, Volume 87, Issue 4, pp 311-318

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

Rent the article at a discount

Rent now

* Final gross prices may vary according to local VAT.

Get Access

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 εNd=+4 to +5 at 1.8 Ga; (2) migmatites, paragneisses and an early granitoid have εNd close to zero; (3) post-tectonic norites have εNd∼ +1.5, while spatially associated more-abundant granitoids have εNd=0 to +1. The metatholeiites show that a normal depleted mantle (εNd=+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 εNd=0. Rather than invoking depleted and undepleted mantle sources whose products did not mix, we infer the εNd=0 value to be caused by mixing of a component derived from depleted mantle (εNd=+ 4 to +5) with Archean crustal material (εNd=-9 to -13). As there are no proven relics of Archean crust beyond the border zone of the Ketilidian, and the εNd= 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.