青岛垭口—八仙墩变质海相碎屑岩的属性和构造意义

青岛垭口—八仙墩变质海相碎屑岩分布面积大,保留了原生的各种沉积构造,为一套浊积岩系,应归属于扬子板块上奥陶统,建议命名为八仙墩组。浊积岩系中的砾岩含花岗质砾石、角闪岩砾石和长英质砾石,可以为复原上奥陶世扬子板块大陆边缘沉积环境和研究物源区大地构造背景提供良好的研究素材。岩石地球化学成份判别为硬砂岩和页岩,也佐证了本套地层为浊积岩系。弱的Ce/Ce~*负异常,说明了海相沉积成因,也表明陆源碎屑物对沉积岩的主导作用。(Fe_2O_3+MgO)-TiO_2图解、La-Th-Sc图解和Th-Sc-Zr/10图解表明本套碎屑岩具有活动大陆边缘背景和大陆岛弧的共同特征,但从比较高的La丰度、Ce丰度和∑REE丰度,可以判断八仙墩碎屑岩为活动大陆边缘背景下发育的浊积岩系。通过露头的构造地质分析,推断本套地层是在扬子板块陆壳物质向华北板块下深俯冲过程期间或其后扬子板块陆壳上部盖层继续向华北板块仰冲就位的,在超高压变质岩折返时经历了由北向南的逆冲。本套碎屑岩明确指示在南黄海盆地深部应该有古生界奥陶统的地层,盆地深部的同一套地层应该具有油气储层和盖层的性能。

Metamorphosed marine clastic rocks in Qingdao;Tectonic attribute and implication

A set of turbidite-bearing sedimentary rocks widely seen between Yakou and Baxiandun,Qingdao,was recently restudied.The rocks are marine clastic and dolomitic formation and have various original depositional structures well preserved.The turbidite is similar to that of the Upper Ordovician in the Yangtze Plate.Naming it as Baxiandun Formation is recommended by the authors.The turbidites contained many pebbles of granitoid,amphibolite,and felsic rocks,indicative of the provenance during the deposition time and providing hints to the tectonic reconstruction of continental margin of the Yangtze Plate in the late Ordovician. In geochemistry,these rocks are greywacke and shale,which reflect the turbidite nature from another aspect. Weak negative Ce/Ce~* anomaly indicate the marine origin,as well as terrigenous contribution. In addition,the turbidite formation shows a combined nature of active continental margin and island arc in tectonic setting,as shown by diagrams of(Fe_2O_3+MgO)TiO_2,La-Th-Sc,and Th-Sc-Zr/10,and should be formed more closely to an active continental margin indicated by higher La,Ce,and ΣREE abundance.The rocks were probably situated in the follow-up obduction of split upper crust cover of the Yangtze Plate onto the North China Plate in or after the subduction of the Yangtze Plate's continental materials towards the North China Plate,or were placed by southward thrusting when ultra-high-pressure metamorphic rocks underpinned.The research result of the clastic rocks imply possible existence of corresponding Ordovician beds with potential oil-gas reservoir and capping beneath the South Yellow Sea.