CHARACTERISTICS OF ULTRAMAFIC XENOLITHS FROM CENOZOIC BASALTS IN ABAGAQI AREA, INNER MONGOLIA

Abstract

The ultramafic xenoliths in various quantities and scales were found in the. Quaternary alkali basalts at Abagaqi area of Inner Mongolia These ultramafic xenoliths are composed of spinel Iherzolite, saxonite, dunite, olivine websterite, spinel pyroxenite, pyroxene megacrystal, and crust xenoliths which include gabbro, granodiorite, quarfelsic-gueiss. Petrography characteristics of ultramafic xenoliths indicate that they are not accumulation material of fractional crystallization from adventive basalt magma; characteristics of major element and rare earth element of ultramafic xenoliths are akin to Alpine type peridotite, composition of ultramafic xenoliths are similar to primary mantle; Sm-Nd and Rb-Sr isotopic characteristics indicate that ultramafic xenoliths with adventive basalt magma are not cognate; The equilibration temperature and pressure of the spinel Iherzolite xenoliths are estimated to be 1056.9°C ~ 1103.1°C and 1.50 ~ 1.84GPa and this range of depth should be 49.5 ~ 60.7km; olivine websterite and spinel pyroxenite are estimated to be 887.1°C ~ 958.6°C and 1.25 ~ 1.4GPa and this range of depth should be 41.25 ~ 46.2km. The equilibration temperature and pressure of adventive basalt are estimated to be 1390°C ~ 1450°C and 1.75 ~ 2.2GPa and this range of depth should be 57.7 ~ 72.6km.. It is considered that an upper mantle diaper occurred since late Tertiary in study area, giving rise to the thinning of the lithosphere and the formation of extensional environment in this area, the basalts magma direct ejected along structural thoroughfare. The ultramafic xenoliths bear no relation to origin of fused mass and residulal body between adventive basalt magma. The ultramafic xenoliths is caught fragment from lithosphere mantle in uplifting process of adventive basalt magma.