吉林东部敦(化)—密(山)断裂带内碱性粗面岩同位素年代学、岩石化学及地质意义

Geochronology,geochemistry and its geological implications of alkaline trachyte from Dunhua--Mishan fault belt,eastern Jilin

吉林敦化—密山断裂带北段琵河口—黄泥河镇之间新发现有多处的霓辉粗面岩,岩石以普遍富含碱性矿物霓辉石为特点,均呈整合接触覆盖于船底山期碱性玄武岩之上,具高位喷发、低位充填特点,全岩K--Ar年龄测定结果 (25. 64±0. 53 Ma、27. 36±0. 45 Ma、28. 45±0. 56 Ma、28. 55±0. 43Ma、28. 63±0. 45 Ma、28. 65±0. 61 Ma)属渐新世。地球化学数据显示,霓辉粗面岩SiO_2质量分数均值为59. 05%,富碱、高钾,亏损大离子亲石元素(Sr、Ba、V、Cr),富集高场强元素(Nb、Ta、Hf、Zr)和轻稀土元素(LREE),∑REE含量很高,具有右倾型稀土分配型式,显示明显Eu负异常。研究表明,霓辉粗面岩是由碱性玄武质岩浆经强烈分离结晶作用形成的,揭示该区渐新世处于一种强烈的拉张环境,幔源岩浆经历了由碱性玄武岩-碱性粗面岩的演化过程。

Aegirine trachytes were newly recognized in northern Dunhua--Mishan fault belt,between the Bahekou and Huangnihe towns,Jilin Province. Aegirine trachytes are characterized by enrichment in alkaline minerals represented by aegirine,conformably overlying the alkaline basalt of Chuanfishan Period. Whole-rock K--Ar isotopic dating reveals ages of 25. 64 ± 0. 53 Ma,27. 36 ± 0. 45 Ma,28. 45 ± 0. 56 Ma,28. 55 ± 0. 43 Ma,28. 63 ±0. 45 Ma and 28. 65 ± 0. 61 Ma,which belong to Oligocene. The geochemical analysis shows average SiO2 content of 59. 05 % for the aegirine trachytes,with high alkaline and enriched K2O contents. They are depleted in LILEs( Sr,Ba,V,Cr) and enriched in HFSEs( Nb,Ta,Hf,Zr) and LREE,with high ∑REE abundances,right-inclined REE distribution patterns and obvious negative Eu anomalies. Integrating all the results,it is indicated that the aegirine trachytes were derived from strongly fractioned crystallization of alkaline basaltic magma,under an intensive extensional setting during Oligocene and the mantle-derived magma evolved from alkaline basalt to aegirinetrachyte.