渝东地区寒武系第四阶龙王庙组古环境演化的稳定同位素与主、微量元素证据
Isotopic and Elemental Evidence for Paleoenvironmental Evolution of Cambrian Stage 4 Longwangmiao Formation, East Chongqing, China

寒武系第四阶是生命演化史上重要的时间节点.渝东地区寒武系第四阶龙王庙组以碳酸盐岩为主要岩石类型，地层中、下部混积岩发育，生物化石发育程度低.∑(CaO+MgO+LOI)含量分布范围广(74.64%～99.18%)，Na2O、K2O、Fe2O3与MnO含量较高(平均值分别为0.20%、0.27%、0.27%和0.014%)，∑(SiO2+Al2O3+Fe2O3+ P2O5+TiO2)在地层中下部具较高含量(最大值24.00%)；Sr元素含量较低(135×10-6～407×10-6)，∑REE+Y在地层中部含量较高，∑LREE/∑HREE分布在2～4，具显著正铈异常(δCe = 5.6～6.2)，配分曲线为平缓右倾模式.全岩与方解石的δ13C值分别为-1.5‰～ 1.1‰ VPDB、-3.4‰ ～ +0.5‰ VPDB，δ18O值分别为-9.7‰ ～ -3.5‰ VPDB、-9.8‰～-5.0‰ VPDB；垂向上，两者的δ13C值在地层中下部均以负值为主，中上部多为正值.依据δ13C、δ18O、MgO/CaO(0.03%～0.12%)、100 × MgO / Al2O3(93.1～ 4 715.4 )、B校 (278.7～1 494.2)、Sr/Ba(4.05～58.25)、1 000×Sr/Ca(5.41～14.43)、V/(V+Ni)(0.61～0.78)及U/Th(0.21～ 1.45 )等与沉积环境的关系，推断龙王庙期水体温度在17～25℃，属半咸水-咸水环境；水体深度在早-中期波动频繁；以贫氧-缺氧环境为主，在早-中期、末期为阶段性富氧环境.整体上龙王庙组沉积早-中期受河流注入影响较为明显、陆源碎屑供给充足，具海陆过渡环境特征，沉积中后期为稳定的海相环境.咸水、缺氧的条件及海平面波动频繁并伴有大量陆源碎屑注入所产生的环境压力，是引发渝东地区寒武系第四阶生物消亡的主要环境因素.
The Cambrian Stage 4 represents a key point in the history of life in Longwangmiao Formation, carbonate is main rocks and diamictite widely distributed at its lowermiddle part, with the slight occurrence of bioclast. The major and trace element analyses show that Longwangmiao Formation carbonate has various contents of ∑(CaO+MgO+LOI) from 74.64% to 99.18%, high Na2O, K2O, Fe2O3 and MnO concentrations (with an average of 0.20%, 0.27%, 0.27% and 0.014%, respectively), low Sr contents (135×10-6-407×10-6), gradually decreased contents of ∑(SiO2+Al2O3+Fe2O3+ P2O5+TiO2) upward section, extremely positive Ce anomalies (δCe=5.6-6.2) and a distinctive LREEenriched pattern all the section (∑LREE/∑HREE=2-4), coupled with relatively high contents of ∑REE+Y at its middle part. The whole carbonates possess δ13C values from -1.5‰ to 1.1‰ VPDB and δ18O values from -9.7‰ to -3.5‰ VPDB, while calcites have relatively high δ13C values (-3.4‰ to +0.5‰ VPDB) and approximate δ18O values (-9.8‰ to -5.0‰ VPDB), alongside a gradual increase in carbon isotopic composition of both whole carbonates and calcites from its lower part to the upper section. Geochemical indictors of different palaeoenvironmental factors, involved palaeoclimate (MgO/CaO), seawater temperature and salinity (δ13C, δ18O, 100 × MgO/Al2O3 and Bproof-read), relative seawater depth (Sr/Ba, 1 000×Sr/Ca) and redox condition (U/Th, V/(V+Ni)), etc., imply that Longwangmiao Formation deposited at saltbrackish water under 17-25 ℃ during the transition mainly from the relative oxidizing and paralic environment to the dysaerobicanaerobic and shallow marine environment, with an abundant riverine influx at its early period and frequent fluctuations of paleoclimate from relatively humid to arid, which would build the