International Geologiical Congress - Oslo 2008

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HPP-05 Evolution of Archean crust - Part 2

 

Formation of Paleoarchean continental crust in a non-subduction setting: The East Pilbara example

 

Martin Van Kranendonk, Geological Survey of Western Australia (Australia)
Hugh Smithies, Geological Survey of Western Australia (Australia)
David Champion, Geoscience Australia (Australia)
David Huston, Geoscience Australia (Australia)
 

 

The Paleoarchean East Pilbara Terrane was initiated as an oceanic plateau from 3.72 Ga and transformed into mature continental crust over a 500 My period, via plateau-like magmatism and intracrustal melting.
Processes involved in this transformation involved multiple (or pulsed) episodes of mantle melting that produced a thick crust comprised of basaltic lavas and intraplated and underplated material. The presence of komatiitic lavas and ultramafic sheeted sills throughout the succession reflects the role that mantle plumes played throughout this stage. Early melting at the base of the thickened mafic crust produced minor TTG melts. These mixed with, or were assimilated by, large mafic magma batches emplaced into the crust, which then fractionated to produce tholeiitic basalt and andesite, sufficiently enriched in LREE, LILE and Th to be suitable source regions for later TTG magmatism, up to c. 3.5 Ga.
Subsequent high-pressure melting of mafic crust (particularly the enriched tholeiitic basalt and andesite sources), produced voluminous TTG magmas. High La/Nb ratios in these rocks reflect rutile stability during source melting, which we infer to have occurred at the base of a magmatically thickened crust (to 60 km), rather than in a subduction zone on the basis that the felsic magmas have low Mg#s, and because preceding, contemporaneous and post-dating mafic magmatism has no subduction-related geochemical signature. There is also no independent structural, stratigraphic, or geological evidence supporting subduction at this stage. Incorporation of significant amounts of TTG into 3.4 Ga large mafic magma batches led to the formation of andesitic to dacitic calc-alkaline magmas. This succession formed the basement to, and was again recycled, during 3.35-3.29 Ga and 3.27-3.24 Ga plume events that produced autochthonous, unconformity-bound supracrustal successions and voluminous K-rich granites. The younger granites include transitional TTG-type rocks that range from Al-rich and Y-poor to Al-poor and Y-rich compositions, reflecting melting over a wide range of crustal depths.
In each magmatic event, crustal overthickening through voluminous magmatism was accommodated by extension of the upper to middle crust, as indicated by synvolcanic growth faults and dyke swarms. This extension facilitated periods of partial convective overturn of the upper and middle crust at the end of major volcanic episodes. Extreme depletion of the subcontinental mantle lithosphere accompanied progressive fractionation of the crust and formed a thick, stable and buoyant lithospheric keel.
Together, these contemporaneous and complementary/reciprocal processes resulted in the stabilization of some of Earth's earliest lithosphere - in a non-subduction setting.

 

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