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This paper is about a report on a dyke feature found at Binge Binge point during stop6 (day4) at a clear Hill Point. The feature consists of a dark colored dyke cutting through the tonalite. The eastern side of the dyke conveys an onglvar basalt that suggests that the basalt there was almost solid hence resulting to the material fragmenting into angular pieces. The scenario suggested that ejecting the solidified material of the basalt from the host material, would require an extremely enormous amount of force.
The western end of the dyke exudes a basalt dyke that had intruded partially molten tonalite magma. At this end, the basalt is still hot hence some mixing between the basalt and tonalite. This end consists of an intact belt shaped basaltic dyke and loose pieces of gloopy basalt.
The purpose of this paper is to explain the formation of the dyke, discussing in particular the nature of the host granite at the time of the intrusion.The dyke feature at Binge Binge point satisfies the ‘Theory of chocolate tablet boudinage whereby a competent (high relative strength) layer is segmented into equine dimensional structure by a layer parallel extension into two directions. The boudinage can be considered as a slip boudinage due to the break ups in the parallel extensions that are belt shaped in forming.
The Plutonic Order suggests that the Tonalite was there first, soon followed by the diorite magma. During the formation, both the Diorite and Tonalite are partly molten. (GRIFFIN, 1978)
The Diorite and Tonalite do not mix due to viscosity differences and the limited time they had before becoming fully cooled and solid. The Diorite is denser, and hence it sinks into the tonalite. The Tonalite is ejected back up into the diorite due to limited space (volume). Elongated enclaves are formed which are 10 -15 cm long, and fairly abundant. There is a foliation within the tonalite enclaves. This foliation was NOT caused by a deformation event (folding), thus the diorite magma was still flowing when it mingled with the tonalite magma and cooled.
The Dyke Order suggests that older dykes are cut by younger dykes. This is displayed in (R.A, 1996) where Dykes are listed in order from the oldest to youngest:
- The North-South Basaltic dyke where there was evidence of similar mineralogy to the plutonic material and finer grained at the margin than the centre due to quenching.
- The Micro-granite dyke Contains Quartz Tourmaline nodules (Tourmaline and quartz in the centre with a ring of quartz bleaching on the outside) which are approximately 2 cm wide suggesting they occurred in the late stage of magma cooling.
- The Dacite dyke contains mineralogy consistent with mostly of plagioclase feldspar with biotite, hornblende, and enstatite. It contains quartz elements, corroded phenocrysts, or as a ground mass element.
- The South-west and North-east dykes are Basalt dykes, and hence, assume all other Basaltic dykes that run in the same direction.
- Andesitic and Basaltic Andesitic Dykes are Assumed to be the youngest, but we cannot tell, as they are not cut by another dyke or cut through other dykes.
The above feature is a ubiquitous micro granular enclave due to its overall fine grain size (0.2 to 1 mm) and abundant acicular apatite inclusions in plagioclase rims are consistent with a period of rapid cooling of enclave magma. (Nelson, 1998)
The magma flow extended from the far east of the feature. It also extended parallel south east and up parallel to North West forming ‘chocolate tablet badinage’. This is affirmed by the solidification process whereby, while the west end the basalt dyke was still hot, the east end basalt was almost fully solidified, even beginning to fragment into angular pieces. These results conclude that the magma flow had begun from the east since the solidification process was already being concluded there while still in early stages at the west side. The difference in material temperature and mixture also confirms that the tonalite was there first, followed by the diorite magma. This is reaffirmed by the ‘cross cutting relations’ theory which dictates that the geological feature that cuts another is the young of the two features.
After the magma flow had been concluded, the basalt dyke intruded partially molten tonalite magma. On the western end of the dyke, the basalt was still hot, allowing some mixing between the basalt and tonalite due to the difference in viscosity and density. An irregular shaped ball and pillow structure formed at the western end. The eastern end of the dyke was almost solid and the material fragmented into angular pieces. Ejecting this solidified fragment of the basalt from the host material would have required an enormous amount of force. (VERNON R.H, 1988)
The margin of the dyke is finer grained than the center due to quenching.