川中地区茅口组两期流体叠合控制下的白云石化模式

岩相学特征、地化分析揭示:川中地区下二叠统茅口组白云岩存在三种白云石:①平直晶面细晶白云石(δ13C_PDB=3.06‰,δ18O_PDB=-6.81‰;Fe:1×10-6,Mn:未检出,Sr:150×10-6);②非平直晶面粗晶鞍状白云石(δ13C_PDB=3.22‰,δ18O_PDB=-7.82‰;Fe:149×10-6,Mn:185×10-6,Sr:85×10-6);③非平直晶面细晶白云石(δ13C_PDB=3.49‰,δ18O_PDB=-9.45‰;Th=123℃,S=133~139‰NaCl;Fe:58×10-6,Mn:59×10-6,Sr:76×10-6)。研究结果表明存在两期白云石化流体:①早期压实作用形成的埋藏白云石化流体(T=37.4℃;S=29.8‰NaCl);②晚期受构造控制的热液白云石化流体(Th=114.8℃;S=153~226‰NaCl)。明确了三种白云石的成因:平直晶面细晶白云石是由早期埋藏流体交代泥晶基质形成的;非平直晶面细晶白云石是平直晶面细晶白云石在受到热液流体改造后所形成的;而非平直晶面粗晶鞍状白云石则是由热液流体直接沉淀出的。基于研究结果建立了相应的白云化模式。

The Model of Dolomitization Jointly Controlled by Two-episode Fluids in Maokou Formation in Central Sichuan Basin

Petrographic characteristics and geochemical analysis reveal that there exist three types of dolomites in dolostones in Maokou Formation, Lower Permian, central Sichuan Basin:①idiotopic fine crystal-dolomite(IFD)(δ13C_PDB=3.06‰, δ18O_PDB=-6.81‰; Fe:1×10-6, Mn:negative, Sr:150×10-6);②xenotopic coarse crystal-saddle dolomite(XCSD)(δ13C_PDB=3.22‰, δ18O_PDB=-7.82‰; Fe:149×10-6, Mn:185×10-6, Sr:85×10-6); ③xenotopic fine crystal-dolomite(XFD)(δ13C_PDB=3.49‰, δ18O_PDB=-9.45‰; Th=123℃, S=133‰~139‰NaCl; Fe:58×10-6, Mn:59×10-6, Sr:76×10-6).This study demonstrates that two episodes of dolomitization fluids in the study area:①early burial-dolomitization fluid formed by compaction(T=37.4℃; S=29.8‰NaCl); ②late hydrotherm-dolomitization fluid controlled by structure(Th=114.8℃; S=153‰~226‰NaCl).The origin of the dolostone is illustrated that:①IFD is replacive by early burial-dolomitization fluid; ②XFD is the product of hydrothermal alteration on IFD; ③XCSD is precipitated directly from hydrothermal fluid.Based on the achievement, corresponding model of dolomitization has been established.