Geochronology and Geochemistry of Volcanic Suites from the Middle Sulu Orogenic Belt, Eastern China: Implication for Petrogenesis and Tectonic Setting

Abstract:

The widely distributed Late Mesozoic igneous events in North China have been intensively studied in the past decades and proposed to be accounted by lithospheric thinning. However, mechanism of the lithospheric thining is still in debate. In this study, we report U-Pb zircon ages, geochemical data and Sr-Nd isotopic data for the Cretaceous mafic and associated felsic intrusions from the middle Sulu orogenic belt in the Shandong Peninsula. The ICP-MS U-Pb zircon analyses show consistent crystallization ages ranging from 120.2 ± 2.1 Ma to 123.3 ± 1.6 Ma for four representative samples. These rocks are characterized by LREE and LILE enrichment, HREE depletion, high initial ⁸⁷Sr/⁸⁶Sr values ranging from 0.70401 to 0.70966, low negative εNd(t) values from -22.0 to -12.2. These features suggest that they were derived from a common enriched lithospheric mantle source. Geochemical and isotopic data indicate that the original magma of these rocks was produced by partial melting of an ancient lithospheric mantle, which was variably mixed with lower crustal eclogite derived melts. The lamprophyres, andesitic porphyrites and syenogranite may have been attributed to subsequent fractional crystallization of olivine and pyroxene, whereas the rhyolite resulted from K-feldspar and plagioclase fractionation, without any crustal contamination affection. Tectonically, the petrogenesis process of these rocks favors an intense lithospheric extensional regime beneath the Sulu orogenic belt at 120 ~ 124 Ma, which was resulted by the widespread removal of the upper mantle and lower crust that driven by the abrupt change of the subducting direction of the Pacific plate.