Mafic And Ultramafic Rocks And Their Regional Structural Significance In Donggouzi Area,Fengzhen City,Inner Mongolia

The Paleoproterozoic tectonic setting and spatial-temporal evolution of the northern North China and Central Orogenic belt constrain the assembly of the North China craton.Here we describe a new Paleoproterozoic mafic-ultramafic intrusions located between the Khondalite belt in Jining and Central Orogenic belt in Fengzhen City,Inner Mongolia,which we refer to as Donggouzi mafic-ultramafic complex,and discuss its regional structural significance.The Donggouzi mafic-ultramafic complex trends NE-SW and is nearly elliptical in shape with ca.700-m long and 200-300 m wide,it has an exposed area of ca.18,000 m~2.The Donggouzi mafic-ultramafic rocks can be divided into two parts:ultramafic rocks and mafic rocks.The ultramafic rocks are composed of peridotite,amphibole pyroxenite,and websterite blocks,with associated syenite dikes.The mafic rocks are mainly composed of gabbro dikes,which intrudes into the ultramafic rocks in the form of dikes.Field observations show that the Donggouzi ultramafic rocks intruded into Paleoproterozoic granulites.Zircon U-Pb geochronology of the granulite and intruding syenite dikes together with the field observations constrain the formation of the Donggouzi mafic-ultramafic complex at ca.1849-1845 Ma.Geochemical and Sr-Nd isotopic data indicate that the Paleoproterozoic mafic-ultramafic rocks were sourced from the subcontinent lithospheric mantle that was metasomatized by the fluid from a subduction slab.Emplacement of the complex occurred in a post-orogenic extensional setting,possibly associated with the collision along the northern margin of the North China craton at ca.1.9 Ga.The ca.1767 Ma gabbroic dike was emplaced in an intraplate extensional setting and experienced crustal contamination,which suggests that an upwelling mantle plume produced magmas that interacted with the lithosphere during the late Paleoproterozoic.