⁴⁰Ar/³⁹Ar laser incremental-heating geochronology of whole-rock fragments and minerals from primitive subalkaline mafic dykes from the Cabo Frio Tectonic Domain, southeastern Brazil, reveal a complex combination of intrusive ages, apparent ages affected by excess argon, and ages that record late-stage hydrothermal alteration. Incremental-heating analysis of encapsulated magmatic amphibole-biotite clusters reveals a minimum intrusive age of 132.83 ± 0.30 Ma. Sericitized plagioclase crystals show the percolation of K-rich fluids at ∼ 106 Ma. Albitized plagioclase phenocrysts suggest the percolation of Na-rich fluids in the 70-40 Ma interval. Hydrothermal alteration events overlap in time with pulses of alkaline magmatism in the extending Brazilian crust, suggesting a link between magmatic activity and generation of fluid circulation cells along the margin and adjacent sedimentary basins. Evidence for pervasive hydrothermal re-setting of the K-Ar system in the Cabo Frio Tectonic Domain dykes suggests that young ⁴⁰Ar/³⁹Ar geochronology results obtained for mafic dykes here and elsewhere along the continental margins may not record multiple intrusive events, reflecting instead partial or total resetting of the K-Ar system by water-rock interaction during fluid circulation.

⁴⁰氩/ ³⁹来自巴西东南部 Cabo Frio 构造域的原始亚碱性基性岩脉的全岩碎片和矿物的 Ar 激光增量加热地质年代学揭示了侵入时代、受过量氩影响的表观时代和晚期记录时代的复杂组合热液蚀变。封装岩浆角闪石-黑云母簇的增量加热分析显示最小侵入年龄为 132.83 ± 0.30 Ma。绢云母斜长石晶体显示出富钾流体在 106 Ma 时的渗透。Albitized 斜长石斑晶表明富钠流体在 70-40 Ma 区间渗流。热液蚀变事件与延伸的巴西地壳中的碱性岩浆作用脉冲在时间上重叠，表明岩浆活动与沿边缘和相邻沉积盆地的流体循环细胞的产生之间存在联系。Cabo Frio 构造域岩脉中 K-Ar 系统普遍热液重新设置的证据表明，年轻的沿大陆边缘这里和其他地方的基性岩脉获得的⁴⁰ Ar/ ³⁹ Ar 年代学结果可能没有记录多次侵入事件，而是反映了流体循环过程中水-岩相互作用对 K-Ar 系统的部分或全部重置。