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.