Petrogenesis , UPb and Sm-Nd geochronology of the Furna Azul Migmatite : partial melting evidence during the San Ignácio Orogeny , Paraguá Terrane , SW Amazon Craton

O Migmatito Furna Azul e um complexo de ~10 km2 localizado em Pontes e Lacerda, Mato Grosso, Brasil. Pertence ao Terreno Paragua, proximo ao limite com o Terreno Rio Alegre, sudeste da Provincia Rondoniana - San Ignacio - Craton Amazonico. O migmatito consiste de metatexitos transicionais com enclaves anfiboliticos e injecoes dioriticas. Os metatexitos sao distinguidos em ricos em residuo e ricos em leucossoma e exibem tres fases deformacionais marcadas pelo bandamento estromatico dobrado afetado por uma xistosidade espacada e metamorfisados na facies anfibolito alto, representada por granada, biotita e sillimanita, bem como pela formacao de clinopiroxenio nos enclaves. O retrometamorfismo para a facies xisto verde e marcado pela formacao de clorita, muscovita/sericita e prehnita. O metatexito rico em residuo apresenta maiores teores de CaO, Na 2O, separando-os do metatexito transicional enriquecidos em K2O, Ba e Rb. Comparando com produtos de anatexia, nota-se uma afinidade com produtos de protolitos tonaliticos e/ou anfiboliticos. Os dados geocronologicos (U-Pb SHRIMP em zircao e Sm-Nd em rocha total) mostraram que o metatexito rico em residuo teve sua cristalizacao em 1436 ± 11 Ma, com idade modelo TDM de 1,90 Ga e e Nd(1,43) de -0,54, enquanto a injecao dioritica cristalizou em 1341.7 ± 17 Ma com idade modelo TDM de 1,47 Ga e e Nd(1,34) de 3,39. Esses resultados evidenciam que o protolito do Migmatito Furna Azul teria sido formado durante a Orogenia San Ignacio (1.43 Ga) posteriormente retrabalhado, servindo de embasamento para o magmatismo tardi a pos-colisional representado pela Suite Intrusiva Pensamiento.


INTRODUCTION
Several geotectonic reconstructions of the Rondonian-San Ignacio Province in southwestern Amazon Craton take into account the amalgamation of intra-oceanic volcanic arcs and the development of continental arcs, attributed to the closure of oceans and microcontinent-continent collisions, including late to post-collisional stages of orogenic collapse in a tectonically active Mesoproterozoic margin (Cordani et al. 1979, 2009, Teixeira et al. 1989, 2010, Tassinari & Macambira 1999, Geraldes et al. 2001, Ruiz 2005, Bettencourt et al. 2010).In this context of successive continental agglutinations, the Paraguá Terrane, an allochthonous microcontinent, collided with the Amazon proto-Craton during the San Ignacio Orogeny (1.56 -1.30Ga) (Bettencourt et al. 2010, Ruiz et al. 2011).
The stratigraphic sequence proposed by Litherland et al. (1986) consists of the Lomas Manechis Granulite Complex (ca.1900 Ma), followed by an association of migmatitic gneisses of the Chiquitania Gneiss Complex.The younger San Ignacio Schists Group established a stratigraphic sequence in which the Chiquitania Gneiss Complex represents a sedimentary, orogenic event that started after 1690 Ma (Boger et al. 2005).The Lomas Manechis Granulite Complex consists of a granite-gneiss suite bearing amphibole and orthopyroxene originated between 1690 and 1660 Ma.Proposed models consider the sequences to be granulitized and migmatized during the San Ignacio Orogeny, and magmatism to have taken place in a magmatic arc setting represented by several orogenic granitoids emplaced around 1300 Ma (Matos et al. 2008, 2009, Jesus et al. 2010, Ruiz et al. 2012, Nalon et al. 2013, França et al. 2014).
The Furna Azul Migmatite (FAM), as defined by Nascimento et al. (2013), was first correlated to the gneisses of Serra do Baú Complex (Ruiz 2005).This unit is composed by othogneisses stratigraphically correlated to the gneisses of Chiquitania Gneiss Complex, which outcrop mainly close to Santa Barbara cliffs and Ricardo Franco ridges, in Mato Grosso state.
The purpose of this paper was to discuss the petrogenesis of the FAM and to temporally establish the metamorphic episodes that allowed the melting event, based on geochronological data.
Concepts of Mehnert (1968) on migmatite, which evolved from Sederholm (1907) and Holmquist (1916), are first for the migmatite issue.Sawyer (2008) suggested that protholiths that undergo partial melting provide neosome as their main product.Neosome is comprised of a solid residual portion (residuum) and a silicate melt, which is the precursor magma (leucosome).The author proposed that, in a migmatitic system, the portion that does not undergo partial melting and consists essentially of refractory minerals is called paleosome.In this paper, the concepts and systematic classification by Sawyer (2008) have been applied.

REGIONAL GEOLOGICAL CONTEXT
The Amazon Craton is one of the largest cratonic areas in the world.It is located in the north-northwestern region of South America and comprises the largest Precambrian segment of the South American Platform (Fuck et al. 2008, Cordani et al. 2009, Brito Neves 2011).It is divided in Central Brazil and Guyana shields by the Amazon Basin and it is surrounded by Neoproterozoic-Cambrian and Andean mobile belts (Tassinari & Macambira 1999).
The study area is located in the south-southeastern portion of the Rondonian-San Ignácio Province, considered by Bettencourt et al. (2010) as an orogen composed of Paleo and Mesoproterozoic terranes.These terranes are defined as the latest stage of cratonization during the Meso-Neoproterozoic, mainly in the regions where the Sunsás-Aguapeí Orogeny rework was less intense.They enabled the recognition of features of the San Ignácio Orogeny and identified four terranes (Fig. 1B): Jauru (1.78 -1.42 Ga), Rio Alegre (1.51 -1.38 Ga), Alto Guaporé (<1.51 -1.33 Ga) and Paraguá (1.74 -1.32 Ga).
Discussions on the stratigraphic and deformational outline of the Paraguá Terrane have been carried out since the first studies (Litherland & Bloomfield 1981, Litherland et al. 1986), into recent studies (Boger et al. 2005, Santos et al. 2008, Matos Salinas 2010, Bettencourt et al. 2010).In the area, the basement of the Aguapeí Group is composed by migmatitic gneisses, with some occurrences of granulites with ages ranging from 1711 to 1640 Ma (Figueiredo et al. 2013, Matos et al. 2013, Faria et al. 2014).These rocks crop out as mega-xenoliths or as basement windows in the midst of several granitic intrusions from San Ignácio Orogeny (Geraldes et al. 2001, Matos et al. 2009, Jesus et al. 2010, Ruiz et al. 2012, Nalon et al. 2013, França et al. 2014) (Fig. 2A and Tab. 1).The orthogneisses of this region were grouped in the Serra do Baú Complex by Ruiz (2005) and are considered as high-K calc-alkaline protholiths of metaluminous to peraluminous character emplaced in a magmatic arc setting due to subduction of a Paleoproterozoic oceanic crust, and later reworked during the San Ignácio Orogeny (Figueiredo et al. 2013, Faria et al. 2014).

FIELD AND PETROGRAPHIC ASPECTS
The FAM is partially covered by the metasedimentary rocks of the Aguapeí Group and by sediments of the Guaporé Formation.It is in abrupt contact with the Amparo Granite (Fig. 2B).Granitoid dikes, amphibolitic enclaves and dioritic injections are commonly observed.
Migmatites consist of rocks with heterogeneous composition and texture that show evidence of ductile and ductile-ruptile deformation by three deformation phases.The first phase consists of a millimeter-size compositional banding (S 1 ) in residuum-rich rocks.Granitic leucosome occurs parallel to the compositional banding or cross-cutting it.Both structures are deformed, by nearly isoclinal folds that were developed during the second deformation phase.This deformation phase is associated with transposition and development of an schistosity (S 2 ) towards northwest, ranging between 85 and 90º, dipping to northeast and southwest (Fig. 3A).The third deformation phase is marked by a gentle folding, spaced foliation (S 3 ) of kind crenulation cleavage and shear zone with oblique-transcurrent component in the northeast direction and with southeast dips (Fig. 3B).

Residuum-rich metatexite
The residuum-rich metatexite is gray, leucocratic to mesocratic, with centimeter veins of granitic leucosome.It is parallel to the first deformation phase foliation and crosscut by late leucosome veins, which, in turn, are controlled by the main crenulation axis generated during the second deformation phase.It is fine grained with few grains that reach up to 2.0 mm, and the texture is granolepidoblastic composed by quartz (32%), plagioclase (29%), biotite (28%) and garnet (9%).Zircon, orthoclase, apatite, titanite and opaque are the accessory minerals while chlorite, muscovite/sericite, prehnite, clay minerals and epidote compose the retrometamorphic assemblage.The residuum-rich metatexite presents preserved garnet grains, while in other lithotypes its pseudomorphosed, suggesting higher interaction with the leucosome.
Quartz forms xenoblastic grains of different sizes, with irregular borders and strong wavy extinction.Garnet is the main inclusion phase.Plagioclase (andesine) is xenoblastic with irregular borders and orthogonal or oblique twining to the C axis, suggesting boundary consumption.Biotite displays a subidioblastic aspect and evidence of reaction to prehnite, muscovite and chlorite.Garnet is granular and disseminated poikiloblastic, envelops anhedral grains of quartz, and displays strain shadows composed of biotite (Fig. 4A), as well as chlorite parallel to the schistosity.
Quartz is stretched with irregular borders and deformation lamella with wavy extinction.Quartz sometimes occurs as interstitial drops resulting from a melting reaction (Fig. 4B).Orthoclase usually occurs in anhedral to subhedral shapes and rarely displays twinning.Moreover, orthoclase has plagioclase drops and perthite intergrowth.Plagioclases are <1.0 mm, with irregular borders that crosscut albite and pericline twinning terminations.This suggests consumption by perithetic phase reaction.In other situations, plagioclase occurs associated to biotite as a product of garnet reaction, suggesting pseudomorphism (Fic.4C), or as subhedral tabular grains with evidence of sericitization, argillization and carbonation.
Orthoclase is subhedral and larger than 3.0 mm, twinless, with string perthite and granular quartz and biotite inclusions (Fig. 4D).Quartz is interstitial in feldspar grains or occurs as a mineral reaction product.Plagioclase is tabular shaped with albite twinning and locally showing myrmekitic texture.Biotite is interstitial with quartz and sillimanite fringes (Fig. 4E and Fig. 4F).Garnet is relictual xenomorphic and replaced by biotite and plagioclase.

Analytical procedures
Major, minor and trace element concentrations were determined for 26 samples of the FAM.Raw data are shown on Table 2. Due to the structural complexity and  injections and metatexites.Residuum-rich rocks show lower contents of Ba, Rb, and K, suggesting fractionation processes.Dioritic injection samples show a positive correlation of Sr, La, Eu, Y and Yb against SiO 2 , attesting light rare earth element (LREE) and heavy rare earth element (HREE) enrichment.The negative correlation between Ba and K suggests an evolution from intermediate to acidic magmas, which is not the case for rocks with alkaline derivation.Sawyer (2010) demonstrates that migmatites from Opatica Subprovince, Canada, might have been originated from partial melting of a leucogranodiorite.In the K 2 O versus CaO + Na 2 O diagram (Fig. 10A), effects of anatexis can be observed: K 2 O enrichment in leucosome-rich metatexites and CaO + Na 2 O enrichment in residuum-rich rocks.However, FAM metatexites do not overlap with melt fields from granodioritic protholiths, and, yet, they overlap with glass from tonalitic melts, according to experimental works by Castro (2004) and Watkins et al. (2007).
Patiño Douce (1999) features the geochemical patterns of melts from several sources.In Figure 10B and Figure 10C, dioritic injection and residuum-rich metatexite samples match melts from an amphibolite source, while transitional leucosome-rich metatexites match melts from mafic pelites, grauvaques, or calc-alkaline granite.

Rare earth elements
Using the primitive-mantle-normalized values of McDonough & Sun (1995), migmatites show LREE enrichment with Eu depletion, indicating plagioclase fractionation.One sample of leucosome-rich metatexite had low HREE values, suggesting that these elements had an immobile behavior during the melting or that the melting condition did not reach the solubility of these elements.Dioritic injection patterns show intense light and heavy rare earth elements (REE) enrichment, indicating an enriched-mantle source or intense processes of crustal assimilation.Eu depletion suggests plagioclase fractionation (Fig. 11A).variations of neosome proportion, the analyzed samples do not necessarily represent individual portions of a migmatite.However, the residuum metatexites showed a compositional pattern that might matches partial melting fractionation.On the other hand, a typical differentiation pattern can be observed in the transitional leucosome-rich metatexite samples.
Around 1 kg of each sample was crushed, homogenized and pulverized at laboratories of the Universidade Federal do Mato Grosso, Brazil, and sent to Acme Analytical Laboratories for the determination of major and trace elements by inductively coupled plasma-atomic emission spectrometer (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS), respectively.Detailed experimental procedures are available online on the Acme homepage.Geochemical data processing was performed using the GeoChemical Data toolkit (GCDkit) software by Janoušek et al. (2011).

Major and trace elements
Since they display complex structural pattern and distinctive neosome proportions, FAM metatexites are considered composite rocks.The geochemical pattern tends to show progressive melting fractionation characteristics.However, classification diagrams should represent the protholith chemical composition.
In the TAS diagram (Fig. 7A), residuum-rich rocks show granodioritic composition while leucosome tends to granite field.Both are subalkaline.Dioritic injections plot in the syenodiorite field of the alkaline series.All lithotypes plot in the non-tholeiitic and peraluminous fields in the AFM and A/NK versus A/CNK diagrams, respectively (Fig. 7B and Fig. 7C).The tectonic discriminant diagrams, according to Frost et al. (2001), indicate a cordilleran character for the migmatites with a peraluminous affinity for the leucosome (Fig. 7D and Fig. 7E).Dioritic injections are ferroan and plot within the field with alkalic affinity.Metatexites plot in the volcanic arc granite field, and Dioritic injections plot in the syn-collisional field, in the Rb/30-Hf-3Ta ternary diagram (Fig. 7F).
In the Harker (1909) diagrams, all samples show a negative correlation of TiO 2 , Al 2 O 3 and MgO against SiO 2 , CaO, Na 2 O, K 2 O, FeO t and P 2 O 5 (Fig. 8), allowing the recognition of two distinct patterns that correspond to dioritic injections and metatexites.Negative correlations suggest fractionation of mafic minerals and calcic plagioclase.Al 2 O 3 , P 2 O 5 , and Na 2 O increments could be related to a magmatic evolution from alkaline basalts.
The large ion lithophile elements LILE (Ba, Rb, Sr and K) against SiO 2 diagrams (Fig. 9 Using the bulk continental crust normalization pattern by Taylor & McLennan (1995), metatexites show an enriched pattern with negative anomalies for Sr and Ti (Fig. 11B), indicating plagioclase, titanite and/or magnetite fractionation or low melting temperatures.The dioritic injection pattern shows LREE, Th and U enrichment, suggesting monazite concentration, and negative anomalies for Zr and Hf that might indicate a mantle affinity.
The melanosome-rich and leucosome-rich metatexite patterns, when normalized to the residuum rock average, show positive anomalies for Ba and K, suggesting that the k-feldspar concentration and Rb and Nb values are controlled by the biotite concentration in schlieren (Fig. 11C).The less incompatible element pattern can be indicated by zircon and garnet concentration in residual rocks, due to the recurrence of garnet as a peritectic phase in leucosome.

Analytical procedures
For the U-Pb SHRIMP II analyses, zircon mounts of dioritic injection and stromatic metatexite samples were prepared.In order to select the target spots in the zircon crystals, cathodoluminescence imaging (CL) was obtained by scanning eletron microscopy at the Laboratório de Microscopia Eletrônica de Varredura (Scanning Electron Microscopy Laboratory) of the Centro de Pesquisas Geocronológicas (CPGeo -Geochronological Research Center) of the Universidade de São Paulo.Analytical procedures of U-Pb SHRIMP analysis at CPGeo were described in details by Sato et al. (2014).The reference material was zircon Temora 2 (416.8 ± 3.8 Ma) for U/Pb ratio calibration and zircon SL13 (U = 238 ppm) for U composition.Preferential sites for the analyses were selected so that these were carried out in the CL lighter areas of the zircon and avoiding fractures, when possible.Age calculations were performed with the Isoplot/ EX software of Ludwig (1999).Results are shown with 2σ standard deviation.

WHOLE ROCK SM-ND ISOTOPE GEOCHEMISTRY
Analytical procedures NF54B (melanosome-rich metatexite) and NF42C (dioritic injection) whole rock samples were pulverized at the Laboratório Multi-Usuário em Técnicas Analíticas (LAMUTA -Analytical Techniques Laboratory) of the Department of Mineral Resources at the Universidade Federal do Mato Grosso, and analyzed at the Laboratório de Geologia Isotópica (Pará-Iso -Isotope Geology Laboratory) of the Universidade Federal do Pará.Sample dissolutions were provided in Savillex ® capsules with a mixture of HNO 3 , HCl and HF after introduction of a 149 Sm-150 Nd spike solution.Sm and Nd extraction was performed by two-step ion exchange chromatography using Biorad Dowex AG 50 × 8 resin and Ln Eichron, according to procedures by Gioias and Pimentel (2000) and Oliveira et al. (2008).Isotopic data were acquired with a FINNIGAN MAT262 thermo ionization mass spectrometer.Nd isotopic ratios were corrected of mass discrimination effects using a 146 Nd/ 144 Nd ratio of 0.7219.Nd T DM model ages were calculated using the depleted mantle model of De Paolo (1981).

Results
Sm and Nd isotopic results yielded a Nd T DM model age of 1.90 Ga and a ε Nd(1.43Ga) of -0.54 for the metatexite.Dioritic injection data provided a Nd T DM model age of 1.47 Ga and a ε Nd(1.34Ga) of 3.39 (Table 4).

DISCUSSION AND CONCLUSIONS
This paper provides the petrographic, geochemical and isotopic characteristics of the Furna Azul Migmatite and associated dioritic injections.New geological and geochronological (U-Pb SHRIMP e Sm-Nd) data are required due to the lack thereof in a key area to the understanding of the Amazon Craton evolution during the Mesoproterozoic Era.
The recognition of migmatites in the region is also important since these rocks can be closely related to orogenic events.Based on petrographic and structural data, transitional metatexites were classified into leucosome-rich and residuum-rich.Leucosome-rich metatexites pointed to higher contents of mobile elements (e.g.K, Rb and Ba), whereas residuum-rich metatexites showed higher U contents.
Evidence of metamorphic recrystallization, such as poikiloblastic garnet, sillimanite, biotite symplectic texture and clinopyroxene in mafic enclaves, suggests an upper amphibolite facies condition for the metamorphism.Biotite with prehnite intergrowth has been assumed by Field and Roowell (1968) as originating from metamorphic temperatures between 350 and 450ºC and low pressure conditions.These conditions are easily achieved through silicate melts and residual rock interactions, under metamorphic conditions.However, the chlorite + muscovite/sericite + carbonate + clay minerals + biotite/prehnite association can be supported by retrograde metamorphism, representing a retrograde trajectory to the greenschist facies.
The lack of garnet + orthopyroxene + cordierite association suggests that partial melting was below the biotite breakdown.These phases are pointed out by Johnson et al. (2008) as peritectic reactions in a graywacke system, as well as in the FAM system.
Metatexites are similar in composition to the anatectic melts of amphibolite, tonalite and graywacke protholiths, as observed in petrographic and geochemical data.They can be geochemically compared to the metagraywackes used by Grapes et al. (2001) to determine the metamorphic grade of quartz-feldspathic rocks.Under low-pressure metamorphism, such rocks can be distinguished between hornblende + (biotite + muscovite + plagioclase ± garnet) bearing or lacking rocks, while a high metamorphic grade is marked by a lack of muscovite and peraluminous granite melt.Therefore, the Patiño Douce and Harris (1998) eaction matches the garnet + sillimanite + melt through biotite breakdown.
The REE pattern of dioritic injections demonstrates both light and heavy REE enrichments, suggesting likely derived from an enriched mantle source, resembling the Sylvester (1998) model of lithospheric delamination and enriched mantle underplating owing to low pressure, and peraluminous and post-collisional magmatism of the Lachlan Fold Belt.
The integration of geological, geochronological and isotopic data points out an age of 1.43 Ga for the metatexite protholith magmatism.The Sm-Nd data indicate that this very early igneous event of the San Ignácio Orogeny (1.57-1.30Ga) involved the partial melting of an Orosirian continental crust around 1.90 Ga.
The following magmatic event, around 1.34 Ga, generated dioritic injections through the partial melting of a younger continental crust, extracted from the mantle around 1.47 Ga.According to geochemical and isotopic signatures, a genetic link between the dioritic injections and the metatexite protholith could not be demonstrated.
Regarding the metamorphic and magmatic events of the Paraguá Terrane, the data shown point to a correlation between dioritic injections and the Pensamiento Intrusive Suite (Litherland et al. 1986, Bettencourt et al. 2010, Nalon et al. 2013, França et al. 2014), probably related to syn-to tardi-kinematic granites of a continental arc during the San Ignácio Orogeny.
The age of the Furna Azul Migmatite does not match similar rocks from the Chiquitania Complex and Serra do Baú Complex, which ranged from 1.78 to 1.72 Ga (Santos et al. 2008, Faria et al. 2014, among others), and with the granulites of Lomas Manechis Complex, with ages between 1.67 and 1.62 Ga (Boger et al. 2005, Matos et al. 2013).Migmatites show protholith age consistent with the San Ramon Granite (1.43 Ga), Bolivia, as presented by Matos Salinas (2010).
The data herein not only analyzed the petrogenetic processes of the Furna Azul Migmatite but also indicated the occurrence of an unknown magmatic event around 1.43 Ga within the Paraguá Terrane, which is worth to be investigated in future studies.

Figure 2 .
Figure 2. (A) Geological map of the Paraguá Terrane modified from Ruiz et al. (2011).The dashed line represents the boundary between Brazil and Bolivia -southwestern Mato Grosso; (zr) and (mnz) are zircon and monazite, respectively; (B) detailed geological map of the study area.
Figure 4. (A) Pre-cinematic garnet with quartz inclusions and biotite strain shadow; (B) pseudomorphic melting drops indicate the enveloping of muscovite and rounded quartz grains; (C) garnet partially pseudomorphed to biotite and plagioclase; (D) rounded plagioclase inclusions within subhedral orthoclase; (E) biotite with quartz intergrowing as fringes; (F) sillimanite indicating high metamorphic grade.Parallel polarization on (A) and cross polarizationon (B, C, D, E and F).Abbreviations are as in Fettes & Desmons (2008).

Figure 5 .
Figure 5. Photomicrographs of banded amphibolite enclaves (A, B, C, and D) and amphibole schist (E and F) showing: (A) compositional grading among hornblende + plagioclase, diopside + plagioclase, and quartz + hornblende bands; (B) detail of a moderate amount of quartz within the quartz + hornblende band; (C) poikiloblast diopside; (D) nematoblastic texture composed of hornblende and pseudomorphosed plagioclase; (E) nematoblastic texture composed of hornblende, quartz, plagioclase, biotite, and chlorite; (F) tremolite/actinolite and cummingtonite.Parallel polarization on A and E and cross polarization on B, C, D and F.

Figure 6 .
Figure 6.Photomicrographs of dioritic injections showing (A) myrmekitic texture in plagioclase; (B) biotite replaced by chlorite, prehnite, and epidote; (C) fractured quartz with embayments and fractures; (D) fractured garnet associated with chlorite.Parallel polarization on D and cross polarization on A, B, and C.
classification scheme.

Table 1 .
Geochronological and isotopic data for the basement rocks of Paraguá Terrane in the southwestern region of Mato Grosso.

Table 2 .
Geochemical data of Furna Azul Migmatite samples (oxides in percentage and trace elements in ppm).

Table 4 .
Sm and Nd whole rock data for the Furna Azul Migmatite samples.Pb/ 206 Pb values did not permitted to calculate a concordant age from the seven zircon crystals.Excluding the grain 1.1, with the older 207 Pb/ 206 Pb, the other six zircon grains yielded a concordant age of 1341,7 ± 17 Ma (MSWD = 3.0; Fig.12D) which may represent the age of emplacement of the injection.The older 207 Pb/ 206 Pb age of 1448 Ma of grain 1.1 can be related to an inherited component in the zircon crystal.