Paleomagnetic Study of the Leka Ophiolite.

The Leka Ophiolite Complex is of special interest because there is excellent exposure of rocks, which represent both the petrologic and geophysical Moho allowing sampling across both boundaries. A detailed magnetic study was conducted on the Leka Ophiolite Complex (U-Pb zircon age 497±2 Ma from Dunning & Pedersen (1987)) on the island of Leka, Norway (65°05’11’’ N, 11°37’45’’). The purpose of this study is to help constrain the timing of emplacement and serpentinization of the ophiolite. It is comprised of five rock units; (1) strongly deformed harzburgite (2) unconformably overlain by ultramafic cumulates, with a sequence of olivine, chromite, clinopyroxene and orthopyroxene. These cumulates are covered with (3) metagabbros, which are cut by an increasing amount of (4) metabasalt dykes and within the dykes is the (5) pillow lavas forming the ancient oceanic floor (Furnes et al. 1988). Samples were taken from 86 sites as either oriented blocks, or oriented drill cores for laboratory tests. Measurements included natural remanent magnetization (NRM), magnetic hysteresis parameters, temperature-dependent susceptibility and magnetic susceptibility on 567 samples. NRM values range from less than 0.01 to 34 A/m with a mean NRM of 2.58 A/m and susceptibility values ranges from 1×10-4 to 0.22 (SI) with an average magnetic susceptibility of 2.9×10-2 (SI). 121 magnetic hysteresis measurements show a range in behavior from multi-domain to pseudo single-domain states. The range of Mr/Ms is 0.03 to 0.3 and Hcr/Hc is 1.1 to 11.5. Samples were demagnetized using both thermal and alternating field techniques. Hightemperature magnetic moment measurements and temperature-dependent susceptibility confirm a dominant Curie temperature of approximately 580°C, indicating an endmember magnetite as the typical oxide. This is also seen in the thermal demagnetization that magnetite is the primary carrier for the magnetic remanence. Another potential carrier is chromite found in some of the dunitic rocks, though preliminary tests suggest the chromite to be paramagnetic at room temperature. The chromite is of interest as the samples with high concentrations have an average NRM direction that is different from the other dunite samples by approximately 28 degrees. The chromite samples have a stable mean direction that is resistant to AF demagnetization and may retain a primary TRM. During alternating field demagnetization many samples have a median destructive field less than 20 mT and an average NRM direction that is near the present day field, suggesting that the rocks contain multi-domain magnetite that has been remagnetized, or that the original TRM has been altered due to the creation of new magnetite from recent serpentinization. These data will help to constrain the evolution of the Leka Ophiolite Complex: by comparing paleomagnetic poles from both the harzburgite and dunite we should be able to narrow down the age of emplacement.