RICERCA ITALIANA     

Relationships between anorogenic and orogenic magma sources and geodynamic implications for the Europe-Africa system.

Università degli Studi di Firenze

Abstract

Petrological, geochemical and isotopic studies on lavas and hosted mantle xenoliths from Cenozoic key magmatic provinces, from subduction related, collisional and within plate geodynamic settings, are proposed. This research aim to contribute to the understanding of the compositional evolution of upper mantle sources of magmas in complex geodynamic settings. Origin of orogenic and anorogenic signatures will be therefore investigated to elucidate their dependence on possible processes of mantle metasomatism, condition of partial melting, and heritage by previous histories of mantle wedges. The acquired data will be discussed in the framework of the complex geodynamic evolution of the Central, Western Mediterranean Regions, which is driven by Africa-Eurasia plate convergence, and subsequent extension due to slab roll-back processes. The transition from orogenic to anorogenic magmatisms in the Central, Western Mediterranean Regions will be also addressed to better clarify relationships between mantle metasomatic processes and geodynamic evolution. On the other hand, the Madagascan volcanic areas will be investigated in order to understand the effects of late-Precambrian lithospheric resetting. Emphasis wil be given to the compositional and isotopic relationships between the mantle sources of the Madagascan alkaline volcanic rocks and those with similar serial affinities found in the Central- Western Mediterranean area, in order to distinguish possible hot-spot influence in both >>>

Principal Investigator

Sandro CONTICELLI Università degli Studi di FIRENZE

Research Objectives

The goal of this research is to establish petrological, geochemical and isotopic characteristics of lithospheric and sub-lithospheric mantle sources belonging to the European and North-Africa domains, facing one another, with microplates in between, in the Mediterranean.
A complete comparative study between mafic lavas and hosted xenoliths from key localities in the Western Mediterranean and those from other typical magmatic associations related to European and African domains, will be also carried out. Because of the very complex geodynamic setting of the Cenozoic Western and Southern Mediterranean magmatic provinces and because of the existence of mutually excluding petrogenetic and geodynamic models proposed to explain the magmatic activity in this area, this project enlarges its interests to areas away from any subduction system in within-plate geodynamic systems such as Madagascar.
The aim of this study is to define the possible role of pristine lithospheric signatures which might be overprinted by both orogenic and anorogenic metasomatic events controlling the final petrological and geochemical features of generated magmas. In detail the proposed research project will be addressed to:
1) recognise the nature of mantle sources prior to metasomatism in both anorogenic and orogenic settings;
2) define the metasomatic components which affected the concerned mantle sections in both the European and African/Adriatic lithospheres;
3 >>>

First Results

At the end of this phase we will expect as results:
- to have analytical procedures running;
- to have databases available;
- to have the samples to be studied collected;
- to have preliminary petrological, geochemical, isotopic data on mafic lavas and xenoliths obtained;
- to have paleomagnetic data partially obtained;
- to have possible preliminary scientific notes written.At the end of this phase, which coincide with the end of the project, we will expect to have performed the overall scientific work scheduled. With this respect, results obtained can be grouped in several categories as follow:
a) analytical facilities. We will have new and up to date analytical methodologies running in each lab of the different Research Units beside the already present laboratories. Namely: U-series and Pb isotope analytical procedures (TIMS), along with trace element high resolution analytical procedures (HR ICP-MS) running at the labs of Reserch Unit n. 1; in situ trace element analytical procedures (LAM ICP-MS) running at laboratories of Research Unit n° 5;
b) databases. We will have different databases containing geochronological, geochemical, petrological, isotopic, paleomagnetic, structural, geological data of the studied areas, which will be available upon request;
c) analytical data. Major and trace alement data will be available both for whole rocks and minerals from lavas and xenoliths, along with isotope data on >>>

Timescale

24 months

National and international background

Subduction-related magmas range in composition from tholeiitic to calc-alkaline, alkali potassic, and alkaline ultrapotassic. Alkalic potassic and ultrapotassic magmas are usually though to be collision-related, and therefore restricted either to the latest stages of the subduction processes or to post-orogenic collapse, post-dating convergent tectonics and active margin processes (e.g., Prelevic et al., 2004). They are found in many collisional orogens related to Alpine orogenesis (e.g., Venturelli et al., 1984a, 1984b; Conticelli et al., 1992; Conticelli, 1998; Duggen et al., 2004), but they also occurr in some continental arcs such as Andes, Western Mexico, and Eastern Sunda Arc (e.g., van Bergen et al., 1992; Carmichael et al. 1996; Righter and Carmichael, 1996; Carlier et al., 1997). The observed differences in alkali enrichment and in silica-saturation degree, shown by orogenic primary mantle magmas, are though to be variably related to: i) different depth of partial melting, ii) different partial melting degrees, iii) variable fertility of the pristine mantle wedge (prior to subduction); iv) variable degrees of metasomatic enrichments induced by subduction-related fluids/melts (e.g., Foley, 1992; Conticelli et al., 2002).
High Field Strength Elements (HFSE) depletion, usually observed in most orogenic magmas, could be related either to partial melting conditions and persistence of refractory accessory phases, or to the peculiar compositions of slab-derived >>>