Basalt platforms in Inner Mongolia and Hebei Province, northeastern China: New K Ar ages, geochemistries, and revision of palaeomagnetic results

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1 Geophys. J. Int. (22) 151, RESEARCH NOTE Basalt platforms in Inner Mongolia and Hebei Province, northeastern China: New K Ar ages, geochemistries, and revision of palaeomagnetic results Zhong Zheng, 1 Hidefumi Tanaka, 2 Yoshiyuki Tatsumi 3 and Masaru Kono 4 1 Sogokaihatsu Co. Ltd., Nishi-Koiwa , Edogawa-ku, Tokyo , Japan 2 Kochi University, Education, Kochi , Japan. hidefumi@cc.kochi-u.ac.jp 3 Institute for Frontier Research on Earth Evolution, JAMSTEC, Yokosuka, , Japan 4 Institute for Study of the Earth s Interior, Okayama University, Misasa, Tottori-ken, , Japan Accepted 22 May 31. Received 22 May 3; in original form 21 December 4 1 INTRODUCTION Flood basalts of large scale are exposed along the Jiening Zhangjiakou Chifeng area in the southern part of Inner Mongolia and the northern part of Hebei Province, northeastern China. They usually form a platform, a few kilometers across and hundreds of meters height. Based on not very many radiometric ages (e.g. Zhou et al. 1988) and other stratigraphic evidence, it is considered that they were formed mainly in the Neogene period with minor activity in Quaternary. A palaeomagnetic study by Zheng et al. (1991) included 42 sites from 12 basalt platforms near Chifeng in Inner Mongolia and nine sites from three platforms near Zhangjiakou in Hebei Province. In their study, no radiometric ages were available for the basaltic rocks, but a Neogene age was assumed for all the samples from this area. This study reports K Ar ages newly obtained SUMMARY K Ar ages were determined for the basalt samples from northeastern China of which palaeomagnetic results were reported in (Zheng et al. 1991, Geophys, J. Int., 14, 29 4). Samples used for dating were collected at 42 sites from 12 basalt platforms near Chifeng in Inner Mongolia, and 9 sites from 3 platforms near Zhangjiakou in Hebei Province, all of which were stratigraphically dated to the Neogene Period in the previous study. K Ar dating showed that most of the platforms with a supposed age of Neogene were formed in either the Miocene or the Oligocene except one platform, which turned out to be about 9 Ma in age. Hence, minor revision was made to the palaeomagnetic data by discarding the relevant Cretaceous palaeodirections, and updated palaeomagnetic poles were reported. Differences in the new and previous poles are not significant for both Tertiary and Late Cretaceous, but some of the sitemean directions were combined due to their similarity, providing a new data set to be used for the study of palaeosecular variation. Chemical compositions of major and trace elements were measured in some of the Tertiary basalt samples. Secular variation was observed in both the incompatible element concentrations and the normative compositions. The secular variation occurred in a similar trend before and after the volcanic hiatus of 1 25 Ma. It is suggested that the observed secular variation in chemical compositions could be caused by differences both in degrees of partial melting and depths of magma segregation. Key words: chemical composition, China, Hebei, Inner Mongolia, K Ar age, palaeomagnetism. from the samples collection by Zheng et al. (1991). The samples used also include those from additional seven sites from two platforms in the Chifeng area, which are also assumed to be of Neogene age but were not included in the previous palaeomagnetic study. As the age of one basalt platform turned out to be Cretaceous, minor updates will be made to the Tertiary palaeomagnetic pole from this area. This study further included a minor basaltic rock from the Baiqi Formation near Datong, Shanxi Province, which was stratigraphically assigned to Late Jurassic to Early Cretaceous, and this gave a reasonable result of about 12 Ma. Distribution of site localities are plotted on a simplified geographical map in Fig. 1, in which triangles and diamonds indicate those of Tertiary and Cretaceous, respectively. It will be important for the tectonic history of northeastern China to know the origin of these large-scale flood basalts. One of the 654 C 22 RAS

2 Ages and geochemistries from Inner Mongolia 655 Figure 1. Simplified geographical map showing the site localities of basalt platforms (an exception is a minor basaltic rock of the Baiqi Formation near Datong). Closed and open triangles indicate those of the age results in 6 1 Ma and Ma, respectively. Closed and open diamonds indicate those of Late and Early Cretaceous, respectively (the latter is the Baiqi Formation). possibilities would be magmatism caused by extension and continental break up, similar to those observed between two blocks of northern and southern China (Kimura et al. 199). Another possibility would be a small plume within upper mantle, known as a hot region (Miyashiro 1986). It is suspected that the subducted basaltic rocks caused upwelling of these small plumes (Tatsumi et al. 199). This study also reports measurements of chemical compositions for Tertiary basalt samples. Temporal change of chemical compositions will be discussed in terms of mantle dynamics under this region. 2 K A r DATING Dating of the basalt samples was made following a conventional K Ar method (Dalrymple & Lanphere 1969). Measurements of K content were made by a flame photometer (FIP-3M) at the Earthquake Research Institute of University of Tokyo, and those of Ar content were made by a noble gas mass spectrometer (Modified VG54) at the Institute for Study of the Earth s Interior, Okayama University. Samples were selected from the collection of Zheng et al. (1991) in which no thermal treatment was given in the palaeomagnetic measurements. A detailed procedure for measuring K content is given in Tanaka et al. (1997). Reliability of the measurements was confirmed by including two standard samples (JG-1 and JB-1), which gave reasonable agreement with the optimal values by Ando et al. (1971). Most samples gave K content of wt per cent except those from platform CP and a basalt from the Baiqi Formation which gave wt per cent. The K-rich samples from platform CP turned out to be Cretaceous in age. Details of K content are included in Appendix. Measurements of Ar content were done by a conventional isotope dilution method using a spike of 38 Ar, and calculation of K Ar age was made following Nagao et al. (1996) by adopting the decay constants of [y 1 ] and [y 1 ], and the atomic ratio 4 K/K of.1167 (Steiger & Jäger 1977). The experiments included six measurements of biotite extracted from the Fish Canyon Tuff (FCT), USA, which have been widely used as a standard sample at many dating laboratories. The aim of using the FCT biotite was to check the accuracy of measurements of Ar content and the results were in good agreement with a series of previous measurements at Okayama University. For this reason, measurements of K content were not made for these specific FCT samples. Nevertheless, when ages were calculated by supposing the K content of 6. wt per cent with a relative error of 5 per cent, which is a typical value for FCT biotite, the mean age of 28.1 ±.6Maisin good agreement with the previous measurements (ex., Matsumoto et al. 1989). High repeatability of the measurement of Ar content is also demonstrated by excellent agreement of two determinations to the same sample which was made at 6 sites (see the detailed results of K Ar dating summarized in Appendix). Microscope observation of thin sections of the samples was also done. The freshness of some samples was not good and in some cases there was even alteration. Although it is difficult to recognize differences in the obtained ages between the good and altered samples, the ages from the altered samples were finally rejected. Level of sample freshness, indicated from excellent to altered, is included in Appendix. The obtained K Ar ages are summarized in Table 1 with reorganized palaeomagnetic data which will be discussed later. They are mostly Tertiary, as expected, except for platform CP and a basalt sample DZ5 from the Baiqi Formation. Although the age of DZ5 of 12.7±1. Ma is in good agreement with the stratigraphy of Late Jurassic to Early Cretaceous, the platform mean age of 91.3±3.4Ma for CP is a major revision because the age of the platform was supposed to be no different from other Tertiary basalt platforms. Fig. 2 shows histograms of site ages (a) and platform ages (b) for 1 Ma, where a site age is mostly a sample age except for three which are a mean of two determinations and a platform age is usually a mean of several determinations but some are actually sample ages. It is noted that there are two age groups in the Tertiary platforms, one at 5 1 Ma and another at 2 35 Ma. This shows that the ages of platforms are older than previously thought, and the number of platforms is actually larger in Palaeogene than in Neogene. Another feature to be noted is that both of the two age groups are found in the Chifeng area while Zhangjiakou area lacks the younger group. It is difficult, however, to show the significance of this feature because the sample collection had not covered all possible basalt platforms. Another important fact revealed in this study is that there are minor basaltic rocks which were formed in Cretaceous.

3 656 Z. Zheng et al. Table 1. K Ar ages and palaeodirections from basaltic rocks from Inner Mongolia and Hebei Province, northeastern China. Site Lat Lon Age Inc Dec N α 95 Plat Plon ( N) ( E) (Ma) ( N) ( E) 6 1 Ma (Chifeng, Inner Mongolia) CJ ± CJ CSZ ± CSZ ± CX ± CX ± CX ± CX ± CX ± CX ± Combined direction from CHD ± CHD ± CHD ± Platform combined direction CMJ ± CMJ CW ± CW ± CW ± Combined direction from Ma (Chifeng, Inner Mongolia) CG ± CG ± CY ±.53 CY ±.29 CM ± CM ± CM ± CM ± CS ± CS ± CB ± CB ± CB ± CB ± CB ± CB ± Combined direction from CD ±.79 CD ±.58 CD ±.29 CD ± 1.6 CD ±.36 CH ± Ma (Zhangjiakou, Hebei Province) ZG ± ZL ZL ZL ±

4 Ages and geochemistries from Inner Mongolia 657 Table 1. (Continued.) Site Lat Lon Age Inc Dec N α 95 Plat Plon ( N) ( E) (Ma) ( N) ( E) ZL ZL ± ZL ZY ± ZY ± Platform combined direction Late Cretaceous (Chifeng area, Inner Mongolia) CP ± CP ± CP ± CP ± CP ± CP ± CP ± CP ± Combined direction from Combined direction from Early Cretaceous (minor basalt in Datong, Shanxi Province) DZ ± Note: Site: site number which is grouped by each platform; Lat, Lon; latitude and longitude of site locality; Age: K Ar age with its error; Inc, Dec, N: inclination and declination of the site-mean direction, and the number of samples involved to calculate the mean; α 95, 95 per cent confidence circle of the mean direction; Plat, Plon: latitude and longitude of virtual geomagnetic pole. Frequency Frequency (a) Site Ages K-Ar Ages (Ma) (b) Platform Ages K-Ar Ages (Ma) Figure 2. Histograms of K Ar ages for the basaltic rocks in Inner Mongolia and Hebei and Shanxi Provinces for 1 Ma. Site ages (a) are mostly a single sample age, and platform ages (b) are mostly a mean of several sample ages. Ages of about 12 Ma from a basaltic rock of the Baiqi Formation are not included in the histogram. 3 CHEMICAL COMPOSITIONS Major and trace element compositions (Table 2) were measured by RIGAKU Symaltics 355 and 37 X-ray fluorescence (XRF) spectrometers on fused glass beads and pressed powder pellets, respectively. Detailed analytical procedures were described in Goto & Tatsumi (1994, 1996). The basalt samples analysed can be divided into four groups based on their incompatible element concentrations (Fig. 3a). In terms of normative compositions, Group A includes alkali olivine basalts and

5 658 Z. Zheng et al. Table 2. Major, trace, and normative compositions of Tertiary basalts from Inner Mongolia. Group A A A A A B B B C C D D D Sample CJ-1 CJ-2 CSZ-1 CSZ-2 CX-7 CMJ-1 CW-1 CY CD-2 CHD-3 CB-3 CH-1 CS-1 SiO 2 (wt per cent) TiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K 2 O P 2 O Total Rb (ppm) Ba Th Nb Pb Sr Zr Y Ni Q (wt per cent) or ab an ne di hy ol mt il ap Total Age (Ma) Fe 2 O 3, total Fe as Fe 2 O 3. Normative compositions are calculated by assuming Fe 2+ /(Fe ) =.9. Group D is quartz tholeiites, whereas Groups B and C samples are classified as olivine tholeiites (Table 2). It should be stressed that clear secular variation in incompatible element concentrations and magma types are observed for the present samples. The K 7. -value, which is defined as K 2 O content at 7. wt per cent MgO for each group (Fig. 3a), increased with Table 3. Revised palaeomagnetic data from northeastern China. decreasing age both towards and after the volcanic hiatus at 2 1 Ma (Fig. 3c). Along with the variation in incompatible element concentration, the normative compositions exhibit such secular variation (Fig. 3c); normative hypersthene increased, or the degree of silica-saturation decreased, with decreasing ages both at pre- and post-hiatus stages. Study area Site Age N Mean direction Pole Remarks Lat ( N) Lon ( E) (Ma) Inc Dec α 95 Lat ( N) Lon ( E) A 95 Chifeng, Inner Mongolia Chifeng, Inner Mongolia Zhangjiakoiu, Hebei Province combined for Tertiary M-O Chifeng, Inner Mongolia ± Datong, Shanxi Province K Zet91 combined for Late Cretaceous K Note: M: Miocene; O: Oligocene; K 2 : Late Cretaceous; Zet91: previous study by Zheng et al. (1991).

6 K 2 O (wt%) K 7. normative nepheline normative hypersthene Nb/Y (a) MgO (wt%) Volcanic Hiatus Volcanic Hiatus Group-A Group-B Group-C Group-D Age (Ma) (b) Rb/Y (c) (d) Age (Ma) Figure 3. (a) An MgO-K 2 O variation diagram showing the presence of four groups of magmas in the region. The number in the diagram indicates the K 7. -value for each group. (b) Secular variation in K 7. -value. (c) Secular variation in normative compositions. (d) Rb/Y-Nb/Y relations for four groups of magmas. Higher ratios in Groups A and B may be caused by lower degrees of partial melting at deeper levels in the presence of residual garnet. Ages and geochemistries from Inner Mongolia 659 One of the possible explanations that could account for the above secular variations may be decreasing degrees of partial melting, since it has been well established that incompatible element concentrations decreased and degrees of silica-saturation increased with increasing degrees of partial melting of an identical peridotite (e.g. Hirose & Kushiro 1993). Lavas of Group A and B possess both Rb/Y and Nb/Y ratios higher than those for Group C and D lavas (Fig. 3d). These observations may be explained by the presence of garnet in the melting residue for Groups A and B magmas, as Y is an element preferentially partitioned into garnet whereas Rb and Nb are not. Such garnet signatures are consistent with lower degrees of partial melting at higher pressures for Groups A and B than Groups C and D magmas. The occurrence of tholeiitic and alkalic rocks within a single basalt field in NE China has been repeatedly demonstrated (e.g. Zhou & Armstrong 1982; Song et al. 199; Fan & Hooper 1991; Hoang & Flower 1998). One of the characteristic geochemical features for such basalts is a more enriched isotopic signature for tholeiitic lavas. This, together with shallower segregation depths for tholeiitic magmas, has led the above authors to the conclusion that melting within the upwelling asthenospheric mantle creates alkalic magmas whereas tholeiitic magmas result from partial melting of enriched continental lithosphere that was heated by the ascending asthenosphere. If so, then the observed secular variation in chemical compositions (Figs 3b and c) may be attributed to the temporal variation in the mantle activity. Intensive asthenospheric upwelling may cause thermal erosion of lithosphere and may segregate more isotopically enriched magmas at shallow depths and by high degrees of partial melting. Decreasing such mantle activity, on the other hand, may cause separation of magmas, having great contribution from isotopically more depleted asthenosphere and little contribution from enriched subcontinental lithosphere, at deeper depths and by lower degrees of partial melting. 4 REVISION OF THE PALAEOMAGNETIC RESULTS As previously mentioned, the formation age of one basalt platform was in Late Cretaceous, and this is much older than the Neogene age supposed in Zheng et al. (1991). Hence, re-calculation of the palaeomagnetic pole was made for Tertiary by discarding the relevant 8 site-mean directions, which were then combined to previous Late Cretaceous results to calculate a new pole. Re-evaluation was also made on the similarity of site-mean directions within the same platform by the method of McFadden & Lowes (1981). The reason for this is that the merging of similar palaeomagnetic directions was avoided in Zheng et al. (1991) in spite of the fact that possible rapid extrusion of basaltic rocks was suspected in some of the platforms. Site-mean directions were combined when the null hypothesis of having a common mean direction could not be rejected on a significance level of 5 per cent. This procedure gave 33 site-mean or combined directions for Tertiary. The combined directions and VGP positions are indicated by an asterisk in Table 1. Mean palaeomagnetic directions and poles averaged according to area and age group are summarized in Table 3. A Tertiary (Miocene Oligocene) palaeomagnetic pole of (85.5 N, E, A 95 = 8.7 ) was obtained by combining all 33 data. A palaeomagnetic pole for 91 ± 3 Ma averaged from four site-mean or combined directions of platform CP are included in Table 3, and this pole is in good agreement with the previous Late Cretaceous results by Zheng et al. (1991) which were obtained at four sites from a limestone near

7 66 Z. Zheng et al. Datong. A Late Cretaceous palaeomagnetic pole of (79.4 N, E, A 95 = 6.3 ) was obtained by combining the data from the two areas. On the other hand, although the mean palaeomagnetic pole for Tertiary is not significantly different from the previous result, the revised site-mean directions will form a new data set to be used for the study of palaeosecular variation in the Tertiary. From this Tertiary data set, an angular standard deviation (ASD) of 2.1 (N = 28) with upper and lower confidence limits of 24.7 and 16.9, respectively, was obtained after excluding four low latitude VGPs which are apart from the mean by 45 (within site dispersion and mean sample number are 6.6 and 7.3, respectively). The obtained ASD of 2.1 for 42 N is not significantly different from the predicted value from the ASD-latitude curves for Tertiary by McFadden et al. (1991). 5 CONCLUSIONS (1) K Ar ages of basalt platforms in northeastern China indicate that there were two periods of extended basaltic magma activity in 5 1 Ma and 2 35 Ma. (2) Similar secular variation was observed in chemical compositions of major and trace elements contained in the basaltic rocks during these two periods of magma activity. (3) It was found that one platform in Inner Mongolia was formed in Cretaceous. Accordingly, minor revisions were made to the palaeomagnetic poles of Zheng et al. (1991). The difference between the new and old poles are not significant, but the revised Tertiary VGPs will provide a new data set to be used for the study of palaeosecular variation. ACKNOWLEDGMENTS Measurements of potassium were made at the Earthquake Research Institute of the University of Tokyo, and we thank Ichiro Kaneoka for instruction to and use of the facilities. Measurements of argon were made at the Institute for Study of the Earth s Interior, Okayama University. Instruction and help in the experiments by Keisuke Nagao (now at University of Tokyo) are acknowledged. Microscope observations of thin sections were aided by Eiichi Takahashi of Tokyo Institute of Technology, and Kozo Uto and Isoji Miyagi of Geological Survey of Japan. Instructions and professional advice given by them are acknowledged. We thank Martin Flower of University of Illinois and an anonymous reviewer for their professional comments which improved the manuscript. REFERENCES Ando, A., Kurasawa, H., Ohmori, T. & Takeda, E., compilation of data on rock standards JG-1 and JB-1 issued from the Geological Survey of Japan, Geochem. J., 5, Dalrymple, G.B. & Lanphere, M.A., Potassium-Argon Dating Principles, Techniques and Applications to Geochronology, p. 258, W.H. Freeman and Company, San Francisco. Fan, Q.C. & Hooper, P.R., The Cenozoic basaltic rocks of eastern China: petrology and chemical composition, J. Petrol., 32, Goto, A. & Tatsumi, Y., Quantitative analysis of rock samples by an X-ray fluorescence spectrometer (I), The Rigaku Journal, 11, Goto, A. & Tatsumi, Y., Quantitative analysis of rock samples by an X-ray fluorescence spectrometer (II), The Rigaku Journal, 13, Hirose, K. & Kushiro, I., Partial melting of dry peridotites at high pressures: Determination of compositions of melts segregated from peridotite using aggregates of diamond, Earth planet. Sci. Lett., 114, Hoang, N. & Flower, M., Petrogenesis of Cenozoic basalts from Vietnam: Implication for origins of a diffuse igneous Province, J. Petrol., 39, Kimura, G., Takahashi, M. & Kono, M., 199. Mesozoic collision-extrusion tectonics in eastern Asia, Tectonophysics, 181, Matsumoto, A., Uto, K. & Shibata, K., K Ar dating by peak comparison method new technique applicable to rocks younger than.5 Ma, Bull. Geol. Surv. Japan, 4, McFadden, P.L. & Lowes, F.J., The discrimination of mean directions drawn from Fisher distributions, Geophys. J. R. astr. Soc., 67, McFadden, P.L., Merrill, R.T., McElhinny, M.W. & Lee, S., Reversals of the earth s magnetic field and temporal variations of the dynamo families, J. geophys. Res., 96, Miyashiro, A., Hot regions and the origin of marginal basins in the western Pacific, Tectonophysics, 122, Nagao, K., Ogata, A., Miura, Y.N. & Yamaguchi, K., Ar isotope analysis for K Ar dating using two Modified-VG54 mass spectrometers I: Isotope dilution method, J. Mass Spectrom. Soc. Jpn., 44, Song, Y., Frey, F.A. & Zhi, X., 199. Isotopic systematics of Hannuoba basalts, eastern China: implications for their petrogenesis and the composition of subcontinental mantle, Chem. Geol., 85, Steiger, R.H. & Jäger, E., Subcomission on geochronology: Convention on the use of decay constants in geo- and cosmochronology, Earth planet. Sci. Lett., 36, Tanaka, H., Kawamura, K., Nagao, K. & Houghton, B.F., K Ar ages and palaeosecular variation of direction and intensity from Quaternary lava sequences in the Ruapehu volcano, New Zealand, J. Geomag. Geoelectr., 49, Tatsumi, Y., Maruyama, S. & Nohda, S., 199. Mechanism of back arc opening of the Japan sea: role of asthenospheric injection, Tectonophysics, 181, Zheng, Z., Kono, M., Tsunakawa, H., Kimura, G., Wei, Q., Zhu, X. & Hao, T., The apparent polar wander path for the North China block since the Jurassic, Geophys. J. Int., 14, Zhou, X.H. & Armstrong, R.L., Cenozoic rocks of eastern China secular and geographic trends in chemistry and strontium isotopic composition, Earth planet. Sci. Lett., 58, Zhou, X.H., Zhu, B.Q., Liu, R.X. & Chen, W.J., Cenozoic basaltic rocks in Eastern China, in Continental Flood Basalts, pp , ed., Macdougall, J.D., Kluwer Academic, Dordrecht.

8 Ages and geochemistries from Inner Mongolia 661 APPENDIX A: K AR DATING RESULTS Table A1. Detailed results of K Ar dating of basaltic rocks from Inner Mongolia, Hebei and Shanxi Provinces, northeastern China. Sample Lat Lon Weight K 36 Ar 4 Ar/ 36 Ar 4 Ar rad Air Age Freshness ( N) ( E) (mg) (wt. per cent) (1 1 cc g 1 ) (1 8 cc g 1 ) (per cent) (Ma) Chifeng, Inner Mongolia CJ ± ± ± ± ±.7 E CJ ±.5 E CSZ ± ± ± ± ±.22 E CSZ ± ± ± ± ±.2 E Platform Mean ± ±.14 CX ± ± ± ± ±.8 G CX ± ± ± ± ±.9 G CX ± ± ± ± ±.12 G CX ± ± ± ± ±.15 G CX ± ± ± ± ±.1 G CX ± ± ± ± ±.7 G Platform Mean 1.95 ± ±.31 CHD ± ± ± ± ±.24 G CHD ± ± ± ± ±.11 G CHD ± ± ± ± ±.1 G Platform Mean.848 ± ±.3 CMJ ± ± ± ± ±.13 E CW ± ± ± ± ±.12 E CW ± ± ± ±.12 Mean of two determinations 9.92 ±.12 CW ± ± ± ± ±.56 E CW ± ± ± ± ±.14 E CW ± ± ± ±.15 Mean of two determinations 9.64 ±.56 Platform Mean 1.5 ± ±.15 CG ± ± ± ± ±.96 G CG ± ± ± ± ±.6 E Platform Mean.317 ± ± 1.48 CY ± ± ± ± ±.53 CY ± ± ± ± ±.29 Platform Mean.727 ± ± 1.33 CM ± ± ± ± ±.32 P CM ± ± ± ± ±.37 P CM ± ± ± ± ±.54 P CM ± ± ± ± ±.59 P CM ± ± ± ±.59 Mean of two determinations ±.59 Platform Mean.62 ± ±.94 CS ± ± ± ± ±.4 G CS ± ± ± ± ± 1.7 P Platform Mean.27 ± ± 1.34 CB ± ± ± ± ±.28 C CB ± ± ± ± ±.62 G CB ± ± ± ± ±.73 G CB ± ± ± ± ± 1.16 G CB ± ± ± ± ±.9 P CB ± ± ± ± ±.85 P Platform Mean.38 ± ± 1.3 CD ± ± ± ± ±.79 CD ± ± ± ± ±.58

9 662 Z. Zheng et al. Table A1. (Continued.) Sample Lat Lon Weight K 36 Ar 4 Ar/ 36 Ar 4 Ar rad Air Age Freshness ( N) ( E) (mg) (wt. per cent) (1 1 cc g 1 ) (1 8 cc g 1 ) (per cent) (Ma) CD ± ± ± ± ±.29 CD ± ± ± ± ± 1.6 CD ± ± ± ± ±.36 Platform Mean.299 ± ± 3.2 CH ± ± ± ± ± 1.17 C CH ± ± ± ± 1.19 Mean of two determinations ±.25 CP ± ± ± ± ± 1.9 E CP ± ± ± ± ± 1.1 E CP ± ± ± ± ± 1.12 E CP ± ± ± ± ± 1.86 G CP ± ± ± ± 1.87 Mean of two determinations ± 1.87 CP ± ± ± ± ±.98 E CP ± ± ± ± 1. Mean of two determinations ±.99 CP ± ± ± ± ± 1.56 E CP ± ± ± ± ±.87 E CP ± ± ± ± ± 1.4 E CP ± ± ± ± ±.95 E CP ± ± ± ± ±.97 E Platform Mean ± ± 3.39 Zhangjiakou, Hebei Province ZG ± ± ± ± ±.48 G ZL ±.465? ZL ± ± ± ± ±.54? ZL ± ± ± ± ±.31? ZL ± ± ± ± ±.38 C ZL ± ± ± ± ±.29 G Platform Mean.969 ± ±.4 ZY ± ± ± ± ±.31 P ZY ± ± ± ± ±.41 P Platform Mean.99 ± ±.46 Datong, Shanxi Province DZ ± ± ± ± ± 1.6 C DZ ± ± ± ± 1.62 Mean of two determinations ±.98 Standard samples JB ±.2 JB ±.3 JB ±.4 JB ±.9 JB ±.1 JB ±.3 JB-1 Mean ±.11 JG ±.4 JG ±.5 JG-1 Mean 3.44 ±.28 FCT ± ± ± ± 1.42 FCT ± ± ± ± 1.42 FCT ± ± ± ± 1.43 FCT ± ± ± ± 1.4 FCT ± ± ± ± 1.39 FCT ± ± ± ± 1.47 FCT Mean 659. ± ±.58 Note: 4 Ar rad, concentration of radiogenic 4 Ar; Air, fraction of atmospheric 4 Ar in total 4 Ar; Freshness, level of freshness of rock based on observation of thin section (E: excellent, G: good, C: common, P: poor,?: altered);, K Ar ages from altered samples were rejected;, age and error was calculated supposing K content of 6. ±.3 wt. per cent which is a typical value for Fish Canyon Tuff biotite.