High-temperature metamorphism in a major strike-slip shear zone: the Ailao Shan-Red River, People's Republic of China
Philippe H. Leloup a and Jean-Robert Kienast b
a Laboratoire de tectonique, Institut de Physique du Globe
de Paris, 4 place Jussieu, 75252, Paris Cedex 05, France
b Laboratoire de pétrologie, Université Paris VII,
4 place Jussieu, 75252, Paris Cedex 05, France
Received 29 June 1992; accepted 30 April 1993. Available online 22 October 2002.
Abstract
Petrographic and thermobarometric analysis provides constraints
on the PT path of the mylonitic gneisses of the left-lateral
Ailao Shan-Red River shear zone which has accommodated the lateral
extrusion of Indochina during the Tertiary. Two different paragenesis,
P1 and P2, are coeval with this deformation and correspond respectively
to the amphibolite and greenschist facies. Microprobe analysis
reveals that P1 garnets bear a chemical zonation from core to
rim. This zonation indicates a temperature increase during garnet
growth. Conditions of formation of garnet rims (P1b), which are
estimated using biotite-garnet and plagioclase-garnet thermobarometers,
are close to the granitic solidus (710 ± 70°C
and 4.5 ± 1.5 kbar). P2 conditions are estimated to be
approximately 500°C and < 3.8 kbar. Both P1b and P2 conditions
correspond to much higher temperatures than expected for their
depths in the continental crust, suggesting a perturbed geothermal
gradient during strike-slip deformation along the Ailao Shan-Red
River shear zone. Thermochronology results suggest that cooling
between P1b and P2 was fast ( 100°C/Ma) and may have been
associated with significant uplift. Uplift during the left-lateral
shearing may have resulted from a slight reverse or, more probably
normal, component of movement along the strike-slip fault. A simple
numerical model suggests that the high temperatures in the shear
zone at the time of deformation may be explained by shear heating
in the more competent upper mantle and by advection of this heat
along the shear zone by ascent of magmas and/or fluids. In this
hypothesis, the medium-pressure and temperature schists bounding
the mylonitic gneisses to the southwest previously interpreted
as resulting from collision-related metamorphism result instead
from `contact' metamorphism of the shear zone at mid-crustal depths.
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Insigths from Tertiary deformation of SE Asia. |