Long and complex thermal history of the Song Chay metamorphic dome (northern Vietnam) by multi-system geochronology.
Françoise ROGER1, Philippe Hervé LELOUP2, Marc
JOLIVET3, Robin LACASSIN2, Phan Trong TRINH4, Maurice BRUNEL3,
Diane SEWARD5.
1 Laboratoire de Géochronologie, Univ. Paris7- IPG Paris,
CNRS-UMR 7578, 75252 Paris France
2 Laboratoire de Tectonique, mécanique de la lithosphère
IPG Paris, CNRS-UMR 7578, Paris France
3 GGP, Université Montpellier 2, CNRS UMR 5567, Montpellier,
France.
4 Institute of Geological Sciences, NCNST, Hanoi, Vietnam
5 Geology Institute, ETH Zentrum, Sonneggstrasse 5, CH-8092 Zürich,
Switzerland
Abstract
Multi-system geochronology was used to unravel the long and
complex thermal history of the Song Chay range (Northern Vietnam),
a high-grade granitic and metamorphic dome in the vicinity
of the Cenozoic, Ailao Shan - Red River fault zone. It was considered
to be Proterozoic South China basement, but its geological history
was basically unknown. Scattered field observations suggest three
episodes of high temperature deformation: firstly at the time
of granite emplacement, secondly a décollement with top
to the north shear and thirdly anticlinal doming of the foliations
formed during the two first stages. P-T estimates suggest that
metamorphism coeval with the second deformation phase culminated
at 580°C and 4.5kb (16 km depth). Multi-system geochronology
is applied to a two-micas granite sample, slightly deformed within
the décollement. U/Pb dating of zircon yields an age of
428 ± 5 Ma (± 2s) interpreted as the time of granite
crystallization within the South China "Caledonian"
belt. Rb/Sr on white micas and biotite yields ages of 206 ±
10 and 176 ± 5.3 Ma, respectively (2s); while 39Ar/40Ar
ages of the same minerals are 210 ± 9 and 190 ±
8 Ma. These ages suggest an Upper Triassic episode of rapid cooling
interpreted as due to doming a few million years after the end
of movement on the décollement. The K-feldspar irregular
39Ar/40Ar age spectrum can, to the first order, be explained by
a cooling history with two episodes of rapid cooling: one at 140
Ma and a second around 41 to 25 Ma. Apatite fission tracks central
age (33.6±3.6 Ma, 1s) confirms a Tertiary rapid cooling
event interpreted as the final exhumation of the Song Chay dome.
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P.H. Leloup page |
Insigths from Tertiary deformation of SE Asia. |