Magnetic and radiometric signatures of alkaline rocks and gabbros from the Ponta Grossa Arch, southeastern Paraná Basin, Brazil

Abstract We present airborne magnetic and radiometric characteristics of some igneous suites from the Ponta Grossa Arch Alkaline Province (southern Brazil), namely the Bairro da Cruz, Banhadão, Barra do Itapirapuã, Mato Preto and Tunas complexes, Barra do Teixeira Phonolite, Itapirapuã Nepheline syenite, and José Fernandes Gabbro. We applied magnetic susceptibility 3D inversions using the total magnetic anomaly data together with its analytic signal of the vertical integral and the vertical integral of the analytic signal for the Bairro da Cruz, José Fernandes, and Tunas intrusions. Gamma-ray spectrometric methods involved analyses of basic, ternary, and ratio maps, thorium-normalized parameters, and radioelement concentrations at sampled data points. Stacked profiles from geophysical data were generated for each rock. The Bairro da Cruz, José Fernandes, and Tunas intrusions are associated with dipolar anomalies with normal polarity whereas their inversions show low magnetic susceptibility values. Gamma-ray spectrometric results allowed us to verify that all complexes are enriched mainly in eTh. Carbonatite bodies showed the highest contents of eTh and eU. Our findings are consistent with geophysical responses of alkaline bodies and impact the understanding of geophysical signatures, especially radiometric ones, of alkaline provinces in Brazil.

摘要 本研究报道了巴西南部蓬塔格罗萨穹窿碱性省（Ponta Grossa Arch Alkaline Province）内部分火成岩套的航空磁法与放射能谱特征，涉及克鲁斯街区（Bairro da Cruz）、班哈当（Banhadão）、伊塔皮拉普阿河（Barra do Itapirapuã）、马托普雷托（Mato Preto）与图纳斯（Tunas）杂岩体，以及特谢拉河响岩（Barra do Teixeira Phonolite）、伊塔皮拉普阿霞石正长岩（Itapirapuã Nepheline syenite）与何塞·费尔南德斯辉长岩（José Fernandes Gabbro）。针对克鲁斯街区、何塞·费尔南德斯与图纳斯侵入体，我们以总磁异常数据（total magnetic anomaly data）为基础，结合其垂直积分（vertical integral）的解析信号（analytic signal）与解析信号的垂直积分，完成了磁化率（magnetic susceptibility）三维反演（3D inversions）。本次γ射线能谱法（Gamma-ray spectrometric methods）的分析内容包括基础图、三元图与比值图解析，钍归一化参数（thorium-normalized parameters）计算，以及采样点处的放射元素浓度（radioelement concentrations）测定。针对每种岩性，我们均生成了对应地球物理数据的堆叠剖面（Stacked profiles）。克鲁斯街区、何塞·费尔南德斯与图纳斯侵入体均伴生正常极性（normal polarity）的偶极异常（dipolar anomalies），但其三维反演结果显示其磁化率值偏低。γ射线能谱分析结果证实，所有杂岩体均主要富集等效钍（eTh）。碳酸岩岩体（carbonatite bodies）的等效钍（eTh）与等效铀（eU）含量均为所有岩体中最高。本研究结果与碱性岩体的地球物理响应特征一致，且有助于加深对巴西境内碱性省地球物理识别标志（尤其是放射能谱类识别标志）的认知。