Eocene age of eclogite metamorphism in Pakistan Himalaya: implications for India‐Eurasia collision
作者:
Sonia Tonarini,
Igor M. Villa,
Felix Oberli,
Martin Meier,
David A. Spencer,
Ugo Pognante,
John G. Ramsay,
期刊:
Terra Nova
(WILEY Available online 1993)
卷期:
Volume 5,
issue 1
页码: 13-20
ISSN:0954-4879
年代: 1993
DOI:10.1111/j.1365-3121.1993.tb00221.x
出版商: Blackwell Publishing Ltd
数据来源: WILEY
摘要:
ABSTRACTA 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 excess40Ar 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 Centra
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