U–Pb geochronology of 3810–3630 Ma granitoid rocks south of the Isua greenstone belt, southern West Greenland

Abstract

Abstract The oldest well preserved granitoid rocks on Earth envelope the Isua greenstone belt, southern West Greenland. The timing and nature of geological events that affected granitoid rocks south of the Isua belt (“southern gneisses”) are refined with new mapping and U–Pb geochronology. These results and previous work allow for a detailed comparison between the southern gneisses and similar granitoid rocks north of the arcuate Isua belt (“northern gneisses”). Zircons from tonalitic gneiss and meta-tonalite that yielded ages of 3810–3795 Ma are interpreted as having grown during igneous crystallisation of their protoliths based on the consistency of ages within and between samples, and the oscillatory internal zoning that is typical of magmatic zircons. Two dated tonalitic rocks are particularly important because they occur as concordant layers within supracrustal rocks: amphibolite with deformed pillow lava structures in the southwestern part of the Isua belt and quartz-rich gneiss (likely derived from a chemical sediment) that forms an enclave in the southern gneisses. Although these layers lack cross-cutting intrusive relationships, they are interpreted as having intruded as sheets into >3800 Ma supracrustal rocks. Most tonalite was converted into gneiss before intrusion of diorite dated at 3658.3±1.2 Ma and deformed again during or shortly after intrusion of 3650–3630 Ma granitic rocks, whereas a few small lenticular zones of meta-tonalite escaped nearly all strain. The timing of amphibolite facies metamorphism may be recorded by zircon and titanite growth in amphibolitic gneiss at 3632–3620 Ma. Other thermal events were recorded by zircon growth at ∼3550 and 2660 Ma, and titanite growth or Pb loss at ∼2650 and 2600 Ma. These new data and interpretations suggest that the southern gneisses underwent many geological events along with the northern gneisses, including >3658 Ma deformation, 3658 Ma diorite intrusion and 3655–3630 Ma crystallisation of granitic rocks during significant metamorphism and deformation. The major differences are that most tonalite in the southern gneisses is 100 my older than the northern tonalite, and the intensity of the Neoarchaean tectonothermal activity increased southward. Any large-scale juxtaposition of the southern gneisses against the northern gneisses, across the Isua belt, likely occurred before intrusion of the 3658 Ma diorite.