At these sites, MIG2 migmatites developed, which are characterized by a higher degree of melt segregation than MIG3 migmatites and by microstructures indicating higher deformation intensities intermediate CPO of cordierite and SPO of biotite. In outcrops lacking strain heterogeneities, mainly MIG3 migmatites are observed, characterized by lower degrees of melt segregation and deformation.
In summary, melt segregation in migmatites of the Bayerische Wald was probably controlled by bulk composition and by strain partitioning as a result of bulk strength contrasts. The latter are probably dependent on pre-migmatitic structures and compositional heterogeneities that result in heterogeneous melt distribution.
The SPO of biotite and garnet, as well as the CPO and SPO of cordierite, indicate different mechanical behaviour of each of these three minerals in the different migmatite types. The change in mechanical behaviour is related to the degree of melt removal segregation from the melting sites mesosomes, melanosomes , expressed by the depletion of these sites in quartz and feldspar.
These results show that the melt fraction remaining at the melting sites and hence melt segregation control structures and deformation mechanisms of the minerals.
Index of Abstracts
In MIG1 migmatites, the euhedral to subhedral shapes and large grain sizes of cordierite, biotite and garnet, and the lack of CPO of cordierite and SPO of biotite exclude deformation and indicate melt-controlled growth of cordierite, garnet and biotite. In contrast to the undeformed MIG1 migmatites, MIG3 migmatites show a strong SPO of biotite and cordierite, but the CPO of cordierite is weak and there are no microstructural indications of intracrystalline plasticity i. Development of the strong SPO of biotite and cordierite, along with the absence of intracrystalline plasticity, can only be explained by passive rotation of solid particles in a weak matrix e.
If the weak matrix were also solid, deformed matrix minerals would be recognizable, which is not so with MIG3 migmatites. Hence, the weak matrix during the development of the strong SPOs of biotite and cordierite is inferred to have been a melt, as is also evident from petrological data.
The presence of melt in MIG3 mesosomes is in line with their very limited depletion in quartz and K-feldspar. Therefore, microstructures in MIG3 mesosomes are mainly melt controlled. Bulk structures, for example, the formation of K-feldspar- and quartz-dominated leucosomes, can be best explained by local melt coalescence during foliation development. With increasing deformation intensities, the connectivity of melt increased. The process corresponds to the development of an IWL in solid rocks foliation weakening; e. Jordan, ; Handy, Therefore, mesostructures in MIG3 migmatites are also melt controlled.
Concentration parameters are calculated from the U-stage data with eigenvalue methods; the degree of CPO increases with increasing values. A completely different situation is encountered in MIG2 and MIG4 migmatites, where considerable melt segregation has to be inferred.
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During melt segregation, a framework LBF must have been built up by the coexisting cordierite, biotite and garnet, as a result of the low percentage of melt remaining in mesosomes and melanosomes. Within an LBF, solid particles can only be deformed by a solid-state deformation mechanism, which is suggested by the evidence of intracrystalline plasticity of cordierite in MIG4 migmatites Figs 4 and 7.
Hence, microstructures in mesosomes and melanosomes of MIG2 and MIG4 migmatites are controlled by the mechanical behaviour of individual minerals.
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In summary, microstructures of the four migmatite types are both melt controlled and mineral controlled. Which process dominates depends on the degree of melt segregation. Melt-controlled behaviour is encountered at migmatite sites that are not considerably depleted in melt. Mineral-controlled behaviour is observed at sites of considerable melt removal. As outlined in the two previous sections, the melt volume fraction as a function of bulk composition, the degree of melt segregation and the deformation mechanisms of the minerals controlled the structural evolution of the migmatites from the Bayerische Wald.
A complex interplay between those factors led to the development of different migmatite types. MIG1 bodies had higher bulk strengths during partial melting, as a result of their homogeneous structure and perhaps slightly lower melt volume fractions when compared with the surrounding MIG3 migmatites. During deformation, the resulting strain partitioning into MIG3 migmatites enhanced melt segregation and the development of stromatic structures in MIG3 migmatites but impeded melt segregation and the development of deformation-related structures in MIG1 migmatites.
Melt was segregated from MIG3 mesosomes, but because of the lack of strain heterogeneities, a considerable amount of melt stayed in the mesosomes. Therefore, as in MIG1 migmatites, no LBF developed and cordierite and biotite growth was melt controlled, resulting in euhedral to subhedral shapes.
However, in contrast to MIG1 migmatites, deformation played a considerable role. In MIG2 and MIG4 migmatites, the comparatively large melt fractions and the position of MIG2 migmatites in sites of strain heterogeneities enhanced melt segregation into leucosomes and thus the development of stromatic structures. Melt removal from the sites of production mesosomes and melanosomes was effective to the degree that an LBF of minerals could form. Before melt segregation started, passive rotation of minerals in the melt took place, producing a strong SPO of biotite in both migmatite types.
This can also be deduced from aligned biotite inclusions in cordierite. Numerous studies have discussed the contribution of melt volume fraction to the bulk mechanical behaviour of partially molten rocks. Experiments e. Although melt volume fractions do not differ dramatically from each other, MIG1—MIG4 migmatite structures indicate very different mechanical behaviour.
This observation implies that there is no simple relationship between melt volume fraction and the mechanical behaviour of migmatites. Schematic line drawings of the possible structural evolution of the different migmatite types. In all line drawings, only four phases are distinguished for the sake of clarity: 1 quartz and feldspar; 2 biotite as the main reactant of the melt-producing reactions; 3 cordierite as the main solid product of the melt-producing reactions; 4 melt. On the left-hand side possible precursors of the migmatites are shown that differ in composition and structure.
In the centre, possible intermediate evolutionary stages are suggested. Several other observations on migmatites from the Bayerische Wald help to shed light on factors that also control the mechanical behaviour of migmatites, but that have not been or cannot be considered in experiments and theoretical models; for example, the effects of melt segregation, melt distribution and time. The obtained results indicate that the mechanical behaviour of migmatites during partial melting depends strongly on the degree of melt segregation.
As melt segregation is partly controlled by melt volume fraction, the mechanical behaviour of migmatites will probably change with time as melt volume fractions become larger in the course of prograde metamorphism. As long as the degree of melt segregation is low and much of the melt resides at its production site, IWL structures will dominate. As soon as considerable amounts of melt have been segregated, LBF structures can form in the restites.
The results obtained here indicate that melt segregation is favoured along strain heterogeneities occurring at sites of pronounced strength contrasts. The effect of strain localization on melt segregation has been described also from Namaqualand South Africa by Kisters et al.
Strain localization results in a heterogeneous distribution of melt and enhances bulk strength contrasts even more with time. In their analogue experiments, shear zone patterns and deformation sites were strongly controlled by melt distribution. As in the migmatites of the Bayerische Wald, the mechanical behaviour and melt distribution changed during melt segregation. In the proposed evolutionary scheme for migmatites of the Bayerische Wald Fig.
However, in nature, differences in bulk composition including fluids probably result in different solidus temperatures. Therefore, during prograde metamorphism, some parts of a migmatite protolith will start to melt earlier than others and this will also result in bulk strength contrasts. The observation that the bulk mechanical behaviour of migmatites may be controlled by the minerals e. MIG4 migmatites emphasizes the importance of the mechanical behaviour of individual minerals during deformation and partial melting. Various minerals can be produced in the course of dehydration melting e.
Therefore, the results obtained here cannot be applied to natural migmatites in general. More data on the mechanical behaviour of other minerals during partial melting and deformation need to be gained. Apart from time, all of the factors controlling the mechanical behaviour of natural migmatites melt volume fractions, bulk strength contrasts, degree of melt segregation, melt distribution, minerals are dependent on the composition and the compositional heterogeneity of the migmatite protolith.
Many migmatites have sedimentary or metamorphic precursors that are probably heterogeneous in terms of modes and bulk compositions on various scales. Therefore, as shown in the Bayerische Wald, distinct parts within a forming migmatite complex show different mechanical behaviour during partial melting and deformation.
Thus, the mechanical behaviour of an entire crustal section is very hard to assess and cannot be described by a single flow law, even if temperature, pressure and melt volume fractions are known. In the future, the combination of three different approaches may serve to derive quantitative models of the bulk mechanical behaviour of partially molten crustal sections, namely, 1 detailed mapping of migmatite areas, along with qualitative and quantitative microstructural observations, as presented in this study, 2 deformation experiments considering various relevant minerals and heterogeneous melt distribution in space and time, from which flow laws for melt—mineral mixtures in relation to melt segregation can be derived, and 3 numerical models that integrate different flow laws for various stages of partial melting.
Migmatites of the Bayerische Wald can be divided into four types MIG1—MIG4 based on geometric relationships, modes and microstructures of mesosomes, melanosomes and leucosomes.
Granulites and Migmatites
The degree of melt segregation is not only a function of melt volume fraction but is strongly enhanced by strain heterogeneities related to strength contrasts. Quantitative data on the SPO of biotite and the CPO of cordierite indicate that the deformation mechanisms of the minerals are mainly controlled by the degree of melt segregation. Passive rotation within interconnected weak layers of melt at low degrees of melt segregation MIG1 and MIG3 changes to deformation within a solid framework of minerals at high degrees of melt segregation MIG2 and MIG4.
The development of microstructures in migmatites of the Bayerische Wald, and thus their mechanical behaviour, is controlled by a complex interplay of melt volume fraction, melt segregation, melt distribution, bulk strength contrasts and mechanical properties of different minerals in a dynamic system. Therefore, the mechanical behaviour of migmatites during partial melting varies in space and time and cannot be modelled by a single flow law. Detailed mapping of migmatite areas, along with microstructural observations, deformation experiments considering heterogeneous melt distribution, and numerical models integrating different flow laws for various stages of partial melting, may serve to derive quantitative models for the bulk mechanical behaviour of a crustal section in the future.
Mark Handy, Ron Vernon and Jean-Louis Vigneresse are thanked for detailed and constructive reviews that helped to improve the quality of the paper. Simon Harley is thanked for having taken over the editorial handling of the manuscript and for numerous helpful comments. Thanks go to Rainer Altherr and Ian Fitzsimons for critical comments on the paper and its earlier versions. Oxford University Press is a department of the University of Oxford.
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Migmatite and metamorphism
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