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52 12B-Zhang+Nuna.Pdf Earth and Planetary Science Letters 353–354 (2012) 145–155 Contents lists available at SciVerse ScienceDirect Earth and Planetary Science Letters journal homepage: www.elsevier.com/locate/epsl Pre-Rodinia supercontinent Nuna shaping up: A global synthesis with new paleomagnetic results from North China Shihong Zhang a,n, Zheng-Xiang Li b,c, David A.D. Evans d, Huaichun Wu a, Haiyan Li a, Jin Dong a a State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing, 100083, China b Department of Applied Geology, ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS), Curtin University, GPO Box U1987, Perth, WA, Australia c Department of Applied Geology, The Institute for Geoscience Research (TIGeR), Curtin University, GPO Box U1987, Perth, WA, Australia d Department of Geology and Geophysics, Yale University, New Haven, CT 06520-8109, USA article info abstract Article history: The existence of a pre-Rodinia Precambrian supercontinent, variously called Nuna or Columbia, has Received 23 June 2012 been widely speculated in the past decade, but the precise timing of its existence and its configuration Received in revised form have been uncertain due to the lack of unequivocal paleomagnetic and geological constraints. Here we 20 July 2012 report high-quality paleomagnetic results from the well dated 1780 Ma Xiong’er Group in southern Accepted 21 July 2012 North China Block (NCB). A total of 110 paleomagnetic samples from 14 sites were collected and Editor: T.M. Harrison subjected to stepwise thermal demagnetization. After removing a low temperature component (CL) of viscous magnetic remanence acquired in recent geomagnetic field, a high temperature component (CH), Keywords: carried by hematite and magnetite in redbeds and volcanic samples, has been isolated. It gives a mean paleomagnetism direction of (D¼18.41, I¼3.71, a ¼7.61, N¼14) after bedding correction, and a corresponding Xiong’er Group 95 paleomagnetic pole at 50.21N, 263.01E(A ¼4.51). The CH passed a reversal test and was interpreted as North China Block 95 Nuna a primary remanence. This new pole plus three other high-quality poles from the NCB that have been supercontinent more precisely dated at 176973 Ma, 1560–1440 Ma and 1437721 Ma define a 1780–1440 Ma reconstruction apparent polar wander path (APWP) for the NCB. This, together with an update of global high quality paleomagnetic dataset, allows us to demonstrate that the pre-Rodinia supercontinent Nuna likely existed at least between 1780 Ma and 1400 Ma. Our paleomagnetism-based global reconstruction, for the first time, quantitatively assembles all major cratons together; it encompasses previously proposed regional links including the SAMBA connection between Baltica, Amazonia and Western Africa (Johansson, 2009), connections between Laurentia, Baltica and Siberia at the core of Nuna (Evans and Mitchell, 2011), the proto-SWEAT connection between Laurentia, East Antarctica and Australian blocks (Payne et al., 2009), and the NCB–India connection (Zhao et al., 2011). & 2012 Elsevier B.V. All rights reserved. 1. Introduction unique. Early attempts of paleomagnetically examining Nuna- related continental reconstructions (e.g. Idnurm and Giddings, Although it has long been speculated that the Earth’s evolution 1995; Payne et al., 2009; Wu et al., 2005) were hampered by the may have been dominated by cycles of supercontinent assembly lack of high-quality paleomagnetic results, or merely regional rather and breakup (e.g., Nance et al., 1988), our understanding of past than global considerations. Evans and Mitchell (2011) provided a supercontinents has mainly been limited to the last 1000 Ma, with recent update on the assembly and breakup of the centre of Nuna, more confidence being placed on the history of the ca. 320–180 Ma between Laurentia, Baltica and Siberia. Here we use new paleomag- supercontinent Pangea (Veevers, 2004) than the ca. 900–700 Ma netic results from North China, as well as recent results from supercontinent Rodinia (Li et al., 2008a). Our knowledge of an even Australia, India, and Amazonia, to examine the positions of these older supercontinent, possibly formed by global ca. 1800 Ma oro- continental blocks in Nuna. Together with the proposed Laurentia– genic events and variably named Nuna (Hoffman, 1997)orColumbia Baltica–Siberia connection (Evans and Mitchell, 2011), we are now (Rogers and Santosh, 2002; Zhao et al., 2002; Zhao G. et al., 2004), able to present a geologically and paleomagnetically viable config- has been more tenuous, and early papers were mostly based on uration for the bulk of Nuna. intercontinental geological correlations that are intrinsically non- 2. New paleomagnetic results from North China 2.1. Geological background n Corresponding author: State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China, Tel.: þ86 10 82322257; fax: þ86 10 82321983. The North China Block (NCB) has an Archean to Paleoproterozoic E-mail address: [email protected] (S. Zhang). metamorphic basement, covered by post-1.85 Ga, little-deformed 0012-821X/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.epsl.2012.07.034 146 S. Zhang et al. / Earth and Planetary Science Letters 353–354 (2012) 145–155 sedimentary and volcanic successions that are amenable to paleo- Our paleomagnetic sampling was carried out for the Xushan and magnetic study (e.g. Zhang et al., 2006). Recently available high- Majiahe formations in the Waifang Mountain region (331570N, precision U–Pb zircon ages for the volcanic units and tuff beds 1121270E, Fig. 1b), because these formations not only have tight within the sedimentary units (Li et al., 2010; Su et al., 2008, 2010) age constraints of ca. 1780 Ma, they are also better outcropped and provide the first precise age constraints on paleomagnetic results more fresh in that area. Outcrops without clear geological contact obtained from these successions, enabling us to use those results for and concrete paleo-horizontal markers, such as those in the quantitative global paleogeographic reconstructions. Jidanping Formation, were not considered in this study. A total of Our new paleomagnetic investigation was carried out on the 110 oriented core samples were obtained from 13 sites in dark-gray well-dated Xiong’er Group, which unconformably overlies the or purple-gray basaltic andesites or andesites, and one site in purple Archean and Paleoproterozoic basement rocks of the NCB. It is mudstones of the Majiahe Formation. They were oriented using a found in southern Shanxi Province and western Henan Province, magnetic compass, combined with a sun compass when possible. with an outcropping area of 60,000 km2 (Fig. 1). It consists Orientation differences between the two devices are within 711 in mainly of andesites and basaltic andesites, with minor dacites, most cases, and the differences are less than 731 for all samples. rhyolites and red mudstones (He et al., 2009). Volcanic strata of The strata tilt is gentle; most sites have dips of 5–201 toward the the Xiong’er Group are traditionally classified into three parts: the southwest. Xushan Fomation at the bottom, the Jidanping Formation in the middle, and the Majiahe Formation at the top. However, this subdivision was mainly based on petrological compositions and 2.2. Experimental condition has been challenged by recent geochronological work (He et al., 2009). The Xushan Formation and the Majiahe Formation have All samples were cut into 22-mm-long specimens and subjected similar petrological compositions. They both are dominated by to stepwise thermal demagnetization. Remanence measurements basaltic andesites and andesites, but the Majiahe Formation were made with an AGICO JR-6A spinner magnetometer and contains more sedimentary red-beds. Zircon U–Pb ages of the demagnetization was made using an ASC TD-48 furnace in volcanic rocks from these two formations are both 1780 Ma, 5–20 1C steps up to 685 1C, or until the remanence became too indistinguishable within uncertainties (Fig. 1b). They may repre- weak to measure. The instruments in the paleomagnetism labora- sent rapid magmatic activity in the south margin of the NCB in tory of China University of Geosciences (Beijing) are housed in late Paleoproterozoic, slightly older than the Taihang dyke swarm room-size Helmholtz coils with a low magnetic field of 200 nT in in the interior of this craton (ca. 1770 Ma, Halls et al., 2000; Wang the sample loading region. The internal residual field in the furnace et al., 2004). is lower than 10 nT. During the experiments, samples were further Age constraints for the Jidanping Formation are more complex. protected from magnetic contamination by m-metal shielding. The Jidanping Formation contains fewer basaltic rocks but more Magnetic hysteresis parameters (i.e., saturation magnetization, dacites and rhyolites and subvolcanic rocks than the Xushan and Ms, saturation remanence, Mrs, coercive force, Hc, and coercivity Majiahe formations. Some volcanic rocks of the Jidanping Forma- of remanence, Hcr) were measured for representative samples tion gave the same ages within errors as the Xushan and Majiahe using an alternating gradient magnetometer (MicroMag 2900). formations, but others gave ages as young as 1450 Ma (He et al., The magnetic field was cycled between 71.8 T for each sample. 2009), suggesting that the lithostratigraphically defined Jidanping Ms, Mrs, and Hc were determined after correction for the para- Formation has a wide range of ages. magnetic contribution. Hcr was obtained when each sample was 111o 00' 125° 115° 105° N Beijing 40° 1783±20 1778±6.5 Xiao Mt. 1778±5.5 Zhengzhou 35° Luoning Fig.1 b Ry208 Ry211-214 1450±31 Ry2156 Xiong'er Mt. Ry217-221 Ry224,225,231 Waifang Mt. Lushi Ry243 34o 00' Paleomagnetic site Ruyang and Guandaokou Gps Majiahe Fm 1783±13 1750±65 1644±14 Xiong' er Jidanping Fm (Ren et al.,2000) (Ren et al.,2000) 1778±8 Gp 1776±20 Xushan Fm 1751±14 (Zhao T.P.
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