西秦岭礼县地区新生代钾霞橄黄长岩系地球化学特征及地质意义

Geochemical characteristics and geological implications of the Cenozoic kamafugites from Lixian County, West Qinling

西秦岭礼县地区新生代钾霞橄黄长岩系具有贫SiO2 、Al2 O3,富MgO、CaO、TiO2 及K2 O +Na2 O的特征 ,矿物组合中除橄榄石、透辉石外 ,普遍含有霞石 /钾霞石、黄长石 /白榴石和钛金云母等矿物。火山岩系的Sr、Nd示踪同位素和微量元素特征显示其与洋岛玄武岩 (OIB)的地球化学体制相似 ,源区具有由软流圈流体交代导致的EMⅠ型地幔端员成分。根据尖晶石相二辉橄榄岩地幔包体温、压条件估算 ,结合区域地球物理资料和区域大地构造环境以及火山岩的形成时代推断 ,钾霞橄黄长岩系的形成机制与本地区新生代岩石圈伸展作用密切相关 ,原始岩浆源于 >92km的岩石圈底部热边界层的低度部分熔融。

The Cenozoic kamafugites in Lixian County of West Qinling are poor in SiO 2 and Al 2O 3 but rich in MgO, CaO, TiO 2 and K 2O+Na 2O. The modal mineral assemblage is composed mainly of olivine, clinopyroxene, nepheline/ kalsilite, melilite/leucite and Ti_phlogopite. The trace isotopic compositions of Sr, Nd and trace element characteristics of kamafugitic series show distinct geochemical features similar to those of N_OIB and MORB as well as an EMⅠ end member caused by metasomatism of asthenospheric fluids. Based on the estimation results of p and T from lherzolites, regional geophysical information, regional tectonic settings and the formation ages of volcanic rocks, it is held that the formation of kamafugitic series was closely related to the Cenozoic lithospheric extension in this area, and that the primitive kamafugitic magma was derived from the low_degree partial melting of the thermal boundary layer at the basement of lithosphere beneath the depth of 92 km.