Geochemical constraints on the origin of Neoarchean magmatic rocks in the Lüliang Complex, North China Craton: Tectonic implications

Abstract

The North China Craton suffered a strong tectonothermal event in the late Neoarchean, with its crystalline basement dominantly composed of ca. 2.6–2.5 Ga granitoid gneisses and metamorphosed volcano-sedimentary sequences. However, controversial issues have long existed on the geodynamic setting of this event. We present new in-situ LA-(MC-) ICP-MS zircon U-Pb and Lu-Hf isotope data, and whole-rock geochemical data for Neoarchean magmatic rocks from the Lüliang Complex. Zircon U-Pb dating reveal that the basalts, andesites and felsic volcanics yield crystallization ages of 2535–2486 Ma, followed by crustal anatexis and metamorphism at ∼2489 Ma. The basalts and andesites yield positive εHf(t) values of 5.1–11.6 and 2.2–6.5 for zircons, corresponding to average Hf model ages of 2547 Ma and 2660 Ma, respectively. The felsic volcanic rock yields positive zircon εHf(t) values of 1.4–6.3, with Hf model ages averaging 2775 Ma. Such ages are consistent with 207Pb/206Pb ages of 2.8–2.7 Ga for relict zircons, indicating reworking of crustal rocks. The basalts and andesites have geochemical compositions of island arc tholeiite (IAT) and calc-alkaline basalt (CAB), suggesting that they were both derived from partial melting of subarc mantle sources that were generated by metasomatic reaction of the mantle wedge peridotite with subducting crust-derived fluids. Compared with the basalts, the andesites were also originated from a subarc mantle source with incorporation of more subducting sediment-derived melts. Thus, both the basalts and andesites would most likely form in a continental arc setting. In this regard, plate subduction and arc magmatism in the North China Craton can be traced back to at least 2.65 Ga in the Lüliang area. The 2.6–2.5 Ga tectonothermal event manifested as widespread accretionary orogeny in the North China Craton, resulting in significant growth of the juvenile mafic crust as well as extensive reworking of the old basement dominated by 2.8–2.7 Ga crustal rocks.