Reworking of Hadean crust in the Acasta gneisses, northwestern Canada: Evidence from in-situ Lu-Hf isotope analysis of zircon

摘要

The Acasta Gneiss Complex of northwestern Canada comprises early Archean orthogneisses and includes the oldest known terrestrial rocks (3.94-4.03 Ga). Previous zircon geochronological studies revealed the presence of zircon xenocrysts with ages up to 42 Ga in the oldest rocks, indicating that the source of early Archean granitoids contained Hadean (>4.03 Ga) crust. In this study, we have determined the Lu-Hf isotopic compositions of zircon grains extracted from the early Archean Acasta gneisses to evaluate the extent of the Hadean crust contribution to the formation of these granitoids. Analyses of the Lu-Hf isotopes were carried out using in-situ laser ablation inductively coupled plasma mass spectrometry combined with cathodolu_minescence images of the internal structure of the grains. Two ca. 3.59 Ga granitoids have ε_(Hf)(T) of -2.4±2.2 and -32±2.5, suggesting that the source of the granitoids was extracted from the mantle as far back as 3.8 Ga. This is consistent with the presence of abundant zircon xenocrysts with ages up to 3.9 Ga. The Hf isotopic composition of a 3.72 Ga granitoid is significantly less radiogenic, with an ε_(Hf)(T) of -6.1 ±2.5. This indicates that the granitoid formed by remelting of very old crust. The ε_(Hf)(T) values for the two oldest rocks, the ca. 3.97 Ga tonalites, are -12 t3.3 and -3.3±1.7, respectively, indicating that even the oldest known granitoids contain reworked older crustal materials. These results suggest that Hadean crust had significantly contributed to the genesis of some of the early Archean Acasta granitoids.