Research program
Relationships between anorogenic and orogenic magma sources and geodynamic implications for the Europe-Africa system.
University Co-ordinator
Universita' degli Studi di ROMA -
SCIENZE DELLA TERRA - ROMA(RM)
Research Unit Leader
Michele LUSTRINO
Description
The Research Project is focused to a mineralogical, petrographic, geochemical and petrological characterization of the Cenozoic igneous rocks of Sardinia. These studies are part of a cooperation project started since 1994 between this research unit and other Italian and foreign Universities. This Project aims, therefore, to better define and to conclude the tasks on the petrogenesis of Cenozoic igneous rocks of Sardinia begun some years ago. Results will be compared with other "anorogenic" and "orogenic" basaltic (s.l.) continental products.From a tectonic point of view, the settings of many of the Cenozoic igneous rocks of Sardinia, as well as those of other circum-Mediterranean Cenozoic igneous products (Spain, Maghrebian Africa, Turkey, Italy, Pannonian Basin) are "orogenic" (i.e. related to ocean-continent or continent-continent collision). However some of these products (emplaced in "orogenic" settings) show "anorogenic" (or "within-plate") geochemical features. To complicate the scheme, some (many) "orogenic" settings (evidenced by geochemical "tracers") of European Cenozoic igneous rocks have been alternatively interpreted as shallow level crustal contamination (AFC-like processes). Moreover, "orogenic" geochemical features of igneous rocks do not necessarily imply orogenic setting during the magma production, but can be linked to ancient modification (e.g., Tamburelli et al., 2000) of mantle sources. In conclusion the geochemical classification "anorogenic" does not imply setting far away from orogenic settings but rather mantle sources not sensibly modified by crustal recycling occurring at plate margins. Similarly, the term "orogenic" does not imply orogenic settings. This term may indicate also crustal contamination of mantle melts (not mantle sources) or modification of mantle sources somewhere in the geological past. Evidence of old (Panafrican/Hercynian) subduction-related modifications in the European subcontinental mantle come mainly from late Paleozoic lamprophyres (e.g. Massif Central, France, and Black Forest, Germany), and from mantle and crustal xenoliths associated to the Cenozoic European alkaline volcanic rocks. Such evidence seems to lack or be diluted in the Cenozoic European volcanic record (Wedepohl and Baumann, 1999). Therefore, the chemical and isotopic composition of the UPV would represent the lacking lithospheric source end-member among the Cenozoic European Volcanic Province (Lustrino, 1999; Lustrino et al., 2002).The chemical and isotopic characteristics of the RPV sub-group of the Sardinian PQ rocks (and, by analogy, of the Cenozoic sodic anorogenic mafic volcanic rocks of Italy and the entire CEVP), could have been inherited by source modifications that occurred during or shortly after the Hercynian Orogeny, among several of which, recycling of dehydrated, subducted, oceanic lithosphere of the Paleozoic Proto-Tethys Ocean, could be a possibility. The relatively uniform HIMU-like composition of the Cenozoic European anorogenic Volcanic rocks and of the RPV could be viewed, at the light of recent experimental works (e.g. Cordery et al., 1997; Yaxley, 2000; Kogiso et al., 2003), as the product of asthenospheric mantle modified by small amounts of subducted, residual, oceanic lithosphere, characterised by low Rb, Pb and Rb/Sr, and high Nb/U, Ce/Pb and U/Pb ratios. To test this second hypothesis Sr-Nd-Pb-O-Os and major and trace element data acquisition is scheduled.The reason of why lithospheric melts derived by sources with ancient subduction-related modifications appear to be relatively rare in the Cenozoic European Volcanic Province is yet an intriguing and unresolved aspect, as is the geodynamic significance of coexisting EMI-like and HIMU-like geochemical characteristics in the Sardinian PQ volcanic region.The evolution of the OM igneous rocks seems to be mostly governed by crustal contamination at shallow depths. The genesis of the OM ignimbrites (dacite and rhyolite) has been related to anatectic processes in the continental crust (e.g., Dupuy et al., 1974), while other Authors (Morra et al., 1994, 1997; Brotzu et al., 1997; Conte, 1997; Lonis et al., 1997) interpreted the positive correlation of 87Sr/86Sr ratios with the differentiation of magmas as results of fractional crystallization coupled with more or less intense crustal assimilation. In contrast with the above hypothesis, Downes et al. (2001), on the basis of Sr, Nd and O isotope study, hold that the strong isotopic variation of rocks from Arcuentu are the effect of source contamination by subducted sediments rather than shallow depth contamination in magma chambers.The research on the Cenozoic igneous rocks from Sardinia is scheduled as follows:1. To complete the collection of the rock samples of Montiferro volcanic complex and Abbasanta-Paulilatino basaltic plains (Plio-Quaternary rocks) and Bosa (Oligo-Miocene rocks);2. To characterize the petrography of the samples. The petrographic characterization is necessary in order to classify the rock samples and to ascribe them to the alkaline and tholeiitic suites and to choose the samples to analyze;3. To analyze the samples via X-ray Fluorescence spectrometer (XRF), atomic absorption spectrophotometer (AAS) and inductively plasma mass spectrometry (ICP-MS). These analyses will be carried out in the laboratories of this research unit and using the facilities of the units number 3 and 5;4. To carry out electron microprobe analyses on mineral phases. These analyses will be carried out in this research unit, using a Cameca SX 50 microprobe with EDS and WDS spectrometers;5. 40Ar/39Ar dating of the most representative samples (about 10 analyses). A 39Ar/40Ar dating is actually considered through collaboration with Prof. Robert A. Duncan (College of Oceanic & Atmospheric Sciences, Oregon State University);6. To obtain Sr, Nd and Pb isotope analyses via thermal ionization mass spectrometry (TIMS) on selected samples at the isotope facilities of the research unit number 1 laboratories;7. To measure oxygen isotopic ratios of mineral separates and whole rock in the laboratories of this research unit via laser fluorination technique (in collaboration with dr. L. Dallai);8. To measure isotopic ratios of Re/Os system via N-TIMS at the University of Leoben (Austria) and CNRS of Nancy, in collaboration with prof. T. Meisel. In particular this systematic is an innovative task because no Os isotopic values of the Cenozoic European Volcanic Province products are currently available.9) To build a database of the most important geochemical data available on the Cenozoic circum-Mediterranean igneous rocks; part of this step is already running;10) To propose a new model of dynamics in the wide scenario of evolution of the Alpine Orogeny;11) To publish the results in international scientific journals.Most of the analytical processing (e.g., 40Ar/39Ar dating, Sr-Nd-Pb-Os isotopic ratios measurements, ICP-MS analyses) will be carried out in other Italian and foreign laboratories with which a scientific cooperation is currently in progress. It is noteworthy to evidence that this research unit is still working on preliminary results of these systematics.All the data acquired, will be compared to the roughly coeval volcanic rocks of the circum-Mediterranean area. In particular will be considered the causes responsible for the peculiar geochemical and isotopic features of the Cenozoic igneous rocks of Sardinia. These characteristics will be considered in a large geodynamic point of view, taking into account the possible variables that could be potential in their capability to change the geochemistry of the European subcontinental mantle. In particular, will be considered all the modification of the European lithospheric mantle occurred during the previous orogenesis: subduction of the proto-Tethys oceanic crust during the middle Paleozoic before the continent-continent collision between Laurussia and Gondwana; lower crustal and lithospheric delamination after the Hercynian orogeny during the Permian-early Triassic; formation of the Tethys oceanic crust; following subduction of this crust during the Alpine orogeny; formation of several oceanic basin in the Mediterranean area within the general compressive contest during the last 30 Ma (e.g., Lustrino, 2000a).The main question which need to find an answer are:* What are the causes responsible for the peculiar geochemical and isotopic features of the Plio-Quaternary volcanic rocks of Sardinia?* Why these modification did not add substantial heterogeneity to the sources of the Oligo-Miocene cycle?* What is the role of the Alpine and Hercynian orogeny in the petrogenesis of teh Plio-Quaternary volcanic rocks of Sardinia?* Why te composition of the Plio-Quaternary volcanic rocks of Sardinia are so unique throughout Europe?* Why are so rare the crustal xenoliths borne in Plio-Quaternary volcanic rocks of Sardinia, while they are relatively abundant in the other European volcanic provinces?* What is the nature of the lower continental crust beneath Sardinia and what is it relation with the European lower crustal estimate of Wedepohl (1995)?* What is the petrogenetic significance of the high U/Pb (HIMU) and low U/Pb (EMI) mantle components in the sources of the Plio-Quaternary volcanic rocks of Sardinia?* Is there a relation between the overall HIMU-like composition of the Cenozoic European Volcanic Province and the Proterozoic-Paleozoic subduction and recycling of oceanic slabs?* Are the tholeiitic and alkaline volcanic rocks cogenetic? And what are the main processes responsible for the magmatic differentiation?* What are the thermo-baric conditions of formation of the Plio-Quaternary volcanic rocks of Sardinia? And are the Plio-Quaternary mafic volcanic rocks in equilibrium with a spinel- or a garnet-facies peridotite?* What relation does exist between the metasomatism recorded in the Sardinian lithospheric mantle (presence of phlogopite laths and glassy patches in lherzolitic mantle xenoliths borne in alkaline rocks from Gerrei) and the processes responsible for the geochemical and isotopic features of the host magma?In particular, the attention of this research unit will be focused on the following first-order debated topics:1) - GEODYNAMIC MEANING OF THE GEOCHEMICAL TERMS "OROGENIC" AND "ANOROGENIC".Too often geochemical features of igneous rocks have been considered true evidences of geodynamic setting of emplacement for igneous rocks belonging to a given district. This research unit will investigate the significance of the mantle heritage in terms of what the upper mantle can "remember" of its previous geodynamic history in a very complex area such as the Mediterranean Sea.2) - GEOCHEMICAL PROBES OF THE EXISTENCE OF DEEP-ROOTED MANTLE PLUMES.Recently, the existence of deep-rooted mantle plumes in the Mediterranean realm have been proposed on geochemical (e.g., Wedepohl, 2000; Bell et al., 2003) and geophysical grounds (e.g., Goes et al., 1999) as well as on geological consideration s.l. (e.g., Golonka and Bocharova, 2000). Also the PQ volcanic activity of Sardinia has been considered related to the presence of a mantle plume (e.g., Gasperini et al., 2000). However, the effective existence of such kind of thermal anomalies is highly debated and a consensus on this topic is far to be achieved. The research project will investigate in detail the tectonic scenario and the geodynamic meaning of some alleged mantle plume pathfinders (e.g., the existence of carbonatitic magmatism, peculiar Sr-Nd-Pb isotopic ratios, specific abundance and ratios among incompatible elements, and so on). Do effectively exist mantle plumes? Is there a relation between geochemistry and mantle dynamics in very complex tectonic areas such as the Mediterranean real? These are two of the most important question that this research unit will try to give some answers.3) - TEMPORAL TRANSITION BETWEEN "OROGENIC" TO "ANOROGENIC" MAGMATISM.The transition from igneous activity with "orogenic" geochemical features to "anorogenic" characteristics is relatively common throughout the entire circum-Mediterranean area (e.g., Lustrino, 2003; Coulon et al., 2002; Duggen et al., 2004). One of the most important aims of this research is to investigate the causes of this shift and the much rarer transition from "anorogenic" to "orogenic" magmatic activity (e.g., Schiano et al., 2001). In some provinces the shift of chemical compositions from "orogenic" to "anorogenic" has been related to "slab detachment" and/or "slab windows" processes, whereas in other cases the involvement of different mantle sources (asthenospheric mantle slightly modified by subduction-related metasomatism and lithospheric mantle strongly modified during ancient times) has been proposed.4) - THE MEANING OF THE HIMU COMPONENT IN THE CIRCUM-MEDITERRANEAN REALM.It is long time that a relatively common and uniform geochemical reservoir has been identified in many CEVP products from central Europe (Wilson and Downes, 2001; Granet et al., 1995; Wilson and Patterson, 2001). In some cases such a uniform reservoir (see Lustrino et al., 2002 for a review) has been related to a giant mantle plume (e.g., Hoernle et al., 1995; Bell et al., 2003) or to smaller finger-like plumes (e.g., Wilson and Patterson, 2001). The tomographic images of the circum-Mediterranean mantle are quite different according to different reduction models and a clear evidence for the presence or absence of a large plume head beneath this area is not available. In some cases (e.g., Lustrino et al., 2000) the HIMU-like features of sodic alkaline CEVP rocks have been related to the recycling of ancient (Hercynian age or even older) oceanic lithosphere in the deep mantle.In doing this, this research unit is intentioned to enlarge the area of investigation also to other circum-Tyrrhenian areas interested by Cenozoic igneous activity like Poland (in cooperation with prof. Marek Michalik; Jagiellonian University, Krakow) and/or Serbia (in cooperation with prof. Dragan Milovanovic, University of Beograd).
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