Precambrian geology of China

Abstract

China contains three major Precambrian blocks, the North China, South China and Tarim cratons, separated and sutured by Phanerozoic orogenic belts. The North China Craton (NCC) contains rocks as old as 3.8Ga, but is dominated by Neoarchean igneous rocks that were formed during two magmatic events at 2.8–2.7Ga and 2.55–2.50Ga. The 2.8–2.7Ga magmatic pulse is considered as a major phase of juvenile crustal growth in the craton, though exposure of these rocks is limited. The 2.55–2.50Ga rocks make up ∼80% of Archean basement in the NCC, but their rock associations, structural patterns, and metamorphic age and P–T paths in the eastern and western parts (Eastern and Western blocks) are different from those in the central part (Trans-North China Orogen). In the Eastern and Western blocks, the end-Neoarchean rocks are exposed as gneissic domes and dominated by tonalitic–trondhjemitic–granodioritic (TTG) gneisses and mafic to komatiitic rocks that were metamorphosed at ∼2.5Ga, along anticlockwise P–T paths involving isobaric cooling, that is interpreted to reflect underplating of mantle-derived magmas. In the Trans-North China Orogen, the end-Neoarchean rocks occur as linear structural belts and are composed of arc-related granitoids and volcanic rocks that formed in a supra-subduction zone setting and were metamorphosed at ∼1.85Ga along a clockwise P–T paths involving isothermal decompression in a continent–continent collisional setting. Although magmatic arc models can explain the origin of the 2.55–2.50Ga TTG rocks in the Eastern and Western blocks, a mantle plume model is favored because it best explains many other features, including the formation of komatiitic rocks. The 2.55–2.50Ga rocks and associated Paleoproterozoic rocks in the Trans-North China Orogen exhibit the same structural and metamorphic characteristics that typify continental margin arcs and collisional belts. Spatially, Paleoproterozoic rocks in the NCC are related to collisional assembly of the disparate parts of the Eastern and Western blocks including a ∼1.95Ga collisional event that led to amalgamation of the Yinshan and Ordos blocks to form the Western Block, which then collided with the Eastern Block to form the Trans-North China Orogen at ∼1.85Ga. Following the final assembly at ∼1.85Ga, the interior of the NCC underwent on-going extension, leading to widespread emplacement of 1.80–1.75Ga mafic dyke swarms, 1.75–1.68Ga anorthosite–mangerite–granite–rapakivi suites, and deposition of Mesoproterozoic and Neoproterozoic strata. Meanwhile, a large Paleo-Mesoproterozoic volcanic belt (Xiong’er Group) developed on the southern margin of the NCC, and is variously interpreted as an intracontinental rift zone or an Andean-type continental margin arc. In the late Mesoproterozoic, the northern margin of the NCC underwent rifting (Zhaertai-Bayan Obo rift zone), coincident with breakup of the Columbia (Nuna) supercontinent.The South China Craton consists of the Yangtze and Cathaysia blocks, which are considered to have collided along the Jiangnan Belt in the Neoproterozoic. Archean and Paleoproterozoic basement rocks in the Yangtze Block are only locally exposed. The late Mesoproterozoic to early Neoproterozoic folded belts in the Yangtze Block are divided into the Jiangnan Belt in the southeast and the Panxi-Hannan Belt in the west and north. The former is dominated by early Neoproterozoic metamorphosed volcanic-sedimentary strata intruded by middle Neoproterozoic peraluminous (S-type) granites and unconformably overlain by the middle Neoproterozoic Banxi Group and its equivalents. The Panxi-Hannan Belt consists of late Mesoproterozoic to early Neoproterozoic metamorphosed volcanic-sedimentary units and plutonic complexes. A number of mutually exclusive models (e.g. plume-rift, slab-arc, plate-rift, etc.) have been proposed for these belts. The Cathaysia Block is composed predominantly of Neoproterozoic basement rocks with Paleoproterozoic rocks only exposed in southwest Zhejiang and north Fujian, and Mesoproterozoic rocks limited to Hainan Island. The Paleoproterozoic rocks consist of 1890–1830Ma granitoids and 1850–1815Ma supracrustal rocks, which were metamorphosed at 1.89–1.88Ga and locally reworked at 250–230Ma. Neoproterozoic rocks make up ∼90% of the Precambrian basement in the Cathaysia Block and mainly consist of volcanic-sedimentary strata metamorphosed from greenschist to granulite facies. Some of the volcanic rocks have arc affinities, suggesting the existence of a Neoproterozoic magmatic arc in the Cathaysia Block. The Neoproterozoic rocks were metamorphosed at 460–420Ma. The tectonic setting of this event has been related to both intracontinental orogeny and continental margin subduction and collision.The Precambrian basement of the Tarim Craton consists of Neoarchean and Paleoproterozoic rocks and late Mesoproterozoic to early-middle Neoproterozoic sedimentary and volcanic strata metamorphosed at greenschist and blueschist facies, which are unconformably overlain by unmetamorphosed late Neoproterozoic (Sinian) cover. The Neoarchean and Paleoproterozoic rocks are exposed in the Kulukatage and Dunhuang complexes on the northern and northeastern margins of the craton, respectively. The Neoarchean rocks consist of granitoid rocks and minor supracrustal rocks, including TTG gneisses, calc-alkaline granites and Kf-granites, most of which were emplaced at 2.6–2.50Ga. The Paleoproterozoic basement rocks are also composed of granitoid and supracrustals rocks (Xingditage and Dunhuang groups), of which the granitoids were emplaced in two stages at 2.45–2.35Ga and ∼1.9Ga. In the Kulukatage Complex, the Neoarchean and Paleoproterozoic rocks underwent metamorphic events at 1.9–1.8Ga and 1.1–1.0Ga that are related to assembly of the Columbia (Nuna) and Rodinia supercontinents, respectively. In the Dunhuang Complex, Archean rocks underwent metamorphic events at ∼2.5Ga and 1.9–1.8Ga, similar to two major metamorphic events occurring in the Western Block of the NCC, leading to speculation that the complex is the western extension of the Alax Complex of the NCC. Late Mesoproterozoic to early-middle Neoproterozoic metamorphosed strata are exposed on the peripheral margins of the Tarim Craton and are considered to have formed in Andean-type continental margins that were deformed and metamorphosed between 1.0Ga and 0.9Ga, probably related to the assembly of Rodinia. During middle Neoproterozoic to Cambrian time, the Tarim Craton became a stable platform overlain by middle-late Neoproterozoic to Cambrian unmetamorphosed cover, of which the middle-late Neoproterozoic units contain four sequences of tillite correlated with the global snowball Earth events. Widespread middle to late Neoproterozoic ultramafic–mafic complexes and mafic dyke swarms with the Tarim Craton are related to a mantle plume event that led to the final breakup of Rodinia.