The timing of ultrahigh-temperature metamorphism in Southern India: U-Th-Pb electron microprobe ages from zircon and monazite in sapphirine-bearing granulites

摘要

We report here U-Pb electron microprobe ages from zircon and monazite associated with corundum- and sapphirine-bearing granulite facies rocks of Lachmanapatti, Sengal, Sakkarakkottai and Mettanganam in the Palghat-Cauvery shear zone system and Ganguvarpatti in the northern Madurai Block of southern India. Mineral assemblages and petrologic characteristics of granulite facies assemblages in all these localities indicate extreme crustal metamorphism under ultrahigh-temperature (UHT) conditions. Zircon cores from Lachmanapatti range from 3200 to 2300 Ma with a peak at similar to 2420 Ma, while those from Mettanganam show similar to 2300 Ma peak. Younger zircons with peak ages of similar to 2100 and similar to 830 Ma are displayed by the UHT granulites of Sengal and Ganguvarpatti, although detrital grains with similar to 2000 Ma ages are also present. The Late Archaean-aged cores are mantled by variable rims of Palaeo- to Mesoproterozoic ages in most cases. Zircon cores from Ganguvarpatti range from 2279 to 749 Ma and are interpreted to reflect multiple age sources. The oldest cores are surrounded by Palaeoproterozoic and Mesoproterozoic rims, and finally mantled by Neoproterozoic overgrowths. In contrast, monazites from these localities define peak ages of between 550 and 520 Ma, with an exception of a peak at similar to 590 Ma for the Lachmanapatti rocks. The outermost rims of monazite grains show spot ages in the range of 510-450 Ma.