Influences of Sedimentary Environments and Volcanic Sources on Diagenetic Alteration of Volcanic Tuffs in South China

Permian-Triassic (P-Tr) altered volcanic ashes (tuffs) are widely distributed within the P-Tr boundary successions in South China. Volcanic altered ashes from terrestrial section-Chahe (CH) and marine section-Shangsi (SS) are selected to further understand the influence of sedimentary environments and volcanic sources on diagenetic alterarion on volcanic tuffs. The zircon 206Pb/238U ages of the corresponding beds between two sections are almost synchronous. Sedimentary environment of the altered tuffs was characterized by a low pH and did not experience a hydrothermal process. The dominant clay minerals of all the tuff beds are illite-smectite (I-S) minerals, with minor chlorite and kaolinite. I-S minerals of CH (R3) are more ordered than SS (R1), suggesting that CH also shows a higher diagenetic grade and more intensive chemical weathering. Besides, the nature of the volcanism of the tuff beds studied is derived from different magma sources. The clay mineral compositions of tuffs have little relation with the types of source volcanism and the depositional environments. Instead, the degree of the mixed-layer clay minerals and the REE distribution are mainly dependent upon the sedimentary environments. Thus, the mixed-layer clay minerals ratio and their geochemical index can be used as the paleoenvironmental indicator.

The Permian-Triassic boundary (PTB) mass extinction event was the most profound biotic crisis in the Phanerozoic, severely affecting most land and marine taxa [1][2][3] . However, the trigger that prompted the PTB mass extinction event continues to be intensely debated and has yet to reach a consensus 4 . The most popular hypothesis is that contemporaneous extensive volcanic activities initiated the mass extinction 1,[5][6][7][8] , while other triggers that have been proposed include oceanic anoxia, bolide impact and sea-level change 9 . The geographically widespread altered tuffs, especially those that are potassium enriched, can also be called K-bentonites. The tuffs represent significant stratigraphic markers for regional correlation, permitting geochemical characterization of the parent magmas and tectonic background of the source volcanism 10,11 .
In south China, the PTB ash beds are well-exposed and continuous with relatively significant diagenetic and metamorphic overprints 12,13 . The PTB ash beds have been frequently studied, primarily for their paleontological significance, REE and Hf-isotope analyses of zircons, and for clay mineralogical investigations 2,14-16 . However, there are very few studies focused on the diagenetic influences of different depositional environments based on mineralogy and geochemistry of the tuffs and the details of clay minerals. As clay minerals are sensitive indicators of their depositional environments, analyses of the volcanic altered ashes are necessary to further understand the role of tuff beds 17 . Hong et al. (2017) studied the littoral and interactive marine-terrestrial sections' PTB ashes in Guizhou Province and suggested that a stratigraphic correlation can be identified from the geochemical fingerprinting of the ash beds.
Volcanic ashes are usually preserved in aqueous environments and further altered into several types of clay minerals 18 . The transformation process is relatively rapid, even within several days to a few years, with the in-situ

Geological background
The studied sections from the Lower Yangtze Block were located in the eastern part of the Palaeo-Tethys Ocean during the P-Tr transition, which connected the Qinling paleo-ocean and the Cathaysian paleo-block 15 . The studied terrestrial section is situated at Chahe (CH), Heishitou town in western Guizhou Province, and is located in the upper part of the Xuanwei Formation. The Permian Xuanwei Formation is comprised of fluvial and lacustrine units, and the change of the formation is from meandering fluvial to lacustrine (Beds 56-80) for a deepening and transgressive process 2 . According to the previous stratigraphic studies 27 , layers 68-70 are the Permian-Triassic Transition Beds (PTTB). Two typical K-bentonite layers, bed 66 f (10 cm) and 68a (8 cm), are interbedded with coal measures and yield abundant Permian terrestrial fossil plants, β-quartz, apatite and zircon. Bed 68 has been identified as the PTB, on the basis of zircon SHRIMP technique zircon 206 Pb/ 238 U date of 252.3 ± 0.07 Ma, correlating with bed 25 at the Meishan section 2,28 . Beds 66 f and 68a closest to the PTB were chosen as the samples and numbered CH-1 and CH-2 (Fig. 1).
The marine Shangsi (SS) section consists of the Dalong Formation and Feixianguan Formation, located at Shangsi town, Sichuan Province, South China. The SS section preserves a continuous depositional environment from an upper slope sedimentary environment in the Late Permian to a tidal flat environment in the early Triassic 29,30 . Beds 23, 25, 27 and 28 are intercalated with K-bentonites presumed to be volcanogenic in origin 31 . The PTB has tentatively been placed on the top of bed 27 on the basis of zircon U-Pb age, 252 ± 0.13 Ma, presented by Shen et al. 27 . Besides, according to the conodont stratigraphy, the PTB was placed ~22 cm above the  32 . Based on the PTB studied before, beds 25 and 28 were selected in this study and numbered SS-1 and SS-2 ( Fig. 1). The bed 27 are primarily composed of pale yellow K-bentonites, and the bed 28 are mainly dolomitic mudstones intercalated with the K-bentonites 27 .

Results
Zircon U-Pb ages. The cathode luminescence (CL) images indicate that the dominant zircons are euhedral to sub-euhedral accompanied by prismatic, especially long prismatic shapes. The internal structures of the zircons are homogenous, oscillatory-zoned, or the oscillatory-zoned with homogeneous core in the volcanic ashes, suggesting their magmatic origin 16,33,34 . The weighted mean 206 Pb/ 238 U ages of the four samples are 264.0 ± 2.6 (CH-1), 253.4 ± 1.8 (CH-2), 260.0 ± 2.4 (SS-1) and 253.6 ± 2.3 Ma (SS-2), respectively (Fig. 2). For purpose of discussion, the zircon ages are interpreted to be inherited from a magmatic source and these represent the crystallization age of the original materials.
The ages of CH-2 and SS-2 are almost equal, which is near the PTB age. It is similar with the evidence of the sporopollen and conodont biostratigraphy which helps to confirm the PTB position. The ash beds of CH-1 and SS-1 also occur in the same time period within the error range of measurement. Clay mineral composition. XRD spectra of the air-dried and EG clay fractions show that clay fractions from different depositional faces consist dominantly of mixed-layer illite-smectite (90-100%) (Fig. 4). I-S is identified by a relatively broad (001) peak between 10-12 Å in the air-dried fraction, and after EG solvation, it separated into two peaks at 11 Å and ~9.5 Å. Both of the CH samples display a strong peak at 11.0 Å, and separate into two peaks at 12 Å and ~10 Å after EG solvation (Fig. 4). The SS samples show a strong peak ~12 Å, and split into ~13 Å and ~9 Å after EG solvation (Fig. 4). Besides, the peaks of I-S are sharper and more intense in the case of CH, showing a more ordered structure. Except for the dominant I-S minerals, the SS clay samples contain trace amounts of kaolinite (~6%), while those of CH present with minor amounts of chlorite (7.04~, 3.52 and 14 Å) (<1%).   The determination of I-S species was carried out by the NEWMOD program simulation of the XRD patterns 35 . The NEWMOD program generally produces a steeper background (calculated pattern) than the experimental pattern at the low-angle region 36 , as is also true for our samples. The accuracy of the fitting procedure was displayed by the content accordance between the experimental and calculated patterns. The simulation result shows that the I-S minerals of CH are more ordered than SS' samples. The order of the I-S is also consistent with the percent of illite (I%): the layer structural order is R3 ordered for CH-1 (I% = 86%) and CH-2 (I% = 84%), R0 and R1 ordered for SS-1 (I% = 58%), and R1ordered for SS-2 (I% = 68%), where R is the Reichweite parameter (Fig. 5).  Elemental geochemistry. The major-element contents of the four selected bulk samples are listed in Table 1.
The Loss on ignition (LOI) value of tuffs of CH-1 and CH-2 are 9.42 wt% and 9.17 wt%, while the SS-1 and SS-2 are 31.28 wt% and 24.56 wt%. SS-1 and SS-2 show a higher content of Ca, Mg, but lower in Si, Fe, K, and Al. Results of the trace-element analyses of the two sections' clay minerals are presented in Table 2. Chondrite-normalized REE patterns 37 of the investigated samples are shown in Fig. 6. The normalization of the study samples shows a generally large REE fraction and sloping curves with an integrally enrichment ratio of LREE (light rare earth elements) and depletion of HREE (heavy rare earth elements). The the clay minerals of CH-1 and CH-2 present slightly negative Eu anomalies (Eu/Eu* = 0.9493 and 0.8263), whereas the SS-1 and SS-2 show strongly negative Eu anomalies (Eu/Eu* = 0.4015 and 0.4596). Besides, the marine samples are relatively  weakly enriched in HFSE (high field strength elements), and Zr and Hf show a slightly positive anomaly. ΣREE concentration is especially higher in CH-2 (805 ppm, which is 2.4-3.8 times higher than other samples (from 207 ppm to 334 ppm). The ΣLREE/ΣHREE ratios of terrestrial and marine sections are 5.02 (CH-1), 11.6 (CH-2) and 7.2 (SS-1), 9.2 (SS-2), respectively. Given the mobility of major elements during the argillization of the parent rock, the chemical discrimination diagram of the studied tuffs was used by Winchester & Floyd (1977), based on the concentrations of immobile elements, to differentiate the origin of the parent rock (Fig. 7).  The original magma chemistry information can be principally obtained from immobile trace elements. All the beds have great ranges of Al 2 O 3 /TiO 2 ratio from 8.24 to 52.59. TiO 2 , the HFSE (Nb, Ta, Zr, Hf), and REEs which are generally considered as immobile elements during diagenesis and weathering, are important indices for magmatic origin 14,20 . The Zr/TiO 2 and Nb/Y ratios are commonly used as indicators of differentiation and alkalinity. Explosively erupted ashes tend to be characterized by high Nb content indicative of their high, silicic volatile feature 38 . As Al and Ti are immobile chemical components in the chemical weathering process 39 , their contents will remain constant in materials with different weathering degrees 40 . Thus, the Al 2 O 3 /TiO 2 ratio has been proven to be a reliable proxy to indicate the provenance 41 . The nature of volcanic ashes of the study samples are classified as felsic and intermediate based on Al 2 O 3 versus TiO 2 diagram (Fig. 7). This is also confirmed by a cross plot of Nb/Y versus Zr/TiO 2 (Fig. 7). The plot suggests that CH-1 and CH-2 are derived from andesite and rhyodacite, respectively, and SS-1 and SS-2 are from a rhyolite source. For the Nd isotopic composition of the tuffs, the chemical weathering process exerts a negligible influence in terms of its primal parent rock undergoing a supergene environment. The 143 Nd/ 144 Nd ratio (Table 3) is another useful and sensitive indicator for the source rock. The 143 Nd/ 144 Nd ratio shows a difference between CH-1 and CH-2 (0.512376 and 0.512424), and a slight difference between SS-1 and SS-2 (0.512034 and 0.512043). Also, the CH section presents a significantly higher 143 Nd/ 144 Nd value than that of the SS section. Significantly different Nb isotopic compositions, together with the Nb/Y-Zr/TiO 2 discrimination diagram (Fig. 7), suggesting that the tuffs were derived from different magma sources for the beds of the CH and SS sections. The ash beds in the SS section present slightly different values of 143 Nd/ 144 Nd and fall into the field of rhyolite indicating that the volcanic ashes mostly likely have been derived from the same local volcanism and source rocks.

Sr-Nd and
The isotopic composition of Sr of the marine section samples depends primarily on two major sources: the Sr isotope signatures in the oceans and the parent rock of the tuffs. Brachiopods and conodonts are identified as the most reliable material for measuring Sr isotope ratios in the oceans. The 87 Sr/ 86 Sr ratio was peaked in the PTB interval from late Permian to early Triassic 42,43 . The Sr isotope composition of conodont of Guandao section located in Guizhou is a plateau with minor fluctuations around 0.70710 44 , which is far below the Sr value of the SS tuffs (0.721708, and 0.717225). Therefore, the source of the rock played a dominant role on the high 87 Sr/ 86 Sr ratio of the samples in this study. The high 87 Sr/ 86 Sr ratio of voluminous silicic rocks is also mostly associated with ignimbrite flare-up eruptions and exhibits a high value, corresponding to available involvement of continental crust in arc magmatism 45 .

Evaluation of diagenesis in marine and terrestrial sections.
The changes in the bulk sample composition of tuffs from different sedimentary facies are controlled by parent-rock composition, pH, organic matter, pore-water, and microbial activity 18,21 . Clay-mineral compositions of the samples are essentially I-S, with only just other minor clay minerals showing a few differences. The pyroclastic materials deposited in a marine environment were primarily altered into an association of K-feldspar and I-S 23 . The SS section samples show a trace amount of kaolinite. However, it is almost not detected in CH, while a minute amount of chlorite is detected in CH section (Fig. 8). Kaolinite and chlorite are the sensitive environmental indicators denoting different factors during its deposition and initial transformation. The minor detrital minerals of chlorite suggest relatively lower speed deposition of volcanic ashes 46 in CH section. The kaolinite formation is typically formed from weathering, altered acidic igneous and metamorphic rocks or detrital products 47,48 . The presence of kaolinte suggests that it is from acidic igneous source rocks and its formation environment is reducing pH and well-drained leading conditions during alteration processes. The stability of clay minerals is inclined to increase along with the decreasing pH 23 . Because, the Al-OH species depends mainly on pH value, and aluminum cations tend to be produced under condition of pH ≤ 6 49 . Si is released gradually by smectite at relatively low pH which further promotes the transformation from smectite to illite. The slightly negative Eu in CH and strongly negative Eu in SS (Fig. 6) indicate that the lower pH of crustal environment induces the host-rock towards clay-dominated products. The strongly negative Eu anomaly of the SS may be inherited from the reduced environmental conditions during the formation of I-S, Eu 3+ , therefore, converted to Eu 2+ and retained. The source of K in illitization can be provided from acidic magmas or high pH seawater 50 . Obviously the former can be considered as the main source of K to the transformation processes from smectite to illite.
In different sedimentary environments, volcanic glass in tuff alters into various minerals, such as the assemblages of the clay minerals, the type of mix-layer clay minerals and the REEs. Smectite converts to I-S at relatively slow and long-lasting time and low temperatures 51 . Illitic clays in volcanic ashes commonly replace the smectite precursor and inherit the texture with low porosity and permeability 52,53 . Different degrees of I-S interstratification in the smectite-to-illite transformation are controlled by primary rock lithology or the later sedimentary environment 20,54,55 , as identified by water/rock ratios, fluid chemistry and temperature. Evidence that the CH and   (Fig. 7) and the content of Nb (Table 3). Clearly, it is indicated that the sedimentary environments mainly influence the order and content of illitization, and the variable types of authigenic illite in the I-S mixed layer attribute to variations in porosity and permeability of the host rock 56,57 .
The degree of I-S and the content of illite in CH (R3) are higher than that of SS (R1) which have close relations with the sedimentary environment. The δ 18 O value of clay minerals is inherited from the water in the sedimentary environment, while the parent rock has little impact on the oxygen isotopic composition of altered clay minerals 58 . Water/rock ratios show an inverse relationship with the δ 18 O value, and the depletion of the oxygen isotope composition of the clays suggests a relatively high water/rock ratio related to fluid flow 59 . The influence of water/ rock ratios occurs when the volcanic ashes are not greatly compacted during deep burial, and the lower water/ rock ratio should theoretically correspond to a weaker leaching condition 56 . The SS I-S minerals have a much higher δ 18 O than that of CH I-S (Table 3), indicating a relatively low water/rock ratio. This is also consistent with the conventional wisdom that higher illitic I-S minerals precipitate at higher water/rock ratios 60 . In addition, the Fe content of the CH section is higher than that of SS integrally, also suggesting a deeper degree of illitization 61 . This reaction driven by Fe reduction. J.M. Huggett et al. (2005) suggest that Fe 2+ promote the illite content.
There is a gradual increase in K 2 O and Al 2 O 3 with a corresponding decrease in SiO 2 and CaO, so the K 2 O/ Al 2 O 3 ratio (Table 1) can be used as a proxy linked for chemical weathering with Ca and Na removal. The K 2 O/ Al 2 O 3 value indicates the loss of K in mineral species during the weathering process, and therefore the relatively higher K 2 O/ Al 2 O 3 ratio of CH than that of SS indicates a higher diagenetic grade and more intensely chemical weathering and alteration.
Unlike other homogeneous Nd isotopic compositions of I-S, the Sr isotopic compositions are more variable. The Sr isotope ratio can also be used to indicate the degree of chemical weathering which is proportional to the 87 Sr/ 86 Sr ratio 62 . Chemical weathering and alteration give rise to the leaching of Sr and increasing of the Rb/Sr ratio, and thus the 87 Sr/ 86 Sr ratio rises accordingly. The 87 Sr/ 86 Sr ratios (Table 3) 2,16 . Magmatic zircons exist widely in the tuff beds in south China. The hypothesis of the Siberian origin is questionable, because the Traps mainly consisted of mafic basalts, tuff in the bottommost and felsic at the upper part. Nevertheless, the PTB beds are primarily comprised of felsic tephras. Besides, the age of the Siberian Traps (251.7 Ma) 1 is younger than the GSSP (252.28 Ma) 27 , and the PTB tuffs are correlated with crustal origin while the Siberian Traps is correlated with mantle origin. Thus, the causal relationships between the PTB volcanic ash and the Siberian Traps can be excluded. The thickness of the ash layers ranges from 13 cm to 30 cm in CH and SS, which is the same as other sections in Guizhou, such as the Xinmin, Duanshan and Zhejue sections. If the ashes were transported the long distances from the tectonic regions proposed above, this may not have resulted in ash layer thicknesses to this degree. The CH and SS section are situated closely in Lower Yangtze Regions, and the tuffs of the corresponding layers were formed contemporarily. The parent rocks of the studied tuff beds fall into the rhyodacite-andesite and rhyolite field separately whereas Meishan section located in the Middle-Upper Yangtze and falls into the rhyodacite-dacite field, indicating somewhat diverse source rocks. According to the studied sections, it can be assumed that the P-T transformation volcanism had a separated and regional magmatic source, not generated generally and identically. The volcanism in south China appears to be heterogeneous and not associated with a predominant and unitary source but derived from local volcanism or adjacent volcanism 17,65 .  (1) The tuff beds of the CH section, CH-2 from rhyodacite and CH-1 from andesite indicate that altered ash layers can be derived from different parent rocks in the same section. The same parent rocks can come from different depositional environments as the source of CH-2 and SS-2 are both rhyodacite. (2) The tuff of heterogeneous parent rocks in marine and non-marine sections both altered into I-S clay minerals, suggesting that the depositional environment plays a critical factor for the trend of alteration. The degree of illitization and the ordering of I-S, moreover, depend on the sedimentary environment mainly including the pH, ambient temperature and the water/rock ratios. Correspondingly, the degree of illitization and the I-S ordering, combining REEs, Nd isotopic and 18 O, can be used as proxies to reflect the paleo-sedimentary environment. The terrestrial (CH) section reflects a higher diagenetic grade and more intense chemical weathering and alteration than the marine (SS) section, and both sections indicate that the relatively low pH environment did not experience the hydrothermal process. (3) Based on the studied terrestrial and marine facies occurring in almost the same period, the volcanism during the PTB transition can be regarded as heterogeneous in south China according to the geochemical fingerprinting of CH and SS. Thus, it is assumed that horizontal strata correlation of the tuff cannot be a reliable role of the source volcanism.

Analytical techniques
Mineralogical compositions. The mineral compositions were conducted by X-ray powder diffraction (XRD). Bulk samples that were strictly avoided from the detrital contamination and weathered portion during sampling were collected from the sections mentioned above. The air-dried whole-rock samples were crushed and ground to powders with a mortar and pestle, and then the clay mineral fractions (<2 μm) were extracted from the powder samples after centrifugation in distilled water using the sedimentation method 66 . The oriented mounts were prepared carefully by smearing the clay suspension onto glass slides and air dried. Air-dried (AD) oriented glass slides were treated in a container with ethylene glycol at 65 °C for 8 h in an electric oven. Ethylene glycol-saturated (EG) samples were to determine the occurrence of smectite, and mixed-layer illite-smectites. ZnO is used as the internal standard to analyze the clay minerals quantitatively by XRD. X-ray diffraction analyses were performed using a Rigaku D/MAX-IIIA diffractometer (35 kV, 35 mA,) with Ni-filtered Cu Kα radiation from 2-65°2θ at a scan rate of 4° 2θ/min and with 1° divergence slit, 1° anti-scatter slit and 0.3 mm receiving slit.
Zircon U-Pb dating. To separate the zircon crystals from K-bentonites, we picked zircon crystals located in the samples by hand under a binocular microscope. The potential target sites of zircons were selected via cathode luminescence (CL) images. Zircon U-Pb ages is dated through an inductively coupled plasma source mass spectrometer (LA-ICP-MS, Agilent 7500a) at GPMR of China University of Geosciences. External standard zircon, GEMOC GJ-1 (608.5 ± 15 Ma) 67 and 91500 (1062 ± 4 Ma) 68 were repeated to rectify the instrumental error and figure the element fractionations of Pb, Th, and U. The Si in NIST 610 glass is conducted as the internal standard to calibrate trace element compositions of zircon particles against multiple-reference materials 69 . The correction of Pb element adopts the method proposed by Andersen 70 . Off-line calculations, zircon U-Pb dating, and time-drift corrections were performed by the ICPMSDataCal program 69,71 . XRF analysis. The major-element concentrations of bulk samples were analyzed by wavelength dispersive X-ray fluorescence (XRF). Samples were powdered to <200 mesh size in an agate mortar, diluted 1:5 with dilithium tetraborate and fused using Philips Perl' X3 automatic bead machine at 1200 °C. Measurements were carried out using a SHIMADZU XRF-1800 sequential X-ray fluorescence spectrometer. A pre-ignition was conducted to confirm the loss on ignition (LOI) prior to major element analyses. The detection limit was about 1 wt% and precision is <1% for major elements.
Trace element and REE analysis. Trace-element and rare-earth-element (REE) analyses of bulk samples were conducted using an Agilent 7500a inductively coupled plasma mass spectrometer (ICP-MS), adopting the techniques presented by Liu et al. 71 . The analytical accuracy is <4% for REEs and Y, and 5-10% for other elements.
Sr and Nd isotopic composition analysis. The composition analysis of Sr and Nd isotopic was measured as a Triton Ti thermal ionization mass spectrometer. Rb, Sr, and trace elements were detected by Resin columns AG50w × 8 and Dowe × 50w × 8 with 2.5 N HCl from the powdered samples. Isotope ratios of 87 Sr/ 86 Sr and 143 Nd/ 144 Nd were standardized respectively based on 86 Sr/ 88 Sr = 0.1194 and 146 Nd/ 144 Nd = 0.7219. The calibration of the instrument was repeatedly tested by standards NBS987 and La Jolla Nd 2 O 3 . Average values of the isotope ratios of 87 Sr/ 86 Sr and 143 Nd/ 144 Nd were 0.71026 ± 5 (2 SD) and 0.511845 ± 10 (2 SD).