The origin of Ba-rich mica and alkali feldspar from the Ponte Nova alkaline mafic-ultramafic massif (Southeastern Brazil) explained by major-and trace-elements and isotopic analyses
Abstract
The Ponte Nova alkaline mafic-ultramafic massif (87.6 Ma) SE Brazil was formed by different magmatic pulses of basanite magma in a chamber environment. The rocks are potassic, with normative nepheline and olivine, and they range in composition from olivine clinopyroxenites and olivine melagabbros to nepheline-bearing monzonites. The massif exhibits cumulate, porphyritic and equigranular textures. The textural features are evidential of hybrid magma, and the presence of partially resorbed crustal xenoliths indicate that open system processes occurred during the evolution of the massif. Whole-rock isotope data (87Sr/86Sri =0.70432-0.70641 and 143Nd/144Ndi =0.512216-0.512555) indicate that each magmatic pulse of the massif represents different levels of crustal contamination. Ba-rich mica (Phl50-80Ann50-61) and alkali feldspar (Ab3-58An0-17Or33-97) are intercumulus phases in olivine melagabbros, interstitial phases in monzogabbros or well-shaped macrocrysts in monzonite, the most crustally-contaminated pulse of the massif. The BaO contents reach up to 9 wt.% in alkali feldspar and 7 wt.% in mica of the monzonite. Ba concentration profiles in alkali feldspar macrocrysts indicate crystal growth by fractional crystallization (increasing Ca/Sr and decreasing Ba/Sr). This produces crystals with Ba-rich cores and Ba-poor rims with a step function decrease in Ba in the middle that suggests changing crystallization conditions. Some alkali feldspar crystals also show enrichment in Ba in the intermediate zone. Ba is positively correlated to An content, Sr (242-3782 ppm) and Ti (274-2020 ppm) and is negatively correlated to Pb (3-23 ppm) and Rb (3-193 ppm). Ti-rich mica crystals with Ba-rich cores also display step-like Ba zoning. The entrance of Ba into the site I is related to the increase of Al2O3 content and the decrease of K2O and SiO2 content of the mineral. Micas from Ponte Nova massif and of country rock form the same compositional trend with respect to Mn (357-2451 ppm), Zn (68-521 ppm) and Ni (11-657 ppm) versus Mg# of the micas. In situ isotopic analyses of alkali feldspar zones is the next step in our strategy to investigate whether the origin of the Ba-rich minerals of the Ponte Nova massif is associated with the crustal contamination processes.
CNPq (205705/2018-9), CAPES, FAPESP (2017/03768-8)- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V23E0232A
- 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