Journal ArticleDOI
Genesis of the Skagit Gneiss migmatites, Washington, and the distinction between possible mechanisms of migmatization
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Some possible chemical and morphological criteria to distinguish different migmatization mechanisms are considered and examples of the different processes discussed in this article, and although the temperature of metamorphism was high (<700 °C), an anatectic model is rejected, as in earlier studies.Abstract:
Some possible chemical and morphological criteria to distinguish different migmatization mechanisms are considered and examples of the different processes discussed Leucosomes formed by anatexis are likely to contain K-feldspar and should have a more sodic plagioclase than the restite rocks Metamorphic segregation can cause a slight fractionation of albite into the leucosome only Anatectic leucosomes might form irregular bodies or might agmatize the restite; close-spaced planar veins are improbable Leucosome veins formed by hydrothermal processes are likely to be coarse grained or pegmatitic, whereas water-rich anatectic melts may freeze readily to aplites
The criteria developed are applied to the Skagit Gneiss migmatites, and although the temperature of metamorphism was high (<700 °C), an anatectic model is rejected, as in earlier studies Much of the migmatization that accompanied regional deformation and metamorphism was due to metamorphic segregation Many other leucosomes are believed to be related to an extensive suite of plagioclase pegmatite dikes, probably of hydrothermal origin, that were emplaced at a late stage These dikes are of uniform composition (plagioclase ≃ An20), irrespective of the host rock, and the plagioclases of some feldspathized schists tend toward this composition
Recent fluid-inclusion studies have shown that high-grade rocks have a mixed CO2-H2O fluid Therefore, partial melting is unlikely to begin until temperatures well in excess of the water-saturated granite solidus are reached Melts so formed might readily coalesce and migrate out of the source rock without producing an extensive migmatite zone Many migmatite terranes may have formed by predominantly hydrothermal processes, notably metamorphic segregation, and it is possible that this might represent a particular type of tectonic environmentread more
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Journal ArticleDOI
Criteria for the recognition of partial melting
TL;DR: Partial melting changes rocks from single phase (solid) to two-phase (solid+melt) systems as discussed by the authors and this effect raises the rate of deformation and heat transfer, as well as causing crustal differentiation.
Journal ArticleDOI
Partial melting, partial melt extraction and partial back reaction in anatectic migmatites
TL;DR: In this paper, the final textures, mineral modes and mineral chemistries are affected by four successive processes: (i) prograde partial melting and small-scale segregation into melt-rich domains and restitic domains; (ii) partial melt extraction; (iii) partial retrograde reactions (back reaction) between in situ crystallizing melt and the restite; (iv) crystallization of remaining melt at the solidus, releasing volatiles.
Journal ArticleDOI
Theory of metamorphic segregation and related processes
TL;DR: In this article, the authors show that the formation and growth of bands or domains of different bulk compositions within an originally unbanded rock can result from an instability arising in some deforming rocks when diffusion transfer is significant.
Journal ArticleDOI
On the role of diapirism in the, segregation, ascent and final emplacement of granitoid magmas
TL;DR: In this paper, it was shown that many granitoids can ascend through the crust as mushes with at least 50% melt, and that such magmas are rheologically able to ascend through a thickness of crust.
Journal ArticleDOI
Petrology of basement-dominated terranes: I. Regional metamorphic T–t path from U–Pb monazite and Sm–Nd garnet geochronology (Central Damara orogen, Namibia)
Stefan Jung,Klaus Mezger +1 more
TL;DR: In this paper, the authors used thermobarometric data to identify the peak metamorphic conditions of metapelites, migmatites and granites from the high-grade basement-dominated part of the Damara orogen.