Abstract Magnetic signals in basalts from the Sylhet traps (ST), Shillong Plateau, India is investigated using rock magnetism as a proxy to explain the potentiality of the ST basalts as an indicator of magnetic mineralogy. A paleomagnetic study was carried out in order to unravel the Paleopole. In detail, rock magnetic and Anisotropy of Magnetic susceptibility (AMS) results have been reported for the first time on the ST basalts. A total of 38 oriented block samples were collected from 8 sites along Cherrapunji-Shella bazaar road section, Meghalaya, India. Samples were subjected to rock magnetic measurements such as magnetic susceptibility (K), Isothermal Remanent Magnetization (IRM), hysteresis loops, backfield (coercivity remanence) and k-T (Susceptibility vs. Temperature) analysis to identify the magnetic properties of basalts. Advanced Variable Field Translation Balance (AVFTB) was used for some rock magnetic studies. A few samples were also subjected to Anisotropy of Magnetic susceptibility (AMS) study to understand the petrofabric orientations and shape anisotropy in the ST basalts. The Remanence ratio (Mrs/Ms) and the Coercivity ratio (Bcr/Bc) ranging from 0.105 to 0.333 and from 1.25 to 1.87 respectively indicate sizes of magnetic grains are mainly in the range of Pseudo Single Domain (PSD) limits with minority of them in the ranges of Single Domain (SD) limits. The results of k-T study in combination with other rock magnetic results shows that magnetite (Fe3O4) [Curie Temperature = 565-590 °C] as the dominant magnetic carrier with a subordinate content of titanomagnetite Fe2+(Fe3+Ti)2O4 [Curie Temperature = 380–450 °C]. From the AMS study, parameters like lineation, foliation are calculated, and results define all the three shape anisotropies present in the ST basalts. The ST basalts display paleomagnetic directions with mean declination = 255°, mean inclination = -57° (α95 = 3.44°; k = 16.74). The pole position of the ST at 117 Ma is -26.79 °N, 330.75 °E (dp = 3.6; dm = 5) with a paleolatitude 37.6 °S. This infers that the ST is a later volcanic event than Rajmahal traps (RT) which is considered contemporary to the ST.

中文翻译：

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印度东北部西隆高原锡尔赫特圈闭的岩石磁学和古地磁研究

摘要 以岩石磁性为代表，研究了印度西隆高原锡尔赫特圈闭 (ST) 玄武岩中的磁信号，以解释 ST 玄武岩作为磁性矿物学指标的潜力。为了解开古极地，进行了古地磁研究。详细地，首次报道了 ST 玄武岩的岩石磁性和磁化率各向异性 (AMS) 结果。从印度梅加拉亚邦 Cherrapunji-Shela 集市路段沿线的 8 个地点收集了总共 38 个定向块样品。对样品进行岩石磁测量，例如磁化率 (K)、等温剩磁 (IRM)、磁滞回线、背场（矫顽力剩磁）和 kT（磁化率与温度）分析，以确定玄武岩的磁性。高级可变场平移平衡 (AVFTB) 用于一些岩石磁性研究。一些样品还进行了磁化率各向异性 (AMS) 研究，以了解 ST 玄武岩中的岩石结构取向和形状各向异性。剩磁比 (Mrs/Ms) 和矫顽力比 (Bcr/Bc) 范围分别为 0.105 至 0.333 和 1.25 至 1.87，表明磁性晶粒的尺寸主要在伪单畴 (PSD) 范围内，其中少数在单域 (SD) 限制范围内。kT研究结果结合其他岩石磁性结果表明，磁铁矿（Fe3O4）[居里温度= 565-590°C]为主要磁性载体，次要含量为钛磁铁矿Fe2+（Fe3+Ti）2O4[居里温度= 380–450 °C]。从 AMS 研究来看，计算线状、叶理等参数，结果定义了 ST 玄武岩中存在的所有三种形状各向异性。ST 玄武岩显示古地磁方向，平均偏角 = 255°，平均倾角 = -57°（α95 = 3.44°；k = 16.74）。ST 在​​ 117 Ma 的极点位置为 -26.79 °N, 330.75 °E (dp = 3.6; dm = 5)，古纬度为 37.6 °S。这推断ST是比Rajmahal圈闭（RT）晚的火山事件，后者被认为是与ST同时代的。