Abstract

Based on field investigation and microscopic study, the Late Paleoproterozoic mafic dyke swarms in the western Shandong, Eastern Block of the North China Craton (NCC) have been researched. These dykes have similar features with those in the northern Shanxi and Luliang areas of the NCC.

(1) Their strikes change gently from NS-trending or even NNE-trending in the southern part to NNW-trending in the northern part, and are vertical to the Yanliao-Zhongtiao aulacogen if eliminating the influence of the extension of Bohai Basin;

(2) The sequence stratigraphy and isotope dating indicate that their ages range from 1.83Ga to 1.71Ga in the Late Paleoproterozoic. These undeformed and unmetamorphic dykes are vertical and their compositions are typical diabase;

(3) The calculation results of extension ratio show that the average extension ratio of the crust at that time was 0.43%;

(4) The chilled margins of these dykes have reserved abundant flow structures which show the characteristics of the magma activity including the direction and way of the dyke emplacement. For example, the bottom of the dyke is not batholith or stock that shows the magma may not intrude vertically from below. Most of the dykes extend from NNW to SSE for hundreds of meters or even tens of kilometers, and their widths may diminish gradually and pinch out at last. There are some dyke branches in the southern segments of the dykes and intruded into the country rock along the dominant orientation. These features of the dykes indicate that the magma flowed from NNW to SSE.

There are also some flow structures in the contact zone between the dyke and the country rock, such as scour marks, mineral lineations, orientational xenoliths and so on, which are formed by the magma flow friction. The orientations of these marks were controlled by the laminar flow of the magma, and could reflect the direction of magma flow.

The flow of magma inside fractures is helpful for us to understand how continental dyke swarms developed. The research on the dyke swarms of the NCC indicate that the paleostress field offered the space for the magma and the flow structures and also indicate the magma intruded subhorizontally from the aulacogen to the pre-existing fractures.