Allen P. Nutman

≥ 3850 Ma BIF and mafic inclusions in the early Archaean Itsaq Gneiss Complex around Akilia, southern West Greenland? The difficulties of precise dating of zircon-free protoliths in migmatites

Abstract The southern part of the early Archaean Itsaq Gneiss Complex (southern West Greenland) on Akilia and adjacent islands consists of polyphase dioritic–tonalitic–granitic injection components with inclusions of metavolcanic amphibolites, chemical sediments such as banded iron formation (BIF), gabbros and ultramafic rocks. Incipient in situ partial melting and strong deformation during several Archaean tectonothermal events strongly modified these injection components, so that they are now mostly banded, schlieric migmatites with neosome produced during several events. The margins of many inclusions have been loci for either segregation of neosome and/or higher strain—obliterating the relationship between the inclusions and the older components of the migmatites. An added complication is that none of the inclusion lithologies in the southern part of the complex contain protolith zircon, which would provide precise, direct dates. Instead, minimum ages of inclusions are obtained by dating invasive components of the granitic (sensu lato) migmatites. Previous age determinations of the inclusions were centred on a ∼200 m long body of amphibolites, BIF and ultramafic rocks on the southwestern corner of Akilia. Geochronology of this locality has been controversial, with our proposed age of ⩾3850 Ma making the inclusion the worlds oldest-dated sedimentary and mafic rocks. We continue the debate on the age of the inclusion on southwest of Akilia, and we broaden it by presenting mapping and zircon dating studies of inclusions on islands with 20 km of Akilia. This (1) addresses the contentious ages of these rocks, (2) examines the broader issue of how the complex field relationships in migmatites can lead to geochronological controversies, and (3) addresses the problem of precise dating of rocks in gneiss complexes which do not carry zircon from their protoliths. On Qilanngaarsuit (island), a gneiss sheet cutting a metaperidotite inclusion has an age of ∼3850 Ma. On an islet ∼2 km west of Akilia, tonalite and quartz-diorite which intrude and envelop two mafic amphibolite and hornblendite inclusions have dates of ∼3850 Ma also. With our proposed ages of ⩾3850 Ma, these inclusions are older than similar rocks in the 3700–3800 Ma Isua supracrustal belt. At other investigated localities ∼3850 Ma zircon dates have not been obtained from the orthogneisses enveloping or intruding inclusions. At two localities invasive leucogranite neosome and a discordant diorite sheet cutting inclusions have ages of 3600–3660 Ma—a period of known tectonothermal events including crustal melting and intrusion of gabbros and diorites. At three localities, banded-schlieric orthogneisses adjacent to supracrustal, gabbroic and ultramafic rocks contain 3800–3700 Ma igneous components. In these cases, the inclusions could still be ⩾3850 Ma and be equivalent in age to similar rocks on Akilia, but this has been masked by the local tectonothermal history and greater amount of neosome development. An alternative explanation is that some younger (

The Atuba Complex, Southern South American Platform: Archean Components and Paleoproterozoic to Neoproterozoic Tectonothermal Events

Abstract The Atuba Complex consists of an Archean terrane, containing rocks differentiated from the mantle and incorporated into the continental crust between 3.1-2.7 Ga. This terrane was later reworked several times. The main metamorphism occurred in the Paleoproterozoic at about 2.1 Ga and can be related to the amalgamation of the Paleoproterozoic supercontinent “Atlantica”. Ages around 1.7 Ga may be related to the break-up of Atlantica. During the Mesoproterozoic, there was little activity in the Atuba Complex and also in the entire South American Platform. Isolated U-Pb zircon ages of 1.4 Ga and Sm-Nd model ages of about 1.2 Ga may be associated to the assembly of Rodinia. The evidence from the South American Platform indicates that the Rodinia rifting pulses started as early as the latest collisional events of the Grenville Cycle, instead of at ca. 0.7 Ga as commonly believed. Finally in the Neopropterozoic at 0.52-0.65 Ga (U-Pb, Rb-Sr, Sm-Nd and K-Ar dates) there were high-grade tectonothermal events associated with Gondwana assembly.