North China Craton and Tarim Block

The North China craton occupies about 1 million square miles (1.7 million km2) in northeastern China, Inner Mongolia, the Yellow Sea, and North Korea, and apparently shares an early geological history with the poorly known Tarim block to the west. It is bounded by the Qinling-Dabie Shan orogen to the south, the Yinshan-Yanshan orogen to the north, the Longshoushan belt to the west, and the Qing-long-Luznxian and Jiao-Liao belts to the east. The North China craton includes a large area of intermittently exposed Archean crust, including circa 3.8-2.5 billion-year-old gneiss, tonalite, trondhjemite, and granodiorite. Other areas include granite, migmatite, amphibolite, ultramafite, mica schist and dolomitic marble, graphitic and other metasedimentary gneiss, banded iron formation (BIF), and metaarkose. The Archean rocks are overlain by the 1.85-1.40 billion-year-old Mesoproterozoic Changcheng (Great Wall) system. In some areas in the central part of the North China craton 2.40-1.90 billion-year-old Paleoproterozoic sequences deposited in cratonic rifts are preserved.

The North China craton is divided into two major blocks separated by the Neoarchean Central orogenic belt, in which virtually all isotopic ages on the rocks fall between 2.55 and 2.50 billion years. The Western block, also known as the ordos block, is a stable craton with a thick mantle root, no earthquakes, low heat flow, and lack of internal deformation since the Precambrian. In contrast, the Eastern block is atypical for a craton in that it has numerous earthquakes, high heat flow, and a thin lithosphere reflecting the lack of a thick mantle root. The North China craton is one of the world's most unusual cra-tons in that it had a thick tectosphere (subcontinental lithospheric mantle) developed in the Archean, which was present through the ordovician, as shown by deep xenoliths preserved in ordovician kimberlites. The eastern half of the root, however, appears to have delaminated or otherwise disappeared during Paleozoic, Mesozoic, or Cenozoic tectonism. This is demonstrated by Tertiary basalts that bring up mantle xenoliths of normal "Tertiary mantle" with no evidence of a thick root. The processes responsible for the loss of this root are enigmatic but are probably related to the present-day high-heat flow, Pha-nerozoic basin dynamics, and orogenic evolution.

The Central orogenic belt includes belts of tonal-ite-trondhjemite-granodiorite, granite, and supra-crustal sequences metamorphosed from granulite to greenschist facies. It can be traced for about 1,000

miles (1,600 km) from west Liaoning to west Henan. Widespread high-grade regional metamorphism including migmatization occurred throughout the Central orogenic belt between 2.6 and 2.5 billion years ago, with final uplift of the metamorphic terrain at 1.9-1.8 billion years ago associated with extensional tectonism or a collision on the northern margin of the craton. Amphibolite to greenschist-grade metamorphism predominates in the southeastern part of the Central orogenic belt, but the northwestern part of the orogen is dominated by granulite-facies to amphibolite-facies rocks, including some high-pressure assemblages (10-13 kilobars at 850 ± 50°C). The high-pressure assemblages can be traced for more than 400 miles (700 km) along a linear belt trending east-northeast. Internal (western) parts of the orogen are characterized by thrust-related horizontal foliations, flat-dipping shear zones, recumbent folds, and tectonically interleaved high-pressure granulite migmatite and metasediments. It is widely overlain by sediments deposited in rifts and continental shelf environments, and intruded by several dike swarms (2.4-2.5, and 1.8-1.9 billion years ago). Several large anorogenic granites with ages of 2.2-2.0 billion years are identified within the belt. Recently two linear units have been documented within the belt, including a high-pressure granulite belt in the west and a foreland-thrust fold belt in the east. The high-pressure granulite belt is separated by normal-sense shear zones from the Western block, which is overlain by thick metasedimentary sequences younger than 2.4 billion years that metamorphosed 1.86 billion years ago.

The Hengshan high-pressure granulite belt is about 400 miles (700 km) long, consisting of several metamorphic terrains, including the Hengshan, Huaian, Chengde, and west Liaoning complexes. The high-pressure assemblages commonly occur as inclusions within intensely sheared tonalite-trond-hjemite-granodiorite (2.6-2.5 billion years) and granitic gneiss (2.5 billion years) and are widely intruded by K-granite (2.2-1.9 billion years) and mafic dike swarms (2.40-2.45 Ga, 1.77 billion years). Locally, khondalite and turbiditic slices are interleaved with the high-pressure granulite rocks, suggesting thrusting. The main rock type is garnet-bearing mafic gran-ulite with characteristic plagioclase-orthopyroxene corona around the garnet, which shows rapid exhumation-related decompression. A constant-temperature decompressive pressure-temperature-time path can be documented within the rocks, and the peak pressures and temperatures are in the range of 1.21.0 GPa, at 1,290-1,470°F (700-800°C). At least three types of geochemical patterns are shown by mafic rocks of the high-pressure granulites, indicating a tectonic setting of active continental margin or island arc. The high-pressure granulites were formed through subduction-collision, followed by rapid rebound-extension, recorded by 2.5-2.4 billion-year old mafic dike swarms and rift-related sedimentary sequences in the Wutai Mountai-Taihang Mountain areas.

The Qinglong foreland basin and fold-thrust belt is north- to northeast-trending and is now preserved as several relict-folded sequences (Qinglong, Fuping, Hutuo, and Dengfeng). Its general sequence from bottom to top can be further divided into three subgroups of quartzite-mudstone-marble, turbidite, and molasse, respectively. The lower subgroup of quartzite-mudstone-marble is well preserved in central sections of the Qinglong foreland basin (Taihang Mountain), with flat-dipping structures, interpreted as a passive margin developed before 2.5 Ga on the Eastern block. It is overlain by lower-grade turbi-dite and molasse-type sediments. The western margin of Qinglong foreland basin is intensely reworked by thrusting and folding and is overthrust by the overlying orogenic complex (including the tonalitic-trondhjemitic-granodiorotic gneiss, ophiolites, accre-tionary prism sediments). To the east its deformation becomes weaker in intensity. The Qinglong foreland basin is intruded by a gabbroic dike complex consisting of 2.4 billion-year-old diorite and is overlain by graben-related sediments and flood basalts. In the Wutai and North Taihang basins, many ophiolitic blocks are recognized along the western margin of the foreland thrust-fold belt. These consists of pillow lava, gabbroic cumulates, and harzburgite. The largest ophiolitic thrust complex imbricated with foreland basin sedimentary rocks is up to 5 miles (10 km) long, preserved in the Wutai-Taihang Mountains.

Several dismembered Archean ophiolites have been identified in the Central orogenic belt, including some in Lioning Province, at Dongwanzi, north of Zunhua, and at Wutai Mountain. The best studied of these are the Dongwanzi and Zunhua ophiol-itic terranes. The Zunhua structural belt of eastern Hebei Province preserves a cross section through most of the northeastern part of the Central orogenic belt. This belt is characterized by highly strained gneiss, banded iron formation, 2.6-2.5 billion-year-old greenstone belts, and mafic to ultramafic complexes in a high-grade ophiolitic mélange. The belt is intruded by widespread 2.6-2.5 billion-year-old tonalite-trondhjemite gneiss and 2.5 billion-year-old granites, and is cut by ductile shear zones. The Neoarchean high-pressure granulite belt (Chengde-Hengshan HPG) strikes through the northwest part of the belt. The Zunhua structural belt is thrust over the Neoarchean Qianxi-Taipingzhai granulite-facies terrane, consisting of high-grade metasedimentary to charnockitic gneiss forming several small dome like structures southeast of the Zunhua belt. The Zunhua structural belt clearly cuts across the domelike Qian'an-Qianxi structural patterns to the east. The Qian'an granulite-gneiss dome (3.8-2.5 billion years old) forms a large circular dome in the southern part of the area and is composed of tonal-itic-trondhjemitic gneiss and biotite granite. Meso-archean (2.8-3.0 billion years old) and Paleoarchean (3.50-3.85 billion years old) supracrustal sequences outcrop in the eastern part of the region. The Qin-glong Neoarchean amphibolite to greenschist-facies supracrustal sequence strikes through the center of the area and is interpreted to be a foreland fold-thrust belt, intruded by large volumes of 2.4 billion-year-old diorite in the east. The entire North China craton is widely cut by at least two Paleoproterozoic mafic dike swarms (2.5-2.4, 1.8-1.7 billion years old), associated with regional extension. Mesozoic-Cenozoic granite, diorite, gabbro, and ultramafic plugs occur throughout the NCC and form small intrusions in some of the belts.

The largest well-preserved sections of the Dong-wanzi ophiolite are located approximately 120 miles (200 km) northeast of Beijing in the northeastern part of the Zunhua structural belt, near the villages of Shangyin and Dongwanzi. The belt consists of prominent amphibolite-facies mafic-ultramafic complexes in the northeast sector of the Zunhua structural belt. The southern end of the Dongwanzi ophiolite belt near shangyin is complexly faulted against granulite-facies gneiss, with both thrust faults and younger normal faults present. The main section of the ophio-lite dips steeply northwest, is approximately 30 miles (50 km) long, and is 3-6 miles (5-10 km) wide. A U/ Pb-zircon age of 2.505 billion years for two gabbro samples from the Dongwanzi ophiolite shows that this is the oldest, relatively complete ophiolite known in the world. Parts of the central belt, however, are intruded by a mafic/ultramafic Mesozoic pluton with related dikes.

A high-temperature shear zone intruded by the 2.4 Ga old diorite and tonalite marks the base of the ophiolite. Exposed ultramafic rocks along the base of the ophiolite in the ophiolite include strongly foliated and lineated dunite and layered harzburgite. Aligned pyroxene crystals and generally strong deformation of serpentinized harzburgite resulted in strongly foliated rock. Harzburgite shows evidence for early high-temperature deformation. This unit is interpreted to be part of the lower residual mantle, from which the overlying units were extracted.

The cumulate layer represents the transition zone between the lower ultramafic cumulates and upper mafic assemblages. The lower part of the sequence consists of layers of pyroxenite, dunite, wehrlite, lherzolite and websterite, and olivine gabbro-lay-

ered cumulates, all formed by heavy crystals sinking through the magma and settling on the bottom of the magma chamber. Many layers grade from dunite at the base, through wehrlite, and are capped by clinopyroxene. Basaltic dikes cut through the cumulates and are similar mineralogically and texturally to dikes in the upper layers.

The gabbro complex of the ophiolite is up to three miles (5 km) thick and grades up from a zone of mixed layered gabbro and ultramafic rocks to one of strongly layered gabbro that is topped by a zone of isotropic gabbro. Thicknesses of individual layers vary from centimeter to meter scale and include clinopyroxene and plagioclase-rich layers. Layered gabbros from the lower central belt alternate between fine-grained layers of pyroxene and metamorphic biotite that are separated by layers of metamorphic biotite intergrown with quartz. Biotite and pyroxene layers show a random orientation of grains. Coarsegrained veins of feldspar and quartz are concentrated along faults and fractures. Plagioclase feldspar shows core replacement and typically has irregular grain boundaries. The gabbro complex of the ophiolite has been dated by the u-Pb method on zircons to be 2,504 ± 2 million years old.

The sheeted dike complex is discontinuous over several kilometers. more than 70 percent of the dikes are unusual in that only one half of each dike is preserved. In most tectonic settings dikes have finegrained chill margins on both margins where they intruded and cooled against country rocks, crystallizing quickly and forming two parallel chill margins with finer-grained crystals than in the slower-cooling interior of the dikes. Dikes in ophiolites often show dikes with only one chill margin, where new dikes successively intrude the center of the previously intruded dike, in a setting where the crust is being extended and filled by new dikes. When the next dike intruded, it intruded along the center of the last dike, and this happened several times in a row, leaving a dike complex with chill margins preserved preferentially on one side. one-way chill margins are preferentially preserved on their northeast side of the dikes in the Dongwanzi ophiolite. Gabbro screens are common throughout the complex and increase in number and thickness downward, marking the

Tectonic map of the North China craton showing the eastern and western blocks, 2.5 billion-year-old Central Orogenic Belt, and the 1.9 billion-year-old North Hebei Orogen (modeled after T. Kusky and J. H. Li)

transition from the dike complex to the fossil magma chamber. In some areas the gabbro is cut by basaltic-diabase dikes, but in others it cuts through xenoliths of diabase, suggesting comagmatic formation.

The upper part of ophiolite consists of altered and deformed pillow basalts, pillow breccias, and interpillow sediments (chert and banded iron formations). Many of the pillows are interbedded with more massive flows and cut by sills; however, some well-preserved pillows show typical lower cuspate and upper lobate boundaries that define stratigraphic younging. Pyroxenes from pillow lavas from the ophiolite have been dated by the Lu-Hf method to be 2.5 billion years old, the same age as estimated for the gabbro and mantle sections.

The base of the ophiolite is strongly deformed, and intruded by the 2.391 billion-year-old Cuizhangzi diorite-tonalite complex. The Dongwanzi ophiolite is associated with a number of other amphibolite-facies belts of mafic plutonic and extrusive igneous rocks in the Zunhua structural belt. These mafic-to-ultramafic slices and blocks can be traced regionally over a large area from Zunhua to West Liaoning (about 120 miles or 200 km). Much of the Zunhua structural belt is interpreted as a high-grade ophiolitic mélange, with numerous tectonic blocks of pillow lava, BIF, dike complex, gabbro, dunite, serpentinized harzburgite, and podiform chromitite in a biotite-gneiss matrix, intruded extensively by tonalite and granodiorite. Cross-cutting granite has yielded an age of 2.4 billion years. Blocks in the mélange correlate with the Dong-wanzi and other ophiolitic fragments in the Zunhua structural belt. This correlation is supported by the isotopic system of Rhenium (Re)-Osmium (Os), since Re-Os age determinations on several of these blocks reveal that they are 2.54 billion years old.

The Eastern and Western blocks of the North China craton collided at 2.5 billion years ago during an arc/continent collision, forming a foreland basin on the Eastern block, a granulite facies belt on the Western block, and a wide orogen between the two blocks. This collision was followed rapidly by post-orogenic extension and rifting that formed mafic dike swarms and extensional basins along the Central orogenic belt, and led to the development of a major ocean along the north margin of the craton. An arc terrane developed in this ocean and collided with the north margin of the craton by 2.3 Ga, forming an 850-mile (1,400-km) long orogen known as the Inner Mongolia-Northern Hebei orogen. A 1,000-mile (1,600-km) long granulite-facies terrain formed on the southern margin of this orogen, representing a 120-mile (200-km) wide uplifted plateau formed by crustal thickening. The orogen was converted to an Andean-style convergent margin between 2.20 and 1.85 billion years ago, recorded by belts of plutonic rocks, accreted metasedimentary rocks, and a possible back arc basin. A pulse of convergent deformation is recorded at 1.9-1.85 billion years across the northern margin of the craton, perhaps related to a collision outboard of the Inner Mongolia-Northern Hebei orogen, and closure of the back arc basin. This event caused widespread deposition of conglomerate and sandstone of the basal Changcheng Series in a foreland basin along the north margin of the craton. At 1.85 billion years the tectonics of the North China craton became extensional, and a series of aulacogens and rifts propagated across the craton, along with the intrusion of mafic dike swarms. The northern granulite facies belt underwent retrograde metamor-phism, and was uplifted during extensional faulting. High-pressure granulites are now found in the areas where rocks were metamorphosed to granulite facies and exhumed two times, at 2.5 and 1.8 billion years ago, respectively, exposing rocks that were once at lower crustal levels. Rifting led to the development of a major ocean along the southwest margin of the craton, where oceanic records continue until 1.5 billion years ago.

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