Lithotectonic elements of Precambrian basement in the North China Craton: Review and tectonic implications

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The North China Craton (NCC) consists of Archean to Paleoproterozoic basement overlain by Mesoproterozoic to Cenozoic cover. Minor Eoarchean to Mesoarchean basement rocks are locally present in the eastern part of the NCC, but little is known about their extent, nature and tectonic evolution due to widespread reworking by later events. The Neoarchean basement in the NCC was formed during two distinct periods: 2.8–2.7 Ga and 2.6–2.5 Ga, of which the former is considered as a major period of juvenile crustal growth in the NCC as evidenced by Nd and zircon Hf isotopic data, though the 2.8–2.7 Ga rocks are not widely exposed. The 2.6–2.5 Ga rocks make up ~80% of the Precambrian basement of the NCC and can be divided into high-grade gneiss complexes and low- to medium-grade granite-greenstone belts that are widespread over the whole NCC, seeming to support a notion that the cratonization of the NCC occurred at ~2.5 Ga. However, the 2.6–2.5 Ga rocks in the eastern and western parts of the NCC (Eastern and Western Blocks) are different from those similar-aged rocks in the central part (Trans-North China Orogen), with the former dominated by gneiss domes and metamorphosed at ~2.5 Ga, characterized by anticlockwise P–T paths involving isobaric cooling, reflecting an origin related to the underplating of mantle-derived magmas, whereas the latter, which are defined by strike-slip ductile shear zones, large-scale thrusting and folding, and transcurrent tectonics locally with sheath folds, were metamorphosed at ~1.85 Ga, characterized by clockwise P–T paths involving isothermal decompression, consistent with subduction and continent-continent collision settings. In addition, komatiites/komatiitic rocks are present in the granite-greenstone belts in the eastern and western parts of the NCC, but generally are absent in the central part. These differences imply that the 2.6–2.5 Ga basement rocks in the eastern and western parts of the NCC formed under different tectonic settings from those in the central part. Although both magmatic arc and mantle plume models can be used to explain the tectonic setting of the 2.6–2.5 Ga basement rocks in the eastern part of the NCC, a mantle plume model is favored as it can reasonably interpret: (1) the exceptionally large exposure of granitoid intrusions that formed over a short time period (2.55–2.50 Ga), without systematic age progression across a ~800 km wide block; (2) generation of komatiitic magmas with eruption temperatures as high as ~1650 °C; (3) dominant domal structures; (4) bimodal volcanic assemblages in the greenstone sequences; (5) affinities of mafic rocks to continental tholeiitic basalts; and (6) metamorphism with anticlockwise P–T paths involving isobaric cooling. In contrast, the 2.6–2.5 Ga high-grade gneiss terranes and low-grade granite-greenstone belts in the central part of the NCC exhibit the same structural and metamorphic characteristics as those of Paleoproterozoic lithological elements that typify active continental margin arcs and continent–continent collisional belts.Paleoproterozoic lithological assemblages in the NCC are mainly restricted to three Paleoproterozoic linear tectonic belts in the western, central and eastern parts of the NCC, which were, respectively, named the “Khondalite Belt (Fengzhen Belt/Inner Mongolia Suture Zone)”, “Trans-North China Orogen (Central Orogen Belt)” and “Jiao-Liao-Ji (Liaoji) Belt”. The three belts display some of the following lithotectonic elements that are classical indicators of subduction and collision tectonics in plate tectonic regimes: (1) arc-related juvenile crust; (2) linear structural belts defined by strike-slip ductile shear zones, large-scale thrusting and folding, and sheath folds and mineral lineations; (3) high-pressure (HP) mafic and pelitic granulites, retrograde eclogites and ultrahigh temperature (UHT) rocks; (4) clockwise metamorphic P–T paths involving near-isothermal decompression; (5) possible ancient oceanic fragments and mélange; and (6) back-arc or foreland basins. These lithotectonic elements indicate that subduction- and collision-related orogenic processes must have been involved in the development of the three Paleoproterozoic belts in the NCC. Different models have been proposed for the formation and evolution of these three Paleoproterozoic orogenic belts, and one of the models suggests that the Khondalite Belt was a continent–continent collisional belt along which the Yinshan and Ordos Blocks amalgamated to form the Western Block at ~1.95 Ga, which then collided with the exotic Eastern Block along the Trans-North China Orogen at ~1.85 Ga, whereas the Jiao-Liao-Ji Belt represents a rifting-and-collision belt within the Eastern Block which underwent rifting to form an incipient oceanic basin that was closed upon itself through subduction and collision at ~1.9 Ga. An alternative model proposes that all of the three Paleoproterozoic orogenic belts in the NCC were initialized from continental rifting on a single continent, which was cratonized through fusing Achaean microcontinental blocks at ~2.5 Ga, followed by the development of incipient oceanic basins which themselves were closed in the Paleoproterozoic through subduction and collision.