Geochemistry of Paleoproterozoic (~1770 Ma) mafic dikes from the Trans-North China Orogen and tectonic implications

Paleoproterozoic (similar to 1770 Ma) mafic dikes occur throughout the Trans-North China Orogen. These dikes can be divided into three geochemically distinct groups. Group 1 with FeOt of 12.73-18.06%, (Nb/La)(N) 0.18-0.39, (Th/La)(N) of, 0.60-1.24, epsilon(Nd)(t) of -5.1 to -2.4, and an enrichment in LILE's and a depletion in HFSE's. Group 2 has flat REEs and incompatible elements patterns, and possesses (Nb/La)(N) = 0.64-0.87, (Th/La)(N) = 0.61-1.20 and epsilon(Nd)(t) = - 1.7 to 1.1. Group 3 has low FeOt contents (8.19-11.57%) and (Th/La)(N) ratios (0.29-0.72), but similar (Nb/La)N ratios and epsilon(Nd)(t) values to those of Group 1. Petrological and geochemical data suggest that these dikes may have originated from different sources. Group I could be derived from a "re-fertilized" lithospheric source incorporating a continental basaltic component trapped from a earlier phase of subduction. Group 2 was likely a product of a subduction-modified lithospheric mantle hybridized by recycled gabbroic lower crust. By contrast, Group 3 originated from a mixture of subduction-modified lithospheric mantle with a MORB component. In combination with the available data, we propose that the sources of the mafic dikes were initially modified by the similar to 1.85 Ga subduction/collision event that assembled the North China Craton, and partially melted under subsequent rifting in response to the upwelling convective mantle. (C) 2007 Elsevier Ltd. All rights reserved.