Accepted Manuscript Chronological constraints on the Paleoproterozoic Francevillian Group in Gabon Yusuke Sawaki, Mathieu Moussavou, Tomohiko Sato, Kazue Suzuki, Cédric Ligna, Hisashi Asanuma, Shuhei Sakata, Hideyuki Obayashi, Takafumi Hirata, Amboise Edou-Minko PII:
S1674-9871(16)30135-9
DOI:
10.1016/j.gsf.2016.10.001
Reference:
GSF 494
To appear in:
Geoscience Frontiers
Received Date: 31 March 2016 Revised Date:
16 September 2016
Accepted Date: 3 October 2016
Please cite this article as: Sawaki, Y., Moussavou, M., Sato, T., Suzuki, K., Ligna, C., Asanuma, H., Sakata, S., Obayashi, H., Hirata, T., Edou-Minko, A., Chronological constraints on the Paleoproterozoic Francevillian Group in Gabon, Geoscience Frontiers (2016), doi: 10.1016/j.gsf.2016.10.001. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Mn-deposits
Black shales in FB unit Macroscopic fossils Black shales in FD unit
in Gabon
10 Lomagundi Excursion
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Huronian glaciations
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Chronological constraints on the Paleoproterozoic Francevillian Group in Gabon
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Yusuke Sawaki a, *, Mathieu Moussavou b, Tomohiko Sato c, Kazue Suzuki a, Cédric
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Ligna b, Hisashi Asanuma a, Shuhei Sakata d, Hideyuki Obayashi e, Takafumi Hirata f,
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Amboise Edou-Minko b, *
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b
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c
O-okayama Meguro-ku, Tokyo 152-8551, Japan University of Sciences and Technology in Masuku, BP:901, 90025 Franceville, Gabon
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Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, 2-12-1 O-okayama
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Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1
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Meguro-ku, Tokyo 152-8551, Japan Department of Chemistry, Gakushuin University, Mejiro 1-5-1, Toshima-ku, Tokyo
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Geochemistry Research Center, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan
* Corresponding authors
Y. Sawaki - Tel.: +81-3-5734-2617; Fax: +81-3-5734-3538.
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E-mail address:
[email protected] A. Edou-Minko - Tel.: +241-07 17 33 41. E-mail address:
[email protected]
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Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
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Abstract The Francevillian Group in Gabonese Republic was recently established as a
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typical sedimentary sequence for the Paleoproterozoic. However, its age is rather poorly
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constrained, mainly based on Rb-Sr and Nd-Sm datings. This study reports new zircon
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data obtained from Chaillu massif and N’goutou complex, which constrain the protolith
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age of the basement orthogneisses and the igneous age of an intrusive granite,
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respectively.
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Most zircons from the orthogneisses are blue and exhibit oscillatory zoning in
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cathode-luminescence images. Zircons with lower common lead abundances tend to be
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distributed close to the concordia curve. Two age clusters around 2860 Ma and 2910 Ma
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are found in zircons plotted on the concordia curve. Based on the zircons’ Th/U ratios,
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these ages correspond to the protolith ages of the orthogneisses, and the zircons are not
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metamorphic in origin.
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Syenites and granites were collected from the N’goutou complex that intrudes
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into the FA and FB units of the Francevillian Group. The granitoids exhibit chemical
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composition of A-type granite affinity. Half of zircons separated from the granite are
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non-luminous, and the remaining half exhibit obscure internal textures under
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cathode-luminescence observation. All zircon grains contain significant amounts of
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common lead; the lead isotopic variability is probably attributed to the mixing of two
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components in the zircons. The zircon radiogenic
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corresponding to a
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depositional age of the FA and FB units. Furthermore, the FB unit consists of
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Pb/206Pb ratio is 0.13707 ± 0.0010,
Pb/206Pb age of 2191 ± 13 Ma. This constrains the minimum
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manganese-rich carbonate rocks and organic carbon-rich black shales with macroscopic
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fossils. Based on our age constraints, these organisms appeared in the study area just
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after the last Paleoproterozoic Snowball Earth event, in concert with global scale
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oxidation event encompassing the Snowball Earth.
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Keywords:
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Gabon, Francevillian Group, Paleoproterozoic, N’goutou complex, zircon, MC-ICP-MS
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1. Introduction The Paleoproterozoic is one of the most important periods through Earth’s history
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and is characterized by numerous geological events such as the emergence of
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eukaryotes (e.g., Han and Runnegar, 1992), Snowball Earth events (e.g., Kopp et al.,
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2005), and the oxygen level increase in the ocean-atmosphere system (e.g., Holland,
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1999, 2005; Rey and Holland, 1998). Recently, macroscopic structures, which can be
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interpreted as colonial organisms and microbial/algal consortia, were reported from
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Paleoproterozoic sedimentary rocks (Francevillian Group) in Gabonese Republic
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(Albani et al., 2010; Moussavou et al., 2015). Several geochemical proxies were
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measured in the sediments in order to decipher the surface paleoenvironmental
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conditions at that time (Gauthier-Lafaye and Weber, 2003; Préat et al., 2011; Canfield et
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al., 2013). The Francevillian Group, in particular, attracted significant attention from
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many scientific fields, because it contains the well-known Oklo natural reactors (Neuilly
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et al., 1972; Gauthier-Lafaye et al., 1989) and excellent manganese-rich carbonate rocks
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(Weber, 1968) in the FA and FB unit. Nevertheless, the chronological constraints on the
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sedimentary sequences are still insufficient. Especially, the maximum and minimum
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depositional ages of sediments containing the above macroscopic structures and natural
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reactors have been only poorly constrained by the ages of the basement rocks and
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intrusive granitoids (Bonhomme et al., 1982; Caen-Vachette et al., 1988; Moussavou
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and Edou-Minko, 2006). Here, we report new precise protolith zircon ages of the
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basement gneiss and the igneous age of the intrusive granite in the N’goutou complex.
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These more precise documentations will allow correlating the Francevillian Group with
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sedimentary sequences in other areas, which is important to understand the order of
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events occurring in the Paleoproterozoic. The basement beneath the Francevillian Group is mainly composed of granitoids
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including granite, monzonite, syenite, diorite, and gneiss. The extensive work by
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Caen-Vachette et al. (1988) has considerably enhanced our knowledge of the geological
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history of these basement rocks. Based on the Rb-Sr isochron method and U-Pb isotope
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systematics, they suggested the following phases: a) 3000–2850 Ma; high-grade
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metamorphism in the eastern Makokou area and formation of the Chaillu gneisses; b)
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2850–2700 Ma; high-grade metamorphism of the western Makokou gneisses and the
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main magmatic phase in the Chaillu area; c) 2700–2600 Ma; epizonal metamorphism
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and later magmatism in the Chaillu area. The data from all zircon grains, however, were
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plotted far from the concordia curve. Therefore, the ages for the individual granitoids,
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based on the discordia lines, are still rather uncertain. Moreover, this previous work did
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not include descriptions of the internal zircon structures obtained through
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cathode-luminescence (CL) observations and consideration for common Pb.
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Plutonic rocks of the N’goutou complex intrude into the Paleoproterozoic
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Francevillian Group. Therefore, determining its intrusive age is important to constrain
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the minimum sedimentary age of the Francevillian Group. The age of the N’goutou
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complex was estimated through Rb-Sr (Bonhomme et al., 1982) and U-Pb datings
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(Moussavou and Edou-Minko, 2006) of syenites and pegmatite. The former gave an
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isochron age of 2143 ± 143 Ma, but a possibility of mixing (erroneous isochron) was
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not well discussed in this previous work. In addition, initial
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Sr/86Sr ratio of 0.7006
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calculated from the isochron is too low for the 2.1 Ga in view of the secular evolution in
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the mantle
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provided an upper intercept age of 2027 ± 55 Ma based on discordant zircon U-Pb data.
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Some zircon grains included a certain level of common Pb (204Pb/206Pb > 0.0002), and
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internal structures in zircon were not checked by conventional CL image.
Sr/86Sr ratio. On the other hand, Moussavou and Edou-Minko (2006)
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In the present study, we collected basement orthogneisses of Chaillu massif
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from pluri-metric blocks enclosed in the FA units, northwest of Francevillian basin
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(GPS position: S 1° 43' 14,4 ''; E 13 ° 18' 24,5 ''), and an intrusive syenite and granite
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from the N’goutou complex in order to acquire chronological data that are more precise
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(Fig. 1). Our in situ U-Pb isotopic data were obtained through multi-collector,
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inductively coupled plasma mass spectrometry (MC-ICP-MS). The age data are
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discussed here together with CL observations and an evaluation of the common Pb
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abundance.
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2. Geological background
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2.1. Geological setting and isotopic history
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Paleoproterozoic sedimentary sequences, named the Francevillian Group, are
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widespread in eastern Gabon Republic and rest unconformably on an Archean basement
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referred to as the East Gabonian block (Fig. 1a). The basement rocks are mainly
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composed of Mesoarchean granitoids and greenstone (Bonhomme et al., 1978;
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Caen-Vachette et al., 1988; Thiéblemont et al., 2014). Granitoids near Koulamoutou and
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Chaillu are dated at 2888 ± 40 Ma, 2765 ± 39 Ma, and 2637 ± 33 Ma based on Rb-Sr
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isochron (Fig. 1a; Caen-Vachette et al., 1988). The Paleoproterozoic Ogooué orogenic
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belt is located west of the Francevillian Group. The tectono-thermal overprint in the Francevillian basin tends to attenuate
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eastward, and the Francevillian and Okondja sub-basins were less affected by
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metamorphism and deformation. The Francevillian Group is subdivided into five
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formations; named FA to FE from bottom to top, respectively (Weber, 1968). The basal
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FA unit is mainly composed of clastic sedimentary rocks. The depositional environment
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is considered as fluvial in the lower part and marine in the upper part (Gauthier-Lafaye
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and Weber, 2003). The FA unit is characterized by the occurrence of uranium deposits
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(e.g., Neuilly et al., 1972; Gauthier-Lafaye, 2006), which were dated at 2050 ± 30 Ma
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through the classical U-Pb methods on uraninites (Gancarz, 1978). The date of the
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fission reactions was determined by comparing the fluence of the fission reaction to the
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amount of fission elements produced, which provided an age of 1950 ± 30 Ma
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(Ruffenach, 1978; Holliger, 1988; Naudet, 1991). Oil-bearing fluid inclusion in quartz
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grains (within C1 layer) yielded hydrocarbon, including hopanes, 2α-methylhopanes,
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terpanes, and steranes (Dutkiewicz et al., 2007); poly aromatic hydrocarbon species
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were discovered in solid bitumen around Oklo natural fission reactors (Nagy, 1993;
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Nagy et al., 1993). The FB unit consists largely of shales and black shales with
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subordinate manganese-rich carbonate rocks and coarse-grained clastic rocks.
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Macroscopic structures, interpreted as colonial organisms (Albani et al., 2010), and
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complex ductile nodules, interpreted as microbial/algal consortia (Moussavou et al.,
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2015), have been reported from the upper FB unit. In addition, hydrocarbon species,
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such as alkanes and alkyl benzene, and organic matter of cyanobacterial affinity have
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been reported from the same unit (Nagy, 1993; Nagy et al., 1993; Amard and
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Bertrand-Sarfati, 1997). Diagenetic illites in the middle of the FB unit provide a Sm-Nd
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age of 2099 ± 115 Ma (Bros et al., 1992). The FC unit includes shallow marine
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dolostones and thin-banded cherts with stromatolite structures. The FD unit consists of
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black shales with ignimbrite tuff at the top. The youngest zircon U-Pb age from the tuff
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is 2083 ± 6 Ma (Horie et al., 2005), which constrains the maximum depositional age of
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the FD unit. The FE unit comprises epiclastic sandstones and intercalating shales.
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In the northern part of the Okondja basin, the Francevillian rifting was
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accompanied by the emplacement of the N’goutou complex (Fig. 1). The N’goutou
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complex is a subvolcanic ring complex, which includes three successive intrusive units
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(Moussavou and Edou-Minko, 2006): (1) an external unit (N1) including syenites and
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microsyenites in the north and alkaline granite in the south; (2) an internal unit (N2)
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consisting of microgranites; and (3) a later internal unit (N3) made of porphyritic to
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pegmatitic syenites. These rocks exhibit an alkaline within-plate signature (Moussavou
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and Edou-Minko, 2006). Recently, Thiéblemont et al. (2014) identified N’goutou
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complex intrusions into the FA and FB units, which are overlain by the sediments of the
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FC unit (Fig. 1b).
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To constrain the protolith age of the basement rocks, we collected orthogneiss
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samples (Gneiss) from the East Gabonian block at the northwest of Franceville basin
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(Fig. 1a).
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2.2. Granitoid petrology Rock samples for age determination were collected from the N1 unit of the
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N’goutou complex (Fig. 1b; GPS positions: S0°1’59.3” E13°24’2.5” (NG12) and
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S0°1’46.5” E13°24’11” (NG18)). The potassium-rich granite (NG12) and alkaline
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syenites (NG18) are mainly composed of medium- to coarse-grained K-feldspar and
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display equigranular textures (Fig. 2). The NG18 sample consists of aegirine, together
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with local quartz and ilmenite, and accessory fluorite and siderite (Fig. 2a–f). The NG12
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sample consists of aggregated quartz crystals and illite (Fig. 2g, h) together with rutile
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and bastnäsite (Fig. 2i–l). These mineral assemblages are largely consistent with those
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reported by Moussavou and Edou-Minko (2006). Elemental maps, acquired using a
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Hitachi S-3400N scanning electron microscope (SEM; Hitachi High Tech. Corp., Japan)
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with a an X-ray energy dispersive spectroscope (EDS; Bruker Corp., USA), show that F
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is rich in some minerals such as aegirine, fluorite, and bastnäsite (Fig. 2d, j).
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3. Sample preparation and analytical methods
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3.1. Whole-rock compositions
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The granite and syenite samples were prepared for whole rock chemistry. Thin
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(1–2 cm thick) rock chips weighing about 20 g were cut from fresh parts of the samples.
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Visible veins were avoided during this process. The chips were washed with distilled
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water using an ultrasonic device and dried completely at 120 °C overnight. Rock
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powders were prepared using a tungsten-carbide mill and an agate-ball mill. Then, they
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were dried at 110 °C and 950 °C (over six hours), respectively.
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Major element compositions (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, and P) were
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determined with an X-ray fluorescence spectrometer (XRF; RIX 2100, RIGAKU) at the
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Tokyo Institute of Technology, Japan. Fused glass discs were prepared with a lithium
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tetraborate (Li2B4O7) flux in a 1:10 dilution ratio at 1050 °C. During the XRF analysis,
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the accelerating voltage was 50 kV and the current was 50 mA. Calibration methods
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using GSJ standards were based on Machida et al. (2008). Repeated analyses of the
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same standard indicated that the reproducibility of this analysis was better than 1 %,
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except for Na and P ( 0.0001; black and gray
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circles in Fig. 4a), and zircons with lower common lead abundances tend to be
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distributed close to the concordia curve. Zircons plotted on the concordia curve and
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including a low common Pb concentration (red circles in Fig. 4a) exhibit two age
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clusters around 2860 Ma and 2910 Ma. The Th/U ratios of the orthogneiss zircons range
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from 0.05 to 0.72, but most have values ranging from 0.1 to 0.5.
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4.3. Granite zircon U-Pb age All zircon grains from the granite contain significant amounts of common Pb
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(204Pb/206Pb > 0.007; Table 3). The 207Pb/206Pb ages of zircons show a wide range from
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2347 to 4593 Ma. In the Tera-Wasserburg diagram, all zircons are plotted off the
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concordia curve, but are distributed along two trends (Fig. 4b). The first trend is shaped
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by 16 zircon grains intersecting the
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concordia curve at ca. 2.1 Ga (orange line in Fig. 4b). The second trend, which is
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formed by four zircon grains, has a steep gradient and intersects the concordia curve at
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ca. 2.0 Ga (purple line in Fig. 4b). There is no systematic difference between the above
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two zircon groups under CL observation; they form apparent regression lines in the
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204
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3.08, independently of their common Pb abundance.
Pb/206Pb axis (238U/206Pb = 0) at ca. 1.1 and the
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Pb/206Pb-207Pb/206Pb diagram (Fig. 5). The zircons’ Th/U ratios vary from 0.22 to
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5. Discussion
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5.1. Protolith age of the orthogneisses
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CL observations indicate that most zircons from the orthogneiss show zoning
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from the core to rim and do not exhibit highly luminescent margins, as seen in
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metamorphic zircons (Fig. 3a). Most zircons have Th/U ratios of 0.1–0.5 (Table 3),
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which is consistent with a magmatic origin (Williams et al., 1996; Rubatto and Gebauer,
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2000), rather than a metamorphic origin, as suggested by Williams and Claesson (1987).
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Due to the pre-ablation process during the analyses, we succeeded in analyzing the
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non-metamictized parts in some zircons. This enabled us to obtain some data plotted on
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the Tera-Wasserburg concordia curve with low common Pb abundance (Fig. 4a),
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although most analyses indicate a certain level of
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concordia curve show two age populations at ca. 2860 Ma and 2910 Ma. There is no
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systematic difference between zircons in these two populations under CL observation.
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Because of the scattered distribution pattern in the concordia diagram, we cannot
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identify a discordia line.
Pb (Table 3). The data on the
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Previously, the Rb-Sr isochron method applied on the granitoids near
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Koulamoutou and Chaillu yielded the ages of 2888 ± 40 Ma, 2765 ± 39 Ma, and 2637 ±
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33 Ma (Fig. 1a; Caen-Vachette et al., 1988). The 2888 ± 40 Ma age is comparable to the
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age obtained from the U-Pb analyses in the present study, whereas the latter two ages
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(2765 ± 39 Ma and 2637 ± 33 Ma) are clearly younger than the ones obtained here. In
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view of the CL observations of the zircons together with the Rb-Sr isochron ages, the
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protolith age of the orthogneiss in East Gabonian block ranges from 2860 to 2910 Ma.
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Based on a previous chronological report (Caen-Vachette et al., 1988), a coeval age was
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reported from diorites in the Nounah-Ivindo area. Epizonal metamorphism and later
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magmatism in the Chaillu area at 2700–2600 Ma (Caen-Vachette et al., 1988) did not
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appear to influence the zircon U-Pb isotope system in the East Gabonian block.
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5.2. Intrusion age of granite (N’goutou complex) The individual
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Pb/206Pb ages are considered meaningless due to the high
abundances of common lead in all zircon grains (Table 3). In the Tera-Wasserburg
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207
Pb/206Pb axis
diagram, a regression line formed by 16 zircon grains intersects the
322
(238U/206Pb = 0) at ca. 1.1 and the concordia curve at around 2.1 Ga (orange line in Fig.
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4b). In addition, another regression line shows a gradient steeper than that of the orange
324
line and intersects the concordia curve at ca. 2.0 Ga (purple line in Fig. 4b). It is likely
325
that these distribution patterns reflect a mixing between igneous (radiogenic) and
326
contaminated (unradiogenic) end members. Based on the evolution model of the Pb
327
isotopic composition in the crust by previous studies (Stacey and Kramer, 1975;
328
Gancarz and Wasserburg, 1977; Allégre et al., 1995), the
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been ca. 1.0 before ca. 2.0 Ga. Therefore, we suggest that common-lead-contaminated
330
zircons before 2.0 Ga, or in an extreme case, the common lead was primarily involved
331
in zircon crystallization, as commonly observed in zircons from syenite (e.g., Peng et al.,
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2008; Krasnobaev et al., 2011). The experimental study by Watson et al. (1997)
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indicated that significant amounts of Pb can be incorporated into zircons grown from a
334
melt with high P abundance. Low phosphorus contents in studied granitoids (Table 2),
335
however, are inconsistent with this interpretation. In another case, tetravalent lead can
336
substitute zirconium site in zircon. The syenite used in this study includes ilmenite (Fig.
337
2a–f). Thus, the oxygen fugacity of the parent magma was not high, as lead is oxidized
338
to tetravalent. The whole-rock chemical compositions of some syenites from the
339
N’goutou complex indicate high Pb abundances up to 132 ppm (Moussavou and
340
Edou-Minko, 2006). It is generally suggested that most A-type granites formed from
341
relatively high-temperature magmas (Clemens et al., 1986; Whalen et al., 1987). The Pb
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diffusion rate in zircon increases with temperature (e.g., Cherniak and Watson, 2003).
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Pb/206Pb ratio in crust had
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Therefore, high common-lead abundances in zircons are possibly attributed to Pb
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diffusion at a high temperature of the parent magma. The reason for the erroneous value of the purple line on the 207Pb/206Pb axis (Fig.
346
4b) can be attributed to the differences in the mechanical strength and crystal structure
347
of zircons, which could have led to an error in the correction of U/Pb ratios during the
348
LA-ICP-MS analysis. The analyses plotted on the purple line may have been largely
349
influenced by metamictization; however, no significant difference between the analyses
350
of the purple and orange groups under CL observation can be seen (Fig. 3b). The U/Pb
351
ratios shown on the orange line were also potentially affected by metamictization in
352
varying degrees, as shown by the low accuracy of the fitting line (MSWD = 5.6; Fig.
353
4b). Therefore, we use lead isotopes to facilitate discussions about obtaining a more
354
accurate igneous age.
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In order to estimate the apparent radiogenic 207Pb/206Pb value, we plotted the data
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on a crossplot of 204Pb/206Pb vs. 207Pb/206Pb (Fig. 5). In analogy with Fig. 4b, two trends
357
are clearly recognizable, both showing high linearity (r2 > 0.9997). In these mixing lines,
358
the values on the 207Pb/206Pb axis represent radiogenic Pb isotopic ratios, which is least
359
affected by the unradiogenic component. One regression line intersects the
360
axis at 0.13707 ± 0.0010 (orange line) and another at 0.13606 ± 0.0011 (purple line).
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The former corresponds to the 207Pb/206Pb age of 2191 ± 13 Ma (MSWD = 1.15) and the
362
latter is equivalent to the
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the ISOPLOT program of Ludwig (2003). Apart from a few data, Th/U ratios in most
364
zircons are within the range of representative igneous zircons (Table 3; Williams et al.,
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Pb/206Pb
Pb/206Pb age of 2178 ± 14 Ma (MSWD = 1.12), based on
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1996; Rubatto and Gebauer, 2000). We prefer the age of 2191 ± 13 Ma (orange line) as
366
the granite igneous age because zircon data yielding 2178 ± 14 Ma contain an erroneous
367
value at the 207Pb/206Pb axis in the Tera-Wasserburg diagram (purple line in Fig. 4b).
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The igneous age obtained in this study is greater than that estimated from the
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upper intercept in the U-Pb concordia diagram (2027 ± 55 Ma; Moussavou and
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Edou-Minko, 2006), but falls within that estimated from the Rb-Sr isochron (2143± 143
371
Ma; Bonhomme et al., 1982) due to its large error. In addition to the volume of
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analytical data, because common lead was not evaluated by Moussavou and
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Edou-Minko (2006), 2191 ± 13 Ma is considered a more accurate igneous age for the
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granite.
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5.3. Implications
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The N’goutou complex, including the syenite and granite, intrudes into the FA
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and FB units in the Francevillian Group (Fig. 1b). Therefore, the ages of the two units
379
could be greater than 2191 ± 13 Ma. This improves the correlations of the Francevillian
380
Group with sedimentary sequences in other areas (Fig. 6). The largest manganese ore
381
(Hotazel Formation) in Earth’s history was deposited just after the last Paleoproterozoic
382
Snowball Earth event (after ca. 2.22 Ga; Cornell et al., 1996; Bekker et al., 2010;
383
Bekker and Holland, 2012), and is possibly contemporary with the manganese-rich
384
carbonate rocks in the FB unit. Above the manganese deposits, the FB unit consists of
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macroscopic structures interpreted as colonial organisms and microbial/algal consortia
386
(Albani et al., 2010; Moussavou et al., 2015). The appearance of these organisms was
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older than previously proposed and likely dates back to the aftermath of the last
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Paleoproterozoic Snowball Earth event. These large-sized organisms might have
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evolved in concert with the oxidation of the atmosphere-ocean system after the
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Snowball Earth event. The deposition of black shales in the FB-FC units was previously
391
considered concurrent with a worldwide deposition of organic carbon-rich black shales
392
(Shunga event; Melezhik et al., 1999) between 2.05 and 1.9 Ga (Gauthier-Lafaye, 2006).
393
However, the newly obtained igneous age for the N’goutou complex does not support
394
this idea, but indicates that the deposition of FB unit likely preceded the Shunga event.
395
The organic carbon-rich black shales of the FD unit are more suitable as sediments
396
during the Shunga event than the FB unit. This correlation is further supported by the
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chemostratigraphic study wherein a negative carbon isotope excursion in the FD unit
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was associated with so-called shungite deposits in Fennoscandia (Canfield et al., 2013).
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6. Conclusions (1) The protolith ages of orthogneiss in the East Gabonian block range from 2860 Ma to
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2910 Ma. Epizonal metamorphism and later magmatism at 2700–2600 Ma did not
404
influence the U-Pb isotopic composition of zircons in the orthogneiss.
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(2) The lead isotopic variation in zircons from the granite can be explained by the
406
mixing of two components, with an igneous age of 2191 ± 13 Ma. This age constrains
407
the minimum depositional age of the FA and FB units in the Francevillian Group.
408
(3) The deposition of manganese-rich carbonate rocks, organic carbon-rich black shales,
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and the emergence of the macroscopic organisms in the FB unit in Gabon Republic
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occurred just after the last Paleoproterozoic Snowball Earth event, before ca. 2.2 Ga.
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Acknowledgements This work was partly supported by a grant for “Chronostratigraphy for the
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Mesoproterozoic strata in Jixian, North China (No. 26800259)” and “Hadean
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BioScience (No. 26106002)” from the Ministry of Education, Culture, Sports, Science
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and Technology, Japan. We thank the reviewer for the comments and appreciate Prof.
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M Santosh for the editorial handling.
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Figure captions
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Figure 1. (a) Geological map of SE Gabon, including the distribution of the
602
Francevillian Group (modified after Thiéblemont et al., 2014). (b) Simplified
603
geological map around the N’goutou complex, showing the locations of granite
604
and syenite (NG12 and NG18).
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Figure 2. Microphotographs in plane and cross-polarized light and elemental maps
607
obtained with SEM-EDS for syenite (a–f; NG18) and potassium-rich granite (g–l;
608
NG12). Qz: quartz. Sd: siderite. Kfs: potassium feldspar. Ilm: ilumenite. Aeg:
609
aegirine. Fl: fluorite. Rt: rutile. Ilt: illite.
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Figure 3. CL images of representative zircons from (a) an orthogneiss (Gneiss) and (b) a
612
granite (NG12). Red circles mark analytical spots. 238U/206Pb and 207Pb/206Pb ages
613
are expressed as yellow and blue characters, respectively. (c) CL image together
614
with elemental maps of a locally metamictized zircon.
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Figure 4. Tera-Wasserburg concordia diagrams of in situ U-Pb ages of zircons with
617
LA-ICP-MS for (a) orthogneiss (Gneiss) and (b) a granite (NG12). These figures
618
were constructed with an ISOPLOT (Ludwig, 2003). The error ellipses on the
619
individual spots are at the 2σ level. Red circles represent zircons with few 204Pbs.
620
Zircons with
621
gray circles, respectively.
204
Pb/206Pb ratios over 0.0001 and 0.0005 are shown as black and
28
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Figure 5. Isotopic crossplot showing
204
Pb/206Pb vs.
207
Pb/206Pb for zircons from a
granite (NG12). Colors of analytical data are based on the distribution pattern in
625
Fig. 4b. Gradients and y-intercepts (207Pb/206Pb-axis) of fitted lines are also shown
626
in black (for all 20 grains), orange (for 16 grains), and purple (for 4 grains).
627
Regression lines for each group are illustrated using the ISOPLOT program of
628
Ludwig (2003).
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Figure 6. Redox indicators of the ancient atmosphere-ocean system (modified from Rye
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and Holland, 1998; Melezhik et al., 1999; Bekker et al., 2006, 2010;
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Pierrehumbert et al., 2011; Bekker and Holland, 2012) and secular carbon-isotope
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variations in the early Paleoproterozoic seawater (modified from Bekker and
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Kaufman, 2007).
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Table 1. Instrumentation and operational settings Laser ablation system Instrument
NWR193 excimer laser (ESI, Portland, USA)
Cell type
Two volume cell
Laser wave length
193 nm
Pulse duration
