On the journey descending from the summit (although you may notice it in the dark while ascending) you will come across white streaks of what might appear to be paint, cutting across the grey granite close to the summit of the mountain. These are strikingly seen from the path to Low’s Peak, with an abundant light coloured straight-sided bands of rock crossing the granite. Sub-vertical narrow planar bodies of intrusive rock are called dykes, and sub-horizontal planar bodies are called sills. On the face of St John’s Peak there are abundant dykes and sills, and similar bodies are observed in many places. They are chilled against the granite, are typically white and very fine grained, and are composed mainly of quartz and feldspar when identified under the microscope or by using X-ray diffraction techniques. These are aplites.
These are aplite dykes. Aplite itself is a fine grained acid igneous rock composed predominantly of quartz and feldspar and therefore white in colour. Aplites represent the final fraction of the granite melt and on Kinabalu are common as dykes intruding the older granites
Aplite and granite are similar in composition but differ greatly in grain size. Granite formed before and its crystals as it seems had ample time to grow. The vein intruded the already solidified granitic rock body and solidified quickly due to rapid heat loss. It forms a narrow intrusive rock body within other rocks. Aplite is fine-grained because it solidified relatively quickly due to rapid heat loss to the surrounding cooler countryrock.
Aplites are often associated with pegmatites. Pegmatitic magma is rich in volatile constituents which makes it significantly less viscous. Chemical elements are relatively free to move in such magma which means that once formed crystals can keep growing because necessary building blocks are right there. When such magma loses its volatiles because they migrate higher, it gets much thicker and solidifies as normal granite does. However, because it loses heat rapidly, very fine-grained texture forms instead of normal appearance of granitic rock.
How to claim this cache
1. Describe the Aplite Dykes seen here - how do they differ from the surrounding granite?
2. Why do the Aplite Dykes on Mt Kinabalu appear white in colour?
3. Observe the Aplite here. Is it fine or coarse grained. Explain why.
4. Take a photo of yourself near South Peak or St John's Peak