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A Study of Sr, Nd and O Isotopes of the K‐rich Melanocratic Dykes in the Late Mesozoic Gold Field in the Jiaodong Peninsula

Abstract  Geologic, petrographic and petrochemical studies of the late Mesozoic K‐rich melanocratic dykes, including lamprophyres, andesite porphyrites and dacite‐porphyry in the gold field system in the Jiaodong Peninsula, China, have shown that these dykes are characterized by rich potassium and alkali but poor titanium. They belong to an ultra‐high potassic, shoshonitic and high potassic calc‐alkaline rock series. The parental magma has relatively high initial strontium ratios ((87Sr/86Sr),=0.70895‐0.71140) and low (143Nd/144Nd)1 ratios (varying from 0.51135 to 0.51231); and its δ18Osmow, whole rock values vary from +5.8%c to +10.6%c with a mean of +7.1%c. These features suggest that the source region of the magma is an enriched mantle wedge transformed from a continental lithosphere mantle which has experienced metasomatism by mantle‐derived fluids with H2O‐dominated fluids that were provided during the underthrusting of an ocean crust. The initial magma was generated by low‐degree partial melting of the enriched mantle in its mature stage in the back‐arc spreading environment. The evolution of magmas is associated with two trends, i.e., fractional crystallization and mixing with or intensive contamination by palaeo‐crust materials or metamorphic rocks. The former process is evident in the gold field system of quartz‐vein type, whereas the latter is dominated in the gold field system of the altered‐rock type. This conclusion is very important for more detailed study of petrogenesis and mineralization through the crust‐mantle interaction (exchange) in the Mesozoic in this region.