S Hoernes
Articles written in Journal of Earth System Science
Volume 106 Issue 4 December 1997 pp 299-311
M Raith A Bhattacharya S Hoernes
The massif-type anorthosite complex at Bolangir in the northern part of the Eastern Ghats belt occurs in a milieu of predominantly supracrustal granulite-grade rocks. The massif is separated from the host gneisses by coarse-grained garnetiferous granitoid gneisses which are interpreted as coeval crustallyderived melts. Melanocratic ferrodiorite rocks occur at the immediate contact with the anorthosite massif which they intrude in cross-cutting dikes and sheets. The emplacement age of the anorthosite diapir and the associated igneous suites is deemed to be pre-D2. Recrystallization of the igneous assemblages of the ferrodiorite suite (750–800°C, 7–8kbar,$$a_{H_2 O}< 0.25, logf_{O_2 }< - 18 bar$$) during a period of near-isobaric cooling from the igneous crystallization stage to the regional
Volume 113 Issue 4 December 2004 pp 619-648
S Sensarma S Hoernes D Mukhopadhyay
New mineralogical, bulk chemical and oxygen isotope data on the Palaeoproterozoic Bijli Rhyolite, the basal unit of a bimodal volcanic sequence (
The Bijli melt is neither formed by fractionation of a basaltic magma, nor does it represent a fractionated crustal melt. It is shown that the mantle-derived high temperature basaltic komatiitic melts/high Mg basalts triggered crustal melting, and interacted predominantly with deep crust compositionally similar to the Average Archaean Granulite (AAG), and a shallower crustal component with low CaO and Al2O3 to give rise to the hybrid Bijli melts. Geochemical mass balance suggests that ∼ 30% partial melting of AAG under anhydrous condition, instead of the upper continental crust (UCC) including the Amgaon granitoid gneiss reported from the area, better matches the trace element concentrations in the rocks. The similar Ta/Th of the rhyolites (0.060) and average granulite (0.065) vs. UCC (0.13) also support a deep crustal protolith. Variable contributions of crust and mantle, and action of hydrothermal fluid are attributed for the spread in δ18Owhole-rock values. The fast eruption of high temperature (∼ 900°C) rhyolitic melts suggests a rapid drop in pressure of melting related to decompression in an extensional setting.
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