Abstract Magnetic fabric and rock-magnetism studies were performed on 153 dikes from diabase, lamprophyre and alkaline dike swarms that outcrop on the island of Ilhabela, NE Sao Paulo State. The dikes crosscut Archean and Proterozoic polymetamorphic rocks. Their thicknesses range from a few centimeters to 4 or 5 m. They trend predominantly N30-60E with a very steep (~60°) to vertical dip. Magnetic fabrics were determined using anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). Rock-magnetism measurements reveal that the magnetic mineral of the three swarms is magnetite mainly with grains in the range of 2–5 μm. For a few dikes, these grains are the carriers of bulk magnetic susceptibility but are not responsible for the AMS, which is due to Fe-bearing minerals, as shown by AARM. The main AMS fabric recognized in the swarms is due to magma flow, in which the Kmax–Kint plane is parallel to the dike's plane, and the magnetic foliation pole (Kmin) is perpendicular to it. The analysis of the Kmax inclination showed that the dikes were fed by horizontal to vertical flows. However, for the minority of the dikes, the AMS and AARM tensors are not coaxial. The AARMmax is oriented N30–60 W, approximately perpendicular to the Kmax. The AARMmax orientation is similar to the direction of a fault system located mainly in the Santos marginal basin which, was formed in the Cretaceous rifting during the South Atlantic opening. The AARM fabric is tectonic in origin, and is parallel with the extensional paleostress responsible for the Gondwana break-up. The comparison of AMS and AARM fabrics suggests that diabase and lamprophyre dikes were emplaced in three distinct events in the earliest stages of the South Atlantic opening. Later, the alkaline dikes were emplaced in the final stages of the Atlantic opening.

中文翻译：

---

来自伊利亚贝拉岛（巴西东南部）的堤防群的就位及其与磁性织物揭示的南大西洋开口的关系

摘要 对来自圣保罗州东北部伊利亚贝拉岛上露头的辉绿岩、萤石和碱性岩脉群的 153 条岩脉进行了磁性结构和岩石磁性研究。堤防横切太古代和元古代多变质岩。它们的厚度范围从几厘米到 4 或 5 m。它们的趋势主要是 N30-60E，垂直倾角非常陡峭（~60°）。使用低场磁化率 (AMS) 的各向异性和无磁滞剩磁 (AARM) 的各向异性来确定磁性织物。岩磁测量表明，三个群的磁性矿物主要是磁铁矿，颗粒在2-5μm范围内。对于一些岩脉，这些颗粒是大块磁化率的载体，但不负责 AMS，这是由于含铁矿物，如 AARM 所示。在群中识别出的主要 AMS 结构是由于岩浆流，其中 Kmax-Kint 平面平行于堤防平面，磁叶理极 (Kmin) 与其垂直。Kmax 倾角的分析表明，堤坝是由水平到垂直的流动供给的。然而，对于少数堤防，AMS 和 AARM 张量不是同轴的。AARMmax 的方向为 N30–60 W，大致垂直于 Kmax。AARMmax 方向与主要位于 Santos 边缘盆地的断层系统的方向相似，该断层系统形成于南大西洋开放期间的白垩纪裂谷。AARM 构造起源于构造，与造成冈瓦纳大陆破裂的伸展古应力平行。AMS 和 AARM 织物的比较表明，辉绿岩和灯藻岩脉在南大西洋开放的最早阶段出现在三个不同的事件中。后来，碱性堤防被安置在大西洋开放的最后阶段。