Moumita Talukdar

Abstract Chromite deposits associated with layered anorthosite complexes in the Archaean high-grade terranes are rare in the world. The late Archaean Sittampundi Layered Magmatic Complex, Tamil Nadu, India is one of the few such deposits in the world where layers of Fe-Al rich chromites are associated with extremely calcic (An>95) anorthosite. ‘Frozen in’ magmatic mineralogy of the chromitite and the enclosing anorthosite suggest successive crystallization of chromite + clinopyroxene and chromite + clinopyroxene + anorthite from a hydrous Al-rich basaltic melt that was emplaced in a suprasubduction zone setting. Intense deformation and upper amphibolite facies metamorphism at ∼2.45 Ga converted the magmatic assemblages to hitherto unreported hornblende + gedrite + Mg-Al rich spinel ± chlorite bearing assemblages. During metamorphic reconstitution, chromite was pseudomorphically replaced by green spinel in the domains rich in secondary amphiboles. Mass-balance calculation and algebraic analyses of the observed mineralogy suggest that a number of chemical species including chromium became mobile during the formation of spinel pseudomorph in response to infiltration driven metamorphism. Aluminium became mobile in the length scale of chromite grain but remained immobile in the length scale of a thin section.

South Purulia Shear Zone (SPSZ) of the East Indian shield area separates two Palaeoproterozoic terrains-the high grade Chotanagpur Granite Gneiss Complex (CGGC) in the north and the low-grade (greenschist-amphibolite facies) North Singhbhum Fold Belt (NSFB) in the south. In the SPSZ, the major rock types include alkaline–carbonatite rocks (Carbonatite, apatite deposits, ultramafic rock-Nephiline syenite, and alkali granite), quartzite and acid tuffs with minor mafic dykes. These rocks are folded and sheared in both ductile (early phase) and brittle (late phase) regimes. Brittle shearing is manifested by brecciation of the ductile deformed quartzite and apatite deposits. Extensive alteration of carbonatite, ultramafic rocks and granite has produced the thick kaolinite deposits that occur as vertical sheets. Along the brecciated zones in apatite deposits and quartz veins, massive goethite forms veins and network around apatite and quartz clasts respectively. Within the massive goethit...

Chromite deposits associated with layered anorthosite complexes in the Archaean high-grade terranes are rare in the world. The late Archaean Sittampundi Layered Magmatic Complex, Tamil Nadu, India is one of the few such deposits in the world where layers of Fe-Al rich chromites are associated with extremely calcic (An>95) anorthosite. 'Frozen in' magmatic mineralogy of the chromitite and the enclosing anorthosite suggest successive crystallization of chromite + clinopyroxene and chromite + clinopyrox-ene + anorthite from a hydrous Al-rich basaltic melt that was emplaced in a suprasubduction zone setting. Intense deformation and upper amphibolite facies metamorphism at $2.45 Ga converted the magmatic assemblages to hitherto unreported hornblende + gedrite + Mg-Al rich spinel ± chlorite bearing assemblages. During metamorphic reconstitution, chromite was pseudomorphically replaced by green spinel in the domains rich in secondary amphiboles. Mass-balance calculation and alge...

The Palghat-Cauvery Shear Zone (PCSZ) of the Granulite Terrane of South India is perceived to be a late Neoproterozoic transcontinental shear zone that develops extensive mylonite and sheath folds. The Mahadevi Layered Complex (MLC), the area of present study, is situated in the eastern part of the PCSZ. The studied area exposes vast expanse of felsic orthogneisses with large enclaves of mafic granulite interlayered with meta-BIF. Our study records two distinct deformational events (D 1-2) that produced E-W closing regional folds with gently curved fold axis. The curved fold axes results in the regional dome-and-basin structure. Kinematic markers suggest that a weak shear couple was accompanied with the D 2 deformation. Preponderance of the granoblastic fabric in the rocks, restriction of mylonites only in local conjugate shear zones, and the structures produced during D 1-2 deformations are at variance with the view that (a) existence of a sheath fold as previously reported from the area and (b) the PCSZ marks the zone of intense shear deformation. Preliminary petrological information from the studied area shows that D 1-2 deformational events were accompanied by high-pressure granulite-facies metamorphism that was followed by a steeply decompressive retrograde P-T path. The U-Pb zircon ages from the felsic orthogneiss fixes the date of emplacement of its magmatic protolith and the regional high-pressure metamorphism at ≥2,530 and ca. 2,479 Ma, respectively. Integrating all the information it is suggested that the evolution of the MLC occurred in a continent-continent/arc collisional setting at the dawn of the Proterozoic Eon. This study does not support the existence of a "mega sheath fold" in the MLC as has been published in the recent literature. KEYWORDS continent-continent/arc collision, Early Palaeoproterozoic, Granulite Terrane of South India, high-pressure metamorphism, Mahadevi Layered Complex, structure, U-Pb zircon chronology

The Palghat-Cauvery Shear Zone (PCSZ) of the Granulite Terrane of South India is perceived to be a late Neoproterozoic transcontinental shear zone that develops extensive mylonite and sheath folds. The Mahadevi Layered Complex (MLC), the area of present study, is situated in the eastern part of the PCSZ. The studied area exposes vast expanse of felsic orthogneisses with large enclaves of mafic granulite interlayered with meta-BIF. Our study records two distinct deformational events (D 1-2) that produced E-W closing regional folds with gently curved fold axis. The curved fold axes results in the regional dome-and-basin structure. Kinematic markers suggest that a weak shear couple was accompanied with the D 2 deformation. Preponderance of the granoblastic fabric in the rocks, restriction of mylonites only in local conjugate shear zones, and the structures produced during D 1-2 deformations are at variance with the view that (a) existence of a sheath fold as previously reported from the area and (b) the PCSZ marks the zone of intense shear deformation. Preliminary petrological information from the studied area shows that D 1-2 deformational events were accompanied by high-pressure granulite-facies metamorphism that was followed by a steeply decompressive retrograde P-T path. The U-Pb zircon ages from the felsic orthogneiss fixes the date of emplacement of its magmatic protolith and the regional high-pressure metamorphism at ≥2,530 and ca. 2,479 Ma, respectively. Integrating all the information it is suggested that the evolution of the MLC occurred in a continent-continent/arc collisional setting at the dawn of the Proterozoic Eon. This study does not support the existence of a "mega sheath fold" in the MLC as has been published in the recent literature. KEYWORDS continent-continent/arc collision, Early Palaeoproterozoic, Granulite Terrane of South India, high-pressure metamorphism, Mahadevi Layered Complex, structure, U-Pb zircon chronology

South Purulia Shear Zone (SPSZ) of the East Indian shield area separates two Palaeoproterozoic terrains-the high grade Chotanagpur Granite Gneiss Complex (CGGC) in the north and the low-grade (greenschist-amphibolite facies) North Singhbhum Fold Belt (NSFB) in the south. In the SPSZ, the major rock types include alkaline–carbonatite rocks (Carbonatite, apatite deposits, ultramafic rock-Nephiline syenite, and alkali granite), quartzite and acid tuffs with minor mafic dykes. These rocks are folded and sheared in both ductile (early phase) and brittle (late phase) regimes. Brittle shearing is manifested by brecciation of the ductile deformed quartzite and apatite deposits. Extensive alteration of carbonatite, ultramafic rocks and granite has produced the thick kaolinite deposits that occur as vertical sheets. Along the brecciated zones in apatite deposits and quartz veins, massive goethite forms veins and network around apatite and quartz clasts respectively. Within the massive goethite deposits, angular clasts of apatite and quartz are replaced by goethite. Interpreting the information from field and petrography it has been demonstrated that the massive goethite deposits were formed by a low temperature (below 200oC), acidic, circulating hydrothermal fluid. Fluid migration was facilitated by brittle fractures.

The Palghat‐Cauvery Shear Zone (PCSZ) of the Granulite Terrane of South India is perceived to be a late Neoproterozoic transcontinental shear zone that develops extensive mylonite and sheath folds. The Mahadevi Layered Complex (MLC), the area of present study, is situated in the eastern part of the PCSZ. The studied area exposes vast expanse of felsic orthogneisses with large enclaves of mafic granulite interlayered with meta‐BIF. Our study records two distinct deformational events (D1–2) that produced E‐W closing regional folds with gently curved fold axis. The curved fold axes results in the regional dome‐and‐basin structure. Kinematic markers suggest that a weak shear couple was accompanied with the D2 deformation. Preponderance of the granoblastic fabric in the rocks, restriction of mylonites only in local conjugate shear zones, and the structures produced during D1–2 deformations are at variance with the view that (a) existence of a sheath fold as previously reported from the area and (b) the PCSZ marks the zone of intense shear deformation. Preliminary petrological information from the studied area shows that D1–2 deformational events were accompanied by high‐pressure granulite‐facies metamorphism that was followed by a steeply decompressive retrograde P‐T path. The U–Pb zircon ages from the felsic orthogneiss fixes the date of emplacement of its magmatic protolith and the regional high‐pressure metamorphism at ≥2,530 and ca. 2,479 Ma, respectively. Integrating all the information it is suggested that the evolution of the MLC occurred in a continent‐continent/arc collisional setting at the dawn of the Proterozoic Eon. This study does not support the existence of a “mega sheath fold” in the MLC as has been published in the recent literature

Chromite deposits associated with layered anorthosite complexes in the Archaean high-grade terranes
are rare in the world. The late Archaean Sittampundi Layered Magmatic Complex, Tamil Nadu, India is
one of the few such deposits in the world where layers of Fe-Al rich chromites are associated with extremely
calcic (An>95) anorthosite. ‘Frozen in’ magmatic mineralogy of the chromitite and the enclosing
anorthosite suggest successive crystallization of chromite + clinopyroxene and chromite + clinopyroxene
+ anorthite from a hydrous Al-rich basaltic melt that was emplaced in a suprasubduction zone setting.
Intense deformation and upper amphibolite facies metamorphism at
2.45 Ga converted the
magmatic assemblages to hitherto unreported hornblende + gedrite + Mg-Al rich spinel ± chlorite bearing
assemblages. During metamorphic reconstitution, chromite was pseudomorphically replaced by green
spinel in the domains rich in secondary amphiboles. Mass-balance calculation and algebraic analyses
of the observed mineralogy suggest that a number of chemical species including chromium became
mobile during the formation of spinel pseudomorph in response to infiltration driven metamorphism.
Aluminium became mobile in the length scale of chromite grain but remained immobile in the length
scale of a thin section.

The Archaean Sittampundi Layered Magmatic Complex (SLC) of south India is interpreted as a part of the oceanic crust that formed in a suprasubduction zone setting. The assemblage corundum + anorthite + amphibole (magnesiohornblende to tschermakite to pargasite) + clinozoisite (C1PACz) developed in highly calcic anorthosite of the SLC at the culmination of a ca. 2.46 Ga tectonothermal event. Changing physicochemical conditions during this early Paleoproterozoic (Siderian) event produced spinel + anorthite + second generation amphibole through destabilization of corundum + first generation amphibole. Spinel retains the shape of the corundum that it replaces (spinel pseudomorphing corundum, SCP) and is surrounded by a rind of plagioclase that separates spinel from the matrix amphibole. Development of the assemblage chlorite + clinozoisite + secondary corundum after spinel + anorthite + amphibole marks the terminal metamorphic event in this area. Mass-balance calculations on pseudomorphs and modeling of preserved reaction textures show that Na, Mg, Ca, and silica were mobile during the formation of the SCP. Al and Fe were mobile at the grain scale but remained immobile in the scale of a thin section. Activity adjusted partial petrogenetic grid in the systems Na2O-CaO-Al2O3-SiO2-H2O (NCASH) and Na2O-CaO-MgO-Al2O3-SiO2-H2O (NCMASH) along with the mineralogy of the rocks that were co-metamorphosed with the anorthosite show that (1) the assemblage C1PACz was formed during high-pressure metamorphism (11 ± 1 kbar and 725 ± 25 °C), (2) SCP and the plagioclase rind around it were formed during exhumation of these higher pressure rocks to lower pressure (7 ± 1 kbar, 675 ± 50 °C) along a steeply decompressive retrograde P-T path, and (3) post decompression cooling and hydration at near isobaric condition triggered the formation of the chlorite–clinozoisite–secondary corundum assemblage (6–8 kbar, <620 °C). The inferred P-T path is consistent with the view that the studied area that represents Archaean oceanic crust of SSZ affinity was subducted and subsequently exhumed during the early Paleoproterozoic orogeny.

The Southern Granulite Terrane of India exposes remnants of an interbanded sequence of orthoquartzite–metapelite–
calcareous rocks across the enigmatic Palghat–Cauvery Shear Zone (PCSZ), which has been interpreted as a Pan-African terrane boundary
representing the eastward extension of the Betsimisaraka Suture Zone of Madagascar. Zircon U–Pb geochronology of metasedimentary
rocks from both sides of the PCSZ shows that the precursor sediments of these rocks were sourced from the Dharwar Craton and the
adjoining parts of the Indian shield. The similarity of the provenance and the vestiges of Grenvillian-age orogenesis in some metasedimentary
rocks contradict an interpretation that the PCSZ is a Pan-African terrane boundary. The lithological association and the likely
basin formation age of the metasedimentary rocks of the Southern Granulite Terrane show remarkable similarity to the rock assemblage
and timing of sedimentation of the Palaeoproterozoic to Neoproterozoic shallow-marine deposits of the Purana basins lying several
hundred kilometres north of this terrane. Integrating the existing geological information, it is postulated that the shallow-marine sediments
were deposited on a unified land-mass consisting of a large part of Madagascar and the Indian shield that existed before Neoproterozoic
time, part of which was later involved in the Pan-African orogeny.