International Geologiical Congress - Oslo 2008

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HPP-05 Evolution of Archean crust - Part 1

 

The Earth's oldest granite

 

Rajesh HM, University of Johannesburg (South Africa)
Joydip Mukhopahdyay, Presidency College (India)
Nicolas J Beukes, University of Johannesburg (South Africa)
Jens Gutzmer, University of Johannesburg (South Africa)
Richard A Armstrong, Australian National University (Australia)
 

 

An important question in understanding the composition of the continental crust is how the crust has grown with time, especially during early Archaean for which the geologic record is fragmental. Although there are detrital zircons as old as 4.4 Gyr, the rocks in which they crystallized remain elusive. Importantly, these zircons are relatively uranium-rich, suggesting they come from silica-rich rocks typical of true continental crust rather than more mafic rocks. Significant volumes of new crust in the Archaean were associated with the emplacement of felsic rocks of the tonalite-trondhjemite-granodiorite (TTG) series. With an average of ∼70 wt.% SiO2, the TTGs are among the oldest silica-rich igneous rocks on Earth and are different from evolved granitoids in their high Na2O and Al2O3, low K2O, and steep rare earth element (REE) patterns. The appearance of evolved granites is of considerable importance as they are believed to mark a significant change in the crustal behavior, since their emplacement is thought to imply that the continental crust attained a sufficient thickness for intracrustal melting to occur. Although constituting large areas of all Archaean cratons, evolved granites typically postdate TTG granitoids, with no record thus far of a granite (sensu stricto) older than 3.4 Gyr. We report a 3.6 Gyr granite from the Archaean Bastar craton in India. In contrast to the typical early Archaean granitoids, which are commonly deformed into gneisses, this granite is relatively undeformed. The dated sample is SiO2-rich, mildly peraluminous, corundum normative, ferroan and calc-alkalic. The age and composition of the granite implies that continental crust of the Bastar craton attained sufficient thickness to permit intracrustal melting at 3.6 Gyr. We relate the oldest granite on Earth to a collisional tectonic setting involving crustal thickening, like the Himalayas, supporting the presence of rigid plates in the early Archaean and operation of conventional plate tectonic processes by this time.

 

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