Provenance records of the North Jiangsu Basin,East China: Zircon U-Pb geochronology and geochemistry from the Paleogene Dainan Formation in the Gaoyou Sag

Chun-Ming Lin , Xia Zhang Ni Zhang Shun-Yong Chen Jian Zhou Yu-Rui Liu

1. State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University,Nanjing 210093, China

2. Institute of Geological Sciences, Jiangsu Oil field Branch Company, SINOPEC, Yangzhou 225009, China

Abstract Detailed zircon U-Pb dating and whole-rock geochemical analyses were carried out on the sedimentary rocks of the Paleogene Dainan Formation from Gaoyou Sag in the North Jiangsu Basin, East China. Whole-rock rare earth element characteristics suggest that the provenance was mainly from the Late Proterozoic low-grade metamorphic felsic rocks in the Dabie-Sulu orogenic belt, with the parent rocks probably being the I-type high-potassium granite gneiss. Cathodoluminescence images indicate that most of the detrital zircons are originally magmatic. A few zircons show overgrowths, indicating multiple-episode tectonic events. The U-Pb age distribution patterns of the detrital zircons suggest four main magmatic episodes in the provenance: Late Archean-Early Proterozoic (2450-2600 Ma), Early Proterozoic (1700-1900 Ma), Late Proterozoic (700-850 Ma), and Late Paleozoic-Mesozoic(100-300 Ma). These zircon U-Pb age and whole-rock geochemical results suggest that the sediments of the Dainan Formation were mainly sourced from the recycled orogenic belts within and/or around the North Jiangsu Basin, including the basement of the Yangtze Block,the Neoproterozoic rocks in the Dabie-Sulu orogenic belt, and the Mesozoic igneous rocks in the south part of Zhangbaling Uplift.

Key words zircon U-Pb dating, Paleogene, provenance analysis, Dainan Formation,North Jiangsu Basin, East China

1 lntroduction＊

The North Jiangsu Basin is one of the richest regions in oil and gas of East China, with widely distributed and thickest Mesozoic-Cenozoic strata. The Paleogene Dainan Formation is one of the most productive reservoir intervals in the Gaoyou Sag, North Jiangsu Basin (Qiuet al., 2006). It shows a well exploration prospect as a structure-lithologic reservoir (Zhanget al., 2005). There is a close relationship between the reservoir distribution and sedimentary facies, which in turn is controlled by a combination of the tectonic system and sediment supply. Previous studies on the Dainan Formation were predominantly focused on the paleontology, sequence stratigraphy, reservoir and sedimentary facies (Zhanget al., 2005; Qiuet al.,2006; Zhouet al., 2010; Gao and Lin, 2012), however, little attention has been paid to the provenance studies (Shuet al., 2005).

At present, provenance analysis for sedimentary rocksis conducted mainly by traditional methods such as mineral components, heavy mineral analysis, and sedimentology. In contrast, there are few studies on the sedimentary basin using zircon U-Pb chronology and whole-rock geochemistry to recover sediment provenance characteristics (e.g., the types of parent rocks, formation stage and the related tectonic events) and to identify the different provenance contribution (Zhouet al., 2011; Zhanget al.,2012a, 2012b). The zircon U-Pb geochronology has its unique advantage in tracing the provenance of various sediments and sedimentary rocks, on which we can reconstruct palaeogeographic patterns, define related tectonic processes, and reveal the early continental evolution, and as a result the utility in provenance analysis has become one of the international research hot spots (Rainbirdet al.1997; Wysoczanskiet al., 1997; Sircombe and Freeman,1999; Cawood and Nemchin, 2000; Wildeet al., 2001;Vermeesch, 2004; Yueet al., 2005; Weislogelet al., 2006;Zhenget al., 2006; Xuet al., 2008; Zhouet al., 2008; Liuet al., 2012). In this paper, the provenance of the Paleogene Dainan Formation from Gaoyou Sag in the North Jiangsu Basin was studied in detail, based on the zircon U-Pb dating and whole-rock geochemistry analysis,which will be helpful to study the information of source rocks and to reconstruct the coupled relation between the sedimentary basin and provenance.

2 Geologic setting

The basement of the Yangtze Block experienced a series of tectonic movements since it formed, and it was eventually stablized after the formation of the unified craton by the Jinning Movement. It is characterized by a dual structure including the low-grade metamorphic rock series and plutonic metamorphic rock series, in which the latter is characterized by high seismic velocity, resistivity, magnetic field strength and density. The plutonic metamorphic rock series is present as clumps surrounded by the low-grade metamorphic rock series, and the biggest one is located in the east of Jianhu, north of Dafeng, which stitches together with the positive gentle magnetic fields of the central South Yellow Sea to form the “South Yellow Sea continental nucleus” in the Early Archean-Late Proterozoic; there are a number of small blocks scattered in the nuclear periphery (Figure 1; Zhang, 1991).

The North Jiangsu Basin is a large Cretaceous-Tertiary fault depressed basin developed on Proterozoic metamorphic rocks and Early Mesozoic carbonate, turbidite, and clastic rocks basement in the northeast corner of the Yangtze Block and southern edge of the Sulu Orogen (Figure 2a; Shuet al., 2005). Geographically, it is located in the north of Jiangsu Province, with a small part lying in the Tianchang area of the East Anhui Province. It is 260 km in length and tapers from the east (220 km) to the west(110 km); it could be divided into three nearly east-west trending first-order tectonic units: the Dongtai Depression,the Jianhu Uplift, and the Yanfu Depression, from south to north; it is bounded by the Binhai Uplift in the north, the Sulu upheavals in the west and the Tongyang Uplift in the south, and it penetrates into the South Yellow Sea in the east, being the onshore part of the North Jiangsu-South Yellow Basin (Figure 2a, 2b; Shuet al., 2005). Before the Cretaceous Taizhou Formation, the North Jiangsu Basin was mainly controlled by the ancient Pacific tectonic regime; then it was affected by the combination of the Pacific Plate and the India Plate (Shuet al., 2005). The North Jiangsu Basin has experienced large-scale nappes during the Yinzhi-Yanshan period, forming a series of reverse faults with the cross-section west-dipping and the strike in a NE direction. At present, the oil and gas exploration layers in the North Jiangsu Basin include the Late Cretaceous Taizhou Formation (E2t, 65-83.5 Ma), and the Paleogene Funing (E1f, 53-65 Ma), Dainan (E2d, 46-53 Ma), and Sanduo Formations (E2s, 37-46 Ma), which are characterized by the lacustrine clastic-sedimentary rocks (Qiuet al.,2006; Chen, 2010; Liu, 2010).

The Gaoyou Sag is located in the central part of the Dongtai Depression and it is a dustpan-shaped faultdepression that is faulted in the south and overlapped in the north. It was formed by the differential elevation and subsidence of fault blocks during the Late Cretaceous-Late Paleocene Yizheng and Wubao movements. There are three fault systems (ENE, NE, and NW) formed in the Cenozoic in the study area; from south to north they are Zhen 1, Zhen 2, and Hanliu faults, which present as the boundary of the sag and divide the Gaoyou Sag into three secondary tectonic units (southern step-fault zone, central deep depression zone, and northern slope zone; Figure 2c). Hanliu fault is a south-dipping contemporaneous fault with strong activity in the western depression.

3 Samples and analytical methods

Figure 1 Distribution of the metamorphic basement in the North Jiangsu Basin and the adjacent regions (modified from Zhang,1991).

Figure 2 Sketch map showing the tectonic setting of Gaoyou Sag and sample locations for zircon U-Pb dating of the Paleogene Dainan Formation.

This study collected 60 mudstone samples, from 52 wells of Zhouzhuang, Huazhuang, Fumin, Yong’an, Caozhuang,Zhenwu, Shaobo, Lianmengzhuang, and Majiazui regions,for the rare earth element (REE) analysis in the Gaoyou Sag, North Jiangsu Basin. Four representative sandstone samples were obtained for detrital zircon U-Pb dating, located respectively at 2727.26 m in Well Z27 of Zhouzhuang(Zr1), 3165.0 m in Well F83 of South Fumin (Zr2), 3215.5 m in Well Sx14 of Shaobo (Zr3), and 2339.19 m in Well H32 of Huangjue (Figure 2c). Sandstone samples were firstly prepared by crushing, washing and magnetic separation to detach zircons, which were in turn purified by handpicking under a binocular microscope. These zircon grains were put on a double-sided adhesive and placed into a ring mold mounted by epoxy, and then polished about one-third of an individual grain diameter. Cathodoluminescence (CL)images of the zircon grains were taken at the State Key Laboratory of Continental Dynamics of Northwest University, using scanning electron microscopy (Quanta 400 FEG) with Mono CL3+ (Gatan, USA). Zircon U-Pb dating was carried out at the State Key Laboratory for Mineral Deposits Research of Nanjing University using an Agilent 7500a type ICP-MS attached with a New Wave UP213 laser ablation system (wavelength 213 nm; laser beam spot diameter 21-32 μm; frequency 5Hz; energy density 9J/cm2). The standard zircon GEMOC GJ-1 of Australia(207Pb/206Pb age=608.5±1.5 Ma) was used to correct the U-Pb fractionation, and standard zircon Mud Tank (intercept age of 732±5 Ma) as an internal standard to control the accuracy of analysis. More detailed analytical methods and procedures are described by Jacksonet al. (2004) and Wanget al. (2013a). In addition, a key problem that using detrital zircon U-Pb geochronology method for source tracing is the choice of the statistical-particle number. In this study, the zircon number per sample is 90-110 in consistent with the requirements of mathematical statistics (cf.,Vermeesch, 2004; Weislogelet al., 2006). REE analysis is obtained using high resolution inductively coupled plasma mass spectrometry (Finnigan Mat Element 2) according to a standard method described by Gaoet al. (2003).

4 Results

4.1 Rare earth elements (REE) characteristics

The chondrite-normalized REE patterns for the studied sedimentary rocks are characterized by the significant enrichment of light REE, strong depletion in heavy REE,and obviously negative anomalies of Eu (the average δEu of 0.4). The total REE (∑REE) concentrations fall in the range 50.01-250.70 μg/g (average at 174.00 μg/g), which is higher than that of the North American shale (mean of 163.94 μg/g), but lower than the average post-Archean sedimentary rocks (184.77 μg/g) (Taylor and McLennan,1985; McLennan, 1989). In addition, the REE parameters are similar to those of active continental margin, indicating that the tectonic background of parent rocks for the studied sedimentary rocks belongs to an active continental margin setting, with the parent rocks from the uplifted basement(Zhanget al., 2012a). However, comparing with the (La/Yb)Nand LREE/HREE values (9.1 and 8.5, respectively)of the active continental margin sedimentary rocks (Bhatia, 1985), the values of (La/Yb)Nand LREE/HREE in the study area are relatively high (11.55 and 11.29, respectively), illustrating near-source characteristics, and the parent rocks are comparable with the felsic material of the average continental crust (Zhanget al., 2012a).

4.2 Zircon U-Pb ages

4.2.1Zircon morphology

Zircons from sample Zr1 are mainly misty rose, secondarily deep rose, and the morphology is dominantly subrounded granular with a little sub-angular prismatic, irregular shape and equigranular, euhedral columnar, showing the character of detrital zircons. Zircons from this sample are generally 30-70 μm in length. Zircons from sample Zr2 is mainly deep rose to pale pink, and the morphology is mainly sub-angular prismatic to sub-rounded granular.They are generally 50-100 μm in length, with a few of 100-150 μm. Zircons from sample Zr3 is dominantly deep rose, secondarily pale pink, and the morphology is mainly sub-rounded granular, with a little prismatic and irregular equigranular, occasionally angular columnar. The length is generally 50-120 μm. Zircons from sample Zr4 is mainly pale pink, with a bit deep rose and colorless; the morphology is predominantly sub-rounded granular and elongated oval granular, secondly angular, sub-angular and prismatic. They are mainly 50-120 μm in length, with a small part of 120-150 μm (Figure 3).

4.2.2Zircon U-Pb age

In general, the radiogenic components are less in young zircon (＜1000 Ma), and the radiogenic207Pb abundance is lower than the206Pb by about an order of magnitude for the differences in half-life and abundance between235U and238U, as a result, the higher precision206Pb/238U age of the young zircon is selected as the crystallization age of the rock, while the206Pb/207Pb age generally shows more uncertainty for the older zircon (＞1000 Ma) (Compstonet al., 1992). In addition, the computational-concordant ages of young zircons with discordance (%) more than 20% and of older ones with discordance (%) more than 15% are unreliable (Konget al., 2009). Thus, the test samples of Zr1-Zr4 respectively got 97, 94, 100 and 85 valid data.In206Pb/238U-207Pb/235U concordia diagram, all measured points are plotted on or near the concordia (Figure 4).

Figure 3 Representative CL images for the detrital zircons from the Dainan FormationinGaoyou Sag (the circles show the number of analysis spots and the age results are indicated below the zircons).

The Th/U ratios for all the spot analyses vary significantly, from 0.11 to 3.56 (Table 1), consistent with the high Th/U ratio of typical magmatic zircon (Wu and Zheng,2004). Most zircon grains show oscillatory zoning in CL images (Figure 3), indicating a magmatic origin (cf.,Connelly, 2000). A few zircons are composed of a dark core and a bright overgrowth, showing the metamorphic growth (overgrowth) or recrystallization, which indicates the sources of the Dainan Formation sediments have experienced multiple tectonic-thermal events. For this case, we focus on the age of zircon cores. In fact, the analyses with ages ＞2500 Ma are generally from the zircon cores with higher psephicity (Figure 3), indicating that some of the sediments in the study area came from the Late Archean provenance through long-distance and/or multiple transportations.

1) Sample Zr1: Zircon U-Pb ages of this sample vary significantly from 118±7 Ma to 2710±12 Ma, indicating the multi-sources for the detrital zircons (Figure 4; Table 2). A total of 30 analyses (accounting for 30.9%) are distributed in the Paleoproterozoic (Pt1); There are separately 12 analyses belonging to the Archean (Ar) and Carboniferous (C), accounting for 12.4% of the number of statistics;there are 11 and 10 analyses distributed in the Triassic (T)and Permian (P), accounting for 11.3% and 10.3% of the number of statistics, respectively; the analyses with ages in the Neoproterozoic (Pt3), Jurassic (J) and Cretaceous(K) are 6.2%, 6.2% and 5.2%, respectively, of the number of statistics; the other analyses are less than 5.0% of total zircons.

Figure 4 Concordant diagrams of zircon U-Pb ages for clastic rocks from the Dainan Formation in Gaoyou Sag.

2) Sample Zr2: Zircon U-Pb ages of this sample range from 108±2 Ma to 3068±11 Ma (Figure 4; Table 2), with the main proportions at Pt1and Ar, accounting for 22.9%and 19.8% of the number of statistics, respectively. The analyses at P, T and Pt3, occupying 11.5%, 9.4% and 9.4%,respectively; zircons at Mesoproterozoic (Pt2) made up about 8.4% of the number of statistics; C and K zircon grains are generally 5.2% of the number of statistics; Ordovician (O), Silurian (S) and Devonian (D) zircon grains are less than 5.0% of the number of statistics (Table 2).

3) Sample Zr3: Zircon U-Pb ages of this sample range from 126±7 Ma to 2715±48 Ma (Figure 4; Table 2). They are mainly of Pt1(39.0%) and Pt3(26%). The Ar analyses only occupy 12.0% of statistics; while the Permian (P) and other analyses account for about 6.0% and less than 5.0%,respectively (Table 2).

4) Sample Zr4: The zircon U-Pb ages are between 94±2 Ma and 3656±17 Ma with good coordination (Figure 4; Table 2). They are mainly distributed in the K and Pt3,accounted for 25.9% and 24.7% of statistics, respectively;secondly in the Pt1, about 15.3% of statistics; there are separately 6 zircons from the C and T, about 7.1% of statistics.The zircon of other ages is all less than 5.0%, and the O and D zircons were not observed (Table 2).

Table 1 LA-ICP-MS zircon U-Pb dating results of the Dainan Formation in Gaoyou Sag

Table 1 (continued)

Table 2 Frequency data of zircon U-Pb ages for clastic rocks from the Dainan Formation in Gaoyou Sag

Cenozoic (Cz) zircons are absent in the study area (Table 2).

5 Discussion

5.1 Provenance comparison

A variety of rocks of the Early Mesozoic, Paleozoic and Neoproterozoic are exposed around the North Jiangsu Basin. Previous studies suggested that the sediments of the North Jiangsu Basin are mainly derived from five provenance regions since the Cretaceous: (1) the western Dabie orogen and the east section of the Tan-Lu fault zone characterized by Proterozoic metamorphic and volcanic rocks; (2) the northeastern Binhai Uplift; (3) the northwestern Sulu orogenic belt composed mainly of metamorphic and igneous rocks; (4) the southwestern Zhangbaling Uplift with Neoproterozoic metamorphic rocks and Mesozoic igneous rocks; (5) the southern Jiangdu and Ningzhen mountains characterized by Paleozoic-Early Mesozoic clastic and carbonate rocks (Shuet al., 2005). This paper will discuss how the bed rocks or intrusive rocks from the above peripheral large provenances affect the weathering deposits in the study area by the comparison of the REE pattern (Figure 5).

The Neoproterozoic high-potassium I-type granitic gneiss of the Jiaodong Section in the Sulu orogenic belt is characterized by the strong REE fractionation ((La/Yb)N=7.4-13.9) and the obviously negative Eu anomaly(δEu=0.47-0.61), similar to the granites in active continental margins and clastic rocks of the Dainan Formation in the Gaoyou Sag (Xueet al., 2006; Figure 5a). Figure 5b shows that the REE distribution patterns of the mudstones from the Dainan Formation in the study area is very similar to the granitic gneiss of Dabie Group in Dabie Orogen,which is characterized by a significant deficit of Eu with the ΣREE of 126-311 μg/g and LREE/HREE of 10.5 (Wuet al., 1998). In addition, the REE distribution patterns for the Mesozoic-Cenozoic sandstones in the southeastern Dabie Orogen are coincident with the Proterozoic blueschist belt of the Susong and Dabie groups in the southern Dabie Orogen, indicating the feasibility that the blueschist belt of the Susong Group and part of the Dabie Group had been denuded as a source in the Middle-Late Triassic (Liet al., 2005). Therefore, the provenance of the Dainan Formation has genetic relationship with the Proterozoic lowgrade metamorphic felsic igneous rocks in the Dabie-Sulu orogenic belt, with the parent rocks as the high-potassium I-type granitic gneiss.

The Zhangbaling Uplift is characterized by the Proterozoic epimetamorphic rocks with some Mesozoic igneous rocks, lying between the Dabie and Sulu orogens extending along the Tan-Lu fault zone in a NNE direction (Figure 2b). The Proterozoic Zhangbaling Group is exposed at the north section of the uplift, and the exposed south section of the uplift consists dominantly of the Archean-Paleoproterozoic Feidong Group and the Neoproterozoic Zhangbaling Group. Feidong metamorphic complex belongs to part of the Dabie-Jiaonan orogenic belt which is characterized by the occurrence of high-potassium calcalkali granites. The REE distribution pattern (ΣREE 32.7-161 μg/g, LREE/HREE 3.87-7.94, (La/Yb)N2.45-6.38,δEu 0.62-0.97) of the upper part of the Zhangbaling Group composed mainly of spilite-quartz keratophyre series (also known as the blueschist), is obviously different from that in the study area (Guo and Wang, 1995; Figure 5c), which shows that the Neoproterozoic spilite-quartz keratophyre series in the Zhangbaling Uplift has had little impact on the study area. The lower part of the Zhangbaling Group is distinguished by greenschist-series metamorphic rocks with a small amount of blueschist. Given that the greenschist were primarily formed by blueschist through the ductile deformation and metamorphism in a high pressure, and relatively low oxygen fugacity environment, the greenschists widespread in the Zhangbaling region may be unlikely the provenance of the study area. Although there are a small amount of Mesozoic igneous rocks exposed in the Zhangbaling Uplift, previous studies on the Mesozoic igneous rocks in the northern Zhangbaling Uplift showed that, the REE distributions own similar ΣREE (112-126 μg/g) and LREE /HREE values (14-18) to those in the Gaoyou Sag, but present positive Eu anomalies; REE patterns of the igneous rocks in the southern section are similar to those of the studied area with close δEu (0.53-0.74),whereas higher ΣREE (215-323 μg/g) and LREE/ HREE ratios (19-22) (Niu, 2006; Caoet al., 2010; Figure 5d).Therefore, the in fluence of the igneous rocks in the Zhangbaling Uplift on the deposits of the Dainan Formation in the Gaoyou Sag needs further detailed study, which will be discussed later.

Figure 5 Chondrite-normalised REE distribution patterns of studied rocks and the possible provenances surrounding the North Jiangsu Basin (Chondrite-normalised data from Taylor and McLennan, 1985). The shadow zones represent the REE distribution patterns of the Dainan Formation in the study area. a-High-potassium I-type granitic gneiss of the Sulu Orogen, with data from Xue et al., 2006; b-Proterozoic granite gneiss in eastern Dabie Mountains with data from Wu et al., 1998; c-Neoproterozoic spilite-quartz keratophyre of Zhangbaling Uplift with data from Guo and Wang, 1995; d-Mesozoic intrusive rocks of Zhangbaling Uplift with data from Niu, 2006 and Cao et al., 2010.

Most exposed-rock masses in the middle Ningzhen Mountains are I-type intermediate-acidic granite, with the enrichment of LREE, the absence of Eu negative anomaly, and occasionally a Ce negative anomaly in some rock masses (Zhanget al., 2012a). Therefore, the intrusive rocks in the Ningzhen Mountains have little in fluence on the source of sedimentary rocks of the Gaoyou Sag.

In summary, the sedimentary rocks of the studied Dainan Formation were predominantly derived from the Proterozoic low-grade metamorphic rocks in the Dabie-Sulu orogenic belt, with the parent rocks as I-type high-potassium granitic gneiss. However, the Proterozoic blueschist belt in the Zhangbaling Uplift, the large-scale eclogite distributed in the southern Dabie Orogen, and the Mesozoic intermediate-acid intrusive rocks widely distributed in the Ningzhen Mountains have negligible effect on the sedimentary rocks of the Gaoyou Sag.

5.2 Geochronology

It is noteworthy that this study simultaneously captured the latest and oldest zircon grains in the Huangjue area.The zircon grain with the youngest age is subhedral and homogeneous with clear rhythm zonation, indicating a magmatic origin. Its age (94±2 Ma) suggests that the Late Cretaceous igneous rocks may have partly presented as a provenance in the study area. In addition, it also shows that the tectonic movements after the Late Cretaceous in the North Jiangsu Basin have not been significantly recorded in zircons, but are just characterized by the basin filling and sediment redistribution. The oldest zircon of this study belongs to metamorphic type with core-rim structure. The magmatic core gave an age of 3656±17 Ma, which corresponds to the ancient detrital zircons (＞3500 Ma, up to 3900 Ma) found in the sedimentary rocks in the Yangtze Craton, further con firming the existence of very old Early-Archean crust material in Yangtze Craton (cf., Jiaoet al.,2009; Wanget al., 2013b). At the same time, a zircon with the age of 546±8 Ma is found in the Fumin region, corresponding to the global Pan-African orogenic event referring to the close of the Mozambique Ocean, progressive polymerization of east and west Gondwana, and the eventual formation of Gondwanaland (550-600 Ma; Luet al.,2004). As a whole, zircons from the studied samples can be divided into four episodes based on their U-Pb ages,which reflect the multiple sources of the sedimentary rocks(Figure 6; Table 1).

1) 2450-2600 Ma (Paleoproterozoic-Neoarchaean):This period yields a weighted average age of 2516 Ma(N=49, MSWD=0.34), consistent with one of the main age peaks for the Yangtze Block basement (2500 Ma; She,2007), which reveals the recycling of the Neoarchaean crystalline basement of the Yangtze Block in the provenance in the study area.

2) 1700-1900 Ma (Paleoproterozoic): The weighted average age for this period is 1855 Ma (N=74, MSWD=5.6).Since the Yangtze Block is characterized by the age peaks at 2500 Ma, 2000 Ma and 800 Ma and the absence of 1800-1900 Ma age peak, the zircons of this group are unlikely to be derived from the crystalline basement of the Yangtze Block (She, 2007). Although the Archean basement of the Yangtze Block is occasionally exposed (such as the Kongling complex), recent studies have showed that the Yangtze Block has the records of a 1800-2100 Ma tectonothermal event. This age group (1700-1900 Ma) is after the polymerization between the Yangtze Block and the Columbia Supercontinent that happened in 1900-2000 Ma (Penget al., 2009), more close to the formation age of the ma fic dikes (1850 Ma) in response to the regional extension found in the Kongling high-grade metamorphic body. This indicates the magmatism in the Yangtze Block may have occurred in relation with the breakup of the Columbia Supercontinent during the conversion period(about 1850 Ma) from the collision compression to extension (Zhanget al., 2011).

3) 700-850 Ma (Neoproterozoic): The weighted average age for this period is 789 Ma (N=33, MSWD=4.9).This age group corresponds to the dating results of the Neoproterozoic granitic gneiss in the Dabie-Sulu orogenic belt, which are mainly concentrated in the 700-800 Ma with a peak age of 750 Ma (cf., Zhenget al., 2006; Huet al., 2010; Liuet al., 2012). Available studies suggested that the Neoproterozoic granitic magma event and associated ma fic magmatic events lasted from 780 Ma to 680 Ma in the south Dabie-Sulu ultrahigh pressure metamorphic belt, corresponding to the breakup of the Rodinia Supercontinent and the 800-850 Ma orogenic event before the breakup (Xuet al., 2006; Huet al., 2007).

4) 100-300 Ma (Mesozoic-Late Paleozoic): This group occupies the largest proportion of the zircons in the Gaoyou Sag. It can be further subdivided into three subgroups. Firstly, the 100-200 Ma subgroup corresponds to the Jurassic-Cretaceous (Yanshanian period), with a weighted average age of 108.3 Ma (N=21, MSWD=3.0),consistent with the multi-period magmatic activity in the south section of the Zhangbaling Uplift (Niuet al., 2008).Secondly, the 200-250 Ma subgroup refers to the Late Permian-Triassic (Indosinian period), with a weighted average age of 228 Ma (N=28, MSWD=3.4), in accordance with the formation age for the ultrahigh pressure metamorphic rocks (225-240 Ma) in the Dabie-Sulu orogenic belt(Zheng, 2008), reflecting the Triassic collisional event between South China and North China recorded in the study area (Liuet al., 2012). Thirdly, the 250-300 Ma subgroup corresponds to the Permian (late Hercynian period), with a weighted average age of 267 Ma (N=37, MSWD=1.6),corresponding to the first phase of the felsic magmatic activity (the end time of which is about 260 Ma) of the Emeishan large igneous province in the western margin of the Yangtze Block (Taoet al., 2008; Xuet al., 2008).

Combined with the regional tectonic evolution and sedimentary history, the sediments of the Dainan Formation in the study area are mainly derived from the internal part of sedimentary basin (crystalline basement) and the recycled orogenic belts around the basin. That is to say, the sediments are predominantly from the Neoarchaean-Paleoproterozoic crystalline basement of the Yangtze Block and Neoproterozoic low-grade metamorphic basement of the Dabie-Sulu orogenic belt, with the parent rock as the high-potassium I-type granitic gneiss. The Mesozoic intrusive rocks of the south Zhangbaling Uplift may have also provided a part of the sediments for the Dainan Formation.

Figure 6 Frequency spectrum diagram of zircon U-Pb ages for clastic rocks from the Dainan Formation in Gaoyou Sag.

6 Conclusions

Provenance comparative analysis of the REE shows that the provenance of the study area is mainly low-grade metamorphic acidic igneous rocks of the Neoproterozoic basement in the Dabie-Sulu orogenic belt, and the specific parent rock type may be the high-potassium I-type granitic gneiss. Zircon U-Pb dating results show that the detrital zircons of the Dainan Formation in the North Jiangsu Basin are dominated by magmatic zircons, and the ages could represent the crystallization age of their parent rocks. A small proportion of zircon grains belong to the hyperpla-sia-mixed zircon, which indicates the provenance area had experienced multi-phase tectonic-thermal events. Zircons from the studied samples can be divided into four groups:(1) Neoarchaean-Paleoproterozoic (2450-2600 Ma), indicating the Yangtze Block crystalline basement existed in the study area; (2) Paleoproterozoic (1700-1900 Ma),in response to the breakup of the Columbia Supercontinent, indicating the conversion process from the collision compression to extension of the Columbia Supercontinent may have occurred in the Yangtze Block in about 1850 Ma; (3) Neoproterozoic (700-850 Ma), in relation with the Neoproterozoic granitic gneiss in the Dabie-Sulu orogenic belt and responding to the assembly and breakup of the Rodinia Supercontinent; (4) Late Paleozoic-Mesozoic(100-300 Ma), indicating that the provenances are igneous rocks formed by multi-stage magmatic activities in the south section of the Zhangbaling Uplift (100-200 Ma),the ultrahigh-pressure metamorphic rocks of the Dabie-Sulu orogenic belt (200-250 Ma), and a few from the first phase of felsic magmatic activity of the Emeishan large igneous province in the western margin of the Yangtze Block(250-300 Ma).

Acknowledgements

Thanks are extended to Jian-Sheng Qiu, Xiao-Lei Wang, and Bing Wu of Nanjing University for their help in sample analysis and useful suggestions. Thanks are also given to the relevant staff of the Jiangsu Oil field for providing the borehole data.