In the southwestern Tianshan,the geologic ages of many strata and their depositional environments are still poorly constrained because of complex structures.The Mada＇er area is located in the Kuokesaleling belt,South...In the southwestern Tianshan,the geologic ages of many strata and their depositional environments are still poorly constrained because of complex structures.The Mada＇er area is located in the Kuokesaleling belt,Southwestern Tianshan.The cherts from the former Wupata＇erkan Group contain abundant radiolarian fossils,including 10 species which are identified as late Devonian to early Carboniferous in age.Eleven chert samples have SiO_2 contents ranging from 88.80 wt%to 93.28 wt%, and 2.02 wt%to 3.72 wt%for Al_2O_3.The SiO_2/Al_2O_3 ratios of all samples vary from 23.84 to 46.11, much lower than those of the pure cherts（80-1400）.These values suggest that the cherts contain high ratios of terrigenous materials.The Al_2O_3/（Al_2O_3＋Fe_2O_3）ratios vary between 0.64 and 0.77,whereas V and Cu concentrations range from 10.92 ppm to 26.7 ppm and from 2.15 ppm to 34.1 ppm respectively. The Ti/V ratios vary from 25.53 to 44.93.The total REE concentrations of the cherts are between 30.78 ppm and 59.26 ppm,averaging 45.46 ppm.The（La/Ce）_N ratios range from 0.81 to 1.12,and 0.88-1.33 for（La/Yb）_N,averaging 1.09,which suggests a continental margin environment.Consequently,it is inferred that the cherts formed in a residual sea environment during the late Devonian to early Carboniferous time,which suggests that the collision between the Karakum-Tarim and Kazakhstan-Junggar plates did not occur at the time.In addition,the regional geological information indicates that the study area experienced a post-collision stage during the early Permian,and thus it is likely that the collision between the two plates took place in the late Carboniferous.展开更多
Bedded cherts occurring in the Early Permian Gufeng Formation in the Lower Yangtze region, eastern China, are nearly 20-80 m in thickness and contain varying amounts of radiolarians and sponge spicules. There are thre...Bedded cherts occurring in the Early Permian Gufeng Formation in the Lower Yangtze region, eastern China, are nearly 20-80 m in thickness and contain varying amounts of radiolarians and sponge spicules. There are three types of section for the Gufeng Formation: chert, carbonate and chert-carbonate mixing types. Bedded cherts mainly occur in the first and third types of section. The depocentres of silica are marked by some small (not larger than a few thousand square kilometres in area) rhombic or elliptic hollows and their formation was controlled by faults. Argillaceous volcanic rocks in cherts occur as numerous thin layers. Bedded cherts are characterized by higher Fe and lower Al contents, enrichment in such trace elements as As, Sb, Bi, Ga, Au, Ag and Cr, lower total REE abundance, negative Ce anomaly and varying degrees of HREE enrichment. These characteristics are important evidence for hydrothermal cherts. Minor amounts of substances of non-hydrothermal origin are mixed in the cherts. The temperatures of sea water range approximately from a few tens of degrees to over 160℃. during the deposition of siliceous sediments.展开更多
Granitoids,volcanic rocks and cherts,mainly of early Paleozoic,in northern Altyn Tagh(Fig.1) are analyzed here for the purpose of the determination of the tectonic environments and their development since Early Paleoz...Granitoids,volcanic rocks and cherts,mainly of early Paleozoic,in northern Altyn Tagh(Fig.1) are analyzed here for the purpose of the determination of the tectonic environments and their development since Early Paleozoic.(1) Granitoids\ The collection of 128 main\|element petrochemical data from North Altyn Tagh area shows that most of the granitoids here are granites and granodiorites of calc\|alkaline series.They consist mainly of metaluminous and peraluminous in Shand’s index,and only one of them,which belongs to Mesozoic,is peralkaline.Most of the granitoids plot in the IAG+CAG+CCG fields in the Maniar and Piccoli’s diagrams (1989) for tectonic discrimination of granitoids.IAG (Island arc granitoids)and CAG (Continental arc granitoids) can be distinguished for Early and Late Paleozoic granitoids,and maybe some CCG (Continental collision granitoids) for Early Paleozoic.Granitoids of Mesozoic and Cenozoic inherited the characteristics of those of Paleozoic.Destructive active plate margin (pre\|plate collision)and Anatectic magmatism(syn\|orogenic,S\|type granites)are distinguished (Fig.2)for Paleozoic granitoids using de la Roche R 1\| R 2 multicationic diagram (Batchelor and Bowden,1985).Mesozoic and Cenozoic granitoids inherited the characteristics of Anatectic magmatism (syn\|orogenic)of these early ones,and post\|orogenic(A\|type)granites occurred in Mesozoic.The mechanism for magma formation is mainly partial melting.展开更多
Through microscopic and SEM observations and X-ray diffraction and energy spectrum analyses, the oldest sponge spicules and radiolarians have been found in bedded cherts and it is proposed that microbial activity of a...Through microscopic and SEM observations and X-ray diffraction and energy spectrum analyses, the oldest sponge spicules and radiolarians have been found in bedded cherts and it is proposed that microbial activity of algae and bacteria played an important role in silica precipitation. The microbial activity resulted in the formation of a series of single crystals and crystal aggregates with peculiar shapes. Biogenic single crystals usually occur in spherical, rod, platy, colloidal and granular forms. The most common aggregates are framboidal aggregates composed of dense spherical quartz crystals, which are considered to be of typical bacterial origin. Other aggregates, such as favositic, double-spherical, chain-spherica'l and coccoid ones, are considered to be probably the result of silicification of algal cells and mainly of primary sedimentary origin. This paper also discusses the ultrastructural features and diagnostic indications of jasper rocks formed in a submarine hydrothermal environment and cherts formed by replacement and diagenetic recrystallisation.展开更多
The sedimentary sequence of the Lower Cambrian is a key interval to reveal the early evolution history of the Earth and there occur widespread cherts worldwide. These cherts possibly carry important information to dec...The sedimentary sequence of the Lower Cambrian is a key interval to reveal the early evolution history of the Earth and there occur widespread cherts worldwide. These cherts possibly carry important information to decipher the breakup process of the Rodinia Continent. Black rock series at the bottom of the Lower Cambrian in the Northern Tarim Basin, China, is composed of black shales interbedded with thin-bedded cherts. Ten chert samples were systematically collected from two outcrops at Xiaoerbulak and Sogatbulak, 8.8 and 7.5 m thick respectively. The cherts were crushed, and were analyzed for trace element and rare earth concentrations. Trace elements such as V, Cu, Zn, U, Pb, Ba, Cd, Ag, Mo, As and Sb are highly enriched, and others such as Rb, Zr, Cs, Hf, Ta, W, Tl, Bi and Th are highly depleted in the cherts. These trace element patterns suggest that the cherts may be of deep crustal origin. The low ratios of Th/U and Rb/Sr further suggest that the cherts are of earth interior sources or received hydrothermal input during their deposition. Chondrite- normalized Eu/Eu* value markedly decreases upward in the section from 5.54 at the lowermost to 0.73 at the top, and NASC-normalized Eu/Eu* value decreases from 8.05 to 1.03. The relatively high Eu/Eu* ratio for the cherts from the northern Tarim Basin is most likely due to a hydrothermal input (e.g., Eu/Eu* ~10). The systematic decrease of Eu/Eu* ratio from the bottom to the top of the section reflects that the hydrothermal input is the largest in the lowermost portion of the section and gradually decreases upward. The chondrite-normalized Ce/Ce* ratio ranges from 0.42 to 0.83, with an average of 0.60. North American Shale Composite (NASC)-normalized Ce/Ce* ratio ranges from 0.42 to 0.79, with an average of 0.57. Negative Ce anomalies are distinct. ΣREEs in the cherts generally increase from 10.50 ppm at the bottom to 35.97 ppm at the top of the sampled section. NASC-normalized (La/Lu) N ratio decreases from 2.72 at the bottom to 0.67 at the top. NASC-normalized (La/Ce) N ratio increases from 1.36 at the bottom to 3.13 at the top. These REE patterns are very similar to those for the cherts deposited in the pelagic ocean-basin floor in the Franciscan Complex exposed at Marin Headlands, California (F-MH chert) (Murray et al., 1991). These geochemical signatures are inconsistent with our previous sedimentological data, which suggests a continental shelf setting. Based on multiple lines of evidence including high TOC content in the concomitant black shales, phosphorite at the bottom of black rock series, regional rise of sea level, and beginning of the southern Tianshan Ocean geotectonic cycle, the authors infer that the hydrothermal fluid was carried to the continental shelf by upwelling from a divergent pelagic ocean floor setting.展开更多
Chert clast-bearing epi-continental neritic-terrestrial Jurassic and Cretaceous sequences are sporadically distributed in southwestern Japan. Typical and geological entities are the Torinosu Group and Tetori Group. Ra...Chert clast-bearing epi-continental neritic-terrestrial Jurassic and Cretaceous sequences are sporadically distributed in southwestern Japan. Typical and geological entities are the Torinosu Group and Tetori Group. Radiolarian dating of chert clasts enables us to discuss denudation stages of mid Mesozoic accretionary complexes. Chert clast-dominated conglomerate can be used for identifying marine beds within terrestrial sequences.展开更多
The Xialu chert, which contains abundant biological information, were investigated by major element analysis, micro-Raman, SEM and EPMA. The results show that SiO2 content of chert is 84.12%-93.08%, averaging 89.84%. ...The Xialu chert, which contains abundant biological information, were investigated by major element analysis, micro-Raman, SEM and EPMA. The results show that SiO2 content of chert is 84.12%-93.08%, averaging 89.84%. The close packed structures of low degree crystallinity of quartz indicate the hydrothermal origin. SiO2 of modern hot springs exhibit loose silica pellets and nodular, beaded structures. Under polarization microscope, the presence of biological skeleton structures indicate that biological activities are involved in the hydrothermal deposition, which correspond to the geochemical characteristics: w(SiO2)/ w(K2O+Na2O), w(SiO2)/w(Al2O3) and w(SiO2)/w(MgO), with average values of 295.29, 68.88 and 284.45, respectively. SiO2 is enriched in the organism(radiolarian) centers, the degree order of SiO2 within the biologic structures is much higher than that of outside. The impurity minerals albites are formed earlier than the original deposition. Kaolinites, feldspars and mixture of organic materials display lower degree of crystallinities and accumulate as vermicular aggregates.展开更多
Rock-magnetic methods prove that the cultures in Moravia (Europe) 15,000 - 11,500 years ago might know the technique how to enhance knapping properties of Jurassic chert and Cretaceous flint in order to make stone tools.
The extensive bedded cherts deposited during the Ediacaran-Cambrian(E-C)transition period play a crucial role in understanding the geological evolution of this period,yet the origin of these cherts remains disputed.He...The extensive bedded cherts deposited during the Ediacaran-Cambrian(E-C)transition period play a crucial role in understanding the geological evolution of this period,yet the origin of these cherts remains disputed.Here,we present new geochemical data for cherts of the Piyuancun(PYC)Formation deposited during the Late Ediacaran and the Hetang(HT)Formation deposited during the Early Cambrian in northern Jiangxi region,Lower Yangtze region,South China.The PYC cherts contain a small amount of monaxons sponge spicules and radiolarian fragments,while the HT cherts lack siliceous organism evidence.Major and trace element analysis,coupled with discriminant diagrams,indicate a possible shift in redox conditions of seawater during the E-C transition in the northern Jiangxi region.The shift suggests a change from weakly-moderately restricted euxinic conditions to strongly restricted euxinic conditions.Furthermore,the location of both cherts are distant from the source area of siliceous organisms.Fossil evidence,as well as the values of Fe/Ti and Fe/(Mn+Ti),Eu anomalies,Post-Archean Australian Shale(PAAS)normalized REE+Y patterns,and various discriminant diagrams,support the conclusion that the PYC and HT cherts originated primarily from direct seawater precipitation,with the PYC cherts exhibiting weak hydrothermal evidence.Upwelling contributes to the formation of HT cherts and organic matter(OM)accumulation.Ocean acidification,triggered by OM degradation and biodegradation processes during the E-C transition period,leads to the extensive silica precipitation and preservation.These results enhance our understanding of the geological processes during the E-C transition.展开更多
The Ediacaran to early Cambrian Blovice accretionary complex,Bohemian Massif,hosts abundant chert bodies that formed on an oceanic plate and were involved in subduction beneath the northern margin of Gondwana.Field re...The Ediacaran to early Cambrian Blovice accretionary complex,Bohemian Massif,hosts abundant chert bodies that formed on an oceanic plate and were involved in subduction beneath the northern margin of Gondwana.Field relationships of cherts to their host,their microstructure and elemental as well as isotopic compositions revealed diverse processes of chert petrogenesis reflecting depositional environment and position on the oceanic plate.The deep-water cherts formed through a hydrothermal precipitation of silica-rich gels on outer trench swell of the subducted slab with none or only minor addition of terrigenous material.On the contrary,the shallow-water cherts formed in lagoons on seamount slopes,and at least some of them represent a product of hydrothermal replacement of former carbonate and/or evaporite precursors.For both chert types,the hydrothermal fluids were of low temperature and continuous pervasive hydrothermal alteration of oceanic crust,together with an elevated Si content in Neoproterozoic seawater,served as the major source of silica.On the other hand,minor carbon enrichment in chert is mostly linked to variable incorporation of organic matter that was deposited on the seafloor.Rare earth element(REE)systematics of the cherts indicate predominantly oxygenated environment for the shallow-water cherts whereas the deep-water cherts were deposited in diverse redox conditions,depending on their distance from hydrothermal vent.Using these data,we demonstrate that the cherts once formed a part of Ocean Plate Stratigraphy(OPS)now dismembered and mixed with terrigenous siliciclastic material to form OPS mélanges.Combining our data with those from the existing literature,we show that cherts can serve as significant markers of OPS since the Archean,recording a complex interplay between seafloor-related volcanic(production of MORB-and OIB-like magmas)and sedimentary processes,hydrothermal activity at mid-ocean ridges and seamount chains as well as at outer slopes of subducting slabs.However,the cherts also exhibit a secular change in composition and petrogenesis most profoundly affected by an overturn in seawater silica cycle across the Precambrian-Phanerozoic boundary.展开更多
The Caledonian North Qilian orogenic belt lies between the North China plate and the Qaidam mi-croplates, and resulted from the collision among the Qaidam microplate, mid-Qilian block and the North China plate. The or...The Caledonian North Qilian orogenic belt lies between the North China plate and the Qaidam mi-croplates, and resulted from the collision among the Qaidam microplate, mid-Qilian block and the North China plate. The orogen initiated from the rifting of the Late Proterozoic Rodinia, and then it experi-enced stages of Cambrian rift basin and Ordovician archipelagic oceanic basin, and foreland basin during Silurian to Early-Middle Devonian. The average ratios of Al/(Al+Fe+Mn), Al/(Al+Fe), δ Ce, Lan/Ybn and Lan/Cen from cherts of Cambrian Heicigou Formation are 0.797, 0.627, 1.114, 0.994 and 1.034 re-spectively. In the NAS standardized REE distribution pattern, the cherts from Xiangqianshan is slightly HREE enriched, and the cherts from Ganluci and Shiqingdong are plane. All of these features indicated that Cambrian cherts of the Heicigou Formation originated from a continental margin rift background. On the contrary, the average ratios of Al/(Al+Fe+Mn), Al/(Al+Fe), δ Ce, Lan/Ybn, Lan/Cen of the Ordovician chert from Dakecha, Cuijiadun, Shihuigou, Laohushan, Heicigou, Maomaoshan, Bianmagou, Da-chadaban, Baiquanmen, Jiugequan and Angzanggou, are respectively 0.72, 0.58, 0.99, 1.09 and 0.96 respectively. Their NAS standardized REE distribution patterns of most Ordovician cherts are plane mode or slightly HREE enriched. The REE distribution pattern of few samples of cherts are slightly LREE enriched. Characteristics of sedimentary geochemistry and tectonic evolution demonstrated that the Cambrian-Ordovician cherts, associated with rift, oceanic, island arc and back-arc volcanic rocks, was not formed in a typical abyssal oceanic basin or mid-oceanic ridge. On the contrary, they formed in a deepwater basin of continental margin or a archipelagic ocean tectonic setting. Several Early Paleo-zoic ophiolite belts in North Qilian and adjacent periphery Qaidam microplate imply that an archipelagic ocean during Ordovician existed in the east of Pro-Tethys.展开更多
Major and trace elements are presented for the late Paleozoic radiolarian cherts, which were spatially associated with the NE Jiangxi ophiolite melange. These chert samples show relatively low SiO2 (78.40%-89.28%) and...Major and trace elements are presented for the late Paleozoic radiolarian cherts, which were spatially associated with the NE Jiangxi ophiolite melange. These chert samples show relatively low SiO2 (78.40%-89.28%) and high Al2O3 (3.42%-11.02%). Low Si/Al ratios (6.3-23) and tight negative correlation between Si/Al and Al2O3 of the samples indicate that they are muddy cherts containing high and variable contents of pelitic detritus. Geochemically, they are characterized by Al2O3/(Al2O3+Fe2O3) = 0.51-0.90, shale-normalized Lan/Cen = 0.76-1.11, Ce/Ce* = 0.91-1.22, V<20μg/g, V/Y<2.6 and Ti/V>40, resembling those of cherts formed in the continental margin regimes. It is therefore concluded that these late Paleozoic radiolarian muddy cherts were most likely formed in a continental margin regime, and not genetically related to the ophiolite suite in NE Jiangxi. It is also unlikely that an oceanic basin existed between the Yangtze and Cathaysia blocks during the late Paleozoic.展开更多
The first data of geochemical study of the Benevka Section cherty rocks belonging to the Taukha terrane of the Sikhote-Alin Late Jurassic-Early Cretaceous accretionary prism, Russia Far East are presented. These data ...The first data of geochemical study of the Benevka Section cherty rocks belonging to the Taukha terrane of the Sikhote-Alin Late Jurassic-Early Cretaceous accretionary prism, Russia Far East are presented. These data demonstrate essential distinctions of major, trace and rare earth element concentrations in different parts of the measured stratigraphic section. The lower chert horizons exhibit high Fe2O3 and MnO contents, low concentrations of Al2O3 and TiO2, relatively high V/Y ratio, and extremely low value of negative Ce anomaly. In contrast the upper horizons composed of clayey cherts and siliceous mudstones are characterized by high Al2O3, TiO2 and K2O contents, low Fe2O3 and MnO values, low V/Y ratio, and slightly negative Ce anomaly. In the middle part of the Benevka Section, in which cherts gradually changed to clayey cherts, intermediate geochemical characteristics are present. Based on these data obtained the depositional environments correspond to proximal to the spreding ridge, open-ocean and near continental margin regimes were successfully reconstructed from bottom to top of the Benevka Section, that indicate that significant horizontal movement took place of the sea-floor, on which the cherts were deposited.展开更多
Paleozoic cherts from the Mianl and the Erlangping ophiolite zones of the Qinling orogenic belt are characterized by low Si/Al ratios (52.14-683.52 in the Mianle cherts, 12.29-58.62 in the Erlangping cherts), Fe2O3 (0...Paleozoic cherts from the Mianl and the Erlangping ophiolite zones of the Qinling orogenic belt are characterized by low Si/Al ratios (52.14-683.52 in the Mianle cherts, 12.29-58.62 in the Erlangping cherts), Fe2O3 (0.01-0.35 and 0.02-1.24) and high Al2O3/(Al2O3+Fe2O3) ratios (0.82-0.99 and 0.83-0.99). The negative correlation between Si2O and Al2O3 in the cherts reflects the important role of terrigenous components. The Erlangping cherts have Lan/Cen=0.9-1.15 and Ce/Ce*=0.95-1.15 with low contents of V, Ni and Cu, consistent with those of cherts forming on the continental margin. In contrast, the Ce/Ce* ratios of the Mianle cherts range from 0.71 to 1.18 and Lan/Cen from 0.88 to 1.43 with slightly high V, Ni and Cu, which are similar to cherts found in the mid-ocean ridges and pelagic basins. Combined with the features of basic lavas associated with the cherts, it is suggested that during the Paleozoic, when the back-arc basin represented by the Erlangping ophiolite commenced shrinking in size in the mid-Ordovician, the southern Qinling was still in an extensional regime and finally grew into a new limited oceanic basin in the early Carboniferous.展开更多
At Xiaodong and Bancheng counties of Qinzhou area, SE Guangxi, there is a suite of strata which is composed of various cherts, siliceous shales and mudstone, about several ten square kilometers in area, distributed NE...At Xiaodong and Bancheng counties of Qinzhou area, SE Guangxi, there is a suite of strata which is composed of various cherts, siliceous shales and mudstone, about several ten square kilometers in area, distributed NEE in the shape of bands. The two granite belts separated from adjacent depositional region lie to the northwest and southeast sides of these cherts which are unconformably covered by the展开更多
Geode, boudinage, and undulation structures are widely distributed in the siliceous beds of the Upper Cretaceous/Tertiary rocks in Jordan. Their formation was attributed to tectonic forces, syngenetic processes, organ...Geode, boudinage, and undulation structures are widely distributed in the siliceous beds of the Upper Cretaceous/Tertiary rocks in Jordan. Their formation was attributed to tectonic forces, syngenetic processes, organic disintegration processes, subaquatic gliding, compaction and settlement, and meteoritic impacts. In this work, the structural features in the siliceous beds of Jordan are attributed to an interplay of load and directed pressures, and mineralogical transformation processes (opal-A to opal-CT to quartz), governed by pH changes. Tectonic directed pressure was acting in an ESE-WSW direction and is common in the silicified limestone of Upper Cretaceous.展开更多
Here we report a detailed trace element study of the cherts from Liuchapo Formation, which is a terminal Ediacaran (551-542 Ma) succession in South China deposited in deep-water basinal setting. The REE of Liuchapo ch...Here we report a detailed trace element study of the cherts from Liuchapo Formation, which is a terminal Ediacaran (551-542 Ma) succession in South China deposited in deep-water basinal setting. The REE of Liuchapo cherts shows similar features as observed for anoxic modern seawater (but not for hydrothermal fluids), characterized by positive La anomaly (LaN/CeN = 0.83–1.91, average 1.37), moderately negative Ce anomaly (0.53–1.1, average 0.73), positive Gd anomaly (average 1.08), positive Y anomaly (average 1.21), and depleted LREE and MREE. In addition, the Liuchapo cherts have low ΣREE (3.36–56.13 ppm, average 20.6 ppm), low Al2O3, Ti, Th and Zr concentrations, and high Y/Ho ratios (up to 43.9). The redox-sensitive trace elements concentrations in the cherts do not correlate with detrital input proxies. All of these features suggest that the redox-sensitive trace elements in the cherts were authigenically concentrated in water column and their concentrations thus are excellent indicators of ancient redox conditions. Very low Th/U ratios, high V/(V+Ni) and Fe?/Al ratios, enrichments of redox-sensitive trace elements (U, V, Mo), and low concentration of Mn in the cherts imply anoxia in the deep seawater. Our data reveal that the terminal Ediacaran ocean was not completely oxidized and the deep ocean was still anoxic, at least in South China. We propose that although the oxidative events existed in the terminal Ediacaran oceans, decomposition of organic matter prolonged anoxia in the deep ocean.展开更多
Geochemical analysis reveals that Middle Triassic radiolarian cherts from northern Thailand,including Chiang Dao,Lamphun and Den Chai,are of biogenic origin.These cherts present slightly high SiO2 content which was po...Geochemical analysis reveals that Middle Triassic radiolarian cherts from northern Thailand,including Chiang Dao,Lamphun and Den Chai,are of biogenic origin.These cherts present slightly high SiO2 content which was possibly modified by diagenetic alteration and migration processes as indicated by negative correlation between SiO2 and most of the other major elements.The relatively high content of Cr,Zr,Hf,Rb and Th and high positive correlation of these elements with Al and Ti from the majority of cherts suggest a close relation to terrigenous component.The Ce anomaly（Ce/Ce＊） with geometric means ranging from 0.85 to 0.93 is compatible with that of continental margin composi-tion（0.67-1.52） from Murray et al.（1990） which is also consistent with low Eu anomalies（Eu/Eu＊,0.91-0.94）.Moreover,the slightly low ratios of La and Ce NASC normalized（Lan/Cen,0.91-0.94） and the low LREE and HREE ratios in most of our samples（Lan/Ybn,0.62-0.85） are in agreement with the continental margin.The result from Lan/Cen vs.Al2O3/（Al2O3＋Fe2O3） discrimination diagrams also supports the continental margin（residual basin,s.str.） interpretation.These geochemical results are compatible with geological evidence,which suggest that during the Middle Triassic,radio-larian cherts were deposited within a deeper part of a residual basin in which an accommoda-tion space was possibly controlled by faults un-der extensional regime subsequent to Late Variscan（Permian） orogeny.Paleogeographi-cally,the main Paleotethys which closed during Late Triassic should be located further to the west of these study localities.This scenario is in agreement with the current view of the Tethys in this part of the world.展开更多
Four layers of cherts were found for the first time in the Yangla copper deposit, western Yunnan Province. The cherts possessed the following geochemical characteristics: 1 Low TiO2 and Al2O3 contents, but high ore-fo...Four layers of cherts were found for the first time in the Yangla copper deposit, western Yunnan Province. The cherts possessed the following geochemical characteristics: 1 Low TiO2 and Al2O3 contents, but high ore-forming element (e.g. Cu, Au, Ag) contents; 2 low total REE contents and clear negative Eu anomalies when normalized to chondrite similar to the REE contents and distribution patterns of associated massive sulfide ores; 3 silicon isotopic compositions of cherts in the Yangla deposit being the same as cherts and geyserite of hot-water sedimentary origin; 4 lead and sulfur isotopic compositions of cherts in the Yangla deposit being similar to those of the massive sulfide ores in the Yangla deposit; 5 Rb-Sr isochron age of cherts from the Yangla deposit being identical with that of host strata. Hence, we conclude that the cherts in the Yangla deposit are of hot-water sedimentary origin, which have a close relationship with the massive sulfide ores. The discovery of hydrothermal cherts from the Yangla copper deposit provides further evidence for the hydrothermal exhalative origin of the massive sulfide deposits.展开更多
The existence of the Paleo-Tethys in western Yunnan has generally beenrecognized as the progress in geological studies there during the last decade.Recently, Japanese geologists proposed that the Carboniferous-Permian...The existence of the Paleo-Tethys in western Yunnan has generally beenrecognized as the progress in geological studies there during the last decade.Recently, Japanese geologists proposed that the Carboniferous-Permian volcanic arcand seamount rock associations in Japan originated from the low-latitudeProto-Pacific realm, and their northward migration seems to have started in or afterthe Triassic times. Then the very interesting problems are facing us: how does展开更多
基金supported by National 305 Project(2007BAB25B05,2011BAB06B02- 04)NSFC grant(40925006,40772045,40572047)111 Project(B07011)
文摘In the southwestern Tianshan,the geologic ages of many strata and their depositional environments are still poorly constrained because of complex structures.The Mada＇er area is located in the Kuokesaleling belt,Southwestern Tianshan.The cherts from the former Wupata＇erkan Group contain abundant radiolarian fossils,including 10 species which are identified as late Devonian to early Carboniferous in age.Eleven chert samples have SiO_2 contents ranging from 88.80 wt%to 93.28 wt%, and 2.02 wt%to 3.72 wt%for Al_2O_3.The SiO_2/Al_2O_3 ratios of all samples vary from 23.84 to 46.11, much lower than those of the pure cherts（80-1400）.These values suggest that the cherts contain high ratios of terrigenous materials.The Al_2O_3/（Al_2O_3＋Fe_2O_3）ratios vary between 0.64 and 0.77,whereas V and Cu concentrations range from 10.92 ppm to 26.7 ppm and from 2.15 ppm to 34.1 ppm respectively. The Ti/V ratios vary from 25.53 to 44.93.The total REE concentrations of the cherts are between 30.78 ppm and 59.26 ppm,averaging 45.46 ppm.The（La/Ce）_N ratios range from 0.81 to 1.12,and 0.88-1.33 for（La/Yb）_N,averaging 1.09,which suggests a continental margin environment.Consequently,it is inferred that the cherts formed in a residual sea environment during the late Devonian to early Carboniferous time,which suggests that the collision between the Karakum-Tarim and Kazakhstan-Junggar plates did not occur at the time.In addition,the regional geological information indicates that the study area experienced a post-collision stage during the early Permian,and thus it is likely that the collision between the two plates took place in the late Carboniferous.
基金This study was supported by China National Natural Science Foundation Grant 4880131
文摘Bedded cherts occurring in the Early Permian Gufeng Formation in the Lower Yangtze region, eastern China, are nearly 20-80 m in thickness and contain varying amounts of radiolarians and sponge spicules. There are three types of section for the Gufeng Formation: chert, carbonate and chert-carbonate mixing types. Bedded cherts mainly occur in the first and third types of section. The depocentres of silica are marked by some small (not larger than a few thousand square kilometres in area) rhombic or elliptic hollows and their formation was controlled by faults. Argillaceous volcanic rocks in cherts occur as numerous thin layers. Bedded cherts are characterized by higher Fe and lower Al contents, enrichment in such trace elements as As, Sb, Bi, Ga, Au, Ag and Cr, lower total REE abundance, negative Ce anomaly and varying degrees of HREE enrichment. These characteristics are important evidence for hydrothermal cherts. Minor amounts of substances of non-hydrothermal origin are mixed in the cherts. The temperatures of sea water range approximately from a few tens of degrees to over 160℃. during the deposition of siliceous sediments.
文摘Granitoids,volcanic rocks and cherts,mainly of early Paleozoic,in northern Altyn Tagh(Fig.1) are analyzed here for the purpose of the determination of the tectonic environments and their development since Early Paleozoic.(1) Granitoids\ The collection of 128 main\|element petrochemical data from North Altyn Tagh area shows that most of the granitoids here are granites and granodiorites of calc\|alkaline series.They consist mainly of metaluminous and peraluminous in Shand’s index,and only one of them,which belongs to Mesozoic,is peralkaline.Most of the granitoids plot in the IAG+CAG+CCG fields in the Maniar and Piccoli’s diagrams (1989) for tectonic discrimination of granitoids.IAG (Island arc granitoids)and CAG (Continental arc granitoids) can be distinguished for Early and Late Paleozoic granitoids,and maybe some CCG (Continental collision granitoids) for Early Paleozoic.Granitoids of Mesozoic and Cenozoic inherited the characteristics of those of Paleozoic.Destructive active plate margin (pre\|plate collision)and Anatectic magmatism(syn\|orogenic,S\|type granites)are distinguished (Fig.2)for Paleozoic granitoids using de la Roche R 1\| R 2 multicationic diagram (Batchelor and Bowden,1985).Mesozoic and Cenozoic granitoids inherited the characteristics of Anatectic magmatism (syn\|orogenic)of these early ones,and post\|orogenic(A\|type)granites occurred in Mesozoic.The mechanism for magma formation is mainly partial melting.
基金This study is a project (No.489700115)supported by the National Natural Science Foundation of China
文摘Through microscopic and SEM observations and X-ray diffraction and energy spectrum analyses, the oldest sponge spicules and radiolarians have been found in bedded cherts and it is proposed that microbial activity of algae and bacteria played an important role in silica precipitation. The microbial activity resulted in the formation of a series of single crystals and crystal aggregates with peculiar shapes. Biogenic single crystals usually occur in spherical, rod, platy, colloidal and granular forms. The most common aggregates are framboidal aggregates composed of dense spherical quartz crystals, which are considered to be of typical bacterial origin. Other aggregates, such as favositic, double-spherical, chain-spherica'l and coccoid ones, are considered to be probably the result of silicification of algal cells and mainly of primary sedimentary origin. This paper also discusses the ultrastructural features and diagnostic indications of jasper rocks formed in a submarine hydrothermal environment and cherts formed by replacement and diagenetic recrystallisation.
基金the National NaturalScience Foundation of China(Project Nos.40172042,40472064 , 40228004)the Ministry of Science and Technology of China(Project No.G1999043304) the Special Foundation for Doctor Subjects in China(Project No.200049107).
文摘The sedimentary sequence of the Lower Cambrian is a key interval to reveal the early evolution history of the Earth and there occur widespread cherts worldwide. These cherts possibly carry important information to decipher the breakup process of the Rodinia Continent. Black rock series at the bottom of the Lower Cambrian in the Northern Tarim Basin, China, is composed of black shales interbedded with thin-bedded cherts. Ten chert samples were systematically collected from two outcrops at Xiaoerbulak and Sogatbulak, 8.8 and 7.5 m thick respectively. The cherts were crushed, and were analyzed for trace element and rare earth concentrations. Trace elements such as V, Cu, Zn, U, Pb, Ba, Cd, Ag, Mo, As and Sb are highly enriched, and others such as Rb, Zr, Cs, Hf, Ta, W, Tl, Bi and Th are highly depleted in the cherts. These trace element patterns suggest that the cherts may be of deep crustal origin. The low ratios of Th/U and Rb/Sr further suggest that the cherts are of earth interior sources or received hydrothermal input during their deposition. Chondrite- normalized Eu/Eu* value markedly decreases upward in the section from 5.54 at the lowermost to 0.73 at the top, and NASC-normalized Eu/Eu* value decreases from 8.05 to 1.03. The relatively high Eu/Eu* ratio for the cherts from the northern Tarim Basin is most likely due to a hydrothermal input (e.g., Eu/Eu* ~10). The systematic decrease of Eu/Eu* ratio from the bottom to the top of the section reflects that the hydrothermal input is the largest in the lowermost portion of the section and gradually decreases upward. The chondrite-normalized Ce/Ce* ratio ranges from 0.42 to 0.83, with an average of 0.60. North American Shale Composite (NASC)-normalized Ce/Ce* ratio ranges from 0.42 to 0.79, with an average of 0.57. Negative Ce anomalies are distinct. ΣREEs in the cherts generally increase from 10.50 ppm at the bottom to 35.97 ppm at the top of the sampled section. NASC-normalized (La/Lu) N ratio decreases from 2.72 at the bottom to 0.67 at the top. NASC-normalized (La/Ce) N ratio increases from 1.36 at the bottom to 3.13 at the top. These REE patterns are very similar to those for the cherts deposited in the pelagic ocean-basin floor in the Franciscan Complex exposed at Marin Headlands, California (F-MH chert) (Murray et al., 1991). These geochemical signatures are inconsistent with our previous sedimentological data, which suggests a continental shelf setting. Based on multiple lines of evidence including high TOC content in the concomitant black shales, phosphorite at the bottom of black rock series, regional rise of sea level, and beginning of the southern Tianshan Ocean geotectonic cycle, the authors infer that the hydrothermal fluid was carried to the continental shelf by upwelling from a divergent pelagic ocean floor setting.
文摘Chert clast-bearing epi-continental neritic-terrestrial Jurassic and Cretaceous sequences are sporadically distributed in southwestern Japan. Typical and geological entities are the Torinosu Group and Tetori Group. Radiolarian dating of chert clasts enables us to discuss denudation stages of mid Mesozoic accretionary complexes. Chert clast-dominated conglomerate can be used for identifying marine beds within terrestrial sequences.
基金Projects(41273040,41303025)supported by the National Natural Science Foundation of China
文摘The Xialu chert, which contains abundant biological information, were investigated by major element analysis, micro-Raman, SEM and EPMA. The results show that SiO2 content of chert is 84.12%-93.08%, averaging 89.84%. The close packed structures of low degree crystallinity of quartz indicate the hydrothermal origin. SiO2 of modern hot springs exhibit loose silica pellets and nodular, beaded structures. Under polarization microscope, the presence of biological skeleton structures indicate that biological activities are involved in the hydrothermal deposition, which correspond to the geochemical characteristics: w(SiO2)/ w(K2O+Na2O), w(SiO2)/w(Al2O3) and w(SiO2)/w(MgO), with average values of 295.29, 68.88 and 284.45, respectively. SiO2 is enriched in the organism(radiolarian) centers, the degree order of SiO2 within the biologic structures is much higher than that of outside. The impurity minerals albites are formed earlier than the original deposition. Kaolinites, feldspars and mixture of organic materials display lower degree of crystallinities and accumulate as vermicular aggregates.
文摘Rock-magnetic methods prove that the cultures in Moravia (Europe) 15,000 - 11,500 years ago might know the technique how to enhance knapping properties of Jurassic chert and Cretaceous flint in order to make stone tools.
基金financially supported by the National Natural Science Foundation of China (Grant No.42207293)the Natural Science Foundation of Anhui Province (Grant No.2208085QD110)+4 种基金the Key Scientific Research Foundation of the Education Department of Anhui Province (Grant No.2022AH051748,KJ2021ZD0125)the Key Programs of the Tongling University (Grant No.2021tlxy ZD03)the Tongling University Starting Research Project (Grant No.2021tlxyrc16)the Anhui University Excellent Research and Innovation Project (Grant No.2022AH010094)the National College Student Innovation and Entrepreneurship Training Program of Tongling University (Grant No.202310383117)。
文摘The extensive bedded cherts deposited during the Ediacaran-Cambrian(E-C)transition period play a crucial role in understanding the geological evolution of this period,yet the origin of these cherts remains disputed.Here,we present new geochemical data for cherts of the Piyuancun(PYC)Formation deposited during the Late Ediacaran and the Hetang(HT)Formation deposited during the Early Cambrian in northern Jiangxi region,Lower Yangtze region,South China.The PYC cherts contain a small amount of monaxons sponge spicules and radiolarian fragments,while the HT cherts lack siliceous organism evidence.Major and trace element analysis,coupled with discriminant diagrams,indicate a possible shift in redox conditions of seawater during the E-C transition in the northern Jiangxi region.The shift suggests a change from weakly-moderately restricted euxinic conditions to strongly restricted euxinic conditions.Furthermore,the location of both cherts are distant from the source area of siliceous organisms.Fossil evidence,as well as the values of Fe/Ti and Fe/(Mn+Ti),Eu anomalies,Post-Archean Australian Shale(PAAS)normalized REE+Y patterns,and various discriminant diagrams,support the conclusion that the PYC and HT cherts originated primarily from direct seawater precipitation,with the PYC cherts exhibiting weak hydrothermal evidence.Upwelling contributes to the formation of HT cherts and organic matter(OM)accumulation.Ocean acidification,triggered by OM degradation and biodegradation processes during the E-C transition period,leads to the extensive silica precipitation and preservation.These results enhance our understanding of the geological processes during the E-C transition.
基金the Czech Science Foundation through project no.20-13644S(to Lukas Ackerman)The Scientific Program RV067985831 of the Institute of Geology of the Czech Academy of Sciences+3 种基金Charles University through Cooperation Programme(Research Area GEOL)Center for Geosphere Dynamics(UNCE/SCI/006)Operational Programme Prague-Competitiveness(Project CZ.2.16/3.1.00/21516)Strategic Research Plan of the Czech Geological Survey(DKRV02018-2022)
文摘The Ediacaran to early Cambrian Blovice accretionary complex,Bohemian Massif,hosts abundant chert bodies that formed on an oceanic plate and were involved in subduction beneath the northern margin of Gondwana.Field relationships of cherts to their host,their microstructure and elemental as well as isotopic compositions revealed diverse processes of chert petrogenesis reflecting depositional environment and position on the oceanic plate.The deep-water cherts formed through a hydrothermal precipitation of silica-rich gels on outer trench swell of the subducted slab with none or only minor addition of terrigenous material.On the contrary,the shallow-water cherts formed in lagoons on seamount slopes,and at least some of them represent a product of hydrothermal replacement of former carbonate and/or evaporite precursors.For both chert types,the hydrothermal fluids were of low temperature and continuous pervasive hydrothermal alteration of oceanic crust,together with an elevated Si content in Neoproterozoic seawater,served as the major source of silica.On the other hand,minor carbon enrichment in chert is mostly linked to variable incorporation of organic matter that was deposited on the seafloor.Rare earth element(REE)systematics of the cherts indicate predominantly oxygenated environment for the shallow-water cherts whereas the deep-water cherts were deposited in diverse redox conditions,depending on their distance from hydrothermal vent.Using these data,we demonstrate that the cherts once formed a part of Ocean Plate Stratigraphy(OPS)now dismembered and mixed with terrigenous siliciclastic material to form OPS mélanges.Combining our data with those from the existing literature,we show that cherts can serve as significant markers of OPS since the Archean,recording a complex interplay between seafloor-related volcanic(production of MORB-and OIB-like magmas)and sedimentary processes,hydrothermal activity at mid-ocean ridges and seamount chains as well as at outer slopes of subducting slabs.However,the cherts also exhibit a secular change in composition and petrogenesis most profoundly affected by an overturn in seawater silica cycle across the Precambrian-Phanerozoic boundary.
基金Sponsored by the National Natural Science Foundation of China (Grant Nos. 40672080 and 40621002)the Developing Plan of Innovation Group of Ministry of Education of China (Grant No. IRT0546)
文摘The Caledonian North Qilian orogenic belt lies between the North China plate and the Qaidam mi-croplates, and resulted from the collision among the Qaidam microplate, mid-Qilian block and the North China plate. The orogen initiated from the rifting of the Late Proterozoic Rodinia, and then it experi-enced stages of Cambrian rift basin and Ordovician archipelagic oceanic basin, and foreland basin during Silurian to Early-Middle Devonian. The average ratios of Al/(Al+Fe+Mn), Al/(Al+Fe), δ Ce, Lan/Ybn and Lan/Cen from cherts of Cambrian Heicigou Formation are 0.797, 0.627, 1.114, 0.994 and 1.034 re-spectively. In the NAS standardized REE distribution pattern, the cherts from Xiangqianshan is slightly HREE enriched, and the cherts from Ganluci and Shiqingdong are plane. All of these features indicated that Cambrian cherts of the Heicigou Formation originated from a continental margin rift background. On the contrary, the average ratios of Al/(Al+Fe+Mn), Al/(Al+Fe), δ Ce, Lan/Ybn, Lan/Cen of the Ordovician chert from Dakecha, Cuijiadun, Shihuigou, Laohushan, Heicigou, Maomaoshan, Bianmagou, Da-chadaban, Baiquanmen, Jiugequan and Angzanggou, are respectively 0.72, 0.58, 0.99, 1.09 and 0.96 respectively. Their NAS standardized REE distribution patterns of most Ordovician cherts are plane mode or slightly HREE enriched. The REE distribution pattern of few samples of cherts are slightly LREE enriched. Characteristics of sedimentary geochemistry and tectonic evolution demonstrated that the Cambrian-Ordovician cherts, associated with rift, oceanic, island arc and back-arc volcanic rocks, was not formed in a typical abyssal oceanic basin or mid-oceanic ridge. On the contrary, they formed in a deepwater basin of continental margin or a archipelagic ocean tectonic setting. Several Early Paleo-zoic ophiolite belts in North Qilian and adjacent periphery Qaidam microplate imply that an archipelagic ocean during Ordovician existed in the east of Pro-Tethys.
文摘Major and trace elements are presented for the late Paleozoic radiolarian cherts, which were spatially associated with the NE Jiangxi ophiolite melange. These chert samples show relatively low SiO2 (78.40%-89.28%) and high Al2O3 (3.42%-11.02%). Low Si/Al ratios (6.3-23) and tight negative correlation between Si/Al and Al2O3 of the samples indicate that they are muddy cherts containing high and variable contents of pelitic detritus. Geochemically, they are characterized by Al2O3/(Al2O3+Fe2O3) = 0.51-0.90, shale-normalized Lan/Cen = 0.76-1.11, Ce/Ce* = 0.91-1.22, V<20μg/g, V/Y<2.6 and Ti/V>40, resembling those of cherts formed in the continental margin regimes. It is therefore concluded that these late Paleozoic radiolarian muddy cherts were most likely formed in a continental margin regime, and not genetically related to the ophiolite suite in NE Jiangxi. It is also unlikely that an oceanic basin existed between the Yangtze and Cathaysia blocks during the late Paleozoic.
基金financially supported by the President of the Russian Federation (No. 1159.2014.5)the Far Eastern Branch of the Russian Academy of Sciences (No. 15-1-2-013-2)
文摘The first data of geochemical study of the Benevka Section cherty rocks belonging to the Taukha terrane of the Sikhote-Alin Late Jurassic-Early Cretaceous accretionary prism, Russia Far East are presented. These data demonstrate essential distinctions of major, trace and rare earth element concentrations in different parts of the measured stratigraphic section. The lower chert horizons exhibit high Fe2O3 and MnO contents, low concentrations of Al2O3 and TiO2, relatively high V/Y ratio, and extremely low value of negative Ce anomaly. In contrast the upper horizons composed of clayey cherts and siliceous mudstones are characterized by high Al2O3, TiO2 and K2O contents, low Fe2O3 and MnO values, low V/Y ratio, and slightly negative Ce anomaly. In the middle part of the Benevka Section, in which cherts gradually changed to clayey cherts, intermediate geochemical characteristics are present. Based on these data obtained the depositional environments correspond to proximal to the spreding ridge, open-ocean and near continental margin regimes were successfully reconstructed from bottom to top of the Benevka Section, that indicate that significant horizontal movement took place of the sea-floor, on which the cherts were deposited.
基金This project was jointly supported by the NationalNatural Science Foundation of China (Grant Nos. 40372039,40032010-C, 40133020 and 49732080)the Foundations of Senior Visiting Scholarship of Colleges in Western areas and Backbone Teacher of Ministry of Education, China (Grant No. 2000-143).
文摘Paleozoic cherts from the Mianl and the Erlangping ophiolite zones of the Qinling orogenic belt are characterized by low Si/Al ratios (52.14-683.52 in the Mianle cherts, 12.29-58.62 in the Erlangping cherts), Fe2O3 (0.01-0.35 and 0.02-1.24) and high Al2O3/(Al2O3+Fe2O3) ratios (0.82-0.99 and 0.83-0.99). The negative correlation between Si2O and Al2O3 in the cherts reflects the important role of terrigenous components. The Erlangping cherts have Lan/Cen=0.9-1.15 and Ce/Ce*=0.95-1.15 with low contents of V, Ni and Cu, consistent with those of cherts forming on the continental margin. In contrast, the Ce/Ce* ratios of the Mianle cherts range from 0.71 to 1.18 and Lan/Cen from 0.88 to 1.43 with slightly high V, Ni and Cu, which are similar to cherts found in the mid-ocean ridges and pelagic basins. Combined with the features of basic lavas associated with the cherts, it is suggested that during the Paleozoic, when the back-arc basin represented by the Erlangping ophiolite commenced shrinking in size in the mid-Ordovician, the southern Qinling was still in an extensional regime and finally grew into a new limited oceanic basin in the early Carboniferous.
基金Project supported by the National Natural Science Foundation of Chinapartly by the Laboratory Foundation of Palaeobiology & Stratigraphy, Nanjing Institute of Geology and Palaeontology, Academia Sinica
文摘At Xiaodong and Bancheng counties of Qinzhou area, SE Guangxi, there is a suite of strata which is composed of various cherts, siliceous shales and mudstone, about several ten square kilometers in area, distributed NEE in the shape of bands. The two granite belts separated from adjacent depositional region lie to the northwest and southeast sides of these cherts which are unconformably covered by the
文摘Geode, boudinage, and undulation structures are widely distributed in the siliceous beds of the Upper Cretaceous/Tertiary rocks in Jordan. Their formation was attributed to tectonic forces, syngenetic processes, organic disintegration processes, subaquatic gliding, compaction and settlement, and meteoritic impacts. In this work, the structural features in the siliceous beds of Jordan are attributed to an interplay of load and directed pressures, and mineralogical transformation processes (opal-A to opal-CT to quartz), governed by pH changes. Tectonic directed pressure was acting in an ESE-WSW direction and is common in the silicified limestone of Upper Cretaceous.
基金National Natural Science Foundation of China (Grants Nos. 40532012, 40873007, 40603021)Chinese Academy of Sciences (Grant No. KZCX3- SW-141)
文摘Here we report a detailed trace element study of the cherts from Liuchapo Formation, which is a terminal Ediacaran (551-542 Ma) succession in South China deposited in deep-water basinal setting. The REE of Liuchapo cherts shows similar features as observed for anoxic modern seawater (but not for hydrothermal fluids), characterized by positive La anomaly (LaN/CeN = 0.83–1.91, average 1.37), moderately negative Ce anomaly (0.53–1.1, average 0.73), positive Gd anomaly (average 1.08), positive Y anomaly (average 1.21), and depleted LREE and MREE. In addition, the Liuchapo cherts have low ΣREE (3.36–56.13 ppm, average 20.6 ppm), low Al2O3, Ti, Th and Zr concentrations, and high Y/Ho ratios (up to 43.9). The redox-sensitive trace elements concentrations in the cherts do not correlate with detrital input proxies. All of these features suggest that the redox-sensitive trace elements in the cherts were authigenically concentrated in water column and their concentrations thus are excellent indicators of ancient redox conditions. Very low Th/U ratios, high V/(V+Ni) and Fe?/Al ratios, enrichments of redox-sensitive trace elements (U, V, Mo), and low concentration of Mn in the cherts imply anoxia in the deep seawater. Our data reveal that the terminal Ediacaran ocean was not completely oxidized and the deep ocean was still anoxic, at least in South China. We propose that although the oxidative events existed in the terminal Ediacaran oceans, decomposition of organic matter prolonged anoxia in the deep ocean.
基金supported mainly by the National Natural Science Foundation of China (No. 40772143)the Special Fund of the State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciencesthe Faculty of Science,Mahasarakham University
文摘Geochemical analysis reveals that Middle Triassic radiolarian cherts from northern Thailand,including Chiang Dao,Lamphun and Den Chai,are of biogenic origin.These cherts present slightly high SiO2 content which was possibly modified by diagenetic alteration and migration processes as indicated by negative correlation between SiO2 and most of the other major elements.The relatively high content of Cr,Zr,Hf,Rb and Th and high positive correlation of these elements with Al and Ti from the majority of cherts suggest a close relation to terrigenous component.The Ce anomaly（Ce/Ce＊） with geometric means ranging from 0.85 to 0.93 is compatible with that of continental margin composi-tion（0.67-1.52） from Murray et al.（1990） which is also consistent with low Eu anomalies（Eu/Eu＊,0.91-0.94）.Moreover,the slightly low ratios of La and Ce NASC normalized（Lan/Cen,0.91-0.94） and the low LREE and HREE ratios in most of our samples（Lan/Ybn,0.62-0.85） are in agreement with the continental margin.The result from Lan/Cen vs.Al2O3/（Al2O3＋Fe2O3） discrimination diagrams also supports the continental margin（residual basin,s.str.） interpretation.These geochemical results are compatible with geological evidence,which suggest that during the Middle Triassic,radio-larian cherts were deposited within a deeper part of a residual basin in which an accommoda-tion space was possibly controlled by faults un-der extensional regime subsequent to Late Variscan（Permian） orogeny.Paleogeographi-cally,the main Paleotethys which closed during Late Triassic should be located further to the west of these study localities.This scenario is in agreement with the current view of the Tethys in this part of the world.
基金This work was jointly supported by key project of Chinese Academy of Sciences (KZ952-S1-407) state Key Project of China (G1999043208 and 1999043210) China Postdoctoral Science Foundation.
文摘Four layers of cherts were found for the first time in the Yangla copper deposit, western Yunnan Province. The cherts possessed the following geochemical characteristics: 1 Low TiO2 and Al2O3 contents, but high ore-forming element (e.g. Cu, Au, Ag) contents; 2 low total REE contents and clear negative Eu anomalies when normalized to chondrite similar to the REE contents and distribution patterns of associated massive sulfide ores; 3 silicon isotopic compositions of cherts in the Yangla deposit being the same as cherts and geyserite of hot-water sedimentary origin; 4 lead and sulfur isotopic compositions of cherts in the Yangla deposit being similar to those of the massive sulfide ores in the Yangla deposit; 5 Rb-Sr isochron age of cherts from the Yangla deposit being identical with that of host strata. Hence, we conclude that the cherts in the Yangla deposit are of hot-water sedimentary origin, which have a close relationship with the massive sulfide ores. The discovery of hydrothermal cherts from the Yangla copper deposit provides further evidence for the hydrothermal exhalative origin of the massive sulfide deposits.
基金Project supported by the National Natural Science Foundation of China the Laboratory of Lithosphere Tectonic Evolution, Institute of Geology, Academia Sinica.
文摘The existence of the Paleo-Tethys in western Yunnan has generally beenrecognized as the progress in geological studies there during the last decade.Recently, Japanese geologists proposed that the Carboniferous-Permian volcanic arcand seamount rock associations in Japan originated from the low-latitudeProto-Pacific realm, and their northward migration seems to have started in or afterthe Triassic times. Then the very interesting problems are facing us: how does
