【摘要】 40Ar/39Ar同位素定年分析显示滇西新生代高镁富钾火山岩均形成于始新世晚期(约36Ma),富含MgO(>9%)和K2O(3.07%～7.28%),绝大部分K2O/Na2O比值大于2,均属超钾质系列岩石。较高的Cr、Ni含量和较低的Sc(<30μg/g)、TiO2(<1%)含量暗示了源区为尖晶石相方辉橄榄岩;高镁富钾火山岩与区内同时代的粗面岩具相似的地球化学组成特征,均富集LILE和LREE,亏损高场强元素Nb-Ta和Ti等,具有极低的Nb/La比值(0.2～0.3)、较高的87Sr/86Sr初始值(0.7056～0.7072)、负的εNd初始值(-0.97～-4.87)和相对较高的206Pb/204Pb(18.556～18.695)、207Pb/204Pb(15.609～15.630)比值,具有明显的岛弧型钾质火山岩的性质。高镁富钾火山岩来源于受古特提斯俯冲带流体交代的尖晶石相方辉橄榄岩的部分熔融岩浆,并在深部岩浆房经历了橄榄石的结晶分异和堆晶作用,而粗面岩则还在相对浅部岩浆房经历了较大程度的长石与单斜辉石的分离结晶。更多还原

【Abstract】 40Ar/39Ar dating results show that all the Paleogene high magnesian ultrapotassic lavas of western Yunnan are formed in late Eocene (～ 36 Ma). The lavas are enriched in MgO (> 9% ) and K2O (3.07% ～ 7.28% ) with high K2O/Na2O ratios (> 2 in most of the samples). Very high Cr, Ni and low Sc (< 30 μ g/g), TiO2 (< 1% ) contents in the high magnesian ultrapotassic lavas imply the mantle sources of spinel harzburgite. Both high-Mg lavas and trachytes are enriched in LILE and LREE, but depleted in HFSE, such as Nb, Ta and Ti. The lavas have very low Nb/La ratios (0.2～ 0.3), high initial 87Sr/86Sr ratios (0.7056～ 0.7072), negative initial ε Nd values (- 0.97～ - 4.87), and relatively high 206Pb/204Pb (18.556～ 18.695) and 207Pb/204Pb (15.609～ 15.630) ratios, which are geochemically similar to K-rich volcanism in subduction settings. It is suggested that the primary magma was likely generated from partial melting of spinel harzburgitic source metasomatized by Paleo-Tethyan subduction-related fluids. The high magnesian ultrapotassic lavas had undergone fractional crystallization and accumulation of olivines in a deep magma chamber, while the fractional crystallization of plagioclases and clinopyroxenes in relatively shallow magma chamber extensively occurred in the evolution of the trachytes.更多还原