Layered anorthosite, chromiferous pyroxenite, and spatially associated mafic and felsic rocks of the 2.9 Ga Sittampundi layered complex (SLC), South India, underwent, high-grade metamorphism at ca. 2.5 Ga and were subjected to amphibolite-facies metamorphism accompanied by intrusion of granitoid plutons during late-Neoproterozoic teetonothermal activity (0.72-0.45 Ga). During the latter event, anorthosite developed millimeter- to centimeter-thick compositional layers rich in clinopyroxene+amphibole+clinozoisite+chlorite±chromite and corundum+spinel+chlorite. Tiny grains of högbomite replaced only the grains of corundum and spinel and are in textural equilibrium, with chlorite. The studied högbomite contains appreciable TiO2 (&gt;4.4 wt%) but insignificant NiO and ZnO. Cr2O3 content, reaches up to 0.35 wt% only in chromite-bearing samples. Systematic partitioning of Fe and Mg between högbomite and associated Fe-Mg minerals demonstrate attainment of chemical equilibrium among these phases. Integrating textural relations and algebraic analyses of the phase compositions, several reactions were constructed involving the associated oxide phases (spinel, corundum, högbomite), calcite, and silicates (amphibole, chlorite, anorthite, clinozoisite). Interpretation of the reaction reveals that (1) Mg2+ and Ti 4+ were mobile for more than 2 cm during the formation of högbomite and chlorite, and (2) chloritization of amphibole in the clinopyroxene+amphibole-bearing layers released Ti that was transported to the spinel+corundum-bearing layers to develop högbomite. Stability fields of some critical mineral assemblages in,P-XCo2 and T-Xco2 space combined with, geothermobarometry in, the associated rocks tightly constrain the growth of högbomite in the presence of aqueous fluids (X Co2 &lt; 0.15) to theP-Trange of 7 ± 1 kbar, 650 ±50 °C. These aqueous fluids, presumably derived from the Pan-African granitoid batholiths, chloritized amphibole grains, and transported the released Ti 4+ to the spinel+corundum-bearing layers to develop högbomite. Topological relations in isothermal-isobaric fugacity diagrams (log f o2-log fs2and log fo2 -log fH2o) in the system FeO-Al2O3TiO2-O2-S 2-H2O-CO2 (+MgO, Cr2O3) indicate that the stability and compositional characteristics of natural högbomite are strongly influenced by fo2fs2fH 2o, and concentrations of other soluble species (Ti, Mg, Cr, etc.) in the metamorphic fluids.

P SENGUPTA

U K BHUI

U DUTTA

D MUKHOPADHYAY

University of Calcutta (informally known as Calcutta University or CU) is a collegiate public state university located in Kolkata, West Bengal, India.&nbsp;Within India it is recognized as a &quot;Five-Star University&quot; and accredited &quot;A&quot; Grade by National Assessment and Accreditation Council.

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The University of Calcutta has secured the Fifth position among the Universities of India in the prestigious &quot;Indian University Ranking 2019&quot; list, released by the NIRF of the Ministry of Human Resource Development, Government of India.

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It was established on 24 January 1857, and was one of the first institutions in Asia to be established as a multidisciplinary and Western-style university. Its alumni and faculty include several heads of state, heads of government, social reformers, prominent artists, only academy award winner and Dirac medal winner in India, many Fellows of the Royal Society and five Nobel laureates as of 2019 which is highest in South Asia.

University Of Calcutta

American Mineralogist

The ca. 2.9Ga old Sittampundi layered complex (SLC) of South India belongs to a special group of deformed and metamorphosed Archaean rocks where chromitites are interlayered with anorthosite. Detailed field studies have revealed that the SLC comprises of a sequence of magmatic rocks consisting of clinopyroxenite, anorthosite and mafic rocks of tholeiitic composition. Minor BIF (Banded Iron Formations) intercalated with the mafic rocks are the only sedimentary rock within the SLC. Chromitite occurs as continuous to discontinuous bands (a few mm up to 6m thick) within clinopyroxenite and highly calcic anorthosite (An 99-100 ) but never in the enclosing mafic rocks. Chromite occurs broadly in three textural types: (a) cumulus grains in chromitite layers; (b) disseminated grains in clinopyroxenite and anorthosite; and (c) small grains included in clinopyroxene and plagioclase. A suite of felsic magma intruded the SLC at ca. 2.51Ga. Subsequently the entire rock sequence underwent polyphase deformation and metamorphism at ca. 2.50-2.45Ga (granulite facies) and at 0.72-0.50Ga (amphibolite facies). During the latter event, extensive alteration of clinopyroxene to amphibole and chlorite, of plagioclase to clinozoisite and of chromite to Cr-poor green spinel took place.Chromite compositions show large variations (Cr#=26-60, Mg#=4-45, Fe 3+ #=2-24) depending upon textural type and nature of the associated phases. The most conspicuous compositional variation took place when chromite was metasomatically replaced by green spinel (Cr#=3-21; Mg# 45-61) during amphibolite facies metamorphism. On a Cr# vs. Mg# plot, pristine magmatic compositions of chromite define an array that suggest increasing Cr# with concomitant decrease in Mg#, presumably during magmatic fractionation. Highly calcic plagioclase in anorthosite and Fe-Al rich chromite in chromitite in the SLC are consistent with a hydrous parental magma of tholeiitic composition. This hydrous magma was generated in an oceanic arc environment and crystallized close to the surface of the earth. In terms of mineralogy and field relations, chromitite-bearing rocks of the SLC show remarkable similarity with the Archaean anorthosite-hosted chromitite deposits in layered magmatic complexes at Fiskenaesset (Greenland) and Messina (South Africa) but contrasts sharply with the chromitite deposits of Proterozoic and Phanerozoic age. A change in the style of arc magmatism appears to be the controlling factor for marked change in chromite compositions across the Archaean-Proterozoic boundary. © 2011 Elsevier B.V.

Ore Geology Reviews

Magmatic and metamorphic imprints in 2.9Ga chromitites from the Sittampundi layered complex, Tamil Nadu, India

The Pan-African tectonothermal activities in areas near Sittampundi, south India, are characterized by metamorphic changes in an interlayered sequence of migmatitic metapelites, marble and calc-silicate rocks. This rock sequence underwent multiple episodes of folding, and was intruded by granite batholiths during and subsequent to these folding events. The marble and the calc-silicate rocks develop a variety of skarns, which on the basis of mineralogy; can be divided into the following types: Type I: wollastonite+clinopyroxene (mg#=71-73)+grandite (16-21 mol% Adr)+quartz+calcite, Type II: grandite (25-29 mol% Adr)+clinopyroxene (mg#=70)+alcite+quartz, and Type III: grandite (36-38 mol% Adr)+clinopyroxene (mg#=55-65)+epidote+scapolite+calcite+quartz. Type I skarn is 2-10 cm thick, and is dominated by wollastonite (&gt; 70 vol%) and commonly occurs as boudinaged layers parallel to the regional foliation Sn1 related to the Fn1 folds. Locally, thin discontinuous lenses and stringers of this skarn develop along the axial planes of Fn2 folds. The Type II skarn, on the other hand, is devoid of wollastonite, rich in grandite garnet (40-70 vol%) and developed preferentially at the interface of clinopyroxene-rich calc-silicates layers and host marble during the later folding event. Reaction textures and the phase compositional data suggest the following reactions in the skarns: 1. calcite+SiO2→wollastonite+V, 2. calcite+clinopyroxene+O2→grandite+SiO2+V, 3. scapolite+calcite+quartz+clinopyroxene+O2→grandite+V and 4. epidote+calcite+quartz+clinopyroxene+O2 →grandite+V Textural relations and composition of phases demonstrate that (a) silica metasomatism of the host marble by infiltration of aqueous fluids (XCO2&lt;0.15) led to production of large volumes of wollastonite in the wollastonite-rich skarn whereas mobility of FeO, SiO2 and CaO across the interface of marble and calc-silicate and infiltration of aqueous fluids (XCO2&lt;0.35) were instrumental for the formation of grandite skarns. Composition of minerals in type II skarn indicates that Al2O3 was introduced in the host marble by the infiltrating fluid. Interpretation of mineral assemblages observed in the interlayered metapelites and the calcareous rocks in pseudosections, isothermal P-XCO2 and isobaric T-XCO2 diagrams tightly bracket the 'peak' metamorphic conditions at c.9±1 kbar and 750°30°C. Subsequent to 'peak' metamorphic conditions, the rocks were exhumed on a steeply decompressive P-T path. The estimated 'peak' P-T estimates are inconsistent with the "extreme" metamorphic conditions (&gt;11 kbar and &gt;950°C) inferred for the Pan-African tectonothermal events from the neighboring areas. Field and petrological attributes of these skarn rocks are consistent with the infiltration of aqueous fluid predominantly during the Fn1 folding event at or close to the 'peak' metamorphic conditions. Petrological features indicate that the buffering capacity of the rocks was lost during the formation of type I and II skarns. However, the host rock could buffer the composition of the permeated fluids during the formation of type III skarn. Aqueous fluids derived from prograde metamorphism of the metapelites seem to be the likely source for the metasomatic fluids that led to the formation of the skarn rocks. © Springer-Verlag 2008.

Mineralogy and Petrology

Genesis of wollastonite- and grandite-rich skarns in a suite of marble-calc-silicate rocks from Sittampundi, Tamil Nadu: Constraints on the P-T-fluid regime in parts of the Pan-African mobile belt of South India

Corundum-leucosome-bearing aluminous gneiss from Ayyarmalai, Southern Granulite Terrain, India: A textbook example of vapor phase-absent muscovite-melting in silica-undersaturated aluminous rocks

Lithos

Anticlockwise evolution of ultrahigh-temperature granulites within continental collision zone in southern India

Journal of Structural Geology

Constrictive deformation in transpressional regime: field evidence from the Cauvery Shear Zone, Southern Granulite Terrain, India

Earth and Planetary Science Letters

First report of garnet–corundum rocks from southern India: Implications for prograde high-pressure (eclogite-facies?) metamorphism

Chemical substitutions, paragenetic relations, and physical conditions of formation of hÃ¶gbomite in the Sittampundi layered anorthosite complex, South India

Journal	American Mineralogist
Publisher	-
ISSN	0003-004X