非热液成因的鞍形白云石:来自加拿大萨斯喀彻温省东南部奥陶系Yeoman组的岩石学和地球化学证据

加拿大萨斯喀彻温省东南部上奥陶统Yeoman组碳酸盐岩中发育有少量的鞍形白云石胶结物。这些鞍形白云石仅局限于Yeoman组上部厚约20~30 m的白云岩带中，上覆及下伏碳酸盐岩地层中均明显缺失这类鞍形白云石，表明其形成于一个相对封闭的体系中。此类奥陶系鞍形白云石胶结物以具有与宿主交代白云岩相似的碳同位素δ13C值（-0.2‰~0.9‰PDB）及锶同位素比值（0.708 2~0.709 0）为特征，表明前期的白云石围岩通过压溶作用形成的碳和锶是鞍形白云石胶结物的主要来源。另外，测得的鞍形白云石胶结物均一温度范围为99~105℃，可以由该区域的正常埋藏温度解释。基于上述资料和观察，我们认为萨斯喀彻温省东南部上奥陶统Yeoman组鞍形白云石胶结物与早期交代白云石的自调节白云石化作用（埋藏过程中相对封闭的体系中通过化学压实作用形成）有关，而与加西盆地其它地方已经证实的热液活动无关。因此，鞍形白云石的分布未必指示热液活动或热流体，也并不是所有的鞍形白云石都与热流体有关。

Non-hydrothermal Saddle Dolomite: Petrological and Geochemical Evidence from the Ordovician Yeoman Formation, Southeastern Saskatchewan, Canada

Minor saddle dolomite cements are identified in the Upper Ordovician Yeoman carbonate rocks in southeastern Saskatchewan, Canada. These saddle dolomite cements are restricted to upper part of the Yeoman Formation, in a 20~30 m zone of dolomite rocks, and they are conspicuously absent both in the overlying and underlain carbonate strata, suggesting their precipitation in a relatively closed system. These Ordovician saddle dolomite cements are characterized by δ13C values ( 0.2‰ to 0.9 ‰ PDB) and Sr isotopic ratios (0.7082 to 0.7090) similar to those of their host replacement dolomites, indicating the sources of carbon and strontium for the saddle dolomite cements were mostly from precursor host dolomite rocks via pressure dissolution. In addition, the measured homogenization temperatures from the saddle dolomite cements vary from 99 to 105℃, which could be accounted for by the normal burial temperatures in the region. Based on these data and observations, the saddle dolomite cements in the Upper Ordovician Yeoman Formation in southeastern Saskatchewan are interpreted to be related to cannibalization of earlier replacement dolomite through chemical compaction in a relatively closed system during burial, rather than related to hydrothermal activities documented elsewhere in the Western Canada Sedimentary Basin. The occurrences of saddle dolomite, therefore, are not necessarily indicative of hydrothermal activity or fluids; nor are all saddle dolomites definitely related to hydrothermal fluids.