Petrology and Stable Isotopes of Patchy Dolostone of Ediacaran Dengying Formation in South Qinling, Central China: Implication for the Diagenetic History

Abstract

Dolostone, with its complex diagenetic history, has long been debate. The formation of patchy dolostone, which consists of various types of dolomites, is particularly controversial due to the superimposed diagenetic events. In this study, we conducted mineralogical and stable isotope geochemical analyses of the Ediacaran patchy dolostone from the Dengying Formation in South Qinling to elucidate the diagenetic processes involved. Microscopic examination reveals that the patchy dolostone can be classified into three types: Type I, composed of micritic to powdery crystalline dolomite (D1) and fine crystalline dolomite (D2); Type II, primarily consisting of medium-coarse crystalline dolomite (D3) along with D2; and Type III, characterized by the presence of saddle dolomite (SD) and D1. Cathodoluminescence and electron probe microanalysis indicate that D2 and D3 are enriched in Mn and Fe, whereas SD is depleted in Sr and Na compared to D1. Backscattered electron images reveal a prevalence of apatite, particularly in Type III dolostone. Calcite and quartz extensively replace the dolomites. Carbon and oxygen isotopes indicate that D1 has the highest δ13C and δ18O values, while D2 and D3 show negative shifts, and SD exhibits the lowest values. These observations suggest that the transition from D1 to D2 and D3 is due to recrystallization processes during burial. The formation of SD is likely the result of hydrothermal activity. Consequently, the patchy dolostone experienced three main stages of diagenesis, which included the recrystallization of D1 and the formation of SD. Considering the vertical distribution of the patchy dolostone and the occurrence of hyperkarst breccia dolostone, it is reasonable to infer that sea level variations are the primary cause of the formation of patchy dolostone.The Ediacaran represents a pivotal era in Earth’s history, characterized by remarkable geological, oceanic, and biological changes [1, 2]. During this period, various types of dolostones were extensively developed [3]. Dolostone, which serves as a rock unit documenting the intricate history of sedimentation and diagenesis, has long been a subject of debate and extensive research in geology [4-8]. Various conceptual models, such as the evaporation pump, brine seepage reflux, mixed-water dolomitization, and hydrothermal dolomitization, have been proposed based on these investigations [9-16]. The development of thick dolostone is controlled by external factors, including the paleoclimate background, relative sea-level changes, and tectonic activity [17-20]. This is particularly significant when studying shallow-marine carbonate, where multiple occurrences of these factors may occur during penecontemporaneous or early burial periods, leading to various dolomitization processes. Patchy dolostone, a unique sedimentary fabric characterized by “patchy” sedimentary records and a complex diagenetic evolution [21-24], is controversial for hydrothermal genesis [25, 26] or earlier diagenesis [27-29].Ediacaran patchy dolostone of Dengying Formation is widely developed in the South Qinling (SQL) region and the Yangtze Craton [30-33]. This dolostone appears heterogeneous and consists of various types of dolomites, recording the evolutionary history of distinct dolostone types [34, 35]. Therefore, a comprehensive investigation into the sedimentary and diagenetic evolution of the dolostone will enhance our understanding of the superimposed diagenetic events and offer insights into the sedimentary and environmental changes at the end of the Neoproterozoic. Based on field observations, microscopic petrological analysis, and combined cathodoluminescence, electron probe microanalysis, X-ray analysis, and carbon and oxygen isotope analyses, this study provides constraints on the diagenetic environment of the patchy dolostone and reveals its diagenetic history.The SQL region is located in the northern part of the Yangtze Craton, along the Mianlue fault [31, 36] (Figure 1(a) and (b)). The tectonic structure of the study area includes a series of NWW-trending faults with thrust and nappe structures originating from south to north (Figure 1(c)). Paleoproterozoic metamorphic rocks form the basement, which is unconformably overlain by greenschist-facies metamorphosed Mesoproterozoic to Neoproterozoic clastic and volcanic rocks. SQL was shaped by intense extension during the Ediacaran period in response to the final breakup of the Rodinia supercontinent [37, 38]. Ediacaran clastic rocks, dolostones, and early Paleozoic clastic rocks and carbonates form the sedimentary cover of the basin, which is subdivided from base to top into three formations: Doushantuo, Dengying, and Shuigoukou [39-43].The Xichuan area is located north of the SQL region [44]. Neoproterozoic mafic and felsic volcanic rocks of the Yaolinghe Group (680, 650 Ma) are believed to record the latest rift-related magmatic activity in the SQL [45-48]. The Doushantuo and Dengying Formations unconformably overlie the Neoproterozoic volcanic rocks (Figure 1(c) and (d)). The Doushantuo Formation primarily consists of sandstones and schist, which is overlain by the Dengying Formation, composed mainly of dolostone [31, 49-51] (Figure 1(d)). During the Ediacaran, the sedimentary environment evolved from a mixed carbonate-siliciclastic ramp characteristic of the Doushantuo Formation to a rimmed carbonate platform typical of the Dengying Formation [41] (Figure 1(b)). The platform can be divided into subfacies, including the sublittoral, interlittoral, and suplittoral zones [42, 52]. The upper section of the Dengying Formation is predominantly composed of a ca. 23-m thick matrix dolostone, which consists of a homogeneous formation of micritic to powdery crystalline dolomite. In contrast, patchy dolostone is primarily developed in the middle and lower sections, interbedded with 3-m thick matrix dolostone and hyperkarst breccia dolostone. The lower Cambrian Shuigoukou Formation, which mainly consists of mudstone and siliceous rock, unconformably overlies the Dengying Formation and is indicative of an inner-shelf environment [31] (Figure 1(d)).This study involved measuring and sampling the Huwo (HW) and Daquangou (DQG) geological profiles in the Xichuan area (Figure 1(c)). Detailed descriptions of the sedimentary environments can be referred to in Zheng et al. [42] and Liu et al. [52]. Samples were collected at 2–7 m intervals up-section. Dolostone samples were selected for cleaning, drying, and grinding alizarin-red stained flakes and probe sheets at the Hebei Regional Geology and Mineral Resources Survey laboratory. Cathodoluminescence (CL) analysis was performed using a CL8200MK5 instrument (CITL coupled with a AxioScope). The selected beam voltage was 12 kV and the beam current was set at 250 μA.Electron probe micro-analysis (EPMA) and X-ray diffraction (XRD) were conducted at the Laboratory of China University of Geosciences (Beijing). EPMA was conducted based on CL’s results. The instrument model used was EPMA-1720, with an experimental voltage of 15 kV. The incident electron beam current was 10 set to nA, and the beam spot diameter was 1 μm. Additionally, X-ray maps were obtained from wave-dispersive spectrometer (WDS) analyses, which were conducted at a voltage of 15 kV and a beam current of 20 nA. The detection limits in terms of weight percent oxides for the elements analyzed in these experiments are estimated as follows: Na2O at 0.05 wt%, SrO, Fe2O3, and MnO at 0.03 wt% [53].Isotope analysis was conducted at the Stable Isotope Laboratory of Louisiana State University using the phosphoric acid method. This method involves the addition of 1 mL of high-concentration phosphoric acid and allowing it to react fully for 1–2 hours at 75°C. Subsequently, the CO2 gas produced was sent to MAT253 stable isotope gas mass spectrometer via GasBench for the determination of carbon and oxygen isotopes. The accuracy of δ13C analysis is better than ±0.1‰, and the accuracy of δ18O analysis is better than ±0.3‰. All analysis standards were V-PDB standards [54].The patchy dolostone of the Dengying Formation in the Xichuan area can be categorized into Type I, Type II, and Type III based on the crystal shape and size of the dolomite [55]. Type I patchy dolostone is macroscopically present in ca. 2-m thick layer, primarily characterized by a light fleshy red color with interspersed gray patches (Figure 2(a) and (b)). This type of dolostone is composed of micritic to powder crystalline dolomite (D1) and fine crystalline dolomite (D2) (Figure 2(c)). The surface of the D2 crystals appears cloudy and predominantly features subhedral shapes that are tightly packed (Figure 2(d)).The weathered surface of Type II patchy dolostone is gray-black, and the fresh surface is fleshy red. There are light fleshy red color patches (Figure 2(e) and (f)). The grain size of dolomite crystals varies greatly, and medium-coarse crystalline dolomite (D3) is dominant. D3 crystals are predominantly subhedral to euhedral and are tightly intergrown with D2 crystals (Figure 2(g) and (h)).Type III patchy dolostone is characterized by a gray, 3-m-thick layer that exhibits an apparent hackly cleavage, a feature typically associated with the occurrence of saddle dolomite (SD) (Figure 2(i)). SD is a milky white or gray sparry dolomite crystal with a distinctive pointed, curved crystal face [56]. This dolomite appears milky or off-white, forming strips or patches with dissolution holes that remain unfilled (Figure 2(j)). The SD grains, which contain micro-cracks, display a noticeable undulating extinction when compared to the D1 crystals, owing to their cloudy, rough, and curved surfaces (Figure 2(k) and (l)).D1, D2, and D3 show closer paragenetic affinities (Figure 2(c), (d), (g), and (h)). CL imaging reveals that D1 within Type I patchy dolostone appears as a dark tangerine color, while D2 displays a dark red hue (Figure 2(m) and (n)). In Type II patchy dolostone, D3 also presents a dark red color, similar to D2 overall, but with distinct bright red edges (Figure 2(o) and (p)). The bright red coloration of the SD in Type III patchy dolostone stands in stark contrast to the darker tones of D1 (Figure 2(q) and (r)). EPMA analysis indicates that D1 of Type I patchy dolostone is characterized by high Sr and Na content and comparatively low levels of Mn and Fe (Figure 3(a) and (b)). Compared to D1, D2 shows a slight decrease in Sr and Na and an increase in Mn and Fe (online supplementary Table S1 and S2). D3 displays variable Fe values from the core to the rim of the crystals (Figure 3(c) and (d)). X-ray mapping reveals that Fe content varies significantly between D2, D3, and D1, while Ca and Mg concentrations remain relatively constant (Figure 3(e)–3(x)). Backscattered electron (BSE) images reveal that quartz and calcite are extensively distributed within the patchy dolostone (Figure 4). D1 is observed within the core of the gypsum grain (Figure 4(d)). Flaky rhodochrosite and spotted fluorite are scattered along the edge of calcite from Type I and are found within calcite veins in Type II dolostones, respectively (Figure 4(f) and (m)). Apatite is commonly found throughout Type III dolostone (Figure 4(p)).Carbon and oxygen isotope analysis reveals a broad range of isotopic values for D1 to D3 and SD (Figure 5). The content of δ13C in the D1 ranges from 1.03‰ to 5.17‰, with an average of 2.67‰. δ18O ranges from −6.11‰ to −0.79‰, with an average of −4.29‰, which is identical to that of the matrix dolostone in this study. D2 and D3 have negatively shifted δ13C and δ18O values compared to D1. δ13C range in D2 is from 0.84‰ to 2.33‰, with an average of 1.53‰, and δ18O is from −7.85‰ to −5.85‰, with an average of −6.85‰. In the case of D3, the δ13C content in the D3 varies from −0.07‰ to 3.19‰, with an average of 1.94‰, while the δ18O content ranges from −9.46‰ to −7.36‰, with an average of −8.40‰. SD has the most negative δ13C and δ18O values compared to the dolomites mentioned above. δ13C values in SD range from −0.02‰ to 1.31‰, with an average of 0.43‰, while δ18O from −12.68‰ to −6.97‰, with an average of −8.98‰ (online supplementary Table S3 and S4). Carbon and oxygen isotope values of carbonate rocks can be utilized to estimate the temperature, salinity, and diagenetic environments [57, 58]. The results are presented in online supplementary Table S4.The content of divalent Mn and Fe ions in dolomites is highly correlated with the corresponding values of pore fluids in sedimentary or diagenetic environments [59-63]. The mineral luminescence features and intensity are related to the contents of divalent Mn and Fe ions in the crystals, as well as the ratio between these ions [64-66]. D2 in Type I patchy dolostone shows a dark red color in CL images compared to D1, indicating increased Mn and a weak oxidization–reduction in the burial environment (Figure 2(m) and (n)) [67, 68]. Despite its bright red edges, D3 of Type II patchy dolostone exhibits a dark red color and has a composition that is similar to D2 (Figure 2(o) and (p)). Compared to D1, the formation of D2 and D3 may be associated with a diagenetic environment characterized by increased reduction and deeper burial. In Type III patchy dolostone, SD exhibits a brighter red color in the CL image than D1 (Figure 2(q) and (r)). This indicates that the SD also formed in the burial environment.Carbon isotope fractionation during the diagenetic process significantly decreases the δ13C value, and both atmospheric freshwater and hydrothermal environments can lead to negative values of δ13C and δ18O [69-76]. Compared to D1, D2 and D3 display more pronounced negative shifts in δ13C and δ18O values, with D3 showing an especially substantial degree of these negative shifts. In conjunction with petrologic data, it is inferred that D2 and D3 may have recrystallized from the early-formed D1 and D2. During this recrystallization process, carbon isotope fractionation notably decreased the δ13C value. The general trend suggests that as burial depth increases, there is a corresponding rise in temperature and reducibility, an enhancement of recrystallization, and a progressive thickening of dolomite crystal sizes (Figure 6) [77-79]. Furthermore, the trace element content of the D3 core is identical to that of D2, which further proves that D3 was formed through the diagenetic transformation from D2 (Figure 3(c) and (d)).CL and negative δ18O values indicate that SD was formed at a higher temperature and with improved closure [80]. δ13C of SD shows decreased values, and the presence of the associated hydrothermal mineral, apatite, indicates that SD was formed through hydrothermal processes (Figure 4(p) and Figure 6) [81-84].During the deposition of the Dengying Formation, loose calcium deposits underwent a transformative process, resulting in the formation of matrix dolostone through the evaporative influx of magnesium ions in the Xichuan area [42]. The grain size of the matrix dolostone varies from micrite to powder, that is, from micritic to powdery D1. The association of dissolution vugs and breccias with SD suggests that basinal-derived hydrothermal fluids may have been involved in the dolomitization and associated dissolution processes [7, 49]. During the burial, hydrothermal fluids were transported into the dense matrix dolostone layers, resulting in the formation of “dendritic” SD within the D1 formation and leading to the development of Type III patchy dolostone.Therefore, the overall diagenetic environment and mineral paragenetic relationship can be shown in Figure 7. The formation of matrix dolostone, predominantly composed of D1, is indicated by the presence of spotted gypsum, which suggests a sedimentary phase close to the surface. Following compaction and recrystallization, Type I patchy dolostone formed in a shallow burial environment, while Type II patchy dolostone emerged in a medium-deep burial environment, marked by hydrothermal dissolution. Type III patchy dolostone was formed through hydrothermal metasomatism in a deep burial environment [56]. Late-stage mineral filling, consisting of fluorite, rhodochrosite, and calcite, occurred at various burial stages. Concurrently, rupture and the development of hyperkarst breccia dolostone took place during the periods of exposure. Throughout most phases of diagenesis, quartz and calcite replaced dolomite, serving as the cementing agents (Figure 7).An overall paleogeographic configuration in SQL was a platform during the Ediacaran (Figure 1(b)) [42]. The recrystallization of matrix dolostone in a shallow burial environment resulted in the formation of Type I patchy dolostone. With the continued accumulation of deposits and the burial depth increased (the phase of medium-deep burial), pre-carbonate deposits underwent a subsequent diagenetic stage characterized by the formation of dolomite typical of intermediate depths, leading to the development of Type II patchy dolostone (Figure 7). When sea levels dropped, preformed dolostone layers in exposed environments were subjected to leaching by atmospheric freshwater and erosion by weathering. This can be confirmed by the presence of three layers of hyperkarst breccia dolostone, which are associated with meteoric water percolation and dissolution, as observed in the HW and DQG sections [49]. The formation of Type III dolostone is caused by the infiltration of hydrothermal fluid into the shallow D1 strata. The alteration of the diagenetic environment caused by sea level change resulted in the formation of different types of dolomites.The salinity of three types of dolostones decreased gradually while the temperature increased continuously (online supplementary Table S4). By comparing the lithology of the HW and DQG sections, it can be inferred that there were three main periods of low sea level during the sedimentary and diagenetic periods of the Dengying Formation (Figure 8).The Dengying Formation in the Xichuan area exhibits three distinct types of patchy dolostone. Type I is composed of micritic to D1 and D2, whereas Type II displays significant grain variation and is primarily constituted by D2 and D3. The δ18O values of D1, D2, and D3 in both Type I and Type II decrease with increasing dolomite grain size, and the core trace elements of D3 are analogous to those of D2. Petrographic, mineralogical, and isotopic analyses suggest that D1, which constitutes the matrix dolostone, formed near the surface. In a burial environment characterized by increased reduction and temperature, D2 was formed through recrystallization of D1, which resulted in the formation of Type I patchy dolostone. D3 formed through recrystallization of both D1 and D2 in a medium-deep burial environment, leading to the development of Type II patchy dolostone. Type III is distinguished by SD patches, dissolution holes, and hydrothermal minerals resulting from hydrothermal fluids infiltrating dense matrix dolostone layers. Quartz and calcite commonly replace dolostone as cement in various diagenetic stages. Cyclical fluctuations in sea level and variations in the thickness of overlying strata deposits have contributed to a specific evolutionary relationship within the patchy dolostone of the diagenetic environment. The overall lithology evidence indicates that sea level variations during the deposition of the Dengying Formation influenced burial and diagenesis.All data supporting the results can be found in the supplementary tables; further inquiries can be directed to the corresponding author.The authors declare that they have no conflicts of interest.We appreciate Prof. Jinhua Hao, Deshun Zheng, and Yongbo Peng for their kind guidance in fields and experiments. Sicong Liu and Yuan Zhang are especially acknowledged for their help in fieldwork. Special thanks to the editors and anonymous reviewers for providing constructive comments that improved the quality of the manuscript. This research is supported by the National Natural Science Foundation of China (42202101), Outstanding Youth Fund Project of Natural Science Foundation of Henan Province (242300421146), International Scientific and Technological Cooperation Project of Henan Province (232102520009), and Fundamental Research Funds for the Universities of Henan Province (NSFRF210323).