Melts from continental crust at mantle depths: a metasomatic agent still preserved as melt inclusions in eclogite/pyroxenite garnets
Abstract
Glassy inclusions have been identified for the first time along with nanogranitoids in garnets of UHP eclogite of the Erzgebirge, Bohemian Massif. These eclogite s occur in lenses/blocks in diamond-bearing gneisses. Garnet, in the same microstructural position of the melt inclusions, contains also polycrystalline/palisade quartz inclusions surrounded by radial cracks, which are interpreted as quartz pseudomorphs after coesite. The melt inclusions, 5-25 μm in diameter, occur as clusters in the inner part of the garnet and range from glassy to polycrystalline, i.e. nanogranitoids. The glassy inclusions, generally smaller than the nanogranitoids, may contain a shrinkage bubble. The nanogranitoids mineral assemblage has been investigated with Raman spectroscopy and EDS mapping and is biotite, calcite, kumdykolite /rare albite , quartz/rare cristobalite, white mica, kokchetavite and variable amounts of graphite and glass. The melt is granitic, hydrous, high in alkalis and mildly peraluminous. The Cs, Rb, Pb, Th, U, Li, B and LREE enrichment of the melt suggests the involvement of a phengite, i.e. a crustal component, in the melt-producing reaction. To better constrain the melt composition re-homogenization experiments of nanogranitoids have been performed using a piston cylinder apparatus and multi-anvil press at 1000°C and 4 - 4.5 GPa - condition expected for the garnet formation and thus of melt entrapment. The composition of the melt trapped in garnets of the Erzgebirge eclogite has been compared with the one trapped in garnets of the Granulitgebirge garnet clinopyroxenite and they are similar in both major and trace elements. The Granulitgebirge garnet clinopyroxenites as a whole have been interpreted as the result of metasomatic interaction between an infiltrated external crustal melt and the peridotite. It is likely that metasomatism is responsible also for the formation of the Erzgebirge eclogite but at UHP conditions as confirmed by the presence of the former coesite inclusions.
These two case study represent unique examples of crust-mantle interaction during subduction of continental crust at mantle depth with the metasomatic agent preserved in the garnet as melt inclusion. This is one of the first study in which the nature and chemistry of the metasomatic agent has been directly measured.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V52B..07B
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRY;
- 8031 Rheology: crust and lithosphere;
- STRUCTURAL GEOLOGY;
- 8150 Plate boundary: general;
- TECTONOPHYSICS;
- 8178 Tectonics and magmatism;
- TECTONOPHYSICS