The Huolinhe Formation is a Lower Cretaceous terrestrial coal-bearing unit distributed in the Huolinhe Basin, one of several important coal-bearing, fault-bounded basins formed during Late Jurassic and Early Cretaceous in the region of the Greater Khingan Range in northeastern China (Li et al., 1982). The Huolinhe Basin in Jarud Banner, eastern Inner Mongolia is ~60 km long and ~9 km wide and is oriented along a northeast to southwest line. Within the basin the Huolinhe Formation rests unconformably on the Upper Jurassic volcanic Xing'anling Group, and comprises a sequence of terrestrial and fluvio-lacustrine-swamp sediments that are up to 1700 m thick (Li et al., 1982). The formation is composed of conglomerate, sandstone, siltstone, mudstone, as well as thick coal and lignite seams. In ascending order the Huolinhe Formation is divided informally into a "conglomerate-sandstone member", a "lower mudstone member", a "lower coal-bearing member", an "upper mudstone member", an "upper coal-bearing member", and a "top mudstone member" (Li et al., 1982). Most of the exploitable coal/lignite seams are in the "lower coal-bearing member", which is up to 720 m thick and has yielded abundant well-preserved plant megafossils (Deng, 1995).
The Huolinhe Basin is among the most extensively studied areas for Mesozoic plant fossils in China (Sun, 1987(Sun, , 1993;;Sun & Shang, 1988;Chen & Deng, 1990;Deng, 1991Deng, , 1995Deng, , 1997;;Mei & Cui, 1994;Cui, 1995;Guo, 1995;Sun et al., 2003;Dong & Sun, 2012;Xu et al., 2013Xu et al., , 2017;;Li et al., 2014Li et al., , 2016Li et al., , 2021;;Guignard, 2019). The plant fossil assemblage from the "lower coal-bearing member" of the Huolinhe Formation is diverse and dominated by ferns, including bryophytes, lichens, Lycopodiales, Equisetales, Filicales, Cycadales, Ginkgoales, Bennettitales, Coniferales and Czekanowskiales, along with some seeds and cones of uncertain affinity (Deng, 1995). No angiosperm megafossils have been reported so far. The floral composition suggests an early Early Cretaceous age, most probably Valanginian to early Hauterivian for the "lower coal-bearing member" of the Huolinhe Formation (Deng, 1995). However, more precise age constraints from the radioisotopic analyses have not been obtained for the Huolinhe Formation so far.
During recent fieldwork in the Huolinhe Basin, we identified an ash layer near the bottom of the "lower coal-bearing member" of the Huolinhe Formation at the Zhahanaoer open-cast coal mine in Jarud Banner. Here we report the U-Pb zircon ages for the ash layer and the associated palynological assemblages.
The ash samples for U-Pb zircon geochronology (059-2b, 059-2c) were collected at an outcrop exposed on the margin of the Zhahanaoer open-cast coal mine (45°21′38.5″N, 119°25′04″E), Jarud Banner, eastern Inner Mongolia, China. Boreholes and stratigraphic analyses suggest that the ash layer occurs below a considerable thickness of coal toward the bottom of the "lower coal-bearing member". The investigated section is ca. 11 m thick and composed of mudstone, silty mudstone, carbonaceous mudstone, siltstone, ash, and chert (Fig. 1). We divided the section into six units based on lithological features, and each of the six units was sampled for palynological analysis with sample numbers 059-1~059-6.
The zircon grains were separated using conventional techniques of heavy liquid separation and magnetic separation after ash samples were crushed and sieved. The grains for U-Pb isotopic analyses were selected by hand-picking under a binocular microscope, mounted in a transparent epoxy disk, and then polished to expose the interior of crystals for analyses. All zircon grains were photographed with transmitted and reflected light photo-microscopy and cathodoluminescence (CL) images to reveal their internal structure (Fig. 2) (Rubatto & Gebauer, 2000). The selected grains were analyzed with the Secondary Ion Mass Spectrometry (SIMS) and Laser Ablation Inductively Coupled Mass Spectrometer (LA-ICP-MS) techniques.
Measurements of U-Pb zircon age by SIMS were conducted with a Cameca IMS-1280 ion microprobe at the Institute of Geology and Geophysics, Chinese Academy of Sciences (CAS) in Beijing, China. Details of the analysis and data-processing methods were described in Li et al. (2009Li et al. ( , 2010)). The zircon mount was vacuum-coated with high purity gold before SIMS measurements. During the analyses, the primary O 2 -ion beam was accelerated at 13 kV, with an intensity of ~10 nA and an analysis spot size of 20 μm × 30 μm. The mass resolution used to measure Pb/Pb and Pb/U isotopic ratios was 5400. U-Th-Pb ratios were determined relative to the standard zircon Plešovice (337.13±0.37 Ma, Sláma et al., 2008), and the absolute abundances of U/Th were determined relative to standard zircon 91500 (Wiedenbeck et al., 1995) (Li et al., 2013). The age calculation was made using Isoplot/Ex_ver3 at 95% confidence (Ludwig, 2003).
Measurements of U-Pb zircon age by LA-ICP-MS were conducted at the Wuhan Sample Solution Analytical Technology Co., Ltd. in Wuhan, China. Details of the analyses and data-processing are described in Zong et al. (2017). Laser sampling was conducted using a GeolasPro laser ablation sys-tem that is composed of a COMPexPro 102 ArF excimer laser (wavelength of 193 nm and maximum energy of 200 mJ) and a MicroLas optical system. An Agilent 7700e ICP-MS instrument was used to acquire ion-signal intensities. Helium was applied as a carrier gas. Zircon 91500 (Wiedenbeck et al., 1995) and glass NIST610 were used as external standards for U-Pb dating and trace element calibration respectively. Each analysis consisted of a background acquisition of (~)20~30 s followed by a further 50 s of data acquisition from the sample, with laser spot size of 32 µm and frequency of 5 Hz. Off-line data processing was performed using ICPMSDataCal software (Liu et al., 2008). The age calculation was made using Isoplot/Ex_ver3 at 95% confidence (Ludwig, 2003).
The palynological samples were prepared using standard palynological techniques at the Experimental Technologies Center of Nanjing Institute of Geology and Palaeontology (NIGP), CAS in Nanjing, China. The samples were first treated with dilute hydrochloric acid (~10% HCI) and 40% hydrofluoric acid (HF) to remove carbonates and silica, and then sieved with a 10 μm nylon sieve. The prepared pollen and spores were mounted on thin slides with phenol-glycerin jelly, sealed with wax, and examined and photographed on a Zeiss Axio Scope A1 microscope with an AxioCam HRc digital camera system at NIGP. All palynological slides are deposited in NIGP.
The zircon grains extracted from the studied ash layer of the Huolinhe Formation in most cases ex-hibit euhedral to subhedral morphologies, and oscillatory zoning patterns under CL images (Figs. 2A, 3A), indicating their igneous origin. The grains are commonly 40~200 μm in length, with a length/width ratio of 1~3.5.
A total of 27 zircon grains were analyzed from the sample 059-2c, but six analyses with unexpected error were discarded (Fig. 2A; Tab. 1). Since the present zircons have high common Pb content, for which the 204 Pb-correction method is inaccurate, we adopted the 207 Pb-correction method of common Pb for age calculation. The isotopic data of the remaining 21 analyses were projected on a mixing line between common 207 Pb/ 206 Pb and the radiogenic 238 U/ 206 Pb on the inverse Tera-Wasserburg Concordia diagram (Tera & Wasserburg, 1972), and yield a lower intercept U-Pb age of 125.6±1.0 Ma (MSWD= 0.53; Fig. 3A), which is interpreted as the age of the crystallization of the zircons. Since the reproducibility (accuracy) of the SIMS analysis is ~1% (Li et al., 2015), we consider 1.0 Ma a reasonable uncertainty of the present SIMS U-Pb age.
A total of 40 zircon grains were analyzed from the sample 059-2b (Fig. 2B; Tab. 2). Among them three analyses with high uncertainties were excluded A-zircons for SIMS analyses, yellow ellipses (20 μm × 30 μm) indicating the locations of SIMS analyses. B-zircons for LA-ICP-MS analyses, the yellow rings (32 µm in diameter) indicating the locations of laser spots from the age calculation (Tab. 2). Three analyses with distinct older ages were interpreted as clastic grains (Tab. 2). The remaining 34 analyses yield a lower intercept U-Pb age of 126.2±0.9 Ma (MSWD = 1.6; Fig. 3B) in the Tera-Wasserburg plot. However, although the internal error of LA-ICP-MS zircon U-Pb dating can be less than 1%, the reproducibility (accuracy) of the technique is relatively poor and is ~4% (Li et al., 2015). This indicates that uncertainty of the present LA-ICP-MS U-Pb age could be ~5 Ma.
Among the six samples only samples 059-2 (ash layer), 059-3 and 059-5 were productive for pollen and spores (Tab. 3;Figs. 4,5). The palynological assemblage of the ash layer is diverse and includes 54 taxa. Gymnosperm pollen dominates the assemblage, with a total percentage of 54.5% and 21 taxa; spores of pteridophytes and bryophytes account for 45.1% of the assemblage, but they are more diverse with 32 taxa. Angiosperm pollen is very rare (0.4%), and represented by only Clavatipollenites hughesii. Gymnosperm pollen is dominated by bisaccate pollen of conifers, which accounts for approximately half of the gymnosperm component, and also includes monosulcate pollen of cycadophytes and ginkgophytes, and inaperturate pollen of Cupressaceae sensu lato. Among the diverse spores of pteridophytes and bryophytes, those of probable Lygodiaceae and Schizaeaceae are most common. The palynological assemblages from samples 059-3 and 059-5 are similar in composition to the assemblage from the ash layer (Tab. 3), and thus these three assemblages can be discussed together.
In general the palynological assemblages characterized by abundant gymnosperm pollen and diverse spores of pteridophytes and bryophytes is typical for palynofloras from the Early Cretaceous of North China (Song, 1986). Some typical Cretaceous elements including Cicatricosisporites, Concavissimisporites, Densoisporites, Impardecispora, Jiaohepollis and Pilosisposrites are very common in the assemblage, but are absent or almost absent in strata older than Cretaceous in age. These taxa are rare and low in diversity in early Early Cretaceous assemblages, and become more common and abundant in the late Early Cretaceous assemblages. In addition, monocolpate angiosperm grains resembling Clavatipollenites first appeared in the Valanginian-Hauterivian of the paleoequatorial regions (Friis et al., 2011). In middle and high latitude regions of the northern hemisphere, however, the earliest appearance of Clavatipollenites is later, commonly in the Barremian (Song, 1986;Zhang, 1999). In the palynomorph assemblage from the ash layer, Clavatipollenites is very rare. Tricolpate pollen, which typically is first recorded at around the Barremian-Aptian boundary, is absent from the samples studied so far from the Zhahanaoer open-cast coal mine, suggesting most likely a Barremian age for the palynological assemblages.
The U-Pb zircon age of the layer of the Huolinhe Formation from the SIMS analyses (125.6±1.0 Ma) is generally consistent with that from the LA-ICP-MS analyses (126.2±0.9 Ma), and both are consistent with the age inference based on palynostratigraphic analysis. All the evidence currently available suggests that the depositional age of the ash layer is late Barremian-earliest Aptian. Guo (1995) investigated the palynomorphs from the "lower coal-bearing member" of the Huolinhe Formation at the very large South Open-cast Mine from the Huolinhe Basin ~10 km northeast of the Zhahanaoer mine. These palynomorphs are characterized by the Cicatricosisporites-Pilosisporites-Osmundacidites assemblage. Gymnosperm pollen (57.4%) dominates the assemblage, followed by the spores of pteridophytes and bryophytes (40.8%). Angiosperm pollen accounts for 0~2.0% of the assemblage and includes Clavatipollenites, Asteropollis and Tricolpites (Guo, 1995). The greater diversity and abundance of angiosperm pollen, including the presence of tricolpate eudicot pollen, suggests that the depositional age of the palynological assemblage from the South Open-cast Mine is Aptian-Albian (125-100.5 Ma), although Guo (1995) interpreted the assemblage to be of Hauterivian to early Barremian age. The palynological assemblage of the "lower coal-bearing member" from South Open-cast Mine appears younger than that from the ash layer of the Zhahanaoer mine reported here, in which angiosperm pollen is very rare (0.4%) and includes only grains similar to Clavatipollenites and no tricolpates. This is consistent with our conclusion that the ash layer is near the bottom of the "lower coal-bearing member" of the Huolinhe Formation based on stratigraphic analyses and borehole data. Therefore, our results provide a maximum age for the lower part of the Huolinhe Formation that is above the ash layer. The palynological sequences through the entire formation remain to be studied.
Early Cretaceous lignite/coal-bearing basins are extensively developed in Northeast China and also in adjacent Mongolia. These include Fuxin and Tiefa basins in Liaoning (Chen et al., 1988), the Hailar, Erlian, Dayangshu and Huolinhe basins in eastern Inner Mongolia (Deng, 1995;Deng et al., 1997), and the Choyr, Tamutsag and several other small basins in eastern and central Mongolia (Ichinnorov, 2003;Nichols et al., 2006;Shi et al., 2014Shi et al., , 2016;;Wang et al., 2014;Herrera et al., 2015Herrera et al., , 2017)). Many of these basins include important paleobotanical localities and beds that yield abundant, diverse plant fossils, but their ages are not well constrained due to lack of associated marine deposits and volcanic beds in these basins. Stratigraphic correlations based on floristic and palynological assemblages generally refer to these lignite/coal-bearing basins as of Valanginian to Albian age, but more secure age estimates for the individual basins are lacking. The more precise U-Pb ages for the palynomorph assemblages preserved associated with the ash layer of the Huolinhe Formation, together with future investigations of the palynological sequence of the entire Huolinhe Formation would provide a more refined chronological framework for the Early Cretaceous lignite/coal-bearing basins in Northeast China and Mongolia. Such research would also be of great value for constraining the age of key fossils, including the fossil plants and fossil vertebrates preserved in these basins.