BGS Rock Classification Scheme
Basalt, tholeiitic - A subgroup of subalkali basalt. Tholeiitic describes a variety of basalt which is saturated or slightly oversaturated in silica with respect to alkalis so that hypersthene appears in the CIPW norm and hypersthene or pigeonite amongst the modal pyroxenes. Olivine may be present in widely varying amounts, or absent. The term is also used for a suite of rocks showing marked iron-enrichment on AFM diagrams. British Geological Survey Research Report RR/99/06, section 8.4.
| Code | BATHL |
|---|---|
| Description | Basalt, tholeiitic - A subgroup of subalkali basalt. Tholeiitic describes a variety of basalt which is saturated or slightly oversaturated in silica with respect to alkalis so that hypersthene appears in the CIPW norm and hypersthene or pigeonite amongst the modal pyroxenes. Olivine may be present in widely varying amounts, or absent. The term is also used for a suite of rocks showing marked iron-enrichment on AFM diagrams. British Geological Survey Research Report RR/99/06, section 8.4. |
| Qualifiers | tholeiitic |
| Definition | Fine-grained, av. <0.25mm normal crystalline extrusive/intrusive rock; CI >35 vol% >40 wt%; quartz <20% or feldspathoid <10% of QAPF, felds >65% plag; TAS SiO2 45-52wt%, Na2O+K2O <5wt%. Silica saturated so hypersthene in CIPW norm + labradorite +/-glass. |
| Linked data | http://data.bgs.ac.uk/id/EarthMaterialClass/RockName/BATHL |
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.