Zircon U-Pb-Hf isotopes and geochemistry of two contrasting Neoarchean charnockitic rock series in Eastern Hebei, North China Craton: Implications for petrogenesis and tectonic setting

The Zunhua-Qinglong microblock in Eastern Hebei, North China Craton, exposes two contrasting chamockitic rock series in the Yuhuzhai-Taipingzhai and Cuizhangzi areas: charnockitic plagioclase gneiss series with chemical compositions of dioritic and tonalitic rocks, and charnockite series showing tonalitic and granodioritic components. Petrographic observation reveals that two types of orthopyroxenes are preserved in these rocks. Medium-fine grained anhedral orthopyroxenes are preserved in both series, and coarse grained subhedral orthopyroxenes are preserved only in the charnockite series. In situ trace element analyses reveal that the coarse grained subhedral orthopyroxenes exhibit typical chemical features of magmatic orthopyroxene, whereas the medium-fine grained anhedral orthopyroxenes display lower medium to heavy rare earth elements (REE) and no obvious Eu anomalies, resulting in left-inclined REE patterns. Combined with petrographic features, these new data indicate that the medium-fine grained anhedral orthopyroxenes were formed by granulite-facies metamorphism caused by dehydration reactions, whereas the coarse grained subhedral orthopyroxenes were directly crystallized from their magmatic precursors. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb isotopic dating reveals that the magmatic precursors of the charnockitic plagioclase gneiss series were emplaced between 2530 +/- 17 and 2523 +/- 12 Ma, and those of the chamockite series were emplaced between 2527 +/- 28 and 2515 +/- 22 Ma. We conclude that these magmatic precursors are contemporaneous emplacements of two Neoarchean granitoid magmatic events, which were followed by regional granulite-facies metamorphism at similar to 2.45 Ga. The charnockitic plagioclase gneiss series and charnockite series exhibit similar epsilon(Hf)(t) values of -2.4 to +5.3 and -3.2 to +5.5, indicating either depleted mantle or juvenile crust affinity, and involvement of ancient crustal components. The charnockitic plagioclase gneiss series exhibit relatively low SiO2 contents (54.1-65.2%), with various MgO contents (1.93-5.36%), Mg# (0.36-0.55), and (La/Yb)(N) values (3.49-56.00); the charnockite series exhibit higher SiO2 contents (60.1-69.2%), with similar (La/Yb)(N) values (14.61-42.69), MgO contents (2.10-3.99%), and Mg# (0.29-0.52). These geochemical and zircon Lu-Hf isotopic features, together with chemical modeling, suggest that the magmatic precursors of the charnockitic plagioclase gneiss series may have been primarily derived from the partial melting of a depleted mantle that was metasomatized by slab-derived melts/fluids, and experienced fractional crystallization processes with hornblende and plagioclase as primary fractionated phases. The aforementioned features further suggest that the chamockite series originated from a partial melting of the subducted slab, and it was strongly contaminated by mantle peridotites. These new data, combined with the previous studies of Neoarchean basement rocks in the Zunhua-Qinglong microblock in Eastern Hebei, suggest that the magmatic precursors of these two contrasting charnockitic rock series were formed in a back-arc-related tectonic environment at a convergent plate margin. (C) 2015 Elsevier B.V. All rights reserved.