Prograde and retrograde reactions, garnet zoning patterns, and accessory phase behaviour in SW Finland migmatites, with implications for geochronology
Annika I. Nyström, Leo M. Kriegsman, 2003. "Prograde and retrograde reactions, garnet zoning patterns, and accessory phase behaviour in SW Finland migmatites, with implications for geochronology", Geochronology: Linking the Isotopic Record with Petrology and Textures, D. Vance, W. Müller, I. M. Villa
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Rates of melt production and segregation in migmatites can be assessed using geochronology of accessory phases. Here we report on the distribution and growth patterns of accessory phases and their coupling with major phase microtextures and chemical zoning patterns in garnet. We propose that migmatization in SW Finland involved partial melting and subsequent moderate retrograde re-equilibration. The latter process has three major effects: (i) it partially obscures geochemical signatures formed during earlier equilibrium or disequilibrium melting; (ii) it excludes the possibility of very fast (< 100 years per batch) melt segregation rates in migmatites where restite-melt back reaction has operated; (iii) trace element distributions between leucosome and host rock cannot be used to infer melt segregation rates.
Garnet MnO patterns show flat cores, suggesting high-grade equilibration, and sharply increasing concentrations at the rims, attributed to garnet-melt back reaction. Trace element patterns (Zr, Y and heavy rare earth elements) also document retrogression at the rims, but in addition preserve earlier growth zoning related to incongruent melting. We report retrograde zircon and xenotime growth associated with garnet resorption, here related to restite-melt back reaction in leucosome, melanosome and mesosome. Hence, geochronological studies of migmatite terranes should take into account that the youngest zircon material is not restricted to leucosome, but can also form overgrowths on corroded grains in melanosome and mesosome.
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Isotope geochemistry has produced many technical developments in the past decade or so that have revolutionized the potential information available on the tectonics of metamorphic belts from geochronology. These include the ability to date minerals and rocks on small spatial scales, scales that at last approach those from which other types of information — structural and petrological — are obtained. However, interpreting the new data, and their integration with the other datasets available, is not straightforward and requires careful chemical and textural observations that go hand-inhand with the geochronology. The increasing realization of the importance of this approach has led to a number of symposia at international conferences devoted to this topic in recent years. The set of papers in this book emanates from one such symposium and describes recent progress in integrating this new information with other datasets from metamorphic petrology on a mineral and sub-mineral scale.