Mechanism of dyke emplacement

The chilled margins of these dykes have reserved abundant flow structures, such as scours, mineral lineations, orientational xenoliths and so on. The orientations of these marks were controlled by the laminar flow of the magma, and could reflect the characteristics of the magma activity including the direction and way of the dyke emplacement (Hou et al., 2003).

Morphology of individual dyke

Most of the dykes in the EB are vertical and pinch out downward, and the bottom of the dyke is not batholith or stock, as a result 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.

Scour marks

As the high temperature magma intrudes into the channel, the contact part with the cold host rock cools faster than other part, resulting in increasing the viscosity and decreasing the velocity of this part. As a result, the middle part with higher velocity is hindered by the contact part causing scour marks on the inner wall of the dyke. These scour marks are wedge or tabular shaped grooves, and mostly not smooth. Besides, if the contact part cools fast enough, scratches may be caused on the inner wall of the dyke. The orientation of the scratches and the heads of the scour marks indicates the flow direction of the magma.

The dykes in the EB have plenty of scour marks and scratches, which indicate that the magma intruded subhorizontally.

Mineral lineation

Due to the difference of the velocity between the middle and the margin of the magma, simple shear may happen resulting in the rotation and orientation of some columnar or tabular minerals, such as plagioclase. The acute angle between the long axis of the minerals and the chilled margin of the dyke points an opposite direction with the magma flow. In addition, the long axes of the phenocrysts plunge to the magma source at about 20°.

The dykes in the EB developed abundant orientated phenocrysts, and these flow microstructures have no trace of tectonism. The observation of thin sections suggests that the magma flow direction indicated by these flow structures is from the North to the South.

Orientational xenoliths

As has been discussed above, the dyke swarms in the EB intrude pre-existing tensional fractures. In this process, the broken fragments of the host rocks may be captured by the magma, and become xenoliths. Similarly as the rotation and orientation of the mineral lineations, the long axes of the xenoliths indicate the flow direction of the magma.

The dykes in the EB also developed some orientated xenoliths. The field evidence suggests that the magma flow direction indicated by these flow structures is from the North to the South.

The flow of magma inside fractures is helpful for us to understand how continental dyke swarms emplace. The research on the dyke swarms of the EB indicates that the paleostress field offered the space for the magma, and the flow structures of the dykes indicate the magma intruded subhorizontally from the North to the South.

Figure 3. Flow structures of the dyke

Flow structures of the dyke

(a) Scour marks. (b) Mineral lineation. (c) Orientational xenoliths. (d) Orientational xenoliths under microscope.