Geochemical 40Ar/39Ar geochronological and Sr-Nd isotopic constraints on the origin of Paleoproterozoic mafic dikes from the southern Taihang Mountains and implications for the ca. 1800 Ma event of the North China Craton

The wide occurrence of Paleoproterozoic mafic dikes in the basement of the Trans-North China Orogen provides important constraints on the major similar to 1800 Ma tectonothermal event of the North China Craton. These matic dikes form a tholeiitic series dominated by basalts and basaltic andesites, and can be geochemically classified into three groups: The Groups I and 2 rocks are characterized by high total FeO contents (12.69-15.64 %), of which the Group I rocks are more enriched in LILE and LREE contents and depleted in HFSE ((Th/Nb)n = 2.5-3.8, (La/Yb)cn = 3.0-5.8; (Nb/La)n = 0.27-0.38) and have more radiogenic Nd isotopic ratios (E-Nd(t) = -3.40 to -5.14), than the Group 2 rocks ((Th/Nb)n = 0.8-1.2, (La/Yb)cn = 1.5-2.1, (Nb/La)n = 0.65-0.87, E-Nd(t) = -0.60 to -1.67). In contrast, the Group 3 rocks are typified by low FeOt (7.86-11.04%) and high MgO (5.62-9.56%) contents. Higher (La/Yb)cn (4.22-7.13) ratios, less radiogenic Nd isotopic ratios (E-Nd(t) = -2.75 to -5.52), and more significant Th-U and Nb-Ta depletion are also apparent features. 40Ar/39Ar geochronology of three representative samples from each group yielded plateau ages of 1780.7 +/- 0.5 Ma, 1765.3 +/- 1.1 Ma and 1774.7 +/- 0.7 Ma, respectively. These elemental and isotopic data suggest that the geochemical variations of these rocks cannot be simply explained by crystallization from a common parental magma involving crustal contamination during emplacement. Instead, they most likely originated from variable sources under different degrees of partial melting. The Group I rocks were derived from relatively low degrees of partial melting of a refractory lithospheric mantle previously metasomatised by subduction-related fluids, whereas the Group 2 rocks originated from a hybridized source involving ca. 40% subduction-modified lithospheric mantle, similar to the Group 1, and ca.60% (C) 2004 Elsevier B.V. All rights reserved.