Geochemical Characteristics of the Late Cretaceous Lower Zhoutian Formation in the Jitai Basin, Southeast China: Implications for palaeoenvironment and provenance

Major, trace and rare earth element geochemistry of the late Cretaceous lower Zhoutian Formation from the Jitai Basin of Southeast China were measured by inductively coupled plasma mass spectromentry (ICP-MS) analysis. The paleoclimate, paleo-environment and provenance of lower Zhoutian Formation in the Jitai Basin are analyzed in detail with these data.According to the research, the range of Sr/Cu values is wide, which indicates the changeable palaeoclimate, the curves of FeO/MnO and Al 2 O 3 /MgO are negatively correlated with the Sr/Cu curves. These changes indicate that the palaeoclimate of the late Cretaceous lower Zhoutian Formation in the Jitai Basin was divided into two parts. The lower part experienced two cooling events; the upper part was dominated by warm humid climate.The changes of B/Ga and Sr/Ba curves are similar to Sr/Cu curves. According to these values, the salinity of the late Cretaceous lower Zhoutian Formation in the Jitai Basin was consistent with palaeoclimate. The salinity of lower part changed from salt water to fresh/brackish water. The upper part was mainly fresh/brackish water, and there were many changes from fresh/brackish water to salt water. The values of Ni/Co, V/Cr, V/(V + Ni) and Ce/Ce* are relatively stable, indicating a long-term oxidation environment. The diagrams of La-Th-Sc, Th-Sc-Zr/10, La/Th-Hf and sandstone-siltstone background function show that the provenance in lower Zhoutian Formation mainly is a mixture of upper crust felsic sediments and old components. X-ray uorescence spectrometry to determine oxides of major , An inductively coupled plasma mass spectrometer (ICP-MS) was used to determine the contents of trace and rare earth element. The samples were fused into by alkali fusion and tested on a sequential X-ray uorescence spectrometer (AB104L, Axios-mAX). The dissolved trace and rare earth elements were measured using an ICP-MS instrument (ELEMENT XR).

inductively coupled plasma mass spectrometer (ICP-MS) was used to determine the contents of trace and rare earth element. The samples were fused into glass sheets by alkali fusion and tested on a sequential X-ray uorescence spectrometer (AB104L, Axios-mAX). The dissolved trace and rare earth elements were measured using an ICP-MS instrument (ELEMENT XR).
All geochemical analysis samples were crushed and ground to less than 200 mesh. The powered samples were rst heated and then fused with the mixture of anhydrous lithium tetraborate, lithium uoride and ammonium nitrate heating in a mu e furnace to determine LOI. The fused samples were heated to 800°C for XRF analysis. Calibrations of accuracy and reproducibility were conducted by GB/T 14506.14-2010 and GB/T 14506.28-2010. Powered samples were heated with HF and HNO 3 . After evaporated, HNO 3 was added to dissolve the samples. And HNO 3 was added to the beaker again heating to 130°C. Finally, the solution was diluted by distilled water for trace analyses, according to GB/T 14506. 29 (Table 3). According to the plot of trace element compositions normalized against UC (Fig. 2c and d), the contents of various elements varied greatly. In the plot of silty mudstones normalized to UC, Cr contents are enriched relative to UC in most samples, but Sr contents show strong depletion (Fig. 2c). In Fig. 2d, Cr contents of some calcilutites are enriched. According to the The values of Sr/Cu vary widely, ranging from 0.99 to 106.56 (Table 5 and Fig. 3). This indicates that the Jitai Basin experienced relatively large climatic change during the late Cretaceous period. According to these values and curves, the Cretaceous palaeoclimate in the lower Zhoutian Formation is roughly divided into two parts, 1435-1270m and 1270-1100m respectively. The values of Sr/Cu in most sections between 1435 and 1270m is greater than 5.0. The values are less than 5.0 at 1321 to 1306m and 1360m. So, the palaeoclimate type in the 1435-1270m segment was a hot arid climate, and there was an obvious cooling event between 1321 and 1306m.The Sr/Cu values of the 1100-1270m segment are between 1.3 and 5.0 on the whole, and are greater than 5.0 at 1106m, 1121m, 1152m, 1205m and 1209m. Hence, the palaeoclimate type in the 1270-1100m segment was mainly warm humid climate, and there was a short period of climate uctuation in some segments. In Fig. 3 Table 5 and Fig. 3, the palaeoclimate of the late Cretaceous lower Zhoutian Formation in the Jitai Basin was mainly divided into upper and lower parts. The lower part (1435-1270m) experienced two cooling events and had the palaeoclimate characteristic of wet-dry cycling; the upper part (1270-1100m) was dominated by warm humid climate, which experienced many climatic uctuations.
Salinity. Boron is one of the most concentrated trace elements in sea water and is mainly attached by illite in coastal to marine environments 24,25 . By comparison, Gallium is principally come from terrestrial environments and is chie y deposited in the form of smectite. According to studies of Wang et al. 26 and Lan et al. 27 , the value of B/Ga is a reliable indicator of salinity. The value less than 3.0 indicates fresh water, the value ranging between 3.0 and 5.0 indicates brackish water, and the value great than 5.0 indicates salt water 28,29 .
The combination of SO 4 2− in the sea/saline water and Ba 2+ in the fresh water produces the precipitation of BaSO 4 . In contrast, the Sr is assumed to migrate Redox condition. Several trace elements, such as U, Ni, V, Mo, Cr, Co, are commonly enriched in anoxic sediments because of their solubility is controlled by the redox [34][35][36][37] . In the study of late Jurassic redox environment in northwest Europe, Jones et al. 38 concluded that the values of Ni/Co, V/Cr and V/(V + Ni) were very reliable by comparing many parameters, and thus summarized a set of trace indexes for judging the redox environment ( sensitively re ect the redox condition in the sedimentary environment. The Ce/Ce* value greater than 1 is a positive anomaly, indicating a reduction environment; while the value less than 0.95 is a negative anomaly, indicating an oxidation environment. According to the Table 5 Table 5, Table 6 and Fig. 3, all values indicate that the late Cretaceous lower Zhoutian Formation was in oxidized environment, in which there were some small uctuation in lower part (1435-1270m). The La-Th-Sc discriminant diagram can better distinguish the continental island arc, while the Th-Sc-Zr/10 discriminant diagram can distinguish the active continental margin from the passive continental margin. In Th-Sc-Zr/10 discriminant diagram, the samples are mainly located in the continental island arc, some in the passive margin, and a few points not in any region (Fig. 4a). In La-Th-Sc discriminant diagram, most of the samples are located in the continental island arc, some in the region of active/passive continental margin, and only one sample in the oceanic island arc (Fig. 4b). In La/Th-Hf discriminant diagram, most of the samples are located in the areas of increasing old sediment component, some in the regions of felsic source and felsic/basic source, and only one sample in the andesitic arc source (Fig. 5). And the values of Th/Sc range from 0.58 to 3.23, with an average of 1.31 ( Table 5)

Conclusions
This study represents a rst relatively systematic study on major, trace and rare elements of the late Cretaceous lower Zhoutian Formation in the Jitai Basin.
The main conclusions are as follows: The changes of palaeoclimate were mainly divided into upper and lower parts. The lower part (1435-1270m) was dominated by hot arid climate, and a cooling event occurred. And the upper part (1270-1100m) was dominated by warm humid climate, which experienced many climatic uctuations.
The salinity of lower part (1435-1270m) changed from salt water to fresh/brackish water with the changes of palaeoclimate. The upper part (1270-1100m) was mainly fresh/brackish water, and there were many changes from fresh/brackish water to salt water. And the late Cretaceous lower Zhoutian Formation was in oxidized environment, in which the lower part (1435-1270m) had some small uctuation.
The lower Zhoutian Formation in the Jitai Basin shows a major tectonic setting of continental island arc, follows by continental margin. And its provenance is a mixture of upper crust felsic sediments and old sediment components.   Table 2, Table 3 and Table 4.