扬子地块四川江油地区上泥盆统—下石炭统古地磁研究
摘要
四川北部江油－北川地区上泥盆统灰岩的古地磁学实验研究表明在逐步热退磁过程中，大多数标本的剩磁强度先增大，后减小；通常当加热温度达到500℃时，剩磁强度衰减至仪器的躁声水平。大多数标本的剩磁具有明显的双分量特征，低温分量（100～300℃）在地理坐标系下与现代地磁场基本一致；中温分量（300～480℃）在地层坐标系中具有高负倾角的特征；部分标本具有三分量特征，从62块灰黑色生物泥晶灰岩和浅灰色砂屑灰岩中分离出的高温分量（400～520℃）在95%置信度下可通过倒转检验和99%褶皱检验，应该代表岩石形成时的原生剩磁，对应的古地磁极位置为46.50N，188.30E（dp=6.30，dm=10.10），位于已有的扬子地块极移曲线内，相应的古纬度为26.10，表明晚泥盆世扬子地块位于北半球中纬度地区。
本区下石炭统灰岩标本的古地磁学实验研究表明标本剩磁具有明显的双分量特征，低温（100～300℃）分量在地理坐标系下与现代地磁场方向基本一致，且不能通过褶皱检验，应是现代地磁场的重磁化，中温（300～480℃）反极性分量在地层坐标系中具有高负倾角的特征，与晚泥盆世中温反极性分量系同一磁成分，其形成可能与早侏罗世岩层的褶皱弯曲有关。
岩石磁学研究表明：上泥盆统三分量样品的主要载磁矿物为磁铁矿，且以假单畴、多畴和少量单畴状态存在，下石炭统灰岩样品的载磁矿物则主要是假单畴和多畴磁铁矿的混合物。从样品中分离出的中温分量由假单畴、多畴磁铁矿携带，上泥盆统三分量样品的高温分量则由记录了岩石原生剩磁的单畴磁铁矿携带。
测量并分离出岩芯中的沉积剩磁（DRM），依据其在层面坐标系中的平均方向与岩石形成时的地磁场方向一致的原则，参照已发表的块体古地磁极移曲线（APWP），可以计算赋存岩芯的隐伏地层产状。对于那些当初未考虑定向问题，而现在又迫切需要重定向的钻井取芯来说，本文提出的算法十分有用。
We have obtained new late Devonain paleomagnetic pole for Yangze Block. A total of 472 paleomagnetic samples were collected from the upper Devonain carbonates from the Jiangyou section in Sichuan province.During thermal cleaning ,intensity of most samples increased progressively(below 300℃) and decayed rapidly (beyond 300℃) till to the noise level of magnetometer at about 500℃.Two magnetic components can be separated from most of samples.The low temperature component(100～300℃) is consistent with the present geomagnetic field. The middle temperature component(300～480℃) is characterized by deep reversed inclination(D=210.20,
I=-80.40,κs=27.5,α95=7.40) The high temperature(420℃～520℃) component(D=49.60,I=44.40,
κs=33.3,α95=8.00)isolated from 62 samples passes fold test and reverse test,yielding a pole position(46.50N,188.30E,dp=6.30,dm=10.10)different from the other poles for Yangze Block.It is therefore suggested that the high temperature was probaly primary magnetization and the Yangze Block was situated at middle latitudes in the northern Hemisphere in the late Devonian.
The paleomagnetic investigation has been carried out on the Lower carboniferous limestone in the hope of identifying the early carboniferous paleomagnetic pole. In general, two magnetic components can be isolated after the removal of a soft viscous overprint by temperature step 100℃ from all samples studied.The low temperature (100~300℃) magnetic component is close to the present day Earth field and fails fold test , which corresponds to a recent overprint. The reversed middle temperature (300~480℃) component (Ds=216.10,Is=-75.50, κs=31.8, α95=4.70) passes fold test and yields a pole position(309.90E,51.20S, dp=7.9,dm=8.6, paleolatitude:62.7),which is completely consistent with MTC revealed from the Upper Devonian limestone in the aera.On the basis of paleomagnetic data and geological evidences , we suggest that the reversed middle temperature component is probably acquired at the time of the Early Jurassic folding in the west part of South China Block.
Rock magnetic studies suggest magnetite presenting as pseudo-single、multidomain and little single domain,carrying the middle(300℃～420℃)and high(420℃～530℃) temperature components respectively, is the main magnetic mineral in the upper Devonian limestone.However,the middle temperature(300℃～480℃) is carried by the mixture of pseudo-single and multidomain magnetite which is also the main magnetic carrier in the lower Carboniferous limestone.
Based on the principle that the depositional remanent magnetization(DRM) is identical to the direction of primary geomagnetic field, reading confirmed polar data from paleomagnetic database, the spatial occurence of buried layers can be calculated.
本区下石炭统灰岩标本的古地磁学实验研究表明标本剩磁具有明显的双分量特征，低温（100～300℃）分量在地理坐标系下与现代地磁场方向基本一致，且不能通过褶皱检验，应是现代地磁场的重磁化，中温（300～480℃）反极性分量在地层坐标系中具有高负倾角的特征，与晚泥盆世中温反极性分量系同一磁成分，其形成可能与早侏罗世岩层的褶皱弯曲有关。
岩石磁学研究表明：上泥盆统三分量样品的主要载磁矿物为磁铁矿，且以假单畴、多畴和少量单畴状态存在，下石炭统灰岩样品的载磁矿物则主要是假单畴和多畴磁铁矿的混合物。从样品中分离出的中温分量由假单畴、多畴磁铁矿携带，上泥盆统三分量样品的高温分量则由记录了岩石原生剩磁的单畴磁铁矿携带。
测量并分离出岩芯中的沉积剩磁（DRM），依据其在层面坐标系中的平均方向与岩石形成时的地磁场方向一致的原则，参照已发表的块体古地磁极移曲线（APWP），可以计算赋存岩芯的隐伏地层产状。对于那些当初未考虑定向问题，而现在又迫切需要重定向的钻井取芯来说，本文提出的算法十分有用。
We have obtained new late Devonain paleomagnetic pole for Yangze Block. A total of 472 paleomagnetic samples were collected from the upper Devonain carbonates from the Jiangyou section in Sichuan province.During thermal cleaning ,intensity of most samples increased progressively(below 300℃) and decayed rapidly (beyond 300℃) till to the noise level of magnetometer at about 500℃.Two magnetic components can be separated from most of samples.The low temperature component(100～300℃) is consistent with the present geomagnetic field. The middle temperature component(300～480℃) is characterized by deep reversed inclination(D=210.20,
I=-80.40,κs=27.5,α95=7.40) The high temperature(420℃～520℃) component(D=49.60,I=44.40,
κs=33.3,α95=8.00)isolated from 62 samples passes fold test and reverse test,yielding a pole position(46.50N,188.30E,dp=6.30,dm=10.10)different from the other poles for Yangze Block.It is therefore suggested that the high temperature was probaly primary magnetization and the Yangze Block was situated at middle latitudes in the northern Hemisphere in the late Devonian.
The paleomagnetic investigation has been carried out on the Lower carboniferous limestone in the hope of identifying the early carboniferous paleomagnetic pole. In general, two magnetic components can be isolated after the removal of a soft viscous overprint by temperature step 100℃ from all samples studied.The low temperature (100~300℃) magnetic component is close to the present day Earth field and fails fold test , which corresponds to a recent overprint. The reversed middle temperature (300~480℃) component (Ds=216.10,Is=-75.50, κs=31.8, α95=4.70) passes fold test and yields a pole position(309.90E,51.20S, dp=7.9,dm=8.6, paleolatitude:62.7),which is completely consistent with MTC revealed from the Upper Devonian limestone in the aera.On the basis of paleomagnetic data and geological evidences , we suggest that the reversed middle temperature component is probably acquired at the time of the Early Jurassic folding in the west part of South China Block.
Rock magnetic studies suggest magnetite presenting as pseudo-single、multidomain and little single domain,carrying the middle(300℃～420℃)and high(420℃～530℃) temperature components respectively, is the main magnetic mineral in the upper Devonian limestone.However,the middle temperature(300℃～480℃) is carried by the mixture of pseudo-single and multidomain magnetite which is also the main magnetic carrier in the lower Carboniferous limestone.
Based on the principle that the depositional remanent magnetization(DRM) is identical to the direction of primary geomagnetic field, reading confirmed polar data from paleomagnetic database, the spatial occurence of buried layers can be calculated.
引文
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[2] Zhao X ，R S Coe．Paleomagnetic constraints on the collision and rotation of North and South China．Nature，1987，327：141～144
[3] 翟永建，周铫秀．华南和华北陆块显生宙古地磁及构造演化．地球物理学报，1989，32(3)：292～307
[4] 吴汉宁，常承法，刘椿等．依据古地磁资料探讨华北和扬子地块运动及其对秦岭造山带构造演化的影响．地质科学，1990，(3)： 201～214
[5] 吴汉宁，常承法，刘椿等．华北和扬子地块古生代至中生代古地磁极移曲线与古纬度分布变化．西北大学学报(自然科学版)，1991，21(3)：99～105
[6] Enkin R J， Z Y Yang，Y Chen．Paleomagnetic constraints on the geodynamic history of the major blocks of China from the Permian to present．J Geophys Res，1992，97，13953～13989
[7]程国良，孙宇航，孙青格等．显生宙中国大地构造演化的古地磁研究．地震地质．1995，17(1)：69～78
[8] Wang Z M ， Van der Voo．Pervasive remagnetization of Paleozoic rocks acquired at the time of Mesozoic folding in the south China block．J Geophys Res，1993，98，1729～1741
[9] Kent D V，Xu G，Huang K．Paleomagnetism of Upper Cretaceous rocks from south China．Earth and Planetary Science Letters，1986，79，179～184
[10] 朱日祥，扬振宇，吴汉宁等．中国主要地块显生宙古地磁视极移曲线与地块运动．中国科学(D辑)， 1998，28(增刊)：1～16
[11] 吴汉宁，朱日祥，白立新等．扬子地块显生宙古地磁视极移曲线与地块运动特征．中国科学(D辑)， 1998， 28(增刊)：69-78
[12] 吴汉宁，朱日祥，邢建孝等．扬子地块湖北兴山－秭归剖面古生界至中生界构造古地磁研究．中国科学(D辑)，1999，29(2)： 144～154
[13] 白立新，朱日祥，吴汉宁等．川北旺苍－南江地区晚侏罗世古地磁学研究与真极移的探讨．地球物理学报，1998，41(3)：311～324
[14] 白立新，吴汉宁，朱日祥等．四川北部旺苍地区早三叠世古地磁结果及构造演化．中国科学(D辑)， 1997，27(6)： 514～518
[15] 白立新，吴汉宁，朱日祥等．扬子地块中寒武世古地磁新结果．中国科学(D辑)，1998，28(增刊)：57～62
[16] 白立新，朱日祥，吴汉宁等．四川旺苍地区中三叠世雷坡口组重磁化机理的初步研究．中国科学(D辑)，1998，28(增刊)： 63～67
[2] Zhao X ，R S Coe．Paleomagnetic constraints on the collision and rotation of North and South China．Nature，1987，327：141～144
[3] 翟永建，周铫秀．华南和华北陆块显生宙古地磁及构造演化．地球物理学报，1989，32(3)：292～307
[4] 吴汉宁，常承法，刘椿等．依据古地磁资料探讨华北和扬子地块运动及其对秦岭造山带构造演化的影响．地质科学，1990，(3)： 201～214
[5] 吴汉宁，常承法，刘椿等．华北和扬子地块古生代至中生代古地磁极移曲线与古纬度分布变化．西北大学学报(自然科学版)，1991，21(3)：99～105
[6] Enkin R J， Z Y Yang，Y Chen．Paleomagnetic constraints on the geodynamic history of the major blocks of China from the Permian to present．J Geophys Res，1992，97，13953～13989
[7]
[8] Wang Z M ， Van der Voo．Pervasive remagnetization of Paleozoic rocks acquired at the time of Mesozoic folding in the south China block．J Geophys Res，1993，98，1729～1741
[9] Kent D V，Xu G，Huang K．Paleomagnetism of Upper Cretaceous rocks from south China．Earth and Planetary Science Letters，1986，79，179～184
[10] 朱日祥，扬振宇，吴汉宁等．中国主要地块显生宙古地磁视极移曲线与地块运动．中国科学(D辑)， 1998，28(增刊)：1～16
[11] 吴汉宁，朱日祥，白立新等．扬子地块显生宙古地磁视极移曲线与地块运动特征．中国科学(D辑)， 1998， 28(增刊)：69-78
[12] 吴汉宁，朱日祥，邢建孝等．扬子地块湖北兴山－秭归剖面古生界至中生界构造古地磁研究．中国科学(D辑)，1999，29(2)： 144～154
[13] 白立新，朱日祥，吴汉宁等．川北旺苍－南江地区晚侏罗世古地磁学研究与真极移的探讨．地球物理学报，1998，41(3)：311～324
[14] 白立新，吴汉宁，朱日祥等．四川北部旺苍地区早三叠世古地磁结果及构造演化．中国科学(D辑)， 1997，27(6)： 514～518
[15] 白立新，吴汉宁，朱日祥等．扬子地块中寒武世古地磁新结果．中国科学(D辑)，1998，28(增刊)：57～62
[16] 白立新，朱日祥，吴汉宁等．四川旺苍地区中三叠世雷坡口组重磁化机理的初步研究．中国科学(D辑)，1998，28(增刊)： 63～67