U-Pb baddeleyite ages, distribution and geochemistry of 925 Ma mafic dykes and 900 Ma sills in the North China craton: Evidence for a Neoproterozoic mantle plume
Numerous Neoproterozoic mafic dykes, referred to as the Dashigou swarm, are identified in the central and southeastern parts of the North China craton (NCC). They are 305-010° trending dykes, with widths of ~ 10-100 m and exposed lengths of several to > 10 km. Precise U-Pb isotope dilution thermal ionization mass spectrometry (ID-TIMS) measurements on baddeleyite grains separated from three dykes yield 207Pb/206Pb average ages of 924.0 ± 3.7 Ma (Dashigou dyke), 921.8 ± 2.6 Ma (Yangjiaogou dyke) and 925.8 ± 1.7 Ma (Taohuagou dyke). Baddeleyite grains from a late-stage pegmatite vein in the Dashigou dyke were analyzed by secondary ion mass spectrometry (SIMS) methods. These yield a 207Pb/206Pb average age of 920.4 ± 5.7 Ma. The Dashigou dyke swarm exhibits an overall radiating geometry (the overall fan angle is about 60°), with a focal point located along the southern margin of the eastern NCC, where a large ca. 900 Ma sill swarm was previously recognized. The rift system hosting these slightly younger sills, named the Xu-Huai Rift System, could represent two breakup-parallel arms of a rift-rift-rift triple junction related to the initiation of the magma center that produced the Dashigou dykes. The sills have similar characteristics and could be cogenetic with the Dashigou dykes.The Dashigou dykes are coarse-grained, composed mainly of clinopyroxene and plagioclase feldspar, with or without olivine. One of the most primitive dykes has 47.79 wt.% SiO2, 6.41 wt.% MgO, 1.38 wt.% TiO2, 17.77 wt.% Al2O3, 10.47 wt.% CaO and 0.62 wt.% K2O. It shows slight enrichment in light rare earth elements and a slightly positiveEu/Eu* anomaly (1.1), and is slightly depleted in high field-strength elements compared to neighboring elements on a primitive mantle-normalized spidergram. The Dashigou dykes show some similarities with enriched-mid ocean ridge basalts (E-MORB) or ocean island basalts (OIB) and have εNdt values of + 1.8 to + 3.1 and 87Sr/86Srt values of 0.7019-0.7047 (t = 920 Ma). All these characteristics indicate that they are not likely derived from the ancient lithospheric mantle under the NCC, but rather from a mantle source below, in the asthenosphere. This 925-900 Ma magmatism represents a second sub-lithospheric mantle upwelling event following the 1780-1730 Ma event that occurred shortly (~ 70 Ma) after the formation of the subcontinental lithospheric mantle (SCLM). Therefore the SCLM of the NCC was multiply metasomatized during asthenospheric upwellings (at least at 1780-1730 and 925-900 Ma), before most of its eastern part was finally removed during the Mesozoic. Collectively, these ~ 925 Ma dykes and ~ 900 Ma sills constitute a large igneous province (LIP) with an areal extent of about 0.5 Mkm2 and a diameter of about 1000 km. This LIP probably resulted from a Neoproterozoic mantle plume centered along the present southern margin of the eastern NCC, and probably resulted in the break-off and rifting away of a separate crustal block. We speculate that this conjugate block could have been the combined São Francisco-Congo craton on the basis of precisely matched ages for the Bahia dykes (São Francisco craton) and Gangila-Mayumbian volcanic associations (western part of the Congo craton).