摘要
对华北山东地区不同构造单元中生代白垩纪基性-中基性岩浆岩进行了系统的Re-Os同位素研究.侵入岩样品采自鲁西济南岩体、鲁中沂南岩体和胶东的观水岩墙群,火山岩样品采自鲁西蒙阴、费县、方城青山群和鲁中安丘青山群.所有样品均以低Re(0.076×10-9～0.711×10-9)和Os(0.006×10-9～1.030×10-9)含量以及高放射成因Os同位素比值为特征,其相应的初始187Os/188Os比值和γOs(t)值分别为0.1316～0.5291和4.30～319.00(安丘粗面安山岩分别高达1.463和1060.000),并具有较大变化范围(0.125～2.090Ga)的Os同位素模式年龄.在Os-Nd和Os-Sr同位素关系图上,这些样品分布于典型的岩石圈地幔与地壳端元组成之间,显示壳-幔源岩浆混合特征.然而,除1件观水岩体(MGS0610)和费县玄武岩之外,各地质单元样品之间或所有的中生代岩浆岩作为整体,样品的γOs(t)与(La/Ta)N和(Th/Nb)N比值间缺乏明显的相关关系,说明样品的混源Os同位素并非来自岩浆作用过程中的地壳围岩混染.区内基性岩浆岩Os同位素模式年龄TMA明显小于古生代地幔橄榄岩包体的Re亏损模式年龄(TRD),可能为地幔条件下源岩与再循环地壳物质发生交代作用后Re/Os比值升高所致.蒙阴高镁安山岩和沂南闪长岩在Yb-Re/Yb图解中大致位于中生代辉石岩包体与地壳端元之间,也指示了下地壳拆沉后产生的熔体与地幔岩石可能发生了交代作用.蒙阴安山岩MMY-12(SiO2=56.4%)具有古老壳源特征的Os同位素模式年龄(2.090Ga)、高Mg#(64.6)和高Os含量(0.089×10-9),为这一解释提供了有力的佐证.
A Re-Os isotopic study of Mesozoic igneous basic to intermediate basic rocks from the western-, eastern- and Yishu fault belt tectonic units in Shandong province, North China Craton （NCC）, is carried out. The samples consists of 13 intrusive rocks from the Jinan, Yinan plutons and Guanshui dykes, and 13 volcanic rocks from the Anqiu, Mengyin, Feixian and Fangcheng volcanic successions. Most samples have relatively low Re （0. 076 × 10-9 --0. 711× 10 9） and Os （0. 006 × 10 9_ 1. 030 × 10 9） contents and high radiogenic 187 Os/188 Os ratios, with corresponding initial 18r Os/188 Os ratio of 0. 131 6--0. 529 1 and yos （t） of 4, 30--319. 00, except for rock MAQ0609 of the Anqiu trachyandesite with initial 187Os/lSSOs ratio of 1. 463 and ）＇o, （t） of 1 060. 000. The igneous rocks show a wide range of 0. 125 to 2. 090 Ga in Os isotope modal age （TMA）. On the y{, （t） vs. eNa （t） and （sT Sr/S6 Sr）i vs. ）o （t） diagrams, these rocks are positioned between the lithospheric mantle and crust fields, in- dicative of a crust-mantle interaction-related origin. All but one Guanshui gabbro （MGS0610） and Feixian basahs of the sam- ples display no clear positive correlation between ？% （t） and （La/Ta）N or （Th/Nb）N, which suggests that the role of crustal as similation or contamination during the magma ascending was insignificant. Compared with the Paleozoic mantle xenoliths, these mantle-derived rocks have evidently younger Os modal ages, higher Re/Os and 187 Os/188 Os ratios. These crustal signs are likely to have been resulted from interactions of partial melts derived from delaminated lower crust rocks with ambient mantle under an upper mantle depth. Besides, on the Re/Yb vs. Yb diagram, samples of the Mengyin andesites and Yinan diorites are plot- ted between the Mesozoic pyroxene xenoliths and crust components, which may be best explained by mantle source rocks meta- somatized by partial melts of delaminated eclogitic lower crust. Furthermore, a Mengyin andesite MMY-12 （SiO2 = 56. 4 %） has a modal age as old as 2. 090 Ga, and high Mg# （64. 6） and Os content （0. 089× 10-9）, which provides an additional evidence for lithospheric thinning-related interaction between the delaminated lower crust and the convecting upper mantle.
出处
《地球科学（中国地质大学学报）》
EI
CAS
CSCD
北大核心
2013年第3期529-540,共12页
Earth Science-Journal of China University of Geosciences
基金
国家自然科学基金委创新研究群体科学基金(No.40821061)
国家自然科学基金重点项目(No.90914007)
教育部和国家外国专家局高等学校学科创新引智计划(No.B07039)
地质过程与矿产资源国家重点实验室开放基金(No.GPMR200609)
国土资源部公益性行业科研专项(No.200911043-19)
中国地质调查局地质调查评价专项(No.1212011121102)
