Two-types of Early Cretaceous adakitic porphyries from the Luxi terrane, eastern North China Block: Melting of subducted Paleo-Pacific slab and delaminated newly underplated lower crust

Abstract

The origin and tectonic setting of Early Cretaceous adakitic rocks from the Luxi terrane in the eastern North China Block (NCB) remain debated. To resolve this issue, we determined whole-rock geochemistry, zircon U–Pb ages, and in situ Hf–O isotopes of the Mengyin and Liujing adakitic porphyries from the Luxi terrane. Zircon U–Pb dating results reveal that both the Mengyin and Liujing plutons were emplaced during the Early Cretaceous, with weighted mean 206Pb/238U ages of 130±1Ma (2σ) and 131±2Ma (2σ), respectively. In addition, abundant Neoarchean–Paleoproterozoic inherited zircon cores are identified in the Mengyin adakitic porphyry with 207Pb/206Pb ages ranging from 2.53 to 2.42Ga. Rocks of both plutons are silicic (SiO2=65.4–70.2wt.%), metaluminous, and alkaline in composition, comprising mainly quartz syenite porphyries. Samples from both plutons are enriched in large ion lithophile elements (LILEs) (e.g., Rb, Sr, and Ba), and light rare earth elements (LREEs), depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, and Ti), and heavy rare earth elements (HREEs), and have either positive or no Eu anomalies. In addition, both adakitic porphyries have high Mg# values (51–64), high Sr and La contents, low Y and Yb contents, and high Sr/Y (Mengyin=149–264; Liujing=58–110) and (La/Yb)N (Mengyin=32.4–45.3; Liujing=43.8–53.1) ratios, similar to adakitic rocks worldwide. The Mengyin adakitic porphyry has higher whole-rock εNd(t) values (–5.8 to −4.1), more radiogenic Pb [(206Pb/204Pb)i=18.35–18.39, (207Pb/204Pb)i=15.55–15.56, (208Pb/204Pb)i=38.20–38.23], higher zircon rim εHf(t) values (+3.3 to +8.8) and δ18O values (+6.5‰ to +7.9‰), and lower (87Sr/86Sr)i ratios (0.7049–0.7050) than the Liujing adakitic porphyry [εNd(t)=−12.4 to −12.2, (206Pb/204Pb)i=17.63–17.72, (207Pb/204Pb)i=15.56–15.58, (208Pb/204Pb)i=37.76–37.94, εHf(t)=−14.8 to −11.2, δ18O=+5.9‰ to +7.1‰, (87Sr/86Sr)i=0.7090–0.7091]. The Mengyin adakitic porphyry was most likely derived from partial melting of subducted oceanic slab with some input of NCB Neoarchean–Paleoproterozoic lower crust components. The Liujing adakitic porphyry was probably derived from partial melting of delaminated newly underplated thick lower crust, which then interacted with above asthenospheric mantle peridotite. Slab rollback together with the ridge subduction of the Paleo-Pacific slab was the most likely geodynamic mechanism for formation of the Early Cretaceous Mengyin and Liujing adakitic porphyries.