MAFIC INTRUSIONS IN SOUTHWESTERN SIBERIA AND IMPLICATIONS FOR A NEOPROTEROZOIC CONNECTION WITH LAURENTIA

Abstract

Geochemical and geochronological studies reveal three distinct groups of Neoproterozoic to Early Paleozoic mafic intrusions in the Biryusa metamorphic massif of southwestern Siberia. Group 1 includes mafic sills that intruded Neoproterozoic passive-margin sediments, and were sourced from mantle-derived melts contaminated by crustal components. One sill yielded a plagioclase 40Ar-39Ar age of 741 ± 4 Ma, similar to ages of sills in the adjacent Sharyzhalgai metamorphic massif (all ages are ±2σ or 95%). Group 2 is represented by north-trending dolerite dykes, one of which furnished a 40Ar-39Ar age of 612 ± 6 Ma. Group 2 dykes may have formed by passive asthenospheric upwelling and decompression during lithospheric stretching within the Neoproterozoic passive margin, a suggestion supported by the occurrence of coeval carbonatites and alkaline ultramafic rocks in the adjacent Sharyzhalgai massif. Group 3 consists of NW-trending dolerite dykes produced by melting of a lithospheric mantle source metasomatised by subduction-related fluids. Zircons from one Group 3 dyke provided a SHRIMP U-Pb zircon age of 511 ± 5 Ma, similar to the age of regional metamorphism during Early Paleozoic orogenesis. Our results show that mafic intrusions, broadly similar in age to the Franklin intrusions of northern Laurentia, exist along the southwestern margin of Siberia. Paleomagnetic data permit a reconstruction in which southern Siberia is located opposite northern Canada, but at a sufficient distance to accommodate other crustal blocks (e.g. northern Alaska, Chukchi Peninsula), and in which Group 1 intrusions could be distal products of the mid-Neoproterozoic Franklin mantle plume that affected northern Laurentia. © 2006 Elsevier B.V. All rights reserved.