Geochronology and geochemistry of Early Cretaceous igneous units from the central Sulu orogenic belt: Evidence for crustal delamination during a shift in the regional tectonic regime

Abstract

Widespread Late Mesozoic igneous events in northern China have been intensively investigated over the past decades and provide evidence for regional lithospheric thinning. The underlying mechanism causing lithospheric thinning remains unclear however. This study reports U–Pb zircon ages, geochemical data and isotopic ratios for Cretaceous igneous units from the central Sulu orogenic belt as it occurs in the Shandong Peninsula. LA-ICP-MS U–Pb analyses of magmatic zircons identified a relatively restricted population of ages ranging from 123±2Ma to 120±2Ma from four representative samples of igneous units in the study area. Geochemical analysis of the samples revealed LREE and LILE enrichment, HREE depletion, high initial 87Sr/86Sr values ranging from 0.7040 to 0.7096, and low negative εNd(t) values from −22.0 to −12.2. The data suggest lamprophyres derived separately from heterogeneous mantle inputs, which experienced crust–mantle interaction, andesitic porphyrites and syenogranites derived from enriched lithospheric mantle and rhyolites derived from partial melting of an ancient crustal component. Assimilation and fractionation processes did not contribute to lamprophyre formation, but did play an important role in generating andesitic porphyrites, syenogranites and rhyolites. The petrogenetic history of these rocks indicates intensive lithospheric thinning of the upper mantle and lower crust beneath the Sulu orogenic belt at 123–120Ma. Given the timing and regional tectonic framework in which these units formed, thinning was likely caused by an abrupt change in the direction of the subducting Pacific plate.