PETRO-GEOCHEMISTRY AND GENESIS OF THE PUMICES AT DONGDAO OF XISHA ISLANDS
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摘要:
对采自西沙群岛东岛的2件浮岩样品进行了岩石学和地球化学初步研究，结果表明, 研究区样品全岩化学组成落入粗面岩区域，稀土配分曲线明显呈右倾，具Eu负异常，富集不相容元素，微量元素标准化曲线特征与中下地壳相似。斜长石及辉石斑晶矿物具有中心熔蚀结构，橄榄石呈熔蚀状。认为研究区浮岩是由西沙地块中地壳减压熔融产生粗面质岩浆后经历短暂的岩浆房冷凝时期，由于板块继续拉张形成东岛西侧的狭窄海槽，而引起岩浆通道突发性张裂，导致压力的突然释放，岩浆沿拉张裂谷急速上升，在浅水中猛烈喷出而形成。
Abstract:In this paper, two pumices samples collected from the Dongdao Island are studied for petrography and mineral geochemistry. With chemical composition, the samples are defined as trachyte, and their rare earth distribution curves are both dipping rightward, with negative Eu anomalies, enriched incompatible elements, and a trace element normalized curve similar to the lower crust. The porphyritic minerals of plagioclase and pyroxene show a central melting structure, and olivine is molten. It is believed that the pumices are formed by an explosion of a short-term condensed magma chamber after the melting and pressure reduction of the crust at the Xisha block, due to the continuous spreading of the plate and the formation of the narrow trough on the west side of the Dongdao Island. The process caused the sudden release of pressure, and rapid eruption of magma along the stretching rift into the shallow water.
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Key words:
- pumice /
- petro-geochemistry /
- magmatic activity /
- Dongdao
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表 1 研究区浮岩主量元素(w)和微量元素(×10-6)分析测试结果及相关参数
Table 1. Contents of major oxides (w) and trace elements(×10-6) of Dongdao pumice and related parameters
样品号 S1 S2 样品号 S1 S2 样品号 S1 S2 SiO2 60.85 60.87 Y 36.2 36.2 La 62.7 63.1 TiO2 0.96 0.99 Nb 8.74 8.59 Ce 123.8 123.7 Al2O3 14.79 15.63 Ta 0.44 0.43 Pr 15.27 15.4 TFe2O3 2.86 2.94 Ba 1 170 1 170 Nd 58.8 59.7 MnO 0.39 0.38 Cu 17.5 14.6 Sm 10.2 10.3 MgO 1.66 1.64 Sr 493 515 Eu 2.54 2.62 CaO 4.1 3.94 V 83.8 91.8 Gd 8.24 8.31 Na2O 5.14 5.13 Zn 138.2 130.5 Tb 1.16 1.17 K2O 4.15 4.08 Li 22.3 21 Dy 6.19 6.29 P2O5 1.57 1.4 Be 2.22 2.08 Ho 1.32 1.32 LOI 3.41 2.87 Sc 9 9.4 Er 3.75 3.78 Total 99.88 99.89 Cr 17.9 10.8 Tm 0.58 0.59 DI 80.02 78.83 Co 8.7 9.3 Yb 3.86 3.88 SI 12.15 12.02 Ni 9.79 5.32 Lu 0.6 0.61 AR 2.94 2.78 Ga 20.5 20.3 бEu 0.82 0.84 σ43 4.61 4.56 Rb 72 72.4 бCe 0.95 0.94 Q 9.46 9.16 Mo 3.47 3.48 LREE 273.31 274.82 A 62.89 58.97 Cd 1.17 0.34 HREE 25.7 25.95 P 12.88 18.51 W 0.52 0.47 ∑REE 299.01 300.77 液相密度 2.45 2.45 Pb 13.5 14.9 LREE/HREE 10.63 10.59 干黏度 5.64 5.61 Th 11.1 11.1 (La/Yb)N 11.65 11.67 湿黏度 4.72 4.74 U 3.48 3.4 液相线温 974 980 K 32 294.5 30 837.8 
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