Multiple enrichment of subcontinental lithospheric mantle with Archean to Mesozoic components: Evidence from the Chicheng ultramafic complex, North China Craton

Abstract

Numerous Late Paleozoic to Early Mesozoic mafic-ultramafic intrusions occur along the northern margin of the North China Craton (NCC). The timing, magma source, genetic types and tectonic evolution of these intrusions still remain controversial. Here we conduct a systematic petrological, mineral chemical, whole-rock geochemical, isotopic (zircon Lu–Hf and whole-rock Sr–Nd isotopes) and geochronological study on representative samples from the Chicheng ultramafic complex in the northern NCC, our results provide new insights into multiple melt-fluid interactions in the subcontinental lithospheric mantle through time. Zircon U–Pb data from the ultramafic rocks (pyroxenite and serpentinized dunite) show a wide range of ages (2721–129 Ma), indicating zircon growth within an evolving and metasomatized mantle wedge during multi-stage melt-fluid interactions. The youngest zircon population (145–129 Ma) possibly marks the timing of formation of the Chicheng ultramafic complex at the Early Cretaceous. Moreover, these ultramafic rocks also witnessed tectonic transition from Paleo-Asian Ocean to Paleo-Pacific Plate subduction during the Paleozoic to Early Mesozoic and subsequent progressive lithospheric thinning and craton destruction at the northern margin of the NCC, which is also attested by the newly discovered diorite porphyry dyke from this complex reported in this study with zircon U–Pb ages in the range of 447 to 128 Ma. Zircon Lu–Hf isotopic data show εHf(t) values in the range of −19.9 to +15.4 with TDM of 219–3012 Ma and TDMC of 240–3650 Ma. Whole-rock Sr–Nd isotopic data display εNd(t) values ranging from −12.3 to −1.6, indicating heterogenous sources involving both depleted mantle and reworked ancient crustal components. Whole-rock geochemical data show enrichment in LILEs (Rb, Sr, Ba, Pb) and depletion in HFSEs (Nb, Ta, Ti), with positive anomalies at Ba, K, Pb and Nd and strongly negative Zr and Ti anomalies, suggesting geochemical affinities related to subduction-related island arc setting within an active continental margin. Mineral chemical data on clinopyroxenes do not correspond to “Alaskan-type” affinities. Integrating geochemical, geochronological and isotopic studies from this study and those from previous studies related to the mafic-ultramafic complexes in the northern Hebei Province along the northern margin of the NCC, we propose that these complexes might be composite plutons formed during the Early Cretaceous, which were derived from the interaction of melts and fluids released from subducted slab with metasomatized mantle peridotite, triggering different degree of partial melting of mantle wedge.