Geochemical implications Isotopic geochronology

that the metamorphic age of these high-grade metamorphic rocks, exposed in the Suwalong area, are pre-Devonian, and possibly as old as Early–Middle Proterozoic. The redefined Eaqing Complex might represent the metamorphic base- ment remnant micro-continental fragment of the Jinsha- jiang area or high-grade metamorphic accretionary complex rocks. 2.5. Correlation In the Ailaoshan Suture Zone two sets of metamorphic rocks with characteristic rock assemblages, metamorphism and deformational structure were established by YBGMR 1990, 1996: the Madeng Lithogroup and another, unnamed, lithogroup. The former is characterized by meta- morphic rocks of lower greenschist facies with strong defor- mation. The latter is further subdivided into the Meiziqing, Kudumu, Shuoshan, Nazhuang and Maoheshan lithoforma- tions. We deduce that these two sets of metamorphic rocks were formed in different tectonicpalaeogegraphic positions. Integrated studies on the mixed metamorphic rocks of the Jiangshajiang and Ailaoshan suture zones suggests that the Eaqing Complex is equivalent to the Ailaoshan Complex and that the redefined Jiaren “Formation” equates to the Madeng “Group”, excluding the Shuanggou Ophiolitic Melange. The Yangla “Formation” equates to the Maohe- shan, Nazhuang and Suoshan Lithoformations, and the Zhongxinrong “Group” is similar to the Ganbatang Lithogroup in lithological association. The Jinshajiang and Shuanggou ophiolitic melanges appear to have been formed at the same time and to be contiguous with each other see below. A correlation of the tectono-stratigraphy of the Jinshajiang and Ailaoshan suture zones is shown in Table 1.

3. Geochemical implications

A detailed study on the basalts exposed in the Jinshajiang Ophiolitic Melange from the Baimaxueshan, Shusong and Gongka areas was made by Han et al. 1996. The results indicated that the basalt composition falls mainly into the MORB on the TiO 2 –MnO–P 2 O 5 diagram, only a few data points falling in the oceanic island tholeiite OIT field Fig. 7. This suggests that at least a part of the remnant oceanic crust in the Jinshajiang Suture might be derived from ocea- nic ridge spreading ORS. On the other hand, element distribution from these basalts also shows a transitional character from MORB to OIB. A similar result is also obtained from basalts of the Ailaoshan Ophiolitic melange.

4. Isotopic geochronology

4.1. Jinshajiang Ophiolitic Melange Plagiogranite is considered a typical oceanic granitoid, being the product of magmatic differentiation, and is more widely found in the Jinshajiang Ophiolitic Melange. Samples for isotopic study were collected from separated plagiogranites, occurring in the metamorphic peridotites of Shusong, Xuedui and Rongjiaoxishan. Preliminary results are here presented from the Shusong plagiogranite and Xuedui plagiogranite. The former is exposed in a road X. Wang et al. Journal of Asian Earth Sciences 18 2000 675–690 683 Table 1 Tectono-stratigraphic subdivision and correlation of the Jinshajiang–Ailaoshan Suture Zone Age T T P C D Pre D Pt? 3 1-2 neritic shelf Tectonostratigraphic unit Jinshajiang Zone continental slope continental slope ocean basin ocean basin residual ocean basin residual ocean basin neritic shelf Jiapila Formation Yiwanshui Formation Zhonxing -rong Gr Yangla Fm Gajinxue- shan Gr Jiaren Fm Gajinxue- shan Gr Jinshajiang Ophiolitic Melange Maohesan Fm Nazhuang Fm Saoshan Fm Maden Gr Shuanggou Ophiolitic Melange Ganbatang Gr Eaqing Complex Ailaoshan Complex Ailaoshan Zone Tectonic event Molasse formation Basin closure Syn-collisional granite emplacement Subduction Formation of ocean lithosphere Basin spreading Intracontinental rift cutting between Deqing and Banzilan Fig. 3. The latter is found in the Xuedui Ophiolitic assemblage. Both plagiogra- nites occur in tectonic contact with metamorphic peridotite, which is a part of the Jinshajiang Ophiolitic belt. Zircons were extracted from a fresh 15 kg sample of the Shusong plagiogranite SA 9738. U–Pb dating was performed in the isotopic laboratory of the Tianjin Institute of Geology and Mineral Resources, China. Isotopic analyses were undertaken on a VG 354 Mass Spectrometer. The mixed spike of 205 Pb 1 235 U was used. Total blanks were Pb 0.05 ng and U 0.02 ng. The ages were calculated using the pbdat and isoplot programs Ludwig, 1987. All the ages obtained are at the 95 confidence level. Zircons from the Shusong plagiogranite are a very homo- geneous population with short prismatic crystals. Three analyses give 206 Pb 238 U ages ranging from 339–347 Ma and 207 Pb 235 U ages of 323–346 Ma. These results show a slight discordance. The weighted mean 206 Pb 238 U age is 340 2 : 5 Ma Fig. 8, Table 2. This age probably repre- sents the emplacement age of the Shusong plagiogranite, indicating that the Jinshajiang Ocean was already actively spreading during the latest Devonian to earliest Carboniferous. Zircons extracted from the Xuedui plagiogranite SA 9722 are elongated and prismatic in appearance. Three analyses give 206 Pb 238 U ages ranging from 289–296 Ma and 207 Pb 235 U ages from 289–294 Ma Table 2. The analyses show basic concordance. The 294 4 Ma average age of 206 Pb 238 U would represent the formation age of the Xuedui plagiogranite, indicating that the Jinshajiang ocea- nic spreading occurred in the Carboniferous. 4.2. Gajinxueshan Group So far, no isotopic ages nor any fossils have been obtained from the clastic flysch deposits in the redefined Jiaren “Formation”, which equates to most of the Gajinxue- shan “ Group”. Some new isotopic results, however, were obtained from the redefined Yangla “Formation”, which includes the original Yangla and Linong lithoformations Zhan et al., 2000. Two zircon-bearing samples Zr 1 and Zr 2 were collected from the bedded metamorphic basalt in the Linong mining area. A sample of 60 kg provided only 30–50 zircons with rounded shapes, probably caused by strong solution. Three analysis give 206 Pb 238 U ages of 523–1354 Ma and 207 Pb 238 U ages of 757–1786 Ma, show- ing obvious discordance. These zircons have obvious magmatic corrosion, and it is suggested that they were derived from the basement rocks, and are probably crustal relics, caught up in the basalt magma. In association with other inherited zircons of 911 31 and 2119 78 Ma Jian et al, 1998b, this data further supports the possibility of a Proterozoic metamorphic basement existing in the Jinsha- jiang Suture Zone. 4.3. Eaqing Complex A fresh sample SA 9730 sillimanite-bearing two mica schist was collected from a 20 m exposure at Gangda, Batang, West Sichuan Fig. 3. Rb–Sr analysis was under- taken in the Isotopic laboratory of Yichang Institute of Geology and Mineral Resources YIGMR. Six analyses X. Wang et al. Journal of Asian Earth Sciences 18 2000 675–690 684 Fig. 8. U–Pb concordia diagram of zircons from the Shusong plagiogranite. Table 2 U–Pb analyses of zircons from the Jinshajiang plagiogranites Sample no. Atomic ratios Age Ma 2 s Ma 206 Pb 204 Pb a 206 Pb 238 U 207 Pb 235 U 207 Pb 206 U 206 Pb 238 U 207 Pb 236 Pb 207 Pb 206 Pb SA 9738-1 1723 0.5437 0.00059 0.3923 0.0051 0.5232 0.00035 341.3 336 299.6 SA 9738-2 1225 0.05384 0.00064 0.3873 0.0059 0.05217 0.00045 338.1 322.4 292.8 SA 9738-3 1339 0.05366 0.00083 0.3885 0.0085 0.05329 0.00169 346.8 346.1 307.7 SA 9738-4 0.05527 0.00183 0.4061 0.0322 0.5033 0.0035 346.8 346.1 341.2 SA 9722-1 183 0.04702 0.00294 0.3263 0.0322 0.05033 0.0035 296.2 286.7 210.1 SA 9722-2 395 0.04580 0.00169 0.3226 0.0179 0.05109 0.00193 288.7 283.9 244.8 SA 9722-3 1004 0.04665 0.00062 0.3355 0.0058 0.05216 0.00055 294 293.8 292.3 a Blank corrected. X. Wang et al. Journal of Asian Earth Sciences 1 8 2000 675 – 690 685 Fig. 9. Rb–Sr isochrons for: a Gangda two-mica schist; b Xumai foliated granite; c Zhongmu granite; and d Zhongmu gabbro. give a slightly scattered isochron. The Rb–Sr age is 423 40 Ma Fig. 9a, Table 3. This age is interpreted as the age of the high-grade metamorphic event which produced the metamorphic minerals mica, sillimanite, kyanite, etc. The dispersed data points are likely related to modifications that did not affect all minerals in the primary rocks. The Eaqing Complex is derived from the Eaqing Group, which is corre- lated with the original Gajinxueshan Group by CIGMR and SBGMR 1991 and was interpreted as Permian in age by the SBGMR 1997. However, a new study of the Eaqing Complex shows that its rock assemblage and metamorphic grade are similar to that of the Ailaoshan Complex exposed in the Ailaoshan area. The metamorphic age of the Eaqing Complex, more- over, is dated at 423 40 Ma : Its protolith age should be therefore older than the Devonian. A zircon sample SA 9731 from the Gadi plagioclase–amphibolite Figs. 3 and 4 is dated by the U–Pb method, and shows a more compli- cated pattern Table 4. It gives an upper intercept age of 1627 192 Ma ; providing a minimum age for the volcanic protolith. This indicates the presence of a Meso- to Neo- Proterozoic remnant metamorphic basement, or micro- continental fragment, in the Jinshajiang area, with an age older than 1627 192 Ma : If the time of opening of the Jinshajiang Ocean corresponds to that of the Ailaoshan Ocean, the Eaqing Complex is likely to be equivalent to the Ailaoshan Complex. An analogous high-grade metamorphic series to the Eaqing Complex, exposed in the Batang area, is also found in both sides of the Jinshajiang River in the Xumei area, Deqing. Based on the rock assem- blage and high metamorphic grade, this series in Xumei should be referred to the Eaqing Complex. X. Wang et al. Journal of Asian Earth Sciences 18 2000 675–690 686 Table 5 Rr-Sr data of granite and gabbro from Jinshajiang Suture Zone Sample no. Rock WSr 26 10 WSr 26 10 87 Rb 86 Sr 87 Sr 86 Sr SA 9717-1 Granite 21.9 446.3 0.1415 0.70948 0.00002 SA 9717-2 Granite 124.7 320 1.125 0.71307 0.00003 SA 9717-3 Granite 55.24 388.8 0.4098 0.71051 0.00003 SA 9717-4 Granite 90.86 351.1 0.7464 0.71178 0.00004 SA 9717-5 Granite 97.06 353.9 0.7095 0.71176 0.00001 SA 9717-6 Granite 34.77 437 0.2294 0.70998 0.00002 SA 9725-1 Foliated granite 34.92 474.9 0.212 0.70484 0.00015 SA 9725-2 Foliated granite 85.49 447.4 0.5508 0.70585 0.00010 SA 9725-3 Foliated granite 29.75 453.9 0.1889 0.70487 0.00001 SA 9717-4 Foliated granite 0.86 557.7 0.2112 0.70507 0.00004 SA 9717-5 Foliated granite 79.33 265.6 0.8612 0.70724 0.00006 SA 9717-6 Foliated granite 49.23 473.6 0.2996 0.70524 0.00004 SA 9715-1 Gabbro 28.83 50.56 1.644 0.70990 0.0006 SA 9715-2 Gabbro 7.672 202.1 0.1094 0.70508 0.00004 SA 9715-3 Gabbro 23.05 219.9 0.3021 0.70558 0.00014 SA 9715-4 Gabbro 4.217 165.4 0.07346 0.70477 0.00005 SA 9715-5 Gabbro 3.174 82.18 0.1113 0.70485 0.0025 Table 3 Rb–Sr data for the kyanite-bearing schist from Suwalong area Sample no. WRb 26 10 WSr 26 10 87 Rb 86 Sr 87 Sr 86 Sr SA 9730-1 183.3 58.03 9.171 0.78780 0.00007 SA 9730-2 203.1 69.55 8.48 0.78054 0.00003 SA 9730-3 180.6 108.5 4.824 0.76414 0.00003 SA 9730-4 120.9 105.9 3.305 0.75213 0.00029 SA 9730-5 137.1 127.7 7.196 0.77780 0.00065 Table 4 U–Pb data of zircon from Gadi plagioclase–amphibolite Sample no. Atomic ratio Age Ma 206 Pb 204 Pb a 206 Pb 238 U 207 Pb 235 U 206 Pb 238 U 207 Pb 235 U SA 9731-1 1658 0.09298 0.00084 1.052 0.011 573.1 729.1 SA 9731-2 1138 0.08477 0.00139 0.9260 0.0181 524.5 565.5 SA 9731-3 1162 0.04733 0.00106 0.2568 0.0106 250.8 232.1 SA 9731-4 781 0.03628 0.00126 0.2389 0.0129 229.8 215 SA 9731-5 1260 0.1034 0.0024 1.294 0.035 634.5 543.1 SA 9731-6 766 0.0963 0.00172 0.9640 0.0208 592.7 585.4 a Blank corrected. 4.4. Geochronology of granites 4.4.1. The Xumai foliated granite SA 9725 Samples were collected from foliated granites, intrud- ing the Eaqing Complex, about 1 km to the east of Xumai village Figs. 3 and 4. The metamorphic complex there attains epidote amphibolite facies to amphibolite facies and underwent several episodes of migmatization. The intrusive relationship between the Xumai granite and its country rocks can be seen in the field. The foliation of the granite is concordant with that of the metamorphic complex. Six whole rock analyses define an isochron age of 238 18 Ma : The age prob- ably reflects the time of a collisional orogenic event. The 87 Sr 86 Sr initial value is 0 : 70421 0 : 00012 ; which is significantly lower than the average value of the crust, X. Wang et al. Journal of Asian Earth Sciences 18 2000 675–690 687 Fig. 10. Schematic diagram illustrating the tectonic evolution of the Jinshajiang–Ailaoshan Suture Zone. implying that the granite probably originated from the mantle Fig. 9b, Table 5. 4.4.2. The Zhongmu granite SA 9717 The Zhongmu granite, intrudes the flysch formation of the Zhongxinrong “Group ” as a stock. The sample was collected in the river bank of the Jinshajiang near Zhongmu village Figs. 3 and 4. The isochron age defined by six whole rock analyses is 255 8 Ma ; with 87 Sr 86 Sr initial value of 0 : 709079 0 : 00008 Fig. 9c, Table 5. The higher 87 Sr 86 Sr initial value indicates that the studied granite was related to crustal melting. The Rb–Sr ages of the Jiaren and Linong granites, indi- cate that they separately intruded into the redefined Jiaren and Yangla “formations”. These Rb–Sr ages are 208 6 and 227 2 Ma with 87 Sr 86 Sr initial values of 0.71027– 0.70640, respectively, Zhan et al., 2000. These data show that these granites, occuring along the Jinshajiang Suture, formed during Late Triassic to earliest Jurassic times, and that the Jinshajiang ocean was already closed at this time. 4.4.3. Geochronology of high-level gabbro SA 9715 Diabase and gabbro are widespread in the Zhongmu area and are in tectonic contact with the Zhongxinrong “Group”. However no associated ultramafic rocks metamorphic peri- dotite, etc. and pillow basalt are found in this area. These rocks, therefore, appear not to be part of the ophiolitic assemblage. At present limited petrological evidence is available to determine the petrogenesis of these rocks. Gabbro samples were collected on the eastern bank of the Jinshajiang near Zhongmu village. The isochron age of five whole rock Rb–Sr analyses is 227 5 Ma ; with 87 Sr 86 Sr initial value of 0 : 70459 0 : 00006 Fig. 9c, Table 5, further indicating that the Jinshajiang ocean closed during the Indosinian orogenic episode.

5. Tectonic evolution of Jinshajiang–Ailaoshan Suture Zone