Identification of the relics of paleo-seamount in orogens: A case study from the geological mapping in Qingshashan and Donggou areas within the Lajishan suture zone
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摘要:
造山带内海山/洋岛残片的识别是确定古缝合带和古洋盆存在的直接地质证据。祁连造山带被普遍认为是原特提斯洋盆俯冲和闭合的产物，然而南祁连地区是否存在古洋盆长期存在争议。通过对拉脊山关键地段早古生代岩石开展大比例尺地质填图和室内综合研究，在青沙山和东沟地区识别出连续的由洋岛型枕状玄武岩、火山碎屑岩、泥岩、凝灰岩和灰岩组成的火山-沉积组合序列，它们分别呈构造窗和构造残片产出，是拉脊山增生杂岩的重要组成部分，代表中寒武世—早奥陶世原特提斯洋内海山残片。这些海山残片的识别不仅表明拉脊山地区存在早古生代洋盆和缝合带，同时为造山带古洋盆构造演化研究提供了新思路。
Abstract:Identification of the relics of seamount/oceanic island in orogens can provide critical geological evidence for the presence of the suture zone and paleo-ocean basin.The Qilian Orogen is widely considered as the product of subduction and closure of the Proto-Tethyan Ocean.However, whether there exists a paleo-ocean basin in the South Qilian is still hotly debated.Based on large-scale geological mapping and comprehensive study on the Early Paleozoic rocks in the key area of Lajishan, a successive volcanic-sedimentary sequence composed of OIB-type pillow basalt, pyroclastic rock, mudstone, tuff, and limestone was recognized in the Qingshashan and Donggou areas.The rocks are outcropped as the tectonic slice and tectonic window respectively.The volcanic-sedimentary sequence is the major components of the accretionary complex within Lajishan, and represents the relics of Middle Cambrian to Early Ordovician seamounts within the Proto-Tethyan Ocean.The identification of these relics of seamount not only indicates that there exist Early Paleozoic ocean basin and suture zone in the Lajishan area but also provides a new idea for the study of tectonic evolution of the paleo-ocean basin in orogens.
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Key words:
- seamount /
- suture zone /
- paleo-ocean basin /
- large-scale geological mapping /
- Lajishan
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表 1 青沙山枕状玄武岩全岩主量、微量和稀土元素组成
Table 1. Major, trace and rare earth elements compositions of the pillow basalts in the Qingshashan area
样品号 12QSS1 12QSS2 12QSS3 12QSS4 12QSS5 12QSS6 12QSS7 SiO2 42.72 42.24 43.51 39.86 40.29 42.22 44.57 TiO2 2.17 2.01 2.04 1.82 1.80 1.89 2.24 Al2O3 14.34 14.27 13.34 12.26 11.74 12.96 14.67 Fe2O3 11.74 12.82 12.09 11.79 10.27 10.74 12.07 FeO 1.53 1.10 1.60 1.56 2.03 2.14 1.31 MnO 0.13 0.11 0.12 0.13 0.12 0.12 0.10 MgO 4.40 3.99 6.14 5.99 5.47 5.34 4.91 CaO 7.76 7.80 8.23 11.69 12.94 9.59 5.60 Na2O 4.78 4.98 4.45 3.14 3.77 3.75 4.42 K2O 1.45 1.14 0.90 1.48 0.91 1.62 2.58 P2O5 0.44 0.44 0.48 0.46 0.40 0.46 0.48 烧失量 7.68 8.27 6.12 8.90 9.51 8.18 6.52 总计 99.14 99.17 99.02 99.08 99.25 99.01 99.47 TFeO 12.09 12.64 12.48 12.17 11.27 11.80 12.17 Mg# 39 36 47 47 46 45 42 La 22.9 22.2 31.7 28.9 26.0 32.3 25.1 Ce 48.6 47.7 64.3 58.3 54.5 63.7 53.4 Pr 6.30 6.21 8.16 7.47 6.93 8.11 7.08 Nd 26.0 26.3 32.6 31.0 27.8 34.0 29.2 Sm 5.40 5.47 6.35 6.29 5.50 6.59 6.10 Eu 1.66 1.70 1.94 1.93 1.73 2.01 1.89 Gd 5.04 5.33 6.09 5.88 5.33 6.22 5.83 Tb 0.78 0.78 0.88 0.88 0.77 0.92 0.88 Dy 3.91 3.90 4.30 4.31 3.85 4.57 4.49 Ho 0.74 0.72 0.79 0.81 0.70 0.85 0.84 Er 1.86 1.81 1.92 2.00 1.69 2.12 2.14 Tm 0.27 0.26 0.28 0.28 0.24 0.29 0.32 Yb 1.57 1.45 1.51 1.60 1.36 1.69 1.86 Lu 0.22 0.21 0.23 0.24 0.20 0.25 0.28 δEu 0.97 0.96 0.95 0.97 0.98 0.96 0.97 REE 125.25 124.04 161.05 149.89 136.60 163.62 139.41 LREE/HREE 7.70 7.58 9.07 8.37 8.66 8.68 7.38 (La/Yb)N 10.46 10.98 15.06 12.96 13.71 13.71 9.68 Li 17.0 15.6 15.7 14.5 11.9 14.1 18.5 Sc 21.8 21.8 21.2 20.1 19.5 20.6 22.5 V 239 258 223 205 191 199 227 Cr 335 414 354 327 306 306 311 Ni 185 220 229 210 203 190 148 Co 47.4 47.8 52.7 50.8 46.6 51.7 42.9 Cu 81.3 50.0 83.1 103.0 69.9 101.0 100.0 Pb 3.17 4.12 3.42 3.93 4.10 4.76 2.41 Cs 4.67 7.41 3.24 2.48 3.57 3.45 3.36 Ga 18.6 18.2 19.2 20.0 16.6 19.1 17.6 Rb 21.0 19.8 15.6 31.0 14.9 29.1 34.4 Ba 327 253 289 419 237 520 435 Sr 475 446 504 442 511 448 393 Th 2.21 2.10 3.22 2.94 2.77 3.08 2.19 U 0.40 0.43 0.57 0.61 0.55 0.62 0.43 Nb 23.6 22.1 30.3 28.5 26.7 30.3 22.9 Ta 1.30 1.26 1.84 1.67 1.60 1.74 1.34 Zr 159 152 185 180 163 185 173 Hf 3.87 3.70 4.33 4.18 3.89 4.37 3.94 Y 17.7 17.8 20.0 20.9 17.0 22.2 20.2 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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表 2 青沙山枕状玄武岩全岩Rb-Sr和Sm-Nd同位素组成
Table 2. Rb-Sr and Sm-Nd isotopic compositions of the pillow basalts in the Qingshashan area
样品号 年龄/Ma 87Rb/86Sr 87Sr/86Sr 2σ (87Sr/86Sr)t 147Sm/144Nd 143Nd/144Nd 2σ (143Nd/144Nd)t εNd(0) εNd(t) fSm/Nd 12QSS2 510 0.1285 0.705828 0.000015 0.70489 0.1257 0.512462 0.000011 0.512042 -3.4 1.2 -0.36 12QSS3 510 0.0896 0.705452 0.000011 0.70480 0.1178 0.512455 0.000012 0.512062 -3.6 1.6 -0.40
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