2. College of Earth and Planetary Sciences, University of the Chinese Academy of Sciences, Beijing 100049, China;
3. School of Earth Sciences, Center for Global Tectonics, China University of Geosciences, Wuhan 430074, China
It is well known that the Qinling-Dabie orogenic belt sutured the North China Craton (NCC) and Yangtze Craton during Triassic. However, in the Qinling Mountains, a belt known as the North Qinling belt (NQB) is bounded by the Luonan-Luanchuan and Shangdan faults, consisting of the Taowan, Kuanping, Erlangping, Qinling and Danfeng groups (e.g., Mattauer et al., 1985). The NQB is characteristic of (1) a plenty of granitoid plutons with the intrusive ages from Neoprotozoic to Middle Devonian (e.g., Wang et al., 2013; Lu et al., 2003), (2) HP and UHP rocks in the Qinling Group with the metamorphic age of ~500 Ma (e.g., Gong et al., 2016; Chen et al., 2015; Cheng et al., 2012, 2011; Wang H et al., 2011; Yang et al., 2003), and (3) widespread detrital zircon populations with the affinity of the Yangtze Craton (e.g., Shi et al., 2013; Diwu et al., 2012), (4) unconformable NCC-type C2–P stratum on the Kuanping and Taowan groups (e.g., Meng and Zhang, 2000). These features suggest different tectono-metamorphic history from that of southern Triassic subducted continental UHP metamorphic belt.
This paper doesn't try to discuss the evolution of the NQB, but provides some zircon U-Pb data to limit the tectonic boundary and ceasing time of amalgamation between the North China Craton and the North Qinling belt. These data include the volcanic eruption timing of the Luanchuan Group, the age spectra of detrital zircons from the Taowan and Kuanping groups and intrusive timing of several plutons ranging from Neoprotozoic to 390 Ma.1 GEOLOGICAL SETTING, SAMPLING AND ANALYTICAL METHOD
The studied region is located in the Qinling Mountains in Shaanxi and Henan provinces, as shown in Fig. 1. The NQB is traditionally divided as the part that is bounded by the Shangdan and Luonan-Luanchuan faults. The Taowan and Luanchuan groups were considered as the geological units forming in the southern margin of the NCC.
The Luanchuan Group is a sedimentary-volcanic sequence. Its uppermost Dahongkou Formation is made up of meta- trachytic volcanic rocks, phyllite, schist and marble, and was intruded by abundant gabbroic dikes with about 830 Ma magmatic crystallization age (Wang X L et al., 2011). However, the eruptive age of the volcanic rocks is still unknown so far, and thus trachytic volcanic agglomerate was sampled to the northwest of Luanchuan City (Fig. 1). The Luanchuan Group also occurs to the northeast of Nanzhao City (Henan Institute of Geological Survey, 2001). Some mylonized K-feldspar-rich granites can be identified in strongly deformed zone (Fig. 1), in which a sample was collected for determining the magmatic crystallization age. The Taowan Group, separated from the Kuanping Group by Luonan- Luanchuan fault, is exposed to the west of Luanchuan City and mainly composed of phyllite, slate, schist, and marble. A pelitic- clastic rock was sampled to the west of Luanchuan City in order to reveal the age spectrum of detrital zircons (Fig. 1).
The Kuanping Group, as the northernmost unit of the NQB, consists of mica schist, meta-sandstone, greenschist, amphibolite and marble. Two metamorphic clastic samples were collected in the west and east of the study region, respectively (Fig. 1). To the south of Lushi City, a granitoid complex that intruded into the Kuanping Group consists of diorite in the core and granite in the rim (Fig. 1). This complex was not or weakly deformed (Fig. 2a) and cut regional schistosity, and thus dating the complex can constrain the timing of regional deformation.
In the south, the NQB includes the Kuanping, Erlangping, Qinling and Danfeng groups. The Erlangping and Danfeng groups have similar rock association, being mainly made up of amphibolite with less ultramafic rock, marble, mica schist, and chert. The Qinling Group is a high-grade metamorphic terrane, consisting of migmatite, granitic gneisses, granulites, amphibolites, marbles and eclogites. A lot of metamorphosed and deformed plutons, ranging from gabbro, diorite to granodiorite, intruded into those groups. For constraining deformation timing, a diorite with striking schistosity parallel to regional schistosity (Fig. 2b), which intruded into the Erlangping Group, and a granodiorite with no deformation from a granitoid batholith in the east part were sampled, respectively (Fig. 1).
Zircon grains were extracted from crushed samples by using heavy liquid and magnetic separation, and then mounted in epoxy resin and polished until suitable exposure for LA-ICPMS analysis. Cathodoluminesce was used to reveal zircon internal structure. All the analyzed zircon grains in this study show magmatic oscillatory zoning. Zircon U-Pb analyses were carried out by using ICP-MS at the State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, and the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. Zircon 91500 was used as the reference, and Spot size was set to be 30–50 μm. The analytical techniques were described in detail by Yuan et al. (2008).2 RESULTS
In all the analyses, only U-Pb concordant-age data were selected to determine significant geological ages. The data with the ratio of apparent 206Pb/238U age/apparent 207Pb/235U age > 0.95 and < 1.05 is defined as concordant data. The analytical results were listed in Tables S1 and S2.2.1 Luanchuan, Taowan and Kuanping Groups
Twenty-one analytical results of zircons from a trachytic volcanic agglomerate from the Luanchuan Group were plotted in concordia diagrams (Figs. 3a, 3b, LCG). There are two populations: ten analyses give a range from 1 700 to 1 850 Ma, and other eleven results from 1 450 to 1 650 Ma with a mean age of 1 593±28 Ma. The age of 1 593±28 Ma represents the volcanic eruptive age.
Fifty-five concordant-age data of detrital zircons from the Taowan Group were shown in Fig. 3c (TWG). The zircons yield an age range from 920 to 2 600 Ma with three populations of 920–1 400, 1 600–2 100 and > 2 200 Ma ages (Fig. 4a TWG).
Ninety-five concordant-age data of detrital zircons from the Kuanping Group (KPG) were displayed in Fig. 3a. The samples are from two localities (Fig. 1). The detrital zircons give an age range from 580 to 3 300 Ma with six populations with the peaks of ~590, ~767, ~952, ~1 590, ~2 485, and ~3 200 Ma (Fig. 4b, KPG).2.2 Granitoid Crystallization Ages
The studied granitoid rocks show strongly deformed and less or not deformed. The analytical results were listed in Table S2. The granites intruding into the Luanchuan Group were mylonized. Thirteen concordant analyses yield an age ranging from 800 to 900 Ma with an average age of 851±16 Ma (Fig. 5). The zircon grains show oscillatory zoning, and thus 851±16 Ma represents magmatic crystallization age.
The results from the strongly deformed diorite in the Erlangping Group and two undeformed granitoid plutons were displayed in Fig. 6. The zircons from the strongly deformed diorite yield an average age average ages of 431.5±8.5 Ma (Figs. 6a, 6b, KP8-08), whereas those from the two undeformed plutons give the average ages of 386.7±6.8 and 391.6±5.4 Ma, respectively (Figs. 6c–6f, QL8-13 and NZ8-3). These ages represent zircon crystallization ages.3 DISCUSSIONS
It's generally accepted that the Luonan-Luanchuan fault is the north boundary of the NQB, and the Taowan and Luanchuan groups were divided as the tectonic units of the southern margin of the NCC. In fact, the Taowan and Luanchuan groups were deformed and metamorphosed during the amalgamation between the NCC and NQB, which leads to the difficult for recognition of the boundary. This study shows that the volcanic rocks of the Dahongkou Formation from the Luanchuan Group had the Mesoproterozoic eruptive age of ~1 600 Ma, similar to the ages of the Longwangzhuang A-type granite and the Dahongyu volcanic rocks in the NCC (Lu et al., 2008). However, the gabbros in the Dahongkou Formation had the Neoproterozoic intrusive age of ~830 Ma (Wang X L et al., 2011), and the mylonized granites in the east part of the Luanchuan Group give the magmatic crystallization age of 851±16 Ma. This casts a suspicion whether or not the magmatic events with the Neoprotozoic ages (700–900 Ma) can be used as a proof of the affinity and provenance to the Yangtze Craton. However, our mylonized granitic sample comes from the Luonan-Luanchuan fault. This is a large shear zone with the width of several kilometers. Tectonically, the granitic rocks are possibly located in a tectonic mélange, and thus it is necessary for further structural study to determine the occurrence of the granite.
The detrital zircons from the Taowan and Kuanping groups display the same age spectra as the results of previous studies (e.g., Diwu et al., 2012; Figs. 4a, 4b and 4c). A recent study for the Taowan Group have similar zircon populations to our samples (Cao et al., 2016; Fig. 4a). Our samples give the youngest age of ~920 Ma (Fig. 4), whereas that from Cao et al. (2016) is ~820 Ma. There are several studies for the detrital zircons from the Kuanping Group (e.g., Cao et al., 2016; Shi et al., 2013; Diwu et al., 2012, 2010). The youngest age in our samples is ~570 Ma (Fig. 4b), while other studies give the ages of ~500 (Cao et al., 2016) and ~450 Ma (Shi et al., 2013). The detrital zircons with ~450 Ma age or younger is probably from contamination during the process of zircon separation since the granitoid plutons of 420–460 Ma ages widely intruded into the NQB (e.g., Wang et al., 2013). The zircon populations from the Taowan Group show large difference from those of the Kuanping Group (Figs. 4a, 4b), suggesting that they have different sediment suppliers. However, the zircon populations with ~900 to 1 200 Ma peaks are very unique, which neither the NCC nor the Yangtze Craton as a single sediment supplier could provide such zircon populations in terms of known geological events recorded in the two cratons (Fig. 4; Zhao and Cawood, 2012; Lu et al., 2008). Therefore, the NQB probably is an independent block/terrene before accretion to the NCC (e.g., Diwu et al., 2012, 2010).
The presence of ~500 Ma high-pressure and ultrahigh- pressure metamorphism recorded in the rocks of the Qinling Group (e.g., Gong et al., 2016; Chen et al., 2015; Cheng et al., 2012, 2011; Wang H et al., 2011; Yang et al., 2003) suggests a deep continental subduction with unknown polarity, and rules out the possibility that the NQB was the southern active continental margin of the NCC in the Early Paleozoic (Dong et al., 2011; Meng and Zhang, 2000; Zhang et al., 1996) since the active continental margin is located above subduction zone, and doesn't take part in subduction process. On the other hand, stratigraphic-sedimentary sequence from Late Neoproterozoic to Middle Ordovician in the southern continental margin of the NCC consists of conglomerates, siltstones and platform carbonates (Meng and Zhang, 2000). The lack of volcanic activity in this period suggests that the southern continental margin of the NCC was a passive margin. The later hiatus of sediments and continental deposition of C2–P stratum reflects an uplift that is related to the amalgamation between the NCC and NQB. It is difficult to define the initial time of collision between the two continental blocks. However, it is possible for constraining the ceasing time of orogenic deformation.
Numerous granitoid had intruded into the NQB, and their crystallization ages range from Neoprotozoic to Devonian (e.g., Dong et al., 2014; Wang et al., 2013; Lu et al., 2003). Zircon U-Pb dating for the strongly deformed diorite that intruded into the Erlangping Group yields ~430 Ma crystallization age, whereas one for the two undeformed granitoid plutons give ~390 Ma crystallization age, indicating that the deformation of amalgamation between the NCC and NQB ceased between ~390 and ~430 Ma. Paleozoic granitoid plutons in the NQB formed in three stages, i.e., 507–470, 460–422, and 415–400 Ma according to the study of Wang et al. (2013). The latest one was dated as ~390 Ma in this study, slightly younger than the results of previous studies. In terms of the deformation feature, the granitoid plutons of 430 Ma formed in collision setting, whereas those of ~390 Ma in post- collision setting.4 CONCLUSION REMARKS
The boundary between the NCC and NQB is a large-scale ductile zone. In the 1 : 250 000 Geological Map (Henan Institute of Geological Survey, 2001), it was divided into the Luanchuan Group. Some Neoprotozoic mylonized granites with ~850 Ma crystallization age occur in this ductile zone. The zircons of volcanic agglomerate from the Luanchuan Group give 1 593±28 Ma crystallization age, consistent with the ages of the Longwangzhuang A-type granite and the Dahongyu volcanic rocks in the NCC. The zircon populations with ~900 to 1 200 Ma peaks in the sedimentary rocks from the Taowan and Kuanping groups are very unique, which cannot be explained as the provenance of either the NCC or the Yangtze Craton, suggesting that the NQB probably is an independent block/ terrene before accretion to the NCC. Zircon U-Pb dating for the strongly deformed and undeformed granitoid plutons indicates that the deformation of amalgamation between the NCC and NQB ceased between ~390 and ~430 Ma.ACKNOWLEDGMENTS
This study was supported by the National Natural Science Foundation of China (No. 41372080). Professor Zhendong You guided the first author to finish his thesis studies on the metamorphism of the Qinling-Dabie orogenic belt, as a master student from 1985 to 1988 and as a PHD student from 1991 to 1994. He is a knowledgeable, gentle and kind teacher. This paper is dedicated to Prof. Zhendong You for his birthday of 90 years old. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0847-8.
Electronic Supplementary Materials: Supplementary materials (Tables S1 and S2) are available in the online version of this article at https://doi.org/10.1007/s12583-018-0847-8.
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