Linking rare earth element zoning in major and accessory minerals to better understand metasedimentary migmatites
Abstract
Trace element signatures from major and accessory minerals are increasingly used to place constraints on the ages of tectonothermal events and gain insights into processes operating in the deep continental crust. Minerals such as garnet, zircon or monazite represent sensitive recorders of events if a solid understanding exists of what trace element signatures represent. A study underway on granulite facies migmatites is providing insights into trace element signatures in garnet, and when integrated with in situ, microbeam analysis of zircon and monazite, is allowing a complex polymetamorphic history to be deciphered. The Larsemann Hills, east Antarctica, are composed of felsic orthogneisses and metasediments that were affected by high-grade metamorphism and partial melting at c. 7 kbar and >800 °C. Initially interpreted to have experienced a single cycle of metamorphism at c. 550-530 Ma, recent ages suggest that the area was affected by an earlier high-grade event at c. 900 Ma; the timing of migmatization relative to these two events remains uncertain. To address this, a detailed study has targeted metasedimentary migmatites. Garnet, zircon and monazite were analysed from melanosome, leucosome and selvage domains. Garnet (predominantly almandine-pyrope) preserves little, if any, major element zoning. In contrast, it records systematic within-grain variations in heavy rare earth element (HREE) concentration depending on textural context. Garnet within residuum domains displays broad cores with flat HREE slopes in chondrite-normalized plots, but are HREE-depleted (negative slopes) at boundaries adjacent to leucosome. In contrast, garnet boundaries adjacent to cordierite-rich selvages are typically HREE-enriched, with positive-sloping patterns. It is interpreted that core-rim depletions in HREE may reflect evolution of the whole-rock reservoir towards depletion during migmatization and garnet growth. Enrichment in garnet HREE adjacent to selvage domains likely resulted from local reaction between solids and melts either during melt transit and/or crystallization. Monazite, which is preferentially located in leucosome domains, preserves ages that reflect growth at c. 900 Ma and c. 530 Ma, although trace element signatures are ambiguous. However, zircon, which is preferentially located in selvage domains, preserves c. 530 Ma rims with depleted, flat to negatively sloping HREE patterns. It is interpreted that these zircon rims grew in, or near to equilibrium with garnet cores and rims during partial melting, and may date at least part of migmatite formation in the Larsemann Hills. They do not date the final crystallization of melts and development of selvage domains. Preliminary results of this study have shown that the trace element composition of garnet from high-temperature terrains can be a powerful tool in tracking metamorphic and melting processes where major elements zoning is no longer preserved. This study also demonstrates the utility of textural context when interpreting trace element patterns and ages from dateable accessory minerals in rocks from polycyclic metamorphic terrains.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V13G..02M
- Keywords:
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- 1065 GEOCHEMISTRY / Major and trace element geochemistry;
- 3619 MINERALOGY AND PETROLOGY / Magma genesis and partial melting;
- 3625 MINERALOGY AND PETROLOGY / Petrography;
- microstructures;
- and textures;
- 3660 MINERALOGY AND PETROLOGY / Metamorphic petrology