Late Paleozoic magnetization of mineralized and unmineralized ordovician carbonates from east Tennessee: Evidence for a post-ore chemical event
Late Paleozoic magnetization of mineralized and unmineralized ordovician carbonates from east Tennessee: Evidence for a post-ore chemical event
复制标题
田纳西州东部矿化和未矿化奥陶系碳酸盐的晚古生代磁化：矿石后化学事件的证据
DOI：
--
复制
发表时间：
1987
期刊：
影响因子：
--
通讯作者：
S. Kesler
中科院分区：
文献类型：
--
作者：
V. Bachtadse;R. Voo;F. Haynes;S. Kesler
田纳西州东部下奥陶统诺克斯群矿化和未矿化岩石样品的热退磁研究表明，60%展开后的平均方向为D/I=15 7°/+18°(α95=7.7°，k=5 2.9)，结合稳定终点和重磁循环数据。尽管在95%的置信度水平上，折叠测试并不重要，但折叠测试暗示了磁化的同折叠(阿勒格尼亚)年龄。磁化作用不可能是后褶皱的，即二叠纪或更年轻，因为原地方向与后石炭纪的方向不同。磁化作用不太可能比褶皱更古老，因为与层理有关的方向产生了具有早二叠世特征的古极，但不同于晚石炭世之前的两极。矿化和非矿化崩塌角砾岩的特征磁化特征一致，未能通过砾岩测试。这项研究得出的最好的古极点估计位于40°N，126°E，在北美古生代视极移路径的晚石炭世段内。因此，由于广泛的Kiaman再磁化作用，磁化作用被解释为同褶皱和二次磁化。高达520°C的阻挡温度和获得的等温剩磁表明磁铁矿是磁化的载体。扫描电子显微镜和能量色散分析表明，石灰岩样品中含有丰富的磁铁矿球体。次生白云岩具有与原生石灰岩相同的磁性特征，推测类似的磁铁矿球体是其磁化的载体。由于这种磁化作用及其载体发生在闪锌矿和次生白云石中，因此它必须与矿石同龄或比矿石年轻，而矿石本身被认为是中晚古生代。认为这种磁化作用是在磁铁矿生长过程中获得的，与晚古生代的一次化学事件有关。
Thermal demagnetization studies on samples from mineralized and unmineralized rocks in the Lower Ordovician Knox Group of east Tennessee reveal a mean direction of D/I = 157°/+18° (α95 = 7.7°, k = 52.9) after 60% unfolding, combining stable endpoint and remagnetization circle data. Although not significant at the 95% confidence level, the fold test is suggestive of a synfolding (Alleghenian) age of the magnetization. The magnetization cannot be postfolding, i.e., Permian or younger, because the in situ directions do not resemble those for post-Carboniferous times. The magnetization is unlikely to be much older than the folding because the directions with respect to bedding yield a paleopole characteristic of the Early Permian, but unlike poles for pre-Late Carboniferous times. Characteristic magnetizations from mineralized and unmineralized collapse breccias are in good agreement and fail the conglomerate test. The best estimate for the paleopole resulting from this study falls at 40°N, 126°E, within the Late Carboniferous segment of the North American Paleozoic apparent polar wander path. Thus the magnetization is interpreted as synfolding and secondary due to widespread Kiaman remagnetization. Blocking temperatures of up to 520°C and the acquisition of isothermal remanent magnetization suggest magnetite as the carrier of the magnetization. Scanning electron microscopy and energy dispersive analysis of magnetic extracts from limestone samples reveal abundant magnetite spheres. Secondary dolomites have the same magnetic characteristics as the primary limestones, and it is inferred that similar magnetite spheres are the carriers of their magnetization. Because this magnetization, and therefore its carrier, occurs in sphalerite and secondary dolomite, it must be coeval with or younger than the ore, which itself is thought to be of middle to late Paleozoic age. It is argued that the magnetization was acquired during growth of magnetite, associated with a late Paleozoic chemical event.