Geochronology And Geochemistry Of Deep-time Volcanic Ash And Its Geological Implications

Volcanic ash is a key clue connecting the Earth's internal system and the surface system.It originates from the deep magmatic activities of the Earth and directly affects the various spheres of the Earth's surface through volcanic eruption.In recent years,volcanic ash is a hot frontier field in Earth Science,and some great progresses have been made in many aspects,such as the chronology of strata and major geological events,volcanic geological disasters,global change and biological extinction events.The rapid eruption,large-scale dispersion and subsidence of volcanic ash make it an accurate"time marker bed"and"isochron stratum"and zircon and other dating minerals in volcanic ash make them ideal dating objects.The study of volcanic ash is mainly concentrated in the modern and Quaternary Geosciences,while the study of deep time volcanic ash before Quaternary mainly focuses on the ages of stratigraphic boundary and major geological events.However,the concept of deep-time volcanic ash is not clear,and its identification and geochemistry are rarely studied.It is suggested by this paper that the deep-time volcanic ash is referred to the pyroclastic rocks with fragment size less than 2 mm in the stratum before Quaternary,including tuff and volcanogenic claystone.Based on the characteristics of pyroclastic fragment and its alteration,the identification characteristics of deep-time volcanic ash are summarized in this paper.Deep-time volcanic ash samples of different ages and sedimentary environments from three regions in China were selected for analysis of petrography and mineralogy,geochemistry,isotope geochemistry and isotope chronology.The lithologic identification,genetic analysis and source tracing of these deep-time volcanic ashes were carried out,and the relationship between these deep-time volcanic ashes and major global or regional geological events is explored.The Permian"vegetational Pompeii"in Wuda area,North China,has in situ preserved the largest early Permian plant community so far,which provides rich materials for the study of Early Permian Cathaysian plant community and the comparison of global plant communities.In order to accurately determine the age of this layer of plant communities and explore the relationship between the formation process of the vegetational Pompeii and volcanism,this paper discusses the relationship between the formation of the vegetational Pompeii and volcanism.Based on the study of petrography and mineralogy of five samples of volcanic claystones from Taiyuan Formation of Upper Permian in Wuda coalfield,zircon geochronology,in situ Hf-O isotope,trace element and element geochemistry were analyzed.The zircon SIMs U-Pb ages of four samples from the lower part of the Permian vegetational Pompeii volcanic claystones in Wuda area all show a single peak age,and the age of the pooled weighted mean ²⁰⁶Pb/²³⁸U age is 296.2±1.0 Ma(MSWD=0.82,n=79),constraining the age of the Wuda Tuff Flora to the earliest Permian.The volcanic claystones is dominated by kaolinite(74.3%?86.5%)and quartz(10.7%?25.7%).Under the scanning electron microscope,the alteration of K-feldspar into kaolinite can be observed.The Al₂O₃/Ti O₂ ratios of the whole rock for samples are high,ranging from 65.8 to 103.5,the REE(Rare Earth Element)distribution pattern shows LREE enrichment and Eu negative anomaly(0.27?0.38),indicating that the source area is felsic volcanism.Nb and Ta depletion in the whole rock spider diagram shows the characteristics of magmatic arc,Zircon?Hf(t)ranges from+0.11 to+5.15,except for two zircons showing negative values(-0.1 and-2.45),which is inconsistent with the characteristics of continental magmatic arc in the northern margin of North China.The?¹⁸O range of zircons ranges from 5.05‰to 6.17‰,which is consistent with mantle source.The zircon Hf-O isotopic characteristics indicate that the source of the early Permian volcanic claystones in Wuda Pompeii is a magmatic arc volcanism related to the subduction of the Palaeo-Asian Ocean.According to the thickness of volcanic claystones and the grain size of pyroclastic rocks,the volcanic source area is not far from the Wuda coalfield.Based on the analysis of the geological background of the North China Craton and its surrounding areas,it is suggested that the North China Craton may not have collided with the Alax block during the early Permian,and there was still subduction of the Palaeo Asian Ocean between the NCC and the Alax block.It supports the view that the Alxa block does not belong to the North China Craton at early Permian.The deep-time volcanic ash of the Permian Triassic boundary(PTB)in South China is highly correlated with the extinction events,which may be one of the triggers of the extinction events.It is of great scientific significance to study the origin,source and extent of the PTB volcanic ash.The“tuff”at the top of Dalong formation in Penglaitan section,Laibin area,Guangxi is identified as tuffaceous sandstone in this study.Based on the petrographic study,detrital zircon geochronology,Hf isotopic elements and trace elements are analyzed on the tuffaceous sandstone.Based on the temporal and spatial distribution characteristics of PTB deep-time volcanic ash in South China,the source area and scale of PTB volcanic ash are discussed and defined.The detrital zircon U-Pb ages of 81%of the tuffaceous sandstone samples from the top of Dalong formation in Penglaitan section are 235±2.7 Ma?258±4.8 Ma,showing a single peak value of?252 Ma.The Hf isotopes of the late Permian detrital zircons in the samples are of crustal origin:except for one zircon with an?Hf(t)value of+10.3,the others fall between-16.5 and-5.0.The U-Th-Hf-Nb trace element characteristics of detrital zircon show that they are derived from continental magmatic arc.There are a large number of 0.5?2 mm grain sizes in the tuffaceous sandstone,and some of them are even 3 mm in size.The zircon U-Pb age and stratigraphic location of the pyroclastic materials in Penglaitan section are basically consistent with those of Permian Triassic volcanic ash in South China,and have similar zircon Hf isotope and trace element characteristics,indicating that they may come from the same source area.The pyroclastic sedimentary rocks and coarse tuff near PTB in South China only occur in the southwest of South China,while fine-grained volcanic ash with grain size less than 0.3mm is mainly distributed in the northern part of South China.The grain size of pyroclastic decreases gradually from southwest to northeast,indicating that the volcanic materials came from the southwest margin of South China.Many evidences,including Permian granites in Hainan Island and acid volcanic rocks in southwestern Guangxi,indicate that there is a Permian continental magmatic arc in the southwestern margin of the South China.Therefore,it is suggested that the PTB volcanic ash in South China may have originated from the palaeo-Tethys continental magmatic arc on the southwestern margin of the South China plate.No volcanic ash of the same period has been found or reported in North China and Northwest China,indicating that the volcanic eruption of arc magma related to this subduction is limited,and it is speculated that it is not the main cause of global PTB extinction event.Taklimakan Desert is the largest desert in China and the second largest mobile desert in the world.Its formation is related to the uplift of Qinghai-Tibet Plateau,and it is an important landmark event in the Cenozoic geological evolution history of Asia.As for the formation age of Taklimakan Desert,it is generally believed to be 3.5 Ma.However,Zheng et al(2015a)reported that a set of volcanic ash sections were identified in the upper part of the extremely thick molasse of the Cenozoic Xiyu Formation in Southwest Tarim,and based on the limitation of the volcanic ash age(11.18±0.11 Ma),combined with palaeomagnetic study,the desert formation age is defined as late Oligocene-early Miocene,which is much earlier than the previous conclusion of late Miocene-Pliocene.However,some scholars believe that this"volcanic ash"is a sedimentary clastic rock,and the dating results of the minerals in the strata cannot represent the age of the strata.In this paper,petrography,mineralogy,elemental geochemistry and zircon geochronology of the"volcanic ash"and the sandstones above and below the volcanic ash are studied in detail.The results support the conclusion that the"volcanic ash"is sedimentary clastic rock.The main evidences are as follows:(1)the mineral composition of the"volcanic ash"sample is plagioclase(48%),potassium feldspar(33%),montmorillonite(15?47%).The content of feldspar is much higher than that of the common deep-time volcanic ash,which should be the clastic rocks deposited in the forehill.(2)In the zircon age spectrum of"volcanic ash"samples,81%of zircons fall in the range of 9?14 Ma,showing a single peak,while 19%of zircons fall outside the single peak age range,showing the characteristics of zircon age spectrum of sedimentary clastic rocks.(3)The age spectrum of detrital zircon in the"volcanic ash"and the upper and lower strata have the same age range,with a peak value of?11 Ma and the detrital zircon of Palaeogene,Mesozoic and Palaeozoic with different proportion,which indicates that the provenance supply is continuous,and the provenance may have strong tectonic or denudation effects.Therefore,the dating results of minerals in the clastic sedimentary strata cannot be used to determine the age of the strata.The conclusion that the Taklimakan Desert was formed in late Oligocene early Miocene remains to be discussed.To sum up,it is concluded in this paper that the identification of deep volcanic ash should be based on the integration of the field observations,petrography and mineralogy,element geochemistry and zircon chronology.The following conclusions are drawn in this paper:(1)This paper identified and studied the volcanogenic claystone in Wuda coalfield,North China,and accurately defined the age of the Permian"vegetational Pompeii".At the same time,the implication of Hf-O isotopes from volcanogenic clay rocks on the nature of the Alashan block is discussed;(2)This article identified the Guangxi Penglaitan PTB“tuff”to be tuffaceous sandstone.And the pyroclastic component in the tuffaceous sandstone are proved to be the same origin with other PTB sections in South China.Comprehensively considering the distribution of deep-time volcanic ash of multiple PTB sections in South China,we consider that the source of PTB deep-time volcanic ash in South China is the Palaeo Tethys continental magmatic arc on the southwestern margin of the South China plate,and the scale of volcanic eruption related to the arc is limited,so it is speculated that it is not the main cause of global PTB extinction event.(3)This paper falsifies the set of"volcanic ash"of Xiyu Formation in Kekeya area,Xinjiang,which is considered as normal sedimentary clastic rock in Piedmont.This study does not support the conclusion that the Taklimakan Desert was formed in late Oligocene early Miocene.