李　冰,张耀玲,王超群,胡道功

北祁连缝合带油葫芦沟玄武岩地球化学特征

李冰1,2,张耀玲1,王超群1,胡道功1

(1.中国地质科学院地质力学研究所,北京100081；2.中国地质科学院,北京100037)

对油葫芦沟蛇绿岩中玄武岩进行了地球化学研究,以探讨其形成环境。油葫芦沟蛇绿岩中玄武岩的主量和微量元素具有低钾富钠的特征,属低钾 (拉斑)系列。玄武岩的球粒陨石标准化稀土元素分配模式图表现为近平坦型,轻重稀土富集不明显,属于亏损地幔 (N型)。大离子亲石元素 (LILE)Rb、K和Sr出现负异常,高场强元素 (HFSE)Nb、Ta、Hf、Zr无负异常,表现出大洋中脊幔源岩浆作用的特征。构造环境判别图显示,油葫芦沟玄武岩属于典型的亏损型大洋中脊玄武岩 (MORB)。研究结果表明油葫芦沟蛇绿岩中玄武岩为北祁连洋洋壳的组成部分,北祁连洋在晚元古代—晚寒武世为典型的大洋中脊环境。

北祁连缝合带;油葫芦沟;玄武岩;地球化学;洋中脊

北祁连缝合带呈北西向分布于中祁连陆块和阿拉善陆块之间,西端被阿尔金断裂切割,向东与秦岭造山带相联接,缝合带内主要地层为寒武—奥陶系,并发育多条蛇绿混杂岩带[1～3]。北祁连蛇绿岩带作为中国最早发现的蛇绿岩之一,研究程度较高[4～6]。蛇绿岩体主要沿南北两条带分布,南带沿熬油沟、玉石沟、油葫芦沟、东草河、达坂山到永登一带展布；北带沿甘肃九个泉、大岔大阪、扁都口至老虎山一线,全长超过1000 km。前人对北祁连蛇绿岩的地球化学特征做了不同程度的研究工作,但对于其形成环境为大洋中脊[7～10]还是弧后盆地[11～12]仍存在争议。本文对油葫芦沟蛇绿岩中玄武岩进行了地球化学研究,以探讨其形成环境。

1　地质概况及油葫芦沟玄武岩特征

北祁连加里东造山带[11,13～14]主体经托莱山、大通北山、达坂山、白银、陇县等地与商丹缝合带相连,大致以中祁连北缘深断裂为主断裂。主断裂带西起托莱河谷,东经托莱南山、达坂山南坡入甘肃境内,呈北西—北西西向延伸,构成中祁连陆块与北祁连缝合带的分界线。油葫芦沟蛇绿岩出露于祁连县西约50 km(见图1),位于北祁连蛇绿岩带南带西段,北祁连加里东期俯冲增生楔的浅部单元内[15]。缝合带内地层主要为早奥陶世阴沟群、中寒武世黑刺沟组及长城系朱龙关群。

图1　北祁连主要蛇绿岩带分布及年龄图(据文献[16]修编)Fig.1　Geological map of the North Qilian Orogen with localities of major ophiolite suites and their ages

油葫芦沟蛇绿岩主要由灰绿色玄武岩组成,劈理发育 (见图2)。镜下观察表明,岩石由斑晶和基质组成,具少斑状结构 (见图3),基质似交织结构,块状构造,少量杏仁状构造。斑晶为斜长石 (<1%)和辉石 (<1%)；基质为斜长石 (55%～70%)、辉石 (25%～40%)和隐晶质 (<5%)。本文沿油葫芦沟 (N 38°13′57.72″,E 99°46′10.48″)采取5个玄武岩样品 (B063-1—B063-4,B063-6)进行地球化学分析。

图2　玄武岩岩石特征Fig.2　photography of basalts

图3　玄武岩显微结构Fig.3　Microstructure of basalt samples

2　玄武岩的岩石地球化学特征

玄武岩样品的主量和微量元素分析均在国家地质实验测试中心完成,结果见表1、表2。

表1 油葫芦沟玄武岩主量元素分析结果Table 1 Major elements of basalts in Youhulugou area　%

表2 油葫芦沟玄武岩微量元素分析结果Table 2 Trace elements of basalts in Youhulugou area　μg/g

2.1主量元素地球化学特征

由表1可见,玄武岩中K2O+Na2O在3.79%～4.75%,Na2O含量 (3.75%～4.63%)大于K2O含量 (0.02%～0.14%),具有相对低钾富钠的特征；且K2O/Na2O比值为0.01～0.03,属于低碱性火山岩系列。在TAS分类图解 (见图4a)中,样品落入玄武岩及玄武安山岩区；在K2O-SiO2图解 (见图4b)上,玄武岩属于低钾拉斑系列。

在Zr/Y-Zr构造环境判别图 (见图5a)中,样品落入洋中脊玄武岩 (MORB)区,部分与岛弧玄武岩 (IAB)区重叠；TiO2-Zr分类图 (见图5b)中,样品落入洋中脊玄武岩(MORB)、岛弧玄武岩 (IAB)和板内玄武岩 (WPB)的重叠区域。在P2O5-TiO2(见图5c)和Th/Yb-Ta/Yb(见图5d)构造环境判别图解中,5个玄武岩样品全部落入大洋中脊玄武岩 (MORB)区域。

2.2稀土和微量元素地球化学特征

由表2可知,玄武岩稀土元素总量 (ΣREE)为81.49～94.89 μg/g,平均值87.16 μg/g；轻稀土总量 (ΣLREE)为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值为0.42～0.71,平均0.54,显示二者无明显分异。Δ(Eu)值为0.77～0.97,平均0.91；δ(Ce)值为0.98～1.03,平均1.00；(La/Yb)N值0.63～1.26,平均0.86；(La/Sm)N值为0.69～1.20,平均0.86,小于1。在球粒陨石标准化稀土元素分配模式图 (见图6a)上,样品的稀土元素配分模式为平坦到略亏损型,属典型的亏损型大洋中脊玄武岩 (N-MORB)的稀土元素分布模式。微量元素原始地幔标准化图解 (见图6b)显示,玄武岩从强不相容元素到弱不相容元素同样表现向左倾斜的特征。大离子亲石元素 (LILE)Rb、K和Sr出现负异常,高场强元素 (HFSE)Nb、Ta、Ti元素无负异常,排除了玄武岩形成于岛弧环境的可能性。总体表现出大洋中脊幔源岩浆作用的特征,进一步表明该玄武岩形成于大洋中脊构造环境。

图4　油葫芦沟地区玄武岩岩性判别图Fig.4　Discrimination diagrams of basalt in Youhulugou area

图5　油葫芦沟地区玄武岩类构造环境判别图Fig.5　Tectonic environment discrimination diagrams of basalts from Youhulugou area

图6　北祁连油葫芦沟玄武岩类稀土元素球粒陨石标准化配分模式图和微量元素原始地幔标准化多元素蛛网图 (原图据文献 [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　讨论与结论

3.1油葫芦沟蛇绿岩形成的构造环境

蛇绿岩被认为是洋中脊扩张或板块俯冲消减过程的产物[29～30],玄武岩是蛇绿岩岩石组合中最为常见的一类,因此蛇绿岩内玄武岩可为蛇绿岩的形成环境提供直接证据。地球化学研究表明,油葫芦沟蛇绿岩中玄武岩低钾富钠,属于低钾 (拉斑)系列；构造环境判别图显示,玄武岩属于典型的亏损型大洋中脊玄武岩 (N-MORB)；稀土和微量元素分析结果显示,岩浆具有明显的大洋中脊幔源岩浆作用的特征,均表明油葫芦沟蛇绿岩形成于大洋中脊的构造环境。

3.2北祁连洋早古生代构造演化

北祁连造山带的形成与演化经历了复杂的过程,华北地块与柴达木地块间的洋盆经历了陆缘拉张、形成大洋到闭合造山的过程[5～6,9,13,31]。研究表明,北祁连南部蛇绿岩带在晚元古代到晚寒武世 (距今550～497 Ma)发育洋盆[7,10,32],处于洋脊扩张、洋壳生长阶段,油葫芦沟蛇绿岩即为此阶段形成；北部蛇绿岩带在早—中奥陶世 (距今490～448.5 Ma)处于弧后盆地或火山弧构造环境,说明北祁连洋在该时代处于岛弧火山作用阶段,洋壳向北俯冲并形成岛弧及弧后盆地[12,16]。北祁连洋向北俯冲的俯冲杂岩带高压变质榴辉岩年龄为463～468 Ma,反映出俯冲作用及其引发的岛弧火山活动主要发生在中奥陶世[33]。晚奥陶世—志留纪,陆源碎屑岩沉积的出现,标志着北祁连洋洋盆已趋于闭合阶段[34]。至此,从晚元古代到志留纪,北祁连洋经历了大陆裂解、洋盆扩张、俯冲消减到洋盆闭合的全过程,进入碰撞造山阶段。

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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

中国地质科学院基本科研业务费专项项目 (YYWF201401)；中国地质调查局 “祁连山及邻区天然气水合物资源勘查 (控矿因素综合研究及编图)”项目 (GZHL20120301)

李冰 (1987-),男,硕士研究生,构造地质学专业。E-mail:libing@cags.ac.cn