Vertical tectonics and formation of Archaean crust

Since Pentti Eskola’s recognition in 1949 that many Precambrian granitic rocks form domes surrounded by cusp-like synclines of supracrustal rocks, such mantled gneiss domes have been found in most cratons.  Probably the best example characterizes the 3.5 Ga Pilbara province of the West Australian Shield.  How they formed has long been a vexed topic, the most popular views being as a result of low-density basement rising through denser cover that contains abundant volcanic rocks, or as a result of regional-scale fold interference.  Precise dating of the Pilbara granitic rocks and greenstones shows a common age range, with some older greenstones,  The age data suggest that the dome and cusp structure is a product of the co-evolution of both, probably from a primary oceanic-like crust of mafic composition (Zegers, T.E. and van Keken, P.E. 2001.  Middle Archean continent formation by crustal delamination.  Geology, v. 29, p. 1083-1086).

Archaean rocks of broadly granitic composition (dominantly tonalites, trondhjemites and granodiorites, or TTG) have geochemical features setting them apart from post-Archaean varieties.  Rather than signifying their origin by supra-subduction melting of the mantle wedge with fractional crystallization and crustal assimilation in the lower crust (the dominant crust-forming process in post-Archaean times), all Archaean TTG seem to have formed by partial melting of a garnet-rich mafic source.  One of several possibilities is that their source was eclogite.  Based on the peculiar regional structure of the Pilbara and its dominance of the whole crust, as shown by maps of gravitational potential and magnetic field strength, Zegers and van Keken revisit earlier ideas of dominantly vertical tectonics that underlay early crust formation.  They suggest that efficient cooling by hydrothermal circulation allowed thick mafic crust (similar in some respects to that formed in the Mesozoic beneath ocean plateaux) to enter the field of eclogite stability at its base to form a layer denser than ultramafic mantle.  Once sufficiently thick, this layer would begin to founder, or delaminate, to be replaced by hot mantle.  Rebound of the remaining crust would set in motion rapid crustal uplift and extension, together with decompression melting of rising mantle (to form high-magnesium basalts high in the crustal sequence)and melting induced in the remaining mafic crust (to generate TTG magmas).  Indeed, the kimberlites that puncture other Archaean cratons carry abundant eclogite xenoliths from mantle depths.  Seemingly well-documented, this tectonic model does not explain all Archaean crust formation, for other cratons, such as that of west Greenland, are more readily accounted for by seemingly familiar subduction-zone processes.


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