Journal of Earth Science, Vol. 27, No. 3, p. 435–443, June 2016 Printed in China DOI: 10.1007/s12583-016-0678-2
ISSN 1674-487X
Zircon U-Pb Geochronology of the Cenozoic Granitic Mylonite along the Ailaoshan-Red River Shear Zone: New Constraints on the Timing of the Sinistral Shearing Xiaofei Guo1, 2, Yuejun Wang*3, 4, Huichuan Liu3, Jianwei Zi5 1. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China 2. University of Chinese Academy of Sciences, Beijing 100049, China 3. School of Earth Science and Geological Engineering, Sun Yat-sen University, Guangzhou 510275, China 4. CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China 5. School of Earth and Environment, University of Western Australia, Crawley 510275, Western Australia ABSTRACT: The Ailaoshan-Red River (ASRR) shear zone in SW China represents an important discontinuity believed to have accommodated eastward extrusion of the Tibetan Plateau in response to the collision of the Indian and Eurasian plates. The onset timing and duration of the ASRR sinistral strike-slip shearing have been hotly disputed. In this paper we present new zircon LA-ICP-MS U-Pb geochronological data from six syntectonic granitic mylonite and leucosomes samples from the ASRR shear zone. Our data reveal a metamorphic age of ~40 Ma, most likely suggesting the maximum age of the shearing initiation. Rocks showing syn-kinematic signatures yield crystallization ages of 38–22 Ma, with inherited components ranging from 716 to 108 Ma. These results, together with existing geological and geochronological data, indicate that the sinistral shearing along the ASRR zone probably began at 40 Ma, mainly activated at 29–22 Ma and lasted at least to ~22 Ma. Our data suggest a continuous extrusion between the Indochina and South China blocks during ~35–17 Ma. The ASRR sinistral shearing has accommodated large scale eastward displacement of the southeastern Tibetan syntaxis, and is likely responsible for the opening of the South China Sea. KEY WORDS: Ailaoshan-Red River, zircon U-Pb dating, mylonite and migmatite, sinistral shearing, eastward extrusion. 0
INTRODUCTION Tibetan Plateau and surrounding mountains are mostly regarded as a result of the Cenozoic collision of the Indian and Eurasian plates. The Himalayan orogen and its adjacent regions have been hot research areas in the past several decades (e.g., Yang et al., 2015; Xu et al., 2014; Wang et al., 2010; Yeh et al., 2008; Yin and Harrison, 2000; Lacassin et al., 1998; Harrison et al., 1997, 1996). The NW-trending Ailaoshan-Red River (ASRR) shear zone in SW China and NW Vietnam along the southern margin of the Yangtze Block or northern margin of the Indochina Block, plays a very important part in accommodating eastward extrusion of the Tibetan Plateau (e.g., England and Molnar, 1990; Tapponnier et al., 1990, 1986, 1982). In addition, the opening of the South China Sea is considered to be likely related to the eastward extrusion of the Tibetan Plateau and the resultant sinistral shearing along the ASRR zone (e.g., Briais et *Corresponding author:
[email protected] © China University of Geosciences and Springer-Verlag Berlin Heidelberg 2016 Manuscript received September 12, 2015. Manuscript accepted January 15, 2016.
al., 1993; Cande and Kent, 1992; Peltzer and Tapponnier, 1988; Taylor and Hayes, 1983). A better understanding of tectonic evolution of the ASRR zone will shed light on the IndianEurasian interactions and Cenozoic evolution of Southeast Asia. Available data show that the ASRR zone has undergone intensive shearing in response to the Cenozoic Indian-Eurasian interactions (Liu et al., 2012; Searle, 2006; Zhang and Schärer, 1999). The southeastward displacement of the Indochina Block relative to the Yangtze Block along the ASRR zone is estimated to be greater than 500 km (e.g., Liu J L et al., 2012, 2010, 2007; Searle et al., 2010; Leloup et al., 2007, 2001, 1995; Gilley et al., 2003; Wang et al., 2001, 2000, 1998; Harrison et al., 1996, 1992; Schärer et al., 1994, 1990). With the exception of the structural pattern of the zone, the shearing time is still debated (e.g., Sassier et al., 2009; Zhang and Schärer, 1999; Schärer et al., 1994). Previous studies provide a wealth of information for structural and thermochronological data (e.g., Leloup et al., 2007; Gilley et al., 2003; Wang et al., 2000; Zhang and Schärer, 1999). Distinct viewpoints on the sinistral shearing time have been proposed, e.g., >35, ~35, ~32, ~27, ~21, ~22, ~17, and 1 000 km and a width of up to 30 km (Fig. 1b; e.g., Zhong et al., 1990; Tapponnier and Molnar, 1977). Numerous rocks of different ages are exposed in the ASRR zone (e.g., Leloup et al., 2007, 2001, 1995; Yunnan BGMR, 1990). These rocks display strong shear deformation and develop subvertical foliation bearing stretching lineation and isoclinal folds with lineation-parallel hinges (Figs. 2a–2d), also see the descriptions in Cao et al. (2011) and Leloup et al. (2007, 2001, 1995). In the high-strain zone, S- and S-L tectonites are commonly developed (Figs. 2a–2h). Mylonitic foliation is defined by the alignment of mica, tabular quartz grains, pressure shadow beards and seams of opaque materials. Stretching lineation is defined by mineral aggregates of elongated quartz, feldspar rods, streaks of micas and preferably oriented NW-SE or NNW-SSE with a pitch angle of
