于亮亮, 許德如, 王智琳, 吳傳軍, 侯茂洲

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海南島西部石炭系砂巖地球化學(xué)及Sm-Nd同位素特征: 對(duì)源區(qū)特征和沉積環(huán)境的制約

于亮亮1, 2, 許德如1*, 王智琳3, 吳傳軍1, 2, 侯茂洲1, 2

(1. 中國(guó)科學(xué)院 廣州地球化學(xué)研究所 礦物學(xué)與成礦學(xué)重點(diǎn)實(shí)驗(yàn)室, 廣東 廣州?510640; 2. 中國(guó)科學(xué)院大學(xué), 北京?100049; 3. 中南大學(xué) 地球科學(xué)與信息物理學(xué)院 有色金屬成礦預(yù)測(cè)教育部重點(diǎn)實(shí)驗(yàn)室, 湖南 長(zhǎng)沙?410083)

沉積巖化學(xué)組分和同位素組成特征能有效反映碎屑沉積巖的源區(qū)和沉積環(huán)境。海南島西部石碌地區(qū)出露下石炭統(tǒng)南好組和上石炭統(tǒng)青天峽組砂巖, 這兩套層序主要由海相-淺海相、淺變質(zhì)陸緣碎屑巖組成。為探討它們的物質(zhì)來(lái)源和沉積環(huán)境, 以期對(duì)海南島晚古生代的構(gòu)造演化提供制約, 對(duì)這兩套砂巖進(jìn)行了主元素、微量元素含量(含稀土元素)和Sm-Nd同位素組成分析。分析結(jié)果表明, 石炭系砂巖均具有與澳大利亞后太古宙平均頁(yè)巖(PAAS)接近的主元素和微量元素含量及配分形式; REE球粒隕石標(biāo)準(zhǔn)化分布模式均顯示出LREE富集的右傾形。南好組和青天峽組砂巖的La/Yb值分別為9.17~16.65(D12為33.9)和8.88~12.10,Eu值分別為0.55~0.68和0.45~0.72。Sm-Nd同位素分析顯示, 南好組砂巖Nd(330Ma)值和模式年齡分別在?12.37 ~ ?3.07(平均值?8.86)和1339~1937 Ma之間, 青天峽組砂巖Nd(310Ma)值和模式年齡分別在?8.35 ~ ?0.35(平均值?5.15)和1136~1651 Ma之間。南好組砂巖具有較高的Th/Sc值, 分布在0.58 ~ 2.38之間(平均值1.58), 絕大部分樣品Th/Sc值大于1, 顯示沉積物再循環(huán)的特征。青天峽組砂巖的Th/Sc值分布在0.61～1.07之間(平均值0.86), 顯示物源中火山物質(zhì)的增加。Co/Th-La/Sc和Nd-Th/Sc圖解表明, 南好組砂巖物源以上地殼物質(zhì)再循環(huán)為主, 年輕火山巖物質(zhì)貢獻(xiàn)不明顯, 物源更為分散; 青天峽組砂巖的物源以上地殼火山巖為主。從Ti/Zr-La/Sc和Th-Sc-Zr/10構(gòu)造判別圖解可以發(fā)現(xiàn), 下石炭統(tǒng)南好組砂巖落在被動(dòng)大陸邊緣和大陸島弧區(qū)域內(nèi); 上石炭統(tǒng)青天峽組砂巖分布在大陸島弧或大陸島弧和被動(dòng)大陸邊緣, 相對(duì)更靠近大陸島弧環(huán)境。根據(jù)上述地球化學(xué)對(duì)比研究推斷, 自早石炭世至晚石炭世, 海南島石碌地區(qū)處于被動(dòng)大陸邊緣向大陸島弧環(huán)境演化的環(huán)境。在石炭紀(jì), 海南島處于被動(dòng)大陸邊緣向大陸島弧環(huán)境轉(zhuǎn)換的環(huán)境, 這一構(gòu)造背景與印支板塊向華南板塊俯沖和古特提斯洋消減這一構(gòu)造事件相吻合。

石炭系; 地球化學(xué); Sm-Nd同位素; 物源; 海南島

0 引 言

海南島位于華夏板塊的西南緣, 以瓊州海峽與華南大陸相連[1]。大地構(gòu)造位置上, 海南島位于太平洋板塊、印度-澳大利亞板塊和歐亞板塊的結(jié)合部位, 具有特殊的構(gòu)造環(huán)境和復(fù)雜的大地構(gòu)造演化歷史。關(guān)于海南島晚古生代的大地構(gòu)造格局, 國(guó)內(nèi)外學(xué)者進(jìn)行了大量研究, 并對(duì)海南島古生代的大地構(gòu)造屬性提出了各種不同的看法。汪嘯風(fēng)等[2]根據(jù)晚古生代古生物組合認(rèn)為, 海南島屬于華夏-特提斯動(dòng)物區(qū)或華夏植物區(qū)的一部分, 主張晚古生代海南島屬于華夏構(gòu)造域范圍。夏邦棟等[3?4]認(rèn)為分布于海南西部軍營(yíng)一帶上古生界地層中的基性火山巖具有雙峰式火山巖的特點(diǎn), 認(rèn)為晚古生代海南島處于大陸裂谷環(huán)境。李獻(xiàn)華等[5]和Li.[6]通過(guò)對(duì)出露于邦溪-晨星農(nóng)場(chǎng)一帶的晚古生代變質(zhì)基性火山巖的研究, 提出邦溪-晨星農(nóng)場(chǎng)一線(xiàn)很可能是古特提斯洋的最東延伸部分的認(rèn)識(shí)。晚古生代海南島的大地構(gòu)造屬性一直是亟待解決的科學(xué)問(wèn)題之一[7]。海南島古生代地層出露面積有限, 不足全島面積的18%, 加上第四系覆蓋及后期構(gòu)造的疊加等, 這給海南島晚古生代沉積環(huán)境和構(gòu)造背景的研究帶來(lái)一定困難[1]。以往的工作都集中在對(duì)巖相學(xué)和古生物或者是出露的變質(zhì)基性巖體的研究方面, 缺乏晚古生代沉積地層的地球化學(xué)和同位素方面的制約。海南島西部的石碌地區(qū)出露較完整的晚古生代地層(圖1)[8], 其中包括下石炭統(tǒng)南好組和上石炭統(tǒng)青天峽組、下二疊統(tǒng)峨查組、峨頂組和南龍組(圖2a)[8]。這套晚古生代地層, 沉積連續(xù)、厚度大, 露頭良好, 是海南島晚古生代地層的代表性剖面[1]。這為探討海南島晚古生代大地構(gòu)造背景和構(gòu)造演化提供了良好的條件。

利用碎屑沉積巖的地球化學(xué)特征來(lái)判別其沉積物來(lái)源和沉積環(huán)境已獲得廣泛的應(yīng)用[9?18]。結(jié)合碎屑巖的主元素及微量元素特征, 可以有效識(shí)別出大洋島弧、大陸島弧、活動(dòng)大陸邊緣或被動(dòng)大陸邊緣等構(gòu)造環(huán)境[10]。同時(shí), 構(gòu)造環(huán)境控制著沉積盆地的物源、風(fēng)化作用、搬運(yùn)分選作用、成巖及成巖后變質(zhì)作用[15], 因此, 在探討碎屑巖沉積大地構(gòu)造背景同時(shí), 也可對(duì)物源及其經(jīng)歷的風(fēng)化作用、搬運(yùn)分選作用和成巖作用進(jìn)行研究。釹同位素可以很好地反映物源性質(zhì)及物源年齡, 并且不容易受到后期變質(zhì)作用的影響, 因而被廣泛用于探討碎屑巖物源[15]。本次研究采取了12個(gè)陸源碎屑巖樣品, 其中7個(gè)采自下石炭統(tǒng)南好組、5個(gè)來(lái)自上石炭統(tǒng)青天峽組, 并對(duì)其進(jìn)行主元素和微量元素(含REE)含量與Sm-Nd同位素組成分析測(cè)試。擬根據(jù)這些樣品地球化學(xué)數(shù)據(jù)和Sm-Nd同位素組成對(duì)海南島晚古生代構(gòu)造環(huán)境和沉積盆地物質(zhì)來(lái)源等進(jìn)行探討, 為海南島晚古生代構(gòu)造屬性提供制約。

圖1 海南島區(qū)域地質(zhì)簡(jiǎn)圖[8]

1 區(qū)域地質(zhì)背景及巖層地質(zhì)特征

海南島是我國(guó)第二大陸緣島, 它經(jīng)歷了多期次復(fù)雜的構(gòu)造運(yùn)動(dòng)[19]。海南島特殊的大地構(gòu)造位置為了解東亞地區(qū)的構(gòu)造演化和華南板塊的形成都具有重要意義。以九所-陵水?dāng)嗔褳榻? 海南島被劃分為南部的三亞地體和北部的五指山地體, 它們分別屬于印支板塊和華南板塊的東南地層大區(qū)[1]。本文研究區(qū)位于石碌Fe-Co-Cu-Au多金屬礦區(qū) (圖1), 區(qū)內(nèi)以產(chǎn)石碌式Fe-Co-Cu礦床而著稱(chēng)[8,19]。石碌礦區(qū)的面積約77 km2 [8], 位于昌江縣城南部, 處于東西向昌江-瓊海斷裂帶和北東向戈枕剪切帶交匯處(圖2a)。海南島內(nèi)發(fā)育4條東西向貫穿全島的斷裂, 從北向南分別是王五-文教斷裂、昌江-瓊海斷裂、尖峰-吊羅斷裂和九所-陵水?dāng)嗔? 同時(shí), 海南島內(nèi)發(fā)育兩條NE-SW向斷裂, 一條是貫穿整海南島的白沙斷裂, 另一條是位于昌江-瓊海斷裂和尖峰-吊羅斷裂之間的戈枕韌性斷裂(圖1)。

海南島內(nèi)火山巖和侵入巖廣泛發(fā)育,占全島面積的60%[1]。加之亞熱帶氣候條件, 海南島內(nèi)巖石風(fēng)化嚴(yán)重、第四系大面積發(fā)育, 地層出露程度較差 (圖1)。石碌礦區(qū)內(nèi)出露的地層相對(duì)齊全, 為研究海南島的構(gòu)造演化提供了良好場(chǎng)所(圖2)。礦區(qū)及外圍出露有中元古代抱板群、中-新元古代石碌群、晚新元古代震旦系石灰頂組、下志留統(tǒng)空村組、下石炭統(tǒng)南好組、上石炭統(tǒng)青天峽組、下二疊統(tǒng)峨查組、下二疊統(tǒng)額頂組、下-上二疊統(tǒng)南龍組[1]。

目前海南島已經(jīng)被確認(rèn)的最古老地層是中元古代的抱板群[20], 主要由泥質(zhì)巖、泥質(zhì)砂巖、薄層狀硅質(zhì)巖組成, 夾少量的硬砂巖, 有基性、中基性巖脈和火山碎屑巖夾層[21]。Li.[22]通過(guò)抱板群中的1450 Ma的花崗巖體的U-Pb年齡譜, 推測(cè)海南島(包括華夏板塊)在Rodinia大陸形成之前很可能是勞倫大陸的一部分。在中元古代末期至新元古代早期, 受四堡造山運(yùn)動(dòng)影響(格林威爾造山運(yùn)動(dòng)同期), 華夏地塊與揚(yáng)子地塊拼合, 形成統(tǒng)一的華南板塊, 并成為Rodinia超大陸的組成部分[8,22]。這一構(gòu)造事件使抱板群發(fā)生了角閃巖相-麻粒巖相的變質(zhì)作用[23?24]。石碌群自下而上被分成6層, 因第6層含有Fe-Co-Cu-Au多金屬礦而聞名于世, 是目前海南島研究程度最高的地層, 更多細(xì)節(jié)可以參照文獻(xiàn)[8,19]。在新元古代四堡造山運(yùn)動(dòng)末期, 石碌群發(fā)生了綠片巖相的變質(zhì), 同時(shí)石灰頂組不整合沉積在石碌群之上[8,19]。

圖2 石碌Fe-Co-Cu礦區(qū)區(qū)域地質(zhì)地形圖[8](a)和采樣地區(qū)剖面圖(b)

海南島中生代受印支-燕山運(yùn)動(dòng)影響, 沉積地層出露不多, 花崗巖大量侵入[1]。海南島中生界地層包括下三疊統(tǒng)峨文嶺組, 下白堊統(tǒng)的六羅村組、湯他大嶺組、嶺殼村組、鹿母灣組, 上白堊統(tǒng)報(bào)萬(wàn)組[1]。這些地層出露面積很小, 其中下三疊統(tǒng)峨文嶺組與抱板群呈不整合接觸, 頂部與印支期花崗巖侵入接觸。下白堊統(tǒng)地層被印支期花崗巖強(qiáng)烈切割, 除鹿母灣組頂部與上白堊統(tǒng)報(bào)萬(wàn)組整合接觸外, 其余地層均未見(jiàn)頂和底, 均與花崗巖侵入接觸。上白堊統(tǒng)報(bào)萬(wàn)組只在長(zhǎng)矛水庫(kù)一帶有出露, 與下伏下白堊統(tǒng)鹿母灣組整合接觸, 頂部與印支期花崗巖侵入接觸。海南島出露大面積新生代的地層, 主要分布在?？诘貙有^(qū)之內(nèi)。中生代以來(lái)海南島及華南東部大陸受太平洋板塊和印度板塊俯沖影響, 印支板塊碰撞引起的走滑斷層, 可能在新生代將海南島從華南大陸分離開(kāi)來(lái)[2]。

2 樣品和分析方法

本次研究的12個(gè)樣品分別采自石碌礦區(qū)東部下石炭統(tǒng)南好組(7個(gè))和上石炭統(tǒng)青天峽組(5個(gè)), 采樣剖面如圖2b所示。在分析測(cè)試時(shí), 首先將新鮮砂巖樣制成200目的粉末樣。粉末樣品在105 ℃烘箱里烘干12 h。然后進(jìn)行主元素和微量元素含量分析及釹同位素組成分析。主元素在測(cè)試時(shí)先將0.5 g左右粉末樣與7倍的Li2B4O7混合, 采用飛利浦PW1500進(jìn)行玻璃片熔樣, 熔好玻璃片采用X射線(xiàn)熒光光譜(XRF)進(jìn)行測(cè)試。XRF主元素分析精度優(yōu)于1%, 并且以氧化物的形式進(jìn)行表示。微量元素采用PE-Elan 6000型電感耦合等離子體質(zhì)譜儀(ICP-MS)進(jìn)行測(cè)試, 采用GSR1花崗巖標(biāo)樣和GRS3玄武巖標(biāo)樣, 微量元素的分析精度優(yōu)于5%。上述實(shí)驗(yàn)都在中國(guó)科學(xué)院廣州地球化學(xué)研究所同位素地球化學(xué)國(guó)家重點(diǎn)實(shí)驗(yàn)室完成, 主元素分析過(guò)程和方法參照文獻(xiàn)[32?33], 微量元素分析過(guò)程和方法參照文獻(xiàn)[34?36]。對(duì)D11樣品外的其余樣品進(jìn)行了釹同位素分析。在釹同位素分析同時(shí)使用W-2a和BHVO國(guó)際標(biāo)樣作為外標(biāo)參考, 使用VG-354表面熱電離同位素質(zhì)譜儀進(jìn)行樣品測(cè)定。釹同位素具體分析方法和具體過(guò)程參考Halliday.[37]和梁細(xì)榮等[38]。分析過(guò)程采用國(guó)際標(biāo)樣JNdi-1 (推薦值:143Nd/144Nd= 0.512115±7), 測(cè)得國(guó)際標(biāo)樣的143Nd/144Nd值為0.512123±10 (2 SD)。

3 分析結(jié)果

3.1 主元素

海南島石炭系碎屑砂巖樣品的主元素和微量元素含量如表1所示。南好組砂巖的SiO2含量在64.44%~89.30%之間, 平均值為76.54%; 青天峽組砂巖的SiO2含量分布在55.21%～78.38%之間, 平均值為66.10%。南好組砂巖的Al2O3含量分布在4.35%～17.90%之間, 平均值為10.28%; 青天峽組砂巖的Al2O3含量分布在7.87%～17.20%之間, 平均含量12.86%。南好組的SiO2/Al2O3和K2O/Na2O值分布在3.6～20.54和0.96～54.5之間, 青天峽組SiO2/Al2O3和K2O/Na2O值分布在3.21～9.96和0.50～2.32之間。下石炭統(tǒng)南好組TiO2、Fe2O3、MnO和CaO的含量從南好組到青天峽組有明顯的升高。相比之下, MgO、Na2O和K2O含量從南好組到青天峽組呈現(xiàn)明顯降低。

相比于澳大利亞后太古宙平均頁(yè)巖(PAAS)[14], 南好組碎屑砂巖富集SiO2, 虧損TiO2、Al2O3、Fe2O3、MnO、MgO、K2O和P2O5, 而CaO和Na2O含量則與PAAS相當(dāng)(表1); 青天峽組砂巖明顯富集CaO, 虧損K2O, TiO、Al2O3、Fe2O3、MnO、MgO和P2O5, SiO2含量與PAAS相當(dāng)(表1)。根據(jù)Herron[40]的碎屑巖砂巖主元素分類(lèi)圖解, 南好組砂巖樣品主要落在巖屑砂巖和長(zhǎng)石砂巖范圍內(nèi)。青天峽組樣品落在巖屑砂巖和硬砂巖區(qū)域內(nèi), 其中樣品D54、D56和D57的CaO分別為14.88%、14.81%和7.70%, 為鈣質(zhì)硬砂巖(圖3)。

3.2 微量元素

海南島石炭系砂巖樣品的微量元素含量如表1所示。從原始地幔標(biāo)準(zhǔn)化蛛網(wǎng)圖可以發(fā)現(xiàn), 這些不同時(shí)代碎屑巖的微量元素分布特征與PAAS相似(圖4a)。相比于PAAS, 石炭系砂巖相對(duì)虧損U、Th、Nb和Ta這些高場(chǎng)強(qiáng)元素。球粒隕石標(biāo)準(zhǔn)化REE分布模式顯示(圖4b), 石炭系砂巖均表現(xiàn)LREE富集而HREE虧損、中等的Eu異常、無(wú)Ce異常。其中南好組砂巖的REE含量分布在112.3～307.1 μg/g之間, 平均含量為171.3 μg/g, LREE/HREE值分布在6.1～9.9之間(D12為15.3), (La/Yb)N值分布在9.2～16.6之間(D12為33.9),Eu值為0.55～0.68(表1)。青天峽組砂巖的REE含量分布在108.1～205.7 μg/g之間, 平均含量為143.0 μg/g, LREE/HREE值分布在5.6～8.5之間, 平均值為7.2, (La/Yb)N值分布在8.9～12.1之間, 平均值為10.6,Eu值為0.45～0.72, 平均值為0.64(表2)(圖4b)[39]。

圖3 石炭系砂巖的lg(Na2O/K2O)-lg(SiO2/Al2O3)分類(lèi)圖解(據(jù)Herron[40])

3.3?Sm-Nd同位素

南好組和青天峽組砂巖樣品的Sm-Nd同位素?cái)?shù)據(jù)如表3所示, 在計(jì)算模式年齡時(shí), 對(duì)于147Sm/144Nd值在0.10～0.13之間的樣品采用虧損地幔模式年齡, 對(duì)于147Sm/144Nd值大于0.13或者小于0.10的樣品采用二階段模式年齡。Nd()、DM和DM2計(jì)算公式和引用參數(shù)均在表后進(jìn)行標(biāo)注。南好組砂巖樣品的147Sm/144Nd值分布范圍為0.099570～0.130881(D10樣品147Sm/144Nd值為0.130881), 平均值為0.113817, 與大陸地殼平均147Sm/144Nd值0.118相近[41],143Nd/144Nd值的變化范圍是0.511813~0.512338。在計(jì)算Nd()時(shí)采用的330 Ma作為下石炭統(tǒng)南好組沉積年齡,Nd(330 Ma)的分布范圍為?12.37 ~ ?3.07。南好組砂巖的模式年齡布范圍為1339~1937 Ma。青天峽組砂巖樣品的147Sm/144Nd值分布范圍為0.102425～0.139967 (D45為0.139967),143Nd/144Nd值相對(duì)均一, 變化范圍是0.512036~0.512469。采用310 Ma的沉積年齡, 計(jì)算得出其Nd(310 Ma)的分布范圍為?8.35 ~ ?0.35。青天峽組模式年齡分布范圍為1136~1651 Ma, 其顯示一個(gè)更為年輕的模式年齡。上述地層由于目前尚沒(méi)有精確的年代學(xué)數(shù)據(jù), 其沉積成巖年齡采用的是古生物和地層學(xué)估算的地層年齡[1], 但對(duì)其計(jì)算結(jié)果沒(méi)有決定性影響。

圖4?石炭系砂巖微量元素原始地幔標(biāo)準(zhǔn)化蛛網(wǎng)圖(a)和稀土元素球粒隕石標(biāo)準(zhǔn)化分布模式(b)

球粒隕石數(shù)據(jù)引自Masuda.[39], 原始地幔數(shù)據(jù)引自McDonough.[41]

Chondrite data from Masuda.[39]; primitive mantle data from McDonough.[41]

表1 石碌地區(qū)石炭系砂巖樣品主元素(%)和微量元素(μg/g)含量

Table 1?Major element (%) and trace element concentrations (μg/g) of the Carboniferous sandstones in the Shilu region

樣號(hào)C1nC2q PAASD09D10D11D12D13D14Y63D53D54D55D56D57 SiO262.8071.1065.4889.3080.8064.4486.4578.2467.9955.2178.3864.4264.50 TiO21.000.620.740.320.490.790.500.420.690.700.610.730.68 Al2O318.9013.1815.054.358.4117.905.187.8715.5117.207.8711.8911.81 Fe2O37.234.897.261.853.724.722.533.755.496.364.363.933.89 MnO0.110.060.090.040.040.010.050.080.040.190.060.110.10 MgO2.202.181.980.291.021.611.152.072.530.700.960.910.92 CaO1.300.180.390.100.230.060.392.450.4314.882.4214.8114.70 Na2O1.202.121.790.270.290.090.051.440.730.752.340.080.25 K2O3.702.012.980.831.864.681.051.581.690.380.650.120.14 P2O50.160.110.170.030.120.090.120.120.060.190.130.030.03 LOI1.403.173.622.192.585.272.081.504.443.011.782.552.57 CIA69.3468.7368.5974.0374.2977.0372.7747.9380.2537.4446.7730.3930.16 SiO2/Al2O33.325.394.3520.549.603.6016.679.944.383.219.965.425.46 K2O/Na2O3.080.951.663.076.3854.4919.141.102.320.500.281.560.57 Sc16.0013.3716.564.068.3619.245.334.8412.0418.368.2612.2912.64 V150.089.6118.632.165.3167.349.837.052.4102.452.576.086.2 Cr110.048.550.029.456.8102.557.631.834.9107.856.866.674.5 Co23.002.5911.022.103.604.451.965.3910.916.0010.5211.5912.61 Ni55.0011.4117.799.4822.2932.5419.2915.5811.4322.3622.9620.2722.05 Cu50.0022.3323.9710.1014.5042.458.2410.029.245.358.618.148.93 Zn85.075.374.619.250.9151.677.533.577.429.143.346.650.4 Ga20.0014.9118.195.7712.7327.427.097.8318.0521.338.9415.2517.05 Ge1.601.661.841.481.872.341.431.252.245.411.251.962.03 Rb160.080.7106.741.499.6257.253.046.7108.022.556.48.18.7 Sr200.039.954.711.143.321.514.563.6161.7540.6178.3396.9432.2 Y27.0021.9030.4417.4924.0434.4828.7815.8822.6151.4720.5522.7524.44 Zr210.0132.9170.9303.9288.9174.5931.3150.2215.5194.6484.4251.5248.5 Nb19.008.4210.286.4511.8017.709.927.048.5011.908.9310.2310.82 Cs3.703.214.961.133.2110.041.971.0917.871.5114.020.870.92 Ba5504683601777007972125181003375267153160 La38.0022.7326.2124.9975.8354.0732.0323.7721.4040.8221.9830.2831.64 Ce80.045.653.648.6137.4108.764.346.942.778.344.559.461.8 Pr8.905.666.685.5714.6412.367.445.545.289.055.166.857.18 Nd32.0021.8426.9020.6850.5544.9627.4720.5920.8436.5218.9125.2626.64 Sm5.604.455.823.978.338.045.053.754.228.463.484.284.53 Eu1.100.851.180.771.531.560.870.780.991.320.700.920.97 Gd4.703.925.553.636.767.174.893.384.219.803.423.974.19 Tb0.770.580.870.560.931.060.790.510.651.500.530.610.64 Dy4.403.555.243.204.826.104.782.894.028.623.423.814.04 Ho1.000.741.100.640.911.251.040.600.841.710.750.830.88 Er2.902.223.071.732.443.432.971.612.424.502.182.442.64 Tm0.400.350.460.260.360.530.450.240.390.630.350.400.42 Yb2.802.192.861.552.243.252.901.512.413.862.272.502.64 Lu0.430.350.470.250.350.500.470.240.390.600.380.400.43

(續(xù)表1)

注:Eu=EuN/(SmN×GdN)1/2, REE球粒隕石標(biāo)準(zhǔn)化數(shù)據(jù)引用Masuda.[39], PAAS數(shù)據(jù)引用McLennan[14]

表2 石碌地區(qū)石炭系砂巖樣品稀土元素組成和不同構(gòu)造背景砂巖稀土元素特征[42]

表3 石碌地區(qū)石炭系砂巖Sm-Nd同位素組成

注:Nd(t)=[(143Nd/144Nd)S/(143Nd/144Nd)CHUR–1]×10000, 計(jì)算方法見(jiàn)文獻(xiàn)[43]。虧損地幔模式年齡DM= ln(1+(143Nd/144Nd–0.51315)/ (147Sm/144Nd–0.2137))/0.00000654, 計(jì)算方法參文獻(xiàn)[44]。二階段模式年齡DM2(Ma)計(jì)算公式DM2=1/0.00000654×ln(143Nd/144Nd– (147Sm/144Nd–0.118)×(exp(0.00654×Age/1000)–1)–0.51315)/(0.118–0.2137)), 其中S代表樣品, (143Nd/144Nd)CHUR= 0.512638和(147Sm/144Nd)CHUR= 0.1967, 計(jì)算方法參文獻(xiàn)[43]。?Sm/Nd=147Sm/144Nd/0.1967–1, 計(jì)算方法參文獻(xiàn)[43]

4 討?論

4.1 物源分析

碎屑沉積巖的地球化學(xué)特征受到源區(qū)、源區(qū)風(fēng)化作用、沉積分選作用、成巖作用和成巖后變質(zhì)作用影響[10,12,18, 45,46]。碎屑巖的源區(qū)巖在沉積之前, 會(huì)經(jīng)歷一定程度的風(fēng)化作用?；瘜W(xué)風(fēng)化指數(shù)(CIA)被廣泛用于衡量物源風(fēng)化作用強(qiáng)度[47]。CIA表達(dá)式為CIA = Al2O3/(Al2O3+K2O + Na2O +CaO*), 式中氧化物含量均以分子數(shù)含量表示, 其中CaO*表示硅酸鹽中CaO分子數(shù)含量[15]。下石炭統(tǒng)南好組砂巖CIA值大部分介于69~77之間, 其中Y63為48, 顯示南好組砂巖經(jīng)歷了中等風(fēng)化。青天峽組含有少量碳酸鹽, 其CIA會(huì)明顯降低不能真實(shí)反映其風(fēng)化程度。在風(fēng)化過(guò)程中, Cs、Rb和Ba容易被黏土礦物吸附而富集, Na、Ca 和Sr則相對(duì)容易流失[44]。風(fēng)化過(guò)程中易風(fēng)化礦物的分解, 會(huì)導(dǎo)致SiO2含量的相對(duì)增加。下石炭統(tǒng)南好組砂巖的Rb/Sr、SiO2/Al2O3和K2O/Na2O值分別為0.74～11.95、3.60～20.54和0.96～54.5, 明顯高于上石炭統(tǒng)青天峽組Rb/Sr、SiO2/Al2O3和K2O/Na2O值0.02～0.67、3.21～9.96和0.50～2.32。這說(shuō)明青天峽組砂巖物源的風(fēng)化程度比南好組砂巖物源的風(fēng)化程度低。

從圖5可以看出, 下石炭統(tǒng)南好組砂巖具有再循環(huán)沉積物特征; 上石炭統(tǒng)青天峽組砂巖則以上地殼物質(zhì)為主, 沉積物再循環(huán)物特征不明顯。在巖漿系統(tǒng)中, Th和Zr是典型的高度不相容元素, Sc相對(duì)Th和Zr有較高相容性, 因此Th/Sc和Zr/Sc值會(huì)隨著巖漿分異演化程度而增加[14]。在搬運(yùn)分選過(guò)程中, 鋯石的密度大而容易富集, 因此Zr/Sc值明顯增加, 同時(shí)Th/Sc值增長(zhǎng)相對(duì)較小。南好組砂巖的Th/Sc值分布在0.59～2.38之間, 平均值為1.44, 明顯高于上地殼Th/Sc值(大約為1)[15], 具有沉積物再循環(huán)特征[10]。其Zr/Sc值最小值為9.94, 最大值為174.89, 顯示物源有再循環(huán)物質(zhì)加入。青天峽組砂巖的Th/Sc值分布在0.61～1.07之間, 平均值為0.86, 略低于上地殼Th/Sc值, 其Zr/Sc值分布范圍為10.60～58.64, 推測(cè)其物源以上地殼巖漿巖石為主。

圖5 石碌地區(qū)石炭系砂巖Th/Sc-Zr/Sc圖解[15]

稀土元素由于其穩(wěn)定的地球化學(xué)特征, ?？捎靡灾甘境练e巖的物源特征[14,42]。不同構(gòu)造環(huán)境和物源沉積巖的稀土元素特征[42]與海南島石炭系砂巖的稀土元素特征見(jiàn)表2中所列。表2顯示, 南好組砂巖稀土元素含量相對(duì)略高于青天峽組砂巖, 其分布范圍為115～307 μg/g, La/Yb和LREE/HREE值分別為10.4～16.6(D12為33.9)和6.1～15.3、Eu值為0.55～0.68, 這些特征與古老克拉通物源性質(zhì)較為接近。青天峽組砂巖稀土元素含量相對(duì)均一, 其La (21.4～40.8 μg/g)、Ce (42.7～78.3 μg/g)、REE (110～206 μg/g)相比于南好組砂巖, 其La/Yb (8.8～12.0)和LREE/HREE值(5.6～8.5)明顯偏低, 接近切穿大陸島弧源區(qū)特征, 其Eu值(0.45～0.72)除D54外大部分高于石炭系南好組, 說(shuō)明物源分異程度相對(duì)偏低, 其物源含有切穿的島弧巖漿巖。

1）符合喘證診斷標(biāo)準(zhǔn)，目前處于穩(wěn)定期；2）年齡在30～80歲之間，性別不限；3）患者知情同意，可按研究要求堅(jiān)持檢查和治療及隨訪(fǎng)。

許多微量元素由于具有穩(wěn)定的地球化學(xué)性質(zhì), 為研究源區(qū)類(lèi)型以及構(gòu)造背景提供了的重要工具[48]。Co/Th-La/Sc圖解(圖6)[49]能很好地反映出砂巖的物源特征。

圖6顯示, 下石炭統(tǒng)南好組砂巖的分布范圍比較大, 具有明顯低于上地殼的Co/Th值, 主要位于長(zhǎng)英質(zhì)火山巖-花崗巖這一區(qū)域內(nèi), 顯示其物源是分異良好的上地殼物質(zhì)或這些物質(zhì)經(jīng)過(guò)多次循環(huán)為主。上石炭統(tǒng)青天峽組砂巖分布范圍向安山質(zhì)和玄武質(zhì)物源區(qū)域移動(dòng), Co/Th值分布在上地殼1.27左右范圍, 在長(zhǎng)英質(zhì)火山巖和安山巖之間的區(qū)域, 說(shuō)明島弧物質(zhì)增加。以上說(shuō)明, 從下石炭統(tǒng)南好組砂巖到上石炭統(tǒng)青天峽組砂巖, 再循環(huán)組分的逐漸減少、安山質(zhì)/玄武質(zhì)火山巖物質(zhì)逐漸增加。

釹同位素不但可以反映物源組分還可以反映物源年齡, 因此可以更好反映物源組成[15]。釹同位素結(jié)合微量元素可為研究沉積巖潛在的源區(qū)提供有力工具。從圖7a可以發(fā)現(xiàn), 下石炭統(tǒng)南好組砂巖因具有較高的Th/Sc值和較低Nd值, 分布范圍相對(duì)較大, 其物源范圍介于上地殼物質(zhì)和安山質(zhì)巖石之間, 更靠近古老上地殼物質(zhì)。上石炭統(tǒng)青天峽組砂巖Th/Sc比值小于南好組,Nd值相對(duì)較大, 這反映其物源中年輕島弧物質(zhì)有增加的趨勢(shì)。因此, 下石炭統(tǒng)南好組砂巖物源以上地殼物質(zhì)再循環(huán)為主, 上石炭統(tǒng)青天峽組砂巖物源含有更多島弧巖石。從?Sm/Nd-Nd圖上也可得出類(lèi)似的結(jié)論(圖7b), 南好組砂巖分布更靠近上地殼物質(zhì), 青天峽組分布在古老上地殼物質(zhì)和年輕島弧物質(zhì)之間。這說(shuō)明隨著時(shí)間推移其Nd值逐漸增大反映物源年輕組分增加, 而Th/Sc值變小和?Sm/Nd值逐漸增大則說(shuō)明源區(qū)有分異程度較低的物質(zhì)加入。

圖6 石碌地區(qū)石炭系砂巖Co/Th-La/Sc源區(qū)判別圖解 (據(jù)Condie[49])

圖7?石碌地區(qū)石炭系砂巖eNd-Th/Sc源區(qū)判別圖解(a)和?Sm/Nd-eNd源區(qū)判別圖解(b)(據(jù)McLennan et al.[15])

Fig.7Nd-Th/Sc provenance discrimination diagram of the Carboniferous sandstones in the Shilu region (a) ?Sm/NdNdprovenance discrimination diagram of the Late Paleozoic sandstones in the Shilu region (b) (after McLennan.[15])

南好組DM(330Ma)虧損地幔段模式年齡分布范圍為1339~1937 Ma; 青天峽組砂巖DM(310Ma) 虧損地幔模式年齡分布在1136~1651 Ma范圍內(nèi), 顯示一個(gè)更為年輕的模式年齡。南好組和青天峽組模式年齡DM()都明顯大于其成巖年齡, 其模式年齡具有明顯的降低趨勢(shì), 這說(shuō)明青天峽組源區(qū)有年輕的島弧物質(zhì)加入。綜上所述, 下石炭統(tǒng)南好組碎屑巖的物源以古老上地殼再循環(huán)物質(zhì)為主, 上石炭統(tǒng)青天峽組碎屑巖物源以島弧巖漿巖物質(zhì)為主, 可能含有一定比例的古老上地殼再循環(huán)物質(zhì)。

4.2 構(gòu)造背景分析

Ti/Zr-La/Sc圖解(圖8a)表明, 上石炭統(tǒng)青天峽組砂巖樣品分布在大陸島弧范圍, 下石炭統(tǒng)南好組砂巖分布區(qū)域較廣。Th-Sc-Zr/10圖解進(jìn)一步表明(圖8b), 下石炭統(tǒng)南好組砂巖樣品落在被動(dòng)大陸邊緣和大陸島弧環(huán)境, 上石炭統(tǒng)青天峽組砂巖落在大陸島弧環(huán)境。Bhatia[10]提出可以用稀土元素的分布特征, 如REE含量、LREE/HREE值和Eu值等[42], 來(lái)識(shí)別構(gòu)造環(huán)境。從表2可以發(fā)現(xiàn), 下石炭統(tǒng)南好組砂巖的稀土元素特征顯示其沉積的構(gòu)造背景更接近被動(dòng)大陸邊緣。上石炭統(tǒng)青天峽組砂巖的REE含量、La/Yb和LREE/HREE值具有與大陸島弧相類(lèi)似的特征。這說(shuō)明從早石炭世到晚石炭世, 海南島五指山地區(qū)處于被動(dòng)大陸邊緣向大陸島弧轉(zhuǎn)變的環(huán)境。由于兩套地層之間連續(xù)沉積, 反映其沉積環(huán)境的逐漸變化。在早石炭世, 被動(dòng)大陸邊緣環(huán)境占據(jù)主導(dǎo)地位, 到晚石炭世則處于大陸島弧沉積環(huán)境。

根據(jù)盆地的沉積基底的屬性不同, 前人將大地構(gòu)造背景劃分為大洋島弧、大陸島弧、活動(dòng)大陸邊緣和被動(dòng)大陸邊緣[10]。每種大陸構(gòu)造環(huán)境又包括若干個(gè)具體的盆地類(lèi)型, 如大陸島弧構(gòu)造環(huán)境包括了拆離背景下弧間盆地、弧后盆地、弧前盆地、減薄陸殼上裂谷盆地和靠近大陸一側(cè)的弧后盆地[9]等。前人研究表明在晚古生代末期, 海南島受到古特提斯洋俯沖的影響: 海南島屯昌晨星和昌江邦溪地區(qū)的上古生界地層中變質(zhì)基性巖, 其Sm-Nd等時(shí)線(xiàn)年齡為(333±12) Ma, 具有MORB地球化學(xué)特征, 很可能古特提斯洋的殘片[5,50]。因此邦溪-晨星很可能是華夏板塊和印支板塊縫合帶, 屬于金沙江-雙溝- Song Ma縫合帶的延伸[50,51]。古地磁研究顯示晚古生代時(shí)期海南島南部(新州-文昌斷裂以南)可能屬于印支-南海板塊[52]。海南島邦溪基巖地球化學(xué)特征和年代學(xué)研究表明, 在270 Ma海南島經(jīng)歷了古特提斯洋閉合的影響[28]。瓊中海西期鉀玄質(zhì)侵入巖研究表明, 在272 Ma, 海南島受到華南板塊向印支-南海板塊俯沖的影響[53]。以上證據(jù)均表明, 在晚古生代, 海南島處于俯沖消減的構(gòu)造環(huán)境。而本文的數(shù)據(jù)表明上石炭統(tǒng)青天峽組具有大陸島弧環(huán)境沉積特征, 結(jié)合上述研究, 上石炭統(tǒng)青天峽組很可能沉積在靠近大陸一側(cè)的弧后伸展盆地環(huán)境。綜上所述, 在石炭紀(jì)海南島處于被動(dòng)大陸邊緣向大陸島弧環(huán)境轉(zhuǎn)換的環(huán)境, 這與印支板塊向華南板塊的俯沖和古特提斯洋的消減事件相吻合。由此可以推測(cè), 在早石炭世(330 Ma), 海南島石碌地區(qū)處于被動(dòng)大陸邊緣沉積環(huán)境, 下石炭統(tǒng)沉積物物源以上地殼物質(zhì)再循環(huán)為主, 此時(shí)華南板塊和印支板塊以古特提斯洋相隔; 在晚石炭世(310 Ma), 古特提斯洋開(kāi)始俯沖消減, 海南島處于大陸一側(cè)弧后盆地, 離火山弧較遠(yuǎn), 屬于大陸島弧環(huán)境, 上石炭統(tǒng)青天峽組沉積于弧后伸展環(huán)境下的弧后盆地, 其物源含有大量島弧火山巖物質(zhì), 也可能有部分上地殼物質(zhì)的加入。

圖8 石碌地區(qū)石炭系砂巖的Ti/Zr-La/Sc (a)和Th-Sc-Zr/10 (b)構(gòu)造判別圖解(底圖據(jù)Bhatia et al. [10])

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

A?ocean island arc; B?continental island arc; C?active continental margin; D?passive continental margin

5 結(jié)?論

(1) 海南島西部石碌地區(qū)的下石炭統(tǒng)南好組砂巖具有相對(duì)較高的成熟度和中等的風(fēng)化程度。上石炭統(tǒng)青天峽組砂巖相對(duì)下石炭統(tǒng)南好組砂巖具有較低的SiO2/Al2O3、K2O/Na2O、Rb/Sr值, 顯示較低的成熟度和風(fēng)化程度。

(2) 下石炭統(tǒng)南好組砂巖具有較高且較分散的Th/Sc值, 結(jié)合REE特征、Co/Th-La/Sc圖解和Nd-Th/Sc圖解等, 推測(cè)其物源以上地殼物質(zhì)再循環(huán)為主。相比之下, 石炭統(tǒng)青天峽組砂巖的物源含有大量大陸島弧物質(zhì)。

(3) 從釹同位素特征可以發(fā)現(xiàn), 南好組砂巖的Nd值分布在?12.37 ~ ?3.07, 模式年齡分布在1339~ 1937 Ma之間, 青天峽組砂巖Nd(310Ma)值和模式年齡分別在?8.35 ~ ?0.35和1136~1651 Ma之間。青天峽組砂巖的物源相對(duì)更為年輕。

(4) 下石炭統(tǒng)南好組沉積于被動(dòng)大陸邊緣環(huán)境, 上石炭統(tǒng)青天峽組砂巖沉積于大陸島弧環(huán)境。自早石炭世到晚石炭世, 海南島石碌地區(qū)處于由被動(dòng)大陸邊緣環(huán)境向大陸島弧環(huán)境轉(zhuǎn)換的過(guò)程。

(5) 結(jié)合前人對(duì)晚古生代海南島構(gòu)造環(huán)境的研究, 筆者認(rèn)為上石炭統(tǒng)青天峽組砂巖沉積的大陸島弧環(huán)境形成于大陸俯沖產(chǎn)生的弧后盆地。這一構(gòu)造背景與印支板塊向華南板塊俯沖和古特提斯洋消減構(gòu)造事件相吻合。海南島石炭紀(jì)構(gòu)造演化為華南的構(gòu)造演化提供佐證。

感謝兩位匿名審稿專(zhuān)家和編輯老師對(duì)本文提出的寶貴意見(jiàn)！野外地質(zhì)工作得到海南省資源環(huán)境勘查院的支持, 在此一并表示感謝。

[1] 陳哲培, 鐘盛中, 何圣華, 陳怡川. 海南省巖石地層[M]. 武漢: 中國(guó)地質(zhì)大學(xué)出版社, 1997: 1–114. Chen Zhe-pei, Zhong Sheng-zhong, He Sheng-hua, Chen Yi-chuan. Stratigraphy (Lithostratic) of Hainan Province[M]. Wuhan: China University of Geosciences Press, 1997: 1–114 (in Chinese).

[2] 汪嘯風(fēng), 馬大銓, 蔣大海. 海南島地質(zhì)(一): 地層古生物[M].北京: 地質(zhì)出版社, 1991: 1–280.Wang Xiao-feng, Ma Da-quan, Jiang Da-hai. Geology of Hainan Island (1): Stratigraphy and Paleontology[M]. Beijing: Geological Publishing House, 1991: 1–280 (in Chinese).

[3] 夏邦棟, 施光宇, 方中, 于津海, 王賜銀, 陶仙聰, 李惠民. 海南島晚古生代裂谷作用[J]. 地質(zhì)學(xué)報(bào), 1991, 65(2): 103–115. Xia Bang-dong, Shi Guang-yu, Fang Zhong, Yu Jin-hai, Wang Ci-yin, Tao Xian-cong, Li Hui-min. The Late Palaeozoic rifting in Hainan Island, China[J]. Acta Geol Sinica, 1991, 65(2): 103–115 (in Chinese with English abstract).

[4] 夏邦棟, 于津海, 方中, 王賜銀, 施光宇. 海南島石炭紀(jì)雙峰式火山巖及其板塊構(gòu)造背景[J]. 巖石學(xué)報(bào), 1991 (1): 54–62. Xia Bang-dong, Yu Jin-hai, Fang Zhong, Wang Ci-yin, Shi Guang-yu. The Carboniferous bimodal volcanic rocks in Hainan Island and their plate tectonic setting[J]. Acta Petrol Sinica, 1991 (1): 54–62 (in Chinese with English abstract).

[5] 李獻(xiàn)華, 周漢文, 丁式江, 李寄嵎, 張仁杰, 張業(yè)明, 葛文春. 海南島“邦溪-晨星蛇綠巖片”的時(shí)代及其構(gòu)造意義——Sm-Nd同位素制約[J]. 巖石學(xué)報(bào), 2000, 16(3): 425–432. Li Xian-hua, Zhou Han-wen, Ding Shi-jiang, Lee Chi-yu, Zhang Ren-jie, Zhang Ye-ming, Ge Wen-chun. Sm-Nd isotopic constraints on the age of the Bangxi-Chenxing ophiolite in Hainan Island: Implications for the tectonic evolution of eastern Paleo-Tethys[J]. Acta Petrol Sinica, 2000, 16(3): 425–432 (in Chinese with English abstract).

[6] Li X H, Zhou H W, Chung S L, Ding S J, Liu Y, Lee C Y, Ge W C, Zhang Y M, Zhang R J. Geochemical and Sm-Nd isotopic characteristics of metabasites from central Hainan Island, South China and their tectonic significance[J]. Island Arc, 2002, 11(3): 193–205.

[7] 許德如, 陳廣浩, 夏斌, 陳濤. 海南島幾個(gè)重大基礎(chǔ)地質(zhì)問(wèn)題評(píng)述[J]. 地質(zhì)科技情報(bào), 2003, 22(4): 37–45.Xu De-ru, Chen Guang-hao, Xia Bin, Chen Tao. Comment on several important basic geological problems in Hainan island, China[J]. Geol Sci Technol Inf, 2003, 22(4): 37–45 (in Chinese with English abstract).

[8] Xu D R, Wang Z L, Cai J X, Wu C J, Nonna B C, Wang L, Chen H Y, Baker M J, Kusiak M A. Geological characteristics and metallogenesis of the shilu Fe-ore deposit in Hainan Province, South China[J]. Ore Geol Rev, 2013, 53: 318–342.

[9] Bhatia M R. Plate tectonics and geochemical composition of sandstones[J]. J Geol, 1983, 91(6): 611–627.

[10] Bhatia M R, Crook K A. Trace element characteristics of graywackes and tectonic setting discrimination of sedimentary basins[J]. Contrib Mineral Petrol, 1986, 92(2): 181–193.

[11] Ghosh S, Sarkar S. Geochemistry of Permo-Triassic mudstone of the Satpura Gondwana basin, central India: Clues for provenance[J]. Chem Geol, 2010, 277(1/2): 78–100.

[12] Gu X, Liu J, Zheng M, Tang J, Qi L. Provenance and tectonic setting of the Proterozoic turbidites in Hunan, South China: Geochemical evidence[J]. J Sediment Res, 2002, 72(3): 393– 407.

[13] Mader D, Neubauer F. Provenance of Palaeozoic sandstones from the Carnic Alps (Austria): Petrographic and geochemical indicators[J]. Int J Earth Sci, 2004, 93(2): 262–281.

[14] McLennan S, Taylor S. Sedimentary rocks and crustal evolution: Tectonic setting and secular trends[J]. J Geol, 1991, 99(1): 1–21.

[15] McLennan S, Hemming S, McDaniel D, Hanson G. Geochemical approaches to sedimentation, provenance, and tectonics[J]. Spec Papers Geol Soc Am, 1993, 284: 21–40.

[16] Miall A D. Sedimention and Plate Tectonics[M]//Miall A D. Principles of Sedimentary Basin Analysis. Berlin: Springer, 2000: 467–593.

[17] Raza M, Khan A, Bhardwaj V R, Rais S. Geochemistry of Mesoproterozoic sedimentary rocks of upper Vindhyan Group, southeastern Rajasthan and implications for weathering history, composition and tectonic setting of continental crust in the northern part of Indian shield[J]. J Asian Earth Sci, 2012, 48: 160–172.

[18] Bauluz B, Mayayo M J, Fernandez-Nieto C, Lopez J M G. Geochemistry of Precambrian and Paleozoic siliciclastic rocks from the Iberian Range (NE Spain): Implications for source- area weathering, sorting, provenance, and tectonic setting[J]. Chem Geol, 2000, 168(1/2): 135–150.

[19] Xu D R, Wang Z L, Chen H Y, Hollings P, Jansen N H, Zhang Z C, Wu C J. Petrography and geochemistry of the Shilu Fe-Co-Cu ore district, South China: Implications for the origin of a Neoproterozoic BIF system[J]. Ore Geol Rev, 2014, 57: 322–350.

[20] 許德如, 林舸, 梁新權(quán), 陳廣浩, 唐紅峰. 海南島前寒紀(jì)巖石圈演化的記錄: 基性巖類(lèi)巖石地球化學(xué)證據(jù)[J]. 巖石學(xué)報(bào), 2001, 22(4): 598–608. Xu De-ru, Lin Ge, Liang Xin-quan, Chen Guang-hao, Tang Hong-feng. The records of the evolution of Precambrian lithospher: The evidences of petrology and gecohemistry of basic rocks on Hainan Island[J]. Acta Petrol Sinica, 2001, 22(4): 598–608 (in Chinese with English abstract).

[21] 張業(yè)明, 張仁杰, 姚華舟, 馬國(guó)干. 海南島前寒武紀(jì)地殼構(gòu)造演化[J]. 地球科學(xué), 1997, 22(4): 59–64. Zhang Ye-ming, Zhang Ren-jie, Yao Hua-zhou, Ma Guo-gan. The Precambrian crustal tectonic evolution in Hainan Island[J]. Earth Sci, 1997, 22(4): 59–64 (in Chinese with English abstract).

[22] Li Z X, Li X H, Li W X, Ding S. Was Cathaysia part of Proterozoic Laurentia? –– New data from Hainan Island, south China[J]. Terra Nova, 2008, 20(2): 154–164.

[23] 梁新權(quán), 侯威, 陳惠芳. 海南島戈枕脆、韌性疊加剪切帶基本特征及其成礦意義[J]. 大地構(gòu)造與成礦學(xué), 1990, 14(3): 255–265.Liang Xin-quan, Hou wei, Chen Hui-fang. Characteristics of the gezhen brittle-ductile superimposition shear zone and its significance for ore formation[J]. Geotecton Metallogen, 1990, 14(3): 255–265 (in Chinese with English abstract).

[24] 戰(zhàn)明國(guó), 張樹(shù)淮, 劉國(guó)慶. 海南島西部戈枕含金剪切帶及其金礦成礦系列[J]. 礦床地質(zhì), 1996, 15(4): 2–10. Zhan Ming-guo, Zhang Shu-hai, Liu Guo-qing. The Gezhen auriferous shear zone and related metallogenic series in western Hainan Island[J]. Mineral Deposit, 1996, 15(4): 2–10 (in Chinese with English abstract).

[25] Li Z X. Tectonic history of the major east Asian lithospheric blocks since the mid-Proterozoic―A synthesis[M]//Flower M F J, Chung S-L, Lo C-H, Lee T-Y. Mantle Dynamics and Plate Interactions in East Asia. Washington DC: American Geophysical Union, 1998: 221–243.

[26] Liu Bao-jun, Xu Xiao-song. Atlas of the Lithofacies and Palaeogeography of South China: (Sinian-Triassic)[M]. Beijing: Science Press, 1994: 1–188.

[27] Metcalfe I, Shergold I H, Li Z X. IGCP321 Gondwana dispersion and Asian accretion: Fieldworkon Hainan Island[J]. Episodes, 1994, 16: 443–447.

[28] Xu D R, Xia B, Li P C, Chen G H, Ma C, Zhang Y Q. Protolith natures and U-Pb sensitive high mass-resolution ion microprobe (SHRIMP) zircon ages of the metabasites in Hainan Island, South China: Implications for geodynamic evolution since the late Precambrian[J]. Island Arc, 2007, 16(4): 575–597.

[29] Hsü K J, Li J, Chen H, Pen H, Sengor A M C. Tectonics of South China: Key to understanding West Pacific geology[J]. Tectonophysics, 1990, 183(1): 9–39.

[30] 張仁杰, 馮少南, 徐光洪, 楊德驪, 鄢道平, 李志宏, 蔣大海, 吳煒. Chuaria-Tawuia生物群在海南島石碌群的發(fā)現(xiàn)及意義[J]. 中國(guó)科學(xué)(B輯), 1989, 19(3): 304–312. Zhang Renjie, Feng Shaonan, Xu Guanghong, Yang Deli, Yan Daoping, Li Zhihong, Jiang Dahai, Wu Wei. Discovery of Chuaria-Tawuia biota and its significance in Hainan Island[J]. Sci China (B), 1989, 19(3): 304–312 (in Chinese).

[31] 陳哲培, 李孫雄, 云平, 林義華, 汪焰華, 陳方穎, 龔丹. 海南省南好地區(qū)南好組地質(zhì)特征及時(shí)代[J]. 中國(guó)地質(zhì), 2012, 39(3): 651–661. Chen Zhe-pei, Li Sun-xiong, Yun Ping, Lin Yi-hua, Wang Yan-hua, Chen Fang-ying, Gong Dan. Geological characteristicsand age of Nanhao Formation in Nanhao Area, Hainan Province[J]. Geol China, 2012, 39(3): 651–661 (in Chinese with English abstract).

[32] Goto A, Tatsumi Y. Quantitative analysis of rock samples by an X-ray fluorescence spectrometer (II) [J]. The Rigaku Journal, 1996, 13(2): 20–38.

[33] Li X H, Qi C S, Liu Y, Liang X R, Tu X L, Xie L W, Yang Y H. Petrogenesis of the Neoproterozoic bimodal volcanic rocks along the western margin of the Yangtze Block: New constraints from Hf isotopes and Fe/Mn ratios[J]. Chinese Sci Bull, 2005, 50(21): 2481–2486.

[34] 李獻(xiàn)華, 劉穎, 涂湘林, 胡光黔, 曾文. 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 Xian-hua, Liu Ying, Tu Xiang-lin, Hu Guang-qian, Zeng Wen. Precise determination of chemical compositions in silicate rocks using ICP-AES and ICP-MS: A comparative study of sample digestion techniques of alkali fusion and acid dissolution[J]. Geochimica, 2002, 31(3): 289–294 (in Chinese with English abstract).

[35] Liang Xi-rong, Li Xian-hua, Liu Ying, LEE Chee-yu. Laser ablation microprobe-inductively coupled plasma mass spectr-ometry: A new method for rapid determination of multiple elements in rock samples[J]. J Instrum Anal, 2000, 19: 33–36.

[36] 劉穎, 劉海臣, 李獻(xiàn)華. 用ICP-MS準(zhǔn)確測(cè)定巖石樣品中的40余種微量元素[J]. 地球化學(xué), 1996, 25(6): 552–558. Liu Ying, Liu Hai-chen, Li Xian-hua. Simultaneous and precise determination of 40 trace elements in rock samples using ICP-MS[J]. Geochimica, 1996, 25(6): 552–558 (in Chinese with English abstract).

[37] Halliday A N, Lee D C, Chirstensen J N, Rehk?mper M, Yi W, Luo X Z, Hall C M, Ballentine C J, Pettke T, Stirling C. Applications of multiple collector-ICPMS to cosmochemistry, geochemistry, and paleoceanography[J]. Geochim Cosmochim Acta, 1998, 62(6): 919–940.

[38] 梁細(xì)榮, 韋剛健, 李獻(xiàn)華, 劉穎. 利用MC-ICPMS精確測(cè)定143Nd/144Nd和Sm/Nd比值[J]. 地球化學(xué), 2003, 32(1): 91–96. Liang Xi-rong, Wei Gang-jian, Li Xian-hua, Liu Ying. Precise measurement of143Nd/144Nd and Sm/Nd ratios using multiple-collectors inductively coupled plasma-mass spectrometer (MC-ICPMS)[J]. Geochimica, 2003, 32(1): 91–96 (in Chinese with English abstract).

[39] Masuda A, Nakamura N, Tanaka T. Fine structures of mutually normalized rare-earth patterns of chondrites [J]. Geochim Cosmochim Acta, 1973, 37(2): 239–248.

[40] Herron M M. Geochemical classification of terrigenous sands and shales from core or log data[J]. J Sediment Petrol, 1988, 58(5): 820–829.

[41] McDonough W F, Sun S-s. The composition of the Earth[J]. Chem Geol, 1995, 120(3): 223–253.

[42] Bhatia M R. Rare earth element geochemistry of Australian paleozoic graywackes and mudrock: Provenance and tectonic control[J]. Sediment Geol, 1985, 45(1/2): 97–113.

[43] Nesbitt H W, Young G M. Early Proterozoic climates and plate motions inferred from major element chemistry of lutites[J]. Nature, 1982, 299(5885): 715–717.

[44] 陳駿, 王鶴年. 地球化學(xué)[M]. 北京: 科學(xué)出版社, 2004: 152–155. Chen Jun, Wang He-nian. Geochemistry[M]. Beijing: Science Press, 2004: 152–155(in Chinese).

[45] Tao Huifei, Wang Qingchen, Yang Xiaofa, Jiang Lin. Provenance and tectonic setting of Late Carboniferous clastic rocks in West Junggar, Xinjiang, China: A case from the Hala-alat Mountains[J]. J Asian Earth Sci, 2013, 64: 210–222.

[46] 李雙應(yīng), 李任偉, 岳書(shū)倉(cāng), 王道軒, 劉因, 孟慶任, 金福全. 安徽肥西中生代碎屑巖地球化學(xué)特征及其對(duì)物源制約[J]. 巖石學(xué)報(bào), 2004, 20(3): 667–676. Li Shuang-ying, Li Ren-wei, Yue Shu-cang, Wang Dao-xuan, Liu Yin, Meng Qing-ren, Jin Fu-quan. Geochemistry of Mesozoic detrital rocks and its constrains on provenance in Feixin are, Anhui Provence[J]. Acta Petrol Sinica, 2004, 20(3): 667–676 (in Chinese with English abstract).

[47] Fedo C M, Wayne N H, Young G M. Unraveling the effects of potassium metasomatism in sedimentary rocks and paleosols, with implications for paleoweathering conditions and provenance[J]. Geol Soc Am, 1995, 23(10): 921–924.

[48] Tran H T, Ansdell K, Bethune K, Watters B, Ashton K. Nd isotope and geochemical constraints on the depositional setting of Paleoproterozoic metasedimentary rocks along the margin of the Archean Hearne craton, Saskatchewan, Canada[J]. Precamb Res, 2003, 123(1): 1–28.

[49] Condie K C. Chemical composition and evolution of the upper continental crust: Contrasting results from surface samples and shales[J]. Chem Geol, 1993, 104(1): 1–37.

[50] 唐紅峰. 海南島晨星玄武巖地球化學(xué)特征及其構(gòu)造環(huán)境含義[J]. 大地構(gòu)造與成礦學(xué), 1999, 23(4): 323–327. Tang Hong-feng. Geochemical characteristics of the basalt from Chenxing, Hainan Island and their tectonic implication[J]. Geotecton Metallogen, 1999, 23(4): 323–327 (in Chinese with English abstract).

[51] 李獻(xiàn)華, 周漢文, 丁式江, 李寄嵎, 張仁杰, 張業(yè)明, 葛文春. 海南島洋中脊型變質(zhì)基性巖: 古特提斯洋殼的殘片?[J]. 科學(xué)通報(bào), 2000, 45(1): 84–89. Li Xianhua, Zhou Hanwen, Ding Shijiang, Lee Chiyu, Zhang Renjie, Zhang Yeming, Ge Wenchun. Metamorphosed mafic rocks N-type MORB geochemical features in Hainan Island, Remnants of the Paleo-Tethys oceanic crust?[J]. Chinese Sci Bull, 2000, 45(1): 84–89 (in Chinese).

[52] 殷鴻福, 吳順寶, 杜遠(yuǎn)生, 彭元橋. 華南是特提斯多島洋體系的一部分[J]. 地球科學(xué), 1999, 24(1): 3–14. Yin Hong-fu, Wu Shun-bao, Du Yuan-sheng, Peng Yuan-qiao. South China defined as part of Tethyan archipelagic ocean system[J]. Earth Sci J China Univ Geosci, 1999, 24(1): 3–14 (in Chinese with English abstract).

[53] 謝才富, 朱金初, 丁式江, 張業(yè)明, 付太安, 李志宏. 瓊中海西期鉀玄質(zhì)侵入巖的厘定及其構(gòu)造意義[J]. 科學(xué)通報(bào), 2006, 51(16): 1944-1954. Xie Caifu, Zhu Jinchu, Ding Shijiang, Zhang Yeming, Fu Taian, Li Zhihong. Stipulation and tectonic significance of Hercynian shoshonitic intrusive rocks in central Hainan[J]. Chinese Sci Bull, 2006, 51(16): 1944–1954 ( in Chinese).

Nd isotopic and geochemical constraints on the provenances and tectonic settings of the Carboniferous sandstones in western Hainan Province, South China

YU Liang-liang1, 2, XU De-ru1*, WANG Zhi-lin3, WU Chuan-jun1, 2and HOU Mao-zhou1, 2

1. Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou?510640, China;2. University of Chinese Academy of Sciences, Beijing?100049, China; 3. MOE Key Laboratory of Metallogenic Prediction of Nonferrous Metals, School of Geosciences and Info-Physics, Central South University, Changsha? 410083, China

Geochemical compositions and Sm-Nd isotopic characteristics can effectively identify the provenances and tectonic settings of clastic sedimentary rocks. This paper focuses itself on two Carboniferous successions, i.e., the Lower Carboniferous Nanhao Formation and the Upper Carboniferous Qingtianxia Formation in the Shilu region, western Hainan Province, South China. Major and trace element compositions and Sm-Nd isotopic compositions were analyzed for 12 sandstone samples from the Nanhao Formation (7) and the Qingtianxia Formation (5) in order to investigate their provenance and tectonic environment, and to provide constraints on the tectonic evolution of the Hainan Island during the Carboniferous period. The analytical results show that the sandstones of different ages have the characteristics similar to those of the post-Archean Australia Average Shale (PAAS) in major and trace element compositions. In the chondrite-normalized REE patterns, both Nanhao and Qingtianxia groups are enriched in LRRE and depleted in HRRE. The La/Yb ratios of the Nanhao and Qingtianxia groups are 9.17~16.65 (D12=33.9) and 8.88~12.10, and theEu ratios are 0.54~0.66 and 0.45~0.72, respectively. The Nd isotope data show that theNd(310Ma)values andDMages of the Nanhao Group sandstones range from ?12.37 to ?3.07 (averaging ?8.86) and from 1339 Ma to 1947 Ma. TheNd(310Ma)values andDMages of the Qingtianxia Formation sandstones range from ?8.35 to ?0.35 (averaging ?5.15) and from 1136 Ma to 1651 Ma. Most of the Nanhao Formation sandstones have higher Th/Sc ratios (> 1.0), showing the recycling provenance of the main upper continental crust. In comparison with the Nanhao sandstone samples, the Qingtianxia Formation sandstones have lower Th/Sc ratios ranging from 0.61 to 1.07 (averaging 0.86), suggesting that a larger proportion of arc materials was inputted into the provenance. The Co/Th-La/Sc andNd-Th/Sc plots also reveal that the provenances for the Nanhao Formation sandstones were dominated by old upper continental crust components, and the provenances for the Qingtianxia Formation were dominated by younger arc materials. The Ti/Zr-La/Sc and Th-Sc-Zr/10 plots further reflect that the Nanhao Formation sandstones were deposited on the passive continental margin and the Qingtianxia Formation sandstones were deposited in a continental island arc. In combination with the results of former studies, the Qingtianxia Formation sandstones were likely deposited in a back-arc basin setting closer to a continental margin. The Hainan Island most likely underwent a tectonic transformation from a passive margin setting to a continental island arc setting during the Carboniferous period. This transformation might be the result of subduction of the Paleo-Tethys ocean to the South China continent.

carboniferous; geochemistry; Sm-Nd isotopic characteristics; provenance; Hainan Island

P595; P597

A

0379-1726(2016)03-0235-14

2015-10-16;

2015-12-29;

2016-01-28

中國(guó)地質(zhì)調(diào)查局老礦山深部和外圍找礦計(jì)劃項(xiàng)目(1212011220711)

于亮亮(1983–), 男, 博士研究生, 構(gòu)造地質(zhì)學(xué)專(zhuān)業(yè)。E-mail: shushengbeibei@136.com

XU De-ru, E-mail: xuderu@gig.ac.cn; Tel: +86-20-85292713