Archean granitoid magmatism and the chemical evolution
of the cratonic lithosphere
Robert Rapp1, Herve Martin2, Didier Laporte2, and J-F.
1 Research School of Earth Sciences, Australian National
University, Canberra, ACT 0200, Australia
2 Laboratoire Magmas et Volcans, Universite de Blaise Pascal, Clermont-Ferrand,
3 Department of Geology, University of Stellenbosch, Matteland, South Africa
Although there is indirect evidence for the existence
of continental crust on Earth more than 4.0 Ga ago (Harrison et al.,
2005), no intact, preserved fragments of continents have been found. This
begs the question when and how did the first truly "nondestructible" continents
form? The development of deep (>200 km), old and chemically
refractory roots to the continents in the underlying lithospheric mantle
appears to be a critical stage in the physical and chemical evolution
of Earth's cratons, the old and stable nuclei of the continents. Without
roots in the underlying sub-cratonic lithospheric mantle, the preservation
of large continental masses over billions of years may not have been
possible. Ongoing experimental and field-based petrologic research
over the past several years has led to an improved understanding of
the genetic links between granitoid magmatism on the early Earth and
the development of their roots in the cratonic lithosphere.
It is well established from studies of Archean (~2.5-4.0
Ga old) granite-greenstone and high-grade gneiss terranes around the
world that the granitoid plutons comprising the "continental" component
in these areas are dominated by rocks of the trondhjemite-tonalite-granodiorite
(TTG) suite of granitoids. A number of experimental studies have
previously shown that TTG "magmas" can be generated by low-moderate
degrees of partial melting of hydrous "metabasaltic" crust
in the garnet-amphibolite-eclogite facies (e.g., Rapp and Watson, 1995;
Rapp et al., 2003), and thus tectonic processes that lead to overthickening
or recycling (subduction?) of secondary basaltic (oceanic?) crust could
also culminate in TTG-forming dehydration melting reactions. In
the meantime, detailed field-based petrologic and geochemical studies
in a number of granite-greenstone terranes (e.g., the Superior Province
of Canada and the Pilbara of Australia; see Smithies and Champion,
2000) had identified another suite of Late Archean "post-kinematic" granitoid
intrusives (the "sanukitoid" suite), that possessed "primitive" (i.e.,
mantle-like) characteristics overprinted onto an overall "TTG-like"
geochemical signature, suggesting a hybrid lineage with a significant
mantle contribution somewhere along the way.
In an effort to constrain the petrogenesis of sanukitoid magmas, we
began a series of high-pressure laboratory experiments at 3-5 GPa in
which TTG melts were allowed to react with (and assimilate) a peridotite
mineral assemblage (Rapp et al., 1999). Our latest results show
that primitive (high-magnesium) granitoids (andesites) comparable to
Late Archean sanukitoids result from the equilibration of TTG melts
with olivine-bearing mantle phase assemblages (Rapp et al., 2009). The
resulting olivine-free garnet pyroxenite and garnet websterite reaction
residues are currently being characterized in terms of their major-
and trace-element compositions, for subsequent comparison with mantle
xenoliths from the subcratonic mantle lithosphere.
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Mojzsis, SJ (2005) Heterogeneous Hadean Hafnium: evidence of continental
crust at 4.4 to 4.5 Ga. Science 310, 1947-1950.
Rapp, RP, Watson, EB (1995) Dehydration melting of metabasalt at 8-32
kbar: Implications for continental growth and crust-mantle recycling,
Journal of Petrology 36, 891-931.
Rapp, RP, Shimizu, N, Norman, MD, Applegate, GS (1999) Reaction between
slab-derived melts and peridotite in the mantle wedge: experimental
constraints at 3.8 GPa. Chemical Geology 160, 335-356.
Rapp, RP, Shimizu, N, Norman, MD (2003) Growth of early continental
crust by partial melting of eclogite. Nature 425, 605-609.
Rapp, RP, Yaxley, GM, Norman, MD (2008) Genetic relations between
Archean granitoid magmatsim and the chemical evolution of subcratonic
lithospheric mantle: experimental constraints at 3-4 GPa. Lithos Special
Volume: 9th International Kimberlite Conference (submitted).
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links to tonalite–trondhjemite–granodiorite magmatism and implications
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