Multistage dolomitization in the Qal'eh Dokhtar Formation (Middle‐Upper Jurassic), Central Iran: petrographic and geochemical evidence
Multistage dolomitization in the Qal'eh Dokhtar Formation (Middle‐Upper Jurassic), Central Iran: petrographic and geochemical evidence
复制标题
DOI：
10.1002/gj.2876
复制
发表时间：
2018-01
影响因子：
1.8
通讯作者：
Mahnaz Sabbagh Bajestani;A. Mahboubi;I. Al-Aasm;R. Moussavi-Harami;M. Nadjafi
中科院分区：
文献类型：
--
作者：
Mahnaz Sabbagh Bajestani;A. Mahboubi;I. Al-Aasm;R. Moussavi-Harami;M. Nadjafi
晚侏罗世Qal‘eh Dokhtar组由三部分组成，自下而上为下砂岩单元、中页岩单元和上碳酸盐岩单元，沿N-S方向延伸至Shotori山脉以东和Lut地块(伊朗中部)以西的大片地区。该序列沉积在碳酸盐-硅质碎屑混合斜坡上。Qal‘eh Dokhtar组碳酸盐岩从未白云化到部分白云化，再到完全白云化。两个测量剖面(类型剖面，厚955m，索朗德剖面，厚639m)的野外观察，结合详细的岩石学和地球化学分析，揭示了该组中不同类型的白云岩。划分出5种交代白云石和1种白云石胶结物。交代白云岩(RD)包括：(1)细晶面-S(RD1)；(2)中晶面-S(RD2)；(3)中晶面-E(RD3)；(4)粗晶面-S(RD4)；(5)粗晶面-E(RD5)。粗晶平面白云石胶结物(DC)丰度低，充填溶蚀空洞和裂缝。白云岩类型的变化主要与早晚成岩作用导致白云化流体成分的变化有关。交代白云石属非化学计量比(Ca43-56-Mg34-45)，锶、锰和铁的含量分别为41-138ppm、168-919ppm和5000-21000 ppm。这些白云岩的δ18O值为0~-11.8vpdb，δ13C值为+1.1vpdb~+3.2vpdb。相对于与海水平衡沉积的晚侏罗世白云岩的推定值，这些值在δ18O中是亏损的，而δ13C值在侏罗纪海水白云岩的值范围内。RD4、RD5和DC的流体包裹体均一温度为72~118°C。根据岩相学、流体包裹体测温和地球化学结果，Qal‘eh Dokhtar组的交代白云岩形成于浅至中埋藏深度的地下。然后，这些白云石在增加的埋藏深度和温度下重新结晶。海水是早成岩白云岩(DR1)的主要镁离子来源，而晚成岩白云岩的镁离子主要来自页岩中粘土矿物的成岩作用和Qal‘eh Dokhtar组泥岩的机械压实作用。白云石胶结物被粗晶马赛克方解石胶结物延迟，表明成岩流体最终相对于白云石变得不饱和，而相对于方解石变得过饱和。版权所有©2016 John Wiley&Sons，Ltd.
The late Jurassic Qal'eh Dokhtar Formation lithologically comprises three parts, from bottom to top, a lower sandstone unit, middle shale unit and an upper carbonate unit, which extend in a N–S direction over a wide area to the east of the Shotori Range and west of the Lut Block (Central Iran). This succession was deposited on a mixed carbonate–siliciclastic ramp. Carbonate rocks of the Qal'eh Dokhtar Formation vary from undolomitized, to partly dolomitized, to completely dolomitized. Field observations from two measured sections (the type section, 955 m thick, and the Sorond section, 639 m thick), combined with detailed petrographic and geochemical analyses, revealed the diverse types of dolomite in this formation. Five types of replacement dolomite and one type of dolomite cement were distinguished. Replacement dolomites (RD) consist of: (1) fine crystalline planar‐s (RD1); (2) medium crystalline planar‐s (RD2); (3) medium crystalline planar‐e (RD3); (4) coarse crystalline planar‐s (RD4); and (5) coarse crystalline planar‐e (RD5). Coarse crystalline planar dolomite cements (DC) were observed in low abundance and filling dissolution voids and fractures. Variation in dolomite types is mainly related to early to late diagenetic processes leading to changes in composition of the dolomitizing fluids. Replacement dolomites are non‐stoichiometric (Ca43‐56–Mg34‐45) with Sr, Mn and Fe concentrations of 41–138 ppm, 168–919 ppm and 5000–21000 ppm, respectively. These dolomites are characterized by δ18O values ranging from 0.0 to –11.8 ‰ VPDB and δ13C values of +1.1 to +3.2 ‰ VPDB. These values are depleted in δ18O relative to the postulated values for late Jurassic dolomites precipitated in equilibrium with seawater, while δ13C values are within the range of Jurassic seawater dolomite values. Fluid inclusion data of RD4, RD5 and DC yield homogenization temperatures of 72 to 118 °C. Based on petrographic, fluid inclusion microthermometric data and geochemical results, the replacement dolomites in the Qal'eh Dokhtar Formation are interpreted to have formed in the subsurface at shallow to intermediate burial depths. These dolomites were then recrystallized at increased burial depths and temperatures. Seawater was the major source of Mg2+ for early diagenetic dolomite (DR1), while Mg2+ for late diagenetic dolomites was provided from diagenesis of clay minerals in shales and mechanical compaction of mudstone in the Qal'eh Dokhtar Formation. The dolomite cement is postdated by coarsely crystalline mosaic calcite cement indicating that diagenetic fluids eventually became undersaturated with respect to dolomite and oversaturated with respect to calcite. Copyright © 2016 John Wiley & Sons, Ltd.