四川盆地二叠—三叠系碳酸盐岩和砂岩中的鞍形白云石——岩石学、形成温度与流体

鞍形白云石是沉积岩中的一种重要成岩矿物。显微镜下鞍形白云石具有特殊的弧形晶面和波状消光,广泛分布于热液环境和其他相对高温的成岩环境中,也常被作为半定量的地质温度计和某些特殊流体的指示矿物。本文综合讨论了四川盆地二叠—二叠系碳酸盐岩和砂岩中鞍形白云石的岩石学特征,铁、锰、钙含量和阴极发光特征,形成温度和矿物的氧同位素组成,以及推算的源流体的氧同位素组成和盐度;同时对照研究了鄂尔多斯盆地上古生界砂岩中的鞍形白云石、塔里木盆地寒武—奥陶系的鞍形白云石以及南海珠江口盆地新近系的鞍形白云石,得到如下认识:(1)碳酸盐岩中大多数的鞍形白云石的晶体大小都超过1mm,具有粗晶—极粗晶结构。砂岩中鞍形白云石胶结物的晶体相对较小,主要具中—粗晶结构。碳酸盐岩中的鞍形白云石主要分布在晶洞或裂缝中,砂岩中的鞍形白云石主要是晚期的胶结物。(2)碳酸盐岩中的鞍形白云石普遍具有阴极发光和很低的铁、锰含量,在钙、镁含量上接近理想组成;砂岩中的鞍形白云石胶结物普遍具有较高的铁、锰含量,因铁的猝灭作用而经常没有阴极发光,同时因富钙而偏离白云石的理想组成。(3)存在溶解现象(或去白云化现象)是一些鞍形白云石的重要特征,与热液有关的鞍形白云石的溶解可能与热液作用后温度和(或)盐度的降低及镁离子的消耗有关,较大埋深条件下形成的砂岩中的鞍形白云石胶结物的溶解可能与构造抬升造成的温度降低有关。(4)大多数鞍形白云石都是在较高的温度和盐度的流体中沉淀的,四川盆地二叠—三叠系碳酸盐地层中鞍形白云石包裹体均一化温度为100~270℃,源流体的盐度(NaCl质量分数)为4.7%~9.7%,盐度最大值约为当时海水的5倍;三叠系须家河组砂岩中的鞍形白云石胶结物包裹体均一化温度为120~170℃,流体盐度在2.6%~6.6%,也超过当时海水的盐度值。(5)南海珠江口盆地新近系碳酸盐岩中的一些鞍形白云石晶体较小,以细晶为主,缺乏可进行均一温度分析的两相包裹体,其形成温度可能较低,因而鞍形白云石形成的最低温度可能低于80℃,但这仍然需要资料的进一步积累。

Saddle dolomite in Permian-Triassic carbonate rocks and sandstones of Sichuan Basin:petrology,formation temperature and palaeofluids

Saddle dolomite is an important diagenetic mineral of sedimentary rocks.Saddle dolomite often shows undulatory extinction and curved crystal boundaries under microscope and is widely distributed in a hydrothermal environment or other environments with relatively high temperature.It is often taken as a semiquantitative geothermometer and an indicator mineral of some peculiar fluids. This paper synthetically discusses the petrology,Fe,Mn and Ca content,cathodoluminescence, formation temperature,oxygen isotope composition of minerals,oxygen isotope composition and salinity of the palaeofluids of the saddle dolomite in the Permian-Triassic carbonate rocks and sandstones of Sichuan Basin.It also comparatively studies the saddle dolomite in the Upper Paleozoic sandstones of Ordos Basin,that in the Cambrian-Ordovician carbonate rocks of Tarim Basin and that in Neogene of Zhujiangkou Basin.The understandings obtained are as follows.(1)Most of the saddle dolomite in carbonate rocks often occurring as fillings in vugs or fractures has the crystal size more than 1 mm,with coarse to very coarse crystalline texture.The saddle dolomite cement in sandstone often occurs as late cement and has relatively small crystal size,normally medium to coarse.(2)The saddle dolomite in carbonate rocks generally has cathodoluminescence to various degrees and very low Fe and Mn concentration and nearly ideal composition in Ca/Mg ratio,and that in sandstone often has no cathodoluminescence (quenched by iron)and has high Fe and Mn concentration and deviates ideal composition in Ca/Mg ratio since it is rich in Ca.(3 )Some saddle dolomite develops dissolution or dedolomitization,probably resulted from the falling of temperature and/or salinity after the hydrothermal process for the hydrothermal originally saddle dolomite and after tectonically uplift for the late saddle dolomite cement in sandstone.(4)The parent palaeofluids of most saddle dolomite have the characteristics of high temperature and salinity.The saddle dolomite hosted in carbonate has the homogenization temperature of 100 ~ 270℃ and the salinity of 4.7% ~ 9.7% by a maximum salinity of 5 times of the palaeoseawater,and the late saddle dolomite cement in sandstone has the homogenization temperature of 120~170℃ and the salinity of 2.6%~6.6%,also higher than that of palaeoseawater.(5)The saddle dolomite in the Neogene carbonate rocks of Zhujiangkou Basin has small crystal size with fine crystalline texture.Being lack of two phase inclusions for homogenization temperature test,there may exists some saddle dolomite with lower formation temperature,but this still needs to accumulate more data.