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A review of the first eclogites discovered in the eastern Himalaya

Bruno Lombardo, Franco Rolfo and William C. Mcclelland
A review of the first eclogites discovered in the eastern Himalaya
European Journal of Mineralogy (July 2016) 28 (6): 1099-1109

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 degrees C), two granulite-facies stages at decreasing pressure (P = 0.8-1.0 GPa, T > 750 degrees C; P approximately 0.4 GPa, T approximately 750 degrees C), an amphibolite-facies stage (T = 700 degrees C, P = 0.4-0.6 GPa) and a late retrogression stage (T < 690 degrees 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 (super 206) Pb/ (super 238) U 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.


ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 28
Serial Issue: 6
Title: A review of the first eclogites discovered in the eastern Himalaya
Affiliation: CNR, Istituto de Geoscienze e Georisorse, Turin, Italy
Pages: 1099-1109
Published: 20160717
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
References: 59
Accession Number: 2016-085306
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, geol. sketch maps
N28°04'60" - N28°15'00", E87°30'00" - E87°34'60"
Secondary Affiliation: University of Iowa, USA, United States
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201641
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