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Late Palaeoproterozoic depositional age for khondalite protoliths in southern India and tectonic implications

Abstract:

SHRIMP dating of detrital zircons from garnet–sillimanite–cordierite paragneisses (khondalites) of the Trivandrum Block, southern India, revealed nanometer-scale Pb isotopic inhomogeneity in most grains that we ascribe to annealing processes during ultra-high-temperature metamorphism at ca. 570 Ma. Our age data for zircons from six representative khondalite samples do not document any Neo- or Mesoproterozoic detrital grains, and we conclude from the concordant ages and discordant minimum 207Pb/206Pb ages that the khondalite precursor sediments were deposited more than 2.1 Ga ago and were subsequently intruded by granitoid rocks at ca. 1765–2100 Ma. Some detrital zircons in the khondalites contain late Palaeoproterozoic metamorphic domains, suggesting that these grains are derived from an unknown crustal source that most likely experienced late Palaeoproterozoic high-grade metamorphism. Both the metasedimentary assemblage and granitoids were severely ductilely deformed, metamorphosed and migmatized during the pervasive Pan-African event at ca. 550–580 Ma. This caused many detrital zircons in the khondalites to become variably recrystallized and to develop metamorphic rims. Proper interpretation of cathodoluminescence images and zircon morphology is important in interpreting detrital populations. The southern Indian khondalites were part of an extensive Palaeoproterozoic metasedimentary assemblage that may have extended from southern Madagascar via the Trivandrum Block to the Highland Complex of Sri Lanka and seems to reflect a continental margin sequence that was deposited on an Archaean to early Palaeoproterozoic continental terrane, possibly the southern margin of the Indian Dharwar craton. The tectonic history of this terrane remains obscure due to the pervasive overprint during the Pan-African event.