北黄海盆地中—上侏罗统火山岩岩石地球化学特征及构造背景

Geochemistry and tectonic background of Middle—Upper Jurassic volcanic rocks in the North Yellow Sea basin

对北黄海盆地X井钻遇中—上侏罗统暗色岩组进行研究。研究结果表明:北黄海盆地X井钻遇中—上侏罗统暗色岩组中有2层火山岩,以夹层产出;火山岩为钙碱性系列,且两层火山岩Na2O含量差别较大,下层是钠质流纹岩,上层是钾质流纹岩英安岩;2层火山岩的微量元素、稀土元素地球化学特征相似;稀土元素配分型式为轻稀土富集的右倾型,轻、重稀土分馏不明显,具微弱铕异常;Rb,Sr,Ba和K等不相容的大离子亲石元素以及和Hf和Zr等不相容的高场强元素富集,Ti,Nb和Yb等难溶元素呈现较强亏损,且(w(Rb)/w(Yb))N>1,岩浆具有地壳物质属性;Nb和Ta相对亏损,Ba相对富集,岩石具有火山弧火山岩地球化学特征;结合w(Ti)/w(Zr)-w(Ti)/w(Y)岩石成因以及w(Nb)-w(Y)和w(Rb)-w(Y+Nb)构造环境判别图,认为研究区火山岩形成于与俯冲有关的火山弧或弧后伸展环境。下层火山岩岩浆是盆地裂陷较深时深部地壳部分熔融的产物,源区受幔源物质混染,Na2O质量分数高;上层火山岩岩浆是盆地裂陷较浅时浅部地壳部分熔融的产物,Na2O质量分数低。

Middle-Upper Jurassic Dark sandstone and mudstone rocks was studied. The results show that there are two layers of volcanic rocks interbeded in Middle-Upper Jurassic Dark sandstone and mudstone rocks revealed by Well X in the North Yellow Sea basin. The volcanic rocks are Calc-alkaline series and acidic rocks.The lower layer is the sodic rhyolite while the upper layer is potassium rhyolite-dacite based on the contents of Na2O and K2O. Geochemistry characteristics of trace and rare earth elements of these volcanic rocks are similar： Light REE elements are rich, and light and heavy REE elements are not singificantly fractionated. Chondrite normalized REE distribution patterns are the Right Type with weak Eu abnonxaality. Incompatible large ion lithophile elements such as Rb, St, Ba and K and incompatible high field strength elements such as Hf, Zr are enriched, but the hardly contain elements such as Ti, Nb and Yb have large loss with （w（Rb）/w（Yb））N〉1, which suggests the source magma comes from crustal materials. Clear relative losses of Nb and Ta, and relatively rich of Ba indicate that these volcanic rocks have a typical characteristics of the volcanic arc. Petrogenesis analysis of w（Ti）/w（Zr）-w（Ti）/w（Y） and tectonic discrimination diagram analysis of w（Ti）/w（Zr）-w（Ti）/w（Y）,w（Nb）-w（Y）, w（Rb）-w（Y＋Nb） suggest that these volcanic rocks in study area are formed in volcanic arc or back-arc extension environment relates to subduction. The source magma of the volcanic rocks of the lower layer is the partial melting of the deep crust while the basin is at strong rifting with contamination of mantle source （high Na2O content）. The source magma of those volcanic rocks of the upper layer is the partial melting of the upper crust and comes while the basin is light rifting with low Na2O content.