Geochemistry, isotope systematics and petrogenesis of the volcanic rocks in the Zhongtiao Mountain: An alternative interpretation for the evolution of the southern margin of the North China Craton

Abstract

The Xiong'er volcanic rocks constitute a large Paleo-Mesoproterozoic volcanic belt along the southern margin of the North China Craton, which are composed primarily of basaltic andesites and andesites, with minor dacites and dacitic rhyolites. The basaltic andesites and andesites from the Xiong'er volcanic rocks in the Zhongtiao Mountain show consistent e{open}Nd(t), La/Nb and Th/Nb values irrespectively of SiO2, which preclude significant crustal contamination during ascent. Both of these volcanic rocks share similar incompatible ratios (e.g. La/Sm, Zr/Nb, La/Nb), indicating that the basaltic andesites and andesites originated from the same source. The basaltic andesites are characterized by high Mg-number (45-55%), precluding the possibility of their derivation from the melting of the lower crust. Variable Ti/Eu, Zr/Sm and Nb/La ratios for the basaltic andesites with higher partial melting degree suggest that amphiboles remained as residues in the magma source, indicating that the Xiong'er volcanic rocks were derived from hydrous magma. Non-radiogenic Nd isotopes for the Xiong'er volcanic rocks are inherited from a mantle source which had been enriched by the subduction-related crustal recycling during Archean to Paleoproterozoic time. Basaltic andesites show HFSE enrichments (especially Nb > 4.7 ppm) and high Fe-Ti contents, comparable with Nb-enriched basalts, suggesting that the Xiong'er volcanic rocks were derived from a metasomatized mantle source, similar to that of Nb-enriched basalts. Therefore, the mantle source of the Xiong'er volcanic rocks in the Zhongtiao Mountain has been enriched in Fe-Ti oxides and HFSE by equilibrium reactions with the slab melts during late Archean and Paleoproterozoic subduction processes, as evidenced by Nd isotopes. In the primitive mantle normalized trace element diagrams, the Xiong'er volcanic rocks show enrichments in the LILE and LREE and negative anomalies on the Nb-Ta-Ti, similar to arc-related volcanic rocks produced by the hydrous melting of the metasomatized mantle wedge. In the Paleo-Mesoproterozoic, the Xiong'er volcanic rocks in the Zhongtiao Mountain were produced by the slab dehydration-induced melting of an existing metasomatized mantle source, and the fluids from the slab dehydration introduced significant LILE and LREE to the source, masking its inherent HFSE-enriched characteristics. The arc-related characteristics of the Xiong'er volcanic rocks in the Zhongtiao Mountain suggest the Xiong'er volcanic belt constructs a subduction zone along the southern margin of the North China Craton in the Paleo-Mesoproterozoic, implying that the craton may have recorded the outbuilding history of the Columbia Supercontinent during Paleo-Mesoproterozoic time. © 2007 Elsevier B.V. All rights reserved.