Permian radiolarians from the A'nyemaqen mélange zone in the Huashixia area of Madoi County, Qinghai Province, Western China, and their implications on regional tectonism

0. INTRODUCTION

As one of the major mélange zones controlling regional tectonic evolution of the Qinghai-Tibetan Plateau during the Paleozoic, the A'nyemaqen mélange zone facilitates better understanding of evolutionary histories of several important terranes such as the Qiangtang terrane and their consolidation histories (Yin and Zhang, 2003; Zhang et al., 1999). In particular, the opening and closure of the A'nyemaqen paleo-ocean are crucial in understanding of the consolidation process of the Qiangtang Block to the paleo-Asian plate. This mélange zone is largely located in the East Kunlun tectonic complex and next to the Bayan Har complex. It is bounded in the north by the South Kunlun major fault and in the south by the Changshitoushan major fault (Yin and Zhang, 2003; Zhang et al., 1999; Wang G C et al., 1997).

Although it extends up to nearly 1 km in the East Kunkun complex, the A'nyemaqen mélange zone is best preserved and exposed in the Huashixia area of the Madoi County, Qinghai Province, western China (see Fig. 1). Therein, the mélange zone extends in NWW trend, and the complete succession crops out well. The Huashixia area therefore has been intensely studied in terms of paleontology, petrology, and tectonic evolution in revealing the opening-closing history of the A'nyemaqen paleo-ocean (Zhang et al., 1999; Wang Y B et al., 1997).

Figure  1.  Geographic map of the Huashixia area of the Madoi County, Qinghai Province, western China showing the location of the study section. 1. Railway; 2. national road; 3. provincial road; 4. lake; 5. location of the section.

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Previously, the Geology and Mineral Resources Bureau of Qinghai Province (GMRBQP) (1997, 1991) referred the tectonostratigraphic successions of the A'nyemaqen mélange exposed in Huashixia as the Buqingshan Group, which was subdivided into the Shuweimenke and Maerzheng formations. The former unit is dominated by the shallow platform facies carbonates yielding Middle Permian fusulinids, brachiopods, and rugose corals (Zhao et al., 2013; Lin et al., 2001; Wang et al., 2001, 2000; Zhang et al., 1999; GMRBQP, 1997, 1991). The latter is composed mainly of slate with mixture of reddish siliceous shale and cherts, bearing abundant deep-water radiolarian elements. The Maerzheng Formation therefore represents the sedimentation of a deep ocean setting (Zhang et al., 1999). The age of this formation also indicates the development of the A'nyemaqen paleo-ocean.

However, the age constraint of the Maerzheng Formation has long been a focus of debate. The No. 1 Regional Geological Exploration Brigade of the Geology and Mineral Resources Bureau of Qinghai Province discovered the Early Permian fusulinid fossils from the limestone blocks of the Buqingshan Group in 1982, and assigned collectively the entire Buqingshan Group to the Lower Permian. Ji and Ouyang (1996) obtained an Early Triassic spore-pollen assemblage from sandy slate of the Maerzheng Formation, which suggests an Early Triassic age for the formation. Later, Zhang et al. (1999) found for the first time the reddish siliceous shales and cherts, yielding radiolarians. These authors considered that these reddish siliceous rocks represent the sedimentation of a bathyal to abyssal setting, and are of Asselian-Sakmarian (Early Permian) age, equating the late Zisongian-early Longlinian (Early Permian) in South China. Moreover, the mixture of the Early Triassic microflora and Early Permian radiolarian fossils indicates that the Maer- zheng Formation comprises combination of different rocks of various ages, typical of the mélange zone of tectonically complicated complex, and may represent the main part of the eastern Kunlun A'nyemaqen mélange zone.

More recently, we have also sampled the Maerzheng Formation section in Huashixia and extracted abundant radiolarian fossils from siliceous rocks. The newly found radiolarians suggest a more convincing age constraint than the previously reported fossil assemblages for the lithologically complicated Maerzheng Formation. The present finding therefore provides some new insights into the timing of the development of the A'nyemaqen paleo-ocean.

1. GEOLOGICAL AND STRATIGRAPHIC SETTINGS OF THE STUDIED SECTION

The studied section is located at the south of the Huashixia township of the Madoi County, Qinghai Province, western China. Therein the Maerzheng Formation succession is exposed along the road cut of the Qing-Kang Highway (Fig. 1). As stated above, the Huashixia area is the core area of the A'nyemaqen mélange zone in the East Kunlun tectonic complex (Zhang et al., 1999). In Huashixia, both the Shuweimenke and Maerzheng formations contact one another with a fault. The Shuweimenke Formation is beneath the latter unit and comprises mainly large blocks of bioclastic limestone with bioherm buildups (Zhao et al., 2013; Lin et al., 2001; Wang Y B et al., 2001, 2000, 1997; Zhang et al., 1999; GMRBQP, 1997, 1991). The Maerzheng Formation is dominated by grayish to greenish silty slates and reddish radiolarian-bearing siliceous shales and cherts. This formation, however, is poorly defined at its base and top, and contacts other lithologic units with faults. The entire unit is composed of alternating gray medium- bedded sandstone and reddish slate in the lower part, reddish slate mixed with reddish radiolarian-bearing siliceous shales/cherts at the middle part, and dark gray thin-bedded fine-grained sandstone at its upper part (Fig. 2).

Figure  2.  Stratigraphic distributions of radiolarians from the Maerzheng Formation in Huashixia.

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2. RESULTS

2.1 The Newly Found Radiolarian Fauna

A total of 37 siliceous rock samples were collected and processed. Samples 2-14, 2-15, and 2-16 are very productive, and yield abundant radiolarians, belonging to the Albaillellaria, Spumellaria, Entactinaria, and Latentifistularia. Of these, the Albaillellaria contains Pseudoalbaillella rhombothoracata (Ishiga and Imoto), P. scalprata (Holdsworth and Jones), P. sp., Albaillella sp.?, Follicucullus bipartitus (Caridroit and Dewever), and F. sp. cf. F. ventricosus (Ormiston and Babcock) (see Figs. 2, 3). Pseudoalbaillella rhombothoracata occurs in a very small quantity from the basal part of the section. Its specimens, overall, are not well-preserved. P. scalprata was found in association with P. rhombothoracata in Huashixia. They both have also been widely reported from cherty strata elsewhere in China, Japan, Malaysia, Thailand, and Russian Far East (Shi et al., 2016; Xie et al., 2012, 2011; Wang and Yang, 2007; Wang, 1995; Wu et al., 1994; Ishiga, 1990). F. sp. cf. F. ventricosus and Follicucullus bipartitus both occur at the top of the section. These two species are also commonly present in the Permian siliceous sequences in Southwest China, Southwest Japan, and Philippines, and range from the late Capitanian (late Middle Permian) to early Wuchiapingian (early Late Permian) in age (Shi et al., 2016; Xie et al., 2012, 2011; Wang and Yang, 2007; Wang et al., 2006; Wang, 1995; Ishiga, 1990).

Figure  3.  Selected radiolarians from the Maerzheng Formation. All species were collected from the Maerzheng Formation in the Huashixia Section and their stratigraphic distributions are shown in Fig. 2. All radiolarian specimens are housed at the Geobiology Department, China University of Geosciences (Wuhan). Scales are 100 μm. 1. Follicucullus sp.; 2. Follicucullus bipartitus (Caridroit and Dewever); 3. Follicucullus sp. cf. Follicucullus ventricosus (Ormiston and Babcock); 4 and 5. Pseudoalbaillella scalprata (Holdsworth and Jones); 6. Pseudoalbaillella rhombothoracata (Ishiga and Imoto); 7, 8, 10, and 11. Ruzhencevispongus uralicus m. triradiatus (Kozur); 9. Latentifistula impella (Ormistone and Lane); 12 and 14. Ruzhencevispongus uralicus m. subtriangulus (Kozur); 13. Latentifistula asperspongiosa (Sashida and Tonishi); 15 and 16. Ruzhencevispongus rotundus (Feng); 17-26. Entactinia itsukaichiensis (Sashida and Tonishi).

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Latentifistularia include Ruzhencevispongus rotundus (Feng), R. uralicus m. subtriangulus (Kozur), R. uralicus (Kozur), R. uralicus m. triradiatus (Kozur), Latentifistula impella (Ormistone and Lane), and L. asperspongiosa (Sashida and Tonishi) (see Figs. 2, 3). All of these species occurred in the middle and upper parts of the section. In particular, Ruzhencevispongus uralicus is well preserved and contains a great quantity of specimens. This species has been reported worldwide (Xie et al., 2012, 2011; Wang and Yang, 2007; Wang, 1995, 1993; Ishiga, 1990; Nazarov and Ormiston, 1986). Although, it spreads through the Lower to Upper Permian in southern Urals, and South China (Feng et al., 2006a, b), R. uralicus is very common and usually abundant in the Capitanian (late Middle Permian) worldwide (Xie et al., 2012, 2011; Wang and Yang, 2007; Wang, 1993).

2.2 Radiolarian Biostratigraphy

Based on their stratigraphic distributions, these Maerzheng Formation radiolarians can be categorized into four assemblage zones in ascending order: Pseudoalbaillella rhombothoracata, Pseudoalbaillella scalprata, Ruzhencevispongus uralicus, and Follicucullus sp. cf. F. ventricosus-Follicucullus bipartitus zones (Fig. 2; Table 1). Startigraphic ranges of each radiolarian zone are shown in Fig. 2.

Table  1.  Correlations of radiolarian zones from the Maerzheng Formation in Huashixia with their counterparts from South China and Southwest Japan

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(1) Pseudoalbaillella rhombothoracata Zone: This zone starts with the first occurrence of P. rhombothoracata and ends with the first occurrence of P. scalprata. The zonal species is only element found in this zone. Its exact base remains unclear because P. rhombothoracata may extend downward but the base of the studied section is bounded by a fault. The same radiolarian zone has been recognized in the Permian siliceous successions in South China (Wang and Yang, 2011, 2007). This zone correlates well with the Pseudoalbaillella scalprata m. rhombothoracata Zone established by previous workers from Southwest Japan (Mikiko and Yao, 2006; Yao and Kuwahara, 2004; Ishiga, 1990, 1986). The P. rhombothoracata zone is also commonly present in the Lower Permian siliceous sequences in Qinzhou of Guangxi, South China, as well as the late Wolfcamp (Early Permian) strata in Malaysia, Thailand, and Russian Far East (Xie et al., 2012, 2011; Wang and Yang, 2007; Wang, 1995, 1993). Wang and Yang(2011, 2007) assigned this radiolarian zone to the late Artinskian in age.

(2) Pseudoalbaillella scalprata Zone: Its base is marked by the first occurrence of Pseudoalbaillella scalprata, which is diagnostic of this zone and also associated with P. rhombothoracata (Fig. 2). Its top, however, remain uncertain due to lack of diagnostic elements. Of these, P. scalprata also occurs in cherty strata of the lower Gufeng Formation in Jiangsu and Anhui provinces, South China, the lower Bancheng Formation in Qinzhou of Guangxi, South China, and the Zhongdian in Yunnan Province, Southwest China (Xie et al., 2012, 2011; Wang and Yang, 2007; Wang, 1995). Outside China, this species is also widespread in the Permian cherty successions in Southwest Japan, Malaysia, Thailand, and Russian Far East (Wang and Yang, 2007; Wang, 1995; Ishiga, 1990). The same radiolarian zone was also proposed in the above regions and was usually assigned to the late Artinskian- early Kungurian (late Early Permian) in age (Mikiko and Yao, 2006; Yao and Kuwahara, 2004).

(3) Ruzhencevispongus uralicus Zone: The base of this zone is marked by the first occurrence of R. uralicus, and its top is defined by the first occurrence of Follicucullus sp. cf. F. ventricosus (Fig. 2). Characteristic species include Ruzhencevispongus rotundus, R. uralicus m. subtriangulus, R. uralicus m. triradiatus, Latentifistula impella, Latentifistula aspersongiosa, and Entactinia itsukaichiensis. Of these, R. uralicus is the predominant species of this radiolarian zone and has been reported from the Capitanian stage, especially in the Gufeng Formation, in the upper Yangtze platform regions (Wang, 1995, 1993), in which the comparable fauna was assigned to the Follicucullus sp. cf. F. ventricosus-Follicucullus bipartitus Zone, although it ranges through the entire Permian (Feng et al., 2006a, b). This zone correlates well with the same zone established from the Gufeng Formation of Jiangsu, South China (Wang, 1995, 1993; Wang and Qi, 1995), the Ruzhencevispongus uralicus-Follicucullus scholasticus Zone established in Anhui (Kuwahara et al., 2007, 1997; He et al., 1999), the F. scholasticus Zone erected from the western Hubei Province, South China (Shi et al., 2016), and the same zone from the Urals of the former Soviet Union (Nazarov and Ormiston, 1986). The present zone is constrained as Maokouan in age in South China (Wang, 1995, 1993) and diagnostic of the Capitanian worldwide (Kuwahara et al., 2007, 1997).

(4) Follicucullus bipartitus-Follicucullus sp. cf. F. ventricosus Zone: The base of this zone is defined by the appearance of F. sp. cf. F. ventricosus, and its top remains uncertain, but is marked tentatively by the disappearance of Follicucullus bipartitus. The associated element includes Entactinia itsukaichiensis. Of these, Follicucullus ventricosus is the zonal element of the Follicucullus scholasticus-Follicucullus ventricosus Zone established in South China (Wang and Yang, 2011, 2007), which is also diagnostic of the late Maokouan radiolarians in South China. Follicucullus bipartitus is also a marker of the Follicucullus bipartitus- Follicucullus charveti-Follicucullus orthogonus Zone in South China (Wang and Yang, 2011, 2007; Feng et al., 2009; Wang et al., 2006; Wang and Li, 1994). The Follicucullus bipartitus- Follicucullus charveti Zone is typical of the early Wuchiapingian faunas in Southwest Japan (Ishiga, 1990, 1986) (Table 1). This zone therefore is constrained as the late Capitanian to early Wuchiapingian in age.

To sum up, four radiolarian zones are recognizable from the Maerzheng Formation in Huashixia. The first two radiolarian zones suggest a late Artinskian to early Kungurian age, while the latter two indicate a late Capitanian to early Wuchiapingian age. The Maerzheng Formation therefore ranges from late Artinskian to early Wuchiapingian in age.

3. DISCUSSION

3.1 Age of the Maerzheng Formation

The siliceous rocks exposed in the study section are about 60 m thick. The cherty sequence, however, bears such a long age range from the Artinskian to early Wuchiapingian. This could be due to (1) an extremely low sedimentation rate that the siliceous rocks may represent, and (2) various faults that may occur within the siliceous successions in the study section and resulted in loss of some sedimentary strata. Given the complicated contact relationships among beds in the studied section, the second interpretation may be more plausible. In addition, these four radiolarian zones are tentatively established when compared with the same zones proposed from South China and Japan (Wang and Yang, 2007; Ishiga, 1990) (Table 1). Their bases and tops are poorly defined due to uneven stratigraphic distributions of radiolarians. The latter is probably, in part, due to the existence of many faults within the Maerzheng Formation succession, which may cause some gaps within the siliceous successions.

3.2 Habitats of the Newly Found Radiolarians

Radiolarians usually live in oceans and are mainly subjected to the effects of seawater temperature and depth. In general, in warm low-latitude areas, there are abundant species of radiolarians and numerous individuals. They possess thin shell walls, delicate structures, long main spines, and developed by-spines. In contrast, there are fewer species and individuals inhabiting in cold high-latitude areas, in which radiolarians usually have thick shell walls, dense structures, and short and thick radiated spikes (Feng, 1992). In moderate-latitudes, the mixed radiolarian faunas also appear (Du et al., 2012; Chen et al., 2008; Feng, 1992). They usually form a high-abundance and high-diversity fauna. The individual radiolarians usually embrace thin shell walls, delicate structures, long radiated spikes, and developed lateral spikes. Besides, the moderate-latitude radiolarian faunas share some features with the warm, low-latitude assemblages. The newly found radiolarians from the Huashixia area are dominated by the high- abundance species that possess thin shell walls, delicate structures, long main spines, and developed by-spines, indicative of low latitude assemblage. Such an ecologic analysis is reinforced by the paleogeographic reconstruction that the Huashixia area was located at 14.01°N in Early Permian indicated by paleomagnetic analysis (Huang et al., 1999).

The variety of morphologic features of radiolarian fauna is also closely related with water depth of the habitats where radiolarians live. Traditionally, the radiolarian-bearing environment has been considered as a deep water habitat. However, growing evidence shows that radiolarians may inhabit a shallow-water niche in the Paleozoic and present day (Itaki, 2003; Kozur, 1993; Feng, 1992). What's more, various radiolarian assemblages inhabit niches with different water depths (He et al., 2011, 2008; Itaki, 2003; Feng, 1992). Based on the previously established schemes (He et al., 2008; Itaki, 2003; Kozur, 1993), the Maerzheng Formation radiolarians, consisting mainly of the Albaillellaria, Spumellaria, Entactinaria, and Latentifistularia, could be assigned to the deep water assemblages, living in open deep sea with a water depth of greater than 500 m (Fig. 4). This radiolarian ecologic inference is strengthened by the rare-earth element and trace elemental analyses of the reddish slate by Wang et al. (2001) who clarified that the siliceous rocks of the Maerzheng Formation are consistent with modern abyssal clays. As such, we infer that the study area was an abyssal environment during the Early Permian to early Late Permian.

Figure  4.  Cartoon diagram showing sedimentation model of waterdepth- related radiolarian assemblages (after He et al., 2008; Itaki, 2003; Kozur, 1993).

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3.3 Evolution of the A'nyemaqen Paleo-Ocean

Several studies concerning the ophiolitic mélange of the A'nyemaqen area have enhanced greatly our understanding of the evolution of the A'nyemaqen paleo-ocean. Previously, the radiolarians obtained from the Huashixia area suggested an Early Carboniferous to Early Permian age (Bian et al., 2004, 1999; Zhang et al., 1999), and fusulinids from the same area constrained the ophiolitic mélange as the Early-Middle Permian in age (Zhao et al., 2013). In addition, the radiometric ages of Early Carboniferous (Liu et al., 2011; Chen et al., 2001; Bian et al., 1999) and Late Carboniferous (Yang et al., 2004) were also obtained from the eastern Kunlun A'nyemaqen mélange zone. All of these published paleontologic and geochronologic data indicate that the A'nyemaqen paleo-ocean may have opened during the Early Carboniferous. The newly found radiolarians suggest both the late Early Permian and Middle- Late Permian transition for the lower and upper parts of the Maerzheng Formation, respectively strengthening the scenario that the paleo-ocean may have expanded unceasingly through the Permian, lasting approximately 100 million years. Furthermore, if the hypothesis that the A'nyemaqen ophiolite developed in the context of rapidly expanded mid-oceanic ridge (Yang et al., 2004) is followed, then the A'nyemaqen paleo- ocean should be rather large and reached its peak during the early Late Permian. The microfloral evidence shows that the paleo-ocean may have shrunk greatly in late Late Permian to Early Triassic, and finally was closed during the Middle Triassic (Zhang et al., 1999; Ji and Ouyang, 1996; Jiang, 1995).

4. CONCLUSIONS

Total 19 species in 10 radiolarian genera were extracted from the siliceous rocks of the Maerzheng Formation in the Huashixia area of the Qinghai-Tibetan Plateau, western China. Four radiolarian zones: Pseudoalbaillella rhombothoracata, Pseudoalbaillella scalprata, Ruzhencevispongus uralicus, and Follicucullus bipartitus-Follicucullus sp. cf. F. ventricosus zones were established. The first two suggest a late Artinskian to early Kungurian age, while the latter two zones indicate a late Capitanian to early Wuchiapingian age. The Maerzheng Formation therefore ranges from the late Artinskian to early Wuchiapingian in age. The loss of some zones and uneven distributions of radiolarians are probably due to the presence of many faults within the Maerzheng Formation succession. The newly found radiolarians are dominated by the elements of the Albaillellaria, Spumellaria, Entactinaria, and Latentifistularia, and indicate a deep water fauna of low latitudes. Integration of the new radiolarian fauna, and previously reported assemblages and microfloras as well as radiometric dating data indicates that the A'nyemaqen paleo-ocean may have opened initially in Early Carboniferous, expanded through the Early to Middle Permian, reached its acme in early Wuchiapingian, shrunk in latest Permian to Early Triassic, and eventually was closed during the Middle Triassic.