Zircon captures melting of an ultrahigh-pressure terrane on the exhumation path, Greenland Caledonides

Jane A Gilotti; William McClelland; J. L Wooden; H. M Lang

Zircon can grow at any stage on the pressure-temperature path related to deep continental subduction, as has been demonstrated by U-Pb ion microprobe geochronology on distinct CL domains that contain index mineral inclusions suites. Zircon from the North-East Greenland ultrahigh-pressure (UHP) terrane formed over a 45 million 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 captured in zircon from samples that preserve the exhumation history of these rocks. Peak UHP conditions from 365-350 Ma are derived from coesite-bearing samples, while a suite of progressively retrogressed quartzofeldspathic host gneisses and late-stage, leucrocratic 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. Isochemical phase equilibrium modeling of anatectic metapelites shows that phengite melting took place below the coesite to quartz transition at approximately 2.2-2.4 GPa and 900 C. Zircon did not grow in the pelitic bulk composition, but a garnet-bearing leucosome in the neck of a kyanite-eclogite boudin gives an age of 347 Ma and this is taken as the beginning of phengite melting. Leucosomes formed in HP granulite to amphibolite facies gneisses between 350-330 Ma, and fluid assisted melting continued from 330-320 Ma in the form of late, cross-cutting pegmatites. Changes in the zircon trace element patterns are linked to decreasing temperature using Ti thermometry, 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 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 break down at the end stages of amphibolite facies metamorphism. Given that the mineral assemblages in continental UHP terranes are commonly dry, HP dehydration melting of phengite begins below the coesite to quartz transition and changes to fluid assisted melting later in the exhumation process.

Zircon captures melting of an ultrahigh-pressure terrane on the exhumation path, Greenland Caledonides

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