Zircon U–Pb geochronology of the Konggar granitoid and migmatite: Constraints on the Oligo-Miocene tectono-thermal evolution of the Xianshuihe fault zone, East Tibet

Tectonophysics 606 (2013) 127–139

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Zircon U–Pb geochronology of the Konggar granitoid and migmatite: Constraints on the Oligo-Miocene tectono-thermal evolution of the Xianshuihe fault zone, East Tibet Li Hailong a,b,c, Zhang Yueqiao a,c,⁎ a b c

Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China Key Laboratory of Neotectonic Movement & Geohazard, Ministry of Land and Resources, Beijing 100081, China

a r t i c l e

i n f o

Article history: Received 30 August 2012 Received in revised form 31 May 2013 Accepted 4 July 2013 Available online 11 July 2013 Keywords: Zircon SHRIMP U–Pb dating Konggar massif Granitoid Migmatite Xianshuihe fault zone

a b s t r a c t The Konggar massif, about 120 km long and 13 – 18 km wide, developed along the eastern segment of the Xianshuihe fault zone in East Tibet. It consists of two rock units: the Konggar granitic pluton and an elongate migmatite zone about 70 km long and 1 – 3 km wide. A metamorphic event was well recorded by the growth rims of zircons in the migmatite, their SHRIMP U–Pb dating of two rock samples taken from leucosome and melanosome yields respectively the ages of ca. 31.75 Ma and ca. 26.9 Ma. The SHRIMP U–Pb dating of zircons from two rock samples of the granitic pluton yields respectively the crystalline ages of 17.35 Ma and 14.4 Ma. These new data, together with the previously zircon U–Pb ages, the mica Ar–Ar dating ages and whole rock Rb–Sr age of the granitoid, allows to constraining the tectono-thermal evolution history of the Xianshuihe fault zone. It is inferred that this fault zone suffered along its eastern segment from high temperature metamorphism and migmatization during the Oligocene (in 32 – 27 Ma), which was followed by magma intrusion during the Miocene (in 18 – 12 Ma); sinistral shearing began to occur at about 10 Ma and continues to present-day. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The linkage and temporal relationship between faulting and magmatism or metamorphism are key factors for understanding the mechanism of continental deformation (Miller et al., 1988; Paterson and Schmidt, 1999; Paterson and Tobisch, 1988; Paterson et al., 1989; Phillips and Searle, 2007). One of the key problems in the study of continental deformation concerns how to determine the onset time of large-scale strike-slip or thrust faulting. Thermochronological methods such as mica 40Ar/39Ar and zircon or monazite U–Pb dating have been widely applied to fault zones with partial melting, magmatism and metamorphism, which have been known to occur widely along the large-scale faults that slice the Himalaya–Tibet orogens. This has been demonstrated by the study of the Red River shear zone in southwestern China, where mica 40Ar/39Ar dating gave ca. 27 – 17 Ma, whereas zircon and monazite U–Th–Pb dating yielded ca. 35–32 Ma of the sinistral shearing (Cao et al., 2011; Harrison et al., 1992; Leloup and Kienast, 1993; Leloup et al., 2001, 2007; Schärer et al., 1990, 1994; Tran et al., 1998). The NW–SE striking Xianshuihe fault zone (Fig. 1) is an important active sinistral strike-slip fault which cuts into the Triassic Songpan–Garzê ⁎ Corresponding author at: Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China. Tel.: +86 10 68412311 (O); fax: +86 10 64422326. E-mail addresses: [email protected] (H. Li), [email protected] (Y. Zhang). 0040-1951/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tecto.2013.07.007

Fold Belt (Roger et al., 2004) and slices the eastern part of the Tibetan Plateau. The active features and seismicity and late Cenozoic history of this sinistral strike-slip fault zone have been intensively investigated by numerous authors (Allen et al., 1991; Roger et al., 1995; Tapponnier et al., 2001; Wang et al., 1998; E. Wang et al., 2012; Wen et al., 2003; Xu et al., 1992; Zhang, 2008; Y.Q. Zhang et al., 2004). It is commonly accepted that the sinistral strike-slip faulting began by about 12 – 10 Ma (Roger et al., 1995; Y.Q. Zhang et al., 2004), which accommodated the eastward extrusion of the Chuan-Dian block. Although the fault zone has been well studied in terms of active tectonics and seismotectonics, little knowledge has been gained about its early Cenozoic deformation history. This paper presents the results of our geochronological study of the Konggar massif which developed along the eastern segment of the Xianshuihe fault zone (Fig. 1), with the aim of establishing a well constrained tectono-thermal evolution history of the Xianshuihe fault zone. 2. Geological setting of the Konggar massif 2.1. The Konggar granitic pluton The NE-striking Xianshuihe fault zone bounds the Chuan-Dian block to north and accommodated eastward motion of this block. It continues northwestward along the Yushu–Garze fault, the Dang Jiang fault and the Fenghuoshan thrust fault into central Tibet

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Fig. 1. Simplified structural map of the Xianshuihe fault showing distribution of the Konggar granitic pluton and the migmatite zone. All dated samples and results are indicated. The inset map at low left corner indicates location of the studied area.

(Wang et al., 2008). The fault bends eastward and bifurcates to the N–S striking Anninghe fault and the Daliangshan fault. Together with the Zemuhe–Xiaojiang fault, they form a large-scale convex strike-slip fault zone in eastern Tibet. The Konggar massif develops along the eastern segment of the Xianshuihe fault zone and consists of two rock units: the Konggar granitic pluton and an elongate migmatite zone (Fig. 1). The Konggar granitic pluton has been investigated by several authors (Liu et al., 2006; Roger et al., 1995; Xu et al., 1992). It can be divided into two lithological units, one composed of medium- to fine-grained biotite granite, granodiorite, and monzogranite; another one composed of porphyraceous, porphyry-like medium to coarse-grained biotite granite and monzogranite (Liu et al., 2006). By using single zircon U–Pb dating method, Roger et al. (1995) got the crystallization age about 12.8 Ma for this granitic pluton. Liu et al. (2006) reported a

SHRIMP U–Pb dating result, about 18 Ma, of a rock sample taken from same section as did by Roger et al. (1995) (Fig. 2). This granitic pluton was cut by several branches of the Xianshuihe fault zone. The major active trace of the Xianshuihe fault zone runs along the NE side of the pluton and shows evidence for significant sinistral strike-slip shearing (Wang et al., 2008; Y.Q. Zhang et al., 2004). Thermochronology of the sheared rock was analyzed by using the 40 Ar/39Ar isotope method (Chen et al., 2006; Y.Q. Zhang et al., 2004). The results show two cooling events occurring at 12 – 10 Ma and 5 – 4 Ma, respectively. These dating results are consistent with that of Roger et al. (1995), confirming that the sinistral shearing began by about 12 – 10 Ma. Analysis of zircon and apatite fission track thermochronology showed phased cooling history occurring in the time span of 11 – 2 Ma of this granitic pluton (Lai et al., 2007; Xu and Kamp, 2000). To further constrain the geochronology of this

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Fig. 2. Three structural sections across the Konggar granitic pluton and the migmatite zone. The dated rock samples: Roger et al. (1995), Y.Q. Zhang et al. (2004), Chen et al. (2006), Liu et al. (2006) and Xu and Kamp (2000).

granitic pluton, we collected new samples from the ductile shear zone for zircon SHRIMP U–Pb dating (Fig. 2). 2.2. The migmatite zone The migmatite zone extends about 70 km long and 1 – 3 km wide along the northeastern edge of the Konggar granitic massif (Figs. 1 & 2). It parallels the Xianshuihe fault zone. The characteristic feature of this migmatite zone is the banded composition consisting of leucosome and melanosome (Fig. 3). Typical minerals in the migmatite include garnet, hornblende, plagioclase, K-feldspar, quartz, muscovite, biotite, epidoite, zircons, etc. Relic magmatic texture and felsic component are also observed in the melanosome. We observed, under microscope, quartz polycrystalline ribbons or aggregates in the melanosome, which parallel to the major foliation (Fig. 3d). These textures with serrated grain boundaries on the sides of the feldspars porphyroclasts indicate a grain boundary migration recrystallization formed in high-temperature condition, commonly more than 600 °C (Passchier and Trouw, 2005; Stipp et al., 2002). Plagioclase in the leucosome displays static or substatic recrystallization. These microstructures indicate a dynamic origin of the metamorphism of the migmatite zone. This migmatite zone has been poorly studied and it must contain key information for deciphering the early Cenozoic thermochronology of the Xianshuihe fault zone. We selected two rock samples collected from both the melanosome and the leucosome for zircon SHRIMP U–Pb dating.

and mounted in epoxy resin with standard Temora zircon of 417 Ma (Black et al., 2003). The mount was polished to expose the internal sections of the zircon grains and then gold-coated. Prior to analysis, the internal structures of zircon were documented by cathodoluminescence (CL) using a JEOL scanning electron microscope. Zircon U–Pb isotopic compositions were analyzed using SHRIMP-II instrument at the Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences. The SHRIMP instrumental conditions and data acquisition procedures are the same as described in these articles (Compston et al., 1992; Liu et al., 2003; Song et al., 2002; Williams, 1998; Williams and Claesson, 1987). The measured 206Pb/238U ratios were corrected using a reference Temora zircon. Ages and concordia diagrams were produced using software SQUID 1.03 (Ludwig, 2001) and ISOPLOT 3.00. Correction for common Pb was made by the measured 204Pb. Errors in data table are 1σ; whereas weighted mean ages of individual samples are quoted at 2σ. Mass resolution during the analytical sessions was ~ 5000(1% peak height). Spot sizes were 25–30 μm and the intensity of the primary O− 2 ion beam was 4–6 nA. For our younger zircon populations with high U contents, each spot was rastered for 150 – 200 s prior to analysis. Data were determined by taking 5 mass scans on 90 16 + Zr2 O , 204Pb+, background, 206Pb+, 207Pb+, 208Pb+, 238U+, 232 Thl6O+, and 238U16O+. Standard SL13 (572 Ma, U content = 238 ppm) was used to calibrate contents of U, Th and Pb. The Temora standard was used to calibrate inter-elemental fractionation, Pb/U calibration follows the equation 206Pb+/238U+ = A (UO+/U+)2 (Claoué-Long et al., 1995). Analytical data are listed in Table 1.

3. Analysis procedure and dating results 3.2. The melanosome of migmatite (sample LMS053-1) 3.1. Analysis procedure The location of the dated rock samples is shown in Figs. 1 and 2. The zircons were extracted from separately crushed blocks using standard techniques of combined density and magnetic separation,

Sample LMS053-1 is taken from the melanosome of the migmatite zone exposed west of Kangdian city. Cathodoluminescence (CL) image analysis displays three types of zircons. The first one shows euhedral shape with length/width ratios of 1.5 – 2, and the grain size varying

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Fig. 3. Field views of the outcropped migmatite (a & b) and their microscopic structures (c & d). a: Banded migmatite with vertical foliation. Note a fine grained granite dyke cutting the migmatite zone. b: Well banded and foliated migmatite including garnets in the leucosome. c: Microscopic view of garnets in the migmatite. d: Microscopic view of the foliated migmatite, showing grain boundary migration recrystallization and static or substatic recrystallization subgrains in quartz ribbon.

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Table 1 SHRIMP zircon U–Pb analytical results of 3 samples form the Konggar granitic massif. Point

% 206

Pbc

ppm U

ppm Th

232

Th/238U

Table 1—1: Migmatite (melanosome), LMS053-1 LMS053-1-1.1 1.20 744 97 0.13 LMS053-1-2.1 0.41 1714 117 0.07 LMS053-1-3.1 – 787 115 0.15 LMS053-1-4.1 3.44 822 69 0.09 LMS053-1-4.2 1.59 750 86 0.12 LMS053-1-5.1 1.44 1058 76 0.07 LMS053-1-5.2 1.29 906 65 0.07 LMS053-1-6.1 0.76 1711 394 0.24 LMS053-1-7.1 – 407 40 0.10 LMS053-1-8.1 0.40 1383 242 0.18 LMS053-1-9.1 0.28 1608 123 0.08 LMS053-1-10.1 – 514 54 0.11 LMS053-1-11.1 2.16 540 49 0.09 LMS053-1-12.1 – 1260 101 0.08 LMS053-1-13.1 1.93 727 63 0.09 LMS053-1-13.2 – 1272 120 0.10 LMS053-1-14.1 – 1563 84 0.06 LMS053-1-14.2 1.11 810 44 0.06 LMS053-1-15.1 0.90 743 71 0.10 LMS053-1-15.2 0.02 4734 150 0.03 LMS053-1-16.1 – 1227 75 0.06 LMS053-1-16.2 0.17 2560 785 0.32 LMS053-1-17.1 1.11 868 102 0.12 LMS053-1-18.1 0.27 2765 80 0.03 LMS053-1-19.1 1.28 842 76 0.09 LMS053-1-20.1 0.25 744 75 0.10 LMS053-1-21.1 0.54 1004 61 0.06 LMS053-1-21.2 – 774 155 0.21 LMS053-1-14.3 0.07 1869 76 0.04

206

U/206Pb⁎

Pb/238U Age: Ma

238

3.26 7.74 3.76 3.65 3.50 3.78 3.55 7.29 1.85 6.54 7.29 2.52 2.55 5.77 3.58 4.68 32.1 3.43 3.71 27.9 4.56 12.8 4.36 9.80 3.06 3.61 3.45 3.78 5.92

32.47 33.65 36.03 32.06 34.36 26.41 28.94 31.7 34.69 35.22 33.84 36.90 34.54 34.76 36.13 27.59 152.2 31.35 37.05 44.06 27.92 37.43 37.16 26.47 26.88 36.17 25.56 36.9 23.71

±0.81 ±0.56 ±0.72 ±0.71 ±0.75 ±0.65 ±0.55 ±2.5 ±0.80 ±0.67 ±0.55 ±0.69 ±0.65 ±0.59 ±0.82 ±0.46 ±2.4 ±0.58 ±0.65 ±0.73 ±0.48 ±0.59 ±0.68 ±0.45 ±0.54 ±0.64 ±0.50 ±1.6 ±0.38

198.1 191.1 178.4 200.6 187.1 243.6 222.3 203 185.3 182.5 190.0 174.2 186.2 185.0 177.9 233.1 41.86 205.1 173.5 145.8 230.4 171.7 173.0 243.0 239.3 177.7 251.7 174.4 271.4

ppm 206 Pb*

Pb⁎/206Pb⁎

±%

2.5 1.7 2.0 2.2 2.2 2.5 1.9 7.8 2.3 1.9 1.6 1.9 1.9 1.7 2.3 1.7 1.6 1.9 1.8 1.7 1.7 1.6 1.8 1.7 2.0 1.8 2.0 4.4 1.6

0.0402 0.0480 0.0525 0.0226 0.035 0.044 0.0407 0.0425 0.065 0.0437 0.0460 0.0566 0.0343 0.0596 0.0359 0.0499 0.05021 0.0419 0.0425 0.04745 0.0523 0.0464 0.0415 0.0459 0.0389 0.0472 0.0452 0.0541 0.0465

8.3 5.6 7.2 42 32 27 17 12 17 4.6 5.6 10 18 7.2 26 5.5 1.3 15 11 1.1 7.7 3.2 16 7.6 17 8.7 13 9.2 3.4

±%

207

Table 1—2; Migmatite (leucosome), S051-1 S05-1.1 – 2569 58 S05-2.1 – 4388 135 S05-3.1 – 5676 182 S051-4.1 – 10009 410 S051-5.1 – 1806 31 S051-6.1 – 2145 50 S051-7.1 – 4325 115 S051-8.1 – 2722 62 S051-9.1 – 6048 210 S051-10.1 – 3008 79 S051-11.1 – 2593 60 S051-12.1 – 4407 112 S051-13.1 – 3459 92 S051-14.1 – 6923 205 S051-15.1 – 5036 152 S051-16.1 – 10523 419 S051-17.1 – 4191 128 S051-18.1 – 4892 128 S051-19.1 – 9871 325 S051-20.1 – 4139 116 S051-21.1 – 1972 42 30 S051-22.1 – 1441 S051-23.1 – 4912 155 S051-24.1 – 1539 27 S051-25.1 – 3397 97 S051-26.1 – 1091 22 S051-27.1 – 3345 91 S053-28.1 – 1419 23 S053-29.1 – 1908 41 S053-30.1 – 4310 152 S051-31.1 – 3390 88 S051-18.2 – 5078 354

0.02 0.03 0.03 0.04 0.02 0.02 0.03 0.02 0.04 0.03 0.02 0.03 0.03 0.03 0.03 0.04 0.03 0.03 0.03 0.03 0.02 0.02 0.03 0.02 0.03 0.02 0.03 0.02 0.02 0.04 0.03 0.07

58.9 21.3 32.5 206 38.1 9.26 32.6 30.4 75.3 64.9 10.9 19.5 34.9 28.1 112 204 18.2 35.1 190 17.5 8.48 15.1 46.4 24.8 72.3 18.5 10.9 20.3 33.7 61.1 21.3 25.8

169.4 36.1 42.8 152.5 156.1 32.1 56.2 83.1 92.5 159.4 31.3 33.0 75.1 30.4 164.4 143.7 32.3 53.6 143.1 31.5 32.0 77.7 70.2 119.4 157.6 125.4 24.02 106.3 131.0 105.2 46.8 37.9

±6.1 ±1.3 ±1.6 ±5.5 ±5.6 ±1.2 ±2.1 ±3.0 ±3.5 ±6.2 ±1.2 ±1.2 ±2.7 ±1.2 ±5.9 ±5.2 ±1.2 ±2.0 ±5.2 ±1.2 ±1.2 ±2.9 ±2.8 ±4.4 ±5.8 ±4.6 ±0.89 ±3.9 ±4.8 ±3.8 ±1.7 ±1.5

37.6 178.0 150.2 41.8 40.8 200.6 114.2 77.1 69.2 40.0 205.6 194.9 85.3 211.6 38.7 44.4 199.3 119.9 44.6 204.2 201.2 82.4 91.4 53.5 40.4 50.9 267.9 60.2 48.7 60.8 137.4 169.4

3.7 3.7 3.7 3.7 3.7 3.7 3.8 3.7 3.8 4.0 3.7 3.7 3.7 3.8 3.7 3.7 3.7 3.7 3.7 3.8 3.7 3.7 4.0 3.7 3.7 3.7 3.7 3.7 3.7 3.7 3.7 4.0

0.04634 0.0441 0.04645 0.04807 0.0474 0.0438 0.04675 0.04605 0.04620 0.04639 0.0452 0.0419 0.0465 0.0448 0.0458 0.04776 0.0371 0.04631 0.04770 0.0438 0.0433 0.0429 0.0431 0.0461 0.04763 0.0436 0.0371 0.0454 0.0482 0.0476 0.0443 0.04572

Table 1—3: Granitic mylonite, LMS045-1 LMS045-1-1.1 1.03 1679 883 LMS045-1-2.1 0.66 739 150 LMS045-1-3.1 – 644 358 LMS045-1-4.1 1.57 1180 663 LMS045-1-5.1 0.83 775 376 LMS045-1-6.1 – 2626 1305 LMS045-1-7.1 0.19 5206 2711 LMS045-1-8.1 – 2176 1066

0.54 0.21 0.57 0.58 0.50 0.51 0.54 0.51

4.03 1.73 1.49 2.59 1.50 6.28 11.8 5.07

17.81 17.47 17.31 16.17 14.34 18.00 16.91 17.51

±0.31 ±0.37 ±0.32 ±0.33 ±0.64 ±0.31 ±0.31 ±0.41

361.4 368.5 372.0 398.2 449 357.5 380.6 367.8

1.8 2.1 1.8 2.0 4.5 1.7 1.9 2.4

0.0404 0.0460 0.0550 0.0369 0.050 0.0526 0.0435 0.0470

1.8 2.9 1.9 0.55 2.3 6.6 1.5 2.1 1.8 1.9 4.7 6.7 4.0 2.5 2.2 0.62 7.6 1.8 0.68 4.5 5.0 7.4 3.4 3.6 1.3 3.6 6.0 3.6 2.2 2.9 3.5 2.0

11 20 4.2 20 22 5.5 7.3 6.5

Pb⁎/235U

±%

206

0.0280 0.0346 0.0406 0.0155 0.0258 0.0251 0.0252 0.0289 0.0484 0.0330 0.0334 0.0448 0.0254 0.0444 0.0278 0.0295 0.1654 0.0282 0.0338 0.04488 0.0313 0.0373 0.0330 0.0260 0.0224 0.0366 0.0248 0.0428 0.02361

8.7 5.9 7.5 42 32 27 17 14 18 5.0 5.9 11 18 7.4 26 5.8 2.1 15 11 2.0 7.8 3.6 16 7.8 17 8.9 13 10 3.8

0.1701 0.0341 0.0426 0.1586 0.1602 0.0301 0.0565 0.0824 0.0921 0.1601 0.0303 0.0297 0.0751 0.0292 0.1630 0.1484 0.0256 0.0533 0.1476 0.0295 0.0296 0.0718 0.0651 0.1189 0.1625 0.1180 0.0191 0.1041 0.1363 0.1080 0.0445 0.0372

0.0154 0.0172 0.02040 0.0128 0.0153 0.0203 0.0157 0.0176

207

Pb⁎/238U

±%

Err corr

0.00505 0.005234 0.00561 0.00499 0.00535 0.00410 0.004499 0.00492 0.00540 0.00548 0.005263 0.00574 0.00537 0.005407 0.00562 0.004289 0.02389 0.004875 0.00576 0.00686 0.004341 0.005823 0.00578 0.004115 0.004179 0.005627 0.003972 0.00574 0.003684

2.5 1.7 2.0 2.2 2.2 2.5 1.9 7.8 2.3 1.9 1.6 1.9 1.9 1.7 2.3 1.7 1.6 1.9 1.8 1.7 1.7 1.6 1.8 1.7 2.0 1.8 2.0 4.4 1.6

.289 .283 .266 .053 .068 .093 .109 .544 .132 .383 .279 .177 .103 .229 .088 .289 .764 .122 .157 .830 .220 .439 .115 .219 .120 .199 .147 .428 .428

4.1 4.7 4.1 3.7 4.3 7.6 4.0 4.2 4.2 4.4 6.0 7.6 5.4 4.6 4.3 3.7 8.4 4.1 3.7 5.9 6.2 8.2 5.3 5.1 3.9 5.1 7.1 5.2 4.3 4.7 5.1 4.5

0.02662 0.00562 0.00666 0.02393 0.02452 0.00499 0.00876 0.01298 0.01446 0.02503 0.00486 0.00513 0.01172 0.00473 0.02584 0.02254 0.00502 0.00834 0.02244 0.00490 0.00497 0.01213 0.01094 0.01870 0.02474 0.01965 0.00373 0.01662 0.02053 0.01645 0.00728 0.00590

3.7 3.7 3.7 3.7 3.7 3.7 3.8 3.7 3.8 4.0 3.7 3.7 3.7 3.8 3.7 3.7 3.7 3.7 3.7 3.8 3.7 3.7 4.0 3.7 3.7 3.7 3.7 3.7 3.7 3.7 3.7 4.0

.896 .789 .884 .989 .842 .491 .931 .865 .904 .906 .615 .485 .676 .836 .853 .986 .438 .902 .983 .648 .600 .452 .759 .716 .944 .721 .527 .718 .857 .787 .727 .894

11 20 4.6 20 23 5.7 7.5 6.9

0.002767 0.002714 0.002688 0.002511 0.002226 0.002797 0.002627 0.002719

1.8 2.1 1.8 2.0 4.5 1.7 1.9 2.4

.158 .107 .404 .102 .197 .298 .248 .341

(continued on next page)

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Table 1 (continued) Point

% 206

Pbc

ppm U

ppm Th

232

Th/238U

ppm 206 Pb*

206

Pb/238U Age: Ma

238

U/206Pb⁎

±%

207

Pb⁎/206Pb⁎

±%

207

Pb⁎/235U

±%

206

Pb⁎/238U

±%

Err corr

Table 1—3: Granitic mylonite, LMS045-1 934 1135 976 182 811 326 908 1316 1117 453 4696 2501

0.58 0.22 0.26 0.76 0.27 0.82 0.30 0.27 0.53 0.35 0.62 0.48

3.91 14.1 9.65 0.571 7.81 0.975 7.87 13.4 6.22 3.17 20.9 13.4

17.29 19.69 18.48 14.6 18.80 17.23 18.73 19.34 21.40 17.21 19.78 18.73

±0.31 ±0.31 ±0.30 ±1.2 ±0.31 ±0.61 ±0.30 ±0.32 ±0.37 ±0.37 ±0.31 ±0.30

372.3 327.0 348.4 440 342.5 374 343.8 332.8 300.7 374.0 325.4 343.6

1.8 1.6 1.6 7.9 1.7 3.5 1.6 1.6 1.7 2.1 1.6 1.6

0.0383 0.0438 0.0448 −0.083 0.0429 0.029 0.04661 0.0395 0.0431 0.026 0.0445 0.0458

15 4.3 5.0 84 7.9 82 1.9 8.5 10 41 3.0 4.0

0.0142 0.01846 0.01771 −0.026 0.0173 0.0106 0.01869 0.0163 0.0198 0.0097 0.01887 0.01838

16 4.6 5.3 85 8.0 82 2.5 8.6 10 41 3.4 4.3

0.002686 0.003058 0.002870 0.00227 0.002920 0.002676 0.002909 0.003005 0.003326 0.002674 0.003073 0.002910

1.8 1.6 1.6 7.9 1.7 3.5 1.6 1.6 1.7 2.1 1.6 1.6

.116 .349 .306 .093 .207 .043 .643 .189 .167 .052 .464 .373

Table 1—4: Granitic mylonite, LMS043-1 LMS043-1-1.1 1.50 2186 1351 LMS043-2-2.1 0.96 3397 5121 LMS043-1-3.1 1.46 3941 3443 LMS043-1-4.1 11.91 343 305 LMS043-1-5.1 0.88 2356 1751 LMS043-1-6.1 2.23 1484 826 LMS043-1-7.1 13.70 229 165 LMS043-1-8.1 2.86 3076 3354 LMS043-1-9.1 15.17 155 144 LMS043-1-10.1 23.59 96 68 LMS043-1-11.1 4.67 633 403 LMS043-1-12.1 5.77 1756 1421 LMS043-1-13.1 7.33 200 196 LMS043-1-14.1 11.83 276 167 LMS043-1-15.1 3.97 379 341 LMS043-1-16.1 0.94 1793 1089 LMS046-1-29.1 15.99 530 233

0.64 1.56 0.90 0.92 0.77 0.58 0.74 1.13 0.96 0.73 0.66 0.84 1.01 0.62 0.93 0.63 0.45

4.44 6.33 7.40 0.779 4.50 2.98 0.548 5.87 0.372 0.302 1.47 3.62 0.453 0.657 0.917 3.54 1.30

14.98 13.83 13.88 14.99 14.17 14.69 15.5 13.89 15.3 18.0 16.57 14.56 15.72 15.73 17.42 14.65 15.4

±0.43 ±0.36 ±0.37 ±0.78 ±0.39 ±0.42 ±1.1 ±0.39 ±1.4 ±1.4 ±0.80 ±0.48 ±0.87 ±0.78 ±0.84 ±0.41 ±1.1

430 466 464 430 454 438 416 464 421 357 388 442 410 409 369 439 417

2.9 2.6 2.6 5.2 2.7 2.9 7.0 2.8 9.4 7.9 4.8 3.3 5.5 5.0 4.8 2.8 7.1

0.0504 0.0525 0.0469 0.039 0.0505 0.0519 0.082 0.0372 0.093 0.069 0.036 0.034 0.069 0.079 0.077 0.0531 0.067

15 5.3 9.6 67 10.0 9.6 39 22 80 76 79 45 39 33 34 12 79

0.0162 0.01553 0.0139 0.0126 0.0153 0.0163 0.027 0.0111 0.030 0.027 0.013 0.0105 0.0231 0.0265 0.0286 0.0167 0.022

15 5.9 9.9 67 10 10 40 22 81 76 79 46 39 34 34 13 79

0.002327 0.002148 0.002155 0.00233 0.002201 0.002282 0.00241 0.002157 0.00237 0.00280 0.00257 0.002262 0.00244 0.00244 0.00271 0.002276 0.00240

2.9 2.6 2.6 5.2 2.7 2.9 7.0 2.8 9.4 7.9 4.8 3.3 5.5 5.0 4.8 2.8 7.1

.189 .442 .267 .078 .265 .287 .175 .125 .116 .103 .061 .073 .142 .149 .141 .221 .090

LMS045-1-9.1 LMS045-1-10.1 LMS045-1-11.1 LMS045-1-12.1 LMS045-1-13.1 LMS045-1-14.1 LMS045-1-15.1 LMS045-1-16.1 LMS045-1-17.1 LMS045-1-18.1 LMS045-1-19.1 LMS045-1-20.1

1.47 0.50 0.35 15.69 0.66 3.17 – 1.78 0.68 2.60 0.39 0.27

1667 5336 3901 247 3091 411 3150 5107 2161 1342 7872 5351

Errors are 1-sigma; Pbc and Pb* are the common and radiogenic portions, respectively.

between 100 and 300 μm in length (Fig. 4a-1, 2, 3, 4). These zircons show oscillatory zoning; the Th/U ratios varying in 0.09 – 0.34 (Fig. 4a, b), indicating magma origin. The second type shows euhedral shapes with weak oscillatory or planar zoning (Fig. 4a-5, 6, 7). The Th/U ratios vary in 0.04 – 0.14 (Fig. 4b). The third type shows clear core–rim structure (Fig. 4a-8, 9, 10). The cores have clear oscillatory zoning, while the rims are uniform colored, showing no or planar zoning. These rims were grown by high temperature metamorphism (Andersson et al., 2002). Very low Th/U ratios, 0.02 – 0.10, are a common feature for zircons originating from anatectic melting (Rubatto, 2002; Schaltegger et al., 1999; Wu and Zheng, 2004). Totally 30 points on zircons have been analyzed for this sample. The concordia diagram of these SHRIMP zircon U–Pb ages and their histogram show two sets of ages (Fig. 4c, e, f). Seven points tested on the growth rims of zircons yielded the weighted mean age of 26.9 ± 0.9 Ma (MSWD = 3.09). This age is interpreted to represent the metamorphic age of the rock sample (Fig. 4c, f). The concordia 206 Pb/238U ages of other test points show a range between 31.3 ± 0.6 Ma and 37.4 ± 0.6 Ma (Fig. 4c, e, f). These zircons include the type one, type two and the irregular cores. This age is interpreted to represent the crystallization age of protolith of the migmatite. Eight test points yielded the weighted mean age of 36.75 ± 0.47 (MSWD = 4.6). An inherit zircon yielded the 206Pb/238U age of 152.2 Ma (test point LMS053-1-14.1) (Fig. 4d). 3.3. The leucosome of migmatite (sample S051-1) The sample S051-1 was collected from the leucosome of migmatite zone (Fig. 1). Cathodoluminescence (CL) image analysis shows complex zircon shapes, which can be grouped into three types (Fig. 5a). Type one shows euhedral shape, with length/width ratios of 1 – 2, and the grain length varying between 150 and 300 μm (Fig. 5a-1, 2). These zircons show oscillatory zoning, indicating magma origin. The resorption structure in these zircons indicates that they had been modified by later

metamorphism (Rubatto, 2002; Vavra et al., 1996; Wu and Zheng, 2004; Y.B. Wu et al., 2007). U–Pb dating on these zircons tends to get a mixed age between the inherit age and the metamorphic age (Li, 2009). Type two shows irregular shapes with clear core–rim structure (Fig. 5a-3, 4, 5, 6). The length/width ratio varies between 1 and 1.5, and the grain length between 200 and 300 μm. We observed two irregular cores crowded together with a common growth rim (Fig. 5a-4, 5, 6). These cores have clear resorption structure typical of metamorphic zircons. The rims show planar or sector zoning, suggesting that they are grown by high temperature metamorphism (Andersson et al., 2002). Due to the narrow rims and incomplete metamictization, 206Pb/238U dating on these growth rims may yield ages larger than that of metamorphic event. Type three shows no or planar zoning (Fig. 4a-7,8), typical of metamorphic zircons that formed by metamorphic melt (Hermann et al., 2001). U–Pb dating of these zircons can represent the metamorphic age of the migmatite. Th/U ratios are very low, 0.02 – 0.04, in zircons of this sample, common for anatectic melting rocks (Rubatto, 2002; Schaltegger et al., 1999; Wu and Zheng, 2004). The concordia 206Pb/238U ages of the leucosome range between 160 Ma and 24 Ma, including many mixed ages (Fig. 4b). Six analysis points of the first-type zircons yielded weighted mean 206Pb/238U age of about 160 Ma (Fig. 4b), which can be interpreted to be the crystallization age of the protolith. This inherit age has also been reported previously (Hu et al., 2005; Liu et al., 2006). Other twelve U–Pb ages tested on the growth rims of zircons are concordant. Seven test points have coherent 206Pb/238U ages between 30.4 ± 1.2 Ma and 33 ± 1.2 Ma (Fig. 4b, Tables 1–2), with a weighted mean age of 31.75 ± 0.66 Ma (MSWD = 0.49). It is interpreted to represent the metamorphic or migmatization age. It is worth noting that zircons from the sample S051-1 are characterized by high U content (Fig. 5d). This will produce a highly damaged glassy structure (radiation damage), which is not comparable with the matrix of the zircon standard (Black et al., 1991) and influences the calculation of erroneous Pb/U ratios (McLaren

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Fig. 4. a. Typical CL images of zircons from the sample LMS053-1 showing three types: type one with oscillatory zoning, type two with weak oscillatory zoning or planar zoning, and type three with core–rim structure. b: The 206Pb/238U ages versus Th/U ratios of zircons from the sample LMS053-1. c: Histogram of zircon U–Pb data displaying two sets of ages. d: Concordia diagram with all dated ages. e: Separation of the SHRIMP U–Pb dates. f: Two concordia 206Pb/238U ages of the migmatite: the 36.75 ± 0.47 Ma determined by eight test points of zircons with oscillatory zoning, and the 26.0 ± 0.9 Ma determined by the ages of the growth rims of zircons.

et al., 1994). Only those measured 206Pb/238U ages that are independent with U content can be used for SHRIMP zircon geochronology (Williams and Hergt, 2000). The correlation between the 206Pb/ 238 U ages and U contents for all investigated zircon fractions shows that the same 206Pb/238U age can be obtained from a wide range of U contents of zircons, implying that the variations of U contents have no bearing on validity of the ages. Thus the dated growth rims of zircons could represent the estimate of metamorphic age. 3.4. Granite Two granitic rock samples, LMS045-1 and LMS043-1, were selected for zircon SHRIMP U–Pb dating. They were collected from a ductile

shear zone in NW of the granite pluton (section A–A′ in Figs. 1, 2). This section exhibits mylonitic rocks, sheared pegmatite and fine- to coarse-grained granites. Zircons from the sample LMS045-1 show oscillatory zoning on CL images (Fig. 6a), with length/width ratios varying from 2 to 4, indicating their magmatic origin. The grain size varies between 100 and 200 μm in length. High U content (50bUb8000 ppm) is a dramatic feature in the zircons, with Th/U ratio of 0.21–0.82. High U concentration is partly correlated with the measured 206Pb/238U age of the mylonite (Fig. 7a). Williams and Hergt (2000) found that high U content could cause a wide range of untenable zircon crystallization ages, and that a correlation between Pb/U ages and U content can give a good internal consistency with U content b3000 ppm. These authors

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Fig. 5. a. Typical CL images of zircons from the lecuosome (sample S051-1) of migmatite showing three types of zircons. The first one is featured by oscillatory zoning and resorption structure indicating magmatic origin and later modification by metamorphism (5a-1, 2); the second one displays clear core–rim structure (5a-3, 4, 5, 6), and the rims show planar or nebulous zoning, suggesting metamorphic origin. The third one with no or planar zoning (5a-7,8), formed by metamorphic melt. b: Concordia diagram of the SHRIMP U–Pb dated ages. c: The concordia 206Pb/238U mean ages(31.75 ± 0.88 Ma) determined by the ages of the growth rims of zircons. d: 206Pb/238U ages versus U contents for the sample S051-1, showing no concentration between U content and the 206Pb/238U ages.

(Williams and Hergt, 2000) observed that some high-U grains yielded weighted mean radiogenic 206Pb/238U age similar to that from relatively low U grains and baddeleyite U–Pb age, implying that ages from high U grains of zircon can be meaningful. A correlation between the measured 206Pb/238U age and U content for the sample LMS045-1 was made (Fig. 6a), which shows a good consistency for the grains with U content b 3000 ppm. Seven test points with U b3000 ppm yielded a weighted mean 207Pb/235U age of 17.35 ± 0.43 Ma, MSWD = 2.5 (Fig. 7b), which is consistent with a 206Pb/ 238 U value of 17.5 ± 0.64 Ma, MSWD = 1.9, from the raw U–Pb date (Fig. 7c). This age is interpreted to be the crystallization age of the mylonitic granite. Sample LMS043-1 is taken from the same area as the sample LMS045-1. CL image analysis of zircons from this sample shows oscillatory zoning with aspect ratios varying between 2 and 3, indicating their magmatic origin. The grain size varies between 80 and 150 μm in length (Fig. 6b). U concentration in zircons ranges in 96–3357 ppm, and Th/U ratio in 0.45–1.56. The correlation between the 206Pb/238U ages and the U contents shows variable U concentration for the same ages (Fig. 7d), implying that the measured grains were not affected by the

U-dependent 206Pb/238U bias. 13 of 18 analyses on 13 zircons yielded a weighted mean 207Pb/235U age of 14.4 ± 0.3 Ma (MSWD = 1.4) (Fig. 7e), which is interpreted to be the crystallization age of the rock sample. 4. Discussions and implications 4.1. Oligocene metamorphic event Our study brings new geochronological data for unraveling Oligocene tectono-thermal events of the Xianshuihe fault zone. Leucosome is generally considered as the result of partial melting related to migmatization, and the U–Pb dating of zircons from the leucosome could therefore probably constrain the crystallization age of partially melting materials. However, our SHRIMP U–Pb dating of zircons from both the leucosome and melanosome of the migmatite yielded complex age spectrum. The growth rims of zircons from the melanosome and the leucosome yielded respectively two weighted mean ages of about 26.9 Ma and 31.75 Ma, suggesting that metamorphism may have occurred in a protracted period of 32 – 27 Ma. The zircon

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135

Table 2 A summary of the isochronic dates of the Konggar granite and the migmatite zone. Rock

Sample

Location

Method

Age

Authors

Migmatite melanosome

LMS053-1 S051-1

36.75 ± 0.47 Ma 27.6 ± 0.9 Ma (rims) 31.75 ± 0.88 Ma (rims)

Granite

CX3039-3

18 ± 0.3 Ma

Liu et al. (2006)

Granitic Mylonite

LMS045-1 LMS043-1 CDU143 CDU59

N30°32.580′ E101°37.549′ See Fig. 1a See Fig. 1a See Fig. 1a See Fig. 1a N30°32.580′ E101°37.549′ N30°32.014′ E101°37.13′ N30°32.708′ E101°37.28′ N30°32.760′ E101°37.44′ See Xu and Kamp (2002), Fig. 2, Transect V Tables 1 and 2

17.35 ± 0.43 Ma 14.4 ± 0.3 Ma 11.6 ± 0.4 Ma 12.8 ± 1.4 Ma 9.9 ± 1.6 Ma 10.4 ± 1.2 Ma 11 Ma (estimated) 10.39 ± 0.01 Ma

This study

CDU60 CDU142 CX10

Zircon SHRIMP U–Pb Zircon SHRIMP U–Pb Zircon SHRIMP U–Pb Zircon SHRIMP U–Pb Rb–Sr (biotite) U–Pb Rb–Sr (whole rock) Rb–Sr (feldspar) Rb–Sr (whole rock) 40 Ar/39Ar (mica)

This study

Migmatite leucosome

N29°59.221′ E101°56.263′ N30°16.787′ E101°50.381′ See Fig. 1a

Granite Pegmatite

Breccia

CX09

Pegmatite

CX13

Mylonite

CX14

Granite

2 3 4 5 6 7 8 9 10 11 12 57, same as LMS045-1 13 14 15 16 17 18 19 20 21

Mylonite Proterozoic rock east of the migmatite

U–Pb age of about 36.75 Ma from the melanosome represents possibly an age of protolith of the migmatite. In fact, Oligocene magmatism has been reported to occur alongside the Ganze–Yushu and the Fenghuoshan fault zones (Spurlin et al., 2005; Wang et al., 2002; Z.H. Wu et al., 2007, 2009), accompanied with formation of Pb–Zn ore deposits (Tian et al., 2009). SHRIMP U–Pb isotopic dating of granites from the Hohxili basin yielded two sets of emplacement ages: about 34.5 Ma and about 27.6 Ma (Y.B. Wu et al., 2007; Z.H. Wu et al., 2007). Late Eocene to early Oligocene alkali granites developed along the Anninghe fault (Hou et al., 2006). It is worth noting that Oligocene metamorphism along the eastern segment of the Xianshuihe fault zone may have occurred coevally with the sinistral strike-slip shearing along the Ailao Shan–Red River shear zone: a monazite U–Pb age about 35 Ma for a monzonitic subvolcanic

40

Ar/39Ar (biotite)

10.12 ± 0.02 Ma

40

Ar/39Ar (K-feldspar)

12.02 Ma ± 0.8 Ma 5.18 ± 0.02 Ma 5.47 ± 0.01 Ma

40

39

Ar/ Ar (biotite)

ZFT/AFT

7.1 ± 0.6 Ma/2.7 ± 0.4 Ma 5.4 ± 0.6 Ma/1.9 ± 0.2 Ma 4.7 ± 0.4 Ma/1.6 ± 0.4 Ma 5.4 ± 0.8 Ma/2.3 ± 0.8 Ma 3.7 ± 0.3 Ma/2.6 ± 1.5 Ma 3.0 ± 0.2 Ma/2.9 ± 0.5 Ma 4.0 ± 0.2 Ma/1.8 ± 0.3 Ma 7.3 ± 0.4 Ma/1.9 ± 0.3 Ma 2.8 ± 0.2 Ma/2.6 ± 0.5 Ma 5.3 ± 0.3 Ma/2.0 ± 0.5 Ma 3.2 ± 0.3 Ma/2.1 ± 0.5 Ma 8.7 ± 0.6 Ma/3.1 ± 0.6 Ma 25.1 ± 1.2 Ma/14.3 ± 2.6 Ma 19.9 ± 1.4 Ma/9.8 ± 4.1 Ma 21 ± 1.4 Ma/6.8 ± 1.1 Ma 22.4 ± 2.1 Ma/7.6 ± 1.1 Ma 18.6 ± 1.4 Ma/4.5 ± 1.1 Ma 16.8 ± 1.5 Ma/5.8 ± 1.2 Ma 18.2 ± 0.8 Ma/6.5 ± 1.3 Ma 20.5 ± 1.5 Ma/5.4 ± 1.0 Ma 17.4 ± 1.6 Ma/7.7 ± 1.0 Ma

This study

Roger et al. (1995)

Y.Q. Zhang et al. (2004), H. Zhang et al. (2004)

Xu and Kamp (2000)

intrusion (Schärer et al., 1990), three zircon U–Pb ages about 32 Ma for the pre-kinematic related granites (Cao et al., 2011), and two zircon and titanite U–Pb ages about 26 Ma for monzonites in the Ailao Shan gneisses (Schärer et al., 1990, 1994). However, the kinematic link between the Red River shear zone and the Xianshuihe fault zone remains a subject for further investigation. 4.2. Episodic emplacements of the Konggar granitic pluton New zircon U–Pb ages of the granitic rocks from the Konggar pluton confirm the results of earlier studies, showing that the granitic pluton was massively emplaced in the time span of 18 – 12 Ma (Liu et al., 2006; Roger et al., 1995). These dates further point out that the emplacement was pulse or episodic, occurring at 18–17 Ma,

Fig. 6. Typical CL images of zircons from two granite samples LMS045-1 and LMS043-1, showing oscillatory zoning and indicating magmatic origin.

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Fig. 7. Correlation between 206Pb/238U ages and U contents and concordia diagrams of SHRIMP zircon U–Pb date for the granite samples LMS045-1 and LMS043-1. a: 206Pb/238U ages versus U contents for the sample LMS045-1, showing an increase in 206Pb/238U ages with U content, which suggests existence of U-dependent 206Pb/238U bias in the rock sample. Only those with U content below 3000 ppm show a good internal consistency between the 206Pb/238U age and U content and are used to constrain the crystalline age of the granite (e.g. Williams and Hergt, 2000). b: Concordia diagram of the U–Pb ages of spots with U content b3000 ppm, which yields weighted mean 207Pb/235U age for the rock sample LMS045-1. c: Concordia diagram of the raw U–Pb date with U b3000 ppm for the sample LMS045-1, which yields the same value as the 206Pb/238U age. d: 206Pb/238U ages versus U contents for the sample LMS043-1. Three points out of the dotted box are discordant data. Other 206Pb/238U ages show a good internal consistency with U contents. e: Concordia diagram of the SHRIMP zircon U–Pb date (without the three discordant date), which yields a weighted mean 207Pb/235U age of 14.4 Ma for the sample LMS043-1.

14 Ma and 13–12 Ma. These episodes of Miocene magmatism have been well recognized elsewhere in the Tibetan Plateau, for instance, the Nyainqentanglha range of 18.3–11.1 Ma (Wu et al., 2005), the northern Himalayan leucogranites and the high Himalayan leucogranites of 15–10 Ma and 27.5–10 Ma (Harrison et al., 1995; Zhang et al., 2004). Geochemical analysis suggested that the Konggar granitic pluton may be originated from partial melting of the Pre-Cambrian basement rocks (Liu et al., 2006). The granite suffered from ductile shearing along the Xianshuihe fault zone. The deformation age of this sinistral strike-slip shearing had been constrained by the mica Ar–Ar ages (H. Zhang et al., 2004) and the whole-rock Rb–Sr age (Roger et al., 1995). The quite consistent among these ages indicates that strongly sinistral shearing began at least by about 10 Ma along the fault. This onset time of shear deformation was further constrained by the minimum age of zircon U–Pb geochronology, and suggests that sinistral strike slip

shearing on the Xianshuihe fault occurred after the emplacement of the granitic pluton.

4.3. Cooling history of the Konggar massif Table 2 gathers all dating data by different methods (SHRIMP U–Pb, mica Ar–Ar, Rb–Sr, zircon and apatite fission-track ages), which allow to establish a possible cooling history of the Konggar massif. The cooling path was drawn based on the following closure temperatures: zircon U–Pb system at above 800 °C, even above 900 °C (Lee et al., 1997; Mezger and Krogstad, 1997); whole rock Rb–Sr system about 600 °C (Faure and Mensing, 2005); biotite Rb–Sr system about 300 ± 50 °C (Cliff, 1985); muscovite 40Ar/39Ar system about 400 ± 50 °C; biotite 40 Ar/39Ar system about 300 ± 50 °C (Hames and Bowring, 1994); zircon FT system about 210 ± 20 °C; and apatite FT system at

H. Li, Y. Zhang / Tectonophysics 606 (2013) 127–139

Fig. 8. Possible cooling paths of the Konggar granitic massif (including the migmatite, sheared and undeformed granite). Zr: zircon; W.R: whole rock; Bi: biotite; Ms: muscovite; ZFT: zircon fission track; AFT: apatite fission track.

90 – 120 °C (Hourigan et al., 2004; Laslett et al., 1987; Zaun and Wagner, 1985).

137

Fig. 8 shows possible cooling paths of both the migmatite zone and the granitic pluton. The zircon and apatite fission-track data of the Proterozoic rocks exposed in the area northeast of the migmatite zone (Xu and Kamp, 2000) are used to get a possible cooling path of the migmatite zone. This migmatite zone shows a two-phase exhumation history: a rapid cooling phase, about 46–58 °C/Ma, in the time of 32–20 Ma, which occurred subsequent to the Oligocene migmatization, and a slow cooling phase, about 6–14°C/Ma, during the last 20 Ma. Meanwhile, the Konggar granitic pluton shows a rapid cooling since the granite emplacement, with a cooling rate of about 27 – 100°C/Ma during the last 12–10 Ma. The exhumation history of the migmatite and granitic pluton along the eastern segment of the Xianshuihe fault zone is quite consistent with that of the Pengguan complex in central Longmenshan (Wang et al., 2012b). Thus, the cooling history of the Konggar massif (migmatite and granitic pluton) allows to decipher two major tectono-thermal events (Fig. 9). The Oligocene metamorphism took place in the time span of 32–27 Ma and generated the migmatite zone along the eastern segment of the Xianshuihe fault zone. This dynamic metamorphic zone underwent a fast cooling before 20 Ma and was followed in Miocene by pulse magma emplacements occurring at 18–17 Ma, 14–12 Ma, of the Konggar granitic pluton. This granitic pluton was then cooled rapidly since its emplacement and suffered from strong sinistral strike-slip shearing since 12–10 Ma.

Fig. 9. Sketch showing the tectono-thermal evolution history of the Konggar massif along the eastern segment of the Xianshuihe fault zone. High temperature metamorphism occurred in Oligocene (32–27 Ma) that generated the migmatite zone along the Xianshuihe fault; this metamorphic event was followed during the Miocene (18–12 Ma) by episodic emplacements of the Konggar granitic pluton. Then the Konggar massif including the migmatite and granitic pluton suffered since 12–10 Ma from intensive sinistral strike-slip shearing along the Xianshuihe fault zone.

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5. Conclusions New zircon SHRIMP U–Pb isotopes of the migmatite from the Konggar massif revealed a high temperature metamorphic event occurring at ca. 32 – 27 Ma and suggest that the Xianshuihe fault zone underwent intense migmatization during the Oligocene. Furthermore, zircon SHRIMP U–Pb dating results of the granites confirm that the Konggar granitic pluton was emplaced episodically from 18 Ma to 12 Ma, and suffered subsequently from strong sinistral strike-slip shearing initiated at 12–10 Ma. This geochronological evolution of the Xianshuihe fault zone may shed a new insight into the timing of the Oligo-Miocene orogeny in the southeastern part of the Tibetan Plateau.

Acknowledgments This study was supported jointly by the Fundamental Research Funds for the Institute of Geomechanics, Chinese Academy of Geological Sciences (DZLXJK201210) and the project SinoProbe-08-01, the National Key Basic Project (973, granted number 2008CB425702). We thank Doctor Xian-bing Xu for field assistance, Doctor Jian-hua Li, Jian-hui Liu, Li-lin Du, and Hang-qiang Xie for their help in SHRIMP testing and analysis, and to Doctor Cao Shu-yun for the help in microscopic structures analysis. We acknowledge Professor Ji Shaocheng and Professor Wang Bo for their critic reviews and constructive comments that greatly improve the manuscript.

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