地球物理学报  2015, Vol. 58 Issue (4): 1269-1283   PDF    
黑龙江省饶河枕状玄武岩古地磁学研究及其构造意义
任收麦1, 朱日祥2, 邱海峻1, 周建波3, 邓成龙2    
1. 中国地质调查局油气资源调查中心, 北京 100029;
2. 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, 北京 100029;
3. 吉林大学地球科学学院, 长春 130061
摘要:蛇绿岩中枕状玄武岩的古地磁学研究可为古海洋的恢复与演化提供定量化依据.黑龙江省饶河地区中侏罗世枕状玄武岩的岩石学、岩石磁学研究表明,该岩石具备水下喷出特点,发育辉长结构,载磁矿物为磁铁矿.17个采点181块样品的热退磁实验表明,中侏罗世枕状玄武岩记录了高温分量和中温分量,前者为熔岩喷发记录的原生剩磁方向,平均方向D/I=59.4°/46.3°,α95=6.8°,对应的极位置为40.3°N,224.6°E,A95=7°;后者可能为晚侏罗世—早白垩世岩浆热事件的叠加,平均方向D/I=55.4°/60.6°,α95=3.9°,对应的极位置为50.8°N,210.6°E,A95=5.2°.综合考虑区域地质背景,将这一结果与邻区同时代的古地磁数据对比,推测在中侏罗世之前,在饶河杂岩与佳木斯地体之间存在一定规模的海域,与现今日本海相似;早白垩世时期,该海域封闭,饶河杂岩与华北、西伯利亚板块在动力学上已成为整体.
关键词饶河杂岩     中侏罗世     玄武岩     古地磁学    
Paleomagnetic study on Middle Jurassic lavas of Heilongjiang Province, NE China and its tectonic implications
REN Shou-Mai1, ZHU Ri-Xiang2, QIU Hai-Jun1, ZHOU Jian-Bo3, DENG Cheng-Long2    
1. Oil & Gas Survey, China Geology Survey, Beijing 100029, China;
2. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
3. The College School of Earth Sciences, Jilin University, Chang Chun 130061, China
Abstract: NE China which experienced multi-stages tectonic evolution, is a key area for research into East Asian tectonic evolution. Normally, paleomagnetism data come from rock samples of the interior of a plate while the size of ocean located in both blocks cannot be identified. Here paleomagnetic data from basaltic pillows in Raohe, NE China were used to study the recovery and evolution of paleo-oceans.181 samples of 17 sites were collected using a portable electric-power drill, and were oriented by both magnetic and sun compasses. The methods, including stepwise thermal demagnetization, rock magnetic and petrologic analyses were used to analyze the paleo-location of the Middle Jurassic basaltic pillows of Raohe, Heilongjiang Province, NE China. Rock magnetic experiments include temperature-dependence of saturated magnetization and hysteresis loops measured by VFTB Curie balance. Thin section identification was carried out on some pilot samples. Then, all the samples were subjected to stepwise thermal demagnetization using a MMTD-60 demagnetizer, with 12 temperature steps up to 585℃. Magnetic remanence measurements were performed on a 2G cryogenic magnetometer. Additionally, some paleomagnetic data of adjacent region have been collected and compared with the study area.The Middle Jurassic basaltic pillows of Raohe, experienced underwater eruption with gabbro texture and magnetite, which show both characteristic high-temperature remanence component and middle-temperature remanence component, with the mean directions of D/I=59.4°/46.3° with α95=6.8° and D/I=55.4°/60.6° with α95=3.9°, respectively. We interpret the high-temperature remanence component as primary magnetization based mainly upon the petrographic analysis, and the middle-temperature remanence component as secondary re-magnetization during the Late Jurassic to Early Cretaceous magmatism. The corresponding pole are at 40.3°N, 224.6°E with A95=7° and at 50.8°N, 210.6°E with A95=5.2°. The paleomagnetic study on basaltic pillow developed between two blocks offers a useful method for researching tectonic evolution. Considering the regional geologic background and paleomagnetic poles of adjacent blocks, our results suggest that there exists an ocean similar to the Japan sea nowdays between the Raohe complex and Jiamusi block before the Middle Jurassic period. This Early Cretaceous pole is in good agreement with those for Siberia, North China, and Inner Mongolia, suggesting that these continental blocks had already sutured together in the Early Cretaceous accompanied by the ocean closing.
Key words: Raohe Complex     Pillow lavas     Middle Jurassic     Paleomagnetism    
1 引言

我国东北地区位于西伯利亚板块南部、华北板块北部和太平洋板块西部之间,构造部位处于中亚造山带东端.以新林—喜贵图、贺根山—黑河、牡丹江、滨海等主干断裂为界,东北地区可划分为额尔古纳地块、兴安地块、松嫩地块、兴凯—佳木斯—布列亚地块,称为黑龙江板块群或佳—蒙地块(李双林和欧阳自远,1998葛肖虹和马文璞,2007张兴洲等,2008Wang et al., 2009).前中生代时期该区主要受古亚洲洋构造体系控制,中生代以后受太平洋板块活动的强烈改造,构成环太平洋构造带的重要组成部分,包括那丹哈达—西锡霍特阿林地体,以发育中—晚侏罗世到早白垩世增生杂岩、大范围北东向展布的花岗岩和火山岩带及其压扭断层为特征(图 1)(Wu et al.,2002周建波等,2009刘永江等,2010).这一地区是解决东北地区乃至东亚大陆构造演化的关键地区.

邵济安等(1991)通过对那丹哈达地体上古生物、古地磁资料的对比研究注意到地体上不同时代生物生存的古纬度有从低纬度向高纬度变化的趋势,古地磁数据主要来自地体内部的沉积岩系.在那丹哈达地体西部饶河地区出露一条南北走向50 km的蛇绿岩带,主要由超镁铁质堆积岩(纯橄榄岩、异剥辉石橄榄岩、单斜辉石岩)、堆晶构造发育的层状辉长石、枕状玄武岩组成(邵济安等,1991).众所周知,辉长岩、枕状熔岩和硅质岩被认为是深海远洋环境下形成的,常出露于造山带中,并成为蛇绿岩套的组成部分.作为海洋板块地层层序的一部分,记录了其形成的古地理环境,为古洋壳的残片.尤其是枕状玄武岩携带较为丰富的载磁矿物,具有较强的剩磁,可为研究板块漂移或洋壳消亡的演化过程提供定量化依据(吴汉宁等,1992).如菲律宾海东北缘博宁群岛始新世的熔岩流古地磁研究为西菲律宾盆地的构造演化提供了证据(Kodama et al.,1983),西藏日喀则蛇绿岩的古地磁学研究用于探讨雅鲁藏布江缝合带的闭合方式(Pozzi et al.,1984),钻井获得的印尼爪哇岛高原早白垩世枕状玄武岩构造磁学研究再现了太平洋板块运动轨迹(Riisager et al.,2003).因此,对其进行古地磁学研究,可为恢复古太平洋的演化提供详细的证据.

2 研究区地质概况和样品采集

研究区位于黑龙江省东部饶河地区,东临俄罗斯.区内地层自老到新发育中侏罗世超镁铁质堆积杂岩、镁铁质堆积杂岩和枕状熔岩,侏罗纪黑云母花岗闪长岩和花岗岩、白垩纪花岗岩,还有前人通过牙形刺、放射虫等微体古生物研究确定的中—上三叠统十八响地组、上三叠统大坝北山组、上三叠统大佳河组、上三叠统—下侏罗统大岭桥组,前三组地层基本上是硅质岩为主,夹少量粉砂岩、板岩,大岭桥组是一套浊积特征明显的地层,以砂岩、粉砂岩为主,夹有灰岩、蛇绿岩等岩块.上述地层是在锡霍特阿林外来地块和亚洲大陆东缘之间的一个扩张小洋盆地内形成的.下侏罗统永福桥组、下白垩统皮克山组为一套硬砂岩、粉砂质砂岩、页岩、泥岩(图 1b).

图 1 中国东北及俄罗斯远东地区大地构造单元划分(a)(据Zhou et al.,2011)和黑龙江省饶河地区地质图及采样位置(b)
F1:西拉木伦—长春断裂;F2:延吉断裂;F3:牡丹江断裂;F2+F3:延吉—黑龙江断裂;F4:贺根山—黑河断裂;F5:新林—喜贵图断裂;F6:依兰—伊通断裂;F7:敦化—密山断裂;F8:滨海断裂.
Fig. 1 Tectonic divisions of NE China and Far East Russia(a)(after Zhou et al., 2011),geological map of the Raohe area,Heilongjiang Province and sampling locations(b)
F1:Xar Moron-Changchun zone; F2:Yanji Fault; F3:Mudanjiang Fault; F2+F3:Yanji-Heilongjiang Fault; F4:Hegenshan-Heihe Fault; F5:Xinlin-Xiguitu Fault; F6:Yilan-Yitong Fault; F7:Dunhua-Mishan Fault; F8:Primoria Fault.

古地磁样品采于黑龙江省东部饶河市西侧约20 km处(46.798°N,133.753°E)(图 1b),岩性为紫红色玄武岩,发育枕状结构,岩枕的长轴长度介于0.3~10 m 之间,岩枕的顶部均以气孔和氧化顶为特征,表明后期未发生倒转,野外可观察到岩枕成层排列(图 2a图 2b),代表了枕状玄武岩的流动构造,产状55°/15°(图 2c).利用便携式岩石取样电钻,在两条剖面的较大岩枕上共布置古地磁采样点17个,在每个采样点至少采集8个独立定向岩芯,利用磁罗盘和太阳罗盘定向.野外共采集181个定向岩芯,在室内切制成长度为2 cm的样品181块用于岩石学、岩石磁学和古地磁学研究.

图 2 枕状玄武岩野外照片
(a)枕状玄武岩原地形貌;(b)枕状玄武岩风化边;(c)枕状玄武岩地层产状.
Fig. 2 Pillow basalts of Middle Jurassic lava in situ
(a)Morphology in situ of pillow basalts;(b)Weathering edges of pillow basalts;(c)Formation occurrence of pillow basalts.
3 岩石学、岩石磁学和古地磁结果 3.1 岩石学研究

为了解载磁矿物的成分和岩石结构,选择典型样品开展了EBSD测试分析.实验在中国地质科学院地质研究所大陆构造与动力学国家重点实验室进行,扫描电子显微镜为日本JEOL公司生产的JSM-5610LV,能谱仪为英国OXFORD公司生产的Inca Energy,EBSD为英国OXFORD公司生产的Inca Crystal.野外露头观察,样品具有典型的水下喷发玄武岩特点.显微镜下观察,样品发育辉长结构,样品中透明矿物主要为斜长石和斜方辉石.且含有少量不透明矿物,正交偏光镜下为黑色,反射光镜下为高亮度,暗色矿物晶型发育规则(图 3a和b),主要为磁铁矿,其晶型保持完好,镜下显示为正方形或条形,呈板状和柱状的钛铁矿及布纹状的钛铁晶石镶嵌于磁铁矿(图 3c和d),化合物百分比中Feo和TiO2的比值可达1 ∶ 1(表 1).

图 3 显微镜下典型样品结构图
其中Pl—斜长石;Opx—斜方辉石;Mt—含钛磁铁矿.
Fig. 3 Structural photos of typical samples under microscope
Pl-plagioclase;Opx-orthopyroxene;Mt-Titaniferous magnetite.

表 1 典型样品的能谱成分数据表Table 1 EBSD analysis data of typical samples
3.2 岩石磁学

为进一步了解枕状熔岩中所含磁性矿物的种类和粒度等特征,挑选部分样品进行了等温剩磁、磁化强度随温度变化曲线、磁滞回线等岩石磁学研究.利用中国科学院地质与地球物理研究所古地磁与年代学实验室的磁天平(MMVFTB)测定的等温剩磁曲线表明,典型样品在200 mT场强下呈饱和状态,剩磁矫顽力位于78~93 mT之间(图 4a);磁滞回线结果显示,样品在 400 mT左右均形成闭合的磁滞回线(图 4b图 4c).样品的J-T 曲线显示,在升温过程 中,磁化强度先缓慢降低至300 ℃左右,300~400 ℃ 之间磁化强度快速降低,然后又缓慢降低至550 ℃左右,在550~600 ℃之间磁化强度呈现迅速降低,至 650 ℃左右磁化强度接近零(图 4d和e).其中,300~400 ℃ 之间磁化强度快速降低,为热不稳定的、强磁性的磁赤铁矿受热分解为热稳定性好、弱磁性的赤铁矿造成(Deng et al.,2001);而居里温度(Tc)为600 ℃左右,稍高于磁铁矿的居里温度(580 ℃),这是被低温氧化的粗颗粒磁铁矿的特征.降温曲线明显位于升温曲线,降温至室温后磁化强度不到初始磁化强度的一半,这部分损失的磁化强度为样品中磁赤铁矿受热分解造成.因此,样品的主要载磁矿物应为磁铁矿,其较高的矫顽力为磁铁矿颗粒受低温氧化造成.另外,我们注意到,降温曲线显示较低的Tc,接近580 ℃,这是由于低温氧化的磁铁矿颗粒 的外壳中的磁赤铁矿受热分解(Dunlop and Özdemir,1997),从而显示了其磁铁矿内核的特征.

图 4 典型样品的等温剩磁获得曲线(a)、磁滞回线(b,c)、J-T曲线(d,e)Fig. 4 Behaviors of acquisition of isothermal remanent magnetization(IRM)(a),hysteresis loop(b,c), and J-T(d,e)curves for pilot samples from the Raohe areas of Heilongjiang province
3.3 古地磁结果

样品的热退磁试验是使用英国Magnetic Measurements公司生产的MMTD60型全自动热退磁仪,剩磁测量是用美国2G-755R型超导岩石磁力仪完成的,全部实验在零磁空间中进行.对所有样品进行逐步热退磁(热退磁温度从室温至590 ℃共12步,退磁间隔在25~50 ℃之间)表明,当退磁温度在100 ℃~250 ℃时可分离出稳定的一组剩磁组分,部分样品在250~400 ℃之间可获得第二组剩磁组分,大部分样品300~590 ℃之间可分离出第三组剩磁组分,且第二组剩磁方向和第三组剩磁组分有部分重合现象,在退磁投影图上趋于原点,第三组剩磁组分可能代表了火山岩喷发时的原生剩磁方向,第二组剩磁组分代表后期部分样品后期重磁化不够彻底的记录(图 5).

图 5 代表样品倾斜校正前的系统热退磁结果的正交矢量
实心圆和空心圆分别代表剩磁方向在水平面和铅垂面上的投影.
Fig. 5 Orthogonal vector plots of progressive thermal demagnetization for representative specimens in-situ
Solid and open circles indicate vector endpoints projected onto the horizontal and vertical planes,respectively.

对于火山岩而言,还必须检验是否消除了地球磁场长期变化的影响(Ren et al.,2004). 根据McFadden等人的分析结果(McFadden et al.,1991),采样点的纬度为46.787°时,所记录地球长期变化的VGP散角S落在(15.82°,17.64°)时,在95%置信度下,所得古地磁数据可能反映该地区地球磁场古长期变化;反之,这些结果则可能受到了平滑作用或后期改造等的影响.饶河中侏罗世枕状熔岩特征剩磁对应的S为16.45°,介于15.82和17.64之间,说明由该剖面17个采样点得出的中侏罗世古地磁数据已消除了地球磁场长期变化的影响,可以用于计算古地磁极位置.另外,根据Deenen M H L等人对地磁场古长期变化的研究,如果一个熔岩流古地磁测试中N小于15,A95介于4~5°,K大于90,则表明该熔岩流记录了现场可读的古地磁场,反之亦然(Deenen et al.,2011),研究区仅发育一期中侏罗世枕状熔岩,N为17,大于15,A95为7.0,k值为15.3,满足k介于12.5~25之间中纬度区可与K对比,其值小于90,表明该熔岩流记录了中侏罗世当时的古地磁方向.

根据主向量分析求得每个样品的特征剩磁分量后(图 5),以采样点为单位对特征剩磁分量进行Fisher统计平均,然后再对各采样点的平均方向进行统计平均,求得整个剖面的平均剩磁方向D/I=59.4°/46.3°,α95=6.8(表 2图 6c),对应的古地磁极位置为40.3°N,224.6°E,A95= 7°,古纬度为27.6°N.32个样品大部分在250~400 ℃之间可获得中温分量,平均方向为:D/I=55.4°/60.6°,α95=3.9(表 3图 6b),介于近代地磁场(D=357.7°,I=60°)与该剖面平均特征剩磁方向之间,很可能是后期构造热事件叠加的结果,对应的古地磁极位置为50.8°N,210.6°E,A95=5.2°,古纬度为41.6°N.在12个采点中可分离出低温分量,8个采点的统计的平均方向为:D=6.8°,I=63.3°,α95=6.7°(表 4图 6c),与近代地磁场的方向在95%置信度内较为一致.理论上,随着钛含量的增加磁铁矿的解阻温度逐渐降低(Dunlop and Özdemir,1997),这也是大部分样品中解阻温度介于550~590 ℃之间的原因,但个别样品达到610 ℃,其原因仍需进一步分析研究.

表 2 黑龙江省饶河地区中侏罗世枕状熔岩高温分量数据表Table 2 High temperature component(HTC)of Middle Jurassic lava in the Raohe area of Heilongjiang province

图 6 黑龙江饶河地区中侏罗统枕状玄武岩的特征剩磁方向及采点平均方向的95%置信椭圆的等面积投影
(a)低温特征分量地理坐标;(b)低温特征分量层面坐标;(c)中温特征分量地理坐标;(d)中温特征分量层面坐标;(e)高温特征分量地理坐标;(f)高温特征分量层面坐标;红圈为平均方向.
Fig. 6 Equal-area projections of directions of characteristic remanences isolated from Middle Jurassic rocks from the Raohe areas of Heilongjiang province
(a) and (b)Low-temperature component before and after tilt-correction;(c) and (d)Middle-temperature component before and after tilt-corrcetion;(e) and (f)High-temperature component before and after tilt-correction. Red circles represent the average direction.

表 3 黑龙江省饶河地区中侏罗世枕状熔岩中温分量数据表Table 3 Medium temperature component(MTC)of Middle Jurassic lava in the Raohe area of Heilongjiang province

表 4 黑龙江省饶河地区中侏罗世枕状熔岩低温分量数据表Table 4 Low temperature component(LTC)of Middle Jurassic lava in the Raohe area of Heilongjiang province
4 讨论 4.1 饶河杂岩运动轨迹

综合黑龙江饶河地区及邻区中侏罗世古地磁极数据(表 5图 7a),分析表明内蒙古、华北在中侏罗世在95%置信度已经拼贴为一体,黑龙江饶河地区和西伯利亚地区则与上述地块有一定的距离,这与区域地质事实相符(Zhao et al.,1994),对应的古地 磁极获得的古纬度为27.6°N,与现今纬度46.8°N相差19.2°,估算距离约2000 km.

表 5 黑龙江饶河地区及其相邻块体晚中生代古地磁数据表Table 5 Late Mesozoic paleomagnetism paleopoles of the Raohe area of Heilongjiang province and adjacent blocks

图 7 黑龙江饶河及邻区中侏罗世(a)和晚侏罗世—早白垩世(b)古地磁极的等面积投影图(北半球面投影)Fig. 7 Equal-area projection of the Middle Jurassic(a) and the Late Jurassic-Early Cretaceous(b)paleopoles of the Raohe areas of Heilongjiang province and adjacent regions

晚侏罗世—早白垩世古地磁极数据(表 5图 7b),分析表明西伯利亚、华北和黑龙江饶河地区在此时期已经拼贴为一体,纬度上的差别与现今纬度差一致,包括朝鲜半岛、内蒙古、中国辽西、黑龙江佳木斯在内,已经处于同一纬度上(杨惠心等,1998),偏角的不同可能代表了后期断裂活动所反映块体的局部旋转和运移(Zhu et al.,2002Lin et al.,2003王海龙等,2011).鄂霍茨克海的封闭时间(刘永江等,2010)、三江盆地早白垩世海相地层的发育,表明海退的时间和块体的最后拼合时间为晚侏罗世—早白垩世时期.另一方面,也说明饶河地区中侏罗世枕状熔岩后期受早白垩世岩浆热作用的影响,发生重磁化,记录了当时的古地磁方向.对应的古地磁极获得的古纬度为41.6°N,与现今纬度46.8°N相差5.2°,估算距离约500 km.

4.2 古环境的恢复

饶河杂岩的主体为镁铁质杂岩和深海沉积成因的硅质岩.杂岩中枕状玄武岩年龄为 169.19±6.48 Ma(Rb-Sr法)(赵海玲等,1996),锆石U-Pb法获得的年龄为167 Ma(Zhou et al.,2014),辉长岩锆石U-Pb年龄所获得206Pb/238U加权平均年龄为166±1 Ma(程瑞玉等,2006),硅质岩中最晚出现的放射虫时代约为165 Ma(Bathonian-Callovian阶)(水谷伸治郎等,1989张勤运,1990邵济安等,1991邵济安和唐克东,1995丁秋红等,1997张庆龙等,1997),其陆缘碎屑成因浊积岩的时代约为160—150 Ma(Oxfordian-Kimmeridgian阶).根据饶河花岗岩乃至中国东部广泛分布的早白垩世火山岩年龄的时代制约(~131 Ma)(Wu et al.,2005),推测饶河杂岩的就位时代介于150—131 Ma之间(程瑞玉等,2006).

考虑黑龙江饶河地区中侏罗世和晚侏罗世—早白垩世的漂移轨迹,该地区中侏罗世时期北向漂移2000 km,晚侏罗世—早白垩世在95%置信水平内漂移量可忽略不计,这与华北板块、西伯利亚板块此时期的漂移趋势一致(任收麦和黄宝春,2002).蛇绿岩作为确定古板块边界的重要证据,形成构造环境大致可以分为洋中脊(MOR)和俯冲带上(SSZ)的弧前和弧后盆地(史仁灯,2005).如果是洋中脊成因的,则海洋的范围为蛇绿岩带和大陆之间距离的两倍;如果是俯冲带成因的,则海洋的范围为蛇绿岩和大陆之间的距离.饶河枕状熔岩地球化学具有TiO2含量偏高,高场强元素Nb,Ta,Zr,Hf等富集,大离子亲石元素Rb,Sr,Ba等相对富集,LREE富集,体现出洋岛型玄武岩的特征(田东江等,2006),推测当时所处的地理环境应为洋壳俯冲带.

4.3 区域大地构造环境恢复及其意义

研究表明,蒙古—鄂霍茨克海从西向东呈剪刀状闭合,石炭纪—二叠纪在蒙古中部发生闭合,三叠纪—早侏罗世在蒙古东北部发生闭合(Zorin,1999),晚侏罗世蒙古—鄂霍次克海在俄罗斯远东地区彻底消亡,华北板块—佳蒙地块和西伯利亚板块最终碰撞拼合(Ying et al.,2010).漠河盆地侏罗系地表露头变形反映晚中生代蒙古—鄂霍次克海经历了三期变形:一是中晚侏罗世的向南推覆运动,二是晚侏罗世的向东推覆和平行造山带的走滑活动,三是早白垩世的东南向推覆运动(Li et al.,1999).黑龙江高压变质带作为一个很重要的转换构造单元可能代表了晚三叠世—早侏罗世蒙古—鄂霍茨克海南北闭合以及太平洋板块俯冲造成的西向增生的开始(Zhou and Wilde, 2013).松辽盆地在中—晚侏罗世伴随着依泽纳奇板块NNW斜向俯冲,蒙古—鄂霍次克板块消亡,西伯利亚板块向南推挤作用下发育挤压逆冲、地壳增厚、岩浆活动和大规模左旋走滑,处于前裂谷期,而在侏罗纪末—白垩纪初,受岩石圈底部拆沉,岩浆底侵的影响,热穹窿式双向拉伸,大面积火山活动,处于断陷期(葛荣峰等,2010).另外,三江盆地早—中侏罗世发育海相地层,且以北北东向地堑-半地堑伸展环境体现(Zhang et al.,2012),到早白垩世时期,向东海相沉积加厚,向西逐渐减薄过渡为陆相沉积(Sha et al.,2003).上述现象与饶河枕状玄武岩所记录的研究区中侏罗世由南向北运移动2000 km,对应西伯利亚板块的南向推覆,鄂霍次克海彻底封闭,主体表现为华北板块—佳蒙地块的北向漂移;晚侏罗世—早白垩世由南向北的运动不明显,暗示此时期西伯利亚板块持续向南运动,南北应力增强,伴随太平洋板块的活动,造成松辽盆地构造反转启动(图 8).因此,饶河枕状玄武岩古地磁学研究认识对丰富我国东北地体的形成和演化证据具有一定的意义.

图 8 蒙古—鄂霍茨克海拼合演化历史(据Zoria,1999改编)Fig. 8 Paleogeodynamic reconstructions of the Mongolia-Okhotsk ocean from Early Jurassic to Early Cretaceous(modified after Zoria,1999)
5 结论

(1)黑龙江省东部饶河中侏罗世枕状玄武岩携带了熔岩流喷发冷凝时期的原生剩磁方向以及后期晚侏罗世—早白垩世岩浆热事件的叠加记录方向.饶河地区枕状玄武岩的古地磁学研究为板块构造演化提供了定量化的依据.

(2)中侏罗世,饶河杂岩与华北、西伯利亚板块均有一定的距离,北向漂移了约2000 km;晚侏罗世—早白垩世,饶河杂岩与华北、西伯利亚板块在95%置信水平内动力学上已经成为整体.

(3)饶河枕状玄武岩发育的古地理环境为洋壳俯冲带,其北向漂移的距离代表了洋盆的大小.据此推测,中侏罗世之前,在饶河和佳木斯地体之间存在一类似现今日本海的水域.这一认识对重新研究东北地体的形成和演化具有一定的意义.

致谢 感谢审稿专家对本文所提的修改意见和有益讨论,感谢吴裕根、杨会东、王欣在野外样品采集工作中的帮助,感谢秦华峰博士在数据处理方面给予支持.

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