The Mineral Chemistry of Ultramafic Xenoliths of Eastern China: Implications for Upper Mantle Composition and the Paleogeotherms

Abstract

Ultramafic xenoliths of garnet lherzolite (±rare spinel), spinel lherzolites, spinel harzburgites, clinopyroxenites, and clinopyroxene megacrysts were collected from Cenozoic basalts in all parts of eastern China. From their modal composition and mineral chemistry all the xenoliths may be placed into three types representing: a fertile or more primitive mantle (garnet lherzolite and spinel lherzolite), a refractory or more depleted mantle (spinel harzburgite and dunite), and inclusions cognate with the host alkali basalts at mantle pressures (pyroxenite and megacrysts). There are systematic differences between the mineral compositions of each type. Spinel shows a wide compositional range and the spinel cr-number [ 100Cr/(Cr + Al)] is a significant indicator of the xenolith type. Spinel cr-number and Al2O3 of coexisting minerals (spinel, clinopyroxene, and orthopyroxene) are useful as refractory indicators for spinel peridotite in that the cr-number increases and the percentage of Al2O3 decreases with increasing degrees of melting. In garnet peridotite, however, the same functions vary with pressure, not degree of melting. According to P–T estimates, the various xenoliths were derived from a large range of depths in the upper mantle: spinel peridotite from approximately 11 to 22 kb (37–66 km), spinel/garnet lherzolite from 19 to 24 kb (62–80 km), and garnet lherzolite from 24 to 25 kb (79–83 km). We conclude that the uppermost mantle beneath eastern China is heterogeneous, with a north-northeast zone of more depleted mantle lying beneath the continental margin and a more primitive mantle occurring towards the continental interior.