Clockwise exhumation path of granulitized eclogites from the Ama Drime range (Eastern Himalayas)

Abstract

The metamorphic evolution of a granulitized eclogite from the Phung Chu Valley (Eastern Himalaya) was reconstructed combining microstructural observations, conventional thermobarometry and quantitative pseudosection analysis. The granulitized eclogite consists of clinopyroxene, plagioclase, garnet, brown amphibole, and minor orthopyroxene, biotite, ilmenite and quartz. On the basis of microstructural observations and mineral relationships, four metamorphic stages and related mineral assemblages have been recognized: (i) M1 eclogite-facies assemblage, consisting of garnet, omphacite (now replaced by a clinopyroxene + plagioclase symplectite) and phengite (replaced by biotite + plagioclase symplectite); (ii) M2 granulite-facies assemblage, represented by clinopyroxene, orthopyroxene, garnet, plagioclase and accessory ilmenite; (iii) M3 plagioclase + orthopyroxene corona developed around garnet, and (iv) M4 brown amphibole + plagioclase assemblage in the rock matrix. Because of the nearly complete lack of eclogitic mineral relics, M1 conditions can be only loosely constrained at >1.5 GPa and >580°C. In contrast, assemblage M2 tightly constrains the peak granulitic stage at 0.8-1.0 GPa and >750°C. The second granulitic assemblage M3, represented by the plagioclase + orthopyroxene corona, formed at lower pressures (0.4 GPa and 750°C). During the subsequent exhumation, the granulitized eclogite experienced significant cooling to nearly 700°C, marked by the appearance of brown amphibole and plagioclase (M4) in the rock matrix. U-Pb SHRIMP analyses on low-U rims of zircon from an eclogite of the same locality suggest an age of 13-14 Ma for the M3 stage. The resulting decompressional clockwise P-T path of the Ama Drime eclogite is characterized by nearly isothermal decompression from >1.5 GPa to 0.4 GPa, followed by nearly isobaric cooling from 775°C to 710°C. Modelling of phase equilibria by a calculated petrogenetic grid and conventional thermobarometry on a biotite-garnet-sillimanite metapelite hosted in the country rock granitic orthogneiss extends the inferred P-T trajectory down to 630°C and 0.3 GPa.

On ultrahigh temperature crustal metamorphism: Phase equilibria, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings

“…Baldwin et al, 2007b;Groppo et al, 2007;Dawson and Harley, 2009;Morishita et al, 2009;Berger et al, 2010;Engvik and Austrheim, 2010;Engvik et al, 2011;Janák et al, 2012). Sapphirine is commonly part of a symplectic breakdown product of kyanite, developing during exhumation of high-pressure mafic rocks (e.g.…”

Introduction to Higher Himalaya

“…These conditions were followed by isothermal decompression down to a pressure of about 0.3 GPa. The peak metamorphism occurred around 30 Ma, whereas their isothermic decompression took place from 20 to 12 Ma (Lombardo and Rolfo 2000;Liu et al 2007;Groppo et al 2007;Kali et al 2010). Corrie et al (2010) used garnet geochronology to measure the timing of eclogitization in the Arun Valley, and they obtained an age of 20.7 ± 0.4 Ma, significantly younger than ultra-high-pressure eclogites from the western Himalaya.…”

“…The Ama Drime paragneisses overlie the orthogneisses and they also contain pelitic schists. The migmatitic orthogneisses contain a few hundred meter-thick boudins of metabasics, recording eclogite facies metamorphism (Groppo et al 2007). The orthogneisses are dated at 1.8 Ga, similar to the Ulleri orthogneisses of the Lesser Himalaya in west Nepal Liu et al 2007;Cottle et al 2009).…”

Seismic visibility of a deep subduction channel – insights from numerical simulation of high-frequency seismic waves emitted from intermediate depth earthquakes

“…Schertl et al, 1991;Stöckhert et al, 1997;Reinecke, 1998;Groppo et al, 2007;Groppo and Castelli, 2010;Krebs et al, 2011;Angiboust et al, 2012), geochronological results pose narrow time windows for typical timescales of exhumation, asking for displacement rates comparable to plate velocity (e.g. Gebauer et al, 1997;Rubatto and Hermann, 2001;Baldwin et al, 2004;Little et al, 2011).…”