Crust–mantle interaction in the central North China Craton during the Mesozoic: Evidence from zircon U–Pb chronology, Hf isotope and geochemistry of syenitic–monzonitic intrusions from Shanxi province

Abstract

In-situ zircon U–Pb ages, Hf isotopic compositions and whole rock geochemical and Sr–Nd isotopic compositions are presented for three Mesozoic syenitic–monzonitic intrusions from Shanxi province, central North China Craton. Zircons from these intrusions all show core–rim structures in that the oscillatory rims recorded their intrusive ages, whereas the cores are interpreted as xenocrysts. The U–Pb age data reveal that the northernmost Dishuiyan monzonite was emplaced at 241Ma, while the Huyanshan and Erfengshan syenitic–monzonitic complexes were emplaced at 130Ma and 128–134Ma, respectively. The Dishuiyan monzonite is petrologically and geochemically uniform, it shows LREE enrichment and HFSE depletion and exhibits enriched Sr and Nd isotopic compositions with (87Sr/86Sr)i=0.7065 and εNd(t)=−8.3–−7.9. The zircon rims in the Dishuiyan monzonite show highly varied Hf isotopic compositions with εHf(t)=−27.8–−6.4. The varied Hf isotopic compositions and enriched Sr–Nd isotopic compositions, together with the ubiquitous xenocrystic zircon cores, suggest the Dishuiyan monzonite was produced by the mixing of melts from enriched lithospheric mantle and lower crust. The monzonite and syenite from the Huyanshan complexes exhibit different geochemical features. The εNd(t) values of syenite, which are higher than those of monzonite resemble the enriched lithospheric mantle, and together with the absence of zircon in the syenite, we propose that it was originated by partial melting of enriched lithospheric mantle. Monzonites from Huyanshan and Erfengshan share similar petrological and geochemical characteristics, being enriched in LREE and depleted in HFSE, and they show low εNd(t) values between −18.2 and −13.9. Similar to zircon rims in the Dishuiyan monzonite, those in the Huyanshan and Erfengshan monzonites also exhibit highly varied Hf isotopic compositions. The consistent ages between the xenocrystic zircons in monzonites and the lower crustal basement rocks of the NCC reveal a significant contribution from the lower crust in the formation of the monzonites. We propose that the monzonites from Huyanshan and Erfengshan were formed through hybridization of enriched lithospheric mantle-derived and lower crust-derived magmas. The Mesozoic syenitic–monzonitic intrusions in Shanxi province were produced under extensional environments which linked to different geodynamic settings. The Triassic Dishuiyan monzonite is a product of post-collisional lithospheric extension related to the subduction of the paleo-Asian oceanic plate under the NCC and subsequent slab break-off in the north, while the Cretaceous Huyanshan and Erfengshan syenite–monzonite was produced under a back-arc extensional regime linked to the subduction of the paleo-Pacific slab beneath East Asia.