Nature of 1800–1600 Ma mafic dyke swarms in the North China Craton: Implications for the rejuvenation of the sub-continental lithospheric mantle

Abstract

Several late Paleoproterozoic (1800–1600Ma) mafic dyke swarms in the North China Craton (NCC), including the ca. 1780Ma Taihang, ca. 1730Ma Miyun, ca. 1620Ma Taishan, and a newly recognized ca. 1680Ma Laiwu swarm of this study, are compared. The newly defined Laiwu swarm contains tens of NNW-trending dykes, with widths of 10–40m and lengths of up to 10km in the Taishan Mts. in the eastern part of the NCC. Baddeleyites from a Laiwu dyke analyzed on a CAMECA SIMS 1280 machine yield a 207Pb/206Pb weighted average age of 1680±5Ma. In the Taishan Mts., there are also ca. 1620Ma dykes, which consist of several ∼30m-wide NNW- and NE-trending dykes. Both the Laiwu and Taishan dykes have a similar mineral assemblage of mainly clinopyroxene, plagioclase and Fe–Ti-oxides, except that the Laiwu dykes are coarser-grained and contain more volumes of plagioclase and Fe–Ti-oxides. Both swarms are compositionally tholeiitic, and exhibit significant enrichment in light rare earth elements (Laiwu: (La/Yb)N=5.2–9.2; Taishan: (La/Yb)N=2.3–3.4) and large ion lithophile elements, but variable depletion in high field-strength elements. Their similarity in ɛNd(t=1680Ma) values (from −2 to +2) and trace element patterns indicate that they may have originated from a common slightly depleted source with various degrees of fractionation. These features are more or less similar to those of the ca. 1730Ma Miyun dykes, which were thought to be derived from paleo-plume, but distinguishable from the ca. 1780Ma Taihang dykes, which were generated from the sub-continental lithospheric mantle (SCLM) of the NCC. Thus the Laiwu-Taishan series was derived either from the SCLM, which was (partially) rejuvenated prior to 1680Ma, or from a region other than the SCLM. The first possibility is favored as younger mafic igneous products from the SCLM show consistent characteristics with the Laiwu-Taishan series. This rejuvenation of the SCLM could be due to an upwelling of asthenosphere during the delamination of a Paleoproterozoic orogen or alternatively a plume process at ca. 1730Ma.