Late　Mesozoic　mafic　dykes,　which　are　widely　developed　in　the　North　Qinling　Orogenic　Belt　(NQOB),　include　abundant　geodynamic　information.　This　paper　describes　the　mafic　dykes　that　intrude　the　Late　Jurassic　granite　in　the　Dayu　and　Kuyu　areas,　and　reports　important　petrological　constraints　for　the　late　Mesozoic　tectonic　transition　from　compression　to　extension　in　the　NQOB.　Three　zircon　U-Pb　results　show　that　the　minimum　ages　of　the　mafic　dykes　are　139.8　+/-　1.4　Ma,　137.4　+/-　1.7　Ma　and　133.4　+/-　0.9　Ma,　indicating　that　the　emplacement　age　of　the　Dayu　and　Kuyu　mafic　dykes　is　140-133　Ma.　Petrogeochemical　analyses　suggest　that　the　mafic　dykes　belong　to　the　high-K　calc-alkaline　shoshonite　series　with　low　SiO2　(46.93-56.73　wt%),　MgO　(1.88-9.10　wt%)　and　TiO2　(1.17-1.82　wt%),　and　high　Al2O3　(13.98-17.46　wt%),　TFe2O3　(7.81-10.92　wt%)　and　K2O　(1.28-4.78　wt%).　The　mafic　dykes　are　enriched　in　large　ion　lithophile　elements　(e.g.　Rb,　Ba,　K,　La,　Sr)　and　depleted　in　high-field-strength　elements　(e.g.　Nb,　Ta,　Zr,　Ti).　These　samples　have　the　right-sloping　chondrite-normalised　rare　earth　element　patterns,　which　suggest　light　rare　earth　element　enrichment　and　heavy　rare　earth　elements　depletion　with　no　obvious　Eu　anomalies　(delta　Eu　=　0.94-1.11).　The　I-Sr,　epsilon(Nd)(t),　epsilon(Hf)　(t)　and　T-DM2(crust)　values　are　0.7056-0.7060,　-10.60　to　-5.98,　-14.1　to　-2.8,　and　1382.4　+/-　25.1　to　2081.9　+/-　47.6　Ma,　respectively.　Both　elemental　and　isotopic　geochemistry　show　that　the　formation　of　Dayu　and　Kuyu　mafic　dykes　is　due　to　the　partial　decompression　melting　of　previously　enriched　lithospheric　mantle　during　a　delamination　process.　The　mafic　dykes　have　undergone　fractionation　crystallisation　of　Mg-Fe　phase　minerals　during　magma　ascent,　accompanied　by　some　crustal　contamination.　Combined　with　the　regional　tectonic　setting,　we　suggested　that　the　NQOB　experienced　intra-continental　extension　during　the　Early　Cretaceous.