四川龙门山唐王寨地区中上泥盆统白云岩成因

THE ORIGIN OF MIDDLE AND UPPER DEVONIAN DOLOMITE IN THE TANGWANGZHAI AREA, LONGMEN MOUNTAINS, SICHUAN PROVINCE

本文应用矿物学、岩石学和地球化学等手段和宏观、微观相结合的方法对研究区的白云岩作了系统分析,确定出两种不同成因类型的白云岩。其中由它形微晶白云石构成的白云岩,是泻湖中形成的高镁碳酸盐泥在成岩作用阶段发生晶格调整而成;而自形和半自形粉至粗晶白云石构成的白云岩则是深埋混合水白云化作用产物。

Middle and Upper Devonian dolomite in the Tangwangzhai area of the Longmen Mountains is mainly composed of euhedral and subhedral, silt-sized granular, to macrogranular and anhedral microcrystalline dolomite, of which euhedral dolomite shows a higher degree of ordering(average 0.95), a low CaCO_3 mole content (average 51.6％), a [lower δ^(18)O value(average -5.46‰ PDB) and a lower δ^(13)C value (average 0.84‰ PDB). Its cathodoluminescence colour is mainly dark red to brick-red. The subhedral dolomite has a higher ordering degree (average 0.91) and a lower CaCO_3 mole content (average 50.3％). Its δ^(18)O and δ^(13)C values and the cathodoluminescence colour are samilar to those of euhedral dolomite. The homogenization temperatures of inclusions in the dolomite range from 109 to 134℃. The analysis of trace elements of some dolomite shows that it has a high Fe^(2+) content (average 1142 ppm) and a low Sr^(2+) content (average 54 ppm). On the other hand, the anhedral microcrystalline dolomite has a low ordering degree(average 0.64) a high CaCO_3 mole content (averaga 53.3％), a δ^(18)O value of -2.28％ (PDB) and a δ^(13)C value of +1.17‰ (PDB). The cathodo-luminescence colour is orange. A comprehensive study suggests that there are two origins for the dolomite in the study area: (1) the anhedral microcrystalline dolomite was formed by crystal lattice readjustment of high-magnesium carbonate mud deposited in lagoons during diagenesis; )2) the euhedral to subhedral, silt-sized granular to macrogranular dolomite is the product of dolomitization of deep-buried mixing water.