U-Pb and Re-Os geochronology tracks stratigraphic condensation in the Sturtian snowball Earth aftermath
U-Pb and Re-Os geochronology tracks stratigraphic condensation in the Sturtian snowball Earth aftermath
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DOI：
10.1130/g47246.1
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发表时间：
2020-06
期刊：
影响因子：
5.8
通讯作者：
A. Rooney;Chuan Yang;D. Condon;Maoyan Zhu;F. Macdonald
中科院分区：
文献类型：
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作者：
A. Rooney;Chuan Yang;D. Condon;Maoyan Zhu;F. Macdonald
雪球地球假说预测了一个强烈的滞后性，导致离散的数百万年冰川，随后是全球同步的冰川消融。本文提出了中国南方后斯图特期层序中新的U-Pb锆石和Re-Os沉积岩年代学和Os同位素化学地层学，以检验消冰作用的共时性。高精度化学磨损-同位素稀释-热电离质谱（CA-IDTIMS） U-Pb锆石测年将脱冰的最小年龄细化为660.98±0.74 Ma，比先前报道的年龄大~ 2 m.。我们还提供了一个新的马里诺冰期开始的最大年龄约束（657.17±0.78 Ma）。新的Os同位素化学地层学的全球汇编揭示了一个大于56 m.y的非放射性成因值的大而系统的趋势。目前，地质年代学数据库与两个长寿命的低温冰期一致（Rooney et al., 2015; Zhou et al., 2019）。然而，Spence等人（2016）认为Re-Os年代表没有经过测试，因为没有一个地方在同一沉积物上同时具有U-Pb CA-ID-TIMS锆石和Re-Os日期，相反，Cryogenian冰川记录可以代表第四纪样的冰川-间冰期条件（Allen and Etienne, 2008）。最近关于Sturtian脱冰作用的U-Pb CA-ID-TIMS锆石日期在澳大利亚最上层冰川沉积物内的日期（663.03±0.11 Ma）和来自中国南方据称的帽状碳酸盐岩的日期（658.80±0.50 Ma）之间留下了明显的4毫秒的差距（Cox等人，2018）。这可以解释为异常长时间的轨道强迫消冰（Benn et al., 2015），消冰的历时性（Allen and Etienne, 2008），或中国南方冰期后序列的极端凝结（Kennedy and Christie-Blick, 2011）。本文利用Re-Os和U-Pb地质时计分别对记录中国南方斯图特冰期结束和消冰期后条件的沉积岩和空气沉降凝灰岩沉积物进行了新的时代约束。这些数据为斯图特冰期提供了一个时间框架，并使我们能够探索斯图特冰期和马里诺冰期的不同持续时间与它们的冰期序列的地层和地球化学差异之间的关系。华南克拉通的低温期伸展形成了地堑和地堑构造，形成了东南向的大陆边缘（Yu et al., 2017; Bao et al., 2018）。在盆地环境中，铁斯奥组的斯图特闪晶岩和等效单元被分离。Alan Rooney， 2020年8月25日626 www.gsapubs.org |第48卷|第6号|地质|美国地质学会从南沱组Marinoan二晶岩中发现大唐坡组薄层页岩和粉砂岩，含少量碳酸盐（图1；Bao等，2018；Zhou等，2019）。在没有图斯特系长晶岩的位置（如军山剖面），大塘坡组不整合地覆盖在托尼系板溪群上（图1；Yu et al., 2017）。我们从测量的大唐坡组地层剖面和扬子地块下垫冰川沉积物中采集了Re-Os和U-Pb年代学和Os同位素化学地层学样品（图1；参见GSA数据库1）。湘西板溪群为不整合上覆铁石坳组辉晶岩、砂岩和砾岩。上覆的大塘坡组由2 km长的长安组二晶岩和富鲁组硅质岩组成（Bao et al., 2018）。在广子洞，约10 m的铁四坳组砂岩、粉砂岩、页岩和层状二晶岩互层与穿透床层的dropstone渐变成基底大唐坡页岩（图1）。我们将这些沉积物解释为记录脱冰作用的浮冰碎屑，并对互层页岩进行Re-Os地质年代学采样。结果：从9个泥岩层中分离出锆石，现场解释为脱氮气落灰沉积，并用U-Pb CA-ID-TIMS定年（图1和2；数据存储库中的表DR1）。FDM14-11和FDM14-12样品锆石上最年轻的协和238U-206Pb测年提供了最大沉积年龄（图2）。FDM14-13和FDM14-14样品的锆石分析结果一致，分别为658.97±0.76 Ma（加权偏差均方根[MSWD] = 1.3）和657.17±0.78 Ma (MSWD = 1.3, n = 7)，我们将其解释为喷发和沉积年龄（图2）。FDM14-19样品中2颗锆石晶粒形貌分析认为为异晶，但屈服年龄<665 Ma。17GZGZ01样品的7次锆石分析结果一致，加权平均238U-206Pb年代值为660.98±0.74 Ma (MSWD = 1.5, n = 7)，可解释为沉积年龄。铁斯奥组上部F1408-15.6样品的Re-Os同位素组成数据得出模型1年龄为660.6±3.9 Ma（不确定性包括0.35%的187Re衰变常数不确定性[Smoliar et al., 1996]; n = 8, MSWD = 0.92, 2σ不确定性，初始187Os/188Os [Os]i = 1.55±0.05；图2；图DR1；表DR2）。化学地层学的初始Os同位素值来自于大塘坡组铁斯坳二晶岩和碳质页岩的黑色页岩基质，假定脱冰期为660 Ma（表DR3）。在四个古大陆上，斯图特冰川作用的终结标志着深灰色到黑色的碳质页岩和石灰岩的沉积，这些页岩和石灰岩尖锐而整合地覆盖在冰川沉积物上。在这里，我们提供了一个新的re - o年龄为660.6±3.9 Ma，来自中国南方（上覆大唐坡组下方3 m）的辉长岩最上层单元，与澳大利亚（Kendall等人，2006；Cox等人，2018）、加拿大西北部（Rooney等人，2014）和蒙古（Rooney等人，2015）（图2）的后斯图特纪序列的现有re - o年龄不确定，表明冰川融化是全球同步的。新的CA-ID-TIMS U-Pb测年来自大唐坡组，分别为660.98±0.74 Ma、658.97±0.76 Ma和657.17±0.78 Ma。这些数据系统地显示了年轻的上剖面，并且在远离古高点的部分是最古老的（图1B）。结合我们最新的CA-ID-TIMS U-Pb测年数据，以及加拿大西北部地区716.5±0.2 Ma的起始年龄(Macdonald et al., 2010；Rooney et al., 2014)，我们增加了全球数据集，并确认了Sturtian冰期的持续时间为56毫秒。关键的是，新的CA-ID-TIMS U-Pb日期与Re-Os日期一致，进一步证明了Re-Os沉积岩计时器的有效性（图2）。与dia1GSA数据存储库项目2020184不同，材料和方法、等时线图和数据表可通过http://www.geosociety.org/datarepository/2020/或editing@geosociety.org在线获取。砂岩锰碳酸盐粉砂岩二晶岩黑色页岩砾岩砂岩
The snowball Earth hypothesis predicts a strong hysteresis resulting in discrete multimillion-year glaciations followed by globally synchronous deglaciation. Here we present new U-Pb zircon and Re-Os sedimentary rock geochronology and Os isotope chemostratigraphy from post-Sturtian sequences in south China to test the synchroneity of deglaciation. Highprecision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-IDTIMS) U-Pb zircon dates refine the minimum age of deglaciation to 660.98 ± 0.74 Ma, which is ∼2 m.y. older than previously reported. We also provide a new maximum age constraint on the onset of the Marinoan glaciation of 657.17 ± 0.78 Ma. A global compilation of new Os isotope chemostratigraphy reveals a large and systematic trend to unradiogenic values over 56 m.y. At present, the geochronological database is consistent with two long-lived Cryogenian glaciations (Rooney et al., 2015; Zhou et al., 2019). However, Spence et al. (2016) argued that the Re-Os geochronometer was untested because no locality had both U-Pb CA-ID-TIMS zircon and Re-Os dates on the same deposits, and that instead the Cryogenian glacial record could represent a Quaternary-like period of glacial-interglacial conditions (Allen and Etienne, 2008). Recent U-Pb CA-ID-TIMS zircon dates on the Sturtian deglaciation leave an apparent 4 m.y. gap between dates from within the uppermost glacial deposits in Australia of 663.03 ± 0.11 Ma (Cox et al., 2018) and from a purported cap carbonate in south China of 658.80 ± 0.50 Ma (Zhou et al., 2019). This could be interpreted to represent an exceptionally long orbitally forced deglaciation (Benn et al., 2015), diachroneity of deglaciation (Allen and Etienne, 2008), or extreme condensation of the post-glacial sequence in south China (Kennedy and Christie-Blick, 2011). Here we present new age constraints using the Re-Os and U-Pb geochronometers on sedimentary rocks and air-fall tuff deposits, respectively, from strata that record the termination and post-deglaciation conditions of the Sturtian glacial event in south China. These data provide a temporal framework for the Sturtian deglaciation, and allow us to explore relationships between the different durations of the Sturtian and Marinoan glaciations and stratigraphic and geochemical differences in their deglacial sequences. GEOLOGICAL SETTING Cryogenian extension of the South China craton resulted in horst and graben structure and development of a southeast-facing continental margin (Yu et al., 2017; Bao et al., 2018). In basinal settings, Sturtian diamictite of the Tiesi’ao Formation and equivalent units are separated Published online 13 April 2020 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/6/625/5051191/625.pdf by Yale University, Alan Rooney on 25 August 2020 626 www.gsapubs.org | Volume 48 | Number 6 | GEOLOGY | Geological Society of America from Marinoan diamictite of the Nantuo Formation by thinly bedded shale and siltstone with minor carbonate of the Datangpo Formation (Fig. 1; Bao et al., 2018; Zhou et al., 2019). In locations, where the Sturtian diamictite is absent (e.g., the Jiangjunshan section), the Datangpo Formation unconformably overlies the Tonian Banxi Group (Fig. 1; Yu et al., 2017). We collected samples for Re-Os and U-Pb geochronology and Os isotope chemostratigraphy from measured stratigraphic sections of the Datangpo Formation and underlying glacial deposits from the Yangtze block (Fig. 1; see the GSA Data Repository1). In western Hunan, the Banxi Group is unconformably overlain by diamictite, sandstone, and conglomerate of the Tiesi’ao Formation. The overlying Datangpo Formation consists of 2 km of diamictite of the Chang’an Formation and siliciclastic rocks of the Fulu Formation (Bao et al., 2018). At Guangzidong, ∼10 m of interbedded sandstone, siltstone, shale, and stratified diamictite of the Tiesi’ao Formation with bed-penetrating dropstones grade into shale of the basal Datangpo (Fig. 1). We interpret these deposits as ice-rafted debris recording deglaciation, and sampled the interbedded shale for Re-Os geochronology. RESULTS Zircon was separated from nine mudstone horizons interpreted in the field as devitrified airfall ash deposits and dated with U-Pb CA-ID-TIMS (Figs. 1 and 2; Table DR1 in the Data Repository). The youngest concordant 238U-206Pb dates on zircon from samples FDM14-11 and FDM14-12 provide maximum depositional ages (Fig. 2). Zircon from samples FDM14-13 and FDM14-14 yield concordant analyses of 658.97 ± 0.76 Ma (mean square of weighted deviates [MSWD] = 1.3) and 657.17 ± 0.78 Ma (MSWD = 1.3, n = 7), respectively, which we interpret as the eruptive and sedimentation age (Fig. 2). Two zircon grains from sample FDM14-19 are considered to be xenocrystic based on their morphology but yield ages <665 Ma. Zircon from sample 17GZGZ01 yielded seven concordant analyses with a weighted mean 238U-206Pb date of 660.98 ± 0.74 Ma (MSWD = 1.5, n = 7), which we interpret as the sedimentation age. The Re-Os isotopic composition data from sample F1408-15.6 in the uppermost Tiesi’ao Formation yield a model 1 age of 660.6 ± 3.9 Ma (uncertainty includes the 0.35% 187Re decay constant uncertainty [Smoliar et al., 1996]; n = 8, MSWD = 0.92, 2σ uncertainties, initial 187Os/188Os [Os]i = 1.55 ± 0.05; Fig. 2; Fig. DR1; Table DR2). Initial Os isotope values for chemostratigraphy were generated from the black shale matrix of the Tiesi’ao diamictite and carbonaceous shale of the Datangpo Formation with an assumed deglaciation age of 660 Ma (Table DR3). SYNCHRONOUS DEGLACIATION AND EXTREME CONDENSATION On four paleocontinents, the termination of the Sturtian glaciation is marked by deposition of dark gray to black carbonaceous shales and limestones that sharply and conformably overlie glacial deposits. Here we provide a new Re-Os date of 660.6 ± 3.9 Ma from within the uppermost unit of a diamictite from south China (3 m below the overlying Datangpo Formation) that is within uncertainty of existing Re-Os ages from post-Sturtian sequences in Australia (Kendall et al., 2006; Cox et al., 2018), northwestern Canada (Rooney et al., 2014), and Mongolia (Rooney et al., 2015) (Fig. 2), suggesting that deglaciation was globally synchronous. Our new CA-ID-TIMS U-Pb dates from the Datangpo Formation of 660.98 ± 0.74 Ma, 658.97 ± 0.76 Ma, and 657.17 ± 0.78 Ma are also within uncertainty of the previously cited ages. These dates systematically young upsection and are oldest in sections away from paleo-highs (Fig. 1B). Incorporating our new CA-ID-TIMS U-Pb date from the lowermost Datangpo Formation, together with the onset age of 716.5 ± 0.2 Ma from northwestern Canada (Macdonald et al., 2010; Rooney et al., 2014), we augment the global data set and confirm that the duration of the Sturtian glaciation was 56 m.y. Critically, the new CA-ID-TIMS U-Pb dates for the final stages of the Sturtian glaciation agree with Re-Os dates, further demonstrating the validity of the Re-Os sedimentary rock chronometer (Fig. 2). Rather than a dia1GSA Data Repository item 2020184, materials and methods, isochron figure, and data tables, is available online at http://www.geosociety.org/datarepository/2020/, or on request from editing@geosociety.org. Sandstone Mn-carbonate Siltstone Diamictite Black shale Conglomeratic sandstone