段 俊，錢壯志,2，焦建剛,2，魯 浩，馮延清

1.長安大學地球科學與資源學院,西安 710054 2.西部礦產(chǎn)資源與地質(zhì)工程教育部重點實驗室,西安 710054 3.山東省地質(zhì)礦產(chǎn)勘查開發(fā)局第三地質(zhì)大隊,山東 煙臺 264004

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甘肅龍首山巖帶西井鎂鐵質(zhì)巖體成因及其構造意義

段 俊1，錢壯志1,2，焦建剛1,2，魯 浩3，馮延清1

1.長安大學地球科學與資源學院,西安 710054 2.西部礦產(chǎn)資源與地質(zhì)工程教育部重點實驗室,西安 710054 3.山東省地質(zhì)礦產(chǎn)勘查開發(fā)局第三地質(zhì)大隊,山東 煙臺 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的巖漿事件進行，而本次選擇西井鎂鐵質(zhì)巖體進行了精確的地質(zhì)年代學和地球化學研究，確定了西井巖體巖性主要為橄欖輝石巖和輝長巖，成巖時代為 (421.0±9.0) Ma，可以和北祁連高壓變質(zhì)帶榴輝巖年齡相對應；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；由于熱傳導效應，大地熱流沿著地幔巖石圈上升，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測年；地球化學；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、復理石、磨拉石和上覆蓋層的造山縫合帶。夏林圻等[1-2]運用區(qū)域構造和火山巖漿演化動力學思路研究了北祁連海相火山巖成因；吳才來等[3]確定了北祁連早古生代花崗質(zhì)巖漿事件； Song 等[4-9]對北祁連超高壓變質(zhì)帶進行了年齡分析；這些都從北祁連演化的角度對上述巖石單元進行了很好的解釋。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，構成龍首山巖帶。由于該巖帶中存在著含世界級巖漿鎳-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)巖體與金川巖體進行對比，以證明這些巖體與金川巖體的關系，并試圖在金川外圍繼續(xù)尋找與金川巖漿事件相關的巖漿硫化物礦床。焦建剛等[10-11]通過龍首山巖帶中幾個典型鎂鐵--超鎂鐵質(zhì)巖體巖石地球化學分析，提出龍首山巖帶巖體處于相同的構造環(huán)境，存在連續(xù)的巖漿演化關系；閆海卿等[12]認為野芨里巖體具有與金川巖體相似的地球化學特征；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)巖體為研究對象，進行精確的地質(zhì)年代學和地球化學研究。根據(jù)西井巖體的鋯石U-Pb年齡和地球化學模擬計算，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)巖帶形成時的地球動力學背景。

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)。阿拉善地塊是一個三角形地塊，位于華北克拉通西緣[15]，基底為早前寒武紀石英閃長質(zhì)/花崗質(zhì)片麻巖，上覆蓋層為寒武紀到中奧陶世地層。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個地段：西段以藏布臺巖體為代表，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、西兩個巖體；西巖體長600 m，寬60～100 m；東巖體長80 m，寬65 m。整個巖體在地表露頭共有19個，最小長約20 m，寬約15 m；最大長達30～195 m；巖性主要為橄欖輝石巖和輝長巖。

橄欖輝石巖：黑綠色，中粒結(jié)構，塊狀構造。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，由于構造擾動而略具定向排列(圖3a)。單斜輝石位于橄欖石周圍，少數(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[14]修編。圖1 中國主要構造單元簡圖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)

序號巖體名稱地理位置巖體特征圍巖特征1藏布臺山丹縣城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，鈉長陽起片巖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號異常金川鎳礦Ⅲ礦區(qū)北西2km斜長二輝橄欖巖，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

輝長巖：暗綠色，較為新鮮，輝長結(jié)構(圖3d)。主要礦物組成為：單斜輝石35%，斜長石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。長石表面干凈，雙晶發(fā)育。巖石結(jié)構構造表明礦物結(jié)晶順序為：橄欖石→ 斜方輝石→單斜輝石→斜長石。

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。礦物成分測試用JXI-8100型電子探針完成，測試條件為加速電壓15 kV，電流1.0×10-8A，束斑直徑1 μm。主量元素采用XRF-1800 型X射線熒光光譜儀測試，XRF熔片法依據(jù)國家標準GB/T14506.28-1993。微量元素分析采用美國X-7型ICP-MS完成，上述工作在長安大學西部礦產(chǎn)資源與地質(zhì)工程教育部重點實驗室進行。

Nd-Sr同位素測試在中國地質(zhì)科學院完成，分析方法為同位素稀釋法，測試儀器為MAT262固體同位素質(zhì)譜計，測試方法參見文獻[19]。鋯石SHRIMP測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。將挑選好的待測樣品鋯石與RSES 參考樣SL13及數(shù)粒TEM置于環(huán)氧樹脂制靶。鋯石陰極發(fā)光及SHRIMP U-Pb測年詳細分析流程和原理參考文獻[20-22] 。

4 測試結(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成分見表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行100%計算，然后將全巖主量元素成分和主要堆晶礦物橄欖石、輝石的平均化學成分進行對比(圖4)，巖體全巖主量元素成分并沒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。薄片中可見到斜長石、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。

球粒隕石標準化的稀土元素和原始地幔標準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

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)量分數(shù)過高，故將其剔除。其他16個鋯石的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同位素測定結(jié)果及特征值見表5。(87Sr/86Sr)i值按照421 Ma計算，結(jié)果為0.704 548～0.707 575，平均0.706 125。εNd(421 Ma)的變化范圍相對較窄，4.06～5.52，平均4.88。由于Rb-Sr比Sm-Nd更具有活動性，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應。

表2 西井巖體中主要礦物平均化學成分

注:n為測點數(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)量分數(shù)單位為%；微量元素質(zhì)量分數(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[25]。圖5 西井巖體球粒隕石標準化稀土元素和原始地幔標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

測點號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; 應用204Pb實測值校正普通鉛, 并假設206Pb/238U和207Pb/235U年齡一致。

圖6 西井巖體鋯石CL圖像及各測試點的年齡結(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、共生關系及其變質(zhì)作用的p-T條件，并結(jié)合同位素年代學數(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)時代為464～490 Ma。如圖8b所示：2.6 GPa對應的變質(zhì)深度為80 km，巖石圈熔融溫度為1 110 ℃。1 370 ℃軟流圈通過大地熱流傳播上升到地幔巖石圈可以發(fā)生熔融的80 km深度的濕的橄欖巖層需要幾十個百萬年，而高壓榴輝巖可以在大陸俯沖過程中的任何時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[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[25]; 軟流圈熔體區(qū)域來自于MORB的Nd和Sr同位素數(shù)據(jù)，巖石圈地幔熔體區(qū)域來自文獻[34]，阿爾卑斯鎂鐵質(zhì)侵入體區(qū)域引自文獻[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)的微量元素比值來判斷西井巖體巖漿的混染程度，因為這些比值在部分熔融和巖漿分異過程中不會發(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端元，用不受蝕變影響的微量元素比值進行模擬計算，結(jié)果表明西井巖體在巖漿侵位過程中遭受小于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、下地殼為終點端元，進行同位素模擬計算，計算結(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[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[25]。圖9 西井巖體Nb/Yb-Th/Yb相關圖(a)和Zr/Nb-La/Sm相關圖(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始于祁連洋殼俯沖進入海溝之后(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渡區(qū)存在拉伸力，該拉伸力隨著深度加深呈數(shù)量級增加。Kusznir和Park[44]指出在巖石圈伸展過程中會發(fā)生局部應變，當應變率非常高時，將產(chǎn)生細小斷裂，熱的低黏度軟流圈物質(zhì)進入斷裂(圖10b)。在板塊連續(xù)俯沖過程中，大陸巖石圈隨著溫度升高而變軟，導致浮力增大，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渡區(qū)域巖石圈減薄，這些因素綜合作用于上述細小斷裂，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)巖體形成過程Fig.10 Geodynamic model of Longshoushan mafic-ultramafic intrusion belt

在減壓過程中，軟流圈并不會發(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深度的時候才會發(fā)生熔融，該深度是下地殼深度。板塊破裂導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，由于熱傳導效應，大地熱流沿著地幔巖石圈上升，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應(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，地球化學模擬表明，西井巖體巖漿在上升過程中遭受了約10%下地殼物質(zhì)混染。

由于板塊斷裂導致熱傳導沿著斷裂走向分布，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漿作用的時間將會集中在很窄的時間段，這一機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)帶榴輝巖年齡相對應。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)?？捎冒鍓K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算表明，西井鎂鐵質(zhì)巖體形成過程中經(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。

[1] 夏林圻, 夏祖春, 徐學義. 北祁連山海相火山巖巖石成因[M]. 北京: 地質(zhì)出版社, 1996: 1-149. Xia Linqi, Xia Zuchun, Xu Xueyi. Petrogenesis of Marine Volcanic Rocks in the North Qilian Mountains[M]. Beijing: Geological Publishing House, 1996: 1-149.

[2] Xia L Q, Xia Z C, Xu X Y. Magmageneisis in the Ordovician in Back Basins of the Northern Qilian Mountains, China[J]. Geological Society of America Bulletin, 2003, 115: 1510-1522.

[3] 吳才來, 徐學義, 高前明, 等. 北祁連早古生代花崗質(zhì)巖漿作用及其構造演化[J]. 巖石學報, 2010, 26(4): 1027-1044. Wu Cailai, Xu Xueyi, Gao Qianming, et al. Early Palaezoic Granitoid Magmatism and Tectonic Evolution in North Qilian, NW China[J]. Acta Petrologica Sinica, 2010, 26(4): 1027-1044.

[4] Song S, Zhang L, Niu Y. Ultra-Deep Origin of Garnet Peridotite from the North Qaidam Ultrahigh-Pressure Belt, Northern Tibetan Plateau, NW China[J]. American Mineralogist, 2004, 89: 1330-1336.

[5] 宋述光, 張立飛,Niu Y L, 等. 北祁連山榴輝巖鋯石SHRIMP定年及其構造意義[J]. 科學通報, 2004, 49: 848-852. Song Shuguang, Zhang Lifei, Niu Y L, et al. Zircon U-Pb SHRIMP Ages of Eclogites from the North Qilian Mountains, NW China and Their Tectonic Implication[J]. Chinese Science Bulletin, 2004, 49: 848-852.

[6] Song S G, Zhang L E, Niu Y L, et al. Geochronology of Diamond-Bearing Zircons from Garnet-Peridotite in the North Qaidam UHPM Belt, North Tibetan Plateau: A Record of Complex Histories Associated with Continental Collision[J]. Earth and Planetary Science Letters, 2005, 234: 99-118.

[7] Song S G, Zhang L F, Niu Y L, et al. Evolution from Oceanic Subduction to Continental Collision: A Case Study of the Northern Tibetan Plateau Inferred from Geochemical and Geochronological Data[J]. Journal of Petrology, 2006, 47: 435-455.

[8] Song S G, Zhang L F, Niu Y L, et al. Eclogite and Carpholite-Bearing Meta-Pelite in the North Qilian Suture Zone, NW China: Implications for Paleozoic Cold Oceanic Subduction and Water Transport into Mantle[J]. Journal of Metamorphic Geology, 2007, 25: 547-563.

[9] Song S G, Niu Y L, Zhang L, et al. Tectonic Evolution of Early Paleozoic HP Tamorphic Rocks in the North Qilian Mountains, NW China: New Perspectives[J]. Journal of Asian Earth Sciences, 2009, 35: 334-353.

[10] 焦建剛, 閆海卿, 錢壯志, 等. 龍首山巖帶典型鎂鐵--超鎂鐵質(zhì)巖體巖石地球化學特征[J]. 礦物巖石, 2006, 26(1): 49-56. Jiao Jiangang, Yan Haiqing, Qian Zhuangzhi, et al. Geochemical Characteristics of Typical Mafic-Ultramafic Rocks in Longshou Mountains[J]. Journal of Mineralogical and Petrological Sciences, 2006, 26(1): 49-56.

[11] 焦建剛, 閆海卿, 劉瑞平. 龍首山幾個鎂鐵-超鎂鐵質(zhì)巖體比較[J]. 地質(zhì)與勘探, 2006, 42(5): 60-65. Jiao Jiangang, Yan Haiqing, Liu Ruiping. Comparison of Several Mafic-Ultramafic Intrusions in the Longshoushan Mountain[J]. Geology and Prospectin, 2006, 42(5): 60-65.

[12] 閆海卿, 湯中立, 焦建剛, 等. 內(nèi)蒙古野芨里鎂鐵質(zhì)-超鎂鐵質(zhì)巖體的巖石地球化學特征[J]. 現(xiàn)代地質(zhì), 2005, 19(4): 515-521. Yan Haiqing, Tang Zhongli, Jiao Jiangang, et al. Petrological and Geochemical Characteristics of Yejili Mafic-Ultramafic Intrusion, Inner Mongolia[J]. Geoscience, 2005, 19(4): 515-521.

[13] 李文淵, 湯中立, 郭周平, 等. 阿拉善地塊南緣鎂鐵--超鎂鐵巖形成時代及地球化學特征[J]. 巖石礦物學雜志, 2004, 24(2): 117-126. Li Wenyuan, Tang Zhongli, Guo Zhouping, et al. Petrogenetic Epoch and Geochemical Characteristics of Mafic-Ultramafic Rocks on the Southern Margin of Alxa Massif in Northern China[J]. Acta Petrological Et Mineralogical, 2004, 24(2): 117-126.

[14] Song S G, Niu Y L, Su L, et al. Tectonics of the North Qilian Orogen, NW China[J]. Gondwana Research, 2013, 23: 1378-1401.

[15] 趙國春. 華北克拉通基底主要構造單元變質(zhì)作用演化及其若干問題討論[J]. 巖石學報, 2009, 25: 1772-1792. Zhao Guochun. Metamorphic Evolution of Major Tectonic Units in the Basement of the North China Craton: Key Issues and Discussion[J]. Acta Petrologica Sinica, 2009, 25: 1772-1792.

[16] Wang C Y, Zhang Q, Qian Q, et al. Geochemistry of the Early Paleozoic Baiyin Volcanic Rocks (NW China): Implications for the Tectonic Evolution of the North Qilian Orogenic Belt[J]. Journal of Geology, 2005, 113: 83-94.

[17] Liou J G, Wang X, Coleman R G. Blueschists in Major Suture Zones of China[J]. Tectonics, 1989, 8: 609-619.

[18] Liu Y J, Neubauer F, Genser J, et al.40Ar/39Ar Ages of Blueschist Facies Pelitic Schists from Qingshuigou in the Northern Qilian Mountains, Western China[J]. Island Arc, 2006, 15: 187-198.

[19] 何學賢, 唐索寒, 朱祥坤, 等. 多接收器等離子體質(zhì)譜(MC-ICPMS)高精度測定Nd同位素方法[J]. 地球?qū)W報, 2007, 28(4): 405-410. He Xuexian, Tang Suohan, Zhu Xiangkun, et al. Precise Measurement of Nd Isotopic Ratios by Means of Multi-Collector Magnetic Sector Inductively Coupled Plasma Mass Spectrometry[J]. Acta Geoscience Sinica, 2007, 28(4):405-410.

[20] Compston W, Williams I S, Kirschvink J L, et al. Zircon U-Pb Ages for the Early Cambrian Time-Scale[J]. Journal of the Geological Society, 1992, 149: 171-184.

[21] Williams I S, Claesson S. Isotope Evidence for the Precambrian Province and Caledonian Metamorphism of High Grade Paragneiss from the Seve Nappes, Scandinavian Caledonides, Ⅱ Ion Microprobe Zircon U-Th-Pb[J]. Contributions to Mineralogy and Petrology, 1987, 97: 205-217.

[22] Williams I S. U-Th-Pb Geochromology by Ion Microprobe, Applications of Microanalytical Techniques to Understanding Mineralizing Processes[J]. Reviews in Economic Geology, 1998, 7: 1-35.

[23] 楊言辰, 孫德有, 馬志紅, 等. 紅旗嶺鎂鐵超鎂鐵巖侵入體及銅鎳硫化物礦床的成巖成礦機制[J]. 吉林大學學報: 地球科學版, 2005, 35(5): 594-600. Yang Yanchen, Sun Deyou, Ma Zhihong, et al. The Forming Mechanisms of Hongqiling Mafic and Ultramafic Intrusive Bodies and Cu-Ni Sulfide Deposits[J]. Journal of Jilin University: Earth Science Edition 2005, 35(5): 594-600.

[24] 郝立波, 吳超, 孫立吉, 等. 吉林紅旗嶺銅鎳硫化物礦床Re-Os同位素特征及其意義[J]. 吉林大學學報: 地球科學版, 2014, 44(2): 507-517. Hao Libo, Wu Chao, Sun Liji, et al. Re-Os Isotope Characteristics of Hongqiling Cu-Ni Sulfide Deposit in Jilin Provinece and Its Sibnificance[J]. Journal of Jilin University: Earth Science Edition, 2014, 44(2): 507-517.

[25] Sun S S, McDonough W F. Chemical and Isotopic Systematics of Oceanic Basalts: Implications for Mantle Composition and Processes[J]. Geological Society London, Speciety Publication, 1989, 42: 313-345.

[26] Lassiter J C, Depaolo D J. Plume/Lithosphere Interaction in the Generation of Continental and Oceanic Flood Basalts: Chemical and Isotope Constraints. Mahoney J Large Igneous Provinces: Continental, Oceanic, and Planetary Flood Volcanism[J].Geophy-sical Monography, American Geophysical Union, 1997: 335-355.

[27] 吳元保, 鄭永飛. 鋯石成因礦物學研究及其對U-Pb年齡解釋的制約[J]. 科學通報, 2004, 49(16): 1589-1640. Wu Yuanbao,Zheng Yongfei.The Genetic Mineralogy of Zircon and the Constraint to the Interpretation of U-Pb Age[J]. Chinese Science Bulletin,2004,49(16):1589-1640.

[28] Zhang M, Kamo S L, Li C. Precise U-Pb Zircon-Baddeleyite Age of the Jinchuan Sulfide Ore-Bearing Ultramafic Intrusion, Western China[J]. Mineral Deposit, 2010, 45: 3-9.

[29] 李獻華, 蘇犁, 宋彪, 等. 金川超鎂鐵侵入巖SHRIMP鋯石U-Pb年齡及地質(zhì)意義[J]. 科學通報, 2004, 49: 420-422. Li Xianhua, Su Li, Song Biao, et al. SHRIMP U-Pb Zircon Age of the Jinchuan Ultramafic Intrusion and Its Geological Significance[J]. Chinese Science Bulletin, 2004, 49: 420-422.

[30] Li X H, Su L, Chung S L, et al. Formation of the Jinchuan Ultramafic Intrusion and the World’s Third Largest Ni-Cu Sulfide Deposit: Associated with the Similar to 825 Ma South China Mantle Plume?[J]. Geochemistry, Geophysics, Geosystems, 2005,doi: 10.1029/2005GC001006.

[31] Zhang J X, Meng F C, Wan Y S. A Cold Early Palaeozoic Subduction Zone in the North Qilian Mountains, NW China: Petrological and U-Pb Geochronological Constraints[J]. Journal of Metamorphic Geology, 2007, 25: 285-304.

[32] Zindler A, Hart S. Chemical Geodynamics[J]. Annual Review of Earth Planetary Sciences, 1986, 14: 493-571.

[33] Rudnick R L, Gao S. Composition of the Continental Crust[C]//Rudnick RL. Treatise on Geochemistry. Amsterdam: Elsvier, 2003: 1-64.

[34] Wilson M. Igneous Petrogenesis: A Global Tectonic Approach[M]. London: Unwin Hyman, 1989: 1-466.

[35] Kagami H, Ulmer P, Hansmann W, et al. Nd-Sr Isotopic and Geochemical Characteristics of the Southern Adamello (Northern Italy) Intrusives: Implications for Crustal Versus Mantle Origin[J]. Journal of Geophysical Research: Solid Earth, 1991, 96: 14331-14346.

[36] von Blanckenburg F, Friih-Green G, Diethelm K, et al. Nd-, Sr-and O-Isotopic and Chemical Evidence for a Two-Stage Contamination History of Mantle Magma in the Central Alpine Bergell Intrusion[J]. Contributions to Mineralogy and Petrollogy, 1992, 110: 33-45.

[37] Becker H. Garnet Peridotite and Eclogite Sm-Nd Mineral Ages from the Lepontine Dome (Swiss Alps): New Evidence for Eocene High-Pressure Metamorphism in the Central Alps[J]. Geology, 1993, 21: 599-602.

[38] Christensen J N, Selverstone J, Rosenfeld J L, et al. Correlation by Rb-Sr Geochronology of Garnet Growth Histories from Different Structural Levels Within the Tauern Window, Eastern Alps[J]. Contributions to Mineralogy and Petrollogy, 1994, 118: 1-12.

[39] Tilton G R, Schreyer W, Schertl H-P. Pb-Sr-Nd Isotopic Behavior of Deeply Subducted Crustal Rocks from the Dora Maira Massif, Western Alps, Italy-II: What Is the Age of the Ultra-High-Pressure Metamorphism?[J]. Contributions to Mineralogy and Petrollogy, 1991, 108: 22-33.

[40] Davies J H, von Blanckenburg F. Slab breakoff: A Model of Lithosphere Detachment and Its Test in the Magmatism and Deformation of Collisional Orogens[J]. Earth and Planetary Science Letters, 1995, 129: 85-102.

[41] 宋述光. 北祁連山古大洋俯沖帶高壓變質(zhì)巖研究評述[J]. 地質(zhì)通報, 2009, 28(12): 1769-1778. Song Shuguang. High-Pressure Metamorphic Rocks in the North Qilian Ocean Subduction Zone, China: A Review[J]. Geological Bulletin of China, 2009, 28(12): 1769-1778.

[42] McKenzie D, Bickle M J. The Volume and Composition of Melt Generated by Extension of the Lithosphere[J]. Journal of Petrology, 1988, 29: 625-679.

[43] Mengel K, Green D H. Stability of Amphibole and Phlogopite in Metasomatized Peridotite Under Water-Saturated and Water-Undersaturated Conditions[C]//Fourth International Kimberlite Conference. Kimberlites and Related Rocks-Their Occurrence, Origin and Emplacement. Sydney: Geological Society of Australia, Special Publication, 1989: 571-581.

[44] Kusznir N J, Park R G. The Extensional Strength of the Continental Lithosphere: Its Dependence on Geothermal Gradient, and Crustal Composition and Thickness[C]//Coward M P, Dewey J F, Hancock P L. Continental Extensional Tectonics. London: Geological Society London, Special Publication, 1987: 35-52.

[45] McKenzie D P. Some Remarks on the Movement of Small Melt Fractions in the Mantle[J]. Earth and Planetary Science Letters, 1989, 95: 53-72.

[46] Platt J P. The Uplift of High-Pressure-Low-Tempe-rature Metamorphic Rocks[J]. Philosophical Tran-sactions of the Royal Society of London, 1987, 321: 87-102.

[47] Wheeler J. Structural Evolution of a Subducted Continental Sliver: The Northern Dora Maira Massif, Italian Alps[J]. Journal of the Geological Society, 1991, 148: 1101-1113.

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

國家自然科學基金項目(41072058)；中國地質(zhì)調(diào)查局項目(1212011085061,12120114044401)；國家留學基金委項目(201306560011)

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

10.13278/j.cnki.jjuese.201503115

P588.1

A

段俊，錢壯志，焦建剛，等.甘肅龍首山巖帶西井鎂鐵質(zhì)巖體成因及其構造意義.吉林大學學報:地球科學版,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.