吉林双辽七星山新生代玄武岩的特点及其成因探讨

本文通过岩石学、稀有元素及同位素地球化学等方面的研究,确认吉林双辽七星山火山是在国内含超镁铁岩包体的火山中唯一喷发于早第三纪的钠质系列碱性玄武岩火山群。该火山群中部三座山所产富橄碧玄岩系幔源原生岩浆直接喷发于地表的产物,并携有大量超镁铁岩包体;东部和西部五座山中的碱性橄榄玄武岩和粒玄岩同样来源于上地幔,但曾经历过一定程度的结晶分异作用。文中根据本区的特点和新的参数,重新计算了原生岩浆的几个判别标准。

PETROLOGY AND PETROGENESIS OF THE CENOZOIC BASALTIC ROCKS FROM MT.QIXINGSHAN,JILIN PROVINCE

Mt. Qixingshan is a cluster of eight volcanic cones located in southern Songliao Basin in Northeast China, and the ultramafic xenoliths of various types have been found in four of these cones.The K-At isotope age determinations show that these volcanos erupted in Early Tertiary and even kate Cretaceous era. This fact can be regarded as an important discovery considering that no ultramafic xenoliths have been found so far in the basaltic rocks erupted earlier than Late Tertiary over the territory of China. Among the four types of volcanic rocks which cccur in the volcanos, the most distinctive ones are highly SiO_2-undersaturated and olivine-rich basanite enriched in FeO, MgO and with. quite low Al_2O_3,and high Al and alkali diabase with high Al-2O_3 and Na_2O contents. The other two types are. alkali olivine basalt and alkali doleritc. Ultramafic xenoliths are found mainly in basanite.Petrochemically, rare element geochemical and isotopic geochemical analyses have been made to reveal their possible petrogenesis. The Ni-content in primitive magmas was recalculated according to the Leeman's new equilibrium constant, and the depth of the magma sources was deteimined on the basis of P-T equilibrium conditions of ultramafic xenoliths calculated from thermodynamic models.All these studies show that olivine-rich basanite was consolidated from primitive magma, and its composition depends on the features of upper mantle source and partial melting process. Magmas of alkali olivine basalt and alkali dolerite were also originated from upper mantle, and probably have undergone crystalfractionation to a certain extent on their way to the Earth's surface.The other tonclusion made in. this study is that rocks from Mt. Qixingshan show no similarities to that from. Mt. Erkeshan, Heilongjiang Province and no evidences support the argument made by Japanese authors in 1939.