Chemical and isotopic changes induced by pyrometamorphism in metasedimentary xenoliths at Tongariro volcano, New Zealand

Abstract Andesites erupted from Tongariro volcano, North Island, New Zealand contain feldspathic and quartzose xenoliths derived from basement rocks. New major oxide, trace element and Sr-Nd-Pb isotopic data indicate that both the Waipapa and Kaweka (meta)sedimentary terranes are represented in erupted xenoliths, rather than only the Kaweka terrane as previously thought. Xenolith mineral assemblages differ from their likely source materials, notably through the lack of white mica, illite, chlorite and quartz, which is reflected in contrasting chemical and isotopic compositions. Major and trace element data indicate that most xenoliths underwent bulk mass decreases of about 50% when pyrometamorphosed at temperatures of ~800–980 °C, similar to typical Tongariro magma temperatures of ~800–1000 °C. Bulk Eu concentrations were retained (in restitic plagioclase); however, other rare earth elements are commonly lower in xenoliths than in protoliths. In xenoliths, the 143Nd/144Nd ratios of protoliths were also retained, which indicates that xenoliths were derived from the Kaweka and Waipapa terranes in subequal amounts. Reductions in 87Sr/86Sr ratios by up to 0.003 in xenoliths, relative to their likely protoliths, were accompanied by decreases in Rb/Sr ratios from 0.1–0.8 down to