非熱液成因的鞍形白云石:來自加拿大薩斯喀徹溫省東南部奧陶系Yeoman組的巖石學和地球化學證據①

卿海若 陳代釗

(1.Department of Geology,University of Regina,Regina,SK,S4S 0A2,Canada; 2.中國科學院地質與地球物理研究所 北京 100029)

非熱液成因的鞍形白云石:來自加拿大薩斯喀徹溫省東南部奧陶系Yeoman組的巖石學和地球化學證據①

卿海若1陳代釗2

(1.Department of Geology,University of Regina,Regina,SK,S4S 0A2,Canada; 2.中國科學院地質與地球物理研究所 北京 100029)

加拿大薩斯喀徹溫省東南部上奧陶統Yeoman組碳酸鹽巖中發育有少量的鞍形白云石膠結物。這些鞍形白云石僅局限于Yeoman組上部厚約20～30 m的白云巖帶中,上覆及下伏碳酸鹽巖地層中均明顯缺失這類鞍形白云石,表明其形成于一個相對封閉的體系中。此類奧陶系鞍形白云石膠結物以具有與宿主交代白云巖相似的碳同位素δ13C值(-0.2‰～0.9‰PDB)及鍶同位素比值(0.708 2～0.709 0)為特征,表明前期的白云石圍巖通過壓溶作用形成的碳和鍶是鞍形白云石膠結物的主要來源。另外,測得的鞍形白云石膠結物均一溫度范圍為99～105℃,可以由該區域的正常埋藏溫度解釋。基于上述資料和觀察,我們認為薩斯喀徹溫省東南部上奧陶統Yeoman組鞍形白云石膠結物與早期交代白云石的自調節白云石化作用(埋藏過程中相對封閉的體系中通過化學壓實作用形成)有關,而與加西盆地其它地方已經證實的熱液活動無關。因此,鞍形白云石的分布未必指示熱液活動或熱流體,也并不是所有的鞍形白云石都與熱流體有關。

鞍形白云石非熱液成因壓溶作用奧陶系Yeoman組威利斯頓盆

0 引言

鞍形白云石是指粗晶、乳白色的白云石,通常具有彎曲的晶面,由于晶格變形而呈波狀消光[1]。一般以膠結物的形式分布在孔隙和裂縫中,局部以交代白云石的方式產出[1～3]。由于鞍形白云石通常與油氣藏的分布或密西西比河谷型鉛—鋅礦床具有空間和成因上的聯系,在過去的二十多年中,許多學者對有關鞍形白云石的成因及其性質進行了廣泛的研究[4～15]。盡管鞍形白云石可以形成于不同的地質環境中[7],但最近有人建議把鞍形白云石的出現作為“指示熱液環境”最重要的巖石學標志[3]。這里所說的熱流體是指溫度高于圍巖溫度的流體介質,而對流體來源并沒有特別限定[16]。

在本研究中,我們將展示加拿大薩斯喀徹溫省東南部(圖1)上奧陶統Yeoman組碳酸鹽巖中非熱液成因的鞍形白云石的證據。該類鞍形白云石與其宿主基質白云石的自調節(蠶食)白云石化(埋藏過程中,在一相對封閉的地熱流體系統中通過壓實作用形成)有關,表明并非所有的鞍形白云石都與熱流體有關。

圖1 加拿大薩斯喀徹溫省東南威利斯頓(W illiston Basin)盆地北部研究區地理位置圖Fig.1Schematic map showing location of the study area in the northernW illiston Basin,Southeastern Saskatchewan,Canada

1 地質背景

威利斯頓盆地(圖1)上奧陶統地層是在晚奧陶世覆蓋大部分北美克拉通盆地的陸表海中沉積的[17]。在薩斯喀徹溫省東南部,上奧陶統地層分為Yeoman組和Herald組[17](圖2),地層總厚度100～165 m左右,現今埋藏深度達1 500～3 000 m[18]。本次研究的鞍形白云石采自Yeoman組上部巖心樣品(圖2)。

2 產狀與分布

上奧陶統Yeomen組鞍形白云石的分布與圍巖碳酸鹽巖的結構和巖性有密切的關系,紅河階Yeomen組碳酸鹽巖最顯著的特征是普遍發育寬約0.5～3 cm(絕大多數為1 cm)的斑狀潛穴系統(圖3),這些潛穴斑塊優先發生白云石化[19,20],顏色比圍巖基質暗(圖3)。在這些斑狀潛穴內部,通常發育有更小的(直徑0.2～0.3 cm)、顏色更深的潛穴(圖3A),

圖2 薩斯喀徹溫省東南部奧陶系紅河群(Red River)地層術語及3-8-1-11W2井巖石學和測井曲線特征。鞍形白云石膠結物僅發育在Yeoman組上部20～30 m宿主灰巖被完全白云化的層段中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

圖3奧陶系Yeoman組碳酸鹽巖巖心和薄片照片

A.具暗色潛穴斑塊的Yeoman組碳酸鹽巖特征,更小的(直徑0.2～0.3 cm)暗色潛穴通常分布在潛穴斑塊之中,3-8-1-11W2井,3 191. 6 m;B.白色鞍形白云石以膠結物的形式充填在潛穴晶洞中,部分晶洞發育示底構造,鞍形白云石作為膠結物沉積在晶洞的上部,3-8-1-11W2井,3 190.7 m;C.鞍形白云石作為膠結物充填在示底構造的方形、矩形晶洞中,3-8-1-11W2井,3 194.4 m;D.鞍形白云石膠結物充填于大的不規則的晶洞中,這些大晶洞可能與潛穴溶蝕擴大有關,3-8-1-11W2井,3 204.3 m;E.潛穴晶洞中充填的鞍形白云石膠結物,呈波狀消光(箭頭指向)的顯微照片,3-8-1-11W2井,3 189.2 m;F.潛穴斑塊晶洞中充填的鞍形白云石(呈波狀消光,實心箭頭指向)及粗晶片狀硬石膏(空心箭頭指向)的顯微照片,3-8-1-11W2井,3 191.1 m;G.Yeoman組上部白云巖化巖石中的縫合線,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潛穴中央局部含有未被充填的孔隙,尤其是采自W innipeg(溫尼佩格市)附近采石場的樣品。

在薩斯喀徹溫省東南部的地下巖心樣品中,白色鞍形白云石以膠結物的形式分布于晶洞中,尤其是這些斑狀潛穴中;鞍形白云石膠結物可能最初是從未被充填的和/或部分淋漓的內部潛穴中沉淀而成(圖3B -D)。部分晶洞含有示底構造,鞍形白云石作為膠結物沉積在與潛穴相關的晶洞上部(圖3B、C),局部地區可見沉積在方形或長方形晶洞(圖3D)中的鞍形白云石膠結物,這可能與食鹽晶體的溶蝕有關[17]。偶爾,鞍形白云石和粗晶硬石膏也存在于巨大的不規則晶洞中(圖3D)。正交偏光鏡下,這些鞍形白云石膠結物具有鞍形白云石的特征[1],呈波狀消光(圖3E、F)。

分散狀鞍形白云石膠結物僅發現于距加拿大—美國邊境約5～12 km,薩斯喀徹溫省東南部的幾口深部鉆井巖心中,鉆井中奧陶系Yeoman組現今的埋藏深度略大于3 000 m。鞍形白云石膠結物一般僅見于Yeoman組上部20～30 m、宿主灰巖已完全白云石化的區域(圖2)[19,21,22],此類白云巖中還發育有大量的縫合線(圖3G)。然而,在Yeoman組下部緊臨白云石帶之下(仍然是灰巖)的碳酸鹽巖中(圖2),并沒有觀察到鞍形白云石膠結物;在Yeoman組上覆的淺水白云巖地層中(包括志留系、泥盆系和密西西比統白云巖地層)也同樣缺乏鞍形白云石膠結物,該套白云巖與Yeoman組白云巖之間被一區域性硬石膏層分隔,Herald組中的Lake Al ma和Coronach段直接覆蓋在Yeoman組之上(圖2)。

3 地球化學性質

同位素測試分析是在University of Saskatchewan (薩斯喀徹溫大學)進行。對于C、O同位素分析,大約100μg粉樣置于與Finnigan MAT—253質譜計相連的Finnigan Kiel—Ⅲ樣品處理器中,加熱至70℃與無水磷酸溶液中反應200 s后,形成的CO2直接導入質譜計,測量C、O同位素比值,以VPDB為標準標記同位素比值變化,測量精度通過NBS-18和NBS-19進行監測,δ13C和δ18O值的精度(1σ)都好于± 0.1‰。對于Sr同位素比值(87Sr/86Sr)測量,把50～100 mg樣品與2.5N HCl溶解,通過常規的離子交換程序分離出Sr,在Finnigan MAT—261質譜計上進行測量,測量值通過同測的標準NBS987值進行校正,分析平均誤差(2σ)為±10×10-6。圖4A.鞍形白云石膠結物(空心三角形)、上奧陶統Yeoman組基質白云石(實心菱形)及白云石化斑狀潛穴(空心菱形)的碳、氧同位素交匯圖;B.鞍形白云石膠結物(空心三角形)、上奧陶統Yeoman組基質白云石(實心菱形)及白云石化斑狀潛穴(空心菱形)的碳、鍶同位素交匯圖;C.3個鞍形白云石樣品中的氣液兩相流體包裹體均一溫度直方圖

(白色樣品:99℃;淺灰色: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

鞍形白云石膠結物的氧同位素δ18O值介于-10.2‰～-11.1‰之間,平均-10.7‰(圖4A),比白云石圍巖的氧同位素δ18O值低2‰～4‰(圖4A);碳同位素δ13C值則介于-0.2‰～+0.9‰之間,平均為0.3‰,與宿主白云石的碳同位素δ13C值部分重疊,落在奧陶紀正常海水的碳同位素δ13C值(-0.5‰～+1.5‰)范圍之內[23,24];鍶同位素87Sr/86Sr比值介于0.708 2～0.709 0之間,與宿主基質白云石近似(圖4B),其中一個樣品的鍶同位素87Sr/86Sr比值明顯偏高,達到0.709 7。

本文在12個樣品中選取3個樣品的氣液兩相包裹體進行測試分析(圖4C),每個樣品的均一溫度變化范圍均較窄,介于99oC～105oC之間;由于包裹體較小(絕大部分3～5μm),冰點溫度無法精確測得。

4 討論和解釋

鞍形白云石膠結物僅局限分布在加拿大與美國邊境的深部鉆井Yeomen組上部相對較薄的白云巖化圍巖中。被區域性硬石膏層分隔的上覆白云巖地層(Lake Al ma和Coronach段;圖2)明顯缺失鞍形白云石,下伏的Yeoman組下部灰巖同樣缺失鞍形白云石;這些表明這些鞍形白云石膠結物形成于白云化地層中相對封閉、沒有向外輸送通道(如:裂縫/斷層)的流體系統。大量發育的縫合線(圖3E、F)表明白云巖宿主地層發生了強烈的壓溶作用,為鞍形白云石膠結物的沉淀提供了鎂離子來源。

鞍形白云石膠結物的同位素地球化學證據進一步證實了上述的認識。碳同位素δ13C值與宿主白云巖的完全重疊(圖4A),表明鞍形白云石中的碳主要來自封閉系統中的前期宿主白云巖地層。另外,鞍形白云石的鍶同位素87Sr/86Sr比值與宿主白云巖也具有相似性(圖4B),表明鞍形白云石中鍶也主要來源于前期白云石。

與宿主白云巖相比,鞍形白云石膠結物偏低的氧同位素δ18O值可能與鞍形白云石沉積形成時溫度較高有關,正如所測得的鞍形白云石的均一溫度值(平均99～105℃)。來自磷灰石裂變徑跡研究熱埋藏史[25]表明,雖然薩斯喀徹溫省威利斯頓盆地前寒武紀基底的溫度在晚石炭紀時可以達到124oC,但對上奧陶統Yeoman組地層的影響很小,該時期Yeoman組烴源巖尚未成熟。因此,所測均一溫度值很可能是研究區正常埋藏的溫度。

根據Yeomen組上部鞍形白云石膠結物的產狀、空間分布和地球化學性質,認為Yeomen組上部鞍形白云石膠結物是非熱液成因的產物;而是埋藏過程中,宿主白云巖在相對封閉的體系中通過壓溶作用形成的。這些特征與加西盆地其它地區報道的熱液成因的鞍形白云石有本質區別[3,5～7,11,26～28],形成大規模的熱液成因的鞍形白云巖體通常需要與區域性的疏導體相聯通的開放體系,流體在通過該疏導體系時有大量鎂離子的加入并被加熱,使圍巖發生白云巖化。因此,熱液白云巖體經常沿疏導體系切割若干地層單元[3,5,6,10,27,28]。較之宿主碳酸鹽巖,熱液白云巖的碳同位素δ13C值一般明顯虧損,表明在烴類運移過程中或熱化學硫酸鹽還原作用過程中,在相對開放體系中的白云化流體中有有機碳的加入[7,29]。此外,熱液成因的鞍形白云巖通常以具有比白云巖宿主地層高的鍶同位素比值[3,5,6,10,27,28],表明流體在沿刺穿硅質碎屑含水層的斷裂疏導體系中遷移時,會逐步吸收放射性鍶同位素進入到白云化流體。

非熱液成因與熱液成因鞍形白云石在空間分布和地球化學性質方面的區別可能與構造背景有關。在本實例中,與壓溶作用有關的非熱液成因鞍形白云石分布在穩定的克拉通內部,而加西盆地其它地區大規模的熱液白云巖體則分布在構造活躍的盆地邊緣,在這些地區斷裂疏導體系廣泛發育[3,10,11,27,30]。

5 結論

鞍形白云石的存在不能作為指示熱液流體的巖石學特征,因為并非所有的鞍形白云石都與熱液流體有關,正如我們所研究的加拿大薩斯喀徹溫省東南部奧陶系Yeoman組的鞍形白云石。

非熱液成因的鞍形白云石能夠在埋藏過程中通過埋藏壓實作用和/或宿主白云巖的自調節(蠶食)白云化作用形成,尤其是在構造穩定的克拉通內部。識別非熱液成因的鞍形白云石可根據其局限分布及碳、鍶同位素與宿主白云巖具有相似性的特性加以區別。

非熱液成因的鞍形白云石在地質記錄中可能很常見,尤其是遭受了深埋藏的宿主白云巖地層。然而,非熱液成因的鞍形白云石可能被大規模發育的熱液鞍形白云石所掩蓋,在這種情況下,熱液白云石有可能被過分的強調而非熱液成因的鞍形白云石則被低估了。

致謝 我(HQ)認識劉寶珺院士已逾32載,還在成都地質學院上學期間曾多次有幸聆聽他的講座。劉院士是一位諄諄教誨、啟人心蒂的良師,他一生致力于沉積學的研究,也是我的至交。當我在加拿大學習、爾后在英國倫敦大學和加拿大里賈納大學從事教學、研究工作時,這種友誼也日久彌深。劉院士對我在加拿大的生活和學術生涯的建立曾提供諸多幫助。在此,謹以此文獻給、并祝賀劉院士八十華誕,祝愿他健康、長壽!

感謝薩斯喀徹溫省工業與資源廳地下巖心實驗室對巖心觀察提供的幫助,也感謝薩斯喀徹溫大學提供了同位素測試分析,感謝Kreis K,Haidl F和Kent D對本文的評論。董少峰幫助把原文譯成中文。

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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 沉積學 地球化學E-mail:hairuo.qing@uregina.ca

P588.24+5

A

1000-0550(2010)05-0980-07

①NSERC基金(GrantNo.155012)和油氣藏地質及開發工程國家重點實驗室開放基金(編號:PLC200801)資助。

2010-05-20;收修改稿日期:2010-06-05