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The physico-chemical conditions of crystallization of the Grenvillian arfvedsonite granite of Dimra Pahar, Hazaribagh, India: constraints on possible source regions
A. Basak,
Published in Springer
Volume: 114
Issue: 4
Pages: 329 - 356
The Dimra Pahar Pluton is composed of arfvedsonite ± aegirine - alkali-feldspar granites with homophanous and mylonitic features. The pluton was emplaced along a regional shear zone during the post-collisional stage of the orogeny. Arfvedsonite and aegirine crystallized after alkali feldspar and quartz. The rocks are slightly peraluminous (alkali-index – AI: 0.92) to peralkaline (AI: 1.11). The pluton was emplaced at shallow depth at around 2–3 kbar pressure. The zircon saturation temperature varies between 889 and 1071 °C which is consistent with the liquidus temperature of the granitic melt. However, somewhat lower temperatures of crystallization of zircons (822–836 °C) were obtained by Ti-in-zircon thermometer. Two feldspar thermometry yields crystallization temperature ranging from 647 to 705 °C. F-free arfvedsonite crystallized at around 670 °C. Aegirine also crystallized around 670 °C. Arfvedsonite and aegirine crystallization indicates near solidus temperature of the magma. Zr-poor nature of both arfvedsonite and aegirine and presence of discrete zircon crystals suggest high oxygen fugacity of the magma. During the crystallization of zircon (near liquidus condition of the magma), the oxygen fugacity fluctuated around QFM buffer (ΔQFM = −1.22 to +1.29) as indicated by the Ce-anomalies in zircon. Later on, during the crystallization of arfvedsonite (near solidus condition of the magma), the granitic melt achieved oxygen fugacity condition above QFM buffer as indicated by the high Fe3+/(Fe2++Fe3+) ratios (0.2–0.23) of arfvedsonites. High Si/(Na + Fe + Si) ratios (0.52–0.57) of these arfvedsonites also suggest that oxygen fugacity reached above the NNO buffer during the later stage of crystallization. Pseudosection analysis of the studied rocks suggests that the mineral assemblage was equilibrated within the temperature range from 505 to 515 °C and oxygen fugacity range from −19 to −23 log units (bar), i.e. above NNO-buffer. The nearly Ti-free aegirines crystallized in highly oxidized condition near HM buffer. The development of coronal aegirine around arfvedsonite indicates the crystallization following an oxidizing path, towards the HM buffer. Water content of the granitic melt during crystallization of quartz and feldspar is estimated to be around 6 wt%. During the crystallization of arfvedsonite water content of the melt came down to 0.02 wt%. The high oxidation state and water saturated parental melts of the Dimra Pahar granite require a highly oxidized, hydrated source for the melts. A subduction-modified, metasomatically enriched lithospheric mantle or asthenospheric mantle can be considered as the source for the basaltic parental melt of Dimra Pahar granite. © 2020, Springer-Verlag GmbH Austria, part of Springer Nature.
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JournalData powered by TypesetMineralogy and Petrology
PublisherData powered by TypesetSpringer