北秦岭-祁连结合带扫帚滩辉长岩年代学、地球化学特征及其地质意义

Geochemistry and chronology of the Early-Paleozoic gabbro from the Saozhoutan area in the North Qinling-Qilian junction zone:Its petrogenesis and geodynamic implications

对北秦岭-祁连结合带关山地区扫帚滩辉长岩进行了系统的岩石学、年代学和地球化学研究。扫帚滩辉长岩的LA-ICPMS锆石U-Pb同位素年龄为(436±3.8)Ma(n=15,MSWD=1.5)。岩石地球化学特征显示该岩体属于钙碱性岩石系列:SiO_2含量为46.32%~52.89%,TiO_2含量有两组区间(1.24%~1.37%和2.11%~2.55%),富Na_2O(2.17%~2.48%),贫K_2O(0.36%~0.62%),Na_2O/K_2O比值介于5.43~6.03之间,里特曼指数σ变化于0.79~2.15之间;岩石的Mg#值为42.65~57.28,分异指数DI=20.92~30.89,固结指数SI=23.68~35.04,表明岩浆分异程度较低。稀土元素分布模式和微量元素原始地幔标准化图解均呈现出两种明显的形态:洋中脊玄武岩型(MORB)和岛弧玄武岩型(IAB)。其中MORB型呈LREE富集的右倾型,(La/Yb)N=1.88~2.51,δEu变化于0.84~1.00之间,IAB型表现出明显的La-Nd及高场强元素(HFSE)负异常,(La/Yb)N=1.57~2.26,δEu介于0.96~1.03之间。结合区域地质背景,认为扫帚滩辉长岩是兼具MORB和IAB型组分的共生基性岩,该岩体可能形成于弧后盆地构造环境,这一结论对于研究秦祁结合带构造演化具有重要意义。

This paper focuses on the petrography,geo-chronology,geochemistry and tectonic setting of gabbro from Saozhoutan in the Guanshan area,the junction zone between the North Qinling and Qilian orogenic belts.LA-ICPMS zircon U-Pb dating indicates that the Saozhoutan gabbro has a crystallization age of(436±3.8)Ma(n=15,MSWD=1.5).The gabbro displays calk-alkaline affinity,with SiO2=46.32%-52.89%.TiO2values have two sections(1.24%-1.37%and2.11%-2.55%),high Na2O(2.17%-2.48%)and low K2O(0.36%-0.62%),Na2O/K2O=5.43-6.03,Rittmann index(σ)of0.79?2.15;Mg#=42.65?57.28,the differentiation index(DI)is20.92?30.89and the solidification index(SI)is23.68?35.04,indicating limited differentiation.The chondrite normalized REE and primitive mantle normalized patterns display two distinct patterns:mid-ocean ridge basalts(MORB)and island-arc basalts(IAB).The MORB samples display right slope REE patterns,with(La/Yb)N=1.88?2.51,insignificant Eu anomalies of0.84to1.00;the IAB samples are depleted in La-Nd and HFSE,with(La/Yb)N=1.57?2.26and Eu anomalies of0.96to1.03.From all the above evidence,we have proposed that the Saozhoutan gabbro was probably formed in a back-arc basin,which has potential significance in the study of the Palaeozoic evolution in the Qinling-Qilian junction zone.