柴北缘牛鼻子梁地区闪长–花岗质岩体锆石U-Pb年龄、地球化学特征及地质意义

Zircon U-Pb Age,Geochemical and Geological Characteristics of the Dioritic-granitic Intrusive Rocks in the Niubiziliang Area,Northern Qaidam Basin

柴北缘牛鼻子梁地区出露大面积闪长–花岗质侵入岩,包括英云闪长岩、闪长岩、角闪闪长岩,本文对其进行了锆石U-Pb定年,结合全岩地球化学特征,探讨了英云闪长岩的岩石成因、形成时代及构造环境。LA-ICP-MS锆石U-Pb测年分析获得英云闪长岩、闪长岩和角闪闪长岩的206Pb/238U加权平均年龄分别为346±1 Ma、343±1 Ma和338±1 Ma,限定了岩体的结晶时代为早石炭世。英云闪长岩的分布最为广泛,地球化学组成具有I型花岗岩的特征:①A/CNK都小于1,K2O/Na2O值小于1,显示出准铝质富钠的特征;②全碱含量(Na2O+K2O)介于4.83%~6.53%,里特曼指数(σ)在2.32左右,表现为钙碱性;③轻稀土元素富集,重稀土元素分馏程度较低,基本无Eu异常;④高Sr、低Y,富集K、Rb、La等元素,亏损Nb、Ta、Ba、Ti、P等元素;⑤P2O5与SiO2具明显负相关关系。综合研究认为英云闪长岩是来自下地壳物质部分熔融,属于晚古生代(早石炭世)柴达木地块与祁连地块的俯冲碰撞后伸展环境下岩浆活动的产物。

Dioritic-granitic intrusive rocks including tonalite,diorite and hornblende diorite are widely exposed in the Niubiziliang area of the North Qaidam Basin.This paper reported zircon U-Pb dating results for the tonalite,diorite and hornblende diorite and discussed their genesis and tectonic setting.LA-ICP-MS zircon U-Pb dating gave 206Pb/238U weighted mean ages of 346±0.78 Ma,343±1.0 Ma,and 338±1.0 Ma for the tonalite,diorite,and hornblende diorite,respectively,which showed that these rocks emplaced in the Early Carboniferous.Tonalite is the most widely distributed rock type in the Niubiziliang area,and it shows I-type granite characteristics:1)A/CNK is less than 1,and K2O/Na2O ratio is less than 1,indicating the characteristics of quasi-aluminum sodium-rich;2)the content of(Na2O+K2O)is between 4.83%and 6.53%,the Litman index(σ)is about 2.32,indicative of calc-alkaline affinity;3)less fractionated rare earth element patterns without Eu normally;4)high Sr,low Y,enriched in K,Rb,La,depleted in Nb,Ta,Ba,Ti,P;5)negative correlation between P2O5 and SiO2.Based on the comprehensive research,the tonalite is likely partial melts of the lower crust under extensional environment in the Late Paleozoic(the Early Carboniferous)subsequent to the subduction collision between the Qaidam and the Qilian blocks.