南京南部娘娘山碱性火山杂岩及其成因

<正> 地质概况 娘娘山碱性火山杂岩体位于下扬子火山岩带宁芜断陷盆地中段的西翼,临近北北东向和北西向基底断裂的交汇处。杂岩体由含有假白榴石、黝方石的熔岩、火山碎屑岩和火山一侵入岩所组成,出露面积约15平方公里。碱性火山喷发杂岩不整合于粗面安山质火山岩之上。碱性火山碎屑岩内含有粗面安山岩和安山岩的角砾,在某些地区还见到碱性次火山岩侵入于粗面安山岩。宁芜地区安山质火山岩同位素年龄为136—114×10~6年,而娘娘山碱性火山杂岩年龄为105.7—91×16~6年。这都证实碱性火山杂岩为安山质岩浆火山活动之后(属晚白垩世)的产物。

PBTROLOGY AND ORIGIN OF THE ALKALINE VOLCANIC COMPLEX OF NIANGNIANG SHAN, NANJING

The alkaline volcanic complex of Niangniang shan represents a volcanic caldera of explo-sion and collapse origin. It consists of crater-neck, near crater, subvolcanic and volcano-intru-sive facies, covering an area about 15 km2. The petrologic facies and rock types of this volcaniccomplex are complicated but their compositions are similar. They are chiefly corresponding tothe transational types from trachyte to phonolite, and similar to those rocks occurring in islandarcs. They belong to the potassium. alkaline rock series, but their alkalinity is somewhat lower than those of island are. The rock types of this complex, the essential rock-forming minerals (noselite, pseudoleucite, augite, potash feldspar, plagioclase and biotite) and the geochemistry of some elements all have distinct characteristics, which not only reflect the different conditions of the various facies formation but also indicate the trend of evolution from the early explosion cycle to the later explosion cycle. On the basis of the geological environment of the eruption of the alkaline magma, its evolution history, the chemical continuity from andesitic to alkaline magma, the Rb/Sr ratio and Ce/Y ratio, together with the consideration of the formation history of the alkaline volcanic complex in the andestic formation was derived from differentiation of the basaltic-andesitic magma. There perhaps existed two trends of magmatic differentiation in the Lower Changjiang (Yantze) volcanic basin, resulted in the formation of an alkaline magma (e.g., volcanics in the Nanjing-Wuhu, Lujiang and Jinzhai basin) and an acidic (e.g., volcanics in the Fanchanag, Huaining, Liyang and Lishui basins), both being derived from the andesitic magma. These two lines of magmatic evolution could have been related to the nature of basement of the volcanic basin, the depth of faults and their activities.