916 Ma Pole for Southwestern Baltica: Palaeomagnetism of the Bjerkreim-Sokndal Layered Intrusion, Rogaland Igneous Complex, Southern Norway

burton s. kaliski;

The Rogaland Igneous Complex (RIC) in southern Norway intruded into post-Sveconorwegian granulite facies crust ∼930 Ma. It includes three massif anorthosites, several small leuconorite bodies and the ∼7 km thick norite-quartz mangerite layered Bjerkreim-Sokndal (BKS) intrusion. The intrusion consists of five rhythmic megaunits created by repeated magma influxes topped by a transition zone and more evolved mangerites and quartz mangerites. Over 70 palaeomagnetic sites have been collected in the BKS, sampling all the megacyclic subunits and overlying mangerites. Remanence within the BKS is held in hemo-ilmenite-only rocks (lower parts of the megacyclic units), mixed hemo-ilmenite and magnetite rocks (upper parts of the lower megacyclic units) and magnetite only rocks in the upper highest megacyclic unit and overlying mangerites. Due to the different oxides present magnetic susceptibility varies over four orders of magnitude with a bimodal distribution (mean susceptibility of 6.4 × 10−3 SI for hemo-ilmenite rocks, and 8.7 × 10−2 SI for magnetite rocks). NRM values do not show a strong bimodal distribution as many of the rocks lacking magnetite have hemo-ilmenite with strong lamellar magnetism; average NRM for the entire suite is 8.83 A m−1. All sites within the cyclic part of the intrusion have stable remanence and produce well-clustered site means. Samples from the upper mangerite rocks, dominated by MD magnetite, are commonly unstable and not all sites provide acceptable data. Mean directions for 66 sites spanning the entire intrusion are I=−73.5◦, D = 303.4◦, with α95 = 3.7◦ and k = 24. The resulting pole position is at 35.9◦S and 217.9◦E, with a palaeolatitude for this part of Baltica of −59.4◦. Examination of the magnetic mineralogy combined with geochronology for RIC rocks and cooling rates for the region yields an age of magnetization of 916 Ma. Metamorphic country rocks yield similar directions at least 10 km from the contact, confirming the presence of a contact aureole around the intrusion. Comparison to other early Neoproterozoic palaeomagnetic poles from southern Scandinavia confirms the high southern latitude position of Baltica at this time, and combined with the few other ∼900 Ma poles from Baltica affirm the presence of the Rodinia supercontinent at this time.