EVIDENCE FROM THE EGERSUND DIKES AND OTHER DOLERITES IN SCANDINAVIA FOR THE BREAKUP OF RODINIA

The Egersund dike swarm is located in Rogaland and Vest Agder, SW Norway and formed by intrusion of magmas ranging in composition from olivine tholeiite to trachybasalt. Age data indicate a short duration intrusion event at either 649±7 Ma or 616±1 Ma. The dikes are extremely fresh, intrude Proterozoic basement, and have not been affected by Caledonian metamorphism. The major oxide, trace element and isotopic compositions are very similar to modern plume-related OIB and E-MORB. It is suggested that intrusion of the dikes mark initiation of plume-related rifting between Baltica and Laurentia during breakup of the supercontinent Rodinia in the late Neoproterozoic. Dikes also occur in the Caledonian nappes of Scandinavia. The nappe sheets were emplaced in the late Silurian onto Precambrian basement, and were deformed and variably metamorphosed to greenschist-amphibolite grade. Some of the dikes intrude arkosic and calcareous sandstones suggesting intrusion into continental sediments during intracontinental rifting, and have ages consistent with emplacement during fragmentation of Rodinia. The Ottfjäll dolerites have similar ages and geochemical characteristics to the Egersund dikes. Both dike systems formed by intrusion of tholeiitic, transitional and alkaline magmas. The geochemical characteristics of Egersund and Ottfjäll dolerites combined with that for the Corrovarre, Sarek, Sylarana, Leksdal, and Geita, dolerites appears to define a trend from P-type MORB through E-type MORB to N-type MORB compositions, best seen on plots involving relatively immobile elements such as Ti, P, Zr, Nb, and Y. Moreover, this trend appears to correlate with intrusion age (old to young). We interpret these data to reflect plume-initiated rifting followed by extension and decompression melting of the asthenosphere. The dikes therefore appear to provide a record of the fragmentation of Rodinia, separation of Baltica and Laurentia, and opening of the Iapetus and Tornquist Oceans between about 650 and 550 Ma. Comparison with data for lavas in drill core on the East European Platform indicates that the rift-drift transition to form Iapetus occurred at 580-550 Ma.