福建龙海天马山-牛头山玄武岩中捕虏体和巨晶特征的研究

A STUDY OF THE CHARACTERISTICS OF XENOLITHIC ENCLAVES AND MEGACRYSTS IN THE BASALTS OF TIANMA-NIUTOU SHAN, LONGHAI COUNTY, FUJIAN

本文研究了天马山-牛头山新生代玄武岩中二辉橄榄岩和橄辉斜长岩捕虏体的岩石学、矿物学和地球化学等特征并论及碱性玄武岩中单斜辉石和歪长石巨晶。尖晶石二辉橄榄岩存在于碱性玄武岩和橄榄拉斑玄武岩中,由尖晶石、橄榄石、斜方辉石、铬透辉石等组成,与模拟地幔岩相比,具低的MgO含量和不相容元素Ti丰度而富集Co、Ni相容元素。尖晶石二辉橄榄岩具低的稀土总量和平缓的球粒陨石标准化稀土元素分配模式,形成于温度约1000℃、深度为50—80公里的上地幔。橄辉斜长岩具正堆积结构,Al_2O_3、CaO、∑REE和La/Yb明显高于二辉橄榄岩。但MgO含量远低于后者,反映两者成因的不一致性。普通辉石巨晶以富铝为特征,歪长石以富钙为特征。

The characteristics of petrology, mineralogy and geochemistry are studied for xenolithic enclaves of spinel-lherzolite and olivineaugite anorthosite in basalts of Cenozoic Futan Group in TianmaNiutou shah, Longhai County of Fujian Province. The clinopyroxene and anorthoclase megacrysts are also discussed. The xenolithic en- claves of spinel-lherzolite that consist of olivine, orthopyroxene, Cr-diopside, and spinel exist in both the alkali basalts and olivine tholeiites. The spinel-lherzolite is in lower abundances of Mg and incompatible element Ti and but is in higher concentration of compatible elements (Co, Ni)than that of pyrolite and is thought that it has been derived from the residual mantle material by partial melting at about 1 000℃ and at a depth of approximately 50～80km. The spinel-lherzolite is low in REE with a gentle chondrite normalized REE Pattern. The olivine-augite anorthosites with cumulate texture is obviously higher in Al_2O_3, CaO, REE, La/Yb and lower in MgO compared with the lherzolites. This reflects that they were formed in different conditions. The augite megacryst and anortho clase megacrysts in alkali basalt are rich in Al_2O_3 and CaO respectively.