Appendix: Analytical methods 
Major element compositions of minerals were measured on a JEOL JXA-8100 electron microprobe at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). The operating conditions are: 15 kv accelerating voltage, 10 nA beam current and 3 μm spot diameter. Well-defined natural mineral standards are applied for calibration. 
Zircons were separated from a pyroxene syenite (JN08-1) and a quartz syenite (JN07-19) sample using standard density and magnetic separation techniques and purified by handpicking under a binocular microscope. Cathodoluminescence images were obtained for zircons prior to analyses, using a JXA-8100 microprobe at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), to reveal their internal structures.  
Zircon U-Pb dating was conducted on an Agilent 7500a ICP-MS equipped with 193 nm laser, housed at the State Key Laboratory of Lithospheric Evolution, IGGCAS. Harvard zircon 91500 was used as a standard and the standard silicate glass NIST was used to optimize the machine. The spot diameter was 30 μm. The detailed analytical technique is similar to that described in Xie et al. (2008). The common- Pb corrections were made using the method of Anderson (2002) and the data were processed using the GLITTER (Griffin et al., 2008) and ISOPLOT (Ludwig, 2001) programs. Individual analyses are reported with 1σ uncertainties; while weighted mean ages are reported at the 2σ confidence level. 
For geochemical and isotopic analyses, samples were grounded in an agate mill to ~200 mesh. Major oxides were analyzed with a Phillips PW 2400 X-Ray fluorescence spectrometer (XRF) at the IGGCAS. Trace element abundances were obtained on a VG-PQII ICP-MS also at the IGGCAS. Samples were dissolved in distilled HF+HNO3 in 15 ml high-pressure Teflon bombs at 120℃ for 6 days, dried and then diluted to 50ml for analysis. A blank solution was prepared and the total procedural blank was < 50 ng for all trace elements. Indium was used as an internal standard to correct for matrix effects and instrument drift. Precision for all trace elements is estimated to be 5% and accuracy is better than 5% for most elements by analyses of the GSR-3 standard.  
Sr and Nd isotopic compositions were measured on a Finnigan Mat 262 thermal ionization mass spectrometer at the IGGCAS, following the procedure described in Zhang et al. (2008). Procedural blanks were < 100 pg for Sm and Nd and < 500 pg for Rb and Sr. 143Nd/144Nd was corrected for mass fractionation by normalization to 146Nd/144Nd = 0.7219, and 87Sr/86Sr ratios were normalized to 86Sr/88Sr = 0.1194. The measured values for the BCR-2 Nd standard and NBS-987 Sr standard were 143Nd/144Nd = 0.512613  0.000012 (2σ, n=3) and 87Sr/86Sr = 0.710251  0.000011 (2σ, n=5) during the period of data acquisition. 
  In situ zircon Hf isotopic analyses were conducted using the Neptune MC-ICP-MS, equipped with a 193-nm laser at the IGGCAS. Spot size of 32 μm was used for analysis, with a laser repetition rate of 10 Hz at 100 mJ. The detailed analytical procedure and correction for interferences follow those described by Wu et al. (2006). During analyses, the 176Hf/177Hf and 176Lu/177Hf ratios of the standard zircon (91500) were 0.282270 ± 0.000023 (2rn, n=15) and 0.00028, similar to the commonly accepted 176Hf/177Hf ratio of 0.282284 ± 0.000003 (1r) measured using the solution method (Woodhead et al., 2004). 
Anderson, T. 2002. Corrections of common lead in U-Pb analyses that do not report 204Pb. Chemical Geology, 192, 59-79. 
Griffin, W.L., Powell, W.J., Pearson, N.J. & O'Reilly, S.Y. 2008: GLITTER: data reduction software for laser ablation ICP-MS. In Laser Ablation–ICP–MS in the Earth Sciences (P. Sylvester, ed.). Mineralogical Association of Canada Short Course Series Volume 40, Appendix 2, 204-207. 
Ludwig, K., 2001. User manual for isoplot/EX (2.49). Berkeley Geochronology Center Special Publication No. 1a. 55p. 
Woodhead J.D., Hergt J., Shelley M., Eggins S. & Kemp R. 2004. Zircon Hf-isotope analysis with an excimer laser, depth profiling, ablation of complex geometries, and concomitant age estimation. Chemical Geology, 209, 121-135. 
Wu F. Y., Yang Y. H., Xie L. W., Yang, J. H. & Xu, P. 2006. Hf isotopic compositions of the standard zircons and baddeleyites used in U–Pb geochronology. Chemical Geology, 234, 105-126.
Xie, L.W., Zhang, Y.B., Zhang, H. H., Sun, J. F. & Wu, F.Y. 2008. In situ simultaneous determination of trace element, U-Pb and Lu-Hf isotopes in zircon and baddeleyite. Chinese Science Bulletin, 53, 1565-1573. 
Zhang, X.H., Zhang, H.F., Tang, Y.J., Wilde, S.A. & Hu, Z.C. 2008. Geochemistry of Permian bimodal volcanic rocks from Central Inner Mongolia, North China: Implication for Tectonic setting and Phanerozoic continental growth in Central Asian Orogenic Belt. Chemical Geology, 249, 261-281.