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Volume 23 Issue 5
Oct.  2012
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Yunxu Wei, Songbai Peng, Xingfu Jiang, Zhongqin Peng, Lianhong Peng, Zhihong Li, Peng Zhou, Xiongwei Zeng. SHRIMP Zircon U-Pb Ages and Geochemical Characteristics of the Neoproterozoic Granitoids in the Huangling Anticline and Its Tectonic Setting. Journal of Earth Science, 2012, 23(5): 659-676. doi: 10.1007/s12583-012-0284-z
Citation: Yunxu Wei, Songbai Peng, Xingfu Jiang, Zhongqin Peng, Lianhong Peng, Zhihong Li, Peng Zhou, Xiongwei Zeng. SHRIMP Zircon U-Pb Ages and Geochemical Characteristics of the Neoproterozoic Granitoids in the Huangling Anticline and Its Tectonic Setting. Journal of Earth Science, 2012, 23(5): 659-676. doi: 10.1007/s12583-012-0284-z

SHRIMP Zircon U-Pb Ages and Geochemical Characteristics of the Neoproterozoic Granitoids in the Huangling Anticline and Its Tectonic Setting

doi: 10.1007/s12583-012-0284-z
Funds:

the China Geological Survey Project 1212010710715

the China Geological Survey Project 1212011085340

More Information
  • Corresponding author: Yunxu Wei, wyx1856@163.com
  • Received Date: 2011-08-17
  • Accepted Date: 2012-01-18
  • Publish Date: 2012-10-01
  • SHRIMP zircon U-Pb dating of the Neoproterozoic Maoping (茅坪) series (Sandouping (三斗坪) rock suite) granites exposed in the southern part of the Huangling (黄陵) anticline shows that the formation time of Sandouping biotite-hornblende tonalite intrusion, Jinpansi (金盘寺) hornblende-biotite tonalite intrusion, and Longtanping (龙潭坪) monzogranite are 863±9, 842±10, and 844±10 Ma, respectively. Their geochemical features include A/CNK=0.98–1.06, from metaluminous to weakly peraluminous, δ=1.37–1.53, Sm/Nd=0.17–0.24, and RbN/YbN=1.1–3.62. These indicate that the granite rocks are supersaturated SiO2 calc-alkaline granitoids. The characteristic of Sr-Nd isotopic composition is that the values of ɛNd(t) and εSr(t) are −12.4 to −11.0 and 20.2–32.2, respectively. It also suggests that the material source of the granite rocks mainly originated from the crust, and they formed in a volcanic arc tectonic environment. These facts suggest that the occurrence of Neoproterozoic granitoids in the southern part of the Huangling anticline should be related to an arc environment along an active continental margin caused by southward subduction of oceanic crust beneath the northern Yangtze craton, and the formation age is not later than 863 Ma.
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SHRIMP Zircon U-Pb Ages and Geochemical Characteristics of the Neoproterozoic Granitoids in the Huangling Anticline and Its Tectonic Setting

doi: 10.1007/s12583-012-0284-z
Funds:

the China Geological Survey Project 1212010710715

the China Geological Survey Project 1212011085340

Abstract: SHRIMP zircon U-Pb dating of the Neoproterozoic Maoping (茅坪) series (Sandouping (三斗坪) rock suite) granites exposed in the southern part of the Huangling (黄陵) anticline shows that the formation time of Sandouping biotite-hornblende tonalite intrusion, Jinpansi (金盘寺) hornblende-biotite tonalite intrusion, and Longtanping (龙潭坪) monzogranite are 863±9, 842±10, and 844±10 Ma, respectively. Their geochemical features include A/CNK=0.98–1.06, from metaluminous to weakly peraluminous, δ=1.37–1.53, Sm/Nd=0.17–0.24, and RbN/YbN=1.1–3.62. These indicate that the granite rocks are supersaturated SiO2 calc-alkaline granitoids. The characteristic of Sr-Nd isotopic composition is that the values of ɛNd(t) and εSr(t) are −12.4 to −11.0 and 20.2–32.2, respectively. It also suggests that the material source of the granite rocks mainly originated from the crust, and they formed in a volcanic arc tectonic environment. These facts suggest that the occurrence of Neoproterozoic granitoids in the southern part of the Huangling anticline should be related to an arc environment along an active continental margin caused by southward subduction of oceanic crust beneath the northern Yangtze craton, and the formation age is not later than 863 Ma.

Yunxu Wei, Songbai Peng, Xingfu Jiang, Zhongqin Peng, Lianhong Peng, Zhihong Li, Peng Zhou, Xiongwei Zeng. SHRIMP Zircon U-Pb Ages and Geochemical Characteristics of the Neoproterozoic Granitoids in the Huangling Anticline and Its Tectonic Setting. Journal of Earth Science, 2012, 23(5): 659-676. doi: 10.1007/s12583-012-0284-z
Citation: Yunxu Wei, Songbai Peng, Xingfu Jiang, Zhongqin Peng, Lianhong Peng, Zhihong Li, Peng Zhou, Xiongwei Zeng. SHRIMP Zircon U-Pb Ages and Geochemical Characteristics of the Neoproterozoic Granitoids in the Huangling Anticline and Its Tectonic Setting. Journal of Earth Science, 2012, 23(5): 659-676. doi: 10.1007/s12583-012-0284-z
  • The Neoproterozoic granite batholith in the Huangling anticline (namely, Huangling granitoid complex) is located on the northern margin of Yangtze craton and is the typical representative granite of the Jinning Period in South China. From the 1980s, a series of 1:50 000 regional geological mappings and research on petrology, geochemistry, and petrogenesis have been carried out in the Huangling anticline (1:50 000 geological mappings of eastern Xintan, western Liantuo; and eastern Guohekou, western Sandouping, 1991, unpublished). Based on the regional geological survey, the Huangling granite batholith is divided into four suites (Ma et al., 2002), including the Sandouping, the Huanglingmiao, the Dalaoling, and the Xiaofeng rock suites (super-units), and can be subdivided into 14 units in more detail. Among them, the Sandouping rock suite includes 6 units. Although many researchers have applied different dating methods to determine the formation age of the Neoproterozoic granitoids in the southern Huangling anticline, they also got a series of geochronologic data that vary from 844 to 765 Ma (Gao and Zhang, 2009; Zhang et al., 2009, 2008; Li Y L et al., 2007; Ling et al., 2006; Li Z X et al., 2003; Li Z C et al., 2002; Feng et al., 1991); however, there are apparent contradiction for ages of different rock-masses between isotopic dating data and invasion sequence of geological contact relationships. According to previous research, two completely different views exist for the genesis and tectonic setting of Huangling anticline: one view is that it is formed in a magmatic arc environment and related to southward subduction processes of the Late Jinning Period "Proterozoic Qinling Ocean" on the northern margin of the Yangtze craton (Ma et al., 2002; Gao et al., 1990); the other holds that the granites formation age is related to the Neoproterozoic (~825 Ma) mantle plume activity in South China, which eventually leads to the Rodinia supercontinent breakup (Zhang et al., 2009; Li X H et al., 2003, 2001; Li Z X et al., 2002, 1999; Li Z X, 1998).

    On basis of previous research and the 1:50 000 regional geological mappings of Liantuo-Sandouping, this article focuses on the petrology, geochemistry, and geochronology of the Neoproterozoic granite body in southern Huangling anticline and discusses its genesis and tectonic setting.

  • The Neoproterozoic Huangling granite batholith in the southern Huangling anticline is a composite plutonic complex with characteristics of multistage invasion. In the northern and western parts of the granitoids, it intrudes into the Xiaoyicun and Miaowan formations of the Kongling Group, respectively, whereas it is overlain with an angular unconformity by the Liantuo Formation and Nantuo Formation of Nanhua System and the Doushantuo Formation of Sinian System in the south and east, respectively (Fig. 1). The types of the intrusive rocks are gabbro, quartz-diorite, tonalite, trondhjemite, and monzogranite from west to east in southern Huangling granite batholith. The tonalite, in the southern Huangling granite batholith, is grouped into Sandouping suite (Ma et al., 2002) and is subdivided into Taipingxi, Xidianju, Yanwan, and Xiaoxikou units (rock-mass). However, the research results of 1:250 000 regional geological mapping of Yichang and Jianshi (2006, unpublished) and 1:50 000 regional geological mapping of Liantuo and Sandouping in 2011 (in this study) lead to the Sandouping suite being redivided into two units (Table 1): medium-grained biotite-hornblende tonalite of the Sandouping intrusion and medium-coarse-grained hornblende-biotite tonalite of the Jinpansi intrusion; the pyroxene diorite (metagabbro), which contains no or minor quartz, and the quartz diorite-tonalite were grouped into the Duanfangxi sequence (super-unit) and the Maoping sequence (super-unit), respectively. In addition, the rock name of the Taipingxi unit was revised to medium-coarse-grained biotite-hornblende ~83 km2, extending from north to south. It is the main intrusion in the Maoping superunit, and intrudes into the Xiaoyicun and Miaowan formations of Kongling Group in the north, and is overlain with an angular unconformity by sedimentary rocks from Liantuo Formation of Nanhua System in the south, and is invaded by medium-coarse-grained hornblende-biotite tonalite (Jinpansi intrusion) or medium-fine-grained trondhjemite (Luxiping intrusion) in the east. Moreover, in the internal part of Sandouping intrusion, it is invaded by fine-grained porphyritic biotite granite (Longtanping intrusion) (Fig. 2a), which contains tonalite xenoliths (Fig. 2b), and is invaded by late period veins and small stocks. The Jinpansi intrusion is distribution near Taipingxi with an area of ~5.2 km2 presenting a band shape with a NNW trend. It shows surging intrusive contact relationship with the Sandouping intrusion in the west, and is overlain with an angular unconformity by sedimentary rocks from Liantuo Formation of Nanhua System in the south, and is invaded by Luxiping intrusion in the east.

    Figure 1.  Geological map of granite distribution in Huangling region, western Hubei.

    Table 1.  Division and correlation of the Neoproterozoic granitic batholith in the Huangling anticline

    Figure 2.  (a) Longtanping fine-grained porphyritic monzogranite (L) intrusive Sandouping tonalite (S); (b) Sandouping tonalite xenolith (S) in the Longtanping fine-grained porphyritic monzogranite (L).

  • The lithology of the Sandouping intrusion is medium-grained biotite-hornblende tonalite with dark gray to black alternating white color and shows medium-grained texture and massive structure. The grain size of the felsic minerals is 2–4 mm, but a few reach up to 5 mm. Major minerals include plagioclase (54%–69%), quartz (18%–22%), biotite (5%–13%), and hornblende (7%–15%). The plagioclase with tabular and polysynthetic-pericline twinning is andesine, An=32–33, and shows weak directional alignment, characterized by obvious sericitization on their surface. The quartz mostly is allotriomorphic granular and distributed in other minerals. The hornblende with green-light yellow pleochroism shows directional alignment in long axis and columnar crystals and includes many small quartz inclusions. Magnetite is the most common accessory mineral, which also contains apatite, ilmenite, allanite, and zircon. The color of zircon is miscellaneous and primarily is rose and light yellow.

    The lithology of the Jinpansi intrusion is medium-coarse-grained hornblende-biotite tonalite, which shows coarse-medium-grained texture, weak gneissic, and massive structure. Major minerals include plagioclase (50%–67%), quartz (18%–28%), biotite (8%–22%), and hornblende (2%–10%). The plagioclase is characterized by euhedral-subhedral tabular and polysynthetic-pericline twinning and is andesine-oligoclase, An=28–30. Plagioclase grains are 2–5 mm, which underwent sericitization obviously on their surface, and show weak directional alignment. Quartz mostly are allotriomorphic granular and distributed in other minerals. Biotite grains with scaly-book structure mostly are 2–5 mm (maximum value is equal to 7–10 mm) and have brown color and brown-light yellow pleochroism. Most of the biotite grains are aggregates and include many inclusions (quartz, plagioclase, and apatite). Biotites preserve slightly weak directional alignment. Hornblende grains mostly are 3–6 mm (maximum value is equal to 8–9 mm), show columnar crystal structure, also include many small quartz inclusions, and have green color with green-light yellow pleochroism and two groups of cleavage and simple twinning. The long axis of biotite grains shows weak directional alignment. Accessory minerals include magnetite, apatite, ilmenite, allanite, and zircon. Magnetite always coexists with dark minerals and shows granular structure and inhomogeneous distribution. Apatite is allotriomorphic columnar-granular and shows scattered distribution.

    The lithology of the Longtanping intrusion is fine-grained porphyritic monzogranite, which shows porphyritic fine-grained granitic texture and massive structure. The major minerals include plagioclase (35%–45%), K-feldspar (18%–30%), and quartz (30%–35%), the minor mineral is biotite (±5%). Phenocrysts consist of plagioclase, K-feldspar, and biotite, with a size of 3–4.5 mm, and the content is 10%–15%. Plagioclase phenocryst with tabularprismatic and polysynthetic twinning structure is oligoclase (plagioclase number An=±23) and often includes quartz inclusions. Biotite phenocrysts have characteristic tabular-flake structure and brown-pale yellow pleochroism. Plagioclase grains of matrix with tabular-prismatic structure underwent weak argillization and sericitization; also, twinning development of albite also experienced sericitization and has rare carlsbad-albite compound twinning. Biotite grains show scaly structure and light yellow-brown pleochroism, and a small amount of them are altered to muscovite in retrograde metamorphism. K-feldspar grains mostly are phenocrysts and show grid twinning commonly. Quartz grains mostly are allotriomorphic granular and distributed in feldspar clearance. Accessory minerals include magnetite, monazite, tremolite, hematite and zircon.

  • In this study, the rock samples of isotope geochemistry and major and trace elements analysis were tested in the Ministry of Land and Resources of South Mineral Resources Supervision and Inspection Center, and the results are shown in Table 2. The major elements were analyzed by wet chemical method according to the GB/T 14506.28-1993 standard, whereas the analytical standards of the H2O+, CO2, and LOI were measured according to the GB/T14506.2-1993 standard, the GB 9835-1988 standard, and the LY/T 1253-1999 standard, respectively. Rare earth element (REE) and Y and trace elements of Th, Hf, Ta, Sc, and Co were tested by Plasma MS Excel methods with the standard of DZ/T0223-2001; however, the analytical standards of the trace elements of Sr, Ba, Rb, Nb, Zr, and Ga were followed by the X-ray fluorescence spectrometry method according to the JY/T016-1996 standard. The analysis process of Sr and Nd isotopic composition included three steps: dissolution, separation, and testing. The detailed information was referred to Ma et al. (2007).

    Table 2.  Representative major (wt.%), trace element (ppm), and Sr-Nd isotope compositions for the Huangling granitoids

    Zircon separation of the samples was completed by the Ministry of Land and Resources of South Mineral Resources Supervision and Inspection Center. Analysis samples for dating were crushed to 80–120 mesh, washed dust by water, removed magnetic minerals such as magnetite with a magnet, selected zircons through heavy liquid methods, and picked out the zircons under the binocular microscope finally. Mount making, reflection light, cathodoluminescence (CL) images for zircon dating samples and measurements of U, Th, and Pb were completed at Beijing SHRIMP Center, Chinese Academy of Geological Sciences. The detailed analysis process was referred to the relevant literature (Song et al., 2002; Compston et al., 1992, 1984). In the laboratory, zircon standard samples were M257, which yielded an age of 416.8±1.1 Ma of 206Pb/238U; however, the content of U, Th, and Pb was heterogeneous. The U-Pb isotopic data were calculated by the isoplot software (Black et al., 2003; Ludwig, 2002), and the content of common Pb was corrected by the measured 204Pb content. The errors of all tested zircon points were 1σ, and the weighted mean age 206Pb/238U had 95% confidence interval.

  • Geochemical analysis results of samples from the Sandouping, Jinpansi, and Longtanping intrusions in the Neoproterozoic Huangling granitoids are shown in Table 2. The major elements of samples from the Sandouping and Jinpansi intrusives show that the content of SiO2 is from 62.27 wt.% to 64.27 wt.%, average 63.21 wt.%, and plot in the dioritegranodiorite fields in the Q'-F'-Anor diagram (Fig. 3) (Streckeisen and Le Maitre, 1979); Al2O3 from 16.94 wt.% to 17.46 wt.%, average 17.16 wt.%; K2O+Na2O from 5.31 wt.% to 5.75 wt.%, average 5.46 wt.%; and Na/K from 2.52 to 3.43. The values of consolidation index (SI), which vary from 13.22 to 16.95, average 15.73, and differentiation index (DI), which vary from 63.72 to 69.68, average 66.11, reflect that these samples have moderate-degree crystallization differentiation characteristics. The Rittmann index (δ=1.37–1.53) and SiO2-K2O diagram (Fig. 4a) reflect that the sam-ples belong to calc-alkaline series granites. Samples from the two intrusions have A/CNK values (Al2O3/CaO+Na2O+K2O) of 0.98–1.06, and the A/NK-A/CNK diagram (Fig. 4b) show that the samples appear as silica supersaturated type of metaluminous to weakly peraluminous calc-alkaline granites. Samples from the fine-grained porphyritic monzogranite of Longtanping intrusion, which intrude the tonalite-diorite of Sandouping rock unit, fall in the fields of monzogranite in the Q'-F'-Anor diagram (Fig. 3 (after Streckeisen and Le Maitre, 1979). The content of SiO2 is 73.16 wt.%, Al2O3 is 13.91 wt.%, K2O+Na2O is 7.23 wt.%, and Na/K mostly is 1.2. The values of SI and DI, which are 2.8 and 85.69, respectively, reflect that the Longtanping intrusion have higher-degree crystallization differentiation characteristics. A/CNK=1.06, Rittmann index δ=1.3, show that the samples belong to silica supersaturated type of peraluminous high-K granites.

    Figure 3.  Q'-F'-Anor diagram of the Huangling granitoids (after Streckeisen and Le Maitre, 1979).

    Figure 4.  (a) K2O-SiO2 diagram (solid line: Peccerillo and Taylor, 1976; dotted line: Middlemost, 1985); (b) A/NK-A/CNK diagram of the Neoproterozoic Huangling granitoids (Maniar and Piccoli, 1989). A/CNK. (Al2O3/(CaO+Na2O+K2O) molecular); A/NK. (Al2O3/(Na2O+K2O) molecular).

  • The range of the total REE abundances of biotite-hornblende tonalite from Sandouping intrusion and hornblende-biotite tonalite from Jinpansi intrusion is from 42.62 ppm to 117.12 ppm (Table 2), and they have similar REE patterns. The Sm/Nd value is 017–0.24, and the two intrusions show significant enrichment in LREE, negative anomalies in Ce (Ce/Ce*=0.87–0.89), and weak positive anomalies in Eu (Eu/Eu*=1.0–1.2), except two groups (one group is 0.97 and another is 1.63). The Eu/Sm values range from 0.33 to 0.5, reflecting that the two units are homologous. The total REE abundance of biotite monzogranite from the Longtanping intrusion is low, and the REE distribution pattern is shown in Fig. 5a. There are significant enrichment in LREE, negative anomalies in Ce (Ce/Ce*=0.89), and weak positive anomalies in Eu (Eu/Eu*=1.03). The value of LREE/HREE ratio is 13.9, which is higher than the Sandouping and Jinpansi units, reflecting that the degree of magma differentiation for Longtanping intrusion is higher, and the curve slope of LREE is steeper also.

    Figure 5.  Chondrite-normalized REE distribution patterns (a) and primitive-normalized trace element spider diagram (b) of the Neoproterozoic Huangling granitoids (chondrite and primitive mantle data after Sun and McDonough, 1989).

  • Trace elements analysis results of biotitehornblende tonalite from Sandouping intrusion and hornblende-biotite tonalite from Jinpansi intrusion are shown in Table 2. The RbN/YbN values range from 1.1 to 3.62, are > 1, and suggest that the two intrusions are strongly enriched in compatible elements. From the spider diagram of trace element ratios (Fig. 5b), the granite samples are enriched in LILE of K, Rb, Ba, and Sr and immobile elements such as Zr but depleted in HFSE of Nb, Ta, Hf, Ti, and P and show that the trace element distribution patterns are characteristics of tectonic environment of active continental margin arc; these features suggest that the generation of the granites is related to plate subduction. They are enriched in K, Zr, and Sr and depleted in Nb, P, Th, and Ti, which indicate that the material source of granite is derived from crustal rocks and show the characteristics of crust-derived granites (Li C N, 1992; Pearce et al., 1984). The trace element contents of fine-grained porphyritic monzogranite of Longtanping intrusion are shown in Table 2. The RbN/YbN value is 10.37, which is > 1. It suggests that the intrusion is strongly enriched in compatible elements. There are significant differences of spider diagrams between the Sandouping, Jinpansi, and Longtanping intrusions. The Longtanping intrusion granites are depleted in, Nb, Sr, P, Ti, and Th and enriched in K and Zr, which indicate that the intrusion material source is derived from crustal rocks.

  • The Rb-Sr and Sm-Nd isotopic analysis results are shown in Table 3. The εNd(t) and εSr(t) values of tonalite from Sandouping intrusion range from -12.4 to -11.0 and from 20.2 to 32.2, respectively, and the fine-grained porphyritic biotite monzonitic granite are -16.3 and 82.9. In the εNd(t)-εSr(t) diagram (Fig. 6), all the representative samples fall in the field of Chinese lower crust, which indicates that the source region of granite formation is the lower crust.

    Table 3.  Rb-Sr and Sm-Nd isotopic composition and main parameters of the Neoproterozoic Huangling granitoids

    Figure 6.  εSr(t)-εNd(t) diagram of the Neoproterozoic Huangling granitoids (Zhu et al., 2001).

  • SHRIMP zircon U-Pb dating of granites are shown in Table 4. The characteristics of zircon grains from the granite samples are as follows: the columnar zircon is the predominant type, except a small amount of granular type, and their aspect ratios range mainly from 1:1 to 3:1. The analyzed zircon grains are uniformly transparent in color, and the CL images show that the zircon grains have clear oscillatory zoning (Fig. 7), which is the typical characteristic of magmatic origin (Chen et al., 2007; Wu and Zheng, 2004; Wysoczanski and Allibone, 2004; Corfu et al., 2003).

    Figure 7.  The CL images of the representative zircons (a, c, e) and SHRIMP zircon U-Pb concordia age diagrams (b, d, f) from the Neoproterozoic Huangling granitoids.

    Medium-grained biotite-hornblende tonalite (Sample No.: PM053-11-1, sample location: N30°52′29″, E111°01′09″) from the Sandouping intrusion has 11 zircon grains analyzed. U and Th contents of zircon range from 70 ppm to 232 ppm and from 40 ppm to 114 ppm, respectively. The Th/U ratio is between 0.33 and 0.81, which is consistent with typical magmatic zircon. U-Pb ages of zircon from the tonalite sample are in the range of 833±14 and 905±15 Ma (Table 4). The CL images (Fig. 7a) show that No. 11 zircon grain has local characteristics of planar zoning, and the rim shows a wide alteration edge that cuts the magmatic zoning of the protolith zircon. These features indicate that the zircon grain may be affected by late hydrothermal alteration, and the U-Pb age (833±14 Ma) is younger than others. The core zoning of No. 7 zircon grain is curve-shaped and unsmooth and has erosion structure. The U-Pb age (905±15 Ma) is older than others as the zircon part of laser ablation contains the inherited core composition. The remaining nine grains all plot close to Concordia, yielding an age of 863±9 Ma (MSWD=0.88) (Fig. 7b), which is the formation time of Sandouping intrusion.

    Medium-grained hornblende-biotite tonalite (Sample No.: PM054-2-1, sample location: N30°54′24″, E111°00′36″) from the Jinpansi intrusion has 13 zircon grains analyzed. U and Th contents of zircon range from 48 ppm to 191 ppm and from 25 ppm to 173 ppm, respectively. The Th/U ratio is between 0.46 and 0.96, which is consistent with typical magmatic zircon. U-Pb ages of zircon from the tonalite samples are in the range of 794±11 and 873± 8 Ma (Table 4). The CL images show that the No. 5 zircon grain has "core-mantle" structure, and the position of No. 5.2 point is located at the core of the grain, which shows the oscillatory zoning characteristics of the magmatic zircon, whereas the crystal edge of No. 5.1 analysis point has been rounded (Fig. 7c), and the content of Th and U is low. These features indicate that the zircon grain may be affected by the late hydrothermal alteration, and the U-Pb age is 803± 11 Ma, which could be used as the age of late hydro-thermal alteration. The No. 8 zircon grain with a black circle inside may be reconstructed by the late fluids, which cause lead loss, and the U-Pb age (794±11 Ma) is younger than others. The No. 2 zircon grain shows characteristics of partial curve shape and erosion structure. The U-Pb age is older (873±8 Ma) as the zircon part of laser ablation contains the inherited core composition. The remaining 10 grains all plot close to concordia, yielding an age of 842±10 Ma (MSWD=0.88) (Fig. 7d), which is the formation time of the Jinpansi intrusion.

    Fine-grained porphyritic monzogranite (Sample No.: PM053-2-1, sample location: N30°52′32″, E111°00′28″) from Longtanping intrusion has 16 zircon grains to be analyzed. U and Th contents of zircon range from 70 ppm to 232 ppm and from 40 ppm to 114 ppm, respectively. The Th/U ratio is between 0.33 and 0.81, which is consistent with typical magmatic zircon. The CL images show that the No. 3 zircon grain has characteristics of partial planar zoning (Fig. 7e) and a wide alteration edge on the rim, which indicates that the zircon grain may be affected by late hydrothermal alteration; the U-Pb age (815±34 Ma) is younger than others. The plot of No. 5 zircon grain deviates from concordia. Therefore, these two points are excluded. The remaining 14 grains all plot close to concordia, yielding an age of 844±10 Ma (MSWD=0.71) (Fig. 7f), which is the formation time of the Longtanping intrusion.

  • The Huangling granitic complex (batholith) is considered as the product of repeated magmatic intru-sions, of which the formation age has been studied by some scholars. Ma et al. (1984) determined the zircon age of 819±7 Ma from biotite granodiorites in the Huanglingmiao sequence (suite) using U-Pb isochron by ion microprobe U-Pb analyses. Ma et al. (2002) and Feng et al. (1991) published the formation ages of 832±12 and 834±35 Ma from tonalites in the Sandouping suite using zircon TIMS U-Pb dating and whole-rock Rb-Sr isochron, respectively. Li Z C et al. (2002) obtained the age of 833±29 Ma from tonalite in the Sandouping suite using zircon TIMS U-Pb dating. Ling et al. (2006) reported an age of 795±8 Ma of tonalites from the Sandouping suite and 794±7 Ma of monzogranites from the Dalaoling suite using LA-ICP-MS zircon dating. Li Y L et al. (2007) yielded ages of 837.3±4.2–838.7±4.0 Ma and 844.0±4.2 Ma by 40Ar-39Ar dating from biotites and hornblendes. Zhang et al. (2009) reported LA-ICP-MS zircon ages of 803±11 and 810±15 Ma of tonalites from Maoping sequence (Sandouping suite). Gao and Zhang (2009) published zircon SHRIMP U-Pb dating age of 837±7 Ma from the Maoping sequence (Sandouping suite). Data obtained from the Neoproterozoic Huangling granite show a formation age of 844–795 Ma. Recently, Peng et al. (2012) obtained LA-ICP-MS zircon ages of 813±14–871±16 Ma from granite clasts in the fold-thrust belt of Miaowan ophiolite in the southern Huangling anticline. Zhang et al. (2009) selected six samples from Huangling complex and attained two groups of ages (803±11, 919±15, 810±15 and 911±14 Ma) from each sample picked in the Sandouping suite and suggested that the emplacement age of Sandouping intrusion is 805±9 Ma, whereas the formation ages of Huanglingmiao, Dalaoling, and Xiaofeng suites are 821±2, 817±22, and 817±3 Ma, respectively. However, it is not in conformity with the fact that Sandouping suite is intruded by the Huanglingmiao suite. It needs further research whether the age of 803–811 Ma is the formation age of later intrusion in Sandouping suite. It is reasonable that the formation age of Sandouping suite is between 821 and 911 Ma.

    Based on the detailed 1:50 000 regional geological mapping, we selected zircons from earlier Sandouping and Jinpansi intrusions and later Longtanping intrusion from Maoping sequence (Sandouping suite), which is the oldest sequence in the Huangling anticline for SHRIMP U-Pb dating. The results show that the formation ages of medium-grained biotite-hornblende tonalite from Sandouping, medium-coarse-grained hornblende-biotite tonalite from Jinpansi, and fine-grained porphyritic monzonitic granite from Longtanping are 863±9, 842±10, and 844±10 Ma, respectively, which is in accord with the stratigraphic relationships. Thus, the formation age of Neoproterozoic Huangling granitoid should be no later than 863 Ma, whereas the Neoproterozoic granite magmatism should have begun at least as early as 863 Ma.

  • Recently, there are two viewpoints about the petrogenesis and tectonic setting of Huangling Neoproterozoic granite: one is that it formed in a magmatic arc environment, which is related with the Late Jinningian southward subduction of the Proterozoic Qinling Ocean (Ma et al., 2002; Gao et al., 1990), and the other holds that there was a mantle plume in ~825 Ma in the South China, which resulted in the breakup of the Rodinia supercontinent (Zhang et al., 2009; Li X H et al., 2003, 2001; Li Z X et al., 2002, 1999; Li Z X, 1998).

    However, the major elements of medium-grained biotite-hornblende tonalite from the Sandouping unit and medium-coarse-grained hornblende-biotite tonalite from the Jinpansi unit show enrichment in Na, Na2O > K2O, high Al (Al2O3 > 15 wt.%), and low Ti (TiO2 < 1 wt.%), which belong to calc-alkaline series, whereas the fine-grained monzonitic granite from Longtanping unit has an evolution trend towards to high-K calc-alkaline granite, the A/CNK are 0.98–1.06, and diopside does not occur in the standard minerals that are all attributes of I-type granite. The REE show LREE-enriched and HREE-flat patterns. In the K2O-Na2O diagram (data not shown), the samples fall into the I-type region. In the εNd(t)-εSr(t) diagram (Fig. 6), they plot in the lower crust region that indicates that the sources of the Sandouping tonalite are mostly crustal rocks that belong to I-type granites. The samples all represent volcanic arc tectonic environment in the diagrams of Rb-Y+Nb (Fig. 8a), Ta-Yb (Fig. 8b), and Rb-Hf-Ta (Fig. 8c). Except for the monzonitic granite from Longtanping intrusion falling into the adakite region, others fall into the classic island-arc region or its overlap area with adakite in the (La/Yb)N-(Yb)N diagram (Fig. 8d). It implies that the rock suite formed within a magmatic arc environment along the active margins of plates because it is characterized by high values of Al2O3, La/Nb, Th/Ta, low content of TiO2, and clear negative Nb-Ti anomalies. Consequently, the granite from the Maoping sequence (Sandouping suite), the oldest Neoproterozoic granite in the southern Huangling anticline, should form in the oceanic crust subduction-related island-arc environment.

    Figure 8.  Geochemical tectonic discrimination diagram for Neoproterozoic granitoids. (a) Y+Nb vs. Rb discrimination diagram; (b) Yb-Ta discrimination diagram (after Pearce et al., 1984); (c) Rb-Hf-Ta ternary diagram (after Harris et al., 1986); (d) (La/Yb)N vs. (Yb)N discrimination diagram of adakite (Defant and Drummond. 1990). VAG. Volcanic arc granite; WPG. within plate granite; S-COLG. syn-collision granite; ORG. ocean ridge granite.

  • (1) The SHRIMP U-Pb ages of medium-grained biotite-hornblende tonalite from Sandouping, medium-coarse-grained hornblende-biotite tonalite from Jinpansi, and fine-grained monzonitic granite from Longtanping, which all belong to the oldest Neoproterozoic Maoping sequence (Sandouping suite) in the southern Huangling anticline, are 863±9, 842±10, and 844±10 Ma, respectively, indicating that the formation age of Neoproterozoic Huangling granitoid should be no later than 863 Ma.

    (2) The medium-grained biotite-hornblende tonalite from Sandouping unit and medium-coarse-grained hornblende-biotite tonalite from the Jinpansi unit belong to calc-alkaline granite, whereas the fine-grained monzonitic granite from the Longtanping unit has an evolution trend towards high-K calcalkaline granite. These show that they should be formed in the oceanic crust subduction-related islandarc envi-ronment.

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