Case Study 1: Granite–Gneiss Contact in the Yaoundé Area
While mapping a 4 km² area 30 km north of Yaoundé, your team records a pink granite body about 1.2 km across enclosed in grey gneiss. Gneiss foliation strikes 060° and dips 35° NW. At three of four contact outcrops the granite contains angular gneiss blocks 10–40 cm long, and the gneiss foliation is truncated at the contact. One outcrop shows a 15 cm fine-grained granite margin; the other three show no chilled margin. A 2 m wide dolerite dyke striking 150° cuts both granite and gneiss, and the gneiss within 20 cm of the dyke is baked and reddened. A quartz vein is displaced 3 m across a fracture striking 020°; slickensides plunge 25° south. The field budget covers three more days and 12 samples. Your supervisor asks whether the granite contact is intrusive or a fault, and where to concentrate the remaining work.
1. Decide whether the granite–gneiss contact is intrusive or tectonic. Justify your conclusion using at least three independent lines of evidence from the setup, and state one piece of evidence that would overturn it.
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Conclusion: intrusive. Three independent lines:
- Xenoliths: angular gneiss blocks inside granite mean the granite surrounded pre-existing gneiss. A fault would produce rounded or shattered clasts in a fault rock, not isolated blocks.
- Truncation: gneiss foliation stops at the contact; an intrusive contact cuts across earlier structure.
- Chilled margin: the 15 cm fine-grained margin is a cooling texture against colder country rock. Its absence at three outcrops can reflect deeper erosion, later heating, or poor exposure; it does not overturn the other evidence. A fault would require a mapped shear zone, mylonite or breccia, slickensides, or offset markers along the contact; none is described. Overturning evidence: a continuous 1–2 m wide mylonite along the contact, with foliation parallel to it and shear-sense indicators.
2. Place the granite, dolerite dyke, quartz vein and 020° fracture in order of formation, giving your reasoning. State any age relationship that the evidence does not prove.
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Oldest to youngest: gneiss → granite → dolerite dyke → quartz vein → 020° fracture.
- Gneiss is foliated and contains the other bodies, so it is basement.
- Granite cuts gneiss and contains gneiss xenoliths, so post-gneiss.
- Dolerite cuts both granite and gneiss and bakes gneiss, so post-granite.
- Quartz vein is offset by the fracture, so the vein predates the fracture.
- The fracture is youngest because it displaces the vein. Uncertainty: the setup does not say whether the quartz vein cuts the dolerite, so the vein–dyke order is not proven. Do not overstate it on the map; use a dashed relative-age boundary or note “vein age uncertain relative to dyke”.
3. You can map only one structure in detail in the remaining three days. Choose either the granite–gneiss contact or the 020° fracture, and justify your choice with reference to producing a geological map and assessing resources.
Read the worked answer
Map the granite–gneiss contact. Reasons:
- It is the largest map feature (1.2 km body) and controls the map pattern; the fracture is a single line.
- The contact determines the distribution of xenoliths and any contact-metamorphic or mineralised zone; sampling 12 sites along it can test for related veins.
- The 020° fracture is already quantified (3 m offset, slickensides), so extra mapping adds less new information unless the aim is hazard assessment. Caveat: if the area has seismic risk or a road or foundation crosses the fracture, map the fracture instead and treat the contact as background. State the aim before choosing; for a resource map, the contact wins.