to results of ABS5 software modelings of the less evolved basaltic sample, the origin of these rocks is a peridotitic mantle wedge source at typical subduction settings. 10) Finally, the mantle sources of Triassic igneous rocks reflect previous subduction metasomatism (likely developed during Hercynian times) when continental rifting caused raising of the geotherms and passive upwelling of asthenospheric mantle. 11) The study is a new evidence of the validity of the AMS method for unraveling the mode of emplacement of igneous bodies. 12) AMS, microstructural and petrographic data from the Predazzo body are consistent with a multistage ring-dyke emplacement mode. 13) The Predazzo sheets were emplaced via either upward magma flow or along-strike lateral magma transport. 14) The ENE-WSW elongated shape of the Monzoni body was controlled by the occurrence of strike-slip faults associated with Ladinian-tectonics. 15) The feeder zones are located at the NE and SW part of the Predazzo intrusion and at the NE edge of the Monzoni intrusion. 16) The low degree of anisotropy (Pj) values indicate the existence of low strain during the emplacement of Predazzo and Monzoni. 17) The Predazzo and Monzoni plutons emplacement followed the main Triassic tectonic phase (post-tectonic). 18) Ladinian strike-slip tectonics providing a preferential pathway for post-tectonic magma.