青藏米巴勒地区中新世钠质方沸石(霞石)响岩中单斜辉石环带研究:对岩浆-构造演化的启示

在拉萨地块中部米巴勒地区产出的中新世钠质方沸石(霞石)响岩中的单斜辉石发育大量环带结构,环带结构包括正环带、反环带和韵律环带。其中正环带结构从核部到边部Mg^#、MgO、Cr₂O₃呈逐渐降低趋势,而FeO、Na₂O、Al₂O₃、TiO₂呈逐渐升高的趋势,它反映了岩浆正常结晶的历史。反环带结构中Mg^#、MgO、Cr₂O₃从核部到边部呈升高的趋势,FeO、Na₂O、Al₂O₃的含量总体上呈降低趋势,本文认为响岩中单斜辉石的这种环带结构是受到了和它共存的钾质-超钾质火山岩岩浆混合或混染所致,两种岩石应是各自独立的母岩浆的结晶产物。具韵律环带结构的单斜辉石从核部到边部出现成分规律性波动,Na₂O、Al₂O₃、TiO₂、Cr₂O₃含量的变化在正反环带的范围内,Mg^#值的变化范围较小,在正环带斑晶和基质的范围内,它是岩浆多次混合作用的结果。该地区钠质过碱性岩的出现可能指示青藏高原在这套火山岩喷发时(13~12Ma),构造体制发生了显著的改变,进入到一个典型的陆内伸展阶段。

The zoning structure of clinopyroxene phenocrysts in the Miocene sodium analcime (nepheline) phonolite in Mibale area, Tibet: Implications for the magmatic and tectonic evolution

Sodium analcime (nepheline) phonolites in Mibale area of Tibetan Plateau contain abundant clinopyroxene phenocrysts which show the normal, reverse or oscillatory zoning patterns. The normal zones increase in FeO, Na₂O, Al₂O₃ and TiO₂ but decrease in Mg^#, MgO, Cr₂O₃ from core to rim, which indicates the normal magmatic evolution of fractional crystallization. The reverse zone have increasing Mg^#, MgO and Cr₂O₃ but decreasing FeO, Na₂O and Al₂O₃, indicating that the sodium analcime (nepheline) phonolite had been mixed or contaminated by the ultra-potassic or potassic magmas, and that both phonolite and potassic-rich rocks might have different parental magmas. The clinopyroxenes with oscillatory zones were probably the result of multi-stage magmatic mixing since that Na₂O, Al₂O₃, TiO₂ and Cr₂O₃ vary between normal and reverse zone clinopyroxenes as well as Mg^# within the range of the phenocryst and the matrix clinopyroxenes. The peralkaline sodium magmas erupted at 13~12Ma in the area, corresponding to a temporary tectonic transformation, which suggests a typical intracontinental extensional setting.