岩石学报  2018, Vol. 34 Issue (2): 383-397   PDF    
北祁连西段双龙镁铁质-超镁铁质岩地球化学特征及构造意义
刘懿馨 , 沙鑫 , 马蓁 , 王金荣     
兰州大学地质科学与矿产资源学院, 甘肃省西部矿产资源重点实验室, 兰州 730000
摘要:北祁连西段双龙一带出露的镁铁质-超镁铁质岩主要由蛇纹石化二辉橄榄岩、辉长岩、辉绿岩及玄武岩组成,这些岩石单元与构造卷入的前寒武纪变质岩系和深海沉积的硅质岩一起构成蛇绿混杂岩带。辉长岩LA-ICP-MS锆石U-Pb年龄为464±2 Ma(MSWD=1.3)。Zr/TiO2-Nb/Y图解显示辉绿岩为拉斑玄武岩系列,玄武岩为碱性系列,两者均具有低的MgO和相容元素Cr、Ni含量,指示在岩浆作用过程中发生过橄榄石、辉石的分离结晶作用。球粒陨石标准化稀土元素配分模式图显示,辉绿岩呈平坦型,(La/Yb)N=1.35~1.45,无Eu异常(δEu=1.0~1.1),以及Lu/Yb(~0.15)、Zr/Y(~2.5)、Y/Tb(~39)比值等均类似于N-MORB;但它们又相对富集Ba、Sr,亏损Nb、Ta、Pb,以及Nb/U(~21)、Zr/Nb(~16)、Th/Ta(~5.7)比值则显示了岛弧玄武岩或弧后盆地玄武岩的特征,推测辉绿岩应源于受俯冲流体交代的尖晶石二辉橄榄岩部分熔融的产物。玄武岩地球化学特征明显不同于辉绿岩,表现为相对富集LREE,(La/Yb)N=~8.48,轻微负Eu异常(δEu=0.91),以及Zr/Y(~8)、Y/Tb(~30)、Ta/Yb(~0.6)、Th/Yb(~2.5)比值等类似于OIB,指示玄武岩应源于富集地幔部分熔融的产物,但岩石又表现出富集Rb、Ba、Th、U、Pb、Sr、Zr、Hf,亏损Nb、Ta、Ti、P,以及Th/Ta(~4.8)、Zr/Nb(~14)、Nb/Yb(~10)、Nb/U(~11)比值又具有岛弧或弧后盆地玄武岩的亲缘性,显示了玄武岩应源于地幔深部流体交代的石榴石二辉橄榄岩或尖晶石+石榴石二辉橄榄岩部分熔融的产物,表明早古生代北祁连古大洋在俯冲闭合过程中具有洋岛或海底高原的增生作用。结合区域地质背景,利用全球大数据建立的构造判别图解综合研究认为,北祁连西段熬油沟-玉石沟-双龙-小龙孔-卡瓦-东沟等镁铁质-超镁铁质岩共同构成一条重要的SSZ型蛇绿岩带,为北祁连古大洋向南俯冲形成的弧后盆地的残留体。
关键词: 镁铁质-超镁铁质岩     SSZ型蛇绿岩     弧后盆地     早古生代     北祁连西段    
Geochemical characteristics and tectonic implication of the Shuanglong mafic-ultramafic rocks in western section of the North Qilian
LIU YiXin, SHA Xin, MA Zhen, WANG JinRong     
Key Laboratory of Mineral Resources in Western China(Gansu Province), School of Earth Sciences, Lanzhou University, Lanzhou 730000, China
Abstract: The mafic-ultramafic rocks exposed in Shuanglong region in western section of the North Qilian, composed of serpentinized lherzolite, gabbro, diabase, basalt, together with tectonically involved Precambrian metamorphic rocks and deep sea sedimentary siliceous rocks, are considered as a set of ophiolitic mélange. The LA-ICP-MS zircon U-Pb dating of the gabbros is 464±2Ma (MSWD=1.3). In the diagram of Zr/TiO2-Nb/Y, the diabases in this ophiolite belt belong to the tholeiitic series, however, the basalts are plotted into the alkaline series. Both of them have low MgO and Cr, Ni contents, indicating the fractional crystallization of olivine and pyroxene in the evolution of their magma. In the chondrite-normalized REE patterns diagram, the diabases display a flat REE pattern with (La/Yb)N=1.35~1.45, no significant Eu anomalies (δEu=1.0~1.1), and have ratios of Lu/Yb (~0.15), Zr/Y (~2.5) and Y/Tb (~39), which are similar to those of the normal mid-oceanic ridge basalts (N-MORB), but relatively enriched in Ba and Sr and depleted in Nb, Ta, Pb, and ratios of Nb/U (~21), Zr/Nb (~16) and Th/Ta (~5.7) display the affinities of island arc basalts (IAB) or back-arc basin basalts (BABB), suggesting that the diabases were the products of partial melting of spinel lherzolite source metasomatismed by subducted fluids. The geochemical characteristics of the basalts are obviously different from diabases, relatively enriched in LREE, (La/Yb)N=~8.48, with slightly negative Eu anomalies. Ratios of Zr/Y (~8), Y/Tb (~30), Ta/Yb (~0.6) and Th/Yb (~2.5) for the basalts are similar to those of oceanic island basalts (OIB), indicating that they are derived from the partial melting of deep enriched mantle (similar to OIB source). However, the rocks are enriched in Rb, Ba, Th, U, Pb, Sr, Zr and Hf, depleted in Nb, Ta, Ti, P, and have ratios of Th/Ta (~4.8), Zr/Nb (~14), Nb/Yb (~10) and Nb/U (~11), respectively, display the affinities of IAB or BABB, suggesting that the rocks should be the products of partial melting of garnet or spinel + garnet lherzolite metasomatized by the deep-mantle fluid cycling, and further indicating that the accretion process of oceanic island could exist during the subduction of the oceanic lithosphere in the Early Paleozoic. Together with the analyses on regional geological settings, the tectonic discrimination diagrams based on the global basalt data suggest that the mafic-ultramafic rocks from Aoyougou, Yushigou, Shuanglong, Xiaolongkong, Kawa, Donggou constitute an important supra-subduction zone type (SSZ-type) ophiolite belt indicative of the relics of an back-arc basin formed by the southward subduction of North Qilian ancient ocean.
Key words: Mafic-ultramafic rocks     SSZ-type ophiolite     Back-arc basin     Early Paleozoic     Western section of the North Qilian    

北祁连造山带是一条位于祁连-柴达木地块与阿拉善地块之间的增生型造山带,主要由增生杂岩、蛇绿岩、洋岛、岛弧地体以及镶嵌其中的大小不等的前寒武纪陆壳残片组成,记录了从新元古代到早古生代的大陆裂解、洋盆扩张、大洋俯冲闭合拼贴的完整的威尔逊旋回(冯益民和吴汉泉,1992冯益民等,1994许志琴等,1994冯益民和何世平, 1995a, b张建新和许志琴,1995冯益民,1997宋述光,1997张旗等, 1997a, b, c左国朝和吴汉泉,1997Xia et al., 1999杨经绥等,2001王金荣等, 2003, 2005, 2006宋述光等,2004Song et al., 2009a, b20122013侯荣娜等,2015胡万龙等, 2016a, b)。长期以来,国内外学者从不同角度对祁连造山带及其构造演化过程开展了系统的研究,特别在海相火山岩及蛇绿岩、高压超高压变质岩系等领域取得了受到国内外高度关注的研究成果(许志琴等,1994张建新和许志琴,1995赖绍聪和隆平,1996赖绍聪等,1997宋述光,1997许志琴,1997张旗等, 1997a, b, c左国朝和吴汉泉,1997Lai et al., 1998Xia et al., 1999杨经绥等,2001吴才来等,2002王金荣等, 2003, 2005, 2006宋述光等, 2004, 2009Song et al., 2009a, b20122013)。肖序常等(1978)将北祁连蛇绿岩分出震旦纪、寒武纪和奥陶纪三期蛇绿岩,冯益民和何世平(1996)认为蛇绿岩形成时代主要是晚寒武世-奥陶纪,并把北祁连蛇绿岩划分为洋中脊型、岛弧扩张脊型和弧后扩张脊型三类。张旗等(1997b)研究指出,北祁连蛇绿岩大多表现出MORB的特征,形成于岛弧和弧后环境,侵位于岛弧增生楔或活动陆缘地体上,应属科迪勒拉型。最近,Song et al.(2013)研究认为,北祁连至少有两条蛇绿岩带,以熬油沟、玉石沟、东沟和东草河蛇绿岩构成北祁连蛇绿岩南带,而九个泉、大岔大坂及扁都口蛇绿岩则构成了北祁连蛇绿岩北带,形成时代为新元古代至早古生代,并获得了学术界认可的祁连古大洋闭合的时限(冯益民等,1994张旗等, 1997a, b, c宋述光等,2004王金荣等,2006吴才来等,2006曾建元等,2007夏小洪和宋述光,2010Song et al., 2013)。上述研究成果无疑为重建北祁连古板块构造体制提供了重要的地质依据。然而,目前对祁连古大洋板块构造演化过程,特别是古大洋俯冲极性仍存在较大的争议,多数学者根据蛇绿岩、岛弧火山岩、花岗岩类及俯冲杂岩带的构造特征等认为,古祁连洋于晚寒武世-奥陶纪开始向北俯冲(许志琴等,1994宋述光等,2009Song et al., 2013);也有部分学者依据北祁连中段两侧存在古生代花岗岩类而提出向南俯冲(Gehrels et al., 2003)、双向俯冲(左国朝和刘寄陈,1987左国朝和吴汉泉,1997张旗等,1997a吴才来等,2006)和先向南俯冲后转换为向北俯冲(肖序常和王军,1998吴才来等,2006),以及在奥陶纪时期多岛弧洋的构造格局(Lai et al., 1998)等观点。此外,有关蛇绿岩形成的构造环境问题也存在较大争议,如相振群等(2007)认为熬油沟蛇绿岩形成于弧后环境;闫巧娟(2012)则认为熬油沟蛇绿岩不具备典型大洋蛇绿岩特征,形成的构造环境可能为小洋盆;夏小洪等(2012)则认为它是小洋盆初始扩张时的扩张脊或大陆裂谷后的初始洋壳。本文在北祁连造山带西段双龙一带新确认的一套镁铁质-超镁铁质岩为蛇绿岩,通过其年代学、岩石地球化学的研究,明确蛇绿岩类型,进而结合区域地质背景分析,揭示其形成的构造动力学过程,为深入研究祁连古大洋板块构造演化过程提供重要的岩石学证据。

1 区域地质概况

北祁连造山带是中国板块构造研究的摇篮,北祁连蛇绿岩的发现及其后的研究是中国大地构造研究从固定论向活动论转变的关键时期(王荃和刘雪亚,1976肖序常等,1978)。北祁连造山带北邻阿拉善地块,南接祁连-柴达木地块,西被阿尔金造山带所截,处于重要的大地构造位置(图 1)。北祁连造山带主要由蛇绿岩带、增生杂岩体、岛弧、洋岛以及镶嵌其中的前寒武纪微陆块残片组成。最近Song et al.(2013)研究认为,北祁连至少有两条蛇绿岩带,以熬油沟-玉石沟-东沟-东草河蛇绿岩带构成北祁连南带蛇绿岩(形成时代497~513Ma),为北祁连古大洋闭合的产物;而以九个泉-大岔大坂-扁都口-老虎山蛇绿岩构成了北祁连北带蛇绿岩(时代为448~505Ma),为祁连古大洋北向俯冲形成的弧后盆地闭合增生的产物。野外地质调查表明,在北祁连西段熬油沟、二只哈拉、塔里干沟、卡瓦、双龙、小龙孔等地有出露良好的镁铁质-超镁铁质岩(图 1),主要岩石类型为蛇纹石化辉橄岩、辉长岩、辉绿岩、玄武岩等,均以构造侵位的形式混杂在太古宇-古元古界北大河岩群和中元古界朱龙关岩群中,以NWW-SEE向展布在水峡脑-光滑岭-金龙河断裂带(长度>100km)、桦树沟南-沙梁大断裂(长度>100km)、古浪峡断裂带及其两侧,其中熬油沟镁铁质-超镁铁质岩已被认为是蛇绿岩套(张招崇等,2001相振群等,2007夏小洪等,2012Song et al., 2013),并发育有俯冲板片变质成榴辉岩减压熔融形成的志留纪高硅埃达克岩(陈育晓等,2012),但对蛇绿岩形成时代及成因的认识则有所不同观点(张招崇等,2001相振群等,2007Song et al., 2012, 2013夏小洪等,2012闫巧娟,2014)。

图 1 北祁连造山带主要蛇绿岩带分布图(据Song et al., 2013修改) Fig. 1 Distribution map of the ophiolite zones in the North Qilian Orogenic Belt (modified after Song et al., 2013)

双龙镁铁质-超镁铁质岩位于玉石沟蛇绿岩带南西,距离熬油沟蛇绿岩带南东约50km、古浪峡以南约15km处,是本文新发现的保存较为完好的弧后盆地岩石圈残片。以团块的形式混杂在前寒武纪变质岩系中(图 1),为蛇绿岩套。双龙蛇绿岩层序较全,断续延伸约6km,呈北西西走向展布。区内出露地层主要为长城系朱龙关群桦树沟组,其次为青白口系龚岔群五个山组。在蛇绿岩两侧均发育有中元古界长城系朱龙关群桦树沟组(图 2图 3),两者为断层接触关系。

图 2 北祁连双龙蛇绿岩带地质简图 1-青白口系五个山组肉红色硅化白云岩;2-长城系桦树沟组二岩段灰白色白云岩;3-长城系桦树沟组一岩段灰黑色灰岩;4-长城系桦树沟组一岩段灰绿色-紫红色砂质板岩;5-铁矿层;6-玄武岩;7-辉长岩;8-超基性岩;9-断层;10-地质界线;11-剖面位置;12-采样位置 Fig. 2 The geological sketch map of Shuanglong ophiolite belt in the North Qilian

图 3 北祁连双龙蛇绿岩剖面及采样位置图(剖面位置见图 2) Fig. 3 The cross section and sample location in Shuanglong ophiolite belt in the North Qilian
2 岩石学特征

双龙镁铁质-超镁铁质岩主要由二辉橄榄岩、辉长岩、辉绿岩、玄武岩组成,其中包含有构造卷入的紫红色硅质岩(图 4)及前寒武纪变质岩系。

图 4 双龙蛇绿岩带野外照片 (a)辉长岩团块;(b)辉长岩与玄武岩断裂接触;(c)辉绿岩岩墙;(d)蛇纹石化橄榄岩及构造混杂的硅质岩 Fig. 4 Photos showing field outcrops of different components of Shuanglong ophiolite belt

(1) 二辉橄榄岩,以团块形式产出(图 4a),矿物组成主要为橄榄石(60%~70%)、辉石(斜方辉石和单斜辉石,20%~30%)和少量(~5%)角闪石及暗色铁质矿物(磁铁矿、钛铁矿、铬铁矿和尖晶石),已发生强烈的蛇纹石化,灰黑-墨绿色,块状构造。镜下可见网状、纤维状、鳞片状变晶结构(图 5a),见有斜方辉石和单斜辉石发生一定程度的韧性变形,晶型有明显的破坏残缺(图 5a)。

图 5 双龙蛇绿岩带不同岩石类型显微照片 (a)正交偏光下蛇纹石化橄榄岩,具网状构造,局部出现具堆晶结构的层状橄榄石、辉石;(b)正交偏光下辉长岩(辉石发生蚀变,斜长石已发生钠黝帘石化和黏土化);(c)正交偏光下辉绿岩,辉石呈他形,镶嵌发育于斜长石之间;(d)正交偏光下玄武岩呈间粒-间隐结构. Ol-橄榄石;Opx-斜方辉石;Mag-磁铁矿;Px-辉石;Pl-斜长石 Fig. 5 The microscopic photos showing different components of Shuanglong ophiolite belt

(2) 辉长岩,以团块形式产出(图 4a)。岩石风化面为褐色,新鲜面呈灰黑色,具半自形粒状结构,块状构造。主要矿物由辉石和斜长石组成,辉长结构(图 5b),斜长石含量约为50%~55%,呈板状,发育不同程度的钠黝帘石化,偶见碳酸盐化。辉石含量约为40%~45%,短柱状,半自形或他形,多蚀变,常被细小角闪石集合体或绿泥石替代。副矿物为磷灰石及磁铁矿等。

(3) 辉绿岩,常与玄武岩、辉长岩一起以团块、岩墙状断续产出(图 4c)。岩石风化面为褐色,新鲜面呈灰黑色,由辉石和斜长石组成,辉绿结构(图 5c),多呈半自形粒状结构,块状构造。斜长石含量约为50%~55%,呈板状,不同程度的钠黝帘石化和碳酸盐化。辉石含量约为40%~45%,短柱状,多镶嵌发育于斜长石三角格架中。副矿物为磷灰石及磁铁矿等。

(4) 玄武岩,岩石风化面多呈褐红色,新鲜面为灰黑色,与辉绿岩、辉长岩一起以团块形式产出(图 4b)。岩石具斑状结构和间粒间隐结构(图 5d),多为块状构造,见气孔杏仁状构造,体径0.5~1.0mm不等,多具有碳酸盐化和绿泥石化。斑晶矿物主要是斜长石,多为条状,晶体之间常有辉石充填,他形,晶粒小。副矿物主要为磁铁矿、磷灰石等。

(5) 硅质岩,以构造卷入的形式散布在橄榄岩、玄武岩中(图 4d),呈紫红色,隐晶质结构,致密块状构造。

3 样品及其分析方法

本次野外采集了1件辉长岩年龄样品(15SL-27),岩石地球化学样品包括5件玄武岩(15SL-40、15SL-43、15SL-44、15SL-47、15SL-48)和5件辉绿岩(15SL-14、15SL-17、15SL-18、15SL-19、15SL-20),样品采集位置见图 2图 3。在野外样品采集过程中,尽量选择岩体出露良好,新鲜弱蚀变的岩石进行采集,然后经过室内筛选鉴定,选择具有代表性的样品进行同位素年龄测定及主微量的分析测试。

将新鲜的辉长岩样品送至河北廊坊实验室进行锆石挑选。首先将样品经过粉碎和重选,分选出纯度较高的锆石,然后在双目镜下挑选出锆石样品。将挑选好的锆石送至西北大学大陆动力学国家重点实验室制靶及年龄测试。首先用环氧树脂固定人工精选出的锆石,对锆石进行抛光,之后用浓度为3%的HNO3清洗锆石,并将锆石表面镀碳,然后制成锆石靶,最后运用扫描隧道显微镜对锆石进行阴极发光(CL)照相。比对锆石反射光、透射光及阴极发光照片,选出足量、晶形较好、大小适宜,有环带的锆石进行LA-ICP-MS打点分析。锆石U-Pb定年工作在连接Geolas-193型紫外激光剥蚀系统的Agilient 7500a型ICP-MS上进行,采用He作为剥蚀物质的载气,激光剥蚀束斑直径为30μm,频率为6Hz。激光采样方式为单点剥蚀,详细的分析流程见Yuan et al., (2004)柳小明等(2007)

主量元素分析由中国科学院广州地球化学研究所同位素地球化学国家重点实验室进行测试,其采用X射线荧光光谱分析(XRF,仪器型号为PA Nalytical Axios型)熔融玻璃片法。首先将样品装入小纸袋中(约5g)放入高温烘干箱中烘干,其次将样品放入小坩埚中进行烧失,在称量实验室中准确地称取0.5000±0.0001g烧失后的样品放置于塑料杯中,再称取4.000g助熔剂Li2B4O7粉末倒入杯中,盖好盖后摇至使其混合均匀。之后将混合好的样品倒入铂金坩埚用2mol/L HCl严格按照流程浸煮,并用Milli-Q水清洗,再在铂金坩埚中加入2滴脱膜剂(1%的LiBr溶液)。最后将铂金坩埚放入熔样机,制作成透明均匀的圆形玻璃片进行岩石主量元素分析测试。

微量元素分析由中国地质大学(武汉)生物地质与环境地质国家重点实验室进行测试,采用ICP-MS完成测试(仪器型号为Agilent7500a型),元素的准确度是由GSR-3、AGV-1来确定。首先称取50mg的粉末样品放入Teflon溶样弹中,再将适量的HF+HNO3加入其中,并将其放于恒温箱中,约48h后取出溶样弹。然后将其蒸干(120℃),稀释至2000倍(2% HNO3),将其放于聚酯瓶中待测(Liu et al., 2008)。

4 年代学

由辉长岩锆石阴极发光图像(图 6)可以看出,锆石具有较完好的柱状晶形,少量为近等轴状,锥面和晶面发育,大多数锆石均显示良好的岩浆震荡环带结构特征,且锆石的Th/U比值均介于0.57~2.27(表 1),表明它们是岩浆成因锆石(Vavra and Hansen, 1991Paterson et al., 1992Hoskin and Blank, 2000)。辉长岩LA-ICP-MS锆石U-Pb年龄测定结果见表 1图 7,考虑到普通铅校正对207Pb/235U比值影响较大,本文采用206Pb/238U年龄加权平均值代表岩石形成的时代。双龙蛇绿岩中的辉长岩LA-ICP-MS锆石U-Pb年龄加权平均值为464±2Ma(MSWD=1.3),代表其形成时代。

图 6 双龙蛇绿岩带中辉长岩锆石阴极发光图像 Fig. 6 CL images of zircons of gabbro in Shuanglong ophiolite belt

表 1 双龙蛇绿岩带中辉长岩锆石U-Pb年龄测定数据 Table 1 LA-ICP-MS U-Pb dating data of zircons of gabbro in Shuanglong ophiolite belt

图 7 双龙辉长岩LA-ICP-MS锆石U-Pb年龄谐和图 Fig. 7 LA-ICP-MS zircon U-Pb concordia diagram for gabbro in Shuanglong ophiolite belt
5 地球化学特征 5.1 主量元素

辉绿岩主量元素分析结果见表 2。SiO2含量变化范围在47.63%~50.62%之间,中等的Al2O3(13.84%~14.58%),与N-MORB相比,具有较低的MgO(4.95%~6.46%,Mg#=50~56)、CaO(4.97%~7.17%)含量,但具有较高Na2O(6.27%~7.35%)、K2O(0.31%~0.93%)含量,Fe2O3T(10.23%~15.70%)、TiO2(1.54%~1.64%)含量与N-MORB的相当。在TAS全碱图上(图 8aLe Maitre et al., 1989),样品均落入粗面岩-玄武安山岩区域,这可能是受到晚期蚀变作用导致活动元素Na偏高所致。为了精准确定岩石类型,我们选择稳定的高场强元素进一步进行划分,在Zr/TiO2-Nb/Y图上(图 8bWinchester and Floyd, 1976),样品点均落入拉斑玄武岩域。

表 2 双龙辉绿岩、玄武岩主量元素(wt%)与微量元素(×10-6)组成 Table 2 The contents of major (wt%) and trace (×10-6) elements of the diabases and basalts in Shuanglong ophiolite belt

图 8 双龙玄武岩辉绿岩TAS全碱图(a,据Le Maitre,1989)及Zr/TiO2-Nb/Y图(b,据Winchester and Floyd, 1976) Fig. 8 TAS diagram (a, after Le Maitre, 1989) and Zr/TiO2vs. Nb/Y diagram (b, after Winchester and Floyd, 1976) for the basalts and gabbros in Shuanglong ophiolite belt

玄武岩主量元素分析结果见表 2,玄武岩的SiO2=45.89%~48.07%,低Al2O3(10.18%~12.18%)、MgO(2.87%~5.63%,Mg#平均61)、Na2O(0.52%~1.58%)、K2O(0.96%),高CaO=24.18%~29.47%(可能与后期碳酸盐微细脉贯入有关),TiO2含量较低(0.44%~0.52%),P2O5(~0.09%)。样品烧失量介于3.12%~4.33%,表明岩石已受到晚期蚀变作用的影响,故将样品利用Zr/TiO2-Nb/Y(图 8bWinchester and Floyd, 1976)图解进行岩石系列及类型划分,样品点均落入碱性玄武岩域。

5.2 微量元素

辉绿岩微量元素分析结果见表 2。稀土元素总量较高(58.09×10-6~62.45×10-6),LREE=39.30×10-6~42.60×10-6,HREE=18.45×10-6~19.85×10-6。在球粒陨石标准化稀土元素配分图(图 9a)中,配分曲线为平坦型,LREE/HREE=2.06~2.17,(La/Yb)N=1.35~1.45,无Eu异常或轻微正异常,δEu=1.0~1.1。原始地幔标准化微量元素蛛网图(图 9b)显示,样品分布曲线较为一致,除了Ba、Sr相对相邻元素表现正异常、Nb、Ta、Pb为负异常外,其余元素呈现出平坦型的配分型式。

图 9 双龙蛇绿岩带中辉绿岩(a、b)及玄武岩(c、d)球粒陨石标准化稀土元素配分模式及原始地幔标准化微量元素蛛网图(球粒陨石标准值、原始地幔标准值、N-MORB和OIB值据Sun and McDonough, 1989;大洋弧玄武岩平均成分据Kelemen et al., 2004) Fig. 9 Chondrite-normalized rare earth element patterns and the primitive mantle-normalized trace element spider diagrams for diabases (a, b) and basalts (c, d) in Shuanlong ophiolite belt (chondrites, trace element, PM, N-MORB and OIB values from Sun and McDonough, 1989; the average composition of oceanic arc basalts from Kelemen et al., 2004)

玄武岩微量元素分析结果见表 2。稀土元素总量为71.44×10-6~74.99×10-6,高于辉绿岩,LREE=62.94×10-6~65.83×10-6,HREE=8.32×10-6~9.18×10-6。在球粒陨石标准化稀土元素配分模式图(图 9c)中,LREE相对富集(LREE/HREE均值7.39,(La/Yb)N均值8.48),轻微负Eu异常,δEu=0.90~0.92。在原始地幔标准化微量元素蛛网图(图 9d)中,样品Rb、Ba、Th、U、Pb、Sr以及Zr、Hf等元素明显富集,相对亏损Nb、Ta、Ti、P。

6 讨论 6.1 岩石成因

双龙镁铁质-超镁铁质岩主要由二辉橄榄岩、辉长岩、辉绿岩、玄武岩组成,其中含有构造卷入的紫红色硅质岩(图 4d),呈NWW-EW向断续分布在桦树沟南、沙梁大断裂、古浪峡断裂及其两侧,整体上表现为团块、基质的产出状态,团块主要有蛇纹石化辉橄岩、辉长岩、辉绿岩、玄武岩,基质为强烈蛇纹石化辉橄岩及前寒武纪泥质岩类,其与围岩(前寒武纪变质岩系)均以一系列复杂的南倾逆冲断裂接触(图 3),同时在镜下辉长岩中的矿物定向排列及韧性变形特征清晰可见(图 5ab)。上述岩石组合及其构造变形特征指示双龙镁铁质-超镁铁质岩为一典型的构造混杂岩(图 4)。

6.1.1 辉绿岩

Zr/TiO2-Nb/Y图解(图 8b)指示辉绿岩类似于大洋拉斑玄武岩(Winchester and Floyd, 1976),总体表现出富Fe2O3T、Na2O(6.27%~7.35%),贫K2O(平均0.64%)的特征,富钠可能与后期发生钠化等热液蚀变作用有关。中等的Al2O3(14.10%)、TiO2(1.60%)含量与N-MORB相当(Schilling et al., 1983),相对较低的CaO(6.41%)、MgO(平均5.33%,Mg#平均52,低于原生岩浆,Mg#=68~75,Wilson,1989)以及非常低的相容元素Cr、Ni(平均含量分别为4.40×10-6、17.68×10-6),表明辉绿岩经历了明显的橄榄石、单斜辉石等矿物的分离结晶作用,但无Eu异常及富集Sr表明岩浆作用过程未发生斜长石的分离结晶作用。

球粒陨石标准化稀土元素配分图为平坦型配分模式(图 9a),但在原始地幔标准化微量元素蛛网图中(图 9b),其特征与N-MORB不同,样品富集Rb、Ba、Th、Sr、U,亏损Nb、Ta,表现出岛弧玄武岩的亲缘性。研究表明微量元素,特别是化学性质相近的元素对比值不受岩浆分异作用的影响,能够较好地示踪岩浆源区的性质。辉绿岩Lu/Yb(~0.15)、Zr/Y(~2.5)、Y/Tb(~39)比值均与N-MORB(Sun and McDonough, 1989)一致,而Nb/U(~21)、Zr/Nb(~16)则远低于N-MORB的比值(Hofmann,1988Sun and McDonough, 1989),以及Th/Ta(~5.7,>3)更接近于岛弧玄武岩或弧后盆地玄武岩(Hofmann,1988张旗等,1999),暗示着双龙辉绿岩源区可能为受到俯冲流体交代的软流圈地幔,但高Y/Tb(~39)比值提示没有俯冲沉积物参与其岩浆作用过程(史仁灯等,2004)。在Zr/Nb-Ce/Y(图 10a)和Nb/Yb-TiO2/Yb(图 10b)以及Zr/Nb-La/Yb(图 10c)和La/Sm-Sm/Yb(图 10d)图中,样品表现为由亏损地幔向富集地幔的演化趋势,源区为尖晶石二辉橄榄岩,部分熔融程度小于10%。由此可以推测,双龙辉绿岩兼有N-MORB和IAB地球化学特征,是源于受俯冲流体交代的尖晶石二辉橄榄岩部分熔融的产物。

图 10 双龙蛇绿岩带中辉绿岩及玄武岩的Zr/Nb-Ce/Y图(a)、Nb/Yb-TiO2/Yb图(b)、Zr/Nb-La/Yb图(c)和La/Sm-Sm/Yb图(d) (a, c, 据Göncüoglu et al., 2010; b, d, 据Pearce,2008) Fig. 10 The discriminant diagrams of Zr/Nb vs. Ce/Y (a), Nb/Yb vs. TiO2/Yb (b), Zr/Nb vs. La/Yb (c) and La/Sm vs. Sm/Yb (d) for the diabases and basalts in Shuanglong ophiolite belt (a, c, after Göncüoglu et al., 2010; b, d, after Pearce, 2008)
6.1.2 玄武岩

玄武岩地球化学特征明显不同于辉绿岩,说明两者并非同源岩浆演化的产物。碱性玄武岩低MgO(~4.75%)及Cr(197.6×10-6)、Ni(65.74×10-6)、无明显Eu异常,表明玄武岩经历了明显的橄榄石、辉石等矿物的分离结晶作用,但没有斜长石的分离结晶。K2O含量(~0.78%)高于大洋拉斑玄武岩(< 0.2%)。低TiO2含量(~0.49%)、Al2O3(~11%)、Fe2O3T(~4%)可能与含铁-钛矿物(如磁钛铁矿)分离结晶作用有关。

球粒陨石标准化REE配分模式(图 9c)强烈右倾,LREE富集,(La/Yb)N均值8.48,轻微负Eu异常,配分曲线总体类似于OIB(Sun and McDonough, 1989)。元素对Zr/Y(~8)、Y/Tb(~30)、Ta/Yb(~0.6)、Th/Yb(~2.5)比值亦类似于OIB,指示其源于富集地幔部分熔融的产物(Sun and McDonough, 1989Wilson,1989)。在原始地幔标准化微量元素蛛网图中(图 9d),曲线与N-MORB不同,样品相对富集Rb、Ba、Th、U、Pb、Sr,亏损高场强元素Nb、Ta、Ti,以及Th/Ta(~4.8,在IAB的范围内)、Zr/Nb(~14)、Nb/Yb(~10)、Nb/U(~11)比值又具有岛弧或弧后盆地玄武岩的亲缘性(Wilson,1989Sun and McDonough, 1989)。玄武岩总体特征兼有OIB与IAB地球化学特征。在Zr/Nb-Ce/Y(图 10a)和Nb/Yb-TiO2/Yb(图 10b)以及Zr/Nb-La/Yb(图 10c)和La/Sm-Sm/Yb(图 10d)图中,岩石具有向富集地幔的演化趋势,应源于石榴石二辉橄榄岩或尖晶石+石榴石二辉橄榄岩部分熔融的产物,部分熔融程度大约在5%左右,在地幔深部熔融过程中可能有俯冲板片流体的参与,其形成机理可能类似于中亚造山带内蒙古中段的本巴图组和阿木山组类似于岛弧玄武岩特征的板内玄武岩(Pang et al., 2016Wang et al., 2016),但前者是形成于大洋板内环境。

6.2 构造背景

岩石地球化学研究表明,双龙辉绿岩与玄武岩具有不同的岩石成因:辉绿岩兼有N-MORB和IAB地球化学特征,是源于受俯冲流体交代的尖晶石二辉橄榄岩部分熔融的产物;玄武岩则兼有OIB与IAB地球化学特征,源自更深部的石榴石二辉橄榄岩或尖晶石+石榴石二辉橄榄岩部分熔融的产物,具有富集地幔的演化趋势,但仍带有深部俯冲板片流体参与的岩浆作用过程印迹。本文通过全球玄武岩大数据筛选(王金荣等, 2016, 2017a, b杨婧等,2016a陈万峰等,2017)并建立的构造判别图(杨婧等,2016bLiu et al., 2017)进行判别,在Log(La/Yb)-Log(Cs/Yb)和Log(Cs/Ta)-Log(La/Y)图上(图 11bc),辉绿岩和玄武岩样品均落入在IAB域内,但在Log(Cs/Ta)-Log(La/Yb)和Log(Th/Ta)-Log(La/Yb)图上(图 11df),辉绿岩投在IAB域,玄武岩分别落入IAB和MORB域内;在Log(U/Ta)-Log(La/Yb)和Log(Ba/Th)-Log(La/Yb)图上(图 11ae),辉绿岩在IAB域内,但玄武岩除了在IAB域内外,部分样品落入OIB域内(还包括存在于小龙孔蛇绿岩中的部分玄武岩),其结果与前述的玄武岩源于被俯冲板片流体交代的深部富集地幔部分熔融的产物相一致。此外,我们将北祁连西段熬油沟-二只哈拉-卡瓦-小龙孔以及玉石沟-东沟-东草河蛇绿岩带中的辉绿岩、玄武岩也分别在上述判别图(图 11)进行判别,可以看出上述蛇绿岩的样品基本上都在IAB域内,部分在MORB内,说明该蛇绿岩带应归属于上俯冲带(SSZ)型,形成于弧后环境。值得指出的是,在该蛇绿岩带内,还存在有被深部俯冲板片流体交代的OIB的组分,如双龙、小龙孔的部分玄武岩,表明早古生代北祁连古大洋俯冲闭合过程中发生有洋岛或海底高原的增生作用。

图 11 北祁连蛇绿岩构造判别图(据Liu et al., 2017) 数据来源:玉石沟玄武岩(冯益民和何世平,1995aHou et al., 2006Song et al., 2013);熬油沟-二只哈拉达坂蛇绿岩(夏小洪等,2012闫巧娟,2014);东沟蛇绿岩(武鹏等,2012);东草河蛇绿岩(曾建元等,2007);卡瓦、小龙孔辉绿岩、玄武岩(沙鑫,2017) Fig. 11 Tectonic discrimination diagrams for the ophiolite in the North Qilian (after Liu et al., 2017)

前人研究认为,熬油沟-玉石沟-东沟-东草河蛇绿岩带构成北祁连南部蛇绿岩带(N-MORB型),是祁连古大洋闭合增生的产物(宋述光等,2004Song et al., 2013),而九个泉、大岔大坂及扁都口蛇绿岩则构成了北祁连北部蛇绿岩带,是北祁连古大洋北向俯冲产生的弧后盆地闭合的产物(张旗等, 1997a, c宋述光等,2004王金荣等,2006Song et al., 2013),形成时代为新元古代至早古生代。但是,胡振兴等(2015)对玉石沟蛇绿岩套中的橄榄岩矿物(橄榄石、单斜辉石、尖晶石、铬尖晶石等)包裹体化学研究认为其橄榄岩形成于上俯冲带环境;也有学者认为熬油沟-二只哈拉蛇绿岩(熬油沟蛇绿岩中的辉长岩和二只哈拉蛇绿岩中辉长岩LA-ICP-MS锆石U-Pb年龄分别为480~504Ma和497±3.2Ma(相振群等,2007夏小洪等,2012沙鑫,2017)形成于弧后环境(相振群等,2007),不具备典型的洋中脊蛇绿岩特征,或可能形成于为小洋盆(闫巧娟,2012)或小洋盆初始扩张时的扩张脊或大陆裂谷后的初始洋壳(夏小洪等,2012),同时还发育有与蛇绿岩有着紧密时空关系的俯冲板片变质成榴辉岩减压熔融形成的志留纪高硅埃达克岩(陈育晓等,2012)。结合前人的研究结果,本文认为北祁连西段熬油沟、二只哈拉、塔里干沟、卡瓦(467Ma,沙鑫,2017)、双龙、小龙孔(477.6Ma,沙鑫,2017)、玉石沟-东沟-东草河蛇绿岩应为SSZ型蛇绿岩,不同于N-MORB型的(宋述光等,2004Song et al., 2013),代表着早古生代北祁连古大洋向南俯冲形成的弧后盆地。

7 结论

(1) 北祁连西段双龙镁铁质-超镁铁质岩主要由蛇纹石化二辉橄榄岩、辉长岩、辉绿岩、玄武岩组成,辉绿岩为拉斑系列,兼有N-MORB和IAB地球化学特征,是源于受俯冲流体交代的尖晶石二辉橄榄岩部分熔融的产物;玄武岩为碱性系列,兼有OIB和IAB地球化学特征,源于受深部地幔流体(俯冲板片)交代的石榴石二辉橄榄岩或尖晶石+石榴石二辉橄榄岩部分熔融的产物,部分熔融程度大约在5%左右。

(2) 北祁连西段熬油沟、二只哈拉、玉石沟、卡瓦、小龙孔、双龙、玉石沟、东沟、东草河镁铁质-超镁铁质岩共同构成北祁连南部SSZ型蛇绿岩带,代表着早古生代北祁连古大洋向南俯冲形成的弧后盆地;其蛇绿岩带中含有OIB组分指示着在洋盆闭合过程中存在有洋岛或海底高原的增生作用。

致谢 甘肃省地质矿产开发局第四勘查院院长丁书宏高级工程师、陈世强、付强、胡小春工程师在野外工作期间给予了大力的支持和帮助;参加野外工作的还有研究生霍永豪、侯克选、边鹏等;特别是两位匿名评审对本文提出了许多建设性的意见和建议;在此一并表示衷心的感谢。
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