Geological characteristics and tectonic environment of Middle Permian volcanic rocks in eastern Pingle depression, Jiangxi Province
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摘要:
在萍乐坳陷区东段的鸣山地区发现中二叠世火山岩, 岩石类型主要为碱性玄武岩、亚碱性玄武岩及安山玄武岩，故将其命名为鸣山玄武岩。该套火山岩呈似层状夹于中二叠统鸣山组薄层状硅质岩与硅质页岩之间，可见气孔构造及大量杏仁体，厚约15m。火山岩具有贫硅、富铁、高铝、低镁的特点，Mg#介于4.08~19.48之间，高场强元素Nb、Ta、Zr、Hf相对富集，U、Th及轻稀土元素明显富集，(La/Yb)N=3.58~72.53, Eu弱亏损 (δEu=0.42~0.85)，稀土元素配分曲线呈右缓倾。初步研究认为，鸣山玄武岩可能形成于板内陆缘裂谷环境, 是轻稀土矿有利的矿源层，也是东吴运动在华南地区引发海底火山喷溢活动的证据。关键词：中二叠世；火山岩；萍乐坳陷区东部；东吴运动；江西
Abstract:Middle Permian volcanic rocks were discovered in the Mingshan area of eastern Pingle depression, and the rock type is mainly composed of alkali basalt, sub-alkaline basalts and andesitic basalt, called Mingshan basalt. The volcanic rocks are sandwiched between Mingshan Formation thin siliceous rocks and siliceous shale, with vesicular structure and a large number of amygdales, about 15m in thickness. The volcanic rocks are characterized by poor silicon, rich iron, high aluminum, low magnesium, Mg# content ranging from 4.08 to 19.48, relative enrichment of high field strength elements Nb, Ta, Zr, Hf, significant enrichment of U, Th and light rare earth elements (LREE), and (La/Yb)N content ranging from 3.58 to 72.53, with insignificant Eu depletion (δEu=0.42~0.85) and slightly rightly-oblique REE distribution patterns. Researches show that Mingshan basalt was probably formed in an active continental margin rift basin environment with light rare earth ore source bed; nevertheless, there also exists the evidence arguing that the Dongwu Movement caused the submarine volcanic eruption in South China.
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Key words:
- Middle Permian /
- volcanic rocks /
- eastern Pingle depression /
- Dongwu Movement /
- Jiangxi
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图 图版Ⅰ 鸣山玄武岩露头全貌，浅灰紫色为火山岩，镜头方位北西向；b.火山岩顶界照片，从上往下依次为浅灰色薄层硅质岩层、深灰色薄层硅质岩与灰白色薄层沉凝灰岩互层、灰紫色碱性玄武岩；c.样品YP7碱性玄武岩，气孔构造及杏仁体 (杏仁状玄武岩)；d.样品YP7杏仁体显微照片（正交偏光）；e.样品YP6亚碱性玄武岩，灰白色斜长石斑晶；f.样品YP6显微照片（正交偏光），棕红色伊丁石（伊丁玄武岩）；g.火山岩底界照片，从上往下依次为灰紫色碱性玄武岩、浅灰色薄层硅质岩层、灰白色安山玄武岩（样品YP2），浅灰色薄层硅质岩层；h.样品YQ2显微照片（正交偏光）
表 1 研究区中二叠世火山岩主量、微量和稀土元素分析结果
Table 1. Major, trace elements and REE analyses of Middle Permian volcanic rocks in the study area
样品号 YP2 YP4 YP5 YP6 YP7 YP8 YP9 岩性 安山玄武岩 碱性玄武岩 碱性玄武岩 亚碱性玄武岩 碱性玄武岩 碱性玄武岩 碱性玄武岩 主量元素 SiO2 45.09 42.73 44.59 47.92 46.95 44.17 47.67 Al2O3 37.07 27.25 28.59 32.47 28.25 29.47 30.77 Fe2O3 1.32 14.69 11.29 3.98 9.55 11.07 6.24 FeO 0.065 0.2 0.37 0.28 0.24 0.45 0.32 CaO 0.056 0.043 0.11 0.03 0.041 0.055 0.038 MgO 0.17 0.32 0.36 0.17 0.35 0.3 0.21 K2O 0.006 0.022 0.027 0.014 0.014 0.016 0.014 Na2O 0.016 0.024 0.03 0.032 0.023 0.019 0.05 TiO2 2.3 1.57 1.56 2.1 2.24 1.46 1.97 P2O5 0.2 0.22 0.19 0.15 0.12 0.32 0.087 MnO 0.003 0.11 0.047 0.01 0.16 0.034 0.013 LOI 13.69 12.8 12.76 12.81 12.04 12.58 12.61 Total 99.99 99.98 99.92 99.97 99.98 99.94 99.99 Mg# 19.48 4.08 5.74 7.28 6.60 4.89 5.93 微量元素 Cr 483 258 335 343 487 214 242 Ni 33.9 184 218 140 81.5 95.7 99.3 Co 1.28 71.3 45.3 5.9 37.9 16.6 11.4 Rb ＜10 11.2 ＜10 ＜10 ＜10 ＜10 ＜10 Cs 0.31 2.35 2.48 1.14 1.29 2.19 1.86 Sr 57.8 57.8 62.6 161 10.1 172 23.5 Ba 70 466 545 235 50 891 50.4 Nb 35.2 23.8 22.6 23.9 27 19.6 21.4 Ta 2.3 2.37 1.5 2.36 2.28 1.52 2.22 Zr 338 223 218 322 353 209 289 微量元素 Hf 9.78 8.03 6.77 11.4 11.4 6.38 10.7 U 23.7 4.94 4.27 23 10.8 9.76 15.9 Th 15.2 10.1 9.47 13.6 13.4 9.26 10.3 Y 129 49.3 45 11.8 14.4 21.2 7.11 Nb/Y 0.27 0.48 0.50 2.03 1.88 0.92 3.01 稀土元素 La 50.9 95 114 86.7 26 272 26.1 Ce 97.7 181 206 142 47.5 457 39.1 Pr 14.1 22.7 26 16.6 4.55 56.3 4.63 Nd 68.3 90.8 103 60.6 14.3 212 14.7 Sm 15.4 15.7 17 9.8 2.38 35.7 2.34 Eu 4.16 3.63 4.01 1.84 0.34 7.86 0.49 Gd 14 12.2 12.8 7.52 2.48 25 2.3 Tb 2.85 1.92 1.92 0.98 0.47 3.26 0.36 1.94 Ho 4.28 1.87 1.73 0.6 0.56 1.59 0.29 Er 11.2 5.01 4.73 1.86 1.78 3.96 1.03 Tm 1.71 0.69 0.67 0.28 0.35 0.39 0.22 Yb 10.2 4.41 4.12 2.06 2.45 2.69 1.59 Lu 1.51 0.67 0.64 0.34 0.4 0.4 0.26 ∑REE 315 445 506 335 106 1090 95 (La/Yb) N 3.58 15.45 19.85 30.19 7.61 72.53 11.77 LREE/HREE 3.9 11.3 13.1 18.1 8.4 21.4 10.9 δEu 0.85 0.77 0.80 0.63 0.42 0.76 0.64 注：主量元素含量单位为%，稀土和微量元素含量为10-6；测试数据完成单位为国土资源部南昌矿产资源监督检测中心，主要测试人员为李绪涛、李百球、廖岩鑫，仪器设备主要为ZSXPRIMUS X射线荧光光谱仪2(D408)、Agilent7700x电感耦合等离子体质谱仪 (D483)、PW2403 X射线荧光光谱仪1(D183)、XseriesII等离子质谱仪ICP-MS (D416)，测试时间为2015年3月 
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