The studied dykes are located in a quadrilateral area among Jinan, Zibo, Linyi and Jining in Western Shandong (Figure 1), EB of the NCC. The strikes of the dykes in the southern part are NS-trending or even NNE-trending, while those in the northern part are NNW-trending, and are vertical to the COB if eliminating the influence of the extension of Bohai Basin (Hou et al., 2001). The change of the strikes of the dykes from the southern part to the northern part is gradual.
Field investigation shows that the dyke swarms intrude Archean geniss and granulite, Palaeoproterozoic quartzite and Mesoproterozoic rocks of Changcheng System. They are covered by Neoproterozoic magnetite quartzite, and weathering crusts and fine branches of the dykes are found at the bottom of the caprocks.
Figure 2. Field photographs of the dykes
(a) Cover of the dyke and the weathering crust on the bottom of the cover.
(b) Vertical inclination of the dyke. (c) Chilled margin of the dyke and baked margin of the host rocks. (d) Tensional breccias in the contact zone between the dyke and the host rock.
Many precise ages of the Precambrian mafic dyke swarms in the EB have been published in recent papers. Zhuang et al. (1995) gave a K-Ar age of 1767 Ma for the diabase dykes at Mt. Taishan. A diabase dyke at Xiafeng in western Shandong has yielded a Sm-Nd age of 1718 Ma (Wang, 1991). Similarly, Hou et al. (2006a) gave a SHRIMP zircon U-Pb age of 1830±17 Ma for a large mafic dyke at Mt. Taishan in the EB. From the extensive previous geochronology it can be established that the ages of these mafic dyke swarms span between 1.83 Ga and 1.71 Ga.
These ages suggest that the mafic dyke swarms of the EB were emplaced at the same time as the cf. 1.8 Ga extensional event that followed cratonization of the NCC.
Representative thin and polished sections show that the primary mineralogy in most dykes generally consists of plagioclase, augite and subordinately biotite, and the rocks have typical diabasic texture.
These dykes are undeformed and unmetamorphosed with chilled margins that exhibit sharp contacts with the host rocks. The thicknesses of individual dykes are variable although values of 10 m are most frequent.