Intraplate tectonics in Asia: a precise age for large-scale Miocene movement along the Ailao Shan-Red River shear zone, China
Urs Schärer1, *, Paul Tapponnier2, Robin Lacassin2, Philippe H. Leloup2, Zhong Dalai3 and Ji Shaocheng3
1 Département des Sciences de la Terre et GEOTOP, Université
du Québec à Montréal, C.P. 8888, Succ. A.
Montréal, P.Q. H3C 3P8, Canada
2 Laboratoire de Tectonique, Mécanique de la Lithosphère,
Institut de Physique du Globe, 4 Place Jussieu, 75230, Paris,
France
3 Institute of Geology, Academia Sinica, P.O. Box 634, Beijing,
People's Republic of China
Received 7 June 1989; accepted 3 October 1989. Available online 22 October 2002.
Abstract
Tertiary left-lateral movement along the 1000 km long Ailao
Shan-Red River shear zone appears to have played an important
role in absorbing post-collisional northward penetration of India
into Asia. Crustal strike-slip shear along this zone caused the
formation of a gneiss belt, metamorphosed to amphibolite grade,
including anatectic melting. Both metamorphism and melting were
induced by ductile deformation yielding the possibility to date
the major tectonometamorphic event that shaped the Ailao Shan-Red
River belt. 17 U-Pb isotope analyses were performed on small size-fractions
of zircon, monazite and xenotime, extracted from two different
leucogranitic layers. The two samples are located about 50 km
apart in the central segment of Ailao Shan, in structurally well
controlled settings where the melts crystallized within the strongly
foliated gneisses during late stages of deformation. All mineral
U-Pb analyses had to be corrected for excess or deficit amounts
of radiogenic 206Pb, originating from initial 230Th disequilibrium
in the 238U decay series. In monazite, such disequilibrium 206Pb
reaches 20% of total radiogenic 206Pb. The corrected U-Pb ages
of monazite and xenotime lie between 22.1 and 23.9 Ma, whereas
zircon yields significantly discordant U-Pb ages between 30.5
and 33.9 Ma pointing to Precambrian material in the magma source
region. Inherited components could also be detected in monazite.
The set of U-Pb data shows that monazite and xenotime formed simultaneously
in both localities substantiating an early Miocene age of 23.0
± 0.2 Ma for late kinematic crystallization of anatectic
melts in the metamorphic belt of the Ailao Shan-Red River shear
zone.
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Insigths from Tertiary deformation of SE Asia. |