The Origin and Geochemical Characteristics of Maokou Formation Silicalites in the Eastern Sichuan Basin
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Silicalites are distributed widely in the Middle Permian Maokou Formation in the eastern Sichuan basin.Detailed observation in the field and the contents of major elements,REE and isotope of silicalites suggest that the Maokou Formation silicalites were hydrothermal origin and were deposited under platform-basin facies environments.The silicalites are characterized by relatively high contents (80.09%～97.91%) of SiO2 and enrichment of Fe and Mn,with a ratio of Al/(Al+Fe+Mn) being 0.30 and average total REE of 8.72×10-6.δCe values range from 0.39 to 0.81 with a negative anomly.δEu of 0.35～5.85,changes from no obvious anomaly to positive anomaly.δ30Si is from 0.2‰ to 1.2‰ that is very similar to that of hydrothermal quartz.δ18O calculation also indicates that the paleotemperatures of the sea water was 34～89℃ during the formation of the silicalites.The formation of hydrothermal silicalites is closely related to volcanic activities and faulting,the mixed fluid of seawater and magmatic fluid was heated and dissolved a large voluminous of SiO2,and other minerals,which erupted along faults in the form of spring and then deposited in adjacent areas.Keywords:
Isotopes of strontium
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The Shuicheng-Ziyun-Nandan rift basin (SZNRB) is located in the northern margin of the YoujiangBasin.During late Paleozoic, the basin was dominated by carbonate platform-basin faices deposition including siliceous rock. Sedimentary assemblage and geochemical features of the siliceous rock are different from that of siliceous rocks in the continental shelf and accretionary complexes.Geochemical analysis for siliceous sediments indicates that siliceous deposits from the bottom of the Upper Devonian Liujiang Formation in the Nandan area and the Lower-Middle Permian Sidazhai Formation in the Hechi area were genetically associated with basaltic volcanic eruption and hydrothermal activity, and showed lower Al/(Al+Fe+Mn) and Al2O3/TiO2 values.The siliceous sediments of the Lower-Middle Permian Sidazhai Formation in the Ziyun area have a Al/(Al+Fe+Mn) value of 0.62±0.16 and Al2O3/TiO2 value of 11.70±4.30, suggesting a pyroclast-bearing nonhydrothemal origin, while the other siliceous sediments were precipitated in normal marineenvironments.During the Late Devonian-Early Carboniferous,siliceous sediments were intercalated with mudstone and limestone, and show moderately negative Ce-anomalies (0.6~0.85mostly) and higher Y/Ho values (30~45 mostly), and low ∑REE+Y values three times lower than PAAS-like composition after deducing the effect of SiO2 dilution. With rifting and expanding of the basin and, siliceous sediments was interbedded with dark thick limestoneto the Early-Middle Permian. The siliceous sediments are characterized by large negative Ce-anomalies(0.06~0.61) and a similar Y/Ho values(40~92) as that in open-ocean seawater, and a high ∑REE+Y value twice higher than PAAS-like composition after deducing the effect of SiO2 dilution.
Late Devonian extinction
Devonian
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There are silicolites in the Guantou formation of the Early Cretaceous basin in Xinchang County, Zhejiang Province. The rock types include massive or lenticular silicolite, stromatolite bearing silicolite and fossil wood silicolite. They are characterized by a higher content of SiO 2 and Fe 2O 3, low content of Al 2O 3 and TiO 2, and small amount of REE with slight Ce depletion. The δ 30 Si values of the silicolites vary from -0.4‰ to -1.5‰, the δ 18 O values are in 18.1‰ to 23.1‰. The silicolites can be identified as hydrothermal sediments based on their geochemical features. The source of silicon and origin of the silicolites were discussed: The water come from the surface and sea penetrates downward and is heated by volcanic activity in a certain depth and extracts SiO 2 and other elements out from the surrounding rocks through which it passes. The SiO 2 rich hydrothermal fluid up flows and deposits, forming massive and lenticular silicolites. The fluid also expounds the palaeo trees and stromatolite bearing limestone to form fossil wood silicolite (petrified wood) and stromatolite bearing silicolite. According to the δ 18 O values of the silicolites calculated from the oxygen isotopic geologic thermometer, their forming temperatures ranges from 70.8℃ to 108.4℃.
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Molybdenite
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Terrigenous sediment
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Abstract Lower Paleozoic sedimentary rocks in the northwestern Tarim basin were strongly altered by complicated geofluids, which resulted in the occurrence of various diagenetic minerals (e.g., dolomite). Here, in situ major, trace, and rare earth element geochemistry of Lower Ordovician diagenetic dolomite grains as well as petrography were performed to unravel the geochemical features, the nature, and origin of the diagenetic fluids. The results indicate that different geochemical information can be detected within a single sample, even within a single dolomite grain. Five generations of diagenetic dolomite have been identified based on geochemical signatures, resulting from four distinct types of diagenetic fluids: (1) HREE enrichment (PAAS‐normalized), low ΣREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to early burial dolomitizing fluids; (2) MREE enrichment, high ΣREE, high Mn, Fe, and low Sr content are likely to be associated with Devonian deep‐circulating crustal hydrothermal fluids; (3) flat or LREE enrichment pattern with obviously positive Eu anomaly is inferred to be linked to Permian magmatic hydrothermal fluids; and (4) flat REE pattern, moderate ΣREE, no Eu anomaly, low Mn, Ba, moderate Fe, and high Sr contents are probably due to late burial dolomitizing fluids. The significances of in situ method demonstrated in this study, compared with the whole rock analysis, include not only contamination‐free analysis but also unraveling the internal geochemical variation within a single sample or a mineral grain. Thus, for the geochemical study of complicated diagenetic process, in situ method should be preferentially considered.
Devonian
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Study on Geochemical Characteristics and Depositional Environment of Pengcuolin Chert,Southern Tibet
The geochemical characteristics and depositional environment of Pengcuolin chert are studied based on the field investigations and analysis on major elements,trace elements and rare earth elements.The massive chert is green and brown,being high in SiO2 content,locally enriched in Fe and Mn,Al/(Al+Fe+Mn) ratio being low.In Fe-Mn-Al triangle diagrams,most chert samples fall into the hydrothermal region.Trace elements such as Sr,Zr,Cu,Zn and Ba are higher and ΣREE is lower,with Ce negative anomaly and Eu anomaly.Their NASC-normalized REE distribution patterns are slightly left-leaning,indicating their hydrothermal origin.In sedimentary environment discrimination diagrams such as 100×Fe2O3/SiO2-100×Al2O3/SiO2,Fe2O3/(100-SiO2)-Al2O3/(100-SiO2) and Fe2O3/TiO2-Al2O3/(Al2O3+Fe2O3),chert samples fall into the areas of the mid-ocean ridges or deep-sea.In parameter variation diagrams,samples of PCL-1,PCL-5 and PCL-7 are closer to the hydrothermal center.It is shown that the Pengcuolin chert,southern Tibet was formed by the hydrothermal systems at convergence and subduction sites of plates.
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Organic-rich source rock was closely related to the development of hydrothermal sedimentation in geochemistry and others. By studying the characteristics of the source rocks of Lower Cambrian Niutitang Formation in Guizhou Province of China, it was found that the strata affected by both terrigenous inputs and hydrothermal sediments. There were a few samples with a low gross amount of REEs, which widely varied in others. The influence of the hydrothermal sedimentation was stronger in Yangtiao and Jiumen sections, while relatively weaker in Lianxing and Wenshui sections. The content of As and Sb enriched in Niutitang Formation, which indicated the existence of the hydrothermal activity. The δEu/Al ratio can be used to distinguish and indicate the intensity of the hydrothermal sedimentation.
Terrigenous sediment
Sedimentation
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Banded iron formation
Terrigenous sediment
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The geochemical researches on the upper Permian cherts of the Dalong Formation in the southwest Guangxi have been conducted.And we have the following findings:the chert is rich in the terrestrial elements(for example Al,Ti etc.),and poor in hydrothermal elements(for example Mn,Fe etc.),with high correlation between the terrestrial elements(R0.90) and low correlation between the hydrothermal elements(R=-0.30);The correlation between the terrestrial elements(Al,Ti,Hf,Zr,Th etc.) and ∑REE is high(R=0.70～0.83);Al-Fe-Mn diagram shows that the cherts is non-hydrothermal type.The characteristics above confirm that terrestrial debris is one of the most important provenances.Considering the truth,the high content of the siliceous bio-debris in the cherts,we prove that the cherts were formed by strong bio-or bio-chemical sedimentation,with lets of terrestrial debris and little hydrothermal and volcanic material interfused.Ce/Ce,(La/Yb)Shale,(La/Ce)Shale,Th/U,Ceanom,∑REE and(La/Ce)Shale-Al2O3/(Al2O3 +Fe2O3) diagram make sure that the cherts are deposited on middle-outer shelf in oxic conditions.
Petrogenesis
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