内蒙古四子王旗东八号中新世含深源捕虏体富钾橄榄玄武岩的发现及其意义

Discovery of Mantle Xenoliths Bearing Miocene Potassium-Rich Olivine Basalt and Its Significance in Siziwangqi Area, Inner Mongolia

内蒙古四子王旗东八号地区最近发现含有深源捕虏体的富钾橄榄玄武岩。玄武岩为灰黑色致密块状 ,以富碱 (K2 O +Na2 O 5 .91%～ 6 .4 7% )、高钾 (K2 O 2 .82 %～ 3.12 % )和较高的钛 (TiO22 .2 2 %～ 2 .36 % )为特征 ,属钾玄岩系列 ,全岩K -Ar年龄为 2 1.9± 1.7Ma。捕虏体主要为尖晶石二辉橄榄岩、二辉橄榄岩等。根据矿物平衡温压计算表明 :捕虏体形成温度为 110 0～ 12 5 0℃ ,压力为 1.7～ 2 .0GPa ,应源自于岩石圈地幔。四子王旗橄榄玄武岩较高的TiO2 和K2 O ,暗示其岩浆可能源于幔柱作用。推测华北板块北缘自中生代开始可能受到来自深部的幔柱作用 ,大量上升的岩浆因受到阻挡而保留在岩石圈底部 ,底侵作用和部分继续上升的岩浆 ,将岩石圈地幔的捕虏体携出地表。

There is a suite of mantle xenoliths bearing olivine basalt discovered recently in the south of Siziwangqi area, Inner Mongolia. The rock is grey-black, with compact block structure and is characterized by alkaline-rich (K_2O+Na_2O= 5.91%～6.47%), high K (K_2O= 2.82%～3.12%) and high Ti (TiO_2= 2.22%～2.36%) compositions. The whole rock K-Ar isotopic age is 21.89±1.65 Ma. The rock types of xenoliths are lherzolites, spinel lherzolites and anorthositic gabbro. Geothermometer and geobarometer calculations manifest that the temperature and pressure ranges for the xenoliths are 1 100～(1 250) ℃ and 1.2～1.7 GPa, respectively, which indicates a lithospheric mantle source. Considering that the high Ti character of the olivine basalt is similar to Hannuoba basalt, the basalt magma might come from the asthenosphere. Deep mantle plume possibly began to work since the Mesozoic under the northern margin of North China Platform, which resulted in the accumulation of vast magma remaining under the lithosphere, followed by underplating and ascending of some magma carrying the mantle xenoliths (upwards).