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Volume 23 Issue 5
Oct.  2012
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Yang Cen, Yang Cen, Timothy M Kusky, Xingfu Jiang, Lu Wang. Granulite Facies Metamorphic Age and Tectonic Implications of BIFs from the Kongling Group in the Northern Huangling Anticline. Journal of Earth Science, 2012, 23(5): 648-658. doi: 10.1007/s12583-012-0286-x
Citation: Yang Cen, Yang Cen, Timothy M Kusky, Xingfu Jiang, Lu Wang. Granulite Facies Metamorphic Age and Tectonic Implications of BIFs from the Kongling Group in the Northern Huangling Anticline. Journal of Earth Science, 2012, 23(5): 648-658. doi: 10.1007/s12583-012-0286-x

Granulite Facies Metamorphic Age and Tectonic Implications of BIFs from the Kongling Group in the Northern Huangling Anticline

doi: 10.1007/s12583-012-0286-x
Funds:

the Postdoctoral Science Foundation 20100471203

the Ministry of Land and Resources 1212010670104

the National Natural Science Foundation of China 91014002

the National Natural Science Foundation of China 40821061

the National Natural Science Foundation of China 41272242

Ministry of Education of China B07039

Ministry of Education of China TGRC201024

More Information
  • Corresponding author: Yang Cen, psb200301@yahoo.com.cn
  • Received Date: 2011-11-11
  • Accepted Date: 2012-01-18
  • Publish Date: 2012-10-01
  • We report preliminary results of a geochemical study on banded iron formations (BIFs) in the Zhaojiayangpo (赵家阳坡) area from the Kongling (崆岭) Group in the northern Huangling (黄陵) anticline, on the northern margin of the Yangtze craton. The CL (cathodoluminescence) images of zircons mostly have sector zoning, fir-tree zoning and patched zoning, and a few show core-rim textures with rims having patched zoning. The calculated formation temperatures using the Ti-in-zircon thermometer are 660–808 ℃ (714 ℃ in average), all indicating that the BIFs underwent granulite facies metamorphism. The age of zircons with granulite facies metamorphism is 1 990±14 Ma by LA-ICP-MS U-Pb dating, indicating that there was a significant granulite facies tectonothermal event in the northern Huangling anticline in the Paleoproterozoic, which may be related with tectonic thermal events of the metamorphism caused by the assembly of the Columbia supercontinent with South China. Moreover, the REE pattern is characterized by depletion in LREE while relatively flat in HREE, LaN/YbN=0.26, with a positive Eu anomaly (Eu/Eu*=1.59), which reveals its hydrothermal sedimentation origin and it may have formed in the environment of submarine exhalation.
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Granulite Facies Metamorphic Age and Tectonic Implications of BIFs from the Kongling Group in the Northern Huangling Anticline

doi: 10.1007/s12583-012-0286-x
Funds:

the Postdoctoral Science Foundation 20100471203

the Ministry of Land and Resources 1212010670104

the National Natural Science Foundation of China 91014002

the National Natural Science Foundation of China 40821061

the National Natural Science Foundation of China 41272242

Ministry of Education of China B07039

Ministry of Education of China TGRC201024

Abstract: We report preliminary results of a geochemical study on banded iron formations (BIFs) in the Zhaojiayangpo (赵家阳坡) area from the Kongling (崆岭) Group in the northern Huangling (黄陵) anticline, on the northern margin of the Yangtze craton. The CL (cathodoluminescence) images of zircons mostly have sector zoning, fir-tree zoning and patched zoning, and a few show core-rim textures with rims having patched zoning. The calculated formation temperatures using the Ti-in-zircon thermometer are 660–808 ℃ (714 ℃ in average), all indicating that the BIFs underwent granulite facies metamorphism. The age of zircons with granulite facies metamorphism is 1 990±14 Ma by LA-ICP-MS U-Pb dating, indicating that there was a significant granulite facies tectonothermal event in the northern Huangling anticline in the Paleoproterozoic, which may be related with tectonic thermal events of the metamorphism caused by the assembly of the Columbia supercontinent with South China. Moreover, the REE pattern is characterized by depletion in LREE while relatively flat in HREE, LaN/YbN=0.26, with a positive Eu anomaly (Eu/Eu*=1.59), which reveals its hydrothermal sedimentation origin and it may have formed in the environment of submarine exhalation.

Yang Cen, Yang Cen, Timothy M Kusky, Xingfu Jiang, Lu Wang. Granulite Facies Metamorphic Age and Tectonic Implications of BIFs from the Kongling Group in the Northern Huangling Anticline. Journal of Earth Science, 2012, 23(5): 648-658. doi: 10.1007/s12583-012-0286-x
Citation: Yang Cen, Yang Cen, Timothy M Kusky, Xingfu Jiang, Lu Wang. Granulite Facies Metamorphic Age and Tectonic Implications of BIFs from the Kongling Group in the Northern Huangling Anticline. Journal of Earth Science, 2012, 23(5): 648-658. doi: 10.1007/s12583-012-0286-x
  • The Huangling anticline, which exposes the oldest crystalline basement of the Yangtze craton, is located along the north margin of the Yangtze craton, and is an important area to study the evolution of Archean–Proterozoic crust. Recent research shows that there is significant information about the tectonic-magmatic activity caused by assembly and breakup of the Columbia supercontinent (Peng et al., 2009; Wu et al., 2009; Xiong et al., 2008; Zhang, 2008) except for the Archean tonalitic-trondhjemitic-granodioritic (TTG) gneisses which contain the information of the evolution of early crust. This article focuses on the preliminary studies of petrology, geochemistry and geochronology of banded iron formations (BIFs), showing that the BIFs experienced a Paleoproterozoic granulite facies tectonic-metamorphic-magmatic event which may be related with assembly of the Paleoproterozoic Columbia supercontinent with South China.

  • The Yangtze craton is one of the major Precambrian blocks in China. The Kongling terrain in the Huangling anticline is located along the northern part of the Yangtze craton and is one of the major units where the evolution of Prenanhua period basement can be studied (Qiu et al., 2000; Gao et al., 1999). The Kongling Group in the northern Huangling anticline is mainly exposed at Xingshan in Yichang (Fig. 1), where it consists of dioritic and TTG gneisses (Gao et al., 1999) and is intruded by Paleoproterozoic granitic gneisses as well as K-feldspar granites (Xiong et al., 2008; Yuan et al., 1991). Metasedimentary rocks are exposed as a supracrustal rock series and are domi-nated by metapelites which are characterized by the presence of graphite and Al-rich minerals (sillimanite, garnet). They mainly consist of graphite bearing banded biotite granulite, gneisses, garnet sillimanite quartzite, garnet sillimanite biotite gneiss, graphite schist, marble, quartzite and magnetite quartzite, which are regarded as khondalites. Amphibolite and locally preserved mafic granulite, commonly are preserved as lenses, boudins and layers in the gneisses (Gao et al., 2001; Jiang, 1986).

    Figure 1.  (a) Geological map of Huangling anticline (Peng et al., 2012); (b) sketch geological map of northern Huangling anticline (modified from 1:500 000 geological map of Hubei Province).

    Since the 1990s, there are many published geochronological data to define the timing of Palaeopro-terozoic high-grade metamorphism in the Kongling Group by methods of U-Pb dating of single-grain zircon, K-Ar dating of hornblende and whole rock Rb-Sr isochron dating from biotite granulite, metabasite and mica quartz schist, but the reported ages are variable and imprecise, ranging from 1.8–2.33 Ga, which are mostly explained as the formation ages of metamorphic rocks (Ma et al., 1997; Li and Nie, 1987; Jiang, 1986), but Ma et al. (1997) thought that it might be the age of amphibolite facies metamorphism. Recently, Ling et al. (2000) and Qiu et al. (2000) published metamorphic ages from 1.9 to 2.0 Ga from metapelites and trondhjemites using zircon SHRIMP U-Pb dating. Zhang (2008) did LA-ICP-MS U-Pb dating for zircons from metapelites and garnet-bearing amphibolites, obtained metamorphic ages of 1 943±44 to 1 979±22 Ma. Wu et al. (2009), using LA-ICP-MS U-Pb dating for zircons, published modified metamorphic ages of 2 009±9 to 2 015±9 Ma for the same rock types as Zhang (2008).

    We discovered banded iron formations (BIFs) in the field which were never recognized before, determining to ascertain the ages of Palaeoproterozoic high-grade metamorphism in the Kongling Group.

  • There are two major stratigraphic units in the Yinjiaping, Tandanghe and Jiaozhanya areas in the northeast part of the Huangling anticline. One includes the metasedimentary rocks that principally consist of Al-rich schist and gneisses, the other is trondhjemitic-granitic based granitic gneiss. Banded iron formations (BIFs) are exposed sporadically within the metasedimentary rocks in the areas of Tandanghe and Zhaojiayangpo, and also in the Yinjiaping and Houshansi areas (Bureau of Geology and Mineral Resources of Hubei Province, 1994, 1987). The sample (F5N) studied in this article comes from an exposure of supracrustal rock series in the area of Zhaojiayangpo.

    The metasedimentary rocks in the Zhaojiayangpo area in Yichang belong to the Huanglianghe Formation, which is part of the Paleoproterozoic Shuiyuesi Group (khondalite series). It is a magnetite-bearing volcanic-sedimentary formation containing schist, gneiss, and metabasite. The lithology is mainly composed of layered metabasite, graphite bearing garnet biotite plagioclase gneiss, magnetite garnet metabasite, locally sillimanitebearing emery rock, garnet silli-manite quartzite, magnetite amphibolite quartzite type iron ore deposits and magnetite amphibolite garnet type iron ore. The granitic gneiss in the Paleoproterozoic Zhoujiahe Formation of the Shuiyuesi Group is renamed as the Paleoproterozoic Bashansi gneiss which intrudes into the Huanglianghe Formation. It mainly consists of trondhjemitic-granitic metamorphic plutonic rocks characterized by barren amphibolite and magnetite but is commonly garnet bearing, as distinguished from typical Archean TTG gray gneisses.

  • The banded iron formation is grayish black, and appears maroon after weathering with uniform fine laminations and microbanding ranging from less than 1 to 5 mm. The dark iron-rich bands mainly consist of magnetite and pyrite while light iron-poor bands contain quartz, feldspar and garnet. The boundary line between the different layers is very distinct (Fig. 2a). In addition, phenomenon of associated sulfide mineralization on bedding surfaces of BIFs is typically present. It has typical pegmatitic texture mainly consist-ing of feldspar, quartz, large purple garnet as well as a small number of magnetite and sulfide such as pyrite and chalcopyrite, ordinarily with vesicular structure after weathering (Fig. 2b).

    Figure 2.  (a) Macro-characteristics of BIFs; (b) sulfide mineralization associated with BIFs.

  • We report major and trace element analyses from a fresh sample of BIFs in the Yuanjiadabao-Zhaojiayangpo cross-section from the Kongling Group. Analyses of major elements were done in the comprehensive rock and mineral test center at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan.

    The results of geochemical data analyses of the BIFs are reported in Table 1. Major elements are basically the same as BIFs from the other areas of the world: total Fe (20 wt.%–40 wt.%), SiO2 (43 wt.%–56 wt.%), CaO (1.75 wt.%–9.0 wt.%), MgO (1.20 wt.%–6.7 wt.%), etc. (Klein, 2005). They have low contents of trace elements. The post-Archean-Australian-shale (PAAS)-normalized (Taylor and McLennan, 1985) REE pattern is characterized by: (i) depletion in LREE while relatively flat in HREE, LaN/YbN=0.26; (ii) positive Eu anomalies, Eu/Eu*= 1.59, which reveals its hydrothermal sedimentation origin; (iii) positive Y anomaly, Y/Y*=1.11, which is the characteristic of the ocean itself (Alibo and Nozaki, 1999; Bau et al., 1995); (iv) slightly negative Ce anomaly, Ce/Ce*=0.99.

    Table 1.  Major and trace elements of BIFs

    Compared with the mean value of Archean crust (Taylor and McLennan, 1985), the contents of high field strength elements (Zr, Th, U, Hf, Pb), lithophile elements (Ba, Sr, Rb) and rare earth elements are obviously lower in the BIFs while the contents of transition metal elements (Sc, V, Cr, Co, Ni, Cu) are higher, which may be due to the mineralization of sulfide in BIFs.

  • Zircon U-Pb dating of BIFs was carried out at the LA-ICP-MS Lab in the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. A spot size of 32 μm was applied to all analyses. Helium was used as carrier gas to enhance transport efficiency of ablated material. Standard zircon 91500 was used as the outside calibration. Common Pb has been corrected after Andersen (2002). ISOPLOT software was used to calculate weighted zircon ages and to construct concordia plots.

  • The majority of zircons in our samples are pale yellow and transparent-semitransparent. Most of the grains are granular and elongated while a small part of the others are equigranular. The length of zircon crystals ranges from 40 to 100 μm and the length-width ratio ranges from 1 : 1 to 5 : 1. The CL images show that most zircons are characterized by sector zoning, fir-tree zoning and patched zoning which is attributed to granulite facies metamorphism (Fig. 3) (Wu and Zheng, 2004; Corfu et al., 2003). It suggests that the BIFs probably experienced granulite facies metamorphism.

    Figure 3.  Typical CL images of zircons with granulite facies metamorphism from BIFs.

    The contents of Ti analyzed from zircons range from 0.48 ppm to 36 ppm except for two other data which one have no Ti and the other up to 160.79 ppm probably attributed to either test error or inclusions of minerals (Table 2). The formation temperatures of zircons were calculated by the Ti in zircon thermometer proposed by Ferry and Watson (2007). Due to the existence of quartz in the sample, the activity of SiO2 can be considered as 1, whereas the activity TiO2 can be set to 0.5 on account of the absence of rutile (Watson et al., 2006). As a result, the calculated metamor-phic temperatures of zircons are between 565 and 1 225 ℃, whose mean value may diverge from the exact peak value with such a relatively wide range of variation of the contents of Ti in zircons. In order to warrant the reliability of data, we report the interquartile range which has large variable temperature range, the calculated metamorphic temperatures are between 660 and 808 ℃ and the mean value is 714 ℃, suggesting that the BIFs experienced granulite facies metamorphism.

    Table 2.  Data of LA-ICP-MS zircon U-Pb dating and Ti-in-zircon thermometer

  • Eighteen U-Pb analyses by LA-ICP-MS are presented in Table 2. The data exhibit low Th (7.91 ppm–69.7 ppm), moderate U (496 ppm–946 ppm), low Th/U (0.01–0.10), which are coincident with the features of typical metamorphic zircon (Wu and Zheng, 2004; Rubatto, 2002). The concordia age for the granulite facies metamorphism zircons in BIFs is 1 988±16 Ma, MSWD=1.2, whose mean age is 1 990±14 Ma, MSWD=1.6 (Fig. 4).

    Figure 4.  Concordia diagram of LA-ICP-MS zircon U-Pb dating of BIFs.

  • BIFs are chemical sedimentary rocks that formed most commonly in Precambrian times. In geochemistry, they are characterized by high contents of Fe (e.g., Bekker et al., 2010; Pecoits et al., 2009; Frei et al., 2008; Klein, 2005; Kato et al., 1998; Trendall, 1983). Klein (2005) summarized that all major compositions of typical BIFs formed before 1.8 Ga have a strong resemblance, that the contents of total Fe (20 wt.%–40 wt.%) and SiO2 (43 wt.%–56 wt.%) are high, whereas the content of Al2O3 (0.09 wt.%–1.8 wt.%) is very low, representing pure chemical sediments. However, in the BIFs found in the northern Huangling anticline, which mainly consist of Fe2O3 and SiO2, the content of Al2O3 (9.83 wt.%) is higher, suggesting that it has a component of detritus probably related to submarine volcanic deposition.

    In the research of trace elements (Li et al., 1986), Ti/V usually can be used to distinguish the provenance of deposit-forming materials and genesis of banded iron deposits. Its mean value ranges from 1.33 to 10.9 in ferruginous shales whereas this ratio ranges from 13 to 85 in volcanic rocks (Shen et al., 2011). The source rock for the detrital component in the BIFs of our study probably was volcanic sediment on account of higher Ti/V which is 34.85 in the sample F5N. Another important proxy is Ni/Co which is lower in volcanic sediments than in terrestrial iron deposits. In line with the characteristic of trace elements in volcanic sedimentary iron deposits, Ni/Co of sample F5N is 1.67, similar to marine chemical sediments.

    Rare earth elements (REEs) are helpful to understand the origin and deposition of iron formations, and other iron oxide-rich sedimentary rocks (e.g., Bau and Dulski, 1996; Derry and Jacobsen, 1990; Klein and Beukes, 1989; Fryer, 1977). Iron formations are likely to preserve their primary REE pattern during burial and exhumation. However, our sample from the northern Huangling anticline experienced high grade metamorphism, resulting in uncertainty about the primary depositional redox states for BIFs (Bau, 1993), but we can still gain some information from the PAAS-normalized REE pattern is characterized by a positive Eu anomaly, which means that the hydrothermal fluids strongly influence the dissolved REE load of seawater (Derry and Jacobsen, 1990). Slack et al. (2007) suggested that a large positive Eu anomaly indicates that the iron present in the protolith is hydrothermally derived. Positive Y anomalies are characteristic of the ocean itself (Alibo and Nozaki, 1999; Bau et al., 1995). It can be inferred from the Eu and Y anomalies that these BIFs formed in an environment of a mixture of hydrothermal and seawater with a con-tribution from volcanic sediments and, the Fe-Si hydrothermal fluids came from the eruption and/or exhalation of volcanoes. The slightly negative Ce anomaly may not be a true negative anomaly, because of the large variation in analytical precision (Bau and Möller, 1993). The apparent negative Ce anomaly may be the result of a positive La anomaly as obtained for REE by ICP-MS (Yamaguchi and Ohmoto, 2000). In addition, Bekker et al. (2010) claimed that Ce anomalies in iron formations can arise from analytical artifacts and diagenetic alteration.

    Precambrian iron formations can be divided into two types based on their depositional setting: one is Algoma-type iron formations, which are interlayered with or stratigraphically linked to submarineemplaced volcanic rocks in greenstone belts and, in some cases, with volcanogenic massive sulfide (VMS) deposits. The other is superior-type iron formations that are developed in passive-margin sedimentary rock successions and generally lack direct relationships with volcanic rocks (Bekker et al., 2010). According to the discussion above, the BIFs from the northern Huangling anticline are classified as volcanic sedimentary iron deposits and their formation time is estimated as Neoarchean-Paleoproterozoic because of their syndeposition with Neoarchean-Paleoproterozoic metasedimentary rocks and intruded by Paleoproterozoic Bashansi gneissic granites.

  • The Bureau of Geology and Mineral Resources of Hubei Province (1987) reported Neoarchean– Paleoproterozoic sillimanite-bearing emery rock, and garnet sillimanite quartzite, and speculated that these areas underwent high-amphibolite facies to lowgranulite facies metamorphism when they prospected the supracrustal rocks in the northern Huangling anticline. Li and Nie (1987) and Jiang (1986) respectively reported zircon U-Pb metamorphic ages of 2.33 and 2.17 Ga for banded biotite leptynites. Ma et al. (1997) concluded there was an amphibolite facies metamorphic event at 1.8 Ga using K-Ar and Rb-Sr dating for migmatites and amphibolites in Kongling Group from the Miaowan Formation, southern Huangling anticline. Ling et al. (2000) and Qiu et al. (2000) obtained 1.9–2.0 Ga metamorphic ages for metapelites and trondhjemites by zircon SHRIMP U-Pb dating. Recently, Wu et al. (2009) and Zhang (2008) also published similar ages for metapelites and amphibolites using LA-ICP-MS zircon dating, furthermore, they presumed they were influenced by a metamorphic event in these areas.

    The zircons in sample F5N show low Th (7.91 ppm–69.7 ppm) and moderate U (496 ppm–946 ppm) with low Th/U (0.01–0.10) ratios. The CL images of zircons mostly have sector zoning, fir-tree zoning and patched zoning, in accord with the characteristics of granulite facies metamorphic zircons. Besides, the calculated formation temperatures using the Ti-in-zircon thermometer is 714 ℃ in average and the average age of U-Pb dating is 1 990±14 Ma, all indicating that the BIFs underwent a granulite facies metamorphic event.

    It is widely accepted that the Columbia supercontinent assembled at 2.0–1.8 Ga during global collisional events (Zhao et al., 2011; Rogers and Santosh, 2009, 2003, 2002). It is unclear of the location of the Yangtze craton in the Columbia supercontinent because of the lack of data. However, recent research has demonstrated that there are significant data about the tectonic-magmatic activity caused by assembly and breakup of Columbia supercontinent (Peng et al., 2009; Wu et al., 2009; Xiong et al., 2008; Zhang, 2008). Zhang et al. (2006) suggested that the Paleoprotero zoic high-grade metamorphism and the following magmatism in Kongling Group is caused by an arc-continent collision. Moreover, there are similar ages in other parts of the Yangtze craton, which are mostly high-grade metamorphic zircons or metamorphic overgrowth, or xenocrystic zircons in volcanic or metamorphic rocks (Zhang, 2008). All the studies provide that there was a Paleoproterozoic tectonic thermal event in Yangtze craton, as supported by the research in this paper. We conjecture it is related with the assembly of Columbia supercontinent, and it provides the basis for the joining of South China and the Columbia supercontinent.

  • Based on preliminary studies of petrology, geo-chemistry and geochronology of banded iron forma-tions (BIFs) from the northern Huangling anticline, we draw some important conclusions below.

    1. The features of geochemistry are consistent with a hydrothermal sedimentation origin for the BIFs, and they may have formed in an environment of sub-marine exhalation, with a minor detrital component.

    2. The zircons from the BIFs mostly have sector zoning, fir-tree zoning and patched zoning, which are the typical features of zircons with granulite facies metamorphism. The calculated temperatures of granu-lite facies metamorphism using the Ti-in-zircon ther-mometer are 660–808 ℃ (714 ℃ in average).

    3. The age of metamorphic rims of the zircons with granulite facies metamorphism from BIFs is 1 990±14 Ma by LA-ICP-MS U-Pb dating, indicating that there was significant granulite facies metamorphism in this area, which may be related with tectonothermal events of the metamorphism caused by the assembly of Columbia supercontinent of South China.

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