Granite with Mafic Inclusion

Igneous (Plutonic)

Granite (felsic igneous rock) with a mafic xenolith or enclave

Also known as: Granite with Mafic Xenolith, Granite with Mafic Enclave, Magmatic Enclave in Granite

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Description

Granite with mafic inclusions is a felsic, coarse-grained intrusive igneous rock characterized by the presence of darker, finer-grained, typically ovoid to irregular-shaped bodies of mafic composition. The host granite is predominantly composed of quartz, feldspar (orthoclase, plagioclase), and micas (biotite, muscovite), giving it a light color (pink, white, grey). The mafic inclusions (xenoliths or enclaves) are richer in ferromagnesian minerals like hornblende, pyroxene, and biotite, and plagioclase, resulting in a darker (black, dark grey, dark green) and often finer-grained texture. The contact between the granite and the inclusion can be sharp or diffuse, sometimes showing evidence of magma mingling such as crenulated margins or acicular amphiboles.

How to Identify

Color: Host granite: typically light-colored (white, grey, pink, red) due to quartz and feldspar. Mafic inclusions: dark-colored (black, dark grey, dark green) due to higher concentrations of mafic minerals.
Luster: Vitreous to sub-vitreous for quartz and feldspar in the granite; dull to sub-vitreous for mafic minerals in the inclusion.
Texture: Host granite: phaneritic (coarse-grained), equigranular to porphyritic. Mafic inclusions: typically finer-grained (aphanitic to fine-grained phaneritic) than the host granite, often with a microgranular texture. The overall texture is heterogeneous.
Crystal Form: Anhedral to subhedral crystals of quartz and feldspar in granite; anhedral to subhedral crystals of plagioclase, hornblende, pyroxene, and biotite in inclusions. Mafic inclusions often have rounded or ovoid shapes.
Cleavage: Feldspars exhibit two good cleavages at or near 90 degrees. Micas (biotite, muscovite) exhibit one perfect basal cleavage. Amphiboles (hornblende) exhibit two cleavages at approximately 56 and 124 degrees. Pyroxenes exhibit two cleavages at approximately 90 degrees.
Geological Environment: Intrusive igneous environments, specifically within large batholiths and plutons formed in continental arcs, collisional orogens, and post-collisional settings. Found in areas of active or past magmatism where felsic and mafic magmas coexisted or where granitic magmas assimilated country rock.

Key Facts

Hardness: 6-7 (Mohs scale) for quartz and feldspar in granite; 5-6 for amphiboles and pyroxenes in inclusions.
Specific Gravity: 2.6-2.7 g/cm³ for granite; 2.8-3.2 g/cm³ for mafic inclusions.
Crystal System: Quartz: Trigonal; Feldspars: Monoclinic/Triclinic; Micas: Monoclinic; Amphiboles: Monoclinic; Pyroxenes: Orthorhombic/Monoclinic.
Color: Polychromatic: light-colored host with dark-colored inclusions.
Luster: Vitreous to sub-vitreous.
Transparency: Opaque to translucent.
Fracture: Conchoidal (quartz), uneven to splintery (feldspars, mafic minerals).
Cleavage: Good to perfect in feldspars, micas, amphiboles, and pyroxenes; none in quartz.
Composition: Host granite: >20% quartz, 65-90% total feldspar (plagioclase + alkali feldspar), <20% mafic minerals. Mafic inclusion: <52% SiO2, rich in plagioclase, hornblende, pyroxene, biotite, olivine (rarely).

Quick Check

Color: Light-colored host granite (white, grey, pink) with dark-colored (black, dark grey) inclusions.
Luster: Vitreous to sub-vitreous for granite minerals; dull to sub-vitreous for inclusion minerals.
Streak: White (for quartz and feldspar); generally no streak for mafic minerals on unglazed porcelain plate, or very faint grey/green.

Physical Characteristics

Crystal Habit: Granular, anhedral to subhedral crystals in both granite and inclusions. Mafic inclusions often have a fine-grained, equigranular to porphyritic texture.
Cleavage Type: Perfect basal (micas), good prismatic (feldspars, amphiboles, pyroxenes).
Fracture Type: Conchoidal (quartz), uneven, splintery.
Tenacity: Brittle.
Luster Type: Vitreous, sub-vitreous, dull.

Formation

Granite with mafic inclusions forms when a felsic granitic magma body incorporates fragments of pre-existing mafic rock (xenoliths) or, more commonly, when mafic magma is injected into a granitic magma chamber, leading to mingling and incomplete mixing (mafic microgranular enclaves). The temperature and viscosity contrast between the two magmas often results in the mafic magma solidifying as distinct, darker, finer-grained blobs within the coarser-grained granite.

Usage

Primarily used as a decorative stone in construction (countertops, flooring, cladding), monuments, and landscaping. The presence of mafic inclusions can add unique aesthetic appeal. Geologically, they are crucial for understanding magma mixing processes, crustal assimilation, and the petrogenesis of granitic batholiths.

Age Distribution

Precambrian to Cenozoic, common in all periods of significant continental crust formation and magmatism.

Where to Find

Sierra Nevada Batholith, USA

Extensive granitic intrusions with abundant mafic microgranular enclaves, particularly in the Mesozoic granodiorites and granites of California.

Cornwall Batholith, UK

Permo-Carboniferous granites often contain mafic enclaves, providing insights into magma mixing processes.

Andean Batholith, South America

Numerous examples of granite with mafic inclusions are found along the Andean Cordillera, reflecting active subduction-related magmatism.

Himalayan Granites, Asia

Syn-collisional and post-collisional granites frequently exhibit mafic enclaves, indicating complex crustal interactions.

Canadian Shield, Canada

Precambrian granitic terrains often display mafic inclusions, representing ancient magmatic events.

Finding Tips

Look for Outcrops

Search in areas known for granitic intrusions, such as mountain ranges, eroded batholiths, and quarry sites. Road cuts often expose fresh surfaces.

Identify Color Contrast

The most obvious feature is the stark color contrast between the light-colored host granite and the darker mafic inclusions. Look for distinct, often rounded or elliptical dark blobs within the lighter rock.

Examine Texture Differences

Note the difference in grain size; the inclusions are typically finer-grained than the surrounding granite. The contact may be sharp or show signs of mingling (e.g., crenulated margins, feldspar phenocrysts from the granite within the enclave).

Check for Xenolith vs. Enclave

Xenoliths (fragments of country rock) tend to have sharper, more angular boundaries and may show signs of metamorphism. Mafic microgranular enclaves (mingled magmas) often have rounded, lobate, or crenulated margins and may contain phenocrysts derived from the host granite.

Similar Rocks

Gneiss

Also known as: Banded Gneiss

Diorite

Also known as: Black Granite (commercial term)

Granodiorite

Also known as: Granite (commercial term)

Scientific Classification

Mineral Class: Silicate minerals (tectosilicates, phyllosilicates, inosilicates)
Group: Igneous Rock (Plutonic)
Crystal System: Mixed (Trigonal, Monoclinic, Triclinic, Orthorhombic)
Chemical Formula: Variable, primarily SiO2, Al2O3, K2O, Na2O, CaO, FeO, MgO.
Composition: Host granite: Felsic (high silica, alkali-rich). Mafic inclusion: Mafic (low silica, rich in Fe, Mg, Ca).

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