Petrogensis Of Syn-collisional Granitoids In West Qingling And Origin Of The Mesozoic Dikes In Eastern China: Petrological And Geochemical Constraints

Wuchaba and JCB pluton are earlier cumulates of the same magmatic system.The Mianlue oceanic crust?MORB-like?contributes to the source of the Luchuba,Wuchaba and JCB granitoids,pointing to the significance of melting of oceanic crust for continental crust accretion.We present K-Ar and zircon U-Pb ages and geochemical data on the Mesozoic dikes in the eastern continental China.The K-Ar and zircon age data?130-110Ma?tightly constrain the emplacement time of the dikes.The dikes represent evolved alkaline basaltic melts intruding in the Mesozoic granitoids.Their rare earth element?REE?and multi-element patterns are differ from the present-day ocean island basalts?OIBs?and MORB,but show strong crustal signatures?i.e.,enrichment of Rb and Pb and depletion of Nb,Ta and Ti?.They show high(⁸⁷Sr/⁸⁶Sr)_i?0.7048 to 0.7103?,low?_Nd?t??-12.3 to-5.7?,low?_Hf?t??-16.5 to-8.0?and variable Pb(²⁰⁶Pb/²⁰⁴Pb_i)isotopic ratios?18.79-18.85?.These geochemical features collectively suggest that these Mesozoic dikes may have originated from the enriched sub-continental lithospheric mantle?SCLM?.The elemental and isotopic data of arc-like geochemical signatures are also consistent with melts derived from SCLM that had been metasomatised by fluids.The fluids?in the form of hydrous melt?may come from the subduction of the Paleo-Pacific plate at¹20Ma.The stagnant subduction slab dehydrated and released water in the form of hydrous melt that percolated through and metasomatized the lithosphere mantle,weakened the base of the lithosphere while producing basaltic melt.The basaltic magmas underplated the lower crust,and melted the lower crust to generate the widespread Mesozoic granitoids in eastern China.