Felsic Volcanism In South China Across The Permian-Triassic Boundary

It has an important significance for the living of human now and in future to understand the processes, causes and mechanisms of mass extinction events in the history detailledly. The end-Permian mass extinction (EPME) is the severest mass extinction event since the Phanerozoic, about90%of marine biota and70%of invertebrate disappeared. Although there have been a lot of relative productions published, the episodes, isochronism and mechanism of this event are still in dispute. The fundamental issue of these controversies is the unclearity to the cause of the event. More and more evidences show that the volcanism might be the most likely cause of this event, although others triggers, such as sea level change, oceanic anoxia, bolide impact, methane gas hydrates, have not been excluded. At the same time, it still disputes about the charater of the volcanism. Radiometric dating and numerical modeling suggest that the eruption of the Siberian large igneous province (SLIP) might have been responsible for the EPME. However, volcanism along the convergent continental margins of the Pangea supercontinent was also very active during the EPME.At the end of the Permian, it deposited a suite of marine-facies to marine-terrigenous-facies stratigraphy in South China, and continuous Permian-Triassic stratigraphy sections emerge widely. On these sections, abundant volcanic ash beds which have altered to claystones and tuffs are developed. Although these ash beds and tuffs can not correspond one-to-one with the horizons of the EPME, some ash beds just coincide with the horizons of the EPME. It has an important significance for recovering the volcanic process and exploring the relationship with the EPME to deeply understand the characteristics and origins of the ash beds and tuffs.As far back as1980s, geologists have realized the importance of the ash beds and tuffs near the Permian-Triassic boundary (PTB), and began to research them. Now there are large quantities of productions published. However, their aims mostly focus on volcanic origin of claystones and their petrological characteristics, and researching methods are mainly traditional mineral type analyses and whole-rock geochemistry. There are some discussions about the tectonic setting of the volcanism in recent years, however, there are two totally different standpoints, one suggests the volcanism derived from intra-plate setting based on whole-rock trace elements, and the other suggests the volcanism came from arc-related setting based on zircon trace elements. There is still not the publishment about magmatic material origin of the volcanism.To understand the volcanism clearly, this study chose Daxiakou section in the deep-water basin at the northern margin, Xinmin section in the Nanpanjiang Basin at the southern margin and Zunyi section in the central platform of the Yangtze Block to study contrastively. The aims are to further undersdand the petrological characteristics of the volcanism, more importantly, to discuss tectonic setting and magmatic material origin of the volcanism and trace tuffs at the crucial horizon, and then infer the effect of these volcanisms in the EPME.Due to strong alteration of samples, the emphasis of this study was placed on the stable mineral in the altered tuffs--zircon, and zircon U-Pb chronology, trace elements and Hf-isotope compositions were analyzed. Zircon trace elements were used to discuss the petrological characteristics and tectonic settings, and Hf isotopes were used to discuss material origins of the volcanism. Because there is still not the systemic method to discuss petrological problems using zircon trace elements, it’s needed to establish the method before the discussion. About2420zircon trace element data from different tectonic settings and different rock types were collected in this study. Combined with published diagrams, a suit of diagrams distinguishing volcanic rock types were collected, including Y-Nb/Ta diagram, U-Th diagram, Hf-Y diagram and Y2O3-HfO2diagram. At the same time, U/Yb-Hf, U/Yb-Y, Nb/Hf-Th/U and Hf/Th-Th/Nb diagrams, which were used to distinguish material origins and tectonic settings, were modified and used to distinguish various tectonic settings. And Yb/Y-Yb/Nb and Yb/Y-Hf/Yb diagrams were established. Combining these six diagrams, they can distinguish the volcanism deriving from intra-plate rift, divergent continental margin, island arc setting, continental arc setting and post-collision setting.Daxiakou section is located at Xiakou Town, Xingshan County, Yichang City, Hubei Province. Upper part of the Upper Permian stratigraphy is Tulong Formation, and Lower Triassic is Daye Formation. The PTB is at the bottom of Bed263. It develops ten altered ash beds from C. meishanensis to i. isarcica Zones. They are Beds249,252,255,258,259-b,260,264,266,271and277in an ascending order. Laser Raman analyses show that there are66.7%of plagioclase,7.2%of alkali feldspar and26.1%of quartz in Bed260, suggesting Bed260is a dacitic tuff, deriving from calc-alkaline volcanism. Most zircons from ten ash beds have prismatic, especially long prismatic outlines and oscillatory zoned or homogenous internal structures, indicating their magmatic and volcanic origin. Magmatic zircon ages are227-279Ma, and inherited/xenocrystic zircon ages are673-2424Ma. To calculate the weighted mean age of each bed, magmatic zircons were divided into three groups according to their206Pb/238U age distributions. Group A (≤243Ma) might go through lead-loss after their crystallizations, Group B (244-259Ma) are dominating age peak and used for weighted mean age calculation, and Group C (≥259Ma) are magmatic inherited zircons crystallizing before volcanic eruption. Weithted mean ages of ten ash beds vary from249.4±1.8Ma to252.2±2.2Ma, which are identical with each other in the error, and also identical with published data of Bed25on Meishan section in the error. All the magmatic zircons have similar trace-element compositions. Their Y, Th, U, Hf contents and Nb/Ta ratio show these ash beds have the affinity of calc-alkaline dacitic or rhyolitic-dacitic volcanism, consist with the petrological result of Bed260. Their U/Yb ratio and Hf, Y contents show that most grains derive from continental crust material, just few grains in Beds258-260showing the affinity of oceanic crust material. Diagrams distinguishing tectonic settings show the affinity of post-collision setting. The ζHf(t) values of syn-erupting magmatic zircons (Groups A and B) vary in the range of-11.0-+5.2with calculated two-stage model ages (Tcrust) of0.95-1.97Ga. Magmatic inherited zircons (Group C) have the ζHf(t) values of-8.7-+2.4and Tcrustages of1.13-1.84Ga. Large Hf-isotope variation of magmatic zircons suggests that the volcanism have a mixed material origin. Beds252-259-b have relatively larger variation of ζHf(t) values (-10.9-+5.2) and relatively higher mean ζHf(t) values (-5.4--2.9), suggesting then more active volcanic eruptions and more addition of juvenile crust material in the magma. Bed258has the largest variation of ζHf(t) values (-9.3-+5.2), the most positive ζHf(t) values (26%) and the highest mean ζHf(t) value (-2.9), suggesting the most active eruption and the most contribution of juvenile crust material in the magma, and also the most input of heat.Xinmin section is located in the Xinmin village, Maodong Town, Puding County, Anshun City, Guizhou Province. The lower part of the section is Tulong Formation, and the upper part is Luolou Formation. The PTB is designed at the bottom of Bed XM5-3-3. It develops up to fifteen beds of altered tuffs from the Late Permian C. zhangi-C. yini Zone to the earliest Triassic. Thereof, eleven beds are rather thick, which are XM1-9, XM2-4, XM2-8, XM3-1-1, XM4-1, XM4-10-1, XM5-1-1, XM5-2, XM5-4, XM5-6and XM8-11, respectively, in an ascending order. The lower four beds are in the C. zhangi-C. yini Zone and are absent on the Daxiakou and Zunyi sections. Ten tuffs except XM1-9were studied. Petrological studies indicate that these tuffs are mainly crystal or vitric tuffs, and crystal fragments are mostly light minerals, feldspar and quartz, dark minerals rather little. Published whole-rock geochemical data show that these tuffs are from rhyolitic-dacitic volcanism, and Rb, Nb and Y contents show that the volcanism derived from transitional setting from island arc to intra-plate, with the affinity of post-collision setting. Zircons are mostly prismatic and oscillatory zoned, and their concordant ages are238-274Ma except two older ages (497Ma and1007Ma), indicating they are magmatic zircons. Similar to Daxiakou section, for calculating the weighted mean ages, magmatic zircons were divided into three groups, Groups A, B and C. The weighted mean ages of ten tuffs are250.5±1.2Ma to252.9±2.1Ma, identical with each other in the error. All the magmatic zircons have similar Y content, Nb/Ta, Th/Nb, and Hf/Yb ratios, but different Hf, Th, U, et al. element contents and Th/U, U/Yb, et al. ratios. Comparing with tuffs near the PTB (C. meishanensis Zone to the earliest Triassic), the syn-erupting magmatic zircons from three tuffs (XM2-4, XM2-8and XM3-1-1) in the C. zhangi-C. yini Zone have higher Hf, Th, U contents and U/Yb and Nb/Hf ratios, and lower mean Zr/Hf, Th/U and Hf/Th ratios. Magmatic inherited zircons have transitional trace-element compositions between them. Y, Th, U, Hf contents and Nb/Ta ratio suggest that except that XM4-1might tend to intermediate volcanic rocks, other nine tuffs mostly come from dacitic or rhyolitic-dacitic volcanism. Three tuffs in the C. zhangi-C. yini Zone show the affinity of aluminous or peraluminous rocks, and seven tuffs near the PTB tend to be calc-alkaline series. The U/Yb ratio and Hf、Y contents suggest that three tuffs in the C. zhangi-C. yini Zone derive from continental crust material, and part grains from XM4-1, XM5-1-1, XM5-2and XM8-11show the affinity of oceanic crust material. Diagrams distinguishing tectonic settings show three volcanisms in the C. zhangi-C. yini Zone have the affinity of post-collision setting, and other beds show the transitional characteristic from post-collision setting to island arc setting. The ζHf(t) values of syn-erupting magmatic zircons vary in the range of-12.8-+4.6with calculated two-stage model ages (Tcrust) of0.98-2.08Ga. Magmatic inherited zircons have ζHf(t) values of-8.9-+2.1and Tcrust ages of1.15-1.85Ga, similar to Hf-isotope composition of magmatic zircons from Daxiakou section. Three volcanisms in the C. zhangi-C. yini Zone have rather depleted Hf-isotope composition with mean ζHf(t) values of-1.3--2.1, and seven volcanisms near the PTB have rather enriched Hf-isotope composition with mean ζHf(t) values of-4.6-8.3.Zunyi section is located in the Zunyi City, Guizhou Province. The section is composed of the Upper Permian Changxing Formation and the Lower Triassic Feixianguan Formation. The PTB is designed in the middle of Bed ZY5-2temporarily. It develops three altered ash beds near the PTB, which are ZY4, ZY6and ZY13in an ascending order. Zircons from three ash beds moslty show prismatic outlines and oscillatory zoned or homogenous internal structures. The dating results show most grains are241-279Ma except two older grains (939Ma and2325Ma). Similar strategy was used to calculate weighted mean ages, and the results are250.2±2.4Ma to251.5±1.9Ma, identical with each other in the error. Trace-element compositions of magmatic zircons are similar to those from Daxiakou section, showing the affinity of calc-alkaline dactic or rhyolitic-dacitic volcanism. U/Yb ratio and Hf、Y contents suggest that magmatic materials mainly derive from continental crust material, and diagrams distinguishing tectonic settings show the affinity of post-collision setting. The ζHf(t) values of syn-erupting magmatic zircons are-11.3--1.0, and the ζHf(t) values of magmatic inherited zircons are-8.1-+0.2. Mean ζHf(t) values of ZY4, ZY6and ZY13are-5.1,-6.9and-8.8, respectively, showing more and more enriched Hf-isotope composition from bottom to up.Synthesizing petrological characteristics and zircon trace-element compositions of three sections, tuffs and ash beds derive from rhyolitic-dacitic volcanism. The volcanism in the C. zhangi-C. yini Zone show aluminous or peraluminous affinity; and the volcanism near the PTB tend to be calc-alkaline series.Zircon and whole-rock trace-element diagrams distinguishing tectonic settings both show that these felsic volcanisms have post-collision affinity. However, whole-rock trace-element data comparison shows that the volcanism have identical trace-element characteristic with felsic volcanism from the SLIP. Because the felsic volcanism in the SLIP are very few and the Siberian Craton was very far from South China then, it’s easy to exclude the possibility that the ash and tuffs in South China are from the SLIP. Thus identical trace-element characteristic suggests their similar tectonic setting and magmatic material origin (The SLIP is related with the reactivation of arc, and its magmatic material is the mixture of arc-related material and mantle-plume material). At the same time, ashes and tuffs have transitional whole-rock trace-element composition from felsic magmatism deriving from arc or post-collision setting in the Jinshajiang-Ailaoshan and Song Ma suture belts to felsic volcanism from the mantle-plume-origin Emeishan large igneous province, suggesting there are additions of mantle-plume material in the magmatic materials. Considering that ashes and tuffs have different zircon and whole-rock trace-element characters from felsic volcanism deriving from mantle-plume-origin large igneous province and Combining the geologic background around South China then, it’s explained that these felsic volcanism in South China formed in the transitional setting from volcanic arc to continent-continent collision or local post-collision setting in the convergent continental margins related with the assembly of the Pangea supercontinent. There are additions of mantle-plume material in their magmas.Magmatic zircons of ashes and tuffs from three sections have similar Hf-isotope compositions, suggesting that the magmas of these volcanisms have similar material origins. At the same time, large variation ranges of magmatic zirconδHf(t) values suggest magmas have mixed material origins. Published age spectrum of inherited/xenocrystic zircons and whole-rock trace-element characteristics show that magma materials of these volcanisms have the affinity of South China, suggesting that their magma materials might come from the South China Craton and they are recycling of crust materials. Combining with the crust evolution history of the South China Craton and the oceanic crust-material affinity of magmatic zircon trace elements in several tuffs, it’s explained that material origins of felsic volcanism in South China are complicated, might mainly mixed of Neoproterozoic and Archean crust materials with the addition of Emeishan mantle-plume material, and there are small quantities of additions of oceanic crust materials in several beds. Three volcanisms in the C. zhangi-C. yini Zone have rather depleted Hf-isotope compositions, suggesting that there are more addition of Neoproterozoic and/or Emeishan juvenile crust materials in the magmas. And their aluminous or peraluminous affinities and higher trace element contents in the magmatic zircons suggest that their magmas might be mixed of intermediate-felsic crust materials and have higher evolution degree. There were frequent volcanisms at the end of the Permian (C.meishanensis Zone to the bottom of H, parvus Zone), and these volcanisms have large variation range of mean ζHf(t) values, suggesting that their magmas come from the mixtures of juvenile and ancient crust materials with different proportions, and there are small quantities of additions of oceanic crust materials in several beds. The volcanisms in the earliest Triassic show rather enriched Hf-isotope compositions, suggesting their magmas mainly come from ancient crust materials.Tectonic setting of felsic volcanism near the PTB in South China and their material origins suggest the eruption sites of the volcanism were most likely located in the southeastern to southwestern margins of South China then, which is now Jinshajiang-Ailaoshan and Song Ma suture belts. Evident differences of zircon sizes from different beds in the same section suggest that they might come from volcanisms erupted in different sites. Thus, tuffs near the PTB in South China might come from multi-site and frequent volcanic eruptions along the Jinshajiang-Ailaoshan and Song Ma suture belts. This study provides geochemical and isotopic evidence for the inference of volcanic eruption sites.There are frequent volcanic eruptions (at least seven times) in the main extinction horizon (Beds25-26in Meishan) by biostratigraphy and lithostratigraohy comparisons. And comparing with upper beds in the earliest Triassic, these volcanisms and the volcanisms in the C. zhangi-C. yini Zone show more depleted Hf-isotope compositions and larger ζHf(t) difference-values, suggesting they are more active and intense, and might have released more energy. These frequent and intense volcanic eruptions in short time have resulted in sharp deterioration of ecological environment, and have threatened the living of biota strongly. Although it is not sure yet if these volcanisms are the main cause of the EPME, at least, they have aggravated the degree of mass extinction.