太行山中段新生代玄武岩中高镁橄榄石捕虏晶:残留古老岩石圈地幔样品

High-Mg olivine xenocrysts entrained in Cenozoic basalts in central Taihang Mountains: relicts of old lithospheric mantle.

太行山中段新生代玄武岩(7.8 Ma)中发现具有明显环带结构的高Mg#橄榄石.其中心部分明显富MgO和NiO,边缘FeO、MnO、CaO和Al2O3的含量较高.高Mg#橄榄石中心部分(Mg#=89.8～92.3)与华北新生代玄武岩所携带的橄榄岩捕虏体中高Mg#橄榄石的组成接近,中间部分(Mg#=84.5～89.7)略高于寄主玄武岩中斑晶橄榄石中心部分的组成(Mg#=77.1～81.7),而高Mg#橄榄石的边缘部分(Mg#=55.6～73.6)和寄主玄武岩斑晶橄榄石边缘的组成(Mg#=55.6～73.6)相一致,都接近于寄主玄武岩基质中橄榄石的组成(Mg#=54.8～66.1).橄榄石的化学成分环带及其所具有的熔蚀结构说明它是地幔橄榄岩的解体矿物,其大颗粒的中心部分能够代表地幔橄榄岩的组成,表明在太行山中段新生代时期仍存在古老的岩石圈地幔.对比研究发现,华北地区岩石圈减薄存在时空上的差异.

Cenozoic (7.8 Ma) basalts from central Taihang Mountains contain obviously zoned high-Mg-# olivines. The core is rich in MgO and NiO and poor in CaO and the rim has higher FeO, MnO, CaO and A(2)O(3) contents. High-Mg-# olivines have compositions in the core ( Mg-# = 90.1 similar to 92.3) very similar to the high-Mg-# ones from the mantle peridotitic xenoliths entrained in Cenozoic basalts from the North China Craton, and in the transitional zone ( Mg-# = 84.5 similar to 89.7) slightly higher than those in the core of olivine phenocrysts of the host basalts ( Mg-# = 77.1 similar to 81.7 The compositions in the rim of high-Mg-# olivines ( Mg-# = 55.6 similar to 73.6), identical to those of olivine phenocrysts of host basalts Mg-# = 55.6 similar to 73.6), are close to the olivines in groundmass ( Mg-# = 54.8 similar to 66.1). These compositional features as well as resorption borders and rounded crystal shapes demonstrate that these high-Mg-# olivines are mantle xenocrysts, i. e. disaggregates of mantle peridotites. The core compositions of big grains can represent those of olivines of mantle peridotites, which indicate that old lithospherie mantle should be existent in the Central Taihang Mountains in Cenozoic. Comparative studies show that large temporal and spatial difference exists in the lithosphere thinning in the North China.