Palaeomagnetic study of the Nagssugtoqidian mobile belt in Central-West Greenland

Abstract

This paper reports a palaeomagnetic study of amphibolite and granulite facies basement rocks across a 60 km wide north-south traverse through the Nagssugtoqidian mobile belt in West Greenland, which incorporates the c. 2500 Ma Itivdleq shear belt, the c. 1800 Ma Ikertôq shear belt, and adjoining terrains. It also integrates the results with earlier investigations of the belt to produce a composite record of the magnetic field during uplift and cooling of the terrain through c. 600–500°C at c. 1750–1600 Ma. Progressive a.f. and thermal cleaning of samples from 81 sites identifies a range of magnetic components. Lowest blocking temperature remanences in Nagssugtoqidian (c. 1700 Ma) amphibolite facies rocks yield steep W to N positive components residing in magnetite. Granulite facies rocks formed at c. 1800 Ma generally have a dual record incorporating steep N to NW positive components unblocked by 580°C and shallow E-W directed components of dual polarity only unblocked close to the Curie point of haematite. A common feature of relict c. 2500 Ma amphibolite and granulite facies terrains is a progressive movement of sample directions back along a palaeofield migration path with treatment and is interpreted to represent the recovery of a discrete spectrum acquired during a.p.w. Areas with a distributed spectra through the critical range of blocking temperatures, altitude-controlled sections, and blocking temperature relationships all record consistent a.p.w. during the interval of uplift-related cooling. The collective data define a clockwise palaeofield migration path between D = 174, I = 32°, and D = 348, I = 66° referred to as the ‘A’ magnetisation. The equivalent palaeopoles define one limb of an a.p.w. loop conforming to other Laurentian poles assigned to the interval 1750–1650 Ma and widely represented as a post-Hudsonian overprinting or cooling remanence. Shallow E-W directions, designated ‘B’, predominate in late tectonic diorites and are therefore younger than 1700 Ma; they appear to represent a localised reheating event near the northern margin of the Ikertôq shear belt, possibly related to this magmatism, which produced the hemoilmenite phases. The equivalent palaeopoles are provisionally interpreted to represent part of the return path of the APW loop because they are coincident with other poles dated c. 1630 Ma. Collectively the palaeofield migration data from all studies define a thermal dome broadly coincident with the Ikertôq shear belt, but it is not possible to isolate the contribution of components of vertical movement along faults running through the belt to this regional effect.