Paleomagnetism and proterozoic dyke swarms continental configurations and evolution of the earth's core

古地磁和元古代岩脉群大陆构造和地核演化

• 批准号: 7824-2006
• 负责人: Halls, Henry
• 金额: $ 2.4万
• 依托单位: University of Toronto
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2008
• 资助国家: 加拿大
• 起止时间: 2008-01-01 至 2009-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=389594
• 关键词: Paleomagnetism; proterozoic; dyke; swarms; continental

Mafic dykes, the remains of magma-filled fissures that once fed basaltic lava to the Earth's surface, frequently preserve a remanent or "fossil" magnetism that formed at the time of magma solidification and cooling, and from which the magnetic pole position and latitude of the dyke at the time of cooling can be calculated.  The age of the dyke, and hence its magnetization, can be obtained by precise radioactive (U-Pb) dating of primary U-bearing minerals. Proterozoic dykes of many ages between 600 and 3000 million years old, criss-cross Precambrian shields in abundance. A shield is largely composed of ancient granitic rocks  surrounded by younger mountain-belts or orogens. By virtue of their toughness the granites have helped to buttress the dykes from the effects of these orogens. The dykes, despite their great age, are therefore unusually pristine, especially in the interiors of the shields, and preserve their original mineralogy, and paleomagnetic signature. Therefore they represent the cornerstone upon which Precambrian paleomagnetic apparent polar wander paths for drifting continents are defined. Paleomagnetism  represents the only definitive way to assemble continental fragments in the Precambrian. The question of super-continent cyclicity through geological time can thus be addressed  and links made to presently dispersed continental fragments that may have economic applications, if known mineral provinces on one continent can be extended into adjoining ones. Super-continents are now  at the forefront of Earth Science enquiry with UNESCO International Geoscience Projects (Nos .417 and 509) on the existence of Proterozoic super-continents. Also an upcoming NUNA conference will highlight the role of mafic magmatism, and dykes in particular, in providing a U-Pb "bar-code" for each Precambrian fragment that may help in fitting them together (Bleeker, 2004). The proposed paleomagnetic research on Precambrian dyke swarms will contribute precisely dated reliable paleomagnetic pole positions from Canada, Greenland, India and China and thus will help in this wider global context, and also provide solutions to more local problems relating to the extent that shields have responded to the orogenic events around their margins.

基性岩脉是曾经将玄武熔岩输送到地球表面的充满岩浆的裂缝的遗迹，经常保留在岩浆凝固和冷却时形成的残余或“化石”磁性，从中可以计算冷却时岩脉的磁极位置和纬度。岩脉的年龄，从而计算其磁化强度，可通过对原生含铀矿物进行精确的放射性（U-Pb）测年获得。元古代岩墙的年龄在600到3000万年之间，纵横交错的前寒武纪地盾大量存在。地盾主要由古老的花岗岩组成，周围是较年轻的山脉或造山带。由于花岗岩的韧性，它们有助于支撑岩墙，使其免受造山带的影响。这些岩脉，尽管年代久远，却异常原始，特别是在地盾内部，并保留了其原始的矿物学和古地磁特征。因此，它们代表了确定漂移大陆的前寒武纪古地磁视极移路径的基石。古地磁学代表了在前寒武纪组合大陆碎片的唯一确定方法。因此，如果一个大陆上已知的矿产省可以延伸到相邻的大陆上，就可以解决地质时期的超大陆旋回性问题，并与目前分散的可能具有经济用途的大陆碎片建立联系。联合国教科文组织国际地球科学项目（417和509号）关于元古代超大陆的存在，超大陆现在处于地球科学研究的前沿。此外，即将举行的NUNA会议将强调镁铁质岩浆作用，特别是岩脉的作用，为每个前寒武纪碎片提供U-Pb“条形码”，这可能有助于将它们组合在一起（Bleeker，2004年）。对前寒武纪岩墙群的拟议古地磁研究将有助于从加拿大、格陵兰、印度和中国准确确定可靠的古磁极位置，从而有助于在更广泛的全球范围内开展工作，并为更多的地方性问题提供解决方案，这些问题涉及到地盾对其边缘周围造山活动的反应程度。