华北中、新生代岩石圈地幔的交代作用:含金云母地幔岩提供的证据  

Metasomatism of Mesozoic and Cenozoic lithospheric mantle beneath the North China Craton: Evidence from phlogopite-bearing mantle xenoliths

作　　者：赵新苗[1] 张宏福[1] 朱祥坤[2] 张文慧[1] 杨岳衡[1] 汤艳杰[1] 

机构地区：[1]中国科学院地质与地球物理研究所岩石圈演化国家重点实验室,9825信箱北京100029 [2]中国地质科学院地质研究所国土资源部同位素地质重点实验室,北京100037

出　　处：《岩石学报》2007年第6期1281-1293,共13页Acta Petrologica Sinica

基　　金：国家自然科学基金委大陆动力学重点项目(批准号:40534022);中国科学院知识创新工程重要方向项目(批准号:KZCX2-YW-103)资助成果.

摘　　要：本文对华北克拉通三个不同地区（河北汉诺坝、内蒙古集宁三义堂、河南鹤壁）新发现的含金云母尖晶石二辉橄榄岩和尖晶石橄榄单斜辉石岩捕虏体进行了详细的矿物组成、单斜辉石的微量元素和Sr-Nd同位素研究.通过与相同地区不含金云母尖晶石二辉橄榄岩捕虏体的系统对比发现通常含金云母的地幔橄榄岩比不含金云母的地幔橄榄岩岩富Al2O3、CaO、NaO、K2O、TiO2,但相对贫镁;其单斜辉石的LREE更为富集,但Sr、Nd同位素组成则相对亏损.这说明地幔交代作用不仅能够造成地幔橄榄岩的玄武质组分和稀土元素的富集,而且亦能够造成全岩和橄榄石Mg#的降低和同位素组成的相对亏损.捕虏体的Rb-Sr等时线年龄暗示地幔交代作用发生在中、新生代;其交代熔体来源于软流圈.同时说明华北新生代岩石圈地幔普遍存在的主、微量元素和同位素组成类似于“大洋型”岩石圈地幔的特征很可能是岩石圈地幔橄榄岩与软流圈来源的熔体的大规模反应的结果,而非真正意义上的新增生的岩石圈地幔.This paper reports the mineral compositions, clinopyroxene trace element concentrations and Sr-Nd isotopes for the newlydiscovered phlogopite-bearing spinel lherzolite and olivine clinopyroxenite xenoliths from three different localities （ Hannuoba, Hebei Province; Jining Sanyitang, Inner Mongolia; Hebi, Henan Province） of the North China Craton. Systematic comparison with those phlogopite-free spinel lherzolite xenoliths from the same localities reveals that the phlogopite-bearing peridotitic xenoliths have relatively higher Al2O3, CaO, Na2O, K2O, TiO2 contents and lower MgO contents than those phlogopite-free counterparts. The former also has higher LREE concentrations, but relatively less radiogenic Sr-Nd isotopic ratios. This demonstrates that mantle metasomatism can not only enrich the basaltic components and trace element concentrations, but also make a decrease in Mg^# of the peridotites and olivines and a relative depletion in Sr-Nd isotopes. ^87 Rb/^86 Sr-^S7 Sr/86 Sr isochrons of the phlogopite-bearing xenoliths indicate that mantle metasomatism happened in the Mesozoic and/or Cenozoic time. The metasomatic agent was derived from the asthenosphere. The result also manifests that the widespread similarity of the geochemical features such as major and trace elements and isotopic compositions in the Cenozoic lithospheric mantle beneath the North China Craton to those ＂oceanic＂ lithospheric mantle could be as a result of the ubiquitous presence of the interaction between the old refractory peridotites and the infiltrated asthenospheric melt, rather than the actually newly-accreted lithospheric mantle.

关 键 词：新生代玄武岩 含金云母地幔捕虏体 地幔交代作用 岩石圈地幔 华北克拉通 

分 类 号：P588.145[天文地球—岩石学]