Tectonically controlled fluid flow and water-assisted melting in the middle crust: An example from the Central Alps
Introduction
The formation of major proportions of silicate melt in the continental (middle) crust strongly depends on the availability of volatile components. Wherever major amounts of melt occur, for example in migmatite terrains, substantial amounts of such volatiles, notably H2O, are implied, and their source is an obvious question. Potential source rocks are those containing one or more hydrate phases, which at suprasolidus temperatures may break down to produce a partial melt.2 In the last decades research on migmatites has focused on migmatites that involve such hydrate-breakdown melting (e.g., Waters and Whales, 1984, Montel et al., 1992, Braun et al., 1996, Brown and Dallmeyer, 1996, Kalt et al., 1999, White et al., 2003, Harris et al., 2005). However, there is evidence for melt generation without dehydration reactions. The low porosity (≪ 0.01 vol.%) of metamorphic rocks in the middle crust allows only for very small melt fractions (< 1%) from fluids stored in situ. The origin of migmatites, which have high leucosome fractions but did not undergo hydrate-breakdown reactions, has to be related to water-assisted partial melting. This process has been considered as a possible cause for migmatite formation (Mogk, 1992, Butler et al., 1997, Viruete, 1999, Prince et al., 2001, Garlick and Gromet, 2004, White et al., 2005). One possible location of water-assisted melting is in contact metamorphic aureoles, where the source of water for partial melting has been related to the infiltration of fluids from a crystallizing pluton (e.g., Yardley and Barber, 1991, Berger and Rosenberg, 2003, Johnson et al., 2003, Johnson et al., 2004). However, water-assisted melting also occurs in large-scale migmatite terrains, which are characterized by masses of granitoid gneiss (e.g., Brown, 1979, Burri et al., 2005). Water-assisted melting is an important process in metagranitoids, which have bulk-rock compositions close to the minimum melt composition (Sawyer, 1998).
Fundamental questions related to these large-scale migmatite terrains are:
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In which geodynamic scenarios is fluid assisted melting possible? Such a scenario is difficult to imagine, because either fluids will direct generate melts, or fluid is produced below the solidus and will never hit the melting field.
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What is the petrology during fluid assisted melting? Recent experimental studies and models have improved our understanding of the process of hydrate-breakdown melting, but the phase relations during water-assisted melting as addressed by Castro et al. (2000) and Gardien et al. (2000) are less well known.
This paper reports on phase relations, structures and the fluid evolution in migmatites of the Central Alps (Switzerland, N-Italy). This migmatite belt is well suited for this kind of investigation, because it has zones of mixed lithologies and much is known about its spatial dimensions and internal characteristics (Burri et al., 2005).
In order to have some new insights, we compare detailed observations in the Alpine migmatites with different modelling approaches. The first part of the paper describes the petrography and petrology of migmatites in the Central Alps, with special emphasis on amphibole-bearing migmatites. The next section summarizes the geodynamic and tectonic situation of the Alps during water-assisted melting. This tectonic scenario is used to construct a simple fluid flow model, based on which some numerical results are presented. Finally, we discuss implications of our observations and of these model results with respect to the evolution of the Alpine migmatite belt.
Section snippets
Methods
Whole-rock major elements compositions were determined using conventional XRF methods on glass pellets. Major element equilibration was calculated using TWQ (Berman, 1991). We use phase diagrams and phase variation diagrams computed with the program THERIAK and DOMINO (de Capitani and Brown, 1987), in combination with the Holland and Powell (1998) database and updates for melt bearing systems (Holland and Powell, 2001, White et al., 2001). Leucosome volumes were estimated in 2D using field
Geological setting and field observations
In the Alps, classic basement thrust sheets can be distinguished from heterogeneous tectonic mélange units (e.g. Adula-, Cima Lunga-, Someo- and Orselina-units) that contain eclogite relics (Fig. 1; Heinrich, 1982, Trommsdorff, 1990, Engi et al., 2001). The Southern Steep Belt (SSB) of the Central Alps bears the most convincing evidence of Alpine anatexis (Wenk, 1970, Burri et al., 2005). The zone of anatexis is spatially associated with aplites and small granites of continental origin.
Geodynamic setting during water-assisted melting in the Alps
Water-assisted melting requires two things: an efficient source of water and a force that drives fluids into the rock at the P and T conditions of interest. Fluids can only be released and mobilized at conditions below the solidus, because fluids would otherwise be used up in situ to produce melt. If the permeability distribution is homogeneous and fluid flow is solely driven by density differences due to temperature variations, a vertical flow component would be promoted. Thus an additional
Fluid sources and fluid production
Several potential fluid sources exist. We first discuss the most promising ones, and those of minor importance are addressed at the end of this section. A major possibility for a fluid source is metamorphic dehydration reactions at conditions cooler than where the first melts are produced (see also section on thermal history). The amount of dehydration fluid produced depends on the rock-type and the P–T path (e.g. Thompson, 2001, Burri, 2005). We have used mineral-equilibrium modelling to
Summary and discussion
The combination of petrological and structural observations indicates that water-assisted melting is an important feature in the Tertiary migmatite belt of the Central Alps. Metapelitic rocks incorporated in the SSB are not a realistic fluid source, because, at the conditions of partial melting, the released aqueous fluid would lead to in situ partial melting and no fluid would be released into the directly adjacent or overlying rocks. A major fluid source directly below the migmatite belt can
Acknowledgements
We thank M. Brown and F. Bussy for constructive and careful reviews. Schweizerischer Nationalfonds has supported our research over several years (2000-055306.98, 20-63593.00, 20020-101826, and 200020-109637). Mike Brown also introduce the term hydrate-breakdown melting. He is further acknowledged for this important improvement of terminology.
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