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Eocene age of eclogite metamorphism in Pakistan Himalaya: implications for India‐Eurasia collision
Author(s) -
Tonarini Sonia,
Villa Igor M.,
Oberli Felix,
Meier Martin,
Spencer David A.,
Pognante Ugo,
Ramsay John G.
Publication year - 1993
Publication title -
terra nova
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.353
H-Index - 89
eISSN - 1365-3121
pISSN - 0954-4879
DOI - 10.1111/j.1365-3121.1993.tb00221.x
Subject(s) - geology , phengite , eclogite , metamorphism , omphacite , hornblende , subduction , geochemistry , metamorphic rock , zircon , coesite , continental collision , quartz , paleontology , biotite , tectonics
A geochronological investigation of two rocks with an eclogitic assemblage (omphacite‐garnet‐quartz‐rutile) from the High Himalaya using the Sm/Nd, Rb/Sr, U/Pb and Ar/Ar methods is presented here. The first three methods outline a cooling history from the time of peak metamorphism at 49±6 Ma recorded by Sm/Nd in garnet‐clinopyroxene to the closure of Rb/Sr in phengite at 43±1 Ma and U/Pb in rutile at 39–40 Ma. The Sm/Nd isotopic system was fully equilibrated during eclogitization and has not been disturbed since; its mineral ages may date the peak metamorphic conditions (650±50°C at 13–18 kbar: Pognante and Spencer, 1991). The Ar/Ar data reveal the presence of substantial amounts of excess 40 Ar in hornblende, and yield a statistically acceptable but geologically meaningless phengite plateau age of 81.4±0.2 Ma, inconsistent with Sm/Nd, Rb/Sr and U/Pb. This questions the use of such a chronometer for the dating of high‐pressure assemblages. The results imply a Late Palaeocene or Early Eocene subduction of the northern Indian plate margin in NW Himalaya. The fact that eclogites are restricted to NW Himalaya may be the result of a peculiar p‐T‐t path associated with a high convergence rate during the first indentation, in contrast to the later and slow subduction in Central and Eastern Himalaya.