Paper No. 3
Presentation Time: 9:30 AM


JOHNSTON, Scott M., Physics Department, California Polytechnic State University, San Luis Obispo, CA 93407, KYLANDER-CLARK, Andrew R.C., Geological Sciences, University of California, Santa Barbara, CA 93109 and BRUECKNER, Hannes K., Lamont–Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY 10964,

The East Greenland Caledonides formed in the hangingwall of the Silurian–Ordovician Laurentia–Baltica Caledonian orogeny, and offer an excellent opportunity to study deformation and metamorphism in the overriding plate during continental collisions. Here, we present coupled thermobarometric and geochronologic results from the Jaettedal complex of southern Liverpool Land to investigate the timing and nature of melting in the mid-lower crust of the Caledonian hangingwall.

The Jaettedal complex consists of gneisses of Laurentian affinity that structurally overly high-pressure rocks of Baltican affinity, and includes a series of intercalated paragneisses and orthogneisses that display a variety of migmatitic textures. Paragneiss migmatites are characterized by Grt + Ky/Sil + Bt + Pl + Qz melanosomes and broadly tonalitic leucosomes. High-Ca garnet rims with sillimanite inclusions in paragneiss melanosomes suggest melting via biotite dehydration in the sillimanite stability field, and thermobarometry and phase equilibria suggest peak metamorphic conditions at temperatures of 850°C at pressures of 9–11 kbar. Paragneiss zircons yield Archean-Mesoproterozoic detrital cores with positive HREE slopes, and at least two populations of zircon rims characterized by analyses with flat HREE slopes that cluster at ~440 Ma and analyses with negatively-sloping HREE profiles that cluster at ~430 Ma. Mafic orthogneisses display Grt + Cpx + Pl + Amph melanosomes and plagioclase-rich leucosomes that yield calculated pressures of 10–12 kbar at 830–860°C, and contain zircon cores with ages of 460–455 Ma with positive HREE slopes and 418–410 Ma zircon rims with flat HREE slopes. Zircon rims with flat–negative HREE slopes are interpreted to date the timing of metamorphic zircon growth in the presence of garnet, and are suggestive of at least three melt events within the Jaettedal complex migmatites. This data defines a period of protracted elevated geothermal gradients and a weak middle crust in the Laurentian orogenic hangingwall prior to collision with Baltica.