An Orphaned Baltic Terrane in the Greenland Caledonides: A Sm-Nd and Detrital Zircon Study of a High-Pressure/Ultrahigh-Pressure Complex in Liverpool Land

摘要

Liverpool Land, at the southern tip of the Greenland Caledonides, exposes a composite metamorphic terrane: the midcrustal granulite-facies Jaettedal Complex tectonically juxtaposed against the eclogite-facies, peridotite-bearing Tvaerdal Complex. The Jaettedal Complex is a Laurentian terrane, whereas the Tvaerdal Complex was proposed by earlier investigators to be a Baltic terrane. PT estimates (880 degrees-920 degrees C at 35-40 kbar) and Sm-Nd mineral isochrons from Tvaerdal eclogites indicate that recrystallization occurred under ultrahigh-pressure (UHP) metamorphic conditions approximate to 400 m.yr. ago, the same time and under similar conditions as the Western Gneiss Complex of the Norwegian Caledonides. Detrital zircons from the Tvaerdal Complex, analyzed for U-Pb, Lu-Hf, and trace elements by laser ablation inductively coupled plasma mass spectrometry, give concordant Mesoproterozoic ages but not the Archean and approximate to 1.8 Ga Proterozoic ages characteristic of Laurentian terranes. Most remaining concordant U-Pb ages are 411-375 Ma (i.e., Scandian), which contrast with older (approximate to 460-410 Ma) zircon ages from the Jaettedal Complex as well as other Laurentian terranes. Both the Precambrian and the Scandian age sets confirm the Tvaerdal Complex as an orphaned Baltic terrane. The Jaettedal Complex underwent a lengthy Caledonian history as part of a continental arc system during the closure of Iapetus, whereas the Tvaerdal Complex was a fragment of the approaching Baltic passive margin. UHP metamorphism occurred when this margin subducted into the mantle beneath Laurentia. We propose that the Tvaerdal Complex separated from Baltica and rose through the hot mantle wedge to the base of the overriding Laurentian crust by diapirism, a process that may explain its abundant anatectic granitoid intrusions.