Crustal Origin

Looking at the Earth’s oldest rocks and minerals could provide us the insights about the early Earth’s crust. The oldest preserved rocks occur small, highly deformed terranes tectonically incorporated with Archean crustal provinces. (4 Ga) The oldest know minerals on the Earth are detrital zircons in Western Australia. Detrital zircons have U-Pb ion probe ages ranging from about 3.5 Ga to 4.4 Ga, although only a small fraction of these zircons is older than 4.0Ga. Even if these old zircons are confined to a little portion of the sediments, they are important in that they indicate the presence of felsic source, some of which contained domains up to 4.4 Ga. These domains may have been remnants of continental crust. The oldest isotopically dated rocks are the Acasta gneisses in northwest Canada. These gneisses are a heterogeneous assemblage of centimeter scaled amphibolites, ultramafic rocks, granites, and − at a few locations − metasediments. The amphibolites appear to represent basalts and gabbros, many of which are deformed dykes and sills. Acasta metasediments include calc-silicates, quartizites, and biotite-sillimanite schists. This rare occurrence in ultramafic rocks indicates that the metamorphic temperature was in the range of 400 to 650.