Geochemical characteristics and LA-ICP-MS zircon U-Pb age of the trachytic volcanic rocks in Zhushan area of Southern Qinling Mountains and their significance
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摘要:
南秦岭竹山地区广泛分布的北西-南东向展布的基性岩-粗面岩带，主要赋存于志留纪地层中。竹山地区粗面质火山岩地球化学研究表明，该区粗面质火山岩全碱Na2O+K2O含量较高，为碱性岩系列；大离子亲石元素（Ba、Th、U）和高场强元素（Nb、Ta等）相对富集，Sr、Ti、Yb等元素相对亏损；稀土元素总量（ΣREE）较高，明显富集轻稀土元素，亏损重稀土元素；结合微量元素构造环境判别分析认为，该区粗面质火山岩形成于大陆裂谷环境。采用LA-ICP-MS方法测得该区粗面质火山碎屑岩中锆石的U-Pb年龄为430.6±2.7Ma，该年龄应代表粗面质火山岩主体的结晶年龄。研究结果表明，南秦岭竹山地区在早志留世发生了强烈的裂解活动，并形成一富硅质和炭质岩组合的深水盆地，该洋盆可能为南秦岭勉略洋向东的延伸。南秦岭竹山地区粗面质火山岩岩石化学及锆石U-Pb同位素定年研究，为了解南秦岭早古生代构造演化提供了重要依据。
Abstract:Basic rock-trachyte belts, trending in NW-SE direction, are widely distributed in Zhushan area of Southern Qinling Mountains, intruding in Silurian strata. Previous researchers conducted detailed studies focused on basic rocks. Compared with the ba-sic rocks, the research on trachytic volcanic rocks seems very insufficient, and hence accurate data forming chronologic data are espe-cially lacking. The dating of trachytic volcanic rocks from Zhushan area in Southern Qinling Mountains shows that the trachytic vol-canic rocks are alkaline series rocks with relatively high Na2O+K2O. Lithophile elements(Ba, Th, U)and high field strength elements (Nb, Ta)are relatively abundant, but the elements such as Sr, Ti and Yb are relatively poor. Rare earth elements(ΣREE, which are rich in light REE and deficient in heavy REE)are relatively high. Combined with the trace element characteristics, it is shown that the trachytic volcanic rocks were formed in a continental rifting environment. The crystallization of trachytic volcanic rocks took place at 430.6±2.7Ma, as shown by the LA-ICP-MS zircon U-Pb dating of trachytic pyroclastic rocks in Zhushan area. The studies show that a strong cleavage activity occurred in the period of early Silurian and formed a deep water basin with rich siliceous and car-bonaceous rock combination in the Zhushan area of Southern Qinling Mountains. Combined with the discovery of ocean island ba-salts as well as island arc and oceanic basalts, the authors hold that this region might have developed a small finite ocean basin after Si-lurian, and the ocean basin was probably the eastward extension of the Mianlue Ocean in Southern Qinling Mountains. The study of the geochemical characteristics and zircon U-Pb age of the trachytic volcanic rocks provides an important basis for early Paleozoic tectonic evolution in Zhushan area of Southern Qinling Mountains.
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致谢: LA-ICP-MS锆石U-Pb测年得到中国地质大学（武汉）地质过程与矿产资源国家重点实验室相关工作人员的帮助，锆石U-Pb同位素数据分析得到湖北省地质调查院郭盼、江苏省地质调查院贺新星的帮助，审稿专家对论文初稿提出了宝贵的建设性的修改意见，在此一并表示感谢。
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图 1 南秦岭竹山地区地质简图
Qb—青白口系；Nh—南华系；Z—震旦系；∈—寒武系；∈-S—寒武系-志留系；S1—下志留统；S2—中志留系；K—白垩系；Q—第四系
Figure 1. Generalized geologialc map of Zhushan area, Southern Qingling Mountains
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图 3 竹山地区粗面质火山岩主量元素Harker图解
Figure 3. Harker-type oxides versus SiO2 for trachytic volcanic rocks in Zhushan area
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图 4 粗面质火山岩稀土元素配分模式（a）和微量元素蛛网图（b）
Figure 4. Chondrite-normalized REE patterns (a) and spider diagram (b) of trace elements for trachytic volcanic rocks
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图 5 竹山地区粗面质火山碎屑岩锆石CL图像（年龄单位：Ma）
Figure 5. CL images of zircons from trachytic pyroclastic rocks in Zhushan area
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图 6 竹山地区粗面质火山碎屑岩锆石U-Pb谐和图
Figure 6. U-Pb concordia diagram of zircons from trachytic pyroclastic rocks in Zhushan area
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图 7 竹山地区粗面质火山岩形成构造环境判别
Figure 7. Tectonic discrimination diagrams of trachytic volcanic rocks in Zhushan area
表 1 竹山地区粗面质火山岩主量、微量和稀土元素分析结果
Table 1 Major, trace and rare earth element data for trachytic pyroclastic rocks in Zhushan area
样号 101-33YQ 101-35YQ 203-20YQ 203-37YQ 205-3YQ 205-8YQ 205-24YQ 305-16YQ 305-92YQ 306-19YQ 岩性 粗面质 粗面质岩屑 粗面质晶屑 粗面质晶屑 粗面质 粗面岩 粗面岩 粗面斑岩 粗面斑岩 粗面岩 凝灰岩 晶屑凝灰岩 凝灰岩 凝灰岩 凝灰岩 SiO2 63.11 61.67 58.84 66.29 54.16 59.80 64.48 58.82 59.29 64.09 TiO2 0.33 1.61 2.14 1.13 1.06 1.16 1.09 1.72 1.19 1.04 Al2O3 ：0.66 14.95 14.12 14.12 19.85 17.60 15.35 15.26 18.49 15.96 Fe2O3 1.79 1.41 3.24 1.42 1.58 1.24 0.94 1.67 1.49 1.91 FeO 0.33 4.65 4.78 3.27 3.05 2.75 3.53 5.03 2.73 2.37 MnO 0.22 0.14 0.21 0.14 0.35 0.32 0.28 0.27 0.31 0.24 MgO 0.21 1.82 2.64 1.70 2.67 2.06 1.00 1.85 1.27 0.90 CaO 0.27 2.63 2.44 1.12 2.52 1.55 0.93 2.35 2.02 0.42 Na2O 8.87 3.25 4.64 4.58 4.47 6.09 7.34 5.78 6.74 5.68 K2O 1.85 4.24 3.96 4.03 6.70 5.20 3.43 4.72 4.58 5.91 P2O3 0.06 0.29 0.31 0.18 0.10 0.23 0.16 0.46 0.17 0.18 烧失 1.40 2.43 1.62 1.45 2.89 1.46 0.99 1.27 0.96 0.61 总量 99.10 99.09 98.94 99.43 99.40 99.46 99.52 99.20 99.24 99.31 La 21.58 92.52 110 124 234.6 170.0 201.4 122.6 218.5 163.2 Ce 45.53 183.3 216 224 607.01 293.76 360.96 246.77 414.43 286.27 Pr 5.48 22.57 23.46 24.28 70.57 34.00 43.53 30.52 48.33 33.58 Nd 17.46 80.79 82.43 84.52 199.31 109.92 139.27 105.63 156.56 111.63 Sm 3.97 14.43 15.39 15.50 31.25 18.49 24.68 19.36 25.98 18.68 Eu 1.28 3.21 3.74 3.85 3.97 4.84 5.03 5.48 6.61 4.10 Gd 3.81 15.25 13.46 13.68 23.92 14.37 19.34 14.52 19.69 14.13 Tb 0.67 2.22 2.33 2.37 3.80 2.21 3.00 2.29 2.94 2.13 Dy 3.98 11.42 11.67 12.04 21.50 11.26 15.49 11.80 14.78 11.29 Ho 0.86 2.17 2.26 2.33 3.94 1.97 2.72 2.00 2.55 1.98 Er 2.63 5.20 6.40 6.61 11.26 5.14 7.13 5.11 6.53 5.15 Tm 0.41 0.73 0.95 0.98 1.83 0.77 1.08 0.75 0.99 0.75 Yb 2.65 4.32 5.32 5.71 11.42 4.60 6.43 4.42 5.73 4.27 Lu 0.42 0.53 0.54 0.64 1.70 0.69 0.96 0.65 0.85 0.64 Y 21.57 52.72 60.62 61.06 116.80 55.75 75.27 55.52 66.80 54.22 1REE 110.73 438.66 494.55 520.51 1226.09 672.00 831.01 571.89 924.46 657.80 LREE 95.29 396.81 451.62 476.15 1146.71 631.01 774.87 530.36 870.41 617.45 HREE 15.43 41.85 42.93 44.36 79.39 41.00 56.14 41.53 54.05 40.34 LREE/HREE 6.18 9.48 10.52 10.73 14.44 15.39 13.80 12.77 16.11 15.30 (La/Yb)N 5.50 14.48 14.02 14.67 13.88 25.00 21.15 18.75 25.78 25.82 Rb 13.60 132.40 127.65 101.43 185.20 77.93 56.29 61.70 42.26 75.57 Ba 8155 1549 1063 268 268 1304 272 2062 1917 143 Th 15.53 16.66 16.19 10.48 57.41 13.57 15.49 11.72 17.49 7.11 U 8.61 2.66 2.40 2.40 16.38 2.63 2.19 2.20 3.35 1.35 Ta 19.36 7.43 7.94 5.52 35.52 13.47 16.29 10.15 15.77 10.35 Nb 613.6 147.8 138 103 638.4 237.2 296.5 177.5 273.7 190.4 Sr 84.54 140.2 244 113 353.4 176.2 123.4 272.6 2250.7 49.7 Zr 1541 659 697 621 2503 830 1388 806 1014 780 Hf 24.66 17.91 16.90 15.37 53.51 16.83 26.68 18.14 20.57 15.61 注：主量元素含量单位为%；微量和稀土元素含量为10-6；数据由国土资源部武汉矿产资源监督检测中心完成 
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表 2 竹山地区粗面质火山碎屑岩LA-ICP-MS锆石U-Th-Pb同位素分析结果
Table 2 LA-ICP-MS U-Th-Pb isotope composition of zircons in trachytic volcanic rocks of Zhushan area
分析号 元素含量/10-6 232Th/238U 同位素比值 同位素年龄/Ma Total Pb 232Th 238U 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 207Pb/206Pb 1σ 207Pb/235U 1σ 206Pb/238U 1σ 205-1 81.5 381 178 2.14 0.0663 0.0041 0.603 0.034 0.0681 0.0011 816.7 130 479.2 21.8 424.7 7.0 205-2 56.8 261 157 1.66 0.0626 0.0037 0.604 0.034 0.0708 0.0011 694.5 130 479.7 21.3 440.7 6.6 205-3 89.4 418 235 1.78 0.0746 0.0049 0.684 0.043 0.0667 0.0013 1057.4 130 529.4 26.2 416.3 7.8 205-4 109.7 544 252 2.15 0.0618 0.0032 0.593 0.030 0.0707 0.0010 733.3 110 473.0 18.9 440.4 6.0 205-5 28.9 127 102 1.24 0.0710 0.0092 0.666 0.085 0.0681 0.0021 966.7 270 518.5 51.7 424.8 12.5 205-6 57.0 232 170 1.36 0.0761 0.0048 0.693 0.040 0.0681 0.0012 1098.2 130 534.6 24.2 424.9 7.0 205-7 72.9 328 193 1.70 0.0595 0.0044 0.576 0.044 0.0711 0.0013 587.1 160 461.9 28.4 442.5 7.8 205-8 148.5 741 337 2.20 0.0553 0.0033 0.537 0.032 0.0703 0.0012 433.4 140 436.2 21.0 437.8 7.5 205-9 51.4 231 144 1.61 0.0659 0.0039 0.615 0.033 0.0700 0.0012 803.4 130 486.6 21.0 436.0 7.5 205-10 155.8 693 317 2.18 0.0701 0.0032 0.657 0.031 0.0687 0.0011 931.5 93 512.7 18.7 428.6 6.6 205-11 112.2 542 265 2.04 0.0656 0.0044 0.629 0.050 0.0682 0.0011 794.4 140 495.5 30.9 425.2 6.5 205-12 52.7 246 142 1.74 0.0618 0.0042 0.563 0.033 0.0685 0.0013 664.8 140 453.5 21.7 427.0 7.5 205-13 152.8 749 408 1.84 0.0582 0.0022 0.553 0.021 0.0690 0.0008 538.9 81 446.9 13.5 429.9 5.0 205-14 183.0 893 400 2.23 0.0584 0.0023 0.554 0.021 0.0694 0.0009 546.3 87 447.5 14.0 432.7 5.3 205-15 87.6 406 205 1.98 0.0668 0.0042 0.623 0.037 0.0687 0.0012 831.5 140 491.6 23.1 428.5 7.2 205-16 96.7 449 225 2.00 0.0615 0.0032 0.600 0.032 0.0701 0.0011 657.4 110 477.2 20.5 437.0 6.5 205-17 123.9 565 322 1.76 0.0588 0.0027 0.558 0.025 0.0696 0.0009 561.1 100 450.2 16.0 434.0 5.5 205-18 224.1 998 675 1.48 0.0597 0.0019 0.587 0.019 0.0715 0.0009 590.8 70 469.2 12.4 445.3 5.4 205-19 72.8 339 174 1.95 0.0579 0.0046 0.538 0.043 0.0682 0.0012 527.8 180 437.0 28.3 425.0 7.1 205-20 167.7 842 387 2.18 0.0598 0.0025 0.560 0.024 0.0685 0.0009 594.5 93 451.8 15.6 427.2 5.4 205-21 142.9 758 304 2.49 0.0588 0.0047 0.547 0.048 0.0668 0.0014 566.7 180 443.3 31.5 416.8 8.2 205-22 70.2 243 388 0.63 0.0569 0.0026 0.548 0.024 0.0705 0.0010 487.1 100 443.7 16.0 439.3 5.8 注：数据由中国地质大学（武汉）地质过程与矿产资源国家重点实验室完成
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