宁芜地区中生代火山岩地层划分及其特征

宁芜地区中生代火山岩的地层划分,自上而下为:娘娘山组、山边村组、姑山组、大王山组和龙王山组,并可分出12个岩性段。本区的火山喷发属于裂谷环境,岩石具跨式分异趋势,其中大王山组是裂谷拉张、岩浆演化的特殊阶段,钠质同化混染强烈,促成了铁质活化、转移和富集,从而成为本区玢岩铁矿的主要成矿期和控矿岩。

STRATIGRAPHIC DIVISION OF MESOZOIC VOLCANIC STRATA AND THEIR CHARACTERISTICS IN NANJING-WUHU AREA

In this paper some additional materials are given on the geologic setting, especially on the characteristics and stratigraphic division of the volcanic strata, for mineral-forming pattern of so-called porphyrite iron ore deposit which was established in the area and published in 1977.Conventional stratigraphic division of normal sedimentary rocks or interbeds as the boundary marks can not be always for dividing volcanic formations and their lithologic members perfectly suitable in the area. So a special division method is suggested in this paper in consideration of volcanic structures, petrofacial series and petrologic characteristics of lavas, combining with some other ways, shch as digital processing of satellite images, interpretation of infrared spectra, multi-element statistical analysis and computer processing. Besides, the geological terms, such as conformity, parallel and inclined unconformities, which are suitable to sedimentary region, also can not really express the complex contact relations between volcanic rocks. So the author suggests some terms, such as eruption unconformity, extrusive unconformity, mosaic eruption unconformity and blurred intrusive contact, etc.From the studies, using the above-mentione 1 method, the volcanic stratigraphic sequence may be divided in descending order into 5 formations, namely Niangniangshan, Shanbian cun, Gushan, Dawangshan and Longwangshan formations, and 12 lithologic members. Furthermore, all their typical sections are shown in Fig. 4-8, and such geological characteristics are listed in Table 1-3.Some rock- and mineral-forming environments are also discussed in the paper. They are:1) From the data, as projected points of 98 rock samples concentrated in region C in the logτ-logσ diagram, the curves of chondrite-normalized REE distribution pattern in the volcanic rocks in this area are declined to the right, belonging to a type rich in light REE, and the differentiation trend of these volcanic rocks may be considered to be of the Kenedy type for Niangniangshan and Dawangshan formations, and Coombs type for Shanbiancun and Gu-shan formations. But on the whole the sequence seems to show a straddle trend. All the evidence indicates that the volcanic rocks in this area might be originated in a continental rift. 2) The Dawangshan formation might be formed in a specific stage of the whole mag-matic evolution, during which intense sodium assimilation and mixing occurred, while the iron ore has got a condition for mineral concentration through the activation and transfer of Fe element. So it is the main epoch for mineral-forming for the porphyrite iron ore deposits in the area.