The Petrogenesis And Tectonic Setting Of The Early Cretaceous Basic Dikes In Pulandian Area, Liaodong Peninsula

The tectonic activities and magmatism were active in Liaodong Peninsula during the Mesozoic Era.Large amounts of acidic to basic dikes and intrusive igneous rocks develop.Using these dikes and intrusions as research objects,the main features and causes of tectonic evolution can be understood in the Liaodong Peninsula region.Further,the exact time of tectonic activity can be defined accurately.Four diabase dikes found in suitable outcrops of Pulandian and Wafangdian area,Liaodong Peninsula,were analyzed under field,petrological,geochemistrical and geochronological approaches.The petrogenesis,sources and tectonic history of these dikes are inferred.According to the U-Pb isotopic analyzing results,three ages of DikeⅠ,DikeⅡand DikeⅢare 124±1Ma,121±3Ma,123±2Ma,respectively.From the U-Pb dating result,it can be concluded that these dikes formed in the Early Cretaceous.Radioactive Sr-Nd isotope geochemistry and petrogeochemistry show that:Samples from DikeⅠhave low radioactive Sr composition(initial ⁸⁷Sr/⁸⁶Sr=0.705),depletion of Nd(ε_Nd=+1.2,+2.8),（La/Yb）_N=5.14⁶.66,（La/Sm）_N=1.38¹.79,Th/La=0.076⁰.103,δEu>1,enrichment of large ion lithophile element Sr.It indicates the magma source of DikeⅠis probably the asthenosphere mantle without the contamination of crust during the formation process.Samples from DikeⅡand DikeⅢshow the enrichment of large ion lithophile element Rb,Ba,and K,depletion of high field strength element Th,U,Nb,Ta,δCe<1.Considering together with the REE analyzing result,it is inferred that the magma sources of DikeⅡand DikeⅢcome from enriched mantle added by oceanic crust material with the contamination of crustal material.Samples from DikeⅣhave high composition of radioactive Sr(initial ⁸⁷Sr/⁸⁶Sr=0.709),enrichment of Nd（εNd=-5.9）and large ion lithophile elements K,Rb,Ba,depletion of high field strength element Nb,Ta,（La/Yb）_N=10.25¹1.50,（La/Sm）_N=3.18³.45,δCe=0.92⁰.94.It indicates the magma source of DikeⅣis derived from partial melting of enrichment and ancient mantle under high pressure.According to the comprehensive analysis,the dolerites in research area formed in the Early Cretaceous.Their REE composition shows the traits of intraplate basalt.The magma sources of these dikes are distinct.The development of basic dike proves that the Liaodong Peninsula area was under a stretched tectonic setting during the Early Cretaceous.