Moore, K.R., Ryan, P.D.R.
Department of Earth and Ocean Sciences, School of Natural Sciences,
National University of Ireland Galway, Galway, Ireland.
Key
carbonatite localities that have been investigated as part of INTAS project
05-1000008-7938 ‘Diamond and Graphite in Carbonate Magmas’ are: (1) Western
Greenland, where the distribution of kimberlites, lamproites and carbonatites is correlated with depth to a
fossil subducted slab (personal communication,
Nielsen 2008); (2) the Chagatai Complex of diamond-bearing silicocarbonatite
dykes and pipes at the western end of the Southern Nuratau
Mountains (Tianshan Orogen),
Uzbekistan; (3) Transbaikalia, Russia, where
carbonatites are temporally correlated both with break-up of the Siberian craton by rifting (Windley et
al., 2007) and also potentially with delamination of
the lithosphere beneath Mongolia (Petit et al., 2002); and (4) The Kola
Peninsula in Scandinavia, where the post-orogenic
Baltic Shield is rifted and has been heated at the base of the lithosphere by
mantle upwelling (Bulakh et
al, 2004).š These key localities can be
tectonically summarised as suture zones that have later been subject to
decompression or heating and produced carbonate magmas as a consequence.
Published
(Lapin et al., 2005) and new (personal communication, Savatenkov
and Konopelko 2008) isotopic data for sövite and
silicocarbonatite dykes from Chagatai (Uzbekistan) are dominated by a mantle
component, but have a significant crustal component
regardless of the abundance of xenoliths.š
Some mechanism is required for contamination of the isotopic signature
of carbonate magmas that, given the association of diamonds with silicocarbonatite
(whether as xenocrysts or phenocrysts)
must ultimately have a deep source (Djuraev and Divaev, 1999; Moore et al, 2009).š Carbonatites emplaced into the Central Asian Orogenic Belt have both subducted
oceanic lithosphere and subducted continental rocks (Windley et al., 2007) as potential source components.š Release of CO2- or CO2-H2O-rich
fluids of melts from a subducting slab has been
linked to enhanced eclogitization, crustal isotopic components in a mantle wedge, and diamond
formation in a subduction environment (Ducea et al., 2005; Timm et al.,
2004).š Furthermore, experimental
petrology has shown that melts parental to carbonatite activity can be geochemically related to fusion of eclogitic
mantle (Treiman & Essene,
1983; Dasgupta et
al., 2003).š
An
alternative mechanism to place eclogite in the
lithosphere beneath intra-continental carbonatite activity is the formation of deep crustal roots that are isostatically stable within the mantle if buoyancy is
overcome by conversion to eclogite facies assemblages.š
Such assemblages may be preserved long after the end of the orogenic process (Ryan 2001) and numerical modelling shows
that such roots are subject to pressures > 3GPa and temperatures in excess
of 700oC (Ryan & Dewey 1997), which is considerably lower than
the solidus of carbonated eclogites and potentially
within the conditions of diamond stability.š
If a sufficiently dense eclogite layer forms,
such that it becomes gravitationally unstable, detaches and sinks, it is
replaced by upwelling mantle, providing a heat source
at the base of the lithosphere.
Finite
element modelling is used to investigate the production of carbonate magmas
from carbonated eclogite in tectonic configurations anologuous to the post-orogenic
scenario in Central Asia, and other INTAS localities, where:
1.
Subducted material remains
at depth in the source region of carbonate magmas,
2.
Delamination has caused the
emplacement of a high-temperature peridotite, heating the base of a thermally
re-equilibrated lithosphere, and
3.
Crustal stretching and
thinning decompress post-orogenic lithosphere.
Initial findings
are that maximum preservation of carbonated eclogites occurs in cold thick
continental lithosphere that has undergone minimal overstepping of the solidus, and that emplacement of hot mafic magmas at the
base of the lithosphere causes local, rapid, short-lived melting of carbonated
eclogites without necessarily producing associated silicate magmas.
This presentation was financially supported by INTAS project 05-1000008-7938.
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