临沂方城盆地中生代青山群火山岩地球化学特征及地质意义

Geochemical Characteristics and Geological Significance of Volcanic Rocks in Mesozoic Qingshan Group in Fangcheng Basin of Linyi City

摘要:

临沂方城盆地发育一套粗面质、粗安质火山碎屑岩及熔岩组合，是一套溢流相与爆发相交替的旋回性火山沉积组合，属碱性钾质岩石的钾玄岩，重稀土元素富集，δEu值为0.82～0.93，接近于1，具弱负铕异常，反映该期火山作用是在可能存在山根的条件下，在地幔岩石圈减薄下形成的；该套岩层前人划分为青山群沉积期形成的粗面质潜火山岩。研究发现该套地层与下伏八亩地组安山质沉凝灰岩为喷发不整合接触，界面处存在有短暂的沉积间断，并发育有1~2cm的古风化壳。根据其岩石组合特征，产出层位及地球化学特征等，将其厘定为青山群方戈庄组。

Abstract:

A set of trachyte, coarsely andesite pyroclastic rock and lava assemblage develops in Fangcheng basin in Linyi city. It is a set of cyclic volcanic sedimentary assemblage of the intersection of overflow facies and eruption. It belongs to the sylvite of alkaline potash rock with heavy rare earth element enrichment. The δ Eu value is 0.82~0.93, which is close to 1 with weak negative europium anomaly. It is reflected that the volcanism in this period is under the condition of possible mountain root in the mantle rock. It was formed under the ring thinning, and the strata are previously divided into trachyte subvolcanic rocks formed in the sedimentary period of Qingshan group. It is found that this set of strata is in unconformity contact with the andesitic tuff of the underlying bamudi formation. There are short-term sedimentary discontinuities at the interface, and paleoweathering crust is developed with the depth of 1~2cm. According to the characteristics of rock assemblage, occurrence horizon and geochemistry, it is determined as Fanggezhuang formation of Qingshan group.