Age and temperature-time evolution of retrogressed eclogite-facies rocks in the Paleoproterozoic Nagssugtoqidian Orogen, South-East Greenland: Constrained from U-Pb dating of zircon, monazite, titanite and rutile

摘要

LA-ICP-MS U-Pb dating of zircon, monazite, titanite and rutile was carried out to investigate the temperature-time evolution of eclogite-facies rocks in the Kuummiut Terrane of the Paleoproterozoic Nagssugtoqidian Orogen in South-East Greenland. The terrane is dominated by Archean TTG gneiss and a variety of supracrustal rocks; basic dykes intruded the gneiss during Paleoproterozoic deformation. Detrital zircon in garnet-kyanite schist gives Archean to Paleoproterozoic dates and confines the maximum deposition of the metasediment precursor to 2107?±?21?Ma. Intrusion of the metabasic dykes occurred at 2146?±?63 and 2092?±?22?Ma, within error of the detrital zircon date, possibly indicating near-contemporaneous dyke emplacement and sedimentation. About 200?m.y. after dyke emplacement, the Kuummiut Terrane underwent a clockwise PT-evolution, involving eclogite-facies metamorphism and subsequent exhumation into the mid crust. The majority of zircon, monazite and titanite give metamorphic dates between 1891?±?10 and 1882?±?3?Ma. Although the REE patterns in metamorphic zircon reflect growth at eclogite-facies conditions, the zircons are associated with retrograde mineral assemblages and their dates are indistinguishable from amphibolite-facies titanite. This may be interpreted to indicate that the timing of eclogite- and high-pressure amphibolite-facies metamorphism overlap within error, consistent with rapid and tectonically-controlled exhumation. However, previous studies have shown that the REE and U-Pb systematics in zircon may be decoupled during retrograde metamorphism, and the range in dates is thus best interpreted to reflect mineral growth and recrystallization during high-pressure amphibolite-facies retrogression. Monazite and titanite dates between 1872?±?70 and 1821?±?31?Ma reflect regional medium-pressure amphibolite-facies metamorphism and mark the final stages of compressional deformation in the Kuummiut Terrane. The subsequent thermal evolution was