2D magnetometric modeling of a basic-intermediate intrusion geometry: geophysical and geological approaches applied to the Limeira intrusion, Paraná Magmatic Province (SP, Brazil)

ABSTRACT: The Limeira Intrusion is an occurrence related to the Mesozoic continental flood basalts of Paraná Magmatic Province, cropping out at the eastern São Paulo state. It is made up of basic-intermediate rocks with a great diversity of structures, textures and modal compositions, related to specific evolutionary process in a magma chamber. Previous geological and stratigraphic evidence suggests it has a lopolithic-like geometry rather than a conventional sill-like, assumed for most regional similar occurrences. 2D forward modeling of magnetometric data, based on available aerial and newly obtained ground data integrated with in situ rock magnetic susceptibility measurements, were used to test such hypothesis. The results, even considering intrinsic drawbacks of magnetometric theoretical models, are compatible with the geologic-based presumed intrusion geometry. The determination of intrusive bodies geometry is quite significant to explain their different emplacement mechanisms. In the case of Limeira Intrusion, the results suggest that the emplacement mechanism may occur, apparently, by inner-accretion of fresh magmatic pulses in its central region. Importantly, the magmatic evolution of lopolith-like basic-intermediate intrusions may open the way for extensive differentiation processes, with strong implications to petrology and metallogenesis.摘要：莱梅拉侵入体（Limeira Intrusion）是巴拉那岩浆省（Paraná Magmatic Province）中生代大陆溢流玄武岩相关的地质产出体，出露于巴西圣保罗州东部。该侵入体由基性-中性岩构成，发育丰富多样的结构、构造与模式矿物组成，与岩浆房内的特定演化过程密切相关。此前的地质与地层学证据表明，其形态呈类岩盆状，而非多数区域同类地质体所公认的岩床状。研究基于现有航空磁测数据与新获取的地面实测数据，并结合原位岩石磁化率测量结果，开展磁测数据二维正演模拟，以验证上述假说。即便考虑到磁测理论模型的固有局限性，模拟结果仍与基于地质推测的侵入体形态相符。确定侵入体形态对解释其多样的侵位机制具有重要意义。就莱梅拉侵入体而言，研究结果显示其侵位机制可能为：新鲜岩浆脉冲在其中心区域发生内增生作用。值得注意的是，类岩盆状基性-中性侵入体的岩浆演化可能触发广泛的岩浆分异作用，这对岩石学与成矿作用研究均具有重要启示。