GEOCHEMISTRY AND GEOLOGICAL SIGNIFICANCE OF THE 1.8 GA MAFIC DYKE SWARMS IN THE NORTH CHINA CRATON: AN EXAMPLE FROM THE JUNCTURE OF SHANXI, HEBEI AND INNER MONGOLIA

Abstract

The 1.8 Ga dyke swarms in the North China Craton are among the largest ones in China. In the juncture of Shanxi, Hebei, and Inner-Mongolia, they presented as NNW and EW-oriented vertical to subvertical dykes with chilled margins from a uniform tectonic setting. The dykes are up to tens of kilometers in length, and 0.5 to 100 m in width. Three dyke swarms, S-I, S-II, and S-EW, are identified. They consist of clinopyroxene and plagioclase as rock forming minerals, and accessory amphibole, Fe-Ti oxides, biotite, apatite, alkaline-feldspar, and quartz as minor minerals. S-II is characterized by olivine phenocrysts and orthopyroxene + amphibole rim structures around olivine and clinopyroxene. S-I is tholeiite relatively high in MgO, with relatively low FeO (total) - TiO2 - P:2 05 ; while S-II varies in composition from alkaline to sub-alkaline high-Fe basalt, with high FeO (total) - TiO2 - P2 O5. S-EW includes high-Fe tholeiite basalt and andesite. All dykes have high total rare earth elements, and the light rare earth elements in dykes are enriched. They are relatively enriched in Ba, K, and P, and depleted in Nb and Ta in primitive mantle normalized diagrams. S-II and S-EW are depleted in Sr. S-I exhibits various degrees of partial melting, with slight fractionation and crustal assimilation. And S-II shows an iron-enriched and silica-poor trend (the Fenner trend) and crustal assimilation. While S-EW performs a silica-rich and iron-poor trend (the Bowen trend) with strong crustal assimilation. We suggest that there are multiple sources, and the dyke swarms initiated the continental break-up, possibly associated with a plume.