Geochemistry of ultrapotassic volcanic rocks in Xiaogulihe NE China: Implications for the role of ancient subducted sediments

Abstract

Abstract The unique eruptions of ultrapotassic volcanic rocks in eastern China reported so far took place in the Xiaogulihe area of western Heilongjiang Province, NE China. These ultrapotassic rocks are characterized by extremely high K 2 O contents (> 7 wt.%), abnormally unradiogenic Pb isotopic compositions ( 206 Pb/ 204 Pb = 16.44–16.55; 207 Pb/ 204 Pb = 15.39–15.46; 208 Pb/ 204 Pb = 36.35–36.61), and moderately high 87 Sr/ 86 Sr ratios (0.7053–0.7057), which can be basically correlated with those of ultrapotassic igneous rocks distributed widely in northwestern America and Aldan Shield. The positive correlation between 187 Os/ 188 Os and 1/Os argues that these ultrapotassic rocks have probably experienced negligible lower continental crust addition (less than 1%) during magma ascent. The high contents of K 2 O and negative correlation between 87 Sr/ 86 Sr and 206 Pb/ 204 Pb of these ultrapotassic rocks indicate the presence of a potassic phase, mostly phlogopite, in their mantle source. Their strong fractionation of rare earth elements and lack of Nd–Hf isotopic decoupling reveal a low-degree partial melting of garnet-bearing source rocks. In addition, the low CaO and Al 2 O 3 contents of whole-rock compositions and low Fe/Mn ratios of olivine phenocryst chemistries suggest peridotites rather than pyroxenites as dominant source rocks for the Xiaogulihe ultrapotassic rocks. Based on these distinctive geochemical characteristics, we thus propose that the ultimate mantle source of the Xiaogulihe ultrapotassic volcanic rocks is phlogopite-bearing garnet peridotite within the lower part of the sub-continental lithospheric mantle (SCLM) that had been metasomatized by potassium-rich silicate melts. Combined with the unradiogenic Pb compositions, the most likely source of these potassium-rich silicate melts is the ancient subducted continental-derived sediments (> 1.5 Ga). These ancient subducted sediments, possessing relatively low initial Pb isotopic compositions, had experienced large U/Pb fractionation during a subduction process, resulting in low-μ ( 238 U/ 204 Pb), and then accumulated in the mantle transition zone. The relatively low 87 Sr/ 86 Sr ratios of these ultrapotassic rocks also imply that their mantle source had evolved with low Rb/Sr ratios, which possibly resulted from the metasomatized melts derived from the ancient subducted sediments. This interpretation is quite different from previous hypotheses that attribute their unusual geochemical features to a dominantly asthenospheric source with a contribution from delaminated ancient SCLM, or a SCLM source that has been metasomatized by melts derived from deep asthenosphere or delaminated ancient lower continental crust.