Geochemical characteristics of ultramafic rocks in Sumdo area, Tibet and its enlightenment for the evolution of the Sumdo Paleo-Tethys Ocean
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摘要:
唐加-松多古特提斯缝合带的提出改变了地学界对青藏高原古特提斯演化格局的认识。为进一步约束唐加-松多古特提斯缝合带中蛇绿岩的岩石成因及类型, 以松多地区的超基性岩为研究对象, 对其进行了岩石学和全岩地球化学研究。结果显示, 松多地区超基性岩与原始地幔相比具有较高的MgO和TFe2O3含量, 以及较低的Al2O3和TiO2含量。样品稀土元素总含量介于4.04×10-6~9.31×10-6之间, 大部分低于原始地幔值。稀土元素配分曲线呈较宽缓的"U"型, 轻稀土元素较富集。微量元素分布形式与全球深海橄榄岩的微量元素分布形式基本一致, 具有明显的Th、Nb负异常, 大部分样品具有轻微的Zr、Hf负异常。定量模拟估算表明, 研究区超基性岩主要为尖晶石二辉橄榄岩地幔批式熔融后的难熔残留体, 熔融程度大于25%, 并具一定程度石榴子石相熔融的特征, 未受到后期岩石-熔体反应的改造, 其原岩应为亏损的深海橄榄岩。结合区域研究成果, 松多地区超基性岩可能形成于大洋中脊(MOR)环境, 为典型的大洋中脊玄武岩(MORB)型蛇绿岩端元之一。
Abstract:The proposal of Tangjia-Sumdo Paleo-Tethys suture zone has changed the understanding of Paleo-Tethys evolution pattern in the Qinghai Tibet Plateau.The petrological and geochemistry of ultramafic rocks in Sumdo area are studied to manifest the genesis and type of the ophiolite in the Tangjia-Sumdo Paleo-Tethys suture zone.The results show the ultramafic rocks have higher MgO and TFe2O3 but lower Al2O3and TiO2 contents compared with those of the primitive mantle.The total rare-earth element(REE)contents of the samples are 4.04×10-6~9.31×10-6, which are lower than those of the primitive mantle.The chondrite-normalized REE patterns display a U-type, and they are enrichment in LREE.The primitive mantle-normalized spider diagrams show an abyssal peridotites affinity, there are obvious negative anomalies of Th and Nb.Most of the samples show slight negative Zr and Hf anomalies.Through quantitative modeling, we conclude that the ultramafic rocks are refractory residual formed by more than 25% batch melting of spinel lherzolite mantle, and the inheritance of some garnet signatures was observed in the samples to a certain extent.They are not modified by later rock-melt reaction, the protolith of the ultramafic rocks may be depleted abyssal peridotite.Combining with regional geology, we suggest that the ultramafic rocks in Sumdo area may have been formed in the mid ocean ridge(MOR)setting and are one of the typical end members of mid ocean ridge basalt(MORB)type ophiolites.
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Key words:
- Qinghai-Tibet Plateau /
- Sumdo /
- ophiolite /
- ultramafic rocks /
- geochemistry /
- Paleo-Tethys Ocean
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表 1 松多超基性岩全岩地球化学分析结果
Table 1. Whole rock composition of the ultramafic rocks in Sumdo
样品编号 ST64H1 ST64H2 ST64H3 ST64H4 ST64H5 ST74H1 ST74H2 ST84H1 ST84H2 ST84H3 SiO2 40.36 44.26 37.65 41.34 38.30 43.28 39.15 40.90 37.59 41.14 TiO2 0.04 0.04 0.04 0.03 0.04 0.01 0.02 0.04 0.01 0.01 Al2O3 0.52 0.53 0.65 0.71 0.61 0.67 0.87 0.81 0.72 0.66 TFeO 9.41 9.19 12.98 8.78 11.69 5.63 7.73 8.75 9.19 8.33 MnO 0.17 0.12 0.18 0.15 0.14 0.06 0.23 0.12 0.14 0.10 MgO 39.08 36.52 39.50 39.21 39.83 36.69 42.48 40.91 42.89 39.82 CaO 0.08 0.00 0.01 0.21 0.06 0.09 0.08 0.72 0.35 0.10 Na2O 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.02 0.03 0.02 K2O 0.00 0.00 0.00 0.01 0.01 0.01 0.01 0.00 0.03 0.01 P2O5 0.01 0.00 0.01 0.00 0.01 0.01 0.01 0.01 烧失量 9.33 8.36 8.13 8.55 8.38 12.67 8.59 6.75 8.09 8.80 总计 99.01 99.03 99.14 99.00 99.08 99.12 99.19 99.04 99.05 99.01 Li 0.20 1.00 0.44 0.71 1.13 0.30 0.40 0.82 0.65 3.61 Sc 10.99 10.12 10.02 10.61 11.63 7.94 4.56 9.88 8.65 8.97 V 49.04 35.26 36.18 35.38 65.34 10.54 206.8 52.98 38.42 19.68 Cr 4708 3950 3940 4396 5628 1997 13052 3162 2878 3914 Co 130.9 124.6 142.3 127.6 139.5 87.9 123.3 137.1 124.5 123.0 Ni 1196 1173 1132 1209 1298 2112 2022 2574 2408 2424 Cu 3.24 4.58 4.30 3.28 4.08 4.49 2.94 0.84 1.84 5.66 Zn 65.68 47.90 64.56 57.12 52.02 45.36 113.6 47.90 49.22 62.52 Ga 2.05 2.07 1.93 2.21 3.19 0.73 2.27 1.14 0.98 1.05 Rb 1.68 0.86 1.11 0.88 0.99 0.42 1.40 2.25 19.58 14.08 Sr 0.4 0.3 0.3 0.2 0.4 1.7 428.6 363.2 238.2 354.8 Y 0.57 0.53 0.44 0.68 0.54 0.22 0.42 0.76 0.43 0.33 Zr 1.68 0.86 1.11 0.98 1.10 0.84 1.28 1.64 1.22 0.82 Nb 0.36 0.28 0.27 0.22 0.35 0.23 0.23 0.22 0.22 0.14 Cs 0.13 0.39 0.19 0.21 0.17 0.04 0.04 0.05 0.08 0.13 Ba 2.61 5.02 4.18 6.15 7.08 3.23 3.02 4.48 6.71 6.89 La 0.22 0.14 0.22 0.31 0.18 0.17 0.63 0.36 0.26 0.10 Ce 0.46 0.31 0.43 0.58 0.35 0.38 0.93 0.52 0.31 0.27 Pr 0.06 0.04 0.05 0.07 0.04 0.05 0.09 0.08 0.05 0.03 Nd 0.25 0.20 0.21 0.30 0.19 0.19 0.30 0.33 0.21 0.15 Sm 0.08 0.06 0.06 0.08 0.06 0.04 0.06 0.08 0.06 0.04 Eu 0.03 0.03 0.03 0.04 0.02 0.01 0.02 0.03 0.02 0.01 Gd 0.09 0.08 0.08 0.11 0.09 0.04 0.06 0.11 0.07 0.05 Tb 0.02 0.02 0.01 0.02 0.02 0.01 0.01 0.02 0.01 0.01 Dy 0.11 0.10 0.09 0.12 0.10 0.04 0.07 0.13 0.07 0.05 Ho 0.02 0.02 0.02 0.03 0.02 0.01 0.02 0.03 0.02 0.01 Er 0.06 0.06 0.05 0.07 0.07 0.03 0.04 0.08 0.05 0.04 Tm 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Yb 0.06 0.06 0.05 0.08 0.06 0.04 0.04 0.10 0.06 0.05 Lu 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 Hf 0.04 0.02 0.03 0.02 0.02 0.02 0.03 0.03 0.03 0.02 Ta 0.03 0.02 0.03 0.02 0.02 0.01 0.02 0.02 0.02 0.01 Pb 0.61 0.29 0.38 0.32 0.36 0.37 0.44 0.94 0.41 0.38 Th 0.02 0.01 0.03 0.01 0.01 0.04 0.03 0.04 0.04 0.02 U 0.03 0.02 0.03 0.03 0.03 0.04 0.05 0.02 0.02 0.01 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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