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Volume 23 Issue 5
Oct.  2012
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Xingfu Jiang, Songbai Peng, Timothy M. Kusky, Lu Wang, Junpeng Wang, Hao Deng. Geological Features and Deformational Ages of the Basal Thrust Belt of the Miaowan Ophiolite in the Southern Huangling Anticline and Its Tectonic Implications. Journal of Earth Science, 2012, 23(5): 705-718. doi: 10.1007/s12583-012-0289-7
Citation: Xingfu Jiang, Songbai Peng, Timothy M. Kusky, Lu Wang, Junpeng Wang, Hao Deng. Geological Features and Deformational Ages of the Basal Thrust Belt of the Miaowan Ophiolite in the Southern Huangling Anticline and Its Tectonic Implications. Journal of Earth Science, 2012, 23(5): 705-718. doi: 10.1007/s12583-012-0289-7

Geological Features and Deformational Ages of the Basal Thrust Belt of the Miaowan Ophiolite in the Southern Huangling Anticline and Its Tectonic Implications

doi: 10.1007/s12583-012-0289-7
Funds:

the Postdoctoral Science Foundation 20100471203

the Ministry of Land and Resources of China 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: Songbai Peng, psb200301@yahoo.com.cn
  • Received Date: 2011-11-05
  • Accepted Date: 2012-01-07
  • Publish Date: 2012-10-01
  • The stratigraphic, structural and metamorphic features of the basal thrust belt of the ca. 1.0 Ga Miaowan (庙湾) ophiolite in the southern Huangling (黄陵) anticline, show that it can be divided into three tectono-lithostratigraphic units from north to south: mélange/wildflysch rock units, flysch rock units, and sedimentary rock units of the autochthonous (in situ) stable continental margin. The three units underwent thrust-related deformation during emplacement of the Miaowan ophiolitic nappe, with kinematic indicators indicating movement from the NNE to SSW, with the metamorphic grade reaching greenschist-amphibolite facies. LA-ICP-MS U-Pb geochronology of zircons from granite pebbles in the basal thrust-related wildflysch yield ages of 859±26, 861±12 and 871±16 Ma; whereas monzonitic granite clasts yield an age of 813±14 Ma. This indicates that the formation age of the basal thrust belt is not older than 813±14 Ma, and is earlier than the earliest formation time of the majority of the Neoproterozoic Huangling granitoid intrusive complex, which did not experience penetrative ductile deformation. These results suggest that the northern margin of the Yangtze craton was involved in collisional tectonics that continued past 813 Ma. This may be related to the amalgamation of the Yangtze craton with the Rodinia supercontinent. Through comparative study of lithology, zircon geochronology, REE patterns between granodiorite and tonalite pebbles in the basal thrust-zone conglomerate, it can be concluded that the pebbles are the most similar to the Huanglingmiao (黄陵庙) rock-mass (unit), implying that they may have come from Huanglingmiao rock-mass. Zircon cores yield xenocrystic ages of 2 074±120 Ma, suggesting that the protolith of the Neoproterozoic Huangling granitoid intrusive complex may have originated from partial melting of older basement rocks, that is to say there may be Paleoproterozoic crystalline basement in the southern Huangling anticline. The ages of xenocrystic zircons in the granite pebbles in the basal-thrust conglomerate/wildflysch show a correlation with the age spectra from Australia, implying that the terrain that collided with the northern margin of the Yangtze craton and emplaced the Miaowan ophiolite at ca. 813 Ma may have been derived from the Australian segment of Rodinia.
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Geological Features and Deformational Ages of the Basal Thrust Belt of the Miaowan Ophiolite in the Southern Huangling Anticline and Its Tectonic Implications

doi: 10.1007/s12583-012-0289-7
Funds:

the Postdoctoral Science Foundation 20100471203

the Ministry of Land and Resources of China 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: The stratigraphic, structural and metamorphic features of the basal thrust belt of the ca. 1.0 Ga Miaowan (庙湾) ophiolite in the southern Huangling (黄陵) anticline, show that it can be divided into three tectono-lithostratigraphic units from north to south: mélange/wildflysch rock units, flysch rock units, and sedimentary rock units of the autochthonous (in situ) stable continental margin. The three units underwent thrust-related deformation during emplacement of the Miaowan ophiolitic nappe, with kinematic indicators indicating movement from the NNE to SSW, with the metamorphic grade reaching greenschist-amphibolite facies. LA-ICP-MS U-Pb geochronology of zircons from granite pebbles in the basal thrust-related wildflysch yield ages of 859±26, 861±12 and 871±16 Ma; whereas monzonitic granite clasts yield an age of 813±14 Ma. This indicates that the formation age of the basal thrust belt is not older than 813±14 Ma, and is earlier than the earliest formation time of the majority of the Neoproterozoic Huangling granitoid intrusive complex, which did not experience penetrative ductile deformation. These results suggest that the northern margin of the Yangtze craton was involved in collisional tectonics that continued past 813 Ma. This may be related to the amalgamation of the Yangtze craton with the Rodinia supercontinent. Through comparative study of lithology, zircon geochronology, REE patterns between granodiorite and tonalite pebbles in the basal thrust-zone conglomerate, it can be concluded that the pebbles are the most similar to the Huanglingmiao (黄陵庙) rock-mass (unit), implying that they may have come from Huanglingmiao rock-mass. Zircon cores yield xenocrystic ages of 2 074±120 Ma, suggesting that the protolith of the Neoproterozoic Huangling granitoid intrusive complex may have originated from partial melting of older basement rocks, that is to say there may be Paleoproterozoic crystalline basement in the southern Huangling anticline. The ages of xenocrystic zircons in the granite pebbles in the basal-thrust conglomerate/wildflysch show a correlation with the age spectra from Australia, implying that the terrain that collided with the northern margin of the Yangtze craton and emplaced the Miaowan ophiolite at ca. 813 Ma may have been derived from the Australian segment of Rodinia.

Xingfu Jiang, Songbai Peng, Timothy M. Kusky, Lu Wang, Junpeng Wang, Hao Deng. Geological Features and Deformational Ages of the Basal Thrust Belt of the Miaowan Ophiolite in the Southern Huangling Anticline and Its Tectonic Implications. Journal of Earth Science, 2012, 23(5): 705-718. doi: 10.1007/s12583-012-0289-7
Citation: Xingfu Jiang, Songbai Peng, Timothy M. Kusky, Lu Wang, Junpeng Wang, Hao Deng. Geological Features and Deformational Ages of the Basal Thrust Belt of the Miaowan Ophiolite in the Southern Huangling Anticline and Its Tectonic Implications. Journal of Earth Science, 2012, 23(5): 705-718. doi: 10.1007/s12583-012-0289-7
  • Fold-thrust belts are an important tectonic unit of plate margins (Zhang and Song, 1997), such as the Paleozoic Appalachian orogenic belt in eastern North America, the Meso–Cenozoic Alpine orogenic belt, and the Qinling-Dabie orogenic belt in China (Tremblay et al., 2011; Hans, 2010; Sasseville et al., 2008; Xie et al., 2006; Golonka, 2004; Sun et al., 2004; Song et al., 2002; Peresson and Decker, 1997; Samson et al., 1995; Thakur, 1980). Although the thrust belts have different scales and formation times, they show similar characteristics: (1) fold-thrust belts are located in the outer margin of the orogenic belts, and show zonal distributions; (2) a series of imbricate thrust faults are developed in the belts; (3) sedimentary strata of the craton margin underwent deformation in the late stages of the collision event; (4) formation of the thrust belts resulted from plate subduction and collision (Rodgers, 1991, 1990; Davis et al., 1983; Chapple, 1978). The progressive geometric and kinematic evolution of interactions between structures in the orogen and those in the thrust and fold belt, are important in order to understand the mechanics of an orogen (Simony and Carr, 2011). Studies of U-Pb geochronology and geochemistry features of zircon from granite pebbles in thrust-related flysch and molasse can shed light on the nature of the source terrane, including its age, character, and tectonic setting. Here, we report details of the structure, lithofacies, and source regions for pebbles in conglomerate/wildflysch from the basal thrust zone of the ca. 1.0 Ga Miaowan ophiolite (Peng et al., 2012, 2010) in the southern part of the Huangling anticline on the northern margin of the Yangtze craton. Our results suggest that the northern margin of the Yangtze craton collided with a fragment of Rodinia should be earlier than 813 Ma, perhaps during amalgamation of the Yangtze block with Rodinia.

  • The Yangtze craton is the largest Precambrian block in South China, and is characterized by a Precambrian crystalline basement, in which the most famous and important area is the Huangling anticline (Zheng and Zhang, 2007; Ma et al., 2002; Qiu et al., 2000; Gao et al., 1999; Jiang et al., 1986). The Huangling anticline is the key area to study the pre-Nanhua tectonic evolution of the Yangtze craton. Precambrian metamorphic rocks in the Huangling area are mainly distributed in the middle of the Huangling anticline and dominated by Archean–Paleoproterozoic medium high-metamorphic rock series. They are divided into northern and southern parts separated by a Neoproterozoic granite batholith. Study of the Kongling Group in the southern part of the Huangling anticline shows that it consists of three rock series (Ma et al., 2002); from oldest to youngest these include Archean grey TTG (tonalite-trondhjemite-granodiorite) gneiss and supracrustal rocks, Paleoproterozoic khondalite and felsic gneiss, mafic-ultramafic rocks and metasedimentary rocks (Miaowan ophiolite). The metasedimentary unit also contains metadiabase, marble and other types of rocks. The khondalite unit is divided into two formations: the lower formation consists dominantly of gneiss with graphite and aluminum-rich minerals, biotite granulite, and felsic granulite in the upper part. This article focuses on the formation age, tectonic setting and significance of the basal thrust belt of the Miaowan ophiolite located at Taipingxi-Xiaoxikou in the southern of the Miaowan ophiolite, southern Huangling anticline.

  • The basal thrust belt of the Miaowan ophiolite in the southern Huangling anticline is located on the southern side the Miaowan ophiolite massif, and is best exposed at Xiaoxikou and Taipingxi in Yichang City. The Miaowan ophiolite is an elongate NWW-SEE belt, with a length of about 13 km and nearly 2 km in width, whereas the basal thrust belt is located along the south sides of dike complexes, with an additional width of 0.15–0.5 km (Fig. 1). The basal thrust belt can be divided into three tectono-lithostratigraphic units from north to south: mélange/wildflysch, flysch, and sedimentary rock units of the autochthonous (in situ) stable continental margin. The main features of each rock unit are described below.

    Figure 1.  Geological map of Miaowan ophiolite in the southern Huangling anticline and the cross section of the basal thrust belt. Map modified after Peng et al. (2012).

  • The formation age of the Miaowan ophiolite is about 1.0 Ga. Ultramafic rocks mainly consist of serpentinite, serpentinized dunite, and harzburgite. Mafic rocks mainly consist of layered, fine-grained, plagioclase-amphibolite, layered and massive metagabbro, fine-grained amphibolite and dike complexes (Peng et al., 2012).

  • The rocks immediately below the Miaowan ophiolite comprise a mélange/wildflysch unit, which preserves a fault structure contact relationship with the southern margin of the structurally dismembered Miaowan ophiolite (Figs. 2a and 2b). In addition, a few monzonitic granite veins and mafic veins with weak deformation intrude this unit (Figs. 2g and 2h).

    Figure 2.  Field and microstructure photographs of basal thrust belt. (a) Mélange/wildflysch rock units (field outcrop); (b) metadiabase in mélange/wildflysch rock units (plane light); (c) high-angle thrust fault in flysch rock units (field outcrop); (d) amphibole-plagioclase-gneisses in mélange/wildflysch rock units (plane light); (e) fold structure of meta feldspar-quartz sandstone in sedimentary rock units in situ of stable continental margin (field outcrop); (f) meta feldspar-quartz sandstone in sedimentary rock units in situ of stable continental margin (plane light); (g) granodiorite, quartzite cobbles in mélange/wildflysch rock units (field outcrop); (h) granodiorite in mélange/wildflysch rock units (plane-polarized light). PAQ. paragneiss; MD. metadiabase; MS. metasandstone; Tro. trondhjemite; GP. granite pebble; QP. quartzite pebble; Am. amphibole; Bi. biotite; Pl. plagioclase; Q. quartz.

    The pebbles in the mélange/wildflysch rock unit, are angular to sub-angular and show a directional structure. The cobbles are mostly 2–10 cm in diameter, but range up to 12 cm. These features indicate that these cobbles were derived from a near by source undergoing rapid erosion. Based on their petrology, the pebbles are divided into three types: (1) granodiorite and tonalite; (2) monzonitic granite; and (3) quartzite. Granodiorite and tonalite pebbles have granoblastic textures, and massive structures. Major minerals in the granodiorite and tonalite include plagioclase (45%–50%), K-feldspar (8%–20%), quartz (25%–30%), biotite and amphibole (3%). Monzonitic granite pebbles have medium-coarse granitoid texture, and consist of plagioclase (25%), K-feldspar (40%), quartz (30%), biotite (4%) and amphibole (1%). Most of plagioclase grains underwent serictization, sausseritization and epidote alteration. Polysynthetic twin texture can be observed in some weakly altered areas, K-feldspar has grains that are smaller than the plagioclase grains and show twinning.

    Metadiabase and amphibole-plagioclase-gneisses form the matrix. Metadiabase shows schistose structure, major minerals with directional arrangement, include amphibole (63%), plagioclase (35%), and magnetite (2%). Amphibole is hornblende, characterized by two directions of cleavage and rhombic cross sections, slight sausseritization and epidote alteration. Plagioclase minerals did not develop twinned-crystals, and a few grains underwent moderate to strong serictization, sausseritization and epidote alteration.

  • Rocks in this unit include strongly ductile-brittle deformed and metamorphic amphibole-plagioclasegneisses, mica schist, metasandstone and metagraywacke in flysch rock units, which were also later intruded by monzonitic granite veins and quartz veins with weak deformation. The deformation is mainly manifested as bedding-parallel schistosity and boudinage structures. A seris of northeast-striking highangle thrust faults is present (Fig. 2c), with dip angles greater than 60°.

    Amphibole-plagioclase-gneisses have banding and gneissic structures. Major minerals include plagioclase (60%), amphibole (35%) and quartz (5%); the rock type of metasandstone is meta feldspathic-sandstone (Figs. 2d and 2f), major minerals include quartz (50%), plagioclase (30%) and biotite (20%) with a strong foliation; the trondhjemite has granoblastic texure and prophyritic structures, and the minerals shows weak directional alignment, phenocrysts are plagioclase (5%). The matrix has a fine-grained, fibrous and granular crystalloblastic texture and is composed of plagioclase (80%), amphibole (10%), orthoclase (3%) and magnetite (2%). All of the plagioclase grains show moderate sausseritization and epidote alteration.

  • Medium-thin layered marble is the main sedimentary rock in the sequence, with a few medium-thin layers of meta-quartz sandstone, and quartzite. Marble has a fine-grained granular crystalloblastic texture, and consists chiefly of calcite (70%), plagioclase (15%), and quartz (14%). The calcite forms equigranular crystals, less than 0.5 mm across, with polysynthetic twinning. The rocks of this unit are gently folded, with axial planes that dip shallowly NE-ward.

    As a whole, the mélange/wildflysch and flysch rock units underwent strong deformation and metamorphism with the metamorphic grade reaching high greenschist-amphibolite facies. Contact relationships of the rock units are northwest-striking faults. Sedimentary rock units of the autochthonous stable continental margin underwent obvious weak deformation and low-grade metamorphism to greenschist facies.

  • We conducted a detailed structural analysis along a 150-m-long section across the basal thrust of the Miaowan ophiolite, which shows that the mélange/ wildflysch and flysch rock units underwent strong ductile-brittle deformation and metamorphism, where the sedimentary rock units of the autochthonous sequence are only weakly deformed and metamorphosed. Twenty-three foliation and seven lineation measurements from the geological structural profile through the basal thrust belt are projected on a lower-hemisphere equal area projection (Figs. 3 and 4), showing that the strike of the penetrative ductile deformational foliation is approximately northwest, and that the lineation trends 97°, and plunges 66° (Fig. 4). These data all are consistent with the attitude of a high angle NE direction thrust fault.

    Figure 3.  Lower hemisphere equal angle projection, of foliations in the basal thrust belt (23 groups of data).

    Figure 4.  Lower-hemisphere equal angle projection, of lineation of basal thrust belt (7 groups of data).

    As previously mentioned, there are three types of veins in the basal thrust belt: quartz veins, weakly deformed trondhjemite veins, and monzonitic granite. The quartz veins' strike N6°–84°W, the trondhjemite veins' strike N13°–77°W (one group strikes 88°) and the monzonitic granite veins' strike W56°–34°S (one group strikes 12°). These results reflect that the quartz veins and the weakly deformed trondhjemite veins intruded along NW-trending fractures and faults, whereas the intrusive direction of the monzonitic granite veins was northeast.

  • On the basis of our detailed observations and measurements from the structural profile of the basal thrust belt, we selected eight non-distorted and weakly deformed fresh granite and quartzite pebbles in the mélange/wildflysch rock units for zircon geochronology and geochemical study. Separation of zircon from the samples was carried out in three stages: rock crushing, gravity separation and magnetic separation completed in the Experimental Test Center of Regional Geological Survey, Institute of Geology in Langfang, Hebei Province. Two quartzites yielded no zircons, and two granite pebbles yielded insufficient zircon grains for dating. Four granite pebbles were successfully dated: Gra1, Gra6, Gra9 and Gra11. U-Pb dating and trace element analyses of zircon were conducted synchronously by LA-ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan.

    Laser sampling was performed using a GeoLas 2005. An Agilent 7500a ICP-MS instrument was used to acquire ion-signal intensities. Each analysis incorporated a background acquisition of approximately 20–30 s (gas blank) followed by 50 s data acquisition from the sample. Off-line selection and integration of background and analysis signals, and time-drift correction and quantitative calibration for trace element analyses and U-Pb dating were performed by ICP-MS Data Cal. (Liu et al., 2010a, 2008). Detailed operating conditions for the laser ablation system and the ICP-MS instrument and data reduction are the same as description by Liu et al. (2010a, b, 2008).

    The methods of trace element compositions of zircons were calibrated and time-dependent drifts of U-Th-Pb isotopic ratios were corrected by zircon 91500 are shown in Liu et al. (2010a). Preferred U-Th-Pb isotopic ratios used for 91500 are from Wiedenbeck et al. (1995). Concordia diagrams and weighted mean calculations were made using Isoplot/Ex_ver3 (Ludwig, 2003).

  • Zircon grains, which were separated from non-distorted and weakly deformed granitic pebbles in the basal thrust-related wildflysch, are uniformly yellow in color and have mostly euhedral to subhedral shapes. Some of the grains are ruptured and incomplete. Their size ranges from 35×40 to 110×150 µm, and their aspect ratios are between 1.0 and 4.0. From cathodoluminescence (CL) images of typical zircon (Fig. 5), zircon rims show clear oscillatory zoning, indicating a magmatic origin (Fig. 5c). Individual zircons have narrow rims with bright luminescence, which may reflect retrograde metamorphism (Wu et al., 2003). A few magmatic zircons have inherited cores (Figs. 5a, 5b and 5d). The inherited cores appear dark in CL images and high REE, Th and U concentrations. The REE patterns for typical zircons show significant depletion in LREE, enrichment in HREE, positive anomalies in Ce and negative anomalies in Eu (Hinton and Upton, 1991; Sun and McDonough, 1989), and REE in zircons can indicate their conditions of formation, as can Nb, Ta and other trace elements (Wu et al., 2002). It is commonly believed that different types of zircons have different U/Th ratios. From the trace element LA-ICP-MS analysis of zircons in weakly deformed granitic pebbles from mélange/wildflysch units (Table 1), shows that U and Th contents of oscillatory zone of zircon in the monzonitic granite pebble (Gra 1), is 22–162 and 16–75 μg/g, respectively, except for Gra1-23 (rim) which has 421 μg/g U and 302 μg/g Th. The Th/U ratio is between 0.35 and 1.65. The range of total REE abundances for zircon rims is from 690 ppm to 1 345 ppm. There is significant depletion in LREE, enrichment in HREE (673 ppm–1 303 ppm), positive anomalies in Ce (Ce/Ce*=4–113) and negative anomalies in Eu (Eu/Eu*=0.19–0.53), and the (Yb/Gd)CN values range from 17 to 51. The mass fraction of Nb and Ta is low, in the range of 0.8–4.4 and 0.3–1.9 μg/g, respectively, and the Nb/Ta ratios are between 1.8 and 5.6. The range of total REE abundances for the cores of zircons is from 98 ppm to 601 ppm, which is much less than the abundances for rim. The cores have significant positive anomalies in Ce (Ce/Ce*=24–64) and negative anomalies in Eu (Eu/Eu*=0.19–0.27), the mass fraction of Nb and Ta is low, in the range of 0.66–0.8 and 0.36–0.4 μg/g, respectivley, and the Nb/Ta ratios are between 1.8 and 5.6, which are lower than the ratios of the zircon rims.

    Figure 5.  Cathodoluminescence images of typical zircon in the non-distorted and weakly deformed granitic pebbles from amphibole-plagioclase-gneisses of mélange/wildflysch rock units. (a), (b) Core ring structure of magmatic zircon; (c) sector zoning of magmatic zircon; (d) core ring structure of zircon.

    The U and Th contents of the oscillatory zone of a granodiorite pebble (Gra 6) is in the range of 17–75 and 23–95 μg/g, respectively, and the Th/U ratios are between 0.82 and 1.44. The range of total REE abundances for zircon rims is 882 ppm to 2 237 ppm. There is significant depletion in LREE, enrichment in HREE (859 ppm–2 168 ppm), positive anomalies in Ce (Ce/Ce*=8–71) and negative anomalies in Eu (Eu/Eu*=0.38–0.54). The (Yb/Gd)CN values range is from 27 to 40. The mass fractions of Nb and Ta are low, in the range of only 1.86–5.15 and 0.34– 0.82 μg/g, respectively, and the Nb/Ta ratios are between 3.1 and 6.4. The range of total REE abundances for zircon cores is 324 ppm, which is much lower than for the rims. There are significant positive anomalies in Ce (Ce/Ce*=121) and negative anomalies in Eu (Eu/Eu*=0.30), the mass fractions of Nb and Ta are 2.62 and 1.52 μg/g, respectively, and the Nb/Ta ratio is 1.7, which is also lower than the ratio for the zircon rims.

    The U and Th contents of oscillatory zoned zircon from a tonalite pebble (Gra 9), are in the range of 18–159 and 15–411 μg/g, respectively, the Th/U ratio is between 0.61 and 1.30, except for point Gra 9-2 (rim) which has a Th/U ratio of 0.06. The range of total REE abundances for zircon rims is from 993 ppm to 2 451 ppm. There is significant depletion in LREE, enrichment in HREE (751 ppm–2 444 ppm), positive anomalies in Ce (Ce/Ce*=4–210) and negative anomalies in Eu (Eu/Eu*=0.15–0.51). The (Yb/Gd)CN values range from 17 to 40 (Gra 9-02 pot is 335). The mass fraction of Nb and Ta is low, in the range of 1.50–18.2 and 0.46–3.29 μg/g, respectively, and the Nb/Ta ratios are between 1.1 and 6.0. The range of total REE abundances for the zircon cores is from 437 ppm to 1 209 ppm, which is much less than in the rims. There are significant positive anomalies in Ce (Ce/Ce*=7–100) and negative anomalies in Eu (Eu/Eu*=0.21–0.33), mass fraction of Nb and Ta is low, in the range of 1.56–5.80 and 0.56–2.99 μg/g, respectively, the Nb/Ta ratios are between 1.94 and 5.80, which is also lower than the ratio for the zircon rims.

    The U and Th contents of oscillatory zoned zircons from a granodiorite pebble (Gra 11) are in the range of 19–69 and 13–73 μg/g, respectively, and the Th/U ratios are between 0.66 and 1.45. The range of total REE abundances for zircon rims is from 721 ppm to 1 560 ppm. There is significant depletion in LREE, enrichment in HREE (704 ppm–1 527 ppm), positive anomalies in Ce (Ce/Ce*=9–48) and negative anomalies in Eu (Eu/Eu*=0.29–0.49), and the (Yb/Gd)CN values range from 22 to 47. The mass fraction of Nb and Ta is low, in the range of 1.28–6.19 and 0.45–1.36 μg/g, respectively, and the Nb/Ta ratios are between 2.7 and 6.5. The range of total REE abundances for zircon cores is from 378 ppm to 939 ppm, which is much less than in the rims. There are significant positive anomalies in Ce (Ce/Ce*=51–215) and negative anomalies in Eu (Eu/Eu*=0.07–0.13), mass fraction of Nb and Ta is low, in the range of 1.73–3.30 and 0.69–2.99 μg/g, respectively, the Nb/Ta ratios are between 1.40 and 2.50, which is also lower than the ratio for the zircon rims.

    The REE patterns of zircons with magmatic zoning and inherited cores show different features (Fig. 6). The range of total REE abundances for zircon rims is from 690 ppm to 2 451 ppm, higher than total REE abundances for the zircon cores (98–1 209 ppm). Previous studies have shown that zircons from mantlederived rocks (kimberlites, mantle-derived carbonate, etc.) have low total REE (< 300 ppm) (Hoskin and Ireland, 2000; Li et al., 2000). In this study, the ∑REE (except 98 ppm) for zircon cores indicates same thing that the protoths were probably crustal rocks. The zircon rims have more significant positive anomalies in Ce (Ce/Ce*=4–210) and negative anomalies in Eu (Eu/Eu*=0.15–0.54) than the cores.

    Figure 6.  REE patterns for zircons from weakly deformed granitic pebbles in the basal thrust belt of Miaowan ophiolite.

  • U-Pb ages of zircon from the granite pebbles in the mélange/wildflysch, are in the range of 813±14 and 871±16 Ma (Peng et al., 2012), granodiorite pebbles yield ages of 861±12 and 871±16 Ma, and tonalite cobbles yield an age of 859±26 Ma, whereas monzonitic granite clasts yield an age of 813±14 Ma. Ages of xenocrysitic zircons in the granite cobbles are mainly in the 1 950–2 299 Ma; the eight analyses obtained on inherited cores, fall roughly on the discordia with an intercept age of 2 074±120 Ma (MSWD=15) (Fig. 7). Although the error is great, the data still imply that the granite cobbles in the basal thrust-related wildflysch may have been derived from a Neoproterozoic granitoid massif that intruded and assimilated older xenocrystic zircons from a Paleoproterozoic basement source.

    Figure 7.  Concordia plots showing the results of xenocrystic ages of clasts in granite pebbles from the basal thrust belt.

  • The formation age of non-distorted and weaky deformation granite cobbles with sub-angular and angular shapes, in the basal thrust-related wildflysch, should be earlier than the formation age of the matrix in which they occur. The matrix rocks generally underwent strong ductile-brittle deformation. Mélange/ wildflysch units are closely associated with the overlying Miaowan ophiolite, and are thrust over underlying flysch units, which in turn were deposited on an autochthonous carbonate shelf. Statistical data of foliation, lineation and strike from the basal thrust belt, show that strikes of the shearing foliation and dikes are consistent with the NWW direction strike of the high angle thrust fault in the flysch unit. The direction is the same as the Miaowan ophiolite (Peng et al., 2012, 2010). In addition, the Miaowan ophiolite has the same metamorphic grade (high greenschist-amphibolite facies) as the basal thrust belt, which overrode the passive continental margin. These indicate that the basal thrust belt and Miaowan ophiolite was emplaced on the northern margin of Yangtze craton (Peng et al., 2012; Ye, 2004).

    The zircon ICP-MS U-Pb geochronology of granite cobbles, contained in the metasedimentary of amphibole-plagioclase-gneisses, shows that the protolith formed between 813 and 871 Ma. The implies that the time of deposition in the basal thrust belt was not earlier than 813 Ma (Peng et al., 2012). Peng et al. (2012) also reported the formation time of the Miaowan ophiolite, which represents oceanic crust, in the range from 970 to 1 118 Ma. Therefore, the time of Early-Neoproterozoic ocean-to-continent subduction-collision and orogenic closure, should be later than 970 Ma. So the emplacement age of the Miaowan ophiolite should be slightly later than 813 Ma, and earlier than 786 Ma. The 786 Ma age is the earliest formation time of the majority of the Neoproterozoic Huangling granitoid intrusive complex, which did not experience the penetrative ductile deformation (Zhang et al., 2009; Li Y L et al., 2007; Zhou et al., 2007; Li Z X et al., 2003; Li Z C et al., 2002; Ma et al., 2002; Wang X F et al., 2001; Feng et al., 1991).

  • Geological features of numerous granite cobbles of the wildflysch, which have variable compositions, sizes and shape, show they were derived from a nearby granite. In recent years, a new data have been published on the Neoproterozoic Huangling granitoid intrusive complex, these show that the Huangling batholith is subdivided into four lithological suites (Ma et al., 2002); namely the Huanlingmiao trondhjemite-granodiorite, the Sandouping quartz diorite-tonalite, the Dalaoling monzodiorite-monzogranite, and the Xiaofeng mafic-felsic composite suite. The emplacement age is 832–750 Ma. Li et al. (2002) recalculated the zircon U-Pb data for the granodiorite in the Huanglingmiao unit and tonalite in the Taipingxi unit from Huangling granitoid intrusive complex; the results were 833 and 819 Ma. Li et al. (2007) obtained a cooling age for the tonalite from Huangling granite batholith through 40Ar-39Ar method of biotite and amphibole, the age rangs from 837 to 844 Ma. These geochronological data show that the age of the Huangling granite ranges from 850 to 810 Ma. Recently, on the basis of the detailed 1 : 50 000 regional geological survey mapping in the southern part of the Huangling anticline, Wei et al. (2012) obtained an age of 862 Ma for the Huangling granite by SHRIMP U-Pb zircon dating, which is the oldest formation age of Neoproterozoic Huangling granite intrusive complex so far, providing important geological evidence that the formational age of the granite is older than 850 Ma in Huanlging granite batholith. Although the age fo the granite cobbles in mélange/wildflysch unit are 813–871 Ma, slightly earlier than the age of most of the Huangling granites. It shows that the majority of earlier Huangling granite may have undergone erosion, or have been intruded with melting transformation by the late Neoproterozoic. Therefore, the Neoproterozoic Huangling granite intrusive complex is exposed and preserved currently, may be formed in the middle-late periods.

    The total REE abundances of zircons from the granite cobbles range from 721 ppm to 1 735 ppm, there are significant positive anomalies in Ce (Ce/Ce*=4–47) and negative anomalies in Eu (Eu/Eu*=0.15–0.54), and the (Yb/Gd)CN values range from 17 to 47, massive fraction of Nb and Ta is low, in the range of 1.28–18.2 and 0.34–3.29 μg/g, and the Nb/Ta ratio is between 2.7–6.5, which are similar to the total REE abundances of zircon from Huanglingmiao suit in Huangling granite, ranging from 583 ppm to 1 738 ppm with the significant positive anomalies in Ce (Ce/Ce*=1.41–13.3) and negative anomalies in Eu (Eu/Eu*=0.3–0.8), and the (Yb/Gd)CN values range from 2.2 to 3.6, massive fraction of Nb and Ta is low, in the range of 3.3–23.6 μg/g and 0.6–7.02 μg/g, and the Nb/Ta ratio is between 2.2–10.5 (Zhang et al., 2010; Ma et al., 2002). This implies that the granite cobbles in the basal thrust belt were likely derived from the Huanglingmao unit of the Huangling granite intrusive complex.

    The Yangtze craton has a complex evolution history (Zhao et al., 2010; Li et al., 2008; Zheng and Zhang, 2007; Ling et al., 2003; Chen et al., 2001; Wang J et al., 2001; Gao et al., 1999). The Kongling complex consists of Archean–Proterozoic rocks of Yangtze block, including Archean TTG gneisses, migmatite, metasedimentary rocks, and a small amout of lenticular amphibolite and mafic granulites (Qiu et al., 2000; Gao et al., 1999). Gao et al. (2001) and Qiu et al. (2000) obtained the 1.9–2.0 Ga metamorphic age for metasedimentary rocks. Xiong et al. (2008) and Fu et al. (1993) obtained U-Pb zircon dates on feldspar granite, located at Quanyitang, intruded into Archean TTG gneisses, with 1 840 and 1 850 Ma, suggesting that the Kongling complex experienced tectonicmetamorphic-magmatic events in the Paleoproterozoic (Zhang et al., 2006; Ling et al., 2000), The events may be related to the assembly and break-up of the Columbia supercontinent (Peng et al., 2009; Xiong et al., 2008). Zhang et al. (2009) also obtained ages of 1 997 and 1 840 Ma for inherited zircon cores from the Huanglingmiao unit in the Huangling granite intrusive complex. The age of xenocrysitic zircons in the granodiorite-tonalite cobbles in the basal thrust related-wildflysch, is 2 074 Ma, implying that the protolith of Neoproterozoic Huangling granitoid intrusive complex may have originated from basement rocks, implying that there may be Paleoproterozoic crystalline basement in the south Huangling anticline. The ages of xenocrystic zircons from the granite pebbles in the basal-thrust conglomerate/wildflysch show a correlation with the age spectra from Australia (Condie et al., 2009), implying that the terrane that collided with the northern margin of the Yangtze craton and emplaced the Miaowan ophiolite at circa 813 Ma may have been derived from the Australian segment of Rodinia (Peng et al., 2012).

  • (1) Metasedimentary rocks series of formerly Miaowan Formation of the Kongling Group, is actually a basal thrust belt with thrust-nappe-strle movements in a NNE-SSW direction. This sequence can be divided into three tectono-lithostratigraphic units from north to south: mélange/wildflysch, flysch, and sedimentary rock of the autochthonous stable continental margin.

    (2) The formation time of the basal thrust belt and age of structural deformation is in the range of 813–786 Ma.

    (3) Comparing the aspects of lithology, zircon geochronology, REE patterns between non-distorted granodiorite, tonalite pebbles of near sources rapid deposition, in the basal thrust related- wildflysch, and Huangling granite intrusive complex, it can be concluded that the pebbles are the most similar to Huanlingmiao trondhjemite-granodiorite, implying that they may come from the Huanglingmiao unit. Zircon cores yield 2 074 Ma of relic age, suggesting the prototh of Neoproterozoic Huangling granitoid intrusive complex may have originated from Paleoproterozoic crystalline basement in the south Huangling anticline.

  • Thanks to Mr. Yunxu Wei from Wuhan Institute of Geology and Mineral Resources, who provided the latest geochronology information for Huangling granite batholith, Prof. Ali Polat, from Windsor University in Canada, guided the thin section identification, and Prof. Yuanbao Wu from China University of China, gives the advices about U-Pb zircon geochronology. Thanks to Prof. Paul T Robinson, from the Dalhousie University in Canada, for his helpful suggestions.

    This study was supported by the Postdoctoral Science Foundation (No. 20100471203), the Ministry of Land and Resources of China (No. 1212010670104), the National Natural Science Foundation of China (Nos. 91014002, 40821061, 41272242), and Ministry of Education of China (Nos. B07039, TGRC201024).

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