Palaeogeography, Palaeoclimatology, Palaeoecology 463 (2016) 230–237 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo Geochronologic age constraints on the Middle Devonian Hujiersite flora of Xinjiang, NW China Daran Zheng a,b,HongheXub,JunWanga, Chongqing Feng a, Haichun Zhang b, Su-Chin Chang a,⁎ a Department of Earth Sciences, The University of Hong Kong, Hong Kong Special Administrative Region, China b State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, 39 East Beijing Road, Nanjing 210008, China article info abstract Article history: The Hujiersite Formation of West Junggar, Xinjiang contain fossil plants that represent significant examples of Received 4 July 2016 Middle Devonian flora in North China. However, previous radio-isotopic age constraint for this fossil-rich forma- Received in revised form 19 September 2016 tion is absent. The best-studied section of the Hujiersite flora occurs at a locality referred to as 251 Hill, which ex- Accepted 11 October 2016 poses numerous examples of fossil plants and information on depositional environment. In this study, we Available online 13 October 2016 investigate the geological background of the 251 Hill section and collect two tuffaceous sandstone beds from a Keywords: normal fault near the classic fossil outcrop. U-Pb ICP-MS analysis of detrital zircons from these samples gave a Land plants maximum depositional age of 385 Ma (Givetian stage) for the hanging wall and 380 Ma (early Frasnian stage) Zircon for the footwall of the upper Hujiersite Formation. This study thus reports a novel Givetian–early Frasnian age Hujiersite Formation constraint for Hujiersite flora. This geochronologic constraint is consistent with previous age interpretations de- Geochronology rived from biostratigraphic correlations. Devonian © 2016 Elsevier B.V. All rights reserved. Xinjiang 1. Introduction entire Hujiersite Formation. Our geochronologic analysis (ICP-MS U-Pb) of detrital zircons from tuffaceous sandstone samples from the 251 Hill Major advances in land plant evolution and diversification oc- section gave robust and consistent maximum depositional ages for the curred in the Middle to late Devonian. The fossil records contain upper Hujiersite Formation. These ages offer a novel age constraint for tree-sized plants that comprised early forests of the Middle-late De- the Hujiersite flora and may further help constrain the distribution, evo- vonian at different continental localities including North America lution and migration of early land plants, as well as their influence on (Stein et al., 2007, 2012), Spitsbergen (Berry and Marshall, 2015) Devonian palaeoenvironments. and West Junggar, China (Xu et al., 2012b, 2015). The appearance and development of forest ecosystems imparted dramatic changes 2. Geological setting on the Earth system, as evident from the rapid decline of atmospher- ic carbon dioxide during the Devonian (Berner, 1997; Algeo et al., West Junggar is located in North Xinjiang, northwestern China 2001; Morris et al., 2015). (Fig. 1). The locality is regionally situated in central Asia near the The Hujiersite flora occurs within the Devonian Hujiersite Forma- China-Kazakhstan border, south of the Altai mountain range and tion, West Junggar, Xinjiang, China and contains representative fossil the Yili block (Geng et al., 2009). Palaeogeographically, West Junggar plants including abundant mega-plants that appear to form a small- abutted Siberia, Tarim and Kazakhstan continental fragments (Feng scaled forest inhabiting an alluvial flat or flood plain (Cai and Wang, et al., 1989). Devonian units of West Junggar include the lower 1995; Xu et al., 2015). Based on conchostracans (Liu, 1990) and plant Devonian Hoboksar group, the Middle Devonian Hujiersite Forma- fossils (Cai and Wang, 1995; Xu et al., 2012b, 2014, 2015), most previ- tion and the upper Devonian Hongguleleng Formation (Cai, 2000). ous studies suggested a late Middle Devonian age for the Hujierste For- The Hujiersite Formation was first described and formalized in mation. However, some disagreements exist (Lu, 1997; Cai, 2000; Wang Mangkelu, Hoxtolgay, Hoboksar in 1973 (Lu, 1997) and occurs main- et al., 2004; Xu et al., 2015) and radio-isotopic age determination for this ly in Hoboksar, West Junggar. fossil-bearing formation is absent. The 251 Hill section lies about 20 km north of the town of In order to clarify the age for the Hujiersite flora, we investigate the Hoxtolgay, Hoboksar Mongol Autonomous County, Xinjiang, NW 251 Hill section. This section exposes the most fossil-rich horizon of the China and hosts only the upper member of the Hujiersite Formation (Fig. 1; GPS: 46° 36′ 55″ N, 86° 1′ 6″ E). A small normal fault disrupts ⁎ Corresponding author. the section with the offset of about 0.5 m (Fig. 2). The section consists E-mail address: [email protected] (S.-C. Chang). primarily of tuffaceous conglomerate, sandstone and siltstone, http://dx.doi.org/10.1016/j.palaeo.2016.10.015 0031-0182/© 2016 Elsevier B.V. All rights reserved. D. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 463 (2016) 230–237 231 Fig. 1. Geological map showing the location of 251 Hill in Hoboksar, Xinjiang, NW China. interbedded with thin coal seams (Fig. 3). Abundant plant taxa 3.2. Zircon separation and cathodoluminescent (CL) imaging (megaplants and spores) were reported from the section (Xu et al., 2015)(Figs. 3 and 4). The samples were crushed and separated to isolate the 80–200 μm The West Junggar area probably represent a land bridge or contigu- grain size-fraction. The detailed preparation procedure is described as ous land area that allowed dispersal and expansion of certain Devonian in Wang et al. (2015a, 2015b). A total of 200 inclusion-free zircon grains lycopsids and aneurophytaleans (Xu et al., 2012a, 2015; Jiang et al., from each sample were then picked under a binocular microscope and 2013). The abundance of miospore Cymbosporites and coal seams in mounted in epoxy resin. Hardened mounts were sectioned and polished the section indicate that lycopsids growing in a tropical and humid en- to expose zircon grain midsections at about 2/3 to 1/2 of their width. vironment represented major constituents of Hujiersite ecosystems (Xu Cathodoluminescent (CL) imaging documented grain morphologies et al., 2014). The degree and quality of plant fossil preservation also in- and internal structure for in situ analysis. dicates burial and fossilization near sites of growth (Feng et al., 2014; Xu et al., 2015). 3.3. U-Pb analysis of zircons 3. Materials and methods U-Pb isotopic data from zircons were obtained through the Depart- ment of Earth Sciences, University of Hong Kong, using a Nu Instru- 3.1. Materials ments Multiple Collector (MC) ICP-MS with a Resonetics Resolution M-50-HR Excimer Laser Ablation System. The analyses used a beam di- Two tuffaceous sandstone samples (H-01, H-02), each weighing ameter of 30 μm and 6 Hz repetition rate for a signal intensity of 4 mV at about 3 kg, were collected between fossil-bearing strata of the 251 Hill mass 238U for the standard zircon 91500. Average ablation time was ca. section (Fig. 2). Sample H-01 was collected from a middle section of 40 s and pit depths reached about 30 μm. The GJ-1 zircon standard the normal fault hanging wall (Fig. 2B, D), while H-02 occurred in the (609 Ma, Jackson et al., 2004) and Harvard reference zircon 91500 lower part of the footwall (Fig. 2C, E). Abundant detrital zircons from (1065.4 ± 0.3 Ma, Wiedenbeck et al., 1995) were used for calibration. the samples are suitable for U/Pb dating. Detailed operational procedures are found in Xia et al. (2011).We 232 D. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 463 (2016) 230–237 Fig. 2. Images of 251 Hill and samples of tuffaceous sandstone. (A) 251 Hill normal fault (arrowed). (B) Footwall location of sample H-02 (white box). (C) Hanging wall location of sample H-01 (white box). (D) Sample H-02 in outcrop. (E) Sample H-01 in outcrop. used ICPMSDataCal (Liu et al., 2010) to process the off-line signal selec- and oscillatory zoning patterns indicate igneous origin and limited ex- tion, quantitative calibration and time-drift correction. We used a func- posure to sedimentary processes (Fig. 5C). Eighty-six zircon grains tion given in Anderson (2002) to correct for common Pb in Microsoft gave concordant ages ranging from 483 to 380 Ma (Fig. 5D). Six analyses Excel. Isoplot v. 3.0 (Ludwig, 2003) was used to construct concordia di- gave a maximum depositional age of 380 Ma for H-02. The largest sub- agrams and probability density plots. Within the overall detrital age dis- population of ages generally fell between 390 and 380 Ma. The age dis- tribution, we cite 206Pb/238U ages for zircon grains younger than tribution also included several sporadic Ordovician and Silurian ages 1000 Ma and 207Pb/206Pb ages for older grains. Supplementary table (483–403 Ma). lists U-Pb data. 5. Discussion 4. Zircon morphology and age data 5.1. Biostratigraphic age for the Hujiersite flora One hundred zircon grains were separated from the sample H-01. With a few exceptions, zircon grain size ranged from 100 to 200 μm. The Hujiersite flora consists of a number of plant taxa (Table 1)in- Most grains exhibited euhedral morphologies and oscillatory zoning cluding lycopsids Haskinsia hastata (Xu et al., 2008), H. sagittata (Xu et patterns indicating igneous origin (Fig. 5A). Angular grain facets also al., 2008), Leclercqia cf. complexa (Xu and Wang, 2008), L. uninata (Xu suggest minimal abrasion from sedimentary transport and support et al., 2011a), Colpodexylom gracilentum (Xu and Wang, 2011), the interpretation that the tuffaceous sandstone depocenter lay Hoxtolgaya robusta (Xu et al., 2012a)andDrepanophycus minor (Xu et near the magmatic source of zircons. Ninety-four analyses provided al., 2013).