Classification of gabbros
Gabbroid rocks are a large family of plutonic rocks that contain almost exclusively plagioclasio as the only felsic mineral, hence plotting close to the P vertex of the QAPF classification diagram (quartz < 5% and plagioclase > 90% feldspars). The relatively simple classification through the QAPF diagram hides, however, the true complexity of these rocks, which may contain variable proportion of plagioclase with variable calcium- to sodium-rich composition and a wide range of different femic minerals, including pyroxene (ortho- and clino-), olivine, and hornblende.
Difference between gabbro, diorite, and anorthosite
The content of plagioclase and its composition are the first two parameters that allow to classify gabbroid rocks. Anorthosite contains more than 90% of plagioclase or, in other words, has color index M < 10%. On the other hand, both gabbro e diorite show M > 10% and plagioclase < 90%, although normally gabbros have a plagioclase content of 35 – 65% and diorites of 25 – 50%. The difference between gabbro and diorite lies in the composition of plagioclase, sodic in diorites (An5–50; oligoclase – andesine), calcic in gabbros (An50–100; labradorite – bytownite – anorthite). In the field, a light-colored gabbroid rock is more likely to be a diorite, rather than a gabbro. However, to apply this classification it is necessary to determine the composition of the plagioclase in thin section or with chemical analyses.
Quartz and foid-bearing varieties
Most gabbroid rocks contain plagioclase as the only felsic mineral but non-negligible contents of quartz or feldspathoids may be present. It is important to identify these minerals, because they indicate that the rock is oversaturated (i.e. quartz in excess) or undersaturated (foids present) in silica. If quarzo constitutes between 5 and 20% of the felsic minerals, the gabbroid rock can be called quartz diorite, quartz gabbro, or quartz anorthosite. On the other hand, if foids up to 10% of the felsic minerals are present, it can be classified as foid-bearing diorite, foid-bearing gabbro, and foid-bearing anorthosite, substituting ‘foid’ with the name of the feldspathoid present (e.g. nepheline-bearing gabbro).
Classification of gabbros
Gabbros, in strict sense, can be classified based on the femic minerals that are present along with plagioclase. The most common varieties of gabbro contain pyroxene, plus some olivine, but there are varieties with significant amounts of olivine and/or orneblenda. Once a rock has been determined to be a gabbro based on the QAPF diagram and the presence of calcic plagioclase, it can be further classified using the plagioclase – pyroxene – olivine (Plag – Px – Ol), and the plagioclase – pyroxene – hornblende (Plag – Px – Hbl) diagrams. [See also: how to use a ternary plot]
According to these diagrams, gabbros (gabbro, gabbronorite, and norite) occupy a broad field, defined by the plagioclase content between 10 and 90%, between ultramafic rocks (peridotite, pirossenite, and hornblendite) and anorthosites. Leucocratic or light-colored varieties (e.g. leucogabbro) contain plagioclase > 65%, whereas in melanocratic or dark-colored varieties the plagioclase content is < 35%. These diagrams define some important rock types:
Troctolite: gabbro with plagioclase and olivine (pyroxene < 5% of femic minerals)
Hornblende gabbro: gabbro with plagioclase and hornblende (pyroxene < 5% of femic minerals)
Olivine gabbro (gabbronorite or norite): with olivine > 5% of femic minerals
Pyroxene hornblende gabbro (gabbronorite or norite): with hornblende > 5% and pyroxene > 5% of femic minerals
Difference between gabbro, norite, and gabbronorite
This distinction is possible only at the microscope, because it requires to recognize clinopyroxene (diopside – augite) and orthopyroxene (enstatite), based on its straight or oblique extinction angle. Norites contain plagioclase and orthopyroxene. Gabbros contain plagioclase and clinopyroxene. In between, gabbronorites contain plagioclase and almost the same proportion of clinopyroxene and orthopyroxene. Other varieties of these rocks can be determined using the diagram below.
Related pages
- Gabbro – Gabbro is a plutonic igneous rock and the intrusive counterpart of basalt. Gabbros are essentially constituted by Calcium-rich plagioclase (>An50), clinopyroxene (augite, diopside), and iron oxides (ilmenite, magnetite) and may contain accessory olivine, amphibole, orthopyroxene, and spinel. Clinopyroxene generally represents between 35 and 65% of the rock volume. If the modal content of plagioclase > 95%, the rock is an… Leggi tutto »Gabbro
Bibliografia
Cox, K. G. (Ed.). (2013). The interpretation of igneous rocks. Springer Science & Business Media.
Le Bas, M. J., & Streckeisen, A. L. (1991). The IUGS systematics of igneous rocks. Journal of the Geological Society, 148(5), 825-833.
Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
Streckeisen, A. (1976). To each plutonic rock its proper name. Earth-science reviews, 12(1), 1-33.
Traduzione in corso!
Le pagine in Italiano dovrebbero essere disponibili nuovamente nel giro di qualche mese.
Italiano 
English