“北京人”遗址洞穴碎屑堆积的矿物组分特征及其意义

“北京人”遗址洞穴堆积物的研究,已有五十多年的历史。多方面的研究成果,也先后报道。近年来,作者分层测制了周口店第一地点西壁、南壁、探井以及上砾石层和下砾石层等几个剖面。还在周河口、羊耳峪东岭、三福村采集样品55个。所有样品均进行筛分,分级进行镜下鉴定,单矿物系统折光率测定,个别矿物辅以X-射线分析,分别对轻、重组份物质成分、百分含量做了系统计量,并着重研究重组份特征。本文是在这些工作基础上,对这一地区的地层划分、沉积物来源和古气候变迁等进行讨论。 一、物质来源和蚀源母岩 野外观察和沉积矿物成份分析表明,第一地点洞穴堆积有洞壁崩塌和外部携入两个来源。碳酸岩角砾和碳酸岩岩块大量出现,在特定层位尤其发育,它们是洞壁崩塌的产物。外来物质尽管矿物含量时有变化,但矿物种类大体相近,构成不同矿物组合。轻组份有石英、酸性斜长石、微斜长石、白云母组合,千枚岩、角岩、板岩岩屑组合以及滚圆状石英、石英岩、硅质岩岩屑组僵。

MINERAL COMPOSITION OF CLASTIC CAVE DEPOSITS FROM THE SITE OF “PEKING MAN” AND ITS IMPLICATIONS

For the purpose of dividing the cave deposits and understanding the mother rock in the provenance and paleoclimatic change,this paper attempts to study systematically the mineral composition of the clastic cave deposits with the emphasis on the heavy mineral.Our research covers following three aspects.Firstly,the cave deposits came from cave-wall collapse and of exotic materials.In the light of the features varing with the heavy and light minerals in the exotic materials we were able to trace the provenance of these materials to Fangshan granodiorite bodies locatated not far away to the north of the cave and to rocks of different ages in its vicinity-Secondly the cave deposits show a fair sedimentary rhythm represented by a correlated cyclo-them of percentage contents of heavy composition,contents of stable minerals and ZTR of heavy composition.According to the sedimentary nature and rhythm,the representative section of location No.1 can be divided into six layers.Thirdly,the contents of heavy composition and stable minerals,especially the ZTE can be regarded as an indication of the maturity of minerals.In case the climatic factor was prevailing and the climate was humid and warm,the maturity of minerals was relatively high.In case the climate was cool and arid,the maturity of minerals was relatively low.Therefore,taking the upper part of No.8-9 layer as a boundary,below it the cool-arid climate dominated,whereas above it warm-humid climate dominated.The "anti-synchronous change" relation between the percentage content of heavy composition and ZTR of heavy minerals indicates that the climatic changes from frigid-cold-slightly warm to cold-warm-humid-slightly cold-warm-humid can be followed in ascending order.The mineralogical characteristics of basal conglomerate and the lower part of the lower conglomerate horizon are of a specific paleoclimatic significance.As we know that the higher content of heavy composition and lower value of ZTR,particularly the existence of extremely unstable minerals in large amount,reflected the considerably cold climatic condition.Taking this fact and the geologic condition in the region studied into consideration in combination with the predecessor’s research we cannot rule out the possibility that the cave deposits seem to be glacigenous.