Reconnaissance survey of the Duolun ring structure in Inner Mongolia ...

Meteoritics & Planetary Science 52, Nr 9, 1822–1842 (2017) doi: 10.1111/maps.12890

Reconnaissance survey of the Duolun ring structure in Inner Mongolia: Not an impact structure Xiaoming XU1,2 , Thomas KENKMANN3*, Zhiyong XIAO1,4*, Sebastian STURM3, Nicolai METZGER3, Yu YANG1, Daniela WEIMER3, Hannes KRIETSCH3, and Meng-Hua ZHU4 1

Planetary Science Institute, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China 2 School of Earth and Space Sciences, Peking University, Beijing 100871, China 3 Institute of Earth and Environmental Sciences–Geology, Albert-Ludwigs-University Freiburg, Freiburg 79104, Germany 4 Space Science Institute, Macau University of Science and Technology, Taipa, Macau * Corresponding authors. E-mails: [email protected] or [email protected] (Received 19 August 2016; revision accepted 01 April 2017)

Abstract–The Duolun basin, which is located in Inner Mongolia, China, has been proposed to be an impact structure with an apparent rim diameter of about 70, or even 170 km. The designation as an impact structure was based on its nearly circular topography, consisting of an annular moat that surrounds an inner hummocky region, and the widespread occurrences of various igneous rocks, polymict breccias, and deformed crustal rocks. Critical shock metamorphic evidence is not available to support the impact hypothesis. We conducted two independent reconnaissance field surveys to this area and studied the lithology both within and outside of the ring structure. We collected samples from all lithologies that might contain evidence of shock metamorphism as suggested by their locations, especially those sharing similar appearances with impact breccias, suevites, impact melt rocks, and shatter cones. Field investigation, together with thin-section examination, discovered that the suspected impact melt rocks are actually Early Cretaceous and Late Jurassic lava flows and pyroclastic deposits of rhyolitic to trachytic compositions, and the interpreted impact glass is typical volcanic glass. Petrographic analyses of all the samples reveal no indications for shock metamorphic overprint. All these lines of evidence suggest that the Duolun basin was not formed through impact cratering. The structural deformation and spatial distribution pattern of the igneous rocks suggest that the Duolun basin is most likely a Jurassic–Cretaceous complex rhyolite caldera system that has been partly filled with sediments forming an annular basin, followed by resurgent doming of the central area.

INTRODUCTION

Highlights The impact origin for the Duolun basin in Inner

Mongolia is disproved. Rocks of presumed impact origin do not show a

shock metamorphic overprint. investigations and petrographic analyses indicate a volcanic origin of the structure. Duolun may be a rhyolite caldera system that was active during the Jurassic and Cretaceous. Field

© The Meteoritical Society, 2017.

Impact craters are the most common and abundant surface features on terrestrial planets and solid satellites in the solar system (Melosh 1989). Similar to the other inner solar system bodies such as Mercury, Moon, and Mars, planet Earth has also been continuously bombarded by celestial materials (e.g., Grieve 1990). The importance of impact cratering as a geologic process for both the formation and subsequent evolution of the Earth and also for the evolution of life

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has become increasingly clear in the last decades and is now generally accepted (Alvarez et al. 1984; Chapman and Morrison 1994; Schulte et al. 2010). However, due to the fast surface weathering rate and active plate tectonics, only 190 impact craters (http://www.passc.net/ EarthImpactDatabase/) have been discovered on Earth so far, and most of them are preserved in areas that have been relatively stable during the last ~2 billion years (Grieve and St€ offler 2012; Hergarten and Kenkmann 2015). Until now only one crater, the ~1.8 km diameter Xiuyan crater, has been confirmed in China (Chen et al. 2010), a country with an area occupying ~6.4% of the Earth’s surface. The distribution of the presently identified impact craters on Earth, and also the cratering history of the inner solar system bodies (Strom et al. 2015) indicate that more impact craters should be preserved at Earth’s surface (Stewart 2011). Besides possibly buried impact structures, Hergarten and Kenkmann (2015) estimated the possible number of preserved impact craters on the Earth’s surface, and their method yielded about 13–20 additional impact craters with diameters ranging from a few hundred meters to about 6 km that should be exposed at the surface within the borders of China. The mainland of China is surrounded by the Himalayan-Tibetan tectonic realm, the Pacific tectonic domain, and the Circum-Siberian tectonic area (e.g., Ren 1996). Intense geological activity and surface erosion have erased and/or substantially modified surface structures older than the Paleozoic (e.g., Pan et al. 2009). Mainland China records a complicated geologic history, and many small ancient blocks that assembled the mainland have been recognized (e.g., Ren 1995). For example, the South China plate, the North China plate, and the Tarim plate are three of the relatively stable plates since at least the Phanerozoic (e.g., Ren 1995). Their crystalline basements have ages of ~1000–800 Myr (Wang and Mo 1996), 1900–1800 Myr (Huang 1984), and 1900–1800 Myr (Liu et al. 1997), respectively. Sedimentary coverage in stable depocenters onto the crystalline basements began ~700 Ma (Wang and Mo 1996), 1800 Ma (Huang 1984), and 1800 Ma (Liu et al. 1997), respectively. Therefore, mainland China, especially the relatively stable cratonic areas, has great potential for finding more impact craters. Indeed, several structures in China have been interpreted to be impact craters (Xu et al. 1989), mainly based on morphological similarities, i.e., negative topographic features that have circular or quasi-circular rims. An impact origin of the Taihu Lake structure (D = ~65 km, 120.2° E, 31.2° N), which is located in Jiangsu province, was proposed based on its quasicircular morphology (He et al. 1990). Materials resembling shocked quartz and shatter cones have been

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reported (Wang et al. 1993a, 1993b, 2009), but their identities are equivocal and not conclusive (Huang and Liu 2014). Other quasi-circular structures that have been suggested to be impact craters in China are the Longdoushe structure in the Guangdong province (D = ~3 km, 113.9° E, 24.7° N; Wu 1983b), Duolun in the Inner Mongolia (D = ~170 km, 116.3° E, 42.2° N; Wu 1983a, 1987, 1989; Wu and Zhang 1992), Baisha in the Hainan province (D = ~3.5 km, 109.5° E, 19.2° N; Wang and Li 1993; Wang and Wang 1995), Shanghewan in the Jilin province (D = ~30 km, 126.3° E, 44.5° N; Wu 1988), Hong Kong granitic arc (D = ~11 km, 114.2° E, 22.3° N; Chan et al. 1992), and Luoquanli in the Liaoning province (D = 1.8 km, 123.5° E, 40.1° N; Qin et al. 2001). So far, only the Luoquanli structure has been confirmed as an impact crater, now named as Xiuyan crater, after drilling revealed planar deformation features in quartz (Chen et al. 2010). The Duolun basin was proposed to be the largest impact structure in China (Wu 1987, 1989). Rocks interpreted to be impact breccias, impact glass, and thick impact melt sheets were reported from both within and outside of the ring structure (Wu 1987, 1989), although conclusive evidence for their presence has never been published or confirmed. Geological mapping (Bai et al. 2003), infrared spectra of hypothesized impact glass (Lin and Liu 1993), and siderophile element contents of hypothesized impact melt rocks (Ma 1995) from the Duolun basin found no evidence of shocked material or elevated concentration of extraterrestrial material. Alternatively, the circular morphology and topography of the Duolun basin, and also the various igneous rocks within this area, have been suggested to be caused by a combination of tectonic and volcanic activity since the Late Jurassic (e.g., Bai et al. 2003). While the endogenic origin of the Duolun basin is consistent with the tectonic background of this region (Lin 2009), the impact origin of this basin remains highly controversial. To investigate whether or not the Duolun basin is an ancient, large impact structure, we conducted two independent geological reconnaissance surveys to the Duolun basin: a German expedition led by Thomas Kenkmann was carried out from July 31 to August 8, 2014, and a Chinese expedition led by Zhiyong Xiao was undertaken during August 14–20, 2014. Later on, we merged our findings and data to consolidate this paper. We investigated the regional context both within and outside of the basin. Referring to the theoretical distribution of shocked materials on similar-sized impact structures on the Earth and other planetary bodies (e.g., Spudis 1993; Koeberl and Henkel 2006), we concentrated our attention on locations where target

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materials most likely could have recorded shock metamorphism. We revisited the outcrops that supposedly consist of impact melt rocks and impact breccia (Wu 1987, 1989, 1996; Wu and Zhang 1992). Samples from all the lithologies, especially those resembling various impact materials, were collected for petrographical inspection. REGIONAL GEOLOGICAL BACKGROUND The Duolun basin is located at the boundary between the Hebei province and Inner Mongolia, the eastern section of the Yinshan Mountains (Fig. 1A). This area is part of the continent–continent collision orogenic belt in northeast Asia (Xue et al. 2012). The North China plate is to the south of this area (Fig. 2). In the Early Permian, the North China plate collided with the Mongolia fold belt after the Hercynian movements. The region where the Duolun basin is situated experienced intense folding in the Early Permian, and the strikes of fold axes are nearly east– west (Cheng et al. 1995). Paleomagnetic data suggest that the North China plate started to collide with the Siberia plate along the Mongolia–Okhotsk suture zone in the Early Triassic, and the final consolidation occurred during the Late Jurassic or Early Cretaceous (e.g., Lin 2009). The closure of the Mongolia–Okhotsk Sea induced large-scale volcanism in the Late Jurassic and the Early Cretaceous (e.g., Lin 2009), which was contemporaneous with the decratonization of the North China block in the Mesozoic (Yang et al. 2008). In the Early Cretaceous, this region was in an extensional environment due to the release of the compressive stress caused by collision. Tectonic deformation was dominated by faulting with minor folding. Several sets of NE-, NNE-, and WNW-trending fault structures have been detected in this area (cf. Bai et al. 2003) (Fig. 3). After the Early Cretaceous, the source of tectonic stresses in this area shifted from the circumSiberian tectonic regime to the circum-Pacific tectonic regime. Vertical movements of the crust were the dominant tectonic activity since the Cenozoic. Restricted Paleogene sedimentation in this area is tectonically controlled (Fig. 3). Based on several buried faults detected by magnetic surveying (Fig. 3), Bai et al. (2003) suggested that vertical movements along regional faults might have been most significant in the center of the basin. Subsidence in the eastern and western parts of the basin formed the present circular ring-like basin that is used by the current river systems. The diameter of the Duolun ring structure is ~70 km, and the rim is marked by the outer slopes of the Shandian River and Luan River valleys (Fig. 1A). Based on low-resolution satellite image data, Wu (1987)

described three topographic ring features in this area, namely an 8 km diameter inner ring confined to the Haolaigou and Xigangou villages (Fig. 1B), the 70 km diameter intermediate ring (i.e., apparent basin rim), and a vague ~170 km diameter outer ring. Only the 70 km intermediate ring is clearly visible in both remote sensing images and the elevation map (Fig. 1). The hypothetical ~170 km diameter outer ring is actually a 90° arc segment of hummocky terrain extending from the north to the southwest (Fig. 1B). The hypothetical 8 km diameter inner ring is a low-lying depression surrounding a hill (Fig. 1B) that likely became isolated by erosion of intermittent rivers in this area. According to Wu (1987, 1989), the three hypothetical ring features were interpreted as an indicator for a multiring impact structure. Based on low-resolution satellite images, he postulated that the ~170 km diameter outer ring was the rim of the potential impact basin. The 1:200,000 geological map shows that the lithological structure of this region is composed of a Mesozoic igneous cover resting on ancient basement (Fig. 3). This principal layering exists both within and outside of the Duolun basin (Han and Fang 2013). The basement lithologies are composed of Middle to Upper Archean and Lower Permian rocks, and their outcrops are only visible in small areas near the 70 km diameter intermediate ring (Fig. 3). The Mesozoic igneous cover consists of Upper Jurassic and Lower Cretaceous igneous rocks, which are the dominant outcrops both within and outside of the Duolun basin. Wu (1987, 1989) suggested that the igneous cover is not of endogenic magmatic origin but represents impact melt rock and various types of impact breccias. METHODS AND STRATEGY Impact cratering forms a great diversity of topographic, petrographic, structural, and geophysical features (e.g., Kenkmann et al. 2014). However, most of these features are not uniquely limited to impact processes, so that they are not capable of conclusively proving an impact hypothesis. A danger of confusion particularly exists when comparing impact craters with large explosive volcanoes because of similarities in topography and petrography. The discovery history of the terrestrial impact structures documents quite plainly that many structures that had previously been interpreted as crypto-volcanic features (e.g., Bucher 1936) were later reinterpreted as impact craters based on the finding of shock features. The Ries crater in Germany is probably the best example of a long-lasting scientific debate between volcanologists and “impactologists.” This dispute was concluded after the documentation of high-pressure polymorphs of quartz


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Fig. 1. A) Remote sensing image of the Duolun basin. The white dashed line denotes the boundary between the Hebei province and Inner Mongolia. Data are from the UltraCam boarded on WorldView-2 satellite (113.6 m per pixel resolution, Mercator projection). B) Digital elevation map of the Duolun basin. Data are from the Shuttle Radar Topography Mission (SRTM; Farr et al. 2007). The number of the data sheet is srtm_60_04, and the resolution is 90 m per pixel (UTM projection). The black dashed lines represent the interpreted three rings of the Duolun structure (Wu 1989). The outer ring is only partly traceable to the northwest quarter. The Xigangou and Haolaigou villages are located on the rim of the interpreted 8 km diameter innermost ring. (Color figure can be viewed at wileyonlinelibrary.com.)

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and other shock features within the Ries (Shoemaker and Chao 1960). The proper documentation of shock metamorphic features, including shatter cones (e.g., Baratoux and Reimold 2016), planar deformation features (PDFs; e.g., St€ offler and Langenhorst 1994), planar fractures (e.g., French et al. 2004), feather features (Poelchau and Kenkmann 2011) in quartz, diaplectic quartz, and feldspar glass, and high-pressure polymorphs such as coesite, stishovite, ringwoodite, and diamond in particular settings (e.g., French and Koeberl 2010), is the only method to prove the existence of an impact structure. Additionally, the higher abundance of extraterrestrial elements (e.g., Os and Ir) in some impact-induced lithologies compared with normal strata in the surrounding areas can also suggest the existence of an ancient impact structure (e.g., Koeberl et al. 2012; Koeberl 2014). The region of interest for Duolun covers a large area (nearly 90,000 km2; Fig. 1). With the aid of the 1: 200,000 (Fig. 3) and 1:50,000 regional geological maps (Bai et al. 2003), we designed the routes of the reconnaissance field investigation to cover all the lithologies, and those locations that potentially have

recorded shocked metamorphism. While our surveys concentrated on the central portions of the Duolun ring structure, we also sampled rocks from outside of the morphological ring. We gathered and displayed our data in a multilayer GIS project using ArcGIS version 10.1 provided by ESRI. During the field trips, we (1) systematically investigated the occurrences and lithologies of the Upper Jurassic and Lower Cretaceous rocks from outside of the ~170 km diameter ring to the innermost area; (2) revisited the locations where breccias (Figs. 8–9, 14), melt rocks (Figs. 6 and 10), and glass-rich rocks (Figs. 7, 14A, 15) of supposedly impact origin were described; and (3) searched for possible shatter cones in the Upper Jurassic, Lower Cretaceous, and older lithologies (Figs. 4A, 4B and 11). We collected 133 samples from all the different lithologies in the region of interest, especially the Jurassic, Cretaceous, and older rocks close to the basin center. Petrographic analyses and the search for shock features were carried out utilizing the polarizing microscope in transmission light mode using standard thin sections. For a preliminary assessment of rock composition and mineral percentages, thin sections were visually


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Fig. 3. The geological map of the Duolun basin (revised from the geological maps of Duolun and Zhenglanqi [1:200,000) 1976]. Aeromagnetic anomaly mapping of this region found buried faults striking NE, NNE, and NNW along the rim of the 70 km diameter Duolun basin, i.e., along the river systems (Fig. 1). Q-Quaternary; K1-Lower Cretaceous, mainly trachydacites; K1-2strongly deformed trachydacites; K1-3-airfall and pyroclastic deposits; K1-4-postvolcanic sediments, mainly carbonates; J3-Upper Jurassic; P1-Lower Permian; Ar3-Upper Archean; Pt2-Upper Proterozoic; 1-Main water systems; 2-Ring structures interpreted by Wu (1987, 1989); 3-Observed faults (Bai et al. 2003); 4-Buried faults (Bai et al. 2003); 5-Place names; 6-Sampling sites for the Chinese survey; 7-Sampling sites for the German survey; 8-Lithological boundaries. (Color figure can be viewed at wileyonlinelibrary.com.)

inspected. Ten samples were selected for X-ray fluorescence spectroscopy (XRF) analyses to further confirm the lithology (Fig. 12). The bulk rock composition was measured using melt tablets. We used a PHILIPS PW2404-X-ray spectrometer equipped with a 4 kW Rh-anode, six analyzer crystals (LiF220, LiF200, Ge111, PE002, PX-1, and PX-4), and three detectors to quantitatively determine the bulk rock element abundance of selected samples (Table S3 in supporting information). We used the Total Alkali Silica (TAS) diagram (Fig. 12) for classifying the igneous rock type after Bas et al. (1986). Sampling location, lithology, and description of samples are given in Tables S1 and S2 in supporting information. RESULTS In the following, we describe the major outcropping lithological units and use the subdivision of Bai et al.

(2003). Our field observation and petrographic analyses show that units in the same formation have little variations in composition and petrographic characteristics. In the center of the ring structure, the geologic map shown in Fig. 3 is overhauled based on our field investigation, and additional lithological descriptions are given. The Middle to Upper Archean and Lower Permian Rocks The Middle to Upper Archean rocks are exposed within and outside of the Duolun basin and are composed of yellowish quartzite and quartz schist (e.g., the No. C4 and C5 sampling sites shown in Figs. 3 and 4A–B). Petrographic analyses indicate that quartz schists are composed of small-sized quartz and biotite with a preferred orientation, defining a weak penetrative foliation (Figs. 4C and 4D). The Archean quartzites have larger and comparable-sized grains

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Fig. 4. The Middle to Upper Archean rocks in this region. Thin sections are imaged using crossed polarizers. A–B) The Archean quartzite and a structure that has similar morphology with shatter cones. The diameter of the coin is 2.5 cm, and the sampling site is the No. C4 shown in Fig. 3. C–D) Archean quartz schist (the No. C5 shown in Fig. 3). Biotite (Bi) is filled in the interval between quartz (Q) crystals. E–F) Thin sections of the potential shatter cone. The quartz crystals are well preserved. (Color figure can be viewed at wileyonlinelibrary.com.)

(>~0.5 mm; Figs. 4E and 4F). Lower Permian rocks have limited exposure in this area (Fig. 3). They are predominantly composed of tuff, dacite, and sandstone. Intense fracture cleavage occurs in the outcrop of the sample site C6 (Fig. 5A), and the thin sections are mainly composed of devitrified glass and quartz (Fig. 5B). All of the quartz crystals within the Archean and Permian samples are virtually undeformed and show no signs of shock metamorphism (Figs. 4 and 5). The Upper Jurassic Rocks The Upper Jurassic rocks predominantly occur in the western portion of the ring structure both inside and outside the annular basin (Fig. 3). The sampling sites C1–3, C13, and C15 (Fig. 3) are situated in the northwestern sector outside of the ring, while sample locations C8 and C10–12 are in the northern sector within the ring. Our field investigation and petrographic analyses suggest that the Upper Jurassic rocks are typical effusive lava flows and pyroclastic deposits. They mainly consist of rhyolite, rhyolitic tuff, dacite, tuff, and tuffaceous mudstone. Potential eruption centers are not obvious on the surface. The rhyolite samples (No. C1–3, C8, and C10) are grayish-white in color, and display thin layers that are

~1 cm thick (Fig. 6A). The rocks have a porphyritic texture and rhyolitic structure (Figs. 6B–C). The phenocrysts are mainly composed of tabular K-feldspar crystals (~15%), which are ~0.4–0.7 mm in size. The matrix is composed of devitrified glass, feldspar, and microcrystalline quartz. The rhyolitic layering results from flow banding and different amounts of phenocrysts. Rhyolitic tuff is grayish in color with poor stratification (Fig. 6D). The samples exhibit a tuff texture (Figs. 6E–F). The pyroclastic rocks are composed of mineral fragments (K-feldspar and quartz; ~40%), glass shards (~55%), and lithic fragments (~5%), with grain sizes ~20 cm in lengths are visible within the outcrops (Fig. 8B). The veins are discontinuous in extent, and devoid of brecciated material at the macroscale. Hence, they are different from typical impact-related breccia dikes (Osinski et al. 2011). Subangular clasts that are ~3–5 cm

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Fig. 6. The Upper Jurassic rocks in the Duolun area. A) Rhyolite sampled from the No. C1 location shown in Fig. 3 is layered. The diameter of the coin is 2.5 cm, and the camera azimuth is ENE 83°. Panels (B) and (C) show the rhyolite under planepolarized light and crossed polarizers, respectively. The phenocryst is mainly composed of K-feldspar (Kf). The arrow points to the bands which bypass the K-feldspar phenocryst. D) The tuffaceous rhyolite has poor stratifications. The photo is taken at the No. C2 sampling site shown in Fig. 3, and the camera azimuth is ENE 75°. Panels (E) and (F) show the tuffaceous rhyolite under crossed polarizers. Flint fragment is visible in panel E. Small-sized quartz crystals in panel F are in weak directional alignment and bypass the K-feldspar (Kf) fragment, forming a pseudorhyolitic structure. G) Close view of the dacite. The photo is taken at the No. C13 sampling site shown in Fig. 3, and the camera azimuth is SSE 149°. Panel (H) show the dacite under crossed polarizers. The phenocryst is composed of well-preserved quartz (Q) and plagioclase (Pl), and felsitic structure in the matrix is obvious. I) Tuff sampled from the No. C11 sampling site shown in Fig. 3 occurs in melting form and the camera azimuth is ESE 96°. Panel (J) shows the tuff under crossed polarizers. The mineral fragments are composed of quartz (Q) and Kfeldspar (Kf). Panel (K) shows tuffaceous mudstone under plane-polarized light. Mud component exhibits obvious lined structures. Panel (L) shows dacite clast within the thin section of tuff under crossed polarizers. Phenocryst is not abundant and the matrix has a felsitic structure. (Color figure can be viewed at wileyonlinelibrary.com.)


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Fig. 7. The Upper Jurassic glass-rich rocks in the Duolun area. A) Occurrence of a potential volcanic crater. The arrows point to the rim of an elliptical depression that was interpreted to be a degraded explosive volcanic crater (Bai et al. 2003). The photo is taken at the No. C11 sampling site shown in Fig. 3, and the camera azimuth is 120° SE. B) The glass-rich rock is red–brown in color, and vitreous luster is clear. Panels (C–F) show the minerals within the glass-rich rocks under plane-polarized light and crossed polarizers. The quartz (Q) crystals within the glass are well preserved and do not exhibit planar deformation features. (Color figure can be viewed at wileyonlinelibrary.com.)

in size also occur in some trachyte and trachyandesite outcrops (Figs. 8C and 8D). Thin sections of the samples (e.g., the No. C7 samples, Fig. 3) show that trachyte (Figs. 9A and 9B) and trachyandesite (Figs. 9C and 9D) have a porphyritic texture. The phenocrysts of the trachyte are mainly composed of K-feldspar (sanidine; ~25%), plagioclase (~5%), and amphibole (~5%). The matrix of the trachyte has a cryptocrystalline texture, and feldspar and glass are also visible. The phenocrysts of the trachyandesite are mainly composed of K-feldspar (~15%), plagioclase, (10%) and amphibole (~5%). The matrix is composed of feldspar and devitrified glass. The mineral assemblage is consistent with that of typical volcanic rocks. Planar fractures, feather features, or planar deformation features are absent (Figs. 9A–D). The angular clasts within the trachyte and trachyandesite outcrops have the same lithology as the host rocks, but they have different proportions of minerals (Figs. 9E and 9F). The phenocrysts in the clasts are mainly composed of K-feldspar (~25%), amphibole (~10%), and plagioclase (~5%). Minerals within the clasts show no hints of shock metamorphism (e.g., Figs. 9E and 9F). The clasts may represent a

slightly more mafic precursor melt rock than was incorporated as xenoliths during the magma ascent. The Lower Cretaceous andesite mainly occurs in the eastern and central part of the ring structure (e.g., the No. C9 sampling site, Fig. 3) and near the Qianjinggou village outside the annular moat (e.g., the No. C14 sampling site, Fig. 3). The outcrop at sampling site No. C9 is gray–white in color (Fig. 10A). Layering is clear in the outcrop and deformation is minor (Fig. 10A). The thickness of the layers is about 4–6 cm, and no clasts are visible within the outcrops. Vast amounts of elliptical vesicles are visible within some layers of the outcrops, which are ~2–3 mm in size, and their long axes are preferentially aligned along NW–SE (Fig. 10B). Although impact melt rocks can also host a vast amount of vesicles, the intact parallel and thin layering suggests that the rock represents extrusive lavas. Plagioclase laths in the matrix forms an intersertal texture, and glass occurs in the interstitial matrix, which forms a hyalopilitic texture (Figs. 10C and 10D). Therefore, the Lower Cretaceous andesites have a volcanic origin as proposed by Bai et al. (2003). Signs for strong deformation and shock metamorphism could not be observed within these rocks. Our investigation suggests that the Cretaceous rocks are typical extrusive

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→ Fig. 8. Outcrops of the Lower Cretaceous trachyte and trachyandesite in the Duolun area. Photos are taken at the No. C7 sampling site shown in Fig. 3. A) Longitudinal joints are visible in the trachyte outcrop, and the camera is pointed to 70° ENE. B) Subparallel hydrothermal veins in the trachyte outcrop (arrows), and the diameter of the coin is 2.5 cm. C) Rectangular clasts within the trachyte outcrop are most likely xenoliths (arrows), and the diameter of the coin is 2.5 cm. D) Elliptical clasts within the trachyte outcrop are possibly xenoliths (arrows), and the length of the pencil is 12 cm. (Color figure can be viewed at wileyonlinelibrary.com.)

volcanic rocks composition.

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Occurrences of Shatter Cones? Shatter cones are the only macroscopically visible shock criteria (French and Koeberl 2010). Therefore, they play a fundamental role in proving of an impact hypothesis. We have identified several fracture cleavages and joint surfaces in various-aged outcrops that resemble and may be confused with shatter cones (Figs. 4A, 4B, 11). However, a closer inspection shows that none of them show the required surface characteristics of shatter cones. Shatter cones can be distinguished from other similar geological features by comparing the distribution, shape, morphology, amplitude, and orientation of the striated surfaces (see fig. 4 in Baratoux and Reimold 2016). The striations of shatter cones diverge and represent sharp, narrow grooves between broader convexities (Milton 1977). The striations are intersection lineations delimiting subcones that branch into symmetrically equivalent ridges (Kenkmann et al. 2016). The hierarchical

subcone pattern with the diverging intersection grooves is unique for shatter cone surfaces. The two components cause the so-called horse-tailing effect of shatter cones. For the features shown in Figs. 4 and 11, conical surfaces occasionally develop from the intersection of three joint systems (e.g., Fig. 11C). However, these fracture surfaces are devoid of diverging striations, multiple generations of subcone ridges with horsetailing effects, and an interpenetration of adjacent cones, etc. (e.g., French and Koeberl 2010; Baratoux and Reimold 2016; Kenkmann et al. 2016). Planar and curvy–planar joints and fractures frequently display diverging plumose structures (Bankwitz 1965) (e.g., Figs. 11A, 11B, 11D) that indicate the propagation direction of the tensile fractures. Such plumose structures occasionally terminate into fringe zones (Hodgson 1961; Fig. 11E), which consist of an array of twisted fractures. While the fringes shown in Figs. 4 and 11 are diverging like the grooves of shatter cones, they are often irregular in shape and do not further branch. Figure 11f shows half-conical fracture surfaces that even show a partial intersection of adjacent cones. However, here the typical branching


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Pl

Kf

Am

Kf

Kf

Am

Kf

B

F

D Pl

Am

Kf

Kf

Am

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Am

Fig. 9. Microscope observations of the Lower Cretaceous trachyte and trachyandesite under crossed polarizers. Sericitization of K-feldspar is visible in all panels. Panels (A) and (B) are for the trachyte. The phenocryst is composed of K-feldspar (Kf), plagioclase (Pl), and amphibole (Am). The crystal forms are well preserved, and the directional alignment of feldspar crystals in the matrix exhibits a trachytic texture. Panels (C) and (D) show that the phenocryst of the trachyandesite is composed of Kfeldspar (Kf), plagioclase (Pl), and amphibole (Am). Panels (E) and (F) are thin sections for the clasts within the trachyte outcrop, and the phenocryst is composed of K-feldspar (Kf), plagioclase (Pl), and amphibole (Am). (Color figure can be viewed at wileyonlinelibrary.com.)

characteristics of subcone ridges are missing and the adjacent cones are not arranged in a manner that an enveloping cone could form. Therefore, the fracture surfaces are not shatter cones but consistent with being plumose structures that are formed during fracture propagation and brittle fracturing.

surrounded and overlain by pyroclastic rocks and ashes (Figs. 14A and 14B), which in turn are superimposed by postvolcanic sedimentary rocks (Fig. 3). DISCUSSION Impact Crater Hypothesis

Deformation of Rocks We have measured the attitudes of flow banding in rhyolites, rhyodacites, and trachyites (Table S2). The Upper Jurassic rhyolites form relatively stable and thick sheets that commonly show gentle warping and dips. In the cross section of Fig. 13 that represents a schematic profile from the northwest to the center of the structure, gentle dips toward the center are shown. Local exceptions occur where faults cross the units. Here the flow banding can be locally tilted to an almost vertical attitude. An exceptional situation occurs in the center of the structure. An isolated hill of about 2 km extent (Fig. 3) that is surrounded by a valley network displays intense deformation. The attitudes of flow banding in rhyolites and trachyites change on the decameter scale owing to complex strata folding and faulting. Layering is frequently tilted to vertical incidences. The hill is

We have studied the petrography and deformation inventory of a variety of rocks related with the Duolun ring structure that were previously interpreted to be impact-derived (Wu 1987, 1989, 1996). We have sampled exactly the same outcrops as Wu (1987, 1989) and further probed additional locations to reach a better understanding of the general geology of this ring structure. Rocks that were previously described as impact melt rocks and impact glass are exclusively of volcanic origin, and they are classified as rhyolite, dacite, trachydacite, and trachyte. None of these rocks contains shocked minerals or clasts. Rocks that were previously interpreted as suevites and impact breccias are also exclusively of volcanic origin and represent layers of ashes, tuff, porphyry, and polymict volcanic breccias that indicate multiphases of explosive volcanism (Fig. 14). Rocks that supposedly show

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A

B

C

D

500 µm

200 µm

Fig. 10. The Upper Cretaceous andesite in the Duolun basin. A) The andesite occurs in thin layers that are ~4–6 cm thick. The photo is taken at the No. C9 sampling site in Fig. 3, and the camera azimuth is ENE 80°. B) Vesicles are abundant within some of the Upper Cretaceous andesite. The vesicles are elliptical in shape. Part of the sample is amplified in the down-right corner. Panels (C) and (D) show the andesite under crossed polarizers. The whitish plagioclase microcrystalline in the dark matrix displays a hyalopilitic texture. (Color figure can be viewed at wileyonlinelibrary.com.)

shatter cone surfaces are actually curvy–planar fractures with typical plumose structures (e.g., Fig. 11). The topography of the Duolun ring structure indeed resembles large eroded terrestrial impact structures, such as Siljan, Sweden, 65 km diameter (e.g., Kenkmann and von Dalwigk 2000); Morokweng, South Africa, 70 km diameter (Koeberl et al. 1997); or Manicuagan, Canada, 100 km diameter (e.g., Dressler 1990). It is the morphological similarity that stimulated us to prove or disprove the impact hypothesis. Fresh meteorite impact craters of this size range have a rather flat topography with a hummocky central region representing the remnant of a broad central uplift that underwent outward gravitational collapse, which ultimately leads to peak ring formation (Collins et al.

2002). An annular moat surrounding this central uplift or peak ring is a characteristic feature of this class of complex craters. In structural terms, the moat of large complex impact craters represents a complex ring syncline comprising down-faulted, thickened, and terraced supra-crustals that were originally situated close to the edge of the transient crater cavity (e.g., Kenkmann et al. 2014). The semicircular trending valleys of the Luan and Shandian rivers with their confluence in the north form a broad annular moat (Fig. 1B), which is structurally controlled and circumscribed by faults (Bai et al. 2003). The downfaulted ring-like basin acted as a depocenter for Cretaceous and Cenozoic sediments, and coals are presently mined from this ring basin at several hundred

Fig. 11. Joints and fractures with cone-like surfaces. A) Planar fracture surface with plumose structures (Hodgson 1961). Plumose structures indicate the orientation of tensile fracture growth. Fracture nucleation points are clearly visible at the upper edge of the sample (sample loc. D59). B) Convex–concave shaped fractures with plumose structures (sample loc. D34). C) This cone developed from the intersection of three joint sets. Note that the cone is devoid of subcone ridges and diverging striations (Kenkmann et al. 2016) (sample loc. D62). D) Curvy–planar fracture with plumose structures (sample loc. D38). E) Plumose structures on a planar fracture terminate in a fringe zone with twisted fractures (sample loc. D34). F) Several cones with apex and diverging striations in a fine-grained Proterozoic meta-volcanic rock. These twisted fractures occur in a fringe zone and emanate from a planar joint containing massive plumose structures (sample loc. D26). (Color figure can be viewed at wileyonlinelibrary.com.)


B

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C D

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5 cm

5 cm

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16 Ultrabasic 14

Alkaline

NIM-S

Phonolite

P58

Foidite

Tephriphonolite

Trachyte Trachydacite

P76

10 Phonotephrite 8

6

P25 RGM P24

Rhyolite

Basaltic trachyandesite

Tephrite Basanite

P26

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P11A P18

P07

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P22

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Picrobasalt

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NaO+KO [wt%] 2 2

12

Acid

Intermediate

Basic

Subalkaline/Tholeiitic

0 38

43

48

53

58

63

68

73

78

SiO2 [wt.%] Fig. 12. Total alkali silica diagram showing the bulk rock composition of several samples that is predominantly rhyolitic to trachytic, which is consistent with the petrological analyses. Samples P07, P11A, and P18 are from the Upper Jurassic outcrops, and P22, P24, P25, P26, P58, and P76 are from the Lower Cretaceous outcrops. P78 is sampled from quaternary basalts. See Fig. 3 and Table S2 for the sample locations.

meters depth in the southwestern sector of the basin. Bai et al. (2003) suggested that an extensional tectonic stress regime during the Cretaceous is responsible for the formation of the ring basin, and they explained the circular appearance of the basin by a combination of NE-, NNE-, and WNW-trending fault structures. According to Bai et al. (2003), the differently oriented sections of the faults were developed over an extended geologic time period (without supporting isotopic ages). In contrast, if the faults delimiting the annular basin were activated more or less simultaneously as a circumferential normal fault in a ring-like arrangement, a specific stress situation is required with r1 > r2 = r3 (Anderson 1951), where r1 = rvertical; r2 = r3 = rhorizontal. Such a stress regime exists, e.g., during caldera collapse, pluton emplacement, salt diapirism, etc. To conclude, in the absence of any macroscopic and microscopic shock indicators, we can discard the impact hypothesis as the cause for the formation of the Duolun ring structure. The morphological similarity to large eroded impact structures is misleading. The formation of the Duolun ring structure requires an alternative formation model that is compatible with the regional geological context, morphology, structure, and petrography.

Volcanic Caldera Model In the following, we present a model that is compatible with the observations during our reconnaissance survey. We propose that the Duolun ring structure probably represents a large volcanic caldera system that was later modified to a resurgent dome (Fig. 13). However, as the detailed investigation of a large volcanic complex was beyond the initial scope of the reconnaissance study, this hypothetical model requires further proof by means of geophysical and petrological investigations: 1. The annular moat that hosts both the Luan and Shandian rivers represents a structurally controlled semicircular basin that has been filled by Cenozoic sediments and Late-Mesozoic volcano clastics. We propose that the outer rim of the valley is bounded by a circumferential normal fault system delineating a large caldera (Fig. 13). This scenario suggests a synchronous activation of the faults delineating the Duolun structure. This is in contrast to Bai et al. (2003) who explained the faults being formed by regional extension tectonics. The caldera has a mean diameter of 75 km and a surface area of 4400 km2. The caldera is not exactly circular but


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uplifted inner caldera

NW

annular moat layered rhyolites

pyroclastics

layered rhyolites

SE

pyroclastics

post-sediments

postsediments

basement deformed rhyolite

coals hydrothermal veins (fluorite)

hypothetical ancient magma chamber

caldera rim

35-40 km radius

center

Fig. 13. Highly simplified radial cross section through the Duolun ring structure from the center toward outer regions crossing the annular moat. The cross section particularly shows the geologic structure of a NW–SE transect. However, the fluorite precipitation mainly occurs south of the center and is radially projected onto the cross section. The presence of a solidified magma chamber underneath the inner part of the Duolun structure is not substantiated by gravity data.

has an aspect ratio of 1.15, with the largest diameter of 83 km in NE–SW direction (Fig. 1). If our hypothetical model is correct, it would imply that Duolun ranks among the largest volcanic caldera on Earth (Mason et al. 2004; Bachmann and Bergantz 2008). 2. The morphology of the annular moat and, hence, of the proposed caldera is subcircular to slightly elliptical, and no nested calderas are discernible. This suggests that the caldera probably formed in a short period of time or even in a single event, most likely during the late Jurassic, the main phase of rhyolite extrusion. The large number of Cretaceous volcanoes interpreted by Bai et al. (2003) possibly postdated the major eruption. The size of a caldera commonly correlates with that of the eruption (Smith 1979). This, in turn, would indicate that a super-eruption may have taken place at Duolun during the Upper Jurassic. Super-eruptions are considered to be those that eject magma (molten, or partly molten) with a mass exceeding 1015 kg, equivalent to a volume greater than 450 km3 (Miller and Wark 2008). Explosive eruptions of this magnitude have a volcanic explosivity index (VEI) of 8 and are classified as Plinian. At Duolun, it seems to be impossible to determine the original mass of volcanic extrusion due to later erosion. Measurement of the thickness of air fall deposits for

the reconstruction of the eruption column is somewhat limited at Duolun owing to a limited number of outcrops. The suggested explosivity of the eruptions relies on the proposed size of the caldera, the presence of decameter-sized ejected boulders (Fig. 14E), and the extensive coverage of thick rhyolite flows. 3. In our model, the hummocky region inside the annular moat is caused by a combination of volcanic refilling of the caldera after the main eruption and subsequent resurgent doming of the caldera floor (Fig. 13). The exposure of Proterozoic bedrock and the preferred dip of thick sheets of rhyolite and rhyodacites toward the center (see Table S2) indicate that doming was not evenly distributed across the caldera. It was strongest near the inner rim of the annular moat, in particular in the NW. The region inside the moat is interpreted as a resurgent dome formed sometime after the main collapse when the caldera floor was pushed upward by the rising of residual unerupted magma (Miller and Wark 2008) or by the replenishment of the magma chamber. Resurgence was associated with tilting and deformation of rocks. The upper Jurassic rhyolite sheets commonly show gentle dips except where faults cross the rocks. In contrast, Lower Cretaceous lavas close to the center of the structures display intense, partly chevron-type

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D16

50 cm

B

C

D31

D59

D

1m

E 2m 1m

D60

D61

F

G

D43

10 cm D40

5 cm

Fig. 14. Field evidence of violent eruptions and posteruption hydrothermal activity at the Duolun basin. A) Normally graded units of whitish airfall deposits consisting of glass lapilli and pumice underlain by tuff breccia and blocks (loc. D16). B) Massive unit of pyroclastic blocks (loc. D31). C) Polymict breccia containing meter- to decameter-sized blocks at the base overlain by flow-banded pyroclastic rocks. On top is another block unit indicating a violent deposition mechanism (loc. D59). D) Variegated polymict breccia with flow banding. The embedded competent clasts show rounding by the turbulent flow (loc. D60). E) Airborne material (tuff, lapilli) wraps around an elongated rhyolite megablock (loc. D61). F) Cataclastically deformed rhyolite. (loc. D43). G) Fluorite ores are massively concentrated in hydrothermal vein systems south of the center of the structure (loc. D43).


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Fig. 15. A) Poorly sorted texture of a tuff (sample loc. D18; BF, 409). (Qz) quartz, (Gl) glass, (Fs) feldspar, and (Op) opac particles. Note the red to brownish film on feldspars. B) Ignimbrite welding is indicated by the presence of elongated black fiamme structure (sample loc. D18; BF, 409).

folding with tight hinges. The attitudes of flow layering are complex and change locally within decameters. It seems as if this central region (e.g., D73, D75, Table S2) has been squeezed out from below. The NE–SW-trending valley that crosses the center of the structure is interpreted as a graben system opened at the crest of the resurgent dome. The orientation of this graben suggests that the least principal stress direction was oriented NW–SE. The structure of the resurgent dome indicates the ascent of a wide shallow magma body (Acocella et al. 2001; Lockwood and Hazlett 2010). Locally and in particular along the central graben, volcanoclastic rocks are superimposed by lacustrine sediments (marls and limestones, e.g., D30, D72, Table S2), which indicate that the central area was a local depression and depocenter before uplift took place. 4. The composition of the voluminous sheets of Upper Jurassic volcanic rocks is dominantly rhyolithic to rhyodacitic (Fig. 12; see The Upper Jurassic rocks section). In some outcrops, we found decameter thick pyroclastic flow units consisting of unsorted mixtures of meter- to decameter-sized fragments and abraded pumice blocks that were topped by coarse grained whitish ash layers with a large fraction of them still being glassy (Figs. 14A–E). Ignimbrites with welding textures, fiamme structures, and clasts are observed near the center of the structure, indicating pyroclastic flows (Fig. 15). We assume that the extrusion of acidic rhyolitic to rhyodacitic lavas during the Upper Jurassic led to the caldera formation when the magma chamber was depleted. The phase of resurgence was most likely correlated with the Lower Cretaceous phase of magmatic activity and the extraction of lavas of intermediate trachytic composition. The volcanically active period terminated with a phase of basalt extrusion

(P78, Fig. 12). An isolated basaltic cinder cone is situated within the central area along the NE–SWtrending graben (D83; Figs. 1B and 3). More widespread areas of basaltic outflow are located south of the annular moat (D56–57; Fig. 3). 5. A phase of intense pneumatolytic and hydrothermal ore precipitation followed the main magmatic phases. A number of steeply dipping, meter thick dikes crosscut through Upper Jurassic as well as Lower Cretaceous volcanic rocks south of the NE– SW-trending central graben system. These dikes show intense fluid-assisted brecciation (Fig. 14F) and contain massive precipitation of fluorite (Fig. 14G) along with quartz, calcite, and barite. 6. The plate tectonic scenario during the Mesozoic is compatible with the formation of a large caldera. At that time, the region of Duolun was located at or close to the active continental margin of the North China plate. This suture was closed when collision with the Siberian plate took place. The main volcanically active phase at Duolun occurred from the Upper Jurassic to the Lower Cretaceous, which is consistent with the widespread volcanic activity at the northern margin of North China plate in early Mesozoic (Zhu et al. 2012). CONCLUSION The hypothesis that the Duolun ring structure in Inner Mongolia, China, was formed by a large meteorite impact strike could not be substantiated in the course of two geologic surveys to the structure. Neither macroscopic evidence in the form of shatter cones nor microscopic shock features such as planar fractures, planar deformation features, or feather features could be identified after a large number of samples from all lithologies had been investigated, including the outcrops from which supposedly impact-derived rocks were

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described. The rocks that were previously interpreted as impact melt rocks, impact glass, and impact breccias are actually typical igneous rocks produced by Upper Jurassic and Lower Cretaceous extrusive and explosive volcanism. Although in general only a few percent of rock masses within an impact crater record evidence of shock metamorphism, it is unlikely that our investigation has missed possible evidence as we have particularly investigated the center of the structure. We conclude that the Duolun ring structure is not caused by a large impact cratering event. Instead, the structure results from volcanic activity that lasted from the Upper Jurassic to the Lower Cretaceous. The main volcanic activity started with massive extrusion and explosion of rhylolitic to rhyodacitic lavas and probably led to the collapse of a large caldera of 75 km diameter that covered an area of 4400 km2. Subsequent volcanic activity of more trachytic composition occurred simultaneously with the formation of a large resurgent dome of 40–60 km diameter. Acknowledgments—Zhida Bai provided his 1:50,000 geologic mapping results, which served as the guide for the field work. Yi Sun, Zhiyong Li, Zuoxun Zeng, Jiannan Zhao, and Xiyao Li provided constructive suggestions and field assistance for the Chinese expedition. Furthermore, we are grateful to Gerwin Wulf and Herbert Ickler for their support, and acknowledge reviews of an earlier version of this manuscript by Christian Koeberl and an anonymous reviewer. David Baratoux and Jens Orm€ o provided constructive reviews for the manuscript, which have substantially improved the paper. This study is supported by National Natural Science Foundation of China (No. 41403053) and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUGL150405). The German expedition party could be realized using the prize money of the state teaching award for Thomas Kenkmann by the Ministry of Science, Research and Art Baden-W€ urttemberg. Editorial Handling—Dr. Christian Koeberl REFERENCES Acocella V., Cifelli F., and Funiciello R. 2001. The control of overburden thickness on resurgent domes: Insights from analogue models. Journal of Volcanology and Geothermal Research 111:137–153. Alvarez W., Kauffman E. G., Surlyk F., Alvarez L. W., Asaro F., and Michel H. V. 1984. Impact theory of mass extinctions and the invertebrate fossil record. Science 223:1135–1141. Anderson E. M. 1951. The dynamics of faulting. Edinburgh, UK: Oliver and Boyd. 206 p.

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SUPPORTING INFORMATION

samples. The locations of the samples are listed in Table S2. P07 = Rhyolite; P11A, P22, P24, P25 and RGM = Rhyodacite; P18 = Dacite; P26, P58, P76 = Quartz latite; NIM-S = Alkali feldspar trachyte; BM, P78 = Basalt/Andesite. “*” denotes element content is below the detection limit. References for contents of the standards RGM, NIM-S, and BM are Fabbi and Espos (1976), Verma et al. (1992), and Govindaraju and Roelandts (1988), respectively.

Additional supporting information may be found in the online version of this article: Table S1: Information about the samples collected during the Chinese-led field investigation. Table S2: Stops and sample locations of the German-led survey. Table S3: Bulk rock composition of selected