Geochemical and Sr-Nd-Pb isotopic compositions of Mesozoic mafic dikes from the Gan-Hang tectonic belt, South China: petrogenesis and geodynamic significance

Mesozoic mafic dikes in the Gan-Hang tectonic belt (GHTB) provide an opportunity to explore both the nature of their mantle source(s) and the secular evolution of the underlying Mesozoic lithospheric mantle in the region. The geochronology and primary geochemical and Sr-Nd-Pb isotopic compositions of Group 1 (middle section of GHTB) and Group 2 (the rest of the section) dolerite dikes spanning the GHTB were investigated. K-Ar ages indicate that dikes of both groups were emplaced during the Cretaceous (131-69 Ma). The dikes are doleritic in composition and are enriched in both large ion lithophile elements (LILEs; e. g. Rb, Ba, and Pb) and light rare earth elements (LREEs), with a wide range of Eu anomalies, but are depleted in high field strength elements (HFSEs; e. g. Nb, Ta, and Ti) and heavy rare earth elements (HREEs). Dikes sampled in the middle section of the GHTB (Group 1) show more pronounced REE differentiation and a greater contribution from crustal material than those from the east and west sections (Group 2) and are similar to GHTB volcanic rocks in exhibiting a slight enrichment in LREEs. The dolerites are further characterized by a wide range in Sr-87/Sr-86(i) = 0.7041-0.7110, Nd-143/Nd-144(i) = 0.511951-0.512758, epsilon Nd-t = -10.4 to +5.6, and Pb isotopic ratios (Pb-206/Pb-204(i) = 18.1-18.3, Pb-207/Pb-204(i) approximate to 15.6, and Pb-208/Pb-204(i) = 38.2-38.7). The dikes have undergone fractional crystallization of olivine, clinopyroxene, plagioclase, and Ti-bearing phases, except for dikes from the Anding area, which possibly experienced fractionation of plagioclase. Geochemically, all the dike samples originated from mantle sources ranging in composition from depleted to enriched that contained a component of foundered lower crust; crustal contamination during the ascent of these magmas was negligible. In the context of the late Mesozoic lithospheric extension across South China, mafic dike magmatism was likely triggered by the reactivation of deep faults, which promoted foundering of the lower crust and subsequent mantle upwelling in the GHTB.