On the crystallization conditions of the Neoproterozoic, high-K calc-alkaline, Bragança Paulista-type magmatism, southern Brasília Orogen, SE Brazil

ABSTRACT:

Syn-orogenic Neoproterozoic granitic magmatism (ca. 630-600 Ma) in southern Brasília Orogen includes large batholiths of compositionally expanded high-K calc-alkaline granites (lato sensu) and small occurrences constituted by anatetic leuco-granites associated to the regional metamorphism. We present petrographic, mineralogical and geochemical data for a representative sample of the Bragança Paulista-type granite and estimates for magmatic crystallization conditions. The sample corresponds to a porphyritic hornblende-biotite monzogranite (CI ≈ 20) comprising perthitic microcline megacrysts within a medium-grained matrix with plagioclase (An ₃₀ Ab₇₀ ), quartz, microcline and mafic minerals. The main mafic minerals are biotite (0.51 ≤ mg# ≤ 0.57) and Ca-amphibole (potassic-magnesio-hastingsite, 0.47 ≤ mg# ≤ 0.52), with similar volumes; accessory phases (ca. 2 vol.%) are Fe-Ti oxides (ferrian ilmenite, titanohematite, hematite and magnetite), allanite, apatite, zircon, titanite and sulphides. Our results indicate crystallization at 510 ± 60 MPa and temperatures from ca. 970ºC (close-to-liquidus) to 755 ± 45ºC (close-to-solidus), under oxidizing conditions, compatible with 0 ≤ ΔNNO ≤ +1. Estimated water and halogen fugacity ratios are ƒ_H2O /ƒ_HF ≥ 10 ^5.8 , 10^3.5 ≤ ƒ_H2O /ƒ_HCl ≤ 10 ^3.8 and ƒ_HF /ƒ_HCl ≤ 10 ^-2.2 . The obtained pressure, integrated with available regional geobarometric data, suggests a relative uplift rate about 0.2-0.3 (km/Ma) from the main regional metamorphism to the post-orogenic magmatism in the area.

KEYWORDS:
Intensive crystallization parameters; high-K calc-alkaline granites; Bragança Paulista-type granites, Neoproterozoic magmatism, ­Socorro-Guaxupé Nappe

INTRODUCTION

Neoproterozoic granite magmatism was widespread in Socorro-Guaxupé Nappe System in Eastern São Paulo state and neighboring areas (e.g., Campos Neto et al. 1984, Wernick et al. 1984, Artur et al. 1991, 1993, Haddad 1995, Campos Neto & Caby 1999, Janasi et al. 2009, Vinagre et al. 2014). The most voluminous occurrences are made up of, compositionally expanded, high-K calc-alkaline syn-orogenic granites (lato sensu), which build the Socorro and Ipuiúna batholiths (e.g., Campos Neto et al. 1984, Wernick et al. 1984, Haddad 1995, Haddad et al. 1997). Several efforts were made in the last decades to increase our geologic, geochronological and geochemical knowledge on such magmatism as basic clues to understand the regional geodynamic environment in Southeastern Brazil (Artur et al. 1993, Hackspacher et al. 2003, Vinagre et al. 2014). However, none quantitative, or even qualitative, information on mineral chemistry and crystallization conditions for such granite rocks is available up to now.

As a part of a beginning work with experimental petrology and geochemistry using natural granites as starting samples conducted by us, we characterized in detail the petrography, geochemistry and mineralogy of a typical sample from the Bragança Paulista-type granite in the so-called Socorro Batholith, which allows put some constraints on the main intensive crystallization parameters (pressure, temperature and volatile activities) of the associated magmas. This contribution presents results and discusses them in the light of the current ideas on the evolution of the granite magmatism in the area during the Neoproterozoic. Given the representativeness of the selected sample, the presented inferences may be partial or total extended to the overall Bragança Paulista magmatism with the caution due to inherent limitations of the employed methods and reduced sampling.

GEOLOGICAL SETTING

The southern Brasília Orogen records the Ediacaran convergence of the São Francisco and Paranapanema cratons, acting as passive and active margins respectively (e.g., Campos Neto & Caby 1999). These margins were grouped into three main nappe terranes (Fig. 1), as follows:

• the Carrancas Nappe System, made up mainly by quartzite and graphite-bearing metapelites, which represents a deformed sedimentary pile with passive margin affinity, metamorphosed under greenschist to lower amphibolite facies by 590-570 Ma (Trouw et al. 2000, Campos Neto et al. 2004, 2011);

• the Andrelândia Nappe System, constituted by a great variety of ortho- and paragneisses metamorphosed under mid-pressure amphibolite to high-pressure granulite facies by 610-605 Ma, which is currently interpreted as an accretionary prism (Campos Neto et al. 2004, 2007, 2010, 2011);

• the Socorro-Guaxupé Nappe System, made up mostly of paragneisses, migmatites and granulites formed during the main regional metamorphism at ca. 630-600 Ma, which represents an early magmatic arc related to an older active continental margin (Campos Neto & Caby 1999, Janasi 1999, Del Lama et al. 2000, Janasi et al. 2005, Martins et al. 2009, Campos Neto et al. 2011, Salazar Mora et al. 2014, Rocha et al. 2017). In this nappe terrane, the estimated conditions of the ultra-high temperature metamorphism peak were ca. 1,170 MPa and 1,030°C, with a post-peak cooling at ca. 750 MPa and 865°C (Martins et al. 2009, Rocha et al. 2017).

Figure 1:
Geological map of Socorro-Guaxupé Nappe System area, highlighting the main syn- and post-orogenic granitic occurrences. Simplified from Janasi et al. (2009).

An extensive Neoproterozoic granite magmatism (ca. 2,200 km²) was emplaced in the Socorro-Guaxupé Nappe System, in part controlled by expressive NE-SW shear zones (Campos Neto & Caby 1999, Campos Neto et al. 1984, Janasi et al. 2009). It is divided into two main associations (Fig. 1): a syn- orogenic magmatism (ca. 630-600 Ma), represented by the Bragança Paulista-type (Socorro I) and Nazaré Paulista-type granites, and a post- orogenic magmatism (ca. 590-570 Ma), represented by the occurrences from the Itu Granite Province.

The Bragança Paulista-type granites comprise a high-K expanded calc-alkaline association (I-type in the sense of Pitcher 1982), made up by metaluminous (to slightly peraluminous) porphyritic tonalite, granodiorite and dominant granites (Campos Neto et al. 1984, Wernick et al. 1984). Previous geochronological data points to emplacement ages of ca. 630-610 Ma (zircon U-Pb), under arguably mid-crustal conditions (Artur et al. 1991, 1993, Ebert et al. 1996, Töpfner 1996). However, recent Sensitive High-Resolution Ion Micro Probe (SHRIMP) II U-Pb dating over zircon crystals from the sample herein studied gave a younger concordia age of ca. 605 Ma (V.A. Janasi, personal communication).

The Nazaré Paulista-type granites are composed of peraluminous garnet-, biotite- (± sillimanite, ± andalusite ± muscovite) bearing S-type leucogranites, associated with migmatites developed over metasedimentary gneissic rocks. Field relationships suggest magma generation by decompression melting (Artur et al. 1991, Janasi 1999, Janasi et al. 2005, Martins 2006, Martins et al. 2009).

The Itu Granite Province is made up of post-orogenic sub-alkaline A-type and evolved high-K calc-alkaline metaluminous to slightly peraluminous biotite (± hornblende) granites (Vlach et al. 1990, Janasi et al. 2009) emplaced under relatively shallow crustal levels. The most typical representatives in the area are the Itu Batholith (Pascholati et al. 1987, Galembeck 1997, Galembeck et al. 1997) and most of the so-called Morungaba granites (Vlach 1993).

SAMPLING AND ANALYTICAL METHODS

Several Bragança Paulista granite outcrops were examined in detail and scrutinized in order to find a relatively fresh representative sample and avoid common hydrothermal alteration inputs (usually detected by dark red-colored alkali-feldspars megacrysts) due to solid-state deformations associated with late transcurrent moves along shear zones. The selected sample (BPs02) was collected in the type-area of the magmatism, along the SP-095 highway, which connects the Bragança Paulista and Amparo cities (Fig. 1). It is relatively homogenous, with a slightly oriented planar structure, and corresponds to a leucocratic, porphyritic, amphibole-biotite granite with centimetric-sized alkali-feldspar megacrysts in a medium-grained matrix constituted mainly by plagioclase, quartz and mafic minerals. Titanite and magnetite may eventually be detected in hand samples. In the selected outcrop, the granite shows incipient to strong planar flow structures with local superposition of more or less concordant solid-state deformation. Schlieren structures made up of mafic minerals and some plagioclase, as well as mafic microgranular enclaves with dioritic compositions, are common outcrop features (Fig. 2).

Figure 2:
Some representative structural features of the Bragança Paulista-type granite, as observed in the BPs02 studied outcrop. (A) Magmatic flow structures, superimposed by solid-state deformation, evidenced by the arrangement of the mafic minerals and a bit stretched pink alkali-feldspar megacrysts, in a heterogeneous zone of the outcrop. (B) Mafic microgranular enclaves of dioritic composition, and irregular mafic schlieren zones, with pinkish megacrysts of alkali-feldspar slightly oriented.

The laboratory work was done at the GeoAnalitica core facility (University of São Paulo), except for whole-rock trace element analysis, performed at the Isotope Geology Laboratory (State University of Campinas). Petrographic analysis was done under transmitted and reflected light on polished thin sections using Zeiss Axioplan and Axio Imager optical microscopes. Modal proportions were obtained by point-counting, integrating alkali-feldspar, plagioclase, quartz and total mafic mineral counts on rock slices with mafic minerals counts in thin sections (e.g., Vlach 1993), by totalizing 1,000 points computed on 5.0 cm and 0.1 mm spaced grids, respectively. Weigh mineral proportions were also quantified by the Rietveld method applied to X-ray whole rock powder patterns, obtained with the D8 Advanced Davinci equipment from Bruker (details in Salazar-Naranjo 2016).

Whole-rock chemical compositions were determined by X-ray fluorescence (XRF) with the PANalytical Axios MAX Advanced equipment, following the procedures described by Mori et al. (1999), while trace elements were quantified through inductively coupled plasma mass spectrometry (ICP-MS) with the X series II (Thermo) equipment, with Collision Cell Technology (CCT), according procedures given in Navarro et al. (2008).

Back-scattered electronic (BSE) imaging and spot quantitative analysis were carried out with the JEOL-8530 field emission electron probe microanalyser (FE-EPMA), provided with five wavelength dispersive (WDS) and one energy dispersive spectrometers (EDS). Quantitative WDS spot analyses of the main rock-forming minerals and Fe-Ti oxides were performed with conventional analytical routines (e.g., Gualda & Vlach 2007), with 15 kV, 20 nA and 5 µm for the column accelerating voltage and beam current and diameter, respectively. F and Cl analyses (Kα spectral lines) in biotite and amphibole were carried out with large area TAP(H) and conventional PET(J) analyzer crystals using fluorapatite and sodalite as standards, respectively; halogen detection limits were about 55 ppm in both cases. In the case of the Fe-Ti oxides, due to the thin exsolution lamellae observed in BSE images, the analytical work was done with 12 kV, 10 nA and 2 µm, respectively, for spatial resolution improvement. Natural and synthetic standards from the Smithsonian Institution and the Geller^TM (McGuire et al. 1992) collections were used for the analytical setup. Raw data reduction and matrix effect corrections were computed with the PRZ/Armstrong software from JEOL.

Cationic proportions and mineral formulae were obtained with the MINCAL software (e.g., Gualda & Vlach 2005). In the case of amphiboles, the Fe³⁺ maximum criterion based on the Schumacher method (Leake et al. 1997) was assumed, following the suggestions proposed by Gualda & Vlach (2005). Fe-Ti oxides mineral formulae were computed following Carmichael & Nicholls (1967).

RESULTS

General petrography and whole-rock geochemistry

The main petrographical and litogeochemical features of the studied sample are presented in brief in the following as a background for the next sections.

Petrography

The studied granite has a porphyritic texture given by grayish to slightly pink tabular alkali feldspar megacrysts (23 vol.%), up to 3 cm long, in a medium-grained matrix (77 vol.%) made up of plagioclase, quartz, minor alkali-feldspar, amphibole and biotite. The accessory minerals include Fe-Ti oxides (ilmenite, magnetite and hematite), titanite, allanite, apatite, zircon and some sulphides. Modal data (Tab. 1) allows to classify the sample as amphibole-biotite monzogranite, with M (total mafic) ≈ 22 vol.% and Q (quartz) ≈ 21 vol.%, according to the International Union of Geological Sciences (IUGS) nomenclature (Le Maitre et al. 2002).

Table 1:
Modal (point-counting, volume, and X-ray powder diffratometry - XRD, mass, estimates) and whole-rock geochemical composition of the BPs02 sample from the Bragança Paulista-type granite.

Mineral	Modal data		Major elements (wt. %)		Trace elements (ppm)
	(vol. %)**	(wt. %)***
Plagioclase	29.00	26.0	SiO2	62.89	Li	22.5	Nb	16.12	Tb	1.06
Quartz	16.60	19.0	TiO2	1.02	Be	3.63	Mo	0.96	Dy	4.93
K-feldspar*	32.10	29.7	Al2O3	15.46	Sc	8.61	Cd	0.12	Ho	0.88
Biotite	9.76	12.6	Fe2O3 T	5.16	V	79.0	Sn	3.15	Er	2.37
Amphibole	9.56	12.7	MnO	0.09	Cr	68.1	Sb	0.03	Tm	0.29
Magnetite	0.31	-	MgO	1.96	Co	11.22	Cs	1.40	Yb	1.96
Ilmenite	1.32	-	CaO	3.79	Ni	10.29	Ba	1381	Lu	0.27
Sulphide	0.12	-	Na2O	3.28	Cu	17.96	La	102	Hf	8.44
Titanite	0.12	-	K2O	4.24	Zn	87.7	Ce	161	Ta	1.03
Allanite	0.50	-	P2O5	0.36	Ga	21.7	Pr	16.75	W	3.37
Zircon	0.31	-	LoI	0.72	Rb	154	Nd	62.5	Pb	29.24
Apatite	0.31	-	-	-	Sr	687	Sm	9.96	Bi	0.07
-	-	-	-	-	Y	25.1	Eu	2.26	Th	13.85
Total	100.00	100.0	Total	98.96	Zr	303	Gd	8.42	U	1.10

• *Alkali-felspar megacryts (23 vol.%) and matrix (9 vol.%); **integrate point-counting between in rock slices and thin sections; ***estimated with XRD.

The megacryst alkali-feldspar is made up of sub-idiomorphic crystals and corresponds to a microcline, with well-developed chessboard twining and variable quantities of exsolved albite (Fig. 3A). Apatite and quartz are common inclusions. In the matrix, microcline (averaging ca. 9 vol.% and size about 0.2 mm) is made up of predominant interstitial and xenomorphic clean crystals; exsolved albite is relatively rare (Fig. 3B). Plagioclase (< 2.2 mm) is the main felsic mineral in the groundmass. It appears as prismatic/tabular idiomorphic and sub-idiomorphic crystals showing albite or, to a lesser extent, combined albite-carlsbad twinning (Fig. 3C); slight bending of the lamellar twinning evidences some plastic deformation. The observed crystal sections are optically homogeneous; minor compositionally zoning is evidence solely by some increase of saussurite alteration in crystal cores. Their compositions, estimated through the optical Michel-Lévy method over [010] crystallographic zone’s sections of albite-twinned crystals, correspond to a sodic-andesine. Amphibole and biotite inclusions do appear. Myrmekite intergrowths occur associated with alkali feldspar crystals. Quartz (≤ 2.7 mm) occurs as xenomorphic interstitial crystals usually with undulatory extinction.

Figure 3:
Optical cross-polarized transmitted light (top) and back-scattered electronic (BSE) (bottom) images emphasizing some textural features of the felsic and mafic minerals in the studied sample. (A) Perthitic alkali feldspar megacryst with crosshatched twin; note myrmekite on the left. (B) Exsolution-free, crosshatched twinned, intersticial alkali feldspar in the rock matrix. (C) Idiomorphic, tabular, non-zoned plagioclase with combined albite-carlsbad twinning in the rock matrix. (D) Subidiomorphic, non-zoned, calcium amphibole (dark green), associated biotite, Fe-Ti oxides, zircon, apatite and zoned allanite, the latter partially included in it. Note straight contacts with biotite. (E) Brown pleochroic biotite aggregates with straight contacts among crystals, with inclusions of quartz and apatite. (F) Fine-grained biotite associated with feldspar and quartz crystals. Note the homogeneous BSE mineral patterns. Red points identify wavelength dispersive spectroscopy (WDS) analytical spot locations.

Amphibole (5-13 mm) is idiomorphic to sub-idiomorphic and optically homogeneous, with a pleochroic formula given by α = dark green brownish, β = green and γ = light yellow and a ZΛc extinction angle about 20°, typical of common hornblende (Fig. 3D). The main inclusions are apatite and zircon. Biotite comprises two textural types, the first is made up of well-developed (up to 0.6 mm) plate crystals and appears associated with amphibole, sometimes replacing it (Fig. 3E), while the second is constituted by small crystals (< 0.2 mm) closely associated with the felsic minerals (Fig. 3F). The pleochroic schemes are similar in both cases, with α = dark to medium brown, β = dark-brown and γ = yellow to pale-brown colors.

Fe-Ti oxides are the commonest opaque phases and, among them, ilmenite is dominant. It appears as sub-idiomorphic crystals (average size 0.7 mm) with thin lamellae exsolution patterns parallel to (0001), constituted by two rhombohedral oxides: titanohematite (≤ 0.07 mm) and hematite (≤ 0.03 mm), which are lens- and lath-shaped, respectively (Fig. 4A). Hematite was recognized by its strong bi-reflectance. Magnetite is usually smaller (≤ 0.5mm), idiomorphic and exsolution-free under reflected light. Ilmenite crystals associated with magnetite do not exhibit pure hematite exsolution, and the titanohematite exsolution lamellae are more sinuous- and/or droplet-shaped (Fig. 4B); of note, the area directly in contact with magnetite is exsolution-free. Rarely some minute crystals of sulphide (pyrite and chalcopyrite) are observed.

Figure 4:
Optical reflected light (top) and back-scattered electronic (BSE) (center and bottom) images of Fe-Ti oxides in the studied sample. (A) Fe-Ti oxide textural association I: Sub-idiomorphic hemo-ilmenite grain with ferrian ilmenite host (pink-grey and dark-grey, respectively) with oriented straight lamellae of hematite (pink and light-grey, respectively) and titanohematite (white and light-grey, respectively), which, in turn, presents lamellae and drop-like exolution of ilmenite. (B) Fe-Ti oxide association II: subhedral hemo-ilmenite grain with ferrian ilmenite host (pink-grey and dark-grey, respectively) with sinuous titanohematite (white and grey, respectively), associated with subhedral exsolution-free magnetite (light pinkish-grey and light-grey, respectively). Blue arrows indicate ilmenite exsolutions-free portions in contact direct with magnetite. Red points locate wavelength dispersive spectroscopy (WDS) analytical spot locations. Yellow squares identify detailed BSE image areas (bottom).

Besides the Fe-Ti oxides, allanite is the main accessory mineral. It forms tabular crystals (up to 1 mm), mostly associated with the main mafic silicates (Fig. 3D). It is slightly pleochroic (from light brownish to reddish colors) and shows strong zoning patterns accentuated by late metamictization and related alteration processes. Titanite occurs as rare idiomorphic primary crystals (up to 0.3 mm) and very often as thin mantles partially substituting for the opaque phases, especially ilmenite. Apatite (≤ 0.2 mm) is idiomorphic, clean, and appear mostly as inclusions in the other phases. Zircon (≤ 0.1 mm) is idiomorphic with well-developed zoning patterns; minute inclusions of apatite and some unrecognized phases are arranged according zircon growth faces.

Whole-rock geochemistry

Analytical data for the BPs02 sample is presented in Tab. 1. In the following discussion, our results are integrated with the available geochemical data for the Bragança Paulista Batholith (BPB) from Campos Neto et al. (1984), Wernick et al. (1984) and Artur et al. (1993). Of note, the compiled whole-rock analyses are significantly old and evidence some analytical problems or representativeness issues. Overall, they display relatively well the main general geochemical fingerprints, certainly.

The BPB granites geochemical features are akin to the typical expanded high-K calc-alkali to alkali-calcic granites, with 55 ≤ SiO₂ (wt.%) ≤ 70. They are high-K (2.5-5.9 K₂O wt.%), metaluminous to slightly peraluminous, with 0.82 ≤ ASI [= Al₂O₃/(CaO + Na₂O + K₂O), moles] ≤ 1.02, and magnesian, with 0.51 ≤ fe# [= FeO^T/(FeO^T + MgO), wt.% oxides] ≤ 0.78 (Figs. 5A, 5B and 5C, see also Frost et al. 2001, Le Maitre et al. 2002). As typical features, fe# values are higher than those observed in modern active continental margins batholiths, and Al₂O₃ contents decrease as SiO₂ increases due mainly to the decreasing biotite and plagioclase modal contents in the evolving magmatic trend (Fig. 5D). Our sample is representative of intermediate compositions.

Figure 5:
Typical geochemical features of the syn- and post- orogenic granites from the studied area, as depicted in conventional diagrams. (A) FeO^T/(FeO^T + MgO) vs. SiO₂; (B) Na₂O + K₂O-CaO vs. SiO₂; (C) alumina saturation index (ASI) vs. SiO₂; (D) Al₂O₃ vs. SiO₂. A, B and C according Frost et al. (2001). Note that the studied BPs2 sample represents intermediate compositions within the compiled Bragança Paulista data. See text for discussion.

Trace element patterns are characterized by well-defined negative Nb and Ta anomalies in relation to the large-ion lithophile (LIL) and light rare earth (LRE) elements (Fig. 6), a most typical feature of calc-alkaline rocks associated with subduction environments (e.g., Rollinson 1993). The rare-earth elements (REE) define high light rare-earth elements (LREE)/heavy rare-earth elements (HREE) (La_N/Yb_N ≈ 37), well-defined LREE (La_N/Sm_N ≈ 6.5), moderate HREE (Gd_N/Yb_N ≈ 3.6) fractionation patterns, and negative Eu anomaly (Eu/Eu*~ 0.8). The patterns observed in the compiled data set are variable to some extent, but high La_N/Yb_N ratios and slight negative Eu anomalies are common features to all.

Figure 6:
Normalized multielement diagram, according McDonough & Sun (1995), for the studied BPs02 sample, and rare-earth elements (REE) patterns, for the studied and two other available samples from the compiled data set. See text for discussion.

Mineral textures and chemistry

Representative WDS quantitative compositions of feldspars, amphibole and biotite, and Fe-Ti oxides are shown in Tabs. 2 and 3, respectively. The complete analytical data are available in the Supplementary ^{Tab. A1}. Their main characteristics are summarized in the following.

Table 2:
Representative wavelengths dispersive spectroscopy (WDS) spot analyses (wt.%) and structural formulae for feldspars, amphibole and biotite in the BBs02 sample from the Bragança Paulista-type granite.

	Alkali Feldspar			Plagioclase			Amphibole		Biotite
	Host	Interstitial	Ab exsolution	Core	Rim	Myrmekite
Point ID	18.8	21.3	24.10	22.3	26.2	18.21	Point ID	1.5	Point ID	16.3
SiO2	64.83	64.32	67.82	60.15	60.62	60.55	SiO2	41.77	SiO2	36.60
TiO2	bd	0.01	0.01	0.01	0.02	bd	TiO2	1.71	TiO2	3.76
Al2O3	18.78	19.17	20.20	25.34	25.02	25.00	Al2O3	11.95	Al2O3	14.09
Fe2O3 T	0.08	0.08	0.06	0.20	0.18	0.08	FeOT	17.36	FeOT	18.61
MnO	bd	bd	bd	bd	bd	bd	MnO	0.59	MnO	0.22
MgO	bd	bd	bd	bd	bd	bd	MgO	9.77	MgO	12.69
-	-	-	-	-	-	-	ZnO	0.04	ZnO	0.05
CaO	0.02	0.10	0.27	6.33	6.15	6.09	CaO	11.10	CaO	0.05
SrO	0.07	0.20	0.15	0.12	0.15	0.13	-	-	-	-
BaO	0.34	0.53	0.02	0.03	0.02	0.04	-	-	BaO	0.29
K2O	15.26	14.98	0.28	0.32	0.25	0.18	Na2O	1.53	Na2O	0.05
Na2O	0.87	0.90	11.19	7.49	7.72	7.79	K2O	1.60	K2O	9.69
-	-	-	-	-	-	-	F	0.02	F	0.02
-	-	-	-	-	-	-	Cl	0.12	Cl	0.10
Total	100.26	100.27	100.01	100.01	100.13	99.86	Total	97.56	Total	96.23
-	-	-	-	-	-	-	Fe2O3(c)	5.94	-	-
-	-	-	-	-	-	-	FeO(c)	12.01	-	-
-	-	-	-	-	-	-	O = (F, Cl)	0.03	O = (F, Cl)	0.03
-	-	-	-	-	-	-	Total	98.56	Total	96.20
Structural formulae (32O)							(23O)*		(11O)
Si	11.941	11.865	11.869	10.712	10.775	10.784	Si	6.260	Si	2.779
Ti	0.000	0.001	0.001	0.002	0.003	0.000	IVAl	1.740	IVAl	1.221
Al	4.077	4.168	4.166	5.319	5.241	5.248	Sum T	8.000	Sum T	4.000
Fe3+	0.011	0.011	0.008	0.027	0.024	0.011	VIAl	0.370	VIAl	0.040
Sum T	16.029	16.044	16.044	16.060	16.043	16.043	Ti	0.193	Ti	0.215
Mn	0.000	0.000	0.000	0.001	0.000	0.000	Fe3+	0.670	-	-
Mg	0.000	0.000	0.001	0.000	0.000	0.000	Fe2+	1.506	Fe2+	1.182
Ca	0.004	0.019	0.050	1.208	1.171	1.162	Mn	0.075	Mn	0.014
Sr	0.008	0.021	0.016	0.013	0.016	0.014	Mg	2.183	Mg	1.437
Ba	0.025	0.038	0.001	0.002	0.001	0.003	Zn	0.004	Zn	0.003
K	3.585	3.525	0.063	0.073	0.056	0.040	Sum C	5.001	Sum M	2.890
Na	0.312	0.321	3.797	2.586	2.660	2.690	Ca	1.782	Ca	0.004
Sum M	3.935	3.924	3.928	3.882	3.905	3.909	Na	0.218	Na	0.008
-	-	-	-	-	-	-	Sum B	2.000	K	0.939
-	-	-	-	-	-	-	Na	0.227	Ba	0.009
-	-	-	-	-	-	-	K	0.306	-	-
-	-	-	-	-	-	-	Sum A	0.533	Sum I	0.959
Sum Cat	19.964	19.968	19.973	19.942	19.948	19.952	Sum. Cat.	15.534	Sum. Cat.	7.849
-	-	-	-	-	-	-	F	0.010	F	0.005
-	-	-	-	-	-	-	Cl	0.045	Cl	0.014
Molecular fractions
An	0.11	0.48	1.28	31.22	30.11	29.84			Phl	0.50
Ab	7.95	8.21	97.07	66.84	68.40	69.06			Sid	0.23
Or	91.31	90.33	1.62	1.88	1.45	1.03			Ann	0.27
Cn	0.63	0.97	0.03	0.06	0.03	0.07	mg#	0.50	mg#	0.55

• *Following Shumacher (1997), as modified Gualda & Vlach (2005); mg#: Mg/(Mg + Fe^T); An: anortite; Ab: albite; Or: orthoclase; Cn: celsian; Phl: phlogopite; Sid: Siderophyllite; Ann: annite; bd: below detection limit.

Table 3:
Representative wavelength dispersive spectroscopy (WDS) spot analyses (wt.%) and structural formulae for Fe-Ti oxides in the BPs02 sample from the Bragança Paulista-type granite.

	Association I				Association II
	Hematite	Titano-hematite	Ilmenite host	Ilmenite exolution	Magnetite	Titano-hematite	Ilmenite host	Ilmenite exolution
Point ID	qnt 21	qnt 19	qnt 2	qnt 24	qnt 35	qnt 33	qnt 5	qnt 27
SiO2	0.04	0.05	0.10	bd	bd	0.06	bd	0.05
TiO2	1.00	7.99	48.14	47.81	bd	9.31	48.75	46.76
Al2O3	0.07	0.11	bd	0.01	0.25	0.11	0.01	0.02
Fe2O3 T	100.14	-	-	-	-	-	-	-
FeOT		83.20	48.87	49.13	92.75	82.20	45.07	47.30
MnO	0.05	0.05	1.18	1.50	bd	0.39	3.93	3.81
MgO	bd	bd	0.43	0.26	0.05	bd	0.48	0.42
CaO	bd	bd	0.04	bd	0.02	0.05	0.02	0.02
ZnO	0.04	bd	0.01	0.13	bd	bd	bd	0.12
Nb2O5	bd	bd	0.10	0.17	bd	bd	0.03	bd
Total	101.35	91.40	98.86	99.00	93.06	92.12	98.29	98.50
Fe2O3 (c)	98.17	84.42	8.30	9.13	68.67	82.48	7.87	11.02
FeO (c)	0.87	7.24	41.40	40.91	31.0	7.98	38.99	37.39
Total	100.26	99.83	99.69	99.92	99.94	100.38	100.08	99.61
Structural formulae*
Si	0.002	0.003	0.005	0.000	0.000	0.003	0.000	0.002
Ti	0.040	0.317	1.835	1.822	0.000	0.367	1.850	1.786
Nb	0.000	0.000	0.002	0.004	0.000	0.000	0.001	0.000
Sum T 4+	0.042	0.320	1.842	1.826	0.000	0.370	1.850	1.789
Al	0.005	0.007	0.000	0.001	0.011	0.007	0.001	0.001
Fe+3	3.911	3.353	0.316	0.348	1.989	3.253	0.299	0.421
Sum M 3+	3.915	3.360	0.316	0.349	2.000	3.260	0.299	0.422
Fe+2	0.039	0.320	1.754	1.734	0.997	0.350	1.645	1.588
Mn	0.002	0.001	0.051	0.064	0.000	0.017	0.168	0.164
Mg	0.000	0.000	0.032	0.020	0.003	0.000	0.036	0.032
Ca	0.000	0.000	0.002	0.000	0.001	0.003	0.001	0.001
Zn	0.002	0.000	0.001	0.005	0.000	0.000	0.000	0.005
Sum R 2+	0.042	0.320	1.840	1.822	1.000	0.370	1.850	1.789
Sum. Cat.	4.000	4.000	3.999	3.997	3.000	4.000	4.000	4.000
Molecular fractions
Usp	-	-	-	-	0.00	-	-	-
Mag	-	-	-	-	100.00	-	-	-
Gk	-	0.00	0.02	0.01	-	0.00	0.02	0.02
Hem	1.00	0.84	0.08	0.09	-	0.82	0.07	0.11
Ilm	-	0.16	0.88	0.87	-	0.18	0.82	0.80
Pph	-	0.00	0.03	0.03	-	0.01	0.08	0.08

• *Based on 6O (hematite, titanohematite and ilmenite) and 4O (magnetite), following Carmichael & Nicholls (1967); Usp: ulvoespinel; Gk: geikelite; Hem: hematite; Ilm: ilmenite; Pph: pyrophanite; bd: below detection limit

Alkali feldspar

The host potassic phase of the megacryst alkali-felsdpar compositions lies in the range Or_94.4Ab_5.4An_0.2 - Or_85.4 Ab₁₄An_0.6, with Ba, Sr and Fe³⁺ contents about 0.045, 0.017 and 0.008 cations per formula unit (cpfu), respectively (Tab. 2, Fig. 7). Albite (Ab_96.1Or_3.3An_0.7 - Ab_97.3Or_1.0An_1.7) appears mainly as string-lets and string-rods exsolution patterns (Fig. 7). The integrated alkali-feldspar compositions, based on BSE imaging (e.g., Gualda 2001), allow to recognize three groups: Or_76.8Ab_22.9An_0.3 (mainly string-lets exsolution patterned crystal zones); Or_85.7Ab_14.1An_0.2 (string-lets and string-rods patterns in similar volumes) and Or_93.4Ab_6.5An_0.1 (mainly string-rods pattern). The first one, with high Ab contents, arguably approaches better the expected primary alkali-feldspar compositions.

Figure 7:
Ab-Or-An ternary diagram showing the measured feldspar compositional variations in the BPs02 studied sample. Back-scattered electronic (BSE) image (upper left) details contrasted albite exsolution patterns in alkali feldspar as discussed in the text. Isotherms were plotted according to Fuhrman & Lindsley (1988).

The matrix, relatively exsolution-poor, alkali-feldspar presents compositions between Or_88.8Ab_10.6An_0.5 and Or_92.8Ab_7.1An_0.1, somewhat similar to the megacrysts, but shows a slightly great content in the Celsian molecule (Tab. 2).

Plagioclase

Plagioclase crystals are relatively homogeneous, with very slightly zoning patterns. Their compositions are in the range Ab_65.4Or_2.7An_31.9 - Ab_69.8Or_0.8An_29.4 (Tab. 2, Fig. 7). Crystal cores are somewhat more calcic and richer in the orthoclase molecule and Sr and Fe³⁺ contents as compared with crystal rims. Ba contents are always low (< 0.006 cpuf). In the late, post-magmatic, myrmekite intergrowths, compositions are slightly more sodic and, as typical, Fe³⁺ abundances are lower (ca. 0.015 cpfu) as compared with the primary crystal rims.

Amphibole

Amphibole is also texturally and compositionally homogeneous, with 0.87 ≤ ^BCa/^B(Ca + Na) ≤ 0.92, 0.47 ≤ mg# ≤ 0.52, 1.927 ≤ Al^T ≤ 2.219 (Tab. 2, Fig. 8). Its compositions approaches the typical hastingsite (e.g., Deer et al. 1997). According to Hawthorne et al. (2012), it is classified as a potassic-magnesio-hastingsite; F and Cl contents are low, up to 0.02 and 0.20 wt.%, respectively. Considering full-filled A sites in the structural formulae, an average H₂O content about 1.75 wt.% is estimated.

Figure 8:
Compositional characteristics of amphiboles from syn- to post- orogenic granites. (A) Classification diagram following Hawthorne et al. (2012). (B and C) diagrams depict the ^M4Ca + ^VIAl = ^M4Na + Ti (CANT) and hastingsite-pargasite-type substitutions, according to Vyhnal et al. (1991) and Vlach (1993). See discussion in the text.

The homogeneity of our amphibole compositions does not allow a better evaluation of the main substitution vectors, in common with typical Ca-amphiboles from granitic rocks (e.g., Vyhnal et al. 1991; cf. also Vlach 1993). However, the diagrams depicted in Fig. 8 suggest, at least, that CANT (^M4Ca + ^VIAl = ^M4Na + Ti) and hastingsite-pargasite (^A + 2Si + Mg = ^ANa + 2^IVAl + (Fe³⁺, ^VIAl)) type coupled substitutions were predominant over the Al/Fe tschermakite (2Si + 2Mg = 2^IVAl+2(Fe³⁺, ^VIAl)) and edenite (^A + Si = ^ANa + ^IVAl) type substitutions.

Biotite

Annite and phlogopite end-members are largely predominant in our biotite compositions (Ann₄₃Phl₅₇ - Ann₄₈Phl₅₂, Fig. 9A); siderophyllite and eastonite molecules contents are very low (^VIAl < 0.16 cpfu). Such compositions are common among typical biotites from orogenic calc-alkaline suites, as compared with the data presented by Abdel-Rahman (1994) and Nachit et al. (2005).

Figure 9:
Compositional characteristics of biotites from syn- to post- orogenic granites. (A) Classification diagram following Guidotti (1984). (B) Lines of the two ideal Ti + ^IVAl possible coupled substitutions in biotite. (C) Ti vs. mg# (Mg/(Mg + Fe^T), cationic plot. See text for discussion.

As described for plagioclase and amphibole, biotite is chemical homogeneous. Some variations in Ti contents may be explained by a Ti + ^IVAl = ^VIAl + Si type exchange vector (Fig. 9B). Of note, titanium contents in biotite are negatively correlated with mg# (Fig. 9C), and biotite crystals associated with amphibole presents relatively low Ti contents as compared with the crystals associated with felsic minerals.

An interesting aspect emerging from our biotite compositions is that they evidence a Mg-Cl but not a Fe-F avoidance pattern (Munoz 1984, Mason 1992, Icenhower & London 1997), which contrasts with the observed patterns in most among the late- to post-orogenic regional granites (e.g., Vlach 1993, Vlach & Ulbrich 1994). This feature could be attributed to biotite and amphibole simultaneous crystallization during most of the melt crystallization range (e.g., Teiber et al. 2014).

Fe-Ti oxides

Fe-Ti oxides are represented by ferrian ilmenite, titanohematite, magnetite and hematite. Reflected-light petrography and BSE images suggest two textural associations among these minerals, as detailed below, according the terminology of Balsley & Buddington (1958).

Association I is made up of sub-idiomorphic hemoilmenite grains (Tab. 3, Fig. 4A and 10), constituted by ferrian ilmenite host (Ilm_88-90Hem_5-8Pph_2-3Gk_1-2) with more or less oriented exsolution lamellae of hematite (Hem_≈100, with FeO ≤ 1.3 and TiO₂ ≤ 1.6 wt.%) and titanohematite (Ilm_14-20Hem_80-86Pph₀Gk₀), which, by its turn, presents lamellae and drop-like ferrian ilmenite exsolution (Ilm_67-87Hem_9-31Pph_2-3Gk_0-2).

Figure 10:
R²⁺-T⁴⁺-M³⁺ ternary diagram illustrating Fe-Ti oxides compositional variations in the BPs02 sample, emphasizing the contrasted textural associations described in the text, and the integrated compositions for rombohedral oxides constituting the Fe-Ti oxide associations I and II. See text.

Association II (Table 3, Figs. 4B and 10) is composed of pure magnetite grains (Hc₁Mag₉₉Usp₀Spl₀) associated with hemo-ilmenite grains, which are made up of ferrian ilmenite host (Ilm_78-86Hem_4-12Pph_7-8Gk_1-2), containing exsolved lamellae and very fine drop-like exsolutions (up to ca. 2 µm) of titanohematite (Ilm_16-18Hem_82-84Pph_0-1Gk₀), which contains exsolved ferrian ilmenite (Ilm₈₀Hem₁₁Pph₈Gk₂).

The primary hemo-ilmenite compositions were recalled using the available chemical quantitative spot analysis, the relative volumes, estimates from representative BSE images, and average densities, given by Deer et al. (2011), for the exsolved and host oxide phases. The integrated composition of hemo-ilmenite from Association II (Ilm₆₉Hem₂₃Pph₇Gk₁) presents higher ilmenite and pyrophanite contents as compared with that constituting Association I (Ilm₅₃Hem₄₅Pph₁Gk₁) (see also Fig. 10).

CRYSTALLIZATION CONDITIONS

Amphibole-plagioclase thermobarometry and Al-in-hornblende barometry

Various experimental- and/or empirical-based calibrations proposal by Blundy & Holland (1990), Holland & Blundy (1994) and Molina et al. (2015), based on the equilibrium between plagioclase and Ca-amphibole, were examined and applied to our chemical data. P-T result ranges are compared in Fig. 11A. The models of Holland & Blundy (1994, T_B Model) and Molina et al. (2015), converge well to temperatures and pressures around 756 (± 45)ºC and 510 (± 60) MPa. Thus, they are considered our near-solidus temperatures and emplacement pressures preferred values. Of note, given the relatively homogeneity of our chemical compositions, there are not significant differences, within errors, between the obtained results for core-core or rim-rim mineral compositions pairs (see also Fig. 11D). It is also worth mentioning that Blundy & Holland (1990) and Holland & Blundy (1994, T_A Model) formulations give significant high pressures and/or temperatures; a feature most probably associated with the subordinate contributions from the edenite substitution scheme in our amphibole.

Figure 11:
Selected diagrams depicting the main intensive crystallization parameter estimates. (A) Geothermobarometry based on amphibole-plagioclase equilibrium and Al-in-amphibole barometry, according to the indicated proposed calibrations. (B) Zircon and apatite saturation temperatures (Watson & Harrison 1983, 1984), and Mg-in-liquid temperature vs. SiO₂ (Molina et al. 2015). (C) Semi-quantitative redox conditions as indicated in the Al^T vs. mg# (Mg/Fe^T + Mg) cationic plot for biotite, according to Anderson et al. (2008), except for the reference to the nickel-nickel oxide (NNO) rather quartz-fayalite-magnetite (QFM) buffer, symbols as in Figure 9. (D) Al^T vs. mg# (Mg/Fe^T + Mg) cationic plot for amphibole with qualitative redox conditions, as inferred according to Anderson & Smith (1995), and isobaric dashed lines based on the revised Al-in-amphibole barometer (Mutch et al. 2016) with uncertainties (gray area). Symbols as in Figure 8.

The Al-in-hornblende barometry model was also tested, given its applicability to our sample, with the mineral assemblage quartz + alkali-feldspar + plagioclase + biotite + hornblende + titanite + Fe-Ti oxides. Both the classic linear and exponential models of Hammarstrom & Zen (1986) and the revised version of Mutch et al. (2016) give similar pressures, around 510 (± 50) MPa (Fig. 11D); a similar value was also computed with the Anderson & Smith (1995) method, which include T as a variable (cf. Fig. 11A).

On the other hand, some older calibrations of the Al-in-hornblende barometer (e.g., Schmidt 1992, and reference therein) give somewhat lower values and, importantly, the empirical calibrations based on whole-amphibole compositions coexisting with residual magmatic liquid, gauged with seismic data (e.g., Ridolfi et al. 2010, Ridolfi & Renzulli 2012), for calc-alkaline volcanic rocks from recent active continental margins result in much lower, incompatible pressures (≤ 300 MPa, see also Erdmann et al. 2014) and higher temperatures (902 and 841°C, respectively) for our amphibole compositions.

Accessory-phase saturation and liquidus temperatures

Zircon and apatite saturation thermometry results for the BPB granites are resumed in Fig. 11B. As depicted, apatite saturation temperatures (Watson & Harrison 1984) cover the 920-1,025°C range and are always higher than zircon equivalents, computed according Watson & Harrison (1983), in the 609-820°C range. Of importance, these latter values will still decrease, by ca. 60-90°C, if calculated with the revised formulae of Boehnke et al. (2013). It is worth to mention that the zircon saturation temperature estimated according the latter authors gives a value close to the obtained through the amphibole-plagioclase thermometer (ca. 755°C) in our sample.

As compared with the available previous whole-rock data set, our sample has the highest for zircon saturation temperatures, even considering their intermediate composition within the complete data set (see also Fig. 5). Of note, several of the obtained values for zircon saturation temperatures in this data set are surprisingly lower for their SiO₂ contents, some even lower than the expected temperatures for granite minimum melts, reinforcing the aforementioned concern in older analysis issues. On the other hand, the results for apatite are of easy interpretation in general as compared with zircon results. As in the latter case, the obtained temperatures could represent the temperatures of source melting, in dependence of source zircon partial or complete dissolution in inheritance-rich magmas (Miller et al. 2003).

The difference between apatite (973°C) and zircon (811°C) saturation temperatures is compatible with the early crystallization of apatite, as deduced from textural relations (see petrographic section), and it is very significant. Considering that sample and parental granite melt compositions approaches each other, these values and the amphibole-plagioclase equilibrium close-to-solidus temperature (ca. 755°C) may suggest a considerable magma crystallization temperature span.

Liquidus temperatures were further estimated from whole-rock chemical compositions (including volatile contents, cf. next section) through the empirical geochemical formulation of Molina et al. (2015) and the thermodynamic Rhyolite-MELTS model (Ghiorso & Gualda 2015). Applying the first model (cf.Fig. 11B), 985°C is estimated, a value approaching well our apatite saturation temperature. The second model, in its turn, gives a well higher temperature (1,150°C) however, which may suggest that our sample is not representative in full of the original melt composition from which it crystallized.

Redox conditions

In magnetite-bearing granites, biotite and amphibole compositions, expressed by the mg# parameter, may give semi-quantitative or, at the least, qualitative ƒ_O2 information, because the Mg/(Mg + Fe) ratios of mafic silicate minerals depend, among other factors, on redox conditions during their crystallization (e.g., Wones 1981, Anderson et al. 2008). Our biotite (0.51 ≤ mg# ≤ 0.57) compositions point to ƒ_O2 values close to ΔNNO+1 (Fig. 11C). The negative correlation between Ti contents and mg# in our biotites also suggests an oxidized cooling path, as in the crystallizing system it is expected that Ti-in-biotite would decrease with temperature (Henry et al. 2005, Nachit et al. 2005). The magnesio-hastingsite, with 0.47 ≤ mg# ≤ 0.52, also indicate a relatively high ƒ_O2 (Fig. 11D). The empirical thermobarometric calibration, based on whole-amphibole compositions, developed by Ridolfi et al. (2010), yield similar oxidizing values (≈ ΔNNO + 0). These estimates are compatible, as expected, to the so-called magnetite-bearing series granites (Ishihara 1977), crystallized close or above the nickel-nickel oxide (NNO) buffer, even magnetite being subordinate to hemo-ilmenite in our sample.

Original oxygen fugacity and the T- ƒ_O2 path during cooling are the main parameters controlling the Fe-Ti oxide assemblages, as well as their textural and compositional fingerprints. The obtained integrated composition of hemo-ilmenite from Associations I and II were used to try to reconstruct of the redox state of the early magmatic stage. These compositions may be well representative of the primary ones, as both associations present solely in situ sub-solidus re-equilibration without major remobilization within the whole system, a normal feature in plutonic rocks (Frost 1991a, 1991b).

Assuming such hypothesis, the occurrence of hemo-ilmenite and magnetite in the studied sample indicates initial conditions relatively oxidized in the crystallizing system. The interpretation of the observed Fe-Ti oxide associations is not so straightforward, however. The relatively higher content of hematite in Association I (X_Hem-rich hemo-ilmenite) as compared with Association II (magnetite + X_Ilm-rich hemo-ilmenite) suggests relative higher ƒ_O2 conditions (Ghiorso & Sack 1991, Ghiorso & Evans 2008, Sauerzapf et al. 2008, Broska & Petrík 2011). On the other hand, the pyrophanite molecular content in ilmenite increases with ƒ_O2 (Czamanske & Mihálik 1972, Harlov & Hansen 2005, Mehdilo et al. 2015), and the ilmenite in Association I is relatively Mn-poor. As Mn has great affinity for the ilmenite structure as compared to Ca-amphibole and biotite (e.g., Czamanske & Mihálik 1972, Harlov & Hansen 2005, Deer et al. 2011), this may indicate that Association II had crystallized earlier than Association I and sequestered a significant quantity of the Mn available in the system. In addition, redox conditions in originally oxidized melts may also increase through the reaction 6Fe₂O_3(melt) = 4Fe₃O_4(magnetite) + O_2(melt) (cf. Richards 2015).

The textural and chemical features of our hemo-ilmenite indicate late- to post-magmatic inter- and oxide-oxide re-equilibration, which approaches their pure end-members (ferrian ilmenite and titanohematite/hematite) and left ilmenite exsolutions-free portions at magnetite-ferrian ilmenite contacts, respectively. In the first case, there are not textural evidences indicating which among the hematite and titanohematite lamellae exsolved first in both Fe-Ti oxide associations during cooling. Nevertheless, arguably the hematite lamellae were formed first, as it represents the most pure end-member (cf.Geuna et al. 2008, McEnroe et al. 2009). In the second case, re-equilibrium can be due through the Fe²⁺Ti↔2Fe³⁺ exchange between ferrian ilmenite and magnetite (e.g., Frost 1991b, Korneliussen et al. 2000).

Halogen fugacity

Halogen fugacity ratios (ƒ_H2O/ƒ_HF , ƒ_H2O/ƒ_HCl and ƒ_HF/ƒ_HCl) were estimated through the equilibrium between melt and biotite, using the formalism presented by Munoz (1992), and depicted in Fig. 12. It must be pointed out that the F-in biotite contents are very low in our sample, approaching F detection limits even using TAP(H) analyzer crystals. Using the maximum measured F content (ca. 200 ppm), minimum and maximum values for ƒ_H2O/ƒ_HF and ƒ_HF/ƒ_HCl ratios may be roughly estimated, and we obtain ƒ_H2O/ƒ_HF ≥ 10^5.8, 10^3.5 ≤ ƒ_H2O/ƒ_HCl ≤ 10^3.8 and ƒ_HF/ƒ_HCl ≤ 10^-2.2.

Figure 12:
Octahedral Mg mole fraction vs. halogen mole fraction in hydroxyl sites in biotite from the syn-orogenic granites. Equal ratio lines calculated following Munoz (1992), assuming biotite-fluid equilibrium at 750ºC. Biotite analyses were plotted with F ≥ 110 ppm. Symbols as in Figure 9.

The fluorine and chlorine relative contents in biotite and amphibole from our sample are very similar and may suggest co-crystallization. Biotite in amphibole-bearing granites are relatively F-poor (≤ 0.04 wt.%) and Cl-rich (≥ 0.33 wt.%), in general as compared to muscovite- and biotite-bearing granites (Teiber et al. 2014). For instance, high F/Cl ratios in biotite (10^-2.8 ≤ ƒ_HF/ƒ_HCl ≤ 10^-0.31) are the most typical feature of crustal metasedimentary source rocks as the ones that generated the Nazaré Paulista-type granites. Our biotite, however, has low to medium Cl contents (≤ 0.18 wt.%).

Vlach & Ulbrich (1994) calculated 10^3.5 ≤ ƒ_H2O/ƒ_HF ≤ 10^4.0 for granites from the Morungaba area using a previous model of Munoz (1984). However, as stated by Munoz (1992), his older model yields somewhat lower ratios. Unfortunately, the available biotite compositions from the post-orogenic granites did not include Cl measurements and do not allow the application of the new model. As a reference for comparison, applying the first Munoz’s formulation, ƒ_H2O/ƒ_HF ≥ 10^5.5 is obtained for our sample, suggesting higher ƒ_H2O/ƒ_HF values for the syn-, as compared with the post-orogenic magmatism. In general, comparing also F contents in biotite from syn-orogenic and post-orogenic granites in the studied area, one may conclude the post-orogenic granites (Itu, Morungaba) were generated from relatively F-rich magmas.

DISCUSSION

The results above suggest that the syn-orogenic magmatism Bragança Paulista-type was emplaced under a lithostatic pressure of ca. 500 MPa, corresponding to a ca. 14-16 km depth, compatible with the observed textures and the low-range compositional variations registered in plagioclase, calcic amphibole and biotite, which indicate a relatively slow temperature decreasing gradient during crystallization, as expected in moderate to high P environments.

Barometric data for the regional metamorphism obtained by Martins (2006, see also Martins et al. 2009) through the garnet-Al silicate-plagioclase (GASP) geobarometer point to lithostatic pressures around 750 MPa. Some among these leucogranites were emplaced under local extensive structures and do contain andalusite, while others present sillimanite; no coexistence of andalusite and sillimanite was yet reported. Andalusite-bearing muscovite leucogranites may be formed up to ca. 400-450 MPa from water-satured, low-temperature (ca. 630-640°C) peraluminous melts (Clarke et al. 2005). Of importance, the metamorphic peak was dated about 630-620 Ma, while some leucogranites have emplacement ages down to 610 Ma (Janasi 1999, Vlach & Gualda 2000, Martins et al. 2009), so the generation of these peraluminous melts took place possibly along a clockwise evolutionary P-T-t path (Janasi et al. 2005, Martins et al. 2009).

On the other hand, the late- to post-orogenic magmatism in the area, represented, respectively, by the Ouro Verde Complex and the Jaguari Pluton from the Morungaba granites (Vlach 1993, Vlach & Ulbrich 1994) and the Meridional Morungaba plutons and the Salto and Cabreúva plutons from the Itu Batholith (Vlach 1993, Vlach & Ulbrich 1994, Galembeck et al. 1997, Janasi et al. 2009), were emplaced at relatively shallower crustal levels (corresponding to ca. 300 MPa, late-orogenic plutons; down to ca. 200 MPa, typically post-orogenic plutons). Thus, considering a time span of about 20-30 Ma between the main syn- and post-orogenic granite emplacement events in the area (e.g., Janasi et al. 2009, Martins et al. 2009, Vlach & Gualda 2000, recent unpublished data), post-closure uplift (in the sense of Pitcher 1982) relative ratios averaging 0.2 to 0.3 km/Ma may be estimated since the main syn- until the post-orogenic granite magmatism. A similar relative ratio may be estimated between the main regional metamorphism and the syn-orogenic magmatism in the area and neighboring.

The calc-alkaline syn- and late-orogenic granites, as well as the post-orogenic biotite granites and, among them, typical A-type granites, as the Salto Pluton, in the Itu Batholith, were formed by the crystallization of relatively oxidizing magmas, under ca. 0 ≤ ΔNNO ≤ 1 (Vlach & Ulbrich 1994, Galembeck et al. 1997, Janasi et al. 2009), values akin to the titanite-magnetite-quartz-amphibole-ilmenite (TMQAI) buffer, as defined by Noyes et al. (1983) and Wones (1989). Up to now, the available data do not allow better define measurable ox-red differences among the syn-, late- and post-orogenic granites. However, mineral assemblage suggests the possibility of somewhat higher oxidation states in the case of the relatively evolved titanite-magnetite-bearing biotite granites. A remarkable exception is the Cabreúva Pluton, in the Itu Batholith, with biotite compositions akin to A-type granites with intermediate to reduced character (Galembeck et al. 1997, Janasi et al. 2009). As expected, the anatetic leucogranites (Nazaré Paulista-type) generated by melting of metasedimentary sequences and related occurrences at the Morungaba regions (Areia Branca Pluton and Meridional Occurrences, cf. Vlach (1993) and Vlach & Ulbrich (1994)) were formed under more reduced conditions and comprise mainly granites of the ilmenite-series series of Ishihara (1977). Oxidizing conditions are the most typical feature of metaluminous, magnesian, calc-alkaline granites related to subduction environments and the associated late biotite granites emplaced during the post-orogenic uplift stages, which contain, in variable amounts, contributions from both the continental crust and the mantle in their compositions.

SUMMARY AND CONCLUSIONS

Detailed petrographic and geochemical studies for a typical sample (hornblende biotite monzogranite, with Fe-Ti oxides, allanite, apatite, titanite, zircon and some sulphide), representative of the main Bragança Paulista-type syn-orogenic magmatism from the southern Brasilia Orogen, indicates that:

• granites present geochemical fingerprints akin to high-K, magnesian, metaluminous, calc-alkaline series. The main granite type was emplaced at medium crustal levels, corresponding to ca. 510 ± 60 MPa (≈ 14-16 km depth); the original magma crystallized under relatively low cooling rates, within an arguably temperature interval from ca. 975ºC (close-to-liquidus) to ca. 755ºC (close-to-solidus);

• magmas crystallized under middle- ƒ_H2O and -ƒ_HCl, low- ƒ_HF and oxidizing conditions, compatible with 0 ≤ ΔNNO ≤ 1. Ilmenite-hematite progressive re-equilibrium with decreasing temperatures suggests an oxidizing evolving trend. The estimate ox-red conditions are similar to those regionally observed for most late- to post-orogenic granites from the Itu Province;

• the integration of barometric and age data available for the main regional metamorphism and magmatism in the studied area suggest a post-closure uplift relative ratio about 0.2 to 0.3 km/Ma, from the main regional metamorphism to the post-orogenic granite emplacement periods.

The presented results may be taken, with the deserved caution, as representative of the main Bragança Paulista-type magmatism. However, similar studies considering the whole granite varieties, new whole-rock and mineral geochemical should be carried out to better constrain the emplacement of this extensive magmatism and its geodynamic implications.

ACKNOWLEDGEMENTS

The authors thank the GeoAnalitica/USP staff for support during sample preparation and analysis. Salazar-Naranjo thanks Brazilian National Council for Scientific and Technological Development (CNPq) for a Master scholarship (Process no. 159228/2014-0). S. Vlach thanks CNPq (Process no. 309557/2014-1). Authors thank R. Dal’Agnol for his invaluable comments and criticism and another anonymous reviewer; both help to improve the first manuscript version.

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  Manuscript ID: 20180033.

SUPPLEMENTARY DATA

Supplementary data associated with this article can be found in the online version: Supplementary Table A1.

Publication Dates

• Publication in this collection
  Jul-Sep 2018

History

• Received
  23 Mar 2018
• Accepted
  01 Aug 2018