徐亞軍 杜遠(yuǎn)生 楊江海 陳甲才

(1.生物地質(zhì)與環(huán)境地質(zhì)教育部重點(diǎn)實(shí)驗(yàn)室 武漢 430074;2.中國地質(zhì)大學(xué)地球科學(xué)學(xué)院 武漢 430074; 3.貴州省地質(zhì)礦產(chǎn)勘查局103地質(zhì)大隊(duì) 貴州銅仁 554300)

甘肅靖遠(yuǎn)上泥盆統(tǒng)沙流水組沉積地球化學(xué)特征及其物源分析①

徐亞軍1,2杜遠(yuǎn)生1,2楊江海2陳甲才3

(1.生物地質(zhì)與環(huán)境地質(zhì)教育部重點(diǎn)實(shí)驗(yàn)室 武漢 430074;2.中國地質(zhì)大學(xué)地球科學(xué)學(xué)院 武漢 430074; 3.貴州省地質(zhì)礦產(chǎn)勘查局103地質(zhì)大隊(duì) 貴州銅仁 554300)

甘肅靖遠(yuǎn)上泥盆統(tǒng)沙流水組沉積地球化學(xué)研究表明:砂巖較泥質(zhì)巖含有較高的SiO2含量和斜長石,而泥質(zhì)巖較砂巖含有較高的鐵鎂質(zhì)組分和含鉀礦物。砂巖和泥質(zhì)巖樣品均虧損Nb、Ta、Sr元素。砂巖中部分樣品富集Zr、Hf元素和強(qiáng)烈的Eu的負(fù)異常,部分樣品顯示Zr、Hf虧損和弱的負(fù)Eu異常。泥質(zhì)巖樣品均富集Ni、Cr元素,但具有較強(qiáng)的Eu負(fù)異常。沙流水組巖石地球化學(xué)特征表現(xiàn)出源巖以酸性巖和基性巖混合為特征,且遭受了中等程度的化學(xué)風(fēng)化作用(CIA:74～81),在沉積過程中有少量來自被動(dòng)大陸邊緣的經(jīng)歷了沉積分選和再循環(huán)的古老物質(zhì)加入。構(gòu)造背景判別表明,源巖主要形成于大陸島弧和活動(dòng)大陸邊緣環(huán)境,并有少量來自大洋島弧和被動(dòng)大陸邊緣環(huán)境下形成的巖石。結(jié)合區(qū)域構(gòu)造單元巖石組合特征認(rèn)為,北祁連沙流水組碎屑巖源巖主要為卷入造山帶的北祁連島弧、中祁連地塊、俯沖雜巖帶中的酸性巖石和基性巖石及少量花崗巖,同時(shí)還有少量來自華北板塊的、經(jīng)過沉積分選和再循環(huán)的物質(zhì)。綜合上奧陶統(tǒng)-中、下泥盆統(tǒng)沉積演化特征及區(qū)域巖石學(xué)研究,認(rèn)為晚泥盆世是造山帶強(qiáng)烈擠壓之后轉(zhuǎn)向伸展垮塌的時(shí)期,上泥盆統(tǒng)沙流水組則是北祁連加里東期造山帶由擠壓向伸展轉(zhuǎn)換的沉積記錄。

沙流水組 沉積地球化學(xué) 物源 甘肅 北祁連造山帶

北祁連造山帶位于青藏高原東北緣,是中祁連地塊與華北板塊西部阿拉善地塊于加里東期形成的造山帶。該造山帶的構(gòu)造演化歷史對(duì)于了解我國西部板塊群的拼合過程以及構(gòu)造演化歷史具有重要意義,因而為中外地質(zhì)學(xué)家所關(guān)注。志留紀(jì)-泥盆紀(jì)是北祁連造山帶的構(gòu)造轉(zhuǎn)換時(shí)期,志留紀(jì)復(fù)理石和泥盆紀(jì)磨拉石記錄了該時(shí)期構(gòu)造轉(zhuǎn)換的詳細(xì)細(xì)節(jié),是揭示造山帶形成與盆地演化的沉積記錄。近來,Yang等[1]利用碎屑鋯石年代學(xué)方法研究了造山帶東段靖遠(yuǎn)和西段肅南兩地早志留世碎屑巖的物源特征,并討論了志留紀(jì)早期構(gòu)造演化的特點(diǎn)。閆臻等[2]利用碎屑巖碎屑組成、重礦物組合和沉積地球化學(xué)研究了中下泥盆統(tǒng)老君山組磨拉石的物源特征,討論了泥盆紀(jì)早期的沉積盆地的性質(zhì)。然而,上泥盆統(tǒng)沙流水組碎屑巖物源特征尚無研究報(bào)道,沙流水組與造山帶演化之間的關(guān)系尚不清楚。本研究利用碎屑巖沉積地球化學(xué)方法對(duì)該組地層的物源進(jìn)行了深入分析,并結(jié)合已有研究討論了該組地層在造山帶演化過程中的意義。

1 區(qū)域地質(zhì)概況及研究剖面

北祁連加里東期造山帶位于華北板塊西南緣龍首山與中祁連地塊之間,北界為龍首山斷裂,南緣與中祁連北緣斷裂相接,東端為同心-固原右行走滑斷裂,西端為左行走滑的阿爾金斷裂所截切(圖1)。根據(jù)馮益民和何世平[3]的劃分方案,北祁連造山帶自北向南可以劃分為河西走廊弧后盆地、北祁連島弧、俯沖雜巖和消減洋殼殘片等不同的單元,并可歸并為河西走廊弧后盆地、北祁連島弧、海溝俯沖雜巖3個(gè)構(gòu)造分區(qū)(圖1)。

北祁連泥盆系分為中、下統(tǒng)老君山組和上統(tǒng)沙流水組[4]。中、下統(tǒng)老君山組是我國著名的陸相磨拉石之一,與早中志留紀(jì)復(fù)理石沉積和晚志留紀(jì)海相磨拉石沉積組成了典型的前陸盆地沉積序列[5]。中、下泥盆統(tǒng)老君山組在全區(qū)廣泛分布,巖性為紫紅色塊狀礫巖,礫石磨圓、分選較差,主要呈扇狀體分布于整個(gè)祁連山山前、山間盆地內(nèi),是祁連山強(qiáng)烈隆升階段的沉積記錄;上泥盆統(tǒng)沙流水組主要分布在東段武威-景泰一帶并具一定的分帶性,被認(rèn)為是碰撞造山期后的磨拉石沉積[6]。巖性主要為紫紅色薄層泥質(zhì)粉砂巖、泥巖與灰白色中厚層砂礫巖互層,研究剖面上礫巖中礫石主要為石英巖。砂巖中發(fā)育交錯(cuò)層理,泥巖中發(fā)育水平層理,并含有植物化石。沉積相研究為陸相湖盆沉積,其內(nèi)發(fā)育震積巖、風(fēng)暴巖事件沉積[7,8]。

圖1 北祁連造山帶東段地質(zhì)簡圖(據(jù)文獻(xiàn)[3],修改)A.河西走廊弧后盆地;B.北祁連島弧;C.海溝俯沖雜巖Fig.1 The geological sketch of the eastern part of the North Qilian Orogenic Belt(after Reference[3])

圖2 甘肅靖遠(yuǎn)沙流水地質(zhì)簡圖Fig.2 Simplified geologicalmap of the Shaliushui area

圖3 甘肅靖遠(yuǎn)沙流水組剖面及采樣位置Fig.3 Profile and sampling location of Shaliushui Formation in Jingyuan County,Gansu province

研究剖面位于祁連山東段,地理位置在甘肅省靖遠(yuǎn)縣水泉鄉(xiāng)沙流水村附近(E:36。56'54″;N:104。34' 35″)(圖1,圖2,圖3)。沙流水組與下伏老君山組呈角度不整合接觸,與上覆石炭系呈平行不整合接觸。底部有5 m左右的灰白色巨厚層礫巖,最大礫石直徑達(dá)20～30 cm,粒序?qū)永戆l(fā)育;向上粒度變細(xì),巖性主要為灰白色中-厚層粗砂巖-細(xì)砂巖與紫紅色薄層粉砂巖、泥質(zhì)粉砂巖、粉砂質(zhì)泥巖、泥巖互層。由剖面底部向上部,灰白色砂巖粒度逐漸變細(xì),礫石含量降低并逐漸消失。紫紅色泥質(zhì)巖中砂質(zhì)含量變低直至與石炭系碳酸鹽巖平行不整合接觸。分析選取樣品共16件,巖性主要為粉砂質(zhì)泥巖、泥巖以及中砂巖。主量元素測試在武漢綜合巖礦測試中心采用X熒光光譜分析,獲得13種主量元素含量。微量元素在中國地質(zhì)大學(xué)(武漢)地質(zhì)過程與礦產(chǎn)資源國家重點(diǎn)實(shí)驗(yàn)室采用ICP-MS儀器測試,獲得37種微量元素含量(附表1)。

2 沙流水組地球化學(xué)特征及其影響因素

根據(jù)巖性,把沙流水組的樣品分為兩組。一組為泥質(zhì)巖(MS)(包括粉沙質(zhì)泥巖和泥巖),另一組為砂巖(SS)(主要為中砂巖)。

2.1 地球化學(xué)特征

主量元素中SiO2含量泥質(zhì)巖較砂巖低(泥巖: 39.01%～62.97%,平均55.67%;砂巖:39.15%～ 84.37%,平均63.39%)(表1,圖4a).gO+Fe2O3T含量除一件砂巖樣品(Shl28-2)外,砂巖低于泥質(zhì)巖,表明泥質(zhì)巖中鐵鎂質(zhì)組分高于砂巖(圖4b).2O/Na2O比值泥巖普遍高于砂巖,反映泥質(zhì)巖含有較多的鉀長石或含鉀礦物,而砂巖含有較多的斜長石(圖4c).l2O3/(CaO+Na2O)比值可用來反映樣品中穩(wěn)定組分與不穩(wěn)定組分的相對(duì)含量[9]。所有樣品中Al2O3/(CaO+Na2O)比值普遍低于澳大利亞后太古代頁巖(PAAS),表明樣品中穩(wěn)定組分與不穩(wěn)定組分比值較低,樣品中不穩(wěn)定組分含量較高,而穩(wěn)定組分含量較低,樣品成熟度較低(圖4d)。

圖4 甘肅靖遠(yuǎn)沙流水組碎屑巖主量元素特征值Fig.4 Characteristic values ofmajor elements of clastic rocks from the Shaliushui Formation,Jingyuan,Gansu province

在上地殼標(biāo)準(zhǔn)化蛛網(wǎng)圖中(圖5),泥質(zhì)巖(MS)和砂巖(SS)具有相似的微量元素分配模式,但是,分布范圍砂巖普遍大于泥質(zhì)巖,表明砂巖沒有經(jīng)歷沉積再循環(huán)作用的顯著均一化。所有樣品均顯示高場強(qiáng)元素Nb、Ta和大離子親石元素Sr的虧損。同時(shí)泥質(zhì)巖均顯示 Ni、Cr元素的富集,而砂巖中部分樣品(Shl45-1和Shl35)富集Zr、Hf,而部分樣品(Shl33和Shl33-1)顯示Zr、Hf的虧損。

在球粒隕石標(biāo)準(zhǔn)化分配模式中,稀土元素分配模式均顯示右傾,輕稀土元素(LREE)富集,重稀土元素(HREE)分布平坦(圖6)(砂巖LaN/YbN:6.95～ 16.69,平均值為9.97;泥巖LaN/YbN:7.48～10.37,平均值為8.66)。泥質(zhì)巖均顯示較強(qiáng)的負(fù)Eu異常(Eu/Eu*:0.58～0.67,平均值:0.63)且與PAAS相近(Eu/Eu*:0.65),砂巖Eu負(fù)異常較泥巖弱且分布范圍較大(Eu/Eu*:0.55～0.74,平均值:0.63),亦暗示砂巖沒有經(jīng)歷沉積再循環(huán)作用的顯著均一化。

2.2 影響地球化學(xué)特征的因素

沉積巖的化學(xué)成分受諸多因素影響,如化學(xué)風(fēng)化和沉積分選與再循環(huán)等。因此,在研究沉積物物源特征以前,評(píng)估上述因素的影響非常重要。

2.2.1 化學(xué)風(fēng)化

圖5 甘肅靖遠(yuǎn)沙流水組碎屑巖微量元素上地殼標(biāo)準(zhǔn)化蛛網(wǎng)圖(上地殼標(biāo)準(zhǔn)化數(shù)據(jù)引自文獻(xiàn)[10],圖例同圖4)Fig.5 Upper crust-normalized spider diagram of clastic rock from the Shaliushui Formation,Jingyuan, Gansu Province(Upper crust-normalizing data from reference[10],see symbols in Fig.4 )

化學(xué)風(fēng)化作用常常影響著碎屑巖的化學(xué)組成。在風(fēng)化過程中,穩(wěn)定的陽離子被保存在風(fēng)化產(chǎn)物中(如Al3+、Ti4+等),而不穩(wěn)定的陽離子往往流失(如Na+、Ca2+、K+等)[11]。元素的丟失程度取決于化學(xué)風(fēng)化強(qiáng)度[12].esbitt和Young[13]提出利用化學(xué)蝕變指數(shù)(CIA)來定量評(píng)價(jià)源巖遭受的化學(xué)風(fēng)化強(qiáng)度。其中,CIA=100 X[Al2O3/(Al2O3+CaO*+Na2O +K2O)],式中各元素采用摩爾百分含量,其中CaO*僅指硅質(zhì)礦物中的CaO摩爾百分含量。

圖6 甘肅靖遠(yuǎn)沙流水組碎屑巖稀土元素球粒隕石標(biāo)準(zhǔn)化分配模式(球粒隕石標(biāo)準(zhǔn)化數(shù)據(jù)引自文獻(xiàn)[10],圖例同圖4)Fig.6 Chondrite-normalized REE patterns of clastic rock from the Shaliushui Formation,Jingyuan, Gansu Province(Chondrite-normalized data from document[10],Symbols as in Fig.4 )

盡管Fedo等[11]給出了去除碳酸鹽中Ca2+成分的方法,但是因確定方解石和白云石的比例比較困難,而無法有效去除碳酸鹽中Ca2+成分。所以采用Bock等[14]提出的方法,當(dāng)CaO>Na2O含量時(shí),Na2O=CaO*;當(dāng)CaO≤Na2O含量時(shí),CaO=CaO*。以此計(jì)算獲得CIA值(表1)并在A-CN-K三角圖中投點(diǎn)(圖7).IA值范圍在74～81,平均為76,屬于中等程度的化學(xué)風(fēng)化強(qiáng)度。表明源區(qū)巖石經(jīng)歷了中等程度的化學(xué)風(fēng)化作用。此外在A-CN-K中,泥質(zhì)巖投點(diǎn)近似平行于A-K邊,可能和泥質(zhì)巖中黏土礦物發(fā)生鉀交代作用有關(guān)[15],這也可能是造成泥質(zhì)巖樣品中K2O/Na2O比值高于砂巖樣品的原因之一。

2.2.2 沉積分選與再循環(huán)

沉積分選與再循環(huán)通常會(huì)造成重礦物富集,從而導(dǎo)致某些元素富集[16]。鋯石是Zr元素的主要賦存礦物,且礦物的穩(wěn)定性很強(qiáng),會(huì)隨著沉積再循環(huán)作用而富集于沉積物中.h通常在酸性巖中賦存,而Sc常賦存于基性巖石中,且Th/Sc比值在沉積再循環(huán)作用下不發(fā)生改變因而常被用做物源化學(xué)成分變化研究[17]。而Zr/Sc比值常因沉積再循環(huán)作用過程中鋯石的富集而升高,且不受后期熱液稀釋等作用影響[18],因而是重礦物富集的良好指標(biāo)。因此 Th/ Sc-Zr/Sc圖常被用來評(píng)估沉積再循環(huán)作用。在Th/ Sc-Zr/Sc圖中(圖8),除樣品Shl45-1外,所有砂巖樣品靠近成分演化線(BFG),并接近于大陸上地殼(UCC)和PAAS,顯示砂巖成分受源巖成份的控制。而樣品Shl45-1中可能含有較多經(jīng)過沉積再循環(huán)的物質(zhì)。泥質(zhì)巖樣品部分沿成分演化線分布,但是部分脫離成分演化線而與鋯石富集趨勢線較為接近,顯示了鋯石富集,部分泥質(zhì)巖中可能含有經(jīng)過沉積再循環(huán)的物質(zhì)(如樣品Shl17-1和Shl 51)。

圖7 甘肅靖遠(yuǎn)沙流水組碎屑巖A-CN-K三角圖(據(jù)文獻(xiàn)[11],圖例同圖4)Ka為高嶺石;Gi為水鋁石;Chi為綠泥石;Sm為蒙脫石; II為伊利石;Mos為白云母;Pl為斜長石;Ksp為鉀長石; T為英云閃長巖;Gd為花崗閃長巖;G為花崗巖Fig.7 A-CN-K triangle diagram of clastic rocks from the Shaliushui Formation,Jingyuan,Gansu Province (after the reference[11],Symbols as in Fig.4 )

圖8 甘肅靖遠(yuǎn)沙流水組碎屑巖Zr/Sc-Th/Sc圖(據(jù)文獻(xiàn)[17],圖例同圖4)B玄武巖;F長英質(zhì)巖石;G花崗巖Fig.8 Zr/Sc-Th/Sc diagram of clastic rocks from the Shaliushui Formation,Jingyuan,Gansu Province(after the reference[17].Symbols as in Fig.4 )

此外受后期熱液的影響,巖石中SiO2含量的升高可改變巖石中微量元素的含量。因?yàn)镾iO2含量的升高可造成Al2O3含量的降低,而Al2O3可控制微量元素的含量[19]。樣品Shl33中石英含量(84.4%)顯著大于其它樣品,可能受到后期熱液的影響,造成該樣品的稀土元素成分被稀釋(圖5b),不能反映物源特征,因此在后面的分析中予以排除。

3 源巖屬性及大地構(gòu)造背景

3.1 源巖屬性

通常來講,元素Sc、Ni、Cr、Co傾向于在基性巖石中富集,而La、Th、Hf、Zr以及REE傾向于富集于酸性巖石中[20]。

沙流水組樣品地球化學(xué)數(shù)據(jù)顯示,砂巖SiO2含量除樣品Shl 30-1較低外(39.15%),其它樣品主要在60%～75%。此外,砂巖具有較低的K2O/Na2O比值和MgO+Fe2O3值,說明樣品中含有較少的鉀長石和鐵鎂質(zhì)組份,較多的斜長石。上述主量元素特征顯示砂巖源巖以酸性巖石為主。另外樣品Shl45-1和Shl 35富集Zr、Hf元素和強(qiáng)烈的Eu的負(fù)異常(Eu/ Eu*分別為0.55和0.56),也說明含有較多的酸性物質(zhì)。而樣品Shl 30-1顯示Zr、Hf虧損和弱的負(fù)Eu異常(0.7),則可能更多地含有基性組份。

泥質(zhì)巖SiO2含量較砂巖低(平均55.67%),而K2O/Na2O比值和MgO+Fe2O3值則較砂巖高,且樣品富集Ni、Cr元素,反映樣品中含有較多的含鉀礦物和基性組份,一致于化學(xué)風(fēng)化評(píng)估所獲泥質(zhì)巖中黏土礦物發(fā)生了鉀交代作用的結(jié)論,此外泥質(zhì)巖樣品具有較強(qiáng)的Eu負(fù)異常,表明泥質(zhì)巖源巖形成過程中存在長石分餾作用,源巖中可能也混有酸性巖石碎屑。

稀土元素(REE)和高場強(qiáng)元素以及部分過渡金屬(如Co)通常被認(rèn)為是沉積過程中最穩(wěn)定的元素,因此,利用這些元素能有效地判別碎屑沉積巖源區(qū)成分[21,22]。在Hf-La/Th圖中(圖9),砂巖樣品零散分布,泥巖分布相對(duì)集中。砂巖樣品Shl 45-1位于含有較多古老沉積物的被動(dòng)大陸邊緣物源一側(cè),說明源巖中含有較多來自被動(dòng)陸緣的古老沉積物,一致于該樣品中含有較多經(jīng)歷沉積分選和再循環(huán)物質(zhì)的認(rèn)識(shí)。此外砂巖樣品Shl 35-1和泥質(zhì)巖樣品Shl 17-1和Shl 51中可能也含有一定量的來自被動(dòng)陸緣的古老沉積物,所以位于長英質(zhì)和基性混合物源區(qū)靠近被動(dòng)大陸邊緣一側(cè)。砂巖樣品Shl28-2和部分泥質(zhì)巖位于酸性島弧物源區(qū),反映源巖中含有島弧環(huán)境下形成的酸性巖石。而砂巖Shl 30-1和部分泥巖位于長英質(zhì)和基性混合物源區(qū),并有向安山巖島弧物源演化的趨勢。

總的來說,沙流水組源巖成分較多,以酸性和基性巖石混合的物源為主,并伴有來自被動(dòng)大陸邊緣的古老沉積物。

圖9 甘肅靖遠(yuǎn)沙流水組碎屑巖Hf-La/Th圖(據(jù)文獻(xiàn)[23],圖例同圖4)Fig.9 Hf-La/Th diagram of clastic rocks from the Shaliushui Formation,Jingyuan,Gansu Province(after reference[23].Symbols as in Fig.4 )

3.2 源區(qū)構(gòu)造背景判別

不同的構(gòu)造環(huán)境,形成的沉積巖地球化學(xué)成分不同,可據(jù)此判定沉積巖形成的構(gòu)造背景[24]。根據(jù)Bhatia等[24,25]提出的常量元素、微量元素構(gòu)造背景判別圖(圖10,圖11),沙流水組樣品主要投在大陸島弧環(huán)境和活動(dòng)大陸邊緣環(huán)境并有1件砂巖樣品(Shl 45-1)投在被動(dòng)大陸邊緣環(huán)境,一件泥質(zhì)巖樣品(Shl 20-1)投在大洋島弧環(huán)境。因此,沙流水組沉積物主要來源于大陸島弧和活動(dòng)大陸邊緣環(huán)境下形成的巖石,并有少量來自大洋島弧和被動(dòng)大陸邊緣環(huán)境下形成的巖石。

4 討論

4.1 沙流水組物源區(qū)位置

北祁連造山帶是在Rodinia聯(lián)合大陸基礎(chǔ)上經(jīng)由寒武紀(jì)華北板塊南緣裂解形成裂谷盆地,早、中奧陶世形成洋盆和溝-弧-盆體系,晚奧陶世-早、中泥盆世由中祁連地塊與華北板塊碰撞造山形成的[3,26～29]。北祁連-走廊盆地在奧陶紀(jì)-泥盆紀(jì)經(jīng)歷了由弧后盆地向前陸盆地的構(gòu)造轉(zhuǎn)換過程[5]。

圖10 甘肅靖遠(yuǎn)沙流水組碎屑巖主量元素構(gòu)造背景判別(據(jù)文獻(xiàn)[24],圖例同圖4)a大洋島弧;b大陸島弧;c活動(dòng)大陸邊緣;d被動(dòng)大陸邊緣Fig.1 0 The tectonic setting discrimination ofmajor elements of clastic rocks from the Shaliushui Formation, Jingyuan,Gansu Province(after the reference[24],Symbols as in Fig.4 )

圖11 甘肅靖遠(yuǎn)沙流水組碎屑巖Th-Co-Zr/10(A)、Th-Sc-Zr/10(B)、La-Th-Sc(C)構(gòu)造環(huán)境判別圖據(jù)文獻(xiàn)[25],圖例同圖4)a.大洋島弧;b.大陸島弧;c.活動(dòng)大陸邊緣;d.被動(dòng)大陸邊緣Fig.1 1 Th-Co-Zr/10(A),Th-Sc-Zr/10(B)and La-Th-Sc(C)tectonic discrimination of clastic rocks from the Shaliushui Formation,Jingyuan,Gansu Province(after the reference[25]、Symbols as in Fig.4 )

來自下志留統(tǒng)的碎屑鋯石年代學(xué)研究表明,早志留世,中祁連地塊與華北板塊在靖遠(yuǎn)一帶已經(jīng)碰撞,而且來自中祁連地塊和華北板塊的沉積物覆蓋了北祁連島弧,導(dǎo)致靖遠(yuǎn)一帶同造山盆地內(nèi)沒有來自北祁連奧陶紀(jì)島弧的沉積物。與此同時(shí),造山帶西部肅南一帶剛剛碰撞,形成同碰撞的鹿角溝組礫巖。沉積物除來自中祁連地塊和華北板塊以外,北祁連島弧也提供了大量碎屑[1]。因此,中祁連地塊與華北板塊的碰撞呈現(xiàn)出“東早西晚”的“斜向碰撞、不規(guī)則邊緣碰撞”的特點(diǎn)。而由于這種獨(dú)特的碰撞方式也導(dǎo)致北祁連造山帶在早、中泥盆世的隆升具有“東強(qiáng)西弱”的不均一造山特點(diǎn)[30]以及形成了北祁連南緣大型韌性走滑剪切帶[31]。來自中、下泥盆統(tǒng)老君山組礫巖的地球化學(xué)和碎屑鋯石年代學(xué)研究表明,北祁連西部肅南一帶沉積物主要來自大陸邊緣弧和殘余島弧[2,30],東部靖遠(yuǎn)一帶沉積物主要來自北祁連島弧、中祁連地塊和華北板塊[30]。造山帶東部靖遠(yuǎn)一帶中、下泥盆統(tǒng)中北祁連島弧物質(zhì)的出現(xiàn)表明,早、中泥盆世時(shí)期強(qiáng)烈的造山作用使早志留世時(shí)被沉積物覆蓋的北祁連島弧卷入了造山帶,并在隆升過程中被剝露出來。本文研究結(jié)果顯示,晚泥盆世沙流水組沉積物主要來自活動(dòng)大陸邊緣以及大陸島弧環(huán)境下形成的基性和酸性巖石,并伴有大洋島弧和來自被動(dòng)大陸邊緣的古老沉積物。

區(qū)域地質(zhì)資料顯示,靖遠(yuǎn)一帶南側(cè)緊鄰北祁連奧陶紀(jì)石灰溝島弧,該島弧是北祁連島弧中發(fā)育最為完整的一段,從島弧發(fā)育初期的拉斑玄武巖系到中期的鈣堿性火山巖系,再到晚期的堿性火山巖,同時(shí)伴生與島弧相關(guān)的火山碎屑巖等[27],形成時(shí)代集中在486～445 Ma[27,32]。另外,在祁連山東段也分布有多個(gè)與北祁連洋殼俯沖消減相關(guān)的島弧型火山巖和花崗巖,年齡集中在470～440 Ma[33～37]。根據(jù)沉積物地球化學(xué)特征推斷,上述島弧火山巖以及花崗巖可能是沙流水組沉積物中大陸島弧沉積物的主要源區(qū)。

活動(dòng)大陸邊緣構(gòu)造背景下的沉積物主要包括上隆基底的花崗巖、花崗片麻巖和硅質(zhì)火山巖以及陸緣弧[38]。中祁連地塊基底玄武巖、花崗巖及花崗片麻巖年齡集中在940～700 Ma[39～43],根據(jù)志留系-中、下泥盆統(tǒng)碎屑巖物源分析結(jié)果[1,30],從晚奧陶世-早、中泥盆世,中祁連地塊已與華北板塊碰撞,中祁連地塊東段并已經(jīng)完全卷入造山帶,因而活動(dòng)大陸邊緣沉積物則可能反映了來自中祁連地塊變質(zhì)基底的貢獻(xiàn)。

北祁連島弧南側(cè)緊鄰北祁連俯沖雜巖帶,該雜巖帶中包含具大洋及洋島性質(zhì)的蛇綠巖帶[26,44,45],時(shí)代集中在550～490 Ma[46,47]。其構(gòu)造性質(zhì)一致于沉積物中少量的大洋島弧物質(zhì),因而北祁連俯沖雜巖帶也是沙流水組的物源區(qū)。如果這一推論正確的話,表明北祁連俯沖雜巖帶也被卷入了造山帶。

研究證明,北祁連洋殼在早、中奧陶世俯沖于華北板塊之下[3,27～29,48],因此華北板塊具有被動(dòng)大陸邊緣的構(gòu)造屬性。因此本文地球化學(xué)物源分析中所獲的來自被動(dòng)大陸邊緣的古老沉積物可能反映了來自華北板塊的物質(zhì)。

綜上所述,沙流水組碎屑巖沉積物主要來自于盆地南側(cè)卷入北祁連造山帶的石灰溝島弧以及相關(guān)的島弧型花崗巖、中祁連地塊基底和俯沖雜巖帶,并伴有來自華北板塊的古老的再旋回沉積物。

4.2 沙流水組的構(gòu)造意義

北祁連加里東造山帶奧陶紀(jì)-泥盆紀(jì)經(jīng)歷了從俯沖-碰撞的構(gòu)造演化[3,26～29]。相鄰?fù)焐脚璧匾步?jīng)歷了從弧后盆地到前陸盆地的轉(zhuǎn)換過程[5]。早、中泥盆世盆地處于造山帶強(qiáng)烈隆升的磨拉石盆地階段,中、下泥盆統(tǒng)老君山組作為該期的沉積記錄被保存下來。上泥盆統(tǒng)沙流水組被認(rèn)為是主造山期后再一次抬升形成的山間盆地磨拉石沉積[6,49]。根據(jù)對(duì)北祁連造山帶從晚奧陶紀(jì)-泥盆紀(jì)同造山盆地碎屑巖的物源分析及與造山過程的關(guān)系,結(jié)合近來的巖石學(xué)研究,認(rèn)為沙流水組碎屑巖應(yīng)是北祁連加里東造山帶從擠壓-伸展轉(zhuǎn)換時(shí)期的沉積記錄。

一個(gè)完整的碰撞造山過程包含三個(gè)相互聯(lián)系的階段[50]:(1)碰撞引起的橫向縮短和陸內(nèi)俯沖導(dǎo)致地殼加厚、地表隆升及地殼根和巖石圈根的形成;(2)地殼根(山根)的榴輝巖相變質(zhì)作用;(3)地殼根或巖石圈地幔的拆沉作用/造山帶的伸展塌陷和莫霍面再平衡(山根的毀壞)[51]。

圖12 北祁連造山帶造山作用與前陸盆地演化Fig.1 2 The orogeny and evolution of foreland basin of the North Qilian Orogenic Belt

前陸盆地主要形成于擠壓造山的動(dòng)力學(xué)背景之下[52],主要對(duì)應(yīng)于造山過程的前兩個(gè)階段,其構(gòu)造演化也可劃分為三個(gè)階段:A早期演化階段,造山帶主體位于海平面之下,逆沖載荷導(dǎo)致地殼的撓曲,此時(shí)沉積處于滯后狀態(tài),因而前陸盆地接受深水沉積;B出露于地表的造山帶達(dá)到平衡狀態(tài),風(fēng)化剝蝕雖強(qiáng),沉積載荷致使前陸盆地橫向演化,水深逐漸變淺;C前陸盆地被沉積物填滿[53]。第一階段可稱為復(fù)理石階段,以深水濁流沉積為主;第二階段可稱為海相磨拉石階段,以淺海-濱海沉積為主;第三階段被稱為陸相磨拉石階段,以陸相沉積為主。由下向上表現(xiàn)出沉積物粒度變粗、水體變淺的進(jìn)積序列[54]。

研究認(rèn)為沙流水組為北祁連加里東造山帶從擠壓-伸展轉(zhuǎn)換時(shí)期的沉積記錄的認(rèn)識(shí)主要基于以下兩點(diǎn):

第一,北祁連造山帶東、西兩段的碰撞過程雖然不同步,但同造山盆地內(nèi)上奧陶統(tǒng)-中下泥盆統(tǒng)表現(xiàn)出明顯的從復(fù)理石-磨拉石轉(zhuǎn)換的沉積特征[5,55]。

造山帶東段:上奧陶統(tǒng)可分為天祝組/古浪組、斯家溝組和斜壕組。天祝組/古浪組以紫紅色、黃綠色砂礫巖、砂巖、粉砂巖,底部見厚達(dá)百米的礫巖,礫石欠磨圓、分選;斯家溝組以灰綠色薄層粉砂巖、泥質(zhì)粉沙巖、泥巖夾薄層灰?guī)r為主;斜壕組為黑色頁巖為主,夾薄層灰?guī)r。志留系僅發(fā)育下志留統(tǒng)骯臟溝組,中、上志留統(tǒng)缺失。骯臟溝組巖性為灰綠色薄-中厚層砂巖,可見完整或不完整的鮑馬序列,為典型的復(fù)理石相深水濁流沉積,巖層發(fā)生褶皺彎曲。泥盆系分為中、下泥盆統(tǒng)老君山組和上泥盆統(tǒng)沙流水組。老君山組在平面上呈扇狀分布,沉積相為山麓相沖積扇,是北祁連造山帶強(qiáng)烈隆升時(shí)期的陸相磨拉石建造。巖性為紫紅色塊狀礫巖,礫石欠磨圓、分選,礫石成分包括花崗巖、玄武巖、火山碎屑巖、石英巖和大理巖等造山帶物源。礫巖底面多見沖刷面、局部礫石呈疊瓦狀排列,礫巖、砂礫巖、砂巖主要發(fā)育塊狀層理,砂巖中可見平行層理和槽狀交錯(cuò)層理,頂部砂巖局部見丘狀交錯(cuò)層理。古流方向以北東15°～45°為主。

造山帶西段:上奧陶統(tǒng)主要為巨厚層的碳酸鹽巖、火山巖和火山碎屑巖。志留系發(fā)育完全,劃分為下志留統(tǒng)鹿角溝組和骯臟溝組,中志留統(tǒng)泉腦溝山組,上志留統(tǒng)旱峽組。鹿角溝組以灰綠色中-厚層礫巖、砂礫巖為主,礫石欠磨圓、分選。骯臟溝組巖性類似于東段,為典型的復(fù)理石相深水濁流沉積,但巖層平直,未發(fā)生褶皺彎曲。中志留統(tǒng)泉腦溝山組下段為灰綠色中-厚層細(xì)砂巖、粉砂巖、粉砂質(zhì)泥巖。該段為不完整的鮑馬序列組成的濁流沉積。上段為灰色中厚層灰?guī)r、泥灰?guī)r夾灰綠色中厚層細(xì)砂巖、粉砂巖及泥巖,砂巖中發(fā)育浪成波痕和交錯(cuò)層理。表明泉腦溝山組上段為淺海相沉積到近岸臨濱沉積,反映一個(gè)海退的過程,沉積環(huán)境逐漸由淺海變?yōu)闉I海環(huán)境。旱峽組主要為紫紅色中-厚層砂、泥巖,內(nèi)含豐富的浪成波痕和交錯(cuò)層理、潮汐層理、泥裂等淺水沉積構(gòu)造,代表淺海、濱海潮坪環(huán)境的背景沉積,反映進(jìn)一步的海退過程。西段泥盆系主要發(fā)育中下泥盆統(tǒng)老君山組,巖性特征類似于東段,為山麓相山前盆地或山間盆地的沖積扇沉積。

綜上所述,北祁連造山帶東段上奧陶統(tǒng)-中、下泥盆統(tǒng)和西段下志留統(tǒng)-中、下泥盆統(tǒng)的沉積物表現(xiàn)出粒度變細(xì)再變粗、水體變深再變淺的進(jìn)積序列,呈現(xiàn)出從碰撞擠壓(上奧陶統(tǒng)天祝組和下志留統(tǒng)鹿角溝組)-海相復(fù)理石(上奧陶統(tǒng)斯家溝組和斜壕組-中下志留統(tǒng)骯臟溝組和泉腦溝山組)-海相磨拉石(上志留統(tǒng)旱峽組)-陸相磨拉石(中、下泥盆統(tǒng)老君山組)的前陸盆地沉積序列。而上泥盆統(tǒng)沙流水組主要發(fā)育于造山帶東段,底部為一套礫石層,向上以灰白色含礫粗砂巖、中砂巖和細(xì)砂巖與紅色粉砂巖、泥質(zhì)粉沙巖、泥巖互層為主,其中灰白色砂巖粒度向上逐漸變細(xì),紅色泥質(zhì)粉沙巖轉(zhuǎn)變?yōu)槟鄮r。砂巖中具浪成波痕和浪成交錯(cuò)層理、泥裂等淺水沉積構(gòu)造,粉砂巖、頁巖中主要為水平層理,沉積相研究屬湖泊相沉積,并夾有湖相震積巖和風(fēng)暴巖事件沉積[7,8]。與老君山組相比,沙流水組內(nèi)部巖性由下向上則表現(xiàn)出粒度變細(xì)、水體加深的退積序列,直至以平行不整合與石炭紀(jì)海相碳酸鹽巖地層接觸[56],總體反映了一個(gè)從大規(guī)模海退轉(zhuǎn)向海進(jìn)的沉積演化序列(圖12)。

第二,火山巖石學(xué)研究表明,祁連山東段同碰撞花崗巖侵位年齡在(454±5)～(446±1)Ma[37],西段同碰撞花崗巖侵位年齡在(444±17)Ma左右[57],顯示“東早西晚”的碰撞順序。北祁連地區(qū)反映加厚的下地殼成因的埃達(dá)克巖形成時(shí)間在 450～430 Ma[58],與柴北緣大陸型榴輝巖變質(zhì)時(shí)間(457～420 Ma)[59,60,61,62]比較一致,如此一方面反映了由于柴達(dá)木-中祁連與阿拉善之間的碰撞造山使地殼加厚,發(fā)生榴輝巖相變質(zhì)作用,另一方面似乎反映了柴達(dá)木板塊陸殼發(fā)生榴輝巖相變質(zhì)時(shí)已經(jīng)下插到了北祁連地區(qū),隨后折返回柴北緣一帶,是否如此?尚需進(jìn)一步考證。然而上述巖漿、變質(zhì)事件發(fā)生的順序完全可與碰撞造山過程的前兩個(gè)階段對(duì)比,而與第三階段相對(duì)應(yīng)的伸展垮塌作用,則主要反映在北祁連造山帶大量的碰撞后花崗巖漿作用上,如形成于383 Ma左右的黃羊河巖體[33],其形成時(shí)代屬于晚泥盆世。

上述沉積學(xué)及巖石學(xué)特征表明,晚奧陶世-早中泥盆世是北祁連造山帶碰撞造山時(shí)期,晚泥盆世是造山帶強(qiáng)烈擠壓之后轉(zhuǎn)向伸展垮塌的時(shí)期,而上泥盆統(tǒng)沙流水組是北祁連加里東期造山帶由擠壓向伸展轉(zhuǎn)換的沉積記錄。

因此,結(jié)合已有研究,可總結(jié)北祁連加里東造山過程如下:

北祁連造山帶由于“東早西晚”的“斜向碰撞、不規(guī)則邊緣碰撞”作用,造成前陸盆地發(fā)育也出現(xiàn)東部早于西部的特征。晚奧陶世早期,中祁連地塊與華北板塊首先在東部武威一帶發(fā)生碰撞,形成上奧陶統(tǒng)天祝組和古浪組礫巖。隨著中祁連地塊向華北板塊擠壓,兩個(gè)板塊的碰接部位巖石圈撓曲下沉,形成前陸盆地,但由于沉積物供給滯后,盆地主要接受深水濁流沉積,形成上奧陶統(tǒng)斯家溝組、斜壕組和下志留統(tǒng)骯臟溝組。早志留世初期,中祁連地塊與華北板塊在西部肅南一帶也發(fā)生碰撞,形成鹿角溝組礫巖,隨后接受下志留統(tǒng)骯臟溝組和中志留統(tǒng)泉腦溝山組復(fù)理石沉積。晚志留世,隨著造山帶向華北板塊推進(jìn),造山帶露出水面,風(fēng)化、剝蝕作用加強(qiáng),沉積物供給增多,水體逐漸變淺,形成上志留統(tǒng)旱峽組海相磨拉石沉積。泥盆紀(jì)早期,由于“東早西晚”的“斜向碰撞、不規(guī)則邊緣碰撞”作用,北祁連造山帶沿走向快速不均一隆升,導(dǎo)致“東強(qiáng)西弱”的造山作用[30],造山帶東部向華北板塊腹地侵進(jìn)的程度遠(yuǎn)大于西部,造成東部前陸盆地內(nèi)早期的沉積物部分被移出盆地,因而在造山帶東部缺失中、上志留統(tǒng)。同時(shí),前陸盆地被迅速充填,形成巨厚的陸相磨拉石建造——中、下泥盆統(tǒng)老君山組。此時(shí)造山帶隆升達(dá)到最大高度。晚泥盆世,由于造山帶東段隆起高于西段,因而伸展垮塌作用開始于東段,在造山帶東段形成沙流水組,以角度不整合覆蓋于前期形成的地層之上,而西段可能由于隆起幅度較小,因而伸展垮塌作用較弱,僅在靠近東段的局部地區(qū)形成上泥盆統(tǒng)碎屑沉積。自此,自晚奧陶世開始的加里東期造山活動(dòng)結(jié)束,中祁連地塊與華北板塊縫合在一起,來自秦嶺海域的海水侵入本區(qū),形成下石炭統(tǒng)碳酸鹽沉積。

5 結(jié)論

(1)北祁連造山帶東段靖遠(yuǎn)一帶上泥盆統(tǒng)沙流水組碎屑巖源巖是卷入造山帶的北祁連島弧、中祁連地塊、俯沖雜巖帶中的酸性巖石和基性巖石及少量花崗巖,源巖在沉積前經(jīng)歷了中等強(qiáng)度的化學(xué)風(fēng)化作用(CIA:74～81),沉積搬運(yùn)過程還有少量來自華北板塊的、經(jīng)過沉積分選和再循環(huán)的物質(zhì)加入。

(2)北祁連造山帶上奧陶統(tǒng)-中、下泥盆統(tǒng)沉積演化特征及區(qū)域巖石學(xué)研究證明:晚奧陶世-早中泥盆世是北祁連造山帶碰撞造山時(shí)期,而晚泥盆世是造山帶強(qiáng)烈擠壓之后轉(zhuǎn)向伸展垮塌的時(shí)期,上泥盆統(tǒng)沙流水組則是北祁連加里東期造山帶由擠壓向伸展轉(zhuǎn)換的沉積記錄。

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Geochem istry and Provenance of the Upper Devonian Shaliushui Formation in Jingyuan County,Gansu Province

XU Ya-jun1,2DU Yuan-sheng1,2YANG Jiang-hai2CHEN Jia-cai3
(1.Key Laboratory of Biology and Environment Geology of Education M inistry,W uhan 430074; 2.Faculty of Earth Science,China University of Geosciences,W uhan 430074; 3.103 Geological Team,Guizhou Province Bureau of Geology and M ineral Exp loration,Tongren,Guizhou 554300)

Sedimentary geochemistry of detrital rocks from the upper Devonian Shaliushui Formation in Jingyuan, Gansu suggest that sandstone contain higher SiO2and plagioclase content thanmudstone;Mudstone,however,occupy higher femic mineral and mineral K-bearing than sandstone.All samples are depleted in Nb,Ta and Sr elements.Some of sandstones are enriched with Zr and Hf elements,with strong negative Eu anomalies,and others are depleted in Zr and Hf without negative Eu anomalies.Allmudstones display enrichment of Ni and Cr elements,with strong negative Eu anomalies.Characteristics of samples from the Shaliushui Formation show that rocks in the source consist of the basic and acidic rockswith ancient sediments recycled from passive continentalmargin.And source underwent middle intense chemicalweathering(CIA:74～81).ectonic setting discriminations suggest that rocks in the source weremainly formed in the continental arc and active continentalmargin,withminor the oceanic arc and passive continentalmargin.Evidences combining tectonic discriminations and comparisonswith potential provenances revealed that sediments in the Shaliushui Formation were derived mainly from the North Qilian Continental arc and the Central Qilian Block,with minor the North Qilian subducted complex and North China Plate.Integrated in the sedimentary evolution from the upper Ordovician to Lower and Middle Devonian and igneous rocks,the North Qilian Orogenic Belt completed the transfer from compression to extension during the Late Devonian,which indicate that the upper Devonian Shaliushui Formation is the sedimentary record of this process.

Shaliushui Formation;sedimentary geochemistry;provenance;Gansu province;the North Qilian Orogenic Belt

徐亞軍 男 1978年出生 博士 沉積學(xué) E-mail:xuyajun19@163.om

杜遠(yuǎn)生 E-mail:duyuansheng126@126.om

595

A

1000-0550(2011)01-0041-14

①國家自然科學(xué)基金項(xiàng)目(批準(zhǔn)號(hào):41002030,40672080和40621002),“111”項(xiàng)目和中國地質(zhì)大學(xué)(武漢)優(yōu)秀青年教師基金(編號(hào): CUGQNL0832)聯(lián)合資助。

2009-11-03;收修改稿日期:2010-01-19

附表1 甘肅靖遠(yuǎn)沙流水組碎屑巖地球化學(xué)數(shù)據(jù)Table1 GeochemicaldataofclasticrocksfromtheShaliushuiFormationinJingyuancounty,Gansuprovince