The diagnostic geochemical and geological features of
Cretaceous oceanic plateaus can be used to help distinguish plateau sequences in the geological record from
those originating in other tectonic settings. These diagnostic features are reviewed below and more fully discussed
in Kerr et al. (2000).
Igneous rocks formed in any subduction zone setting
are relatively easily distinguished from oceanic plateau
sequences because arcs possess generally more evolved
lavas, with high La/Nb ratios, and only occasionally
high-MgO lavas. Additionally, oceanic plateaus do not
possess the abundant volcanic ash layers present in volcanic arc sequences. However, a La/Nb ratio ~1 is not an
entirely robust discriminant of an oceanic plateau
sequence, given that the Kerguelen oceanic plateau often
possesses low La/Nb values, due to magma interaction
with the continental lithosphere beneath the plateau. This
illustrates the point that geochemical discriminants of tectonic setting should not be used in isolation and that other
geological evidence should also be considered. In the case
of Kerguelen, the lack of volcaniclastic horizons and the
thickened oceanic crust help confirm its oceanic plateau
affinity.
Most mid-ocean ridge basalts possess light rare earth
element-depleted chondrite-normalized patterns in contrast to the generally flat patterns of oceanic plateau lavas.
Furthermore, high-MgO lavas are much more common in
oceanic plateaus than at mid-ocean ridges.
The geochemistry of igneous rocks is only of limited
use in distinguishing between volcanic successions
formed in back-arc basins from those formed in oceanic
plateaus. However, the mantle temperature below a
back-arc basin (i.e., ambient mantle) is lower than a mantle plume (200–300
C hotter than ambient mantle), and
this results in the eruption of relatively few high-MgO
lavas. Furthermore, because of their proximity to active
subduction zones, back-arc basin sequences will also be
more likely to contain abundant volcaniclastic horizons
than oceanic plateaus.
Examples of accreted oceanic plateaus
On the basis of many of the criteria outlined above, a significant number of accreted oceanic plateaus have been
identified in the geological record (Kerr, 2014). Examples
of some of the more significant plateaus are briefly
reviewed below:
Wrangellia oceanic plateau
The accreted Wrangellia plateau, comprising basalts, with
minor picrites, is discontinuously exposed along the continental margin of Alaska and Canada over a distance of
~2,300 km. The plateau formed in the Panthalassa Ocean
ca. 230–225 Ma, in a progressively shallowing marine
environment, which eventually became subaerial (see
reviews in Greene et al., 2008; Greene et al., 2009a;
Greene et al., 2009b). The plateau accreted to western
North America in the Late Jurassic–Early Cretaceous.
Most of the Wrangellia plateau basalts contain
5–9 wt% MgO and are light rare earth element enriched,
unlike the Ontong Java Plateau and most of the Caribbean
Plateau. In contrast, many of the lower basalts are light
rare earth element depleted with MgO contents that extend
to more magnesian (6–12 wt% MgO) compositions
(Greene et al., 2008; Greene et al., 2009a; Greene et al.,
2009b). Modeling suggests that the magmas were derived
by 23–27 % partial melting of a mantle source ~200
C
hotter than ambient mantle, thus supporting a hot mantle
plume origin for the Wrangellia plateau (Greene et al.,
2009b).
Carboniferous to Cretaceous oceanic plateaus
accreted in Japan
Japan comprises a series of terrains that have been
accreted to the Eurasian plate over the past 400 Ma. The
Chugoku and Chichibu belts in southwest Japan contain
up to 30 % basaltic material. Geochemical data suggests
that these basaltic assemblages are remnants of a Carboniferous plume-derived oceanic plateau that formed in the
Panthalassa Ocean (Tatsumi et al., 2000).
Permian greenstones of the Mino–Tamba accretionary
complex found in southwest Japan comprise pillowed
and massive basalt flows with minor picrites, with similar
isotopic and trace element compositions (Ichiyama et al.,
2008) to the Ontong Java and Caribbean plateaus.
Ichiyama et al. (2008) proposed that these basalts were
formed by partial melting of a shallow mantle plume head
below the thick oceanic lithosphere in the Early Permian.
In contrast to the other accretionary belts in Japan, the
Sorachi–Yezo terrain is dominated by oceanic crust and
lithosphere (Kimura et al., 1994; Ichiyama et al., 2012).
It comprises pillowed basalts with occasional picrites
and dolerite sills of Tithonian age (150–145 Ma), which
have been interpreted to represent an accreted oceanic plateau. This Jurassic–Early Cretaceous plateau (named the
Sorachi Plateau; Kimura et al., 1994) is the same age as
the Shatsky Rise (Figure 1), suggesting that the Sorachi
Plateau and the Shatsky Rise were originally a single plateau that formed near the Kula–Pacific–Farallon triple
junction at ~150 Ma (Kimura et al., 1994; Ichiyama et al.,
2014). The Sorachi part of the plateau was carried northwestward on the Kula Plate and collided with the Eurasian
continental margin in the mid-Cretaceous.
Precambrian oceanic plateaus
As already noted, the accreted oceanic plateau material
has been a significant contributor to crustal growth. Some
of the oldest oceanic plateau sequences comprise pillow
basalts and komatiites found in the ~3.5 Ga greenstone
belts of the Kaapvaal Craton of Southern Africa. The geochemical signatures of these rocks suggest an oceanic plateau origin (Chavagnac, 2004) and, along with mafic
rocks found in the Pilbara Craton of Australia, they represent some of the oldest oceanic plateau material on Earth
(see review in Kerr, 2003).
OCEANIC PLATEAUS
563
Cretaceous oceanic plateaus can be used to help distinguish plateau sequences in the geological record from
those originating in other tectonic settings. These diagnostic features are reviewed below and more fully discussed
in Kerr et al. (2000).
Igneous rocks formed in any subduction zone setting
are relatively easily distinguished from oceanic plateau
sequences because arcs possess generally more evolved
lavas, with high La/Nb ratios, and only occasionally
high-MgO lavas. Additionally, oceanic plateaus do not
possess the abundant volcanic ash layers present in volcanic arc sequences. However, a La/Nb ratio ~1 is not an
entirely robust discriminant of an oceanic plateau
sequence, given that the Kerguelen oceanic plateau often
possesses low La/Nb values, due to magma interaction
with the continental lithosphere beneath the plateau. This
illustrates the point that geochemical discriminants of tectonic setting should not be used in isolation and that other
geological evidence should also be considered. In the case
of Kerguelen, the lack of volcaniclastic horizons and the
thickened oceanic crust help confirm its oceanic plateau
affinity.
Most mid-ocean ridge basalts possess light rare earth
element-depleted chondrite-normalized patterns in contrast to the generally flat patterns of oceanic plateau lavas.
Furthermore, high-MgO lavas are much more common in
oceanic plateaus than at mid-ocean ridges.
The geochemistry of igneous rocks is only of limited
use in distinguishing between volcanic successions
formed in back-arc basins from those formed in oceanic
plateaus. However, the mantle temperature below a
back-arc basin (i.e., ambient mantle) is lower than a mantle plume (200–300
C hotter than ambient mantle), and
this results in the eruption of relatively few high-MgO
lavas. Furthermore, because of their proximity to active
subduction zones, back-arc basin sequences will also be
more likely to contain abundant volcaniclastic horizons
than oceanic plateaus.
Examples of accreted oceanic plateaus
On the basis of many of the criteria outlined above, a significant number of accreted oceanic plateaus have been
identified in the geological record (Kerr, 2014). Examples
of some of the more significant plateaus are briefly
reviewed below:
Wrangellia oceanic plateau
The accreted Wrangellia plateau, comprising basalts, with
minor picrites, is discontinuously exposed along the continental margin of Alaska and Canada over a distance of
~2,300 km. The plateau formed in the Panthalassa Ocean
ca. 230–225 Ma, in a progressively shallowing marine
environment, which eventually became subaerial (see
reviews in Greene et al., 2008; Greene et al., 2009a;
Greene et al., 2009b). The plateau accreted to western
North America in the Late Jurassic–Early Cretaceous.
Most of the Wrangellia plateau basalts contain
5–9 wt% MgO and are light rare earth element enriched,
unlike the Ontong Java Plateau and most of the Caribbean
Plateau. In contrast, many of the lower basalts are light
rare earth element depleted with MgO contents that extend
to more magnesian (6–12 wt% MgO) compositions
(Greene et al., 2008; Greene et al., 2009a; Greene et al.,
2009b). Modeling suggests that the magmas were derived
by 23–27 % partial melting of a mantle source ~200
C
hotter than ambient mantle, thus supporting a hot mantle
plume origin for the Wrangellia plateau (Greene et al.,
2009b).
Carboniferous to Cretaceous oceanic plateaus
accreted in Japan
Japan comprises a series of terrains that have been
accreted to the Eurasian plate over the past 400 Ma. The
Chugoku and Chichibu belts in southwest Japan contain
up to 30 % basaltic material. Geochemical data suggests
that these basaltic assemblages are remnants of a Carboniferous plume-derived oceanic plateau that formed in the
Panthalassa Ocean (Tatsumi et al., 2000).
Permian greenstones of the Mino–Tamba accretionary
complex found in southwest Japan comprise pillowed
and massive basalt flows with minor picrites, with similar
isotopic and trace element compositions (Ichiyama et al.,
2008) to the Ontong Java and Caribbean plateaus.
Ichiyama et al. (2008) proposed that these basalts were
formed by partial melting of a shallow mantle plume head
below the thick oceanic lithosphere in the Early Permian.
In contrast to the other accretionary belts in Japan, the
Sorachi–Yezo terrain is dominated by oceanic crust and
lithosphere (Kimura et al., 1994; Ichiyama et al., 2012).
It comprises pillowed basalts with occasional picrites
and dolerite sills of Tithonian age (150–145 Ma), which
have been interpreted to represent an accreted oceanic plateau. This Jurassic–Early Cretaceous plateau (named the
Sorachi Plateau; Kimura et al., 1994) is the same age as
the Shatsky Rise (Figure 1), suggesting that the Sorachi
Plateau and the Shatsky Rise were originally a single plateau that formed near the Kula–Pacific–Farallon triple
junction at ~150 Ma (Kimura et al., 1994; Ichiyama et al.,
2014). The Sorachi part of the plateau was carried northwestward on the Kula Plate and collided with the Eurasian
continental margin in the mid-Cretaceous.
Precambrian oceanic plateaus
As already noted, the accreted oceanic plateau material
has been a significant contributor to crustal growth. Some
of the oldest oceanic plateau sequences comprise pillow
basalts and komatiites found in the ~3.5 Ga greenstone
belts of the Kaapvaal Craton of Southern Africa. The geochemical signatures of these rocks suggest an oceanic plateau origin (Chavagnac, 2004) and, along with mafic
rocks found in the Pilbara Craton of Australia, they represent some of the oldest oceanic plateau material on Earth
(see review in Kerr, 2003).
OCEANIC PLATEAUS
563