REVIEW OF THE OLDEST (4400-3600 MA) GEOLOGICAL AND MINERALOGICAL RECORD: GLIMPSES OF THE BEGINNING

Abstract

Known occurrences of rocks from the first billion years (>3550 Ma) form a minuscule ∼10,000 km2 of Earth's surface. The largest areas are in Greenland (Itsaq Gneiss Complex), Labrador and Western Australia, with smaller ones elsewhere in Greenland and in Antarctica, China and the Acasta area, Canada (containing the oldest-known terrestrial rocks at ∼4030 Ma). 4000-4400 Ma detrital zircons in sediments at Mt Narryer and the Jack Hills, Western Australia are another important part of the first billion years record. The Itsaq Gneiss Complex (Greenland) has the largest domains of least strain and migmatisation, with the most recognisable (amphibolite facies) sedimentary, volcanic and plutonic structures. These show a "normal" Earth by ∼3800 Ma, with a hydrosphere, life and division of the lithosphere into granitic and mafic components of unremarkable composition. There is no apparent geological evidence for the effects of impacts. Isotopic and petrographic work on ancient Western Australian zircons implies granites and hydrosphere were present on Earth 4400-4200 million years ago. Isotopic studies of first billion years rocks and minerals show that juvenile granitoids were added repeatedly to continental crust in the Archaean, core formation occurred in the first 100 million years and also the mantle differentiated early on into chemically distinct domains.