The Archean North China craton is composed of the Western block, Eastern block, and the intervening Central orogenic belt. A 4–10-km-wide and 85-km-long tectonic mélange belt informally called the Zanhuang tectonic mélange is documented in the Zanhuang Massif of the Central orogenic belt, separating the Eastern block from an Archean arc terrane in the Central orogenic belt. The mélange belt contains a structurally complex tectonic mixture of metapelites, metapsammites, marbles, and quartzites mixed with exotic tectonic blocks of volcanic, mafic, and ultramafic rocks, metabasalts that locally include relict pillow structures, and tonalite-trondhjemite-granodiorite (TTG) gneisses. The Zanhuang tectonic mélange marks the suture of an arc-continent collisional zone between the Western Zanhuang Massif in the Central orogenic belt and the Eastern block of the North China craton, and it is one of the best-preserved Archean tectonic mélanges in the world. Here, using zircon U-Pb dating of various types of blocks from the Zanhuang mélange, we show that the formation and associated deformation of the Zanhuang mélange occurred in the Neoarchean (ca. 2.5 Ga). High-precision (1:20–1:200) lithostructural mapping of three key outcrops reveals details of the internal fabrics and kinematics of the mélange and regional structural relationships along the arc-continent collisional zone. A synthesis of studies on the tectonic evolution of the North China craton, coupled with our new fabric and kinematic analysis of the Zanhuang mélange, further constrains the initial amalgamation timing and geometry of the arc-continent collision between the Fuping arc terrane in the Central orogenic belt and the Eastern block with a northwest-dipping subduction polarity. The asymmetric structures and mixture of different blocks and matrices with folding and thrusting events in the Zanhuang mélange record kinematic information that is consistent with the tectonic setting of an accretionary wedge that was thrust over the passive margin of the Eastern block by 2.5 Ga. Lithostructural mapping shows that the classic mélange and fold-and-thrust structures along the Neoarchean arc-continent collisional zone are broadly similar to Phanerozoic collisional belts.
The definition of “mélange” and the process of mélange formation have been debated for several decades since the term was first proposed by the British geologist Edward Greenly for the “Gwna Group” of the Mona complex in Anglesey, north Wales (Greenly, 1919). In a general sense, mélanges are considered to represent “mappable geological units consisting of blocks of different ages and origin, commonly embedded in an argillitic, sandy, or serpentinite matrix showing high stratal disruption and chaotic internal structures” (Cowan, 1985; Dilek and Thy, 2006; El Bahariya, 2012; Festa et al., 2010; Hsü, 1968, 1969; Raymond, 1984; Silver and Beutner, 1980; Bradley and Kusky, 1992; Kusky and Bradley, 1999; Wakabayashi and Dilek, 2011; Wakabayashi and Medley, 2004). The origins of mélanges are currently considered either from tectonic deformation, sedimentary processes, diapirism, or a combination of these processes (Festa et al., 2010; Raymond, 1984). Understanding the formation processes and significance of mélanges associated with collision-accretion in the geological record is significant in documenting the tectonic evolution of mountain belts. The Zanhuang mélange is a well-preserved Neoarchean tectonic mélange that marks the suture zone between two major tectonic units in the North China craton (Deng et al., 2013, 2014; Wang et al., 2013, 2015). The Zanhuang mélange contains a structurally complex tectonic mixture of metapelites, metapsammites, marbles, and quartzites mixed with exotic tectonic blocks of ultramafic and metagabbroic rocks, metabasalts that locally include relict pillow structures, and tonalite-trondhjemite-granodiorite (TTG) gneisses (Wang et al., 2013).
We previously proposed that the Zanhuang mélange represents the Neoarchean suture zone between the Fuping arc to the west and the Eastern block of the North China craton to the east, with the Eastern block representing a continent capped by a passive margin and foreland basin sequence (Wang et al., 2013). In order to better document the structural relationship between the Fuping arc and the Eastern block continent, we undertook a detailed field investigation along the Neoarchean arc-continent collisional zone with particular focus on analysis of fabrics and kinematics in typical outcrops of the Zanhuang mélange. In this paper, we provide two new detailed transects that cut across all the units of the study area and present comprehensive new 1:20–1:200 high-precision field maps, together with kinematic and geochronologic analyses. The high-precision (1:20–1:200) lithostructural mapping of key outcrops reveals the internal fabrics and kinematics of the Zanhuang mélange and contributes to understanding of the structural relationships along the arc-continent collisional zone, ancient plate convergence, and the Neoarchean tectonic evolution of the North China craton.
The North China craton is surrounded by the Central Asian orogenic belt to the north, the Qinling-Dabie belt to the south, the Qilian orogen to the west, and the Sulu belt and Jiaoliao belt to the east (Fig. 1A; Bai and Dai, 1996; Kusky, 2011; Kusky et al., 2007a). The North China craton consists of two major Archean blocks, the Western block and the Eastern block, separated by the intervening Central orogenic belt (Fig. 1A; Gong et al., 2014; Kusky and Li, 2003; Kusky et al., 2001, 2004, 2007a, 2007b, 2014; Liu et al., 2004; Polat et al., 2005, 2006; Santosh, 2010; Wang et al., 2013), which partly overlaps in space with the so-called Trans–North China orogen (Zhao, 2001; Zhao et al., 1999, 2000a, 2000b, 2001, 2005). The poorly exposed Western block is a typical stable Archean platform with the characteristics of a thick mantle root, low heat flow, and few earthquakes (Kusky and Mooney, 2015); it is very different from the Eastern block, which has high heat flow and more earthquakes, resulting from the root loss of the lithosphere in the Mesozoic beneath the Eastern block of the North China craton (Gao et al., 2004). The Central orogenic belt is defined by Archean structural boundaries (Kusky, 2011; Kusky and Li, 2003; Kusky et al., 2007a) and is generally considered to record the accretion of an arc terrane to the Eastern block, followed by younger amalgamation of the Western block to this tectonic collage, and it is an ideal orogenic belt in which to study the tectonic evolution of the whole North China craton. The Paleoproterozoic Inner Mongolia–Northern Hebei orogen (Kusky and Li, 2003) stretches along the northern margin of the craton and represents an arc system transformed into a continent-continent collision zone during the amalgamation of the North China craton with the Columbia (Nuna) supercontinent. This collision was responsible for the widespread ca. 1.9–1.85 Ga high-grade metamorphic event across the craton (Kusky et al., 2007b; Peng et al., 2014; Wan et al., 2015). There is considerable debate about the patterns of collision, nature of different tectonic blocks, timing of amalgamation, and subduction polarity during the late Archean and Paleoproterozoic tectonic events. TTG gneisses, ca. 2.5 Ga and 2.1 Ga granites, greenschist to granulite facies supracrustal sequences, passive margin to foreland basin sedimentary rocks, and 2.5–2.4 Ga and 1.9–1.7 Ga mafic dike swarms are well exposed and mainly crop out in the Hengshan Massif, Wutaishan Massif, Fuping Massif, Lüliangshan Massif, and Zanhuang Massif (Fig. 1B; Deng et al., 2013, 2014; Kröner et al., 2006; Kusky, 2011; Kusky and Li, 2003; Kusky et al., 2007b; Peng et al., 2014; Y.J. Wang et al., 2003; J.P. Wang et al., 2013; Xiao and Wang, 2011). In this contribution, we focus on the geometric and kinematic properties of the Neoarchean suture zone between the Fuping arc terrane and the Eastern block of the North China craton in the Zanhuang Massif.
ZANHUANG MASSIF GEOLOGY
The Zanhuang Massif, situated at the eastern margin of the Central orogenic belt (Fig. 1B), is one of the significant areas in which to study the collisional orogenesis and structural relationship between the Eastern block and the Central orogenic belt of the North China craton. The internal deformation within the Zanhuang Massif is complex and multistaged, with overturned folds, multiphase folds, and recumbent folds widely developed (Niu et al., 1994). Based on geochronologic studies on the deformation-related thermal events using the biotite 40Ar/39Ar method, Wang et al. (2003) identified four stages of deformation and suggested that the Zanhuang Massif was a metamorphic dome formed in late stages of the Early Proterozoic. Trap et al. (2009) divided the Zanhuang Massif into the Western Zanhuang domain, Central Zanhuang domain, and Eastern Zanhuang domain. Recently, Wang et al. (2013, 2015) and Deng et al. (2013, 2014) described a Neoarchean tectonic mélange that represents the suture zone between the TTG gneiss of the Western Zanhuang domain and the marble-siliciclastic unit deposited on the western margin of the Eastern Zanhuang domain. Many Precambrian rocks are exposed in the Zanhuang Massif, among which the Archean rocks are the most abundant (Fig. 1C). Lithologically, the main rock types are ca. 2.7 Ga TTG gneisses, mainly exposed in the Western and Eastern Zanhuang domains, felsic gneiss, garnet-bearing amphibolite, ultramafic rocks, and a marble-siliciclastic unit that mainly crops out in the Central Zanhuang domain. These rocks are intruded by the ca. 2.5 Ga Wangjiazhuang granitic pluton and undeformed pegmatite (Fig. 1C; Wang et al., 2013; Xiao et al., 2011). All the units in the study area are intruded by a series of mafic dikes formed in a suprasubduction zone setting (Wang et al., 2013; Deng et al., 2013, 2014). Detailed descriptions of the lithologies of the Zanhuang Massif are presented in Wang et al. (2013).
STRUCTURAL TRANSECTS AND TARGETED HIGH-RESOLUTION MAPPING
Wang et al. (2013) presented two interpretive cross sections through the Western Zanhuang domain and Central Zanhuang domain to the Eastern Zanhuang domain of the Zanhuang Massif (Fig. 1C). The regional structures show that the TTG gneisses of the Western Zanhuang domain are thrust to the southeast upon the Zanhuang mélange units, and the mélange unit is thrust to the southeast upon the marble-siliciclastic unit, and there are many internal top-to-the-southeast thrusts within the Central Zanhuang domain (Fig. 1C; Wang et al., 2013). Circa 2.5 Ga granite and pegmatite intrude the mélange unit, and late mafic dikes crosscut all the units of the study area and are not deformed by the penetrative fabric in the mélange (Deng et al., 2013, 2014; Wang et al., 2013, 2015). In this paper, we present the results of new detailed field mapping of the Neoarchean Zanhuang tectonic mélange in the northeastern part of the study area (Fig. 1C). Two transects (transects A-B and C-D), one profile (profile E-F), and high-precision (1:20–1:200) lithostructural mapping of three main mélange outcrops (outcrops 1, 2, and 3) were completed. Transects A-B and C-D cut through the Western and Central Zanhuang domains to the Eastern Zanhuang domain (Fig. 1C). The locations of transect A-B, transect C-D, profile E-F, and outcrops 1, 2, and 3 are provided in Figures 1C and 2A.
The lower part of outcrop 1 was divided into 1 m by 1 m square grids. Each grid block was mapped individually and drawn on paper with 10 cm by 10 cm square grids, which means that the scale for outcrop mapping is one to ten. Outcrop 2 was also divided into 1 m by 1 m square grids. However, each grid of outcrop 2 was mapped on a paper with 5 cm by 5 cm square grids, which means the mapping scale of outcrop 2 is one to twenty. All the grids were combined together and redrawn using Corel DRAW software after finishing the sketches for each grid.
Transects A-B and C-D
Transects A-B and C-D are located in the northeastern part of the study area and cut through the Western Zanhuang domain, Central Zanhuang domain, and Eastern Zanhuang domain (Figs. 1C and 2B). Transect A-B is located near Zhangme village (Fig. 1C), whereas transect C-D is located between Zhaozhuang and Guandu villages (Fig. 1C). From northwest to southeast, the main lithotectonic units are composed of TTG gneisses of the Western Zanhuang domain, the Zanhuang mélange, the marble-siliciclastic unit, and TTG gneisses of the Eastern Zanhuang domain. The TTG gneisses of the Western Zanhuang domain have a protolith age of 2692 ± 12 Ma, have experienced partial melting (Trap et al., 2009; Yang et al., 2013), and have been suggested to represent an arc setting (Fuping arc terrane; Trap et al., 2009). The contact of the Western Zanhuang domain TTG gneisses with the Zanhuang mélange is a thrust with southeast-directed movement of the hanging wall (Fig. 2A). The TTG gneisses of the Eastern Zanhuang domain with late Archean ages are considered to represent a continent block (Kusky and Li, 2003; Wang et al., 2013, 2015). The Zanhuang mélange in between separates the marble-siliciclastic unit deposited on the northwestern side of the Eastern and Western Zanhuang domains (Fig. 2A). The Zanhuang mélange is composed of blocks of marble, and mafic and volcanic rocks distributed in different kinds of matrices including metapelites and locally serpentinite-talc schist (Wang et al., 2013). A ca. 2.5 Ga granitic pluton and undeformed pegmatites cut all the units of the Zanhuang mélange, providing the minimum age for the Zanhuang mélange (Wang et al., 2013).
A detailed 2-km-long profile (profile E-F) has been completed through the marble-siliciclastic unit (Fig. 2B) deposited on the westernmost margin of the Eastern Zanhuang domain. The marble unit and mica schist and paragneiss unit of the marble-siliciclastic unit were imbricated several times during emplacement of the Fuping arc over the Eastern block along the Zanhuang mélange (Figs. 2A and 2B). Mafic dikes intrude the paragneiss and marble and are parallel to the bedding of the paragneiss and the marble layers in the marble-siliciclastic unit (Figs. 3A–3C). Mafic blocks with epidosite cores are dispersed in various types of matrices, including amphibolite (Fig. 3D), metapelite (Fig. 3E), and paragneiss (Fig. 3F), in several locations of the Zanhuang mélange. The pillows are preserved as decimeter- to meter-size epidosite lenses (Fig. 3E). The amount of epidosite lenses increases from the southwest to the northeast in the Zanhuang mélange, and the best outcrops are located in the northeastern section of the Zanhuang mélange near Guandu (Fig. 1C). Deng et al. (2014) have documented that the crystallization age of an undeformed mafic dike in the Zanhuang mélange is 2535 ± 30 Ma age, providing the minimum age for the marble-siliciclastic unit, which is consistent with a Neoarchean age for the Zanhuang mélange (Wang et al., 2013).
High-Precision Mapping of Three Main Mélange Outcrops
High-precision lithostructural mapping of three main mélange outcrops (outcrops 1, 2, and 3) was completed from the two new transects based on detailed field analysis (Figs. 4 and 5). Outcrop 1 is located at the northwestern end of transect A-B, which is close to the margin between the Zanhuang mélange and TTG gneiss of the Western Zanhuang domain (Figs. 1C and 2A). The outcrop is 52 m long and 8 m high (Fig. 4B), of which the lower part has been mapped in detail. Outcrop 2 is contiguous with outcrop 1, but the lower parts of the section are separated by a concrete wall (Fig. 4A). Outcrop 2 is 54 m long and 7 m high (Figs. 4A and 4C). The spatial relationship between outcrop 1 and outcrop 2 is shown in Figure 4A. Combined with the concrete wall in between, outcrops 1 and 2 constitute a continuously vertical profile, which is one part of transect A-B (Fig. 4A). Outcrops 1 and 2 represent two key locations between the Western Zanhuang domain and the Zanhuang mélange (Fig. 1C). Outcrop 3 is located in the southeast section of the Zanhuang mélange in transect C-D, representing another critical location between the Zanhuang mélange and the Eastern Zanhuang domain (Figs. 1C and 2A). Outcrop 3 is ∼50 m long and 10 m high (Fig. 5). The three outcrops best reflect the structural relationships along the two margins of the Zanhuang mélange. Each outcrop was divided into 1 m by 1 m grids, and each grid block was mapped individually.
Composition of Blocks and Matrices
Outcrop 1 is composed of a complex tectonic mixture of blocks of different origin in metapelitic and metapsammitic matrices (Fig. 4B). The blocks and matrices are strongly deformed. Felsic blocks of mica-bearing syenogranite and biotite quartz schist (Figs. 6A and 6B) are present in metapelitic and gneissic metasedimentary matrices. Blocks of mica-bearing syenogranite and biotite quartz schist in outcrop 1 have been sampled for zircon U-Pb geochronologic analysis to constrain the protolith age of the Zanhuang tectonic mélange belt. Biotite is strongly concentrated and oriented (Fig. 6C) in the syenogranitic blocks possibly due to deformation during mélange formation or emplacement over the continental basement. More felsic blocks and quartzofeldspathic veins occur in outcrop 2 than in outcrop 1 (Fig. 4C). Mafic, ultramafic, and felsic blocks are present in outcrop 2 (Figs. 6D and 6E). All the units and fabrics in outcrop 2 are crosscut by late mafic dikes (Fig. 6D). Blocks of mafic blocks with epidosite cores are widely distributed in metapelitic and metabasic matrices (Fig. 5). Both blocks and matrices are strongly deformed and mixed, with preferred northwest-southeast orientation of the long axis of the basaltic blocks (Fig. 5). The mafic blocks are highly altered to epidosite and were sampled for zircon U-Pb geochronology to constrain the protolith age of the mélange.
Folds are widespread in different rock types, and they range from a few centimeters to a few meters (Figs. 6F–6H). Most of these folds are asymmetric and preserved as rootless folds (Fig. 6H) showing high shear strains, and their vergence can be used to represent the overall sense of shear (Cowan and Brandon, 1994). Many quartzofeldspathic veins are folded in metapelitic matrices (Fig. 6G). The occurrence of these quartzofeldspathic veins outlines fold geometry in outcrop 2 (Fig. 4C), showing a consistent pattern of northwest-dipping thrusts. The folded quartzofeldspathic veins on both sides of the main thrust fault in outcrop 2 have a similar regularity of folding (Fig. 4C), suggesting that the folding and thrusting events were synchronous. The axial planes of these folds generally dip to the northwest. Hinges are generally horizontal. Based on high-resolution mapping of these three typical outcrops, at least two generations of faults formed during development of the mélange. The fault planes for both of the generations consistently dip to the northwest (Fig. 4). However, the faults have slight differences in dipping angles, with one generation over 45° and the other one less than 45°. The faults cut different kinds of blocks and matrices (Figs. 4 and 5) and folds (Fig. 4C), suggesting that the folds and faults formed contemporaneously during the mélange formation process. Late normal faults dipping to the southeast cut the older thrusts and earlier fabrics of mélange (Fig. 5). The whole tectonic mixture of different blocks and matrices was sheared together with a top to the southeast sense of movement, resulting in the complex mélange structure. The description and kinematics of mélange fabrics will be discussed in detail in the “Description of Mélange Fabrics and Kinematics” section.
PETROGRAPHY AND GEOCHRONOLOGY OF SYENOGRANITIC, SCHIST, AND EPIDOSITIC BLOCKS
Sampling and Petrography
Three samples (14XT-3a, 14XT-4, 14XT-7-3) of different blocks from the Zanhuang mélange belt were analyzed for zircon U-Pb dating to constrain the formation age of the Zanhuang mélange. One sample of mica-bearing syenogranite (14XT-3a) and one sample of a biotite quartz schist block (14XT-4) were collected from outcrop 1 in transect A-B for zircon U-Pb analysis. The mica-bearing syenogranite block and biotite quartz schist block are close to each other (Fig. 6A) and dispersed in metapelitic matrices (Fig. 4B). One sample of epidosite (14XT-7-3) was collected from outcrop 3 in transect C-D for zircon U-Pb analysis. The blocks of epidosite are dispersed in a metapelitic matrix (Fig. 5). The locations of these three samples are shown in Figures 2A and 4B.
The mica-bearing syenogranite is white in field observation with strong deformation. The main minerals include quartz (60%–65%), feldspar (30%–35%), mica (5%–10%), and minor epidote, zoisite, and accessory magnetite (Fig. 7A). Feldspar grains are mostly microcline (25%–30%) with typical gridiron twinning and plagioclase (5%–10%) with polysynthetic twinning. The mica mainly includes biotite with a small amount of muscovite. Most of the plagioclases are variably sericitized. The biotite quartz schist is gray and preserved as blocks in outcrop 1. The mineral assemblage of biotite quartz schist (Fig. 7B) is dominated by quartz (60%–65%), biotite (25%–30%), and plagioclase (10%–15%). Based on the outcrop relationships, the epidosites are preserved as relict hard cores of mafic blocks (Figs. 3D–3F and 5). The epidosites (Fig. 7C) are green and characterized by main mineral assemblages of quartz (45%–50%) and epidote (50%–55%).
Zircon separation from the three samples was completed at the Rock and Mineral Sorting Technical Services Company in Langfang City of Hebei Province. Zircons were extracted by using conventional heavy liquid and magnetic separation techniques. Cathodoluminescence (CL) photomicrographs of zircons were taken by Gatan MonoCL4+ in the State Key Laboratory of Geological Processes and Mineral Resources (GPMR) of the China University of Geosciences (CUG), Wuhan, to reveal their internal structures before the U-Pb dating. The working conditions for zircon CL imaging were set to be 10 kV with a spot size of ∼5 μm and working distance of ∼14 mm (Wang et al., 2014).
The U-Pb dating and trace-element analyses of zircons were performed using the laser ablation–inductively coupled plasma–mass spectrometer (LA-ICP-MS) at the GPMR-CUG with specific test conditions presented in Liu et al. (2010). Laser sampling was performed using a GeoLas 2005 with a spot size of 32 μm. Laser repetition rate was set at 6 Hz with energy density of 60 mJ. Each analysis incorporated a background acquisition of ∼20–30 s (gas blank) followed by 50 s data acquisition from the sample. The Agilent Chemstation was utilized for the acquisition of each individual analysis. The standard sample zircon 91500 was used for U-Pb dating, and it was analyzed twice every five analyses. The standard zircon sample GJ-1 was also analyzed for zircon U-Pb dating. NIST610 was also applied to correct the time- dependent drift of sensitivity and mass discrimination for the trace-element analysis. ICPMSDataCal (Liu et al., 2010) and Isoplot/Ex_ver3 (Ludwig, 2003) were used for the off-line selection and integration of background and analytical signals, and time-drift correction and quantitative calibration. The weighted mean 207Pb/206Pb is used for the final results with 2σ error (95% confidence level).
Zircon U-Pb Geochronology
The zircons for U-Pb dating were taken from samples 14XT-3a, 14XT-4, and 14XT-7-3 (see sample locations in Figs. 1C and 2A). Zircons from the three samples are mostly clear and euhedral grains that show internal oscillatory zoning structures in CL images (Figs. 7D–7F), and no inherited cores were present, indicating that these zircons have a magmatic origin. The content of Th in the zircons from all three samples is low (Table 1; 0.54–13 ppm); however, the content of U is relatively high (Table 1), resulting in the dark CL images (Figs. 7D–7F; Wu and Zheng, 2004). In general, metamorphic zircons have relatively low Th/U ratios (<0.1). However, some magmatic zircons with rather low Th/U ratios (<0.1) have been reported in many places of the world (Hidaka et al., 2002; Wu and Zheng, 2004). Therefore, we suggest that the analyzed zircon grains point to a magmatic origin. The detailed zircon U-Pb dating and trace-elements results are given in Table 1 and Table DR1, respectively.1 The negatively discordant analyses plotted above the concordia line (Figs. 7D–7F) are due to Pb diffusion during the metamictization process of zircons (Kusiak et al., 2013; Pidgeon et al., 2013). In addition, the values of mean squared weighted deviation (MSWD14XT-3a = 1.9; MSWD14XT-7-3 = 2.1) of the all the dated samples are small, implying that the age data experienced weak influence from later tectonic events, i.e., the age data are credible (Wendt and Carl, 1991).
Zircon grains from 14XT-3a range from 100 to 500 μm in length and 100–300 μm in width. These zircons have variable Th and U contents of 1.88–4.77 ppm and 297–782 ppm and low Th/U ratios of 0.0025–0.0161 (Table 1) and are characterized by light rare earth element (LREE)–depleted and heavy rare earth element (HREE)–enriched REE patterns with negative to positive Eu anomalies and positive Ce anomalies (Fig. 8A). Twenty-two available spot analyses were conducted on zircons from this sample. Eighteen U-Pb dating results of sample 14XT-3a are located on or near Concordia and within analytical errors (Fig. 7D). The weighted mean 207Pb/206Pb age for the 18 measuring points yields an age of 2529 ± 12 Ma (Fig. 7D; MSWD = 1.9). Four analysis spots due to Pb loss (Fig. 7D) were not calculated into the weighted mean.
The zircons of sample 14XT-4 range from 100 to 400 μm in length and 50–150 μm in width. Zircons from 14XT-4 are characterized by variable Th and U contents of 0.54–13.1 ppm and 49.1–646 ppm and low Th/U ratio of 0.0105–0.0355 (Table 1). Their chondrite-normalized diagram displays depleted LREE distributions and relatively enriched HREE patterns with negative Eu anomalies (Fig. 8B). The zircons from this sample are interpreted as detrital zircons. Twenty-three detrital zircon spots were analyzed and show that the age of the protolith’s provenance are from two sources that have ages at ca. 2.5 Ga and 2.75 Ga, and the main source has an age of ca. 2.5 Ga (Fig. 7E).
Zircons from 14XT-7-3 have a length ranging from 50 to 200 μm and width ranging from 40 to 100 μm. These zircons have variable Th and U contents of 0.79–8.91 ppm and 28.6–652 ppm, with low Th/U ratios of 0.0130–0.0334. On chondrite-normalized diagrams, they are characterized by negative LREE distributions and enriched and relatively flat HREE patterns with marked negative Eu anomalies (Fig. 8C). Twenty-three spot analyses on these zircons are concordant and yield a weighted mean 207Pb/206Pb age of 2496 ± 21 Ma (Fig. 7F; MSWD = 2.1). One spot is located under the concordant line because of Pb loss (Fig. 7F).
DESCRIPTION OF MÉLANGE FABRICS AND KINEMATICS
Description of Mélange Fabrics
The mélange rocks are characterized by chaotic mixtures of metapelites, metapsammites, mica schist, paragneisses, and amphibolite as matrices, and exotic mafic and metagabbroic rocks and metabasalts as clasts, representing “block in matrix” (Silver and Beutner, 1980) fabrics. Clasts of marble and quartzite also occur, although they are not as common as the mafic and metabasaltic clasts. Fabrics in the Zanhuang mélange are asymmetric and consist of folds, a scaly cleavage in the metapelitic matrices (Fig. 9A), sheared gneissic and felsic clasts with tails (Figs. 9B and 9C), and sets of subparallel, small-offset faults (Figs. 9D–9F). The felsic clasts are sheared and exhibit a shape preferred orientation. The subparallel and small-offset faults are widespread in the Zanhuang mélange and are spaced at ∼10–20 cm apart (Figs. 9D–9F). These faults are subparallel to the foliations in the mélange and are generally along the tops and bottoms of these clasts, resulting in the asymmetric shapes of clasts (Figs. 9D–9F). The mélange foliation, preferred orientation of long axes of asymmetric clasts, and sets of faults represent fabrics that are similar to the S-C-C′ fabric described in other mélanges (e.g., Kano et al., 1991; Kusky and Bradley, 1999; Fukui and Kano, 2007) and higher-grade and ductile mylonitic rocks (e.g., Berthé et al., 1979; Lister and Snoke, 1984; Passchier and Trouw, 1996). S-C-C′ type fabrics are also common in brittle fault gouge zones, where S planes (foliation) and C- and C′-type shears correspond to P foliation and Y and R1 (Riedel) shears, respectively (e.g., Rutter et al., 1986; Chester and Logan, 1987). The Zanhuang mélange is not a fault gouge zone produced by mechanical milling (Cloos, 1983), so we prefer the S-C-C′ terminology to describe the fabrics in the Zanhuang mélange. Figure 10 illustrates the three-dimensional geometric relationships between the different planar elements. The C planes are typically characterized by sets of small-scale faults parallel to the overall shear (transport) direction. The S planes are represented by the shape-elongation of the clasts parallel to the anastomosing mélange foliation that is defined by the compositional layering. The angle between the C plane and S plane is less than 45° (Fig. 10). Some discontinuous shear zones (P planes) formed parallel to the mélange foliation (S planes; Fig. 10). The C and S planes are offset by small extensional faults (C′ planes; Fig. 10). These extensional faults are R1 (Riedel) shears, and they have the same shear sense of offset as the C planes (Fig. 10). The intersections of any group of S-C, C′-S, or C′-C planes can uniquely determine the sense of shear in the case of simple shear and plane strain (e.g., Krantz, 1988; Kusky and Bradley, 1999). The sense of shear or transport direction (slip vector) is 90° away from the intersections within the C surface on a lower-hemisphere Schmidt diagram after plotting any group of S-C, C′-S, or C′-C planes (Fig. 10).
Kinematics of Mélange Fabrics
Detailed structural data were collected from the most conspicuous part of each outcrop where asymmetric fabrics were clearly visible, and the shear sense was determinable across almost all units of the Zanhuang mélange. The following asymmetric fabric data were used for kinematic analysis: (1) attitudes of S, C, and C′ planes; (2) lineations on these planes; (3) attitudes of axis planes and hinges of folds; and (4) long-axis trends of different clasts. After measurement of these fabric elements in the Zanhuang mélange, data were plotted as a series of lower-hemisphere, equal-area projections (Figs. 11A–11D). All the planar elements, including S planes (S), C planes, and C′ planes, show similar overall northwest-dip orientations, with moderate (∼30°–60°) dip angles (Fig. 11A). The orientation patterns of S, C, and C′ planes show a dominant northwest-dip in most of the studied outcrops (Fig. 9). The poles of axial surfaces show general northwest-dip orientation (Fig. 11B). The lineation on all the planes generally plunges northwest, recording the slip vector direction (Fig. 11C). The fold hinges are folded about a northwest-striking and plunging axis, consistent with fold hinges being deformed in a semisheath manner with northwest-southeast transport (Fig. 11D). The most credible data were used to calculate the slip vectors, assuming that the transport directions were on the C planes and oriented 90° from the intersections between S planes and C planes as shown in Figure 10 (e.g., Moore, 1978; Krantz, 1988; Kusky and Bradley, 1999). The lower-hemisphere, equal-area projections from different locations are placed on the geological map (Fig. 1C). For each of the projections, the great circles for C planes are plotted as solid lines, and S or C′ planes are plotted as dashed lines (Fig. 1C). The slip vector directions are plotted as arrows pointing in the direction of slip of the hanging wall (Fig. 1C). All the data from different kinematic stations show that the Zanhuang tectonic mélange belt preserves generally northwest-southeast–directed slip vectors formed during its formation and emplacement on the continental margin. In conclusion, we suggest the overall shear sense associated with formation of the Zanhuang mélange was directed northwest to southeast.
Timing of Deformation of the Zanhuang Mélange—New Data and a Revisited Study
Zircon grains from the mica-bearing syenogranitic block (14XT-3a) and epidosite block (14XT-7-3) are characterized by good oscillatory zoning (Figs. 7D–7F) and fractionated REE patterns with positive to negative Eu anomalies and positive Ce anomalies (Fig. 8), which are typical characteristics of igneous zircons (Corfu et al., 2003; Rubatto, 2002). Based on field observation that the epidosites are preserved as cores of mafic blocks, and they show typical relict pillow structures in places, we suggest that the epidosites were altered from pillow lavas under the condition of strong seafloor hydrothermal activity (e.g., Wang et al., 2012). Hence, we interpret the zircon ages from the two samples to represent the protolith ages of these rocks. Zircon grains from a biotite quartz schist block (14XT-4) are detrital zircons and are interpreted to represent its protolith’s detrital provenance, which mainly indicates ages of ca. 2.5 Ga and 2.75 Ga (Fig. 7E). Circa 2.5 Ga granitic rocks and 2.7 Ga TTG gneisses have been reported in the study area (Wang et al., 2015; Yang et al., 2013), and ages of 2.75 Ga are known from the Eastern block of the North China craton (Kusky et al., 2007b). These similar-aged rocks possibly provided the provenances for the metasedimentary rocks. In this paper, the weighted mean 207Pb/206Pb ages of the mica-bearing syenogranitic block and mafic block with epidosite core are 2529 ± 12 Ma and 2496 ± 21 Ma, respectively. These weighted mean 207Pb/206Pb ages represent the protolith ages of different kinds of blocks in the Zanhuang mélange. The different blocks in the Zanhuang mélange are suggested have been produced by off-scraping during the processes of subduction and obduction and were later imbricated with the passive continental shelf sequence during emplacement of the mélange complex (e.g., Kusky et al., 2013; Wakabayashi and Dilek, 2011; Wakabayashi and Medley, 2004). The youngest age of the dated blocks is 2496 ± 21 Ma, providing the maximum age for the formation of the mélange.
Previous field observations showed that the Zanhuang mélange was intruded by the Wangjiazhuang granite and crosscut by undeformed pegmatites (Wang et al., 2013). The foliations in the Zanhuang mélange generally strike northeast and dip northwest (Fig. 1C), but the strike of the foliations is rotated into near-parallelism with the pluton margins. In addition, the late Wangjiazhuang granitic pluton truncates the early foliations in the mélange at the northeast and southwest margins of the pluton. A weak foliation defined by the preferred orientation of feldspar grains and elongation of quartz ribbons near the margins of the pluton is interpreted to represent a post-emplacement-related magmatic foliation that mimics the shape of the pluton margins (e.g., Paterson and Tobisch, 1988; Paterson and Vernon, 1995). The zircon U-Pb ages for all dated samples from the Wangjiazhuang granite are 2517 ± 20 Ma (13XT17-1), 2506 ± 10 Ma (13XT19-1), 2513 ± 13 Ma (13XT22-1), 2493 ± 22 Ma (91-1b), and 2540 ± 23 Ma (168-1) (Wang et al., 2013, 2015). The zircon U-Pb age for the undeformed pegmatite is 2539 ± 44 Ma (76-5c; Wang et al., 2013). The youngest formation age of all the dated Wangjiazhuang granites and pegmatites is 2493 ± 21 Ma (Wang et al., 2013, 2015), providing the minimum age for the formation of the Zanhuang mélange. In conclusion, we suggest the formation, and associated deformation, age of the Zanhuang mélange is Neoarchean (ca. 2.5 Ga).
Significance of Mélange Fabrics and Relationship to Plate Convergence Directions
Although mélanges have been traditionally regarded as indecipherable mixtures of different rock types from different origins in a chaotic matrix, asymmetrical mélange fabrics can be taken as indicators of the style of mélange formation processes and directions of ancient subduction zones and convergent plates (e.g., Kano et al., 1991; Bradley and Kusky, 1992; Kusky and Bradley, 1999; Onishi et al., 2001; Ujiie, 2002; Fukui and Kano, 2007; Tokiwa, 2009; Singleton and Cloos, 2012). The structural fabrics in the Zanhuang mélange have potentially significant implications for its formation and the tectonic evolution of the North China craton. Based on high-precision mapping, abundant kinematic fabrics have been obtained from the Zanhuang mélange (Fig. 9). The measurements of asymmetric kinematic fabrics have been collected from different locations in the field observation. The structural fabrics described herein show a consistent trend indicating that the shearing sense or the transport direction was directed northwest to southeast (Fig. 9). Combined with the field observations that the lithostructural units of the Zanhuang mélange show consistent northwest-dipping thrusts, we suggest the overall shear sense or transport direction during mélange formation was southeast directed. The shear sense or transport direction can represent the subduction polarity during the formation process of the Zanhuang mélange. In conclusion, we propose that the arc-continent collision between the arc terrane in the Central orogenic belt and the Eastern block was a northwestward-directed subduction, which is consistent with a previous study showing that the Western Zanhuang domain was thrust upon the Eastern Zanhuang domain (Eastern block) to the southeast (Fig. 11E; Wang et al., 2013).
Structural Relationships along a Neoarchean Arc-Continent Collision Zone
The boundary between the Fuping arc terrane and the Eastern block continent is marked by the Neoarchean Zanhuang mélange and the marble-siliciclastic unit, which is interpreted to represent a passive continental margin sequence (Fig. 11E; Wang et al., 2013). Previous studies of the regional structure show that the TTG gneisses of the Western Zanhuang domain are thrust to the southeast upon the mélange unit, and the highly sheared and deformed mélange unit is thrust to the southeast over the marble-siliciclastic unit (Wang et al., 2013). In this paper, we present detailed mapping of several typical transects and outcrops to better explain the structural relationships along the Neoarchean arc-continent collisional zone. High-precision mapping shows that different units of the Zanhuang mélange have been deformed and folded together and imbricated several times at both small scale and outcrop scale, forming the southeast-vergent imbricated mélange belt. Different units of the Zanhuang mélange section are also characterized by intense folding, shearing, and thrusting, consistent with southeast-directed thrusting. Both blocks of mafic blocks with epidosite cores and metapelite matrices have been deformed and thrust together in a style similar to Phanerozoic accretionary wedges (Fig. 11E; Brown and Spadea, 1999; Kusky et al., 2013). The different lithostructural units and their internal structures suggest the Zanhuang accretionary mélange complex was gradually built by off-scraping during subduction and obduction, and later imbricated with the passive continental shelf sequence during emplacement of the mélange complex over the passive margin during arc-continent collision. The classic fold-and-thrust internal structures (Fig. 11E) in both the marble-siliciclastic unit and mélange section demonstrate that convergent tectonics along the Neoarchean arc-continent collision zone were broadly similar to Phanerozoic collisional belts, as exemplified by several classic cases from the Appalachian-Ouachita orogen (Thomas, 2004) and Alpine-Himalayan orogenic belt (Yin et al., 2010).
Constraints on Archean Tectonic Evolution of the North China Craton
The formation of Precambrian basement and the Archean tectonic evolution of the North China craton remain disputed. Different tectonic models have been proposed to explain the amalgamation ages and tectonic processes that caused the closure of the ocean(s) between the Eastern block and Western block of the North China craton. The main differences of these different models are focused on the following three aspects: (1) Collisional mechanisms include continent-continent collision (Faure et al., 2007; Wu and Zhong, 1998; Zhao et al., 1998, 2000a, 2005) and arc-continent collision (Kusky, 2011; Kusky and Li, 2003); (2) amalgamation ages include late Archean (Kusky, 2011; Kusky and Li, 2003), late Paleoproterozoic (Wu and Zhong, 1998; Zhai and Santosh, 2011; Zhao et al., 1998, 2000a, 2000b, 2001, 2005), and two collisional stages at ca. 2.1 Ga and 1.9–1.88 Ga (Faure et al., 2007); and (3) subduction polarities include west-dipping subduction (Kusky and Li, 2003; Kusky, 2011; Li and Kusky, 2007) and east-dipping subduction (Kröner et al., 2006, 2006; Zhao, 2001; Zhao et al., 2000a, 2000b, 2001, 2005). Our recent study on the newly documented Zanhuang mélange shows that the initial collision was an arc-continent collision between the Fuping arc terrane in the Central orogenic belt and the Eastern block in the Neoarchean (Fig. 11E; Deng et al., 2013, 2014; Wang et al., 2013, 2015). Combined with this study, we further constrain the amalgamation age of the arc-continent collision to be ca. 2.5 Ga with a northwest-dipping subduction polarity.
The following conclusions are drawn from this study:
(1) LA-ICP-MS zircon 207Pb/206Pb ages of various blocks in the Zanhuang mélange provide the maximum age (2496 ± 21 Ma) of final mélange formation. Previous studies provided zircon 207Pb/206Pb ages of the late Wangjiazhuang granite and undeformed pegmatite that intruded the Zanhuang mélange, giving a minimum age of 2493 ± 22 Ma for the formation of the Zanhuang mélange. In conclusion, we propose the formation and associated deformation age of the Zanhuang mélange is Neoarchean (ca. 2.5 Ga).
(2) The presence of pillow lavas with epidosite cores in the Zanhuang mélange indicates it is an ophiolitic mélange.
(3) Systematic kinematic analysis of asymmetric fabrics of the Zanhuang mélange constrains the convergent direction of initial amalgamation between the Fuping terrane in the Central orogenic belt and the Eastern block as northwest-dipping with a top-to-the-southeast transport direction.
(4) High-precision lithostructural mapping of several transects and outcrops reveals at least two sets of faulting and one generation of folding, and the classic fold-and-thrust structures in the Zanhuang mélange along the Neoarchean arc-continent collisional zone are broadly similar to those in Phanerozoic orogenic belts.
This work was supported by the Postdoctoral Science Foundation of China (grant 2015M580676), Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (grant CUG160807, awarded to Junpeng Wang), National Natural Science Foundation of China (grant 41572203, awarded to Timothy Kusky), and the Natural Sciences and Engineering Research Council of Canada (Discovery Grant 250926, awarded to Ali Polat). We appreciate helpful comments from the editor, associate editor, and two anonymous reviewers that substantially improved the manuscript. We thank Jianmin Fu, Zhensheng Wang, Ye Yuan, Zhuocheng Wang, Bo Huang, and Guanzhong Shi from China University of Geosciences (Wuhan) for helping with the field and laboratory work.
- Received 28 December 2015.
- Revision received 13 June 2016.
- Accepted 18 July 2016.
- © 2016 Geological Society of America