江南造山带西段新元古代超基性岩体年代学和岩石地球化学研究及其对源区的约束  

Geochronology and geochemistry of Neoproterozoic ultrabasic rocks in the western segment of Jiangnan orogenic belt and constraints on their sources

作　　者：寇彩化[1] 刘燕学[1] 李廷栋[1] 何万双[2] 张恒[1] 丁孝忠[1] 陆济璞 KOU Cai-hua LIU Yan-xue LI Ting-dong HE Wan-shuang ZHANG Heng DING Xiao-zhong LU Ji-pu(Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China 66259 Troops of Chinese People＇s Liberation Army, Hohhot 010010, China Guangxi Institute of Regional Geological Survey, Guilin 541000, China)

机构地区：[1]中国地质科学院地质研究所 [2]中国人民解放军66259部队 [3]广西壮族自治区地质调查院

出　　处：《岩石矿物学杂志》2016年第6期947-964,共18页Acta Petrologica et Mineralogica

基　　金：国家自然科学基金资助项目(41402052);中国地质调查资助项目(1212011120117;1212011120115)~~

摘　　要：本文研究对象为江南造山带西段湘西通道地区长界橄榄辉石岩。长界橄榄辉石岩锆石LA ICP-MS U-Pb加权平均年龄为701±11 Ma。长界橄榄辉石岩主要由单斜辉石、橄榄石和少量斜长石以及少量钛铁氧化物组成,指示在岩浆形成过程中单斜辉石、橄榄石和少量斜长石为主要分离结晶相。全岩具有低SiO_2(44.01%~47.72%)、Al_2O_3(6.77%~9.10%)、TiO_2(0.49%~0.75%)和全碱(Na_2O+K_2O=0.07%~2.04%)含量,而高MgO(23.97%~30.70%)含量的特征,岩石属于亚碱性拉斑玄武岩系列。长界橄榄辉石岩稀土元素总量(∑REE)较低,为25.7×10-6~55.6×10^(-6),全岩稀土和微量元素标准化图解与洋岛玄武岩(OIB)相似,具有富集轻稀土元素[(La/Yb)N为3.36~6.48]和富集大离子亲石元素(LILE)的特点,同时还显示出"弧岩浆"的地球化学性质,明显的Nb-Ta负异常,较高的Th/Nb值(0.52~0.81)和较低的Nb/La值(0.25~0.44)以及低Nb(1.11×10^(-6)~3.91×10^(-6))的特征。长界橄榄辉石岩具有高初始(87Sr/86Sr)i值(0.707 206~0.708 561)和正的εNd(t)值(0.25~0.41)。微量元素模拟结果表明,长界橄榄辉石岩起源于尖晶石相,其源区为受来自俯冲消减板片脱水或熔融形成的流体或熔体的交代的软流圈地幔,是软流圈地幔上涌并发生低度部分熔融的产物(4%~7%)。结合区域地质特征,推测长界橄榄辉石岩的形成可能与700 Ma时期江南造山带西段裂谷作用导致软流圈物质上涌并发生熔融有关。This paper reports the characteristics of the Changjie olivine pyroxenolite from the Tongdao area in western Hunan Province, western segment of Jiangnan orogenic belt. Laser ablation inductively coupled plasma mass spectrometry （LA ICP-MS） U-Pb zircon dating of the Changjie olivine pyroxenolite yielded an age of 701±11 Ma. The Changjie olivine pyroxenolite contains clinopyroxene, olivine and plagioclase together with a small amount of Fe-Ti oxide minerals, probably suggesting that clinopyroxene, olivine and plagioclase fractionated within the magma chamber. Geochemically, the Changjie olivine pyroxenolite is characterized by relatively low SiO2 （44.01%~47.72%）, Al2O3 （6.77%~9.10%）, TiO2 （0.49%~0.75%）, and total alkali （Na2O＋K2O=0.07%~2.04%）, and high MgO （23.97%~30.70%） content, and these rocks are of tholeiitic series and belong to sub-alkali series. These rocks display ocean island basalt （OIB）_like signatures, characterized by the enrichment of light rare earth elements and large ion lithophile elements （LILE） relative to high rare earth elements （HREE）. Moreover, they also display typical arc magma features, such as significant Nb-Ta troughs, relatively high Th/Nb ratios （0.52~0.81） and low Nb/La （0.25~0.44） ratios, and typical low Nb content （1.11×10-6~3.91×10-6）. These rocks show relatively high initial （87Sr/86Sr）i ratios （0.707 206~0.708 561） and positive εNd（t） values （0.25~0.41）. Furthermore, the geochemical signature also suggests that the Changjie olivine pyroxenolite was produced by low degree of partial melting （4%~7%） of the spinel-facies, asthenospheric mantle peridotite which had been matasomatized by slab-derived fruits/melts. In combination with the regional geology, the authors infer that the deep dynamic mechanism for the formation of the Changjie olivine pyroxenolite was related to the partial melting of upwelling asthenosphere mantle due to the rifting at about 700 Ma in the western segment of Jiangnan orogenic belt.

关 键 词：湘西通道地区 长界橄榄辉石岩 锆石定年 软流圈上涌 裂谷作用 

分 类 号：P597.3[天文地球—地球化学] P588.125[天文地球—地质学]