北祁连造山带大依马龙地区角斑岩的岩石成因及构造背景分析

The Petrogenesis and Tectonic Setting of the Keratophye in the Dayimalong Area of the Northern Qilian Orogenic Belt

本文对北祁连造山带大依马龙地区的角斑岩进行了主量和微量元素地球化学分析,结果表明,角斑岩SiO_2含量介于52. 51%～55. 46%,Na_2O含量高,介于4. 79%～6. 86%,K_2O含量低,介于0. 16%～0. 93%,Na_2O/K_2O值为5. 16～37. 19,具低钾拉斑系列—中钾钙碱性系列钠质岩石特点。岩石固结指数(SI)介于15. 03～23. 59,Mg#值介于39. 9～52. 14。在球粒陨石标准化的稀土元素配分图上表现出轻稀土轻微富集的右倾型,基本没有或具有轻微的Eu异常,δEu介于0. 86～1. 30之间,平均0. 98;在原始地幔标准化的微量元素蛛网图上,普遍亏损Nb、Ta、Zr、Ti、P等高场强元素,角斑岩Y含量介于15. 6×10^(-6)～21. 8×10^(-6),平均18. 77×10^(-6)<20,Th/Yb介于0. 59～1. 38,平均0. 77>0. 6,Ce/Pb值介于4. 31～7. 79,Nb/La值介于0. 28～0. 49,Zr/Nb值介于14. 73～30. 08,均表明其为岛弧火山岩。结合区域地质背景,我们认为角斑岩形成于岛弧环境,其形成机制为:受俯冲板片流体改造的地幔楔物质发生部分熔融形成母岩浆,在岩浆上升过程中,热的岩浆体系发生了分离结晶作用并受到了围岩热卤水的混染,热卤水带来了Na和Si离子,带走了Fe和Ca离子,从而形成了富钠的岩浆体系。

This paper reports whole rock major and trace element from Keratophye in the Dayimalong area of the North Qilian organic belt. The SiO2 of Keratophye is between 52. 51% ~ 55. 46%,The Na2O content is high,ranging from 4. 79% to 6. 86%,The K2O content is low,ranging from 0. 16% to 0. 93%. The Na2O/K2O ratio is 5. 16 ~ 37. 19,With low potassium pull spot series - medium potassium and calcium alkaline series of sodium rock characteristics. The rock consolidation index( SI) is between 15. 03 and 23. 59,and the Mg#value is between 39. 9 and 52. 14. On the rare earth element distribution map standardized by chondrite,the right-inclined type of light rare earth is slightly enriched,and there is almost no or slight Eu anomaly. The δEu is between 0. 86 and 1. 30 with an average of 0. 98. On the trace elements of the original mantle standardized,the high-field strength elements such as Nb,Ta,Zr,Ti,and P are generally depleted,Y content ranging from 15. 6 × 10^-6 to 21. 8 × 10^-6,with an average of 18. 77×10^-6,both less than 20. The Th/Yb ratio is 0. 59~1. 38,with an average of 0. 77,both of which are greater than 0. 6. The Ce/Pb ratio is 4. 31 ~ 7. 79,the Nb/La ratio is 0. 28 ~ 0. 49,and the Zr/Nb ratio is 14. 73 ~ 30. 08,all indicating that it is an island arc volcanic rock. Combined with the regional geological background,we believe that the Keratophye is formed in the island arc environment,Its formation mechanism is:The mantle wedge material modified by the subduction plate fluid partially melts to form the parent magma. During the magma rise,The hot magma system undergoes separation and crystallization and is contaminated by the surrounding rock thermal brine. The hot brine brings Na and Si ions,and takes away Fe and Ca ions,thus forming a sodium-rich magma system.