Western Block (North China Craton)

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[edit] Geology of the Basement

The basement rocks of the Western Block are mainly exposed in the northern part of the block, especially in the Jining, Daqingshan-Wulashan, Guyang-Wuchuan, Sheerteng, Helanshan-Qianlishan, and Alashan areas, whereas the southern part of the block is covered by the Mesozoic to Cenozoic strata of the Ordos Basin. Data from several boreholes reveal the existence of granulite-facies basement beneath the Ordos Basin, and aeromagnetic data also imply the existence of granulite-facies basement beneath the basin. The exposed basement can be further subdivided into two distinct lithotectonic units: the late Archean (3.0-2.5 billion years old) tonalitic-trondhjemitic-granodioritic (TTG) + supracrustal rocks and the Paleoproterozoic Khondalite Belt. The former crops out as granite-greenstone or high-grade terrains in the Guyang, Wuchuan, Sheerteng and Alashan areas in the northern part of the block, whereas the latter is exposed as a linear structural belt along the Jining-Daqingshan-Wulashan-Qianlishan-Helanshan zone. One school of thought considers that the Khondalite Belt is a Paleoproterozoic collision belt, along which the Yinshan Block represented by the late Archean basement in the north, and the Ordos Block covered by the Ordos Basin in the south collided to form the Western Block about 1.93 billion years (Ga) ago.

The late Archean basement of the Western Block has a lithological assemblage, structural style and metamorphic history similar to those of the Eastern Block. It consists of low-grade granite-greenstone and high-grade TTG gneiss and granulite terrains, which underwent a greenschist to granulite facies metamorphism at ~2.5 Ga, characterized by anticlockwise P-T paths involving near-isobaric cooling. The Paleoproterozoic Khondalite Belt in the Western Block is dominated by graphite-bearing sillimanite-garnet gneiss, garnet quartzite, felsic paragneiss, calc-silicate rock and marble, which have previously been referred to as “khondalite series” in the Chinese literature and are considered to represent stable continental margin deposits. Associated with the khondalites are minor TTG gneisses, mafic granulites, syntectonic charnockites and S-type granites. It has long been considered that the khondalites were deposited and metamorphosed in the Archean. However, recent isotopic data suggest that the khondalites were deposited and metamorphosed in the Paleoproterozoic, with depositional ages ranging from 2.30 to 1.98 Ga and a metamorphic age of ~1.93 Ga.