RICERCA ITALIANA     

Research program

Crustal Anatexis: Natural evidence, Experiments and Modelling (C.A.N.E.M)

University Co-ordinator

Università degli Studi di PADOVA - GEOSCIENZE - ()

Research Unit Leader

Bernardo Cesare

Description

Introduction

The gola of this research unit is the detailed study of the pre- and syn-anatectic stages of crustal melting by combining metamorphic petrology and microstructural analysis with the study of melt and fluid inclusions.
In particular, the research will be focused on the following petrologic topics, very important but still poorly known and constrained:
- characterization of partial melting processes in the source region
thermobarometry
mineral chemistry
structural aspects related to melt localization and segregation
- quantification of melting reactions
- geochemical characterization of anatectic melts
- characterization of syn-anatectic fluids.
These objectives will be achieved by developing the following researches on the key outcrops reported below (see also Fig. 4 and following description):
- Field work and meso-structural analysis on migmatitic complexes
- Microstructural and petrologic study of melt and fluid inclusions (Fig. 5) in enclaves of the Neogenic Volcanic Province (NVP; Southern Spain)
- Petrologic study of crystallized melt inclusions and fluid inclusions in the melanosomes of metapelitic migmatites of the Kerala Khondalite Belt (Southern India), Ulten Unit (Italy), and Ronda Massif (Spain)
- Petrologic study of melt and fluid inclusions hosted in xenoliths and xenocrysts in dacitic and rhyolitic lavas cropping out on the Aeolian Islands (Lipari, Vulcano, Panarea) and on the Tuscan Magmatic Province (San Vincenzo e Roccatederighi).
- Thermodynamic modelling of anatexis conditions in the different study areas, constrained by the petrologic data obtained.


Fig. 4: location of study areas: K: Kerala Khondalite Belt, N: Neogenic Volcanic Province, R: Ronda, U: Ulten Unit, T: Tuscan Magmatic Province, E: Aeolian Islands.


Fig. 5: Coexisting fluid and melt inclusions trapped in a plagioclase crystal in El Hoyazo enclaves. Width ofview: 0.4 mm.

Rationale of Research
The rationale behind this research project is justified by the lack of data on real anatectic systems (see State of the Art for details), i.e. systems which did not suffer the typical phenomena accompanying the slow cooling of migmatitic and granulitic complexes.
Since the anatectic enclaves occurring in the NVP offer a superb natural example in which retrograde post-anatexis phenomena are absent [37], they are a rare natural occurrence in which many features related to crustal anatexis can be suitably studied; this gives the opportunity to attempt to discuss and solve some important problems, related to the development of anatectic processes, that still remain unsolved. In addition to the uniqueness of their geologic context, these enclaves have a very tight genetic link with the host lavas [40,49]: an added value that makes them suitable also as proxies of the source region [44].
These rocks are well characterized in many aspects including field occurrence [40], geochronology [49,53], petrology [42,43,45,52], isotope geochemistry [54,55,56], mineralogy [47,48,50], microstructure [39,41], and rock-physics [13]. Therefore, they are suitable for the a continuation of the studies in this research project, which will be focused on in-depth structural, geochemical, thermobarometric, and melt and fluid inclusion investigations.
This multidisciplinary approach, combined with the modelling and the detailed geochemical studies carried out by the research Unit of Perugia, will be also applied to the study of crustal enclaves and xenoliths occurring in similar geologic contexts (Aeolian Islands [57,58,59,60,61] and Tuscan Magmatic Province [62,63,64]). The study of these enclaves will provide information about anatectic processes, and will also contribute to a better understanding of the petrogenetic and geodynamic processes characterising these important circum-Tyrrhenian areas [65,66].
Part of this research project will be also focused on the study of metapelitic granulites and migmatites of the Kerala Khondalite Belt [67,68,69,70], Ulten Unit [71,72], and Ronda Massif [73,74,75] with the aim to understand in detail the processes associated with the segregation and extraction of anatectic melts. Data obtained from these studies will be cross-compared with those obtained from the analysis of crustal enclaves in order to construct a general framework integrating the different knowledge acquired from the different natural contexts.

Structure of the Project
The project will be structured in different phases, not necessarily sequential, partly overlapping in time, and integrated with the activities carried out by the research Unit of Perugia.
1-Survey and sampling
( in tight collaboration with the Unit of Perugia, mostly in the areas of Aeolian Islands and Tuscan Magmatic Province, and the Ulten Unit. Samples from NVP and Kerala Khondalite Belt can be directly investigated as reported at point 3 and following points). Field survey and sampling of metasedimentary enclaves will be carried out on cordierite-bearing acidic anatectic rocks [76], with emphasis for garnet-bearing lithologies, that are suggestive of a mid-deep crustal origin and, hence, of a more likely genetic link with host lavas. Regarding migmatites, the attention will be focused on the structural features related to the segregation of melt from the restite [77], and on the sampling of garnet-bearing melanosomes (being garnet a mineral particularly suitable to preserve re-crystallized melt inclusions).
2-MIcrostructural and petrographic study
Optical microscope study of newly collected and available samples, and selection of those samples which are suitable for more in-depth investigations. The attention will be mainly focused on the identification of: equilibrium assemblages, presence and localization of the melt phase and of its crystallization products, melting reaction microstructures, occurrence of fluid and melt inclusions, evidence of syn-anatectic deformation. The mapping of the spatial distribution of the melt at the thin-section length scale will be one of the most important issues of this part of the research.
3-Scanning electron microscope SEM analyses
A selected number of samples (ca. 15 crustal enclaves from the NVP, ca. 5 granulites from the Kerala Khondalite Belt, and ca. 15 enclaves from the remaining areas) will be utilised to perform detailed petrographic investigations with the aim of defining the occurrence of sub-micrometric phases, compositional heterogeneities at short lengthscale (by performing X-ray maps of chemical elements), and total and/or partial crystallization processes of the melt phase.
4-Re-melting of crystallized melt inclusions in a heating stage
Crystallized melt inclusions in the granulite samples from the Kerala Khondalite Belt and those in the migmatites of the Ulten Zone or in the crustal enclaves of the Tuscan Magmatic Province will be re-melted (homogenized) by using a heating stage to obtain an homogeneous and analysable melt (phases 5-7), and to gain thermometric information about anatectic conditions. At present there are no similar studies in the literature, but preliminary investigations (Fig. 6, [78]) have shown that this approach is very promising.


Fig. 6: Polycrystalline inclusion, likely representing the product of crystallization of an anatectic melt, hosted in a garnet crystal from the Kerala Khondalite Belt migmatites.

5-Major element analysis by electron microprobe (EMP)
The melts in inclusions (preserved as glass or re-melted), the inter-granular melts in crustal enclaves, and the mineral phases coexisting in the studied samples will be analysed in detail for their major element contents. Regarding glasses, the method proposed by Morgan et al. [79,80] will by applied to analyse both hydrous and anhydrous acidic glasses. Regarding minerals, the study will be also focused on the determination of potential internal heterogeneities and/or zoning patterns, and to quantify possible zoning patterns identified at point 3.
6-Trace element analysis by LA-ICP-MS
By starting from the knowledge acquired in the previous investigations, the Unit of Padova will select those samples which are the most suitable for an in-depth study and analysis of trace element variations (in collaboration with the Unit of Perugia).
7- FTIR e/o SIMS analysis of the H ₂O content
Although the method of Morgan et al. [79] will allow us to infer with good approximation the water content of hydrous glasses, additional direct measurements of this parameter, of great magmatologic significance [5, 31], will be carried out. Analyses will performed of the same glasses previously characterized for their major and trace element contents.
8-Fluid inclusion analysis
Fluid inclusions in the studied samples, in particular those trapped immediately before or during the partial melting process, will be analysed by micro-thermometry, SEM investigations, and Raman spectroscopy. This approach will allow us to completely characterize their composition and density and will provide additional thermobarometric and petrologic constraints.
9-Oxygen isotope analysis
Oxygen isotope analyses will be performed by laser fluorination on mineral separates from metapelitic enclaves and granulites. This will allow us to compare results of isotopic geothermometry with conventional geothermometric measurements. Depending on the results emerging from ‘preliminary’ analyses, if more detailed information is needed, additional in situ isotopic investigations will be also carried out by an excimer laser equipment.
10-Thermodynamic modelling
The thermodynamic modelling of anatectic conditions (P-T-X parameters and P-T-t paths) in the different geologic contexts will be carried out by using the constraints obtained from micro-chemical data, petrographic investigations, and whole rock geochemical data on selected samples. To this aim, the software Perple_X and Thermocalc will be utilised simultaneously. In addition, the comparison between the results of modelling and independent thermobarometric estimates will represent a validation of the available thermodyamic datasets, in particular of the model recently proposed for biotitey [34].
11-Cross-comparison of data from crustal enclaves, migmatites, and granulites
During the development and at the end of this research project all collected data will be analysed by cross-comparing the information from the different geological contexts, with the aim to highlight similarities and differences. In particular, it is of great importance to gain the maximum of information from crustal enclaves and to transfer this information to the understanding to the processes developing in migmatitic and granulitic complexes.
12-Data discussion and evaluation
All data will be discussed and analysed in tight collaboration with the Unit of Perugia during the development (i.e. after 8 and 16 months) and at the end of the project. The planned coordination meetings between the two research units during the development of the project will allow us to refine and calibrate the schedule of the project. The final evaluation of results will allow us to establish the best ways for an efficient dissemination of results , as well as posible follow-ups of these researches.
13-Synthesis and conclusions
This phase, developing during and at the end of the research project, will permit to assess the most relevant outcomes and will lead to the production of scientific papers.
14-Dissemination of results
The dissemination of results will mainly take place by publication of scientific papers on international journals, and by communication of results at conferences, scientific meetings, and thematic workshops. Thematic sessions, on the research subjects of this project, will be organized at FIST2009, EGU2010, and AGU2010 meetings. Dissemination of results will also occur at the fifth Marie Curie European Intensive Seminar of Petrology (EURISPET) “High-temperature metamorphism and crustal melting” (www.eurispet.eu), which will take place in June 2010. In addition, a new web site hosting all relevant information about the research project and its results will be designed.
Time schedule
The different phases of the research project will be carried out as reported in the following chronogram:

Fig. 7 Time schedule of the research Project

Activities of components of the research project
The activity carried out by each researcher belonging to the research Unit of Padova, schematically described in Fig. 7, will be as follows:
Cesare: micro-structural study of crustal enclaves, petrology of high-temperature processes, fluid and melt inclusion investigations, coordination of the research Unit of Padova and of the whole research project
Braga: survey, microstructural analysis and petrology of migmatites, and geochemistry of accessory phases
Meli: survey and petrology of crustal enclaves of the Tuscan Magmatic Province and Aeolian Islands, SEM and EMP analysis, isotopic thermometry
Salvioli-Mariani: petrology of crustal enclaves, fluid and melt inclusion investigations
Ferrero: fluid and melt inclusion investigations
Ferri: micro-structural analysis, rock-physics, and experimental petrology
Tajcmanova: thermodynamic modelling, survey, and petrology of migmatites

The research group of the Unit of Padova includes all the relevant expertise to suitably carry out this research project, and the tight collaboration with the research Unit of Perugia on several common topics will allow us to achieve all the targets outlined above.
The several international collaborations already well established by the Unit of Padova will also play an important role for the successful development of the research project and for the production of very high quality results.

Expected Results
Expected results are significant advances of knowledge on the main targets of the research project. These advances will be concretized mainly by the publication of research papers and by conference presentations.
The outcome of the project will be evaluated positively if at least 4 papers will be published on international journals.
As a potential time-schedule, the first papers will be submitted for publication by the end of first year, the last papers are expected to be published (considering normal publication time-scales on international journals) 12-18 months after the end of the research project.



Additional References (not cited at Point 7.12)
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75 Torres-Roldán RL (1983) - Tectonophysics 96, 95-123
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80 Morgan VI GB, London D (2005) - Am Min 90, 1131–1138