扬子地块奥陶系碳酸盐岩重磁化机制探讨

On the mechanism of remagnetization of Ordovician carbonates from the Yangtze Block,southwestern China

碳酸盐岩是记录古地磁场信息的重要载体,然而,广泛存在的重磁化现象制约了碳酸盐岩在古地磁研究中的应用,其重磁化机制亟待解决.本文对采自贵州羊蹬地区的319块奥陶系碳酸盐岩定向样品作了详细的古地磁学和岩石磁学研究,其结果表明,94%样品(A类)记录了单一剩磁分量A,其解阻温度低于450℃;在地理坐标系下的平均方向为Dg/Ig=3.1°/48.1°(α95=2.9°),对应的古地磁极(87.0°N,2.8°E,A95=3.0°)与扬子地块古近纪-第四纪的古地磁极重合.6%样品(B类)记录了两个磁化分量,其高温分量(450℃～585℃)与A分量显著不同,但明显远离扬子块体早古生代古地磁极;低温分量(<450℃)与A分量类似.说明羊蹬剖面奥陶系碳酸盐岩记录了两期重磁化.A分量和B低温分量的主要载磁矿物为磁黄铁矿(胶黄铁矿),B高温分量的主要载磁矿物为磁铁矿.这些磁性矿物都是成岩后的次生矿物.其中,解阻温度高于450℃的磁铁矿可能受晚燕山期造山运动影响生成;磁黄铁矿(胶黄铁矿)等矿物可能与印度板块与欧亚大陆碰撞引起的喜马拉雅造山运动所产生的流体作用有关,以后一期重磁化为主.新生代早期青藏高原隆升产生的流体在流经东南缘的碳酸盐岩等沉积岩层时,与原岩发生相互作用,使磁黄铁矿、胶黄铁矿、磁铁矿等磁性矿物生长并获得化学剩磁,造成了广泛重磁化.

Carbonates are an important source of paleomagnetic field records. However, widespread remagnetization phenomena have questioned the reliability of the paleomagnetic results from carbonates, where the ultimate cause of the remagnetization remains controversial. In this study we investigated the remagnetization mechanism of the Ordovician carbonates from the Yangdeng （YD） area in the Yangtze Block, Guizhou province. The results show that the majority of samples （94°） are dominated by a unique magnetic component （A） up to 400 ℃or 450 °C, generating a mean geographic direction of Dg/Ig3.1°/48.1° （ag =2.9°）. The corresponding geomagnetic pole is 87.0°N, 2.8°E （ ags= 3.0°）, which coincides with the Quaternary-Eocene pole of the Yangtze block, indicating that these samples were remagnetized sometime since Eocene. Theother minority of samples have two magnetic components： one is the low temperature component （450 °C）, coinciding with the A components° the other is the high temperature component （450 °C-585 °C）, which is significantly different from the A component and departs from the early Paleozoic poles of the Yangtze Block. This indicates the YD carbonates recorded two periods of remagnetization. Combining the paleomagnetic results with rock magnetic investigations suggests that the remagnetization is due to the acquisition of a chemical remanent magnetization （CRM） carried by authigenic magnetite and pyrrhotite, possibly together with greigite. The authigenic processes of these minerals are probably influenced by the orogenic fluid resulted from the Yanshanian orogeny in late Cretaceous and Himalaya orogeny in late Eocene, respectively. The influence from the latter one is dominantly recorded in this study. Sedimentary rocks on the southeastern edge of the Tibetan plateau, such as carbonates in this study, are prone to interaction with the orogenic fluids during migration, which promotes the authigenic processes that lead to the significant remagnetization.