A Paleoproterozoic A-type rhyolite

The rhyolites in the upper Luliang Group of Shanxi, China, are Paleoproterozoic weakly alkaline volcanic rocks. They are characterized by high SiO₂, Na₂O + K₂O, Zr, Nb, Ga, Y and REE contents and large FeO*/MgO, Rb/Sr and Ga/Al ratios, and low CaO, Sr and Eu contents, and share much in common with the A-type granitic rocks. They erupted in the rift setting at the continental margin. Chemical features and isotope data, as well as high Nd and low initial Sr ratios, suggest that the original granitic magma was derived from partial melting of Late Archean metamorphic rocks in the lower crust due to the influence of basaltic magma and hot fluid in response to rifting. The A-type rhyolites were finally formed after the fractional crystallization of the dominant mineral feldspar.