Amitsoq Gneisses Research Articles

207Pb/206Pb whole Rock Age of the Archaean Granulite Facies Metamorphic Event in West Greenland

IN the Archaean basement complex of West Greenland there are several widely separated granulite facies areas, up to 100 km across1. Rocks from three such areas, Sukkertoppen, Nord-land and Fiskenaesset (Fig. 1), were originally selected for this study on the assumption that the granulite facies metamorphism represented the oldest metamorphic event observable in West Greenland1. This idea has been shown to be ill founded by subsequent geological and geochronological work2,3. The granulite facies metamorphism is now recognized as a distinctive time marker which probably has the following chronological relationships with other principal events in the Archaean of West Greenland, particularly in the Godthaabsfjord area2,3: (1) Plutonic development ∼ 3,700-3,750 m.y. ago4,5 of a complex of granites, granodiorites and tonalites. Metamorphism, migmatization and deformation, with production of banded gneisses (Amitsoq gneisses). (2) Intrusion of a basic dyke swarm (Ameralik dykes) into the Amitsoq gneisses. (3) Eruption of basic volcanics and deposition of sediments (Malene supracrustals). (4) Emplacement of basic igneous complexes with prominent calcic anorthosites. Possibly broadly similar in age throughout the Archaean of West Greenland. (5) Tectonic interleaving of earlier rock units. (6) Intrusion of major suite of granites, granodiorites, tonalites and diorites at 3,040 ±50 m.y. ago6, the parent rocks of the Nuk gneisses. (7) Deformation and folding, accompanied by migmatization and metamorphism that culminated in amphibolite facies conditions in the Godthaabsfjord area, but reached granulite facies to the north and south (in the areas we discuss in this communication). Earlier felsic rocks converted to gneisses. (8) Emplacement of Qorqut granite and pegmatites, ˜ 2,600 m.y. ago (Oxford unpublished data). Intrusion of Precamb-rian dolerite dykes.

A Discussion on the evolution of the Precambrian crust - The early Precambrian gneisses of the Godthåb district, West Greenland

Intensely metamorphosed and deformed basic dykes, the Ameralik dykes, have been used to divide the amphibolite-facies gneisses of the Godthåb district into the Amitsoq gneisses (older) and the Nuk gneisses. Metavolcanic and metasedimentary rocks (the Malene supracrustals), and stratiform meta-anorthosites are also present and are probably younger than the Amitsoq gneisses, but are older than the Nuk gneisses. The Amitsoq gneisses contain abundant fragments of Ameralik dykes. They appear to have been derived from homogeneous granitic (s.l.) parents, but most of them have been intensely reworked before and/or after the intrusion of the Ameralik dykes and are now banded gneisses. They range from dioritic to granitic in composition and potassic varieties are common. Isotopic data indicate that the parent rocks of the gneisses were emplaced or metamorphosed about 3750 Ma ago. Ameralik dykes are absent from the Nuk gneisses, which are the most abundant rocks in the area. These gneisses are derived from intrusive calc-alkaline parents, mainly tonalites and granodiorites, and represent a massive addition of granitic material to this level of the crust about 3080 Ma ago, after the first supracrustal rocks had been laid down.

Further Rubidium-Strontium Age Determinations on the Very Early Precambrian Rocks of the Godthaab District, West Greenland

A RECENT reconnaissance age and isotope study of quartzo-feldspathic Amitsoq gneisses of the Godthaab area of West Greenland1,2 showed that these rocks are older than any other terrestrial rocks yet dated. An Rb-Sr whole rock date of 3,980 ± 170 m.y. and a Pb-Pb whole rock date of 3,620 ± 100 m.y. on fourteen granitic, granodioritic and tonalitic orthogneisses and evidence that some of these rocks contained the least radiogenic lead so far found on Earth, were presented in ref. 1.

Isotopic dating of very early Precambrian amphibolite facies gneisses from the Godthaab district, West Greenland

Amphibolite-facies feldspathic gneisses (the Amitsoq gneisses) from the Godthaab area of West Greenland give hitherto unique age and isotope data, including (i) a Rb-Sr whole-rock isochron age of 3980 ± 170 my. (t 1/2 = 5.0 × 10 10y) with an initial 87Sr/ 86Sr ratio of 0.6992 ± 0.0010, (ii) a Pb-Pb whole-rock isochron age of 3620 ± 100 my, and (iii) extremely unradiogenic whole-rock leads, with 206Pb/ 204Pb and 207Pb/ 204Pb values extending down to 11.51 and 13.14 respectively. Taken together, these data provide the first direct evidence for the existence of a granitic crust so early in the Earth's history. The isotopic evidence also demonstrates that these rocks could not have had a lengthy (i.e. more than a few tens of millions of years) history prior to the Rb-Sr isochron age. Possible interpretations of the isochron ages are discussed in detail. Several other petrologically distinctive rocks from the same area are shown to have been affected by thermal events somewhere in the range 2000–2800 my ago.