Contribution of crustal materials to the mantle sources of Xiaogulihe ultrapotassic volcanic rocks, Northeast China: New constraints from mineral chemistry and oxygen isotopes of olivine

Abstract

Ultrapotassic igneous rocks can generally be divided into two sub-groups based on the tectonic settings in which they formed. The orogenic sub-group occurs in subduction-related tectonic settings, while the anorogenic sub-group is confined to stable continental regimes. The Pleistocene Xiaogulihe ultrapotassic volcanic rocks outcrop in the western part of Heilongjiang province, northeast China, and are of intraplate origin with respect to its tectonic settings. Previous elemental and isotopic investigations have suggested that the mantle source of these volcanic rocks had been modified by continental-derived sediments resulting from an ancient subduction (at least older than 1.5Ga). In this contribution, we performed in-situ oxygen isotope analysis on olivine grains in these ultrapotassic rocks using secondary ionization mass spectrometry (SIMS). The olivine grains generally have higher δ18O values and CaO contents than those of mantle peridotite xenoliths in the nearby Keluo potassic rocks and show linear correlations between major and trace elements, and Fo, suggesting that they are cognate phenocrysts resulted from fractional crystallization processes. The restricted and non-correlated variations in δ18O with the Fo of these olivine grains imply that the fractional crystallization processes might have negligible influence on their δ18O values. The relatively higher δ18O values of the olivine phenocrysts than the normal mantle imply the addition of an 18O-rich crustal component into their mantle source after ruling out the crustal contamination of the host magmas. We propose that the high-δ18O feature of the olivine phenocrysts was inherited from the subducted crustal component in their mantle source. Given the rapid oxygen isotopic diffusion under high temperature and the long period between mantle metasomatism event and volcanic eruption, it is postulated that the high-δ18O signature could only be preserved in the relatively cold and stable subcontinental lithospheric mantle. Such speculation is consistent with our previous inference that the Xiaogulihe ultrapotassic volcanic rocks were mainly generated from the lower subcontinental lithospheric mantle which had been metasomatized by potassium-rich silicate melts derived from ancient subducted continental-derived sediments.