摘要
雷州半岛地区新生代火山岩由中更新世和晚更新世早、晚两期组成。两期分别以田洋、青铜洋火山岩和湖光岩、乌石岭等地为代表。岩性以石英拉斑玄武岩、橄榄拉斑玄武岩和橄榄玄武岩为主,而碱性橄榄玄武岩仅在田洋、青铜洋破火山口附近发现。岩石化学成分从早期到晚期有碱度降低,酸度增加的趋势,即使同一期岩石也具有这种变化规律。与中国东部玄武岩平均成分相比,该区玄武岩酸度偏高,而碱度偏低,这与一般认为由大陆边缘向大陆内部碱度变大,酸度变小的结论是一致的。微量元素总含量较低,分离程度较差,与夏威夷拉斑玄武岩的微量元素分配型式相似。从玄武岩岩石化学成分研究表明,具有大陆玄武岩特征,可能形成于较稳定的构造基楚上,以断裂为主的构造活动期,应与岛弧中部的构造性质类似。超镁铁岩包体为饱满型包体,与理想地幔橄榄岩成分相近,表明其熔融程度较低。用Mercier(1976)方法估算超美铁岩包体的平衡温度和压力,分别为1100～1200℃,15～20kbar,表明包体来自上地慢,深度为50～70km,以此可推测含包体玄武岩岩浆的来源深度.
The Cenozoic volcanic rocks of the Leizhou peninsula are composed of two periodic volcanic rocks.Middle Pleistocene and Late Pleistoce- nes.They are represented respectively by volcanic rocks exposed at Tianyang & Qintongyang,and Huguangyan & Wushiling.The main rock types are Qz-tholeiite,Ol-tholeiite and Ol-basalt.Alkaline Ol-basalt, however,is found only near a broken volcanic vent at Qintongyang. The chemical composition of the rocks has indicated that there is a tendency of reduction in alkalinity or of rise in acidity from the early to late stages.The tendency is even also shown in the volcanic rocks of the same period.Compared with the mean composition of basalts from East China the lower alkalinity and higher acidity of the basalts in this area are consistent with the widely accepted conclusion that the alkalinity increases while the acidity decreases from a continental margin to the inside of the continent.The microelement distribution in the basalts of this area is similar to that of Hawaiian tholeiites in having a lower content and a poorly separated degree. The study of the chemical composition shows that the basalts have the features of continental basalt,probably formed in a tectonic envi- ronment which is similar to that of central part of an island-arc,hav- ing a more stable tectonic basement during the tectonic movement period dominated by faulting. The inclusions in ultramafic rock belong to the fertile type,being similar in composition to the ideal mantle peridotite.This indicats their melted degree is lower.Estimated by using the Mereier's method the balance temperature and pressure of the inclusions are 1100-1200℃, 15-20 kbar respectively.This shows these inclusions came from the upper mantle and were formed at a 50-70km depth.Besides,the depth from which the inclusion-bearing basaltic magma comes can be calculated.
