Microstructural Constraints on Tonalitic Melt Extraction from Gabbroic Mushes
Abstract
The physics of melt extraction under crustal conditions is poorly understood. To better characterize the controls on pore spaces reduction of crystal-rich mushes, we conducted petrographic, Electron Backscatter Diffraction (EBSD), and QEMScan analyses on oriented samples of amphibole-gabbro-norite cumulates and related daughter tonalite collected from the deep exposures of the Famatinian Arc, Argentina. The Famatinian arc is a Gondwanan paleo-arc that has been rotated and uplifted due to a combination of terrane accretion and Andean uplift.
Despite the relatively equilibrated grain shape of plagioclase crystals, the samples preserve magmatic fabric and a mild shape preferred orientation (SPO). Both petrographic analyses and EBSD show rare low angle boundaries and sparse evidence of recrystallization (e.g. formation of subgrains or core-mantle structures). Furthermore, the quartz in the tonalites records similar dislocations as the plagioclase in the amphibole-gabbro-norite cumulates. Orientations of plagioclase obtained from the EBSD analyses reveal the presence of both lineations and foliations in the amphibole-gabbro-norites. Additionally, the lack of asymmetrical zoning patterns in plagioclase crystals suggest that the crystals may not have experienced any dissolution-reprecipitation reactions. The combined deformation information obtained from detailed microstructural analyses suggests that viscous compaction was not a dominant factor in the reduction of pore space and extraction of tonalitic melt from the gabbro mush. The dislocations present in both the tonalite and gabbro indicate that viscous deformation occurred after melt extraction. When considering both the orientation data and the deformation results, we propose that several different processes (e.g. magmatic currents and mechanical mush reorganization by intrusion of new material) may have produced the observed fabric and initiated extraction of the interstitial tonalitic melt.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V14A..06M
- Keywords:
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- 1036 Magma chamber processes;
- GEOCHEMISTRY;
- 1199 General or miscellaneous;
- GEOCHRONOLOGY;
- 7299 General or miscellaneous;
- SEISMOLOGY;
- 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY