The growth and destruction of continental crust during arc-continent collision in the Southern Urals

The Southern Urals of Russia contain a well preserved example of a Paleozoic arc-continent collision in which the Laurussia margin was subducted beneath the Magnitogorsk island arc in the Devonian, providing an ideal field area for studying the possible growth and destruction of the continental crust during this process. High-pressure rocks derived from the leading edge of the continental margin indicate that it was subducted to a depth of between 70 km (eclogite assemblages) and 120 km (micro diamonds). The vast majority of the high-pressure rocks have a sedimentary protolith, with mafic eclogite having been derived from dikes intruding into the sediments. High-pressure rocks derived from the mafic ganulite that currently makes up the middle and lower crust of the Southern Urals do not occur in outcrop, suggesting that much of the subducted margin remained in the upper mantle. However, extensive geological, geochemical, and geophysical data do not unequivocally clarify its subsequent fate. Crustal delamination and foundering into the mantle can be ruled out, and recycling through the volcanic arc system appears to have been minor. While the metamorphic products of subducted continental crust can remain in the mantle for long periods of time, in the Southern Urals it has not been imaged by the available geophysical data sets. A possible explanation for this is that the progressive metamorphism of mafic granulite to eclogite assemblages would have altered its physical properties to those of typical mantle lithologies, making them difficult to detect by geophysical methods. It is estimated that the volume of continental crust that was subducted and lost to the mantle was approximately one third of the volume that was added to the Laurussia margin by the accretion of the Magnitogorsk arc. (C) 2009 Elsevier B.V. All rights reserved.