胶东崂山早白垩世A型花岗岩成因及对区域构造演化的意义

Petrogenesis of Early Cretaceous Laoshan A-type granites and the implications for the tectonic evolution of Jiaodong Peninsula

早白垩世崂山花岗岩位于苏鲁超高压变质带中部,是中国东部典型的A型花岗岩之一。其岩浆源区和岩石成因目前还存在一定的争议,而且其成岩过程中的物理化学条件及相关研究也比较缺乏。本文在详细的野外工作和岩相学研究的基础上,采用角闪石、黑云母的成分估算了成岩的温度、压力和氧逸度条件,并开展了岩石地球化学、Sr-Nd同位素分析,探讨岩石的源区和成因。结果显示,崂山花岗岩的岩浆形成于高温(>850℃)和高氧逸度条件。岩浆在地壳浅部压力较低的条件下结晶,岩浆固结温度在700℃左右,随后经历了快速降温的过程。岩石地球化学方面,崂山花岗岩富SiO_(2)、富碱,贫Ca、Mg;高场强元素Th、Zr、Nb、Y和大离子亲石元素Rb、K含量高,而Ba、Sr含量低;稀土元素含量高,且具有显著的Eu负异常,呈右倾“海鸥型”分配特征。同位素方面,所有样品均显示出富集的Sr、Nd同位素特征,(^(87)Sr/^(86)Sr)i=0.706447~0.707876,ε_(Nd)(117Ma)值为-14.8~-17.4,全岩Nd两阶段模式年龄(tDM2)为2112~2325Ma。结合胶东地区前人的研究成果,本文认为崂山A型花岗岩是由地壳中富K的碱性岩重熔而成,其岩浆源区很可能与该区三叠纪碱性岩浆活动有关。崂山A型花岗岩的形成标志着碰撞造山带底部的扬子板块已经垮塌完毕,同时岩石圈减薄达到高峰。

Laoshan granite,a typical A-type granite in eastern China,is located in the middle of Sulu ultrahigh pressure metamorphic orogenic belt.The magma source and petrogenesis are still controversial,and the systematic research on their diagenetic conditions is also lacking.On the basis of detailed field work and petrography,the major element compositions of biotite and amphibole were in-situ examined to estimate the temperature,pressure and oxygen fugacity conditions of magma crystallization.Furthermore,the major and trace element compositions,and Sr-Nd isotopic compositions for Laoshan granite were determined to explore the characteristics of magma source and petrogenesis.New results show that Laoshan granite has a high magma formation temperature(>850℃)and oxygen fugacity conditions.Magma crystallized under low pressure in the shallow crust with magma solidified temperature around 700℃and then went through rapid cooling.Laoshan granite is rich in SiO_(2)and alkali,but low in Ca and Mg.The contents of high field strength elements(i.e.,Th,Zr,Nb,Y)and some large ion lithophilic elements(Rb and K)are high,while the contents of Ba and Sr are low.The content of rare earth elements is enriched,with the Eu negative anomaly,showing a right leaning“seagull type”pattern.In terms of isotopic composition,all samples show enriched Sr and Nd isotopic characteristics,with(^(87)Sr/^(86)Sr)i=0.706447~0.707876,ε_(Nd)(117Ma)=-14.8~-17.4,and tDM2=2112~2325Ma.Combined with the former research results in Jiaodong area,the Laoshan A-type granite is considered to be derived from remelting of K-rich alkaline rock,and its magma source might be related to the Triassic alkaline magmatic activity in Jiaodong area.The formation of Laoshan A-type granite indicates that the Yangtze plate,at the bottom of the collision orogenic belt,collapsed completely.Simultaneously,the lithosphere thinning reached its peak.