Protracted deformation during cooling of the Paleoproterozoic arc system as constrained by 40 Ar / 39 Ar ages of muscovite from brittle faults : the Transamazonan Bacajá Terrane , Brazil

Manuscript ID: 20170033. Received on: 03/14/2017. Approved on: 08/08/2017. ABSTRACT: In the Paleoproterozoic Transamazonas Province, synkinematic granitogenesis has taken place synchronously with compressive tectonic stress. The synkinematic character of the granites is marked by their WNW elongate shape, and by the presence of pervasive and concordant synmagmatic foliation. Ductile shear zones are concordant to the previous regional WNW structures, and tend to be accommodated along contacts between Rhyacian synkinematic granitoids and both Archean orthogneisses and Siderian metabasites. Locally phyllonitic shear zones and brittle-ductile shear zones with cataclasites are oriented subparallel to the preexisting ductile foliation. Late orogenic brittle faults N30E-trending strike-slip faults are either sinistral or destral. 40Ar/39Ar dating of muscovite developed on fault planes gave ages of 1977 ± 8 Ma and 1968 ± 11 Ma. Structural and geochronological data from rocks of the Transamazonas Province permit to conclude that most mylonites and brittle structures were controlled by preexisting structures such as geological contacts and petrographic facies boundaries. Compressive tectonic stress would have initiated at ca. 2100 Ma, since the former magmatic arc (Bacajaí complex), still present at 2070 Ma when syntectonic granites were emplaced and remained until 1975 Ma after granite plutonism and regional cooling.


INTRODUCTION
Synkinematic granite bodies show elongated shapes concordant with the regional structural pattern and microstructural evidence of emplacement during tectonic stress field (e.g., Paterson et al. 1991, Miller & Paterson 1994).Mechanisms that could give rise to high temperature magmatic structures (Vigneresse et al. 1996, Barbey et al. 2008, Barbey 2009) are related to the increase in crystallization rates within the magmatic chamber.Once a critical rheological threshold is reached (Arzi 1978), the magmatic chamber obtains a viscosity similar to that of a solid, and ductile deformation structures can be formed in material that has not completely crystallized.Under these conditions, deformation is progressive and controlled by a decreasing temperature regime.The foliation that develops during crystallization is penetrative within magmatic chambers and tends to be homogenous at outcrop scale (Paterson et al. 1991).The formation of bodies, their internal structures and the preferred orientations of minerals constitute potential anisotropies where shear zones or faults might develop (Vauchez et al. 1998).
Differences in thermal gradients control the strength of different materials, with increase in resistance being directly linked to the age of the lithosphere (Vauchez et al. 1998).Vauchez et al. (1998) consider colder cratons to be more rigid than younger orogenic provinces that are dominated by higher temperature regimes.Combined rheological heterogeneity and mechanical anisotropy are fundamental factors in the deformation of continents.As an example, Vauchez et al. (1998) and Corrêa-Gomes et al. (2005) present the São Francisco Craton and the Ribeira Belt, where several regional strike-slip shear zones have preferentially developed on boundaries of cratonic provinces.
The magmatic rocks of the Bacajá Terrane have been studied by several authors (João et al. 1987, Faraco et al. 2005, Vasquez et al. 2005, Barros et al. 2007) who describe belts of metavolcanics and concordant elongate granitic bodies.There are also shear zones in the region that have formed after complete magma crystallization.The present article discusses the nature and time of stress fields during the Transamazonian orogeny and the influence of preexisting structural anisotropy during the formation of ductile and brittle shear zones.

METHODS
This study is based on the recognition of the main structural trends with regards to structures of the Amazon Basin basement of the Bacajá domain (Fig. 1), as revealed by multiscale analysis through digitalized SRTM elevation models, structural analyses on outcrops and thin sections, and geochronological data.The analysis of digitalized SRTM elevation models, acquired from the USGS site (http://eros.usgs.gov,August 2007), was done through the use of ESRI ® ArcMap™ 8.3 software, aided by geological, hydrographic and other maps available in the literature.
Fieldwork in the Bacajá Terrane was done on outcrops along the Transamazônica road and also across the vicinal roads.These latter are oriented N-S direction, almost perpendicular to the regional structural trend.Thin section studies allowed us to understand the rheological conditions, mechanisms of recrystallization and to estimate the temperatures at which the granitoids of the Bacajá Terrane were deformed.
Muscovite crystals collected from brittle faults were dated using the 40 Ar/ 39 Ar method.This analysis was undertaken at the UQ-AGES Laboratory at the University of Queensland (Australia).After a period of irradiation and decay, the samples were analyzed by 40 Ar/ 39 Ar heating via laser.Before the analysis, the grains were subjected to vacuum and 200ºC temperatures for 12 hours.Each sample was gradually heated with a continuous wave Ar ion laser with an unfocused 2mm wide beam.The gas liberated by this process was cleaned by a cryo-cooled system (at T = -125ºC) and two collectors.It was then analyzed for Ar isotopes in a MAP215-50 mass spectrometer equipped with a C-50 SAEZ Zr-V-Fe collector.

GEOLOGICAL SETTING
In South America, Paleoproterozoic terranes are widely distributed throughout the Amazon Craton (Santos et al. 2000, Tassinari & Macambira 2004), along its southeastern portion (João et al. 1987, Vasquez et al. 2008, Macambira et al. 2009), through the southern extent of the Amazon Basin, and also across its north and northeastern domains, running from Amapá up to Surinam, French Guyana and Guyana (Santos et al. 2000, Avelar et al. 2003, Delor et al. 2003, Rosa-Costa et al. 2006).In this work we focus on the Bacajá Terrane (Fig. 1), which is situated in the southeastern part of the Amazon Craton, between the Carajás Archean Province and the Amazon Basin (Santos 2003, Vasquez & Rosa-Costa 2008).Its evolution is linked to crustal growth and formation of synkinematic granites (João et al. 1987, Barros et al. 2007).
The study area is located in the Maroni-Itacaiúnas Province (Transamazonas Province), which evolved during the Paleoproterozoic.Geochronological studies have revealed Archean fragments (the Aruanã Complex), 2.3 Ga Paleoproterozoic metabasites (Itatá Formation), deformed granitoids of 2.2 to 2.1 Ga representing the early accretion and recycling of Archean rocks (Bacajaí and Arapari suites), and the foliated granitoids of the João-Jorge suite, dated at 2.07 Ga (Santos et al. 2000, Faraco et al. 2005, Vasquez et al. 2008, Macambira et al. 2009).South of Transamazonas province, the largest Archean craton nuclei is represented by the Carajás Province (Santos et al. 2000).
Earlier studies developed in this region pointed out the complex evolution of orogenic domains controlled by granites that were emplaced during the course of tectonic compression (João et al. 1987).Several later studies were undertaken at the regional level (Faraco et al. 2005, Vasquez et al. 2005) in what was then called the Bacajá Terrane (Santos, 2003).Regional lithologic layering is marked by the presence of kilometer-long lenses of metabasites oriented along the WNW direction parallel to the elongate granite bodies composed of differently foliated rocks.Barros et al. (2007) identified different structures in granitoids, many of which were interpreted to have formed in the magmatic stage, suggesting that granite emplacement was synchronous to regional compressive tectonic stress.

Paleoproterozoic metabasic rocks
Metabasites and amphibolites compose a N70W-striking belt having dozens of kilometers long and few hundred meters wide (Fig. 2).These rocks belong to the Itatá Formation of the Três Palmeiras Group (João et al. 1987, Vasquez & Rosa-Costa 2008) and are limited to the north and south by granitic bodies.Along the contacts between granitic plutons, the metabasic rocks display a strong N78/75SW foliation marked by preferred orientation of plagioclase and olive green amphibole.Where this foliation is not present, the metabasites show a massive structure and blastophitic texture.Brittle structures are represented by NNE-SSW, NE-SW and NW-SE fractures.En-echelon N45E fractures indicating dextral movement were observed near the contacts to granites.

Paleoproterozoic granitoids
The mesoscale structures are primary igneous layering, magmatic foliation with solid-state component, ductile shear zones, phyllonites, britlle-ductile shear zones, brittle faults and fractures.The schematic structural map (Fig. 2) shows the areas where the main structural features have been described.Among these structures, the first three are similar to those described by Barros et al. (2007).

Igneous layering
The term "layering", sensu Barbey (2009), refers to rhythmic alternation of layers of different compositions or grain sizes.Layering is commonly observed in the study area, and is defined by the rhythmic alternation of quartz-feldspathic and ferromagnesian levels (Fig. 3a).Sometimes the layering is marked by the alternation of fine-to medium-grained levels with coarse-grained domains.Pegmatite or aplite veins are locally found parallel to the layering, emphasizing the rhythmic alternation.In the granitoids from the Bacajá Terrane, magmatic layering shows subvertical and subhorizontal dips.In the first case, the strike varies from E-W to N70W.In the second case, the layering is generally affected by gentle WNW-trending folds with subvertical axial planes.

Quartzo-feldspathic veins
Quartzo-feldspathic veins and their host granitoids commonly present magmatic foliation.Earlier veins may be folded and more strongly foliated than the later veins where foliation is generally weakly developed.Examples of these features can be found in many outcrops in the Bacajaí, Arapari and João Jorge intrusive suites.At least three phases of quartz-feldspathic veins have been identified, many of which have been affected by dextral faults running N80E and by en-echelon sinistral fractures striking N45E.Magmatic foliation with solid-state component (Sn+1) When compressional tectonic stress is imposed during crystallization of magmas, the final structural pattern may be difficult to distinguish from metamorphic gneissic foliation observed in quartzo-feldspathic rocks.Criteria used to clarify these situations have been proposed by several authors (Paterson et al. 1989, 1998, Gower 1993, Miller & Paterson 1994, Pons et al. 1995, 2006, Barbey et al. 2008), and include the preferred orientation of undeformed crystals (mainly quartz), laterally homogenous ductile foliation surrounding xenolithes, presence of late magmatic shear zones that collect residual melts, coexistence of early foliated pegmatites with late weakly-or non-foliated pegmatites, and tectonically-controlled symplectites.
In cases where magmas are progressively deformed during cooling, the resulting fabric vary from preferred orientation of weakly deformed crystals to strongly flattened rocks showing gneissic aspect (Barros et al. 2001, Pawley & Collins 2002).Magmatic foliation with a solid-state component is found in granites throughout the region.N80W to N60W foliation in these cases is moderate to weak and defined by the preferred orientation of ferromagnesian minerals, feldspars and quartz.Outcrops and thin section analyses of granitoids throughout the region permit to suggest that quartz was deformed mainly by flattening.Quartz deformation varies from weak to strong, and is marked by the presence of weakly elongated aggregates of subgrains and new grains.Elongate mafic enclaves are parallel to foliation observed in the host granitoids suggesting that deformation, magma crystallization and development of foliation Sn+1 were synchronous.In general, magmatic foliation with solid state components (Sn+1) is subparallel to the primary subvertical layering.In rocks with porphyritic texture, we find a preferential orientation of the euhedral microcline phenocrysts.Some isolated phenocrystals are disposed obliquely or perpendicularly to the foliation, indicating crystal rotation in the presence of liquid.Locally, foliation detours around amphibolite xenolithes.This demonstrates a large viscosity difference between the xenolith and the partially crystallized acid magma.In weakly foliated granitoids, the quartz is fine-to medium-grained, has weak to moderate preferred orientation and show strong undulose extinction, subgrains and new grains (Fig. 3b).Sutured contacts between new grains of quartz suggest high temperature recrystallization controlled by grain boundary migration.The microcline crystals are inequigranular, fine and medium, occasionally large, anhedral and subhedral, showing weak preferential orientation and sometimes flame perthites.Microcline shows moderate wavy extinction and well-developed crossed twins, while plagioclase crystals are inequigranular, fine and medium, occasionally large, anhedral and subhedral and have weak to moderate preferred orientation.Plagioclase also shows undulose extinction, kink bands and small fractures.
The biotite crystals are fine, anhedral and subhedral and display moderate preferred orientation.Biotite crystals also occur as elongated clots parallel to the foliation.Recrystallization of biotite produced thin levels of new grains disposed close to coarse and deformed crystals.Symplectites of biotite and amphiboles formed by stress-controlled corrosion of faces parallel to the foliation can be observed.Barros et al. (2007) have considered this texture as the result of tectonic stress on non-completely consolidated magma.

Ductile shear zones
The shear zones are decimeter to meterwide, and in general NW-SE trending with a subvertical orientation concordant with the regional structures (Fig. 2).The rocks within the shear zones are gray and more fine-grained than the host rock.In protomylonites (Fig. 3c), quartz crystals form aggregates of new grains and subgrains with strong preferred orientation and anastomosed aspect, especially around prophyroclasts.In some elongated quartz aggregates, subgrain boundaries are oriented oblique to the foliation.The microcline is fine-to coarse-grained, anhedral and subhedral and locally augen-shaped.Microcline has a preferred orientation, flame perthites and shows recrystallized margins with fine new grains.Undulose extinction varies from weak to strong, evolving towards subgrains and micro-faults.The plagioclase crystals are fine-to coarse-grained, anhedral and subhedral and display a good preferential orientation.Biotite forms anastomosed levels and shows recrystallization along its edges, kink bands and fish-shape.
West of Altamira a 50m wide mylonite/ultramylonite is found.Themylonites are fine-grained and dark gray and has a N45W/subvertical orientation.Mineral lineations associated with this plane have a plunge of 05/125.Quartz is fine, anhedral, has irregular contacts, and shows undulose extinction, subgrains and neoblasts.Quartz forms the rock matrix in oriented and anastomosed fine levels.The microcline is fine to coarse-grained, anhedral and shows recrystallized edges, irregular contacts and augen-shaped forms, strong undulose extinction and flame perthites.The new fine grains of microcline, plagioclase and quartz form the matrix.Some plagioclase crystals are augen-shaped and recrystallization along the edges has produced fine new grains (Fig. 3d).
South of Anapu (Fig. 2), meter-wide WNW-trending mylonite zones developed along the contacts between the orthogneisses of the Aruanã Complex and the syntectonic foliated granitoids of the Bacajaí and Arapari suites.

Phyllonites
Between Bom Jardim and Anapu villages, a kilometer long and decameter wide N80E/84SE shear zone crosscuts the granitoids (Fig. 2).This shear zone is located along the contact between the Pacajá orthogneisses and the granitoids of the João Jorge suite.The most striking feature of this structure is the presence of muscovite-chlorite(?)-quartz-rich phyllonites (Fig. 4a).0.5 to 1 meter-wide veins of quartz can be observed in this shear zone.

Brittle-ductile shear zones
Locally, foliated granitoids were affected by decimeter-to meter-wide brittle-ductile faults striking N60W with subvertical dips.In these cases, intense deformation has reduced the size of the grains and altered the color of the rocks to a dark gray.These cataclasites contain up to three centimeters wide quartz veins that may or may not have formed at the same time as the cataclasites.
Under the microscope, the cataclasites are characterized by the presence of strongly strained inequigranular anhedral quartz.The quartz crystals are fine-grained and show strong undulose extinction, deformation lamella and subgrains.Crystal showing irregular forms and sizes varying from very fine to coarse suggest that cataclastic flow with crystal fragmentation was the dominant formation mechanism.Fine veins of quartz or epidote occasionally fill faults and fractures.Microcline has well developed perthites, strong undulose extinction and microfractures and microfaults.Plagioclase crystals are fine-to coarse-grained and display slightly curved twinned crystals, microfaults and undulose extinction.In the most intensely deformed zones, anhedral crystals make up the largest part of the cataclastic matrix (Figs.4b and 4c), which is composed of angular crystals of varying sizes.In the domains where intense deformation has taken place, epidote and opaque minerals are commonly found.Near the shear zone, the feldspar is fractured, shows microfaults and is cut by epidote veins.

Brittle faults
The faults that affect the granitoids have few tens of meter wide and kilometers length.The fault planes show striations and steps (Figs.4d, 4e and 4f ), en-echelon fractures, sigmoidal brittle structures and vein displacements.
Near Anapu N30W/81NE brittle faults and 142/24 striations were observed.These features attest to the strikeslip character of these faults.Some faults have decimeter to meter-sized steps (megasteps) indicating sinistral sense.In this outcrop, there are also N33E/87SE oriented faults with kinematic indicators showing sinistral movement.Muscovite crystals formed on this plane were collected for 40 Ar/ 39 Ar dating (Fig. 4f ).
Crosscutting relationships suggest that the N33E fault could be formed after the N30W faults mentioned above.In many situations, the principal direction of the fractures and the kinematic indicators correspond to the Y, R and R fractures of Riedel's model.The orientation of the N33E/87SE faults is almost parallel to the maximum principal stress direction (N30E) of a tectonic stress field that would have been present since the magmatic arc growth.In the outcrop where phyllonites are found (PMI-11), three main families of later faults have been identified.Here, the repeated patterns and angular relationships of the fault planes suggest dextral strike-slip patterns with an oblique component towards N82E.Another studied outcrop (PMI-19) is located near Anapu (51º13'31''W/3º26'47''S).The age defined by the plateau is 1975 ± 9 Ma, within error of the age of 1968 ± 11 Ma obtained by the ideogram peak (Figs.5c and5d; Table 1).The similar ages obtained by ideogram peaks and plateau indicate on one hand that the ages are real and on the other hand that the ideograms represent have high probability peaks.The similarity of the ages obtained from the outcrops suggests that this phase  of deformation took place on a regional scale.During the Transamazonian Cycle, the voluminous syntectonic granite magmatism that marked the evolution of the Bacajá recorded different moments of compressive stress during crystallization and emplacement of granite bodies.These late structures have taken place in the orogeny evolution at lower temperatures than those of the mylonites (which formed under temperatures higher than 400ºC).Alike studies correlating structural data of granites and 40 Ar/ 39 Ar ages were realized in Late Ordovician units from the Caledonian orogen in Norway (Scheiber et al. 2016) and in the Eocene Adamello massif, Italian Alps (Pennacchione et al. 2006).In both cases brittle structures are related to the final compressional stress that played a role close to the magmatic cooling.

DISCUSSION
During the emplacement of the granite plutons in the Bacajaí, Arapari and João Jorge intrusive suites, magmatic structures (igneous layering and magmatic foliation with solid state components) developed throughout the region.These structures reflect deformation during emplacement and early crystallization of the granite bodies.At this stage, inflation of magma chambers may have enhanced flow and differentiation, resulting in igneous layering.
In the Paleoproterozoic domains of the West African craton Pons et al. (2006) considered most of the structures in the granites to be developed by the mixing of heterogeneous magmas and the movement of crystals and liquids via thermal convection.Pons et al. (2006) also admitted that the segregation of magmas within their chambers were mechanisms responsible for igneous layering.These processes most probably had an impact on the felsic to intermediate magmas of the Bacajá Terrane.According to Pons et al. (2006), the close association of igneous layering with the existence of mafic enclaves in calc-alkaline plutons suggests that the layering formed due to recurrent injection of basic magmas.In this case, disruption and mixing of the mafic enclaves with acid to intermediate magma may be the effective cause of the development of layering.However, layering of igneous rocks may form by a number of mechanisms, and disruption of early-formed enclaves is just one of them in the granites from the Bacajá region.
With the significant changes that occur in the liquid/crystal ratio during crystallization (Vigneresse et al. 1996, Barbey 2009), regional structural behavior of the granitoids in the Bacajá Terrane and widespread magmatic foliation would have been formed during crystallization.These submagmatic structures are recorded by the homogenous preferred orientation of minerals at a regional scale, the scale of outcrops and in thin section.The flattening of quartz crystals commonly found in syntectonic granites (Gapais & Barbarin 1986) is a clear example of magma deformation reaching the rheological threshold.These structures could be formed under conditions of decreasing temperatures and increasing strain rates, as expected in syntectonic granites.
In the Transamazonas Province, the different pulses of granite emplacement in a compressive tectonic stress field are evidenced by the elongated form of the intrusions on aeroradiometric maps (Carneiro et al. 2012).The elongated shapes of the syntectonic granites, their contacts with host rocks, and/or their different rheology, together with their fabric would have resulted in domains of contrasting rheology and structural anisotropy.In the study area some ductile shear zones, phyllonitic shear zones and faults developed along boundaries between granite plutons or along contact between petrographic facies.Cross-cutting relations and differences in thermal regime show that the ductile-brittle (phyllonites) and brittle-ductile shear zones (cataclasites) formed after the regional ductile foliation in the granites.
Other ductile shear zone controlled by rheological differences is found located to the northeast and south of Anapu, along the contacts between the granites of the Arapari and Bacajá Paleoproterozoic suites and the Neoarchean orthogneisses of the Aruanã complex.The Neoarchean domain probably behaved more rigidly and deformation there would have been more easily accommodated by the Paleoproterozoic granitoids of domains characterized by higher thermal gradients.
According to Ranalli (2000), preexisting faults can be reactivated if the difference of necessary stress is lower than the difference of critical stress for the formation of a new shear plane where tectonic stress is high.Weak zones (Holdsworth et al. 1997) may be represented by such structural anisotropies as shear zones, faults and rheological discontinuities.These anisotropies are locations that enhance tectonic reactivation.The Bacajá domain contains the first two of the four types of criteria used by Holdsworth et al. (1997) to identify reactivation features (structural, geochronological and stratigraphic).

CONCLUSIONS
The syntectonic granites of the Transamazonas province exhibit structures formed under different temperature and rheological conditions.In the initial phases of pluton evolution, igneous layering was controlled by the dynamics of the magmatic chambers and by regional compressive forces, so that deformation would have occurred in not completely crystallized magma.Under these conditions, most deformation is recorded by regional homogenous subhorizontal shortening.With a decrease in temperature, recrystallization would have increased, along with the progressive development of submagmatic foliation.The subsequent evolution from ductile, ductile-brittle, brittle-ductile shear zones, and eventually brittle faults testifies to deformation during a period of cooling.The geochronological data available for the Bacajá region point to a protracted history of construction of magmatic arcs from 2200 Ma to 2080 Ma. 40 Ar/ 39 Ar ages obtained on muscovite from brittle strike/slip faults (N33E/87SE) suggest that the brittle conditions were reached at ~1975 Ma, about 100 m.y. after the emplacement of the Arapari (2150 Ma) and João-Jorge (2075 Ma) granite suites.Structural observations suggest that compressive forces persisted during this time interval.Hence it seems that in long duration Paleoproterozoic orogenetic zones, the installation of successive magmatic arcs was associated with stresses that were maintained for a long time after the complete crystallization of the arc-related plutons.The Paleoproterozoic brittle faults present in granites from the Bacajá Terrane are rare examples of preserved old structures that were not reactivated by younger deformation.

Figure 2 .
Figure 2. Schematic geological map of the Bacajá Terrane showing the main structures and the location of the muscovite-bearing faults dated by the Ar-Ar method.
Ar dating was undertaken for coarse-grained muscovite crystals collected along two N30E fault planes 140 kilometers apart.In both cases the rock faulted is granite.One of the points (PTZ-01) is situated south of Brasil Novo (52º33'19''W / 3º37'39''S).The age of 1977 ± 8 Ma is defined by both ideogram peak and plateau (Figs.5a and 5b; Tab. 1).

Figure 5 .
Figure 5. Diagrams of 40 Ar/ 39 Ar dating of muscovite collected on fault planes affecting Paleoproterozoic granites from Bacajá Terrane, Pará state.(A)and (B) Plateau and ideogram from the sample PTZ-01; (C) and (D) Plateau and ideogram from the sample PMI-19.Sample locations are shown in Fig. 2.