Petrogenesis Of The Early Cretaceous Mafic Dikes And Metallogenic Dynamics In Jiaodong Peninsula

Large-scale of the Early Cretaceous mafic dike swarms emplaced in the whole Jiaodong Peninsula,closedly associating with gold veins in the gold desposits.The petrogenesis and geodynamic background of the mafic dikes in Jiaodong Peninsula are constrained in this study,which not only provide the significant informations for geodynamic background of Jiaodong Peninsula in the Late Mesozoic,and also discuss the relationship between mafic dikes and gold deposit.The mafic dikes,mosltly consisted of lamprophyre and dolerite,extend along with the regional faults.The high-precision zircon dating constrains the timing of mafic dikes emplacement to 130-110 Ma.The mafic dikes in Jiaodong show the arc-like features,such as low contents of Si O2 and high contents of Mg O?Cr and Ni,enrichment of large ion lithophile elements(LILE),depletion of high field strength elements(HFSE),high(87Sr/88Sr)i rations,low ?Nd(t)and ?Hf(t)value.These features collectively point to derivation of dike magmas from enriched sub-continental lithospheric mantle(SCLM)of North China Craton(NCC).High Dy/Yb ratios of dike rocks suggest that their magmas were originated from low-degree partial melting of an amphibole-bearing lherzolitic mantle lithosphere in the garnet–spinel transition zone.The low Mg isotope of Jiaodong mafic dikes demonstrated that the mantle source have been significantly metasomatized by carbonate.The continued fluid-related metasomatism induced the occurance of fertile vein in the lithospheric mantle.The reaction of magmas generated by partial melting of fertile veins and the surrounding peridotitic wall rock resulted in the emplacement of Jiaodong mafic dikes with the features of low contents of Os and elevated 187Os/188 Os isotope ratios.In addition,the slight differences between Jiaobei mafic dikes and Sulu mafic dikes show that the mantle source of Sulu mafic dikes has undergone the subduction-related metasomatism during the subduction and collision between NCC and Yangtze Craton(YC)in Triassic.In the Mesozoic,Paleo-Pacific plate subducted beneath eastern NCC,altering the overlying SCLM.In the Early Cretacous,the Izanagi plate changed its subduction direction with slab roll-back,which induce the extension and thinning of SCLM,eastern NCC.Asthenospheric upwelling caused by this inferred lithospheric thinning led to partial melting of the subduction-metasomatized,amphibole-bearing lherzolitic peridotites within the spinel-garnet transition zone,producing the arc-like magmas of the Jiabei and Sulu mafic dikes.Decompression melting of the upwelling asthenosphere produced OIB-type magmas in the easternmost part of the NCC and the Jiadong Peninsula.In the study of mafic dikes and gold deposit,we found that there a number of mafic dikes associated with gold veins.The geochronology of mafic dikes and gold deposits in Jiaodong Peninsula shows the timing of mafic dikes emplacement from pre-,syn-to post-mineralization,however,the peak of mafic dikes intruding is syn-mineralization(110-130 Ma).Additionally,both of the fliud of mineralization and mafic dikes in Jiaodong Peninsula have the similar mantle source.However,the low PGE compositions of the dike rocks further indicate that their magmas were produced from S-saturated melts,derived from low degree partial melting of the inferred mantle source.The low degree partial melting of the inferred mantle source resulted into that the sulfophile element(e.g.PGE and Au)left in the mantle.Therefore,the Mesozoic mafic dikes possibly didn't carry out the Au from the mantle,for the gold mineralization.