2 – Study area and geological setting - Sedimentology, biostratigraphy and diagenesis of Upper

The study area is located in the Sikhote–Alin orogenic belt (Russian Far East), a collage of various terranes attached to the eastern margin of Asia during the Palaeozoic and the Mesozoic (Malinovsky et al., 2006). This complex geological province contains the Taukha Terrane, a late Tithonian–Hauterivian accretionary prism that borders the Japan Sea, abutting the Zhouravlevka–Amur turbidite basin to the west, and the Kema island arc to the north (Kemkin et al., 1997; Kemkin, 2012, Fig. 2). The Taukha Terrane, characterised by various allochthonous marine deposits, subduction mélange and terrigenous deposits of continental margin affinity, is divided into three tectono–stratigraphic units, namely, the Erdagou Unit, the Grobousha Unit, and the Skalistorechenka Unit (Fig. 2) (Kemkin et al., 1999; Khanchuk et al., 2016). The Gorbousha tectonic unit comprises chert, siltstone, sandstone, and the Dalnegorsk limestone (Bragin and Krylov, 2002; Kemkin et al., 2016): 11 limestone outcrops, affiliated to the same carbonate system, situated within a few kilometres of the Dalnegorsk city (Peyrotty et al., 2020a) (Fig. 2; 3). This limestone is also locally associated with high–Ti alkaline basalt, interpreted as palaeoguyot fragments (Khanchuk et al., 1989). The Dalnegorsk limestone is part of an atoll–type system of Norian age, developed in the Panthalassa Ocean (Fig. 1) (Kemkin et al., 1997; Peyrotty et al., 2020a), and later accreted in the southern part of the Sikhote–Alin orogenic belt (Russian Far East, Fig. 2). As a result of the accretion process, in the studied area, the above lithologies occur in tectonic contact, as an extensive mélange consistent with the

79 ocean plate stratigraphy model proposed by Wakita and Metcalfe (2005). Detailed information about the study area (outcrop names, thickness, GPS coordinates) are provided by Peyrotty et al. (2020a).

Fig. 2 – General geological map of the southeast part of the Sikhote–Alin orogenic belt, combined with a section of the Taukha terrane and tectono–stratigraphic complexes. The Dalnegorsk limestone is located in the north–east of the terrane and belongs to the Gorbousha tectonic unit. Bd: Badzhal Terrane, Bu: Bureya Terrane, Ke: Kema Terrane, Kh: Khabarovsk Terrane, Kha: Khanka Terrane, Km: Kiselevka–Manoma Terrane, Nb: Nadanhada–Bikin Terrane, Sm: Samarka Terrane, Sr: Sergeevka Terrane, Zr–A: Zhuravlevka Terrane. Modified after Kemkin and Kemkina, 2014.

Fig. 3 – Location and size of the Dalnegorsk limestone’s outcrops. The samples used for this study come from the outcrops marked in red which present the best preservation of the studied diagenetic features. GPS coordinates are available in Peyrotty et al., 2020. Maps credits: D–Maps, OpenStreetMap.

3 – Sampling

In the course of accretion, the Dalnegorsk limestone became locally subjected to intense tectonic stresses and high temperature, related to local igneous activity and/or burial. In addition, the atoll system was dismantled when it became incorporated into the accretionary prism: as a result, it occurs as tectonized slabs or mélange (Fig. 4A and B) and not as a continuous outcrop. The Dalnegorsk limestone thus comprises isolated outcrops of mostly

81 recrystallized limestone, of various sizes and without lateral continuity (Peyrotty et al., 2020a) (Fig. 3). Due to tectonic deformation, sampled blocks lack bedding or any sign of stratigraphic polarity. Stratigraphic and/or facies–related sampling were thus impossible. Age–targeted sampling was also impossible, since all Dalnegorsk outcrops are of the same age (Norian, based on of foraminifera fauna: Peyrotty et al., 2020a). Consequently, the sample selection for this study was based solely on the preservation of diagenetic features. In total, 146 petrographic thin sections from 11 different outcrops were investigated with optical and cathodoluminescence microscopy in order to identify the diagenetic features. The best–preserved and most representative samples for each diagenetic event were selected for in situ geochemical analysis.

The analyzed sample set comprised nine thin sections from four different sites (Table 1; Fig.

3): seven thin sections from the Verkhnii–Rudnik and Sakharnaya areas (Mountain and Quarry outcrops); and two recrystallized samples from the Karyernaya and Partizansky areas, for comparison with the well–preserved samples. The four samples from the Verkhnii–Rudnik area are open–lagoon and reefal facies, whereas the two samples from Sakharnaya Mountain are open–lagoon facies (see Peyrotty et al., 2020a for microfacies description). The single selected sample from the Sakharnaya Quarry is a carbonate breccia from a mélange deposit.

4 – Methods

Combined with sedimentological analysis (Peyrotty et al., 2020a), cathodoluminescence observations, and consideration of global environmental change (i.e. sea–

level change, climate evolution, biosphere crises, etc.), geochemistry is a powerful tool for inferring the conditions of precipitation of carbonate cements. Moreover, given that cements of similar morphology and luminescence can precipitate in very different environmental conditions, geochemical characterization of carbonate cements is indispensable to depict the cements in terms of depositional environments. δ¹⁸O and δ¹³C ratios of carbonate cements are indeed useful for differentiating between conditions of cement precipitation (vadose, phreatic, open marine, burial, recrystallisation, etc.: Andrieu et al., 2018; Brigaud et al., 2009; Gross, 1964; Swart, 2015); ¹⁸O and ¹³C isotope analysis is, therefore, essential for a diagenetic study.

Moreover, trace, ultra–trace and minor/major elements (in this study: Fe, Mn, Ca, Mg, and rare earth elements + yttrium: REEY) are proxies of redox conditions, detrital input, and diagenetic alteration (Brand and Veizer, 1980; Brandstätter et al., 2018; Della Porta et al., 2015; Lawrence

82 et al., 2006; Li et al., 2017; Liu et al., 2019; Skinner et al., 2019; Smrzka et al., 2019; Swart, 2015; Tostevin et al., 2016). With the development of high–resolution analytical tools, REEY have been explored extensively as environmental proxies in the past decade (Della Porta et al.,

Fig. 4 – Early and late breccias from the Dalnegorsk limestone. A. Polygenic breccia made of carbonate clasts (light grey) and mudstone (dark parts). B. Polygenic breccia made of carbonate clasts associated with chert, sandstone and mudstone. C. Early carbonate breccia composed of angular clasts of various size. D.

Late carbonate breccia, often difficult to distinguish from early breccias. E. Close up view (thin section) of C. Angular reefal and peri–reefal clasts (some pointed by yellow arrows) cemented by sparry calcite (red arrows). F. Close up view (thin section) of D. Angular to sub–angular lagoonal clasts of various size (some pointed by yellow arrows) cemented by sparry calcite (red arrows). See Peyrotty et al. (2020) for the microfacies and sedimentological description. Scale bars: E, F 2.5 mm

83 2015; Lawrence et al., 2006; Tostevin et al., 2016), and their reliability has been adequately demonstrated. In this work, evidence from trace, ultra–trace and major/minor elements is combined with stable isotope results to refine our interpretations. One of the main uncertainties in any diagenetic study is the absolute chronology of diagenetic events. We use in situ U–Pb dating of calcite cements to estimate the age of each cementation phase. The combination of all these lines of evidence permits us to establish a high–resolution, coherent diagenetic history of the Dalnegorsk limestone. Due to the intense fracturing and random recrystallisation of the Dalnegorsk limestone (Peyrotty et al., 2020), bulk analyses or microdrilling are not appropriate:

these methods are applicable only on very well–preserved samples, and/or on cement deposits that are large enough to be hand–sampled.

4.1 – Cathodoluminescence petrography

146 uncovered and polished thin sections (40 µm thick), from eleven Dalnegorsk limestone outcrops (Fig. 3), were prepared for optical and cathodoluminescence (CL) petrography. CL petrography was conducted at the Department of Earth Science, University of Geneva, Switzerland, using an optical cold cathodoluminescence system (Cathodyne, by NewTec Scientific) with beam conditions of 8–12 kV at 180–200 µA, mounted on an OlympusBX41 microscope. Through microscopic examination (optical and CL) of all samples, a paragenetic sequence was established, based on the superposition or overlap of diagenetic features. The selection of the nine samples for in situ geochemical analysis was based on the visible preservation (no recrystallisation or overlap of several diagenetic phases) and size (large enough for geochemical analyses) of the targeted cements.

4.2 – Stable isotope measurements

Carbon and oxygen isotopes were measured at the Centre de Recherches Pétrographiques et Géochimiques (CRPG) in Nancy, France, using an IMS–1280 SIMS, built by CAMECA. This method is not widely used for sedimentological and diagenetic analyses of carbonates, and only a few very specific studies have previously used it (Allison et al., 2007;

Andrieu et al., 2018; Brodie et al., 2018; Cox et al., 2010; Rollion–Bard et al., 2007; Vincent et al., 2017; Warter & Müller, 2017). Nine thin sections (Table 1) were cut to fit the sample carrier, gold–coated, and analysed. The analytical method is detailed by Rollion–Bard et al.

(2007). The impact zone of the primary ion beam is approximately 20 µm in diameter and 1 µm in depth. A calcite crystal (CCcigA: in–house standard of the CRPG) with δ¹⁸O value of 18.94

±0.14‰ (2σ) SMOW (Standard Mean Ocean Water) and δ¹³C value of 1.04 ±0.10‰ (2σ) PDB (Pee Dee Belemnite) was used for calibration (Pfister et al., 2017). δ¹⁸O and δ¹³C were expressed as per–mil deviation from the PDB standard. For oxygen isotope measurements, the conversion from SMOW to PDB values was made using the Morse & Mackenzie (1990) equation (δ¹⁸OSMOW = 1.03086 × δ¹⁸OPDB + 30.86). The 2σ error range varies from 0.17‰ to 0.32‰ for oxygen isotopes, and from 0.38‰ to 1.31‰ for carbon isotopes. Fourteen transects were measured (one or two per thin section, depending on the targeted cement), each one comprising side–by–side analyses of oxygen and carbon isotopes (Figs. 5A, B, Fig. S1, supplementary material). In total, 122 couples of measurements were made, with a total of 244 points. The compilation of stable isotope measurements is available in the supplementary data (Table S2).

4.3 – REEY, trace, and minor/major element analyses

These analyses were made directly on seven thin sections after detailed screening of the calcite cements with CL microscopy. Images of each thin section under reflected light, natural light and CL, and the Chromium 2.1 software, were used to identify areas of interest, and to locate measurement points on samples in the laser ablation system chamber. REEY and other element analyses were performed at the Géosciences Paris Sud (GEOPS) laboratory of the University Paris–Saclay, France, using an 193 nm Photon Machines (TELEDYNE) laser coupled to a Thermo Scientific^TM High–Resolution (HR) ICP–MS ELEMENT XR (ThermoFisher Scientific). Each analysis included initial 30 s background and 30 s sample data acquisition intervals. The laser–induced aerosol was carried by helium from the sample cell to a mixing funnel, where the sample and helium were mixed with 0.950 to 1 l.min^–1 argon to stabilize the aerosol input to the plasma. The HR–ICP–MS signal strength was tuned for maximum sensitivity while keeping Th/U between 0.97 and 1.03, and ThO/Th below 0.3 on NIST612. The measured isotopes were as follows: ²⁴Mg, ²⁷Al, ³¹P, ⁴³Ca, ⁵⁵Mn, ⁵⁶Fe, ⁸⁸Sr, ⁸⁹Y,

137Ba, ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴⁶Nd, ¹⁴⁷Sm, ¹⁵³Eu, ¹⁵⁷Gd, ¹⁵⁹Tb, ¹⁶³Dy, ¹⁶⁵Ho, ¹⁶⁶Er, ¹⁶⁹Tm, ¹⁷²Yb,

175Lu, ²⁰⁸Pb, ²³²Th, and ²³⁸U. Elemental concentrations were calibrated using the NIST612 glass reference, and the MACS–3 carbonate pellet (USGS) as a secondary reference. The laser beam diameter for unknown calcite and MACS–3 reference calcite was 110 µm. Unknown calcite was ablated at a frequency of 8 Hz and a fluence of 4 J.cm^–2. NIST612 glass reference materials (37.38 ppm U and 38.57 ppm Pb) were ablated at a frequency of 10 Hz, a fluence of 6.25 J.cm^–

2, and a beam size of 40 µm. Calcite MACS–3 was ablated at a frequency of 10 Hz, a fluence of 1 J.cm^–2,and a beam size of 110 µm. Measurements were made in sequences of nine spots

85 on reference materials (five NIST612, four MACS–3), ten spots on unknown calcite, four spots on reference materials (two NIST612, two MACS–3), ten spots on unknown calcite, etc., ending with nine spots on reference material (four MACS–3, five NIST612). Data were acquired in fully automated mode overnight, in two sequences of ca. 125 analyses, for about six hours, on 19 December 2018 and 03 June 2019. Data were reduced in Iolite© using the

‘Trace Elements’ data reduction scheme (Paton et al., 2011). The NIST612 glass was used as the primary reference for correcting the baseline, time–dependent drift of sensitivity, and mass discrimination. ⁴³Ca was used as an internal standard, to account for variations in ablation yield, with an assumed calcite CaO content of 52.75 wt.% (Andrieu et al., 2018), lower than the content of CaO in pure calcite (56.08 wt.%). The values of REEY, trace and minor/major elements (shale–normalised) are expressed in ppm. For the MACS–3 (USGS) carbonate reference material, pellets of pressed calcite powder, the relative deviations of REE were generally within 20%. MACS–3 is heterogeneous, and fragments were dislodged during ablation (Lazartigues et al., 2013). In a recent review of MACS–3, Jochum et al., (2019) proposed a better sample preparation method to produce a new calcite reference. In this study, the concentrations of REE and Y were normalised to Post–Archean Australian Shales [(REEY)SN] (McLennan, 1989). La, Ce, Eu, and Gd anomalies were calculated geometrically, in the manner described by Lawrence et al. (2006):

La/La* = [La/(Pr × (Pr/Nd)²]SN

Ce/Ce* = [Ce/(Pr × (Pr/Nd)]SN

Eu/Eu* = [Eu/(Sm² × Tb)^1/3]SN

Gd/Gd* = [Gd/(Tb² × Sm)^1/3]SN

4.4 – U–Pb Dating

U and Pb measurements were performed on four well–preserved calcite samples with U and Pb concentration sufficient for isotopic analysis: (1) GP–138 (n = 61); (2) GP–141 (n = 39); (3) GP–172–B (n = 41); and (4) GP–179–A (n = 32) (Table 1). The U–Pb analytical and data–processing methods are detailed in the supplementary material (Text S1, Table S1). U–Pb laser ablation analysis was done at the Géosciences Paris Sud (GEOPS) laboratory of the University Paris–Saclay, France, using an 193 nm Photon Machines (TELEDYNE) laser coupled to a Thermo Scientific^TM HR–ICP–MS ELEMENT XR (ThermoFisher Scientific). The glass reference material NIST614 (0.823 ppm U and 2.32 ppm Pb: Jochum et al., 2011) and the calcite reference material from the Walnut Canyon Permian Reef Complex (WC–1: Roberts et al., 2017) were used as standards. Measurements were reduced using Iolite®. Ages were

Sampling

Area Sample Grain, matrix or cement REEY, trace and minor/major

elements measurments U/Pb dating Stable isotopes

determined from Tera–Wasserburg lower intercepts using free regressions Isoplot (Ludwig 2003). Ages are quoted at 2σ and include propagation of systematic and analytical uncertainties.

Table 1 – Synthesis of the performed measurements depending on each analyzed cement, grain or matrix.

5 – Results

A total of ten diagenetic episodes, including seven phases of calcite cement, were identified for the eleven Dalnegorsk limestone outcrops (Figs. 3; 5). The characteristics of each cement phase (cement morphology and cathodoluminescence; δ¹⁸O and δ¹³C; U–Pb age;

REEY; trace and minor/major elements), as well as the inferred environment of precipitation, are detailed in Table 2. Only median values of the isotopic data are cited in the text. The mean and standard deviation values (Table 2) were also taken into consideration for our environmental interpretation. The δ¹⁸O and δ¹³C values are presented in Table S2 (supplementary data). Trace, ultra–trace and minor/major elements and associated analytical errors, and U–Pb age estimates are presented in Tables S3 and S4, respectively (supplementary data). The resolved diagenetic events (from early to late) are as follows:

5.1 – Micritisation and mosaic cement (MC)

In the Dalnegorsk limestone, total or partial micritisation of the great majority of grains has been recognised as syndepositional (Peyrotty et al., 2020). A mosaic cement (MC, Fig. 5E)

87 occurs exclusively in open–lagoon facies, where it fills intergranular space. This cement is characterised by dark, non–luminescent, anhedral crystals of regular micrite size (30 to 50 µm).

Fig. 5 – Natural light and cathodoluminescence pictures of the analyzed cements and the related points of analyzes. A. Impact points of LA–ICPMS measurements made on the sample GP–138 (see also the Figure S1, supplementary material for other examples). B. Impact points of SIMS measurements made on the sample GP–138 (see also the Figure S1, supplementary material for other examples). C. Cathodoluminescence image of A. and example of SIMS and LA–ICPMS measurements made on the sample GP–138. The smallest dots correspond to SIMS measurements of oxygen and carbon isotopes. Biggest dots represent LA–ICPMS analyzes for traces elements + REEY and U–Pb dating. D. Cathodoluminescence image of B. and close–up view of the SIMS transect and the related analyzed cements. E. Dissolved foraminifers (involutinids, originally made of aragonite) in Sakharnaya area, filled by the dogtooth cement and the blocky cement. D.

Overview of the 2 early cementation stages observed in Verkhnii–Rudnik area. Note that ZDC present the exact same luminescence and morphology in Sakharnaya and Verkhnii–Rudnik areas. BC: Blocky Cement;

FC: Fibrous Cement; ZDC: Zoned Dogtooth Cement; MC: Mosaic Cement; DC: Dogtooth Cement. Scale bars: B, E 200 µm; A, C, F 500 µm.

88 MC is very rare in the studied samples, as it has been almost completely dissolved. Due to analytical limitations (small size of cement crystals; poor preservation), micritised grains and MC were not subjected to in situ geochemical analysis and dating. Micritisation and MC were followed by an event of pervasive dissolution, that partially removed skeletal grains (mouldic dissolution) and MC.

5.2 – Mouldic dissolution

The great majority of skeletal grains (foraminifera, bivalves, corals, sponges) in the Dalnegorsk limestone were originally composed of aragonite; as such, they are not preserved.

Mouldic dissolution was one of the very first events that affected the Dalnegorsk carbonate system, and was probably linked to a very early diagenetic episode. The Verkhnii–Rudnik area is characterised by reefal and open–lagoon facies (outer atoll setting), whereas the Sakharnaya area is dominated by lagoon and open lagoon facies (inner atoll setting) (Peyrotty et al., 2020).

The fill of skeletal moulds differs between the two areas: in the Verkhnii–Rudnik area, moulds are filled with the following succession of cements: (1) isopachous fibrous cement (FC) (Figs.

5C, D, F; 6A); (2) anisopachous zoned dogtooth cement (ZDC) (Figs. 5D, F; 6A); (3) blocky cement (BC) (Figs. 5C, D; 6A). In most samples from this area, an important event occurred before the precipitation of BC: formation of carbonate breccia (early breccia) (Figs. 4C, E; 7A), that were subsequently cemented by BC. In the Sakharnaya area, mould fill comprises the following succession: (1) anisopachous dogtooth cement (DC) (Fig. 5E); (2) anisopachous ZDC (Figs. 5D, F; 6A); (3) BC (Fig. 5E). Based on cement chronology, morphology and luminescence, ZDC and BC from both areas, are inferred to have formed during the same diagenetic events.

5.3 – Fibrous cement

Fibrous cement (FC), observed only in the Verkhnii–Rudnik area (outer atoll), consists of cloudy, non–luminescent calcitic fibres, 100 to 200 µm long and 10 to 20 µm wide (Figs.

5D, F). These fibres are characterised by negative δ¹⁸O values, ranging between −7.2‰ and

−2.5‰ (median = −4.9‰; n = 9), and δ¹³C values ranging from −3.8‰ and +3.5‰ (median = +0.5‰; n = 6, Fig. 7). Trace and other elements in FC are as follows (median values; n = 5):

Mg = 3290 ppm; Al = 5.31 ppm; Mn = 7.9 ppm; Fe = 260 ppm; and Sr = 163.6 ppm. The total REE concentration is very low: between 0.01 and 0.03 ppm. FC is marked by a notable negative

89 Ce anomaly (Figs. 8; 9B) and a strong negative Gd anomaly (Figs. 8; 10A). These anomalies occur only in this cement type. As shown in Figure 9B, FC presents a significant positive La

Fig. 6 – Natural light and cathodoluminescence pictures of the diagenetic events and analyzed cements. A.

Early carbonate breccia cemented by blocky cement. Note the presence of ZDC and FC on the angular clasts.

B. Bivalve shell neomorphism. Note the growth lines partially preserved on the close–up view (top left of the image). C. Late carbonate breccia cemented by thick isopachous (white arrows) and blocky luminescent (yellow arrow) cements (breccia calcite). D. Recrystallized oolitic grainstone from Karyernaya area. Note that the grains are almost not distinguishable under cathodoluminescence given the homogenization due to recrystallization. E. Recrystallized and intensely fractured lagoonal facies. F. Cathodoluminescence view of E. Note the numerous fracturation phases, represented by different luminescences. BC: Blocky Cement; FC:

Fibrous Cement; ZDC: Zoned Dogtooth Cement. Scale bar: 500 µm.

90 anomaly (median value = 8.76; n = 4), much higher than the La signal in other cement phases.

These REE signals are good markers of differentiation between cements, and suggest a non–

marine origin of the FC. In the sample successfully dated by U–Pb (GP–138), the average U concentration is 0.11 ppm and the average Pb concentration is 0.12 ppm (Table S4, supplementary data). The age of FC in sample GP–138 is 238 ±52 Ma (Fig. 11A). Since the Dalnegorsk limestone was deposited in the Norian (based on biostratigraphy: Peyrotty et al., 2020), between 227 and 208.5 Ma (ICC, v2019/05; Cohen et al., 2013), the maximum age of any cement is 227 Ma (Fig. 12). Consequently, the possible age of FC is between 227 and 186 Ma (Norian to Pliensbachian).

5.4 – Dogtooth cement

Dogtooth cement (DC) was observed only in the Sakharnaya area (inner atoll). It is characterised by non–luminescent, anisopachous, prismatic crystals of variable length (between 50 and 200 µm along the longest axis: Fig. 5E). Based on cementation sequence defined in cathodoluminescence, DC is interpreted to be synchronous to the FC (which is observed only in the outer part of the atoll), but is characterised by very different geochemical values. DC is the only cement type with positive δ¹⁸O values, between +1.3‰ and +2.2‰ (median = +1.7‰;

n = 3). Its δ¹³C values range between −3.0‰ and +2.5‰ (median = −0.3‰; n = 3). The Fe concentration is very high (3100 ppm; n = 1: Fig. 9B). The concentration of other elements is as follows (n = 1): Mg = 364 ppm; Al = 24 ppm; Mn = 12.90 ppm; and Sr = 309 ppm. The total REE concentration is 0.1 ppm (n = 1). DC is also distinguished by a slightly positive Eu anomaly (Figs. 8; 10B) and a Ce anomaly close to 1 (no anomaly) (Figs. 8; 9B). The geochemical difference between DC and its broadly contemporaneous FC is also manifested in the Y/Ho (mass ratio) and Nd/Yb (shale–normalised ratio, calculated to monitor LREE depletion), which are lower and higher in DC, respectively (Fig. 9A).

5.5 – Zoned dogtooth cement

ZDC makes up the second cementation phase, succeeding DC and FC in the Sakharnaya and Verkhnii–Rudnik areas, respectively. ZDC is characterised by subhedral, non–luminescent, anisopachous crystals, marked by several thin, highly luminescent zonations (Fig. 5D) indicating brief, punctuated changes in the composition of pore water. Crystal size is very

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