Reworking of sialic crust as represented in late Archean–age gneisses, northern Labrador

Abstract

Regionally reworked sialic crust has been recognized in the form of ∼ 2,800-m.y.-old nebulitic rocks (Kiyuktok gneisses) with high initial 87Sr/86Sr ratios (∼ 0.7081) and Pb isotope ratios that suggest a history of the source back to at least 3,500 m.y. B.P. The field, geochemical, and isotopic data are consistent with these rocks having formed by in situ mobilization of granitic components in the ∼ 3,600-m.y.-old Uivak gneisses by the introduction of aqueous fluids some 2,800 m.y. ago. Contemporary but subordinate tonalitic-granodioritic orthogneisses derived from discrete intrusive bodies have contrasting Sr isotope compositions. Some display low initial 87Sr/86Sr ratios (∼ 0.7030) that suggest derivation from a source with mantlelike Sr-isotope composition. However, most of the late Archean–age intrusive rocks display higher initial 87Sr/86Sr ratios (∼ 0.7056, 0.7059). Recognition of these contrasting isotopic patterns suggests that processes of late Archean crust formation and stabilization in the North Atlantic craton were more complex than previously realized. Crustal reworking due to interaction of old crust with aqueous fluids must have been accompanied by the introduction of granitic melts derived by melting of ancient deep crustal rocks or contamination of juvenile granitic melts by ancient deep crustal material.