Age and geochemistry of Aksay ophiolite in East Altun Mountains
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摘要:
北阿尔金蛇绿岩带是分布在阿尔金山北缘的一条蛇绿混杂岩带，超基性蛇纹岩岩块、基性岩墙、辉长岩块及基性熔岩组合是该带东段阿克塞蛇绿岩的典型组合形式。地球化学研究结果显示，阿克塞蛇绿岩超基性蛇纹岩以富MgO、贫Al2O3、P2O5、CaO和K2O为特征，球粒陨石标准化稀土元素配分曲线表现较平缓，比值接近于1；基性熔岩类中TiO2为0.86%~1.80%，Al2O3为12.00%~14.85%，CaO介于4.89%~19.23%之间，球粒陨石标准化稀土元素配分型式表现为右倾；基性岩墙中TiO2介于1.08%~2.44%之间，Al2O3含量为11.07%~13.26%，Na2O（1.20%~2.28%）＞K2O（0.80%~1.38%），稀土元素总量介于48.65×10-6~124.04×10-6之间，平均92.95×10-6；轻、重稀土元素比值介于2.22~4.45之间，总体表现为大洋中脊环境，LA-ICP-MS锆石U-Pb年龄显示，阿克塞蛇绿岩形成于514.6±8.8Ma，为寒武纪，表明在早古生代北阿尔金洋已完全打开，并连通了红柳沟—阿克塞。
Abstract:North Altun ophiolite belt is an ophiolite melange zone located on the northern margin of the Altun Mountains. Rock associations of ultrabasic rock blocks, basic dyke swarms, gabbro rock blocks and basic lava composite the representative ophiolite suite located in Aksay of the East Altun Mountains. Geochemical studies of the Aksay ophiolite show that the ultrabasic serpentinite is high in MgO and poor in Al2O3, P2O5, CaO, K2O, the chondrite-normalized REE patterns are relatively flat, and the ratio of sample to chondrite is close to 1. Basic lava rock is recognized by content of TiO2, Al2O3, CaO, with the chondrite-normalized REE patterns being relatively right-inclined. Basic dyke swarms are recognized by content of TiO2, Al2O3, with the features of Na2O higher than K2O, ∑ REE, and ∑ LREE/∑ HREE(2.22~4.45). Geochemical studies indicate that the Aksay ophiolite was formed in a Mid-Ocean Ridges (MOR) environment and was reformed by subsequent Suprs-Subduction Zone (SSZ). Geochronologic studies of zircon U-Pb show that the Aksay ophiolite was generated at 514.6±8.8Ma of Cambrian period.
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Keywords:
- ophiolite /
- East Altun Mountains /
- geochemistry /
- zircon U-Pb geochronology
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图 1 研究区构造位置(a)和阿克塞蛇绿岩带出露地质概况及采样位置(b)
1—超基性岩块；2—基性岩块；3—采样位置；4—地名；5—河沟；6—地质界线；7—阿克塞蛇绿岩带。Pt1B.—古元古代北大河岩群；Chh—长城纪桦树沟组；∈Lp—寒武纪拉配泉岩群；Qp3apl—晚更新世冲洪积层
Figure 1. Tectonic location of the study area(a) and sampling sites in Aksay ophiolite zone(b)
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图 2 阿克塞蛇绿岩带中超基性岩
a—透辉石(Di)呈分散的碎块状，蛇纹石(Sep)则呈连续的筛网状；b—蛇纹石占大部分，少量的蚀变矿物纤蛇纹石(Crt)呈斑状分于左上角；c—单斜辉石(Cpx)与斜长石(Pl)构成典型的辉长结构
Figure 2. Ultrabasic rock in Aksay ophiolite zone
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图 3 超基性岩球粒陨石标准化稀土元素配分型式(标准化数据据参考文献[22])
Figure 3. Chondrite-normalized REE patterns of ultrabasic rock from Aksay ophiolite
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图 4 阿克塞蛇绿岩熔岩类地球化学图解
a—Ti/Y-Nb/La图解；b—不相容微量元素原始地幔标准化蛛网图；c—Nb/Y-SiO2图解；d—Ta/Yb-Th/Yb图解；e—Zr-Zr/Y构造环境判别图解；f—Zr-TiO2构造环境判别图解
Figure 4. Diagrams of Aksay basic lava
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图 5 阿克塞蛇绿岩基性岩墙地球化学图解
a—不相容微量元素原始地幔标准化蛛网图解；b—Ta/Yb-Th/Yb图解；c—Zr-TiO2构造环境判别图解；d—SiO2-TFeO/MgO图解。OIB—洋岛玄武岩；E-MORB—富集型洋中脊玄武岩；N-MORB—正常型洋中脊玄武岩；WPB—板内玄武岩；VAB—岛弧玄武岩；MORB—洋中脊玄武岩
Figure 5. Diagrams of Aksay ophiolite basic dyke swarms
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图 6 阿克塞蛇绿岩玄武质凝灰岩锆石阴极发光(CL)图像(a)和U-Pb年龄谐和图(b)
Figure 6. CL images of zircons from the Aksay ophiolite (a) and 206Pb/238U ages (b)
表 1 阿克塞蛇绿岩主量、微量和稀土元素分析结果
Table 1 Composition of major, trace and rare earth elements of Aksay ophiolite
样品岩性 枕状熔岩 玄武质熔岩 辉长岩 蛇纹岩 基性岩墙 样品号 624Y
Q1624YQ
2624Y
Q3624Y
Q4QSY
Q30828Y
Q8817Y
Q1817Y
Q3707H10 XEBY
Q1XEBY
Q2XEBY
Q3XEBY
Q4XEBY
Q5Na2O 1.75 4.53 3.53 3.25 4.02 2.84 0.33 0.01 0.04 1.20 1.67 2.16 1.65 2.28 MgO 6.52 4.21 5.26 7.17 4.71 6.15 7.99 39.41 33.62 4.58 5.98 6.26 4.78 5.72 Al2O3 10.98 14.85 12.00 14.75 14.65 15.46 13.49 1.55 1.73 11.07 11.72 12.58 12.36 13.26 SiO2 45.49 46.67 46.11 47.15 49.03 46.91 45.41 38.77 39.96 45.55 45.29 46.50 47.01 49.84 P2O5 0.11 0.23 0.20 0.25 0.31 0.10 0.13 0.01 0.01 0.23 0.15 0.09 0.17 0.15 K2O 0.31 0.32 0.71 1.12 0.37 1.21 0.10 0.02 0.08 1.38 0.88 0.80 0.93 0.88 CaO 19.23 4.89 10.21 7.71 8.53 9.04 13.92 0.26 0.85 6.53 5.06 10.37 6.78 7.51 TiO2 0.86 1.80 1.29 1.49 2.00 1.04 1.27 0.03 0.06 2.31 1.84 1.08 2.44 1.89 MnO2 0.14 0.22 0.19 0.18 0.12 0.14 0.48 0.10 0.12 0.22 0.19 0.19 0.23 0.20 Fe2O3 4.14 4.60 7.49 2.89 3.31 5.71 11.62 7.22 5.23 5.31 4.11 3.51 4.88 4.13 FeO 5.35 10.80 5.00 9.20 5.10 3.17 9.51 1.54 1.86 10.70 9.50 7.90 10.80 9.45 H2O+ 3.97 4.54 3.21 3.90 3.41 2.70 2.75 3.42 11.70 4.14 4.16 3.31 3.67 3.31 烧失量 0.82 2.01 3.59 0.61 4.13 5.24 2.87 12.25 13.06 11.55 12.37 7.49 6.58 3.44 La 9.46 23.06 17.51 17.88 26.72 4.22 5.92 0.31 0.86 17.54 14.38 6.23 21.57 17.30 Ce 18.27 41.37 31.33 34.93 52.43 8.88 20.60 22.30 1.57 36.84 30.04 14.00 42.85 35.82 Pr 2.18 4.97 3.94 4.54 6.55 1.58 2.11 0.31 0.19 4.91 3.85 1.83 5.35 4.17 Nd 8.05 17.97 14.49 17.07 23.85 7.98 10.70 0.17 0.74 19.12 14.57 7.89 20.17 15.81 Sm 2.11 4.46 3.56 4.33 5.45 2.58 3.31 0.10 0.16 5.57 3.79 2.64 5.60 4.01 Eu 0.81 1.50 1.25 1.56 1.83 0.94 1.06 0.10 0.05 1.63 1.30 0.95 1.62 1.39 Gd 2.41 4.69 3.74 4.84 4.86 3.47 3.72 0.36 0.16 6.16 4.17 3.38 6.34 4.39 Tb 0.43 0.81 0.63 0.86 0.82 0.65 0.76 0.10 0.03 1.12 0.75 0.65 1.19 0.80 Dy 2.56 4.63 3.55 5.02 4.44 4.44 5.03 0.10 0.22 6.84 4.54 4.19 7.37 4.77 Ho 0.55 0.95 0.70 1.04 0.88 0.96 1.04 0.10 0.05 1.46 0.98 0.91 1.61 1.04 Er 1.56 2.47 1.70 2.81 2.21 2.66 3.05 0.10 0.16 4.24 2.82 2.65 4.61 2.93 Tm 0.23 0.36 0.23 0.40 0.31 0.39 0.55 0.10 0.03 0.65 0.42 0.39 0.68 0.43 Yb 1.60 2.26 1.43 2.63 1.90 2.39 3.20 0.10 0.20 4.37 2.75 2.56 4.44 2.86 Lu 0.27 0.34 0.20 0.39 0.28 0.33 0.48 0.10 0.03 0.66 0.42 0.38 0.64 0.43 Y 13.21 21.76 13.81 24.18 19.93 22.70 26.00 0.70 10.40 31.80 22.60 21.37 37.97 24.15 Zr 69.30 101.40 78.50 96.80 171.00 63.50 71.30 10.00 13.00 142.60 98.70 61.40 129.10 100.50 Nb 9.94 20.63 18.55 17.47 21.00 6.70 4.17 3.00 0.45 10.13 8.12 6.08 9.83 11.12 Hf 2.20 2.50 2.00 2.40 4.60 2.00 2.85 3.89 0.20 3.10 2.40 1.70 2.80 2.70 Ta 0.80 1.49 1.33 1.37 1.39 0.73 0.31 0.10 0.04 0.95 0.76 0.61 0.88 1.09 Ba 63.56 191.10 401.00 932.90 791.60 115.80 358.30 453.30 21.00 203.10 151.00 131.70 188.5 163.70 NiO 291.50 297.20 280.20 95.48 100.60 127.00 25.00 2038.20 1731.0 72.20 66.48 81.00 63.50 67.87 ΣREE 50.49 109.84 84.26 98.30 132.50 41.47 61.53 24.35 4.45 111.11 84.78 48.65 124.04 96.15 LREE 40.88 93.33 72.08 80.31 116.80 26.18 43.70 23.29 3.57 85.61 67.93 33.54 97.16 78.50 HREE 9.61 16.51 12.18 17.99 15.70 15.29 17.83 1.06 0.88 25.50 16.85 15.11 26.88 17.65 LREE/HREE 4.25 5.65 5.92 4.46 7.44 1.71 2.45 21.97 4.06 3.36 4.03 2.22 3.61 4.45 (La/Yb)N 4.24 7.32 8.78 4.88 10.09 1.27 1.33 2.22 3.08 2.88 3.75 1.75 3.48 4.34 δEu 1.09 1.00 1.04 1.04 1.06 0.96 0.92 1.43 0.95 0.85 0.99 0.97 0.83 1.01 δCe 0.95 0.90 0.89 0.93 0.94 0.84 1.43 15.94 0.91 0.96 0.97 1.00 0.95 1.00 注：主量元素含量单位为%，微量和稀土元素单位为10-6 
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表 2 阿克塞蛇绿岩组火山岩样品LA-ICP-MS锆岩U-Th-Pb定年结果
Table 2 LA-ICP-MS zircons U-Th-Pb compositions of Aksay ophiolite basic lava
测定点号 同位素比值 年龄/Ma 207Pb/206Pb 207Pb/235U 206Pb/238U 207Pb/235U 206Pb/238U 比值 ±σ 比值 ±σ 比值 ±σ 年龄 ±σ 年龄 ±σ 半个洼一带玄武质凝灰岩928TW4，采样坐标：北纬39°26′07″、东经94°25′36″ 928tw4-01 0.05772 0.00075 0.68051 0.01004 0.08555 0.00109 527 6 529 6 928tw4-02 0.0569 0.00072 0.6793 0.00981 0.08663 0.0011 526 6 536 7 928tw4-03 0.0584 0.00095 0.65232 0.01136 0.08106 0.00106 510 7 502 6 928tw4-04 0.05765 0.00091 0.64536 0.01107 0.08123 0.00106 506 7 503 6 928tw4-05 0.0571 0.00076 0.66213 0.00999 0.08411 0.00107 516 6 521 6 928tw4-06 0.0571 0.00072 0.63751 0.0089 0.08096 0.00098 501 6 502 6 928tw4-07 0.05722 0.00078 0.65562 0.00973 0.0831 0.00102 512 6 515 6 928tw4-08 0.05707 0.0007 0.65315 0.00901 0.083 0.00101 510 6 514 6 928tw4-09 0.05835 0.00111 0.64027 0.01254 0.07958 0.00104 502 8 494 6 928tw4-10 0.05824 0.00074 0.67808 0.00953 0.08444 0.00103 526 6 523 6 928tw4-11 0.05785 0.00068 0.67962 0.00909 0.0852 0.00103 527 5 527 6
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