Alkali and lime-rich igneous rocks are among the most puzzling magmas known in literature. Several young (Holocene) case studies crop out in central Italy (San Venanzo and Cupaello villages), and slightly older (Pleistocene) or much older (Triassic) rock types cropping out in central Iran (Nawbaran area and surrounding zones) and southern Italian Alps (Dolomites), respectively will be investigated aiming to shed light on this debated topic. Together with these localities, also other volcanic districts in N Egypt, central Europe (e.g., Eifel, Germany) and NE Spain will be used as benchmarks.
The identification of Ca-carbonatitic activity in central Italy and Iran, as well as K-rich lithologies (shoshonite) in the Italian Alps have been considered key features to unravel the geodynamic settings of such three areas. In particular, the presence of carbonatitic magmas has been previously used to deny the existence of active or fossil subduction tectonic settings along peninsular Italy, proposing the existence of a deep seated mantle plume. The finding of ultrabasic compositions in a clear subduction-related setting in central Iran certainly reduces the validity of such an assumption.
The CaO and Na2O+K2O enrichment of these rare rock types can alternatively be related to the presence of carbonates stable at mantle depths or to interaction with limestones and dolostones at very shallow - crustal - pressures. This research study aims to understand which of these two main hypotheses can be considered reliable in these case studies.
Igneous activity has also a strong influence on the deposition, contributing to release large amounts of CO2 to the atmosphere-ocean system. Repeated volcanic CO2 pulses may cause a repeated decrease in pH and weak CaCO3 saturation and decreasing the calcification potential of carbonate biota up to the drowning of marine carbonate factories. Several case studies from central Italy and other circum-Tyrrhenian areas will be investigated.
Paradoxically, the innovativeness of the petrological part of this research project deals with the oldest approach of investigating igneous rocks, i.e., a detailed petrographic study. Indeed, despite the increasing accuracy, the strong decrease of the detection limits, the continue improving of computer modelling and the exponential use of micro-analytical techniques, the most important results obtained in the igneous petrogenesis field are based on the correct identification of the rocks object of the study. Too often petrogenetic models rely only or essentially on geochemical considerations (i.e., the concentration of a given key element or the value of the ratio of a given radiogenic isotopic system). In complex case studies such as those presented in this project, the precise petrographic investigation is the essential tool, too often forgotten, to discriminate among different hypotheses. Of course a detailed petrographic description is not sufficient, but it is the necessary step after which all the other micro- and macro-analytical investigations can be planned.
In this project an integrated isotope (Sr, Nd, C) stratigraphy will be performed. The authigenic (seawater-derived) neodymium (Nd) isotopic composition of marine archives is increasingly used to study changes in ocean circulation in the context of climate change on millennial and longer time-scales. Its usefulness as a tracer of ocean circulation is a consequence of its variable isotopic composition in the oceans, which reflects sources of different crustal age, and its estimated residence time. The global oceanic Sr isotope signature varied over the geological time due to two major controlling factors: volcanism and continental runoff. Lastly mass differences of the two C isotope (12C and 13C) species lead to strong fractionation during photosynthetic incorporation of carbon into organic matter, 13C enrichment of surface water is caused by carbon isotope fractionation during organic matter production, this last increases by rising of atmospheric CO2 concentration. Therefore, on the basis of Sr, Nd and C isotope signatures of the Miocene Central Mediterranean carbonate successions the different factors affecting water chemistry can be identified; furthermore, Nd isotope ratios can provide new insights in the water exchange between the main Mediterranean Basin and the surrounding oceans, as well as within the different sub-basins of the Mediterranean area.