小兴安岭逊克地区第四纪高镁安山岩的岩石学、K-Ar年代学及火山地质特征

Petrological,K-Ar chronological and volcanic geological characteristics of Quaternary Xunke high-Mg~# andesites from the Lesser Khingan Range

黑龙江省小兴安岭北段逊克地区出露大面积的第四纪火山岩,分布面积约3000km^2。岩性主要为玄武安山岩和玄武质粗面安山岩,还有少量的粗面安山岩和安山岩。逊克火山岩的Si O2含量为54.3%~57.4%,MgO含量变化为3.82%~5.80%,镁指数(Mg#=100×Mg/(Mg+Fe^(2+))变化于49.6~57.8之间,属于高镁安山岩。逊克高镁安山岩火山口的位置分布在火山岩区的南面,根据火山岩区南高北低的地势,推测北边的火山岩是由南侧的岩浆向北流动形成的。岩浆流动形成了具有特征性的火山地貌,如沿河谷形成数公里长的石垄以及大面积的翻花熔岩形成的石海景观。火山岩的K-Ar测年结果表明,逊克高镁安山岩可以划分为早更新世(1.12~1.00Ma)和中更新世(0.68~0.25Ma)两期。在第四纪熔岩和河湖相沉积之间还夹有薄层火山灰,推测在岩浆溢流形成大面积熔岩之前有小规模岩浆爆发活动。

The Quaternary Xunke volcanic rocks are located in the northern Lesser Khingan Range, Heilongjiang Province, covering an area of about 3000km2. These volcanic rocks are mainly basaltic andesites and basaltic trachyandesites, with minor trachyandesites and andesites. They have high SiO2 contents （54.3%~57.4%） and Mg# （49.6~57.8） and medium MgO contents （3.82%~5.80%）, falling into the scope of high-Mg# andesites （HMAs）. Based on the distribution of craters and higher terrain of the southern part of Xunke volcanic field, we speculate that the rocks in the northern part were formed by magma flows from the south. Based on their K-Ar dating （1.12~0.25Ma）, the Xunke high-Mg# andesites can be subdivided into two periods：the Early Pleistocene （1.12~1.00Ma） and the Middle Pleistocene （0.68~0.25Ma）. Thin layer of volcanic ash was found between these Quaternary lavas and fluviatile-lacustrine facies sediments, which indicates that small-scale magmatic explosion happened before abundant flood lavas formed.