Changzhougou Formation Research Articles

Paleocurrent analysis on the basal conglomerate of the model Changzhougou formation in Qian’an

The paleoflow direction of the basal conglomerate of the Changzhougou formation is obtained by the paleocurrent analysis of certain sedimentary structures, and the ancient sedimentary environment of the Qian’an Region is determined by observing the psephicity and gradation of gravel in conglomerate. The results show that the paleoflow direction is from northwest to southwest, and the paleocurrent data manifest a single-peak style in a rose diagram. The average paleoflow direction in the Qian’an region is 265°. The basal conglomerate is of littoral facies, and the sediments of conglomerates are mainly sourced from Shanhaiguan.

Diagenetic xenotime dating to constrain the initial depositional time of the Yan-Liao Rift

In the Mesoproterozoic and Neoproterozoic, the North China Craton (NCC) underwent long-lived sedimentation with minor volcanism in the Yan-Liao Rift forming the Mesoproterozoic Changcheng, Jixian and Qingbaikou Groups in sequence. The basal boundary age of the Lower Changcheng Group indicates the initial time of the NCC as a stable cratonic block and can be used to make correlations among several representative basins worldwide. Recent studies have debated the initial depositional time of the Changcheng System, which is the key to understanding the Proterozoic tectonic evolution of the NCC. This study documented the xenotime and monazite from the lowermost Changzhougou and Chuanlinggou Formations of the Changcheng Group. The monazite grains from the Chuanlinggou Formation yielded U–Pb ages between 1810Ma and 2500Ma, which are comparable to the detrital zircon U–Pb ages in the same formation, indicating their detrital origin. Xenotime mostly occurs as overgrowths on the detrital zircon grains, supporting a post-depositional origin. The low U content of the Changzhougou Formation's xenotimes indicates a hydrothermal origin. Its peak age of 1570±15Ma may indicate a hydrothermal event. The xenotime from the Chuanlinggou Formation is characterized by a high U content and a REE pattern with a nonexistent or weak negative Eu anomaly and a higher Gd/Yb ratio, implying a diagenetic origin. These xenotime overgrowths yielded a wide age variation, even on a single grain, due to prolonged xenotime precipitation. The oldest population of 1716±3Ma is interpreted as the minimum depositional age of the Chuanlinggou Formation. The lowest boundary of the Changchengian System must be older than 1716Ma. This work demonstrates that diagenetic xenotime could better constrain the depositional time; however, extensive systematic research is required, especially with higher spatial resolution and more diagenetic xenotime grains.

In‐Situ SHRIMP U‐Pb Dating of Xenotime Outgrowth on Detrital Zircon Grains from the Changzhougou Formation of the Ming Tomb District, Beijing

In‐Situ SHRIMP U‐Pb Dating of Xenotime Outgrowth on Detrital Zircon Grains from the Changzhougou Formation of the Ming Tomb District, Beijing

Recent advances in the study of the Mesoproterozoic geochronology in the North China Craton

The Mesoproterozoic successions in the North China Craton (NCC) and the Qinling–Qilian–Kunlun Orogens have been revised using the new and highly reliable age data. Many Proterozoic strata in the Qinling–Qilian–Kunlun Orogens, such as the Qinling, Jinshuikou and Beidahe groups that have been ascribed to be Paleoproterozoic are actually of Mesoproterozoic Era. The most significant advances are recent geochronological studies on the Mesoproterozoic stratigraphy and magmatic events in the NCC. The boundary age between the Dahongyu Formation and the overlying Gaoyuzhuang Formation is well constrained to be ∼1600Ma, corresponding to the boundary age between Statherian and Calymmian. The boundary between the Tieling Formation and the overlying Xiamaling Formation is best positioned at ∼1400Ma, which is coeval with the boundary between Calymmian and Ectasian, and is about 400Myrs older than the conventional value of 1000Ma originally defined by the All China Commission of Stratigraphy. Hence the Jixianian System, including the Gaoyuzhuang, Yangzhuang, Wumishan, Hongshuizhuang and Tieling formations in ascending order, is comparable with the Calymmian System in the International Stratigraphic Chart. The lower boundary of the Changchengian System, the first system of the Mesoproterozoic in China Regional Stratigraphic Chart, also needs revision from the conventional 1800Ma to ∼1650Ma well constrained by the zircon U–Pb ages 1673±10Ma (LA-MC-ICP-MS) and 1669±20Ma (SHRIMP) of a granite-porphyry dike that was overlain unconformably by the basal conglomerate of the Changzhougou Formation, the first formation of the Changchengian System. Therefore, the earliest Mesoproterozoic sequence in the NCC represented by the Changchengian and Jixianian Systems in the Yanliao Aulacogen is identical to that of the Vindhyan Supergroup in Central Indian and the Riphean Series in Russia. On the other hand, a series of 1.8–1.6Ga anorogenic magmatic records were well-preserved around the NCC, which marked the initial rifting of the Columbia Supercontinent in the NCC. The magmatic events can thus be subdivided into three phases with peaks at ca 1.77Ga, ca 1.70Ga and ca 1.63Ga, respectively. In addition to 1.8–1.6Ga magmatic events, some minor volcanic eruptions at ca 1.56Ga and 1.44Ga, and wide-spread bi-modal magmatic intrusions at 1.35–1.32Ga have been recognized in the northern NCC, marking the continued rifting of the Columbia Supercontinent since ∼1.8Ga.

Age of the Miyun dyke swarm: Constraints on the maximum depositional age of the Changcheng System

A group of NE-trending (∼30°) mafic dykes, termed Miyun swarm herein, are distributed around the Miyun Reservoir, northern China, and have individual widths of over 40 m, and lengths of up to 8000 m. Baddeleyite grains were extracted from a ∼40 m wide dyke. Using an isotope dilution thermal ionization mass spectrometry method, these grains yield an average 207Pb/206Pb age of 1731±4 Ma (n = 4; or 1731±1 Ma, n = 3). Because this dyke is overlain unconformably by conglomerates of the Changzhougou Formation, the lowermost formation of the Changcheng Group, the maximum depositional age of this group is constrained to be no earlier than 1731 Ma. This result also suggests that the Changcheng System, with the Changcheng Group as its standard stratigraphic section, is younger than 1731 Ma. The Miyun dykes possibly have distinct petrogenesis from the subsequent anorthosite-rapakivi intrusions (1730–1680 Ma), which are thought to be responsible for the opening of the Yan-Liao rift, where the Changcheng Group was deposited. Thus it is reasonable to set the initial boundary of the Changchengian Period at 1730 Ma or slightly younger.

Evidence for eukaryotic diversification in the ∼1800 million-year-old Changzhougou Formation, North China

Shales and silty shales of the ∼1800 million-year-old Changzhougou Formation (lowermost Changcheng Group) in the Pangjiapu Region of North China contain well-preserved, acritarchs. These microbial body fossils, although sphaeromorphs, have a size and fine-scale morphological complexity that would attribute them to the eukaryotic domain on the tree of life. Sphaeromorphs range in size from 62 μm to 216 μm. Shapes include spheroidal, ellipsoidal, and fusiform. Some have medial splits indicating excystment and others have a complex wall structure with three or more wall layers. Thirteen distinct morphological entities have been identified. These results indicate that eukaryotic microbial life was well established by close of the Paleoproterozoic and had already undergone a moderate amount of diversification.

A Study of Mesoproterozoic Iron Cosmic Micro‐spherules from 1.8 Ga and 1.6 Ga Old Strata in the Ming Tombs District, Beijing

Abstract: Numerous iron cosmic micro‐spherules have been discovered from Mesoproterozoic strata including the Changzhougou Formation (1.8 Ga) and the Dahongyu Formation (1.6 Ga) of the Ming Tombs district, Beijing. There are 1 to 30 grains of cosmic spherules per 2 kg of a sandstone sample taken from the bottom of a coarse sandstone bed of the Changzhougou Formation and 56 grains per 3.69 kg of a rock sample from silicified carbonate rocks of the Dahongyu Formation. The surface textures of cosmic spherules analyzed by means of the secondary electron imagery are identical with those reported from references either domestic or abroad. So far the geo‐ages of 1.8 Ga and 1.6 Ga of cosmic spherules from the Changzhougou and Dahongyu formations might be older than those reported in the world. Table 1 gives the electron probe analysis data of cosmic spherules for 30 spherule grains and 44 testing points as follows (%): FeO, 80–95; Cr2O3; 0–9.56; NiO, 0‐0.78; CoO, 0‐0.46; indicating that the Cr2O3 content is higher and FeO content lower in the Changzhougou Formation than in the Dahongyu Formation. The helium isotopic data of cosmic spherules as well as their host rocks vary greatly between the Changzhougou and the Dahongyu formations as shown in Table 2. The data of cosmic spherules of the Changzhougou Formation vs the Dahongyu Formation are 57.5/1.23 in 3He/4He (10−8); and 55.54/809.60 in 4He (10−6cm3STP/g); those of coarse sandstone of the Changzhougou Formation vs silicified carbonate of the Dahongyu Formation are 3.39/2.59 in 3He/4He (10−8) and 4.56/2.34 in 3He (10−6cm3STP/g). The ratio of analytic data of helium isotopes are different for cosmic spherules and their host rocks; for example, the 3He/4He (10−8) values are 16.96 and 0.48, and the 4He (10−6 cm3STP/g) are 12.18 and 345.98 for the Changzhougou and Dahongyu formations respectively. It was reported that the world's oldest micrometeorites had been found in the Meso‐Proterozoic Satakunta Formation, Finland. However, the cosmic spherules from the Meso‐Proterozoic Changzhougou and Dahongyu formations are 200 to 400 Ma older than those from the Satakunta Formation. Besides, one carbonaceous chondrite grain was discovered for the first time as the earliest remain formed in the solar nebula from the Dahongyu Formation.

Paleoproterozoic compression-like structures from the Changzhougou Formation, China: Eukaryotes or clasts?

The Paleoproterozoic Changzhougou Formation (Changcheng Group) of North China contains compression-like structures, millimeters to centimeters in diameter and about a millimeter thick that are circular, elliptical, elongate, and irregular in shape. These structures were interpreted to be carbonaceous compressions of megascopic eukaryotes by Zhu et al. (2000). Some of the compression-like structures were reported to contain masses of cellular material with well-preserved cell walls. Further investigation of these structures, including plane light investigations of thin sections and macerates, petrographic study of thin sections, SEM, EDS, CHN, XRD, and biomarker analyses, indicates that they are pseudo-fossils.

SHRIMP ages of detrital zircons from the Changcheng System in the Ming Tombs area, Beijing: Constraints on the protolith nature and maximum depositional age of the Mesoproterozoic cover of the North China Craton

The Mesoproterozoic Changcheng System is widely distributed in the North China Craton. Determining its time of deposition and sources is important to understand the Precambrian crustal evolution of the North China Craton. This paper suggests age distribution patterns for detrital zircons from clastic sediments of the Changcheng System in the Ming Tombs area, Beijing. Samples of feldspar-bearing sandstone (CHc-2) and pure sandstone (CHc-9) were collected from the Changzhougou Formation, which constitutes the basal part of the Changcheng System. Detrital zircons show an age range from 2.35 to 2.60 Ga. However, sample CHc-9 in the upper Changzhougou Formation also contains some zircons with ages of 1.9-1.8 Ga and 2.3-2.1 Ga. The age patterns lead to the following conclusions: (1) Most of the detrital material came from a source area composed predominantly of ~2.5 Ga continental crust of the North China Craton; (2) 1.9-1.8 Ga reflects the age record of Palaeoproterozoic continent-continent collisional event in the North China Craton; and (3) the oldest age for deposition of the Changcheng System is 1.8 Ga.

Discovery of carbonaceous compressions and their multicellular tissues from the Changzhougou formation (1800 Ma) in the Yanshan range, North China

In 1997 and 1998, hundreds of specimens of megascopic carbonaceous compressions or algal fossils were found from ∼ 1800-million-year old Changzhougou Formation, which is the lowermost unit of the latest Palaeoproterozoic Changcheng Group (∼1 600–1 800 Ma) in the Xinglong-Kuancheng areas at the middle Yanshan Range, North China. They are discoid, ellipsoid and sausage-like, namely shaped likeChuaria Shouhsiennia (Ellipsophyta) andTawuia. By adopting HF acid-resistant maceration coupled with scanning electron microscope and petrologic section, the authors made a preliminary research on the histology for some circular and ellipsoid carbonaceous compressions, namelyChuaria- andShouhsienia-like forms, in addition to their morphology. The following three types of multicellular tissues have been found in the fragments of them: colony-like, pseudoparenchyma-like and parenchyma-like. All of the new data about multicelluar tissues not only supply a very important basis in histology to determine the biological affinity of multicellular algal remains for the Changzhougou carbonaceous compressions newly found, but also provide reliable fossil evidence to prove that metaphytes originated at least 1 800 million years ago.

A new Palaeoproterozoic clastic-facies microbiota from the Changzhougou Formation, Changcheng Group, Jixian, north China

Abundant microfossils have recently been discovered in the late Palaeoproterozoic Changzhougou Formation, Changcheng Group, Jixian, north China. The Changzhougou microbiota consists dominantly of sphaeromorph acritarchs and some osillatoriacean cynobacterial filaments. The micro-fossils were recovered from shales and are compressed on bedding surfaces. The sphaeromorph acritarchs, ranging from 8.4 to 237.8 μm in diameter, are morphologically simple. On the basis of their dimensions and morphological considerations, the sphaeromorph acritarchs are interpreted as probably representing the resistant, reproductive cysts of unicellular protists. This assemblage is evidently among the oldest known clastic-facies microbiotas in the geological record and provides evidence of protist eukaryotes being extant at 1848 Ma.

Tidal sedimentary structures from upper precambrian rocks of the Ming Tombs District, Beijing (Peking), China

The Ming Tombs District is a famous historic and scenic site in the vicinity of Beijing. The Proterozoic strata of this region contain many types of tidal sedimentary structures. This paper deals mainly with tidal or shallow water sedimentary structures from carbonates but some examples are also described from silicilclastic rocks. The structures include burrow-like tube fillings (from the nearly 2 Ga old Changzhougou Formation), herringbone cross stratification, double-crested interference ripple marks, and lenticular and flaser bedding from carbonate rocks of the Gaoyuzhuang Formation. Well developed sedimentary structures in these upper Precambrian rocks indicate that tidal conditions prevailed throughout deposition of most of a 4500 m thick succession. Supratidal, intertidal and subtidal environments have all been recognised. Widespread occurrence of tidal sedimentary structures in carbonate rocks suggests that they were mainly deposited under the influence of physical processes.