Geochronology, Geochemistry and Geological Significance of Intrusive Rocks in Qianshao Forest Farm of Mohe Area, Great Xing'an Range
Journal of Jilin University(Earth Science Edition). 2021, 51 (1):
Qianshao forest farm is located at the edge of Mohe foreland basin in the northern part of Erguna block in the north of Great Xing'an Range. The authors studied the petrography, geochemistry, and geochronology of intrusive rocks in Qianshao forest farm, and discussed the formation age, petrogenesis, and tectonic environment of the intrusive rocks. The results of LA-ICP-MS zircon U-Pb dating show that the granite was formed in (199.9-199.3) Ma, gabbro was formed at (201.8±2.6) Ma, in Late Triassic-Early Jurassic. The petro-geochemical studies show that the w(SiO2) of granite is 63.22%-70.10%, w(Al2O3) is 12.43%-14.36%, and the Ritman index(σ) is 0.74-1.65, indicating that they belong to cala-alkaline series. They have low Mg# values (average 39.43), w(TFeO) is 2.80%-4.41%, and w(CaO)is 1.47%-3.38%. The REE is characterized by significant fractionation of light and heavy rare earth elements with enrichment of LREE and depletion of HERR, δEu of 0.48-0.84, enrichment of Rb, Ta, K, La, Nd, Zr, Ti, and depletion of Th, U, Sr, P, Eu. The w(SiO2) of gabbro is 51.42%-51.98%, w(Al2O3) is 17.24%-17.73%, and the Ritman index(σ) is 3.00-3.53, indicating that they belong to cala-alkaline series. They have high Mg# values (average 51.07), w(TFeO)is 9.06%-9.14%, and w(CaO) is 5.81%-6.69%, light fractionation of light and heavy rare earth elements with δEu of 0.86-0.98, enrichment of Rb, Ta, Nb, Nd, and depletion of Th, U, P, Eu. The above geochemical characteristics show that the granite was the product of crystallization differentiation of gabbro. The original magma was sourced from the mantle and subsequently contaminated by crustal materials. The intrusive rocks in the study area were formed in the compressive environment of the subduction and collision orogenic stage between the Mongolian and Okhotsk Sea.
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