The Mianlüe (Mianxian—Lüeyang) paleo ocean was the north branch of the Eastern Paleo Tethys, which was distributed at the southern margin of the Southern Qinling belt, and the typical ophiolite combination was developed in the Mianxian—Lüeyang area. However, due to the transformation of the collision orogenesis since the late of Middle Triassic and later intra continental deformation effect, the break up and development of Mianlüe ocean in the east part was not clear. This paper studies the petrological, geochemical, isotopic and geochronological characteristics on the mafic dikes in the Southern Qinling belt. The mafic dikes are dominated with the gabbro and diabase. And the content of SiO2 for these dikes were range from 4189% to 4975%, which are belonging to the basic—ultrabasic rock type. These dikes were alkaline rock, and characteristics with the rich sodium and high titanium. The REE, trace elements and Sr—Nd isotope characteristics was similar to the within plate ocean island basalt (OIB) for these mafic dikes，and its magma source was originated from the OIB contamination with the crustal. These mafic dykes intruded in the early Paleozoic strata along the bedding planes, and the zircon SHRIMP dating of the gabbro dikes revealed the emplacement age was early of Late Silurian (~422Ma). These mafic dikes representing the Southern Qinling belt experienced the crustal extension and mafic magma intruded in the late of early Paleozoic, which represented the initial break up of the Paleo Tethyan ocean in the eastern of the Southern Qinling belt, and related to the closure and intracontinental subduction of the Shangdan（Shangnan—Danfeng) Proto Tethys ocean at the northern margin of the Southern Qinling belt in the late of early Proterozoic.