The Columbia connection in North China

Abstract

AbstractThe Archaean and Proterozoic geology, structure and metamorphism of the North China Craton (NCC) reveal that the amalgamated Eastern and Western blocks of the craton collided as a single entity with the Columbia supercontinent at 1.93 Ga, and that the Northern Hebei orogen correlates with the Transamazonian-Eburnian belts of Africa and South America, and the Svecofennian of Baltica. This metamorphic belt preserves evidence for extreme crustal metamorphism with diagnostic ultrahigh-temperature (UHT) assemblages such as sapphirine+quartz, spinel+quartz, high alumina orthopyroxene+sillimanite+quartz and high temperature perthites which record temperatures exceeding 1000 °C and pressure above 12 kbar. The metamorphic P–T trajectory is characterized by initial isobaric cooling followed by steep isothermal decompression, defining an overall anticlockwise exhumation history. Electron probe monazite geochronology and precise SHRIMP zircon dating of sapphirine-bearing granulites constrain the timing of the UHT event at c. 1.92 Ga, suggesting that the UHT metamorphism coincided with collisional orogenesis as the North China Craton joined the Columbia supercontinent amalgam along the Northern Hebei orogen during the Palaeoproterozoic. Fluid inclusion studies in the UHT rocks provide evidence for the involvement of synmetamorphic pure CO2, linking the thermal anomaly and fluid flux to underplated mafic magmas during asthenospheric upwelling. Evidence for probable-plume related mafic magmatism is also provided by the extensive mafic dyke swarms cutting the region and elsewhere within the NCC, with geochemical characters testifying to emplacement within rifts that opened up during the extensional collapse of the orogen subsequent to the collisional event. Recognition of the Palaeoproterozoic Columbia suture in the Northern Hebei orogen represents a major paradigm shift, as one popular group of models for the NCC suggests that the Palaeoproterozoic suture resides in an older orogen, the Central Orogenic belt. However, either model for the location of the suture is able to explain metamorphic P–T–t data for crustal thickening at 1.85 Ga, whereas only the model for a Late Archaean collision in the Central Orogenic belt and a Palaeoproterozoic collision in the Northern Hebei orogen can explain the structural, sedimentological, geochronological, and petrological data. The Central Orogenic belt contains several hundred fragments of a c. 2.505 Ga ophiolite suite, a contemporaneous 2.5–2.4 Ga foreland basin deposited on 2.7–2.5 Ga passive margin sediments on the Eastern Block, and contains rare evidence for c. 2.5 Ga granulite facies metamorphism that was largely overprinted by 1.92–1.85 Ga high-grade assemblages. East-directed 2.5 Ga fold-thrust structures are overprinted by 1.92–1.85 Ga south-directed thrusts associated with large-scale thickening of the craton, succeeded by strike-slip shear zones that slice the orogen into numerous fault-bounded terranes that preserve different levels of exhumation.