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青海拉脊山东段白家藏侵入岩锆石U-Pb年代学、地球化学特征及其地质意义
张新远1,2, 李五福1,2, 王春涛1,2, 刘建栋1,2, 欧阳光文1,2     
1. 青海省青藏高原北部地质过程与矿产资源重点实验室, 西宁 810012;
2. 青海省地质调查院, 西宁 810012
摘要: 白家藏侵入岩位于青海省民和县,大地构造位置上处于拉脊山蛇绿混杂岩带东段。通过岩石学、同位素年代学和岩石地球化学方法,对白家藏侵入岩进行了研究,以确定其侵位时代并探讨大地构造环境。结果表明,白家藏侵入岩主要由闪长岩和石英闪长岩组成,分别在闪长岩和石英闪长岩中获得锆石U-Pb年龄为(472.5±1.9)Ma和(467.3±2.5)Ma,二者在误差范围内一致,据此判断白家藏侵入岩的侵位年龄约470 Ma,为中奥陶世。地球化学特征显示:岩体w(SiO2)为55.67%~59.04%,Na2O/K2O值为3.35~6.14,且w(Na2O)高于w(K2O),显示富钠、贫钾特征;A/CNK值主体介于0.76~1.01之间,属于准铝质钙碱性系列岩石;稀土配分曲线呈轻稀土元素相对弱富集((La/Yb)N=2.51~4.57),轻、重稀土元素内部分馏较弱的微右倾型曲线;岩石明显富集大离子亲石元素Rb、Ba和活泼的不相容元素Th、U,相对亏损高场强元素Nb、Ta、P、Ti等;环境判别显示具岛弧花岗岩特征。以上特征表明白家藏侵入岩为拉脊山洋俯冲阶段的产物。
关键词: 地球化学    锆石U-Pb年龄    岩石成因    俯冲环境    中奥陶世    拉脊山    
Zircon U-Pb Dating, Geochemistry and Their Geological Significance of Baijiazang Pluton in Eastern Section of Lajishan in Qinghai Province
Zhang Xinyuan1,2, Li Wufu1,2, Wang Chuntao1,2, Liu Jiandong1,2, Ouyang Guangwen1,2     
1. Qinghai Provincial Key Laboratory of Geological Processes and Mineral Resources of Northern Qinghai-Tibetan Plateau, Xining 810012, China;
2. Qinghai Geological Survey Institute, Xining 810012, China
Abstract: The intrusive rocks of Baijiazang is located in Minhe County, Qinghai Province, and the geotectonic position is located in the eastern section of Lajishan ophiolis melange belt. Through petrology, petrogeochemistry and isotopic chronology, the intrusive rocks of Baijiazang have been studied in order to determine their time of emplacement and to explore the tectonic environment. The results show that the intrusive rocks in Baijiazang are mainly composed of diorite and quartz diorite. Zircon U-Pb age obtained from diorite and quartz diorite is (472.5±1.9) Ma and (467.3±2.5) Ma, respectively, and they are consistent within the error range. Therefore, it can be judged that the emplacement age of Baijiazang rock is about 470 Ma, which is Middle Ordovician. The geochemical characteristics show that the SiO2 content of the rock mass is 55.67%-59.04%. The value of Na2O/K2O was 3.35-6.14, and w (Na2O) was higher than of w (K2O), showing the characteristics of rich in sodium and poor in potassium. The main body of A/CNK value is between 0.76 and 1.01, belonging to the quasi-aluminum calc-alkaline series rocks. The rare earth partition curve presented a slightly right-leaning curve, where light rare earth elements were relatively weakly enriched ((La/Yb)N=2.51-4.57), and light and heavy rare earth elements were weakly fractionated internally. The rocks were obviously enriched with large ion lipophilic elements Rb, Ba and active incompatible elements Th and U, while relatively deficient with high-field strength elements Nb, Ta, P, Ti, etc. Environmental discrimination shows that it has the characteristics of island-arc granite. The above characteristics indicate that the intrusive rocks in Baijiazang were produced at the subduction stage of the Lajishan ocean.
Key words: geochemistry    zircon U-Pb age    petrogenesis    subduction    Middle Ordovician    Lajishan    

0 引言

祁连造山带位于青藏高原东北缘,北与阿拉善地块相连,南与柴北缘—西秦岭造山带相接,东、西分别与鄂尔多斯地块和塔里木地块相邻,是我国中央造山带的重要组成部分[1-2],是典型的增生型造山带[3]。祁连造山带通常被划分为北、中、南3个构造带[4],其中存在多条以蛇绿混杂岩为主的增生杂岩[5-6]。拉脊山蛇绿混杂岩带位于中祁连和南祁连的结合部位,受到了诸多学者的广泛关注。多数学者认为拉脊山蛇绿混杂岩带属于裂陷槽或裂谷环境[7-8];王二七等[9]认为其属于北祁连早古生代蛇绿岩的重要组成部分,是北祁连早古生代蛇绿岩在古生代中期陆内变形阶段,沿祁连中央冲断带向南俯冲至中祁连古元古代结晶地块之下并经过多期构造变形抬升至地表的“构造窗”。通过近年来开展的区域地质填图和科研工作,在拉脊山蛇绿混杂岩带的空间展布、物质组成、构造属性研究方面获得了新的资料,认为其是由前寒武纪地块、增生楔和岛弧火山岩组成的增生混杂带[10-13]

拉脊山蛇绿混杂岩作为中、南祁连微陆块的分界线,两侧被区域性断裂围限,内部以早古生代喷出岩和蛇绿混杂岩为主,并发育晚期各类侵入岩及岩脉。前人研究的重点在于拉脊山蛇绿混杂岩带的物质组成、构造属性及形成演化等方面,并获得了一些新的成果认识[11-13],但对混杂岩带内部中酸性侵入岩的研究相对较少。

本文以拉脊山蛇绿混杂岩带东段白家藏地区侵入岩为研究对象,通过岩石学、同位素年代学和岩石地球化学等方法,探讨其时代归属、岩石成因及形成构造环境,以期进一步丰富祁连造山带早古生代大地构造演化的资料。

1 区域地质背景和岩体地质特征

拉脊山蛇绿混杂岩带位于中祁连和南祁连的结合部位,西起日月山,东至民和官亭镇,全长约200 km,宽10~20 km,是一条呈北西西—南东东走向的弧形山脉(图 1)。拉脊山蛇绿混杂岩带南、北两侧分别由化隆岩群和湟源岩群构成古元古代变质结晶基底,之间主要出露寒武纪—奥陶纪火山-沉积岩系,局部地段出露志留系和泥盆系山间磨拉石堆积[15]。拉脊山蛇绿混杂岩带由蛇纹石化橄榄岩、辉长辉绿岩、枕状熔岩和硅质岩等组成,其中发育大量的加里东期花岗岩侵入体。

据文献[14]修编。 图 1 拉脊山地区地质简图 Fig. 1 Schematic geological maps of Lajishan area

研究区位于拉脊山蛇绿混杂岩带东段白家藏地区,距民和县城约10 km(图 1)。出露地层为寒武纪—早奥陶世峡门蛇绿混杂岩、古近系—中新统西宁组河湖相碎屑岩建造及第四系(图 2)。其中峡门蛇绿混杂岩属区域上拉脊山蛇绿混杂岩带的东延部分,由强变形的基质(糜棱岩、千糜岩、糜棱岩化岩石)和大小不等的构造块体(橄榄岩、蛇纹岩、辉长岩、玄武岩、灰岩等)组成,“网结状”特征明显。区内断裂构造以近东西向或北东向为主,并在峡门蛇绿混杂岩中发育大量韧性剪切带。

图 2 白家藏地区地质简图 Fig. 2 Geological maps of Baijiazang area

岩浆侵入活动的产物以早奥陶世峡门侵入岩和中奥陶世白家藏侵入岩为代表,其呈两个相对独立的岩株分布于峡门和白家藏两地。其中:早奥陶世峡门侵入岩呈楔状分布于峡门南侧,可进一步解体为英云闪长岩和花岗闪长岩,具有埃达克岩特征(另文发表);白家藏中奥陶世侵入岩呈不规则块状,西侧与峡门蛇绿混杂岩侵入接触,东侧与西宁组断层接触,北侧被第四系覆盖,南侧延伸出图,可进一步解体为闪长岩和石英闪长岩(图 3a)。其中,石英闪长岩与峡门蛇绿混杂岩之间的侵入界线清晰(图 3b),岩石中发育棱角状、透镜状安山岩包体(图 3cd),包体与寄主岩石接触界线平直并发育烘烤边。

a.石英闪长岩宏观特征;b.石英闪长岩与混杂岩侵入特征;c.石英闪长岩中棱角状安山岩包体;d.石英闪长岩中透镜状安山岩包体;e.角闪石中包含斜长石微晶;f.斜长石聚片双晶。Qz.石英;Pl.斜长石;Bit.黑云母;Hb.角闪石。 图 3 白家藏侵入岩野外宏观和微观特征 Fig. 3 Macro and macroscopic geological characteristics of the intrusive rocks in Baijiazang
2 岩相学特征

闪长岩    岩石新鲜面为灰白色—灰色,中细粒半自形粒状结构,块状构造。主要矿物为斜长石、普通角闪石和石英。斜长石体积分数为60%~65%,半自形板状,为更长石,受断层影响晶体产生破碎及双晶纹弯曲错开现象;普通角闪石体积分数约为18%,呈短柱状-他形粒状,其中包含有板柱状斜长石微晶(图 3e);石英体积分数约为4%,他形粒状,边部偶见次生加大边,具高岭土化、绢云母化。

石英闪长岩    岩石新鲜面为灰白色或灰绿色,碎屑-残余似斑状结构,块状构造。主要矿物为斜长石、石英、普通角闪石及黑云母。斜长石体积分数为70%~79%,半自形柱粒状,具细密的聚片双晶(图 3f);石英体积分数为5%~10%,呈他形粒状,分布于其他矿物粒间空隙;普通角闪石体积分数为10%~20%,呈短柱状和他形粒状,多色性明显;黑云母体积分数为8%~16%,呈破碎片状,已被绿泥石及绿帘石交代。岩石受构造应力影响,具明显碎裂岩化。

3 样品采集和测试方法 3.1 锆石U-Pb测年

本次工作在白家藏采集U-Pb同位素测年样品闪长岩和石英闪长岩各1件,采样编号分别为PM-21JD17-1、JD301,采样点地理坐标为36°11′12″N, 102°37′19″E和36°10′10″N, 102°38′03″E(采样位置见图 2)。为保证测试数据的真实性和可靠性,测年样品的采集均选取无变形、无或弱蚀变且没有后期岩脉穿插的基岩露头,样品采集质量均大于5 kg,以保证可以分选出足够的锆石。

测年样品的粉碎和锆石分选工作在河北廊坊地调院矿物分离实验室完成。首先将测年样品用清水洗净后晾干,用常规方法粉碎至80~100目,通过粗淘、强磁分选等工序分离锆石;然后在显微镜下人工挑选出晶型相对完整、透明度高且裂纹相对少的锆石颗粒作为测试对象;再利用双面胶和无色透明的环氧树脂制作锆石靶,并对锆石进行抛光至内部结构暴露;最后在北京离子探针中心完成锆石反射光、透射光及阴极发光(CL)照相。

锆石U-Pb同位素组成分析在中国地质大学(北京)科学研究院元素地球化学实验室完成。采用的测试仪器为激光剥蚀等离子体质谱分析仪(LA-MC-ICP-MS),由美国New Wave Research Inc.生产的UP 193 SS型固体激光器和美国Agilent Technologies公司生产的7500a型四级杆等离子质谱仪联合组成。具体测试方法和工作参数可参考文献[16]。测试数据的处理利用ICPMS Data Cal程序[17],年龄计算及谐和图绘制利用Isoplot程序[18]完成。

3.2 主量、稀土、微量元素测试

本次共采集岩石地球化学样品6件,样品岩性为闪长岩和石英闪长岩。为保证分析数据的代表性和可靠性,测试样品均采自代表性岩石基岩露头,且保证样品新鲜、无或弱蚀变,无后期岩脉穿插。样品主量、稀土、微量元素测试工作由国土资源部武汉矿产资源监督检测中心承担。主量元素利用X荧光光谱仪(XRF-1800)分析,分析精度优于1%;稀土元素、微量元素利用ICP-MS法在质谱仪(ThermoelementalX7)上分析[19],分析精度优于5%,符合国家标准。

4 分析结果 4.1 锆石测年

本次在闪长岩(编号PM21JD17-1)、石英闪长岩(编号JD301)中的测年锆石基本呈无色透明状,锆石形态普遍较完整且自形程度高,多数锆石为短柱状,少数锆石为长柱状或半截柱状(图 4)。锆石粒径一般为80~150 μm,长宽比介于1.5:1.0~2.0:1.0之间,个别锆石长宽比达3:1。锆石CL图像均较为清晰地显示出明暗相间的条带状结构,为锆石生长韵律环带,反映岩浆锆石成因的特点[20-23]。部分锆石边部发育暗色增生边,可能为后期变质作用的产物。

白色实线圆圈代表锆石U-Pb分析点位。 图 4 白家藏闪长岩(PM21JD17-1)、石英闪长岩(JD301)代表性锆石CL特征和同位素年龄 Fig. 4 Representative zircon CL characteristics and isotopic age of the diorite (PM21JD17-1) and the quartz diorite (JD301) in Baijiazang

闪长岩、石英闪长岩同位素样品测年数据分别列于表 1表 2

表 1 白家藏闪长岩(PM21JD17-1)锆石U-Pb同位素分析结果 Table 1 Analysis results of zircon U-Pb isotope from the diorite (PM21JD17-1) in Baijiazang
表 2 白家藏石英闪长岩(JD301)锆石U-Pb同位素分析结果 Table 2 Analysis results of zircon U-Pb isotope from the quartz diorite (JD301) in Baijiazang

闪长岩样品PM21JD17-1中获得有效测试点24个,Th/U值仅23号点为0.37,其余测点介于0.46~0.85之间,均大于0.40,为典型岩浆成因锆石[24-25]。所有测点集中分布在谐和线上,显示出良好的谐和性,表明锆石在形成后其U-Pb体系一直保持在封闭状态,基本没有Pb的丢失(图 5a)。206Pb/238U年龄值介于(468±5)~(480±5)Ma之间,加权平均年龄值为(472.5±1.9)Ma(MSWD=0.58)(图 5b),可以代表闪长岩的岩浆结晶年龄。

图 5 白家藏闪长岩(PM21JD17-1)锆石U-Pb年龄谐和曲线(a)和年龄分布(b) Fig. 5 Concordia diagram of zircon U-Pb age(a) and ages distribution (b) of the diorite(PM21JD17-1) in Baijiazang

石英闪长岩样品JD301中获得有效测试点24个,Th/U值介于0.61~0.90之间,均大于0.40,显示锆石均为岩浆成因。其中22号测点可能由于锆石封闭体系遭受破坏,造成放射成因Pb丢失而明显远离谐和线[26],予以剔除。其余23个测试点显示出极好的谐和性,集中分布于谐和线上(图 6a)。206Pb/238U年龄值集中分布在467~468 Ma之间,加权平均年龄值为(467.3±2.5)Ma(MSWD=0.006 1)(图 6b),代表石英闪长岩的岩浆结晶年龄。

图 6 白家藏石英闪长岩(JD301)锆石U-Pb年龄谐和曲线(a)和年龄分布(b) Fig. 6 Concordia diagram of zircon U-Pb age(a) and ages distribution (b) from the quartz diorite(JD301) in Baijiazang

闪长岩和石英闪长岩206Pb/ 238U年龄加权平均年龄值在误差范围内基本一致,均代表岩浆结晶年龄,综合分析认为白家藏侵入岩的侵位年龄可确定约为470 Ma,为中奥陶世。

4.2 岩石地球化学特征 4.2.1 主量元素

白家藏侵入岩主、微量元素分析结果见表 3。岩石w(SiO2)为55.67%~59.04%,平均为57.56%,属于中性岩类。由表 3可知:w(Al2O3)为15.22%~17.10%,平均为16.17%;w(MgO)为3.34%~5.56%,平均为4.09%;w(Na2O)为3.14%~5.56%,w(K2O)为0.63%~1.12%,全碱质量分数中等,w(Na2O+K2O)介于3.90%~6.46%之间,平均为4.77%,其中w(Na2O)均高于w(K2O),Na2O/K2O值为3.35~6.14,平均为4.42,总体显示岩石相对富Na、贫K的特征。里特曼指数σ均小于3.3(0.95~2.78之间),在AFM图解(图 7a)中样品点均落在钙碱性系列岩石区域;在w(K2O)-w(SiO2)图解(图 7b)中样品投影点分布较为集中,其中有4件样品落在钙碱性区域,2件样品落在低钾-拉斑系列与钙碱性系列接触部位,总体显示钙碱性系列岩石特征;A/CNK值除1个样品大于1.1外,剩余5个样品值介于0.76~1.01之间,在A/NK-A/CNK图解(图 7c)中样品投影点相对分散,但总体分布于准铝质范围内。

表 3 白家藏侵入岩主量元素和微量元素分析结果 Table 3 Analysis results of major and trace elements of the intrusive rocks in Baijiazang
样品号岩性SiO2TiO2Al2O3Fe2O3FeOMnOMgOCaONa2OK2OP2O5烧失量总和
PM21GS5-1闪长岩57.990.7716.323.784.750.163.793.305.560.900.141.8499.30
PM21GS5-2闪长岩56.750.9016.234.105.400.194.064.793.881.120.161.6299.22
PM21GS8-1闪长岩58.360.6816.513.854.500.143.345.953.320.990.121.5699.32
PM21GS17-1闪长岩57.550.6015.222.734.300.155.567.573.380.630.131.4899.31
PM21GS6-1石英闪长岩55.670.8417.103.575.500.164.044.213.980.970.153.0299.21
GS301石英闪长岩59.040.6915.603.014.750.183.756.883.140.760.151.3599.30
样品号岩性A/CNKMg#σQAnAbOrCsRbBaThUTa
PM21GS5-1闪长岩1.0145.342.787.4215.8548.265.491.2920.10246.805.411.140.47
PM21GS5-2闪长岩1.0044.311.8210.8923.2833.666.810.8619.67361.902.920.690.71
PM21GS8-1闪长岩0.9542.731.2115.3427.8528.806.011.2326.78312.502.980.450.99
PM21GS17-1闪长岩0.7659.471.1111.3925.0329.293.822.7316.45229.102.360.471.08
PM21GS6-1石英闪长岩1.1245.231.9311.3620.6834.995.960.4221.94276.004.170.840.66
GS301石英闪长岩0.8447.220.9516.7126.7927.154.5920.14187.103.170.840.82
样品号岩性NbSrNdHfZrSmYLaCePrNdSmEu
PM21GS5-1闪长岩6.16300.8012.812.4082.203.1220.5610.7723.363.1012.813.120.92
PM21GS5-2闪长岩9.92287.7014.872.3082.903.8224.6511.1025.943.4914.873.821.06
PM21GS8-1闪长岩9.66252.3013.312.6091.203.3623.339.8923.203.0813.313.360.94
PM21GS17-1闪长岩6.21393.1011.182.3270.692.3914.5510.7720.492.6911.182.390.94
PM21GS6-1石英闪长岩7.39297.4012.703.70117.003.0720.9510.8024.123.1112.703.071.00
GS301石英闪长岩7.50237.8012.104.6084.803.0019.5812.5524.023.0412.103.000.92
样品号岩性GdTbDyHoErTmYbLu∑REEδEu(La/Yb)N(La/Sm)N(Gd/Yb)N
PM21GS5-1闪长岩3.240.593.770.842.460.382.500.3768.210.882.902.171.04
PM21GS5-2闪长岩3.900.694.451.002.850.442.910.4276.940.832.571.831.08
PM21GS8-1闪长岩3.510.654.180.932.780.422.650.3969.280.832.511.851.07
PM21GS17-1闪长岩2.500.452.670.541.540.251.590.2558.251.174.572.831.27
PM21GS6-1石英闪长岩3.130.593.740.842.440.372.510.3768.780.972.902.211.00
GS301石英闪长岩3.210.593.530.732.150.362.300.3468.840.903.682.631.13
注:主量元素质量分数单位为%;微量元素和稀土元素质量分数单位为10-6。Q(石英)、An(钙长石)、Ab(钠长石)、Or(正长石)为CIPW (Cross, Iddings, Pirsson, Washington)岩石化学计算标准矿物,单位为%。
a.底图据文献[27];b.底图据文献[28];c.底图据文献[29]。 图 7 白家藏侵入岩AFM图解(a)、w(K2O)-w(SiO2)图解(b)和A/NK-A/CNK图解(c) Fig. 7 Diagrams of AFM(a), w(K2O)-w(SiO2)(b) and A/NK-A/CNK (c) of the intrusive rocks in Baijiazang
4.2.2 稀土元素

在稀土元素球粒陨石标准化图解(图 8a)中,闪长岩、石英闪长岩具有同源岩浆演化的特点,均显示轻稀土元素相对弱富集,轻、重稀土元素内部弱分馏的微右倾型曲线。岩石稀土元素总量偏低(w(ΣREE)为58.25×10-6~76.94×10-6,平均68.38×10-6),轻稀土元素总量(w(LREE))为48.47×10-6~60.28×10-6,重稀土元素总量(w(HREE))为9.78×10-6~16.66×10-6,(La/Yb)N值为2.51~4.57,显示轻稀土元素弱富集型;(La/Sm)N值为1.83~2.83, (Gd/Yb)N值为1.00~1.27,说明岩石轻、重稀土元素内部分馏较弱。岩石δEu值主体(5件样品)介于0.83~0.97之间,仅1件样品值为1.17,总体显示弱的Eu负异常,表明侵入岩经历了一定的斜长石分离结晶作用,或源区有一定量的斜长石残留[31]

a.球粒陨石标准化数据引自文献[30];b.原始地幔标准化数据引自文献[32]。图例同图 7 图 8 白家藏侵入岩球粒陨石标准化稀土元素配分模式(a)和原始地幔标准化微量元素蛛网图(b) Fig. 8 Chondrite-normalized REE pattern(a) and primitive mantle-normalized trace element spider diagram(b) of the intrusive rocks in Baijiazang
4.2.3 微量元素

在原始地幔标准化微量元素蛛网图(图 8b)上,总体显示各微量元素质量分数较低的特点,但配分模式近于一致,也显示出同源岩浆的特点。相对于原始地幔,岩石中大离子亲石元素Rb、Ba和活泼的不相容元素Th、U表现出明显富集,而高场强元素Nb、P、Ti等亏损明显。P、Ti的亏损暗示岩石可能受到磷灰石和钛铁矿的分离结晶作用的影响[33-34]。岩石中w(Nb)为(6.16~9.92)×10-6w(Ta)为(0.47~1.08)×10-6,Nb/Ta值为5.78~14.01,平均为10.52,低于地幔平均值(17.5)[35]w(Rb)为(16.45~26.78)×10-6w(Sr)为(237.80~393.10)×10-6,Rb/Sr值为0.04~0.11,平均为0.07,介于上地幔值(0.034)与地壳值(0.350)之间[36],总体反映出岩石壳幔混合源的特点。

5 讨论 5.1 岩石成因和岩浆源区

白家藏地区闪长岩、石英闪长岩6件样品中w(SiO2)为55.67%~59.04%,平均为57.56%,属于中性岩。目前普遍认为中性岩的岩石成因有3种:幔源基性岩浆与花岗质岩浆的混合、幔源岩浆在AFC(同化混染-分离结晶)过程中形成及地壳物质部分熔融形成[37-41]。由幔源基性岩浆结晶分异而形成大面积的中性岩,需要大的幔源岩浆,且以形成基性岩为主体[41],而白家藏地区侵入岩为闪长岩和石英闪长岩,未见基性侵入岩产出,基本可以排除幔源岩浆结晶分异的可能。研究区侵入岩富集大离子亲石元素,亏损Nb、Ti等高场强元素,与大陆壳岩石特征相近,w(Ba)(187.10×10-6~361.90×10-6,平均为268.90×10-6)和w(Sr)(237.80×10-6~393.10×10-6,平均为294.80×10-6)与大陆壳岩石的w(Ba)为(390.00×10-6)和w(Sr)为(325.00×10-6)较为接近,但岩石低硅、高Mg#(42.73~59.47之间,平均为47.38)、富Na贫K(Na2O/K2O=3.35~6.14,平均为4.42)的特征也区别于大陆壳岩石,表明侵入岩不可能是单一陆壳来源,可能与幔源基性岩浆与花岗质岩浆混合有关。

钙碱性系列岩石岩浆为壳幔混合岩,花岗岩类型可划分为富钾及钾长石斑状钙碱性花岗岩类(KCG)和含角闪石钙碱性花岗岩类(ACG)两类[42],KCG高w(K2O)、低w(CaO),主要来源于地壳;ACG低w(K2O)、高w(CaO),主要来源于地幔。白家藏侵入岩为准铝质钙碱性岩,具低w(K2O)(0.63%~1.12%之间,平均为0.90%)、高w(CaO)(3.30%~7.57%之间,平均为5.45%)特征,属ACG岩类,指示岩浆源地幔成分是主要的[43]。研究[44-45]表明, Nb、Ta在侵蚀和变质作用过程中比较稳定,有示踪原始岩浆源区的特征,白家藏侵入岩Nb/Ta值(5.78~14.01之间,平均为10.52)和Nd/Th值(2.37~5.08之间,平均为3.92)均较低,与壳源岩石的Nb/Ta值(≈12)和Nd/Th值(≈3)较为接近,而明显低于幔源岩石的Nb/Ta值(≈22)和Nd/Th值(>15)[46];Rb/Sr值(0.04~0.11之间,平均为0.07)略高于上地幔值(0.034),但明显低于地壳值(0.35)[36],显示了壳幔混合源的特点。Ti/Zr(42.80~65.18之间,平均为51.47)、Ti/Y(174.14~248.37,平均为219.55)略高于陆壳岩石(Ti/Zr<30,Ti/Y<200)[47],可能与源区有石榴子石残留有关[48]

实验岩石学研究表明,下地壳部分熔融产生的基性溶体具有低w(SiO2)、高w(Al2O3)、弱或无Eu负异常等特征,但同时具有低Mg#(<44)和高w(Na2O)(>4.3%)的特征[49-51]。白家藏侵入岩与下地壳基性溶体相比,也具有低w(SiO2)、高w(Al2O3)及弱Eu负异常特征,但岩体Mg#值(42.73~59.47之间)平均为47.38,w(Na2O)(3.14%~5.56%之间)平均为3.88%,显示高Mg#和低w(Na2O)的特征,说明侵入岩不可能是由单纯的下地壳部分熔融产生,可能有部分幔源物质的加入。

在反映岩浆演化的w(TFeO)-w(MgO)图解(图 9a)中,所有样品偏离岩浆演化分离曲线,而落在岩浆混合曲线上,表明分离结晶作用不是白家藏侵入岩的主要形成方式,这与Zr/Nb-w(Zr)图解(图 9b)判别结果一致[53]。在(La/Yb)N-δEu图解(图 10a)中,4个样品点落在壳幔混合范围内,2个样品落在壳源范围。而在Sr/Y-w(Y)图解(图 10b)上,6件样品靠近正常弧岩浆岩区域的下地壳部分熔融曲线。综上所述,白家藏侵入岩岩浆源区具有壳幔混合的特点。

a, b底图据文献[52]。图例同图 7 图 9 白家藏侵入岩w(TFeO)-w(MgO)图解(a)和Zr/Nb-w(Zr)图解(b) Fig. 9 Diagrams of w(TFeO)-w(MgO)(a) and Zr/Nb-w(Zr)(b) of the intrusive rocks in Baijiazang
a底图据文献[54];b底图据文献[55]。图例同图 7 图 10 白家藏侵入岩(La/Yb)N-δEu图解(a)和Sr/Y-w(Y)图解(b) Fig. 10 Diagrams of (La/Yb)N-δEu(a) and Sr/Y-w(Y) (b) of the intrusive rocks in Baijiazang
5.2 构造环境及地质意义

与K、Na等活泼型元素相比,Zr、Hf、Ta等高场强元素在岩浆演化过程中能保持良好的稳定性,通常可作为判别构造环境和岩浆源区性质的示踪剂[56]。通常Th/Ta>3.00可看做岛弧环境的特征[57],白家藏侵入岩岩石样品的Th/Ta值除1件样品小于3.00外(2.20),其余样品(3.00~11.54之间,平均为5.78)均明显大于3.00,符合岛弧环境特征。Th/Yb>0.10(1.01~2.16,平均1.47),Nb/La<0.80(0.57~0.98,平均0.72),Hf/Th<8.00(0.45~1.45,平均0.91),表现出与弧相关的构造环境亲缘性。Zr/Y值(3.36~5.58,平均为4.34)介于大陆边缘安山岩的范围之内(Zr/Y=4~12[58]),显示出活动大陆边缘弧的特性。在Th/Yb-Ta/Yb图解(图 11a)中,样品点均落入活动大陆边缘区域;在w(TFeO)-w(MgO)-w(Al2O3)图解(图 11b)中,样品点均落在岛弧及活动大陆边缘环境。

a底图据文献[57];b底图据文献[59]。图例同图 7。IAB.岛弧玄武岩;IAT.岛弧拉斑系列;ICA.岛弧钙碱系列;SHO.岛弧橄榄玄粗岩系列;WPB.板内玄武岩;MORB.洋中脊玄武岩;TH.拉斑玄武岩;TR.过渡玄武岩;ALK.碱性玄武岩。 图 11 白家藏侵入岩Th/Yb-Ta/Yb图解(a)和w(TFeO)-w(MgO)-w(Al2O3)图解(b) Fig. 11 Diagrams of Th/Yb-Ta/Yb(a) and w(TFeO)-w(MgO)-w(Al2O3) (b) of the intrusive rocks in Baijiazang

与俯冲作用有关的火成岩弧,包括岛弧与活动大陆边缘弧[60]。白家藏侵入岩在An-Ab-Or图解(图 12a)中,1个样品落在奥长花岗岩区域,其余样品均落在英云闪长岩区域,为英云闪长岩-奥长花岗岩组合。根据邓晋福等[62-63]提出的与洋俯冲作用相关的镁安山岩/闪长岩类(MA)判别方法,在w(MgO)-w(SiO2)图解(图 12b)中,样品点均落在镁安山质岩石系列(MA)区域;在w(SiO2)-FeO*/MgO图解(图 12c)中,2件样品落在低铁钙碱性系列(LF-CA)区域,3件样品落在钙碱性系列(CA)区域,暗示白家藏侵入岩的形成与洋壳的俯冲作用有关。

a底图据文献[61];b、c底图据文献[63]。图例同图 7。A.正常安山质岩石系列;MA.镁安山质岩石系列;LF-CA.低铁钙碱性系列;CA.钙碱性系列;TH.拉斑玄武岩系列;FeO*=0.8998w(Fe2O3)+w(FeO)。 图 12 白家藏侵入岩An-Ab-Or图解(a)、w(MgO)-w(SiO2)图解(b)和w(SiO2)-FeO*/MgO图解(c) Fig. 12 Diagrams of An-Ab-Or(a), w(MgO)-w(SiO2)(b) and w(SiO2)-FeO*/MgO (c) of the intrusive rocks in Baijiazang

钟林汐[16]对拉脊山西北峰岩体、山隘岩体、主峰岩体的研究表明,拉脊山地区存在一些成岩年龄在465~450 Ma的闪长岩-花岗闪长岩体,侵入体成岩于中奥陶世。岩体为准铝质钙碱性系列I型花岗岩,具有与俯冲相关的岛弧岩浆岩特征,Sr、Nd同位素特征显示母岩浆成分具有壳幔物质叠加的属性,系幔源钙碱性玄武岩浆结晶分异晚期熔浆叠加岩浆房周边地壳岩石熔融物质的混合岩浆结晶产物。本文研究的白家藏侵入岩成岩年龄约为470 Ma,为中奥陶世,与前述拉脊山岩体基本同期。综合分析认为,白家藏闪长岩、石英闪长岩属拉脊山小洋盆俯冲阶段的产物。

6 结论

1) 拉脊山东段白家藏地区闪长岩、石英闪长岩中分别获得(472.5±1.9)Ma和(467.3±2.5)Ma的锆石U-Pb年龄,据此判断白家藏侵入岩形成于中奥陶世(约470 Ma)。

2) 地球化学特征显示为富钠、贫钾的准铝质钙碱性系列岩石。岩石富集大离子亲石元素Rb、Ba和活泼的不相容元素Th、U,相对亏损高场强元素Nb、Ta、P、Ti等,并反映出岩石壳幔混合源的特点。

3) 环境判别显示白家藏地区侵入岩具岛弧花岗岩的特征,为拉脊山洋俯冲阶段的产物。

致谢: 与笔者一同参加野外工作的有青海省地质调查院丁玉进、王伟、马志远、曹锦山,在此表示感谢。

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http://dx.doi.org/10.13278/j.cnki.jjuese.20190113
吉林大学主办、教育部主管的以地学为特色的综合性学术期刊
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文章信息

张新远, 李五福, 王春涛, 刘建栋, 欧阳光文
Zhang Xinyuan, Li Wufu, Wang Chuntao, Liu Jiandong, Ouyang Guangwen
青海拉脊山东段白家藏侵入岩锆石U-Pb年代学、地球化学特征及其地质意义
Zircon U-Pb Dating, Geochemistry and Their Geological Significance of Baijiazang Pluton in Eastern Section of Lajishan in Qinghai Province
吉林大学学报(地球科学版), 2020, 50(6): 1703-1719
Journal of Jilin University(Earth Science Edition), 2020, 50(6): 1703-1719.
http://dx.doi.org/10.13278/j.cnki.jjuese.20190113

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收稿日期: 2019-05-20

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