甘肅龍首山巖帶西井鎂鐵質(zhì)巖體成因及其構(gòu)造意義

段 俊，錢(qián)壯志,2，焦建剛,2，魯 浩，馮延清

1.長(zhǎng)安大學(xué)地球科學(xué)與資源學(xué)院,西安 710054 2.西部礦產(chǎn)資源與地質(zhì)工程教育部重點(diǎn)實(shí)驗(yàn)室,西安 710054 3.山東省地質(zhì)礦產(chǎn)勘查開(kāi)發(fā)局第三地質(zhì)大隊(duì),山東 煙臺(tái) 264004

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甘肅龍首山巖帶西井鎂鐵質(zhì)巖體成因及其構(gòu)造意義

段 俊1，錢(qián)壯志1,2，焦建剛1,2，魯 浩3，馮延清1

1.長(zhǎng)安大學(xué)地球科學(xué)與資源學(xué)院,西安 710054 2.西部礦產(chǎn)資源與地質(zhì)工程教育部重點(diǎn)實(shí)驗(yàn)室,西安 710054 3.山東省地質(zhì)礦產(chǎn)勘查開(kāi)發(fā)局第三地質(zhì)大隊(duì),山東 煙臺(tái) 264004

西井巖體位于北祁連造山帶以北，阿拉善地塊西南緣的龍首山隆起帶。以往的研究多以沿龍首山斷裂分布的鎂鐵-超鎂鐵質(zhì)巖帶作為和金川巖體相關(guān)的巖漿事件進(jìn)行，而本次選擇西井鎂鐵質(zhì)巖體進(jìn)行了精確的地質(zhì)年代學(xué)和地球化學(xué)研究，確定了西井巖體巖性主要為橄欖輝石巖和輝長(zhǎng)巖，成巖時(shí)代為 (421.0±9.0) Ma，可以和北祁連高壓變質(zhì)帶榴輝巖年齡相對(duì)應(yīng)；εNd(t)為4.06～5.52，(87Sr/86Sr)i為0.704 548～0.707 575，具有地幔巖石圈特征；微量元素及其同位素計(jì)算表明西井巖體經(jīng)歷了約10%的下地殼物質(zhì)混染。據(jù)此得出西井巖體及其龍首山巖帶早志留世鎂鐵質(zhì)侵入巖體成因模式為：祁連洋殼連續(xù)俯沖過(guò)程中洋殼與陸殼分離，熱的軟流圈物質(zhì)持續(xù)沖擊地幔巖石圈的底部；由于熱傳導(dǎo)效應(yīng)，大地?zé)崃餮刂蒯r石圈上升，使得80 km深度的濕的橄欖巖層發(fā)生熔融，產(chǎn)生玄武質(zhì)巖漿作用，玄武質(zhì)巖漿上升過(guò)程中與下地殼物質(zhì)發(fā)生約10%混染，形成西井巖體及其龍首山鎂鐵--超鎂鐵質(zhì)巖帶。

SHRIMP測(cè)年；地球化學(xué)；西井鎂鐵質(zhì)巖體；龍首山鎂鐵-超鎂鐵質(zhì)巖帶；北祁連造山帶

0 引言

祁連造山帶是一個(gè)包括蛇綠巖套、高壓變質(zhì)巖帶、島弧火山巖、深成花崗巖類侵入巖、復(fù)理石、磨拉石和上覆蓋層的造山縫合帶。夏林圻等[1-2]運(yùn)用區(qū)域構(gòu)造和火山巖漿演化動(dòng)力學(xué)思路研究了北祁連海相火山巖成因；吳才來(lái)等[3]確定了北祁連早古生代花崗質(zhì)巖漿事件； Song 等[4-9]對(duì)北祁連超高壓變質(zhì)帶進(jìn)行了年齡分析；這些都從北祁連演化的角度對(duì)上述巖石單元進(jìn)行了很好的解釋。在北祁連造山帶以北、阿拉善地塊西南緣沿著龍首山斷裂分布著一系列鎂鐵--超鎂鐵質(zhì)侵入巖體，構(gòu)成龍首山巖帶。由于該巖帶中存在著含世界級(jí)巖漿鎳-銅硫化物礦床的金川超鎂鐵質(zhì)巖體，前人通常將龍首山巖帶的其他鎂鐵質(zhì)巖體與金川巖體進(jìn)行對(duì)比，以證明這些巖體與金川巖體的關(guān)系，并試圖在金川外圍繼續(xù)尋找與金川巖漿事件相關(guān)的巖漿硫化物礦床。焦建剛等[10-11]通過(guò)龍首山巖帶中幾個(gè)典型鎂鐵--超鎂鐵質(zhì)巖體巖石地球化學(xué)分析，提出龍首山巖帶巖體處于相同的構(gòu)造環(huán)境，存在連續(xù)的巖漿演化關(guān)系；閆海卿等[12]認(rèn)為野芨里巖體具有與金川巖體相似的地球化學(xué)特征；李文淵等[13]將阿拉善地塊南緣的鎂鐵--超鎂鐵質(zhì)巖體自北而南分為北大山巖帶、龍首山巖帶和北海子巖帶，龍首山巖帶和北海子巖帶為中元古代早期超地幔柱作用下地幔派生巖漿的產(chǎn)物。

為了確定龍首山鎂鐵-超鎂鐵質(zhì)巖帶成因，本文選擇龍首山巖帶中段的西井鎂鐵質(zhì)巖體為研究對(duì)象，進(jìn)行精確的地質(zhì)年代學(xué)和地球化學(xué)研究。根據(jù)西井巖體的鋯石U-Pb年齡和地球化學(xué)模擬計(jì)算，結(jié)合已有的北祁連榴輝巖的p-T-t軌跡，最終確定西井巖體及龍首山鎂鐵--超鎂鐵質(zhì)巖帶形成時(shí)的地球動(dòng)力學(xué)背景。

1 地質(zhì)背景

西井鎂鐵質(zhì)巖體位于阿拉善地塊西南緣的龍首山鎂鐵--超鎂鐵質(zhì)巖帶中段(圖1)。阿拉善地塊是一個(gè)三角形地塊，位于華北克拉通西緣[15]，基底為早前寒武紀(jì)石英閃長(zhǎng)質(zhì)/花崗質(zhì)片麻巖，上覆蓋層為寒武紀(jì)到中奧陶世地層。阿拉善地塊位于北祁連造山帶以北，Song S G等[14]根據(jù)北祁連島弧巖石年齡[16]和高壓變質(zhì)巖年齡[17-18]提出祁連洋在約445 Ma閉合， 阿拉善地塊和祁連地塊之間發(fā)生碰撞。

如圖2a所示，龍首山地區(qū)鎂鐵--超鎂鐵質(zhì)巖體主要分布于西、中、東3個(gè)地段：西段以藏布臺(tái)巖體為代表，中段以金川和西井巖體為代表，東段以小口子巖體為代表。這些巖體特征見(jiàn)表1。

2 巖體特征

西井鎂鐵質(zhì)巖體侵入于新元古界震旦系硅質(zhì)條帶大理巖--灰?guī)r、千枚巖類石英巖和加里東早期肉紅色似斑狀中粗?；◢弾r中(圖2b)。巖體呈透鏡狀，可分為東、西兩個(gè)巖體；西巖體長(zhǎng)600 m，寬60～100 m；東巖體長(zhǎng)80 m，寬65 m。整個(gè)巖體在地表露頭共有19個(gè)，最小長(zhǎng)約20 m，寬約15 m；最大長(zhǎng)達(dá)30～195 m；巖性主要為橄欖輝石巖和輝長(zhǎng)巖。

橄欖輝石巖：黑綠色，中粒結(jié)構(gòu)，塊狀構(gòu)造。主要礦物組成為：橄欖石30%，單斜輝石60%，鉻榴石5%，斜方輝石5%。橄欖石半自形粒狀，粒徑為0.2～2.0 mm，由于構(gòu)造擾動(dòng)而略具定向排列(圖3a)。單斜輝石位于橄欖石周?chē)?，少?shù)單斜輝石碎塊聚集成集合體，且多發(fā)生透閃石化(圖3b)，后期熱液作用萃取礦物中的金屬形成鉻榴石(圖3c)。

據(jù)文獻(xiàn)[14]修編。圖1 中國(guó)主要構(gòu)造單元簡(jiǎn)圖Fig.1 Schematic map of major tectonic units of China

圖2 龍首山巖帶地質(zhì)圖(a)和西井巖體地質(zhì)簡(jiǎn)圖(b)Fig.2 Geological map of Longshoushan terrane with localities of major mafic-ultramafic intrusions(a)and Schematic geologic map of the Xijing mafic intrusion(b)

序號(hào)巖體名稱地理位置巖體特征圍巖特征1藏布臺(tái)山丹縣城83°方向23．4km單輝橄欖巖，面積約0．61km2炭質(zhì)千枚巖，綠泥石英片巖，綠泥次閃片巖2青井子金川鎳礦297°方向73．5km角閃單輝巖，面積約5．07km2炭質(zhì)石英千枚巖，綠泥次閃片巖夾薄層灰?guī)r3青石窯金川鎳礦305°方向47km橄欖輝石巖，面積約0．13km2炭質(zhì)千枚巖，硅質(zhì)白云巖，綠泥絹云片巖，鈉長(zhǎng)陽(yáng)起片巖4金川河西堡35°方向15km二輝橄欖巖，面積約1．34km2大理巖，條帶-均質(zhì)混合巖，片麻巖5西井永昌縣城351°方向40km橄欖輝石巖，面積約0．053km2硅質(zhì)條帶大理巖-灰?guī)r，千枚巖夾石英巖6塔馬子溝金川鎳礦285°方向10．25km輝石巖，輝石橄欖巖，面積約0．027km2云母石英片巖夾薄層大理巖7毛草泉金川鎳礦283°方向8．5km輝石巖，橄欖輝石巖，面積約0．017km2云母石英片巖，含石墨大理巖8Ⅴ號(hào)異常金川鎳礦Ⅲ礦區(qū)北西2km斜長(zhǎng)二輝橄欖巖，橄欖巖，面積約0．004km2大理巖，混合片麻巖9小口子金川鎳礦110°方向37．5km橄欖輝石巖，橄欖巖，面積0．2km2二云石英片巖、斜長(zhǎng)角閃巖、片麻巖及白云大理巖

a.橄欖輝石巖鏡下顯微照片; b.單斜輝石透閃石化鏡下顯微照片; c.鉻榴石鏡下顯微照片; d.橄欖輝長(zhǎng)巖鏡下顯微照片。Ol. 橄欖石, Cpx. 單斜輝石。圖3 西井巖體巖石顯微照片F(xiàn)ig.3 Photomicrographs of rocks from Xijing intrusion

輝長(zhǎng)巖：暗綠色，較為新鮮，輝長(zhǎng)結(jié)構(gòu)(圖3d)。主要礦物組成為：?jiǎn)涡陛x石35%，斜長(zhǎng)石60%，金屬礦物5%。單斜輝石半自形粒狀，粒徑0.5～1.0 mm，解理發(fā)育，部分輝石邊部蝕變?yōu)榻情W石。長(zhǎng)石表面干凈，雙晶發(fā)育。巖石結(jié)構(gòu)構(gòu)造表明礦物結(jié)晶順序?yàn)椋洪蠙焓?斜方輝石→單斜輝石→斜長(zhǎng)石。

3 采樣位置與分析方法

沿著西井巖體從南往北穿切巖體采樣。礦物成分測(cè)試用JXI-8100型電子探針完成，測(cè)試條件為加速電壓15 kV，電流1.0×10-8A，束斑直徑1 μm。主量元素采用XRF-1800 型X射線熒光光譜儀測(cè)試，XRF熔片法依據(jù)國(guó)家標(biāo)準(zhǔn)GB/T14506.28-1993。微量元素分析采用美國(guó)X-7型ICP-MS完成，上述工作在長(zhǎng)安大學(xué)西部礦產(chǎn)資源與地質(zhì)工程教育部重點(diǎn)實(shí)驗(yàn)室進(jìn)行。

Nd-Sr同位素測(cè)試在中國(guó)地質(zhì)科學(xué)院完成，分析方法為同位素稀釋法，測(cè)試儀器為MAT262固體同位素質(zhì)譜計(jì)，測(cè)試方法參見(jiàn)文獻(xiàn)[19]。鋯石SHRIMP測(cè)年在北京離子探針中心完成。鋯石樣品為較新鮮的橄欖輝石巖，從中挑選巖漿鋯石>20粒。將挑選好的待測(cè)樣品鋯石與RSES 參考樣SL13及數(shù)粒TEM置于環(huán)氧樹(shù)脂制靶。鋯石陰極發(fā)光及SHRIMP U-Pb測(cè)年詳細(xì)分析流程和原理參考文獻(xiàn)[20-22] 。

4 測(cè)試結(jié)果

4.1 主要礦物成分和全巖主量、微量元素成分

西井巖體中主要礦物的平均化學(xué)成分見(jiàn)表2。橄欖石Fo(Fo=100Mg/(Mg+Fe))值為81～86，屬貴橄欖石，位于玄武質(zhì)巖漿結(jié)晶分異形成的橄欖石Fo值范圍(80～90)內(nèi)[23-24]。全巖主量、微量元素成分見(jiàn)表3。為了排除蝕變的干擾，將全巖主量元素扣除燒失量后重新進(jìn)行100%計(jì)算，然后將全巖主量元素成分和主要堆晶礦物橄欖石、輝石的平均化學(xué)成分進(jìn)行對(duì)比(圖4)，巖體全巖主量元素成分并沒(méi)有完全落入堆晶礦物控制的區(qū)域(圖4中灰色區(qū)域)，表明巖石成分代表玄武質(zhì)巖漿成分，而不是堆晶礦物成分。薄片中可見(jiàn)到斜長(zhǎng)石、角閃石、黑云母及鐵-鈦氧化物等礦物，這與全巖樣品中高的Al2O3、TFeO 和TiO2組分相一致。

球粒隕石標(biāo)準(zhǔn)化的稀土元素和原始地幔標(biāo)準(zhǔn)化的抗蝕變影響的微量元素蛛網(wǎng)圖中，稀土元素呈平坦型(圖5)，具有島弧拉斑玄武巖特征[26]。

4.2 巖體年代學(xué)

陰極發(fā)光圖像中，西井巖體鋯石多呈柱狀，半自形--自形晶，晶面平直光滑。鋯石韻律環(huán)帶較發(fā)育(圖6)， Th/U值>0.4，為巖漿鋯石的特征[27]。由于鋯石XJ-7和XJ-15中U質(zhì)量分?jǐn)?shù)過(guò)高，故將其剔除。其他16個(gè)鋯石的U-Pb諧和年齡為(421.0±9.0)Ma(圖7，表4)，代表西井巖體結(jié)晶年齡。

4.3 Sr-Nd同位素

西井巖體的Sr、Nd同位素測(cè)定結(jié)果及特征值見(jiàn)表5。(87Sr/86Sr)i值按照421 Ma計(jì)算，結(jié)果為0.704 548～0.707 575，平均0.706 125。εNd(421 Ma)的變化范圍相對(duì)較窄，4.06～5.52，平均4.88。由于Rb-Sr比Sm-Nd更具有活動(dòng)性，因此受蝕變影響使得(87Sr/86Sr)i變化范圍較大。

5 討論

5.1 年代學(xué)意義

西井巖體的成巖年齡為(421.0±9.0)Ma，金川巖體中含硫化物的超鎂鐵質(zhì)巖石鋯石U-Pb年齡約為830 Ma[28-30]，表明兩者形成于完全不同的巖漿事件。北祁連高壓變質(zhì)帶榴輝巖年齡為463～489 Ma[5, 31]，該事件年齡可以和西井巖體成巖年齡相對(duì)應(yīng)。

表2 西井巖體中主要礦物平均化學(xué)成分

注:n為測(cè)點(diǎn)數(shù)。

表3 西井鎂鐵--超鎂鐵質(zhì)巖石主量、微量元素成分

注：主量元素質(zhì)量分?jǐn)?shù)單位為%；微量元素質(zhì)量分?jǐn)?shù)單位為10-6。

Ol. 橄欖石；Cpx. 單斜輝石；Opx 斜方輝石。圖4 西井巖體主量元素Hark圖解Fig.4 Hark diagrams of major element of Xijing intrusion

西井巖體εNd(t)為4.06～5.52，(87Sr/86Sr)i為0.704 548～0.707 575，具有巖石圈地幔特征，位于阿爾卑斯造山帶鎂鐵質(zhì)侵入巖范圍之內(nèi)(圖8a)。阿爾卑斯造山帶形成于特提斯洋閉合之后碰撞造山，碰撞開(kāi)始于55 Ma。在阿爾卑斯造山帶也發(fā)現(xiàn)了高壓變質(zhì)的榴輝巖，埋深70～100 km，高壓變質(zhì)年齡為35～45 Ma[37-39]。Davies和von Blanckenburg[40]用板塊斷裂模式很好地解釋了阿爾卑斯地區(qū)碰撞造山過(guò)程中地幔源區(qū)巖漿作用和超高壓變質(zhì)作用的共存現(xiàn)象，且地幔侵入巖體在地表沿著Peri-Adriatic線狀構(gòu)造分布。這一模式同樣可以用于解釋北祁連造山帶高壓折返的榴輝巖和沿著龍首山斷裂走向分布的早志留世地幔侵入巖體共存現(xiàn)象。

球粒隕石和原始地幔值引自文獻(xiàn)[25]。圖5 西井巖體球粒隕石標(biāo)準(zhǔn)化稀土元素和原始地幔標(biāo)注化微量元素配分曲線圖Fig.5 Chondrite normalized REE pattern and primitive mantle normalized spider diagrams of Xijing intrusion

測(cè)點(diǎn)號(hào)w(U)/10-6w(Th)/10-6232Th/238Uw(206Pbc)/10-6w(206Pb?)/10-6207Pb?/235U1σ/%206Pb?/238U1σ/%(206Pb/238U年齡)/MaXJ11671671．030．739．900．530003．90．068602．0427．8±8．1XJ2117900．801．346．500．460008．20．063902．1399．5±8．3XJ3107670．651．946．200．420007．50．065802．7411．0±10．6XJ41431100．800．978．500．470007．40．068403．3426．3±13．5XJ56524120．650．7836．400．480003．80．064501．7402．9±6．5XJ61541280．860．899．200．520007．60．068701．9428．2±8．1XJ79954520．470．2452．100．450002．60．060801．8380．3±6．8XJ9142750．551．888．900．4600012．70．071702．1446．2±8．8XJ101541170．781．899．300．4300012．70．069002．1430．4±8．6XJ11117470．420．806．300．480007．30．062602．5391．7±9．5XJ125792660．470．6733．100．490003．40．066101．7412．5±6．7XJ131551501．001．129．500．5100010．20．071002．1442．2±8．8XJ141911810．981．4811．500．420009．30．069201．9431．1±7．9XJ156745050．780．3842．000．530003．10．072401．7450．3±7．4XJ162923281．163．8319．500．5000012．00．074902．0465．5±8．9XJ175031800．370．4429．100．550003．40．067001．7418．2±7．0XJ194142730．680．6923．600．470004．80．065901．8411．3±7．2XJ202012331．200．9811．600．450007．60．066502．0415．1±8．2

注：206Pbc指普通鉛中206Pb;206Pb*指放射成因鉛中206Pb; 應(yīng)用204Pb實(shí)測(cè)值校正普通鉛, 并假設(shè)206Pb/238U和207Pb/235U年齡一致。

圖6 西井巖體鋯石CL圖像及各測(cè)試點(diǎn)的年齡結(jié)果Fig.6 CL images and the results of analyze point for zircons from Xijing intrusion

圖7 西井巖體鋯石U-Pb諧和年齡及加權(quán)平均年齡示意圖Fig.7 U-Pb Concordia age and weighted mean age of zircons from Xijing intrusion

宋述光等[41]根據(jù)榴輝巖相的礦物成分、共生關(guān)系及其變質(zhì)作用的p-T條件，并結(jié)合同位素年代學(xué)數(shù)據(jù)提出榴輝巖相的峰期變質(zhì)階段的p-T條件為2.2～2.6 GPa，變質(zhì)時(shí)代為464～490 Ma。如圖8b所示：2.6 GPa對(duì)應(yīng)的變質(zhì)深度為80 km，巖石圈熔融溫度為1 110 ℃。1 370 ℃軟流圈通過(guò)大地?zé)崃鱾鞑ド仙降蒯r石圈可以發(fā)生熔融的80 km深度的濕的橄欖巖層需要幾十個(gè)百萬(wàn)年，而高壓榴輝巖可以在大陸俯沖過(guò)程中的任何時(shí)間和深度從俯沖板塊脫離，因此西井巖體的成巖年齡和超高壓變質(zhì)年齡相差60 Ma是合理的。

表5 西井巖體Rb-Sr、Sm-Nd同位素組成

a.下地殼(LC)值據(jù)文獻(xiàn)[32-33]; 原始地幔(PM)、洋島玄武巖(OIB)和大洋中脊玄武巖(MORB)值據(jù)文獻(xiàn)[25]; 軟流圈熔體區(qū)域來(lái)自于MORB的Nd和Sr同位素?cái)?shù)據(jù)，巖石圈地幔熔體區(qū)域來(lái)自文獻(xiàn)[34]，阿爾卑斯鎂鐵質(zhì)侵入體區(qū)域引自文獻(xiàn)[35-36]。b.地幔發(fā)生熔融條件，干的軟流圈用干的橄欖石固相線表示[42]，巖石圈熔融用角閃橄欖巖固相線表示[43]。DMM.虧損地幔源區(qū)。圖8 西井巖體εNd (421 Ma)-(87Sr/86Sr)i相關(guān)圖(a)和巖石圈熔融條件示意圖(b)Fig.8 Correlation diagram of εNd (421 Ma) and (87Sr/86Sr)i of Xijing intrusion (a) and conditions for melting of lithosphere (b)

5.2 地殼混染

選擇具有相似配分系數(shù)的微量元素比值來(lái)判斷西井巖體巖漿的混染程度，因?yàn)檫@些比值在部分熔融和巖漿分異過(guò)程中不會(huì)發(fā)生改變。西井巖體Th/U值(3.97)較幾種主要源區(qū)特征比值沒(méi)有明顯的差異，Nb/U值(5.09)、Ce/Pb值(1.98)二者相較典型的洋中脊玄武巖、洋島玄武巖、地殼、典型地幔等環(huán)境特征比值都更接近于地殼值，均顯示西井巖體遭受一定程度的地殼物質(zhì)混染。

由于西井巖體的稀土配分曲線為平坦型，具有島弧拉斑玄武巖特征，筆者選擇原始地幔演化巖漿的微量元素作為模擬起始端元，上地殼和下地殼為終點(diǎn)端元，用不受蝕變影響的微量元素比值進(jìn)行模擬計(jì)算，結(jié)果表明西井巖體在巖漿侵位過(guò)程中遭受小于10%下地殼物質(zhì)混染(圖9)。同樣選擇原始地幔演化的巖漿和上、下地殼為終點(diǎn)端元，進(jìn)行同位素模擬計(jì)算，計(jì)算結(jié)果如圖8a所示，亦表明西井巖體經(jīng)歷了5%～15%的下地殼物質(zhì)混染。

上地殼(UC)和下地殼(LC)值據(jù)文獻(xiàn)[32-33]；原始地幔(PM)、洋島玄武巖(OIB)和E型大洋中脊玄武巖(E-MORB)值據(jù)文獻(xiàn)[25]。圖9 西井巖體Nb/Yb-Th/Yb相關(guān)圖(a)和Zr/Nb-La/Sm相關(guān)圖(b)Fig.9 Plot of Nb/Yb-Th/Yb (a) and Zr/Nb-La/Sm (b) of Xijing intrusion

5.3 西井巖體成因模型

祁連地塊和阿拉善地塊的碰撞開(kāi)始于祁連洋殼俯沖進(jìn)入海溝之后(圖10a)，由于祁連地塊為厚的陸殼具有浮力，與冷的致密的祁連洋殼產(chǎn)生的向下拖拽力方向相反。因此，在祁連地塊和祁連洋殼之間的過(guò)渡區(qū)存在拉伸力，該拉伸力隨著深度加深呈數(shù)量級(jí)增加。Kusznir和Park[44]指出在巖石圈伸展過(guò)程中會(huì)發(fā)生局部應(yīng)變，當(dāng)應(yīng)變率非常高時(shí)，將產(chǎn)生細(xì)小斷裂，熱的低黏度軟流圈物質(zhì)進(jìn)入斷裂(圖10b)。在板塊連續(xù)俯沖過(guò)程中，大陸巖石圈隨著溫度升高而變軟，導(dǎo)致浮力增大，大洋巖石圈繼續(xù)向下拖拽，軟流圈上涌引起局部加熱且過(guò)渡區(qū)域巖石圈減薄，這些因素綜合作用于上述細(xì)小斷裂，最終發(fā)生板塊破裂，祁連洋殼與陸殼分離。隨著大洋板塊斷裂，軟流圈物質(zhì)快速上升進(jìn)入巖石圈破碎空間，且持續(xù)撞擊地幔楔上部厚的地幔巖石圈(圖10c)。

圖10 龍首山巖帶鎂鐵-超鎂鐵質(zhì)巖體形成過(guò)程Fig.10 Geodynamic model of Longshoushan mafic-ultramafic intrusion belt

在減壓過(guò)程中，軟流圈并不會(huì)發(fā)生熔融，如圖8b所示，軟流圈只有在沿著1 280℃絕熱線上升到50 km深度的時(shí)候才會(huì)發(fā)生熔融，該深度是下地殼深度。板塊破裂導(dǎo)致熱的軟流圈和厚的地幔巖石圈共存于斷裂空間。假如地幔巖石圈在早期祁連洋俯沖過(guò)程中被交代而含水，存在如角閃石和金云母等含水礦物，這將使得固相線降低(圖8b)，引起地幔巖石圈小范圍的熔融，這些熔體將先被捕獲[45]。隨著大洋巖石圈和大陸巖石圈的分離，更多的地幔巖

石圈和熱的軟流圈物質(zhì)接觸(圖10d)，使得斷裂附近地幔巖石圈底部被持續(xù)加熱，由于熱傳導(dǎo)效應(yīng)，大地?zé)崃餮刂蒯r石圈上升，使得80 km深度的濕的橄欖巖層發(fā)生熔融，產(chǎn)生玄武質(zhì)巖漿，巖漿的同位素組分位于巖石圈地幔，這與西井巖體的同位素特征相對(duì)應(yīng)(圖8a)。玄武巖質(zhì)巖漿繼續(xù)上升和地殼物質(zhì)發(fā)生混染，地球化學(xué)模擬表明，西井巖體巖漿在上升過(guò)程中遭受了約10%下地殼物質(zhì)混染。

由于板塊斷裂導(dǎo)致熱傳導(dǎo)沿著斷裂走向分布，且地幔巖石圈發(fā)生熔融的深度和斷裂深度之間距離保持一定，這將導(dǎo)致產(chǎn)生的巖漿作用在地殼表現(xiàn)為與板塊斷裂走向一致的方向分布，且?guī)r漿作用的時(shí)間將會(huì)集中在很窄的時(shí)間段，這一機(jī)理可以很好地解釋沿著龍首山斷裂分布的龍首山鎂鐵--超鎂鐵質(zhì)巖帶中早志留世鎂鐵質(zhì)巖體成因。

板塊斷裂使祁連地塊大陸巖石圈和大洋巖石圈分離，熱的低密度的軟流圈物質(zhì)導(dǎo)致俯沖板塊快速升溫并給大陸板塊提供浮力，使得超高壓相巖石作為浮力塊體返回近地表[46-47]。這樣很好地解釋了北祁連榴輝巖相高壓變質(zhì)巖帶與龍首山鎂鐵--超鎂鐵質(zhì)巖帶共存的現(xiàn)象。

6 結(jié)論

1)西井巖體的成巖年齡為(421.0±9.0)Ma，可以和北祁連高壓變質(zhì)帶榴輝巖年齡相對(duì)應(yīng)。εNd(t)為4.06～5.52，(87Sr/86Sr)i為0.704 548～0.707 575，具有巖石圈地幔特征，位于阿爾卑斯造山帶侵入體范圍之內(nèi)。可用板塊斷裂模式解釋西井巖體和龍首山巖帶早志留世鎂鐵質(zhì)侵入巖體成因。

2)微量元素及同位素計(jì)算表明，西井鎂鐵質(zhì)巖體形成過(guò)程中經(jīng)歷了約10%的下地殼物質(zhì)同化混染。

3)西井巖體成因模式為：祁連洋殼連續(xù)俯沖過(guò)程中，洋殼與陸殼分離，熱的軟流圈物質(zhì)持續(xù)沖擊地幔巖石圈的底部，形成玄武質(zhì)巖漿，巖漿上升過(guò)程中和下地殼物質(zhì)發(fā)生約10%混染，形成包括西井巖體在內(nèi)的龍首山鎂鐵-超鎂鐵質(zhì)巖帶。

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Genesis of Xijing Intrusion from Longshoushan Terrane and the Tectonic Significance

Duan Jun1, Qian Zhuangzhi1,2, Jiao Jiangang1,2, Lu Hao3, Feng Yanqing1

1.CollegeofEarthSciencesandResources,Chang’anUniversity,Xi’an710054,China2.MOEKeyLaboratoryofWesternChinaMineralResourcesandGeologicalEngineering,Xi’an710054,China3.TheThirdExplorationInstituteofGeologyandMineralResourcesofShandongProvince,Yantai264004,Shandong,China

The Xijing mafic intrusion is located in Longshoushan terrane in the west of Alxa block and the north of Qilian orogenic belt. Previous interpretation for mafic-ultramafic intrusions along Longshoushan rift suggested that the genesis of these intrusions were related to the magmatism of Jinchuan intrusion. The major petrographic components of Xijing intrusion are olivine websterite and gabbro, and Xijing intrusion was formed in (421.0±9.0) Ma, which is consistent with the age of eclogites from North Qilian orogenic belt. The values ofεNd(t) and (87Sr/86Sr)iare 4.06-5.52 and 0.704 548-0.707 575 respectively, which lie in the field of lithospheric mantle. The calculation of isotope and trace elements indicates that Xijing intrusion was contaminated by the lower crustal materials in about 10%. At last, we interpret the evolution of Xijing intrusion and Longshoushan intrusion belt as follows: during Qilian continental subduction, the ocean lithosphere detached from the continental lithosphere, the hot asthenosphere impinged the base of the overriding mantle lithosphere near the breakoff point. Because of the conduction, the heat flow went up and caused the melting of lithosphere at solidus of a hydrated peridotite at a depth of 80 km, which produced basalt magmatism. During the ascent of magma through the mantle into the crust, the magma was contaminated with lower crust and formed the Xijing intrusion and Longshoushan mafic-ultramafic intrusion belt.

SHRIMP dating; geochemistry; Xijing mafic intrusion; Longshoushan mafic-ultramafic intrusion belt; north of Qilian qrogenic belt

10.13278/j.cnki.jjuese.201503115.

2014-12-10

國(guó)家自然科學(xué)基金項(xiàng)目(41072058)；中國(guó)地質(zhì)調(diào)查局項(xiàng)目(1212011085061,12120114044401)；國(guó)家留學(xué)基金委項(xiàng)目(201306560011)

段俊(1986--)，男，博士研究生，主要從事礦物學(xué)、巖石學(xué)、礦床學(xué)方面研究，E-mail:duanjun108@163.com。

10.13278/j.cnki.jjuese.201503115

P588.1

A

段俊，錢(qián)壯志，焦建剛，等.甘肅龍首山巖帶西井鎂鐵質(zhì)巖體成因及其構(gòu)造意義.吉林大學(xué)學(xué)報(bào):地球科學(xué)版,2015,45(3):832-846.

Duan Jun, Qian Zhuangzhi, Jiao Jiangang，et al. Genesis of Xijing Intrusion from Longshoushan Terrane and the Tectonic Significance.Journal of Jilin University:Earth Science Edition,2015,45(3):832-846.doi:10.13278/j.cnki.jjuese.201503115.