皖南新元古代花岗闪长岩地球化学特征及构造环境

皖南新元古代花岗闪长岩沿祁门-歙县-三阳深断裂呈串珠状出露。本文在对其岩石学、地球化学细致分析的基础上,探讨了岩体的岩石成因和产出环境。皖南新元古代花岗闪长岩主要由石英、钾长石和斜长石组成,普遍含富铝矿物黑云母和堇青石,副矿物包括锆石、磷灰石、钛铁矿、独居石、磷钇矿、极少的磁铁矿等。地球化学分析数据显示,岩石总体具高硅、高钾、高铝和低钠、低镁、低钙的特征;岩石富碱(ALK=6.63%),高K2O/Na2O比值(1.33)。里特曼指数σ为0.8～2.91,碱度率AR为1.56～3.14,属高钾钙碱性系列。岩石铝饱和指数(A/CNK=1.31)大于1.1,具强过铝质S型花岗岩的特征。岩石稀土元素呈轻稀土富集、重稀土亏损的特征,ΣLREE/ΣHREE比值为5.36～8.36,具较强的负铕异常(δEu=0.39～0.7),配分模式为右倾"V"字形态;微量元素明显富集Rb、Th而亏损Ba、Nb、Ta、Sr等,为低Sr高Yb型花岗岩。地球化学特征显示其岩浆源于围岩-中元古代牛屋组浅变质千枚岩的部分熔融,反映陆-陆碰撞挤压造山环境,为晋宁运动晚期华夏板块向北俯冲与扬子板块碰撞造山的火山弧产物。

Geochemical characteristics and tectonic setting of Neoproterozoic granodiorites in South Anhui Province

Neoproterozoic granodiorites in South Anhui Province are developed along QimenShexian-Sanyang deep fracture and outcropped in the form of beaded veins.This paper studied petrographical and geochemical characteristics of these granodiorites as well as its petrogenesis and tectonic setting.These granodiorites mainly consist of quartz,potash feldspar,plagioclase,biotite and cordierite with minor accessory minerals of zircon,apatite,ilmenite,monazite,xenotime.Geochemical analyses show that the granodiorites have high contents of SiO2,K2O,Al2O3 and total alkal(ALK=6.63%),but relatively low contents of Na2O,MgO,CaO.The ratios of K2O/Na2O(1.33) are generally higher than 1.The Rittmann indexes are between 0.8～2.91,and the AR indexes are between 1.56～3.14,belonging to the high-K calc-alkaline series.The values of A/CNK vary from 0.93 to 1.75 with an average of than 1.1,showing the characteristics of strongly peraluminous S type granites.These granodiorites are obviously rich in ΣLREE from 80.44×10-6 to 169.78×10-6,and depleted in ΣHREE from 11.41×10-6 to 28.28×10-6,with ΣLREE/ΣHREE from 5.36 to 8.36.Rare earth elements display right-inclined V-shape REE patterns with strongly negative Eu anomalies(δEu=0.39～0.7).The trace elements show that these granodiorites have relatively high contents of Rb,Th,and relatively low contents of Ba,Nb,Ta,Sr,belonging to low Sr and high Yb type granites.The geochemical characteristics show that the magmatic source of the grandiosities was formed by partial melting of their wall rock—the epimetamorphic phyllite of the Middle Proterozoic Niuwu Group,reflecting tectonic setting of continent-continent collision and presenting volcanic arc product resulted by northward subduction of the Cathaysia plate and collision orogeny of Cathaysia and Yangtze plate in the later Jinning period.