Influences Of Sedimentary Environments And Volcanic Sources On Diagenetic Alteration Of Volcanic Ashes

Alteration of volcanic ash into altered volcanic ashes is not only a record of volcanic events,but also an important indicator of regional stratigraphic correlation.The characteristics of altered volcanic ashes are mainly determined by the deposition conditions of different sedimentary environments.Therefore,it is of great scientific significance to study the characteristics of volcanic ashes of different sedimentary facies for regional stratigraphic correlation and magmatism properties of volcanic ash clay beds.In the strata near the Permian-Triassic?P-T?boundary in South China,a large number of altered volcanic ashes are developed.It is well developed with marine,marine-terrestrial,terrestrial deposits and good continuity.Typical profiles with PTB boundary and high-precision age division are selected in this paper.They are Xinmin profile of deep-sea facies,Shangsi profile of deep-sea slope facies,Chahe profile of lake facies and Philosophical profile of swamp facies.The altered volcanic clay layer is progressed by X-ray powder diffractometer?XRD?,X-ray fluorescence spectrometer?XRF?and inductively coupled plasma mass spectrometer?LA-ICP-MS?.Mineralogy,geochemistry,isotope composition and zircon U-Pb dating were carried out to explore the influence of sedimentary environment on mineralogy and geochemistry of altered volcanic ashes and its mechanism.Zircon U-Pb dating of volcanic ash clay layers in Shangsi and Chahe profiles shows that the older clay layers near PTB in the two profiles are 260.0+2.4 Ma?SS-1?and 264.0+2.6 Ma?CH-1?,respectively,and the newer clays are 253.6+2.3 Ma?SS-2?and 253.4+1.8 Ma?CH-2?,respectively.This shows a good correspondence between the two profiles,which proves that volcanic works recorded in land and sea profiles are of volcanic origin.Usage basically occurs in the same period.Four layers of volcanic ashes near the P-T boundary were selected in the deep-sea.Xinmin profile.XRD test results show that the mineral composition of each layer of clay rock is basically the same,and all of them become I/S minerals with trace quartz minerals.The ratios of SiO₂/Al₂O₃?2.07-2.11?to MgO/K₂O?0.49-0.54?in clay rocks are close,and the values of?¹⁸O?17.3-18.1?vary slightly,indicating that the sedimentary water environment in Xinmin profile is stable during the process of volcanic ashes deposition and alteration.The trace element distribution curve of clay shows that the enrichment of LREEs and HREEs have slight deficit,and there are obvious negative abnormal chemical activity of Eu and relatively active Sr and Ba elements have strong deficit.Four clay samples were mapped by ¹⁴³Nd/¹⁴⁴Nd isotope and Zr/TiO₂ vs.Nb/Y.The results show that rhyolite and are andesite.Shangsi XRD results show that the mineral compositions of the two volcanic ash layers are mainly illite-montmorillonite mixed clay minerals,and contain calcite,trace quartz and kaolinite.The ratio of SiO₂/Al₂O₃?2.20-2.42?to MgO/K₂O?0.78-0.80?in clay rocks is close,and the value of?¹⁸O?20.5-21.6?varies slightly,indicating that the sedimentary water environment in the process of volcanic ash deposition and alteration in Shangsi section is relatively stable.The distribution curve of trace elements in clay shows that the distribution pattern is similar to that of Xinmin profile,and there are obvious negative differences of Eu and strong deficiencies of Sr and Ba elements.The results of ¹⁴³Nd/¹⁴⁴Nd isotope and Zr/TiO₂ vs.Nb/Y mapping of four clay samples show that the volcanic ashes of rhyolite and rhyolite.It proves that the volcanism recorded by the altered volcanic ashes layers in the two continental and marine profiles occurs basically at the same time.Chahe section all show higher order R3-type structure of illite mixed layer minerals.The illite content?84%-86%?is lower than that in Xinmin section and obviously higher than that in Shangsi section.The ratio of SiO₂/Al₂O₃?1.84-2.66?to MgO/K₂O?0.18-0.24?in clay rocks is close,while the value of?¹⁸O?13.2-13.4?varies slightly,and the ratios are lower than those in the marine Shangsi and Xinmin sections.The normalized partition curves of rare earth element chondrites show slight enrichment of light rare earth elements and relatively gentle distribution patterns of heavy rare earth elements,accompanied by negative Eu anomalies of different degrees.Further mapping of two clay samples by Zr/TiO₂ vs.Nb/Y shows that falls into the andesite area and falls into the rhyolite area.XRD measurements of two layers of volcanic ashes layers near the P-T boundary of the marsh facies Philosophy profile show that the profile is mainly composed of illite-montmorillonite mixed clay minerals and kaolinite,and contains trace quartz and anatase.In which the content of kaolinite?36%and 49%respectively?is significantly higher than that of the other three sections.The ratio of SiO₂/Al₂O₃?1.53-1.61?in clay rock is close,and the ratio of MgO/K₂O?0.22-0.34?is lower than that in marine profile.The distribution curves of rare earth elements show a tendency to incline to the right,showing typical chemical weathering characteristics of light rare earth elements enrichment and heavy rare earth elements depletion,and Eu negative anomaly is obvious.The mapping of magma types of parent rocks of sample Ti/Zr vs.Zr/TiO₂ show that samples fall into the alkaline basalt area.Although the parent magma of different altered volcanic ashes in the same sedimentary facies is different,the types of clay minerals and the stacking types of I/S mixed layer are very similar,and the composition of clay minerals in altered volcanic ashes and the stacking types of in I/S mixed layer are obviously different among the four different sedimentary facies.K₂O content is directly proportional to illite content in claystone,which determines the mineral composition of claystone.It is concluded that the hydrodynamic conditions of K₂O and sedimentary environment are related to the leaching of potassium during alteration.In this paper,the ratio of MgO/K₂O?>0.50?in marine clay rocks is obviously higher than that in continental clay rocks?0.34?.It can be seen that the contents of MgO and K₂O in altered volcanic ashes are mainly related to diagenesis and controlled by sedimentary facies.Therefore,the mineral composition of altered volcanic ashes and the stacking type of clay minerals in the mixed layers of I-MM are mainly determined by the sedimentary environment of volcanic ashes.For marine and continental volcanic ash beds,the LREE/HREE ratio has no obvious regularity difference as a whole,and the light and heavy rare earth ratios of different volcanic ash beds in the same section are quite different.The distribution pattern of rare earth elements reflects the comprehensive influence of the source of volcanic ashes,mineral sorting in the process of mineral transport and diagenetic alteration environment.The slight to obvious negative anomalies of Eu appeared in all the profiles studied,and plagioclase was not detected,indicating that the negative anomalies of Eu element were caused by the environmental impact after deposition.In the reductive seawater environment and anoxic pore water,Eu released in the form of divalent Eu ²⁺can cause positive Eu anomaly of pore fluid and negative Eu anomaly of minerals,which indicates that at the turn of Permian-Triassic,the pH of seawater and terrestrial water environment is acidic,and the ocean acidification is more serious.Based on the analysis of the influence of sedimentary environment on the altered volcanic ashes,and using the plot of Ti/Zr vs.Zr/TiO₂ and the ratio of¹⁴³Nd/¹⁴⁴ Nd isotopes to distinguish the entry of the volcanic limestone parent rocks,the results show that the magma sources of altered volcanic ashes of different sections in the same period falls into different source rocks.This paper considers that the volcanic eruption at the turn of Permian to Triassic is multi-regional.domain,common event and multiple eruption points.