Research program
Relationships between anorogenic and orogenic magma sources and geodynamic implications for the Europe-Africa system.
University Co-ordinator
Università degli Studi di FIRENZE -
SCIENZE DELLA TERRA - FIRENZE(FI)
Research Unit Leader
Sandro CONTICELLI
Description
The Research Unit of Florence will be focused on the investigation of Central-Western Mediterranean magmatism, its mantle sources and the relationships with the geodynamic settings.In this frame, we will consider magmatic suites from selected key localities with significant role in the definition of the geodynamic scenario for the magma genesis. A further aim of this Research Unit is to make a comparison between these magmas and other magmatic suites of orogenic and anorogenic origin, within or outside the Western Mediterranean Sea. This comparison will be made in order to define the presence of any signature acquired by mantle sources from its evolution prior to metasomatism and partial melting, which produced the studied magmatic associations. The scientific work will be focused on two main subjects:1) understanding of the relationships between calc-alkaline, shoshonitic and lamproitic magmatism in Western Mediterranen and their significance into the Africa-Europe geodynamic evolution.2) Analysis of the fluids vs. melts input and their role in the metasomatism of the Central-southern sectors of the Italian peninsula. In this frame particular attention will be put on the geochemical and isotopic end-members involved in a geodynamic scenario, where either Estward roll-back of the subducting plate or upwelling of a deep mantle plume have been suggested.Along with these two main scientific subjects, the Research unit of Firenze will carried on the development of analytical and methodological procedure for the measurements of Pb and U-series isotopic systematics through Thermal Mass Spectrometry (TIMS), and for the whole-rock trace element aboundances determination by High Resolution ICP-MS. Both these two instruments are working in laboratories based in our University. TIMS is presently working for the determination of 87Sr/86Sr e 143Nd/144Nd isotopic ratios at the laboratories of the Earth Sciences Department of Università degli Studi di Firenze. HR ICP-MS will be installed in April 2004 at the laboratories of the Chemistry Department of the same universtity. We will set up the HR ICP-MS for determining incompatible trace elements on magmatic rocks, collaborating with people of the Chemistry Department of the University of Firenze, and with people of the Research Unit of the University of Ferrara.Regarding point (1) of the present proposal, it is worthy to note that Neogene magmatic rocks ranging from sub-alkaline (calc-alkaline), through shoshonitic, to alkalic ultrapotassic (lamproites) of the Western Mediterranean region have an unique and constant orogenic signature with strong depletion of HFSE with respect to LILE. We will perform new high resolution incompatible trace elements determination (HR ICP-MS) to verify detailed variations in the fractionation between incompatible trace elements with similar geochemical affinities. This will be focus to understand the role of fluid and/or melts in the enriching agent of the lithospheric mantle source of these magmas. We will also investigate the possible role of pre-existing signatures inherited by extraction of basaltic component prior to last event of mantle metasomatism and partial melting. On these samples will be performed new and accurate isotopic analyses, performed with a Mass spectrometer of last generation (TRITON TI), to obtain the 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb isotopic composition. These data will define the isotopic characteristic of the mantle source of calc-alkaline to lamproitic magmas in orogenic setting, and to select between either crustal enrichment by metasomatism or long term isotopic evolution in a secular isolated mantle reservoir. Preliminar petrologic and petrographic studies will be carried out on the selected samples to define those compositions that can be considered representative of primary or near primary composition. Samples will come from Tuscan Magmatic Region (Amiata, Radicofani, Cimini volcanoes, vel d'Era dykes), from the Western Alps sector (dykes from Val d'Aosta and Piemonte), from South-Eastern Spain (Murcia-Almeria area). Sampling will be brought to completion during the first phase of the project, being many samples already in the collection of the Research Unit.For what concern point (2) of the proposal, the Research Unit will characterise petrologically and petrographically the volcanoes of the Central Southern sectors of the Roman and Lucanian Magmatic Regions, and those of the Aeolian Arc. We will study detailed volcanic succession of the Alban Hills, Middle Latin Valley, Roccamonfina and Vulture districts to select the most primitive samples for trace element and isotope determinations. In addition primitive samples of the Campanian district (Ischia, Procida, Phlegrean Fields and Vesuvius volcanoes) will be supplyed by the Research Unit of Naples. On selected samples (about 30) we will determine the distribution of incompatible trace elements by high resolution ICP-MS and isotopes (87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb e 208Pb/204Pb) with the TRITON TI (TIMS). The obtained results will be used to better define the existing geochemical and isotopic polarity present in the sector South of the 41st parallel (Campanian district and Vulture Volcano), and to characterise the source of magmas North of this tectonic structure (Vico, Sabatini and Alban districts), and the transition between these two mantle source regions found beneath the (Middle Latin Valley district and Roccamonfina volcano). In addition these data will help to better define the presence of the possible HIMU component in the southermost portion of the mantle beneath Italian peninsula (mantle wedge?), and to clarify the origin of this signature. Is this signature either recently overprinted on a subduction related upper mantle, trhough mantle inflow following the subduction roll back, or it was already there before subduction derived fluid/melts? To clarify this point, an integration between petrologic, isotopic geochemistry data and paleomagnetic data will be performed with the aim to clarify the possibility and timing of the possible lateral detachment of the slab, following the rollback of the subduction. This will be done in collaboration with the Roma III Research Unit.To better define the role of possible HIMU component flowing into the mantle wedge of the Southernmost sector of the Italian region (Aeolian Arc), a group, part of our reserach unit, will exclusively work on calc-alkaline to potassic magmatism of the Aeolian Islands. Therefore, the most primitive sample will be selected following the same procedure above described. On these selection (about 20 samples from Stromboli, Vulcano, Filicudi and Alicudi volcanoes) incompatible trace elements with HR-ICP-MS and isotopes ( 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb) with TRITON TI mass spectrometer (TIMS) will be carried out. These data will help to define the role of fluid vs. melts of the metasomating agent, in producing the isotopic and geochemical signatures of magmas West and East, respectively, of the Tindari-Letojanni line; a deep tectonic feature, which dissect the Aeolian Arc in two sectors. In addition possible comparison with magmatism related to the Eastern Mediterranean subduction (Aegean) will be performed to understand the role of fluid vs. melts in an area in which the mantle seems to has not been influenced by any HIMU component.Recently the presence of a FOZO component has been suggested to occurr in the middle of Tyrrhenian basin, influencing pery-Tyrrhenian and Sicily channel magmatisms. This would imply the presence of a large plume, based in the middle of Tyrrhenian Sea since Oligocene, suggesting the possible evolution of the Western Mediterranean through deep mantle upwelling rather than by subduction retreat. In order to investigate these problematics U-Th analises will be performed on a selection of the previously selected samples for the magmatism of Central-Southern Italy. Moreover will be also analysed samples representative of magmas with clear anorogenic signature. These samples will be taken from magmatic suites located both in the nearby areas, such as in the foreland (Pantelleria, Linosa, Sicily Channel) or in transitional zones in the Southern Tyrrhenian Sea (Ustica), and in oceanic intraplate settings, where no subduction-related processes can affect the mantle. In this way will be possible to distinguish in detail the contribution of plume-related end-members from that of subduction-related orogenic processes.The choice of U-Th systematics for this investigation put some constraints, which has to be considered in the selection of the samples. U-series isotopes evolve in a secular equilibrium condition , where the decay rate (often referred as activity = liNi) of all nuclides is the same. In the genesis of magmas many processes can fractionate the elements of the dacay chain producing disequilibrium. When the secular equilibrium condition is lost, the parent-daughter pairs will move towards the equilibrium at constant rate, relating to the half-life of the daughter nuclide. Secular equilibrium is restored after five times this half life.In 238U dacay chain, the longest lived isotope is 230Th with an half-life of 75 ka. In the U-Th system thus the equilibrium is restored after a bit more than 300 ka, which rapresent the upper age-limit in the choice of the samples; samples older than 300ka will have already lost any useful information. On the other hand, samples younger than 300ka with (230Th/238U) activity ratio different from the unity will need an age-correction to obtain initial values to be unsed in the discussion of the differences between the different magmatic suites. Applying this correction, it has to be taken into account the analytical error will increase with age, and that this effect becomes more significant for samples close (but different) to the equilibrium. In order to minimise this effect the Research Unit will try to collect for U-Th analyses only magmas younger than 150 ka; older products will be considered only where nothing younger will be available. The selected sample will be thus the younger volcanics erupted to the North (Amiata, 190 ka; Vico, 100 ka) and the South (Ernici, 100-200 ka; Vesuvio 30ka-presente; Procida <10ka; Ischia < 9 ka) of the 41st parallel, as described at point (1), those from Aeolian Arc (Stromboli, Vulcanello, Filicudi and Alicudi, 60 ka to present), and the alkaline rocks from the Sicily Channel (Pantelleria 120-9 ka).These data will integrated and cross-interpretated along with the tectonic structural and paleomagnetic data (Roma III Operative Research Unit), in order to elucidate the detailed geodynamic contest and the deformation processes in an intraplate-continental area. Structural and tectonic-modeling in the area will be also performed with the aim of asessing whether magma uprise and genesis is related to the peripheral part of an upwelling plume or to trans-tensive and pull-apart-like rifting processes.This research project will be developed through several working phases. Check of milestones will be therefore easily performed. Dissemination of scientific results will be done through compilation of a database, which will be available for scientist, and scientific publications in peer-reviewed journals:1st phase) Month 0-12 – sampling, compilation database, setting up laboratories, preliminary data;a) Compilation of a database of age, geochemical, mineralogical, isotopic data of the Southern Tyrrhenian Sea magmatism, Sicily Channel and Westenmost Mediterranean zone, to be supplied on requests, and to be used for other phases of the research;b) Sampling, when necessary, of the areas intersted by the research; c) Preliminar petrologic study (EPMA, XRF, AAS) to define the most primitive magmatic rocks;d) Setting up of laboratories and analytical methodologies for the analyses of incompatible trace elements by high resolution ICP-MS and of Pb-U-Th isotopes by RPQ-equipped TIMS;e) Isotopic analyses on samples already own by the Research Unit and on those supplyed by the Naples Research Unit.2nd phase) month 6-18 – heavy determination of anlytical data, possible preliminary papers.a) Determination of incompatible trace elements by HR-ICP-MS;b) Completing 87Sr/86Sr, 143Nd/144Nd isotopic data and 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb and U-series isotopic determinations; c) Completing petrological and geochemical investigations (EPMA, XRF, AAS);d) Preliminary dissemination of dat with first papers;e) Exchanges of data among the different Research Units of the present network.3rd phase) month 12- 24 – Finishing the analytical work, data discussion, dissemination of data.a) Completing the data set of incompatible trace elements;b) Completing the data set of isotopes;c) Completing the data set of petrological data;d) Joint discussion with other Research Units of the network and preparation of possible modelling;e) Dissemination of results through submission of manuscript to peer-reviewed journals.
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