A review of the first eclogites discovered in the Eastern Himalaya

Abstract

As a tribute to Bruno Lombardo, our senior co-worker who sadly passed away in 2014, the geologic setting and metamorphic evolution of the first eclogites discovered in the Eastern Himalaya are summarised in the context of U-Pb analytical results from the original granulitized eclogite samples. These eclogites occur at the top of the Main Central Thrust (MCT) zone in the Ama Drime Range (Arun - Phung Chu Valley), southeastern Tibet. The eclogites record five superposed metamorphic stages: a first eclogite-facies stage ( P > 1.5 GPa, T > 580°C), two granulite-facies stages at decreasing pressure ( P = 0.8–1.0 GPa, T > 750°C; P ~ 0.4 GPa, T ~ 750°C), an amphibolite-facies stage ( T = 700°C, P = 0.4–0.6 GPa) and a late retrogression stage ( T < 690°C, P <0.4 GPa). The timing of eclogite-facies metamorphism in the Ama Drime Range is difficult to constrain because of the widespread high-temperature granulite-facies overprint and the poor preservation of high-pressure assemblages. Zircons separated and analysed by the U-Pb SIMS (secondary ionization mass spectrometry) method display complex and varied systematics. Low-U rim domains with low Th/U ratios (0.02–0.03) give young 206Pb/238U ages of 13–14 Ma, which are consistent with age estimates of other workers for the timing of granulite-facies overprint, but do not constrain the age of eclogitization. Proterozoic ages (1.8 Ga) are attributed to inherited components. Discordant analyses define an array toward an upper intercept value of ca . 1000 Ma. Five grains that give ages of ca . 90 Ma may record contaminants incorporated during processing, or may represent the age of the protolith and thus may be the first indication in the Himalayan basement nappes for the widespread Cretaceous Rajmahal Trap basalt volcanism of NE India.