Abstract: This study presents a microscopic image dataset of the Triassic-Jurassic clastic rocks in Mid-Yangtze. Our samples are mainly medium to coarse sandstone. We systematically analyzed and partially counted detrital components of 86 samples, and after a detrital modal analysis, identified eight types of sandstone, including lithoquartzose, quartzolithic, and feldspathoquartzose. Note that 287 microscopic images of them were taken. We mainly deciphered the microscopic characteristics of metamorphic grains and further classified their metamorphism rank and their protoliths. The establishment of this dataset helps enrich the skeleton research of the Mesozoic sedimentary evolution in mid-Yangtze. It provides references for identifying and classifying metamorphic grains, tracking their places of origin, or reconstructing the exhumation process of orogenic belts.
Keywords: Mesozoic; metamorphic grains; mid-Yangtze; microscopic image
|Title||A microscopic image dataset of Mesozoic metamorphic grains bearing sandstones from mid-Yangtze, China|
|Data corresponding author||Du Yuansheng (firstname.lastname@example.org)|
|Data authors||Ma Qianli, Chai Rong, Yang Jianghai, Du Yuansheng, Dai Xianduo|
|Time range||The rock samples were attributable to a stratigraphic age ranging from Triassic to Jurassic. They were collected from 2016 to 2019 and microscopic images of the samples were taken in 2019.|
|Geographical scope||The study covers an administrative region of Yichang-Badong City, Hubei Province. In terms of peripheral tectonic units, it neighbors Shennongjia Terrane and Qinling Orogenic Belt in the north; Sichuan Basin in the west; Jianghan Basin in the east; Jiangnan-Xuefeng Orogenic Belt in the south and southeast. GPS scope: 110°18'1.4″E–111°46'9.48″′E; 31°22'41.61″N–30°52'52.21″N.|
|Polarized microscope resolution||1000*800–2600*1900 pixels|
|Data volume||1.53 GB|
|Data format||*.jpg; *.xls|
|Data service system||<http://www.dx.doi.org/10.11922/sciencedb.j00001.00046>|
|Source of funding||National Natural Science Foundation of China (Grant No. 41672106)|
|Dataset composition||1. The dataset was stored as a compressed package sized 1.54 GB, including two folders and a form file. The two folders are: “A micrograph dataset of Mesozoic metamorphic grains bearing sandstones from mid-Yangtze, China”, including 86 subfolders and 287 images added up to 784 MB, and “A micrograph dataset of Mesozoic metamorphic grains bearing sandstones from mid-Yangtze, China_unlabeled version”, including 86 subfolders and 290 images sized 826 MB, respectively. The information sheet is named “Clastic rock micrograph database information table”, which is a form document sized 27.9KB.|
Sandstones are products of a series of geological processes including weathering, transportation, and sedimentation. Sandstones are also primary archives of the source-to-sink system and its evolution information. Analysis and statistics on sandstone detrital component are the fundamental means to interpret the above information[2–4]. First, figuring out the detrital mode is the basis of terminology naming and description of sandstones, let alone reflecting the possible mineral assemblage of the parent rock. Second, with the development of the micro-petrography and establishment of Dickinson diagrams in the last century[2,5], the detrital mode of medium-coarse sandstone has been associated with tectonic background. Despite subsequent studies[6–9] revealed that the sandstone component was comprehensively controlled by various factors ranging from parent rock properties, weathering intensity, sorting, recycling, diagenesis, and metamorphism, there is no matchup between the detrital mode and tectonic background. The statistics analysis e.g. point-counting is still an indispensable method to provide insight into scientific issues involving origin analysis and sedimentary basin evolution.
Detrital mineral or metamorphic lithic fragments are considered indicative of some specific source regions. For example, abundant metapelite fragments in Mesozoic Zigui-Dangyang Basins were suggested to be derived from the Silurian in South Qinling Belt or the Mianlue Suture Zone. In another case, in Jianghan Basin, which is in juxtaposition to the Dabie Orogenic Belt, the occurrence of phengite in the Lower Jurassic suggested the Ultra High-Pressure rocks (UHP) had been exhumed to the surface during that time. In terms of Dickinson diagrams, Garzanti proposed that several end-members including ophiolite suite and collisional metamorphic zone as well as more kinds of detrital mineral assemblage should be added. Garzanti’s new diagrams were also applied to variation tendency different in various tectonic settings. The metamorphic grains or fragments could trace the exhumation history of orogens. Hence, it has been crucial to recognize and further classify them.
The metamorphic rocks have a wide variety of protolith and metamorphism grade in nature, which makes lithic metamorphic fragments (Lms) difficult to identify and statistics. There is some current controversy to the classification criteria of Lms. First, what parameters should be referred to signify different ranks of Lms. The classification scheme by Garzanti and Vezzoli is recommended in this study Second, whether pseudo-matrix, as well as metasomes, should be counted as part of Lms. Feldspars in sandstones are apt to carbonated metasomatism, albitization, pressure solution, and alteration of kaolinite, sericite, and some epidote minerals[14-15]. Beyond that, diagenesis will also transform some ultramafic to mafic or pelite fragments to pseudo-matrix. Pseudo-matrix and metasoma should be counted as protolith. As for severely altered samples, statistics works are less necessary.
This study reports the Mesozoic clastic rocks distributed along with the Northern Yangtze Block (NYB). With the convergence and collision of South China and North China Plates, and uplift of Qinling Orogenic Belt[18-20], the long history of marine evolution terminated in the South China Plate and a series of peripheral foreland basins including Sichuan Basin, Zigui Basin, and Dangyang Basin formed on NYB. Among them, significantly thick marine-terrestrial successions documented contemporaneous geological processes ranging from continental collision, marine-terrestrial shift, and exhumation of uplifted Qinling Orogenic Belt, which appealed to scholars[22–23]. For the spirit of sharing, open communication, and providing fundamental research material for successors, we systematically compiled microscopic images from Triassic to Jurassic and probed them into metamorphic grains or fragments. We hope our works could enrich the investigation achievements involving NYB and stir up a discussion on micropetrography.
2.1 Data acquisition method
All the microscopic images were developed on Middle Triassic to Middle Jurassic samples collected from 7 sections (Table 1) which were located in the West Hubei Province, China. Figure 1 demonstrates the sampling sites and Figure 2 simply shows the stratigraphic column of the study area. We thoroughly investigated those strata and commissioned the Grinding Studio of the China University of Geosciences to fabricate optical thin slices. First, the less weathered medium to coarse sandstone would be selected. Then, the surface layer will be removed. The sample would be approximately cut into a 30 mm×30 mm sized and 10 m thick dice. Finally, according to the technical requirements, the dice will be processed by a coarse grinding, fine grinding, resin fixation, coarse polishing, and fine polishing into the optical chip of 0.03 mm standard thickness.
|Sampling Sections (number of sections)||GPS|
|Badong Section, Enshi City, Hubei Province (G209)||31°5'7.994"N, 110°24'50.85" E|
|Xiaozhanghe Section, Xiangyang City, Hubei Province (XZH)||31°22'31.566"N, 111°45'23.346"E|
|Lianghekou, Zigui County, Hubei Province (LH)||30°54′09.23″N, 110°35′09.10″E|
|Hongqi Coal Mine Section, Enshi City, Hubei Province (J1X)||31°4′55.3008"N, 110°26′17.51″E|
|Xuanwudong Section, Zigui County, Hubei Province (XW)||30°54′59.5548″N, 10°43′0.6996″E|
|Guojiaba Section, Zigui County, Hubei Province (WH)||30°57′16.99″N, 110°45′26.41″E|
|Xiakou Section, Xingshan County, Hubei Province (DA/DB/DC)||31°8′54.6755″N, 110°47′12.1128″E|
2.2 Data Preprocessing
2.2.1 Naming of the Sandstone
Naming rules of the sandstone are based on the content of quartz, feldspar, and lithic fragments, respectively. Moreover, some coarse grains subject to the Gazzi-Dickinson point counting method contribute to reducing the deviation during the statistical process[17,24–25]. Owing to the complexity of the sandstone composition, the naming scheme should directly reflect the composition information. Thus, in this study, we name our sample according to the criteria established by Standards for digital micrographs of the sedimentary rocks . This paper also listed another scheme of Chinese Standard GB/T 17412.2–1998 Sedimentary Rocks Classification and Naming Scheme as reference (Fig. 3).
2.2.2 Classification of metamorphic lithic fragments
In terms of metamorphic lithic fragments among sandstones, Garzanti and Vezzoli13 initially divided them into metapelite, metapsammite, metafelsite, metacarbonate, and metabasite based on their protolith types. In addition, according to the metamorphic texture and phyllosilicate types, each category will be subdivided into five ranks of metamorphism from low to high. A typical image album of the above grains was also given. This atlas will adopt this scheme and complete the classification (Table 2).
|Rank||Texture||Phyllosilicates||Garzanti and Vezzoli13|
|None||Unoriented||clay mineral||Lv, Ls|
|Very Low||rough cleavage||illite、chlorite||Lmp1||Lmf1||Lmc1||Lmb1|
|Very High||crystal≥62.5 μm||biotite||Rmp5||Rmf5||Rmc5||Rmb5|
2.2.3 The catalog of rock microscopic images
Standard rock thin sections were photographed at China University of Geosciences, Wuhan. The thin section was observed under the Leica microscope at 100 to 200 times magnification. The grain’s type, size, roundness, and matrix composition were also counted or recorded. We emphasized lithic fragments, their protolith, microscopic texture, and mineral assemblage. Whereafter, the lithic metamorphic grains were classified under the scheme of chapter 1.2.2, and an appropriate angle was selected for taking photographs subsequently. The information collection and compilation proceeded in strict accordance with criteria proposed by Standards for taking and information collecting of digital photomicrograph of sedimentary rock . The catalog of images was named regularly by the following rules: ‘serial number of thin sections’ + ‘serial number of viewshed’ + ‘plane-polarized light symbol – or cross-polarized light +’. All digital images under the same field of vision were placed in one folder. This dataset contains 86 thin rock sections and 287 microscopic images. The resolution ratio was between 1000×800 and 2600×1900. Besides, detrital minerals and lithic fragments were labeled. This dataset also included another unlabeled version so that the interference of mineral marker to computer reading and learning would be avoided.
This dataset consists of three parts, which are the stratigraphic column of measured geological sections, a microscopic image dataset, and a sheet of database information. The column map demonstrates lithostratigraphic units and lithologic sequence (Fig. 2). The dataset compiled different vision under both plane-polarized light and cross-polarized light. These photographs were taken from 9 siltstones, 74 sandstones, and 3 conglomerates collectively. As for the sheet of database information, it documented the description of each thin section and introduction of samples.
This dataset is comparatively weighted in lithic metamorphic grains. The following are various microscopic images of some representative lithic metamorphic grains or fragments. For those samples with less lithic metamorphic grains or less typical texture, we cataloged them only for reference. Metamorphic grains or lithic fragments were uniformly marked as ‘Lm’ and will not be discussed in detail.
3.1 Microscopic images of the Lm bearing sandstone
3.1.1 Middle Triassic sandstone
Sample XZH-18-2 (Fig. 4) was medium to fine sandstone and collected from the 1st Member of Badong Formation in Xiaozhanghe Section, Xiangyang City, Hubei Province. It was moderately sorted and contained angular grains. Detrital quartz and feldspar displayed colorless to isabelline. Opaque hematite and few lithic fragments appeared dark red. As for the content of different grains, the detrital quartz was the highest followed by the matrix that was less than 15%. Hence, this sample was assigned to feldspathoquartzose sandstone. Quartz grains were nearly spotless and first-degree grey interference color under the cross-polarized light. The plagioclase grain underwent carbonatization or alteration so that polysynthetic twinning could be dimly seen. The lithic fragments included plate-like volcanic fragments, sedimentary ones represented by carbonate fragments, and plentiful metamorphic ones. Based on Lms protolith type, schistosity degree, and phyllosilicate enrichment, we distinguished several types of Lms that are Lmf1, Lmp3, Lmb1, Lmb2, and Lmc3 (Table 2) respectively.
3.1.2 Upper Triassic sandstone
Sample XZH-77-6 (Fig. 5) was fine-sandstone from the Xiaozhanghe Section but deposited in the 3rd Member, which had already belonged to the Late Triassic. Most of grains were poorly sorted and displayed colorless or isabelline. As for the matrix, deep red hematite took the highest priority. Quartz was the most abundant detrital grains and lithic fragments were the second most. Above all, this sample was named lithoquartzose. Detrital quartz was granulous or angular. A few plagioclase and lithic fragments showed the strip shape and experienced alteration considerably. The Lms of this section were recognized as Lmc1, Lmc2, Lmf2, and Lmb3. Among them, the medium metamorphic grains were distinct for their plagioclase phenocrysts and schistosity.
Sample XW-11-1 (Fig.6) was coarse sandstone of Jiuligang Formation from the Xuanwudong Section, Yichang City. The grains were better sorted and sub-angular to sub-rounded. Detrital monocrystalline quartz was the most abundant mineral, followed by minor plagioclase and polycrystalline quartz. Metaargillaceous (Lmp1) and metafelsite (Lmf2) fragments suggested the low metamorphic grade. As for the latter, tiny detrital quartz was distinct and formed a strong cleavage with arrayed sericite. Hence, sample XW-11-1 was identified as lithoquartzose.
3.1.3 Lower Jurassic sandstone
Sample XW-67-1 (Fig. 7) was medium to fine sandstone from the Xuanwudong Section. The grain size fell in 0.05–0.1 mm and the matrix was greater than 15%. This sample was named lithoquartzose wackestone according to the QFL diagram. The lithic fragments included volcanic (Lv), sedimentary (Ls) and metamorphic (Lm) ones. Among them, the Ls could be divided into several groups ranging from detrital (Ld) and carbonate (Lc), while the Lm could be categorized as Lmf2 and Lmp2. The difference between the above two grains lied to the obvious detrital grains with the former and clay minerals that were too subtle to recognize with the latter.
Figure 7 Microscopic image of Section XW-67-1m1- and XW-67-1m1+
Abbreviation: Qm-Monocrystalline Quartz; Qp-Polycrystalline Quartz; Pl-Plagioclase; Ld-Detrital Lithic Fragments; Lc-Detrital Carbonate Fragments; Lv-Volcanic Lithic Fragments. Detailed information about metamorphic lithic fragments was listed in Table 2.
3.1.4 Middle Jurassic sandstone
Sample DA-27-6 (Fig. 8) was a coarse sandstone from the Middle Jurassic Qianfoya Formation. Geographically, it was collected from Xiakou Section, Xingshan County, Yichang City. The average grain size was above 0.2mm and demonstrated colorless or drab crystals. Feldspar was the highest component and existed as tabular. Some of these underwent albitization or were replaced by carbonated rocks. As for fresh microcline grains, tartan twinning could be observed. The abundance of detrital quartz was comparatively low. This sample was classified into lithofeldspathic sandstone. The lithic fragments include Lmf1, Lmb1, and Lmb3. Particularly, the Lmb3 was characterized as encapsulated epidote, plagioclase phenocrysts, and flake mica minerals.
Sample DA-33-1 was also coarse sandstone from Xiakou Section (Fig. 9). It was grain-supported, and the matrix was less than 15%. Mainly detrital quartz and lithic fragments constituted the sample so that it was named lithoquartzose. The chert fragments and Lmf2 dominated lithic fragments.
Sample DB-42-2 (Fig. 10) was medium to coarse sandstone that was also from Xiakou Section. It was colorless to grey-brown and fairly poor sorted. Quartz grains appeared to be subangular, predominantly followed by elongated feldspar and metamorphic fragments. The K-feldspar underwent kaolinization and seemed earthy. Metamorphic fragments included Lmc1, Lmp1, and Lmf4, respectively. For Lmf4, it represented the high-grade metamorphism, and muscovite was generated after recrystallization.
Sample DC-50-1 and DC-61-1-2(Fig. 11) were both taken from Xiakou Section. They are feldsapthoquartzose and lithoquartzose respectively. In addition, similar types of lithic fragments were incorporated in these two samples. The protolith of the lithic fragments ranging from chert, volcanic rocks, and metamorphic rocks. Particularly, the metamorphic grains could be further divided into Lmp2, Lmf2, Lmf3, Lmb2, and Lmb3. It is worth noting that there are many zoisites in sample DC-61-1-2.
3.2 Database information sheet of clastic rocks microscopic images
All samples involved in this study were collected from the Middle Triassic to Middle Jurassic distributed at the west Hubei Province. Geographical location and GPS coordinates were included accurately. Sampling sites were labeled in Figure 1. Figure 2 documented lithological sequence of measure sections. The above procedures ensured the integrity and repeatability of sampling. Thin sections were processed according to general technological requirements. During the observation and photography, detrital quartz in the same batch of sections possessed uniformly grey to pale inference color. The yellowing phenomenon usually resulted from an inhomogeneous thickness of thin sections and no discoloration was found within all sections. Consequently, the thickness of sections conformed to the national criteria of 0.03 mm.
Photographing work was completed at the China University of Geosciences, Wuhan. To avoid color differences and maintain consistency with unaidedly viewing, automatic exposure and white balance were adopted during this process. Images were saved in jpg format. Distinguishability fluctuated according to the grain size and magnification times. The median was 1805*811 pixels. To sum up, the quality and definition of microscopic images were reliable.
This microscopic image dataset systematically compiled the Middle Triassic to the Middle Jurassic samples from Northern Middle Yangtze Block. We focused on the classification and observation of metamorphic grains. Beyond that, we discussed microscopic features of Lms-bearing sandstones. This set of microscopic data not only successively but also completely documented a series of geological processes including the marine-terrestrial transition and sedimentary evolution of the Middle Yangtze Block. It will be a supplement to succeeding relevant studies. Besides, this dataset also uploaded another version unmarked, which would be materially studied by computers later.
Thanks, Guest Editors-Prof. Hu Xiumian and Prof. Hou Mingcai for their invitation. We appreciate the hard work by Song Delin and Zhao Chunlei while sampling and convenience brought by Ouyang Guang and Yu Yongshun while photographing. The same gratitude was expressed to Shao Hui and Xu Chang for their guidance on authentication, which improved this study.
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MA Qianli, CHAI Rong, YANG Jianghai, et al. A microscopic image dataset of Mesozoic metamorphic grains bearing sandstones from mid-Yangtze, China. Science Data Bank, 2020. (2020-09-16). DOI: 10.11922/sciencedb.j00001.00046.
How to cite this article
MA QL, CHAI R, YANG JH, et al. A microscopic image dataset of Mesozoic metamorphic grains bearing sandstones from mid-Yangtze, China. China Scientific Data, 2020, 5(3). (2020-09-28). DOI: 10.11922/csdata.2020.0053.zh.