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

摘要

The North China Craton (NCC) consists of Archean to Paleoproterozoic basement overlain byMesoproterozoic 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, ofwhich the former is considered as amajor period of juvenile crustal growth in the NCC as evidenced by Nd and zirconHf 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 lowto medium-grade granite-greenstone belts that arewidespread over thewhole 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 andmetamorphosed 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 granitegreenstone 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.