Talk Description
Zircon trace element geochemistry has become an increasingly popular tool to track crustal evolution through time. This may be especially important in early Earth settings where most of the crust hast been lost, but in some fortuitous instances detrital zircons have been preserved. Here we use trace elements in combination with hafnium and oxygen isotopes of 3.65 to 3.22 Ga detrital zircons of the Moodies and Fig Tree Groups from the Barberton Greenstone Belt (BGB), South Africa, to study the evolution of continental crust, and to compare their geochemistry to 4.2 to 3.3 Ga zircons from the Green Sandstone Bed. Detrital zircons from the Fig Tree and Moodies Groups show major age clusters at 3.55 Ga, 3.45 Ga, 3.29 Ga, and 3.26-3.23 Ga. Based on previous work, these zircons have been sourced from within or the vicinity of the BGB, while those of the Green Sandstone Bed were derived from outside the present-day BGB. The zircons trace a crustal evolution from generally more juvenile to more evolved, encompassing two episodes of crustal growth at 3.53 Ga and, previously unrecognized in the felsic igneous record, at 3.29 Ga. Archean BGB zircons show stark differences in their geochemistry to Hadean zircons of the Green Sandstone Bed and are thus not good analogues for Hadean zircon generation. However, similar crustal evolutions between <3.8 Ga Green Sandstone Bed and the BGB zircons suggests that they were neighbouring terranes experiencing similar crustal processes diachronously. Together, they show three phases of continent formation: (1) formation of long-lived protocrust in the Hadean, (2) onset of pervasive hydrous melting and frequent juvenile additions at 3.8 Ga, and (3) transition to crustal processes generating zircon trace element signatures most similar to modern plate-tectonic environments at ~3.3 Ga, though other processes such as partial convective overturn cannot be excluded. Hence, detrital zircons from the BGB provide a complex picture of continent evolution in the Archean that would not be accessible from felsic igneous rocks alone.