BH250-98
Title
BH250-98
Subject
Granite with Basaltic Dike
Description
Major Minerals: plagioclase, biotite
Minor Minerals: chlorite, clinopyroxene
BH250-97 and BH250-98 are excellent teaching specimens that preserve the intrusion of a basaltic dike into a pre-existing granitic host rock. These samples provide a clear example of intrusive relationships, chilled-margin development, magma flow textures, and the influence of cooling rate on crystal size within igneous rocks. The dark mafic dike sharply cuts across the lighter coarse-grained granite, making the intrusive contact easily visible both in hand sample and thin section.
The granitic host rock consists primarily of quartz and feldspar with minor biotite, whereas the basaltic dike is dominated by plagioclase microlites and mafic minerals including clinopyroxene and altered ferromagnesian phases. In thin section, the dike displays a pronounced fine-grained chilled margin adjacent to the granite contact. This chilled zone formed when the hot basaltic magma was rapidly cooled against the comparatively cold granitic country rock. Rapid heat loss at the margin inhibited crystal growth, producing a dark, very fine-grained to nearly glassy texture.
Toward the center of the dike, crystal size increases progressively as cooling became slower and more uniform. The interior contains larger plagioclase laths and more developed intergranular textures, reflecting a lower cooling rate away from the contact. This gradual transition from cryptocrystalline margin to coarser-grained interior provides an excellent natural example of conductive cooling in mafic intrusions.
One of the most striking petrographic features preserved in these samples is the alignment of plagioclase laths along the dike margins. The preferred orientation of the elongate feldspar crystals records magma flow during emplacement. Near the contacts, the plagioclase grains are strongly aligned parallel to the dike walls, indicating laminar flow of the basaltic melt as it moved through the fracture system. Toward the center of the dike, this flow fabric becomes progressively weaker and more random as crystallization proceeded and magma motion diminished.
The large-format thin section spanning the full width of the dike is particularly valuable because it preserves the complete textural transition from granite contact, through the chilled margin, into the coarser central portion of the intrusion, and back to the opposite contact. Such sections allow students to directly observe how cooling gradients influence crystal nucleation, grain size, and texture development across a single intrusive body.
These samples were used extensively in an introductory geology class project at Carleton College in which students investigated cooling-rate variations within basaltic dikes. Students measured grain-size distributions across the dike width and analyzed the relationship between crystal size and distance from the dike margin. Their work demonstrated that crystal size generally increases toward the center of the dike, consistent with slower cooling rates away from the contact zone. The project also explored the role of dike thickness in cooling history and crystal development.
The accompanying student paper, The Cooling Rates of Two Dikes from Granite Falls, MN, developed quantitative approaches for analyzing crystal-size distributions using petrographic images and digital image-processing techniques. The study concluded that both dikes cooled more rapidly at the margins than in the center and that wider dikes produced significantly larger crystals due to prolonged cooling times.
The photomicrographs further illustrate these relationships. The chilled margins contain abundant tiny plagioclase microlites set within a dark fine-grained groundmass, whereas the interior portions contain larger feldspar crystals and more developed intergranular textures. In crossed polarized light, plagioclase laths exhibit bright interference colors and locally show preferred orientation parallel to the dike contact, reinforcing evidence for magma flow during emplacement.
Overall, BH250-97 and BH250-98 are outstanding educational specimens because they connect field relationships, hand-sample observations, petrography, crystallization kinetics, and quantitative cooling-rate analysis into a single integrated teaching example. They demonstrate how intrusive igneous bodies record both thermal history and magma dynamics during emplacement within the crust.
Key Petrographic Features
• Sharp intrusive contact between basaltic dike and granite host
• Chilled margins along dike contacts
• Progressive grain-size increase toward dike center
• Flow alignment of plagioclase laths
• Intergranular to pilotaxitic basaltic texture
• Large-format thin section preserving full dike profile
• Evidence of conductive cooling and magma flow dynamics
Educational Significance
• Demonstrates intrusive cross-cutting relationships
• Excellent example of chilled-margin development
• Useful for teaching cooling-rate effects on crystal growth
• Connects petrography with quantitative image analysis
• Used in student research projects on igneous textures and crystallization dynamics
Minor Minerals: chlorite, clinopyroxene
BH250-97 and BH250-98 are excellent teaching specimens that preserve the intrusion of a basaltic dike into a pre-existing granitic host rock. These samples provide a clear example of intrusive relationships, chilled-margin development, magma flow textures, and the influence of cooling rate on crystal size within igneous rocks. The dark mafic dike sharply cuts across the lighter coarse-grained granite, making the intrusive contact easily visible both in hand sample and thin section.
The granitic host rock consists primarily of quartz and feldspar with minor biotite, whereas the basaltic dike is dominated by plagioclase microlites and mafic minerals including clinopyroxene and altered ferromagnesian phases. In thin section, the dike displays a pronounced fine-grained chilled margin adjacent to the granite contact. This chilled zone formed when the hot basaltic magma was rapidly cooled against the comparatively cold granitic country rock. Rapid heat loss at the margin inhibited crystal growth, producing a dark, very fine-grained to nearly glassy texture.
Toward the center of the dike, crystal size increases progressively as cooling became slower and more uniform. The interior contains larger plagioclase laths and more developed intergranular textures, reflecting a lower cooling rate away from the contact. This gradual transition from cryptocrystalline margin to coarser-grained interior provides an excellent natural example of conductive cooling in mafic intrusions.
One of the most striking petrographic features preserved in these samples is the alignment of plagioclase laths along the dike margins. The preferred orientation of the elongate feldspar crystals records magma flow during emplacement. Near the contacts, the plagioclase grains are strongly aligned parallel to the dike walls, indicating laminar flow of the basaltic melt as it moved through the fracture system. Toward the center of the dike, this flow fabric becomes progressively weaker and more random as crystallization proceeded and magma motion diminished.
The large-format thin section spanning the full width of the dike is particularly valuable because it preserves the complete textural transition from granite contact, through the chilled margin, into the coarser central portion of the intrusion, and back to the opposite contact. Such sections allow students to directly observe how cooling gradients influence crystal nucleation, grain size, and texture development across a single intrusive body.
These samples were used extensively in an introductory geology class project at Carleton College in which students investigated cooling-rate variations within basaltic dikes. Students measured grain-size distributions across the dike width and analyzed the relationship between crystal size and distance from the dike margin. Their work demonstrated that crystal size generally increases toward the center of the dike, consistent with slower cooling rates away from the contact zone. The project also explored the role of dike thickness in cooling history and crystal development.
The accompanying student paper, The Cooling Rates of Two Dikes from Granite Falls, MN, developed quantitative approaches for analyzing crystal-size distributions using petrographic images and digital image-processing techniques. The study concluded that both dikes cooled more rapidly at the margins than in the center and that wider dikes produced significantly larger crystals due to prolonged cooling times.
The photomicrographs further illustrate these relationships. The chilled margins contain abundant tiny plagioclase microlites set within a dark fine-grained groundmass, whereas the interior portions contain larger feldspar crystals and more developed intergranular textures. In crossed polarized light, plagioclase laths exhibit bright interference colors and locally show preferred orientation parallel to the dike contact, reinforcing evidence for magma flow during emplacement.
Overall, BH250-97 and BH250-98 are outstanding educational specimens because they connect field relationships, hand-sample observations, petrography, crystallization kinetics, and quantitative cooling-rate analysis into a single integrated teaching example. They demonstrate how intrusive igneous bodies record both thermal history and magma dynamics during emplacement within the crust.
Key Petrographic Features
• Sharp intrusive contact between basaltic dike and granite host
• Chilled margins along dike contacts
• Progressive grain-size increase toward dike center
• Flow alignment of plagioclase laths
• Intergranular to pilotaxitic basaltic texture
• Large-format thin section preserving full dike profile
• Evidence of conductive cooling and magma flow dynamics
Educational Significance
• Demonstrates intrusive cross-cutting relationships
• Excellent example of chilled-margin development
• Useful for teaching cooling-rate effects on crystal growth
• Connects petrography with quantitative image analysis
• Used in student research projects on igneous textures and crystallization dynamics
Coverage
Location: Granite Falls, Minnesota, USA
Creator
Bereket Haileab
Source
From the rock collection of Bereket Haileab. Sample 98. Housed at Carleton College in Minnesota.
Type
Hand sample and thin section
Large thin sections 2-3 inches
Large thin sections 2-3 inches
Relation
Collection
Citation
Bereket Haileab, “BH250-98,” BH250 Mineralogy Teaching Collection, accessed October 22, 2026, https://bereket-haileab.geology.sites.carleton.edu/items/show/111.