in a warmer and more homogeneous temperature environment than are the basalt
and dolerite, they tend to develop larger mineral grains with well-defined crystal
outlines and they generally lack a matrix.
The importance of studying gabbroic rocks is that their texture and mineral
composition give a great deal of information on the dynamics of crystal-liquid
fractionation. The gabbroic complexes show layered structures due to the
sequential cooling of a magma reservoir. The heavy and first-formed minerals such
as olivine and spinel drop to the bottom of the magma chamber while the lighter
minerals such as plagioclase and clinopyroxene will be crystallized near the top.
Many gabbroic complexes show stratigraphic sedimentation due to crystal-liquid
fractionation. Indeed, when ophiolite complexes are exposed in subaerial environments, the sequential variation of layered gabbroic units is best exposed. The
base of the complex is marked by the presence of an olivine-spinel enriched layer
(dunites), covered by wehrlite, then massive olivine gabbros, gabbronorite and
ferrogabbros.
On the ocean floor, it is difficult to have access to all these sequences because
there is a lack of continuous observations. However, on the basis of drilling and
on-site observations in submersibles, it has been possible to draw an analogy to
subaerial observations. The lighter and most evolved gabbros are called ‘‘anisotropic gabbros’’ and consist of Fe–Ti enriched ferrogabbros as well as silica
(plagio-granite) gabbroic rocks. The most obvious cumulates are the dunites,
wehrlite and pyroxene-olivine enriched gabbros.
Most gabbroic rocks are exposed in fracture zones and are rarely found along
spreading ridge segments (Dixon et al. 1986; Davis and Clague 1990). Occasionally, gabbros occur on the uplifted walls of magma-starved ridge segments
such as on the slow spreading ridge of the south Atlantic and in the Indian ocean in
an area where the crust is thin and close to a ridge-transform (fracture zone)
intersection. On the ocean floor, many gabbroic rocks are found associated with
serpentinized peridotites. Often veins and veinlets of gabbroic rocks are found as
intrusive bodies running sub-parallel to the foliation of peridotites. Usually these
intrusive bodies have a heterogeneous and wide range in their mineral composition
when compared to the more massive gabbroic complexes forming layered bodies
issued from normal crystal liquid-fractionation. This was inferred to be due to
continued compositional change as the percolating liquid reacts with the surrounding solid rocks. Sometimes gabbros are metamorphosed and often show
streaky layers of oriented hydrated minerals around lenticular preserved minerals.
This type of gabbro is also called ‘‘flaser gabbro’’, since the word ‘‘flaser’’ in
German means ‘‘streaks’’. These streaky gabbros are formed during tectonic-stress
generated activities.
It is not always clear whether the gabbro-peridotite-basalt complexes that are
found together are interrelated due to the crystal-liquid fractionation process. The
gabbro-peridotite could also be emplaced a posteriori during a forceful injection or
during thrust-folding tectonic processes. Detailed mineralogical and geochemical
analyses are often required in order to determine their mode of emplacement.
88
4 Sea Floor Rocks
and dolerite, they tend to develop larger mineral grains with well-defined crystal
outlines and they generally lack a matrix.
The importance of studying gabbroic rocks is that their texture and mineral
composition give a great deal of information on the dynamics of crystal-liquid
fractionation. The gabbroic complexes show layered structures due to the
sequential cooling of a magma reservoir. The heavy and first-formed minerals such
as olivine and spinel drop to the bottom of the magma chamber while the lighter
minerals such as plagioclase and clinopyroxene will be crystallized near the top.
Many gabbroic complexes show stratigraphic sedimentation due to crystal-liquid
fractionation. Indeed, when ophiolite complexes are exposed in subaerial environments, the sequential variation of layered gabbroic units is best exposed. The
base of the complex is marked by the presence of an olivine-spinel enriched layer
(dunites), covered by wehrlite, then massive olivine gabbros, gabbronorite and
ferrogabbros.
On the ocean floor, it is difficult to have access to all these sequences because
there is a lack of continuous observations. However, on the basis of drilling and
on-site observations in submersibles, it has been possible to draw an analogy to
subaerial observations. The lighter and most evolved gabbros are called ‘‘anisotropic gabbros’’ and consist of Fe–Ti enriched ferrogabbros as well as silica
(plagio-granite) gabbroic rocks. The most obvious cumulates are the dunites,
wehrlite and pyroxene-olivine enriched gabbros.
Most gabbroic rocks are exposed in fracture zones and are rarely found along
spreading ridge segments (Dixon et al. 1986; Davis and Clague 1990). Occasionally, gabbros occur on the uplifted walls of magma-starved ridge segments
such as on the slow spreading ridge of the south Atlantic and in the Indian ocean in
an area where the crust is thin and close to a ridge-transform (fracture zone)
intersection. On the ocean floor, many gabbroic rocks are found associated with
serpentinized peridotites. Often veins and veinlets of gabbroic rocks are found as
intrusive bodies running sub-parallel to the foliation of peridotites. Usually these
intrusive bodies have a heterogeneous and wide range in their mineral composition
when compared to the more massive gabbroic complexes forming layered bodies
issued from normal crystal liquid-fractionation. This was inferred to be due to
continued compositional change as the percolating liquid reacts with the surrounding solid rocks. Sometimes gabbros are metamorphosed and often show
streaky layers of oriented hydrated minerals around lenticular preserved minerals.
This type of gabbro is also called ‘‘flaser gabbro’’, since the word ‘‘flaser’’ in
German means ‘‘streaks’’. These streaky gabbros are formed during tectonic-stress
generated activities.
It is not always clear whether the gabbro-peridotite-basalt complexes that are
found together are interrelated due to the crystal-liquid fractionation process. The
gabbro-peridotite could also be emplaced a posteriori during a forceful injection or
during thrust-folding tectonic processes. Detailed mineralogical and geochemical
analyses are often required in order to determine their mode of emplacement.
88
4 Sea Floor Rocks