黃華谷 邸鵬飛 陳多福

(1.中國(guó)科學(xué)院廣州地球化學(xué)研究所邊緣海地質(zhì)重點(diǎn)實(shí)驗(yàn)室 廣州 510640;2.中國(guó)科學(xué)院研究生院 北京 100049)

巴顏喀拉盆地長(zhǎng)石頭山二疊紀(jì)-三疊紀(jì)碳酸鹽巖丘的沉積巖石學(xué)和地球化學(xué)特征①

黃華谷1,2邸鵬飛1,2陳多福1

(1.中國(guó)科學(xué)院廣州地球化學(xué)研究所邊緣海地質(zhì)重點(diǎn)實(shí)驗(yàn)室 廣州 510640;2.中國(guó)科學(xué)院研究生院 北京 100049)

青海巴顏喀拉盆地長(zhǎng)石頭山二疊紀(jì)-三疊紀(jì)碳酸鹽巖丘是由罕見(jiàn)的塊狀純灰?guī)r組成,灰?guī)r具團(tuán)塊結(jié)構(gòu),由泥微晶方解石團(tuán)塊和櫛狀生長(zhǎng)的泥微晶方解石膠結(jié)物組成,部分泥微晶方解石發(fā)生了重結(jié)晶作用。灰?guī)r的礦物組成主要為方解石,平均含量達(dá)97%,化學(xué)成分除CaO外,其余均低于5‰?；?guī)r的5%HNO3可溶相(碳酸鹽礦物)稀土元素頁(yè)巖配分模式具有Ce負(fù)異常,與二疊紀(jì)海相灰?guī)r、二疊紀(jì)海水和現(xiàn)代東太平洋海水基本一致。微量元素分析顯示V/Cr比值極低,自生沉積的Uau僅為0.08～0.5μg/g,在還原環(huán)境中富集的元素U、V、Mo、Cr、Co、Cu和Zn含量偏低或缺失,表明碳酸鹽巖可能是在開(kāi)闊海氧化環(huán)境中的快速沉積產(chǎn)物。

碳酸鹽巖丘 純灰?guī)r 沉積巖石學(xué) 地球化學(xué) 巴顏喀拉盆地

0 引言

巴顏喀拉沉積盆地位于青藏高原腹地的東北部,屬于古特提斯構(gòu)造帶東段[1,2]。盆地面積逾70 X104km2,沉積主體由三疊紀(jì)濁積巖組成,其中發(fā)育有一些大小不一的碳酸鹽臺(tái)地[3,4]。

巴顏喀拉盆地長(zhǎng)期以來(lái)是青藏高原大地構(gòu)造和特提斯地質(zhì)研究的熱點(diǎn)和關(guān)鍵地區(qū)之一,但由于地質(zhì)結(jié)構(gòu)十分復(fù)雜和研究程度低,對(duì)于巴顏喀拉沉積盆地構(gòu)造屬性的認(rèn)識(shí),目前還沒(méi)有統(tǒng)一的定論。任紀(jì)舜等[5]認(rèn)為它是在勞亞大陸南緣形成的三疊紀(jì)巨型濁積巖沉積盆地。張國(guó)偉等[6,7]認(rèn)為巴顏喀拉盆地是一個(gè)一直延續(xù)至中晚三疊紀(jì)的小洋盆。殷鴻福等[2,8]認(rèn)為它是相當(dāng)于現(xiàn)代印度-歐亞板塊碰撞后形成的恒河孟加拉濁積扇,是昆侖造山后剝蝕搬運(yùn)的“垃圾堆”。許志琴等[9]認(rèn)為是主動(dòng)大陸邊緣的增生復(fù)理石楔。潘桂棠等[4]認(rèn)為它屬于是前陸盆地。張以茀等[10,11]將其劃入華力西造山后的三疊紀(jì)裂陷盆地或活動(dòng)型內(nèi)陸海盆地。王永標(biāo)等[12]認(rèn)為早二疊世末在阿尼瑪卿洋走向消亡和閉合的過(guò)程中,其南部的巴顏喀拉地塊被引張、擴(kuò)展,而最終形成巴顏喀拉海。張雪亭和朱迎堂等[3,13]認(rèn)為是殘留洋盆。

此外,楊欣德等[14]研究了巴顏喀拉山三疊系復(fù)理石沉積粒度概率累積曲線的特征,從早到晚平均粒徑變粗、分選變好,認(rèn)為巴顏喀拉山群為碎屑流復(fù)理石沉積。杜德勛等[15]研究了巴顏喀拉三疊紀(jì)沉積盆地砂巖的地球化學(xué)特征與物源區(qū)的構(gòu)造背景,認(rèn)為巴顏喀拉三疊紀(jì)沉積盆地物源區(qū)的大地構(gòu)造背景在不同時(shí)期表現(xiàn)為被動(dòng)大陸邊緣,大陸島弧,活動(dòng)大陸邊緣和大洋島弧等。王永標(biāo)等[16,17]研究了鄰區(qū)的東昆侖早二疊世灰白色灰?guī)r塊體或灰?guī)r山的沉積模式,認(rèn)為它們是礁島海沉積。但目前還沒(méi)有對(duì)巴顏喀拉盆地中的臺(tái)地相碳酸鹽巖的沉積巖石學(xué)和地球化學(xué)特征開(kāi)展研究。本文將對(duì)巴顏喀拉盆地長(zhǎng)石頭山二疊紀(jì)-三疊紀(jì)碳酸鹽巖丘的沉積巖石學(xué)和地球化學(xué)特征進(jìn)行研究,結(jié)果表明它們可能是開(kāi)闊海氧化環(huán)境中快速沉積的碳酸鹽巖。

1 區(qū)域地質(zhì)與采樣位置

研究區(qū)位于青海省果洛州花石峽鎮(zhèn)長(zhǎng)石頭山的山口附近,在214國(guó)道北邊,約距青海省果洛州瑪多縣60 km和花石峽鎮(zhèn)20 km,距西寧近440 km(圖1)。碳酸鹽巖丘產(chǎn)于二疊系-三疊系的馬爾爭(zhēng)組(PT m)地層中。馬爾爭(zhēng)組是構(gòu)造混雜巖,主要由碳酸鹽巖巖片、復(fù)理石巖片、硅質(zhì)巖和泥巖巖片、中基性火山巖和玄武巖巖片組成,與上覆地層三疊系巴顏喀拉山群(T B)整合接觸。巴顏喀拉山群由灰綠色、紫灰色中厚層狀具鮑馬序列雜砂巖、灰黑色夾紫紅色薄-中層泥質(zhì)粉砂質(zhì)板巖、含礫屑灰?guī)r、生物碎屑灰?guī)r和玄武巖巖塊(片)組成[18](圖1)。

樣品采集于4個(gè)孤立的碳酸鹽巖丘(圖1),碳酸鹽巖呈丘狀發(fā)育于馬爾爭(zhēng)組產(chǎn)菊石的深灰色薄層狀泥晶灰?guī)r夾灰黑色板巖的地層中。由于第四系洪沖積物覆蓋嚴(yán)重,無(wú)法觀察到碳酸鹽巖丘與圍巖的接觸關(guān)系(圖2)。4個(gè)碳酸鹽巖丘露頭出露的寬度只有十幾米到幾十米,長(zhǎng)度介于三十多米到一百多米之間。

2 實(shí)驗(yàn)方法

圖1 區(qū)域地質(zhì)圖簡(jiǎn)圖及采樣位置[18]1.第四系;2.三疊系巴顏喀拉山群(T B);3.二疊系-三疊系馬爾爭(zhēng)組(PT m);4.二疊系-三疊系丘狀灰?guī)r;5.采樣位置及碳酸鹽巖丘編號(hào);6.同位素年齡;7.斷層.Fig.1 Simplified regional geologicalmap and sampling locations[18]

圖2 碳酸鹽巖丘的野外照片A.1號(hào)碳酸鹽巖丘;B.2號(hào)碳酸鹽巖丘;C.3號(hào)碳酸鹽巖丘;D.4號(hào)碳酸鹽巖丘Fig.2 The photos of carbonatemounds

野外采集的丘狀灰?guī)r樣品在室內(nèi)用蒸餾水沖洗,室溫環(huán)境干燥,將部分樣品制成光薄片,薄片在LEICA-DMR光學(xué)顯微鏡下觀察,圖像用高清晰數(shù)碼相機(jī)LEICA DC500拍攝。部分樣品破碎到1 cm大小的碎塊,選取新鮮部分研磨至200目以下粉末,用于XRD和地球化學(xué)分析。

全巖粉末樣品的物相分析(XRD)由中國(guó)科學(xué)院廣州地球化學(xué)研究所的Rigaku DXR 3000完成,工作參數(shù)為Cu靶Kα射線,石墨單色器,測(cè)試電壓為40 kV,電流為40 mA,掃描角度為5°～70°(2θ),步進(jìn)掃描,步寬為0.02°,發(fā)散狹縫為0.5°,接受狹縫為0.15 mm,防散射狹縫為0.5°,礦物含量是依面積法進(jìn)行半定量分析,結(jié)果由SIROQUANT程序分析完成[19]。

碳和氧同位素分析和全巖的常量元素分析在中國(guó)科學(xué)院廣州地球化學(xué)研究所的同位素年代學(xué)與地球化學(xué)重點(diǎn)實(shí)驗(yàn)室完成,碳和氧同位素用 GV IsoPrime II型穩(wěn)定同位素質(zhì)譜(IRMS)分析,結(jié)果采用δ(‰)表示,相對(duì)標(biāo)準(zhǔn)為Vienna PeeDee Belemnite (VPDB),精度分別優(yōu)于0.05‰和0.08‰,標(biāo)準(zhǔn)偏差均優(yōu)于0.1%(2σ)[20]。常量元素用Varian Vista-PRO ICP-AES分析,使用外標(biāo)法測(cè)定樣品的常量元素含量,HF-HNO3法進(jìn)行樣品進(jìn)行溶樣,樣品溶液加入中Lu-HNO3溶液作為內(nèi)標(biāo)用以矯正系統(tǒng)誤差,標(biāo)準(zhǔn)偏差(RSD)小于1%[21]。

丘狀灰?guī)r的碳酸鹽礦物相的微量元素和稀土元素分析在中國(guó)科學(xué)院地球化學(xué)研究所進(jìn)行,分析流程為稱(chēng)取0.05 g樣品,放入50 m l離心管中,加入5%的HNO3至22.5 ml和Rh內(nèi)標(biāo)500 ng,擰緊蓋子,放平“溶樣管”搖勻,半個(gè)小時(shí)搖一次,共6次。6 h后,加去離子水至50 ml,然后沉淀、離心。最后在Finnigan MAT公司ELEMENT型高分辨率ICP-MS上測(cè)定5%的HNO3可溶碳酸鹽礦物相的稀土元素和微量元素含量。分析平均標(biāo)準(zhǔn)偏差小于10%,平均相對(duì)標(biāo)準(zhǔn)偏差優(yōu)于5%[22]。

3 結(jié)果與討論

3.1 巖石學(xué)

巴顏喀拉長(zhǎng)石頭山二疊紀(jì)-三疊紀(jì)的丘狀灰?guī)r是由淺灰色-深灰色無(wú)層理的塊狀灰?guī)r組成,塊狀灰?guī)r部分表面呈致密狀,在1和2號(hào)巖丘可見(jiàn)微體古生物;部分節(jié)理發(fā)育二組和三組節(jié)理,使得灰?guī)r顯得極為破碎(圖3A)。此外,在2號(hào)丘底部發(fā)育角礫灰?guī)r,角礫呈球狀或橢球狀,粒徑相差較大,2～10 cm(圖3B)。說(shuō)明碳酸鹽巖丘形成于一個(gè)相對(duì)氧化環(huán)境,并受到明顯的后期改造。

灰?guī)r多由灰白色的泥微晶方解石團(tuán)塊和深灰色櫛狀方解石微細(xì)晶膠結(jié)物組成(圖3C),主要有晶粒結(jié)構(gòu)、團(tuán)塊結(jié)構(gòu)、櫛殼等厚邊結(jié)構(gòu)和纖狀結(jié)構(gòu)。晶粒結(jié)構(gòu)可分為泥微晶結(jié)構(gòu)、細(xì)晶結(jié)構(gòu)、中晶結(jié)構(gòu)和粗晶結(jié)構(gòu),泥微晶方解石部分重結(jié)晶形成細(xì)晶方解石、中晶方解石和粗晶方解石;泥微晶方解石標(biāo)本上多呈深灰色和灰黑色,顯微鏡下為濁晶,細(xì)晶方解石、中晶方解石和粗晶方解石多為亮晶方解石。團(tuán)塊主要由微米級(jí)大小的泥微晶方解石組成(圖3D),具不規(guī)則的形狀,大小介于1 cm到幾厘米之間。具櫛殼等厚邊結(jié)構(gòu)的膠結(jié)物沿泥微晶團(tuán)塊呈梳狀生長(zhǎng),顯微鏡下呈纖狀結(jié)構(gòu)(圖3E)。

此外,顯微鏡觀察到有鈣質(zhì)海綿骨針和藻類(lèi)化石,鈣質(zhì)海綿骨針呈網(wǎng)格狀排列(圖3F和G)。碳酸鹽巖丘中發(fā)育有多期次的方解石脈,大小不一,長(zhǎng)介于幾厘米到幾米之間,最長(zhǎng)達(dá)10 m以上,寬1 mm到10 mm,最寬可達(dá)5 cm以上(圖3A和圖3E)。1號(hào)和3號(hào)丘灰?guī)r樣品發(fā)育有少量的石英和重晶石。石英多為單個(gè)顆粒鑲嵌在方解石中,部分呈集合體狀產(chǎn)出,重晶石呈長(zhǎng)條板狀(圖3H)。

3.2 地球化學(xué)

3.2.1 礦物與常量元素

巴顏喀拉長(zhǎng)石頭山二疊紀(jì)-三疊紀(jì)的丘狀灰?guī)r的XRD分析結(jié)果顯示主要由方解石組成,含量為93%～98%,平均為97%,部分樣品含少量的皂石、白云石、重晶石和石英(表1)。常量元素分析顯示丘狀灰?guī)r的CaO含量介于51.33%～54.71%之間,平均值為53.38%,其CaO含量接近于方解石的理論值56.03%,而Al2O3、Fe2O3、K2O、MgO、MnO、Na2O、P2O5和TiO2等含量都低于5‰(表2),表明丘狀灰?guī)r非常純,主要由方解石組成。

3.2.2 稀土元素

巴顏喀拉長(zhǎng)石頭山二疊紀(jì)-三疊紀(jì)的丘狀灰?guī)r的稀土元素總量(∑REE)偏低,7個(gè)樣品的∑REE為7.69～24.68μg/g,平均值為14.35μg/g(表3).REE﹥MREE﹥HREE,具輕稀土元素相對(duì)富集的特征。碳酸鹽礦物(5%HNO3可溶相)頁(yè)巖標(biāo)準(zhǔn)化稀土元素配分模式具有極好的一致性(圖4),均呈中稀土富集的上凸型,Ce/Ce*比值介于0.27～0.38之間(平均值為0.32),Ce呈明顯的負(fù)異常,Eu/Eu*介于0.98～1.33之間(平均值為1.09),Eu呈正?；蛭⒄惓?這與二疊紀(jì)海相灰?guī)r、二疊紀(jì)海水和現(xiàn)代東太平洋海水頁(yè)巖標(biāo)化準(zhǔn)化模式一致(表3和圖4)。此外,巴顏喀拉長(zhǎng)石頭山二疊紀(jì)-三疊紀(jì)的丘狀灰?guī)r的Pr/Pr*和Ce/Ce*與現(xiàn)代開(kāi)放大洋海水相似[23]。表明丘狀灰?guī)r的REE保持原始沉積特征,丘狀灰?guī)r為開(kāi)闊大洋沉積,巴顏喀拉海在晚二疊紀(jì)-三疊紀(jì)初是一個(gè)開(kāi)闊海。

圖3 巴顏喀拉盆地長(zhǎng)石頭山丘狀灰?guī)r沉積結(jié)構(gòu)與構(gòu)造A.無(wú)層理塊狀灰?guī)r;B.箭頭所指為灰?guī)r角礫的橫切面,呈橢圓狀和近圓狀;C.巖石標(biāo)本照片,白色箭頭所指為團(tuán)塊,黑色箭頭所指為櫛狀膠結(jié)物,圖中比例尺每格為1 cm;D.泥微晶方解石團(tuán)塊;E.箭頭所指為膠結(jié)物呈纖狀生長(zhǎng)的方向;F.箭頭所為藻類(lèi)的橫切面;G.網(wǎng)格狀鈣質(zhì)海綿骨針;H.呈長(zhǎng)條板狀的重晶石.Fig.3 The sedimentary textures of limestone of Longstone Mountain in Bayan Har Basin

表1 巴顏喀拉盆地長(zhǎng)石頭山丘狀灰?guī)r的XRD分析結(jié)果Table1 XRD analysis ofmound limestone of Long Stone M ountain in Bayan Har Basin

表2 巴顏喀拉盆地長(zhǎng)石頭山丘狀灰?guī)r的常量元素含量Table2 M ajor elements ofmound limestone of Long Stone M ountain in Bayan Har Basin

表3 巴顏喀拉盆地長(zhǎng)石頭山丘狀灰?guī)r的稀土元素含量(μg/g)Table3 The rare elements ofmound limestone of Long Stone M ountain in Bayan Har Basin(μg/g)

圖4 丘狀灰?guī)r與海水頁(yè)巖標(biāo)準(zhǔn)化REE配分模式對(duì)比圖A.巴顏喀拉長(zhǎng)石頭山7個(gè)丘狀灰?guī)r頁(yè)巖標(biāo)準(zhǔn)化稀土元素配分模式;B.巴顏喀拉長(zhǎng)石頭山丘狀灰?guī)r平均值與二疊紀(jì)灰?guī)r、二疊紀(jì)海水和現(xiàn)代東太平洋海水[23]的頁(yè)巖標(biāo)準(zhǔn)化REE配分模式對(duì)比圖Fig.4 The comparison chart ofmound limestone and seawater shale-normalized REE distribution pattern

在氧化環(huán)境的海水中,Ce3+可以被氧化成為難溶的Ce4+而沉淀,造成海水Ce虧損,因而Ce異常可以作為判別古海洋氧化-還原的指標(biāo)[24,25].au和Nothdurft等[26,27]認(rèn)為當(dāng)Pr/Pr*>1.0,Ce/Ce*<1.0時(shí),顯示正常的Ce負(fù)異常。巴顏喀拉長(zhǎng)石頭山二疊紀(jì)-三疊紀(jì)丘狀灰?guī)r的Ce/Ce*為0.27～0.38,Pr/ Pr*為1.17～1.63,顯示長(zhǎng)石頭山丘狀灰?guī)r形成于氧化的開(kāi)闊海環(huán)境。

巴顏喀拉長(zhǎng)石頭山二疊紀(jì)-三疊紀(jì)的丘狀灰?guī)r稀土總量(∑REE)與Al、Cu、Zn、K和Ti之間的相關(guān)系數(shù)為0.81～0.90,呈明顯的正相關(guān),而與水成元素Ca、Mg和Mn的相關(guān)系數(shù)為0～0.33,沒(méi)有相關(guān)性(表4),說(shuō)明丘狀灰?guī)r的稀土元素主要與沉積環(huán)境背景值有關(guān),與碳酸鹽礦物沒(méi)有關(guān)系。

3.2.3 微量元素

巴顏喀拉長(zhǎng)石頭山二疊紀(jì)-三疊紀(jì)的丘狀灰?guī)r中的Re、Cd、U、V、Mo、Cr、Co、Cu和Zn等氧化-還原敏感元素的含量偏低或缺失./Cr比值平均為0.09,遠(yuǎn)小于2.00。自生沉積的*Uau含量為0.08～ 0.5μg/g,平均為0.22μg/g,U含量明顯高于Th,U/ Th比值的平均值為2.33(表5)。

一般認(rèn)為Re、Cd、U、V、Mo、Cr、Co、Cu和Zn是氧化-還原敏感元素,容易在缺氧的沉積環(huán)境沉積物中富集,在氧化的沉積環(huán)境中易溶不富集,它們的含量低或缺失代表沉積物形成于氧化環(huán)境[31～35]。

雖然V和Cr在氧化環(huán)境中都是溶于水,在還原環(huán)境中易富集于沉積物中,但V的還原出現(xiàn)在反硝化作用界線的下部,Cr的還原出現(xiàn)在界線的上部[36]。因此,V/Cr比值可以作為判別古海洋的氧化還原環(huán)境的指標(biāo)[37]。通常2.00﹤V/Cr﹤4.25代表次富氧環(huán)境,V/Cr﹤2.00代表富氧環(huán)境,V/Cr﹥4.25則代表貧氧或缺氧環(huán)境(表6)[31]。

表4 ∑REE與常量元素的相關(guān)系數(shù)Table4 Correlation between∑REE and major element contents

表5 巴顏喀拉盆地長(zhǎng)石頭山丘狀灰?guī)r的微量元素含量(μg/g)Table5 The trace element contents ofmound limestone of Long Stone M ountain in Bayan Har Basin(μg/g)

U在氧化環(huán)境中常以UO2(CO3存在,并具有高溶解度,在還原環(huán)境中UO2(CO3容易以擴(kuò)散方式從海水進(jìn)入沉積物,還原成UO2、U3O7或U3O8并固定在沉積物中,造成沉積物中U的富集[31,35]。因此,自生沉積的*Uau可以作為判別氧化還原條件的指標(biāo),*Uau﹦總U含量-Th/3。當(dāng)*Uau<5.0μg/g時(shí),代表富氧環(huán)境;當(dāng)5.0μg/g<*Uau<12.0μg/g時(shí),代表次富氧環(huán)境;當(dāng)*Uau>12.0μg/g時(shí),代表貧氧和缺氧環(huán)境(表6)[31]。

巴顏喀拉長(zhǎng)石頭山所分析的7個(gè)丘狀灰?guī)r的U、V、Mo、Cr、Co、Cu和Zn元素含量的偏低或缺失,V/Cr比值小于0.22,*Uau為0.08μg/g～0.5μg/g(表5),均指示丘狀灰?guī)r形成于富氧環(huán)境。

3.3 碳和氧同位素

巴顏喀拉長(zhǎng)石頭山二疊紀(jì)-三疊紀(jì)丘狀灰?guī)r的δ13CPDB為+4.30‰～ +5.93‰,δ18OPDB為-13.72‰～-11.27‰。不同顏色的泥微晶方解石、膠結(jié)物和脈狀方解石的碳和氧同位素值基本一致(表7)。

表6 不同氧化-還原環(huán)境的微量元素判別參數(shù)Table6 Parameters of trace elements under different redox conditions

Derry等[38]認(rèn)為當(dāng)碳酸鹽巖的δ18OPDB﹤-10‰X10-3時(shí),巖石發(fā)生了強(qiáng)烈蝕變.aufman等[39]認(rèn)為當(dāng)碳酸鹽巖的δ18OPDB﹤-11‰時(shí),巖石發(fā)生強(qiáng)烈蝕變。本文分析的丘狀碳酸鹽巖δ18OPDB為-13.72‰～-11.27‰,顯示了成巖后期的改造。

表7 巴顏喀拉盆地長(zhǎng)石頭山丘狀灰?guī)r碳、氧同位素測(cè)試結(jié)果Table7 Stab le carbon and oxygen isotopes ofm ound limestone of Long Stone M ountain in Bayan Har Basin

Bartley[40-42]等認(rèn)為將 δ18OPDB與 δ13CPDB或δ18OPDB和Mn/Sr比值做相關(guān)性分析,如果兩個(gè)因素之間沒(méi)有任何相關(guān)性,則這兩個(gè)因素不受同一成巖作用的控制。所采集丘狀灰?guī)r樣品的碳同位素和氧同位素相關(guān)系數(shù)僅為0.22(圖5),氧同位素和Mn/Sr比值的相關(guān)系數(shù)僅為0.01(圖6),說(shuō)明δ13CPDB和δ18OPDB不受同一成巖作用的控制.udson總結(jié)了一般海相碳酸鹽巖的δ13CPDB均值為-5‰～+5‰[43]。巴顏喀拉長(zhǎng)石頭山二疊紀(jì)-三疊紀(jì)丘狀灰?guī)r的δ13CPDB為+4.30‰～+5.93‰之間,基本屬于正常海相碳酸鹽巖范圍之內(nèi),與易受后期改造的δ18OPDB值不同,它的δ13CPDB受后期改造影響不大。

圖5 碳同位素與氧同位素相關(guān)圖Fig.5 The correlation diagram of stable carbon isotope and oxygen isotope

圖6 氧同位素與Mn/Sr比值相關(guān)圖Fig.6 The correlation diagram of stable oxygen isotope and Mn/Sr ratio

4 結(jié)論

青海巴顏喀拉長(zhǎng)石頭山二疊紀(jì)-三疊紀(jì)的碳酸鹽巖丘是由罕見(jiàn)的塊狀純灰?guī)r組成,灰?guī)r方解石含量高達(dá)93%～98%,平均含量達(dá)97%。此外,碳酸鹽巖丘發(fā)育有鈣質(zhì)海綿骨針和藻類(lèi)化石。碳酸鹽礦物稀土元素總量為7.69～24.68μg/g,呈中稀土元素富集特征,頁(yè)巖標(biāo)準(zhǔn)化配分模式與二疊紀(jì)海相灰?guī)r、二疊紀(jì)海水和現(xiàn)代東太平洋海水基本一致。灰?guī)r具有明顯的負(fù)異常,V/Cr比值小于0.22,自生沉積的Uau為0.08～0.5μg/g,代表還原環(huán)境的U、V、Mo、Cr、Co、Cu和Zn元素含量偏低或缺失,表明丘狀灰?guī)r形成于相對(duì)氧化的環(huán)境。丘狀灰?guī)rδ13CPDB為+4.30‰～ +5.93‰,δ18OPDB為-13.72‰～-11.27‰,δ18OPDB值可能是受成巖后期的影響,而δ13CPDB基本上保持了原始沉積的特征。因此,這種幾乎完全由方解石組成的碳酸鹽巖可能是相對(duì)氧化的開(kāi)闊海環(huán)境中快速沉積的產(chǎn)物。

致謝 感謝中國(guó)科學(xué)院廣州地球化學(xué)研究所陳先沛老師對(duì)薄片觀察和論文撰寫(xiě)提供的指導(dǎo),海南地質(zhì)勘查局朱迎堂教授級(jí)高工提供資料和在野外采樣工作中提供車(chē)輛,中國(guó)科學(xué)院廣州地球化學(xué)研究所王輔亞老師幫助完成XRD的半定量分析,中國(guó)科學(xué)院廣州地球化學(xué)研究所鄧文峰老師幫助完成碳氧同位素的測(cè)試,劉穎老師指導(dǎo)完成常量元素的測(cè)試,中國(guó)科學(xué)院地球化學(xué)研究所漆亮研究員指導(dǎo)完成微量元素和稀土元素的測(cè)試,青海西寧市的張?jiān)扑緳C(jī)在野外采樣的幫助,編輯部和審稿專(zhuān)家給予的建設(shè)性意見(jiàn)。筆者在此一并深表謝意。

References)

1 孫鴻烈,鄭度。青藏高原研究叢書(shū)——青藏高原形成演化與發(fā)展[M]。廣州:廣東科技出版社,1998[Sun Honglie,Zheng Du.Qinghai-Tibet Plateau Study Series-Evolution and Development of the Tibetan Plateau Formation[M].uangzhou:Guangdong Science and Technology Press,1998]

2 殷鴻福,張克信。東昆侖造山帶的一些特點(diǎn)[J]。地球科學(xué)-中國(guó)地質(zhì)大學(xué)學(xué)報(bào),1997,22(4):339-442[Yin Hongfu,Zhang Kexin.Characteristics of the eastern Kunlun orogenic belt[J].arth Science-Journal of China University of Geosciences,1997,22(4):339-442]

3 張雪亭,王秉璋,俞建等。巴顏喀拉殘留洋盆的沉積特征[J]。地質(zhì)通報(bào),2005,(7):613-620[Zhang Xueting,Wang Binzhang,Yu Jian,etal.Sedimentary characteristics of the Bayan Har renmantocean basin,northwestern China[J].eological Bulletin of China,2005, (7):613-620]

4 潘桂棠,陳智梁,李興振等。東特提斯地質(zhì)構(gòu)造形成演化[M]。北京:地質(zhì)出版社,1997[Pan Guitang,Chen Zhiliang,Li Xingzhen, et al.Geological Formation and Evolution of Eastern Tethys[M].eijing:Geological Publishing House,1997]

5 任紀(jì)舜,肖黎薇。1∶25萬(wàn)地質(zhì)填圖進(jìn)一步揭開(kāi)了青藏高原大地構(gòu)造的神秘面紗[J]。地質(zhì)通報(bào),2004,23(1):1-11[Ren Jixun,Xiao Liwei.Lifting themysterious veil of the tectonics of the Qinghai-Tibet Plateau by 1∶250000 geological mapping[J].eological Bulletin of China,2004,23(1):1-11]

6 張國(guó)偉,郭安林,姚安平。中國(guó)大陸構(gòu)造中的西秦嶺-松潘大陸構(gòu)造結(jié)[J]。地學(xué)前緣,2004,11(3):23-32[Zhang Guowei,Guo Anlin,Yao Anping.Western Qinling-Songpan continental tectonic node in China continental tectonics[J].arth Science Frontiers, 2004,11(3):23-32]

7 張國(guó)偉,董云鵬,賴紹聰?shù)?。秦嶺-大別造山帶南緣勉略構(gòu)造帶與勉略縫合帶[J]。中國(guó)科學(xué):D輯,2003,33(12):1121-1135 [Zhang Guowei,Dong Yunpeng,Lai Shaochong,et al.The southern margin of Qinling Mountains-Dabie belt and Mian slightly suture[J].cience in China:Series D,2003,33(12):1121-1135]

8 殷鴻福,張克信。中央造山帶的演化及其特點(diǎn)[J]。地球科學(xué)-地質(zhì)大學(xué)學(xué)報(bào),1998,23(5):437-441[Yin Hongfu,Zhang Kexin.E-volution and characteristics of the central orogenic belt[J].arth Science-Journal of China University of Geosciences,1998,23(5):437-441]

9 許志琴,侯立瑋,王大可等。“西康式”褶皺及其變形機(jī)制:一種新的造山帶褶皺類(lèi)型[J]。中國(guó)區(qū)域地質(zhì),1991,(1):1-9[Xu Zhiqin,Hou Liwei,Wang Dake etal?！癤ikang-type”folding and their deformationmechanism:A new fold type of orogenic belt[J].egional Geology of China,1991,(1):1-9]

10 張以茀,龐存廉,李長(zhǎng)利。可可西里-巴顏喀拉三疊紀(jì)沉積盆地的形成與演化[M]。西寧:青海人民出版社,1997[Zhang Yifu, Peng Cunlian,LiChangli.Formation and Evolution of Hoh Xili-Bayankala Triassic Sedimentary Basins[M].ining:Qinghai People's Publishing House,1997]

11 張以茀。可可西里-巴顏喀拉及鄰區(qū)特提斯海的特征[J]。青海地質(zhì),1991,(1):1-17[Zhang Yifu.Kekexili-Bayan Har and itsadjacent areas and the characteristics of Tethys[J].inghai Geology, 1991,(1):1-17]

12 王永標(biāo),黃繼春。海西―印支早期東昆侖造山帶南側(cè)古海洋盆地的演化[J]。地球科學(xué)-中國(guó)地質(zhì)大學(xué)學(xué)報(bào),1997,22(4):369-372[Wang Rongbiao,Huang Jichun.Paleo-ocean evolution of the south of East Kunlun orogenic belt During Hercy-Early Indosinian [J].arth Science-Journal of China University of Geosciences, 1997,22(4):369-372]

13 朱迎堂?？煽晌骼?巴顏喀拉三疊紀(jì)沉積盆地的形成及演化[D]。成都理工大學(xué),2006[Zhu Yintang.Formation and evolution of the Kekexili-Bayan Har Triassic sedimentary basin[D].hengdu University of Technology,2006]

14 楊欣德,王宗秀,郭通珍等。青海巴顏喀拉山三疊系復(fù)理石沉積粒度概率累積曲線的特征[J]。地質(zhì)通報(bào),2008,27(4):477-490 [Yang Xinde,Wang Zongxiu,Guo Tongzhen,et al.Probability cumulative grain-size curves of Triassic flysch deposition in the Bayan Har Mountains area,Qinghai,China[J].eological Bulletin of China,2008,27(4):477-490]

15 杜德勛,羅建寧。巴顏喀拉三疊紀(jì)沉積盆地巖石地球化學(xué)特征與物源區(qū)構(gòu)造背景的探討:以阿壩一若爾蓋、小金一馬爾康及雅江盆地為例[J]。巖相古地理,1999,19(2):1-20[Du Dexun,Luo Jianning.Tectonic settings of the provenances for the Triassic Bayan Har Basin deduced from petrogeochemical characteristics:examples from the Aba-Zoige,Xiaojin-Barkam and Yajiang Basins in western Sichuan[J].edimentary Facies Palaeogeography,1999,19(2):1-20]

16 王永標(biāo),徐桂榮,林啟祥等。東昆侖地區(qū)早二疊世礁島海的沉積模式[J]。中國(guó)科學(xué):D輯,2001,33(3):243-249[Wang Rongbiao,Xu Guirong,Lin Qixiang,et al.East Kunlun Early Permian depositionalmodel of reef-island ocean[J].cience in China:Series D, 2001,33(3):243-249]

17 王永標(biāo),徐桂榮。東昆侖南坡早二疊世灰?guī)r塊體的來(lái)源與成因探討[J]。地質(zhì)科技情報(bào)。2000,19(4):35-39[Wang Rongbiao,Xu Guirong.Study on genesis of early Permian limestone blocks in the southern slope of East Kunlun[J].eological Science and Technology Information,2000,19(4):35-39]

18 殷鴻福,張克信,陳能松等。1∶25萬(wàn)冬給措那湖幅地質(zhì)[M]。武漢:中國(guó)地質(zhì)大學(xué),2001[Yin Hongfu,Zhang Kexin,Chen Nengsong,etal.Donggeicuona Lake1∶250,000 Measure Geological[M].uhan:China University of Geosciences,2001]

19 Tayor JC.Computer programs for standardless quantitative analysisof minerals using the full powder diffraction profile[J].owder Diffraction,1991,6:2-9

20 鄧文峰,韋剛健,李獻(xiàn)華。不純碳酸鹽碳氧同位素組成的在線分析[J]。地球化學(xué),2005,34(5):495-500[Deng Wenfeng,Wei Gangjian,Li Xianhua.Online analysis of carbon and oxygen isotopic compositions of impure carbonate[J].eochimica,2005,34(5): 495-500]

21 李獻(xiàn)華,劉穎,涂湘林 等。硅酸鹽巖石化學(xué)組成的ICP-AES和ICP-MS準(zhǔn)確測(cè)定:酸溶與堿熔分解樣品方法的對(duì)比[J]。地球化學(xué),2002,31(3):289-294[Li Xianhua,Liu Ying,Tu Xianglin,et al.Precise determinationg of chemical composition in silicate rocks using ICP-AES and ICP-MS:A comparative study of sample digestion techniques of acid fusion and alkali dissolution[J].eochimica, 2002,31(3):289-294]

22 Qi L,Zhou M,Malpass J,etal.Determination of rare earth elements and Y in ultramafic rocks by ICP-MS after preconcentration using Fe (OH)3and Mg(OH)2coprecipitation[J].eostandards and Geoanalytical Ressarch,2005,29:131-141

23 Webb G E,Kamber B S.Rare earth elements in Holocene reefalmicrobialites:a new shallow seawater proxy[J].eochimica et Cosmochimica Acta,2000,64(9):1557-1565

24 Wright J,Schrader H,Holser W T.Paleoredox variations in ancient oceans recorded by rare earth elements in fossil apatite[J].eochimica et Cosmochimica Acta,1987,51(3):631-644

25 HolserW T.Evaluation of the application of rare-earth elements to paleoceanography[J].alaeogeography,Palaeoclimatology,Palaeoecology,1997,132(1-4):309-323

26 Bau M,Dulski P.Distribution of yttrium and rare-earth elements in the Penge and Kuruman iron-formations,Transvaal Supergroup,South Africa[J].recambrian Research,1996,79(1-2):37-55

27 Nothdurft L D,Webb G E,Kamber B S.Rare earth element geochemistry of Late Devonian reefal carbonates,Canning Basin,Western Australia:confirmation of a seawater REE proxy in ancient limestones [J].eochimica et Cosmochimica Acta,2004,68(2):263-283

28 Mclennan SM.Rare earth elements in sedimentary rocks;influence of provenance and sedimentary processes[J].eviews in Mineralogy and Geochemistry,1989,21(1):169-200

29 韓吟文,馬振東,張宏飛,等。地球化學(xué)[M]。北京:地質(zhì)出版社,2003[Han Yinwen,Ma Zhendong,Zhang Hongfei,et al.Geochimica[M].eijing:Geological Publishing House,2003]

30 Nameroff T J,Balistrieri L S,Murray JW.Suboxic tracemetal geochemistry in the eastern tropical North Pacific[J].eochimica et Cosmochimica Acta,2002,66(7):1139-1158

31 Jones B,Manning D a C.Comparison of geochemical indices used for the interpretation of palaeoredox conditions in ancientmudstones[J].hemical Geology,1994,111(1-4):111-129

32 Calvert S E,Pedersen T F.Geochemistry of recent oxic and anoxic marine sediments:Implications for the geological record[J].arine Geology,1993,113(1-2):67-88

33 Colodner D,Sachs J,Ravizza G,etal.The geochemical cycle of rhenium:a reconnaissance[J].arth and Planetary Science Letters, 1993,117(1-2):205-221

34 熊?chē)?guó)慶,江新勝,蔡習(xí)堯,等。藏南白堊系泥、頁(yè)巖微量、稀土元素特征及氧化-還原環(huán)境分析[J]。地球科學(xué)進(jìn)展,2010,25 (7):730-745[Xiong Guoqing,Jiang Xinshen,CaiXiyao,etal.The Characteristics of trace element and REE geochemistry of the Cretaceousmudrocks and shales from southern Tibet and its analysis of redox condition[J].dvances in Earth Science,2010,25(7):730-745]

35 Tribovillard N,Algeo T J,Lyons T et al.Tracemetals as paleoredox and paleoproductivity proxies:An update[J].hemical Geology, 2006,232(1-2):12-32

36 Piper D Z.Seawater as the source ofminor elements in black shales, phosphorites and other sedimentary rocks[J].hemical Geology, 1994,114(1-2):95-114

37 Scheffler K,Buehmann D,Schwark L.Analysis of late Palaeozoic glacial to postglacial sedimentary successions in South Africa by geochemical proxies-Response to climate evolution and sedimentary environment[J].alaeogeography,Palaeoclimatology,Palaeoecology, 2006,240(1-2):184-203

38 Derry L A,Kaufman A J,Jacobsen SB.Sedimentary cycling and environmental change in the Late Proterozoic:Evidence from stable and radiogenic isotopes[J].eochimica et Cosmochimica Acta,1992,56 (3):1317-1329

39 Kaufman A J,Jacobsen S B,Knoll A H.The Vendian record of Sr and C isotopic variations in seawater:Implications for tectonics and paleoclimate[J].arth and Planetary Science Letters,1993,120(3-4):409-430

40 Xiao S,Bao H,Wang H,etal.The Neoproterozoic Quruqtagh Group in the eastern Chinese Tianshan:Evidence for a post-Marinoan glaciation[J].recambrian Research,2004,130(1-4):1-26

41 Le Guerrou E,Allen P A,Cozzi A,et al。50Myr recovery from the largest negativeδ13C excursion in the Ediacaran ocean[J].erra Nova,2006,18(2):147-153

42 Bartley JK,Kah L C,Mcwilliams JL,etal.Carbon isotope chemostratigraphy of the Middle Riphean type section(Avzyan Formation, Southern Urals,Russia):Signal recovery in a fold-and-thrust belt [J].hemical Geology,2007,237(1-2):211-232

43 Hudson JD.Stable isotopes and limestone lithification[J].ournal of the Geological Society of London,1977,133(6):637-660

44 Da Silva A-C,Boulvain F.Carbon isotope lateral variability in a Middle Frasnian carbonate platform(Belgium):Significance of facies, diagenesis and sea-levelhistory[J].alaeogeography,Palaeoclimatology,Palaeoecology,2008,269(3-4):189-204

Sedimentary Petrology and Geochem istry of Perm ian-Triassic Carbonate M ounds in Long-Stone M ountain in Bayan Har Basin

HUANG Hua-gu1,2DIPeng-fei1,2CHEN Duo-fu1
(1.Key Laboratory of M arginal Sea Geology,Guangzhou Institute of Geochem istry,Chinese Academ y of Sciences,Guangzhou 510640,China; 2.Graduate University of the Chinese Academy of Sciences,Beijing 100049,China)

The Bayan Har sedimentary basin has been a hotspot and key area in studying Qinghai-Tibet Plateau geotectonics and Tethys,which is located in the northeast of Qinghai-Tibet Plateau hinderland.The domination sedimention of Bayan Har basin which has some different sizes of limestone platforms or limestone mounds are composed of Triassic turbidite rocks.In this paper,we study the sedimentary environment of hummocky limestone by the way of sedimentary petrology and geochemistry.

Samples of this paper were collected from four isolated carbonatemounds on Long Stone Mountain of Bayan Har Basin,Qinghai province,which was developed in the Permian-Triassic Maerzheng Formation strata with Ammonoid that produced a dark gray thin-bedded micritic limestone and gray-black slate。

XRD analysis showed that the average calcite content of hummocky limestone from Permian-Triassic strata of Long Stone Mountain of Bayan Har Basin is up to 97%.ajor element analysis showed that the CaO average content of hummocky limestone is up to 53.38%,so the CaO content is close to the theoretical value of calcite 56.03%, while Al2O3,Fe2O3,K2O,MgO,MnO,Na2O,P2O5,TiO2and others content are below 5‰.hose indicate that the chemical and minerals composition of carbonate are very pure。

The shale-normalized REE patterns of 5%HNO3 soluble phase of limestone(carbonateminerals)showed total rare earth elements(ΣREE)were low,negative Ce anomaly,Eu was normal or slightly positive anomalies,Pr/Pr*was from 1.17 to 1.63.t is consistentwith those of Permian limestone,Permian sea water,and modern eastern Pacific sea water and illustrated that it formed in oxidation environment of the open sea。

The correlation coefficient between total rare earth(ΣREE)and Al,Cu,Zn,K and Tiwere from 0.81 to 0.90 and showed a positive correlation,and the correlation coefficient between total rare earth(ΣREE)and Ca,Mg,Mn were from 0 to 0.33 and illustrated there was no obvious relevance or relevance.It is said that rare earth elements of hummocky limestonemainly related to the background value of sedimentary environment and has nomatter with carbonateminerals。

Hummocky limestoneδ13CPDBwere from+4.30‰to+5.93‰andδ18OPDBwere from-13.72‰ to-11.27‰。δ13CPDBis basically consistent with the normalmarine carbonate scope and indicatingδ13CPDBremained the original sedimentary characteristics of hummocky limestone.Butδ18OPDBwas significantly affected by diagenesis and did not represent the original sedimentary features。

The V/Cr ratios of limestone are very low,*Uau values are only from 0.08μg/g to 0.5μg/g,and the contents of U,V,Mo,Cr,Co,Cu and Zn elements,thatwere enriched in anoxic environment,are low or absent.All instructed that limestone formed in oxygen-rich environment。

All above sedimentary rocks and geochemical characteristics show that the Permian-Triassic hummocky limestonemight be the product of rapid deposition in relatively oxic environment of open sea。

carbonatemounds,pure limestone,sedimentary petrology,geochemistry,Bayan Har Basin

黃華谷 男 1981年出生 博士研究生 地球化學(xué) E-mail:huanghg@gig.ac.cn

陳多福 E-mail:cdf@gig.ac.cn

P588.24+5

A

1000-0550(2012)01-0054-12

①國(guó)家自然科學(xué)基金項(xiàng)目(批準(zhǔn)號(hào):40872079,40725011)資助。

2011-02-23;收修改稿日期:2011-04-21