Origin of continental crust of 1.9-1.7 Ga age defined by Nd isotopes in the Ketilidian terrain of South Greenland

Abstract

Initial Nd isotope ratios are determined for components of 1.9-1.7 Ga age continental crust in the Ketilidian terrain of South Greenland. The Ketilidian has well-documented ages of migmatization/metamorphism (1.80 Ga) and post-tectonic granitoid intrusion (1.76-1.74 Ga) from U-Pb zircon studies. The Nd results show that: (1) metatholeiites with chondritic ¹⁴⁷Sm/¹⁴⁴Nd have ε_Nd=+4 to +5 at 1.8 Ga; (2) migmatites, paragneisses and an early granitoid have ε_Nd close to zero; (3) post-tectonic norites have ε_Nd∼ +1.5, while spatially associated more-abundant granitoids have ε_Nd=0 to +1. The metatholeiites show that a normal depleted mantle (ε_Nd=+4 to +5) was present beneath this 1.9-1.7 Ga orogenic zone, as is the case in such environments today. However, metatholeiites are an insignificant part of the Ketilidian crust, and the bulk initial ratio of the whole terrain lies close to ε_Nd=0. Rather than invoking depleted and undepleted mantle sources whose products did not mix, we infer the ε_Nd=0 value to be caused by mixing of a component derived from depleted mantle (ε_Nd=+ 4 to +5) with Archean crustal material (ε_Nd=-9 to -13). As there are no proven relics of Archean crust beyond the border zone of the Ketilidian, and the ε_Nd= 0 value appears to be a wellhomogenized feature, we propose that the Archean material was added in the form of sediments transported to the orogenic zone on oceanic crust. The Archean component comprised between 5 and 17% of the Ketilidian, and the most reasonable estimate is 10%. Thus this 1.9-1.7 Ga terrain consisted of ∼90% new mantle-derived crust.