东准噶尔柳树沟组火山岩锆石U-Pb年代学、地球化学特征及其构造意义

Zircon U-Pb dating, geochemistry and tectonic significance of the volcanic rocks in Liushugou Formation, East Junggar

研究东准噶尔地区晚石炭世构造环境对于理解中亚造山带演化具有重要意义。本文选取晚石炭世柳树沟组二段火山岩为研究对象,开展详细的岩石学、岩相学、锆石U-Pb年代学以及地球化学研究。柳树沟组可分为3段,其中一段为陆源碎屑岩夹少量基性火山岩,二段为枕状玄武岩夹少量火山碎屑岩、碎屑岩,三段为陆源碎屑岩、中酸性火山岩夹火山碎屑岩。定年结果显示柳树沟组中玄武岩和流纹质凝灰岩锆石U-Pb年龄分别为312.5±6.5 Ma和316.2±1.4 Ma,表明柳树沟组火山岩形成时代为晚石炭世。主量元素分析显示该玄武岩属亚碱性—拉斑质系列岩石;稀土元素总量为62.58×10^(-6)~100.61×10^(-6),轻/重稀土比值为2.30~5.05(平均值为2.97),样品(La/Yb)_(N)为1.48~4.32,指示轻稀土较重稀土富集。Eu元素总体显示弱的负异常(δEu=0.94~1.08),岩石富集K、Rb、Sr、Ba等大离子亲石元素(LILE),重稀土元素(HREE)和Nb、Zr、Hf、Ti等高场强元素(HFSE)相对平坦,表现为大陆裂谷性质。据此,综合分析认为东准噶尔地区于晚石炭世总体处于大陆裂谷环境。结合前人研究成果,我们认为柳树沟组演化经历初期断陷盆地、中期大陆裂谷和裂谷末期等过程,分别对应了柳树沟组1至3段的形成环境。

The Late Carboniferous tectonic setting of East Junggar is important for understanding the evolution of the Central Asian orogenic belt. In this paper, we carried out detailed field observations, petrography, zircon U-Pb chronology and geochemistry on the pillow basalt from the Liushugou Formation. The lower part of the Liushugou Formation is composed of terrigenous clastic rocks with minor volcanic rocks, the middle part is composed of pillow basalt with minor clastic rocks, and the upper part is composed of terrigenous clastic rocks,intermediate-felsic volcanic rocks and minor volcanic rocks. The zircom U-Pb dating results from pillow basalt and rhyolitic tuff show that the Liushugou Formation forming age is 312.5±6.5 Ma and 316.2±1.4 Ma, respectively. The major element analysis shows that the basalt belongs to sub-alkaline tholeiitic series, the total rare earth element are 62.58×10^(-6)~100.61×10^(-6),the ΣLREE/ΣHREE ratios is 2.30~5.05(mean value is 2.97), the(La/Yb)_(N)is 1.48~4.32,indicating that the LREEs are more enriched than HREEs, Eu is slightly negative(δEu =0.94~1.08), the samples are enriched in large ion lithophile element(LILE), such as K, Rb,Sr, Ba. The HREEs and high field strength elements(HFSEs), such as Nb, Zr, Hf, Ti are relatively flat, showing the nature of continental-rift. These results indicate that the East Junggar was in continental rift setting during Late Carboniferous(~316 Ma). Combined with previous research results, we think the forming process of Liushugou formation including graben basin, continental-rift, end of rift, which correspond to different lithological sections,respectively.