Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

T. R. N. Kutty ¹, G. V. Anantha Iyer ¹, E. De Poli ¹

Affiliations
1 Indian Institute of Science, Bangalore 560012, IN

Abstract

Petrography, mineral chemistry, major elements as well as Rb, Sr, Ba and REE geochemistry of pink granulites and the associated charnockites have been studied. Pink granulites are prograde rocks metamorphosed under granulite facies condition. There is no genetic relation between potassic pink granulites and tonalitic charnockites and their geochemical evolutionary trends are distinctly different. K/Rb ratios are in the same range, although abundance level of K and Rb in pink granulites is high. Charnockites have higher Sr and lower Rb/Sr. Pink granulites have variable Sr level, high Rb/Sr and variable Ba/Sr indicating heterogeneity in the parent rocks. REE abundance level in pink granulites shows wide variation; non-uniform but negative Eu anomaly, strong fractionation with respect to LREE, positive slope after Gd and lower ∑LREE/∑HREE ratio (5 to 26). Charnockites have coherent REE patterns, with strongly fractionated LREE and slight positive Eu anomaly and higher ∑LREE/∑HREE (30-70) ratios. Field relation, mineral paragenesis, whole rock chemistry, Sr-isotope ratios and REE geochemistry strongly support sedimentary parentage for the pink granulites. The highly fractionated REE patterns of the associated charnockites reflect not only igneous inheritance but also the effect of granulite grade metamorphism.

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T. R. N. Kutty ¹

Affiliations
1 Indian Institute of Science, Bangalore, IN

Abstract

No Abstract.

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P. Venkataramana ¹, T. R. N. Kutty ¹, G. V. Anantha Iyer ¹

Affiliations
1 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, IN

Abstract

Previous experimental studies on magnesite formation which are confined to temperatures greater than 250°C, do not explain all the field and petrographic observations in natural situations where magnesite is associated with ultramafic complexes, as at Dodkanya, Mysore district, Karnataka.

A series of experiments were conducted using natural samples (rock and mineral powders) as starting materials. The experimental temperatures were varied from 35° to 300°C at intervals of approximately 50°C and normal and elevated pressures (500 and 1000 bars). At higher pressures, the X_CO2 in the fluid phase was also varied.

These studies have shown that magnesite and quartz can be formed directly by the interaction of CO₂-bearing water vapour and ferromagnesian minerals (olivine, orthopyroxene) without passing through the intermediate serpentine stage. The temperature conditions at which these reactions take place is between 150 and 250 °C at 500 bars total pressure with X_CO2 in the fluid phase greater than 0.015. The presence of Na⁺ salts enhances these reactions.

The presence of calcite-aragonite-quartz-aluminous hydroxide gel-limonite (birbirite) cap rock intersected by a network of magnesite veins in Dodkanya and surrounding areas has been explained on the basis of experiments with natural hornblende, the major Ca²⁺-bearing mineral in hornblende-peridotites. Hornblende, under hydrothermal conditions, breaks down to calcium silicate hydrate (CSH). CSH, being unstable in the presence of CO₂, inverts to CaCO₃ (calcite/aragonite) and quartz. Mg²⁺ in trace quantities stabilises aragonite.

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T. R. N. Kutty ¹, S. R. N. Murthy ¹, G. V. Anantha Iyer ¹

Affiliations
1 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, IN

Abstract

REE geochemistry of plagioclase-free, plagioclase-bearing and titanoclinohumite-bearing ultramafic suite of Chalk Hills is presented. The rare earth elements are determined by spark source mass-spectrography combined with a preanalytical chemical concentration reaching ± 4 accuracy and 0.01 ppm detection limit. Plagioclase-free ultramafics are 10 times LREE depleted. while HREE abundance is 1-2 times those of chondrite. Since the samples are not serpentinised, these REE patterns are considered to be primary and compare well with those of high temperature peridotites, particularly on Ce_N-Yb_N diagram. The plagioclase-bearing ultramafics and the gabbros have LREE contents 4-22 times chondritic, while the HREE varies from 2-12. The REE patterns of ultramafics from the smaller body show uniform increase in (La/Yb)_N ratios from 7 to 15. Higher abundance levels of incompatible alkali elements which do not correlate with the MgO contents, may suggest metasomatic introduction of these elements into the smaller body.

Petrogenetic modeling, based on partial melting of spinel peridotites as the source rocks with 2 times chondritic abundance, points towards the residual nature of the plagioclase-free ultramafics after extracting 8-10% of the melt. On the other band, the observed REE patterns of the plagioclase-bearing ultramafics and the gabbros not only indicate the absence of rocks more siliceous than syenogabbros but also suggest that they represent compositions of a depleted parent melt from which they originated. Thus the ultramafic association of Chalk Hills is distinctly different from the igneous suites associated with ophiolites and alpine peridotites.

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