黑龙江兰西地区三叠系老龙头组沉积岩元素地球化学特征及古环境

Geochemical characteristics and palaeoenvironment of Triassic sedimentary rocks elements of Laolongtou Formation in Lanxi area of Heilongjiang Province

黑龙江省兰西地区Xr7井三叠系老龙头组揭示了一套紫红色、绿灰色和灰色砂泥岩沉积地层,为了获取这套杂色砂泥岩沉积环境信息,对钻井岩石样品的痕量元素、稀土元素进行了检测和综合应用研究。结果表明:老龙头组沉积岩中轻、重稀土元素分异程度较高,δEu表现为显著负异常、δCe无异常,球粒陨石标准化配分模式图总体表现为轻稀土富集、右倾,重稀土亏损、近平坦型;痕量元素Sr、Cu质量分数及其比值w(Sr)/w(Cu)等在剖面上波动变化表明老龙头组沉积时期的古气候经历由早期干旱炎热到晚期温暖湿润的一个气候变迁过程;w(Li)、w(Sr)、w(Ga)以及w(Sr)/w(Ba)等指标综合研究表明,老龙头组沉积时期水体主要为淡水环境;氧化还原环境判定指标w(Ni)/w(Co)、w(V)/w(Cr)、w(U)/w(Th)、δU、δEu和δCe等综合研究表明,研究区三叠系沉积时期主体为氧化环境。研究成果为后续勘探开发提供地质理论指导。

Triassic Laolongtou Formation of Well XR7 in Lanxi area of Heilongjiang Province reveals a set of purplish red,greenish grey and grey sand-mudstone sedimentary formations.In order to obtain sedimentary environment information of this set of variegated sand-mudstone,trace elements and rare earth elements in drilling rock samples are detected and comprehensively applied in the research.The results shows that light and heavy rare earth elements of sedimentary rocks in Laolongtou Formation are are highly differentiated,with δEu presenting significant negative abnormality while δCe without abnormality.Chondrite normalized distribution pattern generally appears as light rare earth enriched and right inclined,while heavy rare earth depleted and nearly flat.Fluctuation variation of mass fractions of trace elements of Sr and Cu and their ratio w(Sr)/w(Cu)on the section show that the paleoclimate during Laolongtou Formation sedimentary period experienced a climate change process from early dry and hot to late warm and humid.Comprehensive study on indicators of w(Li),w(Sr),w(Ga)and w(Sr)/w(Ba)shows that water body during sedimentary period of Laolongtou Formation is mainly freshwater environment.Comprehensive study of oxidation-reduction environment determination indicators of w(Ni)/w(Co),w(V)/w(Cr),w(U)/w(Th),δU,δEu and δCe shows that Triassic sedimentary period in studied area is dominated by oxidation environment.The reasearch provides geological theoretical guidance for further development and exploration.