BH250-1
Title
BH250-1
Subject
Garnet–Sillimanite Schist (High-Grade Metamorphic Rock)
Description
Mineralogy
Major Minerals:
Sillimanite (including fibrolite)
Garnet
Quartz
Minor Minerals:
Muscovite
Biotite
Albite
Orthoclase
Zircon (accessory, often included in garnet)
Texture & Fabric
Strong foliation (schistosity) defined by aligned micas and fibrolitic sillimanite
Slender prismatic and fibrous sillimanite forming mats and bundles
Porphyroblastic texture with garnet crystals embedded in a foliated matrix
Deformed garnets: flattened, elongated, and sometimes fractured
Quartz inclusion-rich garnets, with trails recording earlier fabrics
Growth zoning in garnets, indicating evolving metamorphic conditions
BH250-1 is a striking example of a high-grade metamorphic rock that records both intense deformation and elevated temperature conditions deep within the continental crust. This rock is dominated by abundant sillimanite, occurring both as slender prismatic crystals and as fine fibrous aggregates known as fibrolite, intergrown with quartz and micas.
One of the most distinctive features of this specimen is the presence of garnet porphyroblasts that have undergone significant deformation. While many garnets originally formed as euhedral, equant crystals, they are now commonly flattened, elongated, and stretched parallel to the rock’s foliation. In some cases, the foliation wraps through or even cuts across the garnets, indicating that deformation continued after or during garnet growth. This provides strong evidence for syn- to post-tectonic deformation.
The garnets contain abundant quartz inclusions concentrated in their central cores, often arranged in trails or patterns that preserve earlier fabrics. In contrast, the outer rims of the garnets are largely inclusion-free, suggesting a later stage of crystal growth. These inclusion-rich cores are key to interpreting the rock’s history:
They likely represent pre-existing foliation (S₁) that became trapped during early garnet growth.
Their orientation relative to the external foliation helps reconstruct the rotational and deformational history of the porphyroblasts.
The presence of inclusion-poor rims indicates that the garnets likely continued to grow after peak metamorphic conditions, during a later stage when deformation had diminished and fewer inclusions were incorporated. Many garnets also show growth zoning, reflecting changes in pressure–temperature–chemical conditions during crystallization.
Taken together, the inclusion-rich cores and inclusion-free rims, along with zoning patterns, suggest that these garnets began forming as pre- to syn-tectonic porphyroblasts, and subsequently experienced post-peak (retrograde or late-stage) growth, followed by continued deformation that modified their overall shape and fabric.
Metamorphic Interpretation
This rock formed under upper amphibolite to granulite facies conditions, where temperatures are sufficiently high for the formation of sillimanite. Its mineral assemblage and textures indicate:
High temperature metamorphism (sillimanite stability field)
Likely moderate to high pressures typical of continental collision zones
Prolonged deformation, resulting in:
Rotation and flattening of garnets
Development of strong foliation
Overprinting of earlier mineral growth
This combination suggests a tectonically active environment, such as a continental orogen, where rocks are buried, heated, deformed, and exhumed.
In addition, the original bulk composition of this rock was clearly pelitic, as indicated by the abundance of sillimanite, muscovite, and plagioclase. This mineral assemblage is characteristic of aluminum-rich sedimentary protoliths (such as shale or mudstone), further supporting an origin tied to metamorphism of fine-grained sedimentary material under high-temperature conditions
Geologic Significance
BH250-1 is an excellent teaching example because it preserves a multi-stage metamorphic history:
1. Initial mineral growth (garnet porphyroblasts with inclusion trails)
2. Development of foliation and alignment of sillimanite and micas
3. Continued deformation, flattening garnets and modifying textures
It clearly demonstrates how metamorphic rocks can act as records of both pressure–temperature conditions and deformation history, making it ideal for interpreting tectonic processes in deep crustal environments.
Major Minerals:
Sillimanite (including fibrolite)
Garnet
Quartz
Minor Minerals:
Muscovite
Biotite
Albite
Orthoclase
Zircon (accessory, often included in garnet)
Texture & Fabric
Strong foliation (schistosity) defined by aligned micas and fibrolitic sillimanite
Slender prismatic and fibrous sillimanite forming mats and bundles
Porphyroblastic texture with garnet crystals embedded in a foliated matrix
Deformed garnets: flattened, elongated, and sometimes fractured
Quartz inclusion-rich garnets, with trails recording earlier fabrics
Growth zoning in garnets, indicating evolving metamorphic conditions
BH250-1 is a striking example of a high-grade metamorphic rock that records both intense deformation and elevated temperature conditions deep within the continental crust. This rock is dominated by abundant sillimanite, occurring both as slender prismatic crystals and as fine fibrous aggregates known as fibrolite, intergrown with quartz and micas.
One of the most distinctive features of this specimen is the presence of garnet porphyroblasts that have undergone significant deformation. While many garnets originally formed as euhedral, equant crystals, they are now commonly flattened, elongated, and stretched parallel to the rock’s foliation. In some cases, the foliation wraps through or even cuts across the garnets, indicating that deformation continued after or during garnet growth. This provides strong evidence for syn- to post-tectonic deformation.
The garnets contain abundant quartz inclusions concentrated in their central cores, often arranged in trails or patterns that preserve earlier fabrics. In contrast, the outer rims of the garnets are largely inclusion-free, suggesting a later stage of crystal growth. These inclusion-rich cores are key to interpreting the rock’s history:
They likely represent pre-existing foliation (S₁) that became trapped during early garnet growth.
Their orientation relative to the external foliation helps reconstruct the rotational and deformational history of the porphyroblasts.
The presence of inclusion-poor rims indicates that the garnets likely continued to grow after peak metamorphic conditions, during a later stage when deformation had diminished and fewer inclusions were incorporated. Many garnets also show growth zoning, reflecting changes in pressure–temperature–chemical conditions during crystallization.
Taken together, the inclusion-rich cores and inclusion-free rims, along with zoning patterns, suggest that these garnets began forming as pre- to syn-tectonic porphyroblasts, and subsequently experienced post-peak (retrograde or late-stage) growth, followed by continued deformation that modified their overall shape and fabric.
Metamorphic Interpretation
This rock formed under upper amphibolite to granulite facies conditions, where temperatures are sufficiently high for the formation of sillimanite. Its mineral assemblage and textures indicate:
High temperature metamorphism (sillimanite stability field)
Likely moderate to high pressures typical of continental collision zones
Prolonged deformation, resulting in:
Rotation and flattening of garnets
Development of strong foliation
Overprinting of earlier mineral growth
This combination suggests a tectonically active environment, such as a continental orogen, where rocks are buried, heated, deformed, and exhumed.
In addition, the original bulk composition of this rock was clearly pelitic, as indicated by the abundance of sillimanite, muscovite, and plagioclase. This mineral assemblage is characteristic of aluminum-rich sedimentary protoliths (such as shale or mudstone), further supporting an origin tied to metamorphism of fine-grained sedimentary material under high-temperature conditions
Geologic Significance
BH250-1 is an excellent teaching example because it preserves a multi-stage metamorphic history:
1. Initial mineral growth (garnet porphyroblasts with inclusion trails)
2. Development of foliation and alignment of sillimanite and micas
3. Continued deformation, flattening garnets and modifying textures
It clearly demonstrates how metamorphic rocks can act as records of both pressure–temperature conditions and deformation history, making it ideal for interpreting tectonic processes in deep crustal environments.
Coverage
Location: Alaska, USA
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample BH250-1. Housed at Carleton College in Minnesota.
Contributor
Bereket Haileab
Carl Tape, '01
Carl Tape, '01
Type
Thin section and hand sample
Relation
Collection
Citation
Bereket Haileab, “BH250-1,” BH250 Mineralogy Teaching Collection, accessed April 25, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/1.