DOI: 10.1590/2317-4889201820180099 ARTICLE 2D magnetometric modeling of a basic-intermediate intrusion geometry: geophysical and geological approaches applied to the Limeira intrusion, Paraná Magmatic Province (SP, Brazil) Lucas Martins Lino1*, Francisco de Assis Cavallaro1,2, Silvio Roberto de Farias Vlach1, Daniel Cunha Coelho1 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. KEYWORDS: Limeira intrusion; Intrusion geometry; 2D magnetometric modeling; Lopolith; Paraná Magmatic Province. INTRODUCTION Geophysical modeling is an important tool, which allows defining the geometry of several geologic occurrences. The formation and geometry of igneous intrusions in the The magnetometric method, despite being a relatively more crust are still a subject of debate in the literature due to their complex potential one, is a very efficient technique in the diverse forms of occurrence and to a great amount of tec- detection of geological occurrences with high magnetic sus- tonic environments in which they occur. However, recently, ceptibility contrast in relation to their environments (e.g., the formation and growth of igneous intrusions have been Kearey & Brooks 2002). associated to the amalgamation of several tabular-like intru- The magnetization intensity of a rock strongly depends on sions (Cruden 1998, Menand 2011). The simplicity, larger the modal volume and magnetic characteristics of its miner- distribution and relatively small volumes, as compared with als. For instance, magnetic anomalies produced by a 200 m3 batholiths, turn tabular intrusions an object of preferential body can be the same as those produced by a 2 km3 body, study when dealing with igneous bodies emplacement and depending on its overall magnetic mineral content, while thermal evolution. gravity anomalies are just related to density. This difference 1Instituto de Geociências, Universidade de São Paulo – USP, São Paulo (SP), Brazil. E-mail: [email protected], [email protected], [email protected], [email protected] 2Departamento de Engenharia Civil, Universidade de São Paulo – USP, São Paulo (SP), Brazil. *Corresponding author. Manuscript ID: 20170099. Received on: 07/18/2017. Approved on: 12/02/2017. 305 Brazilian Journal of Geology, 48(2): 305-315, June 2018 2D magnetometric modeling of Limeira intrusion may lead to misinterpretation when dealing with magnetic The majority of lavas are tholeiitic basalts, associated with anomalies; however, there are ways to circumvent this prob- minor andesites, riodacites and rhyolites, these later repre- lem, based on well-designed previous geological models and senting only ca. 3% of the lava flows and occurring mainly the knowledge of the targets magnetic properties. in the southern Paraná Basin area (Comin-Chiaramonti The continental crust presents internal stratification et al. 1988, Nardy et al. 2002). These lava flows comprise associated with variations of the constituting rocks physi- low-Ti geochemical types, which crop out in the base, while cal properties, as so far indicated by geophysical data and high-Ti varieties appear at the intermediate and the upper- theoretical models based on indirect observations. As well most levels of lava sequences (Peate et al. 1992). known, the crust is constituted, at least, by two main lay- The origin of the PMP is certainly related to the opening ers, a deep one composed of mafic rocks (lower crust), and of the South Atlantic Ocean, through the process of litho- a shallow one of granitic composition (upper crust) (e.g., spheric mantle melting under the influence of the Tristan da Rudnick & Fountain 1995). This type of stratification could Cunha Plume (Peate et al. 1990, 1999, Hawkesworth et al. not be explained by considering only the emplacement of 1992, Turner & Hawkesworth 1995, Marques et al. 1999) vertical intrusions. In fact, available seismic data reveal or, alternatively, to heat accumulation under the Pangea the occurrence of intrusive complexes made up of tabular Supercontinent (Hartmann 2014). intrusions at the base of the continental crust, which could In the eastern area of the São Paulo State, the high-Ti be likely mechanisms for the formation of different crustal PMP magmatism is characterized by tabular-shaped intru- layers (Franke 1992). sions, such as dikes, sills, and laccoliths (Zalán et al. 1986). The present work aims to provide geological and geophysical Most dikes intrude basement rocks and form expressive information about the geometry of Limeira Intrusion, a tabu- dike swarms (Peate et al. 1992) while the emplacement of lar-like occurrence related to the Mesozoic Paraná Magmatic sub-horizontal tabular intrusions occur mainly in sedimen- Province (PMP), in southern Brazil. Although the province has tary host discontinuities; estimated total thickness of such been extensively studied concerning its stratigraphy, petrogra- intrusions vary in general between 2 to 200 m, but may phy, elemental and isotope geochemistry, and geochronology reach up to 1,000 m (Marques & Ernesto 2004). (e.g., Bellieni et al. 1984, Ernesto & Pacca 1988, Piccirillo & Melfi 1988, Peate 1997, Gallagher & Hawkesworth 1994, Limeira Intrusion Nardy et al. 2003, and references therein), which allowed to The Limeira Intrusion is one among several PMP intru- draw several petrogenetic and tectonic models (e.g., Peate et al. sions that crop out in the São Paulo State central-eastern 1992, Turner & Hawkesworth 1995, Peate & Hawkesworth area (Fig. 1). It intrudes Permo-Carboniferous sedimentary 1996, Peate 1997), there is still few detailed geological and rocks, mainly sandstones and rhythmic siltstones, of the stratigraphic research emphasizing the physical and geomet- Itararé Subgroup (Faria 2008, Lino 2015). The best fresh- ric aspects of individual intrusions or lava flows. rock expositions are found in the so-called Cavinatto, own- Previous semi-detailed geophysical studies by Seixas et al. er’s family name, and Basalto 4 quarries. Soares (1985) were (2015) showed that the Limeira Intrusion has a more limited the first to recognize a huge intrusion at this region, named lateral spreading than previously considered. In addition, as the Limeira-Cosmópolis Sill. According to him, it would previous geological studies give some support for a lopolith- correspond to a tabular shaped intrusion, with almost 60 km ic-like intrusion form (e.g., Lino 2015). Herein we present of horizontal extension and 280 m of thickness, as suggested the results obtained through a more detailed ground mag- from surface evidences and available drill holes. netometric survey, coupled with geological and rock mag- Several recent works, emphasizing petrological, geochem- netic susceptibility data. ical, stratigraphical and geophysical aspects, were conducted by Faria (2008), Oliveira & Dantas (2008), Lino (2015) and Seixas et al. (2015), among others, and highlighted some GEOLOGICAL FRAMEWORK features relatively uncommon to other intrusion analogs. One of us, Silvio R. F. Vlach, had pointed out the occurrence The PMP, southern Brazil, represents the most important of typical stratiform-like rocks at the medium-to deep-out- Mesozoic continental flood basalt expression, constituting crops at the Cavinatto quarry, given by modal and textural the Serra Geral Formation, one of the uppermost formations centimetric- to decimetric-thick igneous layering. of the Paraná Basin (Bryan & Ernst 2008, Milani 2004). The Limeira rock types include since aphanitic basalts Over 562,000 km3 of lavas (Frank et al. 2009) were erupted and diabases, at the intrusion borders, coarse-grained mon- in a time span between ca. 133 and 119 Ma (Brückmann zodioritic to very coarse-grained quartz-monzodioritic rocks, et al. 2014). inwards. Rhyolites/rhyodacites occur as late dikes, with 306 Brazilian Journal of Geology, 48(2): 305-315, June 2018 Lucas Martins Lino et al. mm- to cm-sized thickness, cutting across monzodiorites site, and proposed that it becomes thinner to southeast. and diabases, and as cm-sized ocelli in the upper diabases. Based on ground magnetometric survey and analytical signal The main rocks are composed of plagioclase (An65-30), amplitude profiles, Seixas et al. (2015) suggest that there are clinopyroxene (augite