Abstract
The Early Cretaceous Shengli River–Changshe Mountain marine oil shale belt, located in the northern Qiangtang Basin, Tibet, is estimated to be the largest marine oil shale resource in China. However, its palaeoweathering, provenance, and tectonic setting during the depositional history have received little interest. In this paper, 19 outcrop marine oil shale samples from the ChangsheMountain (CSM) area were studied to investigate the provenance, palaeoclimate, palaeoweathering, and tectonic setting during the marine oil shale deposition. The total organic carbon and organic sulphur contents of the CSM oil shales range from 2.2% to 13.44% and 0.19% to 1.08%, respectively. The CSM oil shale samples contain abundant carbonate (e.g., calcite and dolomite), quartz, and clay minerals. The clay minerals of the CSM oil shale samples is dominated by illite and then by smectite. Other minerals, including haematite, apatite, pyrite, baryte, and celestite, were also identified by scanning electron microscope–energy‐dispersive X‐ray spectrometer. The total rare earth element (∑REE) contents of the oil shales range from 9.01 to 105.96 ppm (average 57.56 ppm), close to the average REE contents of U.S. coals. The REE contents in oil shale samples are mainly controlled by terrigenous input. The CSM oil shales show LREE‐enriched, HREE‐depleted, with the LREE/HREE ratios ranging from 5.12 to 10.18, negative Eu anomalies (0.56–0.84) and slightly negative Ce anomalies (0.87–1.05). The chemical index of alteration values of the CSM oil shales range from 67 to 76 (average 73), indicating a moderate chemical weathering condition. Sr/Cu and Rb/Sr ratios suggest the CSM oil shales were mainly deposited under a warm and humid climate regime. Sr/Ba ratios, ranging from 0.54 to 7.84, suggest a palaeoenvironment with fluctuant salinity. Major element, trace element, and REE relationships indicate that the CSM oil shales are mainly from the felsic and intermediate igneous rocks. The REE distribution patterns of the CSM oil shales are similar to the Late Triassic Nadi Kangri felsic volcanic rocks in the Qiangtang Basin, indicating that the CSM oil shale may be derived from the Late Triassic volcanic rocks and the oil shales were mainly formed in rift tectonic setting. Favourable preservation condition (e.g., warmer palaeoclimate, higher palaeosalinity, and dysoxic to anoxic water column environment) could be the main controlling factor for organic matter accumulation.
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