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.

Geometry and stratigraphy of the Limeira tholeiitic intrusion, Paraná¡ magmatic province (SP-Brazil)

“…The intrusion was formerly being considered as a classical sill example (e.g., Soares, 1985;Faria, 2008). However, recent detailed geological, petrographic and ground-/arerial-magnetometric data indicate that it conforms better a lopolith-like intrusion, probably formed by a number of individualized magma pulses (Lino et al, 2018). These authors also draw attention to its compositional, structural and textural variability inwards, pointing in evidence a more complex magmatic evolutionary model.…”

“…The Limeira intrusion was emplaced within sandstones and rhythmic siltstones of the Itararé Subgroup (Permo-Carboniferous) and exhibits a great variety of rocks-types. Fresh rocks are exposed at Cavinatto and Basalto 4 quarries, where the first site exhibits the most complete rock exposure, given by a complete section from its upper contact until a depth of 110 meters (Lino et al, 2018). These exposures allows to examine atypical features rarely seen in other analogous intrusions in other analogous intrusions, such as (1) the preservation of structural relationship with host-rocks at the uppermost contact, (2) expanded compositional variation, including since basaltic to rhyolitic rocks, as well as a significant late hydrothermal input, and (3) a significant variety of magmatic structures and textures.…”

“…The distances between samples and the intrusion top were measured directly, while the distances from the intrusion bottom and samples downwards the central region were estimated from available magnetometric modeling (Lino et al, 2018). The obtained values for the τ, G, J, and cooling rates are summarized in Table 8.…”

“…231 to 108.367 days from the intrusion upper border to its central region, which results in contrasting cooling rates ranging from 1.733ºC/day to 0.004ºC/day, respectively. -9 8.15 x 10 -10 0.0389 IC-07b 2400 1.28 x 10 9 14774 6.10 x 10 -10 8.31 x 10 -10 0.0270 LIM-10 2800 1.74 x 10 9 20109 2.08 x 10 -10 1.17 x 10 -8 0.0199 TF-17 3000* 1.99 x 10 9 23084 3.74 x 10 -10 2.61 x 10 -10 0.0173 IC-04 3800 3.20 x 10 9 37037 2.28 x 10 -10 3.26 x 10 -10 0.0108 TF-24 4500* 4.49 x 10 9 51939 1.56 x 10 -10 1.36 x 10 -10 0.0077 IC-04b 4700 4.90 x 10 9 56658 1.42 x 10 -10 3.34 x 10 -10 0.0070 IC-04c 5200 5.99 x 10 9 69355 1.31 x 10 -10 1.72 x 10 -10 0.0057 IC-06 5500 6.70 x 10 9 77579 1.18 x 10 -10 1.04 x 10 -10 0.0051 TF-30 5900* 7.71 x 10 9 89284 1.29 x 10 -10 3.46 x 10 -11 0.0044 LIM-02 6200* 8.52 x 10 9 98594 1.38 x 10 -10 8.25 x 10 -11 0.0040 TF-36 6500 9.36 x 10 9 108367 1.50 x 10 -10 3.00 x 10 -11 0.0037 *Estimated values of edge distance, from geophysical modeling (Lino et al, 2018).…”

“…These exposures allows to examine atypical features, rarely seen in other analogous intrusions, such as (1) the preservation of structural relationship with host-rocks at the uppermost contact, 2expanded compositional variation, including since basaltic to rhyolitic rocks, as well as a significant late hydrothermal input, and (3) a significant variety of magmatic structures and textures. In this sense, Lino (2015) and Lino et al (2018) draw attention to its compositional, structural and textural variability inwards, pointing in evidence a more complex emplacementand magmatic-evolutionary model, rather than a single magma batch that evolved under the process of fractional crystallization, as it had been previously considered. Previous geological studies give some support for a lopolithic-like intrusion form (e.g., Lino, 2015)…”