华北一些新生代玄武岩及其包体的氧同位素初步研究

OXYGEN ISOTOPE COMPOSITION IN SOME CENOZOIC BASALTS AND INCLUSIONS FROM NORTH CHINA

华北新生代玄武岩氧同位素组成δ^(18)O值为5.2～7‰,属幔源成因。拉斑玄武岩和碱性橄榄玄武岩的氧同位素组成不同,表明其岩浆源有差别。包体中斜方辉石和单斜辉石的δ^(18)O值很接近,表明它们处于平衡状态,根据平衡体系中的橄榄石和单斜辉石的δ^(18)O值计算出它们平衡时的温度为1049～1167℃。

This paper deals with some characteristics of oxygen isotope of Cenozoic basalts and inclusions. All these samples were collected from North China, such as Datong alkaline basalts (Quarternary) and tholeiites(Quarternary), Wuan alkaline basalts(Neogene), Xiyang alkaline basalts (Neogene), Zhangbei alkaline basalts and tholeiites (Neogene), Fanshi alkaline basalts and tholeiites (Paleogene), Beijing alkaline basalts and tholeiites(Paleogene). The oxygen isotope characteristics of these Cenozoic basalts and inclusions of this area are as follows:(1)The range of δ^(18)O values of basalts is from 5‰ to 7.58‰, which means the origin of basalts is upper mantle; (2) The mean δ^(18)O values of alkaline basalts(δ^(18)O) are slightly higher than those of tholeiites (δ^(18)0), despite the different period, e.g. the values of (?)_Al ^(18)O and (?)_Th ^(18)O in Quarternary basalts are 5.63‰ and 5.56‰, respectively; the values of (?)_Al ^(18)O and (?)_Th ^(18)O in Neogene basalts are 6.90‰ and 6.13‰; the values of (?)_Al ^(18)O and (?)_Th ^(18)O in Paleogene basalts are 5.80‰ and 5.62‰. However, even at the same area, the values of (?)_Al ^(18)O are also slightly higher than (?)_Th ^(18)O values; (3)From the δ^(18)O values of olivine and clinopyroxene in isotope equilibrium system, the equilibrium temperature (1049～1167℃) can be calculated, which is very similar to the temperature(950～1160℃) calculated from chemical composition of minerals.