Magmatic record of Neoarchean arc-polarity reversal from the Dengfeng segment of the Central Orogenic Belt, North China Craton

Abstract

The Dengfeng segment of the Archean Central Orogenic Belt in the North China Craton (NCC) includes circa 2.66–2.5 Ga metavolcanics, metasedimentary and plutonic rocks. All units are intruded by ∼2.5–2.4 Ga mafic dykes and a suite of coeval circa 2.5–2.45 Ga potassic granites and leucogranitic veins. Mafic dykes are characterized by mixed alkaline and calc-alkaline affinities, enriched LREE and negative Nb and Zr anomalies, indicating a lithospheric mantle source region. Potassic granitic dykes exhibit highly fractionated LREE patterns, and negative HFSE anomalies and positive zircon εHf(t) values, which are similar to those of TTG gneisses in the area. Leucogranitic veins cutting across the diorites are characterized by high contents of SiO2 and K2O, LREE-enriched patterns, positive Eu and Sr anomalies, negative Nb and Ti anomalies. We suggest that the granitic dykes and leucogranitic veins were derived from partial melting of thickened older crusts. Combined with other reported 2.5 Ga mafic dykes and potassic granites from the NCC, we propose that they demonstrate an ancient example of arc-polarity reversal and provide a record of the formation of a new plate boundary in the Archean. The underplating of the mafic dyke source beneath the newly accreted Eastern Block may have generated the widespread 2.5–2.47 HT-LP metamorphism with anticlockwise P-T paths in the Eastern Block.