矿床地球化学国家重点实验室知识库

The ages and source of Precambrian mafic dolerite dykes of the North China Craton (NCC) were determined using geochronological, geochemical, and whole-rock Sr–Nd isotopic data. Laser ablation–inductively coupled plasma-mass spectrometry (LA–ICP-MS) U–Pb analysis of zircons yielded consistent ages of 1621 ± 6 and 1621 ± 7 Ma for two mafic dykes (SYG01 and SYG02) within the NCC. The dykes are classified as the sub-alkaline series according to their K₂O + Na₂O concentrations (2.5–2.9 wt%), and as the calc-alkaline series according to their K₂O concentrations (0.05–0.72 wt%). These dykes also contain low concentrations of the light rare earth elements [(La/Yb)_N = 1.5–2.4], with negligible Eu anomalies (δEu = 0.9–1.1), positive Ba, Pb, and K anomalies, and depletions in Th, U, and high field strength elements (P and Ti). In addition, the dykes contain relatively low amounts of radiogenic Sr[(⁸⁷Sr/⁸⁶Sr)_i = 0.7025–0.7035] and have high ɛ_Nd(t) values (5.6–5.8). These data suggest that the dykes were derived from a depleted mantle source that was hybridized during interaction with foundered lower crustal material. The parental mafic magmas underwent the fractionation of olivine, pyroxene, hornblende, and Fe–Ti oxides (e.g. rutile, ilmenite, titanite) during ascent, with negligible evidence for crustal contamination prior to emplacement at a high crustal level. The NCC extension was related to foundering of lower crust, and there were three episodes of lithosphere extension during Proterozoic time (i.e. 1.8–1.6 Ga, 1.3–1.2 Ga, and 0.8–0.7 Ga).