Petrographic characteristics and several other related problems of the layered basic-ultrabasic intrusives in the Upper Yangtze Valley

Abstract

Three iron, titanium-rich layered basic-ultrabasic intrusives have been described. They occur in an approximately south-north striking belt of tectonic-magmatic complex within an anteklise. Iron and titanium were concentrated in the lower or middle-lower parts of these intrusives at the early and the middle stages of differentiation, during which FeO may have played an important role. Intimate spacial and temporal association as well as common petrochemical features (i.e., high iron and titanium contents) have been found between these layered intrusives and the syenite and alkali-syenite plutons. All of these rocks are considered as hypogene products of the differentiation of the Omeishan basaltic magma. In other words, the hypogene differentiation of the iron (titanium)-rich, sub-alkali Omeishan basltic magma under the same structural-geological conditions resulted in the formation of the rock series of layered intrusives (peridotite → iron, titanium-rich ultramafic rocks → iron-, titanium-rich gabbro, plagioclasite) → syenite, alkali-syenite.