Mantle-Lherzolite Xenoliths from Eastern China: Petrogenesis and Development of Secondary Textures

Abstract

Spinel-lherzolite xenoliths in alkali basalts from eastern China have porphyroclastic to equigranular textures displaying varying degrees of deformation and subsolidus re-equilibration. The proportions of minerals in these xenoliths vary from 52 to 72% homogeneous olivine (Fo88-91); 11 to 26% orthopyroxene (Wo0.9.1.6; En88-90; Fs8.7.10.7), with minor discontinuous variations of Al2O3, FeO, and CaO; 6 to 19% clinopyroxene (Wo43.47; En49.51; Fs3.7.6.7); and 1 to 5% spinel, with similar Mg# (79.6 to 82.6), but wider variations of Al2O3 and Cr2O3 (100Cr/(Cr + Al + Fe3+) = 8.1 to 23.6). Although previous trace-element and isotopic studies have shown that at least two distinctly different mantle sources were sampled by Cenozoic basalts, mineralogical heterogeneities seem to be minor within the spinel-peridotite-facies lithosphere beneath eastern China. These xenoliths experienced limited interaction with the host basaltic magma during eruption. Symplectites of secondary, minute silicates, titanomagnetite, and sulfide have replaced orthopyroxene—and to a lesser extent olivine—at the contact with the basalt. The spinel in the margin of the xenolith is continuously zoned by substitutions of Fe3O4 (magnetite) and Fe2TiO4 (ulvospinel) for MgAl2O3 (spinel), and is rimmed by titanomagnetite with a sharp boundary. However, the compositions of the interior clinopyroxenes were commonly modified by metasomatic partial melting, which resulted in “spongy-textured” rinds on primary clinopyroxene. This secondary assemblage is composed mainly of a refractory, jadeite-poor clinopyroxene, which is largely in optica! continuity with the primary clinopyroxene in addition to interstitial feldspars, with minor titanomagnetite and Fe-Ni sulfides. This assemblage was produced by the introduction of K-rich fluids from the enclosing basaltic magma. The intensity of these secondary reactions appears to have been a function of the residence time of the xenolith in the host basalt. Therefore, all secondary alteration of both external and internal primary minerals in these xenoliths are the result of near-surface metasomatic processes, rather than of mantle phenomena.