青海南山吾口侵入岩体锆石U-Pb年代学、地球化学特征及其地质意义

The intrusive rocks in the Wukou region,Qinghainanshan area,Qinghai:Zircon U-Pb chronology,geochemistry and geological implications

本文对青海南山吾口正长花岗岩和花岗闪长岩进行了岩石学、岩石地球化学及年代学研究。首次测得吾口侵入岩体正长花岗岩的锆石206Pb/238U年龄为253. 8±3. 3Ma(MSWD=0. 96),形成时代为晚二叠世。岩石中Al_2O_3为(14. 13～16. 51)×10^(-2),K_2O为(1. 66～3. 59)×10^(-2),A/NK=1. 15～2. 35,A/CNK=0. 75～1. 03,TFe O/(TFe O+MgO)=0. 53～0. 81,岩石具偏铝质中-高钾钙碱性花岗岩特征;吾口岩体表现为富集大离子亲石元素Rb、K等,亏损高场强元素Nb、Ta、Ba等;轻重稀土分异较明显,具右倾特征,中等负Eu异常(δEu=0. 44～0. 68),具壳幔岩浆混合特征。吾口岩体代表了阿尼玛卿洋盆向北削减俯冲运动在青海南山构造带与西秦岭造山带和南祁连造山带的构造转换衔接部位的构造演化的地质记录。该侵入岩体形成时代及构造环境的确定,为研究印支早期青海南山构造演化及动力学机制提供重要依据。

The present paper deals in detail with the petrology,geochemistry and La-ICP-MS zircon U-Pb chronology of the syenogranite and granodiorite from the Wukou region in the Qinghainanshan area,Qinghai.The syenogranite from the Wukou region gives a zircon 206^Pb/238^U age of 253.8±3.3 Ma(MSWD=0.96),which may be traced back to the Late Permian.Al2O3=(14.13-16.51)×10^-2,K2O=(1.66-3.59)×10^-2,A/NK=1.15-2.35,A/CNK=0.75-1.03,and TFeO/(TFeO+MgO)=0.53-0.81,all of which are characteristic of metaluminous medium to high-K calc-alkaline granites.Both the syenogranite and granodiorite are enriched in large ion lithophile elements Rb and K,and depleted in high field strength elements Nb,Ta and Ba,noticeably differentiated in light and heavy rare earth elements with right-leaning REE distribution patterns,and moderately negative Eu anomalies(Eu=0.44-0.68),indicating that the intrusive rocks in this area have the origins of the mixting of the crust and mantle magmas.This phase of magmatic thermal event represents the geological records of the tectonic transition between the western Qinling orogenic zone and the southern Qilian orogenic zone in the Qinghainanshan tectonic zone against the background of the northward subduction of the A'nyemaqen oceanic basin during the Late Permian.The timing of the intrusive rocks and determination of the tectonic settings may provide important evidence for the study of the tectonic evolution and dynamic mechanisms of the Qinghainanshan area during the early Indosinian.