Himalayan inverted metamorphism constrained by oxygen isotope thermometry |
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| Authors: | Jean-Claude Vannay Zachary D Sharp Bernhard Grasemann |
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| Institution: | Institut de Minéralogie et Pétrographie, Université de Lausanne, CH-1015 Lausanne, Switzerland E-mail: Jean-Claude.Vannay@imp.unil.ch, CH
Institut für Geologie, Universit?t Wien, Althanstrasse 14, A-1090 Wien, Austria E-mail: Bernhard.Grasemann@univie.ac.at, AT
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| Abstract: | Inverted metamorphic field gradients are preserved in two amphibolite facies metapelitic sequences forming the crystalline
core zone of the Himalayan orogen in the Sutlej valley (NW India). In the High Himalayan Crystalline Sequence (HHCS), metamorphic
conditions increase upwards from the staurolite zone at the base, through the kyanite-in and sillimanite-in isograds, finally
to reach partial melting conditions at the top. The structurally lower Lesser Himalayan Crystalline Sequence (LHCS) shows
a gradual superposition of garnet-in, staurolite-in and kyanite + sillimanite-in isograds. Although phase equilibria constraints
imply inverted temperature field gradients in both units, garnet-biotite (GARB) rim thermometry indicates final equilibration
at a nearly uniform temperature around T ≈ 600 °C across these sequences. The P-T path and garnet zoning data show that this apparent lack of thermal field gradient is mainly the consequence of a resetting
of the GARB equilibria during cooling. In order to constrain peak temperature conditions, 20 samples along the studied section
have been analysed for oxygen isotope thermometry. The isotopic fractionations recorded by quartz-garnet and quartz-aluminosilicate
mineral pairs indicate temperatures consistent with phase equilibria and P-T path constraints for metamorphic peak conditions. Together with barometry results, based on net transfer continuous reactions,
the oxygen isotope thermometry indicates peak conditions characterized by: (1) a temperature increase from T ≈ 570 to 750 °C at a nearly constant pressure around P ≈ 800 MPa, from the base to the top of the HHCS unit; (2) a temperature increase from T ≈ 610 to 700 °C and a pressure decrease from P ≈ 900 to 700 MPa, from the base to the top of the LHCS metapelites. Oxygen isotope thermometry thus provides the first quantitative
data demonstrating that the Himalayan inverted metamorphism can be associated with a complete inversion of the thermal field
gradient across the crystalline core zone of this orogen.
Received: 1 April 1999 / Accepted: 12 July 1999 |
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