RICERCA ITALIANA     

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

Università degli Studi di Padova

Abstract

This Research Project will study in detail the processes accompanying crustal melting in its early stages of high-grade metamorphism and beginning of anatexis, and in the following stages during which anatectic melts segregate from the source region, migrate through the continental crust, and emplace as plutonic or volcanic bodies. We will use a multidisciplinary approach integrating metamorphic and igneous petrology, microstructural analysis, thermodynamic and geochemical modelling, also using techniques based on Chaos Theory and fractal Geometry, studies of melt and fluid inclusions, and experimental petrology simulations,
The Project is focused on the following objectives, of fundamental petrological importance but still poorly known or constrained.
Pre-anatectic phases ('gestation’)
-to understand the development of anatectic processes in the source region determining the thermo-baric conditions during the prograde phase of high temperature metamorphism and the onset of melting reactions, by studying the chemical composition of mineral phases at the beginning and after the onset of melting, and the microstructures related to the localization of the anatectic melt in the source region and during the segregation process.
-to quantify melting reactions and their melt productivity by detecting microstructures indicative of melting, and by performing mass balance calculations based on the composition of analysed mineral phases.
Syn-anatectic phases ('birth’)
-to study systematically the chemical composition of intergranular melts and melt inclusions in crustal enclaves and xenoliths, and of recrystallized melt inclusions in granulites and migmatites, with the aim to construct a database of compositions of natural anatectic melts.
-to study, by analysing coexisting fluid and melt inclusions, the composition and density of syn-anatectic fluids.
-to perform the thermodynamic modelling of the anatectic process in the studied rocks by comparing the thermo-baric conditions, the composition of mineral phases and the melting reactions.
-to understand interaction processes among magmas generated either by melting of different source rocks or by different degrees of partial melting of the same source region, and to analyse how these processes may mask the original geochemical features of single melts, by studying the compositional heterogeneity of intergranular glasses in crustal xenoliths (or enclaves) and of leucosomes in migmatitic complexes.
Post-anatectic phases ('migration and emplacement’)
-to understand magma interaction processes among anatectic melts during their migration from the source region and ascent toward the emplacement level by developing new experimental simulations and numerical models based on magma migration through 'Small World’ fracture networks.
-to understand in detail, by studying natural outcrops, the mechanisms acting during accumulation of melts in magma chambers and in volcanic conduits and their kinetics, and to comprehend the influence of the development of chaotic mixing processes on their original geochemical compositions.

To achieve these goals, the studies will be focused on key outcrops of plutonic and volcanic rocks where the coexistence in space and time of anatectic melts, with different geochemical compositions, has been documented, and on crustal enclaves and xenoliths, representing fragments of source region, hosted in volcanic rocks. In particular, the natural case studies include: crustal enclaves and xenoliths hosted in the volcanites of the Neogenic Volcanic Province (Spain), of Aeolian Islands and of the Tuscan Magmatic Province (Italy); metapelitic migmatites and granulites of the Kerala Khondalite Belt (Southern India), of Ulten Unit (Italy), and Ronda Massif (Spain); leucocratic facies of Pulchiana (Italy) and Sithonia and Arnea (Greece) plutons.

One of the most important outcomes of this research project is the development of a general framework linking the different stages involved during crustal anatexis, by continuous and systematic cross-comparisons between studies carried out on crustal enclaves, on typical crustal anatectic contexts (i.e. granulites and migmatites), and on the compositional heterogeneity of magma bodies generated by the emplacement of crustal anatectic melts. The construction of such a framework is fundamental for the development of a conceptual model taking into account the most important processes contributing to the genesis of crustal anatectic melts and their compositional features. In addition, by integrating data resulting from the detailed analyses of different magmatic provinces around the central-western Mediterranean area, this research project will also contribute to a better understanding of the petrogenetic processes and their significance in the recent and/ot active geodynamic evolution of this important geologic context. <<<

Principal Investigator

Bernardo Cesare Università degli Studi di PADOVA

Research Objectives

Due to the lack of a specific section, citations in the following texts are referred to those reported in the B Model of each Research Unit. In particular, the prefix “a” and “b” is utilised to indicate citations given in the Research Unit of Padova and Perugia, respectively.

The target of this Research Project is the detailed understanding of the processes accompanying crustal melting in its early stages of high-grade metamorphism and beginning of anatexis, and the successive stages during which anatectic melts segregate from the source region, migrate through the continental crust, and emplace as plutonic or volcanic bodies. To this aim, a multidisciplinary study integrating metamorphic and igneous petrology, microstructural analysis, thermodynamical and geochemical modelling, also using techniques based of Chaos Theory and Fractal Geometry, studies of melt and fluid inclusions, and experimental simulations, will be carried out.
The main objectives, of fundamental petrological importance but still poorly known or constrained, are:

Pre-anatectic phases ('gestation’)
- to understand the development of anatectic processes in the source region by characterizing the behaviour of natural metapelitic systems during the prograde phase of high temperature metamorphism and the onset of melting reactions. The researches will be focused on the determination of anatectic thermobaric conditions, on the study of the chemical composition of mineral phases at the beginning and after the onset of melting, and on the investigation of microstructures related to the localization of the anatectic melt in the source region and during the segregation process;
- to quantify melting reactions and their melt productivity by detecting microstructures indicative of melting, and by performing mass balance calculations based on the composition of mineral phases.

Syn-anatectic phases ('birth’)
- to study systematically the chemical composition (major and trace elements and isotopes) of inter-granular melts and melt inclusions in crustal enclaves and xenoliths, and of recrystallized melt inclusions in granulites and migmatites, with the aim to construct a unique database of compositions of natural anatectic melts. At present such a database is not available, but its construction may represent an extraordinary advancement in the study of crustal anatectic systems;
- to study, by analysing coexisting fluid and melt inclusions, the composition and density of syn-anatectic fluids with the aim to constrain the fluid-melt-rock interactions in partially melted pelitic systems;
- to perform the thermodynamic modelling of the anatectic process in the studied rocks by starting from their geochemical composition, and to compare the thermobaric conditions, the composition of mineral phases and the melting reactions obtained using existing thermodynamic database with the same features resulting from parallel and independent petrological and microstructural studies;
- to understand interaction processes among magmas generated either by melting of different source rocks or by different degrees of partial melting of the same source region, and to analyse how these processes may mask the original geochemical features of single melts, by studying the compositional heterogeneity of intergranular glasses in crustal xenoliths (or enclaves) and of leucosomes in migmatitic complexes.

Post-anatectic phases ('migration and emplacement’)
- to understand magma interaction processes among anatectic melts during their migration from the source region and ascent toward the emplacement level by developing new Experimental Petrology simulations and numerical models based on 'Small World’ fracture networks, and to understand the influence of fracture network topology in transferring such melts toward shallower crustal levels as a function of chemical exchanges among them;
- to understand in detail, by studying natural outcrops, the mechanisms acting during accumulation of melts in magma chambers and in volcanic conduits, and to comprehend the influence of the development of chaotic mixing processes on their geochemical compositions and on the production of geochemical heterogeneities at different length scales;
- to understand, by detailed studies of chemical diffusion processes, the kinetics associated with the development of compositional heterogeneities within magmatic systems, and to define their timescales as a function of the dynamics occurring during melt segregation, migration, and accumulation in plutonic and volcanic magmatic systems.

To achieve these goals, the studies will be focused on key outcrops of plutonic and volcanic rocks where the coexistence in space and time of anatectic melts, with different geochemical compositions, has been documented, and on crustal xenoliths, representing fragments of source region, hosted in volcanic rocks. A very important point to be highlighted is the possibility to investigate anatectic processes in situ by studying the restitic enclaves hosted in the volcanics of the Neogenic Volcanic Province (Southern Spain) [a37]; these enclaves represent a unique occurrence of natural anatectic metapelites transported to the Earth surface so quickly to preserve the anatectic melt in the glass state [a38,a39].
One of the most important outcomes of this research project is the development of a general framework linking the different stages involved during crustal anatexis, by continuous and systematic cross-comparisons between studies carried out on crustal enclaves, on typical crustal anatectic contexts (i.e. granulites and migmatites), and on the compositional heterogeneity of magma bodies generated by the emplacement of crustal anatectic melts. The construction of such a framework is fundamental for the development of a conceptual model taking into account the most important processes contributing to the genesis of crustal anatectic melts and their compositional features. In addition, by integrating data resulting from the detailed analyses of different magmatic provinces around the central-western Mediterranean area (see B Models), this research project will contribute to a better understanding of the petrogenetic processes and their significance in the recent and active geodynamic evolution of this important geologic context. <<<

First Results

The expected results are the advancement of knowledge relative to the main objectives of the Research Project [further details can be found in section 11 of this A Model and in the B Model of each Research Units (R.U.)]. These are synthetically listed as follows:

Pre-anatectic stages ('gestation’)
- comprehension of anatectic processes in the source region by characterising the behaviour of natural metapelitic systems.
- quantification of melting reactions and their melt productivity.

Syn-anatectic stages ('birth’)
- understanding of the causes of compositional heterogeneities of natural anatectic melts.
- construction of the first geochemical database for natural anatectic melts.
- comprehension of the petrological significance of fluid inclusions coexisting with melt inclusions.
- execution of thermodynamic modelling of the studied rocks.
- understanding of the interaction mechanisms among anatectic melts, on both small and large length scale.

Post-anatectic stages ('migration and emplacement’)
- understanding of the influence of interaction processes among anatectic melts on their original geochemical compositions.
- comprehension of the influence of transport and coalescence on the composition of anatectic melts during their migration through fracture networks.
- understanding of the mechanisms associated with the accumulation of anatectic melts in magma chambers and volcanic conduits.
- comprehension of the causes inducing the development of chaotic mixing dynamics among compositionally different melts.
- understanding of the kinetics and time-scales associated with the development of compositional heterogeneities in anatectic magmatic masses. <br />
On the whole, expected results can be summarized in a better understanding of the causes leading to the genesis of compositionally different anatectic melts and of the mechanisms associated with their segregation and extraction from the source regions. This will allow us to construct a conceptual model which is able to explain the origin of compositional heterogeneities within magmatic masses as a function of the different stages characterizing their geologic history, including pre-, syn-, and post-anatectic stages. An additional result will be a better understanding of petrogenetic processes in relationship with the recent and/or active geodynamic evolution of the central-western Mediterranean region.

The obtained results will be diffused throughout the scientific community in the fundamental activity of DISSEMINATION OF RESULTS (D) which represents the completion and concretization of the researches and which will be carried out by developing the following steps (see also Fig. 2):
D1-Publication and communication of results
Dissemination of results of this Research Project will occur by publication of scientific papers on international journals and presentation at conferences, scientific meetings and thematic workshops. Thematic sessions on the subjects of this research will be proposed at FIST2009, EGU2010, and AGU2010 meetings. Diffusion 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.
D2-Construction of a new web site
Diffusion of results will also occur through a new web site, specifically designed for this Research Project, in which all relevant information about the status of the research, the Research Group, and the results obtained will be reported. The web site will host a forum area in which the results and advancements of researches will be discussed real-time by organising multi-user workshops of researchers interested in the topics of the Project. In addition, a mailing-list service, which will be utilised to inform all interested colleagues about the advancements of the Research Project, will be also developed.
D3-International workshop
In order to increase the diffusion and discussion of results of this Research Project throughout the national and international scientific community, an international workshop will be organized at the end of the Project.

Concerning the concretization of researches as research papers minimum expected results will be the publication of at least three papers on international journals for each research activity (Analysis, Experiments, Modelling), for a total of at least nine articles. <<<

Timescale

24 months

National and international background

In his recent review about the state of the art and problems related to crustal anatexis, Brown (2007) concludes: “We have come far in our understanding of melt generation and extraction from anatectic lower crustal sources, and magma ascent and emplacement into the upper crust of large hot orogens, but much remains to be done”.
Anatexis (partial melting) of the middle-lower crust is tightly associated and connected with granulite facies metamorphism and with the genesis of migmatites [a1,b1,a2]. By generating granitoid magmas [a3,a4,b11], which can segregate from the source region and migrate toward shallower crustal levels [a5-a9,b12,b13], anatexis represents one of the most important agent of the differentiation of the continental crust [a10,a4], influencing its rheological behaviour [a11-a13,a7] and, hence, its geodynamic evolution [a14,a6,b1].
The emplacement of anatectic melts, both in plutonic and volcanic environments, marks the final stage of formation of magmatic bodies [e.g. a7,b13]. These bodies are the product of aggregation of compositionally different melts: i) heterogeneous primary melts due to different degrees of partial melting and/or source variability; ii) interaction among such melts during segregation and ascent processes; iii) interaction in magma chambers and/or in volcanic conduits [e.g. b3,b27,b28]. Aggregation and interaction among the different melts can occur with different efficiencies, masking or completely obliterating the original geochemical features of the single melts [e.g. b3].
Being anatexis in the region of overlap between metamorphic and magmatic processes, its study requires a multidisciplinary approach involving several disciplines, the most important being igneous and metamorphic petrology, geochemistry, structural analysis, and mineralogy. The knowledge of the causes and behaviour of rocks undergoing partial melting, and of the successive segregation of anatectic melts, derives up to now mostly from field studies, experimental simulations, and modelling.
Researches on migmatitic and granulitic complexes allowed direct observation of the products of anatexis. They provided fundamental information on several aspects of the anatectic process including the P-T conditions and characteristic reactions, the geodynamic context, and the structural features associated with melt segregation and ascent [a16-a23]. In addition, it has been highlighted that anatectic melts can be transported according to two main processes [e.g. b10-b12]: i) segregation, marked by small-scale movements of melt (10 ^-3-10 ^-2m), mostly within the source region [e.g. b1, b13], where ductile deformation is the dominant mechanism [e.g. b14-b16]; ii) long-range (10 ²-10 ⁴m) ascent through the brittle continental crust to the site of final emplacement [e.g. b17,b18], a process governed by rising of magmas in conduits, porous spaces and fracture networks [b19-b23]. Besides, recent studies on natural fracture networks have shown that these behave as 'Small-World’ networks [b24], i.e. they have a very high efficiency to drain and transfer magmas from the source region to the emplacement levels [e.g. b25, b26].
Phase relations, rate of melt production and melt composition as a function of P-T-X conditions have been determined by performing partial melting experimental simulations, using both natural and synthetic pelitic compositions [es. a25-a31]. In addition, magma interaction processes have been recently reproduced in laboratory applying experimental petrology methods by using real magmas; this approach allowed to follow the development of mixing processes in space and time, providing further constrains on the understanding of natural systems [b40, b41].
Numerical modelling of partial melting processes and melt interaction, recently performed, has been focused on the understanding of both thermodynamic and geochemical processes. In the first case, due to the construction and continuous upgrading of thermodynamic database, and the availability of specifically designed software, thermodynamic analysis allowed to infer the P-T condition associated to the partial melting of natural rocks, and to interpret their mineralogical, compositional and textural features [es. a32-a34]. In the second case, geochemical modelling based on the application of the new techniques of Fractal Geometry and Chaos Theory demonstrated that the processes of melt coalescence and interaction are governed by chaotic dynamics [b29-b31]. These studies provided information about the dynamics of magmatic systems, their kinetics, and the style of propagation of compositional heterogeneities [e.g. b32-b34], allowing to recognize those portions of magmas that still contain information on the initial compositions of melts involved in the mixing process [e.g. b35-b36]. Noteworthy is the fact that such results cannot be obtained by using only classical Igneous Petrology techniques.
In spite of the fact that many advancements have been done in the understanding of the processes inducing the partial melting process and the mechanisms associated with the separation of melts from the source region and their successive interaction, several important problems still remain unsolved; their solution is of paramount importance for a complete characterization of the complex mechanisms acting during pre-, syn- and post-anatectic stages. Among the most important problems, the following are worth mentioning:
- regarding migmatitic complexes, even Grant [a35] stated that “Although we have been discussing partial melting of pelitic rocks, more of what we now see in a pelitic migmatite is probably attributable to re-crystallisation”; in other words, migmatitic complexes mostly represent an anatectic product which has been affected by further modifications;
- regarding experimental simulations, sample dimensions, assumptions about the studied chemical system, and the short duration of experiments make them difficult to be directly applied to natural contexts without a continuous and systematic cross-comparison between natural data and experimental results;
- regarding thermodynamic analysis, lacunae in database, in particular concerning properties of fundamental phases such as biotite and melt, place strong limitations on its quantitative value [es. a36];
- regarding geochemical modelling, the complexity of magma interaction processes, studied with classical petrological methods, has, de facto, made very difficult a systematic study of behavior of crustal anatectic melts from their genesis to their emplacement as plutonic or volcanic bodies.
One of the major limitations is the extreme paucity of information about real natural contexts (i.e. contexts that did not suffer slow crystallization processes). It follows that the original composition, amount, and micro-structural localization of anatectic melts during the very initial stages of the partial melting process are relatively known. In addition the construction of a conceptual model to understand the influence of interaction among anatectic melts on their geochemical composition during a) separation from the source region, b) migration through fracture networks and c) coalescence during emplacement, is still lacking.
The aim of this Research Project is to contribute to give a solution to the above mentioned problems developing a multidisciplinary and integrated study involving analyses, experiments, and modelling of the processes accompanying crustal anatexis by using as starting materials crustal enclaves, xenoliths, migmatites and outcrops of volcanic and plutonic rocks.

References not reported in the B Models
Brown M. 2007 Journal of the Geological Society, London, 164, 709–730 <<<