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①

卿海若 陈代钊

(1.Department of Geology,University of Regina,Regina,SK,S4S 0A2,Canada; 2.中国科学院地质与地球物理研究所 北京 100029)

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①

卿海若1陈代钊2

(1.Department of Geology,University of Regina,Regina,SK,S4S 0A2,Canada; 2.中国科学院地质与地球物理研究所 北京 100029)

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δ13C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。

鞍形白云石非热液成因压溶作用奥陶系Yeoman组威利斯顿盆

0 引言

鞍形白云石是指粗晶、乳白色的白云石,通常具有弯曲的晶面,由于晶格变形而呈波状消光[1]。一般以胶结物的形式分布在孔隙和裂缝中,局部以交代白云石的方式产出[1～3]。由于鞍形白云石通常与油气藏的分布或密西西比河谷型铅—锌矿床具有空间和成因上的联系,在过去的二十多年中,许多学者对有关鞍形白云石的成因及其性质进行了广泛的研究[4～15]。尽管鞍形白云石可以形成于不同的地质环境中[7],但最近有人建议把鞍形白云石的出现作为“指示热液环境”最重要的岩石学标志[3]。这里所说的热流体是指温度高于围岩温度的流体介质,而对流体来源并没有特别限定[16]。

在本研究中,我们将展示加拿大萨斯喀彻温省东南部(图1)上奥陶统Yeoman组碳酸盐岩中非热液成因的鞍形白云石的证据。该类鞍形白云石与其宿主基质白云石的自调节(蚕食)白云石化(埋藏过程中,在一相对封闭的地热流体系统中通过压实作用形成)有关,表明并非所有的鞍形白云石都与热流体有关。

图1 加拿大萨斯喀彻温省东南威利斯顿(W illiston Basin)盆地北部研究区地理位置图Fig.1Schematic map showing location of the study area in the northernW illiston Basin,Southeastern Saskatchewan,Canada

1 地质背景

威利斯顿盆地(图1)上奥陶统地层是在晚奥陶世覆盖大部分北美克拉通盆地的陆表海中沉积的[17]。在萨斯喀彻温省东南部,上奥陶统地层分为Yeoman组和Herald组[17](图2),地层总厚度100～165 m左右,现今埋藏深度达1 500～3 000 m[18]。本次研究的鞍形白云石采自Yeoman组上部岩心样品(图2)。

2 产状与分布

上奥陶统Yeomen组鞍形白云石的分布与围岩碳酸盐岩的结构和岩性有密切的关系,红河阶Yeomen组碳酸盐岩最显著的特征是普遍发育宽约0.5～3 cm(绝大多数为1 cm)的斑状潜穴系统(图3),这些潜穴斑块优先发生白云石化[19,20],颜色比围岩基质暗(图3)。在这些斑状潜穴内部,通常发育有更小的(直径0.2～0.3 cm)、颜色更深的潜穴(图3A),

图2 萨斯喀彻温省东南部奥陶系红河群(Red River)地层术语及3-8-1-11W2井岩石学和测井曲线特征。鞍形白云石胶结物仅发育在Yeoman组上部20～30 m宿主灰岩被完全白云化的层段中Fig.2 Generalized stratigraphic nomenclatures ofOrdovician Red River strata in southeast Saskatchewan with lithology and log signatures derived from well 3-8-1-11W2.Note saddle dolomite cements are generally restricted to the upper 20～30 m of the Yeoman Formation where the host limestoneswere completely dolomitized

图3奥陶系Yeoman组碳酸盐岩岩心和薄片照片

A.具暗色潜穴斑块的Yeoman组碳酸盐岩特征,更小的(直径0.2～0.3 cm)暗色潜穴通常分布在潜穴斑块之中,3-8-1-11W2井,3 191. 6 m;B.白色鞍形白云石以胶结物的形式充填在潜穴晶洞中,部分晶洞发育示底构造,鞍形白云石作为胶结物沉积在晶洞的上部,3-8-1-11W2井,3 190.7 m;C.鞍形白云石作为胶结物充填在示底构造的方形、矩形晶洞中,3-8-1-11W2井,3 194.4 m;D.鞍形白云石胶结物充填于大的不规则的晶洞中,这些大晶洞可能与潜穴溶蚀扩大有关,3-8-1-11W2井,3 204.3 m;E.潜穴晶洞中充填的鞍形白云石胶结物,呈波状消光(箭头指向)的显微照片,3-8-1-11W2井,3 189.2 m;F.潜穴斑块晶洞中充填的鞍形白云石(呈波状消光,实心箭头指向)及粗晶片状硬石膏(空心箭头指向)的显微照片,3-8-1-11W2井,3 191.1 m;G.Yeoman组上部白云岩化岩石中的缝合线,11-27-1-17W2井,3 062.92 m

Fig.3 Core and thin section photographs ofOrdovician Yeoman carbonate rocks.

A.The characteristic feature of Yeoman carbonate showing burrow mottlingswith a darker tone compared with the host matrix.A much s maller (0.2 to 0.3 cm in diameter)and darker colored burrows commonly occurred inside the burrow mottlings.3-8-1-11W2,3 191.6 m;B.White saddle dolomite occurs as cement in the vugs that are preferentially distributed in burrow textures.Some of these vugs have geopetal structure with saddle dolomite precipitated as cement in the upper part of the vugs.3-8-1-11W2,3 190.7 m;C.Saddle dolomite precipitated as cement in the geopetal structure in the square-,or rectangular-shaped vugs.3-8-1-11W2,3 194.4 m;D.Saddle dolomite cement in a large irregular shaped vug,probably related to a solution enlarged pre-existing burrow texture.3-8-1-11W2,3 204.3 m;E.Thin section photomicrograph showing saddle dolomite with its characteristic undulose extinction(highlightedwith arrows)precipitated as cementpreferentially in a vugwithin the burrow texture.3-8-1-11W2,3 189.2 m;F.Thin section photomicrograph showing saddle dolomitewith its characteristic undulose extinction (highlighted with solid arrows)and coarsely crystalline bladed anhydrite(indicated by the open arrow)precipitated as cements preferentially distributed in a vugwithin the burrow texture.3-8-1-11W2,3 191.1 m;G.A stylolite in a dolomitized zone in the upper part of the Yeoman For mation.11-27-1-17W2,3 062.92 m潜穴中央局部含有未被充填的孔隙,尤其是采自W innipeg(温尼佩格市)附近采石场的样品。

在萨斯喀彻温省东南部的地下岩心样品中,白色鞍形白云石以胶结物的形式分布于晶洞中,尤其是这些斑状潜穴中;鞍形白云石胶结物可能最初是从未被充填的和/或部分淋漓的内部潜穴中沉淀而成(图3B -D)。部分晶洞含有示底构造,鞍形白云石作为胶结物沉积在与潜穴相关的晶洞上部(图3B、C),局部地区可见沉积在方形或长方形晶洞(图3D)中的鞍形白云石胶结物,这可能与食盐晶体的溶蚀有关[17]。偶尔,鞍形白云石和粗晶硬石膏也存在于巨大的不规则晶洞中(图3D)。正交偏光镜下,这些鞍形白云石胶结物具有鞍形白云石的特征[1],呈波状消光(图3E、F)。

分散状鞍形白云石胶结物仅发现于距加拿大—美国边境约5～12 km,萨斯喀彻温省东南部的几口深部钻井岩心中,钻井中奥陶系Yeoman组现今的埋藏深度略大于3 000 m。鞍形白云石胶结物一般仅见于Yeoman组上部20～30 m、宿主灰岩已完全白云石化的区域(图2)[19,21,22],此类白云岩中还发育有大量的缝合线(图3G)。然而,在Yeoman组下部紧临白云石带之下(仍然是灰岩)的碳酸盐岩中(图2),并没有观察到鞍形白云石胶结物;在Yeoman组上覆的浅水白云岩地层中(包括志留系、泥盆系和密西西比统白云岩地层)也同样缺乏鞍形白云石胶结物,该套白云岩与Yeoman组白云岩之间被一区域性硬石膏层分隔,Herald组中的Lake Al ma和Coronach段直接覆盖在Yeoman组之上(图2)。

3 地球化学性质

同位素测试分析是在University of Saskatchewan (萨斯喀彻温大学)进行。对于C、O同位素分析,大约100μg粉样置于与Finnigan MAT—253质谱计相连的Finnigan Kiel—Ⅲ样品处理器中,加热至70℃与无水磷酸溶液中反应200 s后,形成的CO2直接导入质谱计,测量C、O同位素比值,以VPDB为标准标记同位素比值变化,测量精度通过NBS-18和NBS-19进行监测,δ13C和δ18O值的精度(1σ)都好于± 0.1‰。对于Sr同位素比值(87Sr/86Sr)测量,把50～100 mg样品与2.5N HCl溶解,通过常规的离子交换程序分离出Sr,在Finnigan MAT—261质谱计上进行测量,测量值通过同测的标准NBS987值进行校正,分析平均误差(2σ)为±10×10-6。图4A.鞍形白云石胶结物(空心三角形)、上奥陶统Yeoman组基质白云石(实心菱形)及白云石化斑状潜穴(空心菱形)的碳、氧同位素交汇图;B.鞍形白云石胶结物(空心三角形)、上奥陶统Yeoman组基质白云石(实心菱形)及白云石化斑状潜穴(空心菱形)的碳、锶同位素交汇图;C.3个鞍形白云石样品中的气液两相流体包裹体均一温度直方图

(白色样品:99℃;浅灰色:105℃;暗色:99℃)

Fig.4 A.Cross plot ofδ18O andδ13C values for saddle dolomite cement(open triangles)and their host Upper Ordovician Yeoman matrix dolomite(solid diamonds)and dolomitized burrow mottlings(open diamonds);B.Cross plot ofδ18O and87Sr/86Sr for saddle dolomite cement(open triangles)and their host Upper Ordovician Yeoman matrix dolomite(solid diamonds)and dolomitized burrow mottlings(open diamonds);C. A histogram of homogenization temperature(Th)of two-phase aqueous inclusions of 3 saddle dolomite samples,with an average temperature of 99℃(white sample),105℃(light gray), and 99℃(darker gray)respectively

鞍形白云石胶结物的氧同位素δ18O值介于-10.2‰～-11.1‰之间,平均-10.7‰(图4A),比白云石围岩的氧同位素δ18O值低2‰～4‰(图4A);碳同位素δ13C值则介于-0.2‰～+0.9‰之间,平均为0.3‰,与宿主白云石的碳同位素δ13C值部分重叠,落在奥陶纪正常海水的碳同位素δ13C值(-0.5‰～+1.5‰)范围之内[23,24];锶同位素87Sr/86Sr比值介于0.708 2～0.709 0之间,与宿主基质白云石近似(图4B),其中一个样品的锶同位素87Sr/86Sr比值明显偏高,达到0.709 7。

本文在12个样品中选取3个样品的气液两相包裹体进行测试分析(图4C),每个样品的均一温度变化范围均较窄,介于99oC～105oC之间;由于包裹体较小(绝大部分3～5μm),冰点温度无法精确测得。

4 讨论和解释

鞍形白云石胶结物仅局限分布在加拿大与美国边境的深部钻井Yeomen组上部相对较薄的白云岩化围岩中。被区域性硬石膏层分隔的上覆白云岩地层(Lake Al ma和Coronach段;图2)明显缺失鞍形白云石,下伏的Yeoman组下部灰岩同样缺失鞍形白云石;这些表明这些鞍形白云石胶结物形成于白云化地层中相对封闭、没有向外输送通道(如:裂缝/断层)的流体系统。大量发育的缝合线(图3E、F)表明白云岩宿主地层发生了强烈的压溶作用,为鞍形白云石胶结物的沉淀提供了镁离子来源。

鞍形白云石胶结物的同位素地球化学证据进一步证实了上述的认识。碳同位素δ13C值与宿主白云岩的完全重叠(图4A),表明鞍形白云石中的碳主要来自封闭系统中的前期宿主白云岩地层。另外,鞍形白云石的锶同位素87Sr/86Sr比值与宿主白云岩也具有相似性(图4B),表明鞍形白云石中锶也主要来源于前期白云石。

与宿主白云岩相比,鞍形白云石胶结物偏低的氧同位素δ18O值可能与鞍形白云石沉积形成时温度较高有关,正如所测得的鞍形白云石的均一温度值(平均99～105℃)。来自磷灰石裂变径迹研究热埋藏史[25]表明,虽然萨斯喀彻温省威利斯顿盆地前寒武纪基底的温度在晚石炭纪时可以达到124oC,但对上奥陶统Yeoman组地层的影响很小,该时期Yeoman组烃源岩尚未成熟。因此,所测均一温度值很可能是研究区正常埋藏的温度。

根据Yeomen组上部鞍形白云石胶结物的产状、空间分布和地球化学性质,认为Yeomen组上部鞍形白云石胶结物是非热液成因的产物;而是埋藏过程中,宿主白云岩在相对封闭的体系中通过压溶作用形成的。这些特征与加西盆地其它地区报道的热液成因的鞍形白云石有本质区别[3,5～7,11,26～28],形成大规模的热液成因的鞍形白云岩体通常需要与区域性的疏导体相联通的开放体系,流体在通过该疏导体系时有大量镁离子的加入并被加热,使围岩发生白云岩化。因此,热液白云岩体经常沿疏导体系切割若干地层单元[3,5,6,10,27,28]。较之宿主碳酸盐岩,热液白云岩的碳同位素δ13C值一般明显亏损,表明在烃类运移过程中或热化学硫酸盐还原作用过程中,在相对开放体系中的白云化流体中有有机碳的加入[7,29]。此外,热液成因的鞍形白云岩通常以具有比白云岩宿主地层高的锶同位素比值[3,5,6,10,27,28],表明流体在沿刺穿硅质碎屑含水层的断裂疏导体系中迁移时,会逐步吸收放射性锶同位素进入到白云化流体。

非热液成因与热液成因鞍形白云石在空间分布和地球化学性质方面的区别可能与构造背景有关。在本实例中,与压溶作用有关的非热液成因鞍形白云石分布在稳定的克拉通内部,而加西盆地其它地区大规模的热液白云岩体则分布在构造活跃的盆地边缘,在这些地区断裂疏导体系广泛发育[3,10,11,27,30]。

5 结论

鞍形白云石的存在不能作为指示热液流体的岩石学特征,因为并非所有的鞍形白云石都与热液流体有关,正如我们所研究的加拿大萨斯喀彻温省东南部奥陶系Yeoman组的鞍形白云石。

非热液成因的鞍形白云石能够在埋藏过程中通过埋藏压实作用和/或宿主白云岩的自调节(蚕食)白云化作用形成,尤其是在构造稳定的克拉通内部。识别非热液成因的鞍形白云石可根据其局限分布及碳、锶同位素与宿主白云岩具有相似性的特性加以区别。

非热液成因的鞍形白云石在地质记录中可能很常见,尤其是遭受了深埋藏的宿主白云岩地层。然而,非热液成因的鞍形白云石可能被大规模发育的热液鞍形白云石所掩盖,在这种情况下,热液白云石有可能被过分的强调而非热液成因的鞍形白云石则被低估了。

致谢 我(HQ)认识刘宝珺院士已逾32载,还在成都地质学院上学期间曾多次有幸聆听他的讲座。刘院士是一位谆谆教诲、启人心蒂的良师,他一生致力于沉积学的研究,也是我的至交。当我在加拿大学习、尔后在英国伦敦大学和加拿大里贾纳大学从事教学、研究工作时,这种友谊也日久弥深。刘院士对我在加拿大的生活和学术生涯的建立曾提供诸多帮助。在此,谨以此文献给、并祝贺刘院士八十华诞,祝愿他健康、长寿!

感谢萨斯喀彻温省工业与资源厅地下岩心实验室对岩心观察提供的帮助,也感谢萨斯喀彻温大学提供了同位素测试分析,感谢Kreis K,Haidl F和Kent D对本文的评论。董少峰帮助把原文译成中文。

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Non-hydrothermal Saddle Dolom ite:Petrological and Geochemical Evidence from the Ordovician Yeoman For mation, Southeastern Saskatchewan,Canada

Q ING Hai-ruo1CHEN Dai-zhao2
(1.Depar tment of Geology,Un iversity of Regina,Regina,SK,S4S 0A2,Canada; 2.I nstitute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029)

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 For mation, 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 byδ13C 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 cementswere 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 UpperOrdovician Yeoman For mation 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 hydrother mal fluids.

saddle dolomite;non-hydrothermal;pressure solution;Ordovician Yeoman Formation;W illiston Basin; Canada

book=5,ebook=297

卿海若男1955年出生博士教授 沉积学 地球化学E-mail:hairuo.qing@uregina.ca

P588.24+5

A

1000-0550(2010)05-0980-07

①NSERC基金(GrantNo.155012)和油气藏地质及开发工程国家重点实验室开放基金(编号:PLC200801)资助。

2010-05-20;收修改稿日期:2010-06-05