Where is Granite with Mafic Dike found?
Granite (felsic intrusive igneous rock) with mafic dike (igneous intrusion)
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Open the appGranite with Mafic Dike occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Granite with Mafic Dike
Sierra Nevada Batholith, USA
Numerous examples of mafic dikes cutting through granitic rocks are observed throughout the Sierra Nevada, California, providing insights into the post-emplacement tectonic history.
Scottish Highlands, UK
The Caledonian granites are frequently intruded by later mafic dikes, particularly in areas like the Isle of Skye and other parts of the Hebrides.
Fennoscandian Shield, Scandinavia
Ancient granitic terrains often exhibit extensive dike swarms, including mafic dikes, reflecting prolonged periods of crustal extension.
Canadian Shield, Canada
Vast exposures of Precambrian granites and gneisses are commonly cross-cut by mafic dikes, some of which are part of large regional dike swarms.
Collecting tips
- Look for Contrasting Colors and Textures. The most obvious feature is the sharp contact between the light-colored, coarse-grained granite and the dark, finer-grained mafic dike. This contrast is usually visible even from a distance.
- Observe Cross-Cutting Relationships. Dikes are intrusive bodies that cut across pre-existing rock layers or structures. Look for linear features of the darker rock cutting through the lighter granite. This is a fundamental principle of relative dating in geology.
- Examine Grain Size Variations. The mafic dike may show finer grain sizes at its margins (chilled margins) where it cooled rapidly against the cooler granite, and coarser grains towards its center.
- Check for Xenoliths or Apophyses. Occasionally, the dike may contain xenoliths (fragments) of the host granite, or send out small offshoots (apophyses) into the granite, further confirming the intrusive relationship.
Quick Check
- Color:
- Light (granite) with dark (dike) linear intrusion
- Luster:
- Vitreous to dull (granite minerals), dull to sub-vitreous (dike minerals)
- Streak:
- White to colorless (granite minerals), gray to black (mafic dike minerals)
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