BGS Rock Classification Scheme
Mylonitic-rock - A type of foliated mechanically broken rock with primary cohesion. Mylonitic rocks represent the products of dominantly ductile deformation. They generally occur within restricted zones related to faults, thrusts or shear zones. These foliated rocks develop as a result of grain size reduction by a combination of breakage and plastic strain of grains. Plastic deformation increases the aspect ratio of affected minerals producing textures such as quartz ribbons and a foliated fine-grained matrix. Other minerals, for example feldspar and garnet, may resist ductile deformation or fracture in a brittle manner and remain significantly larger than the foliated matrix. These are commonly lens shaped and termed porphyroclasts. As mylonitisation proceeds, the porphyroclasts are progressively wrapped by and then become isolated within the foliated matrix. They also become smaller, either by fracturing or by marginal erosion. Porphyroclasts may develop asymmetrical tails which can indicate the sense of shearing (dextral or sinistral) within the mylonitic rocks. Mylonitic rocks are classified into protomylonite, mylonite, and ultramylonite based on the percentage of visible porphyroclasts within the streaky, platy, fine-grained matrix. British Geological Survey Research Report RR/99/02, section 7.3 and table 5.
| Code | MYLTIC |
|---|---|
| Description | Mylonitic-rock - A type of foliated mechanically broken rock with primary cohesion. Mylonitic rocks represent the products of dominantly ductile deformation. They generally occur within restricted zones related to faults, thrusts or shear zones. These foliated rocks develop as a result of grain size reduction by a combination of breakage and plastic strain of grains. Plastic deformation increases the aspect ratio of affected minerals producing textures such as quartz ribbons and a foliated fine-grained matrix. Other minerals, for example feldspar and garnet, may resist ductile deformation or fracture in a brittle manner and remain significantly larger than the foliated matrix. These are commonly lens shaped and termed porphyroclasts. As mylonitisation proceeds, the porphyroclasts are progressively wrapped by and then become isolated within the foliated matrix. They also become smaller, either by fracturing or by marginal erosion. Porphyroclasts may develop asymmetrical tails which can indicate the sense of shearing (dextral or sinistral) within the mylonitic rocks. Mylonitic rocks are classified into protomylonite, mylonite, and ultramylonite based on the percentage of visible porphyroclasts within the streaky, platy, fine-grained matrix. British Geological Survey Research Report RR/99/02, section 7.3 and table 5. |
| Qualifiers | No qualifiers currently exist. |
| Definition | Not Available |
| Linked data | http://data.bgs.ac.uk/id/EarthMaterialClass/RockName/MYLTIC |
CAVEAT
"Thumbnail" definitions attached to some of the table entries are an interim attempt to provide simple short descriptions of the "unqualified" rock types presented within the BGS Rock Classification Scheme. Every effort has been made to ensure accuracy and consistency but minor contradictions or omissions might be present. The BGS welcomes any feedback pointing out such minor imperfections and/or providing constructive comment and suggestions for improvement of the definitions. In the first instance such feedback should be directed to Dr Tim McCormick.