PERMIAN LAMPROPHYRES OF LIVERPOOL LAND, EAST GREENLAND

Liverpool Land, East Greenland, is a rotated fault block within the Caledonian collisional belt. Central Liverpool Land is characterized by the Krummedal sequence, a Late Mesoproterozoic amphibolite-facies supracrustal sequence containing Grenvillian and Caledonian orogenic effects. The Krummedal sequence is intruded by the Caledonian Hurry Inlet Granite (c.445-438 Ma) and a later Caledonian quartz monzodiorite pluton (c. 425 Ma). Cutting across both the Krummedal sequence and the quartz monzodiorite pluton are several NNE-SSW trending lamprophyre dikes. The lamprophyres are contained within the hanging wall block of the E-W trending, N-dipping, polyphase Gubbedalen Shear Zone. The most recent movement in the Gubbedalen Shear Zone is extensional. Samples M-14C and M-21 were collected from the lamprophyres where they cut the quartz monzodiorite. Phlogopite biotite phenocrysts were analyzed using ⁴⁰Ar/³⁹Ar Single Crystal Total Fusion (SCTF) and Incremental Heating (IH) techniques in the Auburn Noble Isotope Mass Analysis Laboratory (ANIMAL). A single crystal of phlogopite from one dike (sample M-14C) yielded a single-crystal IH plateau age of 261.6±0.4 Ma, and a crystal of phlogopite from another dike (sample M-21) yielded an IH plateau age of 263±0.9 Ma. Regression analysis of these IH data, and also SCTF data, indicate the phlogopite of each sample has a normal distribution of age and only radiogenic ⁴⁰Ar with a minor component of atmospheric argon. Therefore, we interpret that the phlogopite in these dikes crystallized at approximately 262 and 264 Ma, respectively, and that these ages are broadly synchronous with dike intrusion and crystallization.

These Permian ages of lamprophyre crystallization are coincident with ages of muscovite in extensional structures of Lofoten, Norway, and broadly synchronous with other extension in the region (e.g. Oslo rift). Lamprophyres of broadly Caledonian age, and similar orientation to those in this study, have previously been reported in a region of East Greenland north of the present study (Stordalen). We suggest that many of the lamprophyre dikes in East Greenland, and some coincident extensional structures in Lofoten, formed in deep, metasomatically altered crust and intruded during episodic Silurian to Permian extension of the late to post-Caledonian orogen.