Magnetic fabrics and their relationship with the emplacement of the Piracaia pluton, SE Brazil
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- Raposo, M.I.B., Pressi, L.F. & de Assis Janasi, V. Int J Earth Sci (Geol Rundsch) (2012) 101: 773. doi:10.1007/s00531-011-0696-5
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Magnetic fabric and rock-magnetism studies were performed on the four units of the 578 ± 3-Ma-old Piracaia pluton (NW of São Paulo State, southern Brazil). This intrusion is roughly elliptical (~32 km2), composed of (i) coarse-grained monzodiorite (MZD-c), (ii) fine-grained monzodiorite (MZD-f), which is predominant in the pluton, (iii) monzonite heterogeneous (MZN-het), and (iv) quartz syenite (Qz-Sy). Magnetic fabrics were determined by applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanent magnetization (AARM). The two fabrics are coaxial. The parallelism between AMS and AARM tensors excludes the presence of a single domain (SD) effect on the AMS fabric of the units. Several rock-magnetism experiments performed in one specimen from each sampled units show that for all of them, the magnetic susceptibility and magnetic fabrics are carried by magnetite grains, which was also observed in the thin sections. Foliations and lineations in the units were successfully determined by applying magnetic methods. Most of the magnetic foliations are steeply dipping or vertical in all units and are roughly parallel to the foliation measured in the field and in the country rocks. In contrast, the magnetic lineations present mostly low plunges for the whole pluton. However, for eight sites, they are steep up to vertical. Thin-section analyses show that rocks from the Piracaia pluton were affected by the regional strain during and after emplacement since magmatic foliation evolves to solid-state fabric in the north of the pluton, indicating that magnetic fabrics in this area of the pluton are related to this strain. Otherwise, the lack of solid-state deformation at outcrop scale and in thin sections precludes deformation in the SW of the pluton. This evidence allows us to interpret the observed magnetic fabrics as primary in origin (magmatic) acquired when the rocks were solidified as a result of magma flow, in which steeply plunging magnetic lineation suggests that a feeder zone could underlie this area.
KeywordsMagnetic fabricAMSAARMGranitesRock magnetismPiracaia pluton
The complexity of the geometric patterns and physical causes of magma flow in plutons is one of the key problems in understanding magmatic processes. In many cases, the direct field evidence for large-scale flow has been erased from rock record (Trubač et al. 2009). The flow patterns may be inferred from the preserved mesoscopic fabrics which are acquired late in the magma chamber history along migrating crystallization fronts and are easily reset by regional tectonic deformation (Paterson et al. 1998; Benn et al. 2001). Then, the inference on the large-scale flow patterns within a pluton is difficult because of the poor strain memory of magmatic fabrics (Paterson et al. 1998). Late deformation makes new fabrics which may appear locally in large bodies or can be more pervasively in smaller ones. These fabrics may modify or erase the earlier (primary) one depending on temperature and strain intensity. Indeed, the study of granite pluton emplacement and deformation during the regional tectonic events is challenging since granitic rocks do not always develop mesoscopic scale deformation fabrics. However, microstructural observations are commonly used to separate the primary, magmatic imprint, from the secondary, late to post-magmatic fabrics (e.g., Paterson et al. 1989; Bouchez et al. 1990; Pignotta and Benn 1999; Benn et al. 2001; Esmaeily et al. 2007; Njanko et al. 2010, among others). In syn-tectonic intrusions, this distinction may not be possible, considering that the emplacement of magmas is mostly controlled by the regional strain. In such a scenario, magmatic structures may have the same orientation of country rocks, even if a solid sate foliation is not strongly developed.
Today, determination of anisotropy of low-field magnetic susceptibility (AMS) is the most efficient method to obtain the petrofabric orientation in rocks, even in rocks that are visually isotropic (Bouchez 1997), and it has been applied in many granites around the world (Bouchez et al. 1990; Benn et al. 2001; Archanjo et al. 2008; Raposo and Gastal 2009; Njanko et al. 2010, among many others). The magnetic anisotropy techniques are particularly useful where the mineral fabrics are difficult to determine in the field, due to poor outcrop conditions or because the fabric-defining crystals are only weakly anisotropic, as is the case of central-SW of the Piracaia pluton (SE, Brazil). In this pluton, lineations are difficult to measure in the field due to absence of a fissile schistosity within which the linear fabrics can be viewed. The advantage of using AMS in fabric studies lies in the fact that precise and reproducible foliation and lineation measurements can be obtained for any outcrop in a pluton. AMS data yield maximum information when integrated with available information of mesoscopic structures within plutons (foliation, banding, etc.) as well as mesoscopic and map-scale structures in the country rocks.
Magnetic fabric can also be determined using anisotropy of magnetic remanence (AMR), which is not so popular in granite studies. This anisotropy isolates the contribution of remanence-bearing minerals from that of the paramagnetic and/or diamagnetic matrix. Since the ferromagnetic particles, which define the AMR tensor, and the paramagnetic and/or diamagnetic minerals may crystallize at different times with different orientations, the determination of the AMR allows an investigation into possibly overprinted fabrics. The most common AMR is anisotropy of anhysteretic remanence magnetization (AARM), which has been employed for some granite plutons for petrofabric analysis (Trindade et al. 1999; Pignotta and Benn 1999; Raposo and Gastal 2009, among others).
In this study, we have applied both AMS and AAMR techniques to the Piracaia pluton, located in NW São Paulo state (SE Brazil) with an aim to determine its internal fabric, and infer its mode of emplacement.
The AGT is a result of the collage of the Paranapanema and São Francisco cratons completed by 630–610 Ma with the eastward emplacement of high-P nappes such as the high-PT Socorro-Guaxupé Nappe (SGN). The SGN and the low-to-medium-grade Apiaí-São Roque Domain (Fig. 1a) correspond to the Cryogenian-Ediacaran active continental margins developed at the border of the Paranapanema plate (Campos Neto 2000). The IGP was emplaced in these two domains, contemporaneously with the accretion, south of the AGT, of the ENE-NNE-trending Ribeira Fold Belt, locally associated with lateral escape contemporaneous to the frontal collision occurring further north, and related to the closure of oceanic remains east of the São Francisco craton (e.g., Vauchez et al. 1994).
The 578 ± 3-Ma-old Piracaia pluton (U–Pb zircon, Janasi et al. 2009; Fig. 1b) as well as the other IGP plutons was emplaced 25–30 Ma after the end of “orogenic” magmatism and the high-grade metamorphism in the SGN. The emplacement was partly controlled by NNE sinistral strike-slip faults and shear zones (Fig. 1a), which imprinted its foliation. To the west, the country rocks are strongly deformed porphyritic hornblende-biotite, granites which are related to the 630–615 Ma syn-orogenic high-K calc-alkaline magmatism, whereas to the south and east, they are a sequence of migmatitic biotite gneisses and garnet-biotite gneisses of the Piracaia Metamorphic Complex both associated with small anatectic granite bodies (e.g., Martins et al. 2009).
The Piracaia pluton and sampling
The Piracaia pluton has a roughly N35E elongated (32 km²) elliptical shape (Fig. 1b). Most of the pluton exhibits strong magmatic foliation, which evolves into solid state upon cooling of the magmas, mainly in the northern part and borders east and west. It comprises four petrographic units: (i) coarse-grained monzodiorite (MZD-c), (ii) fine-grained monzodiorite (MZD-f), (iii) monzonite heterogeneous (MZN-het), and (iv) quartz syenite (Qz-Sy). Trace-element chemistry and isotopic data indicate that the pluton was built by magma batches of different sources, in which a mixture between them occurred at different degrees (Janasi et al. 1993, 2007). The negative εNd(t) (−7) together with a primitive 87Sr/86Sr(t) (~0.705) indicates an enriched lithospheric mantle source for MZD-c and MZD-f, whereas the high HFSE (high-field-strength elements) contents of Qz-Sy makes it comparable to A-type granites. The MZN-het shows evidence of mixing between mafic and felsic magmas.
MZD-f unit is predominant in the Piracaia pluton. It occurs in the center as well as a thin elongated body on the NW of the pluton (Fig. 1b). The magmatic foliation is evidenced by oriented plagioclase and biotite (Fig. 2b). Toward N–NE of this unit, the foliation evolves to solid state as evidenced by planar orientation of mafic minerals, polygonal texture of quartz, and elongation and incipient recrystallization of the feldspars (Fig. 2c).
The MZN-het unit occurs at the northern part of the pluton and as a small body on the south (Fig. 1b). It results dominantly from mingling process between monzodiorite and syenite magmas and is characterized by the presence of centimeter-sized alkali-feldspar-rich, light-colored, medium-grained portions, set in a dark-gray, fine-grained matrix (Fig. 2d). These felsic parts vary in proportion and commonly contain millimeter-sized crystals or aggregates of hornblende (Fig. 2d).
The Qz-Sy unit comprises coarse-grained quartz syenites and quartz monzonites and is located at the NE and S of the pluton (Fig. 1b). This unit is intrusive into the MZN-het and shows co-mingling structures in the NE and SE contacts with the central MZD-f unit (Fig. 2e). They exhibit granular hypidiomorphic texture, sometimes with a granoblastic matrix due to solid-state deformation (Fig. 2f).
Oriented samples from 32 sites (29 from the pluton and 3 from the country rock), widely distributed throughout the Piracaia pluton (Fig. 1b), were collected. It may be noted that weathering and lack of good in situ exposures did not allow uniform sampling from all units of the pluton. Sample orientations were determined using both magnetic and sun compasses, whenever possible. At least 10–13 cores, using a gasoline-powered rock drill, were collected from each site for magnetic measurements, and at least three specimens (2.5 × 2.2 cm) were cut from each core.
The sites PI-1 to PI-6 were sampled in the same outcrop at the NE contact between Qz-Sy and MZD-f units (Fig. 1b). This outcrop shows meter-sized bodies of quartz syenite mixed heterogeneously with the monzodiorites, forms structures such as monzodioritic enclaves and pillows, and locally generates fine-grained hybrid monzonites.
Anisotropy of low-field magnetic susceptibility
Anisotropy of low-field magnetic susceptibility (AMS) describes the variation of magnetic susceptibility with direction within a material and represents the contribution of all the rock-forming minerals (i.e., dia-, para-, and ferromagnetic). Therefore, its use is not restricted to iron-oxide-bearing rocks. AMS is the tensor that relates the intensity of the applied field (H) to the acquired magnetization (M) of a material through the equation: Mi = KijHj, in which the proportionality Kij is a symmetrical second-rank tensor referred to as the susceptibility tensor (Hrouda 1982; Tarling and Hrouda 1993). This tensor is represented as an ellipsoid in which the three principal axes are maximum (Kmax), intermediate (Kint), and minimum (Kmin) susceptibility. The orientations of these axes correspond to the eigenvectors of the susceptibility tensors and give the magnetic fabric. The Kmax axis represents the magnetic lineation, while Kmin is the pole of the magnetic foliation (Kmax−Kint plane). In rocks in which AMS 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). Otherwise, in rocks in which AMS is carried by ferrimagnetic minerals such as multidomain (MD) (titano)magnetite grains, the origin of AMS is intimately related to the grain shape (shape anisotropy), in which Kmax and Kmin are parallel to the long and short axis, respectively, of the particle. AMS can be due to the distribution anisotropy of magnetite grains crystallized within a preferred flow-oriented silicate template (Hargraves et al. 1991). Also, in some cases, the AMS can be due to magnetic grain interaction within a rock (Cañon-Tapia 1996). Therefore, for MD magnetite grains, the AMS is due to shape-preferred orientation–distribution–interaction anisotropy. However, recent studies have shown that the AMS might still be dominated by the shape effect despite the occurrence of magnetic grain interaction or distribution anisotropy in the rock (Gaillot et al. 2006). On the other hand, if small single domain (SD) (titano)magnetite prolate grains are present in a rock, they provoke an interchange between maximum and minimum AMS axes (Stephenson et al. 1986; Jackson 1991) and can produce anomalous AMS fabrics in rocks (i.e., the magnetic fabric is not coaxial with petrofabric). For this reason, a detailed rock-magnetism study is always required in AMS analysis.
Anisotropy of low-field magnetic susceptibility data for Piracaia pluton
Mean AMS parameters
For each site, the mean magnetic susceptibility Km (SI units) is the arithmetical mean [Km = (Kmax + Kint + Kmin)/3] for each specimen (Table 1). In general, Km is high for all units with an average of 3.31 × 10−2 (SI). However, Km < 2 × 10−2 occurs in the NW of the pluton mainly in the monzonite heterogeneous unit (Table 1).
To better define and characterize the magnetic carriers and their relative contribution to both the mean magnetic susceptibility and the remanence, several rock magnetic experiments were carried out. These experiments were: measurements of continuous low-field thermomagnetic curves (K × T curves, susceptibility versus (high and low) temperature); acquisition of remanent coercivity spectra determined from alternating field (AF) tumbling demagnetization of the natural remanent magnetization (NRM), and partial anhysteretic remanent magnetization (pARM); isothermal remanent magnetization (IRM) acquisition curves; AF demagnetization of saturated IRM (SIRM); and hysteresis loops.
Coercivity remanent spectra from pARM (Jackson et al. 1989) were determined in the same specimens subjected to AF of NRM. They were determined using a Molspin alternating field demagnetizer as the source of the AF field; superimposition of a steady field (DC field) was attained by a small coil (home-made) inside and coaxial to the demagnetizer, and it was controlled by a Molspin apparatus. The pARM acquisition consists of applying a steady field (DC field) in between two chosen values (AF window, H1 and H2, H1 < H2) of a decaying AF peak (H) while the rest of assemblage is demagnetized from a peak field H > H2. Then, specimens were exposed to an AF peak of 98 mT and DC field of 0.1 mT with an AF window width of 10 mT. Tumbling AF demagnetization at 100 mT was applied after each pARM acquisition. Results from this experiment are presented in Fig. 6b and show that the majority of analyzed specimens have, in general, low-coercivity magnetic grains. However, a few samples from Qz-syenite, coarse-grained monzodiorite, and monzonite heterogeneous show higher coercivity (Fig. 6b). Since coercivities are linked to grain sizes (Jackson et al. 1989), the studied units are composed of large (low-coercivity) and fine (high-coercivity) magnetite grains.
After AF tumbling demagnetization of the last pARM acquisition, the specimens were subjected to isothermal remanent magnetization (IRM) in progressively increasing magnetizing fields using a pulse magnetometer (MMPM9, Magnetic Measurements). The IRM pattern for the analyzed samples is shown in Fig. 6c in which just one component is evident for the majority of samples, which saturate at ~100 mT. Only the monzonite heterogeneous shows two components with different contributions to IRM acquisition curves. One of them starts to saturate at 80 mT up to 100 mT (~90% of the magnetization). On the other hand, above 100 mT, samples start to acquire another magnetization, which is saturated by fields <280 mT (Fig. 6c). Therefore, saturation at those fields is consistent with the presence of fully saturated fine-grained magnetite. For a few specimens from the studied units, the magnetic grains are, therefore, coarse and fine magnetite, which is coherent with coercivity spectra (Fig. 6b).
Following the IRM acquisition, samples were AF-demagnetized (Fig. 6d). In general, a minor amount of the initial SIRM remains after demagnetization to 60 mT for the majority of the specimens. For the specimens from monzonite heterogeneous, less than 20% remains after demagnetization to 60 mT, suggesting that fine-grained magnetite has negligible contribution to both remanent magnetization and magnetic anisotropies as compared to coarse-grained magnetite for these specimens.
Based on all magnetic experiments, therefore, we conclude that coarse magnetite, which was also observed in thin sections, and (titano)maghemite (whenever present) grains carry NRM and IRM magnetizations and are responsible for the magnetic susceptibility of the Piracaia pluton.
Anisotropy of anhysteretic remanence magnetization
Anisotropy of anhysteretic remanence magnetization (AARM) (see Jackson 1991 for a review) isolates the contribution of remanence-bearing minerals from that of the paramagnetic and/or diamagnetic matrix. AARM is determined from the intensity of an artificial anhysteretic remanence magnetization acquired when a magnetic field is applied along different directions through the sample. AARM has distinct advantages because it precludes the effect of “inverse” AMS fabric due to (SD) magnetites (Stephenson et al. 1986). The AARM tensor is also a symmetrical second-rank tensor expressed by its principal eigenvectors AARMmax, AARMint, and AARMmin representing the maximum, intermediate, and minimum axes of anhysteretic remanence, respectively, in which AARMmax corresponds to the magnetic lineation and the AARMmin is the magnetic foliation pole (normal to AARMmax−AARMint plane).
The AARM was determined in at least 4 specimens from one site of each studied unit. The procedure consists of cycles of a hysteretic remanence acquisition, measurement, and demagnetization along different positions for each specimen. The AARM tensor was calculated from a seven-position measurement scheme.
AARM tensor was determined exposing the specimens to an AF peak of 50 mT and a DC field of 0.1 mT which was further removed by an AF tumbling demagnetization of 100 mT. Before AARM determinations, the samples were demagnetized by AF tumbling at 100 mT to establish the base level. The best-fit AARM tensors were calculated by the least-squares method, which showed root-mean-squares of less than 5%, indicating that the ellipsoids are well resolved (Jackson 1991). The mean AARM eigenvectors (AARMmax, AARMint, AARMmin) and the 95% confidence regions for each site were also calculated using the bootstrap method of Constable and Tauxe (1990).
The magnetic lineations, Kmax, present mostly low plunges (<30°), in approximately 73% of the sites, most of them located at the northern portion of the pluton (Fig. 9d, Table 1). Five sites display magnetic lineation plunges between 30 and 60°, and in three sites, the plunges are >60°. The majority of these sites is roughly close to each other and is located at the southwestern portion of the pluton (Figs. 1b, 9d).
Fabrics in plutons are regarded as either related to magma emplacement or resulting from syn- to post-tectonic strain. Therefore, fabric patterns in plutons can result from internal magma chamber processes such as magma pulses, convection, magma surges, dike injections, and crystal settling, or may be related to regional deformation, or can involve a combination of these two processes (Paterson et al. 1998), which is characteristic of syn-tectonic regimes. However, this combination can be simultaneous, if a compressive component predominates during intrusion, or subsequent, considering that an intimate association of extension and compression is commonly observed along shear zones. In such a case, an originally vertical lineation may be obliterated by the shearing process. Indeed, fabrics related to emplacement such as the presence of magmatic microstructures and steep lineations are rarely preserved since they are easily overprinted by minor ductile strain during or after cooling of the magma (Clemens et al. 1997). These overprinted fabrics may be observed in the whole pluton, or in some places, which is more common.
Comparing oriented structures from country rocks with internal magnetic and deformational fabrics in plutons allows determining whether the pluton fabrics reflect the effect of regional tectonic strain. On the other hand, if foliations and lineations in country rocks and pluton are different, structures in the pluton will preferentially be considered to be the result of internal processes in the pluton.
The field and petrographic evidence (Fig. 2) shows that rocks from Piracaia pluton were affected by the regional strain during and after emplacement since magmatic foliation evolves to solid-state deformation in the N (PI-8 and PI-1) of the pluton (Fig. 2c, f). Magnetic foliations for these rocks are steeply dipping and parallel to foliation measured in the field (Figs. 1b, 9a). The lineations are mostly subhorizontal. This clearly indicates that magnetic fabrics in this sector of the pluton record this strain, which is probably related to the sinistral minor and major shear zones (Fig. 1a, Extrema and S. B. Sapucaí), which controlled in part the Piracaia emplacement. On the other hand, no deformation is observed in rocks from SW of the pluton (e.g., PI-10, PI-19, PI-23 and PI-30, Fig. 2a). In fact, even magmatic foliation is poorly developed in most sites, and magnetic foliations are not parallel either to magnetic foliations from the northern part of the pluton or to the regional deformation pattern. Also, the plunge of the magnetic lineation (Fig. 9d) is moderate to steep. This indicates that the SW of the Piracaia pluton was not affected by any tectonic deformation either during or after the emplacement, and magnetic fabric in this part of the pluton probably is primary (magmatic) in origin, acquired when the rocks were solidified reflecting magma flow. Therefore, the high plunge lineation (e.g., PI-10 and PI-30) could be linked to a magma feeder zone (magma chamber).
Since the N of the pluton has been affected by regional deformation, it is important to discuss whether the subhorizontal lineation represents an overprint of an initially steep lineation, reworked by shearing during or after emplacement. The AARM results clearly show that there is no fabric overprint (i.e., AMS and AARM tensors are coaxial). As the majority of magnetic lineations point toward the south, it can be reasonably argued that SW section represented an area of extension and, therefore, a preferential place for magma ascent as indicated by steep lineation plunges. Due to the dynamics of the fault zone, the magmas were affected by strain and flow toward north in which the magmatic foliation evolved to solid state upon cooling. This implies that the sites from N were weakly strained by shearing and are likely to preserve fabrics related to magma flow, which are manifested in the magnetic data.
Summary and conclusion
The Piracaia pluton is composed of four petrographic units. Rock-magnetism determinations have allowed the successful characterization of magnetic proprieties and determination of the internal fabric of the pluton. No SD effect was found in the AMS fabrics as shown by rock-magnetism and AARM data. Magnetite is the main mineral responsible for the AMS and AARM fabrics. The AMS and AARM fabrics in the studied rocks are mainly due to the shape-preferred orientation (distribution–interaction) of magnetite grains. Both AMS and AARM tensors are coaxial and are partially related to regional strain, which affected the magma flow in the north of the pluton, whereas the high lineation plunges in the SW of the pluton suggest that this place could be linked to a magma feeder zone. Our data suggest that regional strain did not affect the whole pluton, and it was not able to erase the primary fabric (magma flow) in the N portion of the Piracaia pluton.
We thank FAPESP (95/8399-0 and 07/0935-5 grants) Brazilian agency for its financial support. Pressi L.F. thanks CNPq (Brazilian agency) for MSc scholarship (133629/2009-0 grant). We thank Alex Fortunato Ribeiro for his help in both field work and AMS measurements. We also thank Fátima Martín-Hernández, an anonymous referee, and the Editor Manish A. Mamtani for their comments and suggestions.