Gabbro
Chemical Process (Liquid-phase Growth) - Closed system under ground (Melt precipitation)
Definition
Igneous rocks with equigranular texture having mineral compositions as follows: Quartz in felsic minerals = 0-5 vol%, Plagioclase in feldspar = 90-100 vol%, and An in plagioclase > 50 % Other characters summarized below are those of general gabbros, and the rock type may be inferred but cannot be determined by mafic mineral assemblages.
Mineral composition
Mafic minerals = 60-70 vol% (Mafic), Quartz in felsic minerals = 0-5 vol%, Plagioclase in feldspar = 90-100 vol%, An in plagioclase > 50 %
Chemical composition
SiO2 = 45-52 wt% (Basic).
Texture
Equigranular texture composed of grains more than a few mm.
Required Geological Setting
Plutonic rocks, Accretionary complexes
Occurrence
Rock forming minerals in gabbro, mafic (basic) plutonic rock formed from magma under ground. Gabbroes correspond to M-type granites, and almost all of gabboes are included in magnetite-series. Gabbro distributes as cumulate rocks formed by differentiation in basaltic magma chambers of island arcs. Gabbro also observed in accretionary complexes as fragments of oceanic plates formed by differentiation of Mid-ocean ridge basalt (MORB) on ridges. Japanese M-type granite are Tanzawa body and a part of Kofu body.
Mineral Assemblages
Clinopyroxene (Augite), Plagioclase (Anorthite), Magnesio-hornblende, Orthopyroxene (Hypersthene), Forsterite