A LATE ARCHEAN TECTONIC MÉLANGE IN THE CENTRAL OROGENIC BELT, NORTH CHINA CRATON

Laterally extensive belts of mélange characterize convergent margins, but are remarkably rare in Archean terrains. We document a late Archean mélange in the Zanhuang massif, southern Taihang Mountains, of the North China Craton (NCC). The mélange separates a passive margin 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 mélange belt is 2-10 km wide, and contains a structurally complex tectonic mixture of metapelites, metapsammites, marble, metalimestone, and tectonic blocks of ultramafites and metagabbroic rocks, metabasites that locally include relict pillow lavas, and TTG gneisses. All units in the mélange have been intruded by mafic dikes that were subsequently deformed, and are now preserved as garnet-amphibolite boudins. We interpret the mélange to mark the suture zone between the Eastern Block and the arc terrane in the COB. Field relationships and geochemistry suggest that the exotic ultramafites-metagabbro-metabasaltic blocks are possible slivers of an intra-oceanic arc incorporated into the mélange during the arc/continent collisional process. A circa 2.5 Ga granitic pluton intrudes the mélange and undeformed circa 2.5 Ga pegmatites cut the mélange. Tectonic models for the evolution of the COB of the NCC 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, favoring 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 mélange 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.