BGS Rock Classification Scheme

Gabbro - A type of gabbroic-rock. In the Rock Classification Scheme, it is a coarse-grained crystalline igneous rock whose mineral mode plots in field 10 of the quartz - alkali feldspar - plagioclase - feldspathoid (QAPF) diagram. Diorite, gabbro and anorthosite are separated according to the average composition of their plagioclase and the proportion of mafic minerals (M): if M is greater than 10% and An (the anorthite content of plagioclase) is greater than 50% the rock is gabbro. British Geological Survey Research Report RR/99/06, section 5.1 and figs. 9, 11, 12.

Code GB
Description Gabbro - A type of gabbroic-rock. In the Rock Classification Scheme, it is a coarse-grained crystalline igneous rock whose mineral mode plots in field 10 of the quartz - alkali feldspar - plagioclase - feldspathoid (QAPF) diagram. Diorite, gabbro and anorthosite are separated according to the average composition of their plagioclase and the proportion of mafic minerals (M): if M is greater than 10% and An (the anorthite content of plagioclase) is greater than 50% the rock is gabbro. British Geological Survey Research Report RR/99/06, section 5.1 and figs. 9, 11, 12.
Qualifiers No qualifiers currently exist.
Definition Coarse-grained, av. between 2-16mm normal crystalline intrusive rock; quartz <5% of QAPF, feldspar >90% plagioclase (>50% An) +/- mica/pyroxene/amphibole/olivine
Linked data http://data.bgs.ac.uk/id/EarthMaterialClass/RockName/GB

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.