Geological characteristics of the deposits, protolith restoration and paleoenvironment of the Aobaoshan crystalline graphite deposit concentrated area in Dunhuang block
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摘要:
研究目的 为解决敦煌地块敖包山晶质石墨矿集区晶质石墨矿床研究程度较低的问题，通过对该区变质岩原岩特征及古构造环境进行深入研究，以期为该区的成矿规律研究提供地质依据。
研究方法 本文运用岩石地球化学方法对该区变质岩进行原岩恢复，探讨其形成时的古构造环境。
研究结果 二云石英片岩、黑云斜长片麻岩和斜长角闪岩均具有轻稀土富集的特征。斜长角闪岩K2O＜Na2O，具有弱的Nb、Ta亏损和强烈的Zr、Hf亏损，LREE/HREE=2.21~6.97，轻重稀土元素分馏程度较弱；黑云斜长片麻岩和二云石英片岩K2O>Na2O，富集大离子亲石元素Rb、K等，亏损Ba、Nb、Ta、Zr、Hf、Ti、Sr等元素，具有明显的Eu负异常，LREE/HREE平均值分别为12.44和8.89，轻重稀土元素分异较强烈。
结论 敖包山晶质石墨矿集区变质岩原岩形成于浅水区，水动力环境较弱。斜长角闪岩的原岩为亚碱性基性火山岩，形成于岛弧环境；二云石英片岩和黑云斜长片麻岩的原岩为页岩、黏土岩，构造环境为活动大陆边缘，古沉积环境为干燥气候带条件下富氧的海陆交互相，古水体介质为半咸水。
Abstract:This paper is the result of mineral exploration engineering.
Objective Due to the relatively low research level of crystalline graphite deposits in the Aobaoshan crystalline graphite deposit-concentrated area in Dunhuang block, the protolith restoration and paleoenvironment of this area are studied profoundly, which will provide a geological foundation for metallogenic regularity.
Methods The protolith of the metamorphic rocks in the Aobaoshan crystalline graphite deposit-concentrated area is restored using the method of petrochemistry, and the paleotectonic environment of its formation is discussed.
Results Two-mica quartz schist, biotite plagioclase gneiss and plagioclase amphibolite are enriched in the light rare earth elements. The plagioclase amphibolites have less K2O contents than Na2O. They are weakly depleted in Nb and Ta, and strong depleted in Zr and Hf, and have the ratios of LREE/HREE ranging from 2.21 to 6.97, indicating the weak fractionation degree of light and heavy rare earth elements. The biotite plagioclase gneisses and two-mica quartz schists have higher K2O contents than Na2O. They show the enrichment in large ion lithophile elements, such as Rb and K, and the depeletion in Ba, Nb, Ta, Zr, Hf, Ti and Sr, and obvious Eu negative anomalies. The average ratios of LREE/HREE of the biotite plagioclase gneiss and two-mica quartz schist are 12.44 and 8.89, respectively, indicating strong differentiation of light and heavy rare earth elements.
Conclusions It is inferred that the protoliths of the metamorphic rocks from the Aobaoshan crystalline graphite deposit-concentrated area were formed in shallow water area with weak hydrodynamic environment. The protoliths of the plagioclase amphibolites in the Aobaoshan crystalline graphite deposit-concentrated area are subalkaline basic volcanic rock, and they were formed in an island arc. The protoliths of biotite plagioclase gneisses and two-mica quartz schists are shale and clayrock, and they were formed at active continental margins. The paleosedimentary environment was oxygen-rich interactive marine and terrestrial deposition in dry climate zone, and the paleowater medium was brackish water.
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图 6 敖包山晶质石墨矿集区样品的原始地幔标准化图解（a）和球粒陨石标准化图解（b）（原始地幔和球粒陨石标准化值来自Palme and O'Neill，2014）
Figure 6.
图 7 Zr/TiO2-Ni图解（a, 底图据Winchester et al., 1980）；Ga/Sc-Nb/Y图解（b, 底图据Winchester and Floyd, 1977）；(al+fm)-(c+alk)-Si图解（c, 底图据Simonen, 1953）；La/Yb-∑REE图解（d, 底图据王仁民等, 1987）
Figure 7.
图 10 SiO2-K2O/Na2O（a）和SiO2/Al2O3-K2O/Na2O（b）图解（底图据Roser and Korsch, 1986）
Figure 10.
表 1 敖包山晶质石墨矿集区岩石化学分析结果
Table 1. Petrochemical analyses of Aobaoshan crystalline graphite deposit-concentrated area
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