湘中地区奥陶系和震旦系含锰层中稳定同位素组成的沉积学新解释

A NEW SEDIMENTOLOGICAL EXPLANTION OF STABLE ISOTOPIC COMPOSITIONS IN ORDOVICIAN AND SINIAN MANGANESE-BEARING BEDS IN CENTRAL HUNAN

湘中地区有中奥陶世和早震旦世两个重要的锰碳酸盐层。安化县附近的湘锰组含锰泥灰岩和锰白云岩的碳和氧同位素值分别为—2.69～—4.00‰PDB和—6.87～—14.04‰PDB。湘锰组MnCO_3有δ^(12)C值—7.44～—13‰PDB和δ^(18)O值—2.65～—13.03‰。磨刀溪组黑色页岩(中奥陶世)中的Mn-泥灰岩显示负的δ^(13)C值(—1.7～—6.56‰PDB)和δ^(18)O值(—11.67～—14‰PDB)。但结晶锰矿石有更高的负δ^(13)C(—4.70％～—17.79‰PDB)和δ^(18)O(—11.67～17.27‰)。本区存在三种锰碳酸盐岩C和O同位素演化趋势:(1)安化地区的锰灰岩为稳定的δ^(13)C趋势;(2)湘锰组黑色页岩中的MnCO_3具有同步趋势:(3)中奥陶世黑色页岩中的锰矿石具有强烈反向趋势。碳成岩作用对锰碳酸盐形成过程中C和O同位素组成影响很大。湘锰组碳酸盐来源为非生物成因的海洋CO_2和SR至Me带的有机碳加入有关。磨刀溪组结晶锰碳酸盐的主要来源是热解脱羧基带(D)。δ^(34)S也为生物来源(—8～—27.49‰CDT)。Mn(％)品位与δ^(13)C呈负相关关系。证据表明锰碳酸盐成因与海盆地缺氧事件和海底热水(喷气)沉积有关。

There are two important manganese carbonate horizons in the Middle Ordovician and Early Sinian strata in central Hunan Province. Manganese marl and manganese dolomite in the Xiangmeng Formation (Early Sinian) in the vicinity of Anhua County have carbon and oxygen isotope values ranging from -2.69 to -4.00‰ and from -6.87 to -14.04‰, respectively. MnCO_3 in the Xiangmeng Formation yields δ^(13)C values ranging from -7.4 4 to -13.00‰ and δ^(18)O values from -2.65 to -13.03‰. Manganese marl in the black shale of the Modaoxi Formation (Middle Ordovician ) shows negative δ^(18)C values ranging between -1.70 and -6.56‰, and δ^(18)O values between -11.67 and -14.00‰. Morever, crystalline manganese ores have highly negative δ^(13)C values ranging between -4.70 and -17.79‰, and δ^(18)O values between -11.67 and -17.27‰. There are three kinds of evolutionary trends for carbon and oxygen isotopes in the manganese carbonate rocks. (1)Stable δ^(13)C trend in the manganese limestone of the Early Sinian strata in the Anhua district; (2) sympathetic trend of MnCO_3 in black shale in the Xiangmeng Formation. and (3) strongly antipathetic trend of manganese ores in the Middle Ordovician black shale. Carbon diagenesis has an important effect on carbon and oxygen isotope compositions during the formation of manganese carbonates. Carbonate source in the Xiangmeng Formation probably comes from the inorganic marine CO_2, and is related to the addition of organic carbon from sulphatereduction (SR) zone to methanogenesis (Me) zone. The carbonates in crystalline MnCO_2 in the Modaoxi Formation are derived mainly from the decarboxylation (D) zone. δ^(34)S, ranging from -8.00 to -27.49‰, also has an organic source. The manganese contents in the manganese ores show a negative correlation with δ^(13)C values. The evidences here suggest that the manganese carbonate origin is closely linked to the anoxic events in the marine basin and submarine hydrothermal (exhalative) deposits.