Exhumation History of the Gangdese Batholith, Southern Tibetan Plateau: Evidence from Apatite and Zircon (U-Th)/He Thermochronology Author(s): Jingen Dai, Chengshan Wang, Jeremy Hourigan, Zhijun Li, and Guangsheng Zhuang Source: The Journal of Geology, Vol. 121, No. 2 (March 2013), pp. 155-172 Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/10.1086/669250 . Accessed: 09/09/2013 01:52 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Geology. http://www.jstor.org This content downloaded from 132.239.1.230 on Mon, 9 Sep 2013 01:53:00 AM All use subject to JSTOR Terms and Conditions Exhumation History of the Gangdese Batholith, Southern Tibetan Plateau: Evidence from Apatite and Zircon (U-Th)/He Thermochronology Jingen Dai,1,* Chengshan Wang,1 Jeremy Hourigan,2 Zhijun Li,3 and Guangsheng Zhuang4 1. State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences and Resources, Research Center for Tibetan Plateau Geology, China University of Geosciences (Beijing), Beijing 100083, China; 2. Department of Earth and Planetary Sciences, University of California, Santa Cruz, 1156 High Street, Santa Cruz, California 95064, U.S.A.; 3. Chengdu University of Technology, Chengdu 610059, China; 4. Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, Connecticut 06520, U.S.A. ABSTRACT To test previously suggested exhumation histories of the Gangdese Batholith in the central part of the Transhimalayan plutonic belt, we conducted paired apatite and zircon (U-Th)/He thermochronological investigations of the Yarlung Zangbo gorge in the central part of the batholith. Age-elevation relationships and multisystem thermochronometers showed three periods of accelerated exhumation (∼46–48, ∼22–18, and ∼11–8 Ma). Combining these data with pre- viously published thermochronological ages and synthesizing these ages with regional geological events provides an entire exhumation history. The Cretaceous–Early Paleogene exhumation of the Gangdese Batholith was probably caused by both the northward subduction of the Neo-Tethys and the collision between the Lhasa and Qiangtang blocks. The Early Miocene rapid exhumation might be a response to shortening caused by the Gangdese Thrust or erosion driven by dynamic uplift following lithospheric delamination. In contrast, the Late Miocene exhumation is coincident with both the proposed capture of the Yarlung Zangbo gorge by a foreland draining catchment and the intensification of the Asian monsoon, as well as normal faulting. Hence, the latest stage of exhumation might be attributed to the incision of the Yarlung Zangbo gorge, the activity of a north-south fault, or both. Online enhancements: supplementary tables. Introduction The Transhimalyan Batholith is a product of the 1992; Chung et al. 1998; Rowley and Currie 2006; northward subduction of Neo-Tethyan oceanic Wang et al. 2008; Dai et al. 2012). Thermochrono- lithosphere beneath Asia prior to and during the logical studies employing biotite and K-feldspar, early stages of collision of India and Asia (e.g., Yin 40Ar/39Ar, apatite fission track, and apatite (U-Th)/ and Harrison 2000; Zhu et al. 2012). The surface He reveal that central Tibetan Plateau has experi- uplift and exhumation history of the Tibetan Pla- enced slow exhumation for 45 m.yr. (fig. 1a; Rohr- teau has played a critical role in both global climate mann et al. 2012). However, the Gangdese Batho- (Raymo and Ruddiman 1992) and seawater chem- lith in southern Tibetan Plateau has undergone istry (Richter et al. 1992; Misra and Froelich 2012); discontinuous exhumation since the Late Eocene, thus, many studies have attempted to determine with one pulse of rapid exhumation around 20 Ma the timing of plateau growth (e.g., Harrison et al. (Copeland et al. 1987, 1995; Richter et al. 1991; Pan et al. 1993; Chen et al. 1999a, 1999b). Little is known about when this batholith began to expe- Manuscript received May 13, 2012; accepted November 20, 2012. rience a low exhumation rate (e.g., Copeland et al. * Author for correspondence; e-mail: [email protected]. 1987). [The Journal of Geology, 2013, volume 121, p. 155–172] ᭧ 2013 by The University of Chicago. All rights reserved. 0022-1376/2013/12102-0003$15.00. DOI: 10.1086/669250 155 This content downloaded from 132.239.1.230 on Mon, 9 Sep 2013 01:53:00 AM All use subject to JSTOR Terms and Conditions 156 J. DAI ET AL. Figure 1. a, Topography map of the Tibetan Plateau showing the major suture zones and major terranes (Yin and Harrison 2000). The major sutures are as follows: AKSZ p A’nemaqin Kunlun suture zone, JSSZ p Jinshajiang suture zone, BNSZ p Bangong Nujiang suture zone, YZSZ p Yarlung Zangbo suture zone. The area of low erosion rate by 45 Ma is from Rohrmann et al. (2012). b, Simplified geological map showing the distribution of the Gangdese Batholith (Pan et al. 2004) with the locations of Coqin (Murphy et al. 1997), Dajin (Yan et al. 2005), Lajiazi (Wan et al. 2001), Chazi, and Wenbu (Ding and Lai 2003). The Gangdese Batholith is the central part of the et al. 1987). Furthermore, a number of thermo- Transhimalayan plutonic belt, which also contains chronological studies have been conducted in the the Kohistan-Ladakh Batholith in the west and the Ladakh Batholith, and various models have been Burma Batholith in the east (fig. 1b). The Gangdese proposed for explaining the exhumation history of Batholith is exposed throughout southern Tibet the region (Kirstein et al. 2006, 2009; Kumar et al. from Kailas in the west to Lyingchi in the east (fig. 2007; van der Beek et al. 2009; Kirstein 2011). 1b). Systematic geochronological studies indicate To test previously proposed exhumation histo- that plutonic rocks of this batholith were formed ries, we conducted paired apatite (AHe) and zircon between the Late Triassic and the Eocene, with a (ZHe) (U-Th)/He analyses for most samples from peak in magmatic activity between 65 and 41 Ma one transect along the Yarlung Zangbo gorge in cen- (Chu et al. 2006, 2011; Wen et al. 2008; Zhu et al. tral Gangdese Batholith (fig. 3a). Age-elevation 2011). The Cretaceous-Paleogene sediments of the Xigaze forearc basin, which lies in front of the relationships (AERs) and multisystem thermo- Gangdese Batholith, were mostly derived from the chronometers, in combination with published ther- batholith (Du¨ rr 1996; Wang et al. 2012), indicating mochronological data, geological mapping in the that erosion and exhumation of the Gangdese Bath- Coqin area (Murphy et al. 1997), and leucogranites olith began as early as the Cretaceous. However, in central-northern Gangdese (Ding and Lai 2003), extant thermochronological studies do not show allowed us to obtain the entire exhumation history the Cretaceous exhumation (fig. 2; e.g., Copeland of the Gangdese Batholith. This content downloaded from 132.239.1.230 on Mon, 9 Sep 2013 01:53:00 AM All use subject to JSTOR Terms and Conditions JournalofGeology EXHUMATION HISTORY OF THE GANGDESE BATHOLITH 157 Gangdese Batholith is predominantly diorite and granodiorite (Tafti et al. 2009; Tang et al. 2009). We collected 18 samples from a transect in the Yarlung Zangbo gorge, at elevations ranging from 3756 to 5110 m. This transect spanned a short hor- izontal distance of ∼8 km, along which samples were collected with a typical vertical separation of 1354 m. Only eight samples produced good-quality grains and were analyzed. Apatite (U-Th)/He anal- yses were performed on six samples that yielded datable apatites. Five of the six samples were an- alyzed for ZHe ages, and two additional ZHe from samples XC13 and XC15 were dated. Three samples (XC01, XC07, and XC18) were analyzed for zircon U-Pb ages. Five of these samples were collected from biotite granodiorite (XC01, XC04, XC07, XC15, and 5045-7), whereas the other three samples Figure 2. Histograms of thermochronological data from were from hornblende diorite porphyry (XC09, the Gangdese Batholith. Apatite fission-track data are XC13, and XC18). Sample 5045-7 was obtained compiled from Copeland et al. (1987, 1995), Pan et al. (1993), and Yuan et al. (2002, 2009); biotite 40Ar-39Ar data from a drill core at a depth of 425 m, and the other are from Copeland et al. (1987); K-feldspar data are from seven samples were collected from the surface. The Copeland et al. (1995); and apatite and zircon (U-Th)/He rocks examined in this study do not show any pet- data are from this study. A color version of this figure is rographic evidence for metamorphism or meteoric available online. alteration. Geological Setting and Sampling Methods The samples were crushed mechanically. Apatite The Late Triassic to Eocene Gangdese Batholith is and zircon grains were concentrated by using stan- widely exposed in the Lhasa terrane (Chu et al. dard heavy liquid and magnetic separation tech- 2006, 2011; Wen et al. 2008; Zhang et al. 2010, niques. Apatite and zircon grains were handpicked 2012; Guan et al. 2012), together with Cretaceous and photographed under a polarizing Leica MZ16 to Tertiary terrestrial volcanic rocks of the Linzi- stereographic microscope outfitted with a Q- zong Group (figs. 1b,3a; Mo et al. 2008; Lee et al. imaging 5-megapixel digital imaging system.