【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
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.
赵吉昌,汤庆艳,柳永刚,苏天宝,王振,胡小春,杨镇熙,宋宏,张家和. 敦煌地块敖包山晶质石墨矿集区矿床地质特征、变质岩原岩恢复及形成环境[J]. 中国地质, 2023, 50(2): 557-572.
ZHAO Jichang, TANG Qingyan, LIU Yonggang, SU Tianbao, WANG Zhen, HU Xiaochun, YANG Zhenxi, SONG Hong, ZHANG Jiahe. Geological characteristics of the deposits, protolith restoration and paleoenvironment of the Aobaoshan crystalline graphite deposit concentrated area in Dunhuang block[J]. Geology in China, 2023, 50(2): 557-572(in Chinese with English abstract).