徐淮早白垩世埃达克质岩中含橄榄石单斜辉石岩的成因及其岩石学意义  

Petrogenesis of Olivine-Bearing Clinopyroxenite Xenolith in Early Cretaceous Adakitic Rocks from Xuhuai Area in Eastern China and Its Petrologic Implication

作　　者：周群君[1] 许文良[1] 王清海[2] 杨德彬[1] 裴福萍[1] 于洋[2] 

机构地区：[1]吉林大学地球科学学院,吉林长春130061 [2]吉林大学东北亚国际地学研究与教学中心,吉林长春130026

出　　处：《地球科学（中国地质大学学报）》2014年第2期141-154,共14页Earth Science-Journal of China University of Geosciences

基　　金：科技部"973"项目(No.2009CB825005);国家自然科学基金项目(No.90814003)

摘　　要：报道了徐淮地区早白垩世埃达克质岩中首次发现的含橄榄石单斜辉石岩捕虏体的岩相学与矿物化学资料，该类捕虏体显示堆积结构、块状构造，主要由单斜辉石（～80％）、斜方辉石（～5％）、橄榄石（～5％）和普通角闪石（～10％）组成．橄榄石外侧发育有斜方辉石反应边，角闪石沿辉石粒间分布，呈嵌晶结构．矿物化学分析结果表明：橄榄石的镁橄榄石分子值（Fo）=77．7～79．3，Ni=623×10-6～773×10-6；斜方辉石的Mg#=75．6～80．2，Cr=161×10-6～684×10-6，Ni=79×10-6～708×10-6；单斜辉石的Mg#=84．5～86．4，CaO=21．59％～23．13％，Al2O3=1．72％～2．44％．上述矿物与中、新生代玄武岩中橄榄石、斜方辉石和单斜辉石斑晶以及堆积成因辉石岩中的斜方辉石和单斜辉石成分类似．此外，单斜辉石的稀土配分型式以相对富含中稀土元素的上凸型为特征，稀土元素含量较低（∑REE=10．14×10-6～12．71×10-6），无明显的铕异常（8Eu=0．90～1．16），类似于新生代玄武岩中单斜辉石斑晶．捕虏体中的普通角闪石的Mg#=74．0～80．4、SiO2—43．2％～44．5％、Na2O—2．04％～2．29％，稀土元素分馏不明显，显示亏损高场强元素（HFSEs，如Nb、Ta、Zr、Hf），富集Sr、Rb、Ba的特征，与新生代玄武岩中角闪石捕虏晶成分不同．结合其嵌晶结构，普通角闪石应是寄主岩浆贯入结晶的产物．综合上述特征，可以看出含橄榄石单斜辉石捕虏体为镁铁质岩浆高压堆晶成因．结合华北克拉通东部早白垩世双峰式火山岩组合的出现，推断含橄榄石单斜辉石岩捕虏体可能是早白垩世基性岩浆底侵的产物．The petrography and mineral chemical data of olivine-bearing clinopyroxenite xenolith entrained by the Early Cretaceous adakitic rocks in Xuhuai area, eastern China are reported in this paper. Olivine-bearing clinopyroxenite consists of clinopyroxene （-80%）, orthopyroxene （-5%）, olivine （-5%）, and hornblende （-10%） and displays cumulate texture and massive structure. The orthopyroxene reaction rim can be found around olivine. The hornblendes occur among pyroxenes, and display poikilitic texture. The mineral chemical data indicate that olivines have forsterite （Fo） = 77.7 -- 79.3 and Ni = 623 ×10-6 -- 773 ×10-6 , that orthopyroxenes have Mg# = 75.6 - 80.2, Cr= 161 ×10-6 -- 684×10-6, and Ni= 79 ×10-6- 708 ×10-6, and that clinopyroxenes have Mg# =84.5--86.4, CAO=21.59%-23-13%, Al2O3 =1.72%-2.44%. Chemically, these minerals are similar to the olivine, orthopyroxene, and clinopyroxene phenocrysts from the Mesozoic and Cenozoic basalts and those from the pyroxenites with cumulate origin. Additionally, the clinopyroxenes from the xenolith are characterized by convex upward rare earth element （REE） distribution patterns, low REE contents （ }2 REE= 10.14 ×10-6- 12.71 ×10-6）, and no Eu anomalies （3Eu=0.90--1.16）, similar to the clinopyroxene phenocrysts in the Cenozoic basalts. Hornhlendes in this xenolith have Mg# =74.0--80.4, SiO2 =43.2%-44.5%, Na2O=2.04%-2.29%, and display flat REE patterns and depletion in high field strength elements （HFSEs, Nb, Ta, Zr and Hf） and Th as well as enrichment in Sr, Rb and Ba, different from those hornblende xenocrysts from Cenozoic basalts. Combined with its poikilitic texture, it is suggested that these hornblendes could be formed by injection of the host magma. Taken together, we conclude that the olivine-bearing clinopyroxenite xenolith could be formed by the high-pressure accumulation of basaltic melt. Combined with Early Cretaceous himodal magmatism in eastern China, it is suggested that the olivine-bearing clinopyroxenite could be generat

关 键 词：早白垩世 埃达克质岩石 含橄榄石单斜辉石岩捕虏体 岩石成因 岩石学 徐淮地区 

分 类 号：P581[天文地球—岩石学]