also have occurred. The plume has carried deep-seated material made up of
products enriched in LILE elements from recycled sediment found in subduction
zones. The basalt and the silica-rich lava of the Foundation Seamount Chain is
more enriched in the
206 Pb/
204 Pb ratio than that of the magma erupted on the PAR
(Pacific Antarctic Ridge). This suggests that the lava in these two provinces
originated from different sources. In addition, it was shown that the basalt from
both areas lies on a linear mixing trend.
On the contrary, in the PAR, the andesite and dacite (silica-rich lava) have a
higher
87
Sr/
86 Sr ratio for a given
206 Pb/
204
Pb, which might indicate the assimilation or a contamination of material enriched in the
87
Sr/
86 Sr ratio. Seawater has a
high
87 Sr/
86 Sr ratio (0.709), hence it is not excluded that the assimilation of altered
oceanic crust (metamorphosed to the greenschist and/or amphibolite facies) is
responsible for this type of enrichment. Haase et al. (2005) estimated that about
25 % metamorphic crust would be necessary and would need to be assimilated in
order to produce silica-rich lava in the PAR.
Metamorphosed amphibolite is often found in oceanic crust as well as in
ophiolite complexes, and this product has a high distribution coefficient for Nb
(niobium) relative to La (lanthanum). A melt of amphibolite composition would
yield a relatively low Nb/La ratio. Haase et al. (2005) have shown that two
separate fields of Nb/La ratios can exist for the basalt-silica-enriched lava
(andesite-dacite) association: one with low values (Nb/La \ 0.9) and the second
with higher values (Nb/La = 1.0–1.2). These different fields also coincide with the
locations of the two basalt-silica-enriched flows. Those with lower Nb/La values
are found on the ridge segments south of latitude 39–40°S and the others, with
higher Nb/La values, occur at 37–38°S.
It is also speculated that the partial melting of heterogeneous mantle sources
having variable Nb/La ratios influenced the two different ridge segments. Furthermore, the relatively high chlorine content, and Cl/K ratio is another indication
supporting the assimilation of altered (metamorphosed) oceanic crust for producing
these lavas. In conclusion, silica-rich lavas have probably originated from a combination of partial melting at the source, plus crystal-liquid fractionation and the
assimilation of previously altered oceanic crust. It is also likely that silica-rich lavas
do not have the same parent melt as do other basalts found on the ocean floor.
Gabbroic Rocks
Gabbroic rocks constitute another important component of the oceanic lithosphere
and probably comprise about 20 % of the exposed outcrops on the ocean floor.
They represent the coarse grained equivalent of a basalt which has solidified in a
magma reservoir and/or conduit. Along with the dolerite-dyke components of the
oceanic crust, gabbroic rocks are very significant because they suggest the presence of a magma chamber. Gabbros differ from the dolerite formed in dykes due
their grain size (larger crystals) and texture. Because gabbros are generally formed
Origin of Silica-Rich Lavas
87
products enriched in LILE elements from recycled sediment found in subduction
zones. The basalt and the silica-rich lava of the Foundation Seamount Chain is
more enriched in the
206 Pb/
204 Pb ratio than that of the magma erupted on the PAR
(Pacific Antarctic Ridge). This suggests that the lava in these two provinces
originated from different sources. In addition, it was shown that the basalt from
both areas lies on a linear mixing trend.
On the contrary, in the PAR, the andesite and dacite (silica-rich lava) have a
higher
87
Sr/
86 Sr ratio for a given
206 Pb/
204
Pb, which might indicate the assimilation or a contamination of material enriched in the
87
Sr/
86 Sr ratio. Seawater has a
high
87 Sr/
86 Sr ratio (0.709), hence it is not excluded that the assimilation of altered
oceanic crust (metamorphosed to the greenschist and/or amphibolite facies) is
responsible for this type of enrichment. Haase et al. (2005) estimated that about
25 % metamorphic crust would be necessary and would need to be assimilated in
order to produce silica-rich lava in the PAR.
Metamorphosed amphibolite is often found in oceanic crust as well as in
ophiolite complexes, and this product has a high distribution coefficient for Nb
(niobium) relative to La (lanthanum). A melt of amphibolite composition would
yield a relatively low Nb/La ratio. Haase et al. (2005) have shown that two
separate fields of Nb/La ratios can exist for the basalt-silica-enriched lava
(andesite-dacite) association: one with low values (Nb/La \ 0.9) and the second
with higher values (Nb/La = 1.0–1.2). These different fields also coincide with the
locations of the two basalt-silica-enriched flows. Those with lower Nb/La values
are found on the ridge segments south of latitude 39–40°S and the others, with
higher Nb/La values, occur at 37–38°S.
It is also speculated that the partial melting of heterogeneous mantle sources
having variable Nb/La ratios influenced the two different ridge segments. Furthermore, the relatively high chlorine content, and Cl/K ratio is another indication
supporting the assimilation of altered (metamorphosed) oceanic crust for producing
these lavas. In conclusion, silica-rich lavas have probably originated from a combination of partial melting at the source, plus crystal-liquid fractionation and the
assimilation of previously altered oceanic crust. It is also likely that silica-rich lavas
do not have the same parent melt as do other basalts found on the ocean floor.
Gabbroic Rocks
Gabbroic rocks constitute another important component of the oceanic lithosphere
and probably comprise about 20 % of the exposed outcrops on the ocean floor.
They represent the coarse grained equivalent of a basalt which has solidified in a
magma reservoir and/or conduit. Along with the dolerite-dyke components of the
oceanic crust, gabbroic rocks are very significant because they suggest the presence of a magma chamber. Gabbros differ from the dolerite formed in dykes due
their grain size (larger crystals) and texture. Because gabbros are generally formed
Origin of Silica-Rich Lavas
87