准噶尔盆地西北缘风城组喷流岩稀土元素地球化学特征

Characteristics of Rare Earth Elements of Exhalative Rock in Fengcheng Formation,Northwestern Margin of Jungger Basin

准噶尔盆地西北缘乌尔禾地区下二叠统风城组深湖—半深湖相"白烟型"云质喷流岩中,广泛发育的纹层状、条带状、网脉状、同生变形和喷爆角砾及内碎屑等热水沉积的结构构造,具备国内外已公布的海相和陆相"白烟型"喷流岩的基本特征,但其富含火山物质和硅硼钠石、碳钠钙石等复杂盐类矿物的性质则明显有别于已知的"白烟型"喷流岩类。在岩石学、矿物学和稀土元素地球化学特征研究的基础上,发现风城组不同喷流岩的∑REE变化范围较大,LREE/HREE、La_N/Yb_N、La_N/Sm_N、Gd_N/Yb_N有不同的特征和多种稀土元素配分形式,结合Y/Ho特征,认为热流体具有分异的多源和多阶段喷流活动性。又据大部样品具有与同层位安山岩及周缘晚石炭世火山岩相似的稀土元素配分模式,其间具有很强的亲缘性,反映地壳深部的岩浆房不仅是热液喷流活动的驱动源,而且有岩浆源物质组分参与热液喷流活动,同时热流体还受到陆壳混染作用的影响。喷流岩中云质、火山质和复杂盐类矿物纹层与条带的韵律层及其稀土元素地球化学特征的规律性变化,指示热液喷流和沉积具有交替变化的脉动性。有意义的是,风城组云质喷流岩极弱的Ce异常和中等负Eu异常明显不同于海相喷流岩,反映正常的喷流沉积发生在很少外来流体(湖水)混入的深湖底碱性—还原的中—低温卤水池环境中,而盐脉及受水—岩反应影响强烈的围岩表现出重稀土富集的稀土配分模式,结合盐脉偏高的Y/Ho比值,反映其形成时流体更多地具备深部海相地层来源热卤水流体性质。

Objectives： Hydrothermal sedimentary structures,textures and complex saline minerals widely distributed in the deep—semideep lacustrine facies ＂white smoke type＂dolomitic exhalative rock in Lower Permian Fengcheng Formation in Urho area,the northwestern margin of Jungger Basin. This article prove that the dense oil reservoir of dolomitic rock in Fengcheng Formation belong to ＂white smoke type＂exhalative rock with special genesis by way of presenting a deep analysis of REE geochemical characteristics based on the study about petrology and mineralogy.In addition,this paper discuss about the characteristic of hydrothermal fluids and sedimentary environment.Methods：This paper investigate the genesis of hydrothermal sedimentary structures and textures such as lamellar,banded,veined—network,contemporaneous deformation,eruptive breccia and intraclastic textures,and discuss the genesis of special saline mineral such as reedmergnerite. Based on the discussion,we try to explain the origins and factors of hydrothermal fluids,characteristics of sedimentary environment by exploring the parameter of REE of Fengcheng Formation such as LREE / HREE,La_N/ Yb_N,La_N/ Sm_N,Gd_N/ Yb_N,Y/Ho,δEu and δCe in detail,and compare the REE patterns of Fengcheng Formation with andesite ＇s in same horizon and peripheral volcanic rocks ＇ of Late Carboniferous. At the same time,we study the regularity of hydrothermal activity by analyzing the REE characteristics of samples derived from cyclothem.Results： Hydrothermal sedimentary structures and textures have widely distribution in the deep- semideep lacustrine facies ＂white smoke type ＂dolomitic exhalative rock of Lower Permian Fengcheng Formation in Urho area. This type rock have the basic characteristics of marine and continental ＂white smoke type＂exhalative rockpublished home and abroad. But its character of having rich volcanic material and complex saline minerals composition such as reedmergnerite,shortite etc. is obviously different from the known ＂white smoke type ＂exhalative rocks. We found that the range of ∑REE from different exhalative rocks in Fengcheng Formation is large,and characteristics of LREE / HREE,La_N/ Yb_N,La_N/ Sm_N,Gd_N/ Yb_Nare different and have a variety of REE distribution pattern. Combined with the feature of Y / Ho,it is believed that hydrothermal fluids have differentiation of multi-source and multi-phase eruption activity. There are similar REE distribution patterns between most samples and andesite in same horizon and peripheral Late Carboniferous volcanic rocks,which reflects the magma chamber in deep crust is not only the drive source of hydrothermal fluids activity but also participate in the hydrothermal fluids activity. Meanwhile the hydrothermal fluids is also affected by continental crust contamination. The lamellar and banding composed of dolomite,volcanic material and complex saline minerals in exhalative rock and the regular change of geochemical characteristics of REE,indicate that the alternation between hydrothermal activity and sedimentation. Significantly,the extremely weak Ce anomaly and moderate negative Eu anomaly of dolomitic exhalative rock in Fengcheng Formation is apparently different with marine exhalative rock,which reflect the normal exhalative sedimentation occurred in the bottom of deep lake with alkaline-reducting and low-temperature brine pool environment free of foreign fluids. While salt-veins and surrounding rock affected by the water- rock interaction strongly show REE distribution pattern of enrichment of HREE,which indicate the fluid is with the characteristics of thermal brine from deep marine strata during formation. in combination with higher Y / Ho value of salt veins.Conclusions： Fengcheng Formation of Lower Permian in Urho area belong to ＂white smoke type＂exhalative rocks,but is different from the known ＂white smoke type＂exhalative rocks published home and abroad obviously for owning special minerals. Characteristics of REE indicate that hydrothermal fluids have multiple sources,with endogenetic fluids playing a dominant role. Hydrothermal fluids have strong connection with structures and magmatic evolution,and affected by continental crust contamination and mixing of fluid from marine strata buried deeply. The fluid actived with stages and pulsation. Hydrothermal deposition occurred in hot brine pond with alkaline,reductive,middle- low temperature environment,and affected weakly by external fluids.