鲁西临朐地区早白垩世青山群火山岩的年代学、地球化学及岩石成因

Geochronology,Geochemistry and Petrogenesis of the Early Cretaceous Volcanic Rocks of the Qingshan Group in the Linqu Area,Western Shandong Province

本文对鲁西临朐青山群火山岩进行了系统的年代学、元素地球化学和Sr-Nd-Hf同位素组成研究,据此探讨其岩石成因。临朐早白垩世青山群火山岩主要为一套中性岩(SiO2=52.43%~67.81%),岩石组合为安山岩-粗安岩-粗面岩-粗面英安岩,属于钙碱性岩类。锆石LA-ICP-MS U-Pb定年获得粗安岩的年龄为121.3±1.5 Ma,形成于早白垩世。岩石地球化学表现出富集轻稀土元素和Rb、Ba、K等大离子亲石元素,明显的亏损Nb、Ta、Ti等高场强元素的特征。临朐火山岩具有富集的Sr-Nd同位素特征,其(87Sr/86Sr)t值在0.704346~0.708656之间,εNd(t)值在-14.6^-9.2之间,并且具有低的Hf同位素组成,其锆石εHf(t)值在-29.3^-21.6。岩石具有与变基性岩熔融产生熔体相一致的Mg~#值,但镜下存在显著的矿物不平衡结构和变化范围较大的Cr(26.7×10^(-6)~239×10^(-6))和Ni(10×10^(-6)~69.7×10^(-6))含量。研究表明,临朐青山群火山岩形成于早白垩世伸展的构造背景下,是华北古老下地壳部分熔融熔体与富集地幔熔体发生岩浆混合形成的。

In this paper,we report the geochronology,major and trace element geochemistry and Sr-Nd-Hf isotope data for the early Cretaceous volcanic rocks of the Qingshan Group in the Linqu area,with an objective to constrain the petrogenesis.The volcanic rocks are mainly intermediate rocks,with a SiO2 content varying between 54.41%~67.81%.Rock association consists mainly of andesite,trachyandesite,trachyte and trachydacite.These volcanic rocks belong to calc-alkaline series.Zircon LA-ICP-MS U-Pb dating yields an age of 121.3 ± 1.5 Ma for trachyandesite,indicating that the rocks crystallized in Early Cretaceous.The rocks are geochemically characterized by enrichment of LREE and LILE (e.g.,Rb,Ba,K),and depletion of HFSE (e.g.,Nb,Ta,Ti).These volcanic rocks have enriched Sr-Nd isotopic compositions ((87Sr/86Sr)t =0.704346~0.705831,εNd (t) =-14.6^-9.5),and have low Hf isotopic composition (εHf(t)=-29.3^-21.6).The rocks have the similar Mg# values as the melts produced by metabasite,with large range of Cr (26.7×10^-6~239× 10^-6) and Ni (10× 10^-6 ~69.7× 10^-6) content,and show significant mineral textural disequilibrium.The study further indicates that the Linqu volcanic rocks of the Qingshan Group may be the product of magma mixing between lower crust-derived melts of ancient North China Craton and mantle-derived melts of enriched mantle under the lithospheric extension setting.