Major and trace-element geochemistry of Late Cretaceous clastic rocks in the Jitai Basin, southeast China

Major, trace and rare earth element (REE) geochemistry of the late Cretaceous lower Zhoutian Formation from the Jitai Basin of Southeast China were measured by inductively coupled plasma mass spectrometry (ICP-MS) analysis to infer the provenance of the sediments and to reconstruct the palaeoenvironment and palaeoclimate. The wide range of Sr/Cu ratios point to a fluctuating palaeoclimate, and the negative correlation between the FeO/MnO and Al2O3/MgO ratios and the Sr/Cu ratio indicates that the late Cretaceous climate during the lower Zhoutian Formation in the Jitai Basin can be divided into two parts. The lower part experienced two cooling periods, whilst the upper part was dominated by warm-humid climate. Mostly corresponding trends of the B/Ga, Sr/Ba and Sr/Cu ratios show that the salinity changed consistently with the late Cretaceous climate during the lower Zhoutian Formation in the Jitai Basin. During the lower part, the salinity changed from salt water to fresh/brackish water. In the upper part, water was mainly fresh/brackish, and there were many changes from fresh/brackish water to salt water. The relatively stable Ni/Co, V/Cr, V/(V + Ni) and Ce/Ce* data indicate a long period of oxic conditions. The La-Th-Sc, Th-Sc-Zr/10 and La/Th-Hf data of the silt- and sandstones of the lower Zhoutian Formation show that its provenance was mainly a mixture of felsic upper crust sediments and older sedimentary rocks.

As an important carrier of geological information, the geochemical characteristics of clastic rocks record the significant information of provenance, structure, environment and ecological evolution in a reliable and detailed way. During the process of deposition, the distribution, circulation and differentiation (deficit and enrichment) of trace elements sensitive to redox conditions in water and sediments are not only related to their own chemical properties, but also controlled by the physical and chemical conditions of the deposition medium and the palaeoclimate conditions [1][2][3][4] . Hence, some major and trace elements that dissolve in water are sensitive to climatic change, and they can be used as a valuable proxies of palaeoclimate evolution 5 .
The Mesozoic was a period of drastic tectonic changes in South China and even the whole East Asian continent, and it was also a crucial period in the development of East Asian tectonics [6][7][8][9][10] . During the late Cretaceous-Paleogene, a series of rift basins were formed in the central part of South China, such as the Jianghan and Jitai basins. At this time, most of the lakes in these basins evolved into saline lakes and deposited huge quantities of halite and other saline minerals [11][12] . Deep brine is found in the late Cretaceous strata of the Zhoutian Formation in the Jitai Basin, which is rich in potassium, lithium, boron, rubidium, cesium, bromine, iodine and other high-value and emerging strategic mineral resources 13,14 .
The formation of deep brine deposits is controlled by the material sources, and the tectonic and climatic conditions during the sediment-formation period [15][16][17] . So far, basic geological research is rarely conducted in the Jitai Basin, and the existing data are not sufficient to study the formation mechanisms of the deep brine. In order to accumulate more geological information and to better understand the late Cretaceous characteristics of the Jitai Basin, we conducted geochemical analyses of major, trace and rare elements of silty mudstones and calcilutites of the lower Zhoutian Formation. Based on our geochemical data and previous studies, we discuss

Materials and methods
Study area. The Jitai Basin is located in the transition zone between central uplift and southwest depression of Jiangxi Province. The basin is about 120 km long and 10-30 km wide, with an area of about 1850 km 218 . The basement of the basin varies from region to region with late Paleozoic strata dominating in its northern part and early Paleozoic epimetamorphic series in its southern part (Fig. 1). There is a set of Cretaceous continental red strata in the basin with a thickness of several thousand meters. The lower Cretaceous is exposed at the western edge of the basin. The lower strata are mainly composed of magenta-coloured coarse clastic formations, the middle strata are mainly composed of magenta-coloured medium fine clastic formations, and the upper strata mainly include argillaceous deposits, forming a sedimentary cycle from coarse to fine. The upper Cretaceous is  Trace elements. The three trace elements with the highest average content are Ba, Zr and Sr, and their average contents are 451.20 ppm, 255.93 ppm and 148.85 ppm, respectively ( Table 3). The contents of various trace elements vary over a large range if they are normalized to Upper Crust (UC) data (Fig. 4c,d) 21 . The Cr contents are enriched relative to UC in most non-carbonate samples, but Sr contents show strong depletion (Fig. 4c). In contrast, Cr contents of some carbonate samples are enriched (Fig. 4d). The B/Ga ratios range from 1.03 to 42.78, with an average of 6.80 (Table 5). The Sr/Ba ratios range from 0.04 to 11.13, with an average of 0.99. The Ni/Co ratios range from 1.23 to 6.97, with an average of 3.48. The V/Cr ratios range from 0.24 to 2.21, with an average of 0.75, and the V/(V + Ni) ratios from 0.43 to 0.88, with an average of 0.70.
The B/Ga ratios in the lower Zhoutian Formation of the Jitai Basin range from 1.03 to 42.78 ( Table 5). The B/Ga ratios show a similar pattern as the Sr/Cu ratios over the sequence which can be divided into two parts Table 4. Contents of rare elements from Well M2, Jitai Basin (unit in ppm).

No
La www.nature.com/scientificreports/ (Fig. 5). The B/Ga ratios are generally larger than 5 in the lower part from 1435-1270 m, indicating a saltwater environment. The ratios are mostly between 3-5 in the upper part above 1270 m, which includes also some samples with B/Ga ratios lower than 3 or higher than 5. The salinity during the formation of the upper part of the sedimentary sequence was mainly brackish, but including several changes to fresh/brackish and salt water environments. The Sr/Ba ratios range from 0.04 to 11.13 and their changes display similarities with those of the B/Ga and Sr/Cu ratios (Table 5). Compared with the B/Ga curve, the Sr/Ba curve follows more closely the Sr/ Cu curve (Fig. 5). The similar patterns of the Sr/Cu and Sr/Ba ratios in the lower Zhoutian Formation implies that the salinity evolution in the basin was controlled by climate change. When the palaeoclimate as a whole was warm-humid, the salinity of the water was fresh/brackish, and when climate conditions were hot-arid, saline water prevailed in the basin. During the lower part (1435-1270 m) of the investigated lower Zhoutian Formation, salt water dominated in the Jitai Basin when hot-arid conditions prevailed, interrupted by fresh/brackish water periods during more warm-humid periods. During the formation of the upper part (1270-1100 m) of the investigated sequence, mainly fresh/brackish water existed in the basin under warm-humid climate conditions, interrupted by salt water periods when climate became hot and arid.  40 concluded that the ratios of Ni/Co, V/Cr and V/(V + Ni) are reliable proxies of redox conditions. They established a set of trace-element indexes for the assessment of the redox environment (Table 6). Following their work, the trace-element ratios of Ni/Co, V/Cr and V/ (V + Ni) were widely applied to reconstruct past redox conditions 3,[40][41][42][43] .
In addition, Cerium anomalies in REE distribution patterns are generally used to investigate the redox conditions 21,32,[44][45][46][47] . Ce is present as Ce 3+ under reducing conditions and separates from other REE 3+ in the form of Ce 4+ under oxidized conditions. The Ce/Ce* ratio is defined as 2(Ce) PAAS /((La) PAAS + (Pr) PAAS ), with PAAS referring to the normalization of element concentrations against the PAAS. The ratios of Ce/Ce* can sensitively reflect the redox conditions in the sedimentary environment. A Ce/Ce* ratio larger than 1 or a positive anomaly indicates a reducing environment, whilst a ratio below 0.95 or a negative anomaly indicates an oxidized environment.
The Ni/Co ratios in the investigated lower Zhoutian Formation range from 1.23 to 6.97, the V/Cr ratios range from 0.24 to 2.21, the V/(V + Ni) ratios range from 0.43 to 0.88, and the Ce/Ce* ratios range from 0.70 to 1.07 (Table 5). All determined Ni/Co ratios are below 7.0, indicating an oxidation/weak oxidation environment. The Ni/Co ratios fluctuate over a wide range in the lower part (1435-1270 m), but even the maximum Ni/Co ratio of 6.97 indicates a weak oxidation environment. The Ni/Co ratios are relatively constant in the upper part   Provenance. The chemical composition of terrigenous clastic rocks is an integrated reflection of the nature, denudation and transport processes of the source area. The elements Cs, Zr, Th, Hf, Ti, La and Yb are relatively stable with respect to weathering, transportation and diagenesis. Because they are not intensively affected by migration, they are often used to assess the tectonic setting and the types of rocks in the source area 21,[48][49][50][51] . The composition of REE may change slightly during deposition. The REE abundances in source rocks and the weathering conditions in source areas are the major factors controlling the REE in the accumulating sediments. Hence, REE in clastic sedimentary rocks are widely used as the main indicator to identify the provenance 21,52 . In general, the La-Th-Sc, the Th-Sc-Zr/10 and the La/Th-Hf compositions are used to identify the tectonic setting of the source area 49,[53][54][55] . The La-Th-Sc discriminant diagram is predominantly used to distinguish between continental and oceanic island arcs, whilst the Th-Sc-Zr/10 discriminant diagram can be used to distinguish between active and passive continental margins. The samples from the lower Zhoutian Formation mainly cluster in the continental island arc field in the Th-Sc-Zr/10 discriminant diagram, a minority falls into the field of the passive margin, and a few samples are located outside the four predefined fields (Fig. 6a). In the La-Th-Sc discriminant diagram, most of the samples are located in the continental island arc field, some in the region of active/passive continental margins, and only one sample in the oceanic island arc field (Fig. 6b). In La/Th-Hf discriminant diagram, most of the samples are located in the field of the increasing old sediment component, some in the regions of the felsic and the mixed felsic/basic sources, and only one sample in the field of the andesitic arc source (Fig. 7). The Th/Sc ratios range from 0.58 to 3.23, with an average of 1.31 ( Table 5). Most of the Th/Sc ratios are located in the range of felsic rocks (0.84-20.5) and are significantly larger than those of mafic rocks (0.05-0.22). The La/Sc ratios range from 0.33 to 11.51, with an average of 4.20 (Table 5). Similar to the Th/Sc ratios, the La/Sc ratios mostly lie within the felsic rock range (2.5-16.3) [56][57][58] . Hence, most of the samples from the lower Zhoutian Formation were formed in a continental island arc or passive margin environment. The sediments' provenance is mostly mixing of felsic material of the upper continental crust and old sediments.

Conclusions
This study represents a first systematic investigation of major, trace and rare elements of the late Cretaceous lower Zhoutian Formation in the Jitai Basin. The main conclusions are the following: The climate history during the formation of the sedimentary sequence is divided into a lower and an upper part. The lower part (1435-1270 m) was dominated by hot-arid climate, interrupted by two cooling events. In contrast, warm-humid climate prevailed during the formation of the upper part (1270-1100 m), further characterized by frequent climate fluctuations.
The salinity in the Jitai Basin was high during the formation of the lower part, and it was reduced to fresh/ brackish conditions during two cooling events. During the formation of the upper part, fresh/brackish water www.nature.com/scientificreports/ conditions prevailed, interrupted by saline water periods during hot-arid climate conditions. Sediment accumulation during the late Cretaceous lower Zhoutian Formation occurred in an oxidized environment, with periods of weaker oxidation during the formation of its lower part. The sediments of the lower Zhoutian Formation in the Jitai Basin were mainly deposited in a continental island arc to passive margin setting. Their provenance was a mixture of upper crust felsic sediments and old sediment components.