Geochemical and Sr–Nd–Pb–Hf–O isotopic compositions of the Tiezhai complex: Implications for lithosphere destruction of the North China Craton

Abstract

It is generally accepted that in the Late Mesozoic, >100 km of ancient refractory lithospheric mantle beneath the eastern part of the North China Craton (NCC) was removed and replaced by young and fertile mantle material. However, the mechanisms and a tectonic model for this lithospheric destruction remain subjects of debate. To elucidate this lithospheric destruction, an investigation that included whole-rock geochemical analyses, zircon U–Pb age determinations, and in situ Hf–O isotope analyses of samples from the Tiezhai syenite porphyry and mafic rocks from western Shandong Province (the Luxi terrane) was conducted. Zircon U–Pb dates reveal that the Tiezhai syenite porphyry, gabbro, and monzonite were emplaced during the Early Cretaceous (weighted-mean 206Pb/238U ages of 130–129 Ma). Abundant Mesoarchean–Paleoproterozoic inherited zircons/cores were identified in both the syenite porphyry and the gabbro, yielding 207Pb/206Pb ages of 2.95–2.34 Ga. Both the syenite porphyry and mafic rocks are metaluminous, high-K calc-alkaline rocks enriched in large ion lithophile elements (e.g., Rb, Sr, and Ba) and light rare earth elements, and depleted in high field strength elements (e.g., Nb, Ta, and Ti) and heavy rare earth elements. The samples and have no obviously Eu anomalies. The syenite porphyry has high Sr and La contents, high Sr/Y (196–292) and (La/Yb)N (24.8–36.2) ratios, and low Y and Yb contents, similar to adakitic rocks. The Tiezhai syenite porphyry has a relatively narrow range of (87Sr/86Sr)i ratios (0.7057–0.7059), εNd(t) values (−8.9 to −6.8), and relatively high radiogenic Pb isotopic compositions [(206Pb/204Pb)i = 18.36–18.37, (207Pb/204Pb)i = 15.54–15.55, and (208Pb/204Pb)i = 38.25–38.31]. The Tiezhai mafic rocks can be divided into two groups (1 and 2) based on their whole-rock Sr, Nd, and Pb isotopic compositions. Group 1 has lower (87Sr/86Sr)i ratios than group 2 (0.7046–0.7047 vs. 0.7072–0.7096), lower radiogenic Pb isotopic compositions [(206Pb/204Pb)i = 17.68–17.71 vs. 18.08–18.12; (207Pb/204Pb)i = 15.44–15.45 vs. 15.52–15.53; (208Pb/204Pb)i = 37.72–37.98 vs. 40.17–40.60], and slightly higher εNd(t) values (−8.0 to −7.1 vs. –10.8 to −8.2). The syenite porphyry and gabbro from group 2 have similar narrow ranges of zircon rim εHf(t) values (mostly between −4 and + 2), and high zircon rim δ18O values (syenite porphyry = 6.9‰–9.0‰; gabbro = 7.7‰–9.1‰), and low inherited zircon/core δ18O values (syenite porphyry = 5.8‰–6.9‰; gabbro = 5.7‰–6.9‰). The Tiezhai mafic rocks were probably derived from partial melting of the Luxi EM1-asthenospheric mantle previously metasomatized by fluids derived from subducted oceanic crust and sediments. The melts probably assimilated ancient NCC upper crust material (~5% assimilated by group 1 mafic rocks, 20%–30% assimilated by group 2 mafic rocks) during magma ascent. The Tiezhai syenite porphyry was most likely derived from partial melting of the newly underplated thick lower crust and assimilated ~10% of the ancient NCC upper crust material during ascent. The addition of fluids derived from oceanic crust and sediments to the overlying subcontinental lithospheric mantle during subduction was an important factor contributing to lithosphere destruction in the eastern NCC. Slab rollback, together with the ridge subduction of the paleo-Pacific slab beneath the eastern NCC, is the most reasonable tectonic model for NCC destruction and formation of the Early Cretaceous large-scale magmatic rocks in the eastern NCC.