Abstract Anisotropy of magnetic susceptibility (AMS) and anhysteretic remanent magnetization (AARM) are tools to characterize flow structures and emplacement conditions of volcanic and subvolcanic bodies, directly related to magnetic fabrics. An AMS and AARM study was performed to contribute to the understanding of the processes involved in magma transportation and emplacement dynamics of the volcanic and subvolcanic rocks of Trindade Island, and their implications on the evolution of the volcanic field. Trindade Island is located in the South Atlantic Ocean, at 1260 km to the east of the Espirito Santo state coast in Brazil. The island is composed of lava flows, intrusions and pyroclastic rocks of alkaline, SiO2 undersaturated nature, forming five geological units aged 3.9–0.25 Ma. During fieldwork, 17 sampling sites were established in the phonolitic necks and melanephelinitic dyke of the oldest unit (Trindade Complex) and in the melanephelinitic ‘a'a flows of the youngest units (Morro Vermelho, Valado and Paredao Volcano formations), totaling 160 oriented mini-cores and 504 rock specimens. The analysis of petrofabric, magnetic mineralogy, AMS and AARM diagrams suggests the dominance of a low-coercivity magnetic mineral phase, represented by low-Ti titanomagnetite/titanomagnetite in the phonolitic necks and melanephelinitic dyke. These bodies display normal fabric (multi-domain and vortex grains) and sub-vertical/high-angle magnetic foliation and lineation. The melanephelinitic ‘a'a lavas present at least two magnetic mineral phases of high and low-coercivity (titanomagnetite, maghemite and hematite), with normal, intermediate or inverse fabrics (vortex or single domain grains). Mean magnetic foliation and lineation suggest that lavas flowed to NE. Data interpretation indicates that the subvolcanic bodies were emplaced vertically during the initial stages of island formation, followed by lavas flowing to the northeast. The orientation of the dyke and lava feeders along NNW-SSE leads to the conclusion that the tensional field during the evolution of volcanism remained the same, with NW direction for σ during approximately 4 My. This study is an example for the understanding of physical processes in the evolution of world oceanic islands.

中文翻译：

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巴西特林达德岛碱性火山岩和亚火山岩的就位动力学

摘要 磁化率各向异性 (AMS) 和非滞回剩磁 (AARM) 是表征火山和次火山体的流动结构和侵位条件的工具，与磁性结构直接相关。进行了 AMS 和 AARM 研究，以有助于了解特林达德岛火山岩和次火山岩的岩浆输送和侵位动力学过程及其对火山场演化的影响。特林达德岛位于南大西洋，距巴西圣埃斯皮里图州海岸以东 1260 公里。该岛由熔岩流、侵入体和碱性、SiO2 欠饱和性质的火山碎屑岩组成，形成五个年龄为 3.9-0.25 Ma 的地质单元。在实地考察期间，在最古老单元（Trindade Complex）的音质岩颈和黑质岩脉以及最年轻单元（Morro Vermelho、Valado 和 Paredao 火山地层）的黑质岩脉'a'a 流中建立了 17 个采样点，共计 160 个定向微型岩芯和504个岩石标本。对岩石织物、磁性矿物学、AMS 和 AARM 图表的分析表明，低矫顽力磁性矿物相占主导地位，以声质岩颈和黑斑岩脉中的低钛钛磁铁矿/钛磁铁矿为代表。这些物体显示出正常的结构（多域和涡旋颗粒）和亚垂直/高角度的磁性叶理和线状。黑斑岩 'a'a 熔岩至少存在两个高矫顽力和低矫顽力的磁性矿物相（钛磁铁矿、磁赤铁矿和赤铁矿），具有正常、中间或反向织物（涡流或单域晶粒）。平均磁叶理和划线表明熔岩流向 NE。资料解释表明，在岛屿形成的初始阶段，火山下体垂直侵位，随后熔岩流向东北。沿 NNW-SSE 的岩脉和熔岩馈线的方向得出的结论是，火山活动演化过程中的张力场保持不变，σ 在大约 4 My 期间的 NW 方向。这项研究是理解世界海洋岛屿演化物理过程的一个例子。资料解释表明，在岛屿形成的初始阶段，火山下体垂直侵位，随后熔岩流向东北。沿 NNW-SSE 的岩脉和熔岩馈线的方向得出的结论是，火山活动演化过程中的张力场保持不变，σ 在大约 4 My 期间的 NW 方向。这项研究是理解世界海洋岛屿演化物理过程的一个例子。资料解释表明，在岛屿形成的初始阶段，火山下体垂直侵位，随后熔岩流向东北。沿 NNW-SSE 的岩脉和熔岩馈线的方向得出的结论是，火山活动演化过程中的张力场保持不变，σ 在大约 4 My 期间的 NW 方向。这项研究是理解世界海洋岛屿演化物理过程的一个例子。