Evolution of in-situ geochemical records in hydrothermal recrystallization of dolomite: A case study from Upper Cambrian dolostones in the Bachu area, Tarim Basin
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摘要:
塔里木盆地巴楚地区上寒武统白云岩遭受了大量的热液改造。基于岩石学、原位微量元素特征分析及前人的研究成果,本研究从BT5井上寒武统白云岩中识别出了3种组构的白云石:基质白云石(MD)、环带状鞍形白云石(SD1)和巨晶鞍形白云石(SD2)。其中,SD1自形程度较好,其核部(SD1-1)具有污浊的晶面和亮红色阴极光,边缘(SD1-2)具有明亮的晶面并在阴极光下显示无规则明暗环带;SD2具有波状消光和亮红色阴极光。原位微量元素分析显示,从MD到SD1和SD2核部(SD2-1),样品呈现Sr元素含量逐渐下降、稀土元素总含量逐渐增加且轻稀土元素的富集程度逐渐增加的规律。综合上述研究,推测SD2-1是从热液中沉淀的白云石充填物,而SD1与热液导致的MD重结晶改造有关。本研究显示热液导致的重结晶形成的白云石具过渡性质稀土元素构成(介于MD和SD2之间),这种特征可以用来识别热液导致的重结晶过程。
Abstract:The Upper Cambrian dolostones in the Tarim Basin have undergone widely alteration by hydrothermal fluids. In this study, based on petrological and trace element analyses, three types of dolomites were identified in the Upper Cambrian dolostones of Well BT5 in the Bachu area: matrix dolomites (MD), dolomite cements with cathodoluminescence zonation (SD1), and blocky dolomite cements (SD2). SD1 occurs as euhedral crystals, consisting of a core (SD1-1) and a rim (SD1-2). SD1-1 is characterized by cloudy crystal surfaces and bright red luminescence under cathodoluminescence (CL), while SD1-2 displays bright crystal surfaces with irregular bright-dark zoning under CL. And SD2 exhibits wavy extinction and shows bright red under CL. From the host dolostone (MD) to the last generation of dolomite cements (SD1, and SD2's core SD2-1), the Sr concentrations decreased, and the total rare earth element concentrations (ƩREE) as well as light rare earth element enrichment increased. Consequently, SD2-1 is interpreted to be dolomite fillings precipitated from hydrothermal fluids, while SD1 results from hydrothermal recrystallization of MD. This study suggests that the dolomites formed from hydrothermal recrystallization show transitional REE characteristics, which can serve as a useful tool to identify recrystallization process in dolomites.
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0. 引言
沉积盆地内各种构造演化都可能伴随着热液流体的活动(Machel and Lonnee,2002; Davies and Smith,2006; Goldstein,2012),研究表明,热液流体能对油气运聚产生巨大影响,包括影响烃源岩的热演化历史、改造储层的储集空间物性,以及促进油气运移过程等(金之钧等,2006,2013; Zhu et al.,2015; 万友利等,2020)。前人对全球范围内发现的与热液活动相关的碳酸盐岩储层进行了大量研究,例如西加拿大盆地泥盆系、美国东北部密歇根盆地奥陶系,以及中国塔里木盆地寒武—奥陶系储层等,发现优质储层的发育与热液流体的改造有一定关系(Qing and Mountjoy,1992; Duggan et al.,2001; Davies and Smith,2006; 胡九珍等,2009; 金之钧等,2013)。热液流体进入碳酸盐岩储层后可能造成溶蚀、胶结、交代(白云岩化或硅化)、重结晶等现象(Ye et al.,2019,2022; Liu et al.,2020),但是由于热液驱动机制、来源的多样性以及运移过程中的复杂性,导致其往往难以示踪,使得储层的演化程度难以精确评估。前人研究发现,热液流体可能来源于大气淡水、埋藏孔隙水以及与断裂沟通的深部流体,这些流体在温度、盐度、氧化还原环境和水岩比上可能存在差异,而流体的微量和稀土元素特征可能受控于这些因素(Qing and Mountjoy,1992; Wang et al.,2015)。因此,利用原位元素地球化学方法可以较好地示踪热液活动规律(Qing and Mountjoy,1992; Wang et al.,2015; Liu et al.,2017)。
随着塔里木盆地巴楚地区油气勘探不断深入,大量的研究揭示了上寒武统大套形成于准同生期的白云岩遭受了后期热液改造(谢世文等,2013; 黄擎宇等,2016; Jiang et al.,2016; 赵文智等,2018),对巴楚地区白云岩样品的大量研究表明缝洞充填的鞍形白云石具有热液特征,例如较高的均一温度(Zhang et al.,2009; 谢世文等,2013; Dong et al.,2013b)、高于同时期海水的87Sr/86Sr比值(周波等,2012; 谢世文等,2013)以及伴生的热液矿物(黄擎宇等,2016; 景帅,2020)。热液流体对碳酸盐岩的改造可能显著地影响储层物性(金之钧等,2006; Zhang et al.,2009; 景帅,2020),其中热液的溶蚀、胶结和交代对储层储集物性的影响比较显著,但是热液导致的白云石重结晶对储集空间演化的影响争议较大,有关破坏性(刘伟等,2016; 刘红等,2022)和建设性作用(胡九珍等,2006; 朱东亚等,2008; 谢世文等,2013; Zhu et al.,2015)的研究结论,学界各执一词。相关争议的根源主要在于深埋藏白云岩中热液活动引起的重结晶难以从微观角度进行识别。重结晶作用会导致原始白云岩的结构和地球化学特征发生改变,其结构特征上的变化包含“雾心亮边”结构的出现、晶体尺寸变大、非平直晶面的发育等(Gregg and Sibley,1984; 白璇等,2022);地球化学特征主要体现在多种同位素和微量元素的变化上,包括氧同位素降低、87Sr/86Sr值升高、锶含量降低等(Li et al.,2019; 白璇等,2022)。最近,部分学者利用激光剥蚀手段对成岩矿物进行了更加精准的原位微量元素分析,揭示了成岩流体的地球化学性质(Zhang et al.,2014; Lu et al.,2022),但重结晶过程中原位元素地球化学信号的变化规律仍然模糊。
因此,本研究通过岩石学,以及原位主、微量元素方法,对巴楚地区上寒武统白云岩特征进行了研究,并结合前人在该地区发表的锶同位素数据,揭示了该区域热液导致的白云岩重结晶过程,以期为利用微量元素性质识别热液导致的白云岩重结晶过程、评价热液对围岩影响程度提供参考,也为塔里木盆地巴楚地区油气勘探提供地质依据。
1. 地质背景
研究区巴楚隆起位于塔里木盆地中西部,东北紧邻柯坪断隆,东南与阿瓦提坳陷和卡塔克隆起相接,其西南部紧邻麦盖提斜坡,东南部则是塘古巴斯坳陷,是盆地内最大的一个隆起单元(图1;马庆佑等,2015; 景帅,2020)。该隆起是一个从震旦纪开始就发育的古隆起,经历了加里东运动、海西运动、印支–燕山运动和喜马拉雅运动,形成了“东西分区、南北分带”的构造格局(丁文龙等,2008; 宁飞等,2021)。研究区在寒武纪—中奥陶世发育了一套稳定的台地相碳酸盐岩沉积,寒武系由下至上依次可划分为盐下的玉尔吐斯组(Є1y)、肖尔布拉克组(Є1x),膏盐岩段的吾松格尔组(Є1w)、沙依里克组(Є2s)、阿瓦塔格组(Є2a)和盐上白云岩段下丘里塔格组(Є3xq)。之后受加里东构造运动影响,巴楚隆起发育大量的逆冲断层,使得隆起持续发育,并在海西构造运动和印支–燕山构造过程中遭受了大量剥蚀,形成了巴楚隆起东高西低、中生界大量缺失的构造格局(景帅,2020)。塔里木盆地经历了四次地质热事件,其中影响程度最大、影响范围最广的是二叠纪的岩浆作用。研究表明,早二叠世大火成岩省导致热液流体沿着断裂裂缝等通道对基质白云岩进行改造(Dong et al.,2013b)。
图 1 塔里木盆地构造图(据王家豪,2012修编)和BT5井上寒武统综合柱状图A. 巴楚隆起区域位置图;B. BT5井上寒武统综合柱状图Figure 1. Location of the Tarim Basin (modified after Wang, 2012) and stratigraphic column of the Upper Cambrian in Well BT52. 样品和方法
本研究从塔里木盆地上寒武统白云岩中采集了BT5井的15个样品,并制备了厚度为30 μm的标准薄片用于岩相学观察,厚度为80 μm的专用激光分析薄片用于原位主量和微量元素分析工作。阴极发光观察在西南石油大学地球科学与技术学院实验中心CI8200 MK5平台上进行,工作环境为束电压15 kV,束电流320 μA。原位主量元素分析在西南石油大学电子探针实验室完成,使用JXA-8230进行,工作环境为:加速电压15 kV,电流10 nA。原位微量元素分析使用ESL 193UC激光剥蚀系统(ArF 193 nm气体准分子激光)和Agilent 7800 ICP-MS在西南石油大学碳酸盐岩沉积–成岩地球化学实验室完成。在本研究中,激光光斑大小和频率分别设置为80 µm和5 Hz。标准NIST 612和MACS-3分别用于校正漂移和作为基体匹配的外标,Ca元素含量作为内标(来自电子探针分析结果)进行测试和数据校正。其他测试步骤和方法参考Gong et al. (2021)。为减少稀土元素奇偶效应、了解沉积物来源和分馏情况(Taylor and McLennan,1985),测试样品的稀土元素含量需要进行标准化。前人针对碳酸盐岩样品通常采用后太古代澳大利亚页岩(PAAS)(McLennan,1989)以及太平洋表层海水进行标准化(Bau et al.,2003; Wang et al.,2014; Hood et al.,2015; 万友利等,2017; Li et al.,2019),本研究中则采用PAAS对数据进行标准化。为了解流体氧化还原性质和热液改造特征,分别采用Ce异常和Eu异常指标进行示踪,计算公式分别为:(Ce/Ce*)N = 2CeN/(LaN + PrN)和(Eu/Eu*)N = 2EuN/(SmN + GdN)(Webb and Kamber,2000; Bau et al.,2010),轻、重稀土比值(LREE/HREE)N = 4 (LaN + PrN + NdN)/3 (ErN + TmN + YbN + LuN),中、重稀土比值(MREE/HREE)N = 4 (SmN + GdN + DyN)/3 (ErN + TmN + YbN + LuN)(据Tostevin et al.,2016修改)。
3. 岩相学和元素地球化学特征
3.1 岩相学特征
巴楚地区BT5井上寒武统岩心发育大量孔洞和裂缝,孔洞直径不均一,大的可达3 cm,小的仅2 mm。裂缝和较大的孔洞被白云石半充填至全充填,部分白云石充填物晶体比较粗大(图2A-B),针状孔中未见白云石充填物(图2A)。
图 2 BT5井上寒武统白云石照片集A. 浅灰色白云岩,白云石胶结物中发育孔洞(Є3xq,4806.99 m);B. 浅灰色—灰色白云岩,孔洞发育,孔洞大小不一(Є3xq,4809.43 m);C. 中—粗晶白云岩,可见鞍形白云石充填物,从内到外依次为基质白云石(MD)、环带状鞍形白云石核部(SD1-1)、环带状鞍形白云石边缘(SD1-2)和巨晶鞍形白云石(SD2)(Є3xq,4806.99 m);D. 中—粗晶白云岩,可见鞍形白云石充填物,从内到外依次为MD、SD1-1、SD1-2和SD2,图C的阴极发光照片(Є3xq,4806.99 m);E. 白云岩孔洞中的巨晶块状鞍形白云石(SD2),可见波状消光(Є3xq,4810.02 m);F. 白云岩孔洞中充填的巨晶鞍形白云石(SD2),可见浑浊的核心(SD2-1)和清晰的边缘(SD2-2)(Є3xq,4810.02 m)Figure 2. Micrographs showing petrography of the Upper Cambrian dolomites in Well BT5研究区上寒武统白云岩类型多样,本研究对BT5井上寒武统白云岩及其缝洞充填物进行了透射光和阴极发光观察。将样品中的白云石划分为基质白云石(MD)、环带状鞍形白云石(SD1)和巨晶鞍形白云石(SD2)(图2C-D),其中,样品中MD以中—粗晶白云石为主,晶体大小介于200~800 μm,通常大于300 μm,呈半自形—他形。晶体无波状消光,阴极光照射下表现为发暗红色光(图2C-D)。SD1沿孔洞和裂缝壁发育,主要为粗晶鞍形白云石(500~
1000 μm),晶体自形程度好,呈自形—半自形,单偏光下观察发现,晶体核部浑浊边缘干净(图2C)。根据SD1的单偏光和阴极光特征,可以将SD1细分为SD1-1和SD1-2:(1)SD1-1为SD1核部,单偏光下晶面较污浊,与MD间界限模糊,呈过渡接触;阴极光照射下表现为发均匀亮红色光。(2)SD1-2为SD1边缘部分,单偏光下晶面干净,阴极光照射下观察到明暗环带(图2D)。SD2为巨晶鞍形白云石(>2 mm),呈他形,具有波状消光特征(图2E),晶体表面较脏,阴极光照射下表现为均一亮红色光(图2C-D)。根据SD2的透射光特征,可以将SD2细分为SD2-1和SD2-2:(1)SD2-1为SD2核部,在透射光下较污浊,这是SD2的主体部分,占SD总体积的90%以上;(2)SD2-2为SD2边缘,单偏光下较干净(图2F)。根据不同组构白云石的岩相学特征和矿物的空间接触关系,发现研究区白云岩形成经历了基质白云石(MD)、环带状鞍形白云石核部(SD1-1)、环带状鞍形白云石边缘(SD1-2)、巨晶鞍形白云石核部(SD2-1)和巨晶鞍形白云石边缘(SD2-2)形成等阶段(图3)。
3.2 主量元素特征
本次研究对MD、SD1-1、SD1-2、SD2-1和SD2-2分别进行了白云石颗粒主量元素电子探针分析,结果见附表1
1 。MD中CaO的含量非常稳定,在30.268%~30.829%之间(均值30.544%),MgO的含量为21.127%~21.891%(均值 21.572%),SD1白云石和SD2白云石的CaO含量介于29.848%~31.078%(均值 30.512%),MgO的含量为21.203%~21.938%(均值 21.546%),几乎不含FeO和MnO,其中,SD1白云石的CaO的含量介于29.848%~30.918%(均值 30.464%),MgO的含量为21.275%~21.938%(均值 21.544%);SD2白云石的CaO的含量介于30.029%~31.078%(均值 30.583%),MgO的含量为21.203%~21.818%(均值 21.550%)。3.3 原位微量元素特征
对MD、SD1-1、SD1-2、SD2-1和SD2-2等组构进行了原位微量元素测试,具体数据见附表2
1 和附表31 。MD的Fe元素含量为139.5×10-6~289.9×10-6(均值205.0×10-6),Mn元素含量为21.1×10-6~44.8×10-6(均值29.8×10-6),Sr元素含量为18.0×10-6~48.8×10-6(均值33.2×10-6),稀土元素总含量(ƩREE)范围为2.3×10-6~4.2×10-6(均值3.1×10-6)(图4,图5A-B)。MD的稀土元素和钇(REY)配型表现出轻稀土元素(LREE)相对重稀土元素(HREE)亏损(LREE/HREE)N = 0.61~0.95(均值0.75),中稀土元素(MREE)相对重稀土元素(HREE)无明显亏损(MREE/HREE)N = 0.84~1.25(均值1.02),无明显的Eu和Ce负异常(δEu = 0.78~1.06,均值0.90;δCe = 0.92~1.00,均值0.97)和较低的Y/Ho值(21~30,均值23)等特征(图4,图5C-E,图6A-B)。SD1-1白云石的Fe元素含量为187.5×10-6~245.5×10-6(均值217.3×10-6),Mn元素含量为41.7×10-6~130.8×10-6(均值69.9×10-6),Sr元素含量为16.7×10-6~30.5×10-6(均值24.6×10-6),ƩREE范围为6.4×10-6~13.5×10-6(均值10.6×10-6)(图4,图5A-B)。SD1-1的REY配型表现出LREE相对HREE富集((LREE/HREE)N = 0.96~2.26,均值1.51),MREE相对HREE富集((MREE/HREE)N = 1.18~2.16,均值1.51),无明显的Eu和Ce负异常(δEu = 0.84~1.30,均值0.84;δCe = 0.97~1.03,均值1.01)和较低的Y/Ho值(23~29,均值26)的特征(图5C-E,图6A-B)。
SD1-2白云石的Fe元素含量为235.8×10-6~366.7×10-6(均值292.6×10-6),Mn元素含量为45.4×10-6~85.8×10-6(均值63.8×10-6),Sr元素含量为12.2×10-6~49.2×10-6(均值22.1×10-6),ƩREE范围为6.8×10-6~22.3×10-6(均值14.9×10-6)(图4,图5A-B)。SD1-2的REY配型表现出LREE相对HREE富集((LREE/HREE)N = 1.32~4.00,均值2.38),MREE相对HREE富集((MREE/HREE)N = 1.53~2.89,均值2.09),无明显的Eu和Ce负异常(δEu = 0.91~1.17,均值1.00;δCe = 1.03~1.10,均值1.07)和较低的Y/Ho值(22~28,均值26)的特征(图5C-E,图6A-C)。
SD2-1白云石的Fe元素含量为132.7×10-6~166.3×10-6(均值152.5×10-6),Mn元素含量为50.0×10-6~58.9×10-6(均值53.5×10-6),Sr元素含量为17.7×10-6~26.2×10-6(均值21.4×10-6),ƩREE范围为27.2×10-6~42.0×10-6(均值33.4×10-6)(图4,图5A-B)。SD2-1的REY配型表现出LREE相对HREE富集((LREE/HREE)N = 7.64~11.79,均值9.64)、MREE相对HREE富集((MREE/HREE)N = 3.34~4.38,均值3.83)、无明显的Eu和Ce负异常(δEu = 0.87~1.01,均值0.93;δCe = 1.02~1.05,均值1.03)和较低的Y/Ho值(29~35,均值32)的特征(图5C-E,图6A-D)。
SD2-2白云石的Fe元素含量为121.8×10-6~147.8×10-6(均值138.2×10-6),Mn元素含量为64.8×10-6~88.6×10-6(均值74.7×10-6),Sr元素含量为76.7×10-6~100.2×10-6(均值86.3×10-6),ƩREE范围为7.2×10-6~8.8×10-6(均值7.8×10-6)(图4,图5A-B)。SD2-2的REY配型表现出LREE相对HREE无明显亏损((LREE/HREE)N = 0.75~1.07,均值0.90)、MREE相对HREE富集((MREE/HREE)N =3.44~5.14,均值4.04)、无明显的Eu和Ce负异常(δEu = 1.01~1.04,均值1.02;δCe = 0.90~0.98,均值0.93)和较低的Y/Ho值(26~33,均值29)的特征(图5C-E,图6A-D)。
利用原位元素地球化学手段对基质白云石及其孔洞裂缝中充填的鞍形白云石进行了对比,发现MD的Mn含量比SD低,并且不同阴极发光特征的白云岩组构,其Fe、Mn元素具有不同的含量,发暗红色阴极光的MD、SD1-2具有更高的Mn/Fe比值,而发亮红色光的SD1-1、SD2-1和SD2-2具有更低的Mn/Fe比值(图4)。而且在同一区域内,从MD、SD1-1、SD1-2一直到SD2-1,ƩREE和(LREE/HREE)N逐步增加,且稀土元素配型也逐渐发生变化(图6,图7)。
图 7 BT5井白云石样品原位微量元素变化特征A. 中—粗晶白云岩,基质白云石(MD)的孔洞中充填环带状鞍形白云石(SD1)和巨晶状鞍形白云石(SD2)(Є3xq,4806.99 m);B. 白云石原位微量元素变化柱状图,取点来自图A;C. 中—粗晶白云岩(与A为同一样品),基质白云石(MD)的孔洞中充填环带状鞍形白云石(SD1)和巨晶状鞍形白云石(SD2)(Є3xq,4806.99 m);D. 白云石原位微量元素变化柱状图,取点来自图CFigure 7. Variations of in-situ trace element concentrations in dolomites of Well BT54. 讨论
4.1 白云石充填物成因
白云石充填物SD2是本研究中最主要的充填物类型,它们在孔缝充填物中所占的体积比率达到90%以上,其中SD2-1又是SD2的主体部分(图2)。据前文所述,SD2-1的稀土元素特征主要表现为高ƩREE(27.2×10-6~42.0×10-6)和LREE强烈富集((LREE/HREE)N = 7.64~11.79)。
SD2-1的ƩREE显著高于正常海水成因的碳酸盐矿物(通常海相碳酸盐岩中ƩREE小于4×10-6,Hood and Wallace,2015; Li et al.,2019)。稀土元素在流体中主要以配合物的形式进行搬运,SD2-1中较高的ƩREE说明流体对稀土元素具有较强的负载能力,这可能与流体具有较高的温度、盐度等因素有关(Debruyne et al.,2016)。前人开展的部分原位微量元素研究也发现了塔里木盆地寒武系中热液成因的白云石具有更高的ƩREE(Zhang et al.,2014; Liu et al.,2020; Lu et al.,2022),这与本研究中的SD2-1特征相似。此外,由于在温度低于300℃、偏碱性的流体环境中,OH−和CO32−是稀土元素的主要配体,这种条件下,HREE的配合物稳定性高于LREE的配合物,会导致HREE的相对富集;反之,在温度高于300℃、偏碱性的流体环境中,LREE的配合物稳定性高于HREE的配合物,进而导致LREE的相对富集(Louvel et al.,2022)。 因此,LREE在流体中的富集可能有两种情况:(1)流体对磷灰石等LREE富集矿物的淋滤,和(2)温度较高(>300℃)的条件下轻、重稀土元素配合物稳定性不同造成的分馏(Migdisov et al.,2016; Louvel et al.,2022)。这两种情况均指示流体可能具有较高的温度和/或外源的供给,所以,页岩标准化后LREE相对HREE富集是一种外源热液流体的典型特征(Bau,1991; Bau et al.,2010)。综上所述,本研究中的SD2-1表现出的特征与前人报道的热液流体特征一致,推测SD2-1可能形成于外源的热液流体。
前人研究发现,巴楚地区热液作用强烈,该地区的鞍形白云石87Sr/86Sr值(
0.7091 ~0.7192 ,Zhang et al.,2009; 周波等,2012; Dong et al.,2013a; 谢世文等,2013),普遍高于同时期的海水(0.7089 ~0.7092 ,Veizer et al.,1999)。另外,SD2-1的REY配型与前人在塔里木盆地报道的热液白云石REY配型一致(图8;Zhang et al.,2014; Liu et al.,2020; Lu et al.,2022)。这些证据进一步支持了本文中SD2-1热液成因的认识。图 8 白云石稀土元素配型对比图A. BT5井白云石稀土元素配型;B. 塔里木盆地阿克苏地区下寒武统热液白云石稀土元素配型图,包含细—中晶直面白云石充填物(FD1)和粗晶鞍形白云石充填物(FD2),数据来自Liu et al.,2020;C. 塔里木盆地柯坪地区寒武—奥陶系热液白云石稀土元素配型图,包含T2-SD(MREE富集、高REE含量)和T3、T4-SD(LREE富集、高ƩREE、正Eu异常),数据来自Zhang et al.,2014;D. 塔里木盆地库鲁克塔格地区上寒武统热液白云石(KZ-SD)稀土元素配型图,数据来自Lu et al.,2022Figure 8. Comparison of REY profiles from this study with those of hydrothermal saddle dolomites reported in previous publications4.2 热液导致的白云石重结晶作用
在热液白云石SD2-1与围岩MD之间,还存在一期鞍形白云石充填物SD1。该期白云石在阴极发光下可见生长环带,碳酸盐矿物中决定阴极发光强度的是Fe和Mn的矿物学和化学性质,本研究中Fe和Mn的含量与白云石阴极发光规律吻合(图4)。
岩石学上,SD1核部(即SD1-1)在单偏光下较污浊,且MD与SD1-1之间的界线在单偏光下模糊,呈过渡接触;而SD1-2截然不同,单偏光下干净,这种现象与前人报道的白云石经重结晶后晶体表面呈现的“雾心亮边”相似(Flügel and Munnecke,2010; 白璇等,2022);同时,从MD到SD1晶体表现出尺寸变大、自形程度逐渐变高的特征,这与前人报道的白云石重结晶现象相似(Machel,1997; 朱东亚等,2010; 白璇等,2022)。元素地球化学分析可见,从MD、SD1-1到SD1-2的Sr元素含量逐渐下降(从33.2×10-6下降至22.1 ×10-6),由于Sr2+的离子半径大于Ca2+离子半径,碳酸盐矿物在重结晶过程中会伴随着Sr元素的丢失(Machel et al.,1996; Fantle and DePaolo,2006; Fantle,2015),这说明Sr元素含量的下降可能是白云石重结晶造成的。综上所述,SD1很可能是由MD重结晶形成。
一般而言,埋藏过程中的升温及深部热液作用都易导致白云石发生重结晶作用(白璇等,2022),其地球化学信号往往能反映流体的性质(Lonnee and Machel,2006; Dong et al.,2013a; Lukoczki et al.,2019; Huang et al.,2021)。焦存礼等(2011)在塔里木寒武系中也识别出与SD1相似的白云石,他认为这种白云石是热液刚进入白云岩后在裂缝中沉淀的白云石胶结物,与后一期鞍形白云石在流体环境上是连续过渡的。但在本研究中,原位微量元素结果显示,从MD、SD1-1到SD1-2,ƩREE及(LREE/HREE)N值均逐渐升高(图6A,图7B-D),在ƩREE与(LREE/HREE)N值的交会图上,这种逐渐变化的趋势外延至热液白云石SD2-1(图6A),表现出向热液过渡的流体特征。本研究认为SD1出现这种特征是由于热液导致MD重结晶造成的。此外,对SD1而言,从核部的SD1-1至边缘的SD1-2表现出LREE的逐渐富集的配型特征,如前文所讨论的,PAAS标准化后的LREE富集特征很难在碳酸盐岩内部形成。因此,SD1-1至SD1-2表现出的LREE逐渐富集的信号很大可能是由沉淀SD2-1的流体提供的,热液流体在进入缝洞系统后使MD发生重结晶作用形成了SD1,且稀土元素信号被记录在了随后沉淀的SD2-1中。
本次研究结果表明,热液导致的白云石重结晶过程中,岩石学和稀土元素地球化学构成均出现了由围岩向热液流体过渡的特征:在靠近围岩一侧发生重结晶的白云石(SD1-1)与围岩相似;而在远离围岩、靠近直接沉淀的热液白云石一侧(SD1-2),重结晶白云石则与热液白云石相似。由此看来,运用稀土元素特征识别热液过程中的重结晶现象是可行的。另外,结合岩石学特征,可以判断研究的样品中热液导致的白云石重结晶程度总体较低,深部热液流体活动可能仅局限于先存的缝洞体系,对围岩的改造规模有限,更多的是热液作用下的白云石直接沉淀,因此,该方法同样可应用于评价热液对围岩的改造程度。
5. 结论
(1)通过对BT5井上寒武统白云岩进行岩石学观察,识别出三种不同组构的白云石:基质白云石(MD)、环带状鞍形白云石(SD1)和巨晶鞍形白云石(SD2)。
(2)各类白云石样品的岩石学及原位微量元素特征表明,SD2-1具有总稀土元素含量高、轻稀土元素相对重稀土元素富集的特征,为热液流体中直接沉淀的产物;SD1表现出由围岩向热液白云石过渡的特征,是由MD发生热液重结晶形成。
(3)热液重结晶使得不同类型的白云石在岩石学和稀土元素特征上呈现出明显的过渡性,MD、SD1-1、SD1-2到SD2-1的总稀土元素含量逐渐增加且轻稀土元素逐渐富集,这种特征可以作为热液重结晶的一种指示。
致谢:在原位微量元素数据的收集和分析中,碳酸盐岩沉积–成岩地球化学实验室的谭秀成老师和赵东方老师对本研究给予了指导,在此表示衷心感谢。编辑和两位审稿专家在论文评审过程中提出了宝贵的意见和建议,在此致以诚挚的谢意。
1 *数据资料请联系编辑部或登录期刊网站https://www.cjyttsdz.com.cn/获取。 -
图 1 塔里木盆地构造图(据王家豪,2012修编)和BT5井上寒武统综合柱状图
A. 巴楚隆起区域位置图;B. BT5井上寒武统综合柱状图
Figure 1. Location of the Tarim Basin (modified after Wang, 2012) and stratigraphic column of the Upper Cambrian in Well BT5
图 2 BT5井上寒武统白云石照片集
A. 浅灰色白云岩,白云石胶结物中发育孔洞(Є3xq,
4806.99 m);B. 浅灰色—灰色白云岩,孔洞发育,孔洞大小不一(Є3xq,4809.43 m);C. 中—粗晶白云岩,可见鞍形白云石充填物,从内到外依次为基质白云石(MD)、环带状鞍形白云石核部(SD1-1)、环带状鞍形白云石边缘(SD1-2)和巨晶鞍形白云石(SD2)(Є3xq,4806.99 m);D. 中—粗晶白云岩,可见鞍形白云石充填物,从内到外依次为MD、SD1-1、SD1-2和SD2,图C的阴极发光照片(Є3xq,4806.99 m);E. 白云岩孔洞中的巨晶块状鞍形白云石(SD2),可见波状消光(Є3xq,4810.02 m);F. 白云岩孔洞中充填的巨晶鞍形白云石(SD2),可见浑浊的核心(SD2-1)和清晰的边缘(SD2-2)(Є3xq,4810.02 m)Figure 2. Micrographs showing petrography of the Upper Cambrian dolomites in Well BT5
图 7 BT5井白云石样品原位微量元素变化特征
A. 中—粗晶白云岩,基质白云石(MD)的孔洞中充填环带状鞍形白云石(SD1)和巨晶状鞍形白云石(SD2)(Є3xq,4806.99 m);B. 白云石原位微量元素变化柱状图,取点来自图A;C. 中—粗晶白云岩(与A为同一样品),基质白云石(MD)的孔洞中充填环带状鞍形白云石(SD1)和巨晶状鞍形白云石(SD2)(Є3xq,4806.99 m);D. 白云石原位微量元素变化柱状图,取点来自图C
Figure 7. Variations of in-situ trace element concentrations in dolomites of Well BT5
图 8 白云石稀土元素配型对比图
A. BT5井白云石稀土元素配型;B. 塔里木盆地阿克苏地区下寒武统热液白云石稀土元素配型图,包含细—中晶直面白云石充填物(FD1)和粗晶鞍形白云石充填物(FD2),数据来自Liu et al.,2020;C. 塔里木盆地柯坪地区寒武—奥陶系热液白云石稀土元素配型图,包含T2-SD(MREE富集、高REE含量)和T3、T4-SD(LREE富集、高ƩREE、正Eu异常),数据来自Zhang et al.,2014;D. 塔里木盆地库鲁克塔格地区上寒武统热液白云石(KZ-SD)稀土元素配型图,数据来自Lu et al.,2022
Figure 8. Comparison of REY profiles from this study with those of hydrothermal saddle dolomites reported in previous publications
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