Magnetic fabric data on interactive syntectonic magmas of contrasting composition in composite dikes from south Brazil

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Highlights

  • The study reports a case of syntectonic contrasting magmas in a composite dike.

  • Magnetic fabric data reflects the flow of mafic and felsic magmas during emplacement.

  • Ferromagnetic mineral grainsize varies due to interaction of contrasting magmas.

  • Magmatic interaction in mafic borders contrasts with its absence in individual dikes.

  • The absence of magnetite crystals is interpreted as due to magmatic interaction.

Abstract

The composite dikes of the Zimbros Bay in the Porto Belo-Bombinhas region, southern Brazil, are emplaced in the late stages of post-collisional magmatism, at ca. 590 Ma as NE-striking, subvertical tabular bodies intrusive in basement orthogneisses. The composite dikes are formed by a felsic core containing a swarm of mafic microgranular enclaves, bordered on both sides by mafic material of irregular thickness, and individual mafic dikes parallel to the main intrusion. The felsic and mafic rocks contain a well-developed magmatic foliation with solid-state deformation near the contacts. Stretching lineation is horizontal to sub-horizontal and better developed in the felsic rocks. The absence of a well-developed linear fabric in the mafic rocks hinders investigation of their emplacement based on conventional structural data. In order to investigate/explore the relationship between contemporaneous mafic and felsic rocks, as well as to understand the emplacement of these intrusions in relation to the transcurrent tectonics, a study on the magnetic fabric was made using anisotropy of magnetic susceptibility (AMS) and rock magnetism techniques for composite and individual mafic dikes. Magnetic mineralogy of both dikes is predominantly controlled by ferromagnetic minerals (e.g. magnetite and Ti-poor tinanomagnetite), with little contribution of paramagnetic minerals. The rock magnetism study shows that the magnetite grains have varied sizes. Individual mafic dikes have small grain sizes, whilst in the mafic borders, sizes vary from small to large. Felsic cores have large grain sizes. SEM data show oxyexsolution textures in the felsic core samples and in samples of individual mafic dikes. Only Ilmenite is found in the mafic border samples, either as anhedral grains or as thin lamellae, interpreted to result from intense oxyexsolution of magnetite. Increasing oxyexsolution degrees observed in samples from individual mafic dikes to felsic cores and mafic borders suggest different degrees of interaction between the original magmas. The studied intrusions show normal magnetic fabrics, with subvertical magnetic foliation (k1-k2 plane) and subhorizontal (k1) lineation. Magnetic data are concordant with structural field measurements and indicate sub-horizontal magma flow during emplacement. The much larger volume and size of enclaves found in the southern portion of the dike is compatible with its northward directed emplacement. The obtained magnetic data are also in agreement with the syntectonic emplacement of the composite dikes relative to the transcurrent movement of the Major Gercino Shear Zone.

Introduction

Composite dikes are formed by different magmas, commonly of contrasting composition, emplaced along the same fissure. This type of intrusion is widely known and has been studied for many years (e.g. Judd, 1893; Harker, 1904; Gibson and Walker, 1963; Walker and Skelhom, 1966; Wiebe, 1973; Wiebe and Ulrich, 1997; Snyder et al., 1997; Katzir et al., 2007; Eriksson et al., 2011). According to Snyder et al. (1997), composite dikes may show either felsic core and mafic border, or mafic core with felsic border. They most frequently consist of mafic borders and almost always appear to be associated with larger, apparently contemporaneous intrusive complexes (Wiebe and Ulrich, 1997). Studying composite dikes is especially important because these intrusions can provide information about plutonic and subvolcanic plumbing systems, and may record the parallel evolution of contrasting magmas that may or may not come from different sources.

Studies of anisotropy of magnetic susceptibility (AMS) in granitic rocks have been used to characterize their magnetic fabric which may in turn be correlated to magma emplacement mechanisms and regional tectonics (e.g. Archanjo et al., 1994, 2002; Trindade et al., 1999; Bouchez, 2000; Salazar et al., 2008; Lyra et al., 2018). Studies of AMS have also been widely applied to dikes, especially mafic ones, in order to investigate magmatic flow geometry and emplacement in rocks whose texture is practically isotropic at macroscopic scale (e.g. Knight and Walker, 1988; Ernst, 1990; Rochette et al., 1991, 1992; Ernst and Baragar, 1992; Raposo and Ernesto, 1995; Raposo, 1997; Tauxe et al., 1998; Raposo and D’Agrella-Filho, 2000; Cañón-Tapia, 2004; Craddock et al., 2008; Wiegand et al., 2017; Raposo, 2017). Knight and Walker (1988), when comparing the magnetic fabric with the macroscopic surface dike lineaments of the Koolau Complex, show that if there is a good grouping of the maximum axis (k1) and if the minimum axis (k3) is perpendicular to the dike plane, then k1 may represent the magma flow direction. Therefore, the resulting correlation between field data and magnetic axes has made AMS a powerful technique to infer magma flow direction in dikes.

AMS is used in petrofabric and structural studies because of the correlation between rock mineral fabric and the magnetic susceptibility ellipsoid reported in the literature (e.g. Tarling and Hrouda, 1993; Borradaile and Henry, 1997; Féménias et al., 2004; Borradaile and Jackson, 2010; Ferré et al., 2014). Most of these studies are carried out either in granitic plutons or dikes of various compositions. However, AMS studies in composite dikes, especially those involving coeval magmas of contrasting composition, are scarce (e.g. Craddock et al., 2008; Eriksson et al., 2011), and no study is yet reported from syntectonic composite dikes in shear zones.

The composite dikes described in this paper are part of a dike swarm emplaced along an active transcurrent shear zone in southern Brazil. We have investigated the internal magnetic fabric of these intrusions by AMS and rock magnetism techniques in order to: (i) compare the magnetic planar fabric with the planar structural fabric of mafic and felsic rocks; (ii) determine the linear magnetic fabric of the mafic rocks, since their structural lineation is poorly developed, and to compare the linear magnetic fabric with the structural lineation in the felsic rocks; (iii) understand which minerals are responsible for the AMS in each case; and (iv) understand the relationship between the magnetic and structural fabric marked in the felsic and mafic rocks and discuss the relative timing of these intrusions.

Section snippets

Regional geology

The southern segment of the Mantiqueira Province (MP) (Almeida et al., 1981) comprises shield areas from Uruguay and southern Brazil (states of Rio Grande do Sul and Santa Catarina). This segment is part of a Neoproterozoic orogenic system known as Dom Feliciano Belt (DFB), limited by Archean and Paleoproterozoic rock sequences best exposed in Uruguay. In southern Brazil, the DFB comprises, from west to east, juvenile magmatic arc rocks, low to medium-grade metamorphic sequences partly covered

The studied composite dikes

The NE-trending intrusions where this work was carried out are the most complete examples of composite dikes found in the Zimbros Bay (Fig. 2a). They correspond to two discontinuous exposures (points 198 and 288, Fig. 2b), referred here as south and north segments, respectively (Fig. 2c). The central felsic portion has an average thickness of 3.5–4.5 m along the two segments, whilst the mafic borders range in thickness from 0.2 to 2 m. Parallel to the composite intrusion, 1−2 m thick individual

Sampling

Oriented samples were collected at 9 sites along composite dikes and individual mafic dikes near the composite intrusions in the south and north segments (points 198 and 288). At each site, at least six cores approximately 10 cm long and 2.5 cm in diameter were extracted using a gasoline-powered portable rock drill. The cylinder cores were oriented in situ with a Brunton-type compass coupled to a hollow cylindrical rod and, when possible, with a solar compass as well. In the laboratory, each

Magnetic fabric

In samples of the composite and SE mafic dikes from the south segment, k1, k2 and k3 axes are well grouped and show angular dispersion (e/z) below 16° (Fig. 4). Magnetic foliation is given by the plane formed by k1 and k2 axes which is perpendicular to k3. Magnetic lineation is given by the clustered k1 axes. The dominant magnetic strike of the dike plane is NE-SW, with local variation to NNE-SSW. Both magnetic lineation and foliation of the SE mafic dike and composite dike are concordant.

Discussion and interpretations

In both studied segments, the composite intrusion shows similar proportion of mafic borders and felsic cores, but the volume and size of mafic enclaves is much larger in the south segment (Fig. 3a and b). Despite such morphological differences, corresponding data for magnetic mineralogy and magnetic fabric of each core-border pair are obtained in both segments. The individual mafic dike SE of the composite intrusion outcrops also in the south segment (Fig. 2c), and the data for this segment are

Conclusions and implications

The magnetic fabric of the studied intrusion is controlled mainly by magnetite and/or Ti-poor titanomagnetite, with little contribution of paramagnetic minerals. The type of normal fabric in the studied composite dikes, identified as primary flow fabric, is confirmed by the average orientation of the maximum axis (k1) matching the lineation found commonly in the felsic cores of the composite dike and locally in the mafic rocks. This type of fabric reflects magma flow during dike emplacement,

CRediT authorship contribution statement

Camila Rocha Tomé: Writing - original draft, Visualization, Formal analysis, Conceptualization, Investigation, Methodology. Maria de Fátima Bitencourt: Writing - review & editing, Conceptualization, Project administration, Funding acquisition, Resources, Supervision, Investigation, Methodology. Maria Irene Bartolomeu Raposo: Methodology, Software, Supervision, Visualization. Jairo Francisco Savian: Writing - review & editing, Validation, Conceptualization, Methodology.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research received financial support from the Brazilian National Research Council (CNPq, Universal Program 471266/2010-8), and Rio Grande do Sul State Research Foundation (FAPERGS, 10/0045-6) granted to M.F. Bitencourt. The paper is part of Camila Rocha Tomé’s MSc thesis. We are grateful to Luis Alva-Valdivia and an anonymous reviewer for their careful reviews and suggestions that helped to significantly improve the manuscript.

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