Results and discussion Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol102.Issue3-4.2000:

dard deviation is usually B 1 for major ele- ments except H 2 O + , B 4 for REE and Y, and 5 – 10 for trace elements. Sr and Nd isotopic analyses were performed at the Isotope Laboratory of the Yichang Institute of Geology and Mineral Resources, by procedures described by Ma et al. 1998. Repeated measure- ments n = 6 of the NBS987 and La Jolla Nd 2 O 3 standards were taken throughout the analytical period and yielded the following average ratios: 87 Sr 86 Sr = 0.71026 9 5 2 SD, and 143 Nd 144 Nd = 0.511845 9 10 2 SD. 87 Rb 86 Sr and 147 Sm 144 Nd ratios were determined to a precision of 9 0.7 and 9 0.2, respectively. Total proce- dural blanks are insignificant: B 1 ng for Sr, and 86 pg for Nd. Parameters used for calculations are: 143 Nd 144 Nd CHUR, today = 0.512638, 147 Sm 144 Nd CHUR, today = 0.1967. DM values for Nd are 143 Nd 144 Nd today = 0.51315, 147 Sm 144 Nd today = 0.2137. Decay constant l: for 87 Rb = 1.42 × 10 − 5 Ma − 1 , and for 147 Sm = 6.54 × 10 − 6 Ma − 1 . The nota- tions of o Nd 0 and o Nd t are defined as: o Nd 0 = [ 143 Nd 144 Nd meas. 143 Nd 144 Nd CHUR, today − 1] × 10 4 , o Nd t = [ 143 Nd 144 Nd initial. 143 Nd 144 Nd CHUR, t − 1] × 10 4 . Where, meas = measurement, 143 Nd 144 Nd initial is the initial ratio of the sample suite at the time of its formation t and is calculated from the expression 143 Nd 144 Nd initial = 143 Nd 144 Nd meas − 147 Sm 144 Nd meas e lt − 1. 143 Nd 144 Nd CHUR, t is the isotope ratio of CHUR at time t and is given by the expression: 143 Nd 144 Nd CHUR, t = 0.512638 − 0.2137 × e lt − 1. Sm – Nd model ages T DM were calculated using a linear isotopic ratio growth equation Peu- cat et al., 1989: T DM = l − 1 ln {1 + 143 Nd 144 Nd meas. − 0.51315 147 Sm 144 Nd meas. − 0.2137}.

4. Results and discussion

VSB 4 . 1 . Geochemical characteristics of felsic and grey gneisses Representative analyses of major, trace and rare earth elements for felsic and grey gneisses are presented in Tables 1 and 2. Among them, the Kongling grey gneisses were mainly analyzed by Li Fuxi and Ma Daquan 1991, unpublished data and Ma et al. 1997, while a few data for the Dabieshan felsic gneisses are from You et al. 1996, Sang et al. 1997 and Wu et al. 1998. The Kongling grey gneisses are high-Al TTG rocks, according to the criteria of Barker 1979 and Drummond and Defant 1990, with Al 2 O 3 \ 15 wt. at the 70 wt. SiO 2 level, high Sr \ 300 ppm, and low RbSr B 0.15, KRb typically B 550, Y B 15 ppm, and Nb B 10 ppm, ex- cept in sample H44. The average concentrations of Rb 36 9 14ppm, Sr 594 9 63 ppm and Ni 11 9 2 ppm are also similar to the values pro- posed by Condie 1981 for ‘high Al 2 O 3 ’ Archean gneisses. Leat et al. 1986 have argued that the TiO 2 versus Zr plot of Pearce 1980 can effec- tively separate peralkaline from sub-alkaline felsic volcanics, and that the low-Zr group is subalka- line B 350 – 700 ppm Zr Fig. 3. The Kongling gneisses fall into the intermediate and silicic ‘vol- canic arc’ field below line A-B on Fig. 3, and are all depleted in TiO 2 and Zr relative to ‘within plate’ rocks. Being low in Zr content B 350 ppm, rocks from the KLC are subalkaline Leat et al., 1986, and fall into the calc-alkaline field of Fig. 3. Their REE patterns are highly fractionated [LaYb n = 48.85 − 94.60], and may be divided into two subgroups with negative and positive Eu anomalies Fig. 4. In the chrondrite normalized LaYb versus Yb diagram Fig. 5A, the Kongling grey gneisses display typical Archean features Jahn et al., 1981; Martin, 1986. Immobile trace elements are also used to test the tectonomag- matic affinity of the grey gneisses on the discrimi- nation diagram of Pearce et al. 1984. The data confirm the orogenic affinity of their protoliths, and all the grey gneisses from the KLC fall in the field of volcanic arc granitoids Fig. 5B. The Dabieshan felsic gneisses display a large variation in chemical composition. Compared with high-Al trondhjemitic gneisses fron the KLC, they are richer in Zr and TiO 2 Fig. 3, and are not high-Al granitoids. However, their low Zr contents B 500 ppm also indicate a subalkaline C . Ma et al . Precambrian Research 102 2000 279 – 301 285 Table 1 Chemical compositions of the Dabieshan felsic gneisses and granulite a Dome region Eclogite units Z083 94095 94084 Sh2-3 TP5-1 TS23-1 SG-2 LJW-1 Sample 109-1 § Htl-M4 XXW-2 XXW-1 M9 M16 M16 M23 M24-1 M25 M27 M28 M29 Nos. in Fig. 2 M3 M4 M9 2 3 3 1 4 4 1 1 Refs. 1 1 1 1 SiO 2 wt. 68.99 67.04 72.41 65.44 68.57 68.75 72.98 75.48 71.93 66.21 60.77 66.31 0.49 0.30 0.70 0.35 0.40 0.43 0.69 0.26 0.37 0.69 TiO 2 0.82 0.49 14.75 14.66 14.97 12.88 15.65 15.89 14.96 13.47 11.71 13.48 16.14 13.37 Al 2 O 3 2.07 1.58 1.17 1.32 1.69 0.81 1.18 1.17 0.87 0.71 0.70 0.77 Fe 2 O 3 1.29 1.87 3.09 2.02 2.31 1.29 2.77 1.45 FeO 7.84 2.52 2.05 3.05 MnO 0.03 0.10 0.06 0.10 0.05 0.05 0.05 0.08 0.05 0.06 0.08 0.15 0.92 0.43 1.01 1.12 1.18 0.89 1.93 0.76 1.74 MgO 7.63 2.18 1.27 3.28 3.21 2.32 1.33 2.17 2.50 3.47 2.33 1.37 2.68 4.26 1.58 CaO 4.69 4.44 4.37 3.46 4.15 5.40 4.59 4.10 3.56 4.42 5.17 2.84 Na 2 O 4.02 4.55 4.38 2.11 2.06 1.42 2.15 3.62 K 2 O 2.17 1.22 1.79 1.93 0.21 0.07 0.17 0.24 0.12 0.10 P 2 O 5 0.06 0.27 0.17 0.13 0.09 0.09 0.40 0.29 0.55 0.64 0.70 0.93 0.75 0.43 0.65 1.50 H 2 O + 0.76 0.49 0.04 0.07 0.04 0.20 0.19 0.03 0.20 0.14 0.04 0.11 0.20 0.25 CO 2 99.65 99.67 99.21 99.25 99.29 99.74 99.97 99.30 99.74 99.34 99.81 99.58 Total 69.9 132.2 110.4 45.4 40.6 83.0 34.9 49.7 91.7 36.1 50.9 75.4 Rb ppm Sr 941 363 111 227 469 742 238 184 481 507 178 252 2780 682 1547 765 1563 534 1066 767 536 874 Ba 603 978 0.90 0.28 1.11 4.10 3.00 0.50 0.45 0.64 0.94 0.24 0.07 0.79 Ta 8.9 8.2 7.7 22.3 35.9 9.5 4.8 4.7 13.1 7.0 6.0 10.2 Nb 7.0 nd. nd. 7.9 2.7 2.1 4.7 8.2 Hf 4.9 5.2 7.4 4.1 192 243 538.2 304 137 152 Zr 221 126 243 156 149 151 7.93 41.00 63.80 14.21 7.61 15.82 33.77 25.50 12.91 13.88 Y 10.54 7.21 7.2 5.2 7.9 nd. nd. 19.6 9.1 5.5 9.1 4.3 3.1 11.3 Th 0.20 0.57 0.29 2.40 2.60 4.50 0.59 0.71 1.65 0.27 0.78 0.21 U 21.8 nd. nd. 8.9 22.9 B 2 55.9 115.5 Cr 1014.9 115.5 26.5 15.6 11.0 8.9 12.4 4.9 14.9 3.9 4.1 Ni 6.6 6.9 68.5 376.3 12.9 8.4 6.0 6.1 6.7 12.1 6.1 14.0 5.6 9.7 7.7 Co 44.1 12.6 16.15 26.51 58.50 54.90 66.70 97.11 55.58 36.07 31.38 9.41 11.28 La ppm 19.66 35.71 43.33 100.80 104.00 122.70 186.2 90.43 66.55 71.90 21.37 24.87 Ce 39.27 10.73 11.90 15.20 20.92 8.71 7.58 4.90 8.46 Pr 4.42 3.49 2.95 4.40 32.57 40.60 59.10 73.95 25.56 27.22 Nd 31.44 17.71 11.57 13.16 15.92 19.18 4.78 8.80 14.20 9.89 3.60 4.86 4.11 5.67 2.81 2.93 Sm 3.15 2.04 1.15 1.03 1.34 0.91 1.82 2.40 0.92 0.97 0.88 0.75 0.88 0.94 Eu 5.04 3.09 3.03 7.21 12.80 5.26 2.43 3.66 5.26 2.01 2.38 2.51 Gd 0.29 1.14 1.89 0.64 0.25 0.47 0.89 0.82 Tb 0.48 0.44 0.43 0.28 C . Ma et al . Precambrian Research 102 2000 279 – 301 Table 1 Continued Dome region Eclogite units Z083 Sample 94095 XXW-1 94084 Sh2-3 TP5-1 TS23-1 LJW-1 XXW-2 Htl-M4 109-1 § SG-2 M16 M23 M24-1 M25 M27 M28 M16 M29 Nos. in Fig. 2 M9 M9 M4 M3 1 1 2 3 3 1 4 4 1 1 Refs. 1 1 1.64 7.02 11.30 2.59 1.60 2.49 2.86 Dy 4.34 5.47 2.32 1.50 2.13 0.44 1.25 0.31 1.57 2.46 0.59 0.32 0.57 0.89 0.30 0.45 Ho 0.51 0.71 4.50 6.95 Er 1.57 1.08 0.74 1.74 2.44 0.76 1.32 1.48 3.67 0.11 0.78 1.03 0.29 0.11 0.29 0.59 0.38 Tm 0.27 0.23 0.11 0.14 0.60 4.72 6.29 1.89 0.59 1.94 Yb 2.17 0.71 0.68 1.24 1.61 3.32 0.10 0.77 0.98 0.27 0.09 0.32 0.54 0.34 Lu 0.25 0.20 0.12 0.11 215.51 S REE 248.32 86.28 323.42 403.54 190.93 155.10 166.32 53.84 65.05 92.69 119.95 65.89 7.86 7.17 34.72 63.66 8.25 12.56 6.15 9.77 10.53 LaYb n 15.37 9.35 EuEu 1.01 1.00 0.34 0.41 0.92 0.90 0.68 0.46 1.11 1.01 1.03 0.77 a For notes see Table 2. affinity, and may be classified further into high-K or calc-alkaline types Fig. 3. Their REE patterns are relatively flat Fig. 4. Most felsic gneisses from the dome region do not show negative Eu Table 2 Chemical compositions of the Kongling grey gneisses a H45 H46 H34 Sample H36 H37 H44 5 5 5 5 5 Refs. 5 71.44 67.84 70.82 SiO 2 wt. 69.40 68.69 71.62 0.18 0.30 0.19 0.35 0.26 TiO 2 0.58 15.42 15.19 16.95 16.41 Al 2 O 3 14.98 15.85 0.31 0.90 0.67 Fe 2 O 3 0.59 0.36 0.63 1.85 1.21 1.55 1.85 2.11 3.06 FeO 0.05 0.02 MnO 0.03 0.02 0.04 0.03 0.85 0.51 MgO 0.51 0.45 0.85 0.73 2.36 1.89 2.20 2.83 CaO 2.99 2.67 4.67 5.06 5.56 5.06 Na 2 O 5.26 5.44 1.22 K 2 O 1.37 2.73 1.52 2.30 1.52 0.81 0.07 0.08 0.11 0.08 0.21 P 2 O 5 0.81 0.86 H 2 O + 0.64 0.58 0.70 0.93 100.28 99.67 99.33 99.71 99.70 99.47 Total 28 53 53 Rb ppm 38 33 20 620 540 760 490 505 Sr 380 700 390 600 Ba 740 440 780 0.8 Ta 0.8 0.4 1.6 0.8 1.2 6.1 1.8 4.3 5.9 13.9 8.1 Nb 133 128 118 141 164 466 Zr 3.39 4.41 3.39 2.14 Y 2.84 1.52 20.7 4.7 Th 2.7 6.7 3.7 8.9 1.1 1.0 1.0 U 0.7 1.0 0.8 134 95 143 66 107 113 Cr 10 56 Ni nd 10 14 9 5 3 3 5 2 7 Co 33.00 140.00 47.80 La ppm 19.10 24.50 12.00 21.00 49.90 35.20 27.30 78.90 Ce 339.00 19.50 5.23 1.98 Pr 3.65 2.77 8.23 101.00 Nd 21.00 6.98 30.70 13.10 9.94 3.48 1.10 2.17 1.57 5.21 13.50 Sm 0.69 0.56 0.66 0.54 0.81 0.83 Eu 4.92 2.33 1.62 0.86 Gd 1.15 0.67 0.25 0.13 0.19 0.16 0.37 0.78 Tb 0.47 Dy 1.09 3.60 0.80 1.76 0.63 0.20 0.09 0.16 0.12 0.26 0.51 Ho 1.08 0.40 Er 0.33 0.20 0.23 0.51 0.17 0.07 Tm 0.05 0.03 0.04 0.09 1.00 0.40 0.34 0.23 Yb 0.31 0.17 0.05 0.03 0.05 0.04 0.08 0.17 Lu 63.53 626.05 117.32 S REE 177.44 45.40 82.32 48.85 64.82 53.06 56.12 81.57 LaYb n 94.60 1.84 0.78 1.15 1.29 0.61 EuEu 0.26 a Data sources for Table 1 and Table 2: 1 this study; 2 Sang et al. 1997; 3 Wu et al. 1998;4 You et al. 1996; 5 Li Fuxi and Ma Daquan 1991, unpublished report. § Sample 109-1 is an intermediate granulite. nd, not determined. Chondrite normalizing values for REE are from Taylor and McLennan 1985. Fig. 3. Plot of Zr vs TiO 2 for the Dabieshan felsic gneisses and the Kongling grey gneisses. Fields of ‘volcanic arc’ and ‘within plate’ rocks from Pearce 1980. Rocks below line A-B, draw from co-ordinate TiO 2 = 0.2, Zr = 10 ppm to TiO 2 = 1.0, Zr = 100 ppm Pearce, 1980, are interpreted as having inter- mediate or silicic compositions. It is suggested that Zr content ranging from 350 to 700 ppm can effectively divide intermedi- ate and silicic rocks into peralkaline and subalkaline types, and subalkaline rocks can be further divided into high-K and calc-alkaline Leat et al., 1986. In a plot of Nb versus Y, most of the Dabieshan felsic gneisses display volcanic arc affinity except for two gneisses of the UHP eclogite unit, which fall in the field of within-plate granites Fig. 5B. Fig. 6 shows that chondrite-normalized REE patterns and primitive mantle-normalized incom- patible element patterns for two felsic gneiss sam- ples from the eclogite units match those of the Kongling grey gneisses well, implying that some of the felsic gneisses in the eclogite units may have been derived from the KLC perhaps during Meso- zoic exhumation of the UHP metamorphic rocks. 4 . 2 . Rb – Sr and Sm – Nd isotopic characteristics Twenty-four felsic gneiss, amphibolite, eclogite and other metamorphic rock samples, and twelve samples of Mesozoic intrusive rocks were col- lected from the Dabieshan terrane. Published Nd and Sr isotope data for the DBC, and for the KLC have been compiled from available literature sources. The isotopic data are given in Table 3 and Table 4, and sample locations for the Da- bieshan rocks are shown in Fig. 2. anomalies, and they display Archean features Fig. 5A. In contrast, the majority of felsic gneisses from the eclogite units show significant negative Eu anomalies Fig. 4, and they straddle both Archean and post-Archean fields in Fig. 5A. Fig. 4. Chondrite-normalized REE patterns for selected samples of Dabieshan and Kongling gneisses. Chondrite normalizing values used are from Taylor and McLennan 1985. C . Ma et al . Precambrian Research 102 2000 279 – 301 289 Table 3 Rb–Sr and Sm–Nd isotopic data for metamorphic rocks and selected granitoid rocks from the Dabieshan 87 Rb 86 Sr No. of samples 87 Sr 86 Sr 9 2s Sm ppm Nd ppm 147 Sm 144 Nd T DM 143 Nd 144 Nd Rock types o Nd0 Rb ppm o Nd at 760 Sources b Sr ppm No. in Fig. 2 Ma Ga 9 2s 0.70923 9 3 5.98 39.16 0.0923 0.511525 9 8 − 21.7 131.20 2.0 472.10 − 11.6 1 Felsic gneiss 96-58 M1 0.8015 746.30 0.3408 0.70876 9 3 7.01 52.81 0.0803 0.511821 9 19 − 15.9 1.5 − 4.6 1 M2 Felsic gneiss 96-53 88.20 0.70846 9 2 3.43 19.09 0.1088 0.512276 9 12 − 7.1 1.3 0.4100 1.5 XXW-1 1 M3 363.66 75.59 Felsic gneiss X-1 3.87 12.72 0.1838 0.512738 9 13 2.0 2.1 3.2 1 Amphibolite M31 0.70851 9 7 4.35 27.40 0.0961 M4 0.512198 9 7 X-2 − 8.6 1.2 1.2 1 Felsic gneiss 47.47 378.62 0.3610 0.70774 9 6 4.74 14.93 0.1920 0.512000 9 1 − 12.4 7.9 0.1270 − 12.0 M41 1 347.87 15.38 Amphibolite XXW110 0.71017 9 1 9.03 34.55 0.1582 0.512383 9 6 − 5.0 2.1 M5 − 1.2 96-12 1 Amphibolite 8.64 287.50 0.0867 0.70705 9 1 4.70 22.48 0.1266 0.511895 9 8 − 14.5 2.2 0.2176 − 7.7 M6 1 276.90 36.00 Felsic gneiss 96-48-1 0.8647 96-63-2 0.71215 9 24 7.29 40.48 0.1090 0.511616 9 5 − 19.9 2.2 − 11.4 1 Felsic gneiss 139.40 464.60 M7 0.71298 9 10 5.08 29.45 0.1043 0.511364 9 7 − 24.9 2.5 0.3913 − 15.9 96-110 1 641.50 M8 87.02 Felsic gneiss 213.80 Intermediate 1.1800 0.74642 9 3 5.19 27.95 0.1123 0.511044 9 13 − 31.1 3.1 − 22.9 1 109-1 87.13 M9 granulite 0.74189 9 8 7.20 28.01 0.1555 0.511721 9 8 − 17.9 3.7 0.5815 − 13.9 Felsic gneiss 1 M91 a 96-M4 286.50 57.58 283.10 Amphibolite 0.3338 0.71243 9 21 6.57 30.42 0.1305 0.511569 9 9 − 20.9 2.9 − 14.4 1 96-M2 32.77 M92 a xenolith 3.8 15.45 0.1489 0.512347 9 8 − 5.7 M10 1.9 d144 − 1.0 2 Felsic gneiss 0.7065 9 4 5.96 24.21 0.1490 0.512511 9 11 − 2.5 1.5 0.1819 2.2 96-238 1 M11 870.00 54.89 Felsic gneiss 0.4368 96-111 0.70727 9 1 4.88 22.90 0.1290 0.512122 9 12 − 10.1 1.8 − 3.5 1 Felsic gneiss 57.73 381.00 M12 0.70594 9 2 4.63 17.29 0.1620 0.512334 9 7 − 5.9 M13 2.4 96-212 − 2.6 1 Amphibolite 30.23 467.80 0.1863 0.70638 9 4 7.58 48.44 0.0947 0.511810 9 9 − 16.2 1.7 0.2738 − 6.2 91.36 1 962.00 M14 LG2 Felsic gneiss 0.0590 SG-1 0.70661 9 5 10.04 50.38 0.1205 0.512032 9 10 − 11.8 1.8 − 4.4 1 Felsic gneiss 17.42 849.53 M15 0.70874 9 4 4.05 19.51 0.1689 0.512339 9 9 − 5.8 2.7 M151 − 3.1 SG-2 1 Amphibolite 70.8 286.56 0.7120 7.47 40.36 0.1118 0.512011 9 7 − 12.2 1.7 − 4.0 d118 2 M16 Felsic gneiss 0.001 DB-91-34 0.70494 9 2 4.74 20.08 0.143 0.512173 9 10 − 9.1 2.1 − 3.9 3 Eclogite 0.01 47.4 M17 0.70886 9 1 3.88 18.72 0.1255 M18 0.511895 9 11 Bh-12 − 14.5 2.2 − 7.6 1 Felsic gneiss 76.57 529.60 0.4169 0.70647 9 1 2.52 8.04 0.1894 0.512098 9 10 − 10.5 6.5 0.1473 − 9.8 M181 1 117.50 6.01 Eclogite Bh-9b 0.70611 9 11 5.94 26.13 0.1376 0.512028 9 9 − 11.9 2.2 M182 a − 6.2 Bh-7 1 Eclogite 11.76 570.70 0.0594 0.70698 9 2 10.94 79.47 0.0833 0.511410 9 9 − 24.0 2.0 0.1329 − 13.0 M19 1 1859.00 85.71 Felsic gneiss 96-218 d143 12.6 89.09 0.0855 0.511335 9 7 − 25.4 2.1 − 14.6 2 Felsic gneiss M20 6.58 40.37 0.0984 0.511589 9 9 − 20.5 2.1 − 10.9 d105 2 M21 Felsic gneiss d123 9.04 59.25 0.1088 0.511379 9 6 − 24.6 2.6 − 16.0 2 Felsic gneiss M22 0.418 BJ93-01 0.710847 9 8 8.0 43.53 0.1111 0.512407 9 5 − 4.5 1.1 − 3.8 4 Felsic gneiss 28.65 198.35 M23 0.703755 9 5 1.96 7.51 0.1578 0.512520 9 4 − 2.3 1.7 0.001 1.5 0.14 4 456.33 M231 BJ93-03 Eclogite 0.072 BJ93-04 0.704026 9 8 1.63 5.55 0.1778 0.512608 9 6 − 0.6 2.3 1.3 4 Eclogite 5.81 232.31 M232 a 0.710768 9 6 4.0 21.5 0.1125 0.512129 9 8 − 9.9 1.5 M24 − 1.7 BJ93-26 5 Felsic gneiss 49.61 361.23 0.398 0.724031 9 9 3.28 14.82 0.1338 0.511420 9 18 − 23.8 3.3 1.041 − 17.7 BJ93-32 6 M25 247.21 88.85 Felsic gneiss 0.005 BJ93-31 0.710781 9 6 4.57 16.89 0.1636 0.511695 9 7 − 18.4 4.4 − 15.2 6 Eclogite 0.19 112.12 M251 M252 0.704889 9 7 BJ93-36 3.72 14.89 0.151 0.512229 9 7 − 8.0 2.2 − 3.5 6 Eclogite 2.7 487.28 0.016 7.33 34.89 0.1272 0.512176 9 9 − 9.0 1.7 − 2.3 T16-1 7 M26 Felsic gneiss 1.9370 LJW-1 0.71949 9 7 5.13 22.76 0.1363 0.511941 9 38 − 13.6 2.4 − 7.7 1 Felsic gneiss 91.40 182.52 M27 0.71325 9 2 1.62 7.36 0.1330 0.512153 9 15 − 9.5 1.9 M28 − 3.3 D12 3 Felsic gneiss 1.71 39.30 1.1120 7.14 37.70 0.1144 0.512005 9 9 − 12.3 1.8 − 4.4 D132 2 M29 Felsic gneiss D142 5.47 24.07 0.1374 0.512362 9 12 − 5.4 1.6 0.4 2 Susong schist M30 0.4842 0.70963 9 2 8.39 46.41 0.1093 0.512127 9 7 − 10.0 1.5 G1 Quartz 8 XXW-N11 80.84 481.43 monzonite 0.72340 9 1 5.57 31.86 0.1058 0.511612 9 7 − 20.0 124.03 2.2 Granite 8 243.57 5.6703 G2 B11-2 B1-2 3.0430 0.71675 9 4 9.80 58.58 0.1012 0.511520 9 7 − 21.8 2.2 8 Granite 218.60 G2 207.30 0.70598 9 35 2.73 12.96 0.1275 0.511928 9 10 − 13.8 2.2 G3 109-2 1 Tonalite 73.13 606.90 0.3474 0.70841 9 14 3.43 10.25 0.2022 0.512773 9 7 2.6 4.9 0.398 Mafic enclave 1 G31 a 96-M11 221.9 30.63 1.6030 96-63-1 0.71861 9 2 7.39 50.73 0.0811 0.511402 9 8 − 24.1 2.0 1 Granite 147.80 266.10 G4 C . Ma et al . Precambrian Research 102 2000 279 – 301 Table 3 Continued 87 Sr 86 Sr 9 2s No. in Fig. 2 Sm ppm No. of samples Nd ppm 147 Sm 144 Nd Rock types 143 Nd 144 Nd Rb ppm o Nd0 T DM Sr ppm o Nd at 760 87 Rb 86 Sr Sources b Ma Ga 9 2s 0.70804 9 4 9.27 59.14 0.0949 G5 0.511665 9 8 96-34 − 19.0 1.9 1 Granite 154.30 637.30 0.6981 0.70855 9 6 8.41 47.55 0.1070 0.511617 9 10 − 19.9 2.2 0.4378 96-116 1 G6 731.20 1110 Granite 0.5268 96-117 0.70875 9 1 2.78 15.08 0.0914 0.511688 9 19 − 18.5 1.8 1 Granite 110.90 607.10 G7 1096.60 0.2140 0.70838 9 2 14.54 94.08 0.0935 0.511604 9 8 − 20.2 2.0 1 Granite G8 81.41 94-1 0.71446 9 8 6.68 40.23 0.1004 0.511841 9 40 − 15.5 1.8 3.3160 Granite 1 187.43 215.42 G9 94-6 1123.50 Quartz 0.1951 0.70771 9 1 5.18 32.52 0.0964 0.511443 9 7 − 23.3 2.2 8 089 76.04 G10 monzonite D102 6.37 44.23 0.0870 0.511375 9 5 − 24.6 2.1 2 Granite G11 0.70832 9 5 12.54 76.52 0.0991 0.511729 9 8 − 17.7 1.8 0.3828 G12 8 713.50 94.73 Monzonite YH61 0.70946 9 3 12.38 82.55 0.0907 0.511699 9 7 − 18.3 1.8 G121 a A12 8 Quartz syenite 153.75 948.47 0.4674 10.08 65.53 0.0959 0.511695 9 4 − 18.4 1.9 Granite 1 G13 D1211 302.80 Quartz 0.4577 0.70952 9 4 3.90 17.81 0.1326 0.512341 9 13 − 5.8 1.5 1 Shiguan1 48.06 G14 monzonite 7.05 55.97 0.0800 0.511530 9 3 − 21.6 1.8 G15 D1351 1 Granite 0.1238 0.70806 9 12 9.86 44.13 0.1352 0.512196 9 8 − 8.6 523.90 1.9 96-217 8 G16 22.47 Quartz monzonite A1 0.0818 0.70802 9 2 9.75 48.73 0.1211 0.511629 9 9 − 19.7 2.5 8 Monzodiorite 24.91 G17 878.36 G171 a 0.70780 9 4 A5 6.26 33.17 0.1141 0.511571 9 6 − 20.8 2.4 8 Monzodiorite 42.43 736.37 0.1660 0.332 0.70889 9 11 5.16 33.77 0.0925 0.511354 9 7 − 25.0 700.07 2.3 Quartz 1 80.61 G172 a 103-2 monzonite 94-2 0.2721 0.70677 9 2 2.64 16.62 0.0961 0.511641 9 7 − 19.4 2.0 8 Granite 81.70 G18 865.71 G19 0.70622 9 4 MC-1 4.68 29.31 0.0966 0.511778 9 6 − 16.8 1.8 8 Monzonite 81.24 879.45 0.2663 10.12 67.34 0.0909 0.511338 9 11 − 25.4 2.2 2 G20 D1412 Granite a The sample has not been located in Fig. 2, but it has a sampling location similar to that before it. b Sources of isotopic data: 1 This study; 2 Xie et al. 1996; 3 Ames et al. 1996; 4 Chavagnac and Jahn 1996; 5 Chen and Jahn 1998; 6 Liou et al. 1997; 7 Li et al. 1993 and 8 Ma et al. 1998. C . Ma et al . Precambrian Research 102 2000 279 – 301 291 Table 4 Rb–Sr and Sm–Nd isotopic data for the Kongling complex from the Yangtze basement a Rock types b 87 Rb 86 Sr 87 Sr 86 Sr 9 2s Sm ppm Nd ppm 147 Sm 144 Nd 143 Nd 144 Nd 9 2s o Nd0 T DM Ga Rb ppm No. of Sr ppm samples 9.08 70.70 H45 0.0776 gg 0.510315 9 17 − 45.3 3.2 60.56 460.87 0.3879 0.71911 9 9 3.96 26.40 0.0906 0.51057 9 16 − 40.3 0.71552 9 2 3.2 gg H44 0.33166 560.79 64.37 0.71252 9 5 gg 1.40 8.63 0.0983 0.510511 9 12 − 41.5 3.5 45.42 506.20 0.21608 H37 0.10096 0.70744 9 4 7.07 0.09 0.0907 0.510548 9 17 − 40.8 3.2 H34 24.29 gg 693.47 6.07 0.15 0.1528 0.511789 9 17 − 16.6 0.70688 9 3 3.4 363.96 0.08737 HvI-7 amph 11.03 amph 2.63 8.43 0.1888 0.512621 9 12 − 0.3 3.2 B17 amph 2.79 8.96 0.1888 0.512578 9 26 − 1.2 3.5 B19 4.27 15.28 0.1691 0.512142 9 25 − 9.7 3.4 B20 amph amph 3.30 10.72 0.1860 0.51253 9 7 − 2.1 3.4 B21 5.35 24.94 0.1299 0.511289 9 27 − 26.3 3.4 amph B22 amph 3.68 11.76 0.1894 0.51265 9 9 0.2 3.1 B56 6.15 30.21 0.1231 0.511211 9 8 − 27.8 B57 3.2 amph 2.15 6.83 0.1896 0.512653 9 13 0.3 3.1 amph B61 amph 2.81 9.45 0.1800 0.512427 9 7 − 4.1 3.3 B62 4.66 17.54 0.1608 0.512071 9 9 − 11.1 3.1 B65 amph 4.80 15.47 0.1879 0.512531 9 14 − 2.0 3.6 B66 amph amph 3.01 11.72 0.1553 0.511838 9 15 − 15.6 3.4 B70 3.15 24.19 0.0786 0.51031 9 16 − 45.4 3.2 B73 gg a Sources of isotopic data: H45 to Hv17 from Li Fuxi and Ma Daquan 1991, unpublished report and B17 to B73 from Ma et al. 1997. b Rock types: gg, grey gneiss, and amph, amphibolite. Fig. 5. Plots of Yb n vs. LaYb n A and Nb vs. Y B for the Dabieshan felsic gneisses and Kongling grey gneisses. For Yb n vs. LaYb n plot: Chondrite normalizing values used are from Taylor and McLennan 1985, and fields for Archean and Post-Archean granitoids are indicated after Jahn et al. 1981 and Martin 1986. Data points from the KLC plot in the Archean field, whereas felsic gneisses from the Dabieshan straddle both Archean and post-Archean fields. The Nb vs. Y plot shows tectonomagmatic grid of Pearce et al. 1984. VAG = volcanic arc granitoids, WPG = within-plate grani- toids; ORG = ocean-ridge granitoids. Symbols as in Fig. 3. amphibolite sample HVI-7 from the KLC shows similar 87 Sr 86 Sr and 143 Nd 144 Nd ratios to the Dabieshan amphibolites. Fig. 6. Chondrite-normalized REE patterns and primitive mantle-normalized incompatible element patterns, showing that some felsic gneisses in the eclogite units have geochemical features which match those of the Kongling grey gneisses. Percentages in brackets represent SiO 2 contents of the samples. Chondrite normalizing values used are from Taylor and McLennan 1985, and primitive mantle normalising factors are from Hofmann 1988. Fig. 7. 143 Nd 144 Nd vs. 87 Sr 86 Sr isotopic variation for felsic gneisses, amphibolites and eclogites from the Dabieshan and Yangtze basement. In a plot of 87 Sr 86 Sr versus 143 Nd 144 Nd Fig. 7, the Dabieshan felsic gneisses have a higher average 87 Sr 86 Sr ratio of 0.712 than the Da- bieshan eclogites and amphibolites : 0.707. The felsic gneisses from the eclogite units show a wide range, especially in 87 Sr 86 Sr ratio, probably reflecting fractionation between Rb and Sr. How- ever, the 87 Sr 86 Sr ratios for the majority of meta- morphic rocks from the DBC and KLC are lower than an average ratio of continental crust 0.72; Hofmann, 1997. Felsic gneisses from the Da- bieshan terrane have much higher 143 Nd 144 Nd ratios than the Kongling grey gneisses, but an Fig. 8. Plot of 87 Rb 86 Sr vs. 147 Sm 144 Nd for felsic gneisses, amphibolites and eclogites from the Dabieshan and Yangtze basement. Also shown for comparison are the parentdaughter isotopic ratios for average continental crust CC and depleted mantle DM. For Sm – Nd, CC value is taken from Taylor and McLennan 1985, DM value from Peucat et al. 1989. Rb – Sr ratios for CC and DM are taken from Mo¨ller et al. 1998. Symbols as in Fig. 7. ing the formation of new continental crust from depleted mantle, that processes in the crust in- cluding partial melting and high-grade metamor- phism did not affect the SmNd systematics on a whole-rock scale, and that all the material in the sample was derived from the upper mantle during a single event Arndt and Goldstein, 1987; Milisenda et al., 1994. If this is the case, measurement of 143 Nd 144 Nd and 147 Sm 144 Nd in a crustal rock would yield a Nd model age T DM which represents the latest time that Nd was in isotopic equilibrium with depleted mantle DM. A complication in interpreting the Nd model ages of orthogneisses is the possible effect of mixing between juvenile material and old crust. If that has occurred, the model ages would be intermediate between the actual times of derivation of crustal material from depleted mantle and the model residence age of the old crust, and would not correspond to crust-forma- tion events Arndt and Goldstein, 1987. Assuming that the Sm – Nd components of a crustal sample did not fractionate since addition of the sample to the continental crust from the mantle, calculated Nd model ages of felsic gneisses provide a powerful tool for insights into the premetamorphic crustal history of a high- grade terrane. However, the significance of Nd model ages for mafic rocks is ambiguous, as many of them have mantle-like SmNd ratios Fig. 8. It is clear that most amphibolites and eclogites in Tables 3 and 4 have high 147 Sm 144 Nd ratios of \ 0.15, which would give unre- alistically high Nd model ages with no geological significance. We have thus limited ourselves to Nd model ages of the Dabieshan felsic gneisses in this paper Fig. 9. Grey gneisses from the KLC have a mean Nd model age of 3.3 9 0.1 Ga at 1 S.D., which is distinctly older than Nd model ages of the major- ity of the Dabieshan felsic gneisses Fig. 9, most of which range from 1.0 to 2.9 Ga Fig. 9. A Susong schist from the blueschist-bearing fold- thrust belt has a model age of 1.6 Ga. However, an intermediate granulite sample 109-1 from the Luotian dome and a felsic gneiss BJ93-32 from Thirty-six felsic gneiss samples from the Da- bieshan yield a mean 147 Sm 144 Nd ratio of 0.12 9 0.02, which is within the typical range 0.09 – 0.13 for crustal rocks Taylor and McLennan, 1985. 147 Sm 144 Nd ratios of the Kongling amphibolites 0.12 – 0.19, see Table 4 and most Dabieshan eclogites \ 0.12, Fig. 8 are higher than the typical range of crustal rocks, while grey gneisses from the KLC have lower 147 Sm 144 Nd ratios 0.08 – 0.10 than the mean 147 Sm 144 Nd ratio 0.12 of crustal rocks Fig. 8 Milisenda et al., 1994; Mo¨ller et al., 1998. The Dabieshan felsic gneisses display 87 Rb 86 Sr ratios ranging from 0.059 to 1.937 with a mean value of 0.546 9 0.102, which is higher than average continental crust 0.4098, Faure, 1986, whereas the Dabieshan eclogites show a relatively wide range in 87 Rb 86 Sr values Fig. 8. A major application of Sm – Nd isotope sys- tematics to old orogens is to delineate provinces of different ages of ‘crust-formation’, ‘crustal ex- traction’, or ‘crustal residence’ indicated by Nd model ages e.g. DePaolo, 1981; Arndt and Goldstein, 1987; Dickin et al., 1990; Milisenda et al., 1994; Mo¨ller et al., 1998. Basic assump- tions in using such model ages are that major fractionation between Sm and Nd occurred dur- Fig. 9. o Nd 0 vs. T DM Ga variation for felsic gneisses from the Dabieshan and Yangtze basement. Symbols as in Fig. 3. bieshan yield Proterozoic model ages Table 3, Fig. 2, possibly suggesting that they came from, or were contaminated by, crustal sources and rocks of the Dabieshan and Yangtze basement discussed blow. Rowley et al. 1997 and Ames et al. 1996 have argued that an episode of rift-related mag- matism took place between 600 and 800 Ma. We calculated initial o Nd values of the Dabieshan fel- sic gneisses, amphibolites and eclogites at 760 Ma Table 3, and also show the o Nd at 760 Ma values of a few samples of the Dabieshan amphi- bolites and eclogites in Fig. 2. The o Nd values of the amphibolites and eclogites, ranging from + 3.2 to − 15.2, are similar to those of the Da- bieshan felsic gneisses + 3.8 to − 17.7. 4 . 3 . Geochronological significance On a conventional Rb – Sr isochron diagram, most of the felsic gneisses, amphibolites and eclogites from the Dabieshan plot close to a 493 Ma reference line Fig. 10. In contrast to the Qinling and Tongbai metamorphic rocks, which experienced high-temperature metamorphism be- tween 480 and 430 Ma zircon Pb data by Kro¨ner et al., 1993; Ar – Ar hornblende data by Zhai et al., 1998, the geological significance of the Or- dovician chronological results from the Da- bieshan terrane has been a matter of some debate. Ames et al. 1996 suggested that the Dabieshan metamorphic rocks did not experience an early Paleozoic metamorphic event at all. However, You et al. 1996 obtained a U – Pb zircon lower intercept age of 471 9 2 Ma MSWD = 0.3 and a Sm – Nd garnet-whole rock isochron age of 481 9 25 Ma for the Dabieshan eclogites, and they considered these ages as another eclogite-facies metamorphic event in addition to the well-known Triassic HP and UHP metamorphism Yang et al., 1994; You et al., 1996. Given the relative mobility of Rb and Sr, the Rb – Sr isotopic system might readily be disturbed either by influx of fluids or by a later thermal event. Thus it appears that the Rb – Sr errorchron of 493 Ma in Fig. 10 may indicate an early Paleozoic metamorphism in the Dabieshan terrane. the eclogite units yield Archean model ages simi- lar to those of the KLC. The regional distribution of the Nd model ages for the Dabieshan felsic gneisses and Mesozoic intrusive rocks is shown in Fig. 2. Most younger model ages of 1.0 – 1.6 Ga occur near the north- ern and southern margins of the Luotian dome, and the northern margin of the eclogite units. Granulite facies rocks with a depleted-mantle model age of 3.1 Ga are exposed in the core of the Luotian dome, and the oldest model age of 3.3 Ga was found in a felsic gneiss that encloses UHP diamond-bearing eclogite blocks from the Shuanghe region Xu et al., 1992; Liou et al., 1997. All Mesozoic intrusive rocks in the Da- Fig. 10. Rb – Sr isochron diagram for felsic gneisses, amphibo- lites and eclogites from the Dabieshan. Reference age is calcu- lated with the Isoplot program Version 2.95 of Ludwig 1997. Symbols as in Fig. 7. Fig. 11. Sm – Nd isochron diagram for felsic gneisses, amphi- bolites and eclogites from the Dabieshan and Kongling com- plex. Ages and error are calculated with the Isoplot program Version 2.95 of Ludwig 1997. Discussion see text. Symbols as in Fig. 7. with o Nd T = + 2.8. The high MSWD value of 30.2 indicates that the variation along the regres- sion line is much larger than that caused by analytical error. This implies that the Sm – Nd isotopic system of these samples was not in equilibrium. The scatter in the measured Sm – Nd ratios of the Dabieshan felsic gneisses indicates that either the Sm – Nd system was differentially mobilized or the rocks did not have the same initial ratios. The isotopic variation of gneiss samples on the mixing line 1 may be associated with mixing between Neoproterozoic? mantle-derived magmas and Kongling gneisses, because crustal assimilation by a primary mafic magma during ascent and em- placement combined with fractional crystalliza- tion AFC, presumably generated the protolith of felsic gneisses Ma, 1999. Three samples of Da- bieshan eclogites and amphibolite on mixing line 2 possibly represent a mixing relationship between a younger mafic magma and Kongling gneiss. Among the Dabieshan metamorphic rocks, the Huangtuling intermediate granulite sample 109-1, Table 3 has the oldest Nd model age of 3.1 Ga, and falls on line L2 in Fig. 11. In order to constrain the age of its protolith and the timing of granulite – facies metamorphism, Sm – Nd isotopic analyses were performed on a set of samples including intermediate granulite, host tonalite fel- sic gneiss and two mafic enclaves from the core of the Luotian dome point M9 in Fig. 2. A few mineral separates from the granulite were also analyzed. The data are given in Table 5 and Table 3. Interestingly, the majority of data points in- cluding granulite, mafic enclaves, tonalite felsic gneiss, plagioclase and orthopyroxene define an The majority of Sm – Nd data for the DBC displays rather a broad array on a Sm – Nd isochron diagram Fig. 11. Five samples of Da- bieshan eclogites and amphibolites yield a 2147 9 456 Ma reference line L1 with an initial o Nd of + 4.9 age calculated using model 1 of Ludwig 1997 in which scatter from a straight line is attributed to analytical errors alone, and indicat- ing derivation from a LREE-depleted mantle reservoir. Although the reference line does not meet the critical isochron prerequisites of White- house et al. 1996, we consider that the age provides an approximate estimate of the protolith age for some Dabieshan eclogites and amphibo- lites. In contrast to the Dabieshan eclogites, thir- teen data points for the Kongling amphibolites define a reference line L2 whose slope corre- sponds to an age of 3244 9 145 Ma 95 conf Table 5 Sm–Nd isotopic data for the constituent minerals of the Huangtuling intermediate granulite and an amphibolite enclave Mineral or rock Samples Sm ppm 143 Nd 144 Nd Nd ppm 147 Sm 144 Nd 9 2s 2.70 Garnet from 109-1 a Grt 0.3564 0.512475 9 12 4.59 Biotite from 109-1 Bt 0.511419 9 22 0.1302 1.29 0.28 Orthopyroxene from 109-1 1.67 Opx 8.70 0.1165 0.511125 9 9 Plagioclase from 109-1 1.52 9.85 0.0934 0.510785 9 21 Pl Amphibolite enclave 3.43 10.25 0.2022 0.512773 9 7 96-M11 a Isotopic data of the intermediate granulite 109-1 are given in Table 3. Fig. 12. Sm – Nd isochron diagram for the Huangtuling inter- mediate granulite sample 109-1, its constituent minerals, a host felsic gneiss of the granulite sample 96-M4 and amphi- bolite xenolith and enclave 96-M2 and 96-M11 from the core of the Luotian dome. Data and abbreviations are given in Table 3 and Table 5. 4 . 4 . Contribution of the Archean Kongling gneiss to the Mesozoic Dabieshan mafic monzodiorite The Mesozoic mafic monzodiorites of Group I e.g. A1 and A5, Table 3 are characterised by high ThYb ratios, relative enrichment in incom- patible elements, negative Nb anomalies, low o Nd , high initial 87 Sr 86 Sr ratios and Pale- oproterozoic Nd model ages, suggesting either interaction between crustal material and mafic magmas during their ascent through the crust, or enrichment of the mantle source by subduc- tion and recycling of crustal material Ma et al., 1998. In order to examine the nature and ex- tent of possible crustal components which were involved in the petrogenesis of the Liujiawa mafic monzodiorites Group-I, we have per- formed isotopic mixing calculations. The element concentrations and isotope ratios of mixtures of two components are described by the following equations Boger and Faure, 1976; Gray, 1984: C I = f · C C + 1 − f · C M 1 and C I · R I = f · C C · R C + 1 − f · C M R M 2 where C I , C C and C M are the concentrations of element, ppm Nd, Sm, Sr, or Rb in isotopically mixed intrusion rocks I, a crustal component C and a mantle component M, and R I , Rc, and R M are their respective isotopic ratios 87 Sr 86 Sr or 143 Nd 144 Nd. The factor f is the weight- ing factor of the crustal component in any given mixture. For example, for the ratio 143 Nd 144 Nd in a mixture, Eq. 2 becomes: C NdI 143 Nd 144 Nd I = f · C NdC 143 Nd 144 Nd C + 1 − f · C NdM 143 Nd144Nd M 3 To examine the effects of mixing on the Nd model ages T DM , a parentdaughter isotopic ratio 147 Sm 144 Nd in the mixture must be cal- culated. An equation for the calculation can be given by finding the molar concentrations of 147 Sm and 144 Nd in the mixture, following the procedure outlined by White et al. 1967 and using the atomic fractions atom reported by Rosman and Taylor 1998: errorchron whose slope corresponds to an age of 2789 9 650 Ma 95 conf. Fig. 12, but a data point from garnet in the granulite does not lie on the line. Since biotite is most likely to be a non-primary phase, it was excluded from re- gression calculations. The very high MSWD value of 511 implies that the samples cannot be regarded as cogenetic or as equilibrium assem- blages. Thus line L1 may reflect only a mixing relationship. The inset in Fig. 12 shows a whole- rock WR- orthopyroxene-plagioclase isochron age of 2238 9 300 Ma with an initial o Nd = − 6.6 for sample 109-1 MSWD = 1. The non- linearity of data points from the granulite sam- ple, orthopyroxene and plagioclase in a 143 Nd 144 Nd – 1Nd plot not shown suggests that the straight line L2 is a true isochron and not a mixing line. Consistent within the bounds of uncertainty with a Pb-Pb zircon age of 2456 9 7 Ma for the same granulite exposure re- ported by Jian et al. 1999, the isochron age of 2238 9 300 Ma probably represents the timing of granulite – facies metamorphism. Therefore, the age of the protolith to the granulite must be older than 2238 9 300 Ma. Petrographical study shows that garnet occurs as rim around a ca. 2 mm wide felsic vein cutting the granulite, imply- ing that the garnet resulted from anatexis after granulite – facies metamorphism. 147 Sm 144 Nd I = [C SmI 532.78 + 142.91 143 Nd 144 Nd I ] 1003.09C NdI 4 where C SmI , C NdI , and 143 Nd 144 Nd I in the mixture can be calculated using the Eq. 1 and Eq. 3. In order to numerically model isotopic mixing, an average isotopic composition for four Permian gabbro samples from the Xinxian block, ca. 70 km west of the Dabieshan terrane Zhang et al., 1995 and three Cenozoic basalt samples from the Tan – Lu fault belt Zhi and Chen, 1992 is used as the mantle component. The other crustal compo- nents that may contribute to the final composition of the Mesozoic Dabieshan mafic intrusive rocks include those of the Yangtze basement repre- sented by the Kongling gneisses and Dabieshan intermediate and felsic gneissesgranulite. Thus, two felsic gneisses and one intermediate granulite with distinct Nd model ages are examined in turn as possible crustal end members H44 from the Kongling complex; 96-110 and 109-1 from the Dabieshan dome region. Mixing calculations show that the addition of 27 Archean Kongling gneiss will yield a magma composition which closely matches the compositions of the Liujiawa mafic monzodiorites in Nd and Sr isotopes Fig. 13, and that calculated T DM of the resulting magma ca. 2.5 Ga agrees well with the Nd model ages of the samples A1 and A5 Table 3, whereas mixing with components from the Da- bieshan terrane samples 96-110 and 109-1 would not result in isotopic compositions of the Group I mafic monzodiorite stock. Fig. 13 shows that half the samples of the Group II lie on the trend line between the mantle-derived component and the Kongling grey gneiss. It appears that the Kongling grey gneisses have been involved in the Group II magmatism, and that the Dabieshan complex may have played a significant role in the petrogenesis of the Group III intrusive rocks Fig. 13. Distinguishing between the effects of crustal contamination and subduction enrichment i.e. source contamination is critical for assessing the implications of our results for crustal structure. The most compelling evidence for crustal contam- ination would be a correlation between isotopic ratios of Sr and Nd and chemical compositions with a fractionation indicator e.g. SiO 2 , MgO, or Mg-number, because such a correlation would imply that changes in isotopic composition were produced during differentiation, while the magma was ascending and emplaced in the crust. Accord- ing to the Nd model ages, rocks of the Kongling complex are not a major rock type exposed in the Dabieshan terrane and therefore composition ef- fects of the Kongling grey gneiss on the mantle- derived mafic magmas may be indicative of recycling of the Yangtze basement into the mantle via continental subduction. However, an alterna- tive possibility is that the KLC underlies the Dabieshan terrane, so that interaction between Fig. 13. o Nd 0 vs. 87 Sr 86 Sr diagram showing the isotopic mixing relationship between mantle-derived material and crustal components. Also shown are composition fields of the Kongling complex in the Yangtze basement, Permian gabbros from the Xinxian block Zhang et al., 1995 and Paleogene basalts from the Tan-Lu fault belt Zhi and Chen, 1992. Trace element and isotopic composition of mantle-derived material used in the mixing calculation is the average composition of four Permian gabbros Zhang et al., 1995 and three Cenozoic basalts Zhi and Chen, 1992: Rb: 8.048 ppm, Sr: 424.101 ppm, Sm: 2.311 ppm, Nd: 9.394 ppm, 87 Rb 86 Sr = 0.01641, 87 Sr 86 Sr = 0.70405; 147 Sm 144 Nd = 0.17644, and 143 Nd 144 Nd = 0.51267. Three crustal end members include the Yangtze basement gneiss H44, the Dabieshan felsic gneiss 96-110 and the Huangtuling granulite 109-1 see Table 4 and Table 3 for their compositions. Figures on the mixing lines refer to mass fractions wt. of the crustal components in the mixed intrusive magmas. Other symbols as in Fig. 3. Kongling grey gneiss and the Group-I magma could occur at a lower crustal depth Ma et al., 1998. By comparing the two Liujiawa mafic mon- zodiorite samples Table 3, it becomes clear that a sample A5 with more ‘primary’ chemical com- position, e.g. a Mg-number of 0.56 and a SiO 2 Al 2 O 3 ratio of 3.5 Ma et al., 1998, close to those of primary mantle melts Kempton et al., 1997, does have less contaminated isotopic ratios of Nd and Sr than the other sample A1 with a lower Mg-number of 0.45. The possibility of subduction enrichment therefore seems to be more plausible. Regardless of which mixing mechanism is pre- ferred, the Archean KLC may have been present below the Dabieshan terrane.

5. Implications for the crustal structure of the Dabieshan terrane