Melting of enriched Archean subcontinental lithospheric mantle: Evidence from the ca 1760Ma volcanic rocks of the Xiong'er Group, southern margin of the North China Craton

The Xiong'er Group is an important geologic unit in the southern margin of the North China Craton. It is dominated by the volcanic rocks, dated at 1763±15Ma, that have SiO₂ contents ranging from 52.10wt% to 73.51wt%. These volcanic rocks are sub-alkaline and can be classified into three subgroups: basaltic andesites, andesites and rhyolites. They unexceptionally show enrichment of light rare earth elements (LREE) and share similar trace element patterns. Depletions in Nb, Ta, Sr, P and Ti relative to the adjacent elements are evident for all the samples. The volcanic rocks are evolved with low MgO contents (0.29-5.88wt%) and accordingly low Mg^# values of 11-53. The Nd isotopes are enriched and show a weak variation with e{open}_Nd(t)=-7.12 to -9.63. Zircon Hf isotopes are also enriched with e{open}_Hf(t)=-12.02±0.45. The volcanic rocks of the Xiong'er Group are interpreted to represent fractional crystallization of a common mantle source. The volcanic rocks might have been generated by high-degree partial melting of a lithospheric mantle that was originally modified by the oceanic subduction in the Late Archean. This brings a correlation with the subduction-modified lithospheric mantle in an extensional setting during breakup of the Columbia supercontinent in the late Paleoproterozoic, rather than in an arc setting. The elevated SiO₂ contents and evolved radiogenic isotope features indicate the possible incorporation into their source of lower crustal materials that have similar Nd isotopic characteristics to the subcontinental lithospheric mantle. The existence of extensive Xiong'er volcanic rocks (60,000km²) indicates an early large-scale subduction-related metasomatism in the area and probably suggest a flat subduction model for the plate-margin magmatism in the Late Archean.