along gullies, which suggested that the Median Ridge is an unstable structure that
must have undergone vertical uplift in order to expose mantle material. In addition,
the observations of more striated and schistose serpentinized peridotites and the
presence of consolidated breccias indicated tectonic stress along the general east–
west strike-slip motion of the Garrett Transform as well as in another direction
(WNW-ESE). The dives were able to define the limits of the active tectonic zone
(ATZ), which seemed at the time to be located on the northern slope of the Median
Ridge. This ATZ is about 2.5 km wide to the East at 111°29
0 W and about 300 m
wide at 111°30
0 W. The ATZ was responsible for the emplacement of the mantle
peridotite on the northern slope of the Median Ridge. In addition, these three dives
permitted us to observe the circulation of magmatic melt within exposed peridotite
in the form of light colored lances and dykes, made up of plagioclase and clinopyroxene minerals. At a distance to the west from the ATZ (that is to say: to the
west of dive GN15 at 13°28’S-111°34’W), the peridotite disappears and volcanic
outcrops made up of basalt were observed on tectonically undisturbed terrain.
Ultramafic rocks consisting essentially of peridotite (harzburgite) and dunite
plus mafic gabbroic rocks were only discovered on the north-facing slope of the
Median Ridge. The first hand description of the collected samples, which was
performed on board the ship, helped us classify the rocks and gave us valuable
information on the composition and the mode of emplacement of the Median
Ridge (Hébert et al. 1983).
Peridotite, consisting essentially of serpentinized harzburgite residues, is
associated with dunites and troctolites which have textures inherited from crystal
accumulation during crystal growth and metasomatism in magma-lenses. The
dunites and troctolites are formed during magmatic injections into the peridotite.
They show mineral lenses of clinopyroxene that are elongated to a faint foliation
of serpentine in a subparalel direction. Some of the plagioclase-bearing dunites
and troctolites show kink-bands and altered (saussuritized) plagioclase. The plagioclase-bearing dunites essentially contain olivine (69 %), plagioclase (28 %)
and minor amounts of orthopyroxene, spinel and clinopyroxene. An important
aspect of the residual harzburgite samples is that they have been affected by
metasomatism (magmatic alteration) during melt circulation and their subsequent
accumulation within fissures and voids. Thus, melt percolation has formed
localized and discontinuous trails (\1 mm) and droplets of heterogeneous rocks
(i.e. sample GN03-14) within the host peridotite as well as in larger veins and
dykes (Figs. 8.10 and 8.11).
No continuous gabbroic formation, which might have suggested the presence of
a magma chamber, was found. The only occurrence of gabbroic rocks is in metric
sized veins, veinlets and dykes (\2 m wide) and in metric sized intrusive lenses
within the serpentinized peridotites. These magmatic intrusions range from
troctolite, olivine gabbro, gabbronorite and ferro-gabbros (ilmenite-bearing gabbros). From their texture and mineral composition two sets of gabbroic rocks were
identified:
270
8 Fracture Zones and Transform Faults
must have undergone vertical uplift in order to expose mantle material. In addition,
the observations of more striated and schistose serpentinized peridotites and the
presence of consolidated breccias indicated tectonic stress along the general east–
west strike-slip motion of the Garrett Transform as well as in another direction
(WNW-ESE). The dives were able to define the limits of the active tectonic zone
(ATZ), which seemed at the time to be located on the northern slope of the Median
Ridge. This ATZ is about 2.5 km wide to the East at 111°29
0 W and about 300 m
wide at 111°30
0 W. The ATZ was responsible for the emplacement of the mantle
peridotite on the northern slope of the Median Ridge. In addition, these three dives
permitted us to observe the circulation of magmatic melt within exposed peridotite
in the form of light colored lances and dykes, made up of plagioclase and clinopyroxene minerals. At a distance to the west from the ATZ (that is to say: to the
west of dive GN15 at 13°28’S-111°34’W), the peridotite disappears and volcanic
outcrops made up of basalt were observed on tectonically undisturbed terrain.
Ultramafic rocks consisting essentially of peridotite (harzburgite) and dunite
plus mafic gabbroic rocks were only discovered on the north-facing slope of the
Median Ridge. The first hand description of the collected samples, which was
performed on board the ship, helped us classify the rocks and gave us valuable
information on the composition and the mode of emplacement of the Median
Ridge (Hébert et al. 1983).
Peridotite, consisting essentially of serpentinized harzburgite residues, is
associated with dunites and troctolites which have textures inherited from crystal
accumulation during crystal growth and metasomatism in magma-lenses. The
dunites and troctolites are formed during magmatic injections into the peridotite.
They show mineral lenses of clinopyroxene that are elongated to a faint foliation
of serpentine in a subparalel direction. Some of the plagioclase-bearing dunites
and troctolites show kink-bands and altered (saussuritized) plagioclase. The plagioclase-bearing dunites essentially contain olivine (69 %), plagioclase (28 %)
and minor amounts of orthopyroxene, spinel and clinopyroxene. An important
aspect of the residual harzburgite samples is that they have been affected by
metasomatism (magmatic alteration) during melt circulation and their subsequent
accumulation within fissures and voids. Thus, melt percolation has formed
localized and discontinuous trails (\1 mm) and droplets of heterogeneous rocks
(i.e. sample GN03-14) within the host peridotite as well as in larger veins and
dykes (Figs. 8.10 and 8.11).
No continuous gabbroic formation, which might have suggested the presence of
a magma chamber, was found. The only occurrence of gabbroic rocks is in metric
sized veins, veinlets and dykes (\2 m wide) and in metric sized intrusive lenses
within the serpentinized peridotites. These magmatic intrusions range from
troctolite, olivine gabbro, gabbronorite and ferro-gabbros (ilmenite-bearing gabbros). From their texture and mineral composition two sets of gabbroic rocks were
identified:
270
8 Fracture Zones and Transform Faults