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

Abstract

The wide occurrence of Paleoproterozoic mafic dikes in the basement of the Trans-North China Orogen provides important constraints on the major ∼1800 Ma tectonothermal event of the North China Craton. These mafic dikes form a tholeiitic series dominated by basalts and basaltic andesites, and can be geochemically classified into three groups: The Groups 1 and 2 rocks are characterized by high total FeO contents (12.69–15.61%), of which the Group 1 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 ( ɛ 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, ɛ 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 ( ɛ 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 1 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% N-MORB component. In contrast, the Group 3 rocks show significant Th–U depletion that may mark the involvement of portions of gabbroic lower crust trapped in the source region during subduction. Based on all the available data, we propose a tectonic model of a full subduction-syncollision-postcollision-rifting cycle for the evolution of the NCC between 1870 and 1765 Ma. A variety of source regions beneath the southern Taihang Mountains initially developed during the period of subduction/collision, and subsequent extension/rifting resulted in the melting of the subduction-modified lithospheric mantle, which produced widespread Paleoproterozoic mafic dikes on the Trans-North China Orogen in response to upwelling of convective mantle.