G
GABBRO
Jürgen Koepke
Institute of Mineralogy, Leibniz University Hannover,
Hannover, Germany
Definition
Gabbro is a medium- to coarse-grained, mafic intrusive
igneous rock, mostly grayish to greenish in color, and is
the most abundant rock in the oceanic crust. Oceanic
gabbros are mainly composed of three minerals: olivine,
plagioclase, and clinopyroxene, sometimes associated
with orthopyroxene, amphibole, and Fe-Ti oxides. They
are formed in magma chambers (qv. “Mid-ocean Ridge
Magmatism and Volcanism”) under the mid-ocean ridges,
where primitive MORB melts (mid-oceanic ridge basalt;
qv. “mid-ocean ridge magmatism”) differentiate by fractional crystallization. The accumulated minerals in dense
crystal mushes and subsequent slow cooling lead to the
formation of gabbros in the lower oceanic crust.
Introduction
Gabbro is the most common rock of the oceanic crust and
forms its lower part (for review see Coogan, 2014). The
name “gabbro” was suggested from the German geologist
Leopold von Buch after a town in the Italian Tuscany
region. Gabbros from the oceanic crust are named “oceanic gabbros” in order to distinguish them from gabbros
from continents. Oceanic gabbros mostly consist of three
minerals: forsterite-rich olivine, calcium-rich plagioclase
feldspar (usually labradorite or bytownite), and
clinopyroxene, which has the composition of augite.
Orthopyroxene, amphibole, and Fe-Ti oxides might also
occur, but only in small amounts, and usually crystallized
late forming interstitial crystals.
Oceanic gabbros are regarded as the frozen crystal
mushes accumulated during fractional crystallization of
primitive MORB melts in the magma chambers under
the mid-ocean ridges, forming – after subsequent
cooling – the layer 3 of the oceanic crust (qv. “Layering
of Oceanic Crust”). In contrast, the corresponding differentiated melts rise upward to erupt as sheeted dikes (layer 2b)
or as basaltic sheet flows or pillow basalts (layer 2a).
Owing to the scarcity of natural exposures of the deep
gabbroic basement, especially from fast-spreading oceanic crust, complementary studies on ophiolites
(qv. “Ophiolites”) have helped establish models for the
structure of the oceanic crust.
Classification of oceanic gabbros
Gabbroic rocks can be classified using the modal abundances of the constituent minerals olivine, plagioclase,
clinopyroxene, and orthopyroxene, based on the International Union of Geological Sciences (IUGS) system
shown in Figure 1 (Le Maitre, 1989). Figure 1 is composed of four individual triangular plots, the three outer
ones define gabbroic rocks (or mafic plutonic rocks),
while the inner triangle defines ultrabasic (or ultramafic)
rocks. The presence of more than 5 vol% plagioclase in
Figure 1 divides ultrabasic from gabbroic rocks. In addition to the major rock names given in Figure 1, the plutonic part of the oceanic crust may contain:
Diorite: in the IUGS classification, diorite is distinguished from gabbro by the anorthite content (An) of plagioclase. Diorites have plagioclase containing <50 mol%
An, while gabbros have plagioclase containing >50 mol%
An. The vast majority of oceanic gabbroic rocks are
gabbros.
Oxide gabbros: gabbros with Fe-Ti oxides >2 vol%
Dolerite: is applied to fine- or medium-grained gabbroic rocks with dominant ophitic or subophitic textures
J. Harff et al. (eds.), Encyclopedia of Marine Geosciences, DOI 10.1007/978-94-007-6238-1,
© Springer Science+Business Media Dordrecht 2016
GABBRO
Jürgen Koepke
Institute of Mineralogy, Leibniz University Hannover,
Hannover, Germany
Definition
Gabbro is a medium- to coarse-grained, mafic intrusive
igneous rock, mostly grayish to greenish in color, and is
the most abundant rock in the oceanic crust. Oceanic
gabbros are mainly composed of three minerals: olivine,
plagioclase, and clinopyroxene, sometimes associated
with orthopyroxene, amphibole, and Fe-Ti oxides. They
are formed in magma chambers (qv. “Mid-ocean Ridge
Magmatism and Volcanism”) under the mid-ocean ridges,
where primitive MORB melts (mid-oceanic ridge basalt;
qv. “mid-ocean ridge magmatism”) differentiate by fractional crystallization. The accumulated minerals in dense
crystal mushes and subsequent slow cooling lead to the
formation of gabbros in the lower oceanic crust.
Introduction
Gabbro is the most common rock of the oceanic crust and
forms its lower part (for review see Coogan, 2014). The
name “gabbro” was suggested from the German geologist
Leopold von Buch after a town in the Italian Tuscany
region. Gabbros from the oceanic crust are named “oceanic gabbros” in order to distinguish them from gabbros
from continents. Oceanic gabbros mostly consist of three
minerals: forsterite-rich olivine, calcium-rich plagioclase
feldspar (usually labradorite or bytownite), and
clinopyroxene, which has the composition of augite.
Orthopyroxene, amphibole, and Fe-Ti oxides might also
occur, but only in small amounts, and usually crystallized
late forming interstitial crystals.
Oceanic gabbros are regarded as the frozen crystal
mushes accumulated during fractional crystallization of
primitive MORB melts in the magma chambers under
the mid-ocean ridges, forming – after subsequent
cooling – the layer 3 of the oceanic crust (qv. “Layering
of Oceanic Crust”). In contrast, the corresponding differentiated melts rise upward to erupt as sheeted dikes (layer 2b)
or as basaltic sheet flows or pillow basalts (layer 2a).
Owing to the scarcity of natural exposures of the deep
gabbroic basement, especially from fast-spreading oceanic crust, complementary studies on ophiolites
(qv. “Ophiolites”) have helped establish models for the
structure of the oceanic crust.
Classification of oceanic gabbros
Gabbroic rocks can be classified using the modal abundances of the constituent minerals olivine, plagioclase,
clinopyroxene, and orthopyroxene, based on the International Union of Geological Sciences (IUGS) system
shown in Figure 1 (Le Maitre, 1989). Figure 1 is composed of four individual triangular plots, the three outer
ones define gabbroic rocks (or mafic plutonic rocks),
while the inner triangle defines ultrabasic (or ultramafic)
rocks. The presence of more than 5 vol% plagioclase in
Figure 1 divides ultrabasic from gabbroic rocks. In addition to the major rock names given in Figure 1, the plutonic part of the oceanic crust may contain:
Diorite: in the IUGS classification, diorite is distinguished from gabbro by the anorthite content (An) of plagioclase. Diorites have plagioclase containing <50 mol%
An, while gabbros have plagioclase containing >50 mol%
An. The vast majority of oceanic gabbroic rocks are
gabbros.
Oxide gabbros: gabbros with Fe-Ti oxides >2 vol%
Dolerite: is applied to fine- or medium-grained gabbroic rocks with dominant ophitic or subophitic textures
J. Harff et al. (eds.), Encyclopedia of Marine Geosciences, DOI 10.1007/978-94-007-6238-1,
© Springer Science+Business Media Dordrecht 2016