大兴安岭北段扎兰屯哈多河“花岗质糜棱片麻岩”年代学及地球化学特征研究

Geohronology and Geochemistry Characteristics of the Granitic Mylonitic Gneiss in the Zhalantun Haduohe Area of the Northern Great Xing′an Range

哈多河"花岗质糜棱片麻岩"为一套变质变形的花岗质岩石,其岩石组合为细粒黑云母二长花岗质糜棱片麻岩、细粒黑云母花岗质糜棱片麻岩和细粒石榴二云母花岗质糜棱片麻岩。SiO_2含量在66.4%~74.52%之间,K_2O+Na_2O介于7.36%~8.84%之间,Al_2O_3含量在13.43%~15.74%之间,A/CNK=0.974~1.092,属于高钾钙碱性、准铝质到过铝质岩石;具有较高的稀土总量(ΣREE=94.15×10^(-6)~164.30×10^(-6)),稀土元素配分模式表现为轻稀土元素(LREE)富集、重稀土元素(HREE)相对亏损的右倾型曲线,具有弱的负铕异常;富集Rb、Ba、U、K等大离子亲石元素,相对亏损Ta、Nb、P、Ti等元素。锆石U-Pb测年结果表明,哈多河"花岗质糜棱片麻岩"的岩浆侵入期次可分为两期,第一期在320Ma左右,代表性岩石为细粒黑云母花岗质糜棱片麻岩和细粒石榴二云母花岗质糜棱片麻岩;第二期在304Ma左右,代表性岩石为细粒黑云母二长花岗质糜棱片麻岩。综合岩相学、年代学及岩石地球化学特征,哈多河"花岗质糜棱片麻岩"原岩为高钾钙碱性I型花岗岩,形成于晚石炭世兴安地块与松嫩地块的后碰撞构造环境下,之后在早二叠世末遭受"后碰撞作用"的影响,发生较强的韧脆性变质变形。

The granitic mylonitic gneiss in Haduohe area is a set of metamorphic granitic rocks is mainly composed of fine grained biotite monzonitic granitic mylonitic gneiss, fine grained biotite granitic mylonitic gneiss and fine garnet mica granitic mylonitic gneiss. The results of geochemical study indicate that SiO2 values are in the range of 66.4% -74.52%, K20-b-Na20 between 7.36%~8.84%, A1203 values in the range of 13.43%~15. 74%, A/CNK=0. 974~1. 092, suggesting that it belongs to high-K calc-alkaline and quasi-aluminous to weakly peraluminous rocks. The chondrite-normalized REE patterns demonstrate right-oblique linear REE patterns with weak negative Eu anomalies, higher total rare-earth content （Y, REE = 94.15 X 10 -6 ~ 164. 30 X 10-G）, and LREE enrichment and HREE depletion. The trace element is characterized by a strong enrichment in large ion lithophile elements, such as Rb, Ba, U, K et al, depletion in Ta, Nb, P, Ti et al. The zircon U-Pb dating results indicate that the magma formation can be divided into two periods, the first period is around 320Ma, the representative rock is fine grained biotite granitic mylonitic gneiss and fine garnet mica granitic mylonitic gneiss; and the second period is around 304Ma, the representative rock is fine grained biotite monzonitic granitic mylonitic gneiss. Combined with petrography, chronology and rock geochemical characteristics, the protolith of the granitic mylonitic gneiss is high K calc-alkaline I type granite which formed in the post-collision environment of Xing＇an block and Songnen block during Late Carboniferous, and then suffered relatively strong ductile brittle metamorphism and deformation during Late Early Permian by the post-collision of Xing＇an block and Songnen block.