Massive and thick-layered gabbroic units, which are comparable to subaerial
ophiolite complexes, are found in deep-seated troughs such as the Terevaka
transform fault, the Pito Deep and the Hess Deep in the Eastern Pacific as well as
in the Atlantic (Gorringe Bank, Vema transform fault). These gabbros are tabular
blocks with angular contour lines and show a granular texture. Usually these
massive gabbros are olivine gabbros, which are found in the lower part of the
gabbroic complex. These massive gabbros are more homogeneous in composition
than the gabbros observed intruding into the peridotites. The massive gabbros form
layered complexes going from evolved ferrogabbros and/or isotropic gabbros to
olivine gabbros and plagioclase dunite cumulates. Gabbroic cumulates are formed
by crystal fractionation which accumulates in the lower part of a solidifying
magma pool. The cumulate crystals have well-defined separate crystalline outlines.
The crystals are large in size, varying from 2–3 cm in diameter to a few millimeters across. The more fractionated types of gabbros, such as ferrogabbros and
plagioclase-enriched gabbros, abound in the upper part of the gabbroic complex as
well as forming intrusions within peridotites.
The order of mineral crystallization for gabbroic rocks is spinel, followed by
olivine, plagioclase, clinopyroxene and ilmenite. This is in agreement with the
order of crystallization observed in basaltic rocks.
In addition to land-based ophiolite complexes, deep sea drilling has provided
other direct evidence on the stratigraphic relationship of gabbros with other units.
It was during the GLOMAR CHALLENGER leg 37 on the Mid-Atlantic Ridge near
37°N–34°W at about 30–40 km west of the Mid-Atlantic Ridge rift valley
(Aumento and Loubat 1971; Melson et al. 1977) that a basalt-gabbro-peridotite
complex was drilled at 2619 m depth. It is at this site (# 334) that 50 m of basalt
overlaying 68 m of gabbro and peridotite were drilled (Clarke and Loubat 1997).
The drilled core penetrated 376 m into the crust and recovered 254 m of sediment
and 118 m of hard basement rock.
Based on textural and compositional variations, the major gabbroic sequences
observed in the oceanic environment consist of Fe-bearing gabbros (ferrogabbro),
isotropic gabbros, gabbronorite, olivine gabbro, troctolite, werhlite and dunite.
Ferrogabbros consist of plagioclase (an 30-55 ), clinopyroxene and Fe–Ti magnetite—ilmenite. Isotropic gabbros are crystalline rocks with a homogenous crystal
orientation. Gabbronorites consist of olivine, calcic-pyroxene and plagioclase
(anorthite content of an 55-75 ). They could contain orthopyroxene but some gabbronorites do not have any of this mineral. Olivine gabbros are coarse-grained
rocks made-up essentially of olivine, plagioclase, clinopyroxene and othopyroxene. They have a plagioclase composition of an 65-80 , calcic-clinopyroxene and
olivine with Fo 75-87 . Troctolites have a patchy green and white appearance and are
a variety of gabbroic rocks composed of plagioclase (anorthite-bythonite) and
olivine-pyroxene. They are either cumulates from a gabbroic complex and/or late
impregnated melts. Wehrlites differ from dunites by their higher content in
pyroxene, diopside and clinopyroxene. The olivine is commonly serpentinized.
Dunites are cumulates with an equigranular texture (same-sized crystals) consisting essentially of magnesian-olivine (Fo 89-94 ) altered into serpentine and
Gabbroic Rocks
89
ophiolite complexes, are found in deep-seated troughs such as the Terevaka
transform fault, the Pito Deep and the Hess Deep in the Eastern Pacific as well as
in the Atlantic (Gorringe Bank, Vema transform fault). These gabbros are tabular
blocks with angular contour lines and show a granular texture. Usually these
massive gabbros are olivine gabbros, which are found in the lower part of the
gabbroic complex. These massive gabbros are more homogeneous in composition
than the gabbros observed intruding into the peridotites. The massive gabbros form
layered complexes going from evolved ferrogabbros and/or isotropic gabbros to
olivine gabbros and plagioclase dunite cumulates. Gabbroic cumulates are formed
by crystal fractionation which accumulates in the lower part of a solidifying
magma pool. The cumulate crystals have well-defined separate crystalline outlines.
The crystals are large in size, varying from 2–3 cm in diameter to a few millimeters across. The more fractionated types of gabbros, such as ferrogabbros and
plagioclase-enriched gabbros, abound in the upper part of the gabbroic complex as
well as forming intrusions within peridotites.
The order of mineral crystallization for gabbroic rocks is spinel, followed by
olivine, plagioclase, clinopyroxene and ilmenite. This is in agreement with the
order of crystallization observed in basaltic rocks.
In addition to land-based ophiolite complexes, deep sea drilling has provided
other direct evidence on the stratigraphic relationship of gabbros with other units.
It was during the GLOMAR CHALLENGER leg 37 on the Mid-Atlantic Ridge near
37°N–34°W at about 30–40 km west of the Mid-Atlantic Ridge rift valley
(Aumento and Loubat 1971; Melson et al. 1977) that a basalt-gabbro-peridotite
complex was drilled at 2619 m depth. It is at this site (# 334) that 50 m of basalt
overlaying 68 m of gabbro and peridotite were drilled (Clarke and Loubat 1997).
The drilled core penetrated 376 m into the crust and recovered 254 m of sediment
and 118 m of hard basement rock.
Based on textural and compositional variations, the major gabbroic sequences
observed in the oceanic environment consist of Fe-bearing gabbros (ferrogabbro),
isotropic gabbros, gabbronorite, olivine gabbro, troctolite, werhlite and dunite.
Ferrogabbros consist of plagioclase (an 30-55 ), clinopyroxene and Fe–Ti magnetite—ilmenite. Isotropic gabbros are crystalline rocks with a homogenous crystal
orientation. Gabbronorites consist of olivine, calcic-pyroxene and plagioclase
(anorthite content of an 55-75 ). They could contain orthopyroxene but some gabbronorites do not have any of this mineral. Olivine gabbros are coarse-grained
rocks made-up essentially of olivine, plagioclase, clinopyroxene and othopyroxene. They have a plagioclase composition of an 65-80 , calcic-clinopyroxene and
olivine with Fo 75-87 . Troctolites have a patchy green and white appearance and are
a variety of gabbroic rocks composed of plagioclase (anorthite-bythonite) and
olivine-pyroxene. They are either cumulates from a gabbroic complex and/or late
impregnated melts. Wehrlites differ from dunites by their higher content in
pyroxene, diopside and clinopyroxene. The olivine is commonly serpentinized.
Dunites are cumulates with an equigranular texture (same-sized crystals) consisting essentially of magnesian-olivine (Fo 89-94 ) altered into serpentine and
Gabbroic Rocks
89