多金属结核内部生长层组年代研究

DATING STUDY ON THE GROWTH GENERATIONS WITHIN THE POLYMETALLIC NODULE

根据结构构造、矿物组分、微体古生物组合不同，多金属结核内部可划分为三个生长层组，从内到外依次为：第Ⅰ生长层组、第Ⅱ生长层组、第Ⅲ生长层组。运用古生物地层学方法，对其中微体古生物化石（硅藻、超微、放射虫）进行综合分析，确定三个生长层组的生长年代为：第Ⅰ生长层组形成年代为渐新世到早中新世，其顶界年龄不晚于１８Ｍａ，第Ⅱ生长层组形成年代为早中新世末期到上新世早期。

A polymetallic nodule can be divided into three growth generations based on its structure,mineral assemblage and micropaleontologic assemblage,that is,growth generation Ⅰ,growth generation Ⅱ and growth generation Ⅲ from inside to outside.Using paleontologic stratigraphic method,we contrast different paleontologic assemblages in each growth generation and give the following ages: Growth generation Ⅰ contains calcareous nannofossils such as Ericsonia cf. robusta,Coccolithus pelagicus,T.Carinatus and Discoaster druggii and also some diatom fossils such as Actinocyclus ingens varnodus Bald and Nitzschia porteri Freng.The ages of some fossils are significant.The last appearance of T. Carinatus is at the top of NN2 with the age of 18Ma.The first appearance of D. druggii is in 22Ma,mainly in the middle of Early Miocene.The last appearance of Sphenolithus conicus is not beyond NN3,and the last appearance of Theocyrtis annosa in about 2 3Ma.A comprehensive study on above characteristics tells that the paleontologic assemblages are mainly in Early Miocene,and appeared the earliest in about Oligocene(?). Growth generation Ⅱ contains diatom fossils ( Actinocyclus ingens varnodus Bald, Nitzschia porteri Freng)and radiolaria fossils( Liriospyris parkerae and Axoprunum pierna ).The first appearance of Actinocyclus ingens varnodus Bald is in 17Ma and last appearance in 4 8Ma.The first appearance of Nitzschia porteri Freng is in 11 5Ma and last in 7Ma.After studying the above fossil assemblage characteristics,we found that the age of growth generation Ⅱ was mainly from the end of Early Miocene to Early Pliocene,in other words,from 17Ma to 4 8Ma. Growth generation Ⅲ contains calcareous nannofossils as Gephyrocapsa oceanica,Coccolithus pelagicus, diatom fossils as Thalussinonem nitzschoides Grum, Nitzschia sp.and Coscinodiscus sp.and radiolaria fossils as Spongodiscus sp. Cenosphaera sp.and Dictyocoryne sp.These fossils represent the assemblages from Early Pliocene to Quaternary.