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

A. M. Nair ¹, Narayanaswamy ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum, IN

Abstract

Comparative morphometric analysis of zircons from sediments and charnockite-quartzo-feldspathic gneiss-pegmatite rock units along the Manimala river valley in Kottayam and Alleppey districts of Kerala has brought to light a unimodal-bimodal size frequency distribution of zircons transported via a major meander near Komalam in Alleppey district. The slope variation in the up-mid (meander)-downstream zircons is 25°28' to 30°17'. The trapping of heavy minerals (0.002%-0.004%) by the meander has also affected the anticipated coastward enrichment of zircons. RMA plots for the rocks reveal the self-nucleated origin of zircons and the dispersion of trend lines suggests phases of zircon crystallization. Heavy minerals released from the rocks as revealed from zircon morphometry, are not carried through to the coast. Nevertheless, the coastward enrichment of Cu-Pb-Zn concentration indicates a steady flow right up to the coast of only the fine sediment fraction with these absorbed elements.

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S. Sinha-Roy ¹, John Mathai ¹, Narayanaswamy ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum, IN

Abstract

Cordierite-bearing gneiss of the Quilon district, Kerala, occurs in a 8-10km wide zone within quartzo-feldspathic gneiss, being bordered on the north by a charnockite massif. These gneisses are deformed and are polymetamorphic in character. The cordierite-forming metamorphism is the second in metamorphic chronology. The reaction producing the bulk of cordierite was: biotite + sillimanite + quartz ⇌ cordierite + garnet + K-fetdspar + H₂O. The K_D (Mg-Fe) values of coexisting garnet-cordierite and garnet-biotite pairs indicate that the PT conditions of metamorphism (∼ 7 kb, ∼ 720°C) were those of granulite facies. Under these conditions cordierite was produced from Fe-Mg enriched melt restite, being trapped at the fringe of the charnockite massif.

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R. Parthasarathy ¹, A. V. Murali ¹, M. Sankar Das ¹, A. M. Nair ², Narayanaswamy ²

Affiliations
1 Analytical Chemistry Division, BARC, Bombay 400085, IN
2 Geosciences Division, Centre for Earth Science Studies, Trivandrum, Kerala 695010, IN

Abstract

Zircon separates from a few whole rocks and sediments along with the parent sources have been analysed for Zr, Hf, Se, REE, Th and U concentrations by the neutron activation analysis technique. The major, minor and trace analyses of rock samples and their modal analyses are also presented. The chondritic normalised REE patterns of rock samples show positive Eu anomalies, while the zircon samples show negative Eu anomalies. The data have been used to explain the genetical relationship among the rocks under study. This study supports the hypothesis proposed from this laboratory that the detrital zircons of Kerala beach are time integrated cumulates from the whole-rocks of this region. The scandium data indicate the paragenetic sequence of crystallization of zircons in relation to cpx and magnetite.

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C. P. Rajendran ¹, G. Rajagopalan ², Narayanaswamy ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrurn 695031, IN
2 Birbal Sahni Institute of Palaeobotany, Lucknow 226007, IN

Abstract

An event of transgression (8000-6000 Y.B.P.) and another event of regression (30Q0..5000 Y.B.P.) which gave rise to extensive peat and shell deposits during Holocene in Kcrala is recorded. The initiation of the backwater system in Kerala is linked to these events.

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Narayanaswamy ¹, K. Soman ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum 695031, IN

Abstract

Description and size data are presented for zircons separated from Precambrian rocks of south Kerala, comprising garnet-sillimanite-graphite gneiss (khondalite), charnockite, garnet-biotite gneiss and garnet-Quartz-feldspar neosome.

Zircons from khondalite are rounded, do not show much overgrowth features and are typical of granulites of sedimentary parentage. Zircons in charnockite show unimodal frequency curves and self-nucleated RMA nature. suggestive of magmatic origin. Those from garnet-biotite gneisses do not show any linear. RMA trends, and the terminally rounded zircons in them do not show any angle between length and c-axis suggesting a mixed source. Zircons from garnet-quartz-feldspar neosome show features, suggestive of migmatitic origin.

Keywords

Zircon Morphology, Petrology, Igneous and Metamorphic, Kerala Khondalite-Charnockite.

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K. Soman ¹, Narayanaswamy ², W. R. Van Schmus ³

Affiliations
1 Centre for Earth Science Studies, Trivandrum-695 031, IN
2 Centre for Earth Science Studies, Trivandrum-695 031
3 Department of Geology, University of Kansas, Lawrence, Kansas 66045, US

Abstract

South Kerala in southwestern India offers interesting khondalite-charnockite assemblages suitable for detailed multidisciplinary case studies on granulite petrogenesis. Arrested chamockite formation is also characteristic of the terrain. Absolute ages of the khondalites are poorly constrained; we have obtained limited U-Pb zircon data for various rock types of south Kerala khondalite belt. Zircons from khonddite sensu-stricto and garnet-biotite gneiss of possible sedimentary origin give a concordia upper-intercept apparent age of ca. 2200 Ma, suggesting that the khondalites contain detrital material that is atleast Early Proterozoic in age; the data yield a tower intercept apparent age of ca. 400-600 Ma, but it is not clear if this represents new zircon growth andepisodic Pb loss at that time (preferred) or is an artifact of norrnal diffusive loss of radiogenenic Pb. Zircons from retrograded granulitic gneisses have zircon upper-intercept apparent ages of ca. 1550 Ma, indicating either a younger protolith age or, more probably, greater radiogenic Pb loss during subsequent migmatite-grade metamorphism that caused retrogression of the granulite. Since all the zircon populations are complex, more detailed anaIyses will need to be pursued to resolve age relationships of these rocks with greater certainty. Monazite from a retrograded granulitic gneiss, yields a concordant age of ca. 520 Ma; this age is probably related to late Proterozoic-Early Cambrian regional thermal event in south Kerala and is correlative with a similar event in Sri Lanka.

Keywords

Geochronology, Zircons, Granulites, Kerala.

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G. R. Ravindra Kumar ¹, Narayanaswamy ¹

Affiliations
1 Centre for Earth Science Studies, PB 2235, Trivandrum 695010, IN

Abstract

A conglomerate horizon separates the Early Precambrian high-grade supracrustals from the low-grade supracrustal rocks in north Kerala. The conglomerate consists dominantly of quartzite pebbles. In addition quartz. granite and quartz-mica schist pebbles are also recognised. Tbe matrix is of arkosic composition. The petrochemistry of pebbles and matrix indicate a dominant sialic provenance. The presence of quartzite pebbles suggests a cycle of sedimentation prior to the deposition of Vengad conglomerates.

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