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Geology is the Way

Monzodiorite and monzogabbro

Monzodiorite and monzogabbro are intermediate plutonic igneous rocks containing plagioclase, alkali feldspar, biotite, hornblende and/or pyroxene. These rocks, together with monzonite, used to be called respectively syenodiorite e syenogabbro, due to their composition, lying halfway between syenites e diorite/gabbro in the QAPF diagram. However, the IUGS commission discourages the use of these terms. Modally, monzodiorite and monzogabbro contain more alkali feldspar (between 35 and 10% of all feldspars) with respect to their gabbroic counterparts. The difference between monzodiorite and monzogabbro is the composition of plagioclasio: more sodic in monzodiorite (< An50) and calcic in monzogabbro (> An50). Both may contain up to 20% quarzo or up to 10% feldspathoids over the felsic minerals. If quartz is more than 5% (but less than 20%) of the felsic minerals, these rocks can be classified as quartz monzodiorite/quartz monzogabbro. Alternatively, if foids up to 10% are present, they are foid-bearing monzodiorite (or monzogabbro). In the latter case, the term ‘foid’ in the name must be substituted with the most abundant foid present (e.g. nepheline-bearing monzogabbro). The content of mafic minerals (color index) of monzodiorites is typically between 15 and 50% and up to 25 – 60% in monzogabbros.

orbicular monzodiorite

Orbicular monzodiorite with orbicles of (mostly) hornblende and pyroxene in a groundmass of light-colored andesine plagioclase and microcline feldspar. Plantation Lake, Davie County, North Carolina, USA. Width: 13.7 cm. Photo © James St. John.

orbicular monzodiorite

Orbicular monzodiorite with orbicles of (mostly) hornblende and pyroxene in a groundmass of light-colored andesine plagioclase and microcline feldspar. Plantation Lake, Davie County, North Carolina, USA. Width: 5.5 cm. Photo © James St. John.

Monzodiorite/monzogabbro
Plutonic igneous rock
Felsic minerals:
plagioclasio
alkali feldspar
quarzo
feldspathoids
Mafic minerals:
hornblende
biotite
pyroxene

QAPF classification:
Plagioclase composition: monzodiorite(< An50), monzogabbro (> An50)
Q = 0 – 20% or F < 10%
Plagioclase/feldspars = 65 – 90%
Colored varieties:
• leucomonzodiorite (M < 15%), leucomonzogabbro (M < 25%)
• melamonzodiorite (M > 50%), melamonzogabbro (M > 60%)
Other varieties: quartz monzodiorite/monzogabbro, foid-bearing monzodiorite/monzogabbro
Extrusive equivalent: andesite, basalt

Bibliografia

        

Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies

 

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