Ishiwatari mainly considered petrological and geochemical
criteria in gabbros and basalts, whereas Boudier and
Nicolas applied a structural and petrological approach,
largely based on the mantle component of ophiolites. This
mantle contribution to the fate of any ophiolite and its structural approach being essential, we wish to incorporate these
wider criteria in Table 1.
In a rising mantle with an average mantle composition,
mantle depletion increases with spreading rate of the ridge
of origin. This approach does not account for ophiolites
that depart much from the average norm. For instance,
the present day Reykjanes Ridge is significantly hotter
than other ridges because it is located close south to the
Iceland hot spot.
Figure 1 emphasizes the contrast between fastspreading (HOT) and slow-spreading (LHOT-LOT)
ridges. The copious thermal budget in HOT supports the
continuous activity at ~1,200
C of a crustal magma chamber beneath the ridge axis (see “Oceanic Spreading Centers”). The asthenosphere/lithosphere upper limit
(~1,100
C) is the wall of this permanent magma chamber.
At the Moho level, this limit flattens. As a consequence,
the fast rising and melting mantle diapir rising below the
ridge axis can diverge in nearly all directions. In contrast
in LHOT, the thermal budget does not sustain
a permanent magma chamber. The steepened limit
between lithosphere and asthenosphere along the ridge
axis channels mantle flow parallel to the ridge and generates just beneath Moho and only locally, a thickened MTZ
compared to HOT. The LOT ophiolites ascribed to conditions of continental to oceanic rifts are structurally akin to
LHOT with, as a consequence of a lower thermal budget,
a steep limit between lithosphere and asthenosphere
channeling mantle flow parallel to the ridge axis (see
“Oceanic Rifts”).
Modes of emplacement of ophiolites
Within the frame of plate tectonics, Moores (1982) and
Coleman (1984) divided ophiolites into two groups. The
Tethyan ophiolites are giant thrust sheets resting upon passive margins of continental and adjacent oceanic crust, as
along the Oman margin of Arabia (Figure 2a). In contrast,
the Cordilleran ophiolites are incorporated, often dismembered or incorporated in mélanges, into accretion terranes. This is typically achieved in California Cordillera
where ophiolites are in a situation of forearc (Figure 2b).
Clearly, the Western Pacific and its complex back-arc
Ophiolites, Table 1 The three main ophiolite types reflecting decreasing mantle partial melting related to decreasing spreading
rates. Specific references to the massifs of origin (their number in first line) are presented in Nicolas and Boudier, where
a comparable typology based on information from oceanic environments is also presented
Harzburgite ophiolite type
(HOT)
Harzburgite lherzolite ophiolite type (LHOT)
Lherzolite ophiolite
type (LOT)
Number of Massifs
considered
3
1 2
4
Crustal section
Thickness: 4–6 km
Thickness: 2–3 km
Thickness : 0–1 km
Volcanics overlying
sheeted dikes
Low-alumina tholeiite
High-alumina tholeiite
Alkali basalt
Sheeted dikes
Well expressed and steep
Steep to horizontal, poorly organized
Absent or poorly
organized
Gabbros
Thick, continuous
Thin, continuous, or nestled
Absent, small bodies
Exposure
Lithology
Foliated ol-gabbros locally
gabbronorites
Dominant gabbronorites, ol-gabbros, ferrogabbros
Ol-gabbros
Structure
Lower, layered gabbros
Well to poorly layered-foliated gabbros, isotropic gabbros
Isotropic to foliated
gabbros
Upper, foliated gabbros
Top, isotropic
Penetrative
deformation
Very large, magmatic
Magmatic to plastic (flasergabbro)
Seawater
alteration
Variable, up to high-T
Variable
Variable,
metamorphic
Wehrlite intrusions Abundant
Present
Absent
Mantle section
Moho transition
zone (MTZ)
Dunite-gabbro, websterites
50–400 m
Locally thick (up to 2 km) dunite, interlayered with wehrlite,
websterite
Dunite lenses
Shear zones
Uncommon, HT, vertical
horizontal lineation
Common, mid-T to low-T, steep, steep lineation, oceanic core
complex horizontal detachments
Very common, mid-T
to low-T steep
Diabase
occurrence
Uncommon
Common
Common dikes-sills
Chromite pods
Uncommon
Common to very large
Uncommon, absent
Serpentinization
Lizardite
Lizardite and antigorite
Antigorite
OPHIOLITES
593
criteria in gabbros and basalts, whereas Boudier and
Nicolas applied a structural and petrological approach,
largely based on the mantle component of ophiolites. This
mantle contribution to the fate of any ophiolite and its structural approach being essential, we wish to incorporate these
wider criteria in Table 1.
In a rising mantle with an average mantle composition,
mantle depletion increases with spreading rate of the ridge
of origin. This approach does not account for ophiolites
that depart much from the average norm. For instance,
the present day Reykjanes Ridge is significantly hotter
than other ridges because it is located close south to the
Iceland hot spot.
Figure 1 emphasizes the contrast between fastspreading (HOT) and slow-spreading (LHOT-LOT)
ridges. The copious thermal budget in HOT supports the
continuous activity at ~1,200
C of a crustal magma chamber beneath the ridge axis (see “Oceanic Spreading Centers”). The asthenosphere/lithosphere upper limit
(~1,100
C) is the wall of this permanent magma chamber.
At the Moho level, this limit flattens. As a consequence,
the fast rising and melting mantle diapir rising below the
ridge axis can diverge in nearly all directions. In contrast
in LHOT, the thermal budget does not sustain
a permanent magma chamber. The steepened limit
between lithosphere and asthenosphere along the ridge
axis channels mantle flow parallel to the ridge and generates just beneath Moho and only locally, a thickened MTZ
compared to HOT. The LOT ophiolites ascribed to conditions of continental to oceanic rifts are structurally akin to
LHOT with, as a consequence of a lower thermal budget,
a steep limit between lithosphere and asthenosphere
channeling mantle flow parallel to the ridge axis (see
“Oceanic Rifts”).
Modes of emplacement of ophiolites
Within the frame of plate tectonics, Moores (1982) and
Coleman (1984) divided ophiolites into two groups. The
Tethyan ophiolites are giant thrust sheets resting upon passive margins of continental and adjacent oceanic crust, as
along the Oman margin of Arabia (Figure 2a). In contrast,
the Cordilleran ophiolites are incorporated, often dismembered or incorporated in mélanges, into accretion terranes. This is typically achieved in California Cordillera
where ophiolites are in a situation of forearc (Figure 2b).
Clearly, the Western Pacific and its complex back-arc
Ophiolites, Table 1 The three main ophiolite types reflecting decreasing mantle partial melting related to decreasing spreading
rates. Specific references to the massifs of origin (their number in first line) are presented in Nicolas and Boudier, where
a comparable typology based on information from oceanic environments is also presented
Harzburgite ophiolite type
(HOT)
Harzburgite lherzolite ophiolite type (LHOT)
Lherzolite ophiolite
type (LOT)
Number of Massifs
considered
3
1 2
4
Crustal section
Thickness: 4–6 km
Thickness: 2–3 km
Thickness : 0–1 km
Volcanics overlying
sheeted dikes
Low-alumina tholeiite
High-alumina tholeiite
Alkali basalt
Sheeted dikes
Well expressed and steep
Steep to horizontal, poorly organized
Absent or poorly
organized
Gabbros
Thick, continuous
Thin, continuous, or nestled
Absent, small bodies
Exposure
Lithology
Foliated ol-gabbros locally
gabbronorites
Dominant gabbronorites, ol-gabbros, ferrogabbros
Ol-gabbros
Structure
Lower, layered gabbros
Well to poorly layered-foliated gabbros, isotropic gabbros
Isotropic to foliated
gabbros
Upper, foliated gabbros
Top, isotropic
Penetrative
deformation
Very large, magmatic
Magmatic to plastic (flasergabbro)
Seawater
alteration
Variable, up to high-T
Variable
Variable,
metamorphic
Wehrlite intrusions Abundant
Present
Absent
Mantle section
Moho transition
zone (MTZ)
Dunite-gabbro, websterites
50–400 m
Locally thick (up to 2 km) dunite, interlayered with wehrlite,
websterite
Dunite lenses
Shear zones
Uncommon, HT, vertical
horizontal lineation
Common, mid-T to low-T, steep, steep lineation, oceanic core
complex horizontal detachments
Very common, mid-T
to low-T steep
Diabase
occurrence
Uncommon
Common
Common dikes-sills
Chromite pods
Uncommon
Common to very large
Uncommon, absent
Serpentinization
Lizardite
Lizardite and antigorite
Antigorite
OPHIOLITES
593