张广才岭南段两个侏罗纪花岗岩体的地球化学特征及其地质意义

Geochemical characteristics of two Jurassic granitic plutons from southern Zhangguangcai Range and their geological implications

通过LA-ICP-MS锆石U-Pb定年和岩石化学分析,研究了张广才岭南段上营北岩体和帽儿山岩体的形成年代,地球化学特征和形成环境。上营北岩体为中粗粒钾长花岗岩,帽儿山岩体为中细粒黑云母钾长花岗岩。上营北岩体的加权平均年龄为178.9±2.7 Ma,帽儿山岩体为183.7±2.4 Ma,均为早侏罗世侵入岩。上营北岩体和帽儿山岩体主量元素都具有Si O_2和K_2O含量较高,Ti O_2、Mg O、Ca O含量较低,TFe O/Mg O值较高的特点;上营北岩体A/CNK=0.98~1.08,里特曼指数σ=1.51~2.66;帽儿山岩体含铝指数A/CNK=1.00~1.01,里特曼指数σ=2.12~2.36。上营北岩体稀土元素配分模式为海鸥型,轻重稀土分馏不明显;帽儿山岩体稀土元素配分模式为右倾型,轻稀土较重稀土富集;两个岩体均富集Rb、K,Ba、Nb、Ta、Sr、Ti、P、Ho、Er、U、Eu等元素出现不同程度的亏损。地球化学特征分析显示上营北岩体和帽儿山岩体均为A_2型花岗岩,为后造山型花岗岩,形成于兴蒙造山带后造山的伸展环境。

LA-ICP-MS zircon U-Pb dating and petrochemical analysis were used to study the formation age,geochemical characteristics and diagenetic environment of Shangyingbei pluton and Maoershan pluton from southern Zhangguangcai Range.Shangyingbei pluton consists of mid-coarse-grained moyite and Maoershan pluton consists of mid-fine-grained biotite moyite.The weighted average age of the plutons are 178.9±2.7 Ma for Shangyingbei pluton and 183.7±2.4 Ma for Maoershan pluton,which define them both as Early Jurassic intrusive rock.Both plutons have higher content of SiO2and K2O,and lower content of Ti O2,Mg O,Ca O,and higher TFe O/Mg O ratio.In Shangyingbei pluton the Aluminum index A/CNK=0.98～1.08 and the Rittmann Indexσ=1.51～2.66,whereas in Maoershan pluton the A/CNK=1.00～1.01,σ=2.12～2.36.The rare earth element distribution model of Shangyingbei pluton is in seagull form,indicating an inconspicuous light-heavy REE fractionation,while Maoershan pluton has a rightwards REE element distribution model,and light REE is more enriched than heavy REE.Both plutons are enriched in Rb,K,and depleted in Ba,Nb,Ta,Sr,Ti,P,Ho,Er,U,Eu etc.Geochemical analysis indicates both plutons are A2-type post orogenic granite,emplaced in extensional environment of the post orogenic belt in the Xing-Meng orogenic belt