Geochemical Characteristics of Late Mesozoic Dikes, Jiaodong Peninsula, North China Craton: Petrogenesis and Geodynamic Setting

Widespread Cretaceous dikes in the Jiaodong Peninsula of eastern China are genetically related to gold deposits. Bulk-rock major element analyses show that they are composed of medium-K, subalkaline to shoshonitic rocks characterized by wide ranges of SiO2 (48.98-71.38%) and MgO (0.63-1.0.02%), and extremely high contents of Al2O3 (14.32-16.06%). They are enriched in large-ion lithophile elements (LILE, e.g., K, Rb, Sr, Ba) and light rare-earth elements (LREE), but depleted in high-field-strength elements (HFSE, e.g.,Ta, Nb, Zr, Hf), and display consistent primitive mantle- and chondrite-normalized multi-element patterns on spidergrams, different from Cenozoic basalts in the Peninsula. These data suggest a magma origin by partial melting of an ancient enriched lithospheric mantle. High Th/Yb and Ph/Ce ratios and weak negative Ce anomalies reflect involvement of recycled sediments in the mantle reservoir. Strong fractionation in REE, with LREE >100 times chondrite and HREE <10 times chondrite, indicates the presence of garnet in the mantle source. Nb* values of 1.49-2.38 (>1), K* values of 0.08-0.14 (<1), and similar Zr/Hf (32.15-40.65), U/Th (0.10-0.33), and Hf/Sm (0.45-0.67) ratios in all the dikes, along with published chemical and Sr-Nd isotopic compositions of lamprophyres (Cheng et al., 1998; Sun et al., 2000; Yang et al., 2004) preclude crustal contamination during magma ascent and binary mixing between mantle and crustal magmas. The geochemical features of these dikes can be interpreted as the result of interaction of old lithospheric mantle with melt and/or fluid generated from subducted lower/middle crust of the Yangtze craton and/or Izanagi oceanic crust. The source for the Late Mesozoic dikes was completely reconstructed, enriched lithospheric mantle containing disseminated amphibole- and phlogopite-bearing pyroxenite veins surrounded by refractory peridotite wall rock. The enriched source was modified by subsequent partial melting due to sinistral transtensional strike-slip movement of the Tan-Lu fault that led to widespread dike intrusion. Trace element geochemistry and regional geology indicate that these dikes formed in a back-arc extensional environment.