南京地区新生代玄武岩的期次、层序及时代

THE DIVISION AND AGE OF BASALTS IN THE NANJING AREA

在研究晚第三纪地层层序的基础上,结合玄武岩的古地磁及K-Ar稀释法年龄测试,把南京附近新生代玄武岩划分为三期五次,即早第三纪石金山玄武岩,晚第三纪六合组玄武岩、黄岗组玄武岩、方山组玄武岩,早更新世桂枝山玄武岩。其中六合组玄武岩在距今大约14Ma前喷发,古地磁属反极性;黄岗组玄武岩在距今大约10—12Ma前喷发,古地磁属正极性;方山组玄武岩的磁性特征与吉尔伯特反向极性世大致相当,喷出年龄大约在6—3.5Ma之间。

The upper Tertiary sand-gravel beds near Nanjing consist of the Dongxuanguang Formation, Luhe Formation, Huanggong Formation and Ynhuatai Formation. The basalts related to these sand-gravel beds involve five eruptions. (1) Shijingshan basalt, an olivine-bearing basalt, characterized by a higher SiO_2 content (>54%). It overlies volcanic rocks of J_3-K_1 age and underlies the Yuhuatai Formation. Its isotopic age is about 55—65Ma. (2) Luhe basalt, a greyish-black, compact alkali-olivine basalt with a cryptocrystalline texture. It occurs in greyish-green mudstone on top of the Luhe Formation. The polarity is reversed and its isotopic age is about 14Ma. (3) Huanggong basalt, a greyish-black alkali-olivine basalt with a porphyritic texture. It appears in the middle and upper parts of the sand-gravels of the Huanggong Formation and covered by the Fangshan basalt or the Yuhuatai-Formation sand-gravels. It shows normal polarity. Its isotopic age is about 10—12Ma. (4) Fangshan basalt, a greyish-black alkali-olivine basalt with medium- to thin-grained texture. It overlies the sand-gravel beds of the Huanggong Formation and is covered by the Yuhuatai Formation. Its polarity is reversed and the isotopic age is about 5.9Ma. Therefore its age can be correlated with the Gilbert epoch. (5) Guizheshan basalt, a titanifernus augite basalt. It overlies the Yuhuatai Formation at the Guizheshan section and is overlain by the Xiashu loess(Q_2x). The basalts isotopic age is about 1.0Ma. On the basis of the above information, the authors consider that the Shijingshan basalt of the first eruption can be correlated with the Funing Formation basalt in northern Jiangsu, whose age belongs to early Tertiary. The basalts of the second, third and fourth eruptions are of the same origin and are the late Terti ary product, whose ages are assigned to Miocene and Pilocene, respectively. It is advisable to assign the Guizheshan basalt of the fifth eruption to early Pleistoce ne. Therefore, there are five eruptions belong to three phases for the Cenozoic basalts near Nanjing.