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Authors
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
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 45, No 2 (1995), Pagination: 127-136
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.
