G. V. Anantha Iyer

Sm-Nd ages of two meta-anorthosite complexes around Holenarsipur: Constraints on the antiquity of Archean supracrustal rocks of the Dharwar craton

Whole-rock Sm-Nd isochron ages are reported for two stratiform meta-anorthosite complexes emplaced into the Archean supracrustal-gneiss association in the amphibolite facies terrain around Holenarsipur, in the Dharwar craton, South India. While these metaperidotite-pyroxenite-gabbro-anorthosite complexes are petrologically and geochemically similar, they differ in the intensity of tectonic fabric developed during the late Archean (c. 2.5 Ga) deformation. They also differ in their whole-rock Sm-Nd isochron ages and initial Nd isotopic compositions: 3.285 ± 0.17 Ga,ɛNd0.82 ± 0.78 for the Honnavalli metaanorthosite complex from a supracrustal enclave in the low-strain zone, and 2.495 ± 0.033 Ga, ɛNd = -2.2 ± 0.3 for the Dodkadnur meta-anorthosites from the high-strain southern arm of the Holenarsipur Supracrustal Belt (HSB). We interpret these results as indicating that the magmatic protoliths of both meta-anorthosite complexes were derived from a marginally depleted mantle at c. 3.29 Ga but only the Dodkadnur rocks were isotopically reequilibrated on a cm-scale about 800 Ma later presumably due to the development of strong penetrative fabrics in them during Late Archean thermotectonic event around 2.5 Ga. Our results set a younger age limit at c. 3.29 Ga for the supracrustal rocks of the HSB in the Dharwar craton.

Geochemical Comparison of Archaean Granulites in India with Proterozoic Granulites in Canada

Abstract The charnockitic gneisses of North Arcot, Tamil Nadu, have a bimodal compositional distribution. The basic granulites show a tholeiitic Fe-rich trend, while the intermediate and acid charnockites show a calc-alkaline one, A similar compositional variation is apparent in amphibolite facies gneisses and asscciated mafic rocks and in the low-grade schist belts of Kolar and Holenarasipur of southern Karnataka. However there are notable differences in their respective trace element compositions. The charnockitic rocks are low in largeion lithophile (LIL) elements (i. e. K, Rb, Th, U, Pb) but not in cther normally incompatible alements (i. e. Ba, Sr, Zr), and have higher K/Rb and lower Rb/Sr ratios compared with the lower grade gneisses. The geochemical data suggest that the amphibolite faciss gneisses of South Karnataka are not retrogressed charnockites; on the other hand, the charnockites may be derived from the highgrade gneisses. There is no major chemical distinction between Archaean and Proterozoic granulites. As such, the hypothesis that granulites are derived from an igneous parent magma that contained low levels of LIL trace elements does not seem to be valid. The basic members of the charnockitic suite compare better with the mafic enclaves within the high-grade gneisses than with the mafic components of the schist belts.

Geochemistry of meta-anorthosites from Holénarasipur, Karnataka, South India

Geochemical and Rb---Sr isotope studies indicate that the meta-anorthosites of Holenarasipur, occurring as minor differentiates in ultramafic-mafic complex are igneous intrusives with cumulus character, emplaced around 3095 m.y. ago. The fine-grained nature is secondary; relict cumulus features are preserved in less deformed bodies. In major element chemistry, they compare well with other Archean anorthosites. Abundance levels of Ti, Zr, Y and P indicate the evolution through crystal fractionation of a parental magma; cumulus olivine and pyroxenes dominated chemistry for ultramafites, cumulus plagioclase and possibly clinopyroxene controlled chemistry for anorthosite-gabbros and cumulus magnetite in magnetite-gabbros. Magnetite is not an early cumulate. REE geochemistry is dominated by plagioclase with low abundance levels, slightly LREE enriched and variable positive Eu anomaly. Sr and Image values vary with An content in plagioclase. Isotopic studies show low initial Image (=0.7016) indicating that Rb---Sr isochron age represents the time of intrusion rather than the time of metamorphism.

Electron microscopy of olivine with perfect cleavage

Electron microscopy (SEM and TEM) of unusual olivine (Fo = 88) crystals, with perfect (010) cleavages, from Chalk Hills, Salem, Tamil Nadu has been carried out. SEM studies reveal the effect of compressive stress. Microstructures by TEM showed the abundance of curved dislocations with jogs, kinks and dipoles, indicative of the dominant climb, characteristic of high temperature deformation. The stacking fault fringes observed in olivine are due to mechanical weakening caused by nonstructural chemical constituents. The evidences for this come from fiuid-microinclusions observed and higher amount of K, Na, Rb and Sr in the olivine. Hydrolytic weakening through dislocation glide motion, assisted by H2O containing incompatible elements, may break the metal-oxygen bonds. This leads to perfect parting of crystallographic planes under deformational stress, during solid emplacement of the dunite. The easily cleavable planes are those with the largest interplanar spacings.

Influence of Structural Defects on Gas Adsorption Properties of Anthophyllite Asbestos

The surface area of anthophyllite asbestos variants with different stages of fiber development is found to be not proportional to the fineness of the fibrils. Coarser variants have transitional pores whereas the asbestiform is microporous. HRTEM images show structural intergrowths of

High resolution electron microscopy of chloritoid minerals from different geological melieu

Mg(OH)_2

Copper metallogeny in the Jogimardi volcanics, Chitradurga greenstone belt

in an anthophyllite lattice, whereas the intergrowths are absent in the asbestiform anthophyllite. Conversion of brucite intergrowth into a noncrystalline phase results in the generation of transitional pores.

Rb-Sr and Pb-Pb Whole Rock Isochron Ages of Basement Gneisses in Karnataka Craton

Structural defects of three chloritoid minerals from distinet geologic melieu have been investigated by high resolution electron microscopy. X-ray powder and electron diffraction patterns indicate that the chloritoid from one geological source (A) is2M1+2M2 monoclinic variant while those from another geological source (B) are 2M2 monoclinic variants. In a typical one-dimensional lattice image of a crystal from sourceA, the 2M2 matrix is broken by insertion of triclinic inter-growths. Another crystal with the 2M2 matrix showed single, triple, quadruple and quintuple layers displaying an unusually high degree of disorder. Lattice images of 2M2 monoclinic variants from sourceB yielded more homogeneous micrographs.The important finding from the present studies is that the chloritoid from sourceA is a severely disordered low-temperature intermediate phase in the conversion of the triclinic chloritoid to the high-temperature ordered monoclinic variants of sourceB. Severely disordered chloritoids, marking the beginning of low grade metamorphism, are generated as intermediates between the state of complete disordered arrangement towards the end of low grade metamorphism within the narrow stability range of 400°–500°C.