河北矾山超镁铁岩-正长岩杂岩体中黑云母的特征及其成岩指示意义

摘要

The Fanshan ultramafic-syenitic complex is located on the northern margin of North China Craton (NCC),and crystallized from a SiO2-undersaturated ultrapotassic alkaline-peralkaline parent magma which was originated from an enriched lithospheric mantle source that had previously been metasomatized by melts from carbonated oceanic crust above a subduction zone.The Fanshan complex was emplaced in late Trassic,and is a typical compelx among the alkaline/ultramafic magmatic belt along the northern margin of NCC.Major and trace elements compositions of biotites from the Fanshan complex are obtained using electron microprobe (EMPA ) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS),respectively.The biotites from the Fanshan complex have Fe/(Fe+Mg)in the range 0.32~0.57,and total Al from 1.179 to 1.375,and belong to typical biotite.Biotite compositions indicate that the crystallization temperatures of the garnet-clinopyroxene syenites are between 680℃ and 780℃,and their oxygen fugacity (fO2 )= 10-12.5~10-15 .The biotites from the Fanshan complex have relatively low contents of rare-earth elements (REE)with total REE contents lower than 0.100×10-6~1.077×10-6, indicating that biotite is not a major carrier of REE of the whole rock.However,trace elements compositions of biotites indicate that biotite is a major carrier of Rb,Ba,Nb,Ta,V,Cr,Co and Ni;whereas the contents of Sr,Zr,Hf,Y,Sc,Th and U of biotites are much lower than those of whole rock. Biotite compositions suggest that the Fanshan complex is characterized by high Fe contents and high oxygen fugacity,which may be the reason why the complex hosts an apatite-magnetite deposit.%河北矾山超镁铁岩-正长岩杂岩体侵位于华北克拉通北缘，为硅不饱和过钾质碱性岩，起源于受古亚洲洋俯冲物质改造过的富集岩石圈地幔。矾山杂岩体侵位于晚三叠世，是华北克拉通北缘三叠纪碱性岩带的代表性岩体之一。本文利用电子探针（EMPA）和激光等离子质谱仪（LA-ICPMS）获得了该特殊岩浆体系下黑云母的主量和微量元素组成，并讨论了其成岩指示意义。矾山杂岩体各类型岩石中的云母均为黑云母，Fe／（Fe＋Mg ）为0．32～0．57，∑Al为1．179～1．375。黑云母成分记录石榴石辉石正长岩结晶时体系氧逸度LogfO2＝－12．5～－15，温度为680～780℃。矾山杂岩体的黑云母具有极低的稀土元素含量（小于0．100×10－6～1．077×10－6），对全岩稀土元素配分型式基本上没有影响。在微量元素组成上，黑云母是元素Rb、Ba、Nb、Ta、V、Cr、Co 和Ni 的主要载体，其Sr、Zr、Hf、Y、Sc、Th和U的含量明显低于全岩。黑云母成分反映矾山杂岩体岩浆体系以富 Fe、高氧逸度为特征，这可能是该杂岩体发育具有经济价值铁矿床的原因之一。