韭菜地等地的新生代玄武岩及其超镁铁岩包体的岩石学研究

Petrogenesis of Cenozoic Basaltic Rocks and Their Ultramafic Inclusions from Jiucaide and Jilongshan

韭菜地和鸡笼山两地的玄武岩为新生代的火山锥体,主要岩石为碧玄岩和碱性橄榄玄武岩,其中含有较丰富的超镁铁岩包体。玄武质岩石属碱性系列的钾质亚系,SiO_2不饱和,Al_2O_3较低且富含K、Ti和P等不相容元素,具有Al_2O_3/CaO值较高、M值在60—70之间、固结指数近于40、熔融程度低(4％)等特征,表明玄武质岩石是地幔橄榄岩低度熔融的产物。超镁铁岩包体中易熔组分与不饱和型包体组分相近,其矿物化学特征为:橄榄石的Fo值和斜方辉石的En值较低,单斜辉石的Al_2O_3、TiO_2和Na_2O较高,表明包体相对地富集易熔组分,与原始地幔组分相似。估算包体的矿物平衡温度和压力分别为1050℃和21.6×10~3Pa。

Jiucaide and Jilogshan bodies are located at 6km to the north of Wujingfu town, Jieyang County and 15kin to the northwest of Shantou City respectively. They are both in form of volcanic cones with area less than 0.2 km^2.The volcanic rocks are mainly consisted of basanites and alkali-olivine basalts with abundance of ultramafic inclusions-spinel lherzolites which characterised by inequigranular and porphyroclastic textures.The basaltic rocks in composition belong to the potash subseries of alkaline series and are characterized by higher CaO, K_2O+Na_2O and TiO_2, and lower SiO_2 and Al_2O_3. Except for enrichment of the incompatible elements, such as P, K and Ti, the basalts, have higher Al_2O_3/CaO ratio, with M value=60-70, index of consolidation nearly to 40 and degree of melting=4％. All these features indicate that the basaltic rocks are product driven from the mantle peridotite.The fusible components of the ultramafic inclusions are similar to that of fertile mantle. The olivines and the orthopyroxenes in the ultramafic inclusions have lower Fo and En respectively, and the clinopyroxenes are characterized by higher Al_2O_3, TiO_2 and Na_2O. The calculated temperature and pressure for the lherzolites are confined to 1050℃ and 21.6×10~8 Pa respectively. The basaltic rocks have a high REE abundance with enrichment of LREE, and La/Yb=15.63. The REE abundance of ultramafic inclusions approaches to that of the primary mantle.