SCIENCE CHINA Earth Sciences • RESEARCH PAPER •
October 2013 Vol.56 No.10: 1688–1700 doi: 10.1007/s11430-013-4620-7
Geochemistry of the Middle to Late Permian limestones from the marginal zone of an isolated platform (Laibin, South China) QIU Zhen1,2*, WANG QingChen1 & YAN DeTian3 1
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; 2 PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China; 3 Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, University of Geosciences, Wuhan 430074, China Received March 27, 2012; accepted September 18, 2012; published online April 26, 2013
Major, trace and rare earth elements were measured in 27 samples of the Middle to Late Permian limestones from the Tieqiao section located on the marginal zone of an isolated platform (Laibin, South China). Shale-normalized REE+Y patterns of all samples show notably negative Ce anomalies (0.21–0.66, average 0.33), slightly positive Gd anomalies (1.08–1.30, average 1.20), and positive Y anomalies with superchondritic Y/Ho ratios (36–91, average 55), which are consistent with those of modern shallow seawater. Their relative LREEs enrichment with higher NdSN/YbSN ratios (0.58–1.80) than those of modern shallow seawater (0.21–0.50) suggests complicated sources of REEs for all samples. Compared with geochemical features of sediments influenced by terrigenous particles and hydrothermal fluids, it is concluded that ambient shallow seawater was the primary source of REEs in these limestones. Comparing the indicators of REE+Y elements (REE, NdSN/YbSN, Ce/Ce*, Gd/Gd*, Eu/Eu* and Y/Ho) in limestones with those in bedded cherts from the Tieqiao section, we consider that limestone and bedded chert have similar sources of REE+Y elements: ambient shallow seawater with more or less hydrothermal fluids. In addition, there is a completely negative correlation between CaCO3 and SiO2 contents in limestones and bedded cherts. These results imply that precipitation of CaCO3 was inhibited by that of SiO2 which was derived mainly from hydrothermal fluid, especially in bedded cherts from the Tieqiao section. geochemistry, rare earth element, limestone, the Middle to Late Permian, South China Citation:
Qiu Z, Wang Q C, Yan D T. Geochemistry of the Middle to Late Permian limestones from the marginal zone of an isolated platform (Laibin, South China). Science China: Earth Sciences, 2013, 56: 1688–1700, doi: 10.1007/s11430-013-4620-7
REEs abundances in marine (fine-grained) sediments are controlled dominantly by absorption from ambient seawater and direct inclusion of metalliferous particles (Fe-Mn-oxides relating to hydrothermal fluid) and terrigenous particles [1]. Because REEs are relatively immobile during diagenesis [2–5], the REEs signatures in ancient marine sediments have important implications for the depositional conditions (e.g., input sources, ancient seawater chemistry) of these sediments, such as bedded cherts [3, 4, 6–10] and car-
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© Science China Press and Springer-Verlag Berlin Heidelberg 2013
bonates [5, 8, 11–17]. The chemistry behavior of yttrium (Y) is similar to that of REEs, and particularly close to that of Ho [18, 19]. In recent studies, shale-normalized REEs and yttrium element (REE+Y) patterns from Neoproterozoic to Holocene carbonates (limestones) have been used as proxies of ancient seawater chemical characteristics [5, 8, 12, 13, 15, 17]. Similar to the modern shallow seawater, shale-normalized REE+Y patterns of ancient seawaters recorded in carbonates are characterized by (1) a uniform light REEs (LREEs) depletion, (2) a notable negative Ce anomaly, (3) a slightly positive Gd anomalies, and (4) a positive Y anomaly earth.scichina.com
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with superchondritic Y/Ho ratio [5, 12, 13, 20]. However, carbonates contaminated by terrigenous input display a relatively flat shale-normalized REE+Y pattern with no notable element anomalies [13, 15, 21]. Shale-normalized REE+Y patterns of marine high-temperature hydrothermal fluids are characterized by relative LREEs enrichment, notable positive Eu anomaly and chondritic Y/Ho ratio (Figure 5c in [21], data from [18, 22, 23]). Most carbonates influenced by hydrothermal fluid show typical positive Eu anomalies [8, 15, 17]. During the Permian in South China, an isolated carbonate platform developed around Heshan area [24] and nearby Laibin area was located in the marginal zone between the platform and basin (Figure 1). From the Middle Permian (Maokou Formation) and Late Permian (Heshan Formation), a continuous marine sequence, composed mainly of limestones and bedded cherts [10, 25–28], was deposited in Laibin area. In our previous study [10], geochemical characteristics of the bedded cherts showed that they were deposited in absence of terrigenous particles and the primary source of silica came from marine hydrothermal fluids. However, no systematic study has been carried out on geochemical characteristics (especially REE+Y) of limestones intercalated within those bedded cherts. In this study, major, trace and rare earth elements distributions in these limestones are reported from the Maokou Formation (Middle Permian) and the Heshan Formation (Late Permian) in the Tieqiao section (Figure 2(a) and 2(b)). This study may enhance our under-
Figure 1 Permian.
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standing of the depositional conditions of the Middle to Upper Permian strata in Laibin area by (1) interpreting the sources of REE+Y elements in limestones and (2) comparing shale-normalized REE+Y patterns of limestones with those of bedded cherts.
1 Geological setting Laibin area (Laibin City), Guangxi Autonomous Region of China, was located in the eastern Dian (Yunnan)-Qian (Guizhou)-Gui (Guangxi) Basin (Nanpanjiang Basin or Youjiang Basin) during the Permian (Figure 1). Regional paleogeographical reconstructions have indicated that several isolated carbonate platforms were surrounded by deep-water basins in the Dian-Qian-Gui Basin from the Devonian to the Middle Triassic [29–31] (Figure 1), which suggests persistent extension during the Late Paleozoic. At the same time, volcanism and syn-depositional faulting, accompanied by the rifting activities, widely occurred in the margins and interior of the Dian-Qian-Gui Basin [32–36]. Many strata-bound ore deposits were formed with influencing of hydrothermal fluid extensively in the basin [37]. In recent studies, submarine hydrothermal origin of the Middle to Upper Permian bedded cherts was reported in the DianQian-Gui Basin [9], especially in Laibin area [10]. Hence, from the Middle to Late Permian, submarine hydrothermal fluids could have greatly affected the deposition in this
Paleogeographical reconstruction of South China (modified from ref. [31]) and Dian-Qian-Gui Basin (modified from ref. [24]) during the Late
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Figure 2 (a) Geological location of the Tieqiao section, on the western wing of Laibin-syncline (modified from ref. [40]); (b) lithological column (modified from ref. [28]) and sampling location. Chert samples and their location are from ref. [10].
basin. In the Dian-Qian-Gui Basin, an isolated carbonate platform developed around Heshan area during the Permian [24] and nearby Laibin area was located on the marginal slope between the platform and basin (Figure 1). In these areas, the deposits of the carbonate platform are characterized by limestones and coals [38], whereas the deep-water basins are represented by bedded cherts, mudstones, and siliciclastic submarine fan turbidites [39]. In Laibin area, the Permian strata are extensively exposed along the two wings of the Laibin-syncline (Figure 2(a)), which consist of the Maping, Chihsia, Maokou, the Heshan, and Talung Formations (Lopingian) in ascending order [27]. The Tieqiao section is located on the western wing of Laibin-syncline [40] (Figure 2(a)), where the Maokou Formation and the Heshan Formation consist mainly of limestones and bedded cherts [10, 25–28]. Detailed description about this section was provided by these previous studies [25, 27, 28].
2 Samples and analytical methods In this study, 27 limestone samples (13 samples from the Maokou Formation and 14 samples from the Heshan Formation) from the Tieqiao section (Figure 2(b)) were applied
for major, trace and rare earth elements analysis. Wellpreserved samples were crushed to powders about 200 meshes in the steel vessel and each sample was mixed twice. Major element contents were determined with an automatic X-ray fluorescence spectrometer (XRF-1500) at the Institute of Geology and Geophysics, Chinese Academy of Sciences. Sample powders for major elements analysis were first toasted in an oven at 105°C for about 1 h and then were mixed with flux (Li2B4O7) in the proportion 1:5 to make fusion glasses. Finally, major elements were analyzed by the X-ray fluorescence spectrometer using those glasses. Analytical precision for major element contents is generally better than 1%. Trace and rare earth elements were analyzed by ICP-MS (Agilent 7500a) at the China University of Geosciences using procedures outlined by Liu et al. [41]. Sample powders (50 mg) were weighed into a Teflon bomb with ultrapure 1.5 mL HNO3 + 1.5 mL HF. The sealed bomb was heated at 190°C in an oven for 48 h to ensure complete dissolution of powders. Then the solution of the Teflon bomb was evaporated at about 115°C to dryness and was added by 1 mL HNO3 and dried once again. The resultant salt was re-dissolved by adding about 3 mL of 30% HNO3 and heated in the bomb at 190°C for 12–48 h to ensure complete dissolution. The final solution was diluted to about 100 g
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with mixture of 2% HNO3 for ICP-MS analysis. The precision of the analysis is generally better than 5% of trace and rare earth elements contents.
3 Result 3.1
Major and trace elements
Table 1 lists all contents of major and trace elements of analyzed samples. CaCO3 (calculated as weight-percent from CaO) and SiO2 contents of all samples range between 79.14% and 98.89%, 0.19% and 13.17%, respectively. All samples have very low contents of Al2O3 (0.03%–0.44%), TiO2 (0.008%–0.032%), MnO (0.01%–0.23%), Fe2O3 (0.03%–0.88%), MgO (0.10%–0.55%), K2O (0.01%–0.13%), Na2O (0.01%–0.22%) and P2O5 (0.01%–0.14%) (Table 1). Therefore, CaCO3 and SiO2 are main components of all samples and the total contents of them are over 97%. Sr contents of all samples range from 160 to 1066 ppm with an average of 492 ppm. Zr and Hf contents are very low (
