鄂尔多斯盆地长7段富有机质页岩中磷灰石类型及其成因

Species and formation mechanism of apatites in the 7^(th) member of Yanchang Formation organic-rich shale of Ordos Basin,China

鄂尔多斯盆地延长组长7段是一套富有机质页岩,其中富含草莓状黄铁矿、有机质纹层、磷灰石等。磷灰石是其中非常重要的一种矿物,它与生物生命活动密切相关。通过岩石薄片、扫描电镜及能谱分析研究了长7段富有机质页岩中磷灰石的岩石学特征,发现其主要有两大类,即生屑磷灰石和胶磷矿;并根据胶磷矿的重结晶程度,又将其分为3类:即没有发生重结晶或重结晶程度极低的胶磷矿、重结晶程度较低的胶磷矿和重结晶程度较高的胶磷矿。根据对富有机质页岩和凝灰岩的元素分析,认为形成磷灰石的磷元素主要来源于盆地周缘火山喷发带来的火山灰沉积物。长7段富有机质页岩中磷灰石的形成与生物密切相关,磷灰石的形成机制有2种:一是生物的直接作用;二是生物的间接作用,其中生物的间接作用是研究区磷灰石形成的主要形式。

The organic-rich shale in the 7th member（Chang 7）of Upper Triassic Yanchang Formation in Ordos Basin is rich in authigenic apatites and volcanic ash sediments.Through rock thin section,scanning electron microscopy（SEM）and energy spectrum analyses,two kinds of apatites are found in the Chang 7organic-rich shale：one is formed by bioclastic（bioclastic apatite,BA）,and the other is colloidal aggregation of apatite（colloidal apatite,CA）.According to the degree of recrystallization of CA,it can be further subdivided into three types：（1）CA-Ⅰ：CA has non-recrystallization or very low degree of recrystallization;（2）CA-Ⅱ：CA has lower degree of recrystallization;（3）CA-Ⅲ：CA has higher degree of recrystallization.Elemental analysis shows that the phosphorus（P）mainly came from volcanic ash caused by volcanic eruption.The formation of apatites in Chang 7organic-rich shale is closely related to the life activity of organisms.P provided by volcanic ash sediments was absorbed through the biological metabolism,and buried with organisms into sediments after they died.The P element preserved in organisms would be transformed into apatites during the process of diagenesis.It is thought that the formation mechanisms of apatites have two kinds：one is directly biological effect,and the other is indirectly biological effect.There is close relationship among volcanic ash sediments,organic-rich shale and apatites and we consider that the volcanic ash sediments promote the formation of apatites and control the distribution of P2O5 contents and organic-rich shale.