初谈渤海周边地区的岩浆活动

为了给地震地质工作提供地质背景资料,我们对渤海周边地区构成结晶基底的前震旦纪变质岩系、地质构造及各时代的岩浆活动做了初步分析和探讨。一、本区构造单元的划分本文所涉及的范围大致在北纬36°至42°、东经114°至123°之间。位于中朝地块的中部。区内较著的深断裂有:1.康保-围场-赤峰深断裂,2.赤城-隆化-建平深断裂,3.昌平-遵化-迁西深断裂,4.太行山东侧深断裂,5.沈阳-郯城-庐江深断裂。其中有的已被查明为航磁异常带和重力梯度带。

A PRELIMINARY STUDY OF IGNEOUS ACTIVITIES IN THE REGION AROUND BOHAI SEA

Taking into consideration Pre-Sinian crystalline basement structures in the region around Bohai Sea this paper gives a synthetic analysis of igneous activities of various geologic ages and draws the following conclusions.1. Basic and ultrabasic magmatic activities might have taken place, three times during the Pre-Cambrian (Ari, Pti, Z). According to rock-associations and petrochemical features the ultrabasic intrusives of the first stage may be divided into two kinds, corresponding respectiveily to the Alpine type and Alaska type. .2. Four volcanic eruptions probably took place in the Mesozoic (J1, J2-3, K1, K2) arranged petrographically in succession as basalts, basaltic andesite, andesite, acid and subalkalic volcanics and andesite, all belonging to the lime-alkali series. It may be stated as a general rule that the Mesozoic volcanics are characterized by 1) the increase in the content of potash and alkali, 2) the greater K/Na ratio and 3) the higher FeO(Fe2O3×0.899+FeO) content, as one goes from the Okhotzk Sea margin and the southwestern part of Japan toward the Great Xingan (Khingan) range, including Yinshan and Yenshan subzones. The Mesozoic granite intrusives are evidently controlled by the NE-NNE trending structures (chiefly faults and locally folds). These granites petrographically belong to rock-association of the transitional type between the series of alkali rocks and that of calc-alkalic rocks, which differs entirely from calc-alkalic rocks occurring in the Cir cum-Pacific island arc. Another charac- teristic of this series is its higher content of potash (generally about at 4-5%).3. The Paleogene (Paleocene and Eocene) basalts are found beneath the North China Plain and the Songliao Plain, while Neogene (Pliocene) basalts ooccurred in the Yinshan, west Shandong and Jiaoliao faulted upwarpings. The Quaternary (Pleistocene) basalts occur in mountains bordering the North China Plain and Songliao Plain. Beneath the plains mentioned above Tertiary basalts are intermediate between the alkalicbasalt series and tholeiite series but basically belonging to the latter. This rock series is characterized by the moderate content of alkali, poor in lime and alumina, higher Mg/Ca ratio, as well as weakly saturated silicon or weakly unsaturated silicon. Tertiary basalts in mountainous areas are basically similar in petrochemical features to those of the plains, but they locally belong to the alkalic basalt series. The Quaternary basalts are only weakly alkaline, although they belong to the alkalic basalt series.