北祁連縫合帶油葫蘆溝玄武巖地球化學特征

李　冰,張耀玲,王超群,胡道功

北祁連縫合帶油葫蘆溝玄武巖地球化學特征

李冰1,2,張耀玲1,王超群1,胡道功1

(1.中國地質科學院地質力學研究所,北京100081；2.中國地質科學院,北京100037)

對油葫蘆溝蛇綠巖中玄武巖進行了地球化學研究,以探討其形成環(huán)境。油葫蘆溝蛇綠巖中玄武巖的主量和微量元素具有低鉀富鈉的特征,屬低鉀 (拉斑)系列。玄武巖的球粒隕石標準化稀土元素分配模式圖表現(xiàn)為近平坦型,輕重稀土富集不明顯,屬于虧損地幔 (N型)。大離子親石元素 (LILE)Rb、K和Sr出現(xiàn)負異常,高場強元素 (HFSE)Nb、Ta、Hf、Zr無負異常,表現(xiàn)出大洋中脊幔源巖漿作用的特征。構造環(huán)境判別圖顯示,油葫蘆溝玄武巖屬于典型的虧損型大洋中脊玄武巖 (MORB)。研究結果表明油葫蘆溝蛇綠巖中玄武巖為北祁連洋洋殼的組成部分,北祁連洋在晚元古代—晚寒武世為典型的大洋中脊環(huán)境。

北祁連縫合帶;油葫蘆溝;玄武巖;地球化學;洋中脊

北祁連縫合帶呈北西向分布于中祁連陸塊和阿拉善陸塊之間,西端被阿爾金斷裂切割,向東與秦嶺造山帶相聯(lián)接,縫合帶內主要地層為寒武—奧陶系,并發(fā)育多條蛇綠混雜巖帶[1～3]。北祁連蛇綠巖帶作為中國最早發(fā)現(xiàn)的蛇綠巖之一,研究程度較高[4～6]。蛇綠巖體主要沿南北兩條帶分布,南帶沿熬油溝、玉石溝、油葫蘆溝、東草河、達坂山到永登一帶展布；北帶沿甘肅九個泉、大岔大阪、扁都口至老虎山一線,全長超過1000 km。前人對北祁連蛇綠巖的地球化學特征做了不同程度的研究工作,但對于其形成環(huán)境為大洋中脊[7～10]還是弧后盆地[11～12]仍存在爭議。本文對油葫蘆溝蛇綠巖中玄武巖進行了地球化學研究,以探討其形成環(huán)境。

1　地質概況及油葫蘆溝玄武巖特征

北祁連加里東造山帶[11,13～14]主體經(jīng)托萊山、大通北山、達坂山、白銀、隴縣等地與商丹縫合帶相連,大致以中祁連北緣深斷裂為主斷裂。主斷裂帶西起托萊河谷,東經(jīng)托萊南山、達坂山南坡入甘肅境內,呈北西—北西西向延伸,構成中祁連陸塊與北祁連縫合帶的分界線。油葫蘆溝蛇綠巖出露于祁連縣西約50 km(見圖1),位于北祁連蛇綠巖帶南帶西段,北祁連加里東期俯沖增生楔的淺部單元內[15]。縫合帶內地層主要為早奧陶世陰溝群、中寒武世黑刺溝組及長城系朱龍關群。

圖1　北祁連主要蛇綠巖帶分布及年齡圖(據(jù)文獻[16]修編)Fig.1　Geological map of the North Qilian Orogen with localities of major ophiolite suites and their ages

油葫蘆溝蛇綠巖主要由灰綠色玄武巖組成,劈理發(fā)育 (見圖2)。鏡下觀察表明,巖石由斑晶和基質組成,具少斑狀結構 (見圖3),基質似交織結構,塊狀構造,少量杏仁狀構造。斑晶為斜長石 (<1%)和輝石 (<1%)；基質為斜長石 (55%～70%)、輝石 (25%～40%)和隱晶質 (<5%)。本文沿油葫蘆溝 (N 38°13′57.72″,E 99°46′10.48″)采取5個玄武巖樣品 (B063-1—B063-4,B063-6)進行地球化學分析。

圖2　玄武巖巖石特征Fig.2　photography of basalts

圖3　玄武巖顯微結構Fig.3　Microstructure of basalt samples

2　玄武巖的巖石地球化學特征

玄武巖樣品的主量和微量元素分析均在國家地質實驗測試中心完成,結果見表1、表2。

表1 油葫蘆溝玄武巖主量元素分析結果Table 1 Major elements of basalts in Youhulugou area　%

表2 油葫蘆溝玄武巖微量元素分析結果Table 2 Trace elements of basalts in Youhulugou area　μg/g

2.1主量元素地球化學特征

由表1可見,玄武巖中K2O+Na2O在3.79%～4.75%,Na2O含量 (3.75%～4.63%)大于K2O含量 (0.02%～0.14%),具有相對低鉀富鈉的特征；且K2O/Na2O比值為0.01～0.03,屬于低堿性火山巖系列。在TAS分類圖解 (見圖4a)中,樣品落入玄武巖及玄武安山巖區(qū)；在K2O-SiO2圖解 (見圖4b)上,玄武巖屬于低鉀拉斑系列。

在Zr/Y-Zr構造環(huán)境判別圖 (見圖5a)中,樣品落入洋中脊玄武巖 (MORB)區(qū),部分與島弧玄武巖 (IAB)區(qū)重疊；TiO2-Zr分類圖 (見圖5b)中,樣品落入洋中脊玄武巖(MORB)、島弧玄武巖 (IAB)和板內玄武巖 (WPB)的重疊區(qū)域。在P2O5-TiO2(見圖5c)和Th/Yb-Ta/Yb(見圖5d)構造環(huán)境判別圖解中,5個玄武巖樣品全部落入大洋中脊玄武巖 (MORB)區(qū)域。

2.2稀土和微量元素地球化學特征

由表2可知,玄武巖稀土元素總量 (ΣREE)為81.49～94.89 μg/g,平均值87.16 μg/g；輕稀土總量 (ΣLREE)為24.53～39.43 μg/g,平均30.48 μg/g,重稀土含量 (ΣHREE) 54.10～60.67 μg/g,平均56.68 μg/g。ΣLREE/ΣHREE值為0.42～0.71,平均0.54,顯示二者無明顯分異。Δ(Eu)值為0.77～0.97,平均0.91；δ(Ce)值為0.98～1.03,平均1.00；(La/Yb)N值0.63～1.26,平均0.86；(La/Sm)N值為0.69～1.20,平均0.86,小于1。在球粒隕石標準化稀土元素分配模式圖 (見圖6a)上,樣品的稀土元素配分模式為平坦到略虧損型,屬典型的虧損型大洋中脊玄武巖 (N-MORB)的稀土元素分布模式。微量元素原始地幔標準化圖解 (見圖6b)顯示,玄武巖從強不相容元素到弱不相容元素同樣表現(xiàn)向左傾斜的特征。大離子親石元素 (LILE)Rb、K和Sr出現(xiàn)負異常,高場強元素 (HFSE)Nb、Ta、Ti元素無負異常,排除了玄武巖形成于島弧環(huán)境的可能性?？傮w表現(xiàn)出大洋中脊幔源巖漿作用的特征,進一步表明該玄武巖形成于大洋中脊構造環(huán)境。

圖4　油葫蘆溝地區(qū)玄武巖巖性判別圖Fig.4　Discrimination diagrams of basalt in Youhulugou area

圖5　油葫蘆溝地區(qū)玄武巖類構造環(huán)境判別圖Fig.5　Tectonic environment discrimination diagrams of basalts from Youhulugou area

圖6　北祁連油葫蘆溝玄武巖類稀土元素球粒隕石標準化配分模式圖和微量元素原始地幔標準化多元素蛛網(wǎng)圖 (原圖據(jù)文獻 [27～28])Fig.6　Chondrite-normalized REE distribution patterns and primitive mantle-normalized trace element spider diagram of volcanic rocks in Youhulugou of North Qilian Orogen

3　討論與結論

3.1油葫蘆溝蛇綠巖形成的構造環(huán)境

蛇綠巖被認為是洋中脊擴張或板塊俯沖消減過程的產(chǎn)物[29～30],玄武巖是蛇綠巖巖石組合中最為常見的一類,因此蛇綠巖內玄武巖可為蛇綠巖的形成環(huán)境提供直接證據(jù)。地球化學研究表明,油葫蘆溝蛇綠巖中玄武巖低鉀富鈉,屬于低鉀 (拉斑)系列；構造環(huán)境判別圖顯示,玄武巖屬于典型的虧損型大洋中脊玄武巖 (N-MORB)；稀土和微量元素分析結果顯示,巖漿具有明顯的大洋中脊幔源巖漿作用的特征,均表明油葫蘆溝蛇綠巖形成于大洋中脊的構造環(huán)境。

3.2北祁連洋早古生代構造演化

北祁連造山帶的形成與演化經(jīng)歷了復雜的過程,華北地塊與柴達木地塊間的洋盆經(jīng)歷了陸緣拉張、形成大洋到閉合造山的過程[5～6,9,13,31]。研究表明,北祁連南部蛇綠巖帶在晚元古代到晚寒武世 (距今550～497 Ma)發(fā)育洋盆[7,10,32],處于洋脊擴張、洋殼生長階段,油葫蘆溝蛇綠巖即為此階段形成；北部蛇綠巖帶在早—中奧陶世 (距今490～448.5 Ma)處于弧后盆地或火山弧構造環(huán)境,說明北祁連洋在該時代處于島弧火山作用階段,洋殼向北俯沖并形成島弧及弧后盆地[12,16]。北祁連洋向北俯沖的俯沖雜巖帶高壓變質榴輝巖年齡為463～468 Ma,反映出俯沖作用及其引發(fā)的島弧火山活動主要發(fā)生在中奧陶世[33]。晚奧陶世—志留紀,陸源碎屑巖沉積的出現(xiàn),標志著北祁連洋洋盆已趨于閉合階段[34]。至此,從晚元古代到志留紀,北祁連洋經(jīng)歷了大陸裂解、洋盆擴張、俯沖消減到洋盆閉合的全過程,進入碰撞造山階段。

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GEOCHEMICAL CHARACTERISTICS OF THE YOUHULUGOU BASALTS IN THE SUTURE ZONE OF THE NORTH QILIAN MOUNTAIN

LI Bing1,2,ZHANG Yao-ling1,WANG Chao-qun1,HU Dao-gong1
(1.Institute of Geomechanics,Chinese Academy of Geological Science,Beijing 100081,China；2.Chinese Academy of Geological Science,Beijing 100037,China)

Geochemical study for the basalt of Youhulugou ophiolite was carried out and discussed its genesis environment.Major and trace element in basalt is one of the representative ophiolite fragments in the suture zone of the North Qilian Mountain.In this study,geochemical study and indicates that the basalts from the Youhulugou ophiolite have N-MORB type characteristics.And according to the REE distribution patterns with no anomaly,the basalts are also similar to mid-ocean ridge basalt.Meanwhile,these basalts are generally depleted in large ion lithophile elements (LILE)of Rb、K and Sr,and high field strength elements(HFSE)are normal,indicating the characteristicsofmantle-derivedmagmafrommidoceanicridge.Tectonicenvironment discrimination diagrams also suggest that these basalts are typical mid oceanic ridge basalts (MORB).Combined with regional geological characters and geochemical data,we conclude that the Youhulugou basalts represents a part of the dismembered North Qianlian oceanic crust of the era. And the tectonic setting of the North Qilian Ocean was mid-ocean ridge during the late Proterozoic to late Cambrian.

North Qilian suture zone；Youhulugou basalts；geochemical characteristics；MOR

P632

A

1006-6616(2016)01-0048-08

2015-09-15

中國地質科學院基本科研業(yè)務費專項項目 (YYWF201401)；中國地質調查局 “祁連山及鄰區(qū)天然氣水合物資源勘查 (控礦因素綜合研究及編圖)”項目 (GZHL20120301)

李冰 (1987-),男,碩士研究生,構造地質學專業(yè)。E-mail:libing@cags.ac.cn