Magnetic fabrics of the neoproterozoic piquiri syenite massif (Southernmost Brazil): Implications for 3D geometry and emplacement

Journal Pre-proof Magnetic Fabrics of the Neoproterozoic Piquiri Syenite Massif (Southernmost Brazil): implications for 3D geometry and emplacement Samuel Sbaraini (Conceptualization) (Methodology) (Software) (Investigation) (Formal analysis) (Writing - original draft), M. Irene B. Raposo (Methodology) (Software) (Validation) (Formal analysis) (Investigation) (Resources) (Data curation) (Writing - review and ´ editing), Maria de Fatima Bitencourt (Conceptualization) (Writing review and editing) (Supervision) (Project administration) (Funding acquisition), Camila Rocha Tome´ (Writing - review and editing)

PII:

S0264-3707(19)30178-4

DOI:

https://doi.org/10.1016/j.jog.2019.101691

Reference:

GEOD 101691

To appear in:

Journal of Geodynamics

Received Date:

2 July 2019

Revised Date:

27 November 2019

Accepted Date:

3 December 2019

´ Please cite this article as: Sbaraini S, Raposo MIB, de Fatima Bitencourt M, Tome´ CR, Magnetic Fabrics of the Neoproterozoic Piquiri Syenite Massif (Southernmost Brazil): implications for 3D geometry and emplacement, Journal of Geodynamics (2019), doi: https://doi.org/10.1016/j.jog.2019.101691

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Magnetic Fabrics of the Neoproterozoic Piquiri Syenite Massif (Southernmost Brazil): implications for 3D geometry and emplacement Samuel Sbarainia*, M. Irene B. Raposob, Maria de Fátima Bitencourta, Camila Rocha Toméa a Programa

de Pós Graduação em Geociências, Universidade Federal do Rio Grande do

Sul, Av. Bento Gonçalves, 9500, 91501-970, Porto Alegre/RS, Brazil b

Instituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, 05508-080,

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São Paulo/SP, Brazil * Corresponding author. E-mail addresses: [email protected] (S. Sbaraini),

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[email protected] (C. R. Tomé)

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[email protected] (M. I. B. Raposo), [email protected] (M. F. Bitencourt),

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Abstract

The study of magnetic fabrics and rock magnetic properties, together with geological and

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structural mapping, was carried out in a syenite pluton to investigate its shape and emplacement history. The Piquiri Syenite Massif (PSM) is an alkaline pluton which exhibits

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S >> L magmatic fabric and is interpreted to be part of the last Neoproterozoic postcollisional magmatic episodes in southernmost Brazil. Thermomagnetic curves, hysteresis

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data and coercivity spectra obtained from representative samples of different facies in the massif reveal that magnetic susceptibility is dominated by ferromagnetic minerals, especially magnetite. Magnetic fabric data were determined by using Anisotropy of Magnetic Susceptibility (AMS) and Anisotropy of Anhysteretic Remanence (AARM). Both fabrics are coaxial, and the parallelism of AMS and AARM tensors in more than 84% of the sampled sites rules out the possibility of significant effects of Single Domain (SD) crystals.

The magnetic foliation is concordant with the magmatic foliation field measurements, both parallel to pluton contacts, with high, inward dip angles. The magnetic lineation shows distinct but related behaviour from one facies to another. It is dominantly subvertical in the marginal facies rocks and plunges at moderate to shallow angles in the main facies. It is sub-horizontal in the quartz-syenites and plunges at shallow angles in the granitic rocks. Oxidizing conditions determined from the study of magnetic mineralogy leads to challenge former interpretation of in situ differentiation and crystallization and points to the multiintrusive character of the pluton. Field relations such as fragments of marginal facies rocks

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found within the main facies rocks, which are in turn intruded by quartz-syenites, together with the general absence of contact metamorphism except near the marginal facies, lead to interpret that a sequence of magmatic pulses have built up the pluton. Thus, a first

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magmatic pulse may have heated the host rocks and resulted in the marginal facies which

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was followed by the next pulses to form the main facies and the quartz-rich varieties, therefore constructing the pluton from outside inwards.

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Keywords: magnetic fabric, AMS, rock magnetism, magma flow, emplacement fabrics,

Introduction

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syenite.

Fabric patterns in plutons may result from internal magma chamber processes such

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as convection, magma surges, dike injections, and crystal settling, or may be related to

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regional deformation, or can involve a combination of these processes (Paterson et al., 1998). Comparing oriented structures from host rocks with internal magnetic and deformational fabrics in plutons allows to determine whether the pluton fabrics reflect the effect of regional tectonic strain or of internal chamber processes. Additionally, such study may lead to the unravelling of crystallization conditions and magmatic emplacement as one or more pulses, at constant or variable physical conditions.

The main factor responsible for the generation of primary fabric in igneous rocks is the shape alignment of crystals such as biotite, olivine, pyroxene, plagioclase and maybe (titano) magnetite and (titano)hematite during the magma flow. However, it is also possible that another mechanism in which late crystallizing ferromagnetic minerals fill in the gaps left between earlier formed and aligned plagioclase, pyroxene and olivine crystals takes place. Therefore, these minerals are able to acquire a preferred orientation even if their growth postdates magma flow (Hargraves et al., 1991). AMS has been extensively used as a tool to reconstruct the emplacement history of

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igneous rocks (see Tarling and Hrouda, 1993; Borradaile and Henry, 1997; Special Issues of Tectonophysics, 2006, 2009, 2014, for detailed review). As pointed out by Bouchez

(1997), AMS measurement has increased in the last decades because of its sensitivity and

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speed in determining rock fabrics even in rocks that are visually isotropic, and its use is not

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restricted to iron oxide bearing rocks (Bouchez, 1997). Therefore, the AMS technique is widespread for both directional and semi-quantitative purposes (Tarling and Hrouda, 1993;

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Borradaile and Henry, 1997), particularly where standard petrofabric techniques are inadequate or inefficient, as is the studied case.

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The main goal of this paper is to apply both AMS and AARM techniques to the Piquiri Syenite Massif, to determine the internal fabrics of the four facies of the massif,

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mainly the lineation that is not readily visible in the field, to provide information on their mode of emplacement. In order to better understand the magnetic fabrics, we have also

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performed an extensive rock magnetism study. 2. Geological Setting The Neoproterozoic Dom Feliciano Belt (DFB) in southern Brazil and Uruguay (Fig. 1A) represents mainly the record of collisional events related to the Brasiliano/Pan-African Orogenic Cycle. It preserves several magmatic records related to previous oceanic crust subduction and to collisional events involving the Rio de La Plata, Congo and Kalahari

cratons. The central part of this orogenic belt is exposed in the Sul-rio-grandense Shield (SrgS – Fig. 1A), southernmost Brazil, mostly Neoproterozoic rock associations bound to the west by Paleoproterozoic metamorphic basement, as pointed out by Hartmann et al. (1999), among many other authors. The eastern part of this shield area features mostly post-collisional Neoproterozoic granitoids found along the transpressive Southern Brazilian Shear Belt (SBSB – Bitencourt and Nardi, 2000). The SBSB is defined as a lithospheric-scale structure composed of several km-wide, anastomosing shear zones of dominant transcurrent kinematic formed in

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the late stages of the Neoproterozoic collisional history. The granitoids are found within this megastructure along a NE-trending, 800 km-long and 150 km-wide belt which takes local names as Pelotas Batholith (Rio Grande do Sul state, Brazil), Florianópolis Batholith

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(Santa Catarina state, Brazil) and Aiguá Batholith (Uruguay).

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Most of the shear zones are subvertical, and they are interpreted by Bitencourt and Nardi (2000) to have played an important part in the reactivation of mantle sources

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previously modified by subduction. They have also favoured crustal melting and provided ascent and emplacement channels for a large volume of magma. Mafic igneous rocks are

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also found along the granitic belt, represented by coeval mafic enclaves, synplutonic dikes and diorite intrusions. The study area (Fig. 1B) is part of a structural block limited by two

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shear zones related to the SBSB. Within the limits of this block, several granitic intrusions are found, some of them in association with syenite bodies, which are interpreted as late-

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to post-collisional, A-type magmas (e.g. Nardi et al. 2008).

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Figure 1 – (A) Outline of the main geological and structural features of the Dom Feliciano Belt and its basement in southern Brazil and Uruguay; rectangle indicates location of figure 1B. Notice the elongate red area where Neoproterozoic post-collisional magmatism is concentrated, referred in southernmost Brazil as Pelotas Batholith. Modified from Bitencourt and Nardi (2000). (B) Simplified geological map of the studied region featuring the Piquiri Syenite Massif and its host rocks.

3. The Piquiri Syenite Massif The Piquiri Syenite Massif (PSM) is a slightly elliptical body in map view, with major axis trending 330° and a superficial area of approximately 140 km² (Fig. 1B). The original geometry of its southeastern part is modified by later intrusions (Fig. 2). According to Nardi et al. (2008), the PSM results from the mingling of two mantle-derived magmas emplaced in a post-collisional environment. Additional igneous processes like fractionation and assimilation of wall rocks are mentioned by Stabel (2000). The age of 611 ± 3 Ma (Pb-Pb

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zircon) is reported for the massif (Philipp et al., 2002). Based on textural and compositional criteria, as well as on geometric distribution of varieties, the PSM is presently divided into four subunits: (i) the marginal facies, (ii) the main facies, (iii) the quartz-syenite facies, and (iv) the granitic facies.

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Rocks of the marginal facies form the outer parts of the massif, found mainly in its

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north-northeastern and southwestern part (Fig. 2). They are also found as apophyses in the host-rocks at the northeastern and eastern part of the massif. The rock types of the

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marginal facies are foliated to massive, fine- to medium-grained equigranular syenites and alkali-feldspar syenites, with colour index M’15 to 30. Mafic minerals are pyroxene,

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amphibole and biotite, together with Fe and Ti oxides. Xenoliths of gneissic country rocks are common near the contacts, as well as fragments of very fine grained syenites

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interpreted to come from a disrupted chilled margin. Most apophyses have irregular shape, either concordant or at high angle to the host rock foliation, but internal foliation is usually

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parallel to intrusion limits. At some places, these apophyses are seen to be intrusive. In some locations of these apophyses, it is possible to verify interactions between the main and the marginal facies (Fig. 2).

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Figure 2 – Geological map of the Piquiri Syenite Massif showing sampled sites and structural foliations (magmatic and metamorphic) measured in the field. The main facies, which makes up most of the massif area, comprises medium- to coarse-grained, equigranular alkali-feldspar syenites with colour index M’ 5 to 15 and a

strong planar fabric (Fig. 3A). Mafic mineral concentrations are common, and eventually give rise to schlieren when progressively disrupted by the magmatic flow. At some outcrops, a xenolith-rich, fine-grained rock is found as fragments of variable size in rocks of this facies. These fragments are interpreted to result from dismembering and progressive assimilation of the marginal facies rocks as the magma forming the main facies is sheared (Fig. 3B). Elongate mafic cumulate autoliths and mafic microgranular enclaves are also common in the syenites of the main facies. The innermost part of the massif features quartz-syenites arranged in a boomerang-

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shaped area (Fig. 2). These are medium- to coarse-grained rocks of very low mafic

mineral content (M' 2-8). The outcrops are generally very much weathered, except for a few well-preserved ones. Fragments of the marginal facies varieties are found in the

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quartz-syenites, although much less common that observed in the rocks of the main facies

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(Fig. 3C). A small volume of granites is found in the very center of the massif. They are medium-to coarse-grained rocks with slightly higher colour index (M' 10) when compared

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to the quartz-syenites. Quartz-syenites and granites occasionally show layered structure marked by concentration of mafic minerals along irregular layers which attests to their

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segregational character.

Along the entire massif, the magmatic foliation is marked by shape alignment of K-

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feldspar crystals and enhanced by the alignment of mafic mineral aggregates resulting from co-mingling and flow-segregation, as previously described by Nardi et al. (2007). The

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geometrical distribution of this planar fabric conforms to the outer contacts of the massif, with medium to high, inward dip values. No linear fabric is observed in these rocks in the field. A sub-horizontal magmatic lineation identified by means of single outcrop quantitative analysis is reported by Peternell et al. (2011). At the eastern margin apophyses (Fig. 2), rocks of the main facies are intrusive in those of the marginal facies. At the inner portions of the massif, quartz-syenites are

observed to be intrusive in rocks of the main facies forming tabular bodies that show either sharp or diffuse contacts, suggestive of low-temperature differences between the varieties

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(Fig. 3D e 3E).

Figure 3 – Structural relations and textural variations of the Piquiri Syenite Massif facies. (A) Main facies. (B) Quartz-syenite facies with partially incorporated mafic microgranular enclave. (C) Remnants of marginal facies rocks containing autoliths, xenoliths and mafic microgranular enclaves partially assimilated by the main facies magma. (D) and (E) Photographs and drawings of the intrusive relations of quartz-syenite facies rocks (qsf) on the main facies rocks (mf). The opaque mineralogy is widely dominated by magnetite throughout the massif, except for the rocks of the marginal facies. This variety shows smaller magnetite crystals, as compared to the ones found in the other varieties, as well as hematite-mantled pyrite

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crystals (Fig. 4A), and ilmenite, titanomagnetite and ilmeno-rutile in oxi-exsolution textures (Fig. 4B). Alkali-feldspar syenites of the main facies, as well as quartz-syenites and

granites, have only magnetite (Figs. 4C e 4D), which shows zircon inclusions and tends to

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more pure endmembers towards the intrusion center.

Figure 4 – Scanning Electron Microscope images. (A) Pyrite rimmed by hematite, marginal facies. (B) Ilmenite, titano-magnetite and ilmeno-rutile in the same crystal as oxi-

exsolution texture, marginal facies. (C) and (D) Magnetite grains, commonly showing zircon inclusions, representative of the main facies and quartz-syenite facies, respectively. The most voluminous host rocks are high-grade gneisses and syntectonic syenites with NNW-striking main foliation. The western part of the massif is in contact with Ediacaran sedimentary rocks of the Camaquã Basin (Fig. 2). The southern part of the massif is intruded by the Encruzilhada Granite, dated at 594 ± 5 Ma (U-Pb zircon) by Babinski et al. (1997). Despite its epizonal character, no significant contact metamorphism aureole is found in the host rocks, but local contact metamorphism effects caused by rocks

(2007).

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4. Magnetic measurements: methods and sampling

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of the marginal facies are reported at the northern and southwestern border by Martil

4.1 Anisotropy of Low-Field Magnetic Susceptibility (AMS)

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AMS is a tensor which relates the intensity of applied field (H) to the acquired

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magnetization (M) of a material through the equation: Mi = KijHj, with the proportionality Kij being a symmetrical second-rank tensor referred to as the susceptibility tensor. This tensor

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is expressed by its principal eigenvalues (susceptibility magnitudes) and eigenvectors Kmax>Kint>Kmin (their orientations) representing the maximum, intermediate, and minimum

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axes of susceptibility, respectively. AMS describes the variation of magnetic susceptibility with direction within a material, and represents the contribution of all rock-forming minerals

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(i.e. dia-, para- and ferromagnetic). The Kmax axis represents the magnetic lineation while Kmin is the pole to the magnetic foliation (the plane formed by Kmax and Kint axes). In rocks where K is carried by either Fe-bearing silicate paramagnetic matrix minerals or (titano) hematite or pyrrhotite the AMS is due to the preferred crystallographic orientation of these minerals (magnetocrystalline anisotropy). On the other hand, in rocks where K is carried by ferrimagnetic minerals such as Ti-poor titanomagnetite the origin of AMS is related to

grain shape (shape-anisotropy), in which Kmax is parallel to the long axis of a particle within the rock. 4.2 Rock Magnetism An extensive rock magnetism study was performed to better define the magnetic carriers and their relative contribution to both the mean magnetic susceptibility and the remanence. Rock magnetism properties were investigated through several diagnostic rock-magnetic experiments such as: measurement of continuous low-field thermomagnetic

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curves (K-T curves, susceptibility versus low and high temperature), acquisition of remanent coercivity spectra determined by alternating field (AF) tumbling demagnetization of the natural remanent magnetization (NRM) and partial anhysteretic remanent

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magnetization acquisition curves (pARM), isothermal remanent magnetization (IRM) acquisition curves, and hysteresis loops.

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The K-T curves from low-temperature (from about –195°C to room temperature) susceptibility were recorded using a CS3-L apparatus coupled to a Kappabridge (KLY-4S)

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instrument (Agico, Czech Republic). Corresponding high K-T curves were carried out in an Ar atmosphere using a CS-3 apparatus coupled to the KLY-4S. Specimens were

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progressively heated up to 700°C and subsequently cooled to room temperature. These

Massif.

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experiments were performed on nine samples from different facies of the Piquiri Syenite

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The behavior of MRN was studied by AF tumbling demagnetization in steps of 5 or 10 mT up to 100 mT using Molspin (Molspin, NewCastle-Upon-Tyne, UK) alternating field demagnetizer. All remanences were measured using a JR5A magnetometer (Agico, Czech Republic). Acquisition of remanent coercivity spectra by pARM followed the procedure given by Jackson et al. (1988), which consists of applying a steady field (DC field) in between two chosen values (AF window) of a decaying AF peak (H) while the rest of the

assemblage is demagnetized from a peak field. A Molspin alternating field demagnetizer was employed as source of the alternating magnetic field. Superimposition of a steady field (DC field) was attained by a small coil (home-made) inside and coaxial to the demagnetizer and controlled by a Molspin apparatus. The specimens were exposed to an AF peak of 95 mT and DC field of 0.1 mT with an AF window width of 10 mT. AF demagnetization at 100 mT was applied after each pARM acquisition. This experiment was performed in the same specimens studied by AF-NRM demagnetization. After that, the specimens were subjected to IRM acquisition curves in progressively

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increasing magnetizing fields using a pulse magnetometer (MMPM9, Magnetic

Measurements). Hysteresis measurements at room temperature were performed using a vibrating sample magnetometer (VSM-Nuvo, Molspin, Newcastle-upon-Tyne, UK) in fields

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up to 1 T.

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4.3 Anisotropy of Anhysteretic Remanent Magnetization (AARM)

The AARM is exclusively carried by remanence-bearing minerals. This anisotropy is

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determined from the intensity of an artificial magnetic anhysteretic remanence acquired when a magnetic field is applied along different directions through the sample. AARM has

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distinct advantages because it precludes the effect of inverse AMS fabric due to singledomain titanomagnetite or magnetite (Stephenson et al., 1986). The AARM tensor is also

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a symmetrical second-rank tensor expressed by its principal eigenvectors AARMmax > AARMint > AARMmin representing the maximum, intermediate, and minimum axes of

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remanence, respectively, in which AARMmax corresponds to the magnetic lineation, and AARMmin is the pole to the magnetic foliation (AARMmax-AARMint plane). The AARM was determined at 26 sites in different facies of the Piquiri Syenite

Massif. The procedure consists of cycles of anhysteretic remanence acquisition, measurement, and demagnetization along different positions for each specimen. AARM was determined using seven position measurement scheme. For all sites the AARM tensor

was determined by iteratively magnetizing the specimen in an AF peak of 60 mT with a DC field of 0.16 mT in the desired orientation, measuring the resulting remanence, and AF tumble-demagnetizing in 100 mT before proceeding to the next step. Before AARM determinations, the samples were demagnetized by AF tumbling at 150 mT to establish the base level. 4.4 Sampling Oriented samples from 46 widely-distributed sites throughout the Piquiri Syenite

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Massif were collected along roadcuts and farms mostly northwest of the Encruzilhada do Sul town (Figs. 1B and 2) from outcrops which are certainly in situ. Sample orientations were determined using both magnetic and sun compasses, whenever possible. At least

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10-13 cores for magnetic measurements were collected from each site using a gasoline-

5. Magnetic measurements: results

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5.1 AMS directional and scalar data

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powered rock drill.

AMS measurements were performed on 2.5 cm x 2.2 cm cylindrical specimens cut

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from the oriented cores at each sampled site. At least three specimens from each core were cut. More than one thousand specimens were measured using the KLY-4S

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Kappabridge (Agico, Czech Republic). The scalar and directional AMS data are presented in Table 1.The eigenvectors within the sites are generally well grouped with very low

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values for the 95% confidence regions given by Jelinek´s (1977) statistics (Fig. 5). The AMS directional data show a well-defined magnetic foliation at all sampled

sites, marked by grouping of the minimum magnetic axis (Kmin). The magnetic lineation is also well defined, marked by grouping of the maximum magnetic axis (Kmax) (Fig. 5, Table 1).

Figure 5 – Representative examples of AMS fabric found in the Piquiri Syenite Massif. Squares are maximum susceptibility (Kmax), triangles are intermediate susceptibility (Kint),

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and circles are minimum susceptibility (Kmin). Dashed line ellipses = 95% confidence. Data plotted in lower hemisphere, equal area diagrams.

The magnetic foliation is either vertical or steeply-dipping inwards (Fig. 6A) in most

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cases, especially in the marginal facies rocks. Despite the dominance of steeply-dipping foliations in the other varieties, medium- and gently-dipping magnetic foliations are also

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found. The magnetic lineation patterns vary according to the lithological facies (Fig. 6B).

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Marginal facies magnetic lineations plunge steeply, but tend to lower angles in the apophyses (maximum 15°). In the intrusion main facies, magnetic lineations plunge at

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moderate to low angles, except for its western portion, near tectonic contacts, where higher plunge values are found. The lowest (usually below 15°) and most regular plunge values are found in the quartz-syenite facies, whilst plunge angles may reach 30° in the

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granitic varieties.

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Pr

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Figure 6 – AMS fabric pattern for the Piquiri Syenite Massif. (A) Magnetic foliation (normal to Kmin). (B) Magnetic lineation (parallel to Kmax). The mean magnetic susceptibility, expressed by Km = (Kmax+Kint+Kmin)/3 in SI units is higher than 10-3 SI (Fig. 7A) ranging from 0.35- 61.30 x 10-3 with an average of 15.08 x 10-3 (Table 1). Unfortunately, it is not possible to establish a relation between the magnetic susceptibility and the faciology of the massif. On the other hand, samples with low susceptibility values, around 0.5 x 10-3, are restricted to the syenite apophyses in country rocks east of the main intrusion (Fig. 2).

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The anisotropy degree P = Kmax/Kmin varies in the studied samples between 1.014 and 1.330 (average of 1.110 ~ 11%, Table 1). The highest range of P is found in rocks of the marginal facies, however, there is a predominance of P values > 1.12. On the other

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hand, P values are in the main facies are below 1.18. Constant P values of 1.05 are found

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in the quartz-syenite and granite facies (Fig. 7B).

In general, there is no clear relation between Km and P parameters for the studied

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massif (Fig. 7A). However, there is a certain relationship between the values of Km and P for the marginal and main facies of the pluton suggesting some compositional dependence

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of these parameters. Jelinek´s (1981) ellipsoid shape parameter expressed by T = [2ln(Kint/Kmin)/ ln(Kmax/Kmin)]-1 indicates oblate (T > 0) to triaxial (T ~ 0) shapes for most of

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the rocks (Table 1, Fig. 7B).

S it e

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Localizat ion UTM coordina

Scalar data

K m

S D

P

Direction of principal axes

S D

T

S D

Km ax

e1/ e2

Kin t

e1/ e2

Kmi n

e1/ e2

te

1, 0 7 6

0. 0 1 5

3 5

33 06 18

66 43 62 1

1 3. 9 0

0. 0 0 3

1, 1 7 7

33 07 39

66 42 69 5

3 1. 1 0

0. 0 0 5

3 5

33 12 09

66 42 01 8

1 9. 9 0

2 4

33 16 26

66 40 87 6

2 0

33 17 03

3 5

33 17 00

25 8/ 20

15. 8/8. 3

10 1/ 68

16. 5/7. 5

35 1/ 8

11. 1/5. 3

mar gina l

0. 1 9 1

11 2/ 75

4.5/ 3.9

22 9/ 7

23. 7/4. 0

32 1/ 13

23. 6/3. 8

mar gina l

1, 1 6 6

0. 0 4 1

0. 2 9 3

0. 1 5 6

35 6/ 79

13. 7/8. 5

10 0/ 3

18. 8/1 0.5

19 1/ 11

17. 7/9. 3

mai n

0. 0 0 7

1, 1 2 5

0. 0 2 4

0. 5 0 1

0. 1 6 6

44 /2 9

11. 0/1 0.4

22 5/ 61

11. 0/7. 9

13 4/ 0

10. 9/7. 8

mai n

1. 0 4

0. 0 0 0

1, 0 1 4

0. 0 0 6

0. 5 4 6

0. 2 6 9

32 1/ 9

16 4/ 81

13. 8/1 1.3

mai n

66 41 22 8

2 3. 6 0

0. 0 0 4

1, 1 2 7

0. 0 1 0

0. 5 6 3

0. 1 4 0

66 41 22 5

6 1. 3 0

0. 0 2 1

1, 1 0 4

0. 0 2 9

0. 3 9 6

1, 1 2 2

0. 0 3 1

41. 7/1 1.3

52 /4

41. 7/1 3.8

22 3/ 27

12. 6/4. 7

94 /5 0

13. 7/5. 3

32 8/ 29

7.8/ 4.7

mai n

0. 3 4 5

24 7/ 9

17. 9/6. 4

14 2/ 59

10. 9/5. 5

34 2/ 30

15. 8/5. 5

mai n

0. 9 6 0

0. 1 4 3

10 1/ 72

65. 9/2. 2

22 4/ 10

65. 9/5. 9

31 7/ 15

6.0/ 1.9

mai n

0. 7 8 7 0. 0 0 8 -

0. 1 6 6

52 /2 7

35. 4/8. 5

23 0/ 63

35. 96. 0

32 2/ 1

9.7/ 8.5

mai n

0. 1 9 2 0.

21 2/ 15 12

15. 8/5. 5 11.

10 6/ 48 22

12. 9/1 1.4 11.

31 4/ 39 33

12. 8/1 0.9 12.

qz sye nitic qz

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2 7

0. 2 2 6

0. 0 3 3

0. 4 3 0 0. 6 1 5

66 38 89 3

8. 8 1

0. 0 0 4

3 0

32 71 54

2 1

32 79 74

66 38 97 8

8. 3 6

0. 0 0 2

1, 0 8 0

0. 0 1 3

1 7 2

32 99 06 33

66 39 21 7 66

3. 6 8 1

0. 0 0 3 0.

1, 0 5 0 1,

0. 0 2 4 0.

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0. 0 0 2

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1 9. 2 0

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66 44 17 9

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1 7

33 14 45

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P Q 0 1 P Q 0 2 P Q 0 3 P Q 0 4 P Q 0 5 P Q 0 6 P Q 0 7 P Q 0 8 P Q 0 9 P Q 1 0 P

(1 03 )

0 1 7

3 2 5

0. 0 2 5

0. 0 5 3 0. 1 4 2

2 3

33 53 80

66 38 21 3

1 9. 6 4

0. 0 0 5

1, 0 6 6

2 0

33 63 30

66 36 44 4

5. 1 1

0. 0 0 4

1, 0 5 5

0. 0 1 6

2 1

33 64 44

66 35 33 6

3 0. 7 0

0. 0 0 3

1, 1 6 7

32 52 23

66 34 28 9

2 0. 2 0

0. 0 0 4

32 54 56

66 33 52 1

2 1

3 1

32 93 08

66 30 60 4

2 6

33 15 98

66 38 11 2

33 25 30

9

5/ 6

8/9. 1

7/ 63

8/9. 1

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9/1 0.5

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0. 2 9 1

30 0/ 15

13. 0/8. 4

15 7/ 71

16. 9/7. 0

33 /1 1

17. 0/1 1.9

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0. 2 9 5

0. 2 1 3

11 7/ 12

13. 2/3. 6

24 /1 2

13. 1/7. 2

25 2/ 73

7.2/ 3.8

mai n

0. 0 3 5

0. 8 6 7

0. 1 4 0

18 3/ 18

38. 4/8. 4

29 1/ 44

38. 4/5. 6

77 /4 0

8.7/ 5.4

mai n

1, 0 8 7

0. 0 2 0

0. 3 0 5

0. 2 3 2

18 2/ 35

16. 9/6. 3

57 /3 9

18. 2/9. 0

29 7/ 31

11. 7/5. 7

mai n

7. 3 2

0. 0 0 1

1, 1 2 3

0. 0 1 1

0. 5 3 9

0. 1 0 2

86 /4 3

7.7/ 5.1

18 3/ 7

11. 2/6. 2

28 0/ 46

10. 2/5. 2

mai n

2 6. 5 0

0. 0 0 2

1, 3 3 0

14 4/ 10

14. 2/6. 7

23 6/ 9

12. 6/2. 1

mar gina l

0. 6 2 9

na

0. 0 2 1

2 2. 0 0

0. 0 0 5

1, 0 4 5

0. 0 1 0

66 38 27 3

7. 5 3

0. 0 0 2

1, 0 5 7

0. 0 1 2

1 6. 5 0

0. 0 0 3

1, 0 4 7

0. 0 1 5

0. 8 0 6 0. 0 5 9 0. 0 1 0

1 9. 3 0

0. 0 0 4

1, 1 4 0

0. 0 2 7

0. 8 2 0

2 8

33 41 08

66 38 98 2

3 1

33 39 35

66 43 40 5

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0 6 9

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48 24

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8

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Q 1 1 P Q 1 2 P Q 1 3 P Q 1 4 P Q 1 5 P Q 1 6 P Q 1 7 P Q 1 8 P Q 1 9 P Q 2 0 P Q 2 1

0. 0 6 5

8/ 77

10. 4/2. 8

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19 5/ 28

51. 4/1 6.5

28 /6 1

51. 3/1 1.1

28 8/ 5

18. 9/9. 8

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0. 3 5 9

21 7/ 27

10. 9/6. 0

11 9/ 17

12. 2/1 0.2

0/ 58

11. 9/5. 8

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0. 2 8 8

26 1/ 0

11. 6/8. 6

35 1/ 46

14. 6/9. 3

17 0/ 44

14. 8/1 0.3

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0. 3 3 6

34 /1 4

47. 6/5. 1

30 0/ 13

47. 6/7. 8

16 8/ 71

8.3/ 5.1

mar gina l

33 52 18

66 43 15 7

1 3. 1 0

0. 0 0 4

1, 1 2 1

0. 0 6 1

33 52 13

66 39 87 5

2 2. 3 3

0. 0 0 4

1, 1 0 3

0. 0 1 5

0. 2 3 9 0. 6 9 0

2 6

33 67 66

66 39 03 4

1 9. 8 0

0. 0 0 4

1, 1 0 4

0. 0 2 1

2 6

33 50 23

66 42 10 6

3 5. 7 7

0. 0 0 4

1, 3 1 5

3 1

33 33 47

66 41 12 8

1 3. 8 9

0. 0 0 2

2 7

33 86 72

66 36 17 7

1 1. 7 2

2 2

33 90 09

66 33 43 9

2 3

32 49 67

66 31 09 0

24 1/ 7

18. 2/7. 9

33 5/ 31

16. 1/8. 5

mar gina l

0. 1 4 2

30 5/ 19

10. 4/7. 3

16 6/ 65

10. 4/3. 6

41 /1 5

7.6/ 3.2

mai n

0. 5 8 4

0. 1 8 3

32 2/ 13

21. 9/9. 2

20 6/ 63

22. 9/7. 7

57 /2 4

11. 2/8. 9

mai n

0. 0 3 9

0. 6 6 5

0. 0 7 8

14 1/ 78

3.6/ 1.8

1, 0 5 1

0. 0 0 9

0. 1 8 2

22 4/ 42

6.5/ 3.6

0. 0 0 3

1, 0 9 0

0. 0 1 6

0. 2 0 0 0. 5 3 4

0. 3 1 7

32 2/ 5

1 3. 3 0

0. 0 0 3

1, 1 0 8

0. 0 1 6

1. 1 5

0. 0 0 1

1, 0 2 6

0. 0 0 5

66 35 69 9

5. 6 6

0. 0 0 2

1, 0 4 9

0. 0 1 5

0. 3 1 9 0. 2 2 6 0. 0 4 3

2 6

32 77 20

66 34 30 7 66 32 55 3

7. 9 2 1 1. 6 0

0. 0 0 2 0. 0 0 4

1, 1 0 1 1, 1 1 3

0. 0 1 3 0. 0 2 9

0. 3 9 4 0. 7 8 0

0. 2 2 3 0. 2 1 5

3 0

32 90 23

10. 9/2. 5

mar gina l

11 7/ 17

13. 4/6. 4

10 /4 3

13. 4/3. 6

mai n

13. 4/4. 1

11 5/ 84

31. 7/4. 6

23 2/ 3

32. 4/1 0.2

mai n

0. 2 9 0

32 7/ 6

8.8/ 3.5

23 5/ 17

8.7/ 6.4

76 /7 2

6.6/ 3.4

mai n

0. 2 6 1

35 4/ 58

11. 2/7. 0

33 /2 5

21. 0/1 0.9

23 2/ 18

21. 3/6. 4

mai n

0. 2 0 8

13 1/ 16

16. 0/5. 4

29 /3 5

17. 0/8. 8

24 2/ 50

11. 1/5. 0

mai n

34 6/ 27

15. 4/1 0.6

14 8/ 61

15. 4/5. 9

25 2/ 8

11. 6/4. 0

mai n

32 3/ 4

46. 9/1 0.9

22 2/ 69

47. 1/4. 0

54 /2 0

10. 9/7. 6

qz sye nitic

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30 3/ 11

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33 /4

10. 7/1. 9

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3 2

32 71 06

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14. 0/6. 9

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3 0

14 1/ 58

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1 3

0. 2 1 9

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P Q 2 3 P Q 2 4 P Q 2 5 P Q 2 6 P Q 2 7 P Q 2 8 P Q 2 9 P Q 3 0 P Q 3 1 P Q 3 2 P Q 3

1, 0 5 6

0. 0 2 8

0. 8 4 8

0. 2 7 1

30 1/ 6

28. 0/6. 4

32 /1 3

28. 5/1 3.6

18 5/ 76

15. 2/6. 4

mai n

34 00 52

66 34 29 0

2 4. 2 3

0. 0 0 6

1, 1 3 7

0. 0 2 8

0. 5 1 7

0. 1 7 0

18 /9

9.2/ 3.9

28 3/ 32

8.9/ 4.4

12 3/ 57

5.7/ 3.2

mai n

2 3

34 02 68

66 35 06 5

3. 9 6

0. 0 0 2

1, 1 4 7

0. 0 7 5

0. 2 4 9

0. 3 3 7

15 6/ 15

20. 7/1 2.8

55 /3 6

21. 2/8. 9

26 5/ 49

13. 1/9. 8

mar gina l

2 3

34 02 17

66 34 76 4

1 2. 6 0

0. 0 0 5

1, 1 5 5

0. 0 8 0

0. 4 1 9

0. 1 8 9

16 8/ 4

35. 8/7. 6

1 6

34 02 17

66 34 76 5

1 9. 3 0

0. 0 0 7

1, 1 9 5

0. 0 5 3

0. 0 7 2

0. 3 3 5

15 3/ 21

2 7

33 68 57

66 41 75 5

0. 6 4

0. 0 0 0

1, 0 3 3

0. 0 0 6

0. 1 4 3

0. 3 7 0

30 8/ 5

6.4/ 5.1

2 4

33 68 55

66 41 79 8

0. 3 5

0. 0 0 0

1, 0 4 3

0. 0 1 0

0. 5 7 2

0. 2 3 8

12 2/ 9

2 3

33 84 48

66 41 59 0

9. 1 5

0. 0 0 2

1, 1 4 9

0. 0 4 9

0. 6 3 2

0. 1 6 6

6. 1 0

0. 0 0 3

1, 1 0 8

0. 0 6 4

5. 8 1 1. 8 6

0. 0 0 1 0. 0 0

1, 0 7 6 1, 0 3

0. 0 0 8 0. 0 1

0. 8 0 7 0. 3 3 9 0. 0

2 2

2 5 1 3

33 81 49

66 41 69 0

33 04 81 33 15 09

66 33 04 1 66 32 58

33. 6/6. 8

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26 1/ 33

17. 3/6. 9

mai n

32. 6/1 8.2

34 4/ 69

18. 9/6. 2

mar gina l

20 3/ 71

17. 1/6. 0

39 /1 9

17. 1/5. 4

mar gina l

16. 8/9. 6

3/ 71

18. 9/1 5.8

21 5/ 17

20. 2/5. 6

mai n

15 4/ 1

12. 4/2. 8

24 5/ 71

12. 5/5. 5

64 /1 9

5.7/ 2.9

mar gina l

0. 2 8 9

34 0/ 16

9.9/ 6.4

23 0/ 49

12. 9/9. 9

82 /3 7

12. 9/6. 3

mai n

0. 1 6 8 0. 2 5

17 3/ 8 15 5/ 3

9.9/ 4.8 25. 4/1 2.1

8.3/ 4.8 31. 5/2 1.8

36 4/ 7 28 2/ 85

10. 3/7. 6 31. 0/8. 6

qz sye nitic qz sye nitic

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32. 5/5. 3

71 /5 7

24 5/ 4

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2 8

1 7

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0. 5 8

0. 0 0 0

ur

33 92 60

66 36 26 8

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3 P Q 7 1 P Q 7 8 P Q 9 1 P Q 9 2 P Q 9 3 P Q 9 4 P Q 9 6 P Q 9 7 P Q 9 8 P Q 9 9 P Q -

35 /8 0 64 /4

1 4

1 7

2 2

0

1

1

3 8

6

33 03 32

66 31 25 6

1 6. 4 9

0. 0 0 2

1, 1 3 1

0. 0 1 5

0. 0 7 5

0. 1 4 7

31 6/ 2

33 05 93

66 31 80 4

3 1. 3 0

0. 0 1 6

1, 1 1 3

0. 0 1 5

0. 2 0 6

0. 1 5 0

33 14 25

66 28 64 9

2 1. 5 4

0. 0 1 0

1, 2 8 1

0. 0 7 6

0. 2 1 0

0. 1 0 5

8.1/ 5.1

21 9/ 77

11. 3/4. 1

46 /1 3

11. 8/5. 8

mai n

31 5/ 3

9.1/ 3.4

50 /6 1

10. 1/3. 9

22 3/ 29

6.7/ 3.3

qz sye nitic

31 5/ 46

7.0/ 3.8

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1

13 2/ 43

6.4/ 4.5

22 3/ 1

5.5/ 3.5

mar gina l

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1 0 0 P Q 1 0 1 P Q 1 0 2 P Q 1 0 3

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Table 1 – Anisotropy of low-field magnetic susceptibility data for the Piquiri Syenite Massif.

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Footnote

N is the number of specimens included in the AMS means; Km = (Kmax+Kint+Kmin)/3 is the mean magnetic susceptibility (SI units); P is the degree of anisotropy (K max/Kmin); T =

na

[2ln(Kint/Kmin)/ ln(Kmax/Kmin)]-1 is Jelinek’s shape parameter (Jelinek, 1981). Kmax, Kint and Kmin are mean AMS eigenvectors which represent the maximum, intermediate and minimum susceptibility intensities, respectively. Dec, declination in degrees. Inc, inclination

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in degrees. SD, standard deviation; X 95% are the semi angles of the major and minor

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axes of the 95% confidence ellipse, respectively, calculated by Jelinek´s (1977) method.

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Figure 7 – AMS scalar data for the Piquiri Syenite Massif. (A) Degree of anisotropy (P) vs mean magnetic susceptibility (Km ) plot. (B) P vs shape parameter (T). 5.2 Rock magnetism

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Examples of K-T curves are shown in Fig. 8A. A well-defined peak was observed in

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all analysed specimens at around –150°C which indicates the Verwey transition, characteristic of almost pure magnetite. The high K-T curves show a small Hopkinson

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peak (~530-540°C), and all of them display a decrease in the intensity of susceptibility around 550°C-580°C indicating the presence of Ti-poor titanomagnetite or magnetite. The

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cooling and heating curves are nearly reversible in all specimens (PQ-12). However, in some samples an increase of susceptibility intensity ~180°C to 290°C followed by a steep

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decrease up to 350°C suggests the presence of maghemite (PQ-7, PQ-17). Examples of acquisition of remanent coercivity spectra determined from AF

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demagnetization of both NRM partial ARM are shown in Fig. 8B and C. In general, samples show low coercivity with medium destructive field (MDF - the field necessary to destroy 50% of the magnetization) around 40mT. Results from the pARM curves (Fig. 8C) show that the majority of samples are low coercivity, indicating that magnetite grains are relatively large (maximum peak 15 mT) since coercivity is intimately linked to grain size (Jackson et al., 1989) as also observed in

thin sections (Fig. 4). However, the intensity of pARM in three samples (PQ-6, PQ-7 and PQ-18, Fig. 8C) does not decrease to zero, suggesting either the presence of fine magnetite grains or hematite, as observed in the thin sections. The IRM pattern is shown in Fig. 8D which shows that more than 95% of the magnetization reaches the total saturation in fields < 100 mT, except for those three samples mentioned above, whose total saturation is reached in fields < 200-250 mT (Fig. 8D). The IRM curves indicate the presence of fully saturated, coarse and fine magnetite

A

B 1600

1.0

PQ-12

400

0 -200

0

200

400

C

0.6

0.4

0.2

0.0

20

40

60

80

100

Alternating Field (mT)

D 1.0

Normalized Magnetization

ur

0.8

0.2

0

pARM

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Normalized Magnetization

1.0

0.4

0.0

600

na

Temperature (°C)

0.6

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PQ-17

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K x 10-6 (SI)

800

0.8

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Normalized Magnetization

PQ-7 1200

AF-NRM

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grains.

IRM

0.8

0.6

PQ-2I2 PQ-6C1 PQ-7C3 PQ-12G3 PQ-17C1 PQ-18K2 PQ-20E2

0.4

0.2

0.0 0

20

40

60

Alternating Field (mT)

80

100

0

100

200

300

Applied Field (mT)

400

500

Figure 8 – Representative examples of rock magnetism for specimens from different facies of the Piquiri Syenite Massif: Marginal (PQ-02 and PQ-17), Main (PQ-06 and PQ07); Qz-Syenite (PQ-20) and Granitic (PQ-12 and PQ-18). (A) K-T curves (susceptibility versus low and high temperatures). (B) Remanent coercivity spectra determined from AF demagnetization of NRM. Remanence intensities are normalized to NRM. (C) Remanent coercivity spectra derived from partial anhysteretic remanence acquisition in an AF peak demagnetization at 95 mT with AF window width of 10 mT during DC field application of 0.1 mT. Remanence intensities are normalized to the highest value of partial remanence acquisition. (D) Isothermal remanence magnetization (IRM) acquisition curves. Intensities of remanence are normalized to saturation of IRM (SIRM) versus field strength.

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Some typical hysteresis curves are illustrated in Fig. 9. For most of the samples the shape of the hysteresis curves reveals that ferromagnetic grains carry most of the bulk magnetic susceptibility, with no significant contribution of paramagnetic minerals. In

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general, the curves are narrowly waisted, typical of low-coercivity ferromagnetic minerals,

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na

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which is in agreement with other experiments such as IRM acquisition curves (Fig. 8D).

Figure 9 – Representative hysteresis loops for the different facies of the Piquiri Syenite

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Massif. M is magnetization in Am2 and H is applied field. Loops are not corrected for paramagnetic slopes. PQ-7 - main facies, PQ-12 - granitic facies, PQ-17 - marginal facies. 5.3 AARM data

The AARM tensor was obtained in at least 6 specimens from 26 sites in different facies of the massif. The mean AARM eigenvectors (AARMmax, AARMint, AARMmin) and the 95% confidence regions for each site were also calculated using the Jelinek´s (1977) statistics. For most of the sites AARM and AMS tensors are coaxial (Fig. 10). This

indicates that paramagnetic minerals and magnetite have probably the same shape preferred orientation and probably record the same magmatic event, and also indicates

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na

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that the effect of SD grains in the AMS tensors is precluded.

Figure 10 – Examples of magnetic fabrics determined from AMS and AARM for the same sites. For most sites these magnetic fabrics are coaxial. Squares are maximum susceptibility (Kmax) and maximum remanence (AARMmax). Triangles are intermediate susceptibility (Kint) and intermediate remanence (AARMint) and circles are minimum susceptibility (Kmin) and minimum remanence (AARMmim). Ellipses indicate 95%

confidence. Lower-hemisphere, in equal-area projection. 6. Discussion The PSM magnetic fabric is controlled mainly by relatively coarse-grained magnetite. In plutonic rocks with k values above 5 x 10-3 SI, AMS probably results from the shape anisotropy of magnetite grains (e.g. Archanjo et al., 1995; Bouchez, 1997), and in the PSM this is also shown by the AARM results coaxial with the AMS ellipsoids for most of the analysed samples. Therefore the magnetic fabrics (AMS and AARM) are primary

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(magmatic) in origin, acquired when the rocks were solidified as a result of internal magma chamber processes reflecting magma flow.

The absence of solid-state deformation features at mesoscale as well as in thin

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sections rules out substantial deformation after full crystallization of the studied rocks, and the magnetic fabric probably results from magmatic flow, as observed by several authors

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for similar cases (e.g. Raposo and Gastal, 2009; Trubac et al., 2009; Oliveira et al., 2010). Since the magnetic foliations correspond well with the foliations measured in the field, the

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magnetic fabric should mimic the orientation of K-feldspar and phyllosilicates, and the magnetic fabric is related to pluton emplacement, and the magnetic lineation accounts for

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magma flow direction during emplacement.

As may be seen in the variation of the elipse angular dispersion (Jelinek, 1978), the

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PSM magnetic fabric is well organized, and about 87% of foliations and 70% of lineations

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are well defined (Table 1). The statistically better-organized planar fabric, as compared to linear one, is observed in the predominance of positive values for the shape parameter (T) found in the studied samples, and indicates the dominant oblate tendency of the fabric. This explains why it is so difficult to identify magmatic lineation in the PSM rocks. The magnetic planar fabric of the PSM rocks is mostly parallel to the magmatic fabric, with only minor local variations, and boths are parallel to the external contacts of the intrusion with its host rocks. The magnetic and magmatic fabrics form a concentric pattern

and tend to dip inwards suggestive of an inverted cone 3D shape for the pluton (Fig. 6). The magnetic linear fabric is variable throughout the massif and systematically follows different patterns conditioned to each facies. Steeply-plunging lineations are dominant in the marginal facies indicating the possible location of conduits or feeding zones for the ascent of magmas. In the host rocks on the eastern side of the intrusion (Fig. 1), the internal structures of the PSM apophyses tend to conform to the orientation of the host rocks. Syenites of the main facies show moderate- to gently-plunging magnetic lineations, which indicates a change in magma ascent and emplacement geometry. The quartz

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syenite facies rocks show only gently-dipping to flat-lying magnetic lineations, indicating a definite change of flow pattern related to pluton emplacement. The granitic varieties show slightly higher lineation plunge values that would suggest later emplacement, but the field

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relations do not support such tendency, and rather suggest that these granitic varieties are

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but local composicional variations. The most contrasting lineation patterns in the PSM are found between the external facies - the marginal and main varieties, both richer in mafic

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minerals - and the internal ones – quartz-syenitic and granitic, more leucocratic, which may be spotted also in the scalar data. The marginal and main facies varieties show high

na

values of magnetic susceptibility (K) and anisotropy degree (P), which are related to their larger amount of magnetite and relative preferred orientation (Cruden et al., 1999). The

ur

quartz-syenitic and ganitic facies show lower values of K and P, in agreement with their lower magnetite content and less expressive mineral alignment.

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The rock magnetism data integrated with SEM investigation show that the PSM

magnetic fabric is controlled mainly by ferromagnetic minerals (magnetite and/or titanomagnetite. The K-T curves indicate the presence of magnetite, both in the marginal and main facies rocks. However, the decrease in susceptibility at about 350 °C registered by these curves could be attributed either to maghemite-haematite or maghemite-magnetite inversion in these rocks, depending on the oxidation state of the medium (Ozdemir and

Dunlop, 1993). The presence of maghemite, as indicated by the K-T curves, taken together with SEM data indicative of ilmenite, titano-magnetite and ilmeno-rutile with oxiexsolution texture, as well as pyrite crystals mantled by hematite found only in the marginal facies rocks, suggest more hydrated crystallization conditions for these varieties. SEM data obtained in the rocks of the main facies indicate only the presence of magnetite. The maghemite shown in the K-T curves indicates that the main facies was also a water-rich system during crystallization, but it would have been a less oxidated

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system if compared to the marginal facies. SEM data from the quartz syenitic and granitic rocks are in agreement with the K-T curves, which show well-defined Vervey transitions and Hopkinson peak, both pointing to the presence of pure magnetite. The absence of

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magnetite transformations is in agreement with the less oxidizing conditions for the

crystallization of this magma. Taken together, the dataset points to progressively less

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oxidizing conditions from the margins to the center of the intrusive body. Such trend would be opposite of the one expected for in situ differentiation of a plutonic body. Therefore, the

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data presented here permit to argue against in situ differentiation, and rather suggest that the MSP was built by successive emplacement of multiple intrusive pulses.

na

The presence of two non comagmatic, mantle-derived liquids in the generation of PSM rocks has been identified by several authors (e.g. Plá Cid et al. 2000, 2003; Stabel et

ur

al., 2001; Nardi et al., 2007, 2008). As a result, several compositional and textural

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heterogeneities are found in alll lithological types forming the massif, such as mafic lamprophyric microgranular enclaves in the syenitic and granitic rocks, especially in those of the marginal facies, which become less abundant towards the inner part of the pluton. Evidence of disaggregation and transport of fragments of the marginal facies rocks by the magma of the main facies is seen at several outcrops, where the host magma tends to partly digest the early-formed varieties, leaving relatively large areas of anomalous high concentration of mafic minerals and microgranular enclaves, most commonly near the

contacts with the marginal facies. Quartz-syenite tabular intrusions are found at some outcrops of the main facies, and their sharp contacts suggest relatively high temperature contrast. Additionally, the amount of mafic cumulate autolith and mafic microgranular enclaves in the quartz-syenites is drastically reduced relative to the marginal facies rocks. The rare and local occurrence of contact metamorphism effect of the PSM over the host rocks is worth noting, especially because these are only found related to the marginal facies rocks. Taken together with the fact that most of the magnetic lineations of the

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marginal facies rocks are steeply plunging, this would lead to the hypothesis of the marginal facies rocks representing a magma feeder zone, and also the first pulse to be emplaced and give away heat to the host rocks. After that, space was generated (possibly

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under extension) and successive pulses were emplaced, which is demonstrated by the geometrical variation of the magnetic lineation plunge from subvertical in the external part

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to very shallow in the internal part of the pluton.

The structural features observed in the field, such as large fragments and partly

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dismembered portions of the marginal facies rocks carried by the main facies magma, as well as tabular intrusions of quartz-syenite in the main facies rocks are taken as further

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evidence of the PSM multi-intrusive character. Therefore, the 611 ± 3 Ma age obtained by Philipp et al. (2002) for the main facies rocks should be taken just as a reference age for

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the pluton. The duration of the magmatism involved in the construction of the Piquiri

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Syenite Massif will only be precisely established when the ages of the marginal and granitic facies are determined. CRediT author Statement_Sbaraini Samuel Sbaraini: Conceptualization, Methodology, Software, Investigation, Formal analysis, Writing - Original Draft. Maria Irene Raposo: Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data Curation, Writing - Review & Editing. Maria de Fátima Bitencourt: Conceptualization, Writing - Review & Editing, Supervision, Project administration, Funding acquisition. Camila Rocha Tomé: Writing - Review & Editing.

Declaration of interests 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.

Acknowledegments This paper is part of Samuel Sbaraini’s MSc thesis. The research was supported by the Rio Grande do Sul State Research Foundation (FAPERGS, 10/0045-6) and National

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na

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Research Council (CNPq, Universal Program 471266/2010-8) granted to M.F. Bitencourt.

References Archanjo, C.J., Launeau, P. & Bouchez, J.L., 1995. Magnetic fabric vs magnetite and biotite shape fabrics of the magnetite-bearing granite pluton of Gamelerias (Northeast Brazil). Physics of the Earth and Planetary Interiors, 89: 63-75. Babinski, M., Chemale Jr, F., Van Schmus, W.R., Hartmann, L.A., Silva, S.C., 1997. U-Pb and Sm-Nd geochronology of the Neoproterozoic Granitic-Gneissic Dom Feliciano Belt,

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Southern Brazil. Journal of South American Earth Science, 10: 263-274. Bitencourt, M.F., Nardi, L.V.S., 2000. Tectonic setting and sources of magmatism related to the southern Brazilian shear belt. Revista Brasileira de Geociências, 30(1): 186-189.

anisotropy. Earth Science Review, 42: 49–93.

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