Abstract:

The Hongqiying Complex of northern Hebei in the middle northern margin of the North China Craton (NCC) recorded multiple tectono-thermal events from the Paleoproterozoic to early Mesozoic. This complex potentially holds the crucial geological evidence supporting the Paleoproterozoic oceanic subduction, however, its formation age, metamorphic evolution history and tectonic attributes remain debatable. In this paper, the metamorphic processes and geochronology of the Hongqiying Complex are summarised; its complicated, multi-stage tectonic evolution is discussed. The Hongqiying Complex comprises three rock units: orthogneisses (1.87-1.82 Ga, 2.55-2.53 Ga), (Chicheng) mélange (>1.88 Ga) and supracrustal sequence (2.1-2.0 Ga). The orthogneisses are dominated by Paleoproterozoic granodiorite-monzogranite-granite, with minor Neoarchean tonalite-granodiorite. The mélange exhibits a typical ophiolitic affinity, consisting of retrograded eclogite (amphibolite) and peridotite blocks within a metasedimentary matrix. The supracrustal sequences comprise dominantly carbon-bearing clastic-carbonate rocks interlayered with minor volcanics with different metamorphic grades. Integrated analyses of retrograded eclogites and amphibolite from the mélange and staurolite-bearing garnet mica schists at Fengshan reveal five phases of metamorphism. The first phase (M1) identified from the mica schists at Fengshan is a medium-p type, characterized by clockwise p-T paths, with peak conditions of p between 1.0-1.1 GPa, T>780 ℃ and a metamorphic age of >1.95 Ga. The second phase (M2) recognised from the eclogites is a high-p type, showing a clockwise p-T path, with stages of compression-heating to peak and post-peak isothermal decompression; the peak p-T conditions are roughly p>2.2 GPa, T≈750 ℃ and a geothermal gradient of ~9 ℃/km. Despite zircons predominantly yield late Paleozoic metamorphic ages, the eclogite metamorphism is regarded to have occurred at >1.88 Ga. The third phase (M3) is also revealed from the mica schists at Fengshan and is marked by the overprinting of staurolite assemblages on the high-grade (M1) assemblages; it exhibits a clockwise p-T path, with the peak condition of p between 0.6-0.7 GPa, T between 610-630 ℃ and metamorphic ages of 1.88 Ga. The fourth phase (M4) is identified from amphibolites that are overprinted on eclogites in the Chicheng mélange; it exhibits a ‘greater-than sign’-shaped p-T trajectory, including pre-peak decompression-heating and post-peak decompression-cooling processes, having peak conditions of p between 0.5-0.7 GPa, T between 780-830 ℃ and a geothermal gradient of ~35 ℃/km (low-p type). This low-p metamorphism is interpreted to take place during the late Paleozoic (354-289 Ma), accompanied by extensive anatexis. The fifth phase (M5) is locally developed, represented by overprinting of greenschist facies assemblages; it displays a clockwise p-T path, with probable metamorphic ages of 255-234 Ma. The M1 medium-p metamorphism is interpreted to relate to a crustal thickening orogeny (2.0-1.95 Ga) in the western NCC. The M2 high-p metamorphism may attribute to an oceanic subduction (1.95-1.88 Ga) along the northern margin of the NCC, robustly evidencing that the plate tectonics of the modern style may have operated during the Paleoproterozoic Era. The M3 medium-p metamorphism suggests another crustal thickening orogeny (1.88-1.83 Ga) along the northern margin of the NCC. The M4 low-p metamorphism is related to late Paleozoic extension in the northern NCC. The M5 low-T metamorphism is associated with the early Mesozoic collision event along the Solon Suture Zone. The above five phases of metamorphism recorded complex tectonic processes involving Paleoproterozoic collision-subduction-collision, late Paleozoic extension and early Mesozoic compression. In addition, the Hongqiying Complex might have located in different crustal depths after having undergone the 1.88-1.83 Ga orogeny. The supracrustal rocks at Hongqiying and the Chicheng mélange were situated in mid-lower crustal depths, in which zircons recorded multiple tectono-thermal events at 1.72-1.66 Ga, ~450 Ma, 354-289 Ma and 255-234 Ma. In contrast, the supracrustal rocks of eastern Fengshan may have situated at the mid-upper crust therefore lack both the zircon and monazite records of the later tectono-thermal events.

Key words: multi-phase metamorphism, Paleoproterozoic mélange, Hongqiying Complex, North China Craton