辽宁大石桥菱镁矿床的碳氧同位素组成和成因  

. The C-O isotope geochemistry and genesis of the Dashiqiao magnesite deposit, Liaoning Province, NE China.

作　　者：汤好书[1] 武广[2] 赖勇[1] 

机构地区：[1]北京大学造山带与地壳演化重点实验室,北京100871 [2]中国科学院广州地球化学研究所成矿动力学重点实验室,广州510640

出　　处：《岩石学报》2009年第2期455-467,共13页Acta Petrologica Sinica

基　　金：国家973项目(2006CB403500和2007CB815600);国家自然科学基金项目(40352003和40425006)资助

摘　　要：在2.33～2.06Ga期间,δ^(13)C_(carb)发生了地质历史上最强烈的正异常事件,被称为Lomagundi事件。辽河群形成于2.3～1.85Ga期间,其大石桥组蕴涵超大型的大石桥菱镁矿矿床,在矿区厚约1144m。为检验华北克拉通对Lomagundi事件响应,揭示大石桥菱镁矿的成因,本文报道我们最新获得的地球化学研究结果。大石桥菱镁矿6件下伏白云岩地层样品的CaO/MgO摩尔比为0.87～0.94,δ^(13)C_(carb)和δ^(18)O_(carb)值分别为0.6‰～1.4‰(平均1.2‰,V-PDB标准)和16.4‰～19.5‰(平均 18.2‰,SMOW标准);与世界正常海相碳酸盐岩地层相比,δ^(13)C_(carb)较高,而δ^(18)O_(carb)较低,表明原始沉积物具有类似于Lomagundi事件的δ^(13)C_(carb)正异常,δ^(13)C_(carb)可能高达4.2‰,但δ^(13)C_(carb)和δ^(18)O_(carb)值均在沉积之后的成岩或/和变质过程中显著降低。研究剖面大石桥菱镁矿含矿地层厚逾550m,6件样品的CaO/MgO摩尔比为0.005～0.23,δ^(13)C_(carb)和δ^(18)O_(carb)值分别为0.1‰～0.6‰(平均0.4‰),9.2‰～12.7‰(平均10.9‰),均低于下伏围岩白云岩;推测与区域变质有关的流体交代作用导致岩石发生重结晶作用和同位素交换,使δ^(13)C_(V-PDB)和δ^(18)O_(V-PDB)值降低。而对菱镁矿顶板白云岩和网脉状菱镁矿矿石的研究进一步证明了上述解释的合理性。总之,大石桥菱镁矿的形成经历了初始沉积、成岩作用、区域变质和流体交代作用以及成矿后的局部蚀变作用。The positive delta(13)C(carb) excursion happened in 2.33 similar to 2.06 Ga is unique in the Earth's history, which is called as Lomagundi Event. Liaohe Group, Liaoning Province, was deposited in the time span of 2.3 similar to 1.85 Ga. Its Dashiqiao Formation hosts the giant Dashiqiao magnesite deposit and is 1144 to thick in the Dashiqiao orefield. To insight into the impact of Lomagundi event in the North China Craton and the genesis of the Dashiqiao giant magnesite deposit, this paper reports new results obtained from our geochemical study. The and delta(13)C(V-PDB) and delta(18)O(V-SMOW) values of six dolomite samples whose CaO/MgO(mol) ratios range from 0.87 similar to 0.94, range 0.6 parts per thousand similar to 1.4 parts per thousand and 16.4 parts per thousand similar to 19.5 parts per thousand, with averages of 1.2 and 18.2 parts per thousand, respectively. Compared to the normal marine carbonates in the world, the delta(13)C(V-PDB) values are higher, whereas the delta(18)O(V-SMOW), values are lower, suggesting that the primary sediments should possess positive delta(13)C(V-PDB) anomaly (possibly 4.2 parts per thousand) which reflects the impact of Lomagundi Event, and that the delta(13)C(V-PDB) and delta(18)O(V-SMOW) decreased in post-sedimentation diagenesis or/and regional metamorphism. The magnesite layer is >550 m thick and has CaO/MgO(mol) ratios ranging from 0.005 similar to 0.23. Its delta(13)C(V-PDB) and delta(18)O(V-SMOW) values are 0.1 parts per thousand similar to 0.6 parts per thousand and 9.2 parts per thousand similar to 12.7 parts per thousand, with averages of 0.4 parts per thousand and 10.9 parts per thousand (n = 6), respectively, obviously lower than those of the underlying dolomites. The depletions of (13)C and (18)O in magnesites relative to dolomites were interpreted to be the results of regional metamorphism-related hydrothermal alteration which resulted in rock recrystallization and mass exchange. This interpretation call be furthered by two particular samples, i.e. the hanging

关 键 词：Lomagundi事件 大石桥菱镁矿矿床 辽河群 C-0同位素体系 成因 

分 类 号：P619.237[天文地球—矿床学] P597.2[天文地球—地质学]