PETROGRAPHIC ANALYSIS OF THE ROCKS OF REPULSE BAY, VICTORIA ISLAND HONG KONG:

FORMERLY "OCCUPIED CHINA" NOW PEOPLES REPUBLIC OF CHINA

The rocks around the Repulse Bay area, Hong Kong, are volcanic in origin and rhyolite in composition. Various rock samples were taken from different locations around the area and were petrologically analysed. The rock samples were sliced into thin-sections and examined under a petrographic microscope to determine their composition. It is thought that, in the Jurassic age, about 175 million years ago, the area was once a site of a large volcano.

Previous researches of the geology of Hong Kong such as Davis (1952) and Allen and Stephens (1971) have considered that the Repulse Bay rocks are acid-volcanic rhyolite (see Glossary of Terms Cited) in composition. The rocks of nearby offshore Lamma Island are considered granitic. All the rocks of the area are considered Jurassic in age which is about 175 million years ago (Davis, 1952). This study sought to confirm these findings.

Samples were collected from six locations at rock outcrops around the bay area. At each location a photograph was taken of the exposed rock face and a small sample of the rock was selected and numbered. The first sample was collected at a location on Belleview Road, high on the western side of the bay. The second sample was collected also on the western side of the bay, at an exposure on Repulse Bay Road. The third sample was collected at an exposure on the far eastern side of the bay at South Bay beach at the end of South Bay Road. Three loose rock samples were also taken of different rocks found on South Bay beach. Two further samples were taken at outcrops along South Bay Road. A sixth sample was taken at a location below Hong Kong International School. Two further samples were obtained from Lamma Island near the shoreline when visited by boat.

     All rock samples were labeled, packaged, and made into thin-sections. Thin-sections are wafer-thin slices of rock glued onto glass slides that can be viewed under a petrographic microscope to determine the composition. Photographs of each rock thin-section were made using this petrographic microscope. These photographs of the rock composition were then identified by comparing them to similar rock thin-section photographs found in a textbook of igneous rocks (Mackenzie et al, 1982). The comparisons were made by comparing the shape, size, and color under different types of light, of the crystals of the thin-sections. For example, Dolerite forms long, narrow crystals that appear mainly black and white under plane polarized light, and blue, purple, and orange under crossed polars (Far left, and left)

It is considered that the collected rock samples are representative of the exposed rock around Repulse Bay.

 It was noted that there was a great similarity in all the samples, except for certain samples obtained from the shoreline at South Bay beach and Lamma Island. The major difference between these samples was that of color. It is considered that this color difference is due to weathering. Under the petrographic microscope, the rock thin-sections the samples appeared similar, except for the dolerite and granite samples from Lamma Island and one granite sample found on South Bay beach. (Right and far right)    

From the evidence obtained from the petrographic examination of rock thin-sections it was determined that all the rock samples, except three, were similar, and that these rocks were acid-volcanic in origin and rhyolite porphyry in composition (JPG Images:RB1-4: Petrographic technique: micrographs are taken at 90 degrees to each other in crossed-polarized light of the same specimen), which confirms the findings of Davis (1952). Of the three different samples, one from Lamma Island and one from South Bay beach were granitic, and the third also from Lamma Island was a dolerite. The granitic beach sample may have been swept by storm waves from a different area and deposited on the beach. Therefore, this study confirms the previous work of Davis (1952) and Allen et al (1971) that the rocks of Repulse Bay are volcanic in origin. Figures left to right, below.

       

 

REFERENCES

GLOSSARY OF TERMS CITED