Zircon captures exhumation of an ultrahigh-pressure terrane, North-East Greenland Caledonides

Abstract

Zircon from the North-East Greenland ultrahigh-pressure (UHP) terrane formed over a 45million year period from peak UHP conditions through the amphibolite facies. Our study utilizes sensitive high resolution ion microprobe-reverse geometry (SHRIMP-RG) mass spectrometry to assess the multiple ages and trace element patterns preserved in zircon from samples chosen to capture the exhumation history of these rocks. Peak UHP conditions from 365 to 350Ma are derived from coesite-bearing samples, while a suite of progressively retrogressed quartzofeldspathic host gneisses and late-stage, leucocratic melts emplaced into the gneisses track exhumation. Melting occurred during all stages of exhumation, beginning with H2O-absent dehydration melting of phengite on the decompression path. A garnet-bearing leucosome in the neck of a kyanite-eclogite boudin that gives an age of 347Ma is taken as the beginning of phengite melting. Leucosomes formed in HP granulite to amphibolite facies gneisses between 350 and 340Ma, and fluid assisted melting continued until 320Ma in the form of late, cross cutting pegmatites. Changes in the zircon trace element patterns are linked to decreasing temperature, and show that significant new zircon grew during melting on the exhumation path. Zircon cores recording protolith ages generally preserve magmatic temperatures (700°C) and typical igneous REE patterns (Yb/Gd=10). UHP/HP eclogite-facies zircon records higher T (900°C) and flat HREE patterns (Yb/Gd=1). Granulite to amphibolite facies zircon in quartzofeldspathic gneisses records both flat (Yb/Gd=1) and steep (Yb/Gd=100) HREE patterns at ca 700°C suggesting the variable effects of garnet during decompression. Amphibolite facies pegmatites and leucosomes document a transition from moderate HREE (Yb/Gd=10) at 700°C to steep HREE (Yb/Gd=100–1000) patterns at 600°C. The pronounced steepening of the HREE patterns is attributed to garnet breakdown during amphibolite-facies metamorphism. The 30–50million year spread of ages observed in individual samples records multiple periods of zircon growth and is interpreted as a characteristic signature of slowly exhumed UHP terranes. The data show that zircon ages combined with trace element and textural characterization of zircon from a broad suite of samples can successfully define the exhumation history of UHP terranes.