Zircon ages and geochemistry of amphibolitic rocks from the Paleoproterozoic Erdaowa Group in the Khondalite Belt, North China Craton and their tectonic implications

A currently popular model regards the Khondalite Belt in the North China Craton as a typical continent-continent collisional belt along which the Yinshan and Ordos blocks amalgamated to form the Western Block at ∼1.95 Ga. Although this model is fully supported by the geological data of high-grade metamorphic rocks from the belt, it remains unknown whether it is consistent with the low- to medium-grade units in the belt. One of such low- to medium-grade units from the Khondalite Belt is the Paleoproterozoic Erdaowa Group that consists of low-grade schists, quartzites, amphibolitic rocks, calc-silicate rocks, marbles and minor meta-conglomerates, of which amphibolitic rocks can provide important insights into understanding the tectonic setting and timing of collision of the Yinshan and Ordos blocks along the Khondalite Belt. The Erdaowa amphibolitic rocks can be divided into two groups of which Group1 is assigned to tholeiitic basalts, whereas Group 2 belongs to basaltic andesites, but they both experienced the fractional crystallization of the primary magma that was derived from the partial melting of the lithospheric mantle with minor crustal contamination. Such partial melting of the lithospheric mantle most likely occurred in a subduction zone as evidenced by their REEs and trace elements characterized by depletions in high field strength element (HFSE), Ti, P, Nb and Ta, suggesting that their igneous protoliths have affinities to continental marginal arcs. Metamorphic zircons from two amphibolitic rock samples yield Pb/Pb ages of 1941 + 14 Ma and 1910 + 9 Ma, similar to those metamorphic ages obtained for high-grade pelitic granulites from the Khondalite Belt. This confirms that the Yinshan-Ordos collision occurred at 1.95-1.90 Ga, and provides direct evidence for the involvement of low- and medium-grade rocks during the collision.