Provenance and Tectonic Implications of Paleozoic Strata in the South Yellow Sea Basin, China–Revealed from the Borehole CSDP-2

GAO Xiaohui, ZHANG Xunhua, GUO Xingwei, CAI Laixing, HOU Fanghui and ZHU Xiaoqing

Provenance and Tectonic Implications of Paleozoic Strata in the South Yellow Sea Basin, China–Revealed from the Borehole CSDP-2

GAO Xiaohui1), 2), 3), 4), ZHANG Xunhua3), 4), *, GUO Xingwei3), 4), CAI Laixing3), 4), HOU Fanghui3), 4)and ZHU Xiaoqing3), 4)

1),,266100,2),,266100,3),,266237,4),,266071,

Well CSDP-2 is a fully coring deep drilling hole, penetrating the thick Paleozoic marine strata in the South Yellow Sea Basin (SYSB) in the lower Yangtze Block (LYB). Based on the petrological and geochemical analysis of 40 sandstone samples from the core CSDP-2, the provenance and tectonic features of Paleozoic detrital rocks from SYSB are analyzed and systematically delineated in this article. The results show that the Silurian–Carboniferous sandstones are mature sublitharenite, while the Permian sandstones are unstable feldspathic litharenite. The average CIA (chemical index of alteration) is 74.61, which reflects these sediments were derived from source rocks with moderate chemical weathering. The REE (rare earth element) patterns are characterized by LREE enrichment, flat HREE and obviously negative Eu anomaly, which are similar to that of the upper continental crust. Dickinson QFL discrimination results indicate the recycled orogeny provenance. Various diagrams for the discrimination of sedimentary provenance based on major and trace element data show all the sediments were derived predominantly from quartz sedimentary rocks, of which the Permian strata contain more felsic sedimentary rocks. Geochemical data for these detrital rocks suggest they occur at the passive continental margin and island arc settings, and the Permian sandstone presents active continental margin setting.

South Yellow Sea Basin; Paleozoic; geochemistry; provenance analysis; tectonic setting

1 Introduction

The South Yellow Sea Basin (SYSB) is the offshore extension of the lower Yangtze Block (LYB), which is a large-scale superposed sedimentary basin being composed of pre-Indosinian (Paleozoic–middle Triassic) marine de- position and Mesozoic–Cenozoic continental deposition (Gong, 2017; Pang, 2017a; Zhang, 2017). For a long time, due to a lack of deep drilling and provenance analysis, the Paleozoic sediment source and tectonic setting of SYSB are still poorly understood. The previous studies focus mainly on the Neoproterozoic–Paleozoic orogenesis of Cathaysia Block (Li, 2010; Wang, 2010; Xu, 2012; Wang, 2018) and concluded that the early Paleozoic detritus was derived from the inland Wuyi-Yunkai Massif of the South China Craton. Bao(2011), Li(2013) and Li(2017) studied the Paleozoic detrital zircon and geochemistry fea-tures of samples from the land area of LYB and inferred that the tectonic background of Cathaysia Wuyi Massif between Carboniferous to Permian is most likely to be a continental arc. As is well known that the geochemical characteristics usually reflect the source, denudation and transportation process of the clastic rocks, especially some trace elemental compositions such as rare earth element (REE), Zr, Th and Sc (McLennan., 1980; Taylor and McLennan, 1985). So they are widely used in the provenance discriminant researches (Wang, 2012; Du, 2013; Li, 2014; Han, 2016).

From 2015 to 2016, the Chinese Continental Shelf Dril- ling Program (CSDP) completed the drilling of a continu- ous core named CSDP-2 of more than 2800m in length, which is the first scientific drilling borehole in the Central Uplift of SYSB in order to reveal the Mesozoic–Paleozoic stratigraphic sequences and evaluate hydrocarbon pro- spects (Guo, 2017b; Zhang, 2017; Yuan, 2018; Pang., 2020b). Based on the geochemical analysis of 40 sandstone samples from Paleozoic marine strata in core CSDP-2, the major and trace element characteristics are used to discriminate the Paleozoic provenance and its tectonic environment for the Central Uplift of SYSB. The conclusions of this article will be significant for the study of the regional tectonic evolution, basin modeling and resource evaluation.

2 Geological Setting

The SYSB is located on the lower Yangtze Block. On thenorthwest boundary along the Qinling-Dabie-Sulu Orogen and Tan-Lu Fault, the LYB is adjacent to the North China Craton. To the south, it is bounded by the Jiang- Shao fault and next to the Cathaysia Block in South China. On its west side is the middle Yangtze Block bounded by the Ganjiang Fault, and on its east side is Korean Peninsula (Meng., 2018). The SYSB can be subdivided into five secondary tectonic units from north to south: Qianliyan Uplift, Northern Depression, Central Uplift, Southern Depression and Wunansha Uplift (Zhang, 2014; Pang, 2017b). The central Uplift is a nearly EW-trending zone between approximately 34˚N and 35˚N in latitude and separates the Northern Depression and Su- bei-southern SYSB (Fig.1).

The South China Craton has completed the consolidation and unification of the basement since the Neoproterozoic Jinning movement and thus developed at a relati-vely stable stage of platform development. Under the in fluence of Caledonian Orogeny, the Yangtze Block collided with the Cathaysia Block and formed a foreland basin in the southwest of lower Yangtze area. During this period the SYSB developed black shale deposition in shallow shelf facies (Yang, 2019). At the end of Caledonian Orogeny in late Silurian, the lower Yangtze Uplift was denuded until the late Devonian, before it began to subside and receive deposition again. At this point, it entered the stage of relatively stable marine craton basin, when transitional facies and carbonate platform facies developed in the SYSB (Shu, 2012; Qi, 2013; He, 2014; Pang., 2020a).

The borehole CSDP-2 (34˚33΄18.9˝N, 121˚15΄41˝E) is located in approximately 170km east of Lianyungang City, Jiangsu Province (Fig.1), with a total depth of 2843.18m and an average recovery of 97.7% (Guo., 2017b). It encounters the lower Silurian firstly, filling up the vacantof geological data in this region. Moreover, the core interpretation confirms the inference that the thick marine strata are well developed in the Central Uplift, and for the first time, many oil and gas signs are founded and several sets of high quality source rocks are revealed (Cai, 2018a, 2018b; Zhang, 2018; Pang, 2019). The Paleozoic formation thickness is more than 1970m in this borehole. Beneath the Triassic strata, the well CSDP-2drilled through upper Permian Longtan-Dalong Formation (P2l-d), lower Permian Qixia-Gufeng Formation (P1q-g)upper Carboniferous Huanglong-Chuanshan Formation (C2h-č), lower Carboniferous Gaolishan Formation (C1g), upper Devonian Wutong Formation (D3w) and lower Silurian Gaojiabian-Fentou Formation (S1g-f) (Fig.2).

Fig.1 Tectonic sketch map of the SYSB located in Lower Yangtze Block (modified after Shu, 2012) with the borehole CSDP- 2 in the Central Uplift. The red solid/dashed lines represent main/detective faults. The light gray shadow shows the orogen area, while the dark gray shadow shows the basin area. 1, Tan-Lu Fault; 2, Ganjiang Fault; 3, Jiangnan Mesozoic North Margin Buried Fault; 4, Jiang-Shao Fault; 5, Dongxiang-Dexing Fault; 6, Zhenghe-Dapu Fault.

Fig.2 Paleozoic stratigraphic column section of borehole CSDP-2.

The Silurian is mainly composed of gray-black mudstone in shallow sea shelf environment. A large number of Silurian acritarchs are identified, such asandA set of gray-black and red-brown medium- coarse quartz sandstone is developed at the bottom of S1f with parallel bedding and a thickness of 53.3m, which is interpreted as sandy shore deposits (Figs.3a, e).

The D3w is characterized by a set of highly mature sandstone, siltstone and mudstone with marine fossils, with a parallel unconformity above the S1f. The upper part named Leigutai Formation (2033.6–2087.8m) is a delta front se- dimentary facies, while the lower Guanshan part (2087.8–2319.2m) is composed chiefly of hydrodynamic shoal se- dimentary facies with cross bedding and tidal flat sedimentary facies with positive rhythmic lithological sequence (Figs.2, 3b, 3f). We found several well-preserved spore fossils of late Devonian in South China (assemblage).

From late Carboniferous to early Permian, the study area was dominated by carbonate platform deposition. The C2h and C2č consist of thick bioclastic limestone with abundant Carboniferous-Permian fossils (,belt). The lithology of P1q is mainly gray-black swinestone and mudstone, while P1g is black siliceous mudstone with thin coal seam. Only the C1gdevelops mixed color sand-mudstone and blue-gray fine-grained quartz sandstone (Fig.3c), which is interpre- ted as delta plain sedimentary facies. In the Permian, fine siltstone deposits under the delta environment, and a set of clastic tidal flat facies strata were developed in the lower part of P2l (Gao, 2019; Fig.3d).

Fig.3 Core characteristics of borehole CSDP-2. a, 2608.5 –2608.7m (S1f),gray-black medium-grained sublitharenite; b, 2172.9–2173.2m (D3w), gray sublitharenite with parallel and cross bedding; c, 1986.4–1986.6m (C1g), green- gray fine sublitharenite with parallel bedding; d, 1524.1–1524.3m (P2l), fine feldspathic litharenite with tidal cross bedding; e, 2605.1–2609.6m (S1f), gray-black medium- coarse sublitharenite (occasional gravel); f, 2271.7–2276.3m (D3w), dark-gray fine-medium sublitharenite.

By making lithological comparison between those from borehole CSDP-2 and Nanjing, we found that the SYSB is an extension of onshore lower Yangtze Subei Basin to the sea. In addition, according to the identification of the late Devonian plant fossils in the borehole and their comparison with those from the Korean Peninsula, the LYB can be extended to the central part of Korean Peninsula (Guo, 2017a).

In this article, 40 unaltered or weakly altered sandstone samples were selected from formation S1f, D3w, C1g and P2l (sampling sites are shown in Fig.2). Detrital component identification and geochemical measurement of major element and trace elemental compositions were carried out respectively.

3 Detrital Components

The analysis of sandstone detrital component is widely used in the study of source rock types and their tectonic setting (Dickinson, 1983; Li, 1999; Ma, 2014). The Gazzi-Dickinson method of point counting was developed to get most source-rock data, while using the least time, effort and expense of gathering such data. This me- thod minimizes the variation of grain size, thus eliminating the need for sieving and multiple counting for different size fractions (Dickinson and Suczek, 1979). In this paper, Gazzi-Dickinson method and Zeiss Axio Imager M2m metallographic microscope were applied to manually calculate the content of detrital grains. The total number of grains in each sample was not less than 400. The detrital mineral composition of the Paleozoic sandstone in core CSDP-2 is listed in Table 1, based on which the sandstone type identification is made (Folk, 1980; Fig.4).

Fig.4 Distinction diagram of Paleozoic sandstone types based on the mineral composition (after Folk, 1980).

It can be seen from Fig.4 that the sandstone types of Fentou, Wutong and Gaolishan Formationsare sublitha- renite, while Longtan Formation is feldspathic litharenite. In Fentou, Wutong and Gaolishan Formations, the major detrital component is quartz, with the average contents of 90.1% (85%–93%), 91.8% (90%–94%) and 87.9% (84%–89%), respectively. They contain trace amounts of feldspar (0.6%, 0.2% and 2.5%, respectively). The lithic frag- ments account for 9.3% (6%–13%), 8.0% (6%–10%) and 9.6% (7%–11%) including metamorphic fragments chiefly (average 6.2%, 5.7% and 8.7%). The metamorphic fragments in Fentou Formation consist of weakly metamorphic sedimentary rocks, while those in Wutong Formation contain mainly quartz rocks. The grain size ranges from 0.125mm to 0.5mm, which belongs to medium-fine grains. The sandstone is composed of moderately to well sorted, subangular grains, with particle supporting structure, and point-linear contact relation. The matrix contents in all samples are lower than 15%. The quartz grains present widespread and intense secondary enlargement cementation (Figs.5a, b). All the above show that the compositional and textual maturity of the sandstone is very high, and the average values of/(+) are 9.1, 11.2 and 7.3 respectively for Fentou, Wutong and Gaolishan Formations. It can be inferred that the detrital grains were trans- ported for a long-distance. In addition, the lithic composition indicates that parent rocks may include metamorphic rocks and sedimentary rocks.

The sandstone samples from the Longtan Formation are composed of quartz (42%–60%), feldspar (16%–27%) and lithic fragment (21%–36%). The metamorphic fragments accounts for 24.1% averagely. The grains contain subcorrugate-subangular and moderate sorted detritus around 0.125–0.25 mm in diameter (Fig.5c). These characteristics show that the sandstones in Permian have relatively low maturity with the/(+) value of 1.0, which reflect not too far distance of transport and proximal deposits.

Table 1 Detrital mineral compositions of Paleozoic sandstones from CSDP-2 (%)

Notes: Q, Quartz; F, Feldspar; Lv, Igneous detritus; Lm, Metamorphic detritus; Ls, Sedimentary detritus; L, Total detritus (Lv+Lm+Ls).

Fig.5 Photomicrographs of Paleozoic sandstones from core CSDP-2. a, 2620.9m (No. FT04, S1f), medium-coarse sublitharenite (+); b, 2148.5m (No. WT02, D3w), coarse sublitharenite (+); c, 1556.8m (No. LT09, P2l), ultrafine feldspathic litharenite (+).

4 Whole-Rock Geochemistry

40 fresh and unweathered sandstone samples were selected from the core CSDP-2 for whole-rock geochemical analysis. The samples were physically crushed, then cleaned and crushed again to more than 200mesh. The geoche- mical analysis was carried out by the Experimental & Testing Center of Marine Geology, Ministry of Land and Resources, in which the contents of major oxides were determined by Axios PW4400 X-ray fluorescence (XRF) while the trace element compositions were determined by Thermo X Series 2 plasma mass spectrometer. The operation flow and concrete steps are based on GB/T20260-2006 and DZG20.01-1991. The analytical precisions are better than 5% according to GB/T20260-2006.

Sandstones from Fentou and Wutong Formations have higher SiO2(75.17% and 75.8% respectively) than those from Longtan (61.51%) and Gaolishan(61.15%) ones. However, the contents of Al2O3, Fe2O3T, MgO and K2O in Fentou (10.64%, 4.48%, 1.99%, 3.29%) and Wutong(11.70%,4.16%, 1.58%, 2.73%) are lower than in Longtan (17.78%, 5.49%, 2.27%, 3.45%) and Gaolishan (17.28%, 5.50%, 2.46%, 3.58%). The average contents of Na2O change from high to low in Longtan (1.95%), Gaolishan (0.79%), Wu- tong (0.52%) and Fentou (0.46%). The average contents of CaO are 0.74% in Longtan, 2.11% in Gaolishan, 0.38% in Wutong and 0.66% in Fentou respectively (Table 2).

Table 2 Major element compositions (wt%) of the Paleozoic sandstones from the core CSDP-2

The chemical index of alteration (CIA) is a key index to evaluate the weathering degree of rocks. It is usually expressed by the mole fraction of the whole-rock oxide. The calculation formula is:

CaO* stands for the fraction of CaO in silicate (Nesbitt and Young, 1982). Thein the core CSDP-2 ranges from 58.56 to 85.96, and the average value is 74.61, which is close to Phanerozoic shale (70–75) (Feng, 2003). So the Paleozoic sedimentary rocks in the study area experienced moderate chemical weathering under warm and humid conditions. During Paleozoic, the Yangtze Block was located near the equator (Zhu, 1998). The warm paleoclimate environment was favorable for weathering and alteration.

REEs have always been regarded as non-migrating elements, so the REE features of source rocks can be well preserved in sediments and can effectively be used to judge the tectonic setting of source area (Bhatia, 1985; Bhatia and Crook, 1986). The REE compositions of Paleozoic sandstones from the core CSDP-2 indicate that (Table 3) the total REE contents of sandstones vary from 176.98×10−6to 254.0×10−6(average 209.26×10−6) in the Longtan Formation, 83.30×10−6–208.47×10−6(average 135.46×10−6) in Gaolishan Formation, 55.73×10−6–255.09×10−6(average131.33×10−6) in Wutong Formation and 52.36×10−6–200.48×10−6(average 122.40×10−6) in Fentou Formation, respectively. Thus, the average value of ∑REE decreased from the new formations to the old formations.

Table 3 REE contents of the Paleozoic sandstones in the core CSDP-2 (×10−6)

Notes: LT, Longtan formation; GA, Gaolishan formation; WT, Wutong formation; FT, Fentou formation. δEu = (Eu/0.087)/((Sm/0.231+Gd/0.306)/2).

Fig.6 shows the chondrite-normalized curve of REEs in core CSDP-2 (the standard values of chondrites are based on Boynton (1984). It can be seen that the sedimentary rocks formed during the four periods have similar REE patterns, all of which present moderate enrichment of light REE (LREE) and flat heavy REE (HREE). The average values of LREE/HREE are 10.12, 9.12, 10.03 and 11.11 in Longtan, Gaolishan, Wutong and Fentou Formations, respectively. The samples demonstrate obviously negative Eu anomaly with an average value of 0.63 (0.45–0.77). (La/Yb)Nrepresents the slope of the distribution curve in REE chondrite-normalized diagram, ranging from 5.48 to 17.83, with an average of 10.96. The REE distribution pat- tern is similar to that of upper continental crust (UCC) (Taylor and McLennan, 1985). The difference is that the REE contents in Longtan formation are higher than UCC, while those in the other three formations are generally lower than UCC, especially for the HREEs (Fig.6).

Most trace elements are inactive and show slight changes during the process of sedimentation. The source rocks and weathering condition are the chief factors in controlling the trace elemental compositions of sediments. As a result, some of trace elements can well reflect the tectonic environment of sedimentary basins (Bhatia and Crook, 1986; McLennan, 2001). The average contents of trace element Th, U, Hf, Sc, Zr, and Y in Paleozoic sandstone samples from the core CSDP-2 are 13.76×10−6, 3.21×10−6, 6.38× 10−6, 12.28×10−6, 215.36×10−6and 20.25×10−6. The average values of Th/U, La/Th, Th/Sc, La/Y, Rb/Sr are 5.39, 2.60, 1.37, 1.74 and 1.42 respectively (Table 4).

5 Provenance Analysis

5.1 Detrital Component Discriminant

As the distribution pattern between sedimentary basin and source area is controlled by geotectonics, the detrital component and structural features of sediments in basins are closely related to the geotectonic setting of source areas. The traditional Dickinson triangle diagram shows that all the sandstone samples from well CSDP-2 are projected in the source area of recycled orogen (Fig.7). At the same time, it is found that the ratio of marine components to terrigenous ones in the sediments from the strata during Silurian–Carboniferous is low, and increases in Permian, which suggests that the Paleozoic sediments in the study area were mainly derived from plate collision and foreland uplift in the recycled orogen. In the Permian these materials began to mix with felsic marine substances in the basin.

5.2 Major Element Data

According to the major element discriminant function of sandstones and argillites from New Zealand graywackes terranes (Table 5), Roser and Korsch (1988) divided pro- venance into four groups: P1 (mafic)–first-cycle mafic and lesser intermediate igneous provenance, standing for immature oceanic island arc; P2 (intermediate)–primarily in- termediate igneous provenance, dominantly occurring as andesitic detritus, belonging to mature magmatic arc and immature continental margin magmatic arc; P3 (felsic)–felsic igneous provenance, coming from mature continental margin arc and transitional continental margin, active and dissected magmatic arc; P4 (recycled)–mature polycyclic quartzose continental provenance, coming from passive continental margin, or sedimentary basin within craton and recycled orogen, belonging to mature continental crust source region with the quartz content over 80%. Most of the1–2 values of this study fall within the quartzose sediment source (P4) (Table 2; Fig.8), indicating that the sediments are mainly derived from recycled continental crust. Furthermore, samples from Longtan Formation fall within the field of both the felsic igneous source (P3) and quartzose sediment source (P4), suggesting that the Permian sedimentary source region contains more areas with felsic igneous materials in addition to ancient sedimentary terrane.

Fig.6 Chondrite-normalized REE curves of the Paleozoic sandstones from the core CSDP-2 (red curve represents UCC, the standard values of chondrites are based on Boynton (1984).

Table 4 Trace element compositions of the Paleozoic sandstones in CSDP-2 (×10−6)

Table 5 The coefficients used in Provenance discriminant function (after Roser and Korsch, 1988)

Notes:=11+22+L+ax+, wherexrepresent the content of discriminant variable, and1–aare the corresponding coefficients.

5.3 Rare Earth Element Data

From the Table 3 and Fig.6, we conclude that the total REE contents are highest in Longtan Formation, secondary in Gaolishan Formation, and lowest in Wutong and Fentou Formations. The distribution ranges of REEs in Wutong and Fentou Formations are wider, and normalized pattern curves are various, indicating more than one source for detrital materials during Devonian and Silurian. However, the distribution of REEs in Longtan and Gaolishan is relatively concentrated, and the pattern curves are similar, suggesting that the clastic materials of the Permian and Carboniferous strata have relatively single source with basically same attributes.

Bhatia (1985) summarized the REE contents of sedimentary rocks from four representative tectonic environments (Table 6). We compared the four sorts of chondrite- normalized REE patterns with samples from the well CSDP-2 (Fig.9), and found that REE patterns of the four formations are very similar to that of the passive continental margin environment. Meanwhile, the REE content of the Permian strata is very close to that of the passive continental margin, while that of the Silurian–Carboniferous strata is totally lower than the passive continental margin. We compared the characteristic parameters of sand-stones from representative tectonic settings and the central uplift of SYSB. From the results presented in Table 6, the Permian Longtan Formation is similar to those from continental island arc, active continental margin and passive continental margin, while the other three formations suggest the provenance features of passive continental margin and continental island arc.

Fig.7 QFL discrimination diagram of Paleozoic sandstones in CSDP-2 (after Dickinson, 1983).

Fig.8 F1–F2 discrimination diagram of Paleozoic sandstones in CSDP-2 (after Roser and Korsch, 1988). P1, Pri- marily mafic and lesser intermediate igneous provenance; P2, Primarily intermediate igneous provenance; P3, Felsic igneous provenance (volcanic and plutonic); P4, Quartzose sediments of mature continental provenance.

Table 6 REE characteristic parameter comparison between the core CSDP-2 and various tectonic settings (after Bhatia, 1985)

Notes: OIA, oceanic island arc; CIA, continental island arc; ACM, active continental margin; PCM, passive continental margin.

Fig.9 Average REE chondrite-normalized pattern of rocks from CSDP-2 and various tectonic settings. OIA, oceanic island arc; CIA, continental island arc; ACM, active continental margin; PCM, passive continental margin.

5.4 Trace Element Data

In the La/Yb-REE discrimination diagram (modified after Allegre and Minster, 1978) (Fig.10a), the samples are mainly projected in the source areas of sedimentary rocks and granite. Only the sandstones from Permian Longtan Formation fall near the boundary of sedimentary rocks and granite. La/Th-Hf discrimination diagram was applied to distinguish the source areas of sediments from various tectonic environments (Floyd and Leveridge, 1987). It canbe seen from Fig.10b that the Paleozoic sandstones in well CSDP-2 have lower La/Th ratio (0.92–3.86). The content of Hf ranges from 2.90×10−6to 10.60×10−6. The samples are located in the mixed source region from felsic to passive continental margin, indicating that the source rocks mainly come from continental upper crust and consist of felsic rocks and ancient sedimentary terranes.

The trace elements such as La, Th, Zr, Sc and Co are stable in the process of weathering, transportation and deposition. From oceanic island arc to continental island arc, active continental margin and passive continental margin, the contents of La and Th increase gradually, while those of transitional elements such as Zr and Sc decrease instead. Therefore, La-Th-Sc, Th-Sc-Zr/10 and Th-Co- Zr/10 discrimination diagrams can also be used to identify the tectonic environment (Bhatia and Crook, 1986). In Fig.11a, most of the samples fall in the field of continental island arc, and only several samples from Wutong and Fentou Formations fall in the group of active continental margin and passive continental margin. In Fig.11b, the majority of samples fall in the continental island arc group, while several samples from Wutong and Fentou Formations locate in the group of passive continental margin, and the other four samples from Longtan Formation are projected in the transitional region between continental island arc and active continental margin. In Fig.11c, most of the samples locate in the continental island arc group, and only three samples from Wutong Formation fall in the group of passive continental margin, and two samples from Fentou Formation fall in the transitional zone between continental island arc and passive continental margin. In addition, it must be noted that nine samples from Longtan Formation are entirely projected in the region of active continental margin.

Bhatia and Crook (1986) summarized the trace element characteristic parameters of graywackes in basins with known tectonic genesis, which were compared with Paleozoic sandstones in well CSDP-2 (Table 7). The results show that the Silurian–Carboniferous strata is basicallygenerated in the tectonic setting of passive continental margin and continental island arc, while the Permian tectonic setting is similar to continental island arc, active continental margin and passive continental margin. From oceanic island arc to continental island arc, active continental margin and passive continental margin, their main source rocks range from andesite to dacite, and then to granitic gneiss and sedimentary rocks (Bhatia, 1985). Although there exist differences among the various discri- mination diagrams, the Paleozoic sandstone samples do- minated by quartz in SYSB, are generally derived from the upper continental crust rocks, and have a terrigenous tectonic background.

Fig.10 Trace element discrimination diagrams of CSDP-2 sandstones.

Fig.11 Trace element (×10−6) discrimination diagrams of Paleozoic sandstones in CSDP-2 (after Bhatia and Crook, 1986).

Table 7 Trace element characteristic parameter comparison between CSDP-2 and different tectonic backgrounds (after Bhatia and Crook, 1986)

5.5 Tectonic Background and Provenance

We summarized the Paleozoic sedimentary sources based on various data and proxies (Table 8). The tectonic background revealed from the petrography and geoche- mical data of the Paleozoic sandstones in well CSDP-2 are basically the same, indicating that they come from the non-oceanic island arc area. The geochemical features of the clastic rocks keep consistent with those from typical passive continental margin, active continental margin, continental island arc and their transitional regions. Their parent rocks should be derived from source areas with mature continental quartz detritus, such as ancient orogen and land mass. During the whole Paleozoic, the two plates of South China and North China drifted in the Paleo- Tethys Ocean, and the North China Old Land was far away from the lower Yangtze Block (Meng and Zhang, 1999). So the provenance in this study area could not be the North China. Based on U-Pb dating of detrital zircon, the Early Paleozoic sediments in the LYB are derived from Cathaysia Wuyi Massif in the south, not the LYB itself (Wang, 2010; Xu, 2012; Li, 2013; Li, 2017). From the paleogeographic point of view, the sedimentary facies of LYB during early–middle Silurian to late Devonian were generally shoreline-shallow shelf. Mean- while, the southern Cathaysia Old Land was above the sea-level, and the basement was subjected to the uplift and erosion. Along with part of the ancient sedimentary strata recycling, clastic materials were transported from south to north by rivers, and finally deposited in the shallow sea area of LYB (Fig.12).

The Caledonian Orogeny (Guangxi Movement) occurred from late Ordovician to Late Silurian undoubtedly play an extremely important role in controlling the study area, which made the Cathaysia Block and Yangtze Block collide continuously, forming a uniform South China Old Land. The LYB was uplifted and eroded in a large-scale area, so that a parallel unconformity was shaped between the lower Silurian Fentou Formation and upper Devonian Wutong Formation. However, for the Paleozoic sandstones from SYSB, the REE patterns of the Silurian and Devonian are basically the same (Figs.6 and 9). So, although the study area was once uplifted by Guangxi move- ment at the end of Silurian, it did not undergo intense tectonic deformation. There occurred no essential changes in the provenance from Silurian to Devonian. Based on similar age distribution pattern (the major peaks are 420– 465Myr and 812–838Myr) of detrital zircon from Wutong and Fentou Formations respectively in Chaohu and Nanjing area, Li(2017) proposed that the Devonian se- diments inherited the provenance features of Silurian se- diments, which is consistent with the results in this paper.

The geochemical compositions of Silurian–Carboniferous sandstones in well CSDP-2 reveal that the clastic sediments in the SYSB are from passive continental margin and continental island arc regions. The source rocks include quartzose ancient terrane in upper continental crust dominantly, and some young felsic rocks, accompanied by recycled orogen materials generated during Jinning and Caledonian movements. Moreover, according to age constraints and geochemical characteristics of detrital zircon, Wang. (2016) found that the parent rocks located in the southeast of Yangtze Block formed in 810–830Myr belong to intermediate-acid igneous rocks produced at the continental arc tectonic setting. Meanwhile, the above mentioned 812–838Myr peak of detrital zircon, reported by Li(2017), corresponds to the Neoproterozoic magmatic event in Cathaysia Block, which is con- firmed by the continental island arc background of source rocks in this study. The discriminant indexes of Carboniferous sandstones are essentially similar to those of the Devonian and Silurian, indicating that the Carboniferous sediment source in the SYSB inherited the earlier provenance characteristics and had no obvious changes.

The source of sediments from the Permian Longtan Formation is obviously different from that of the former three. From the petrographic point of view, the Permian sandstone has lower maturity than the previous sedimentary rocks, demonstrating proximal and rapid deposition. In the Dickinson QFL triangle diagram (Fig.7), the Permian strata contains a higher proportion of marine components. From1–2 discrimination diagram based on major element data (Fig.8), more Permian samples fall into the field of felsic igneous provenance (P3) besides a few points in the field of quartz provenance (P4). As presented in comparison with typical tectonic setting (Tables 6 and 7), Permian Longtan sediments in core CSDP-2 are primarily derived from active continental margin, not from continental island arc and passive continental margin. The trace element discrimination diagram (Fig.11) also shows that the Permian source area is closer to active continental margin (C region) than that of Silurian–Carboniferous. Therefore, it can be concluded that the source rocks of Permian sediments present closer relationship with magmatic tectonic setting than that of pre-Permian. Li(2017) propose that the magmatic activity during 341–254Myr in the Cathaysia Wuyi Massif most likely occurred at a continental arc tectonic setting. Under the paleographic background of north-sea and south-land, the denudation area provides a material source for the near lower Yangtze region. Li(2012) also suggested that the start time of the active continental margin located in the southeastern Cathaysia is 280Myr, which is a typical Andean convergence margin with continental magmatic arc developed intermittently (Ramos and Aleman, 2000) (Fig.12). In a word, the provenance attributes of SYSB Permian sandstones deduced in this paper basically coincides with the previous statements.

Table 8 Paleozoic sedimentary sources of SYSB

Fig.12 A sketch map of paleogeographic evolution and sedimentary provenance in South China Craton.

In addition, the REE normalized pattern of Permian Longtan Formation shows an affinity with passive continental margin (Fig.9). However, its REE characteristic parameters (Table 6) and trace element discrimination results (Fig.11; Table 7) indicate that continental island arc and active continental margin are predominant. This may be due to the fact that sediments from active continental margin often have transitional REE patterns between typical andesite and post-Archean average shale (PAAS), even there is no significant difference from PAAS sedimentary rocks to some extent (Chen and Wang, 2017).

6 Conclusions

1) Petrography features of sandstones from the South Yellow Sea Basin of lower Yangtze Block suggest that the Silurian–Carboniferous sandstones, which have high com- positional and textural maturity, are mainly sublitharenite, reflecting long-distance transportation. The Permian sand- stones are mainly feldspathic litharenite, and have low compositional and textural maturity, reflecting proximal deposits.

2) The Silurian–Carboniferous clastic sediments in the SYSB are mainly derived from passive continental margin and continental island arc. The source rocks are do- minated by ancient quartzose terrane and also contain young felsic substances from upper continental crust, ac- companied by significant recycled orogen rocks. The Ca- ledonian Orogen happened in the Early Paleozoic did not lead to the essential change of the sediment source in this area.

3) Continental island arc and active continental margin dominate the Permian sedimentary source areas in the SYSB, which verify the existence of the continental arc magmatic activity on the southeastern coast of Cathaysia during middle–late Permian.

Acknowledgements

This study was financially supported by the National Natural Science Foundation of China (Nos. 41776081, 41210005), the China Geological Survey Project (No. DD 20160147), and Aoshan Science and Technology Innovation Project of Qingdao Pilot National Laboratory for Ma- rine Science and Technology (No. 2015ASKJ03).

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