摘要:
报道了广西钦州海槽早石炭世—早二叠世深水硅质岩的锶同位素测定结果和经年代校正后的87Sr/86Sr初始值,建立了锶同位素演化曲线;通过与同时代上扬子地区台地相碳酸盐岩87Sr/86Sr比值和演化曲线的对比,研究了二者的异同和造成差异的原因。深水硅质岩和同期台地相碳酸盐岩的锶同位素演化曲线在总体特征上具有一定的相似性,反映海平面变化对其影响的一致性。玄武岩喷发等局部事件和硅质岩中无法回避的陆源碎屑的影响是造成87Sr/86Sr比值和演化曲线差异的主要原因。硅质岩锶同位素演化曲线显示,海平面上升发生于早石炭世,在晚石炭世早期达到最大值,晚石炭世晚期,海平面有所下降,在其后的早二叠世,海平面再次上升。
Abstract:
The carbon,oxygen and strontium isotope composition of marine carbonate is mainly controlled by global events,such as relative amount of organic accumulation and oxidation,orogenic events,glacial activity,changes in seafloor spreading rate and palaeoclimate variation.The evolution tendency of isotopes with the same stratigraphy interval is similar.This is the fundamental principle of isotope stratigraphy.However,carbonate sediments are normally absent in deep sea environment,we could not obtain the data of carbon and oxygen isotopes,but the deep sea sediments with few fossils specially need the supports of isotope stratigraphy.For this reason,we collected siliceous rock samples from deep sea environment,researched their strontium isotope and set up evolution curves to seek after their application foreground in isotope stratigraphy. The deep sea siliceous rock samples studied here were collected from the sedimentary section from Earlier Carboniferous to Earlier Permian,Qinzhou,Guangxi.Based on the analysis of X ray diffraction and cathodoluminescence of the collected samples,they should be chemical precipitation from seawater.After age correction based on the Rb/Sr ratio,we obtained the original 87Sr/86Sr values and set up the evolution curve which is similar to that of shallow water carbonate from the same age in Upper Yangtze Platform.It supports the notion of a global consistency in strontium isotope composition of marine interior sediments.However,because we could not eliminate effect from terrigenous clastics with higher 87 Sr content,the average of 87Sr/86Sr values form siliceous rocks is observably higher than that from carbonate rocks.Additionally the lower 87Sr/86Sr ratio record of carbonate in Earlier Permian was probably caused by substantial basalt eruptions on Upper Yangtze Platform at the time.The strontium data exhibit three intervals of relatively low 87Sr/86Sr ratio in Earlier Carboniferous,earlier stage of Late Carboniferous,and Earlier Permian,corresponding to global eustatic high sea level stands.The strontium data exhibit one intervals of relatively high 87Sr/86Sr ratio in Late stage of Late Carboniferous corresponding to global eustatic lower sea level stands.
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