• A Bhattacharya

      Articles written in Journal of Earth System Science

    • A HFSE- and REE-enriched ferrodiorite suite from the Bolangir Anorthosite Complex, Eastern Ghats Belt, India

      M Raith A Bhattacharya S Hoernes

      More Details Abstract Fulltext PDF

      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 regionalP-T regime led to extensive development of coronitic garnet at the interface of plagioclase phenocrysts with the mafic matrix assemblage (opx + fay + cpx + ilm ± amph, bio). Abundant accessory phases are zircon, apatite and thorite. The mafic phases have extremely ferrous compositions (XFe gar: 0.93-0.87, fay: 0.90-0.87, opx: 0.80-0.60, cpx: 0.70-0.47, amph: 0.81-0.71) reflecting the low Mg-number (16-8) of the rocks. Compared to worldwide occurrences of similar rocks, the Bolangir ferrodiorites (SiO2 36–58 wt.%, FeO*: 39-10 wt.%) are characterized by exceptionally high concentrations of HFSE and REE (TiO2: 4.8-1.0 wt.%, P2O5: 1.7-0.5 wt.%, Zr: 5900-1300 ppm, Y: 240-80 ppm, La: 540-100 ppm, Ce: 1100-200 ppm, Yb: 22-10 ppm, Th: 195-65 ppm). Well defined linear variation trends for major and trace elements reflect progressive plagioclase accumulation towards the felsic members of the suite. The ferrodiorites are interpreted to represent residual liquids of anorthosite crystallization which after segregation and extraction from the ascending diapir became enriched in HFSE and REE through selective assimilation of accessory phases (zircon, monazite, apatite) from crustal felsic melts. Ferromonzodioritic rock presumably formed through hybridization between the ferrodiorite and overlying felsic melts.

    • The Turkei anorthosite complex revisited

      A K Maji A Bhattacharya M Raith

      More Details Abstract Fulltext PDF

      Geological investigation in recent years reveals that the anorthosite-leuconorite massif (81 sq km) is much larger than known from previous studies. The massif is bordered by a suite of garnetiferous felsic rocks comprising quartz monzonite gneiss, granite gneiss and megacrystic K-feldspar-bearing granite. Ferrodiorites, hitherto unknown from this area, occur as veins at the massif-felsic suite interface, and as rare apophyses within leuconorites at the massif margin. The massif and the bordering felsic rocks were presumably emplaced during the earliest of the three phases of folding documented by the metasedimentary gneisses that host the massif.

      The petrographic and geochemical characteristics suggest that the low-K anorthosite-leuconoriteferrodiorite suite does not share a common parentage with the bordering high-K felsic intrusives. The anorthosites and leuconorites were derived by polybaric fractionation of mantle-derived melts. The ferrodiorites are anorthosite residual melts that were not entirely segregated from the host solids. By contrast, the granite gneisses and granites originated by incongruent melting of crustal rocks. The chemical differences between quartz monzonite and granite gneisses point to their derivation from different crustal precursors.

    • A reappraisal of polymetamorphism in the Eastern Ghats belt — A view from north of the Godavari rift

      A Bhattacharya S Gupta

      More Details Abstract Fulltext PDF

      Evidence collated from different parts of the Eastern Ghats belt north of the Godavari rift (barring the “Western Charnockite Zone” ) indicates that this sector evolved through a series of compressive structures (F1 to F3), with prolific migmatization in quartzofeldspathic and metapelitic gneisses synchronous with F1 shortening, as was the syn-F1 emplacement of profuse megacrystic K-feldspar-bearing granitoid bodies. Thereafter, melt productivity of the rocks (synchronous withF2– F3 folding) sharply decreased. Mineral parageneses stable in the S1, S2 and S3 fabrics indicate persistence of granulite facies conditions. P-T estimates on orthopyroxene + garnet + plagioclase + quartz assemblages anchored to recrystallized mosaic that overgrow all penetrative fabric elements in mafic granulites, granitoids and quartzofeldspathic gneisses are in the range of 900‡-950‡C and P≅ 8–9 kbar. This estimate is comparable to those retrieved from sapphirine-bearing paragenesis in Mg-Al metapelites that appear to be diachronous in relation to the fabric elements, and arguably disrupt the granoblastic mosaic. These facets in the northern sector of the orogenic belt are compatible with either a single cycle of tectonic events (i.e., F1, F2 and F3 in continuum), or temporally-separate thermo-tectonic events, with the peak of earlier metamorphism (pre- to syn-F1) at lower temperature (in the granulite facies) in comparison to the record of high post-F3-Tmax values.

      It is suggested on the basis of the above evidence that the late Proterozoic/Pan-African granulites in the Eastern Ghats belt north of the Godavari rift, are unlikely to be reworked equivalents of any older granulitic crust, such as the ∼1.6 Ga granulites south of the rift. Instead, the temporally disparate sectors may represent different crustal segments with unconnected pre-amalgamation tectonic history. However, if the ∼ 1.6 Ga granulites of the Western Charnockite Zone continue northwards across the rift, as suggested by recent isotope data, there are serious doubts as to the validity of a north-south division within the Eastern Ghats belt.

  • Journal of Earth System Science | News

    • Editorial Note on Continuous Article Publication

      Posted on July 25, 2019

      Click here for Editorial Note on CAP Mode

© 2017-2019 Indian Academy of Sciences, Bengaluru.