Age and geochemistry of volcanic rocks of Baishan Formation in Yagan tectonic zone, Northern Alxa and their constraints on regional tectonic evolution
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摘要:
阿拉善北部雅干地区古生代火山岩发育,研究其形成时代和地质特征,对探讨中亚造山带中段南缘北山弧盆系的演化具有重要的意义。对雅干地区原划奥陶系火山岩进行了同位素年代学与地球化学研究,获得流纹岩LA-ICP-MS锆石U-Pb年龄为298.4±1.5Ma,时代为早二叠世初期,结合岩石组合特征和区域对比,将其重新厘定为上石炭统—下二叠统白山组。该套火山岩富SiO2、高K2O、低TiO2,属于钙碱性系列; 相对富集Rb、Pb、K等大离子亲石元素,明显亏损Ta、Nb、P、Ti等高场强元素; 呈现为轻稀土元素相对富集、重稀土元素相对亏损的的右倾特征,具有较明显的负Eu异常,显示出陆缘弧火山岩的地球化学特征。上述证据表明,雅干地区白山组火山岩形成于古亚洲洋向明水-旱山地块北缘俯冲的陆缘弧构造环境。
Abstract:Paleozoic volcanic rocks are developed in Yagan area,northern Alxa,which is of great significance to study their formation age and geological characteristics for the evolution of the North Mountain arc basin system in the southern margin of the Middle Central Asian orogenic belt. In this paper,the isotope chronology and geochemical characterization of the proto-Ordovician volcanic rocks of the Yagan region are studied. The results show that the LA-ICP-MS zircon U-Pb age of the rhyolite is 298.4±1.5 Ma,which belongs to the early Early Permian. It is redetermined to the Upper Carbonifeous-Lower Permian Baishan Formation based on the characteristics of the lithological assemblage. This set of volcanic rocks belong to the calc-alkaline series,which have higher content of SiO2,higher content of K2O and lower content of TiO2. They are relatively enriched in large ion lithophile elements such as Rb,Pb,K,and are obviously depleted high field strength elements such as Ta,Nb,P and Ti. The sample apparent right-leaning distribution patterns of the REE with depletion in LREE and enrichment in HREE and negative Eu anomaly,and they have the geochemistry characteristics of continental arc. The new evidence shows that Baishan Formation in Yagan area formed in the subduction of Paleo-Asian Ocean towards the northern margin of Mingshui-Hanshan block,and it was resulted from magmatism of the active continental margin.
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图 1 洪果尔吉乌拉山地区大地构造位置(a，据潘桂棠等, 2016;陈智斌等, 2020修改)和地质简图(b)
Figure 1.
图 6 白山组火山岩TAS图解(a, 底图据Le Bas et al., 2004)和SiO2-K2O图解(b, 底图据Peccerillo, 1976)
Figure 6.
表 1 白山组流纹岩LA-ICP-MS锆石U-Th-Pb同位素测试数据
Table 1. LA-ICP-MS zircon U-Th-Pb isotopic analysis data of rhyolite from Baishan Formation
测点 含量/10-6 同位素比值 年龄/Ma 谐和度/% Pb Th U 206Pb/
238U1σ 207Pb/
235U1σ 207Pb/
206Pb1σ 206Pb/
238U1σ 207Pb/
235U1σ 207Pb/
206Pb1σ 1 20 271 391 0.0475 0.0005 0.3482 0.0064 0.0531 0.0009 299.4 3.4 303 6 334 37 101 2 16 139 329 0.0477 0.0005 0.3531 0.0068 0.0537 0.0009 300.2 3.4 307 6 359 40 102 3 15 134 322 0.0466 0.0005 0.3507 0.0090 0.0546 0.0012 293.8 3.4 305 8 394 50 104 4 14 168 274 0.0467 0.0006 0.3447 0.0089 0.0535 0.0012 294.4 3.9 301 8 350 52 102 5 12 113 245 0.0475 0.0006 0.3487 0.0090 0.0532 0.0012 299.4 3.8 304 8 337 49 101 6 10 128 191 0.0475 0.0006 0.3548 0.0101 0.0541 0.0014 299.4 3.6 308 9 377 59 103 7 9 100 185 0.0472 0.0005 0.3538 0.0089 0.0543 0.0013 297.6 3.4 308 8 384 53 103 8 11 108 226 0.0482 0.0006 0.3517 0.0092 0.0529 0.0013 303.4 3.6 306 8 326 56 101 9 4 34 84 0.0480 0.0007 0.3493 0.0211 0.0528 0.0031 302.0 4.3 304 18 321 133 101 10 10 81 208 0.0474 0.0006 0.3541 0.0091 0.0542 0.0013 298.4 3.6 308 8 380 54 103 11 21 282 395 0.0471 0.0005 0.3469 0.0064 0.0534 0.0009 296.8 3.4 302 6 346 37 102 12 26 214 547 0.0466 0.0005 0.3385 0.0060 0.0527 0.0008 293.7 3.2 296 5 315 36 101 13 8 76 156 0.0476 0.0005 0.3421 0.0110 0.0521 0.0016 299.8 3.5 299 10 290 70 100 14 17 154 318 0.0477 0.0006 0.3512 0.0076 0.0534 0.0010 300.2 3.7 306 7 347 43 102 15 14 129 272 0.0472 0.0006 0.3477 0.0076 0.0535 0.0010 297.1 3.6 303 7 349 44 102 16 11 109 215 0.0479 0.0006 0.3435 0.0110 0.0520 0.0015 301.8 3.6 300 10 284 65 99 17 15 187 283 0.0474 0.0005 0.3455 0.0082 0.0528 0.0011 298.8 3.4 301 7 321 48 101 18 12 113 248 0.0472 0.0005 0.3445 0.0082 0.0530 0.0012 297.1 3.4 301 7 328 51 101 19 17 207 325 0.0472 0.0005 0.3519 0.0070 0.0541 0.0010 297.3 3.4 306 6 374 40 103 20 13 107 255 0.0477 0.0006 0.3450 0.0089 0.0525 0.0012 300.2 3.9 301 8 307 53 100 21 11 74 224 0.0475 0.0006 0.3447 0.0101 0.0526 0.0015 299.3 3.6 301 9 312 64 100 
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表 2 白山组火山岩主量、微量和稀土元素含量
Table 2. Contents of major, trace element and REE of volcanic rocks from Baishan Formation
编号 GS118 GS120 GS124 GS135-1 GS136-2 GS142 编号 GS118 GS120 GS124 GS135-1 GS136-2 GS142 SiO2 73.67 67.74 70.68 71.56 79.89 76.24 Pb 3 6.99 7.67 11.14 6.28 7.1 TiO2 0.37 0.31 0.4 0.3 0.29 0.1 Th 10.65 11.18 8.28 16.51 8.76 17.64 Al2O3 13 14.35 12.57 13.75 10.32 12.66 U 3.75 3.43 2.51 5.29 2.63 4.79 Fe2O3 2.88 4.42 2.7 2.3 1.15 1.29 La 37.9 18.82 18.67 24.14 23.98 47.04 MnO 0.09 0.11 0.07 0.07 0.05 0.02 Ce 96.34 42.34 42.16 52.04 54.74 93.72 MgO 0.32 0.65 0.92 1.23 0.28 0.12 Pr 12.02 5.08 5.19 5.96 7.19 12.16 CaO 1.04 2.23 3.05 2.27 0.61 0.33 Nd 50.5 21.06 22.54 23.24 31.14 49.54 Na2O 4.72 2.44 4.39 5.47 3.36 4.05 Sm 10.46 4.74 5.25 4.43 7.19 9.8 K2O 2.66 4.74 2.44 1.36 3.38 4.43 Eu 1.74 0.91 1.25 0.84 1.35 0.74 P2O5 0.07 0.07 0.08 0.09 0.05 0.02 Gd 10.43 5.28 5.86 4.72 8.04 10.21 烧失量 1.02 2.78 2.58 1.47 0.51 0.61 Tb 1.33 0.73 0.85 0.68 1.16 1.47 Na2O/K2O 1.77 0.51 1.8 4.02 0.99 0.91 Dy 8.93 5.11 5.89 4.12 8.34 8.64 σ 1.77 2.04 1.66 1.62 1.23 2.16 Ho 1.87 1.07 1.22 0.82 1.75 1.67 AR 3.22 1.83 2.55 2.49 4.19 4.31 Er 6.08 3.35 3.79 2.63 5.51 5.42 A/NK 1.221 1.569 1.274 1.313 1.123 1.105 Tm 0.94 0.49 0.55 0.39 0.81 0.77 A/CNK 1.037 1.087 0.816 0.942 1.002 1.05 Yb 6.82 3.52 3.87 2.68 5.55 5.29 Rb 57.9 139.06 56.22 28.32 91.46 88.72 Lu 0.99 0.5 0.56 0.41 0.77 0.79 Sr 146.68 145.64 160.32 356 83.68 69.36 Y 48.88 28.18 32.22 22.06 48.64 43.98 Zr 484.2 175.9 203.6 156.68 233.4 301 ΣREE 246.38 113 117.66 127.1 157.52 247.26 Nb 11.47 5.46 6.02 6.05 6.74 8.82 LREE 208.97 92.95 95.07 110.65 125.6 213 Cd 0.77 0.33 0.37 0.27 0.35 0.52 HREE 37.41 20.05 22.58 16.45 31.92 34.26 Cs 1.21 2.23 0.81 1.44 2.65 3.7 LREE/
HREE5.59 4.64 4.21 6.73 3.93 6.22 Ba 670.8 727.8 594 371.6 699.2 618.4 Hf 12.74 5.24 5.88 4.96 6.6 9.73 LaN/YbN 3.98 3.84 3.46 6.47 3.1 6.38 Ta 0.79 0.45 0.46 0.55 0.48 0.66 δEu 0.5 0.55 0.69 0.56 0.54 0.22 W 1.11 1.47 1.13 0.57 0.54 0.57 δCe 1.1 1.04 1.03 1.03 1.01 0.94 注：主量元素含量单位为%，微量和稀土元素含量单位为10-6
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