青藏东缘马关尖晶石二辉橄榄岩包体中单斜辉石的成分特征及其意义

青藏东缘马关尖晶石二辉橄榄岩包体的矿物组合为橄榄石 (Ol) +斜方辉石 (Opx) +单斜辉石 (Cpx) +尖晶石 (Spl)。单斜辉石主要为透辉石 ,少量为顽透辉石 ,其成分特征表现为高Ca(w(CaO) =18.71%～ 2 0 .78% )、高Al(w (Al2 O3 ) =6 .0 0 %～ 7.30 % )、高Mg# 值 (89.84～ 91.4 8)和低Cr# 值 (5 .18～ 8.72 )。SiO2 ,TiO2 ,FeO 和Na2 O的质量分数变化范围分别是 5 1.71%～ 5 2 .5 0 %、0 .34%～ 0 .6 1%、2 .5 0 %～ 3.14 %和 1.4 6 %～ 1.80 %。单斜辉石的成分特征显示尖晶石二辉橄榄岩包体为A型包体 ,属低度熔融后的大陆地幔橄榄岩。透辉石的w(Y) (18.6 0～ 2 0 .90 μg/g)、w(Yb)(1.97～ 2 .18μg/g)模拟计算表明 ,马关尖晶石二辉橄榄岩包体是原始地幔经低于 3%的部分熔融后的残留地幔岩。

THE CHEMICAL COMPOSITION CHARACTERISTICS OF CLINOPYROXENES FROM SPINEL-LHERZOLITE XENOLITHS IN MAGUAN AREA, EASTERN TIBET AND ITS SIGNIFICANCE

The mineral assemblages of spinel-lherzolite xenoliths in Maguan Area, Eastern Tibet are Ol+Opx+Cpx+Spl. The Cpxs belong to diopside, characterized by high Ca (w(CaO)=18.71%～(20.78%),) Al (w(Al_2O_3)=6.00%～7.30%), and Mg~# value (89.84～91.48), while low Cr~# value ((5.18)～8.72). The contents of SiO_2, TiO_2, FeO~* and Na_2O range from 51.71% to 52.50%, 0.34% to (0.61%,) 2.50% to 3.14%, and 1.46% to 1.80%, respectively. The chemical composition characteristics of Cpxs indicate that the spinel-lherzolite xenoliths belong to the type-A peridotite enclaves and the lower melted continental mantle. The analog computation of Y(18.60～20.90 μg/g) and Yb(1.97～(2.18 μg/g)) values from diopside revealed that the spinel-lherzolite xenoliths were derived from the remnant of a primary mantle after 3% partial melting.