浙东嵊州福泉山地区硅藻土地球化学及其古环境意义

Geochemistry and palaeoenvironment implications of the Fuquanshan diatomite deposit in the Shengzhou region,eastern Zhejiang Province

浙东新昌盆地是我国第二大硅藻土矿富集区,包含一系列中新世硅藻土矿床,是研究新生代古环境的理想对象。本文以嵊州地区福泉山古火山内的硅藻土为研究对象,探讨硅藻土的地球化学特征及其形成时的古环境。福泉山硅藻土具有相对较低的SiO_(2)(61.3%~69.4%)以及高的Al_(2)O_(3)(14.6%~17.9%)及稀土元素含量,表明其中含有较多的碎屑物质。在微量元素组成上,相比伴生的嵊县组玄武岩,明显富集大离子亲石元素、轻稀土元素,并具有Eu、Nb、Ti的负异常,类似于白垩纪酸性火山岩。硅藻土中的碎屑物质可能主要来自于周边白垩纪酸性火山岩的风化剥蚀。根据硅藻土的化学风化指数以及其他参数,结合前人研究成果,认为福泉山硅藻土矿主要形成于中新世潮湿、温暖的淡水湖相环境,这与古近纪浙东地区的干旱环境截然不同,反映了古近纪—新近纪之交中国东部构造地貌的剧烈转折。

The Xinchang Basin in eastern Zhejiang Province contains a series of diatomite deposits interlaid in the basalts of the Miocene Shengxian Formation.The geochemical characteristics of deposits can provide crucial constraints on the Miocene palaeoenvironment of the eastern Zhejiang Province.In this study,three typical diatomites from the Fuquanshan volcano in Shengzhou City have been analyzed.Compared to the typical diatomites worldwide,low SiO_(2)(61.3%~69.4%),high Al_(2)O_(3)(14.6%~17.9%),and REE contents indicate that the Fuquanshan diatomites contain high proportions of fine detritus.Compared with the associated basalt of Shengxian Formation,the composition of the diatom is obviously enriched in large ion lithophile elements and light rare earth elements,with negative anomalies of Eu,Nb and Ti,similar to those in Cretaceous acid volcanic.Based on REE and incompatible elements characteristics,the fine detritus of the Fuquanshan diatomites were mainly derived from the Cretaceous silicic volcanic rocks,rather than coexisting basalt.Chemical weathering index,Sr/Ba ratio,etc.,combined with previous paleontological studies support that the Miocene Fuquanshan diatomite deposits were formed in the freshwater lake under a warm humid climate,which is distinct from the Paleogene drought climate of eastern Zhejiang.Miocene diatom“Boom”in eastern China reflects the regional tectonic and palaeoenvironment transformation in the turn of Paleogene-Neogene.