Strontium isotope and restricted marine basin analysis from thin-bedded limestone at the top of Xujiajuan Formation, Xiangshan Group in Ningxia, China
Xiangshan Group in Ningxia is mainly composed of grayish-green, fine-grained sandstones interbedded with mudstones induced by deep-water turbidity currents. Determining the geological age of the Xiangshan Group is crucial for understanding the tectonic evolution in the northern Qilian area, regional geology in Ningxia, and marine deep-water petroleum exploration in the western Ordos basin. Here we focus on the characteristics of strontium isotope in thin-bedded limestones at the top of the Xujiajuan Formation. We employ sedimentology and sedimentary geochemistry methods to investigate this. The results reveal that: ① the weak degree of diagenetic alteration in the thin-bedded limestones suggests that the measured 87Sr/86Sr values can represent the coeval seawater Sr isotope composition; ② the 87Sr/86Sr ratios range from 0.71057 to 0.71356, with an average value of 0.71189. These ratios are significantly higher than those observed in Cambrian and Ordovician ocean water; ③ there is an average negative linear correlation between the 87Sr/86Sr ratios and sea water salinities (Z values caculated with carbon and oxygen isotopes). Additionally, a strong and average positive linear correlation is observed between the 87Sr/86Sr ratios and sea water temperatures (oxygen isotope temperature) and Mn/Sr ratios, respectively; ④ the 87Sr/86Sr ratios also display a negative nonlinear correlation with silicon contents and a weak negative linear correlation with aluminium and magnesium contents. Considering the Early Paleozoic tectonic evolution in the western Ordos basin and some previous research on the Xiangshan Group, it is plausible to suggest that the thin-bedded limestones at the top of the Xujiajuan Formation were possibly deposited in a deep-water environment within a restricted marine basin. This basin may have been confined by a small paleocontinent or underwater uplift from the open sea. The influence of coeval confined seawater in the basin could have included submarine hydrothermal solutions and surface runoff. Both of these factors may have contributed to the high 87Sr/86Sr ratios observed, owing to strontium isotope exchange with siallite and flow through karst weathering crusts, respectively. Consequently, the restricted marine basin water exhibited higher 87Sr/86Sr ratios compared to coeval oceanic water.
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