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

Abstract

Paleoproterozoic (∼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, ε 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 ε 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 ε Nd( t) values to those of Group 1. Petrological and geochemical data suggest that these dikes may have originated from different sources. Group 1 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 ∼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.