资源描述：
653 International Geology Review, Vol. 44, 2002, p. 653-669. Copyright 2002 by V. H. Winston Dewey et al., 1988. The Kunlun Mountain range lies along the northern margin of the Plateau, and is divided into West and East Kunlun, extending west- ward to the North Pamir Range of Afghanistan Debon et al., 1987. This mountain belt resulted from subduction of Proto- and Paleo-Tethys, and records the earliest stage of plateau ation Pan et al., 1994; Hs et al., 1995. Preliminary studies revealed that both the West and East Kunlun oro- gens have similar evolutionary histories Matte et al., 1996; Yin and Harrison, 2000. However, some geological issues are not well understood, particu- larly in the West Kunlun where steep topography, high elevation, and widespread glaciers make inves- tigation difficult. Therefore it is not easy to elucidate the bewilderingly complex evolution of the West Kunlun solely by analyses of strata and tectonic facies. Metamorphism, deation, and erosion have greatly obscured the original geologic record in this old orogenic belt. Two contrasting tectonic models have been pro- posed for the evolution of the West Kunlun. One is the archipelago model of Yao and Hs 1994 and Hs et al. 1995, which envisages the West Kunlun as a magmatic arc resulting from continuous con- sumption of Tethys sensu lato since the Late Pre- cambrian. The second model, based on geochronological data of granitic intrusions in the West Kunlun Pan et al., 1994; Xu et al., 1994; , invokes two stages of subduction to explain mag- matic quiescence during Middle Paleozoic time. However, the paucity of high-quality geochronologic 1Corresponding author; email minsunhkucc.hku.hk 2YUAN ET AL. data do not allow us to precisely constrain the timing of subduction-related magmatism, and the time of closure of Proto- and Paleo-Tethys. Magmatic rocks in this orogenic belt may provide necessary constraints on tectonic evolution Sengr et al., 1991, 1993, because their compositions characteristically reflect tectonic settings, their ages are related to orogenic events, and they are more resistant to erosion than metamorphic and sedimen- tary rocks Pitcher, 1993; Pearce et al., 1984; Bar- barin, 1999. Here we present new zircon U-Pb isotope results and geochemical data for three repre- sentative granitic plutons from the West Kunlun. These granitoids cover more than half of the Kudi area. The new data, interpreted in combination with field evidence, provide a coherent picture for the Paleozoic–Early Mesozoic evolution of the West Kunlun. Regional Geology The West Kunlun is divided into North and South Kunlun blocks by the Kudi suture Fig. 1 Pan et al., 1994; Matte et al., 1996, which is marked by the Kudi ophiolite made up of ultramafic rocks, basalts, flysch sediments, and turbidites Deng, 1995; Yang et al., 1996; Wang et al., 2000. The age of this ophiolite is constrained by radiolarian fossils recently discovered in the flysch sediments, sug- gesting deposition in the Early Paleozoic Fang et al., 1998; Zhou et al., 1998. The North Kunlun Block is in fault contact with the Tarim craton in the north and the South Kunlun Block is bounded by the Karakash strike-slip fault in the south Matte et al., 1996, which is coincident with the suture of Paleo- Tethys Pan et al., 1994 Fig. 1. The basement of the North Kunlun Block is dom- inated by gneissic and migmatitic rocks, cut by a 2.2 Ga granitic intrusion Pan, 1994; Xu et al., 1994. A 2.8 Ga Nd depleted-mantle model age was reported for the gneissic rock, suggesting that this block probably had an Archean basement similar to that of the Tarim craton Arnaud and Vidal, 1990. The overlying sedimentary cover is composed of clastic rocks and carbonates, including the Sinian to Ordovician Kilian Group Chang et al., 1989, the Devonian Tisnab Group XBGMR, 1985, and a sequence of Late Paleozoic to Cenozoic strata Pan and Bian, 1996; Mattern et al., 1996. The South Kunlun Block consists of a metamor- phic complex primarily composed of schist and gneiss with local, lens-shaped ultramafic rocks Gaetani et al., 1990; Deng, 1995. The overlying strata are dominated by Upper Paleozoic to Meso- FIG. 1. Schematic map of terranes in the Tibetan Plateau. TECTONIC EVOLUTION OF THE WEST KUNLUN3 zoic clastic rocks, carbonates, and calc-alkaline volcanic rocks GITT, 1985; Pan et al., 1994; Wang, 1996 Fig. 2. Although this metamorphic complex is lithologically similar to the basement of the North Kunlun Block, its age is not well con- strained. Isotope data for the gneissic complex of the South Kunlun Block and the associated gra- nitic intrusions both give Nd depleted-mantle model ages of 1.1 to 1.5 Ga Zhou, 1998; Yuan, 1999, suggesting that this block has a much younger basement Yuan et al., 2002. 40Ar/39Ar dates for the hornblende, biotite, and K-feldspar separates from the South Kunlun metamorphic complex range from 451 Ma to 350 Ma Matte et al., 1996; Li et al., 2000; Zhou et al., 2000, which were interpreted to record an Early Paleozoic colli- sional event between the North and South Kunlun blocks Matte et al., 1996. Granitic plutons are widely exposed in the West Kunlun and exhibit a gradual southward younging trend Wang and Fang, 1987. The following three representative granitic plutons in the Kudi area Figs. 1 and 2 were sampled for this study. Yirba pluton The Yirba pluton is located about 20 km north of Kudi. It intrudes the South Kunlun metamorphic complex in the west and is truncated by the Halastan fault in the east Fig. 2. This pluton is composed of medium- to coarse-grained plagioclase 55, K-feldspar 15, quartz 20, and subor- dinate hornblende and biotite. Accessory minerals include magnetite, sphene, zircon, and apatite. The pluton is deed, with well-developed lineation and foliation defined by hornblende and biotite, consistent with the NW/SE shearing in the country rocks Mattern et al., 1996. Previous geological investigations suggested that the pluton intruded the volcanic sequence of the Kudi ophiolite i.e., Yishake Group along its southern margin Pan et al., 1994; Matte et al., 1996. However, more recent field work casts doubt on the intrusive relationship FIG. 2. Geological map of the Kudi area, West Kunlun. 4YUAN ET AL. and has identified a fault contact between the pluton and the Yishake volcanic sequence Mattern and Schneider, 2000; Xiao et al., 2002. Previous geo- chronological studies have not provided consistent data for this pluton. Conventional zircon U-Pb anal- ysis yielded 191253/–52 Ma and 4582.4/–2.5 Ma upper and lower intercept ages, respectively, whereas single-grain zircon evaporation provided Pb-Pb ages of 480 to 510 Ma Li et al., 1995. Horn- blende and biotite separates gave 40Ar/39Ar plateau ages of 476 9 and 496 4 Ma, respectively Xu et al., 1994. North Kudi pluton NKP The NKP intruded the metamorphic complex of the South Kunlun Block Fig. 2. This pluton has a monzogranite composition, with medium-grained K- feldspar 50, plagioclase 70 wt are generally K2O rich and cluster in the monzogranite field. The low-SiO2 group 70 wt includes both relatively Na2O-rich and K2O-rich samples and ranges from granodiorite to monzogranite, with an average composition of granodiorite Table 2, Fig. 4A. Both groups show LREE-enriched patterns FIG. 3. Concordia diagrams for granitoids of the Kudi area. TECTONIC EVOLUTION OF THE WEST KUNLUN7 La/YbN 6–28 with intermediate negative Eu anomalies δEu 0.5–0.8 Figs. 5C and 5D, and are characterized by relatively low Nb/La 0.3–0.6 and high Rb/Nb 5–12 ratios. The ASIC rocks plot in the I-type granite field Fig. 4B. Discussion Ages and Tectonic Settings of the Granitoids Yirba pluton Inasmuch as there is no evidence for multiple magmatic injections for this small pluton, the younger age 471 5 Ma is interpreted to represent the crystallization age of the Yirba pluton, whereas the older age 491 3 Ma is considered to be inher- ited from the magma source. The regional dea- tion in the study area was ascribed to the collision between the North and South Kunlun blocks, after the closure of Proto-Tethys in the Middle Paleozoic Matte et al., 1996; Mattern et al., 1996; Pan, 1996. The regional lineation/foliation crosscuts the Yirba pluton, indicating that the pluton was ed prior to the collisional event Wang and Fang, 1987. The 471 Ma Early Ordovician age further supports this interpretation. In the tectonic discrimination dia- gram of Pearce et al. 1984, samples mainly fall in the VAG field Fig. 6, implying a subduction- related setting. Chemical compositions show affinity to adakites Wang et al., 2000, which are commonly produced by partial melting of subducted oceanic lithosphere beneath island arcs Defant and Drum- mond, 1990. However, relatively radiogenic Sr iso- topic compositions initial 87Sr/86Sr 0.7073– 0.7089 Yuan, 1999 do not favor this comparison. Fault contact with the Yishake volcanic sequence, which was produced in an intra-oceanic arc setting Yuan, 1999, may imply that the Yirba pluton prob- ably intruded in a different environment. We con- sider that the Yirba Pluton was generated in an active continental margin during consumption of Proto-Tethys. North Kudi pluton The new zircon U-Pb age 405 2 Ma is signif- icantly older than previous conventional U-Pb zir- con and biotite 40Ar/39Ar results, suggesting that the NKP was ed in the Early Devonian. The NKP was previously considered to be subduction- related, due to its calc-alkaline nature Yao and Hs, 1994; Li et al., 1995; Ding et al., 1996. How- ever, detailed field studies constrain a post-colli- sional setting, because the regional fabrics do not pass through this pluton Matte et al., 1996. The low Rb/Nb and high Nb/La ratios and alkali- enriched features show its affinity to A-type gran- ites, which usually intrude in anorogenic or post- orogenic environments Eby, 1992. Co lam- prophyre dikes in the Kudi area, with a hornblende 39Ar/40Ar plateau age of 405 3 Ma, were recently recognized Zhou and Li, 2000, suggesting that the area was under extension in Early Devonian time. In the tectonic discrimination diagram, the NKP plots in the WPG field Fig. 6, consistent with our post-orogenic interpretation. FIG. 4. A. Classification for granitoids of the Kudi area after Le Maitre, 1989. Abbreviations Q quartz; A alkali feldspar, P plagioclase. CIPW normative mineral propor- tions were used for plotting. B. Rb/Nb versus Rb diagram for the granitoids from the Kudi area after Christansen and Keith, 1996 8YUAN ET AL. TABLE 2. Major and Trace Element Compositions of Representative Granitic Plutons of the Kudi Area PlutonYirba plutonNorth Kudi pluton Sample96-KL-4396-KL-4596-KL-4896-KL-15896-KL-15996-KL-16096-KL-16196-KL-16296-KL-16396-KL-17496-KL-176 SiO261.756.458.860.359.058.660.354.958.870.771.2 TiO20.740.750.560.590.590.620.490.740.550.440.47 Al2O318.416.616.416.516.216.515.716.816.614.614.7 Fe2O3*6.927.527.396.176.936.556.038.096.342.012.08 MnO0.150.130.140.100.130.120.120.150.110.070.08 MgO2.353.302.942.282.792.402.242.962.440.280.37 CaO3.785.115.884.744.335.685.156.815.690.801.15 Na2O4.202.832.572.813.313.063.003.122.933.623.60 K2O1.324.472.844.123.993.804.003.653.565.995.66 P2O50.210.480.180.360.370.380.340.470.360.090.11 Total99.7397.5297.7397.9697.6997.7597.3597.5897.3198.5799.35 Be1.784.003.574.363.915.60 Rb107167151162146242 Sr470830811864854173 Y19.428.525.034.023.147.2 Zr51.3193180233172437 Nb17.421.017.724.214.557.4 Cs1.131.972.092.222.038.42 Ba8111444122311331425507 La47.574.557.087.565.155.0 Ce92.2139109160116146 Pr11.316.013.118.613.120.6 Nd38.156.447.966.246.376.2 Sm7.0210.29.0112.38.1313.7 Eu1.582.492.242.792.052.15 Gd6.659.097.6110.46.8210.7 Tb0.861.060.941.260.841.48 Dy4.615.374.96.44.247.76 Ho0.820.980.891.170.771.52 Er2.202.592.312.962.064.4 Tm0.330.380.340.450.280.67 Yb2.172.442.182.881.924.43 Lu0.330.380.320.420.290.65 Hf2.095.545.226.334.6711.2 Ta1.051.071.011.270.743.00 Th9.1121.527.824.217.516.2 U1.033.536.485.272.673.95 TECTONIC EVOLUTION OF THE WEST KUNLUN9 SiO269.868.668.170.571.567.368.367.753.957.268.5 TiO20.610.620.570.430.430.760.650.250.840.600.25 Al2O314.815.015.414.213.814.914.516.220.019.515.4 Fe2O312.702.672.521.932.063.363.022.797.836.122.57 MnO0.090.090.080.080.080.090.080.060.160.120.05 MgO0.470.490.510.280.340.730.610.401.801.210.37 CaO1.321.321.181.160.921.861.672.185.555.351.96 Na2O3.543.663.843.603.513.613.503.814.924.873.76 K2O5.685.816.085.595.575.455.504.992.102.644.78 P2O50.130.140.140.100.110.210.180.090.320.240.09 Total99.1698.4498.4697.8798.398.2198.0498.4297.4397.8197.79 Be5.806.417.285.721.33.823.152.46 Rb22426328524317310313019
展开阅读全文
