Discussion on genesis of pyroxenite and granulite xenoliths from Hannuoba
汉诺坝捕虏体辉石岩和麻粒岩的成因探讨

The Cenozoic basalts from Hannuoba contain abundant xenoliths derived from mantle and crust. As pyroxenite and granulite xenoliths appear very similar by naked eye sometimes, it is easy to confound the two kinds of rocks. Here we present our research on the genesis of pyroxenite and granulite xenoliths through petrographical observation, mineral chemistry and trace element and Sr-Nd isotopic compositions of clinopyroxene. The results indicate that the two kinds of xenoliths are remarkably different. Granulite xenoliths have layered cumulate structure and both kinds of pyroxenes (salite and bronzite) are relatively rich in FeO and poor in MgO. Clinopyroxene in granulite xenolith is relatively enriched in REE and characterized by high 87Sr/86Sr ratios and low 143Nd/144Nd ratios. In contrast, pyroxenite xenoliths generally show massive structure. Diopside and bronzite- enstatite of the pyroxenite xenoliths are relatively rich in MgO and poor in FeO. Clinopyroxene in pyroxenite xenolith shows very low REE content and a REE-depleted pattern. Isotopic compositions of pyroxenite xenoliths are between those of peridotite and granulite xenoliths. The above features as a whole suggest that granulite xenoliths result from magma underplating, which was subjected to lower crustal contamination. However, the pyroxenite xenoliths, distinguished from peridotite and granulite xenoliths, are derived from enriched mantle.