Improving the link between accessory phase chronometers and petrological information
2003. "Improving the link between accessory phase chronometers and petrological information", Geochronology: Linking the Isotopic Record with Petrology and Textures, D. Vance, W. Müller, I. M. Villa
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Monazite is the mineral of choice in pelitic rocks for providing time constraints on metamorphic rocks and metamorphic processes. However, unlike rock-forming mineral chronometers such as garnet, the petrogenesis of monazite is relatively poorly understood. Consequently, although it is possible to generate precise monazite ages, the significance of the age in metamorphic rocks is often uncertain. In this contribution, we show how the petrogenesis of monazite can be linked to pressure and temperature information. Four complementary approaches, each illustrated by examples, are discussed: (i) the textural relationships of accessory minerals are used to relate the petrogenesis of monazite to that of the rock-forming mineral assemblage, and through this to P-T; (ii) monazite composition, in particular Y content, is used to relate monazite to the rock-forming mineral assemblage, and thus, to P-T; (iii) the bulk compositional control on monazite stability has been empirically determined and this relationship allows the temperature of initial monazite growth to be estimated in a given bulk composition; (iv) monazite-xenotime thermometry is utilized to provide estimates of the temperature of monazite growth. Either individually or combined, these approaches successfully enable monazite age data to be placed in a P-T framework.
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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.