Discovery of a giant 3.3--3.1 Ga terrane in the Rae craton, Canada: Implications for the timing and extent of ancient continental growth.

We report the discovery of one of the largest ancient (>3.0 Ga) crustal terranes on Earth. Granitoids with crystallization ages >3.0 Ga and/or Sm-Nd depleted mantle model ages ≥3.2 Ga define a -1000 x 100 km belt on the western margin of the Rae craton, Canada, referred to herein as the Perry River terrane (PRT). Zircon U-Pb-Hf-O isotope and whole-rock geochemical data from granitoids show that the PRT is a predominantly juvenile 3.3--3.2 Ga terrane that was partially reworked by more evolved ca. 3.1 Ga magmatism. These findings call for a reassessment of the timing and extent of ancient continental growth on Earth. A global compilation of zircon Hf isotope data from 3.6 to 3.0 Ga igneous rocks reveals clusters of relatively juvenile (initial εHf -2 to +3) rocks at ca. 3.31 and ca. 3.23 Ga, which include samples from the PRT and 13 other terranes worldwide. Other global zircon data sets also document age peaks between 3.3 and 3.2 Ga, and a cluster of broadly chondritic initial εHf values around 3.2 Ga. The 3.3--3.2 Ga period may therefore have been a time of enhanced net continental growth on Earth, and the PRT is one of the largest terranes preserved from that time. Furthermore, zircon Hf isotope data from 3.3--3.1 Ga PRT granitoids and 3.5--3.0 Ga igneous rocks worldwide yield little evidence for parent magmas that interacted with or derived from pre--3.6 Ga continental material. Contrary to some continental-growth models, this latter observation suggests that the volume of continental crust established by 3.6 Ga was relatively small. [ABSTRACT FROM AUTHOR]