Genesis of Ultrapotassic Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 453

the requisite signature. The unique isotopic characteristics of Samoan plume likely
arise from its tectonic setting at the northern terminus of subduction in the Tonga
trench.
Addition of subducted materials to mantle wedge might have caused variation in
Pb isotopic data of K-rich lavas of Ringgit Beser complex (Indonesia, Edwards
et al. 1994) and that both highly and moderately enriched lavas are derived from
heterogeneous mantle source.

15.8 Metasomatic Fluid Source

Nelson (1992) divided the potassic rock suite (with molar K 2 O/Na 2 O > 1) into an
“ orogenic ” subgroup that occur in the subduction — related tectonic settings and an

“ ned to stable continental settings. Represen-

tatives of both sub-groups possess trace element and isotopic signature consistent
with the contamination of their magma sources by isotopically evolved

anorogenic ” sub-group that are con

“ metaso-

matic components enriched in incompatible elements. It is generally argued that

these metasomatic components are principally derived from subducted lithosphere,
containing sediments. Most examples of orogenic potassic magmatism (e.g. Italian
potassic rocks, Spanish lamproites, Sunda arc leucitites) have trace-element and Sr
and Nd and Pb isotopic characteristics consistent with the contamination of their
mantle sources by a component derived from marine sediments. Nelson (1992)
argued that potassic magmas belonging to anorogenic subgroup, have generally
similar incompatible trace elements and Sr and Nd isotopic characteristics to those
of orogenic potassic magmas, but many examples have unusual Pb isotopic com-
positions with unradiogenic 206 Pb/ 204 Pb. Modern marine sediments characteristi-
cally have low U/Pb ratios and the unradiogenic 206 Pb/ 204 Pb of anorogenic potassic
magmas may have evolved during long-term storage of subducted sediments (or
components derived from them) within the subcontinental lithosphere. These
unusual Pb isotopic compositions require substantial time period (>1 Ga) to have
elapsed between the fractionation event and subsequent potassic magmatism. The
fractionation events might have lowered the U/Pb ratio due to erosion and sedi-
mentation at the Earth

”

is surface. It is therefore, not surprising that most examples of
anorogenic potassic magmatism are not associated with recent subduction pro-
cesses. According to Nelson, although the eruption of potassic magmas is com-
monly related to rifting or hotspot activity, these processes do not necessarily play
an important role in the genesis of unusual sources from which potassic magmas are
derived.
Ultrapotassic volcanism related to subduction has been reported from southern
Spain, Italy, Highwood Mountains and Rio Grande Rift, U.S.A., Tibet, Indonesia,
Manchuria and many other localities. Thus, there may be arguments, whether some
or all strongly potassic and ultrapotassic ma

'

c magmas are directly related to
subduction and whether their immediate sources are within lithospheric upper
mantle (also see Thompson et al. 1990).