Gabbroisch

Gabbroisch is a geological term used to describe rocks that have gabbroic characteristics. In English, the corresponding adjective is gabbroic. It refers to coarse‑grained, mafic intrusive rocks whose mineral assemblage and texture resemble the mineralogy of gabbro.

Mineralogy and texture typically define gabbroisch rocks. The dominant minerals are calcic plagioclase feldspar and pyroxene, with optional olivine and amphibole. Accessory minerals may include magnetite, ilmenite, and apatite. The texture is phaneritic, meaning crystals are large enough to be seen with the naked eye. This reflects slow cooling underground, as opposed to the fine-grained, erupted basalt that is its extrusive counterpart.

Formation and relation to basalt describe the origin of gabbroisch rocks. They crystallize from mafic magma that solidifies at depth in plutons, sills, or large magma chambers within the crust. Because cooling occurs slowly, crystals grow large, producing the characteristic coarse texture. Gabbroisch rocks are the intrusive equivalent of basalt; while basalt forms at the surface through volcanic activity, gabbroisch rocks form beneath the surface before being exposed by uplift and erosion.

Occurrence and classification place gabbroisch rocks in various geological contexts. They are common in oceanic crust and in ophiolites, where buried magmatic chambers are brought to light. They also occur in continental settings as part of layered intrusions and other mafic plutonic bodies. Subtypes and related rocks include norite and troctolite, which differ in the proportions of pyroxene, olivine, and plagioclase, as well as metagabbro when subjected to metamorphism.

In summary, gabbroisch describes coarse, gabbro-like rocks formed by slow underground crystallization of mafic magma, the intrusive counterparts to basalt.