Abstract
Pb-isotopic data for magnetite from amphibolites in the Nagssugtoqidian orogen, central West Greenland, have been used to trace their source characteristics and the timing of metamorphism. Analyses of the magnetite define a Pb-Pb isochron age of 1726 ± 7 Ma. The magnetite is metamorphic in origin, and the 1726 Ma age is interpreted as a cooling age through the closing temperature of magnetite at ~600°C. Some of the amphibolites in this study come from the Naternaq supracrustal rocks in the northern Nagssugtoqidian orogen, which host the Naternaq sulphide deposit and may be part of the Nordre Strømfjord supracrustal suite, which was deposited at around 1950 Ma ago. Pb-isotopic signatures of magnetite from the Arfersiorfik quartz diorite in the central Nagssugtoqidian orogen are compatible with published whole-rock Pb-isotopic data from this suite; previous work has shown that it is a product of subduction-related calc-alkaline magmatism between 1920 and 1870 Ma. Intrusion of pegmatites occurred at around 1800 Ma in both the central and the northern parts of the orogen. Pegmatite ages have been determined by Pb stepwise leaching analyses of allanite and monazite, and source characteristics of Pb point to an origin of the pegmatites by melting of the surrounding late Archaean and Palaeoproterozoic country rocks. Hydrothermal activity took place after pegmatite emplacement and continued below the closure temperature of magnetite at 1800– 1650 Ma. Because of the relatively inert and refractory nature of magnetite, Pb-isotopic measurements from this mineral may be of help to understand the metamorphic evolution of geologically complex terrains.
Highlights
Pb-isotopic data for magnetite from amphibolites in the Nagssugtoqidian orogen, central West Greenland, have been used to trace their source characteristics and the timing of metamorphism
Pb-isotopic signatures of magnetite from the Arfersiorfik quartz diorite in the central Nagssugtoqidian orogen are compatible with published whole-rock Pb-isotopic data from this suite; previous work has shown that it is a product of subduction-related calc-alkaline magmatism between 1920 and 1870 Ma
Formations of Palaeoproterozoic age are limited to the Sisimiut igneous suite, Arfersiorfik quartz diorite, and minor supracrustal sequences including the Naternaq supracrustal belt (Connelly et al 2000; Thrane & Connelly 2006, this volume)
Summary
Pb-isotopic data for magnetite from amphibolites in the Nagssugtoqidian orogen, central West Greenland, have been used to trace their source characteristics and the timing of metamorphism. Some of the amphibolites in this study come from the Naternaq supracrustal rocks in the northern Nagssugtoqidian orogen, which host the Naternaq sulphide deposit and may be part of the Nordre Strømfjord supracrustal suite, which was deposited at around 1950 Ma ago. The present study comprises Pb-isotopic analyses of magnetite from amphibolites, hydrothermally altered amphibolites, the Arfersiorfik quartz diorite (see below), skarn, ultramafic rocks and pegmatites. The aims of the study were (1) to use Pb-isotopic signatures of magnetite in an attempt to outline the metamorphic history of the region; (2) to characterise the hydrothermal overprinting in terms of its timing and Pb source; and (3) to place the results within the evolutionary frame of the Nagssuqtoqidian orogen. Formations of Palaeoproterozoic age are limited to the Sisimiut igneous suite, Arfersiorfik quartz diorite, and minor supracrustal sequences including the Naternaq supracrustal belt (Connelly et al 2000; Thrane & Connelly 2006, this volume)
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