A late Archean tectonic melange in the Central Orogenic Belt, North China Craton

Laterally extensive belts of melange characterize Phanerozoic convergent plate margins, but are rare in Archean terranes. We document a late Archean melange in the Zanhuang Massif of the North China Craton (NCC). The Zanhuang melange separates a passive margin to foreland basin sequence developed on the western edge of the Eastern Block of the NCC from an arc terrane consisting of trondhjemitic, tonalitic and granodioritic (TTG) gneisses in the Central Orogenic Belt (COB) of the NCC. The melange belt contains a structurally complex tectonic mixture of metapelites, metapsammites, marbles and quartzites mixed with exotic tectonic blocks of ultramafic and metagabbroic rocks, metabasalts that locally include relict pillow structures, and TTG gneisses. All units in the melange have been intruded by mafic dikes that were subsequently deformed, and are now preserved as garnet-amphibolite boudins. We interpret the melange to mark the suture zone between the Eastern Block and the arc terrane in the COB. Field relationships and geochemistry suggest that the exotic ultramafic-metagabbroic-metabasaltic blocks are possible slivers of an intra-oceanic arc or fore-arc ophiolite incorporated into the melange during the arc-continent collision process. A circa 2.5 Ga granitic pluton intrudes the melange and undeformed circa 2.5 Ga pegmatites cut the melange. Tectonic models for the evolution of the COB are varied, but include models that favor collision at 2.5 Ga, 2.1 Ga, and 1.8 Ga. This work shows clearly, from field structural relationships and geochronology, that the first collision must have occurred prior to 2.5 Ga, consistent with late Archean suturing of the western margin of the Eastern Block with an arc terrane (Fuping terrane) during an arc-continent collision. The presence of an Archean melange with exotic blocks in a suture zone between an Archean arc and continental margin is clear evidence for the operation of plate tectonics at circa 2.5 Ga. Published by Elsevier B.V.