延边开山屯地区二叠纪地质体的构造属性：侵入岩及碎屑锆石证据*

2020-04-01魏敬洋裴福萍周皓焦骥王奕朋

岩石学报 2020年3期

魏敬洋裴福萍, 2, 3 周皓焦骥王奕朋

1. 吉林大学地球科学学院，长春 1300612. 自然资源部东北亚矿产资源评价重点实验室，长春 1300613. 东北亚生物演化与环境教育部重点实验室，长春 1300261.

延边地区位于中亚造山带东段南缘，其东北部为兴凯地块，西北部为佳木斯地块，西南部为华北板块(图1a)。古生代时期，延边地区经历了古亚洲洋构造域的演化过程，中生代经历了古太平洋构造域强烈的叠加改造作用。延边地区晚古生代-早中生代构造演化历史一直是地学界研究的热点问题(邵济安等, 1995; 李锦轶, 1998; 吴汉泉等, 2003; 唐克东等, 2004)。

图1 中国东北地区大地构造略图(a, 据Wu et al., 2007修改)和延边开山屯地区地质略图(b)

延边开山屯地区是解决延边地区晚古生代-早中生代构造演化历史的关键地区之一(图1b)，关于开山屯地区的构造亲缘性及其晚古生代-早中生代演化历史主要有两种观点：一种观点认为该地区的晚古生代地层具有史密斯地层层序，处于华北板块北部大陆边缘(刘爱和赵成弼, 1998; 彭玉鲸等, 1999; 周晓东和殷长建, 2004; 周晓东等, 2009)，古亚洲洋的最终闭合位置位于汪清-珲春一带(Chenetal., 2014)；另一种观点基于构造地层学的理论，认为晚古生代地层属于非史密斯地层，开山屯地区属于兴凯地块的西部大陆边缘，存在浊积岩、混杂堆积以及滑塌堆积(邵济安和唐克东, 1995; 邵济安等, 1995; 唐克东等, 2004; 唐克东和赵爱林, 2007)，认为古亚洲洋最终闭合的位置应在开山屯地区(Jiaetal., 2004)。另外，对于延边地区古亚洲洋的最终闭合时间也存在争议，部分学者认为早二叠世开山屯地区处于古亚洲洋向兴凯地块之下俯冲的构造背景，古亚洲洋的闭合时间为晚二叠世-早中生代(Jiaetal., 2004; 唐克东和赵爱林, 2007; 辛玉莲等, 2011; 曹花花等, 2012; 付俊彧等, 2018)。近年来，基于牡丹江构造带上黑龙江杂岩的研究，部分学者认为佳木斯-兴凯地块为外来块体，于晚三叠世-早侏罗世拼贴于东北亚东部大陆边缘(周建波等, 2012, 2013)。

尽管前人对延边地区晚古生代-早中生代火成岩进行了锆石U-Pb年代学、地球化学及同位素特征的研究(Li, 2006; Caoetal., 2011; 曹花花等, 2012; Wangetal., 2015; Bietal., 2017)，同时对开山屯地区开展了较为详细的构造地层学以及古生物化石的研究(孙恒元, 1990; 彭玉鲸等, 1999)。然而，一方面前人缺乏对开山屯地区火成岩的成因研究，另一方面缺乏对该地区晚古生代沉积岩沉积时限、物源特征及其沉积环境研究，上述研究是解决延边地区晚古生代-早中生代构造演化历史的关键之一。本文对延边开山屯地区晚古生代-早中生代沉积岩进行了锆石U-Pb年代学及物源研究，同时对该地区二叠纪侵入岩的成因进行了研究，进一步与邻区和典型地区进行对比，最终确定了开山屯地区二叠纪地质体的构造属性，进一步探讨了延边地区晚古生代-早中生代的演化历史。

1 地质背景与样品描述

延边地区广泛分布着二叠纪-白垩纪含火山岩地层，同时区内大面积分布着二叠纪-早白垩世的花岗岩。开山屯地区晚古生代沉积岩和侵入岩时代的确定，主要依据碎屑岩中所含生物化石。持有序地层观点的学者们将开山屯地区的晚古生代-早中生代地层划分为山秀岭组、大蒜沟组、寺洞沟组、开山屯组和柯岛群(滩前组和山谷旗组)。大蒜沟组由两部分组成，下部为巨厚层砾岩和角砾岩组成，砾石成分有硅质岩和各种火山岩，存在有纯橄榄岩、辉长岩等基性、超基性岩岩块，上部由砂岩、粉砂岩夹灰岩凸镜体组成。开山屯组以花岗质砾岩为主，夹有砂岩和碳质粉砂岩，属于陆相地层，产丰富的植物化石。砾石成分以花岗岩和火山岩为主，其次为砂岩、板岩和少量灰岩(李东津, 1997)。同时还零星分布晚古生代-早中生代中基性侵入岩(图1b, 唐克东和赵爱林, 2007)。另外邵济安和唐克东(1995)认为开山屯地区的晚古生代地质体为无序的增生杂岩(邵济安和唐克东, 1995; 唐克东等, 2004; 唐克东和赵爱林, 2007)。为了便于理解，本文暂以有序地层的观点来描述地层和样品的相对位置。本文样品采自大蒜沟组(16YB13-2、16YB13-3)、寺洞沟组(16YB23-1)和开山屯组(16YB10-1)以及零星分布的中基性侵入岩(16YB27-1、16YB11、16YB12)。中基性侵入岩采自柯岛群内的岩块，岩块出露面积大致为0.08km2，由于强烈的植被覆盖，中基性侵入岩与围岩的接触关系不清(图2h)。样品的岩相学特征如下。

图2 延边开山屯地区沉积岩及侵入岩的野外照片及镜下岩相学特征

样品16YB13-2为中粗粒岩屑杂砂岩，采自江域村北约5km处(GPS坐标：N42°36′04.46″、E129°42′05.54″)，碎屑物直径大部分在0.2～1mm之间，少数碎屑物直径小于0.2mm。碎屑颗粒分选差，磨圆较差，石英颗粒常呈次棱角状。样品的杂基含量约为50%，呈基底式胶结，以杂基支撑为主。矿物碎屑中石英约占40%，长石含量很少，约占5%，部分斜长石发育聚片双晶。岩屑成分以火山岩和硅质岩岩屑为主，含量约为50%。不透明矿物含量约占5%(图2a)。

样品16YB13-3采自样品16YB13-2西约2m，岩性为中砾复成分砾岩，样品中碎屑直径介于5～20mm之间，少数碎屑直径小于5mm，分选差，磨圆较差，常呈次圆或次棱角状。样品的杂基含量很少，呈孔隙式胶结，颗粒支撑。碎屑石英和长石含量较少，约占10%。岩屑含量在90%左右，成分复杂，存在安山岩、石英岩和硅质岩等岩屑(图2b)。野外可观察到直径介于1～10cm的砾石。

样品16YB10-1为中细粒长石杂砂岩，采自船口村东南约3km处(GPS坐标：N42°46′53.82″、E129°47′28.26″)，碎屑物直径在0.15～0.4mm之间，少量碎屑物直径小于0.15mm。碎屑颗粒分选较差，石英颗粒多呈次棱角状。样品中杂基含量在25%～30%之间，存在绢云母化，呈基底式胶结，基质支撑。石英可占40%左右、长石约占50%，其中碱性长石含量约为30%，斜长石含量约占20%，样品中岩屑含量小于5%，含白云母等矿物(图2c)。

样品16YB23-1为中粒长石砂岩，采自江域村西北咸万洞附近山顶处以花岗质碎屑为主的砂砾岩夹层中(GPS坐标：N42°34′53.76″、E129°43′59.04″)。砾石磨圆中等，多呈次圆状。砾石直径大者可达0.5m左右，一般在10cm左右，大小混杂，砾石成份以花岗岩砾石(二长花岗岩)为主(图2e)。此外还见有粉砂质泥岩、粉砂岩、硅质岩以及燧石砾石，砾石含量达50%左右。样品中碎屑物直径介于0.2～0.3mm之间，少数直径小于0.2mm，碎屑颗粒分选和磨圆中等。杂基含量约为12%，呈孔隙式胶结，颗粒支撑。石英约占50%，碱性长石和斜长石含量各占25%左右(图2d)。

样品16YB27-1为细粒角闪闪长岩，采自开山屯镇西南约6km处(GPS坐标：N42°39′42.98″、E129°41′41.10″)，该处岩石经受强烈的风化作用，呈疏松状。样品呈细粒半自形结构，块状构造。主要矿物组成为斜长石(～81%)和普通角闪石(～15%)，斜长石可见聚片双晶，发生次生绢云母化和粘土化。普通角闪石多转变成黑云母，含少量石英(3%，图2f)。

样品16YB11(16YB11-1、16YB11-3、16YB11-4)采自样品16YB27-1东北约700m(GPS坐标：N42°40′1.57″、E129°41′59.37″)，岩性为闪长岩，细粒半自形结构，受后期变形影响呈现片理化，部分矿物呈碎裂状或发生塑性变形。主要矿物由斜长石(～65%)、黄绿色普通角闪石(～30%)和少量的黑云母和石英(2%)组成，斜长石呈自形-半自形板状，表面显示绢云母化。普通角闪石以他形为主，部分普通角闪石受变质变形影响转变为次生的榍石和绿泥石等。黑云母铁质析出。沿裂隙发育碳酸盐化。另外可见锆石和磷灰石等副矿物(图2g)。

样品16YB12(16YB12-1、16YB12-3、16YB12-6)采自样品16YB27-1东约30m(GPS坐标：N42°39′41.62″、E129°41′49.42″)，岩性为辉长闪长岩，似斑状结构，块状构造。斑晶为斜长石和普通角闪石，斑晶含量为10%。基质矿物组成与斑晶相同，主要由斜长石(～50%)和普通角闪石(～40%)组成。斑晶和基质中普通角闪石呈绿色-浅黄绿色多色性，新鲜的样品中普通角闪石中心见有残留的单斜辉石。含榍石、锆石和磷灰石等副矿物，斜长石见有次生的绿帘石和绢云母化，岩石遭受后期的碳酸盐化(图2i)。

2 分析方法

2.1 年代学分析方法

锆石矿物由河北省廊坊市岩拓地质服务有限公司采用标准的重、磁分选技术从全岩样品中提取，并在双目镜下挑选出晶形较好、无明显裂隙和矿物包体的锆石颗粒，然后将选定的锆石浸入环氧树脂中，冷却凝固后打磨抛光以显示出锆石的内部结构。锆石的透射光、反射光和阴极发光图像的采集工作在中国科学院地质与地球物理研究所完成。锆石U-Pb测年实验在中国地质大学(武汉)地质过程与矿产资源国家重点实验室的Agilent 7500a ICP-MS仪器上应用标准测定程序进行，详细的实验原理和流程见(Liuetal., 2008, 2010a, b)。采用锆石标准91500和玻璃标准物质NIST610作外标分别进行同位素和微量元素分馏校正，实验仪器测试的激光束斑直径和频率分别为32μm和10Hz。采用Andersen (2002)提出的方法进行同位素比值的矫正，以扣除普通铅的影响。SIMS锆石U-Pb定年在中国科学院地质与地球物理研究所离子探针实验室的Cameca IMS-1280型二次离子质谱仪(SIMS)上进行，详细的分析流程见(Lietal., 2009)。离子束斑约为20μm×30μm。利用ISOPLOT宏程序进行了锆石年龄谐和图的绘制和年龄加权平均计算(Ludwig, 2003)。单个分析点的同位素比值和同位素年龄的误差(标准偏差)为1σ，206Pb/238U加权平均年龄以95%的置信度得出。

2.2 锆石Hf同位素分析方法

锆石Hf同位素测试在中国科学院地质与地球物理研究所岩石圈演化国家重点实验室配有193nm激光取样系统的Neptune多接收电感耦合等离子体质谱仪(MC-ICP-MS)上进行，仪器的运行条件和详细的分析流程见(Wuetal., 2006)。测定时用标准锆石91500作外标，分析时激光束斑直径为44μm，所用的激光脉冲速率为6Hz，激光束脉冲能量为100mJ，对每个分析点的Yb和Hf的校正工作则主要是利用该分析点实测得出的平均值。

2.3 主量和微量元素分析方法

全岩主量和微量元素分析均在武汉上谱分析科技有限责任公司完成，主量元素含量利用日本理学 PrimusⅡ X射线荧光光谱仪(XRF)分析完成，微量元素含量则采用Agilent 7700e ICP-MS仪器分析完成。用于XRF分析的样品处理流程如下：先将200目样品置于105℃烘箱中烘干12h，称取～1.0g在1000℃马弗炉中灼烧2h，冷却至室温计算烧失量；分别称取6.0g助熔剂、0.6g样品、0.3g氧化剂(NH4NO3)置于铂金坩埚中，在1150℃熔样炉中熔融14min，冷却后取出以备XRF测试。用于ICP-MS分析的样品处理如下：先将200目样品置于105℃烘箱中烘干12h，称取50mg置于Teflon溶样弹中，依次缓慢加入1mL高纯HNO3和1mL高纯HF，、置于190℃烘箱中加热24h以上，冷却后置于140℃电热板上蒸干，加入1mL HNO3并再次蒸干，然后加入1mL高纯HNO3、1mL MQ水(超纯水)和1mL内标In(浓度为1×10-6)，再次将溶样弹放入钢套，拧紧后置于190℃烘箱中加热12h以上；将溶液转入聚乙烯料瓶中，并用2% HNO3稀释至100g以备ICP-MS测试。对国际标样BCR-2、BHVO-1和AGV-1的分析结果表明，主量元素和微量元素的分析精度和准确度分别优于5%和10%。

3 分析结果

3.1 锆石U-Pb年代学

本文对开山屯地区大蒜沟组岩屑杂砂岩(16YB13-2)和复成分砾岩(16YB13-3)、开山屯组长石杂砂岩(16YB10-1)和长石砂岩(16YB23-1)，以及角闪闪长岩(16YB27-1)和闪长岩(16YB11-1)进行了LA-ICPMS锆石U-Pb定年；对辉长闪长岩(16YB12-1)进行了SIMS U-Pb定年。部分锆石的CL图像见图3，锆石U-Pb定年结果见表1。

图3 开山屯地区沉积岩及侵入岩中部分锆石阴极发光(CL)图像

3.1.1 碎屑锆石U-Pb年代学

采自大蒜沟组的样品16YB13-2(岩屑杂砂岩)和16YB13-3(复成分砾岩)中的锆石多呈半自形或他形，直径介于50～100μm之间，发育岩浆振荡生长环带(图3a, b)，Th/U比值介于0.06～3.78，暗示它们为岩浆成因。定年结果显示，79个测点的206Pb/238U年龄介于628～289Ma之间，最年轻峰值年龄分别为298Ma和293Ma(图4a, b)。另外还存在330Ma、366Ma、438Ma、454Ma和479Ma的峰值年龄以及少量628Ma的碎屑锆石(图5a)。

图4 开山屯地区沉积岩及侵入岩锆石LA-ICP-MS和SIMS U-Pb年龄谐和图

开山屯组中的样品16YB10-1(长石杂砂岩)中的锆石多为半自形或他形粒状，少数锆石呈次圆状，直径在60～120μm之间，多数锆石发育岩浆振荡生长环带(图3c)，Th/U比值介于0.07～1.40。57个锆石测点的分析结果显示，其中最年轻的峰值年龄为259Ma，另外存在279Ma、456Ma、476Ma、500Ma、529Ma、891Ma、940Ma以及974Ma的峰值年龄(图4c, d)。此外，还存在少量395Ma以及新元古代-古元古代的碎屑锆石(728Ma、867Ma、1099Ma、1124Ma、1282Ma、1423Ma、1476Ma、1550Ma和1765Ma)(图5c)。

样品16YB23-1(长石砂岩)中的锆石呈自形或半自形，粒径在60～120μm左右，具有岩浆震荡生长环带，部分锆石具有核边结构(图3d)，Th/U值介于0.16～1.24之间，暗示其为岩浆成因锆石。17个分析点的锆石206Pb/238U的年龄介于250～258Ma之间，加权平均年龄为253±1Ma(MSWD=1.01，n=17)(图4e)。该年龄代表了物源区花岗质侵入体的侵位时代。

3.1.2 侵入岩锆石U-Pb年代学

样品16YB27-1、16YB11-1和16YB12-1中的锆石为自形或半自形，具有条痕状吸收的特点(图3e, f, g)，锆石的Th/U比值介于0.34～1.77，暗示它们为岩浆成因。样品16YB27-1(角闪闪长岩)23个锆石测点的206Pb/238U年龄介于274～279Ma，它们的加权平均年龄为277±1Ma(MSWD=0.31)(图4f)。样品16YB11-1(闪长岩)16个锆石测点的206Pb/238U的加权平均年龄为261±2Ma(MSWD=1.3)(图4g)。样品16YB12-1(辉长闪长岩)24个锆石测点的206Pb/238U的加权平均年龄为256±1Ma(MSWD=0.40)(图4h)。3个样品的加权平均年龄代表了各自的形成时代，分别为早二叠世、中二叠世和晚二叠世。

表1开山屯地区沉积岩及侵入岩锆石LA-ICP-MS和SIMSU-Pb分析结果

Table 1 LA-ICP-MS and SIMS zircon U-Pb analytical data of sedimentary and intrusive rocks in Kaishantun area

测点号含量(×10-6)ThUTh/U同位素比值同位素年龄(Ma)207Pb206Pb±1σ207Pb235U±1σ206Pb238U±1σ207Pb206Pb±1σ207Pb235U±1σ206Pb238U±1σ大蒜沟组岩屑杂砂岩样品16YB13-2-012624900.530.058440.003770.375840.023760.046640.00060546145324182944-021794070.440.057540.001730.564780.016290.070930.0006551247455114424-032104260.490.056170.003250.407050.023060.052560.00062459132347173304-043054990.610.048500.005460.312020.033690.046660.00149124243276262949-051042140.490.053050.004250.381370.030140.052130.00067331184328223284-061322810.470.058450.003460.380620.022100.047230.00055547133327162973-07341090.310.046050.004450.325360.029760.051250.001612022862332210-0881410280.790.058570.002850.383590.018340.047500.00042551109330132993-092405640.430.053970.001370.410800.010190.055120.000453704134973463-10621270.490.060050.007520.433530.053410.052360.00119605283366383297-112344380.530.054200.002500.411170.018190.054980.0006538078350133454-1213303533.770.060900.010540.446940.076780.053230.00112636386375543347-132724930.550.054080.008060.354870.052550.047590.00082374327308393005-1410002643.780.061380.004730.475030.034560.056150.00076652134395243525-15861900.450.067590.003160.493160.022450.053270.0008685667407153355-162432331.040.060260.002470.583210.022610.070560.0008461363467154405-17671080.620.059280.007930.447140.059290.054710.00098577303375423436-181213650.330.056790.002030.372860.012630.047650.000444835832293003-19832690.310.052090.002800.339660.017970.047290.00045289126297142983-203855190.740.057450.001890.375170.011500.047460.000515094832382993-213284420.740.057370.002050.377950.013450.047670.0004250663326103003-221793610.500.056630.003200.416260.024040.053060.00077477102353173335-2329890.320.064210.009680.413920.061100.046750.00143749338352442959-241811930.930.061840.002380.872250.033650.102270.0015266857637186289-252634090.640.056070.002570.488360.023330.062970.0008345583404163945-2628680.420.051030.007470.377900.054510.053710.00133242304325403378-27761820.420.055770.003860.547170.037010.071160.00104443159443244436-2819640.300.048480.010500.342230.073690.051200.00122123381299563227-29891610.550.046320.004940.294330.030960.046090.0008514217262242905-301162530.460.067560.003180.498370.022740.053610.0006885574411153374-315195740.900.065210.004590.434990.030190.048170.00081781118367213035-321241720.720.058100.004180.500750.036840.062710.00131534125412253928-331152960.390.061820.003230.447890.021420.052910.0007366879376153324-3427820.330.047830.005170.342310.036230.051910.0011391233299273267-352044560.450.060720.005290.603680.051560.072100.00127629195480334498大蒜沟组复成分砾岩样品16YB13-3-011132850.400.052700.002240.339530.014100.046420.0004531677297112923-021381460.950.056070.006120.395260.038840.051690.00147455169338283259-033448370.410.058280.001630.594270.016320.073270.0005154048474104563-041313040.430.056900.004280.525040.037520.066480.00076488139429254155-053739300.400.056270.001050.570580.010650.072950.000524632945874543-063234550.710.056810.001600.551800.015540.069840.0004648451446104353-07701100.630.056500.008290.409530.059220.052570.00129472331349433308

续表1Continued Table 1测点号含量(×10-6)ThUTh/U同位素比值同位素年龄(Ma)207Pb206Pb±1σ207Pb235U±1σ206Pb238U±1σ207Pb206Pb±1σ207Pb235U±1σ206Pb238U±1σ-08921520.610.054140.003090.347750.020430.046470.00067377107303152934-091824970.370.057240.001380.614600.014520.077400.000545014048694813-10451210.370.057210.004030.459480.029820.058620.00111499110384213677-112863270.880.057640.001680.558000.015800.069920.0005251649450104363-121082500.430.058390.001750.585870.016060.073040.0006754444468104544-13471050.450.053970.006290.385740.045730.051930.001653702093313432610-142903030.950.063960.003110.683240.035250.076780.0013674079529214778-15731450.510.064590.003930.683560.040140.076940.0012576196529244788-16491240.390.057800.003860.465090.030380.058360.00082522151388213665-17741460.510.056980.004740.412180.036440.051400.00116491157350263237-18581310.440.056280.003550.401460.025780.051300.00079463116343193235-191123460.320.056990.001810.577260.018330.072940.0007249153463124544-202112990.710.059030.001900.595980.018220.073100.0006456851475124554-21741410.520.055050.004930.350980.034190.045830.00144414162305262899-22621010.620.049600.006400.394450.050390.057680.00104176274338373626-23741800.410.056920.002490.573770.024360.072920.0009448871460164546-2444880.500.057170.003990.416120.025660.052010.00107498100353183277-252983780.790.060590.003780.387520.024200.046190.0010062597333182916-261513040.500.061350.002010.650900.020750.076900.0007665151509134785-27661330.500.059660.002830.598190.027100.073110.0008059179476174555-281262730.460.055950.001860.565310.019130.072850.0006845059455124534-292102900.720.058550.002300.414200.017740.050670.0004655078352133193-30471090.430.057780.002970.466940.023620.058720.0007852188389163685-313169540.330.056260.001410.572040.015680.073010.0006046246459104544-32162790.060.058070.001500.616430.015870.076670.0006453342488104764-334065890.690.057440.001360.613760.015170.076810.0005850841486104773-342894130.700.056600.001700.573000.016860.072910.0005647651460114543-3559980.600.061510.003240.491760.024460.058670.0008965781406173685-36662080.320.059480.002040.637790.022270.076970.0007458559501144784-37871680.520.068870.003960.738460.039220.077750.0010889587562234836-387827581.030.053140.001430.376290.009820.050980.000393354532473212-391241860.670.060990.004670.400200.031160.047190.00105639130342232976-40441110.390.063770.009590.468460.059920.054100.001687342223904134010-411111710.650.048540.009100.352920.065840.052730.00094126341307493316-4250840.600.069590.005030.729170.046950.077110.00166916975562847910-431041570.670.046160.003950.321690.027110.050550.000746189283213185-44541400.380.063990.003020.465330.026740.051590.00062741102388193244开山屯组长石杂砂岩样品16YB10-1-011023160.320.083540.001252.575280.043720.223370.00197128220129412130010-021013770.270.063990.001440.709000.015050.080670.000567413354495003-03453440.130.071860.001491.556350.032510.156930.0011598231953139406

续表1Continued Table 1测点号含量(×10-6)ThUTh/U同位素比值同位素年龄(Ma)207Pb206Pb±1σ207Pb235U±1σ206Pb238U±1σ207Pb206Pb±1σ207Pb235U±1σ206Pb238U±1σ-042728000.340.057540.001630.607150.015900.076570.0005351245482104763-052033310.610.060390.002160.668020.023170.080410.0009261855520144995-061661361.220.060540.006570.336140.034360.040960.00131623166294262598-07822220.370.071740.002391.467420.047770.148180.0013197852917208917-08522980.170.072650.001611.639740.038340.163280.00140100433986159758-093555190.680.051610.003040.288100.016700.040490.00044268137257132563-10485160.090.073450.001231.648410.026050.162770.00093102635989109725-1145112530.360.058170.001490.642390.015680.080090.0006053657504104974-124013541.130.053550.003780.323890.022610.043870.00046352163285172773-13781990.390.056730.004570.346290.027230.044270.00076481184302212795-141094100.270.057260.001880.637710.021150.080590.0007150258501135004-152732331.170.062790.004350.353340.023890.041060.00060701120307182594-161312010.650.064250.006300.559330.053660.063140.00130750216451353958-173039670.310.057530.001320.641760.015030.080590.000615123850395004-18421120.370.092440.002323.146750.075570.246890.0017914764914441914229-196539750.670.068140.000921.357840.018620.143950.000818731987188675-201624380.370.056940.001130.603460.011960.076700.000544893147984763-21461860.250.057950.001820.612650.018280.076680.0007752848485124765-222493160.790.054890.003920.328470.023230.043400.00044408164288182743-231423710.380.107950.001784.517470.072210.302140.00210176519173413170210-24614760.130.068850.001401.405120.026880.148010.0010589443891118906-251081560.690.096130.003013.821580.118260.287510.00452155035159725162923-26672080.320.071020.002511.538280.047600.156950.00254958379461994014-271111990.560.057450.002820.321650.014940.040840.0004650982283112583-28615360.110.069610.001021.514660.022830.157450.001149171993699436-292413150.770.055860.003930.310260.021540.040290.00047447161274172553-303245980.540.054140.001320.305040.007270.040930.000313774027062592-312577730.330.051310.002220.287980.011510.040710.0006825510225792574-32511690.300.076160.001501.932890.039120.184110.0015410992710931410898-332235260.430.077100.001092.084680.029840.195770.0011011242011441011536-341282300.560.061660.002370.353000.013180.041680.0004366262307102633-353065410.570.052550.001220.295960.006820.040860.000303093926352582-361734130.420.053310.001470.299890.008040.040980.000363424526662592-371955220.370.071940.001231.619440.032380.162870.00172984249781397310-3894212610.750.054120.001590.331420.009610.044530.000303765329172812-39871850.470.055700.006580.310530.034320.040910.00085440211275272585-401461850.790.060950.002910.676260.033830.080750.0006863794525205014-415195021.040.051140.002740.287100.016170.041150.00032247116256132602-421292010.640.070830.004531.447420.100190.148580.00127952130909428937-43861560.550.056250.004690.655420.057770.085330.00107462176512355286-441243400.360.060060.005630.702090.069450.085490.00094606200540415296-4553811980.450.056560.005560.567930.060880.073240.00049475230457394563-467098990.790.054010.005360.328700.035440.044520.00035371230289272812-47497130.070.072230.006101.551870.142480.157360.00134992177951579427-482537500.340.056400.004120.565800.044650.073220.00051468166455294563

续表1Continued Table 1测点号含量(×10-6)ThUTh/U同位素比值同位素年龄(Ma)207Pb206Pb±1σ207Pb235U±1σ206Pb238U±1σ207Pb206Pb±1σ207Pb235U±1σ206Pb238U±1σ-49443200.140.072740.004391.628180.105970.163390.001091007123981419766-50453330.140.071320.003581.454850.078020.148480.0011996697912328927-514348000.540.074080.002771.593970.063590.156480.00095104471968259375-521751251.400.066400.003260.375360.018590.041360.0004981984324142613-535148400.610.089880.001632.956670.056460.238750.0016414232613971413809-5448518530.260.063770.000931.054180.016380.119620.000847342173187285-55745980.120.060060.001420.711160.017090.085610.0007160637545105304-562555360.480.057330.001250.638590.013300.080790.000615043250185014-574156770.610.055560.001380.316010.007520.041150.000304354027962602开山屯组长石砂岩样品16YB23-1-01250928680.870.051260.002100.288390.011360.040770.000522536725792583-02252520301.240.060380.003610.334730.020630.039860.00051617112293162523-032985350.560.051630.002240.287970.012190.040530.0004026979257102563-04121922120.550.048440.004700.268740.025780.040230.00058121216242212544-05127522730.560.054240.001540.298750.008410.039760.000313814926572512-0695113160.720.052480.002290.294210.013010.040520.0004330681262102563-07177119820.890.054130.002030.297350.010830.039840.000353768626482522-0862221030.300.053720.002640.299290.014520.040410.00033359114266112552-09702870.240.051190.006630.282820.036290.040070.00069249286253292534-101874600.410.052970.004210.291940.022940.039970.00046328183260182533-113475060.690.052500.003010.287530.016140.039720.00047307134257132513-1293121410.430.057040.001910.312520.010260.039570.000394935527682502-13171352170.330.052970.002390.289090.012670.039580.00043327105258102503-1419612500.160.051430.001410.290610.007690.040670.000302604725962572-15571700.340.051740.002860.283630.015290.039800.0005527498254122523-1694710580.900.051780.002090.291060.011280.040320.000352767325992552-1784323610.360.054140.004190.296700.022690.039750.00048377178264182513角闪闪长岩样品16YB27-1-01123618400.670.052010.001180.315500.007570.043780.000422863727862763-022774150.670.050970.001010.309010.006380.043810.000392393127352762-036076550.930.051200.001850.310810.010890.044020.000452506227582783-0469810490.670.050300.001450.306550.009020.044110.000372095327272782-05161412891.250.052000.001220.315060.007360.043820.000352863927862762-0667010490.640.051680.001400.314370.008530.044010.000402714627872782-07109410731.020.058390.003420.347290.019060.043940.0007954488303142775-0882511540.710.054940.001720.330950.010260.043690.000454105129082763-098288450.980.054620.001950.328810.011510.043690.000513975728992763-106609650.680.054010.002120.328300.012370.044270.000503716528892793-11480331201.540.056280.002260.336840.012870.043480.0004546466295102743-128107491.080.051280.002030.312470.011940.044160.000472536827692793-132853390.840.055970.002070.336930.011860.044010.000474515929592783-14734541551.770.057290.001930.345240.012450.043450.000455036130192743-1575911490.660.054080.002040.328430.012260.044020.000473746528892783-166458640.750.055760.004380.335960.026190.043700.00089443138294202766-172903650.790.048350.001970.296780.012080.044300.000481167326492793

续表1Continued Table 1测点号含量(×10-6)ThUTh/U同位素比值同位素年龄(Ma)207Pb206Pb±1σ207Pb235U±1σ206Pb238U±1σ207Pb206Pb±1σ207Pb235U±1σ206Pb238U±1σ-18138814700.940.052670.001570.316840.009260.043580.000423154927972753-19128313380.960.052900.002100.322750.013160.044010.0005032472284102783-20280926831.050.055360.002820.338130.016510.044270.0006442783296132794-213895890.660.052300.001930.318340.012090.044000.000462996728192783-2281712920.630.051290.004020.307600.023660.043490.00064254179272182744-23173315711.100.057020.002680.343940.015080.043620.0004649278300112753闪长岩样品16YB11-1-01232713811.690.054630.007620.316210.043950.041980.00050397315279342653-028768441.040.062920.006180.359860.034990.041480.00056705217312262623-035275450.970.061300.006120.355930.034990.042110.00074650223309262665-04191212061.590.052040.005110.290990.028390.040560.00043287225259222563-05120410111.190.051290.003090.293960.017470.041560.00040254139262142632-065696250.910.048050.004210.270510.023590.040830.00036102197243192582-07128013240.970.047030.004820.265820.027060.041000.0004751218239222593-086679130.730.057800.005820.332070.033300.041660.00036522229291252632-096836601.030.046290.003810.264380.021610.041430.0004013181238172622-104895130.950.055290.009700.317380.055550.041630.00053424368280432633-11575551731.110.051350.002560.294850.014490.041650.00035256117262112632-125517200.770.052720.004460.303060.025500.041690.00040317195269202632-137686741.140.053370.006920.297980.038510.040490.00046345293265302563-1414258891.600.056580.002560.324170.014500.041560.00030475103285112622-154995640.890.050090.006600.276540.036220.040040.00058199281248292534-16205515181.350.046050.004690.261440.026460.041180.00050209236212603辉长闪长岩样品16YB12-1-0194412400.760.051160.001630.285530.008560.040500.000372485225572562-0290015870.570.050470.001580.282810.008670.040530.000362175525372562-0356710440.540.053480.001710.299310.009510.040490.000413495326672563-045206610.790.050890.002950.277860.015280.039800.00045236106249122523-055867220.810.051360.001940.286430.010600.040430.000442576525682563-0679110080.780.050790.002060.285040.011380.040540.000432317325592563-0799212350.800.051130.001850.288820.010850.040540.000412476825892563-08132216460.800.051730.001670.289180.008950.040420.000372745425872552-095277000.750.050670.002190.284130.013100.040230.0005222683254102543-105096860.740.051510.002450.289810.014470.040320.0005026492258112553-115268490.620.051270.002080.287020.011040.040690.000452536825692573-125966970.850.050820.002380.284780.013270.040670.0004923286254102573-139069250.980.051380.002170.288390.011940.040670.000442587525792573-14162517840.910.051790.001460.289720.007910.040380.000332764725862552-152434070.600.051590.003460.285640.019280.040250.00064267126255152544-165478630.630.052080.002700.297410.015490.041140.0004928998264122603-1761011470.530.052670.001710.297070.009730.040710.000453155526482573-1891814840.620.051350.001720.293360.010000.041080.000472565726182603-196719350.720.051120.001920.286340.010070.040650.000442466125682573-20114315390.740.051250.001500.290400.008390.040760.000352525125972582

续表1Continued Table 1测点号含量(×10-6)ThUTh/U同位素比值同位素年龄(Ma)207Pb206Pb±1σ207Pb235U±1σ206Pb238U±1σ207Pb206Pb±1σ207Pb235U±1σ206Pb238U±1σ-214309430.460.052490.001880.293830.010200.040480.000443075926282563-22193421170.910.051520.001210.292330.007080.040790.000342644026062582-2336110590.340.052820.001830.297510.010120.040820.000413215926482583-242524780.530.052620.002810.290950.014180.040440.0006231383259112564

图5 开山屯地区二叠纪地层碎屑锆石U-Pb年龄频谱图

图6 开山屯地区中-晚二叠世侵入岩的TAS图解(a,据Middlemost, 1994)和硅钾图(b,据Peccerillo and Taylor, 1976)

图7 开山屯地区中-晚二叠世侵入岩球粒陨石标准化稀土元素配分图(a, b, 标准化值据Boynton, 1984)和原始地幔标准化微量元素蛛网图(c, d, 标准化值据Sun and McDonough, 1989)

3.2 全岩地球化学特征

开山屯地区闪长岩和辉长闪长岩的主量和微量元素分析结果见表2。

3.2.1 主量元素

开山屯地区的中二叠世闪长岩的SiO2含量介于55.5%～56.7%之间，TiO2含量为0.75%～0.82%，Al2O3含量介于17.4%～18.4%之间，MgO含量介于5.47%～5.84%，其Mg#值介于62～63之间，全碱含量(Na2O+K2O)介于5.22%～5.62%，Na2O/K2O为1.79～2.42，在TAS图解中落入闪长岩范围内(图6a)，样品属于中钾钙碱性系列(图6b)。

开山屯地区的晚二叠世辉长闪长岩(16YB12-1)的SiO2含量介于53.0%～53.7%之间，TiO2含量为0.63%～0.88%，Al2O3含量介于13.6%～16.9%之间，MgO含量介于6.79%～10.3%，其Mg#值介于62～73之间，全碱含量(Na2O+K2O)介于3.17%～5.69%，Na2O/K2O为3.27～10.9，在TAS图解中落入辉长闪长岩范围内(图6a)，属于低钾拉斑系列(图6b)。

3.2.2 微量元素

开山屯地区中二叠世闪长岩和晚二叠世辉长闪长岩富集轻稀土元素(LREEs)、亏损重稀土元素(HREEs)(图7a)，稀土元素总量(∑REE)介于45.6×10-6～77.5×10-6之间，δEu值介于0.79～1.12之间，这与岩相学中显示的自形的斜长石存在一致，暗示斜长石分离结晶作用及堆晶作用的存在。轻、重稀土比值(LREEs/HREEs)介于3.27～5.73之间，(La/Yb)N介于2.56～5.50之间，相对富集大离子亲石元素(LILEs)，亏损Nb、Ta、Ti等高场强元素(HFSEs)(图7c)。与中二叠世闪长岩相比，晚二叠世辉长闪长岩具有较低的强不相容元素以及较高的相容元素(Sc、Cr、Co、Ni等)含量。开山屯地区中-晚二叠世侵入岩稀土及微量元素分配型式与开山屯地区东北部珲春地区的二叠纪火成岩基本一致(Caoetal., 2011; 曹花花等, 2012)，它们不同的微量元素丰度主要与岩浆演化过程中堆晶作用有关(Guoetal., 2016)。

表2开山屯地区中晚二叠世侵入岩主量(wt%)及微量(×10-6)元素分析结果

Table 2 Major (wt%) and trace (×10-6) element data of the Middle-Late Permian intrusive rocks in Kaishantun area

样品号16YB11-116YB11-316YB11-416YB12-116YB12-316YB12-6样品号16YB11-116YB11-316YB11-416YB12-116YB12-316YB12-6岩性闪长岩辉长闪长岩岩性闪长岩辉长闪长岩SiO256.556.755.553.753.053.2TiO20.820.750.760.880.630.81Al2O317.917.418.413.616.816.9Fe2OT36.596.776.287.687.498.27MnO0.100.100.100.160.120.12MgO5.485.845.4710.297.576.79CaO4.674.335.256.938.175.52Na2O3.583.983.354.082.914.36K2O1.851.641.870.570.271.33P2O50.170.150.140.110.090.15LOI2.712.793.352.402.972.88Total100.4100.5100.5100.4100.0100.3Na2O/K2O1.942.421.797.1610.933.27Na2O+K2O5.435.625.224.653.175.69Mg#626363736762Li14.413.716.49.017.515.2Be1.311.171.110.580.861.04Sc21.421.522.038.425.025.7V170164170229169269Cr153152159273281178Co23.824.325.135.531.227.2Ni76.277.278.011911786.1Cu24.120868.117.140.7104Zn60.261.657.048.761.170.7Ga19.917.719.511.918.518.3Rb59.251.163.711.935.2031.1Sr386367389372534504Y16.516.415.517.612.521.2Zr16412868.466.955.4114Nb3.603.112.741.231.442.59Sn1.070.970.960.880.601.22Cs4.173.884.520.851.272.37Ba453391429161144377La13.311.710.46.367.5111.5Ce27.924.622.015.116.127.5Pr3.282.922.692.151.953.62Nd13.712.511.510.018.8616.0Sm3.102.942.742.752.254.07Eu0.920.810.870.870.831.06Gd3.052.902.823.072.244.07Tb0.500.480.450.490.370.63Dy2.932.952.623.212.213.70Ho0.590.580.540.630.440.74Er1.671.701.501.801.262.03Tm0.240.260.220.270.190.28Yb1.631.581.411.681.221.95Lu0.250.260.220.270.190.29Hf4.093.291.971.841.553.23Ta0.240.230.210.090.110.19Tl0.240.220.260.0800.0390.17Pb9.498.2511.21.707.586.78Th4.955.324.160.891.874.20U1.571.381.220.260.610.92∑REE73.066.160.048.745.677.5∑LREE62.255.450.237.337.563.9∑HREE10.910.79.811.48.113.7LREE/HREE5.735.195.133.274.624.67(La/Yb)N5.504.984.942.564.164.00δEu0.910.840.950.921.120.79

3.3 锆石Lu-Hf同位素特征

大蒜沟组中的岩屑杂砂岩(16YB13-2)以及开山屯组中的长石杂砂岩(16YB10-1)和长石砂岩(16YB23-1)中的碎屑锆石以及早二叠世角闪闪长岩(16YB27-1)、中二叠世闪长岩(16YB11-1)和晚二叠世辉长闪长岩(16YB12-1)锆石Hf同位素组成见图8，分析结果见表3。

大蒜沟组样品16YB13-2(岩屑杂砂岩)中早二叠世年龄的锆石(～298Ma)的176Hf/177Hf比值介于0.282953～0.282965之间，εHf(t)值为+12.56～+12.99，二阶段模式年龄(tDM2)为489～512Ma；早石炭世和中泥盆世年龄的锆石(337～393Ma)的176Hf/177Hf比值介于0.282733～0.283035之间，εHf(t)值为+5.60～+15.17，Hf同位素二阶段模式年龄(tDM2)为374～981Ma；早志留世年龄的锆石(～442Ma)的176Hf/177Hf比值介于0.282475～0.282558之间，εHf(t)值为-1.14～+1.74，二阶段模式年龄(tDM2)为1318～1496Ma。

开山屯组的样品16YB10-1(长石杂砂岩)和16YB23-1(长石砂岩)中晚古生代年龄的锆石(253～395Ma)的176Hf/177Hf比值介于0.282460～0.282956之间，εHf(t)值为-5.22～+10.15，Hf同位素二阶段模式年龄(tDM2)为536～1632Ma；早古生代年龄的锆石(456～529Ma)的176Hf/177Hf比值介于0.282369～0.282718之间，εHf(t)值为-3.61～+8.47，Hf同位素二阶段模式年龄(tDM2)为401～1696Ma；新元古代年龄的锆石(891～974Ma)的176Hf/177Hf比值介于0.282147～0.282276之间，εHf(t) 值为-3.10～+3.12， Hf同位素二阶段模式年龄(tDM2)为1701～1960Ma；中元古代年龄的锆石(～1282Ma)的176Hf/177Hf比值为0.282119，εHf(t)值为+5.03，Hf同位素二阶段模式年龄(tDM2)为1764Ma。

表3开山屯地区沉积岩及侵入岩锆石Hf同位素特征

Table 3 Zircon Hf isotopic data of sedimentary and intrusive rocks in Kaishantun area

测点号年龄(Ma)176Yb/177Hf176Lu/177Hf176Hf/177Hf2σεHf(0)εHf(t)2σtDM1 (Ma)tDM2 (Ma)fLu/Hf大蒜沟组岩屑杂砂岩样品16YB13-2-012940.0854390.0029210.2829630.0000286.712.651.0432503-0.91-023300.0639020.0018480.2829500.0000266.313.150.9438499-0.94-032970.0632340.0018180.2829640.0000246.812.990.8416484-0.95-043290.0890700.0023150.2827400.000027-1.15.601.0752981-0.93-053340.2642820.0071560.2829690.0000347.012.741.2479528-0.78-063350.0559690.0020180.2829600.0000306.613.571.0426476-0.94-073000.0675630.0024030.2829650.0000256.812.940.9423489-0.93-082980.0706820.0020290.2829530.0000256.412.560.9435512-0.94-093330.0723010.0020290.2829510.0000236.313.210.8439497-0.94-103940.0442930.0013880.2827330.000025-1.46.920.9744947-0.96-114430.0525880.0013490.2824750.000024-10.5-1.140.911081496-0.96-122900.0628320.0016900.2829630.0000286.712.791.0418491-0.95-133370.2981670.0069750.2830350.0000349.315.171.2365374-0.79-143320.0610380.0020810.2829460.0000266.113.000.9447510-0.94-154490.0542730.0018760.2825580.000026-7.61.740.910061318-0.94开山屯组长石杂砂岩样品16YB10-1-019400.0608100.0017410.2822610.000021-18.11.630.714261701-0.95-0212820.0252440.0008560.2821190.000022-23.15.030.815891764-0.97-034990.0529120.0017220.2826560.000032-4.16.321.18611066-0.95-044760.0279360.0009550.2824270.000035-12.2-2.021.211641578-0.97-058910.0318510.0010920.2822150.000037-19.7-0.641.314651806-0.97-069750.0453770.0013800.2822730.000023-17.73.030.813951640-0.96-072560.0218300.0008150.2825750.000022-7.0-1.470.89531375-0.98-085000.0403630.0012420.2824280.000024-12.2-1.560.811721567-0.96-092790.0272630.0009870.2828160.0000251.57.490.9618822-0.97-105000.0339120.0011460.2823690.000026-14.2-3.610.912511696-0.97-113950.0395100.0013650.2825420.000022-8.10.220.810141374-0.96-124760.0361560.0012810.2824740.000024-10.5-0.450.911081478-0.96-134760.0198960.0007080.2825030.000048-9.50.761.710511401-0.98-142740.0811880.0029810.2828560.0000333.08.471.2592755-0.91-158900.0357070.0011150.2821470.000027-22.1-3.100.915621960-0.97-162580.0458460.0015450.2829060.0000324.710.151.1498635-0.95-172550.0330480.0012020.2826020.000028-6.0-0.601.09251319-0.96-189430.0628260.0017890.2822000.000022-20.2-0.520.815151838-0.95-192580.0484050.0016350.2827840.0000270.45.801.0675913-0.95-202580.0414750.0013780.2828260.0000221.97.330.8610816-0.96-212600.1010680.0031890.2828750.0000193.68.820.7567722-0.90-225010.0624110.0020360.2827180.000029-1.98.471.0777931-0.94-235280.0352520.0011080.2826170.000025-5.55.780.99011122-0.97-245290.0381070.0012560.2825640.000051-7.43.861.89801245-0.96-254560.0197060.0005650.2824620.000021-11.0-1.100.811041504-0.98-262810.0609780.0019510.2824600.000026-11.0-5.220.911481632-0.94-279760.0492200.0014400.2822760.000031-17.53.121.113931635-0.96-282610.0851750.0026720.2828880.0000294.19.371.0541688-0.92

续表3

Continued Table 3

测点号年龄(Ma)176Yb/177Hf176Lu/177Hf176Hf/177Hf2σεHf(0)εHf(t)2σtDM1 (Ma)tDM2 (Ma)fLu/Hf-295300.0912990.0025720.2825980.000026-6.14.630.99661197-0.92-302600.0533340.0015970.2828150.0000221.56.960.8629841-0.95开山屯组长石砂岩样品16YB23-1-012540.0781430.0028580.2829560.0000276.511.710.9441536-0.91-022560.0268880.0008630.2826180.000021-5.4-0.360.78941290-0.97-032520.0451000.0013460.2828650.0000213.38.850.7553723-0.96-042510.0336890.0010630.2826220.000030-5.3-0.281.18941285-0.97-052500.0253150.0008730.2825840.000024-6.6-1.560.89421367-0.97-062550.0339700.0010780.2826080.000026-5.8-0.710.99131314-0.97-072510.0297000.0010380.2825000.000026-9.6-3.990.910641542-0.97角闪闪长岩样品16YB27-1-012760.0409150.0012850.2830050.0000198.314.090.7352397-0.96-022780.0385920.0010390.2830180.0000188.714.610.6332365-0.97-032780.0434750.0013070.2830300.0000269.114.990.9317340-0.96-042770.0285800.0007500.2830220.0000198.914.810.7323351-0.98-052760.0635880.0014870.2830320.0000269.215.000.9315338-0.96-062760.0474850.0012460.2830000.0000238.113.920.8359408-0.96-072780.0457510.0013870.2830120.0000308.514.341.1344382-0.96-082790.0401690.0010340.2829910.0000227.713.680.8370425-0.97-092780.0269360.0007210.2830070.0000228.314.280.8345386-0.98-102740.0239670.0006250.2830130.0000238.514.430.8335373-0.98-112750.0361260.0009100.2829980.0000228.013.870.8359410-0.97闪长岩样品16YB11-1-012660.0313380.0007640.2829840.0000247.513.230.8377444-0.98-022560.0549330.0016680.2829930.0000307.813.171.1373440-0.95-032590.0208270.0006040.2829730.0000267.112.710.9391472-0.98-042620.0404460.0013260.2829640.0000326.812.321.1412499-0.96-052630.0197850.0004880.2829740.0000227.212.850.8388466-0.99-062560.0268950.0007200.2829930.0000337.813.321.2364430-0.98-072530.0321120.0009250.2829910.0000257.713.150.9369439-0.97-082600.0463700.0010880.2829910.0000247.713.270.8371437-0.97辉长闪长岩样品16YB12-1-012560.0245820.0007200.2829820.0000127.412.930.4380456-0.98-022580.0716520.0022690.2829720.0000127.112.350.4411495-0.93-032560.0443560.0012760.2829840.0000117.512.920.4382456-0.96-042570.0219140.0007000.2829720.0000107.112.620.4393476-0.98-052600.0664520.0021230.2829760.0000107.212.560.4403482-0.94-062540.0513280.0016220.2829870.0000197.612.920.7382455-0.95-072600.0416240.0013250.2829470.0000126.211.690.4435538-0.96-082570.0544800.0014210.2830060.0000138.313.670.5353408-0.96-092570.0429850.0012100.2829920.0000127.813.240.4370436-0.96-102550.0369290.0010360.2829860.0000107.613.010.4377449-0.97-112570.0279790.0009520.2829570.0000116.512.020.4418515-0.97-122560.0263790.0007340.2829850.0000107.513.030.3376449-0.98

图8 开山屯地区沉积岩和侵入岩锆石Hf同位素特征(燕山褶皱带范围据Yang et al., 2006)

早二叠世角闪闪长岩(16YB27-1)中代表其形成时代的锆石(～277Ma)的176Hf/177Hf比值介于0.282991～0.283032之间，εHf(t)值为+13.68～+15.00，Hf同位素单阶段模式年龄(tDM1)为315～370Ma。其εHf(t)值与兴凯西南缘晚二叠世闪长岩中的早二叠世岩浆成因的捕获锆石相一致(Caoetal., 2011)，并且明显高于吉中地区早二叠世大河深组火山岩的锆石εHf(t)值(曹花花等, 2012)。中二叠世闪长岩(16YB11-1)和晚二叠世辉长闪长岩(16YB12-1)中岩浆锆石(261～256Ma)的176Hf/177Hf比值介于0.282947～0.283006之间，εHf(t)值为+11.69～+13.67，Hf同位素单阶段模式年龄(tDM1)为353～435Ma。其εHf(t)值与兴凯地块西南缘同时代的闪长岩相似(Caoetal., 2011)。

4 讨论

4.1 开山屯地区二叠纪地质体的形成时限

开山屯地区大蒜沟组和开山屯组沉积岩中的锆石具有震荡生长环带，Th/U比值介于0.11～3.78(除3个点外)，暗示它们为岩浆成因。锆石U-Pb定年结果表明，大蒜沟组岩屑杂砂岩和复成分砾岩中锆石最年轻的峰值年龄为296Ma，该年龄代表了大蒜沟组沉积的下限年龄。前人对开山屯地区大蒜沟组和开山屯组时代的确定主要依据其中的动物和植物化石。大蒜沟组砾岩中存在灰岩凸镜体以及山秀岭灰岩岩块，在山秀岭灰岩岩块中产有Triticites晚石炭世化石，同时在大蒜沟组中产有Parafusulina、Monodiexokina等栖霞晚期-茅口早期的代表性动物化石分子(孙恒元, 1990; 李东津, 1997; 唐克东和赵爱林, 2007)，上述代表性动物化石的产出限定大蒜沟组沉积时限为早-中二叠世，这与本文的定年结果是吻合的。

尽管前人将开山屯地区的寺洞沟组归入中二叠世，但缺少化石及同位素年代学证据。野外观察发现，样品16YB23-1所在砂砾岩层中砾石磨圆度较好，同时出现大量的花岗质砾石，这与大蒜沟组中以碳酸盐岩和硅质岩为主的砾石组成明显不同，而与开山屯组极其相似，这也得到相似的碎屑锆石年龄组成的支持。鉴于上述特征，本文认为该砂砾岩层应属于开山屯组。样品16YB23-1(长石砂岩)和16YB10-1(长石杂砂岩)的碎屑锆石U-Pb定年结果显示，其中最年轻的峰值年龄为256Ma(图5d)，这与前人对开山屯组碎屑锆石年代学研究结果相一致(Duetal., 2019)。表明开山屯组沉积的下限为晚二叠世。该结论也得到了开山屯组粉砂岩和细砂岩中大量晚二叠世植物化石碎片存在的支持(彭玉鲸等, 1999)。延边地区分布有大量晚二叠世-中三叠世(256～237Ma)以及晚三叠世火成岩(228～215Ma)，然而开山屯组中存在晚二叠世(256Ma)碎屑锆石而未见晚三叠世的碎屑锆石，暗示开山屯组的沉积时限应为早-中三叠世。

根据野外地质特征，结合本文锆石年代学研究，开山屯地区的晚古生代-早中生代地质体整体是无序的，这与邵济安和唐克东(1995)的认识一致，即大蒜沟组应为滑塌堆积，柯岛群应属于浊积岩建造的一部分(邵济安和唐克东, 1995)。本文采自柯岛群内的侵入岩块形成于277～253Ma，明显老于柯岛群碎屑锆石最年轻锆石年龄(252Ma, 付俊彧等, 2018)。本文的定年结果也说明大蒜沟组滑塌堆积形成时间早于浊积岩建造。

4.2 开山屯地区二叠纪碎屑岩的沉积物质来源

开山屯地区大蒜沟组以巨厚砾岩、灰岩以及火山岩岩块为特征，砾岩中的砾石成分以硅质岩、碳酸盐岩、火山岩、基性和超基性侵入岩为主。大蒜沟组中的岩屑杂砂岩(16YB13-2)和复成分砾岩(16YB13-3)中碎屑锆石U-Pb定年结果表明，存在6期峰值年龄，分别为296Ma、330Ma、366Ma、438Ma、454Ma和479Ma(图5b)。缺乏与陆块基底有关的前寒武纪年龄锆石，暗示其空间位置远离陆块的内部，这与该组中砾石成份相一致。大蒜沟组碎屑锆石早古生代的峰值年龄(438Ma、454Ma和479Ma)及其Hf同位素组成与开山屯地区以及兴凯地块西缘早古生代侵入岩及沉积岩中的同时代锆石年龄相一致(Wangetal., 2016; Hanetal., 2018)，表明其来源于延边地区早古生代的岛弧火成岩，而不同于华北板块北缘被动大陆边缘环境(Mengetal., 1997; Dobretsov and Buslov, 2007)；大蒜沟组晚泥盆世-早石炭世碎屑锆石具有较为亏损的锆石Hf同位素组成，该特征与华北板块北部大陆边缘同时代的岩浆事件不同(Yangetal., 2006)，进一步根据延边五道沟群中早石炭世和晚泥盆世的碎屑锆石峰值年龄的存在，暗示延边地区存在晚泥盆世和早石炭世岩浆事件(图5b, Chenetal., 2014)。大蒜沟组碎屑岩中存在早二叠世(～296Ma)的峰值年龄，这与兴凯地块西部珲春地区解放村组砂岩的峰值年龄一致(～297Ma，未发表资料)，并存在于五道沟群变质粉砂岩和片岩中(图5b, Chenetal., 2014)。同时该年龄与延边珲春同时代关门咀子组火山岩近似(曹花花等, 2012)。另外，大蒜沟组早二叠世(～296Ma)碎屑锆石强烈亏损的锆石Hf同位素组成也不同于华北板块基底具有亲缘性的碎屑锆石的Hf同位素组成(Yangetal., 2006)。基于上述分析，本文认为，大蒜沟组沉积岩的物源应来自兴凯地块西部大陆边缘。

开山屯组以砾岩为主，砾石成分主要为花岗质砾石，其中产有丰富的植物化石，花岗质砾石的出现暗示其物源区陆壳的成熟度较高，其陆相沉积环境与物源区存在高成熟度陆壳的特征与大蒜沟组明显不同。开山屯组长石杂砂岩(16YB10-1)和长石砂岩(16YB23-1)的锆石定年结果显示，不仅存在早古生代(456Ma、476Ma、500Ma和529Ma)和晚古生代(256Ma和279Ma)峰值年龄，同时存在少量古元古代-中元古代(1099～1550Ma和1765Ma)以及新元古代(891Ma、940Ma和974Ma)峰值年龄(图5c)。其中早古生代年龄锆石的Hf同位素组成(εHf(t)=-3.61～+8.47)与早古生代洋内岛弧相关的火成岩相似(Peietal., 2016; Wangetal., 2016; Shuetal., 2018)，暗示其来自早古生代弧火成岩。开山屯组中新元古代岩浆事件峰值年龄暗示与北部兴凯地块的亲缘性。另外，与大蒜沟组早二叠世年龄锆石以及开山屯地区二叠纪侵入岩相比(图8a)，开山屯组中的早二叠世和晚二叠世年龄的锆石具有相对较为富集和变化较大的εHf(t)值(-5.22～+10.15)，这与兴凯地块和华北板块北部陆缘二叠纪花岗质侵入体中锆石Hf同位素组成相似(Liuetal., 2017; Zhangetal., 2018)。结合古元古代碎屑锆石的存在，说明开山屯组碎屑物源为兴凯地块和华北板块北缘双向物源，这与开山屯组中华夏植物群和安加拉植物群的存在相吻合(彭玉鲸等, 1999)。

4.3 开山屯地区二叠纪侵入岩源区性质

开山屯地区中-晚二叠世闪长岩和辉长闪长岩的SiO2含量分别介于55.5%～56.7%和53.0%～53.7%，具有较高的MgO含量(5.47%～10.29%)和Mg#值(Mg#=62～73)以及高Sc、Cr、Ni和Co含量，上述特征与高镁安山岩相似，暗示其原始岩浆主要来源于地幔物质的部分熔融，这也得到相对变化范围有限的锆石Hf同位素组成的支持。闪长岩和辉长闪长岩属于低钾拉斑系列-中钾钙碱性系列，同时显示相对富集轻稀土和大离子亲石元素，亏损重稀土和高场强元素特征，具有较低的(Ta/La)N(0.24～0.34)和(Nb/La)N(0.18～0.26)比值，与珲春地区二叠纪火成岩的地球化学特征相似，暗示它们均形成于俯冲背景(图7c, Ellam and Hawkesworth, 1988; Caoetal., 2011; 曹花花等, 2012)。进一步结合开山屯地区中二叠世闪长岩和晚二叠世辉长闪长岩中亏损的锆石εHf(t)值(+11.69～+13.67)和年轻的单阶段模式年龄(tDM1=353～435Ma)，表明开山屯地区中-晚二叠世闪长岩和辉长闪长岩的原始岩浆主要来源于受俯冲流体/熔体改造的亏损地幔楔的部分熔融。

4.4 延边地区二叠纪构造背景及演化历史

关于延边地区二叠纪的构造格局及古亚洲洋的闭合时间存在很多争议，尽管邵济安和唐克东提出开山屯地区存在蛇绿混杂岩，应为兴凯地块与华北板块最终闭合的位置(邵济安和唐克东, 1995; Jiaetal., 2004)。然而，其它学者基于火成岩和碎屑锆石的研究，提出了拼合位置位于汪清-珲春一带等不同观点(赵春荆等, 1996; 李锦轶, 1998; Xiaoetal., 2003; 李锦轶等, 2009; Chenetal., 2014; Duetal., 2019)。另外，关于古亚洲洋闭合的最终时间，存在晚二叠世-早三叠世以及中三叠世等不同观点(Wangetal., 2015; Bietal., 2017)。

首先，大蒜沟组的岩石组合特征与滑塌堆积环境相似，其次，碎屑锆石年龄组成暗示沉积环境应为弧前盆地或增生楔环境，进一步结合研究区及其东部珲春地区二叠纪火成岩与俯冲作用有关的地球化学及同位素特征(曹花花等, 2012)，表明开山屯-珲春地区二叠纪火成岩处于活动大陆边缘构造背景(图9; Guoetal., 2016)。

图9 开山屯地区中-晚二叠世侵入岩构造环境判别图解(a,据Wood, 1980；b,据Pearce and Peate, 1995)

开山屯组以砾岩为主，其中碎屑锆石年龄组成及Hf同位素特征与大蒜沟组明显不同，并具有来自成熟陆壳的(二长)花岗岩岩屑，锆石的Hf同位素组成与兴凯地块和华北板块北缘同时代的花岗质岩石相似(Wangetal., 2015; Zhangetal., 2018)，暗示为双向物源，符合造山晚期磨拉石建造特征。开山屯组中锆石年龄组成显示，缺乏230～240Ma的锆石，同时延边地区也缺乏同时期岩浆事件(Bietal., 2017; Yangetal., 2018)，推测该阶段(即开山屯组沉积时期)为古亚洲洋最终闭合的后造山阶段，沉积了开山屯组磨拉石建造组合，同时产出早-中三叠世埃拉克质岩石(闪长岩-英云闪长岩-二长花岗岩，Yangetal., 2018)。综合上述研究，认为延边地区古大洋的闭合时间应为早-中三叠世。根据大蒜沟组中极其亏损的碎屑锆石Hf同位素组成以及古生代碎屑锆石年龄组成(图8)，结合开山屯地区蛇绿混杂岩的存在(邵济安和唐克东, 1995)，认为开山屯地区应为兴凯地块与华北板块的最终拼合位置。

关于长春-延吉构造带的构造属性是近年来地学界争论的热点问题之一。大部分学者认为该构造带西延与西拉木伦河-索伦构造带相连(唐克东等, 1995; Li, 2006; Wuetal., 2007; Caoetal., 2011; 曹花花等, 2012)。然而，近年来，基于黑龙江杂岩的研究，周建波等(2013)则认为长春-延吉构造带应向北与牡丹江构造带相连，从延边地区到牡丹江构造带于晚三叠世-早侏罗世以剪刀式最终闭合(周建波等, 2012, 2013)。同时有学者提出牡丹江洋向西俯冲的观点(孙晨阳等, 2018; Longetal., 2019)。另外，对于延边地区二叠纪古大洋的俯冲方向也存在向华北板块之下俯冲以及向兴凯地块之下俯冲两种观点(Caoetal., 2011; Wangetal., 2015)。目前本文数据尚不能解释上述问题。如果开山屯地区的蛇绿岩组合存在的话，考虑到开山屯地区东北部珲春-五道沟一带地球化学特征相似的二叠纪基性火成岩的存在，说明古亚洲洋向兴凯地块之下俯冲作用是存在的，同时存在向华北板块之下的俯冲作用，即双向俯冲作用。