The Eastern Block of the North China Craton is one of the Earth's oldest pieces of continent. It is separated from the Western Block by the Trans-North China Orogen. It is situated in northeastern China and North Korea. The Block contains rock exposures older than 2.5 billion years (pre-Neoarchean and the Neoarchean Era). It serves as an ideal place to study how the crust was formed in the past and the related tectonic settings.
Geologists are able to deduce the past environments and tectonic events by studying geological records like rocks and geological structures. The oldest components of the Eastern Block were first formed more than 4 billion years ago (the Hadean Eon). It later experienced numerous geological events between 3.8 and 1.85 billion years ago (the Eoarchean to Paleoproterozoic Era), including recurring volcanic eruptions and metamorphic events. Therefore, most of the rocks were reworked and highly metamorphosed with changes in minerals and texture. Due to the sparse exposure of pre-Neoarchean rocks, only the tectonic settings of the rocks generated from 2.7 to 1.85 billion years ago (during the Neoarchean and Paleoproterozoic Era) are thought to be understood. These settings include a Large Igneous Province event, mantle plume activity, continental collisions, rifting and subductions of plates. The Jiao-Liao-Ji Belt joined the two small blocks (the Longgang and Langrim blocks) together as the larger Eastern Block, whereas the Trans-North China Orogen shows the assembly of Eastern and Western Blocks, forming the North China Craton. The root of the craton was collapsed 130–120 million years ago (the Cretaceous Period), resulting in a thinner lithosphere.
Lithology and geological formation Hadean Although there has not been any evidence of Hadean rocks in the Eastern Block, a number of Hadean zircons were dated as >4 billion years ago. They were found in Anshan, eastern Hebei and Xinyang, suggesting the presence of a Hadean crust in the area.
Eoarchean The Eoarchean rocks are very rare in Anshan, covering an area smaller than 20 km2. The basement rock was made of 3.8–3.6 billion-year-old trondhjemitic gneiss. It was emplaced in two phases: Phase I took place at around 3.8 billion years ago, while Phase II at approximately 3.6 billion years ago. The phases are evidenced by the older inclusions of gneiss in younger trondhjemite and younger trondhjemitic veins cross-cutting the older gneiss.
Other than the plutons, plentiful Eoarchean detrital zircons were reported from the metamorphosed sedimentary rocks in eastern Hebei and few in Anshan. Biotite schist, fuchsite quartzite and paragneiss recorded an isotopic age of 3.88–3.55 billion years. This further reinforces the existence of a Hadean-Eoarchean crust, which later became the sedimentary protolith of the metamorphosed sedimentary rocks.
In Xinyang, at the southwestern edge of the Eastern Block, 3.6 billion-year-old zircons from felsic granulite xenoliths were found. It implies that the Eoarchean crust might be present in the western part of Eastern Block as well.
Paleoarchean The rare Paleoarchean rocks are located in Anshan and eastern Hebei, composed of granitoids, metamorphosed sedimentary rocks and amphibolite. Pre-existing, pre-Paleoarchean sedimentary rocks and granitoids were metamorphosed 3.55 billion years ago. They were discovered as trondhjemitic gneiss and metamorphosed sedimentary rocks, including quartzite, paragneiss, calc-silicate rocks and more. In eastern Hebei, minor amphibolites were discovered within the metamorphosed sedimentary rocks. It suggests an eruption of basalts at 3.5 billion years ago, after the metamorphic event. In Anshan, stripes and lenses of trondhjemitic gneiss were observed in granitic and pegmatitic migmatites. They were crystallised 3.45 billion years ago during the trondhjemite magmatism (Phase III). Similar emplacement (Phase IV) at 3.33 billion years ago created granitoids. It also led to the formation of metasedimentary rocks, which are composed of amphibolites, biotite-plagioclase gneiss, quartzite and more.
Mesoarchean The scarce Mesoarchean rocks are generally igneous and metamorphic rocks with the age of 3.2–2.8 billion years. Granites are mainly situated in Anshan and eastern Hebei. They were emplaced and crystallized at around 3 billion years ago. Meanwhile, the parent rocks of the metamorphosed sedimentary rocks, including amphibolite, paragneiss and quartzite, were deposited. In Qixia of eastern Shandong, a local magmatic event 2.9–2.85 billion years ago formed the igneous parent rocks of tonalitic-trondhjemitic-granodioritic gneisses. Besides, other than Anshan and eastern Hebei area, a Mesoarchean crust is confirmed to be existed in Qixia area.
Neoarchean Other than the sparse distribution of pre-Neoarchean rocks in the northeastern Eastern Block, the Neoarchean rocks predominantly cover 90% of the exposed Block's basement. They are made up of tonalitic-trondhjemitic-granodioritic gneiss with a minority of metamorphosed sedimentary rocks. Basically, there are two groups of rocks with various lithologies, metamorphic and geochemical characteristics. They were formed during two geological events 2.75–2.65 and 2.55–2.5 billion years ago. The older group of rocks is locally existed in western and eastern Shandong, while the younger one spreads over the Block.
Early Neoarchean Luxi Complex, also known as the Luxi granite-greenstone terrane in western Shandong, contains gneiss and sheets and lenses of metamorphosed ultramafic-mafic volcanic rocks (greenstone). Metamorphosed komatiite was discovered as serpentinized peridotite and schist with spinifex texture. Such texture is interpreted to be associated with a basaltic volcanic activity 2.74 billion years ago. Similarly, in Qixia, 2.75–2.7 billion-year-old gneiss of Qixia Complex was seen. The rocks in both areas imply an early Neoarchean emplacement of granitoid and the eruption of volcanic rocks about 2.75–2.65 billion years ago. Shortly, a metamorphic event took place 2.65 billion years ago, which altered the rocks into gneiss and metamorphosed sedimentary rocks. Nonetheless, the gneisses in both areas have slightly different geological records. Gneisses in Luxi Complex show records of metamorphic events 2.65, 2.5 and 1.90–1.85 billion years ago. On the other hand, those in Qixia Complex only record the younger metamorphisms. This suggests the record of the metamorphism 2.65 billion years ago was removed and overprinted by later metamorphic events.
On top of that, the formation of greenstone and gneiss in Luxi and Qixia are considered to be linked to a wider geological event, the Large Igneous Province event 2.7 billion years ago. Magma was extracted and led to the formation of a mafic crust.
Late Neoarchean Late Neoarchean rocks spread over the whole Eastern Block. High to medium-grade gneiss and ultramafic extrusive rocks, especially komatiites occur in eastern Hebei, eastern Shandong, northern Liaoning and southern Jilin areas, whereas low to medium-grade granite-greenstone terranes are seen in western Shandong, southern Liaoning and Anshan areas. All rocks were formed in a geologically brief period, between 2.55 and 2.5 billion years ago. In this period, mafic and felsic lava erupted and granitoids intruded the whole Eastern Block, followed by a 2.5 billion-year-old regional metamorphism.
The metamorphic event has an anticlockwise pressure-temperature-time path with nearly isobaric cooling. The anti-clockwise path indicates the metamorphism is related to the intrusion of magma within the Earth's crust. During prograde and peak metamorphism, temperature and pressure are increased and a large amount of mafic material is added to the crust, whereas after the peak metamorphism, the intrusion of magma stops, resulting in isobaric cooling.
Structurally, these Late Neoarchean rocks are dome-shaped, for example, the Jinzhou dome in southern Liaoning and the Huadian dome in southern Jilin. These tonalitic-trondhjemitic-granodioritic gneiss domes are circular or oval and around 10–50 km wide. Some charnockites and granites can be found in the core. Their formation remains controversial that some suggest they were formed from the superimposition of folds, while others think they were caused by the diapirs of granitoid magma.
Paleoproterozoic
Jiao-Liao-Ji Belt The Eastern Block consists of two sub-blocks, which are linked by the Jiao-Liao-Ji Belt. On the northwest of the Belt, there is the Longgang (Yanliao) Block, while on the southeast, there is the Langrim Block.
Within the Belt, there are granitic intrusions and metamorphosed sedimentary and volcanic rock sequences. The emplacement of granitoids happened 2.22 billion years ago and generated A-type granite, alkaline syenite and rapakivi granite. They later became some of the parent rocks of the 2–1.95 billion-year-old sedimentary and volcanic rock sequences. The greenschist to lower amphibolite facies can be found in southern Jilin, eastern Shandong, eastern Liaoning and North Korea. All these rocks were metamorphosed 1.93–1.90 and 1.87 billion years ago.
Due to their difference in pressure-temperature-time paths within the stratigraphy, the Belt is divided into northern and southern zones. The southern zone with anticlockwise pressure-temperature-time path contains Jingshan, South Liaohe and Ji'an groups.He, G. P., & Ye, H. W. (1998). Two type of Early Proterozoic metamorphism in the eastern Liaoning to southern Jilin and their tectonic implication. Acta Petrologica Sinica, 14(2), 152–162.Li, S. Z., Han, Z. Z., Liu, Y. J., Yang, Z. S., & Ma, R. (2001). Regional metamorphism of the Liaohe Group: implications for continental dynamics. Geological Review, 47(1), 9–18. On the contrary, the northern zone with clockwise pressure-temperature-time path includes Fenzishan, North Liaohe and Laoling groups.
Summary of lithology and geological events From the geological formations and the textures, the past geological events can be deduced. The Eastern Block underwent a number of volcanic eruptions, emplacements and metamorphic events. Period Time (billion years ago) Geological event Lithological evidence LocationHadean
(>4 billion years ago)>4Formation of Hadean crustHadean zirconsAnshan, eastern HebeiEoarchean
(3.8–3.6 Ga billion years ago)3.8Emplacement of trondhjemite (Phase I)Tonalitic-trondhjemitic-granodioritic gneissAnshan3.6Emplacement of trondhjemite (Phase II)Tonalitic-trondhjemitic-granodioritic gneiss3.7–3.6Formation of metamorphosed sedimentary rocksMetamorphosed sedimentary rocksPaleoarchean
(3.6–3.2 billion years ago)3.55MetamorphismTonalitic-trondhjemitic-granodioritic gneiss, metamorphosed sedimentary rocksAnshan, eastern Hebei3.5Eruption of basaltsCaozhuang amphibolitesEastern Hebei3.45Emplacement of trondhjemite (Phase III)Tonalitic-trondhjemitic-granodioritic gneiss in Shengousi ComplexAnshan3.33Emplacement of trondhjemite and granite (Phase IV)Tonalitic-trondhjemitic-granodioritic gneiss in Dongshan Complex,
Chentaigou graniteFormation of metamorphosed sedimentary rocksChentaigou metasedimentary rocksMesoarchean
(3.2–2.8 billion years ago)3Emplacement of granitesLishan, Tiejiashan, Eastern and Western Anshan and Yangyashan granitesAnshan, eastern HebeiFormation of metamorphosed sedimentary rocksQianan metamorphosed sedimentary rocksEastern Hebei2.9–2.85Emplacement of trondhjemiteHuangyadi tonalitic-trondhjemitic-granodioritic gneissQixiaNeoarchean
(2.8–2.5 billion years ago)2.7Large Igneous Province eventKomatiite in Luxi greenstone,
tonalitic-trondhjemitic-granodioritic gneiss in Luxi and Qixia ComplexesWhole Eastern Block2.75–2.65Emplacement of tonalite-trondhjemite-granodiorite and eruption of volcanic rocksLuxi granite-greenstone terrane,