Paired metamorphism in the Neoarchean: A record of accretionary-to-collisional orogenesis in the North China Craton

Paired metamorphism is the hallmark of asymmetric subduction at Phanerozoic convergent plate margins, yet spatially and temporally linked paired metamorphic belts have rarely been documented in the Archean rock record. Here, we investigate the Neoarchean Dengfeng Complex, a typical granite-greenstone belt in the southern part of the North China Craton (NCC). Using petrography, geochronology, and phase equilibrium constraints on rocks from different lithostructural units, we suggest the ca. 2.54-2.50-Ga Dengfeng Complex represents a remnant of a spatially and temporally linked Neoarchean paired metamorphic belt. In the western part of the complex, a garnet amphibolite from a dominantly metabasic unit (MBU) records near peak P-T conditions of 6.3-10 kbar and similar to 675-750 degrees C, corresponding to a high thermobaric ratio (TIP) of similar to 720-1200 degrees C/GPa. Tonalite-trondhjemite-granodiorite (TTG) gneisses preserve evidence for partial melting, also consistent with relatively high TIP (similar to 875-1400 degrees C/GPa). In the eastern part of the Dengfeng Complex, two garnet amphibolites and three garnet quartz-mica schists from a dominantly metasedimentary unit (MSU) record peak P-T conditions of >9.8 kbar and similar to 525-655 degrees C, corresponding to intermediate thermobaric ratios of similar to 425-600 degrees C/GPa. Zircon and titanite U-Pb dating, coupled with existing ages of post-kinematic intrusions, constrain the metamorphic age to ca. 2.52-2.50 Ga. The metamorphic P-T data indicate that the MSU was buried to >30 km then exhumed to the near-surface by the early Paleoproterozoic. Our metamorphic results, when combined with recent structural and geochemical data, suggest the Dengfeng Complex records Neoarchean paired metamorphism, in which the higher thermal gradients reflect the arc-forearc region and the lower thermal gradients correspond to the accretionary complex. Our data indicate that the southern NCC experienced accretionary-to-collisional orogenesis with intra-oceanic subduction, forearc accretion (ca. 2.54-2.50 Ga), and subsequent arc-continent collision (ca. 2.50-2.47 Ga). The recognition of paired metamorphism in the NCC is consistent with widespread subduction and the operation of a plate tectonic regime in the late Archean. (C) 2020 Elsevier B.V. All rights reserved.