TIMING AND TRACE ELEMENT GEOCHEMISTRY OF MIGMATITIC HOST GNEISSES IN THE LIVERPOOL LAND ECLOGITE TERRANE, NE GREENLAND CALEDONIDES

The study of ultrahigh- and high-pressure terranes gives unique insight into the dynamics of continental collisional orogens. The Tvaerdal complex of southern Liverpool Land in NE Greenland consists of migmatitic orthogneisses that include eclogite bodies that formed during the Caledonian Baltica-Laurentia collision at ~399 Ma, and was exhumed in the Laurentian hangingwall. Here, we present new trace element geochemistry from migmatite melanosomes and leucosomes, as well as in situ LA-ICP-MS geochronology and geochemistry from zircons.

The Tvaerdal complex orthogneisses display a variety of migmatitic textures, although stromatic migmatites defined by plagioclase + quartz + biotite +/- amphibole +/- garnet melanosomes and plagioclase + K-feldspar + quartz leucosomes are dominant. Rare relict garnet in the melanosome suggests the presence of an earlier high-pressure mineral assemblage that formed during burial that nearly completely retrogressed during exhumation. We analyzed the whole-rock geochemistry of two migmatitic orthogneisses, separating the melanosome from the leucosome. When compared with the melanosome, the leucosome displays elevated K, Ba, and Rb, enriched Sr, depleted REEs, and steeper (negative) HREE slopes. Eu concentrations within the leucosomes were inconsistent showing a positive Eu anomaly in one sample while another leucosome analysis displayed a subtle negative Eu anomaly. Zircons were concentrated in the melanosome with few found in the leucosome, and display bright rims in CL that are typically less than 10 microns in thickness. Zircon cores yield upper intercept ages of 1645-1660 Ma with steep (positive) HREE patterns, while metamorphic rims yield Caledonian ages < 400 Ma with flat HREE patterns. These new data support previous work that indicates the presence of widespread Mesoproterozoic inheritance possibly associated with a Baltican protolith for the Tvaerdal complex. In addition, these data indicate that melting within Tvaerdal complex migmatites likely occurred at garnet-stable conditions during decompression from eclogite facies into granulite facies as the Tvaerdal complex was exhumed.