Dyke morphology as the reflection of dyke intrusion conditions (examples from the Palaeoproterozoic Gridino dyke swarm)

Gromov I.V.

IG, KarRC, RAS, Petrozavodsk, Russia

Mafic dykes, massifs and their fragments (drusites) are widespread in the Belomorian province of the Fennoscandian Shield, mafic fragments being dominant (Stepanov, 1981).

A dyke swarm was discovered in the Gridino area, central Belomorian province (Stepanov & Stepanova, 2005). As the multiple Svecofennian tectonic processes in the Gridino area were not vigorous, the dyke swarm is well-preserved (Fig. 1).

Available geological data, such as direct intersections of compositionally different dykes and dyke-host rock relationship, have provided the basis for distinguishing seven groups of dykes in the Gridino swarm (Stepanov & Stepanova, 2005): olivine gabbronorite dykes of a lherzolite-gabbronorite complex; subalkaline metagabbro; symplectitic metagabbro dykes; high-Mg and high-Fe metadiorite dykes; Vorotnaya Luda coronitic gabbro dykes; and Izbnaya Luda coronitic metagabbro dykes. Each group has distinctive composition characteristics of its own (Stepanov & Stepanova, 2005, 2006, 2009). Olivine gabbronorite dykes of a lherzolite-gabbronorite complex are most common in the Gridino swarm. They form a large part of the ~5 km wide NW-trending swarm traced over ~ 10 km. The gabbronorite dykes vary in thickness from several centimetres to 100 m. One dyke of the complex, located on northeastern Vorotnaya Luda Island (Fig. 1. А), is well-exposed and was, therefore, used for studying the structural characteristics of the dykes.

Fig. 1. А. Gabbronorite dyke distribution in the Gridino swarm

(Stepanov & Stepanova, 2006, simplified version)

B. Structural scheme of a gabbronorite dyke from Vorotnaya Luda Island with a thickness profile (1 = exposed dyke areas; 2 = dyke areas covered with Quaternary sediments; 3 = thickness profile).

The dyke studied consists of fine-grained poorly amphibolized gabbronorites. The rock is coarser-grained in the central, thickest, portion. In the endocontact zone, the gabbronorites are finer-grained and amphibolized. Chill zone relics are common there.

The dyke is 97.5 m long. Its exposed portion is ~ 58 m long. The dyke strikes persistently over the interval 325-340° and dips steeply (~ 80°) NE. At the same time, areas that dip more gently (~ 45°) are encountered in the southern portion of the dyke. The dyke varies in thickness from 2 to 32 cm, as shown in more detail in the thickness profile (Fig. 1 B).

The northern and central portions of the dyke form an integral, practically rectilinear body, which falls into two subparallel bodies in its southern portion. One of the bodies has an articulate bend (Fig. 1. B). Furthermore, bends and pinches that vary in amplitude occur at the southern end, the general dyke orientation being preserved. In the central portion, there is an S-shaped bend, ~ 4 m in length and ~ 50 cm in amplitude, and an area, where the dyke is in undulating contact with host rocks. The dykes on Izbnaya Luda Island are structurally similar (Travin & Dokukina, 2005).

The dyke cuts gneissosity at a near-right angle. The gneissosity commonly passes all the way from the eastern contact to the western one with a shift, whose length depends on dyke thickness and the orientation angle of the gneissosity, the orientation of the gneissosity in the direct contact zone being unchanged. However, the gneissosity is folded on a small-scale in some small country rock zones near the dyke contact, as, for example, in the S-shaped bend area.

Twenty-two apophyses, characteristic structural elements, were revealed in exposed dyke areas. They fall into two groups: 1) apophyses practically parallel to the dyke body; 2) apophyses branching from the dyke body at an angle over 20° (up to 45°). Six apophyses of group 1 are located in the northern portion of the body; four of them are at the western dyke contact and two at the eastern contact. All six apophyses strike NE. Apophyses are scarce in the central portion of the dyke; the only apophyse of group 1 was revealed at the eastern contact. Apophyses are most abundant (seven apophyses of group 1 and eight apophyses of group 2) in the southern portion of the dyke. Four apophyses of group 1are at the eastern contact and three apophyses at the western contact. All of eight apophyses of group 2 are at the western contact. There, most apophyses, in contrast to those discussed above, strike SW.

Apophyses can indicate the orientation of the stress field constituent, in which the dyke was formed, and the angles at which the apophyses extend from the dyke body carry information оn the rheology of the environment into which the melt intruded (Goncharov et al., 2005).

Some signs, such as sharp contacts with host rocks, a regular elongate shape of the dyke and the presence of group-2 apophyses, suggest that magmatic melt intruded into relatively cold and rigid country rocks. The dyke was formed along a tension joint, as shown by an undulating contact, characteristic of tension joints (Goncharov et al., 2005), and the character of its relationship with gneissosity. Variations in the angle at which apophyses branch from the dyke suggest that the melt intruded into a heterogeneous environment (the angle is ~ 45° for brittle rocks and less than 45° for plastic rocks). This assumption is also favoured by bends and pinches of varying amplitude revealed in the southern portion of the body (upon dyke formation, the southern end of the dyke intruded into a rheologically heterogeneous zone). A general trend of the thickness profile suggests that pressure, which arose from the widening of the tension joint by magmatic melt, was uniform (Hoek, 1994). Maximum pressure values were estimated in the central portion of the dyke body, where the dyke thickness is maximum.

Basic conclusions: 1) Magmatic melt intruded into relatively cold and rigid country rocks. However, the environment, into which the melt intruded, was rheologically heterogeneous; 2) The magmatic melt intruded along a tension joint.

The author wishes to thank A.I. Slabunov, V.S. Stepanov and A.V. Stepanova of the Institute of Geology, KarRC, for their contribution to the study and discussion of results.

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