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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Complex remanent magnetization in the Kizilkaya ignimbrite (central Anatolia): Implication for paleomagnetic directions
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Complex remanent magnetization in the Kizilkaya ignimbrite (central Anatolia): Implication for paleomagnetic directions

机译：Kizilkaya火成岩（中部安纳托利亚）中复杂的剩余磁化强度：对古磁方向的影响

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摘要

Pyroclastic flow deposits, known as ash-flow tuffs or ignimbrites, are invaluable materials for paleomagnetic studies, with many applications for geological and tectonic purposes. However, little attention has been paid to evaluating the consistency and reliability of the paleomagnetic data when results are obtained on a single volcanic unit with uneven magnetic mineralogy. In this work we investigate this issue by concentrating on the Kizilkaya ignimbrite, the youngest large-volume unit of the Neogene ignimbrite sequence of the Central Anatolian Volcanic Province in Turkey, bringing evidence of significant magnetic heterogeneities in ignimbrite deposits (magnetic mineralogy, susceptibility, magnetic remanence, coercivity, etc.) and emphasizing the importance of a stratigraphic sampling strategy for this type of volcanic rocks in order to obtain reliable paleomagnetic data. Six sections were sampled at different stratigraphic heights within the devitrified portion of the ignimbrite. Isothermal remanence measurements point to low-Ti titanomagnetite as the main magnetic carrier at all sites; at some sites, the occurrence of oxidized Ti-magnetite and hematite is disclosed. The bulk susceptibility (k(m)) decreases vertically at two out of six sections: its value for the topmost samples is commonly one order of magnitude lower than that of the samples at the base. In most cases, low k(m) values relate to high coercivity of remanence (B-CR) values, which range from 25 to >400 mT, and to low S-ratios (measured at 03 T) between 0.28 and 0.99. These data point to the occurrence of oxidized magnetic phases. We therefore consider the k(m) parameter as a reliable proxy to check the ignimbrite oxidation stage and to detect the presence of oxidized Ti-magnetite and hematite within the deposit. The characteristic remanent magnetization is determined after stepwise thermal and AF demagnetization and clearly isolated by principal component analysis at most sites. For these sites, the site mean paleomagnetic direction is consistent with data from the literature. At a few other sites, the remanence is more complex: the direction moves along a great circle during demagnetization and no stable end-point is reached. The occurrence of oxidized Ti-magnetite or hematite as well as two remanence components with overlapping coercivity and blocking temperature spectra suggest that the Kizilkaya ignimbrite acquired first a thermal remanent magnetization and then, during the final cooling or a short time later, a secondary remanent magnetization component which is interpreted as a CRM acquired during post-emplacement devitrification processes. Notwithstanding the Kizilkaya ignimbrite is a single cooling unit, its magnetic properties suffered substantial variations laterally and vertically within the deposit. The Kizilkaya case shows that thick pyroclastic deposits should be sampled using a stratigraphic approach, at different sites and different stratigraphic heights at each individual sampling location, otherwise, under-sampling may significantly affect the paleomagnetic results. When sampling is performed on a short duration or on very poorly preserved deposits we recommend drilling the lower-central portion in the most strongly welded and devitrified fades. Such sampling strategy avoids complications arising from the potential presence of a pervasive secondary CRM masking the original ChRM. (C) 2017 Elsevier B.V. All rights reserved.
机译：火山碎屑流沉积物，被称为灰凝灰岩或火成岩，是古磁学研究的宝贵材料，在地质和构造方面有许多应用。但是，当在单个具有不均匀磁性矿物学的火山单元上获得结果时，很少有注意力放在评估古磁数据的一致性和可靠性上。在这项工作中，我们着重研究了这个问题，重点研究了土耳其中部安那托利亚火山省新近纪火成岩层序中最年轻的大体积单位Kizilkaya火成岩，为火成岩矿床中明显的磁异质性提供了证据（磁性矿物学，磁化率，磁化剩磁，矫顽力等），并强调了针对此类火山岩的地层取样策略的重要性，以便获得可靠的古磁数据。在火成岩的失透部分内，以不同的地层高度对六个剖面进行了采样。等温剩磁测量结果表明低钛钛磁铁矿是所有场所的主要磁性载体。在某些地方，公开了氧化钛磁铁矿和赤铁矿的发生。总体磁化率（k（m））在六个部分中的两个部分处垂直减小：最顶部样本的值通常比基础样本的值低一个数量级。在大多数情况下，低k（m）值与剩磁矫顽力（B-CR）值（范围从25到> 400 mT）和0.28至0.99之间的低S比（在03 T下测得）有关。这些数据表明发生了氧化的磁性相。因此，我们认为k（m）参数是检查火成岩氧化阶段并检测矿床内氧化的Ti-磁铁矿和赤铁矿的可靠代表。剩余磁化强度的特征是在逐步进行热和AF消磁之后确定的，并且在大多数位置通过主成分分析清楚地将其隔离。对于这些地点，该地点的平均古磁方向与文献数据一致。在其他一些站点，剩磁更复杂：在消磁过程中，方向沿一个大的圆周运动，并且没有达到稳定的终点。氧化的钛磁铁矿或赤铁矿以及两个剩磁成分具有重叠的矫顽力和阻塞温度谱，表明Kizilkaya着火岩首先获得了热剩磁，然后在最终冷却期间或不久之后又获得了二次剩磁。组件，它解释为在贴装后失透过程中获得的CRM。尽管Kizilkaya火成岩是一个单独的冷却装置，但其磁性能在沉积物中横向和垂直方向上都发生了显着变化。 Kizilkaya案例表明，应使用地层学方法在每个单独的采样位置的不同地点和不同地层高度对厚火山碎屑沉积物进行采样，否则，欠采样可能会严重影响古磁结果。当采样时间很短或保存不良时，我们建议在焊接强度最高且失透的玻璃中钻出中心下部。这样的采样策略避免了由于潜在的普遍存在的二级CRM掩盖原始ChRM而引起的复杂性。 （C）2017 Elsevier B.V.保留所有权利。

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