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首页> 外文期刊>Journal of South American earth sciences >Geophysical signatures of the alkaline intrusions bordering the Parana Basin
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Geophysical signatures of the alkaline intrusions bordering the Parana Basin

机译：与巴拉那盆地接壤的碱性侵入体的地球物理特征

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摘要

Alkaline intrusions at the edges of the Parana Basin are mostly grouped in provinces, but occur in a few cases as isolated bodies. All of these intrusions surround the edge of the basin and intrude into weak zones, such as belts and arches. In rare cases, the intrusions are covered by basin sediments, but they are mainly found in the exposed crystalline basement. In this study, we discuss the gravimetric and magnetic signatures of the alkaline intrusions in the Ponta Grossa Arch, Alto Paranaiba and Goias provinces. The provinces ages range from 120 to 60 Ma, and are thus contemporary or subsequent to the Serra Geral magmatic event. The magnetic signature of the province intrusions indicates a strong residual magnetization, as observed on maps of the anomalies reduced to the pole, which were different from the anomalies resulting only from induced field. The residual magnetization was often verified by magnetic measurements in the laboratory. Some of the studied intrusions have reverse magnetization. Other intrusions shows two magnetization directions, normal and reverse, indicating the likely presence of carbonatite. With the exception of the intrusions containing carbonatite, anomalies reduced to the pole are characterized by highly concentrated magnetization and well-defined 3D geometry. Intrusions with carbonatite usually exhibit dual polarization. The gravimetric signature is a positive anomaly of great magnitude. When carbonatite is present, a negative anomaly within the positive anomaly is observed, where the negative anomaly is associated with carbonatite. The gravimetric anomalies are also highly localized, indicative of 3D bodies. Gravimetric and aeromagnetic maps of the provinces show a coincidence between the location of the mapped intrusions and the anomalies. These maps also show anomalies that have the same signature but are not related to the mapped outcrops. This result suggests that these anomalies are indicative of the presence of alkaline intrusions that are covered by sediments (from either the Parana Basin or alluvial sources) or are not emergent from the basement. The analysis of these maps and the inversion of the anomalies allows inference of the subsurface geometry, and thus also calculation of the body volume and its emplacement mechanism, which are elements necessary for mineral deposit evaluation. The maps also allow the identification of pre-existing faults and those formed during the emplacement. We compare geophysical models developed for gravimetric and magnetic data for the intrusions of the Ponta Grossa Arch, Alto Paranaiba and Goias provinces in order to identify physical similarities and differences in the alkaline intruded into lithospheric segments with different tectonic histories.
机译：巴拉那盆地边缘的碱性岩体侵入物大部分集中在各省中，但在少数情况下是孤立体。所有这些侵入都围绕着盆地的边缘并侵入了脆弱的区域，例如皮带和拱门。在极少数情况下，侵入岩被盆地沉积物覆盖，但主要在裸露的结晶基底中发现。在这项研究中，我们讨论了Ponta Grossa拱门，Alto Paranaiba和Goias省的碱性侵入岩的重量和磁学特征。这些省的年龄范围从120到60 Ma，因此是现代的，或者是Serra Geral岩浆事件之后的。如在缩小到极点的异常图上所观察到的，省侵入体的磁性特征表明强烈的残留磁化强度，这与仅由感应场引起的异常不同。残留磁化强度经常在实验室中通过磁测量来验证。一些已研究的侵入体具有反向磁化强度。其他侵入显示出两个磁化方向（正向和反向），表明可能存在碳酸盐。除了包含碳酸盐岩的侵入以外，极点异常的特征在于高度集中的磁化强度和定义明确的3D几何形状。含碳酸盐岩的侵入通常表现出双重极化。重量特征是很大的正异常。当存在碳酸盐岩时，在正异常区内观察到负异常，其中负异常与碳酸盐岩有关。重力异常也高度局部化，表示3D物体。各省的重力图和航空磁图显示了被测侵入物的位置与异常之间的重合。这些图还显示具有相同特征但与映射露头无关的异常。该结果表明，这些异常现象表明存在碱性沉积物，这些沉积物被沉积物覆盖（来自巴拉那盆地或冲积物源）或未从地下室出来。对这些图的分析和异常的反演可以推断出地下几何形状，从而也可以计算出矿体体积及其沉积机理，这是评估矿藏的必要元素。这些图还可以识别先前存在的断层以及在置入过程中形成的断层。我们比较了为重力和磁数据开发的地球物理模型，这些数据用于庞塔格罗萨拱门，上巴拉那巴和戈亚斯省的侵入，以识别侵入到具有不同构造历史的岩石圈片段中的碱的物理相似性和差异。

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