滇西莴中晚始新世高镁富钾火山岩中单斜辉石斑晶环带结构的成因:岩浆补给-混合过程  

Genesis of Compositional Zoning of Clinopyroxene Phenocrysts in the Wozhong Late Eocene High-Mg Ultrapotassic Lavas,Western Yunnan,China:Magma Replenishment-Mixing Process

作　　者：黄小龙[1] 徐义刚[1] 杨启军[1] 陈林丽[1] 

机构地区：[1]中国科学院广州地球化学研究所同位素年代学和地球化学重点实验室,广州510640

出　　处：《高校地质学报》2007年第2期250-260,共11页Geological Journal of China Universities

基　　金：国家自然科学基金项目(40202009);国家重点基础研究发展规划项目(2002CB412603)

摘　　要：滇西莴中晚始新世高镁富钾火山岩中单斜辉石斑晶普遍出现正环带结构、反环带结构或韵律环带结构,少量为具绿色核部的单斜辉石(“绿核辉石”)。反环带斑晶和“绿核辉石”的幔部与正环带斑晶的核部具有相似并且相对较窄的成分范围,相对高Mg#(0.83～0.90),低TiO2(0.13%～0.29%),Al2O3(0.73%～1.68%)和Na2O(0.22%～0.42%),为钾质岩浆平衡结晶的产物。反环带斑晶的核部相对低Mg#(0.77～0.84),但与反环带斑晶的幔部、正环带斑晶的核部均具有相似的Ti/Al比值(0.06～0.16);韵律环带结构斑晶的成分变化均在正、反环带斑晶的成分范围之内。莴中高镁富钾火山岩中的这些环带结构单斜辉石斑晶应来源于相似的岩浆体系,反环带结构表明在岩浆房存在较原始岩浆对较演化岩浆再补给的岩浆混合过程,而韵律环带结构特征揭示曾多次发生这种岩浆混合过程。“绿核辉石”的核部明显低Mg#(0.50～0.74),相对富Al2O3(1.66%～3.63%)和Na2O(0.87%～2.17%),具有明显较低的Ti/Al比值(<0.05)和较高的AlVI/AlIV比值(0.38～0.76),为下地壳捕虏晶来源,证实了在滇西晚始新世富钾岩浆演化过程中存在少量地壳混染作用。Normal, reverse and oscillatory zonings are common in clinopyroxene phenocrysts in the Wozhong late Eocene high-Mg uhrapotassic lavas, western Yunnan province. Both the mantle of reverse zoning and the cores of normal zoning phenocrysts are all characterized by high Mg^# values （0.83 ～ 0.90）, low TiO2 （0.13 ～ 0.29 %）, Al2O3 （0.73 ～ 1.68 %） and Na2O contents （0.22 ～ 0.42 %）. They were equilibrious crystallization products from the host ultrapotassic lavas. The cores of reverse zoning phenocrysts are relatively low in Mg^# （0.77 ～ 0.84）, but have similar Ti/Al ratios （0.06 - 0.16） as the mantle of reversed zoning and the cores of normal zoning phenocrysts. The compositions of oscillatory zoning phenocrysts vary in the ranges of those for normal and reverse zoning phenoxcrysts. Consequently, it is suggested that these zoning phenocrysts were cognate affinity with the host ultrapotassic lavas. The reversed zonation discovers magma mixing process in the chamber in which a partially crystallized magma was replenished by a mafic magma, and oscillatory zoning denotes several episodes of such mixing process. The greencore clinopyroxene phenocrysts are very rare, in some samples are obviously low in Mg^# （0.50 - 0.74）, relatively high in Al2O3 （1.66 - 3.63 %） and Na2O （0.87 ～ 2.17 %）, very low in Ti/Al ratios （〈 0.05） and high in AlⅥ/AlⅣ ratios （0.38 - 0.76）. The green-core phenocrysts are interpreted as wall-rock xenocrysts from the lower crust, indicating the crustal contamination during magma evolution.

关 键 词：单斜辉石 “绿核辉石” 环带结构 岩浆混合 高镁富钾火山岩 滇西 

分 类 号：P588[天文地球—岩石学]