Geochronology (Rb-Sr, Sm-Nd and Pb-Pb) of the proterozoic granulitic and granitic rocks around Usilampatti, Madurai district, Tamil Nadu: Implication on age of various lithounits

摘要

Rb-Sr, Sm-Nd and Pb-Pb systematics on mafic granulites, intermediate-chamockites, chamockites, calcgranulites, orthogneisses, leptynites, granites and two (2) mineral samples (microcline and muscovite) from pegmatite have been studied. Sm-Nd model ages (T-DM) for most of the rocks cluster around 2.1 Ga. Calc-granulites gave Rb-Sr whole rock isochron age of 1339 +/- 110 Ma with initial Sr-87/Sr-86 ratio (I.R.) = 0.709. Mesoproterozoic age for the Metasedimentary Group of rocks (calc-granulites) has been inferred based on their Sr-87/Sr-86 ratio also. Granites and leptynites have yielded Rb-Sr whole rock isochron ages of 823 +/- 38 Ma, with I.R. = 0.713 and 894 +/- 82 Ma, with I.R. = 0.708 respectively which are younger than the calc-granulites. Microcline and muscovite from pegmatite gave Rb-Sr model ages as 532 and 491 Ma respectively. Granites, leptynites and calc-granulites are derived from the crustal source as indicated by their high initial Sr-87/Sr-86 ratios. Most probably the protolith of the granitic and granulitic rocks is of Palaeoproterozoic age in this part of the Madurai Block. The minimum age of granulite grade of metamorphism has been inferred at c. 850 Ma, indirectly on the basis of Rb-Sr ages of leptynites, which normally form during the evolution of granulite facies assemblages. Both granulite facies metamorphism and granitic magmatism probably took place during Neoproterozoic period correlatable to the early phase of Pan-African orogeny. Later decompression, causing mineral scale resetting of the Rb-Sr and Sm-Nd systematics, around 450-550 Ma, may correspond to the final exhumation, which brought the middle to lower crustal granulites to upper crustal levels, during the last phase of Pan-African activity. As per ages obtained on various lithounits in the present study the Metasedimentary Group of rocks (Mesoproterozoic) are younger than the Chamockite Group of rocks (Palaeoproterozoic) followed by the rocks belonging to the Migmatite Complex (Neoproterozoic).