江南东段平水地区桃红闪长岩:早新元古代洋壳消减的证据  

Taohong Diorite from Pingshui Region in Eastern Jiangnan Orogen:Evidence for Early Neoproterozoic Oceanic Crust Subduction

作　　者：谭清立[1] 王岳军[1] 张玉芝[1] 李庶波[1] 周永智 王玉琨[1] 

机构地区：[1]中山大学地球科学与地质工程学院,广东广州510275

出　　处：《地球科学（中国地质大学学报）》2017年第2期173-190,共18页Earth Science-Journal of China University of Geosciences

基　　金：国家自然科学基金项目(Nos.41375198;41402165)

摘　　要：平水一带早新元古代岩浆岩的精细年代学约束及其形成构造背景的厘定是理清华南内部新元古代演化机制的重要突破口.对江南造山带东段平水地区桃红岩体中的闪长岩进行了详细的锆石U-Pb年代学(LA-ICP-MS)、全岩地球化学和Sr-Nd同位素研究,结果显示:闪长岩锆石U-Pb年龄为913±2Ma和898±2Ma,代表其形成年龄;去掉挥发分并百分化处理后,SiO_2、Al_2O_3、Na_2O和K_2O含量分别为58.33%~63.36%、15.76%~17.42%、2.62%~3.12%和0.53%~1.53%,属低钾-中钾系列;铝饱和指数(A/CNK)为0.84~0.92,属偏铝质;轻重稀土分异明显((La/Yb)_N=4.65~6.09)、轻微Eu异常(δEu=0.82~1.01)、高场强元素(Nb、Ta、Ti、P)强烈亏损、大离子亲石元素(Rb、Ba、K、Sr)相对富集,具岛弧岩浆亲缘性;具低(^(87)Sr/^(86)Sr)i值(0.703 060~0.703 076)、高ε_(Nd)(t)值(+6.58^+6.76).综合研究认为桃红闪长岩为扬子与华夏陆块之间的古洋壳南东向俯冲的过程中,被俯冲板片流体和熔体改造的地幔楔橄榄岩部分熔融的产物;双溪坞群及其同期岩浆岩应是扬子与华夏陆块早期拼合阶段的产物,与格林威尔构造事件无关.The fine chronological constraints and the determination of tectonic setting of the Early Neoproterozoic magmatic rocks in the Pingshui area are important breakthroughs for the Neoproterozoic evolution mechanism in South China.This paper presents a synthesis of zircon U-Pb geochronological,whole rock elementaland Sr-Nd isotopic geochemistry study on the Taohong diorite from Pingshui region along the Eastern Jiangnan Orogen.Two representative samples yielded weighed mean 206Pb/238 U ages of 913±2Ma and 898±2Ma,respectively,representing the crystallization ages of the dioritic pluton.In the case of loss on ignition free,the dioritic samples are characterized by SiO2 of 58.33%-63.36%,Al2O3 of 15.76%-17.42%,Na2O of 2.62%-3.12%,K2O of 0.53%-1.53% and A/CNK of 0.84-0.92,classified as low to medium-K metaluminous series.Their chondrite-normalized rare earth elements（REEs）patterns exhibit a right-sloping tendency with more enriched light rare earth elements（LREEs）relative to heavy rare earth elements（HREEs）（（La/Yb）N=4.65-6.09）and slightly Eu abnormality（δ（Eu）=0.82-1.01）.On the primitive mantle-normalized multi-elements patterns,these samples have strong enrichment in large iron lithophile elements（LILEs）（Rb,Ba,K and Sr） and depletion in high field-strength elements（HFSEs）with marked negative Nb,Ta and Ti,similar to those of typical arc magmatites.The samples show low（^87Sr/^86Sr）iof 0.703 060-0.703 076 ratio and high positive ε（Nd）（t） of ＋6.58 to ＋6.76 values.It is inferred that the formation of the rocks is attributed to the partial melting of the mantle wedge peridotite,which was modified by the subducted slab-derived fluid and melt during the southeastwards subduction of ancient oceanic crust between the Yangtze and Cathaysia Block.In conjunction with previous studies,it is proposed that the Shuangxiwu Group along Eastern Jiangnan Orogen resulted from suturing of the Yangtze andCathaysia Block,and is not related to Grenvillian Orogen in petrogenesis.

关 键 词：闪长岩 洋壳消减 锆石U-PB定年 SR-ND同位素 江南造山带 地球化学 

分 类 号：P588.12[天文地球—岩石学] P597.3[天文地球—地质学]