Russian Journal of Earth Sciences
Vol. 5, No. 2, April 2003
Genesis of Archean tonalite-trondhjemite suites:
Plume or
subduction related?
O. M. Turkina and A. D. Nozhkin
Abstract
Discussed are two probable models for lateral growth of continental crust in the
Early
Precambrian: accretion of island arcs and accretion of oceanic plateaus. Important
constraints on
geodynamic settings in which Archean crust was generated can be obtained from the
study of
magmatic products found in granite-greenstone provinces-tholeiitic basalts and rocks
of the
tonalite-trondhjemite-granodiorite series (TTG). The komatiite-tholeiite and tholeiitic-basalt
series are differentiated into island-arc and plume (oceanic plateau) assemblages
based on their
geochemical signatures and the character of associated lithologies. Because of the
geochemical
similarity between Archean TTG suites and adakites (which are generated through oceanic
slab
melting in subduction zone), the tonalite-trondhjemite-granodiorite series is widely
considered to
be subduction related. Our case study in the Early Archean TTG suite of the Onot
block (SW
margin of the Siberian craton) shows that tonalite-trondhjemite suites, unlike adakites,
bear no
major element evidence that their initial melts interacted with mantle wedge peridotites.
The
TTG-associated amphibolites, just like model metabasite sources of tonalite melts,
have trace
element signatures identical to modern and Archean basalts of plume related oceanic
plateaus,
and not to MORB-like abyssal tholeiites. Archean TTG suites were not restricted
solely to subduction related settings, but could also have formed through melting
at the bottom a
of thick crust generated by accretion of oceanic plateaus. The most promising criteria
to
reconstruct geodynamic settings can be provided by (i) compositional characteristics
of
metabasite enclaves in TTG suites and the model source for tonalite-trondhjemite
melts, (ii)
Mg#'s of TTG rocks and their similarity to experimental melts, and (iii) isotope
parameters of
TTG rocks and their associated amphibolites.