An early Precambrian “orogenic belt” exhumed by the Phanerozoic tectonic events: A case study of the eastern North China Craton

Abstract

The operation of early plate tectonics is evidenced by belt-like occurrences of some Neoarchean–Paleoproterozoic (ca. 2.7–1.8 Ga) high-grade basement rocks in ancient continental blocks. Studies on the continental evolution of the North China Craton defined Paleoproterozoic mobile/orogenic belts in this continental block, which are usually regarded as solid evidence for the operation of plate tectonics during the Paleoproterozoic. However, if considering subsequent tectonic overprinting, then whether the current occurrences of these mobile belts can represent the primary tectonic geometry remains unclear. The high-grade Jiaobei Terrane is centrally located within one such Paleoproterozoic mobile belt, namely the Jiao–Liao–Ji mobile belt of the eastern North China Craton. Archean–Paleoproterozoic basement rocks are well preserved in this terrane and its multiple episodes of metamorphism are similar in age with those of the other Paleoproterozoic mobile belts of the craton. Additionally, Phanerozoic tectonothermal overprinting in this terrane also widely influenced much of the whole North China Craton. Most importantly, the basement rocks occurring in different regions of this terrane exhibit different metamorphic grades. Therefore, this terrane is ideal for identifying how the high-grade basement components, after repeated episodes of exhumation, were finally emplaced on the surface in the Phanerozoic. The Archean–Paleoproterozoic lithologies of the Jiaobei Terrane mainly consist of Archean tonalite–trondhjemite–granodiorite gneisses (TTGs) and Paleoproterozoic meta-volcanosedimentary rocks, as well as a small amount of Paleoproterozoic granitoids. The Archean TTGs occupy the central part of the Jiaobei Terrane and exhibit three episodes of magmatism at ca. 2.9 Ga, ca. 2.7 Ga, and ca. 2.5 Ga, respectively. The Paleoproterozoic meta-volcanosedimentary sequences are further divided into the Jingshan Group and the Fenzishan Group, which mainly occur in the southern and northern margins of the Archean gneisses, respectively. Despite this division, both groups have similar protoliths and geochronological frameworks. The Jiaobei Terrane was subjected to two episodes of high-grade metamorphism during the Neoarchean (ca. 2.57–2.46 Ga) and the Paleoproterozoic (ca. 1.98–1.78 Ga). Although the Neoarchean metamorphism has been identified in many places of the Jiaobei Terrane, its geological implications are equivocal because of pervasive and strong Paleoproterozoic metamorphic overprints. The Paleoproterozoic high-grade metamorphism greatly influenced the whole terrane, and pelitic and mafic granulites have been reported along the southern margin of the high-grade metamorphic terrane bordering the North Sulu ultrahigh-pressure (UHP) belt. The metamorphic pressure and temperature conditions and the pressure-temperature (P–T) paths of these granulites are comparable to those of the Phanerozoic, indicating that the continental crust has maintained similar thermal and mechanical characteristics since at least the Paleoproterozoic in certain tectonic settings. A synthesis of previous studies in combination with new petrographic observations reveals clear spatial variations in the metamorphic grade for the Paleoproterozoic metamorphism in the Jiaobei Terrane, with granulite-facies rocks occurring only along the southern margin and the metamorphic grade gradually changing to amphibolite and greenschist facies towards the north. Even though the amphibolite and granulite facies rocks show distinct mineral assemblages, their metamorphic geochronology and metamorphic reaction textures (especially the symplectite around garnet grains) observed in the thinsection are strikingly similar. Therefore, we infer that these rocks showing different metamorphic grades represent a cross-section of the continental crust, with the granulites representing lower crustal components and the amphibolites representing exposed rocks of the shallower crustal levels. The strike direction of the variable metamorphic grades is notably subparallel with the North Sulu orogenic belt. The early Precambrian basement rocks along the southern Jiaobei Terrane and within the Phanerozoic fault zone also show Permo–Triassic metamorphic overprinting associated with the Sulu UHP orogenesis. The spatial variations in metamorphic grade as well as the Permo–Triassic overprint, thus, imply that the final emplacement of Jiaobei Terrane basement components was related to the continental collision process between the North China and South China cratons. Therefore, the belt-like occurrences of the Paleoproterozoic granulites in this region should not be regarded as representing the primary tectonic geometry of the Paleoproterozoic orogenic belt. This inference based on the Jiaobei Terrane should also apply to other high-grade terranes of the North China Craton considering that basement rocks are more often exposed in high-elevation regions caused by Phanerozoic tectonothermal events.