Genesis
of carbonatites: inferences from a world map and
database
Woolley A.R.
Department of Mineralogy,
A recently
published world map and database of carbonatites
(Woolley and Kjarsgaard, 2008) includes data that
provide insights, some new, into the spatial and temporal distribution and petrogenesis of carbonatites. 527
carbonatite occurrences are briefly described and
their plotting on a world geology base map demonstrates (a) that in many
provinces there has been repeated emplacement of carbonatites
over geological time, (b) that there are few carbonatites
in oceanic areas, most of which lie close to continental margins, (c) that
there is an overwhelming concentration in Precambrian cratons
(perhaps as much as 86%), including occurrences of Phanerozoic
age, and (d) that many of these cratonic areas are
‘domed’ i.e. are topographically elevated. The repetition of carbonatite emplacement in restricted areas of continental
lithosphere/plate, which have generally drifted
considerable distances between episodes of carbonatitic
volcanism, must preclude any genesis involving mantle plumes, and underlines
the crucial role that the lithosphere undoubtedly plays in carbonatite
generation. The doming of the carbonatite-bearing cratonic areas is interpreted as reflecting metasomatism of the lithosphere, the metasomatism
being concentrated, initially at least, by structural features leading to
‘lithosphere focussing’.
The database
comprises short descriptions of all 527 carbonatites
together with tabulated information on economic aspects, including working
deposits, and a range of subsidiary maps. Some 50 occurrences include extrusive
carbonatite and these are particularly significant
because 20 of them contain mantle xenoliths and/or xenocrysts,
with none found in intrusive carbonatites, which gives
direct evidence of a mantle origin and metasomatism,
and pressure (depth) and temperature data. The igneous silicate rocks found in
association with carbonatites are tabulated and form
an extensive range of rock types which fall into six series i.e. kimberlite, melilitite (melilitolite), nephelinite (ijolite), phonolite (foid syenite), basanite (alkali gabbro) and trachyte (syenite). About 20% of carbonatites have no associated silicate rocks. The
extrusive carbonatites demonstrate one major
difference from the intrusive ones in that nearly half of them are associated with
melilite-bearing rocks, whereas the equivalent
association in the intrusives is found in <10% of occurrences.
The relative abundance of melilite-bearing rocks
amongst the extrusive carbonatites is believed to
reflect one of the major, if not the main, paths of carbonatite
evolution, while the relative paucity of these rocks amongst intrusive carbonatites is considered to be owing to loss during
multiple high-level emplacement and volcano growth as well as reflecting the
instability of melilite in later melt fractions. Finally,
essentially utilising the evidence of the map and database, a model is
presented in the form of a lithosphere section, metasomatised
in the lower part, in which carbonatites are either
generated at depth directly in the mantle or at higher levels by fractionation
from at least five of the six silicate series. The silicate series vary in depth
of origin from kimberlite at the base of the
lithosphere, or within the uppermost asthenosphere,š through melilitites
and nephelinites to relatively shallow originating phonolites and basanites. Some
cognisance is also taken of known alkali silicate differentiation series. All
the silicate series rocks also occur in carbonatite-free
associations so that the impression is gained that carbonatites
are only generated when a particular type of metasomatism
is superimposed on lithosphere mantle which, in its absence, has the potential to
produce the silicate rocks anyway. However, the silicate rocks, being
essentially alkaline, themselves probably have antecedants
involving mantle metasomatism, albeit chemically
distinct from that leading to carbonatite generation.
Reference:
Woolley,
A.R. and Kjarsgaard, B.A. 2008. Carbonatite
occurrences of the world: map and database. Geological Survey
of