The Qinling- Qilian junction intersects at the Long Shan tectonic belt in the southwestern part of the North China block, and the Longshan tectonic belt is the key area to reveal the spatio- temporal relationship between North China block, North Qinling orogenic belt and North Qilian orogenic belt. A set of Mesoproterozoic quartz diorite in Xinjie determined by predecessors has doubts in terms of rock type, formation age and formation environment. In this paper, petrology, geochemistry and U- Pb chronology of monazite and zircon have been studied for this set of rocks and biotite monzonite gneisses inclusions. This set of rocks has been renamed as Xinjie plagioclase amphibolite schist. According to geochemistry, the protolith is a set of basic magmatic rocks. Zircon U- Pb dating gives the formation age of protolith is 433. 15±3. 42 Ma (Middle Silurian). Metamorphic zircon and monazite U- Pb dating gives metamorphic age of 427.9 ±1.46 Ma. The protolith was metamorphic shortly after its formation. This is an important feature of Xinjie plagioclase amphibolite schist in the history of formation and evolution. Combined with the analysis of regional geological data, it is shown that in the late Ordovician- middle Silurian, Longshan area was in a strong back- arc extensional tectonic environment under the background of northward subduction in the eastern part of North Qilian. Large- scale basic- acid magmatic activity developed in Longshan area under this extensional background. The protolith of Xinjjie plagioclase amphibolite schist is the basic magmatic rock formed under this extensional background. The protolith experienced hydrothermal metasomatism associated with magmatism in the same period and became plagioclase amphibolite schist. The detrital zircon age of biotite monzonitic gneiss enclosed in plagioclase hornblende schist is in good agreement with the detrital zircon age of Huluhe Group (S1H) in southern Longshan area, and its provenance area is closely related to Ordos block and North Qinling orogenic belt. The results of this paper show that the magmatism caused by back- arc extension in Longshan area during the late Ordovician to middle Silurian is large in scale and lasts for a long time, and there is a good time- space relationship between magmatism, metamorphism and sedimentation in this background.