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Granite with a mafic dike is a geological feature consisting of a light-colored, coarse-grained felsic intrusive igneous rock (granite) that has been intruded by a darker, finer-grained mafic igneous rock (the dike). The granite typically exhibits a phaneritic (coarse-grained) texture, composed predominantly of quartz, feldspar (orthoclase and plagioclase), and mica (biotite and/or muscovite), giving it a speckled appearance. The mafic dike, which cuts across the granite, is typically dark gray to black, with a fine-grained (aphanitic) to medium-grained (ophitic or subophitic in diabase) texture. Its composition is rich in ferromagnesian minerals such as pyroxene and olivine, and calcic plagioclase feldspar. The dike's width can vary from centimeters to many meters, and its orientation provides clues about the regional stress field at the time of intrusion. The contact between the granite and the dike is usually sharp and distinct.
How to Identify
- Color
- Granite: Typically light-colored (white, pink, gray, or red) with dark specks. Mafic Dike: Dark gray to black.
- Luster
- Granite: Vitreous to dull for individual mineral grains. Mafic Dike: Dull to sub-vitreous.
- Texture
- Granite: Phaneritic (coarse-grained), interlocking crystals, typically equigranular. Mafic Dike: Aphanitic (fine-grained) to medium-grained (e.g., ophitic in diabase), often with chilled margins near the granite contact.
- Crystal Form
- Granite: Anhedral to subhedral crystals of quartz, feldspar, and mica. Mafic Dike: Typically anhedral, small crystals, sometimes porphyritic with phenocrysts of plagioclase or pyroxene.
- Cleavage
- Granite: Feldspars exhibit good cleavage (two directions at ~90 degrees), micas have perfect basal cleavage. Quartz has no cleavage. Mafic Dike: Pyroxenes have two cleavages at ~90 degrees, amphiboles at ~60/120 degrees. Plagioclase has good cleavage.
- Geological Environment
- Continental crust, typically in stable cratonic areas, mountain belts, or areas of past magmatic activity. Dikes form in extensional or shear stress regimes where fractures allow magma ascent.
Key Facts
- Hardness: Granite: 6-7 (Mohs, due to quartz and feldspar). Mafic Dike: 5-6 (Mohs, due to pyroxene, plagioclase).
- Specific Gravity: Granite: 2.6-2.7 g/cm³. Mafic Dike: 2.8-3.1 g/cm³.
- Crystal System: Granite: Monoclinic (feldspars), Hexagonal (quartz), Monoclinic (micas). Mafic Dike: Monoclinic (pyroxene, plagioclase), Orthorhombic (olivine).
- Color: Granite: White, pink, gray, red. Mafic Dike: Dark gray to black.
- Luster: Vitreous to dull.
- Transparency: Opaque to translucent (individual crystals).
- Fracture: Granite: Uneven to conchoidal (quartz). Mafic Dike: Uneven to sub-conchoidal.
- Cleavage: Granite: Good in feldspars and micas. Mafic Dike: Good in pyroxenes and plagioclase.
- Composition: Granite: Predominantly quartz (20-60%), alkali feldspar (35-90% of total feldspar), plagioclase, biotite, muscovite, hornblende. Mafic Dike: Predominantly plagioclase (calcic), pyroxene (augite), olivine, minor amphibole, biotite, magnetite.
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)
Physical Characteristics
- Crystal Habit: Granite: Granular, anhedral to subhedral. Mafic Dike: Fine-grained, sometimes porphyritic or ophitic.
- Cleavage Type: Granite: Two directions at ~90° (feldspar), perfect basal (mica). Mafic Dike: Two directions at ~90° (pyroxene), two directions (plagioclase).
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to dull.
Formation
Granite forms from the slow crystallization of felsic magma deep beneath the Earth's surface. This magma is rich in silica, alkali feldspar, and quartz. After the granite has solidified and cooled, subsequent tectonic stresses can create fractures or weaknesses in the granite body. Mafic magma, typically basaltic or diabasic in composition, then intrudes into these fractures, forming a dike. The mafic magma is hotter and less viscous than the felsic magma that formed the granite, allowing it to inject rapidly into narrow cracks. The contact between the granite and the dike can show evidence of chilling (finer grain size in the dike near the contact) and sometimes minor alteration of the granite.
Usage
The primary usage is as a geological indicator for understanding tectonic history, stress fields, and magmatic events. In construction, granite is widely used as dimension stone, countertops, and aggregate. While the mafic dike itself is generally not specifically targeted for use, it can be incorporated into aggregate if the entire rock mass is quarried. The aesthetic contrast between the light-colored granite and dark mafic dike can be visually striking and is sometimes sought after for decorative purposes in landscaping or architectural features.
Age Distribution
Granite can range from Precambrian to Cenozoic. Mafic dikes can intrude at various times after the granite's emplacement and cooling, often reflecting later extensional tectonic events. Ages can vary from millions to billions of years.
Where to Find
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.
Finding 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.
Similar Rocks
Granite with Aplite Dike
Granite with Aplite
Also known as: Granite with Felsic Dike
Gneiss with Mafic Dike
Gneiss with Mafic Dike
Also known as: Banded Gneiss with Dike
Granite with Pegmatite Dike
Granite with Pegmatite
Also known as: Granite with Coarse-grained Felsic Dike
Scientific Classification
- Mineral Class
- Igneous Rock (Composite)
- Group
- Felsic Intrusive (Granite) and Mafic Intrusive (Dike)
- Crystal System
- Polycrystalline aggregate, individual minerals vary (e.g., Monoclinic, Hexagonal, Orthorhombic)
- Chemical Formula
- Complex silicate mixtures (no single formula)
- Composition
- Granite: SiO2-rich (>63% SiO2), Al2O3, K2O, Na2O. Mafic Dike: SiO2-poor (45-52% SiO2), FeO, MgO, CaO.
Explore Granite with Mafic Dike
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