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吉林中部小蜂蜜顶子组火山岩的形成时代及其地质意义:锆石U-Pb年代学和Hf同位素组成证据
张海洪1,2, 许文良1, 王枫1, 曹花花1,3     
1. 吉林大学地球科学学院, 长春 130061 ;
2. 吉林省地质调查院, 长春 130021 ;
3. 中国海洋大学海洋地球科学学院, 山东 青岛 266100
摘要: 本文对出露于吉林省中部地区的小蜂蜜顶子组标准剖面的流纹岩(样品号:JH21-1)和非标准剖面小蜂蜜顶子组的安山岩(样品号:JX1-1)进行了锆石LA-ICP-MS U-Pb年代学以及Hf同位素组成研究。2个样品中的锆石均呈自形-半自形,CL图像上显示条痕状吸收或典型的岩浆振荡生长环带,结合其Th/U比值(0.27~0.81),均暗示了它们的岩浆成因。定年结果表明:流纹岩(JH21-1)中锆石206Pb/238U加权平均年龄为(192±2)Ma (MSWD=4.8),其形成于早侏罗世;安山岩(JX1-1)中锆石206Pb/238U加权平均年龄为(172±1)Ma (MSWD=0.27),表明其形成于中侏罗世。前者与标准剖面时代一致,后者与小蜂蜜顶子组并非同一套火山岩。而2个样品中锆石的Hf同位素组成较为均一,εHft)值为+5.25~+9.37,二阶省模式年龄为803~1 177 Ma,暗示它们起源于中-新元古代新增生陆壳物质的部分熔融。综上所述,吉林中部地区侏罗纪岩浆事件至少可以划分为两期——早侏罗世和中侏罗世,结合区域构造演化历史,可以认为吉林省中部地区早-中侏罗世火山岩的形成应与古太平洋板块俯冲于欧亚大陆之下的地质作用相联系。
关键词: 吉林中部     小蜂蜜顶子组     火山岩     锆石U-Pb年代学     岩石组合     构造意义    
Formation Timing of the Volcanic Rocks from the Xiaofengmidingzi Formation in Central Jilin Province and Its Geological Implications: Evidence from Zircon U-Pb Dating and Hf Isotope Compositions
Zhang Haihong1,2, Xu Wenliang1, Wang Feng1, Cao Huahua1,3     
1. College of Earth Sciences, Jilin University, Changchun 130061, China ;
2. Institute of Geologic Survey, Jilin Province, Changchun 130021, China ;
3. College of Marine Geosciences, Ocean University of China, Qingdao 266100, Shandong, China
Supported by Supported by the National Natural Science Foundation of China Project (41072038) and China Geological Survey's Program Project (12120113098300)
Abstract: The data of LA-ICP-MS zircon U-Pb dating and Hf isotopes are reported for the rhyolite (JH21-1) from typical section and the andesite (JX1-1) from another section of the Xiaofengmidingzi Formation in the central Jilin Province, NE China. The zircons from two volcanic rocks are euhedral-subhedral in shape and display striped absorption or oscillatory growth zoning in the CL images, and have high Th/U ratios (0.27-0.81), implying their magmatic origin. The dating results indicate that the zircons from the rhyolite (JH21-1) yield a weighted mean 206Pb/238U age of (192±2) Ma (MSWD=4.8), i.e., the Early Jurassic, and the zircons from the andesite (JX1-1) yield a weighted mean 206Pb/238U age of (172±1) Ma (MSWD=0.27), i.e., the Middle Jurassic. The former is consistent with the timing of the typical section, and the latter is different from the typical Xiaofengmidingzi Formation. Their zircon εHf(t) (172-192 Ma) values range from +5.25 to +9.37, and Hf two-stage model ages vary from 803 Ma to 1 177 Ma,implying that their primary magmas were derived from partial melting of the juvenile lower crust. Taken together, it is suggested that the Jurassic magmatism can be subdivided into two stages, i.e., the Early Jurassic and Middle Jurassic. Combined with regional tectonic evolution history, we conclude that the Jurassic magmatism in the central Jilin Province could be related to the subduction of the Paleo-Pacific plate beneath the Eurasian continent.
Key words: central Jilin Province     Xiaofengmidingzi Formation     volcanic rocks     zircon U-Pb chronology     rock association     tectonic implication    

0 引言

吉林中部地区处于华北克拉通北缘东北部和中亚造山带东南部交汇区域。古生代—中生代早期,该区构造演化受古亚洲洋构造体系控制,以多个微陆块的碰撞拼合为主要特征,包括额尔古纳地块、兴安地块、松辽地块、张广才岭地块、佳木斯地块以及兴凯地块[1-9]。中生代期间,该区主要受环太平洋构造体系控制[5, 10]。对于环太平洋构造体系开始的时间,目前尚无统一认识。翁文灏早在1927年提出燕山运动始于中侏罗世期间,同时认为其标志环太平洋主动陆缘发展的开始[11]。而目前对于环太平洋板块俯冲开始的时间主要存在两种主要观点:一种观点认为俯冲开始于晚三叠世[12-14],另一种观点认为俯冲开始于早—中侏罗世[7, 15]。产生上述争议的主要原因是对该地区中生代火山岩的形成时代、岩石组合及空间展布特征认识不清,这对于解决上述问题带来了制约。鉴于此,本文选取了吉林中部地区小蜂蜜顶子组作为研究对象,通过对出露于该组标准剖面上的流纹岩(J1x)[16]和出露于吉林市大苇子沟地区的该组安山岩(T1x)[17]进行锆石U-Pb年代学和Hf同位素研究,对不同地区小蜂蜜顶子组火山岩的形成时代给予了准确的限定,同时结合区域地质资料,探讨了小蜂蜜顶子组火山岩形成时代及其地质意义。

1 地质背景和样品描述

吉林中部地区在地理位置上,位于吉林省长春市和吉林市之间;在大地构造位置上,位于张广才岭构造带的南缘,西邻松辽盆地,南以西拉木伦河断裂与华北克拉通北缘相连(图 1图 2a)。研究区出露地层主要为石炭系鹿圈屯组、磨盘山组、石咀子组,岩石类型主要为一系列灰黑色角岩和灰岩;二叠系包括大河深组和范家屯组,前者主要为一套火山沉积岩,后者主要由一系列砾岩和砂岩组成;侏罗系主要包括小蜂蜜顶子组和南楼山组,主要为一套中酸性火山岩;白垩系主要为前人所厘定的泉头组,由一系列棕红色砾岩和砂泥岩组成。岩浆岩主要是燕山早期花岗岩,出露广泛。对东北地区显生宙花岗岩的年代学研究表明,其中190、185和175 Ma是岩浆事件发生的高峰,而吉中地区(松辽—张广才岭)花岗岩主要分为172~176,180~187,190~200 Ma三期[15, 18]

图 1 研究区构造位置图 Figure 1 Tectonic sketch map of research area
图 2 吉林中部小蜂蜜顶子组火山岩分布图 Figure 2 Distribution map of the volcanic rocks from the Xiaofengmidingzi Formation in central Jilin Province

小蜂蜜顶子组由吉林省地质矿产局所创立[16],主要出露于长春市双阳以南和吉林市西南地区(图 2),主要由一系列中—酸性火山岩、火山碎屑岩组成,局部夹薄层砂岩、砾岩和煤[17, 19]

样品JH21-1为流纹岩,采于前人所厘定的小蜂蜜顶子组标准剖面,采样地点位于长春市双阳区黑顶村东南黑顶子水库北东约2 km处(东经125°37′59'',北纬43°23′56'')(图 2b)。样品呈浅灰色,具典型的斑状结构和块状构造,斑晶主要由透长石和石英所组成。透长石粒径为0.5~1 mm,最大可达2 mm;石英斑晶粒径在0.25 mm左右(图 3a,b)。基质为隐晶质,具有典型的流纹构造。

a、b. 流纹岩(JH21-1),a 为正交偏光,b为单偏光;c、d.安山岩(JX1-1),正交偏光。Af.碱性长石; Pl.斜长石; Q.石英。 图 3 小蜂蜜顶子组火山岩显微结构特征 Figure 3 Microphotographs showing textures of volcanic rocks from the Xiaofengmidingzi Formation

样品JX1-1为安山岩,采于吉林市西南地区的小蜂蜜顶子组,采样地点位于吉林市永吉县芦苇村南西2 km处(东经126°07′55'',北纬43°37′54'')(图 2c)。样品呈灰绿色,具典型斑状结构和块状构造,斑晶体积分数为20%,主要由斜长石和少量的角闪石所组成。基质为交织结构,局部绿泥石化(图 3c,d)。

2 分析方法 2.1 年代学分析方法

本文锆石LA-ICP-MS U-Pb同位素分析在中国地质大学(武汉)地质过程与矿产资源国家重点实验室的Agilent 7500a型ICP-MS仪器上用标准测定程序进行,采用He作为剥蚀物质的载体,详细的实验原理和流程见参考文献[20-22]。应用标准锆石91500进行分馏校正,标准锆石TEMORA1作为未知样品测定获得的年龄为(415±4)Ma(MSWD=0.112,n=24),该锆石的ID-TIMS年龄为(416.75±0.24)Ma[23]。激光束的束斑为30μm。实验获得的数据采用Andersen[24]的方法进行同位素比值的校正,以扣除普通Pb的影响,然后用ISOPLOT 3.0宏程序进行年龄谐和图的生成和处理[25]

2.2 锆石Hf 同位素分析方法

在LA-ICP-MS 锆石U-Pb 定年的基础上,参照锆石阴极发光(CL) 图像对本文所采样品进行了锆石原位微区Hf 同位素测定。锆石Lu-Hf 同位素测试在中国科学院地质与地球物理研究所配有193 nm 激光取样系统的Neptune 多接收电感耦合等离子体质谱仪(LA-MC-ICP-MS)上进行,详细的测试过程见参考文献[26] ,详细测试流程以及仪器运行条件等参见[27]。测定时用锆石国际标样91500作外标,He气作为剥蚀物质载体,分析时激光束斑直径为44 μm,激光脉冲宽度为15 ns,所用的激光脉冲速率为6~8 Hz,激光束脉冲能量为100 mJ。

3 分析结果 3.1 年代学

长春双阳黑顶子流纹岩样品(JH21-1)和吉林市大苇子沟安山岩样品(JX1-1) 部分测定锆石CL图像如图 4所示,U-Pb年龄谐和图如图 5所示,LA-ICP-MS U-Pb分析结果表见表 1

虚线圈为Hf 同位素分析点;实线圈为U-Pb 年龄分析点,圈内数字为U-Pb 年龄分析点号,上排数字为U-Pb 年龄值,下排数字为εHf(t) 值。 图 4 小蜂蜜顶子组火山岩锆石代表性CL图像 Figure 4 Typical cathodoluminescence (CL) images of zircons from the volcanic rocks of the Xiaofengmidingzi Formation
图 5 小蜂蜜顶子组火山岩锆石LA-ICP-MS U-Pb 年龄谐和图 Figure 5 Concordia diagram showing LA-ICP-MS zircon U-Pb dating data for the Xiaofengmidingzi Formation volcanic rocks
表 1 LA-ICP-MS 小蜂蜜顶子组火山岩锆石U-Pb测试数据 Table 1 LA-ICP-MS zircon U-Pb dating data for the Xiaofengmidingzi Formation volcanic rocks
岩性样号wB/10-6Th/U同位素比值年龄/Ma
207Pb/206Pb207Pb/235U206Pb/238U207Pb/206Pb207Pb/235U206Pb/238U
ThU比值误差1σ比值误差1σ比值误差1σ年龄误差1σ年龄误差1σ年龄误差1σ
JH21-1-0187.33226.220.390.052 660.000 950.258 970.004 200.035 670.000 282135822452252
JH21-1-02240.19534.250.450.053 760.000 920.220 180.003 380.029 710.000 233612120231891
JH21-1-03223.94607.250.370.050 200.000 810.209 210.002 970.030 220.000 232041919321921
JH21-1-04225.01612.840.370.069 740.001 510.295 330.005 890.030 720.000 273228020161912
JH21-1-05231.82631.680.370.050 360.000 730.209 610.002 600.030 190.000 222121619321921
JH21-1-06211.45514.820.410.050 090.000 910.209 430.003 440.030 320.000 241992419331932
JH21-1-08250.60605.820.410.050 220.001 150.205 850.004 400.029 730.000 252053419041892
JH21-1-09120.12362.740.330.050 700.001 160.236 360.005 030.033 810.000 282273421542142
JH21-1-10258.25508.820.510.055 310.001 280.229 800.004 960.030 130.000 264253321041912
JH21-1-11290.52552.820.530.060 810.001 730.248 480.006 690.029 630.000 293449319971862
JH21-1-12361.131 053.540.340.049 380.000 690.205 890.002 460.030 230.000 221661519021921
JH21-1-13535.33790.610.680.050 910.000 840.246 930.003 640.035 170.000 272372022432232
JH21-1-14325.33732.010.440.057 240.000 970.240 190.003 620.030 430.000 244135421041932
JH21-1-15154.64408.280.380.053 210.000 930.218 860.003 420.029 830.000 232515519441891
JH21-1-16196.38423.550.460.050 200.000 800.212 280.003 000.030 660.000 232041919531951
JH21-1-17190.64421.820.450.057 030.001 000.238 150.003 750.030 280.000 243166020141912
JH21-1-18596.93903.580.660.051 380.000 780.212 140.002 820.029 940.000 222581719521901
JH21-1-19152.69475.270.320.049 70 0.001 060.207 580.004 110.030 290.000 251813119231922
JH21-1-20206.80596.730.350.050 280.000 830.213 160.003 130.030 740.000 232082019631951
JH21-1-21372.42790.570.470.054 000.000 700.229 160.002 480.030 770.000 222224819731951
JH21-1-22294.32736.590.400.050 920.000 700.217 080.002 530.030 910.000 232371419921961
JH21-1-23526.68829.570.630.059 910.000 880.247 100.003 150.029 900.000 233426020041882
JH21-1-24228.69782.940.290.049 930.000 700.204 920.002 450.029 750.000 221921518921891
JX1-1-1229.06427.060.540.053 900.003 930.200 670.014 180.027 150.000 51367126186121733
JX1-1-2335.49720.980.470.049 130.002 000.182 190.007 280.027 050.000 291547417061722
JX1-1-3514.63816.260.630.052 920.001 850.195 860.006 650.026 980.000 313255618261722
JX1-1-4500.861 265.490.400.052 270.001 870.192 760.006 580.026 830.000 242976117961712
JX1-1-5667.911 622.600.410.052 530.001 660.195 400.006 010.027 010.000 243085418151722
JX1-1-6326.29820.200.400.049 810.002 190.184 110.007 890.026 930.000 301867917271712
JX1-1-7619.22999.050.620.053 340.003 840.197 150.013 970.026 810.000 35343166183121712
JX1-1-8590.00996.260.590.051 230.004 510.188 930.016 340.026 750.000 44251202176141703
JX1-1-9737.971 807.580.410.050 960.001 460.190 490.005 450.027 030.000 222395117751721
JX1-1-101 270.551 589.760.800.051 300.002 590.192 560.009 350.027 260.000 342548917981732
JX1-1-11199.30679.650.290.046 050.003 870.169 290.013 730.026 660.000 58629185159121704
JX1-1-12493.32750.250.660.054 450.002 540.201 510.009 260.026 910.000 323908218681712
JX1-1-13331.02700.600.470.051 900.002 260.190 010.007 950.026 810.000 312817517771712
JX1-1-14750.731 197.740.630.049 730.001 660.182 680.005 690.026 770.000 241825617051702
JX1-1-15766.142 256.010.340.053 770.003 130.202 880.010 390.027 470.000 433618718891753
JX1-1-16114.13352.690.320.051 550.004 450.193 720.016 390.027 390.000 50266159180141743
JX1-1-17382.58966.200.400.049 310.003 470.181 690.012 420.026 720.000 44163160170111703
JX1-1-18757.811 305.840.580.050 700.001 430.189 920.005 650.027 080.000 292274917751722
JX1-1-19951.571 179.220.810.046 050.003 140.170 050.011 300.026 780.000 40344150159101703
JX1-1-201 173.254 413.250.270.050 110.001 630.188 060.006 490.027 120.000 362005517561722

小蜂蜜顶子组流纹岩(JH21-1)中的锆石呈自形—半自形,多显示岩浆成因的振荡生长环带(图 4a),结合较高的Th/U比值0.29~0.68(表 1),表明了它们的岩浆成因[28]。20个测点的206Pb/238U年龄值主要介于(186±2)~(196±1) Ma之间,加权平均年龄为(192±2)Ma(n=20)(图 5a)。此外,3颗独立锆石分别给出了(214±2)Ma、(223±2)Ma和(225±2)Ma的谐和年龄,代表了捕获锆石的年龄(表 1)。

小蜂蜜顶子组安山岩(JX1-1)锆石CL图像显示,锆石呈自形—半自形,条痕状吸收特征,(图 4b),Th/U比值为0.27~0.81(表 1),暗示岩浆成因[28]。20个测点的206Pb/238U年龄值介于(170±2)~(175±3)Ma之间,加权平均年龄为(172±1)Ma(n=20)(图 5b)。

3.2 锆石Hf 同位素

在锆石U-Pb年代学测定的基础上,选取了部分测点进行了Hf同位素分析(图 4)。流纹岩(JH21-1)样本共计分析了15个点,其中12颗代表形成时代192 Ma的锆石的176Hf/177Hf的比值为0.282837~0.282 912,各点的数据比较均一,以形成年龄192 Ma进行计算,其εHf(t)值为+6.35~+9.01,同时给出了Hf同位素模式年龄,其TDM1=486~591 Ma,TDM2=850~1 091 Ma(表 2)。另外3颗捕获锆石年龄分别(214 ±2)、(223 ±2)和(225±2)Ma,其176Hf/177Hf比值为0.282 854~0.282 913,其εHf(t)值介于+7.45~+9.67。

表 2 吉林中部小蜂蜜顶子组火山岩LuHf 同位素组成 Table 2 LuHf isotopic data for the Xiaofengmidingzi Formation volcanic rocks in central Jilin Province
岩性测点号t/Ma176Yb/177Hf176Lu/177Hf176Hf/177Hf 2σmεHf(0)2σεHf(t) TDM1(Hf)/MaT DM2(Hf)/MafLu/Hf
JH21-1 012240.034 0610.001 1850.282 8540.000 0222.97.630.8567996-0.96
JH21-1 021920.035 8250.001 4280.282 8590.000 0403.17.111.45641 022-0.96
JH21-1 031920.033 5010.001 2120.282 8700.000 0253.57.520.9545985-0.96
JH21-1 041920.040 6490.001 4080.282 8670.000 0313.47.411.1551995-0.96
JH21-1 051920.039 0420.001 3530.282 9120.000 0285.09.011.0486850-0.96
JH21-1 061920.043 4160.001 5160.282 8550.000 0222.96.970.85701 035-0.95
JH21-1 071920.043 8200.001 4530.282 8520.000 0242.86.850.85741 045-0.96
JH21-1 082240.046 8760.001 6190.282 9130.000 0225.09.670.8488812-0.95
JH21-1 092140.028 7910.001 0380.282 8540.000 0242.97.450.85651 006-0.97
JH21-1 101920.027 3600.000 9160.282 8990.000 0264.58.600.9499887-0.97
JH21-1 111920.034 1650.001 2160.282 8770.000 0283.77.781.0534961-0.96
JH21-1 121920.033 4480.001 1660.282 8370.000 0232.36.350.85911 091-0.96
JH21-1 131920.039 0080.001 4290.282 9000.000 0254.58.570.9504890-0.96
JH21-1 141920.029 6450.000 9900.282 8730.000 0213.67.650.8537973-0.97
JH21-1 151920.033 1880.001 1840.282 8920.000 0244.28.300.8513914-0.96
JX1-1 011720.045 8460.001 7720.282 9360.000 0235.89.370.8457804-0.95
JX1-1 021720.034 0810.001 2730.282 9000.000 0244.58.150.8503914-0.96
JX1-1 031720.036 4870.001 3350.282 8930.000 0254.37.910.9513936-0.96
JX1-1 041720.028 6960.001 1350.282 8990.000 0224.58.140.8502915-0.97
JX1-1 051720.039 4670.001 5010.282 8190.000 0241.65.250.96231 177-0.95
JX1-1 061720.045 8840.001 7590.282 9210.000 0235.38.850.8479851-0.95
JX1-1 071720.044 0110.001 6940.282 9130.000 0255.08.560.9490877-0.95
JX1-1 081720.037 1850.001 4840.282 9350.000 0405.89.371.4455803-0.96
JX1-1 091720.023 4360.000 8870.282 9040.000 0254.78.330.9492898-0.97
JX1-1 101720.051 2060.001 9190.282 8960.000 0254.47.940.9517933-0.94
JX1-1 111720.032 5570.001 2270.282 8590.000 0253.16.700.95611 046-0.96
JX1-1 121720.041 0860.001 5610.282 8450.000 0242.66.170.85861 093-0.95

安山岩(JX1-1)样本共计分析了12个代表其形成年龄的锆石(172 Ma),其176Hf/177Hf比值为0.282 819~0.282 936,各点的数据比较均一,εHf(t)值为+5.25~+9.37,同时给出了Hf同位素模式年龄,其TDM1=455~623 Ma,TDM2=803~1 177 Ma(表 2)。小蜂蜜顶子组火山岩的锆石εHf(t)值位于球粒陨石演化线和亏损地幔演化线之间,与兴蒙造山带东段显生宙火成岩的Hf同位素组成相似[29](图 6)。

阴影范围据文献[28] 图 6 吉林中部地区小蜂蜜顶子组火山岩Hf同位素特征 Figure 6 Zircon Hf isotopic compositions for the Xiaofengmidingzi Formation volcanic rocks in central of Jilin Province
4 小蜂蜜顶子组火山岩的形成时代及地质意义 4.1 形成时代

小蜂蜜顶子组是由吉林省地质矿产局在测绘1:20万长春幅地质图时所建立,主要由一系列中—酸性火山岩和火山碎屑岩所组成,同时基于野外接触关系以及全岩同位素年龄将其时代置于早侏罗世(J1x)[16]。然而,在1:20万地质调查报告吉林市幅中,前人依据地层对比则将其置于晚三叠世(T3x)[17]。而王光奇等则认为小蜂蜜顶子组与舒兰图幅中所谓的马达屯组为同一地层单元[30],结合马达屯组被晚三叠世二长花岗岩(年龄219.2 Ma[31])侵入等野外关系,将该组置于晚三叠世[31]。此外,武要红等在通过地层和古生物研究,结合前人年代学资料,将小蜂蜜顶子组归于中—晚侏罗世[32]

值得注意的是,吉中地区恰巧处于多个构造体系叠加改造的地区,受多期构造--岩浆热事件的改造[33-36]。此外,随着东北地区基础地质研究的不断深入以及同位素地质年代学测试技术的不断发展,传统上采用的区域地层对比方法和全岩同位素测年方法对于地层时代的限定缺乏准确性[16-18, 31-32, 37-38]

本次研究中,对出露于双阳地区和吉林大苇子沟地区的小蜂蜜顶子组火山岩分别进行了锆石LA-ICP-MS U-Pb定年。定年结果显示,出露于双阳地区小蜂蜜顶子组典型剖面中火山岩(JH21-1)的锆石具有典型的岩浆成因[28] ,给出的206Pb/238U加权平均年龄为(192±2) Ma,代表其形成时代为早侏罗世;而出露于吉林大苇子沟地区小蜂蜜顶子组中火山岩(JX1-1)的锆石同样具有明显的岩浆成因[28],其206Pb/238U加权平均年龄为(172±1)Ma,为中侏罗世。二者相差20 Ma,暗示二者并非同一套火山岩。此外,后者主要是一套安山岩和中酸性火山碎屑岩,并未见有玄武岩[17],与吉林中部地区桦甸市—永吉县一带广泛发育的南楼山组地理位置接近。南楼山组也是一套中酸性岩石,主要为安山岩、英安质角砾凝灰岩、安山质凝灰岩和流纹岩,并未见有玄武岩[19]。孙罡等在南楼山组建组剖面北大湖二道沟和南大岭所采集的流纹岩和安山岩进行U-Pb测年结果分别为(174.0±3.4)和(179.4±1.1)Ma,所以小蜂蜜顶子组样品JX1-1((172±1) Ma)无论时代还是岩石类型,与南楼山组具有可比性[39-40],建议将该区的小蜂蜜顶子组划归为南楼山组,时代修定为中侏罗世。

上述研究结果表明,吉林中部地区侏罗纪岩浆事件至少可以划分为两期,即早侏罗世(192±2)Ma和中侏罗世(172±1)Ma。这两期火山岩在吉黑东部和小兴安岭—张广才岭均有分布,出露于吉黑东部绥芬河—延边地区早—中侏罗世火山岩包括绥阳镇双桥子组英安岩(184±2)Ma、老黑山罗圈站组玄武安山岩(180±1)Ma,汪清县天桥岭组安山岩(175±3)Ma和满和组安山岩(187±6)Ma,和龙县刺猬沟屯田营组安山岩(176±3)Ma[41-43]。小兴安岭—张广才岭的哈尔滨—伊春地区的早—中侏罗世火山岩则主要包括五道岭组流纹岩,其形成时代为(175±1)Ma,太安屯组的流纹岩、英安岩和宁远村组流纹岩形成时代分别为(187±2)、(190±1)和(190±1)Ma[41, 43]。此外,出露于小兴安岭孙吴县风水沟河群黑云母二长岩((175±1)Ma)、天龙山幸福林场黑云母二长花岗岩体((176±1)Ma)和黑云母正长花岗岩((183±2),(185±2)Ma)[44],与出露于伊春市—五常县地区的早侏罗世(182~186 Ma)的基性侵入岩[45]共同构成了双峰式的岩石组合。

4.2 地质意义

无论是吉黑东部还是小兴安岭地区的火成岩都与本文中所介绍的小蜂蜜顶子组火山岩具有类似的Hf同位素组成[43],即小蜂蜜顶子组两期火山岩中结晶锆石的εHf(t)值全部>0,Hf 同位素二阶段模式年龄值集中在新元古代,少部分为中元古代末期,暗示它们均起源于中元古代—新元古代增生陆壳物质的部分熔融,与兴蒙造山带Hf同位素特征一致[29, 43, 46]

小蜂蜜顶子组标准剖面中火山岩组合为安山岩流纹岩组合[16],和吉黑东部出露的一系列钙碱性火山岩与活动大陆边缘火山岩的岩石组合相似[47]。从吉黑东部向小兴安岭—张广才岭地区,SiO2和K2O含量具有增高的趋势,显示了成分的极性变化特征[41-44] ,表示陆壳的成熟度逐渐增高,揭示了早—中侏罗世古太平洋板块向欧亚板块俯冲作用的开始。而出露小兴安岭孙吴县地区的酸性岩与伊春市—五常县地区早侏罗世基性岩,二者构成典型的双峰式火成岩组合[44-45],揭示了早侏罗世小兴安岭中部地区处于一种类似弧后伸展的构造环境。所以,小蜂蜜顶子组火山岩的形成应与古太平洋板块俯冲的开始相联系。

5 结论

1) 吉林中部长春双阳小蜂蜜顶子组标准剖面中流纹岩形成时代为早侏罗世,而出露于吉林大苇子沟地区小蜂蜜顶子组的安山岩则形成于中侏罗世,后者与区域上的南楼山组相当。吉林中部侏罗纪岩浆事件至少可以划分为早侏罗世(192 Ma)和中侏罗世(172 Ma) 两期。

2) 吉林中部早—中侏罗世火山岩的原始岩浆起源于中—新元古代新增生陆壳物质的部分熔融。

3) 结合区域构造演化历史,小蜂蜜顶子组火山岩的形成应与古太平洋板块俯冲于欧亚大陆之下的事件相联系。

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http://dx.doi.org/10.13278/j.cnki.jjuese.201605112
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

张海洪, 许文良, 王枫, 曹花花
Zhang Haihong, Xu Wenliang, Wang Feng, Cao Huahua
吉林中部小蜂蜜顶子组火山岩的形成时代及其地质意义:锆石U-Pb年代学和Hf同位素组成证据
Formation Timing of the Volcanic Rocks from the Xiaofengmidingzi Formation in Central Jilin Province and Its Geological Implications: Evidence from Zircon U-Pb Dating and Hf Isotope Compositions
吉林大学学报(地球科学版), 2016, 46(5): 1418-1429
Journal of Jilin University(Earth Science Edition), 2016, 46(5): 1418-1429.
http://dx.doi.org/10.13278/j.cnki.jjuese.201605112

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收稿日期: 2015-12-07

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