Jun-ping Liu, Su-mei Tin, Xun-zo Zhu, Jin-hu M, Jing Li,c, Sho-bin Hu,c, Si-ying Yu,c,Hu Zhng,c, Zhong-ming She, Xu-gui Li

a Yunnan Institute of Geological Survey, Kunming 650216, China

b China University of Geosciences, Beijing 100083, China

c Key Laboratory of Sanjiang Metallogeny and Resources Exploration and Utilization, Ministry of Natural Resources, Kunming 650051, China

d Yunnan Department of Natural Resources, Kunming 65000, China

Keywords:

Maolu Formation

Puduhe Group

Late Neoarchean

Zircon U-Pb age

Tectonic-thermal event

Geological survey engineering

Eastern margin of Qinghai-Tibet Plateau

Yangtze Craton

A B S T R A C T

The Mesoproterozoic Dongchuan Group that is widely exposed in Yimen area, central Yunnan Province is a series of sedimentary sort of low-grade metamorphic rocks interbedded with volcanic rocks, which are closely related to the early tectonic evolution of the Earth. However, its formation era, sedimentary filling sequence, and geotectonic characteristics have always been in dispute. In this study, several rhyolitic tuffaceous slate interlayers with a centimeter-level thickness were found in the previously determined Heishan Formation of the Dongchuan Group located to the western part of Yimen-Luoci fault zone. This paper focuses on the study of the rhyolitic tuffaceous slate in Qifulangqing Village, Tongchang Township,Yimen County. LA-ICP-MS zircon dating was conducted, achieving the crystallization age of magma of 2491 ± 15 Ma and the metamorphic ages of about 2.3 Ga, 2.0 Ga, and 1.8 Ga for the first time.Meanwhile, according to in-situ Hf isotope analysis, the zircon εHf(t) values were determined to range from -3.0 to 7.6, with an average of 2.7. Furthermore, the first-stage Hf model age (TDM1) was determined to be 2513-2916 Ma, indicating that the provenance of the rhyolitic tuffaceous slate is the depleted mantle or juvenile crust between the Middle Mesoarchean and the Late Neoarchean. Therefore, it is believed that the strata of the slate were deposited in the Late Neoarchean, instead of the Mesoproterozoic as determined by previous researchers. Accordingly, it is not appropriate to group the strata into the Mesoproterozoic Dongchuan Group. Instead, they should be classified as the Maolu Formation of the Neoarchean Puduhe Group given the lithologic association and regional information. Furthermore, the magma ages of 2491 ±15 Ma are highly consistent with the eras of the large-scale Late Neoarchean orogenic magmatic activities on the northern margin of the Yangtze Craton, and thus reflect the orogenic process consisting of subduction and collision from Late Neoarchean to Early Paleoproterozoic. The magmatic activities during this period were possibly caused by the convergence of the supercontinent Kenorland. Meanwhile, the metamorphic ages of 2.3 Ga, 2.0 Ga, and 1.8 Ga are highly consistent with three metamorphic ages of 2.36 Ga, 1.95 Ga, and 1.85 Ga of the northern margin of the Yangtze Craton, indicating that the strata experienced Paleoproterozoic tectonic-thermal events. The study area is located on the eastern margin of Qinghai-Tibet Plateau, and thus was possibly re-transformed by magmatism subjected to the subduction of the Meso-Tethys Ocean during the Early Cretaceous. The discoveries made in this study will provide strong petrological and chronological evidence for analyzing the early crustal evolution of the Yangtze block.

1. Introduction

The Yangtze Craton is one of the important cratonized continental blocks in South China. It is considered to have been involved in the supercontinent cycle in the Early Precambrian and thus recorded Paleoproterozoic supercontinent Columbia and Neoproterozoic supercontinent Rodinia. Therefore, it has drawn considerable attention. In recent years, increasing Archean and Paleoproterozoic age records have been discovered on the southwestern margin of the Yangtze Craton with the development of high-precision zircon dating method. Accordingly, people realized that the basement on the southwestern margin of the Yangtze Craton was formed not only during the Mesoproterozoic -Paleoproterozoic but also in the Archean (Zhu HP et al., 2011;Zhou BG et al., 2012; Li J et al., 2018; Liu W et al., 2018; Cui XZ et al., 2019; Liu JP et al., 2018b, 2019, 2020b). Magmatic events and tectonic-thermal events of the Late Neoarchean are merely well developed on the northern margin of the Yangtze block, while there is no report yet on these events occurring in central Yunnan Province on the southwestern margin of the Yangtze block.

The central Yunnan Province serves as an ideal place for the investigation of stratigraphic eras, stratigraphic sequence,and tectonic framework of Early Precambrian strata in the Yangtze Craton, and much new isotopic data have been reported recently. For example, the post-collision granitic gneiss with an age of 2.36 Ga was identified in Cuoke complex in Yunnan Province (Cui XZ et al., 2019). Many zircon ages in the range of 2.5-3.0 Ga were obtained in the sandstone cuttings in the Yinmin Formation of the Dongchuan Group in Huili County, Yunnan Province (Liu W et al., 2018),indicating that Archean - Paleoproterozoic basement widely exists on the western margin of the Yangtze block.Meanwhile, a large number of isotope ages in the range of 1.85-3.05 Ga were obtained recently according to theAnnals of Regional Geology in Yunnan Province(Version 2,revision) (Li J et al., 2018; Liu JP et al., 2018b, 2019a,2020b), including the zircon age of 3073-2977 Ma obtained from the volcanics in Chahe Formation of Mesoarchean Yuanjiang Group in Honglongchang area, central Yunnan Province (Li J et al., 2018). Furthermore, LA-ICP-MS zircon U-Pb ages of 2433.1 ± 9.7 Ma were produced from the tonalites in Tongchang Township, Yimen County (Liu JP et al., 2020c). The LA-ICP-MS zircon U-Pb ages of 2.29-2.24 Ga were achieved from the volcanics in Luowadie Formation in Cuoke and Yimen areas, and meanwhile, zircon U-Pb ages of 2175 ± 31 Ma and 2049 ± 32 Ma were obtained from the tuff in the Liangshan Formation of the Paleoproterozoic Yimen Group, Yimen County (Li J et al., 2018; Liu JP et al.,2018a, 2019). The LA-ICP-MS zircon U-Pb ages of 2.30-2.29 Ga were also obtained from the tuff in Luowadie Formation of Niuchangping Village, Dongchang District (Zhu HP, 2011; Zhou BG et al., 2012). The zircon U-Pb ages of 1842 ± 26 Ma and 1860 ± 25 Ma were obtained from the tuff in Shanmujing Formation of Yimen Group in Tongchang Township, Yimen County (Liu JP et al., 2018b). Furthermore,the ages 1858 ± 18 Ma was yielded from the diabase that intruded into Archean Yuanjiang Group in Shizishan Village,Yimen County (Liu JP et al., 2020b). Based on these ages, the epimetamorphic strata beneath Mesoproterozoic Dongchuan Group were re-classified as the Paleoproterozoic Yimen Group, Neoarchean Puduhe Group, and Mesoarchean Yuanjiang Group (Li J et al., 2018; Liu JP et al., 2018b,2019a, 2020b). Among them, the Puduhe Group was divided into Maolu Formation, Longtoushan Formation, and Madi Formation from bottom to top, with conformable contact occurring between two adjacent formations.

In this paper, the rhyolitic tuffaceous slate in Maolu Formation of Neoarchean Puduhe Group (Ar3m) in Yimen County on the southwestern margin of Yangtze Craton was reported, including the latest results of their LA-ICP-MS zircon U-Pb ages, Hf isotopes, and features of lithologic association. These new data along with existing regional information will contribute to the further understanding of:(1) the sedimentary era of the Maolu Formation of Neoarchean Puduhe Group, which will provide chronological constraints on the sedimentary era of the Puduhe Group;(2) the temporal and spatial distribution features of the tectonic-thermal events occurring in the Yangtze Craton from the Late Neoarchean to the Paleoproterozoic; (3) the stratigraphic eras, stratigraphic sequence, and tectonic framework of Precambrian strata on the southwestern margin of the Yangtze Craton.

2. Geological setting

The study area is located in Yimen County, central Yunnan Province, and lies in the area where two third-order tectonic units are bordered with each other, namely the Chuxiong intracontinental basin and Kangdian basement fault-uplift zone of the Yangtze block above the Yangtze Craton. The strata in the study area lie at the junction of two stratigraphic districts Chuxiong and Kunming of the stratigraphic sub-region Kangdian in the stratigraphic region Yangtze in South China (Fig. 1). The outcrops in this area mainly include the Archean and Paleoproterozoic strata (Fig. 1).The Manlin Formation of the Mesoarchean Yuanjiang Group(Ar2m) is a set of “flyschoid” rhythmic interbeds consisting of greyish calcareous dolomitic spherulitic quartzite (quartz sandstones) and dark-grey phyllitic argillaceous slate,interbedded with a small amount of dolomite. The Maolu Formation of Neoarchean Puduhe Group (Ar3m) is a set of black - dark-grey phyllitic slate, phyllite, and carbonaceousargillaceous slate bearing rich pyrite lamina, carbonaceous matters, and calcareous matters, with the interbeds of rhyolitic tuffaceous slate and carbonaceous-argillaceous dolomite being occasionally visible. The Abudu Formation of the Paleoproterozoic Yimen Group (Pt1a) mainly consists of a set of heterogeneous conglomerates with poor sorting, which are interbedded with purplish-red and grey argillaceous slate and siliceous dolomite upwards. As for the gravels in this formation, most of them exhibit poor sorting and poor roundness, some have the characteristics of roche moutonnee and flatiron boulder, and a small number of them show the features of transportation and rounding due to running-water.Meanwhile, oblique beddings are developed in some sections of the formation. The Luowadie Formation of the Paleoproterozoic Yimen Group (Pt1l) is mainly composed of a set of intermediate-acid tuff and sedimentary tuff, with a small amount of dacite being interbedded. The Liangshan Formation of the Paleoproterozoic Yimen Group (Pt1ls)mainly contains grey and dark-grey argillaceous slate bearing multicellular fossils and silty argillaceous slate, and is interbedded with a small number of thin - medium laminated fine sandstones, siltstones, and tuffaceous siltstones (Liu JP et al., 2018a, 2019; Li J et al., 2018). The Xishancun Formation of Paleoproterozoic Yimen Group (Pt1x) mainly contains dark-grey - black slate and carbonaceous-argillaceous slate.The rocks in this formation are pervasively rich in Se and organic carbon. They are likely the parent rocks of massive Se-rich soil in Yimen County. It can be referred that this reflect that the global atmosphere - ocean system was transformed into the reduction environment again after the Great Oxygenation Event (Li J et al., 2018; Liu JP et al.,2018b, 2020b).

Fig. 1. Geotectonic maps (a, b) and sampling points (c) in the study area. Ar2m-Mesoarchean Manlin Formation; Ar3m-Neoarchean Maolu Formation; Pt1a-Paleoproterozoic Abudou Formation; Pt1ls-Paleoproterozoic Liangshan Formation; Pt1x-Paleoproterozoic Xishancun Formation

The Archean rocks in the Yangtze block mainly include Kongling complex in Huangling County, Douling complex in south Qinling Mountains, Zhongxiang complex in Zhongxiang City, Yudongzi complex, and Phan Si Pan complex in north Vietnam. Among them, the rocks that were recorded in 2.5 Ga magma mainly include the 2.5 Ga TTG rocks in Douling complex, the 2.5 Ga potassic granites in Yudongzi complex, and a small amount of Kongling complex(Ling WL et al., 1998; Hu J et al., 2013; Hui B et al., 2017;Zhou GY et al., 2018; Cui XZ et al., 2019). However, there is no record of 2.5 Ga magma on the southwestern margin of the Yangtze block, and only 2.5 Ga tectonic-thermal events have been widely found in rocks in the Archean Yuanjiang Group and Puduhe Group. Therefore, the discoveries achieved in this study will greatly enrich the information on the evolution of early crust tectonics in this region.

3. Sampling and testing

3.1. Sampling

The study area lies in Yimen County, Yunnan Province.Several beds of volcanics were found in a wide range of the previously determined Heishan Formation of Mesoproterozoic Dongchuan Group located to the west of Yimen - Luoci fault.These volcanics mainly consist of hoary tuffaceous slate and a small amount of tuffaceous argillaceous slate. Samples were taken from Qifulangqing Village, Tongchang Township,Yimen County. The study strata contact their surrounding strata by faults and thus occur in the form of structural interbeds. They have clear sequence inside, and can be divided into seven beds along the profile sampled with a thickness of 18 m. The sections bearing volcanics in the strata can be divided into two beds, whose tops and bottoms all consist of carbonaceous siltstones. Meanwhile, tuff is in conformable contact with other lithologic beds (Fig. 2a), and the outcrops of the two beds are 1.2 m thick in total. The seven beds are described as follows.

Fig. 2. Stratigraphic histogram (a), the field outcrops of sample No. D0010 (b), and microscope images (c, d).

⑦ Bottom: tuffaceous argillaceous slate, top: dark-grey epimetamorphic carbonaceous siltstone 1.9 m

⑥ Dark-grey epimetamorphic carbonaceous siltstones 2.1 m

⑤ Hoary rhyolitic tuffaceous slate, interbedded with siliceous-argillaceous slate 3.8 m

④ Dark-grey epimetamorphic carbonaceous siltstones 1.7 m

③ Greyish-black carbonaceous slate 1.8 m

② Grey epimetamorphic quartz sandstones 3.5 m

① Dark-grey thin - medium laminated siliceousargillaceous slate 3.2 m

The sample No. D0010 was taken from the lithologic bed⑤ (Fig. 2a), which consisted of hoary rhyolitic tuffaceous slate. The profile sampled was located on the unsurfaced road from Liangshan Village to Qifulangqing Village, Tongchang Township, with geographical coordinates of 102°00′42″E and 24°39′40″N.

3.2. Sample features and testing

The sample No. D0010 was rhyolitic tuffaceous slate. It was fresh, hoary, and had palimpsest tuffaceous texture and platy structure. Flow-like structures were locally visible and a small number of crystal pyroclasts of palimpsest quartz and sanidines can also be found. The crystal pyroclasts of the quartz mostly took the form of aggregates, with corrosioninduced perfectly round appearance being reserved. The sanidines were colorless, transparent, with part crystal forms being visible and corrosion occurring on the edges of part particles. The crystal pyroclasts varied in grain size, which was 0.2-0.5 mm mostly and even up to 0.8 mm individually.They were subangular in shape mostly and accounted for 5%-10% in all crystal pyroclasts. All of these crystal pyroclasts were cemented by volcanic ash (sericitized mostly). They were continuously distributed in semidirectional - directional arrays, and thus the platy structure was formed. The hand specimen and images under a microscope are shown in Figs. 2b-d.

Two samples were taken for geochemical assay. The separate compound content in these two samples were determined as follows: SiO2: 72.36% and 72.04%; A12O3:13.23% and 14.12%; K2O: 3.69% and 3.31%; Na2O: 3.52% and 4.29%; Fe2O3: 2.91% and 2.97%; FeO: 0.72% and 0.60%; MgO: 0.61% and 0.27%; CaO: 1.34% and 1.40%;P2O5: 0.08% and 0.04%. These results indicate that the tuffaceous slate is composed of acidic volcanics. Meanwhile,the samples fall within the calc-alkalic series zone in the volcanic AFM plot and fall in the rhyolite zone in the TAS diagram. This indicates that the tuff is acidic, which is consistent with the identification results using a microscope.Therefore, the samples should be rhyolitic tuffaceous slate.

The zircon particles were separated from the samples at the Nanjing Hongchuang Exploration Technology Service Co., Ltd. The protolith samples were crushed into powder of 200 meshes, which was then elutriated to remove light minerals. As a result, the heavy concentrate was left.Afterwards, zircon samples containing a small number of impurities were separated by electromagnetic technology.Consequently, zircon crystals were finally picked out under a binoscope. Among them, the zircon grains with good crystal form and without fissures were selected to be dipped in epoxy to prepare testing targets. After that, the targets were ground to expose zircon centers and then were polished. Thereafter,the photos of the reflected light, transmitted light, and cathodoluminescence (CL) of the zircon grains were taken. As for U-Pb isotopic dating and Hf isotopic analysis, it is required to select proper grains and areas based on their features (Hu Z, 2008; Wang HR et al., 2013; Liu JP et al.,2018c). U-Pb isotopic dating was conducted using LA-ICPMS technology at the Hubei Geological Research Laboratory,with GeoLasPro being used as the laser ablation system and Agilent 7700 as the ICP-MS. In the process of the laser ablation, helium gas was used as carrier gas, and meanwhile,argon gas was adopted as complementary gas to regulate the sensitivity. The two types of gases were mixed via a T-shaped joint before they entered ICP. Furthermore, a small volume of nitrogen gas was injected in the central gas flow (Ar+He) in the plasma to improve instrument sensitivity, reduce detection limits, and improve analytic precision. The laser ablation system was equipped with a signal smoothing device, which was used to achieve a pulse frequency as low as 1 Hz. In this manner, smooth analytic signals can be obtained from the laser ablation system. Every piece of time-resolved analytic data included a blank signal of about 20-30 s and sample signals of 50 s. The laser beam diameter was set to 24 µm,and the standard sample for zircon dating was the standard zircon 91500. Meanwhile, Isoplot (Ver. 3.0) was used to calculate weighted means and draw U-Pb concordia diagram(Ludwig KR, 2003).

The Hf-isotope analysis of the zircons was carried out at Wuhan Sample Solution Analytical Technology Co., Ltd. Thein-situHf-isotopes of the zircons were measured with a laserablation (LA) multi-collector (MC) inductively-coupledplasma (ICP) mass-spectrometer (MS), with a NWR213nm solid-state laser being used as the laser sampling system and a multicollector ICP-MS (Neptune Plus) as the analysis system.The laser beam used for laser ablation was typically 40 μm in diameter, 7-8 J/cm2in energy density, and 10 Hz in frequency. The isobaric interference of176Lu and176Yb on176Hf signals was corrected by using175Lu/176Lu = 0.02655 and176Yb /172Yb = 0.5886 based on the intensity monitoring of175Lu and172Yb signals. Meanwhile, instrumental drift was monitored externally by cross-analysis of standard zircons 91500 and GJ-1 and zircon samples. According to the analysis results, the176Hf/177Hf ratios of the standard samples 91500 and GJ-1 were 0.282283 ± 0.000041 (n= 4, 2σ) and 0.282019± 0.000029 (n= 4, 2σ), respectively, which are consistent with the reference values within the error ranges (Wu FY, 2007).For the calculation ofεHf(t), the176Hf/177Hf and176Lu/177Hf ratios of chondrite were set to 0.282772 and 0.0332,respectively. For the calculation of the first-stage Hf model age (TDM1), related values of the depleted mantle were set to176Hf/177Hf = 0.28325 and176Lu/177Hf = 0.0384, while for the calculation of second-stage Hf model age (TDM2), the mean176Lu/177Hf ratios of the crust was taken as 0.015 (Wu FY et al., 2007; Xie SW, 2009).

Two rock samples were selected to separately conduct major element analysis and the analysis of rare earth elements(REEs) and trace elements. They were crushed into 200 meshes and then were tested and analyzed to determine their major elements and trace elements at Kunming Supervision and Inspection Center of Mineral Resources, Ministry of Natural Resources of P.R.C. Major elements were tested using an X-ray fluorescence spectrometer (XRF-1500). Meanwhile,a glass sheet made using 0.6 g of samples and 6 g of lithium tetraborate were imployed to measure the mass fraction of oxides on ShimadzuXRF-1500, with the precision lower than 2%-3%. The trace elements and REEs were determined as follows. Samples were prepared by acid dissolution method and were tested using ICP-MS technology (Element Ⅱ), with the precision being consistent with the test precision of Chinese rock powder reference materials GSR-1 and GSR-2.In detail, the precision was lower than 5% when the element’s mass fraction was over 10×10-6and was lower than 10% otherwise.

4. Analytical results

4.1. Zircon U-Pb ages

Zircon U-Pb dating was conducted on 49 measuring points in the sample No. D0010 (Table 1). As a result, 49 pieces of valid data were obtained, and all of them showed high degrees of concordance (greater than 90%). Meanwhile,the Th/U ratios ranged from 0.1 to 2.6, and the zircon U-Pb concordia diagram is as shown in Fig. 4. The zircon grains in the sample can be grouped into two types according to their CL images. The zircons of the first type were typical magmatic zircons. They exhibited high euhedral levels and took clear morphology and angular shapes, with typical oscillatory zoning being developed (Fig. 3a). The zircons of the second type were argued to be surrounding-rock zircons captured during magma rose or inherited zircons. Most of them were anhedral-subhedra, subangular, with core-mantlerim texture and without apparent zoning. Meanwhile, their cores showed fan-shaped or ellipsoid texture (Fig. 3b). Hoary narrow metamorphic accretionary edges have generally developed on the rims of most of these zircons, which were likely due to the late transformation by tectonic-thermal events (Hoskin PWO, 2003; Hu ZC et al., 2008, 2012; Liu JP,2018c, 2019b).

Table 1. Zircon LA-ICP-MS U-Pb age of the rhyolitic tuffaceous slate in Yimen County, central Yunnan Province.

Fig. 4. U-Pb concordia diagrams (a, b), age probability distribution (c), and age weighted means (d) of the zircons from the rhyolitic tuffaceous slate in the Yimen County, central Yunnan Province.

Forty-nine zircon grains were selected for dating, and all of them fall on or near the U-Pb concordance line (Fig. 4a).The Pb elements of part of zircons were lost due to the transformation by late metamorphism. The data of 31 zircon grains were relatively concentrated (Fig. 4b), and the details are as follows. The zircon Th/U ratios were 0.3-2.6 and the 31 zircon grains showed typical oscillatory zoning, indicating that they were magmatic zircons. As shown from the distribution pattern of chondrite normalized REEs of the zircons (Fig. 5b), the REE distribution pattern declined rightwards, indicating the enrichment in HREEs and the depletion in LREEs. Meanwhile, positive Ce anomalies and negative Eu anomalies can be clearly observed, and the total content of the REEs was high and varied greatly.Furthermore, the Nb content of the 31 zircon grains was less than the Nb content range of magmatic zircons (62×10-6;Hoskin PWO and Schaltegger U, 2003; Lei WY et al., 2013),thus exhibiting the features of the REE distribution pattern of typical magmatic zircons. Consistent weighted means of206Pb/207Pb ages were obtained from the 31 zircon grains, i.e.,2491 ± 15 Ma (MSWD = 1.19,n= 31). These weighted means indicate that the tuffaceous slate was formed in Late Neoarchean and they also represented the sedimentary age of the strata.

Fig. 5. Distribution pattern of chondrite normalized rare earth elements and t (Ma) vs. εHf(t) diagram of the zircon Hf isotopes from the rhyolitic tuffaceous slate in the Yimen County, central Yunnan Province (the chondrite values are from Sun SS and McDonough WF, 1989).

The ages of the 49 zircon grains were less than 2417 Ma,ranging between 110 Ma and 2367 Ma. 16 measuring points fell within the zircon grains with unapparent oscillatory zoning, namely Nos. 1, 12, 15, 16, 18, 20, 35, 37, 38, 41, 42,43, 44, 45, 47, and 48 of the sample No. D0010. Meanwhile,some zircons showed lower Th/U ratios (e.g., 0.2, 0.1, 0.2,and 0.1 of the points Nos. 12, 35, 37, and 48, respectively).All these are the typical features of metamorphic zircons. As shown in the chondrite normalized REE distribution pattern(Fig. 5a), the REE distribution pattern of the zircons were consistent with each other, showing that HREEs were less rich than the MREEs and LREEs. Meanwhile, there were unobvious positive Ce anomalies, negative Nd anomalies, and positive Eu anomalies, and the Nb content of these 16 zircons was greater than Nb content range of magmatic zircons(62×10-6; Hoskin PWO and Schaltegger U, 2003). These are typical features of the REE distribution pattern of typical zircons of metamorphic and hydrothermal origins. The zircon ages of these 16 measuring points may be the ages of tectonicthermal events of several stages or mixed ages between old ages and young ages. In addition, the measuring points Nos. 6 and 27 of the sample No. D0010 fell on the inherited cores of the zircon grains, which showed unclear oscillatory zoning texture and subangular shape with the core-rim texture (Fig. 3b).Therefore, it can be inferred that their ages are the ages of inherited zircons or captured surrounding-rock zircons(Kröner A et al., 1994; Hoskin PWO and Schaltegger U,2003; Lei WY et al., 2013).

4.2. Zircon Lu-Hf isotopes

In-situHf isotopic analysis was conducted for the samples based on zircon U-Pb dating, and the results are shown in Table 2. Among them, the εHf(t) values and the two-stage Hf model age of the zircons was calculated from the concordant U/Pb ages of the rock masses. The initial εHf(t) values ranged between -3.0 and 7.6 (2.7 on average), the first-stage Hf model age (TDM1) ranged between 2513 Ma and 2916 Ma(2700 Ma on average), and the second-stage Hf model age(TDM2) ranged between 2527 Ma and 3173 Ma (2825 Ma on average). Meanwhile, the176Yb/177Hf,176Hf/177Hf, and176Lu/177Hf ratios were 0.012087-0.041613, 0.281144-0.281450, and 0.000477-0.001686, respectively, with averages of 0.023216, 0.281311, and 0.000898, respectively.

Table 2. Data of Hf isotope analysis of rhyolitic tuffaceous slate (D0100) from Yimen county, central Yunnan Province.

5. Discussions

5.1. Chronological implications of zircon U-Pb dating and their temporal constraints on Neoarchean Puduhe Group

The Neoarchean Puduhe Group consists of Precambrian strata as recently determined in theAnnals of Regional Geology in Yunnan Province(Version 2), and is an important part of the rock series in the crystalline basement of the Yangtze block in central Yunnan Province. It is divided into three formations from bottom to top, namely Maolu,Longtoushan, and Madi formations. Among them, the Maolu Formation is a set of black - dark-grey phyllitic slate,carbonaceous-argillaceous slate, and siliceous-argillaceous slate that are rich in pyrite, with interbeds of tuffaceous slate,carbonaceous-argillaceous quartz sandstones, and dolomite being occasionally visible. This corresponds to the lithologic association determined in this study. The ages of 2491 ± 15 Ma were discovered from rhyolitic tuffaceous slate in the Maolu Formation for the first time in this study, providing a temporal basis for accurately determining the sedimentary era of the Maolu Formation. The Maolu Formation may partly overlap with and interpenetrate the Longtoushan Formation temporarily and spatially, respectively (Fig. 6). The ages obtained in this study are highly consistent with the eras of the 2.51-2.47 Ga orogenic magmatic activities on the northern margin of the Yangtze Craton (Hu J et al., 2013; Wu YB et al., 2014; Li J et al., 2018; Liu JP et al., 2018b; Cui XZ et al.,2019). Meanwhile, they correspond to the age 2433.1 ± 9.7 Ma obtained from the tonalite in Tongchang Township,Yimen Count (Liu JP et al., 2020c). Furthermore, 2.54 Ga gabbro rock masses were found in Cuoke Village and Dongchuan District, Yunnan Province (unpublished). The magmatic activities during this period were possibly induced by the convergence of the supercontinent Kenorland, and the Yangtze block may serve as a vital component.

Fig. 6. Stratigraphic framework of the Early Precambrian in central Yunnan Province (modified from Li J et al., 2018)

According to theAnnals of Regional Geology in Yunnan Province(2nd Version), two sets of zircon U-Pb ages 2.55-2.52 Ga and 1.88-2.15 Ga were obtained from the basalt and basaltic tuff in the middle-upper part of Longtoushan Formation of Puduhe Group in Longtoushan-Madi area, Dongchuan district. According to the stratigraphic contact relationship and the feature analyses of zircon morphology and shapes, it is believed that the former may be the protolith age and the latter may reflect late tectonicthermal events (Li J et al., 2018). Meanwhile, the zircon U-Pb ages of 2.45 and 2.44 Ga were achieved from the basalt and intermediary-acid tuff on the top of Longtoushan Formation in the Cuoke- Yuanjiang Cement Plant area. The basalt and intermediary-acid tuff were intruded by 2.35 Ga granites and were covered by the Abudu Formation of the Paleoproterozoic Yimen Group though unconformable contact(Li J et al., 2018; Cui XZ et al., 2019). By combining the isotope data acquired in this study, it can be considered that Puduhe Group was formed at 2.45-2.78 Ga and belongs to the Neoarchean.

5.2. Provenance of magma

Hf isotopes in magmatic zircons can be used to trace the provenance of magma. If the initial ratioεHf(t) value is positive, the first-stage Hf model age approximates to the second-stage Hf model age and these ages are close to apparent ages, indicating that the provenance of magma is the depleted mantle or the young crust as the new accretion from the depleted mantle and that the magma originates from remelting of juvenile crust. In contrast, a negativeεHf(t) value denotes that the crust-derived materials predominated during the formation of rock masses, showing typical genetic characteristics of crust-derived rock masses (Griffin WL et al., 2002; Kinny PD, 2003; Hu J et al., 2013; Liu W et al.,2018). The176Lu/177Hf ratios (＜ 0.002) indicate that extremely low amount of radiogenic Hf has accumulated on the zircons after the zircons were formed, and the176Hf/177Hf ratios determined in this study may represent the Hf isotopic composition in the study area during zircon crystallization(Wu FY et al., 2007; Xie SW et al., 2009; Hu J et al., 2013).The analytical results of the sample No. D0010 in this study are as follows. The zirconεHf(t) values ranged between -3.0 and 7.6, with an average of 2.7. TheTDM1was 2513 Ma,which was nearly equal to the weighted mean of the apparent ages of 2491 Ma within the error range. Moreover, the data of the measuring points exactly fell on the mantle evolution line(Fig. 5b), indicating the provenance of materials is depleted mantle or juvenile crust from the Middle Mesoarchean to the Late Neoarchean.

5.3. Constraints of Early Paleoproterozoic tectonic-thermal events on the southwest margin of Yangtze Craton

The Yangtze block borders the North China Craton in the north with the Qinling-Tongbai-Dabie orogen as the boundary, borders the Songpan-Ganzi terrane in the northwest with the Longmenshan Fault as the boundary, borders the Indo-China continental block in the southwest with the Ailaoshan-Honghe fault zone as the boundary, and borders the Cathaysia Block in the southeast with the Jiangnan orogen as the boundary (Hu J et al., 2013; Cui XZ et al., 2019; Fig. 1a).The recently obtained U-Pb ages of the magma in the tuff,dacite, basalt, and granitic gneiss in central Yunnan Province indicate that Archean basement exists on the southwestern margin of the Yangtze Craton, and meanwhile, the Neoarchean Puduhe Group and Mesoarchean Yuanjiang Group were established (Liu F et al., 2009; Gong JH et al.,2012; Liu JP et al., 2018b, 2020b; Li J et al., 2018; Cui .et al,2019). The age of 2491±15 Ma obtained in this study is consistent with the age of the large-scale 2.5 Ga TTG magmatism on the northern margin of the Yangtze Craton (Hu J et al., 2013; Wu YB et al., 2014). Meanwhile, the ages of the Neoarchean Puduhe Group are comparable to the ages of Yudongzi complex, Kongling complex, and Douling complex on the northern margin of the Yangtze block (Ling WL et al.,1998; Hu J et al., 2013; Hui B et al., 2017; Zhou GY et al.,2018; Cui XZ et al., 2019). All these will provide helpful petrologic and chronological evidence for the analysis of the early crust evolution of the entire Yangtze block.

During the Paleoproterozoic (2.4-1.8 Ga), metamorphic magmatic activities widely developed on the northern margin of Yangtze block and the North China Block, and tectonicthermal events extensively developed on the southwestern margin of the Yangtze block (Liu W et al., 2018). The magmatic activities during this period are crucial to the restoration of the history of global tectonic evolution. The magmatic activities and tectonic-thermal events in the Paleoproterozoic (2.4-1.8 Ga) mainly involve the 2.1 Ga gneiss in Houhe complex in Hanzhong City, south Shaanxi Province (Wu YB et al., 2014), the 2.0 Ga gneissic granite in Lengshui complex (Wang ZJ et al., 2015) and the 1.82-1.84 Ga adamellite and moyite in Huashangguan complex (Zhang LJ et al., 2011) in Zhongxiang City, Hubei Province, the 1.84 Ga latite in Maanshan City, Anhui Province, and the 1.82-1.86 Ga eclogite in Weihai, Wumiao, and Rongcheng cities, Shandong Province (Yang JS et al., 2003; Tang J et al., 2004). Recently,many Paleoproterozoic magmatic rocks were also identified on the southwestern margin of the Yangtze block, including the 2.28-2.19 Ga granitic gneiss in Phan Si Pan complex,north Vietnam (Wang W et al., 2016), the 2.36 Ga granitic gneiss in Cuoke complex, Yunnan Province (Cui XZ et al.,2019), the 2.43 Ga tonalite in Tongchang Township, Yimen(Liu JP, 2020c), the 2.30 Ga diabase in Tong’an Township,Sichuan Province (Lu GM et al., 2019), the 2.05-2.18 Ga and 1.84-1.86 Ga intermediary-acid tuff in Yimen County, central Yunnan Province, the 1.85 Ga diabase in Shizishan Village,Yimen County, Yunnan Province, and 2.04 Ga quartz dikes with tectonic differentiation in Xishancun Formation, Yimen County, Yunnan Province (Li J et al., 2018; Liu JP et al.,2018b, 2020b, 2020d). The magmatic activities during this period are precisely consistent with the events caused by the convergence and rifting of the supercontinent Kenorland and the convergence of the supercontinent Columbia.

Three sets of metamorphic and hydrothermal events obtained in this study, which occurred at 2313-2367 Ma,2025-2103 Ma, and 1825-1872 Ma, respectively. They are consistent with above-mentioned ages, and are comparable to the three stages (2.36 Ga, 1.95 Ga, and 1.85 Ga) of metamorphism on the northern margin of the Yangtze block(Xiao LL et al., 2019; Cui XZ et al., 2020). Therefore, it can be inferred that the northern and southwestern margins of the Yangtze block were involved in convergence - rifting of supercontinents Kenorland and Columbia as crucial components. Meanwhile, they showed different degrees of deformation and metamorphism, which is possibly due to their different locations during the Paleoproterozoic. The Mesoarchean Yuanjiang Group and Neoarchean Puduhe Group in central Yundnan Province on the southwestern margin of the Yangtze block also recorded the impacts of Paleoproterozoic tectonic-thermal events (Li J et al., 2018;Liu JP et al., 2018b; Cui XZ et al., 2019).

5.4. Indicative significance of captured zircons and the zircons transformed by tectonic-thermal events

According to theAnnals of Regional Geology in Yunnan Province(Version 2, revision), the LA-ICP-MS zircon U-Pb ages 3.08-1.85 Ga were recently obtained from volcanics and intrusive rocks in the key areas of central Yunnan Province,indicating that Paleoproterozoic-Archean geological bodies exist on the western margin of the Yangtze block, and meanwhile, three groups were newly established, namely the Mesoarchean Yuanjiang Group, Neoarchean Puduhe Group,and Paleoproterozoic Yimen Group (Li J et al., 2018; Liu JP et al., 2018b, 2019, 2020b). In this study, 49 valid U-Pb isotope ages were acquired. Among the zircon grains used in this study, two grains of slightly higher ages are supposed to be the magmatic zircons captured in the surrounding rocks of magmatic pathways during magma rose. Furthermore, their ages 2673 Ma and 2758 Ma correspond to the ages of the Lvzhijiang Formation of the Mesoarchean Yuanjiang Group(Li J et al., 2018; Liu JP et al., 2018b). Meanwhile, 16 zircon grains in the 49 zircon grains had the ages 110-2367 Ma,which are supposed to be ages of several stages of tectonicthermal events. Furthermore, special attention should be paid to that the ages of three zircon grains in the 16 zircon grains were 118 ± 4 Ma, 122 ± 4 Ma, and 110 ± 4 Ma. Based on this,it can be concluded that a stage of important tectonic-thermal event may have occurred in central Yunnan Province in the Late Mesozoic. The study area is located on the eastern margin of Qinghai - Tibet Plateau, and therefore, it was possibly re-transformed by magmatism in Early Cretaceous induced by the subduction of the Meso-Tethys Ocean. This is consistent with the eras of the Yanshanian mafic dikes in Dongchuan District, Yunnan Province (Wang SW et al.,2014), and is highly consistent with the large-scale magmatic activities in the middle-lower reaches of the Yangtze River and North China in the Early Cretaceous. In addition, this is also highly consistent with the global Pacific supercontinentmantle plume events in the same period (Wang SW et al.,2014). Therefore, it is necessary to conduct further demonstration in the future. The tectonic-thermal events of other stages (at 1825-2367 Ma, the zircon ages of the remaining 13 measuring points) are comparable to critical regional tectonic - thermal events on the western margin of the Yangtze Craton at 2.4-2.3 Ga, 2.0 Ga, and 1.8 Ga (Lu SN et al., 2002; Yin FG et al., 2012; Zhu HP et al., 2011; Zhou BG et al., 2012). Therefore, it can be inferred that (1) the early metamorphism of the Maolu Formation of the Neoarchean Puduhe Group occurred in Early Paleoproterozoic, which may be related to the rifting of the supercontinent Kenorland; (2) the Middle metamorphism of the Maolu Formation occurred in Middle Paleoproterozoic,which possibly correlates with the convergence of the supercontinent Columbia and is consistent with the era when extensive 2.04 Ga hydrothermal quartz dikes with tectonic differentiation developed in Yimen County (Liu JP et al.,2020d); (3) the late metamorphism of the Maolu Formation possibly occurred in Late Paleoproterozoic under intracontinental extensional tectonic background; it was accompanied by the 1.90-1.65 Ga bimodal magmatic activities and may be related to the rifting of the supercontinent Columbia (Zhu HP et al., 2011; Wang DB et al., 2013; Guan JL et al., 2011; Zhou BG et al., 2012; Zhou JY et al., 2011; Liu JP et al., 2020a). All these indicate that the Neoarchean Puduhe Group underwent the main stage of greenschist-facies metamorphism in the Paleoproterozoic and possibly suffered re-transformation by magmatism followed by thermal metamorphism in the Late Mesozoic (early Cretaceous).

6. Conclusions

(i) In this study, the zircon U-Pb age 2491 ± 15 Ma were obtained in intermediary-acid tuff in Qigfulangqing Village,Tongchang Township for the first time, and thus it is believed that the Paleoproterozoic - Neoarchean strata with a thickness of over 2000 m exist to the west of Yimen-Luoci fault zone in the central Yunnan Province. Meanwhile, the strata in the Qigfulangqing Village should be grouped into the Maolu Formation of the Neoarchean Puduhe Group rather than being classified as the Heishan Formation of the Mesoproterozoic Dongchuan Group previously.

(ii) The sedimentary ages of the Maolu Formation of the Neoarchean Puduhe Group are highly consistent with the eras of large-scale orogenic magmatic activities on the northern margin of the Yangtze Craton in the Late Neoarchean. Thus,they recorded the orogenic process consisting of subduction and collision from Late Neoarchean to Early Paleoproterozoic. Meanwhile, the metamorphic ages 2.3 Ga,2.0 Ga and 1.8 Ga are highly consistent with three metamorphic ages 2.36 Ga, 1.95 Ga and 1.85 Ga of the northern margin of the Yangtze Craton. All these will provide strong petrological and chronological evidence for the analysis the early crustal evolution of the Yangtze block.

(iii) The provenance of the rhyolitic tuffaceous slate in the Maolu Formation of the Neoarchean Puduhe Group mainly includes the Middle Mesoarchean-Late Neoarchean depleted mantle or juvenile crust. The plate underwent the main stage of greenschist-facies metamorphism in the Paleoproterozoic,and was possibly re-transformed by magmatism in Early Cretaceous due to the subduction of the Meso-Tethys Ocean.

CRediT authorship contribution statement

Jun-ping Liu, Su-mei Tian, and Xun-zao Zhu conceived of the presented idea. Jun-ping Liu wrote the manuscript in consultation. Su-mei Tian, and Xun-zao Zhu supervised the findings of this work. All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgment

The authors hereby extend the sincere gratitude to Qiuyun Yuan from the Nanjing Hongchuang Exploration Technology Service Co., Ltd., and Dan Zhu and Shi-yang Pan from the mineral laboratory of the Hubei Geological Research Laboratory for their assistance in zircon LA-ICP-MS analysis and CL images. The thanks also go to the reviewers for their valuable comments. The work was financially supported by Project of 1∶50000 Regional Geological Survey of Samaki,Yinmin, Guicheng and Shugu Sheets in Yunnan Province by Land and Resources Department of Yunnan Province(D201905); Project of 1∶50000 Regional Geological Survey of Erjie, Yimen, Mingyihe and Shangpubei Sheets in Yunnan Province (DD20160017) and Regional Geological Survey Area Summary and Service Product Development in Yunnan Province by China Geological Survey (121201102000150012-02).