A CA. 1640 MA MAFIC MAGMATIC EVENT IN SOUTHERN SIBERIA, AND LINKS WITH NORTHERN LAURENTIA

The Biryusa region of the southern Siberian craton hosts the “Nersa” dolerite sill complex, which on the basis of previous geochronology, has yielded multiple (emplacement) ages of 741±2 Ma (Ar-Ar, plagioclase), 612±6 Ma (Ar-Ar, plagioclase) and 511±5 Ma (U-Pb, zircon). However, another sill, results presented here, has yielded a significantly older age of 1641±8 Ma (U-Pb, baddeleyite). This new Mesoproterozoic age for a “Nersa” intrusion, along with similar ages (1674±29 Ma, Sm-Nd) for Khibelen dykes in the Baikal region (hundreds of km to the NE), are preliminary evidence for a possible large intraplate igneous province in southern Siberia which can potentially be linked with the 1635–1620 Ma Melville Bugt swarm of Greenland in northern Laurentia. It is probable that among the rift-related mafic-ultramafic complexes of southern Siberia, traditionally regarded as Neoproterozoic in age, there are also significant volumes of older mafic magmatic units potentially belonging to the 1640 Ma event.

From a paleomagnetic perspective there is a strong similarity between reference (APWP) magnetization directions at three of these emplacement times (1640, ca. 740 and ca. 500 Ma) of “Nersa” magmatism in the Biryusa region, and so paleomagnetism will be of limited use in distinguishing the different generations of intermixed sills. A possible reconstruction of northern Laurentia – southern Siberia from Nuna (Columbia) to Rodinia supercontinent times is allowed by a 150°–165° rotation around an Euler pole with coordinates ca. 77° N, 115° E. The 1641 Ma pole position for the Biryusa region after this rotation is not significantly different from a published 1700-1650 Ma pole for the Sioux quartzite (southwestern Minnesota) of Laurentia.