Crystallization conditions of two adjacent epidote + diopside-bearing granitic stocks, northeastern Brazil

Abstract

The Tamboril and Olho d’Água granitic stocks are part of the abundant calc-alkalic magmatic epidote-bearing granitic rocks in the Cachoeirinha–Salgueiro Terrane (CST) in the Transversal Zone Domain, northeastern Brazil. The equigranular Olho d’Água stock is composed of medium-grained clinopyroxene-amphibole–biotite tonalite; the porphyritic Tamboril stock is medium- to coarse-grained amphibole biotite ± clinopyroxene granodiorite. Abundances of clinopyroxene and epidote vary inversely in both stocks. Amphibole–rich clots are regarded as fragments from the source region captured by granodioritic/tonalitic magma during its ascent. Epidote composition in the Olho d’Água stock (Ps₁₈–₂₆) and in Tamboril stock (Ps_17-20) is consistent with crystallization under oxygen fugacity between QFM and HM buffers. In the Olho d’Água stock, calculated values of pressure range from 5.1 to 6.6 kbar and in the Tamboril stock from 6.2 to 7.0 kbar. Solidification temperatures estimated from plagioclase-hornblende pairs in the Olho D´Água stock range from 637 to 679 °C and for Tamboril from 587 to 641 °C. Zr-saturation temperature estimates are 788 to 819 °C (Olho d’Água) and 807 to 829 °C (Tamboril). Altogether our data suggest that the studied stocks crystallized from two distinct magmatic pulses formed from fractional melting of a single amphibolitic source. These two magma pulses underwent subsequent crystallization, in a convective magmatic chamber, at rather high pressure.

Key words
calc–alkalic granites; magmatic epidote; clinopyroxene; thermobarometry; oxygen fugacity

INTRODUCTION

Neoproterozoic calc-alkalic and high-K calc-alkalic granodioritic to tonalitic plutons intruded Neoproterozoic low-grade metasedimentary rocks of the Cachoeirinha–Salgueiro (CST) and Alto Pajeú (APT) terranes in northeastern Brazil. These plutons are made up of medium–grained equi- to inequigranular rocks that commonly contain magmatic epidote (mEp) crystals up to 2 mm long. They exhibit a texture, informally described as “toad leather”, which is characteristic of granites regionally known as of the Conceição type (Sial 1984). In some of these plutons, samples with lower-abundance of mEp have higher-abundance of clinopyroxene (e.g. Pedra Branca, Tamboril, Angico Torto, Minador plutons; Sial et al. 1999, and also the Coronel João Sá pluton in the Macururé Domain (Long et al. 2005). Extensive reviews on petrography and geochemistry of these plutons are found in Sial et al. (1999, 2008) and Brasilino et al. 2011.

This study concerns two such plutons: Tamboril and Olho d’Água (Fig. 1). Although a single NE—SW trending elongate “Tamboril” pluton was initially mapped, we have distinguished two adjacent stocks, renamed as Tamboril and Olho d’Água, whose P-T conditions of crystallization differ. Both plutons bear magmatic epidote (mEp) but they also carry clinopyroxene, which is normally absent. The contact zone between the two “facies” is severely weathered, and only after an examination of the mineral chemistries and thermobarometric estimates did it become clear that they are actually two adjacent stocks.

Figure 1
(a) Simplified geological map of the study area and the magnetic susceptibility values of Tamboril and Olho d’Água stocks. (b) Location of the Transversal Zone within the Borborema Province, Northeast of Brazil. Modified from Siqueira et al. (2018).

This study aims to: (a) understand the conditions of coexistence of magmatic epidote with clinopyroxene, (b) characterize the mineral chemistry of these mEp–bearing granitoids; (c) estimate crystallization temperatures and pressures, which differ; (d) understand the geologic environment of formation.

MAGMATIC EPIDOTE AND CLINOPYROXENE IN GRANITIC PLUTONS IN THE CST

In the Cachoeirinha–Salgueiro Terrane, only the Pedra Branca and Angito Torto plutons, 100 km apart from one another, are so far known to contain both mEp and cpx. In the Pedra Branca pluton, abundant calcic clinopyroxene crystals up to 3 cm long commonly show rims partially transformed into hornblende, suggesting disequilibrium with the host melt (Sial et al. 1998). In minor epidote, pistacite contents [Ps = molar 100*Fe³⁺/(Fe³⁺+Al³⁺)] vary between Ps₁₉ and Ps₂₄. Not all of the values of pistacite point to a magmatic origin of this epidote (Sial 1990). Similarly, in the Angico Torto pluton, cpx occurs as crystals up to 1 cm long, accompanied by four different types of less-abundant mEp, two of them of probable magmatic origin, displaying chemical zonation and allanite–rich cores (Sial 1990).

In the unfoliated Coronel João Sá batholith, intrusive into the Sergipano Belt, Macururé Domain, magmatic epidote coexists with strongly resorbed clinopyroxene. In this batholith, which comprises two adjacent plutons, clinopyroxene (augite–diopside) is present in the northern pluton and in the western portion of the southern pluton. Partially resorbed clinopyroxene laths, up to 1.5 cm long, are replaced by hornblende, biotite, or epidote, leaving disaggregated clinopyroxene (cpx) grains that are in optical continuity (Long et al. 2005). Textural characteristics of epidote indicate magmatic origin. Below, we compare the commonalities of the Pedra Branca, Angico Torto, and Coronel João Sá bodies with Tamboril and Olho D´Água, the subject of this paper.

PREVIOUS WORKS

The Tamboril and Olho D´Água stocks, exposed for over than 70 km², were intruded into low- to intermediate- grade metasedimentary rocks of the Santana dos Garrotes Formation (Cachoeirinha Group), which consists predominantly of metapelite, metapsammite, metaconglomerate, and to a much lesser extent, metavolcanic rocks, marble, and banded iron formation. TIMS U‒Pb zircon ages in the 660‒620 Ma interval were determined in metavolcanic rocks of this Group (Van Schmus et al. 2011).

According to Siqueira et al. (2018), the Olho d’Água stock is composed of medium–grained, equigranular clinopyroxene–biotite tonalite containing amphibole, magmatic epidote, and titanite. The Tamboril stock is a coarse- grained porphyritic granodiorite, with quartz plagioclase megacrysts, K-feldspar, biotite, amphibole, magmatic epidote, ± clinopyroxene, and titanite. The former stock is faulted in its western contact and the latter in its northern contact, and they were emplaced along a regional SW‒NE trending fracture (Fig. 1). Mafic syn–plutonic dikes are observed only in the Olho d´Água stock. In the Tamboril stock, up to 50 cm wide pegmatitic dikes record multiple injections of felsic magma and in some of than comb structure are present.

Enclaves

The Olho d´Água stock exhibits abundant dioritic enclaves up to one meter long. These enclaves show crenulated margins in contact with tonalitic to granodioritic host rock, indicating that both lithologies, of different magmatic viscosities, had been in a magmatic state. In some of these mafic enclaves, ellipsoidal K-feldspar crystals seem to have been captured from the tonalite magma, suggesting a degree of mixing.

Sial et al. (1998) described two types of amphibole–rich clots (ARCs) in mEp–bearing calc–alkalic plutons intruded into the Cachoeirinha–Salgueiro Terrane. One type consists of dark green calcic amphibole aggregates interpretated to have fractionated from the host magma; the other type, consisting of amphibole aggregates up to 15 cm long, exhibits fine-grained texture (Sial et al. 1998). The second type, with crystals up to 5 cm long, is mainly composed of actinolite amphibole with magnesio-hornblende margins, regarded as source fragments captured by granodioritic/tonalitic magmas (Sial et al. 2008, Sial & Ferreira 2015). Typically a biotite + amphibole layer armors the second type, thus to have inhibited further interaction with the host magma. Both ARC types are present in the Olho d’Água and Tamboril plutons. One of them is angular in shape, suggesting extraction from the source in a solid state (Fig. 2c); the other type is armored by biotite crystallized from the host magma (Fig. 2d).

Figure 2
Geological and petrographic aspects of Tamboril and Olho d’Água plutons: (a) biotite rows at euhedric plagioclase; (b) Plagioclase megacrysts with oscillatory zoning and biotite rows; (c) angular–shaped of calcic amphibole rich clot; (d) amphibole rich clot armored by biotite; (e) euhedral clinopyroxene without inclusions; (f) Clinopyroxene with poikilitic texture; (g–h) euhedral magmatic epidote (mEp) with allanite nucleus and compositional zoning.

Olho D´Água tonalite and Tamboril granodiorite

The studied stocks are high–K calc–alkalic, metaluminous, Mg-type Cordilleran tonalitic to granodioritic rocks, but their data points plot in discrete fields in the same diagrams, exhibiting two independent trends (Siqueira et al. 2018). Both tonalite-granodiorite and enclaves carry clinopyroxene, in both plutons clinopyroxene in host rock is unrelated to clinopyroxene in the enclaves (Siqueira et al. 2018). In the host tonalite, elongate clinopyroxene up to 2 cm long occupies 5 to 10% of the rock volume (Fig. 2e). In the diorite enclaves, elongate poikilitic crystals up to 4 cm long occupy up to 25% of the volume. Abundant small inclusions are comprised of anhedral amphibole, biotite, and titanite (Fig. 2f). Uralitized clinopyroxene in the host tonalites are observed, probably due to subsolidus hydrothermal alteration.

Plagioclase, up to 4 cm long, in the Tamboril pluton exhibits polysynthetic twinning, synneusis, myrmekitic intergrowth lamellae and wavy extinction, all of which resulted from solid–state deformation. Plagioclase megacrysts also display oscillatory zoning in, and biotite rows (double concentric rows in some cases; Fig. 2a), attest that the host magma had experienced multiple pulses of magma injection, possibly associated with convection.

Sial et al. (2008) described four types of magmatic epidote textural relationships: type 1 mEp is embayed or in vermicular contact with unaltered plagioclase; type 2 mEp is rimmed by biotite and exhibits zoned allanite cores; type 3 mEp encloses patches of hornblende and in type 4, mEp is partially enclosed by biotite in the interstices of K-feldspar aggregates. Siqueira et al. (2018) identified only the type 2 mEp in both the Olho d’Água and Tamboril stocks, besides anhedral to subhedral, very elongated crystals of dubious origin in the core of plagioclase megacrysts, and epidote as product of plagioclase saussuritization.

For both plutons, all of these characteristics — presence of amphibole rich clots (ARC), magmatic epidote, euhedral titanite, clinopyroxene, calcic amphibole, and calc-alkalic and metaluminous whole-rock chemistry — have been recognized as diagnostic indicators of I-type granitoides (Siqueira et al. 2018).

ANALYTICAL METHODS

The mineral chemical analyses were carried out at the Institute of Geosciences, University of Brasília, Brazil. Thin sections were covered by a carbon film in a vacuum chamber and analyzed in a JEOL model JXA–8230 electron microprobe, with one EDS and five WDS spectrometers. For major element analysis an acceleration voltage of 15 kV, current of 10 nA, and a diameter of 1 μm of the electronic beam were used for analyzing feldspar, amphibole, biotite, epidote, titanite, and clinopyroxene. Mineral standards were: Na₂O (albite gaspox), MgO (forsterite), F (topaz), Al₂O₃ (microcline), SiO₂ (microcline), TiO₂ (MnTiO₃), Cr₂O₃ (Cr₂O₃), MnO (MnTiO₃), K₂O (microcline), CaO (andradite gaspox), NiO (NiO), FeO (andradite gaspox), V₂O₃ (vanadinite), Cl (vanadinite).

Clinopyroxene structural formula was calculated based on 6 oxygens and 4 cations, as well as the percentages of wollastonite (Wo = Ca₂Si₂O₆), enstatite (En = Mg₂Si₂O₆) and ferrosilite (Fs = Fe₂Si₂O₆). Plagioclase compositions were calculated based on 5 cations and 8 oxygens, biotite on 20 cations and 24 oxygens, epidote on 8 cations and 12.5 oxygens, titanite on 12 cations and 20 oxygens, and amphibole on 13 cations and 23 oxygens. The Tindle & Webb (1990) procedure was used to calculate Li₂O and H₂O of biotite and the Tindle & Webb (1994) methods to partition Fe²⁺ and Fe³⁺, and calculate H₂O contents of amphibole.

MINERAL CHEMISTRY

Feldspar

Tamboril plagioclase is mostly An₁₈–₃₄ (oligoclase–andesine), whereas Olho d’Água plagioclase varies from An₄ to An₄₂ (albite–andesine) (Fig. 3) (Table I, Supplementary material – Table SI). A profile (core–rim) performed on a plagioclase phenocryst of the Tamboril pluton demonstrates a smooth reverse zoning with a nucleus slightly enriched in Na and depleted in Ca.

Figure 3
Isotherms in (a) and (b) are respectively from Green and Watson (1982) and Green and Pearson (1986), with silica versus oxides (in wt.%) diagrams for the Tamboril and Olho d’Água plutns. (c) Douce (1999) experimental diagram.

Table I
Microprobe analyses of plagioclase from Tamboril (TAM) and Olho d’Água (OA) plutons.

Sample	BELRS-2A	BELRS-2A	BELRS-2A	BELRS-2A	BELRS-2A	BELRS-2A	BELRS-13A	BELRS-13A	BELRS-20A	BELRS-20A	BELRS-31	BELRS-31
Position	Center	Rim	Center	Rim	Center	Rim	Center	Rim	Center	Rim	Center	Rim
Pluton	TAM	TAM	TAM	TAM	TAM	TAM	OA	OA	OA	OA	OA	OA
SiO2	59.32	62.66	60.82	62.23	62.76	63.12	57.50	60.52	61.10	66.48	58.01	62.62
TiO2	<dl	<dl	0.06	<dl	0.04	<dl	<dl	0.05	0.07	0.07	<dl	<dl
Al2O3	25.93	23.47	25.31	23.48	24.22	23.17	26.60	23.93	25.14	22.64	26.17	23.80
FeO	<dl	0.09	<dl	<dl	<dl	0.03	<dl	0.15	0.07	0.13	0.13	0.10
MnO	0.04	<dl	0.04	0.03	<dl	0.13	<dl	0.02	0.13	<dl	0.06	<dl
MgO	<dl	<dl	<dl	0.01	<dl	0.01	0.02	<dl	<dl	0.02	<dl	0.01
CaO	7.03	4.87	6.41	4.96	5.41	4.33	8.42	5.13	7.37	4.06	7.95	4.77
BaO	0.08	<dl	<dl	<dl	0.08	0.10	<dl	0.03	<dl	<dl	<dl	0.06
Na2O	7.79	9.17	8.17	8.95	8.67	9.47	6.89	8.94	7.40	9.27	7.08	9.08
K2O	0.13	0.11	0.16	0.19	0.09	0.14	0.09	0.05	0.18	0.13	0.26	0.19
Total	100.32	100.36	100.98	99.84	101.27	100.51	99.52	98.82	101.44	102.81	99.65	100.63
	Cation proportions are based on 8 oxygens
Si	2.631	2.760	2.676	2.757	2.749	2.773	2.581	2.706	2.694	2.867	2.598	2.752
Ti	<dl	<dl	0.002	<dl	0.001	<dl	<dl	0.002	0.002	0.002	<dl	<dl
Al	1.355	1.218	1.312	1.226	1.250	1.200	1.408	1.261	1.306	1.151	1.382	1.233
Fe	<dl	0.003	<dl	<dl	<dl	0.001	<dl	0.006	0.003	0.005	0.005	0.004
Mn	0.001	<dl	0.001	0.001	<dl	0.005	<dl	0.001	0.005	<dl	0.002	<dl
Mg	<dl	<dl	<dl	0.001	<dl	0.001	0.002	<dl	<dl	0.001	<dl	0.001
Ca	0.334	0.230	0.302	0.235	0.254	0.204	0.405	0.246	0.348	0.188	0.382	0.225
Ba	0.001	<dl	<dl	<dl	0.001	0.002	<dl	0.001	<dl	<dl	<dl	0.001
Na	0.670	0.783	0.697	0.769	0.736	0.807	0.599	0.775	0.633	0.775	0.615	0.773
K	0.007	0.006	0.009	0.011	0.005	0.008	0.005	0.003	0.010	0.007	0.015	0.011
Sum	5,000	5,000	5,000	5,000	4,997	5,000	5,000	4,999	5,000	4,996	4,998	4,999
Mol.% An	33.048	22.548	29.957	23.203	25.511	19.997	40.121	24.031	35.134	19.339	37.734	22.260
Mol.% Ab	66.252	76.862	69.142	75.761	73.989	79.222	59.396	75.713	63.873	79.902	60.802	76.662
Mol.% Or	0.700	0.590	0.902	1.036	0.500	0.781	0.482	0.256	0.994	0.760	1.464	1.078

• <dl: below detection limits.

Amphibole

Amphibole is calcic (Leake et al. 1997) (Table II, Table SII). Amphibole in the Tamboril pluton and its dioritic enclaves is mostly ferro-edenite, whereas Olho d’Água amphibole is edenite to Fe-pargasite (Fig. 4a), althougth slightly more magnesian than Tamboril amphibole, that is similar to amphiboles in plutons elsewhere in the Cachoeirinha–Salgueiro Terrane (e.g) Brasilino et al. (2011). Mg/(Mg+Fe) cationic ratios in edenite (0.45–0.70) are in the range proposed by Mason (1985) for amphibole from alkali-calcic granitoids; Fe/(Fe+Mg) ratios correspond to moderately high oxygen fugacity during crystallization of this phase. Alternatively, the availability of Fe can have also played some influence on the Fe/(Fe+Mg) ratio. A positive correlation between Al^IV and Al^T in amphibole (Fig. 4b) is similar to that reported by Hammarstrom & Zen (1986), which reflects perhaps crystallization of amphibole during ascent of magma. In Olho d’Água stock, amphibole has alumina saturation index (ASI = Al₂O₃/(CaO + Na₂O + K₂O) molar) about 0.62-0.94 and in Tamboril stock ASI is around 0.65-0.79.

Figure 4
(a) Anorthite–Albite–Orthoclase ternary diagram (Deer et al. 1992) showing the compositions of feldspars from the Tamboril and Olho d´Água plutons; (b) Classification of amphiboles after Leake et al. (1997) compared with results obtained by Brasilino et al. (2011) (gray field); (c) Relationship between Al^IV and Al^T in the amphiboles of Tamboril and Olho d’Água plutons.

Table II
Microprobe analyses of amphibole from Tamboril (TAM) and Olho d’Água (OA) plutons. Tindle & Webb (1994) methods calculated to Fe partition (Fe²⁺ ˗ Fe³⁺) and H₂O contents.

Sample	BELRS-2A	BELRS-2A	BELRS-2A	BELRS-2A	BELRS-2A	BELRS-2A	BELRS-13A	BELRS-13A	BELRS-13A	BELRS-13A	BELRS-19B	BELRS-19A	BELRS-19A
Position	Center	Rim	Center	Rim	Center	Rim	Center	Rim	Center	Rim	Center	Center	Rim
Pluton	TAM	TAM	TAM	TAM	TAM	TAM	OA	OA	OA	OA	OA	OA	OA
SiO2	43.502	43.674	43.304	43.648	43.751	43.73	48.458	46.128	46.25	42.568	45.421	46.424	41.921
TiO2	1.313	0.682	1.221	0.87	1.066	1.325	1.24	1.007	1.023	2.158	1.595	1.612	1.63
Al2O3	10.902	10.679	10.544	10.899	10.787	11.04	7.477	9.707	8.779	11.397	8.706	7.653	11.097
FeO	17.358	16.816	17.013	17.117	18.477	17.443	15.912	14.625	16.993	17.76	14.136	14.12	16.325
MgO	8.393	8.435	8.347	8.347	8.494	8.342	11.918	11.802	10.25	8.602	11.106	11.234	11.143
MnO	0.229	0.381	0.36	0.354	0.224	0.338	0.301	0.096	0.339	0.243	0.493	0.4	0.223
CaO	11.548	11.425	11.461	11.547	11.39	11.401	11.533	11.517	11.82	11.326	11.406	11.244	6.167
Na2O	1.491	1.393	1.563	1.614	1.557	1.543	1.365	1.672	1.251	1.56	1.507	1.374	0.594
K2O	1.323	1.307	1.274	1.33	1.373	1.336	0.525	0.744	0.962	1.143	0.902	0.604	5.032
F	0.157	0.184	0.15	0.285	0.161	0.135	0.163	0.256	0.16	0.218	0.028	0.045	0.146
Cl	0.014	<dl	0.014	<dl	0.002	0.006	<dl	0.007	0.022	0.005	0.013	0.012	0.009
Sum	96.23	96.10	95.25	96.01	97.28	96.64	98.89	97.56	97.85	96.98	95.31	94.72	94.29
	Cation proportions are based on 23 oxygens.
T-sites
Si	6.650	6.743	6.690	6.696	6.628	6.650	7.033	6.788	6.881	6.457	6.863	7.025	6.614
Aliv	1.350	1.257	1.310	1.304	1.372	1.350	0.967	1.212	1.119	1.543	1.137	0.975	1.386
Sum	8.000	8.000	8.000	8.000	8.000	8.000	8.000	8.000	8.000	8.000	8.000	8.000	8.000
M1.2.3 sites
Alvi	0.615	0.687	0.610	0.667	0.555	0.630	0.312	0.473	0.421	0.496	0.414	0.390	0.678
Ti	0.151	0.079	0.142	0.100	0.121	0.152	0.135	0.111	0.114	0.246	0.181	0.183	0.193
Fe3+	0.096	0.100	0.066	0.064	0.226	0.120	0.329	0.298	0.228	0.250	0.161	0.161	0.311
Mg	1.912	1.941	1.922	1.908	1.918	1.891	2.578	2.588	2.273	1.945	2.501	2.533	2.620
Mn	0.030	0.050	0.047	0.046	0.029	0.044	0.037	0.012	0.043	0.031	0.063	0.051	0.030
Fe2+	2.123	2.071	2.132	2.132	2.115	2.099	1.602	1.502	1.886	2.003	1.625	1.626	1.789
Ca	0.073	0.072	0.081	0.082	0.036	0.066	0.006	0.015	0.035	0.029	0.055	0.055	0.000
Sum	5.000	5.000	5.000	5.000	5.000	5.000	5.000	5.000	5.000	5.000	5.000	5.000	5.000
M4 site
Fe	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	0.676
Ca	1.819	1.818	1.816	1.816	1.813	1.792	1.787	1.801	1.849	1.812	1.792	1.769	1.043
Na	0.181	0.182	0.184	0.184	0.187	0.208	0.213	0.199	0.151	0.188	0.208	0.231	0.282
Sum	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000
A site
Ca	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl
Na	0.261	0.235	0.284	0.296	0.270	0.247	0.171	0.278	0.210	0.271	0.234	0.172	-0.100
K	0.258	0.257	0.251	0.260	0.265	0.259	0.097	0.140	0.183	0.221	0.174	0.117	1.013
Sum A	0.519	0.493	0.535	0.556	0.536	0.506	0.268	0.417	0.392	0.492	0.408	0.288	0.913
OH site
OH	1.920	1.910	1.923	1.862	1.922	1.933	1.925	1.878	1.919	1.894	1.983	1.975	1.925
F	0.076	0.090	0.073	0.138	0.078	0.065	0.075	0.120	0.076	0.105	0.013	0.022	0.072
Cl	0.004	<dl	<dl	<dl	<dl	0.002	<dl	0.002	0.006	0.001	0.003	0.003	0.002
Sum	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000	2.000
Sum cations	15.519	15.493	15.535	15.556	15.536	15.506	15.268	15.417	15.392	15.492	15.408	15.288	15.913
Fe#	0.537	0.528	0.534	0.535	0.550	0.540	0.428	0.410	0.482	0.537	0.417	0.414	0.451
Mg/Fe2+	0.901	0.937	0.901	0.895	0.907	0.901	1.609	1.723	1.205	0.971	1.539	1.558	1.063
Mg/Fe	0.862	0.894	0.874	0.869	0.819	0.852	1.335	1.438	1.075	0.863	1.400	1.418	1.216
Mg/(Mg+Fe2+)	0.382	0.388	0.384	0.382	0.384	0.378	0.516	0.518	0.455	0.389	0.500	0.507	0.524

• <dl: below detection limits.

Biotite

In the Al^T vs. Mg atoms per formula unit (apfu) diagram, the biotite compositions (Table III, Table SIII) from the Tamboril stock plot within the field for biotite of the calc–alkalic series of igneous rocks (Fig. 5a), while biotite composition from the Olho D´Água stock straddles from the sub-alkalic to the calc-alkalic fields. The Al^IV vs Fe# diagram (Deer et al. 1992) (Fig. 5b) shows that trends for the two stocks are distinct, with biotite from the Olho D´ Água stock showing a larger Al^IV variation than of biotite from the other stock, both a with limited Fe# variation. In the MgO vs. Al₂O₃ diagram, the biotite compositions of both plutons plot in the field of calc–alkalic series of igneous rocks (Fig. 5c), compatible with the calc-alkalic nature of host rock (Siqueira et al. 2018). The average value of FeO/MgO (1.76) for biotite in these stocks is similar to ratios reported by Brasilino et al. (2011) from other calc–alkalic granites from the Cachoeirinha-Salgueiro Terrane. The alumina saturation index from the biotite of these stocks between 1.38–1.86 reflects low alumina activity in the crystallizing magma (Zen 1988). Under these conditions, it is common to form Ca-rich and low-Al mineral phases, such as hornblende and epidote.

Figure 5
Chemical classification and discrimination diagrams for biotite (a-c): (a) Al^T vs Mg diagram, fields after Nachit et al. (1985); (b) Al^IV vs Fe/(Fe+Mg) diagram; (c) Diagram proposed by Abdel–Rahman (1994). Symbols: A = alkalic; C = calc–alkalic; P = peraluminous (Deer et al. 1992); (d) Al vs. F correlation of titanites from the Olho d’Água stock.

Table III
Microprobe analyses of biotite from Tamboril (TAM) and Olho d’Água (OA) plutons. Tindle & Webb (1990) approach was used to calculate Li₂O and H₂O contents.

Sample	BELRS-10B	BELRS-10B	BELRS-10B	BELRS-10B	BELRS-10B	BELRS-19A	BELRS-19A	BELRS-27	BELRS-27	BELRS-32	BELRS-32
Position	Center	Center	Rim	Center	Rim	Center	Rim	Center	Rim	Center	Rim
Pluton	TAM	TAM	TAM	TAM	TAM	OA	OA	OA	OA	OA	OA
SiO2	36.80	36.66	36.55	35.53	36.25	36.72	37.00	36.07	36.59	36.37	36.54
TiO2	1.60	1.87	1.22	0.90	1.20	2.34	2.31	3.29	2.72	2.92	2.39
Al2O3	16.10	16.00	15.95	15.76	15.99	15.38	15.60	15.32	15.63	15.20	14.75
FeO	23.16	22.54	23.99	23.77	22.33	16.36	16.57	16.95	16.31	18.87	18.94
MnO	0.26	0.26	0.16	0.39	0.17	0.18	0.16	0.10	0.16	0.33	0.23
MgO	9.00	9.03	9.43	9.94	9.59	10.56	10.96	10.79	11.48	9.98	10.65
CaO	0.01	0.04	<dl	0.03	0.04	0.02	0.01	0.04	0.02	0.07	0.08
Na2O	0.04	0.07	0.09	0.05	0.08	0.06	0.05	0.05	0.07	0.03	0.08
K2O	9.14	9.20	8.99	7.72	8.87	9.46	9.18	9.74	9.89	9.11	9.15
BaO	0.26	0.15	0.25	0.16	0.36	0.18	0.29	0.48	0.04	0.57	0.03
F	0.68	0.94	0.96	0.47	0.62	0.34	0.23	0.31	0.28	0.42	0.39
Cl	0.01	0.01	<dl	0.01	<dl	0.01	0.01	<dl	0.01	<dl	0.01
Cr2O3	0.01	<dl	0.10	0.04	0.08	0.04	0.10	0.09	0.05	0.10	<dl
NiO	<dl	<dl	<dl	<dl	<dl	0.08	0.07	<dl	0.03	<dl	0.08
Li2O*	1.01	0.97	0.94	0.64	0.85	0.99	1.07	0.80	0.95	0.89	0.93
H2O*	3.64	3.50	3.51	3.64	3.61	3.71	3.80	3.74	3.80	3.70	3.70
Subtotal	101.71	101.25	102.14	99.04	100.03	96.42	97.40	97.77	98.02	98.57	97.94
O=F.Cl	0.29	0.40	0.40	0.20	0.26	0.15	0.10	0.13	0.12	0.18	0.17
Total	101.42	100.85	101.74	98.84	99.77	96.28	97.30	97.64	97.90	98.39	97.78
	Cation proportions are based on 24 oxygens
Si	5.564	5.565	5.533	5.516	5.561	5.689	5.664	5.560	5.582	5.596	5.637
Al iv	2.436	2.435	2.467	2.484	2.439	2.311	2.336	2.440	2.418	2.404	2.363
Al vi	0.433	0.427	0.380	0.399	0.453	0.497	0.478	0.343	0.393	0.352	0.319
Ti	0.182	0.213	0.139	0.105	0.138	0.273	0.266	0.381	0.312	0.338	0.277
Cr	0.002	<dl	0.012	0.005	0.009	0.005	0.012	0.011	0.006	0.012	<dl
Fe	2.929	2.861	3.037	3.087	2.865	2.119	2.121	2.185	2.081	2.428	2.443
Mn	0.033	0.034	0.020	0.051	0.022	0.023	0.021	0.013	0.021	0.043	0.029
Mg	2.029	2.043	2.128	2.300	2.192	2.439	2.500	2.480	2.611	2.288	2.449
Ni	<dl	<dl	<dl	<dl	<dl	0.010	0.009	<dl	0.004	<dl	0.010
Li*	0.614	0.592	0.571	0.402	0.525	0.615	0.657	0.496	0.582	0.549	0.580
Ca	0.001	0.006	<dl	0.005	0.007	0.003	0.001	0.007	0.003	0.012	0.013
Na	0.011	0.021	0.028	0.014	0.023	0.017	0.014	0.015	0.020	0.010	0.024
K	1.763	1.782	1.736	1.529	1.736	1.869	1.793	1.915	1.924	1.788	1.801
Ba	0.015	0.009	0.015	0.010	0.021	0.011	0.017	0.029	0.003	0.034	0.002
OH*	3.672	3.546	3.541	3.769	3.698	3.831	3.885	3.850	3.864	3.794	3.807
F	0.326	0.453	0.459	0.229	0.302	0.167	0.111	0.150	0.134	0.205	0.191
Cl	0.002	0.001	<dl	0.002	<dl	0.003	0.004	<dl	0.002	0.001	0.001
Sum	20.012	19.989	20.066	19.908	19.992	19.882	19.889	19.875	19.960	19.855	19.947

• <dl: below detection limits.

Epidote

Most epidote grains are zoned in which Fe⁺³ decreases from center to edge. Euhedral and sub-euhedral epidote present distinct molar pistacite, from Ps₁₇–₂₀, avg. Ps₁₈ (Tamboril) to Ps₁₈–₂₆, avg. Ps₂₁ (Olho d’Água) (Table IV, Table SIV).

Table IV
Microprobe analyses of epidote from Tamboril (TAM) and Olho d’Água (OA) plutons.

Sample	BELRS-2A	BELRS-2A	BELRS-10B	BELRS-10B	BELRS-16B	BELRS-16B	BELRS-20B	BELRS-20B	BELRS-31	BELRS-31	BELRS-32
Position	Center	Center	Center	Rim	Center	Rim	Center	Rim	Center	Rim	Center
Pluton	TAM	TAM	TAM	TAM	OA	OA	OA	OA	OA	OA	OA
SiO2	38.40	39.00	38.38	38.83	37.95	37.93	36.86	36.42	38.17	38.53	36.68
TiO2	0.11	0.20	0.27	0.15	0.13	0.44	0.01	0.17	0.27	0.82	0.20
Al2O3	24.41	26.96	26.65	26.84	25.33	25.81	25.27	24.75	26.42	26.67	23.86
FeO	10.90	7.53	7.91	8.04	9.13	9.07	9.53	10.51	8.32	7.94	10.58
MnO	0.17	0.09	0.13	0.17	0.18	0.19	0.33	0.30	0.15	0.07	0.18
MgO	0.05	<dl	<dl	0.01	<dl	0.03	0.02	0.34	0.02	0.01	0.14
CaO	23.29	23.85	23.47	23.43	23.37	23.13	22.78	22.54	23.26	23.23	22.03
Na2O	<dl	0.02	<dl	<dl	0.02	0.01	0.04	0.04	0.01	0.02	0.01
K2O	0.02	<dl	0.01	<dl	<dl	<dl	<dl	0.02	0.01	0.01	0.04
BaO	0.10	0.05	0.05	0.02	0.06	0.09	<dl	0.08	0.05	0.05	<dl
F	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl
Cl	0.01	0.01	0.01	<dl	<dl	<dl	0.02	<dl	<dl	<dl	<dl
Cr2O3	0.09	0.18	0.07	0.11	0.14	0.13	0.16	0.08	0.05	0.15	0.04
NiO	<dl	0.01	0.01	<dl	<dl	0.05	<dl	0.03	<dl	<dl	0.05
V2O3	0.06	0.17	0.07	0.11	0.05	0.12	0.04	0.05	0.12	0.16	0.11
Total	97.61	98.07	97.02	97.71	96.35	96.98	95.06	95.31	96.82	97.66	93.93
	Cation proportions are based on 12.5 oxygens
Si	3.015	3.018	3.005	3.016	3.006	2.984	2.966	2.934	2.997	2.996	2.990
Ti	0.007	0.012	0.016	0.009	0.008	0.026	0.001	0.010	0.016	0.048	0.012
Al	2.259	2.459	2.459	2.456	2.365	2.393	2.396	2.349	2.445	2.443	2.292
Fe+3	0.716	0.487	0.518	0.522	0.605	0.597	0.641	0.708	0.546	0.516	0.721
Mn	0.011	0.006	0.009	0.011	0.012	0.013	0.022	0.021	0.010	0.005	0.013
Mg	0.006	<dl	<dl	0.001	<dl	0.003	0.003	0.040	0.002	0.001	0.017
Ca	1.959	1.978	1.969	1.950	1.984	1.949	1.964	1.946	1.957	1.935	1.924
Na	<dl	0.002	<dl	<dl	0.004	0.001	0.006	0.006	0.001	0.003	0.002
K	0.002	<dl	0.001	<dl	<dl	<dl	<dl	0.002	0.001	0.001	0.004
Ba	0.003	0.002	0.001	0.001	0.002	0.003	<dl	0.002	0.002	0.002	<dl
Cr	0.006	0.011	0.004	0.006	0.009	0.008	0.010	0.005	0.003	0.009	0.003
Ni	<dl	<dl	0.001	<dl	<dl	0.003	<dl	0.002	<dl	<dl	0.004
V	0.004	0.011	0.005	0.007	0.003	0.007	0.002	0.003	0.008	0.010	0.007
Sum	7.987	7.987	7.987	7.979	7.997	7.988	8.012	8.028	7.987	7.969	7.990
Pistacite	24	17	17	18	20	20	21	23	18	17	24

• <dl: below detection limits.

Titanite

Titanite compositions (Table V, Table SV) in the Olho d’Água stock display positive correlation between Al and F (Fig. 5d), with Al contents in the 0.16 to 0.34 apfu range and fluorine in the 0 to 0.36 range. Based on Franz & Spear (1985), these chemical features argue that titanite crystallized at high pressure, a contention that finds further support in observations by Evans & Patrick (1987) and Tropper et al. (2002).

Table V
Microprobe analyses of titanite from the Tamboril (TAM) and Olho d’Água (OA) plutons.

Sample	BELRS- 21	BELRS-14A	BELRS-14A	BELRS-19A	BELRS-19A	BELRS-20A	BELRS- 27	BELRS- 27	BELRS- 30	BELRS- 30	BELRS- 32
Position	Rim	Center	Rim	Center	Rim	Center	Center	Rim	Center	Rim	Center
Pluton	TAM	OA	OA	OA	OA	OA	OA	OA	OA	OA	OA
SiO2	30.54	30.74	31.38	29.58	30.58	32.22	30.10	30.13	29.70	30.24	30.02
TiO2	36.95	38.52	37.96	41.67	41.20	37.54	42.00	40.93	41.85	42.02	41.20
Al2O3	2.19	1.89	2.21	1.49	1.61	1.67	2.26	2.09	1.62	1.52	1.46
FeO	0.67	0.52	0.54	0.52	0.60	0.54	0.58	0.89	0.49	0.58	0.54
MnO	0.03	0.09	0.11	0.09	0.09	0.09	0.11	0.01	<dl	0.10	0.10
MgO	0.01	<dl	0.01	<dl	<dl	<dl	0.03	0.27	<dl	0.03	0.01
CaO	27.08	27.84	28.02	26.54	27.36	27.20	27.03	27.16	27.09	27.80	27.02
Na2O	0.08	0.02	0.01	0.02	0.01	0.02	0.02	0.05	0.02	0.01	<dl
K2O	<dl	<dl	0.01	<dl	0.01	<dl	<dl	0.10	<dl	0.01	0.01
BaO	0.13	0.15	0.19	<dl	0.35	<dl	0.02	0.12	<dl	0.10	0.01
F	0.34	0.14	0.25	<dl	0.08	0.17	0.24	0.24	0.01	0.18	0.06
Cl	0.01	<dl	<dl	<dl	<dl	0.01	0.01	<dl	0.01	<dl	<dl
Cr2O3	0.04	0.07	0.14	0.01	<dl	0.06	0.07	0.08	<dl	<dl	0.04
NiO	0.03	0.03	0.09	<dl	<dl	<dl	<dl	0.04	<dl	0.02	<dl
V2O3	0.30	0.36	0.26	0.22	0.25	0.30	0.31	0.44	0.37	0.34	0.31
Total	98.24	100.32	101.09	100.14	102.09	99.75	102.68	102.45	101.15	102.87	100.76
	Cation proportions are based on 20 oxygens
Si	4.053	3.999	4.049	3.848	3.912	4.184	3.823	3.848	3.829	1.914	3.885
Ti	3.689	3.770	3.684	4.079	3.965	3.668	4.013	3.932	4.059	2.030	4.011
Al	0.342	0.290	0.337	0.228	0.242	0.255	0.338	0.314	0.247	0.164	0.222
Fe+2	0.075	0.056	0.059	0.057	0.064	0.059	0.062	0.095	0.053	0.053	0.058
Mn	0.003	0.010	0.012	0.010	0.010	0.010	0.012	0.001	<dl	<dl	0.011
Mg	0.002	<dl	0.003	<dl	<dl	<dl	0.006	0.051	<dl	<dl	0.002
Ca	3.850	3.880	3.873	3.700	3.751	3.786	3.678	3.717	3.742	3.742	3.747
Na	0.021	0.006	0.003	0.006	0.003	0.006	0.005	0.013	0.004	0.008	0.001
K	<dl	<dl	0.001	<dl	0.001	<dl	<dl	0.017	<dl	<dl	0.002
Ba	0.007	0.008	0.010	<dl	0.018	<dl	0.001	0.006	<dl	<dl	0.001
Cr	0.004	0.007	0.014	0.001	<dl	0.006	0.007	0.008	<dl	<dl	0.004
Ni	0.003	0.003	0.009	<dl	<dl	<dl	<dl	0.004	<dl	<dl	<dl
V	0.031	0.038	0.027	0.023	0.025	0.031	0.032	0.045	0.038	0.025	0.032
Sum	12.080	12.066	12.080	11.950	11.991	12.005	11.978	12.050	11.972	7.937	11.976

• <dl: below detection limits.

Clinopyroxene

The analyzed clinopyroxene grains (Table VI, Table SVI) from both plutons and their enclaves are generally rich in calcium and have low iron content (from 7.2 to 13.1% FeO), which makes their compositon to plot in the diopside field (Morimoto 1988) (Fig. 6). Clinopyroxene in host rock and diorite enclaves are similar, even though these lithologies are texturally different.

Figure 6
Compositional variation diagram as a function of the molecular components Wo, En and Fs of pyroxenes from the Tamboril and Olho D´Água plutons (Morimoto 1988).

Table VI
Microprobe analyses of clinopyroxene from Tamboril (TAM) and Olho d’Água (OA) plutons.

Sample	BELRS-2A	BELRS-2A	BELRS-13A	BELRS-13A	BELRS-16A	BELRS-16A	BELRS-19A	BELRS-19A	BELRS-19D	BELRS-19D	BELRS- 25	BELRS- 25	BELRS- 32	BELRS- 32
Position	Center	Rim	Center	Rim	Center	Rim	Center	Rim	Center	Rim	Center	Rim	Center	Rim
Pluton	TAM	TAM	OA	OA	OA	OA	OA	OA	OA	OA	OA	OA	OA	OA
SiO2	52.80	53.09	51.47	51.93	53.66	53.14	52.48	52.09	55.72	55.52	52.74	52.20	52.86	52.95
TiO2	0.26	0.25	0.44	<dl	<dl	0.03	0.19	<dl	0.02	0.08	0.08	0.05	0.16	<dl
Al2O3	0.79	0.79	1.35	0.46	0.71	0.57	0.81	0.92	0.67	0.72	1.18	0.59	0.67	0.94
Cr2O3	<dl	<dl	0.05	<dl	0.11	0.08	0.07	0.13	0.12	0.01	<dl	<dl	0.12	0.02
FeO	12.01	11.79	12.88	12.35	10.41	9.97	9.71	9.93	9.38	9.74	11.55	11.27	12.36	11.82
MnO	0.38	0.36	0.44	0.43	0.34	0.43	0.32	0.41	0.42	0.50	0.30	0.50	0.55	0.39
MgO	10.32	10.36	10.29	10.68	12.53	12.55	11.72	11.56	11.45	11.25	10.36	10.58	11.10	11.47
CaO	22.77	22.90	22.38	23.41	23.22	22.94	22.80	22.27	23.02	23.08	23.10	22.96	22.20	22.70
Na2O	0.26	0.43	0.40	0.38	0.30	0.27	0.35	0.38	0.44	0.48	0.44	0.39	0.29	0.38
K2O	0.02	<dl	<dl	<dl	<dl	0.01	0.01	<dl	0.03	<dl	<dl	<dl	0.01	<dl
Total	99.61	99.96	99.71	99.65	101.27	99.99	98.46	97.68	101.28	101.36	99.75	98.53	100.31	100.67
	Cation proportions are based on 6 oxygens
Si	2.019	2.019	1.967	1.980	1.993	1.997	2.007	2.008	2.074	2.067	2.006	2.009	2.003	1.990
Ti	0.007	0.007	0.013	<dl	<dl	0.001	0.005	<dl	0.001	0.002	0.002	0.001	0.005	<dl
Al	0.036	0.035	0.061	0.020	0.031	0.025	0.037	0.042	0.029	0.031	0.053	0.027	0.030	0.041
Fe+3	<dl	<dl	0.009	0.047	0.002	<dl	<dl	<dl	<dl	<dl	<dl	<dl	<dl	0.006
Cr+3	<dl	<dl	0.001	<dl	0.003	0.002	0.002	0.004	0.004	<dl	<dl	<dl	0.003	0.001
Fe+2	0.384	0.375	0.403	0.347	0.322	0.313	0.310	0.320	0.292	0.303	0.368	0.363	0.392	0.365
Mn	0.012	0.011	0.014	0.014	0.011	0.014	0.010	0.014	0.013	0.016	0.010	0.016	0.018	0.013
Mg	0.588	0.587	0.586	0.607	0.693	0.703	0.668	0.664	0.636	0.624	0.587	0.607	0.627	0.643
Ca	0.933	0.933	0.916	0.957	0.924	0.924	0.934	0.920	0.918	0.921	0.942	0.947	0.901	0.914
Na	0.019	0.032	0.030	0.028	0.022	0.020	0.026	0.028	0.032	0.035	0.032	0.029	0.021	0.028
K	0.001	<dl	<dl	<dl	<dl	0.001	0.001	<dl	0.002	<dl	<dl	<dl	<dl	<dl
Sum	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000	4.000
Wo	48.97	49.22	48.09	50.06	47.65	47.60	48.84	48.30	49.75	49.82	49.65	49.40	46.94	47.56
En	30.87	30.99	30.77	31.78	35.77	36.25	34.94	34.89	34.43	33.78	30.97	31.67	32.66	33.44
Fs	20.16	19.79	21.14	18.16	16.58	16.15	16.23	16.81	15.82	16.40	19.38	18.93	20.40	19.00

• <dl: below detection limits.

THERMOBAROMETRIC ESTIMATES

Temperature estimates using hornblende–plagioclase

Blundy & Holland (1990) proposed a thermometer (BH thermometer) based on amphibole–plagioclase relation, applied to temperatures in the 500–1000 °C range and the Si contents < 7.8 apfu. Subsequently, Holland & Blundy (1994) proposed a new calibration (HB thermometer) for the edenite-tremolite reaction, which requires silica saturation, and reduced the level of uncertainty from ± 75 °C to ± 35 °C.

Temperature estimates were calculated by both semi-empirical thermometers of Blundy & Holland (1990) and Holland & Blundy (1994) (Table VII), using the compositions where rims of plagioclase and amphibole crystals in contact to each other, i.e. where the two phases are most likely to have been in exchange equilibrium. Estimated temperature using Blundy & Holland (1990) are 675° to 713 °C (average 692 °C, BH) for the Tamboril, and 696° to 744 °C (average 714 °C, BH) for the Olho d’Água pluton (Table VII). Calibration of Holland & Blundy (1994) geothermometer, based on the reaction “edenite + 4 quartz = tremolite + albite” (Fig. 7) requires silica saturation and presence of Ca-amphibole and Na-rich plagioclase, which is the case for both plutons. Estimated temperatures are 587° to 641 °C (average 612 °C, HB) for Tamboril and 637° to 679 °C (average 652 °C, HB) for Olho d’Água. All these calculated temperatures are lower than expected crystallization temperatures of granitic magmas (650° to 700 °C, at 4-10 kbar; Luth et al. 1964) and lower than temperatures calculated by the BH thermometer. Therefore, using both thermometers, temperature results are distinct for both stocks, although with some overlap. The estimated temperatures represent that of the equilibrium crystallization of plagioclase and amphibole, somewhere between liquidus and solidus.

Table VII
Summary of the estimation of crystallization conditions from Tamboril and Olho d’Água Plutons.

Sample	Belrs-2A	Belrs-2A	Belrs-2A	Belrs-2A	Belrs-2A	Belrs-2A	Belrs-2A	Belrs-2A	Belrs-13A	Belrs-13A	Belrs- 31	Belrs- 31	Belrs- 31	Belrs- 31	Belrs- 31	Belrs- 31
Pluton	TAM	TAM	TAM	TAM	TAM	TAM	TAM	TAM	OA	OA	OA	OA	OA	OA	OA	OA
T1 (H&B. 1994)	587.4	603.7	609.3	606.5	624.3	641.1	601.7	621.4	666.5	679.1	641.9	645.9	651.3	637.0	644.4	650.2
P(T1) (A&S. 1995)	6.83	6.93	6.95	7.06	6.95	6.21	6.95	7.05	5.09	6.64	5.75	6.02	5.26	5.85	5.31	5.79
P (Mutch. 2016)	4.90	5.02	5.05	5.13	5.11	4.59	5.03	5.18	3.88	5.31	4.23	4.47	3.91	4.29	3.91	4.32
T2 (H&B. 1994)	597.0	616.2	618.1	604.7	634.8	652.9	617.5	621.8	683.5	682.6	655.7	665.9	647.7	638.2	671.1	662.3
P(T2) (A&S. 1995)	6.81	6.88	6.91	7.07	6.88	6.11	6.89	7.05	4.91	6.59	5.64	5.84	5.28	5.84	5.08	5.68
T3 (B&H. 1990)	675.0	687.0	687.8	686.3	704.4	713.0	688.7	697.1	719.2	744.7	711.1	717.4	694.5	696.8	716.3	712.8
P(T3) (A&S. 1995)	6.24	6.22	6.24	6.35	6.06	5.35	6.21	6.25	4.42	5.50	4.97	5.14	4.81	5.24	4.50	5.04

• Temperature in °C calculated: T1 (Holland & Blundy 1994) refers to Holland & Blundy (1994) reaction A, T2 (Holland & Blundy 1994) refers to Holland & Blundy (1994) reaction B, T3 (Blundy & Holland 1990) refers to Blundy & Holland (1990) thermometry. P(T1)-P(T2)-P(T3) (Anderson & Smith 1995) refers to Anderson & Smith (1995) Al-in-hornblende barometry for each temperature calculated. P (Mutch 2016) refers to Mutch et al. (2016) barometry. Belrs-2A – Tamboril Pluton. Belrs-13A and Belrs-31 – Olho d’Água Pluton.

Figure 7
Thermobarometry of Tamboril and Olho d’Água plutons. The regions with orange and yellow colors are the data obtained in plutons analyzed by Sial et al. (1999) in the Cachoeirinha–Salgueiro and Seridó terranes. See text for details.

Al–in–hornblende barometry

Anderson & Smith (1995) and Anderson (1996) showed that both temperature and total pressure are sensitive parameters to influence the mineral chemistry of mafic silicates and that all barometers are often affected by variations in intensive parameters such as oxygen fugacity. They have shown that at low oxygen fugacity the barometer fails to record high pressure (as known from independent evidence) for plutons with iron–rich hornblende coexisting with the full barometric assemblage (quartz, iron–magnesium silicates, euhedral titanite and magnetite). Therefore, the barometer is recommended for hornblende Fe/(Fe+Mg) ratio with values between 0.40 and 0.65.

The Anderson & Smith (1995) barometer was applied in this study. This Al-in-hornblende barometer is dependent on temperature, which in this study was that calculated using the thermometer proposed by Holland & Blundy (1994) as discussed above. These results are shown in Table VII and in Figure 7. The pressure ranges obtained are distinct for the two stocks. In the Tamboril pluton, the pressure values are between 6.2 and 7.0 kbar (average 6.9 kbar), while in the Olho d’Água pluton pressure values are between 5.1 and 6.6 kbar (average 5.7 kbar).

For comparison, the Mutch et al. (2016) barometer, which is based only on the total Al, was also calculated (Table VII). These authors caution that their barometer is valid in the range of T = 725 ± 75°C, which are temperatures substantially higher than our calculated temperatures (except for Olho D´Água stock). Values of pressure calculated by the Mutch et al. (2016) barometer are between 4.9 and 5.2 kbar (Tamboril pluton) and and between 3.8 and 5.3 kbar (Olho d’Água pluton).

So, serious discrepancies pertain to pressures calculated by the two barometers based upon the same data. Temperatures determined by the hornblende-plagioclase pair seem to have been affected by solidus re-equilibration, especially in the Tamboril pluton, where they are lower than the temperature range required for use of Mutch’s barometer. Therefore, one assumes here that only pressures estimated for the Olho d’Água pluton should be considered for further discussion.

Near-liquidus temperature estimates using Zr saturation thermometry

If a granitic melt is saturated with Zr, the whole-rock abundance of Zr is related to the temperature of zircon crystallization (Watson 1987). Watson & Harrison (1983) demonstrated that zircon saturation thermometry provides a simple and consistent means of estimating magma temperatures, and showed experimentally that the solubility of zircon is correlated with magmatic SiO₂ contents. Zircon must be the first mineral to crystallize, and solely of magmatic origin (not inherited and not a cumulate phase). According to this methodology, a minimum liquidus temperature can be calculated by the equation of Watson & Harrison (1983) as rearranged by Miller et al. (2003) using the Zr of the total rock (ppm):

$$T_{Zr}=\frac{12900}{2.95+0.85M+\ln (496000/Z{r}_{melt})}$$

Results for the Tamboril stock range from 807 to 829 °C, and for the Olho d’Água stock between 788 and 819 °C (Table VIII). Fig. 8 summarizes T-P relationships; it displays an estimated near-liquidus temperature for mEp-bearing plutons in the region, and an H2O-saturated solidus in relation to calculated intensive data for the Tamboril and Olho d’Água plutons.

Table VIII
Estimated near-liquidus temperature using Zr-saturation equation of Watson & Harrison (1983) rearranged by Miller et al. (2003). Zr concentration in ppm.

Sample	Zr	T (°C)
(A) Tamboril Pluton
BELRS-02	227	819
BELRS-06	233	829
BELRS-06A	211	822
BELRS-07	194	807
BELRS-08	226	824
BELRS-09	233	818
BELRS-10B	215	827
BELRS-21	200	826
BELRS-21M	214	814
(B) Olho d’Água Pluton
BELRS-04A	200	801
BELRS-04B	206	805
BELRS-05	211	800
BELRS-13	208	802
BELRS-14	207	808
BELRS-16	193	790
BELRS-17	201	788
BELRS-18	224	804
BELRS-19A	205	806
BELRS-19B	202	797
BELRS-20	192	790
BELRS-25	204	804
BELRS-26	197	808
BELRS-27	214	802
BELRS-28	212	804
BELRS-29	248	818
BELRS-30	213	804
BELRS-31	215	805
BELRS-32	242	819

Figure 8
P–T diagram for granitoids with magmatic epidote of the Cachoeirinha–Salgueiro Terrane. Curve of the liquidus obtained with the zircon saturation equation and curve for the solidus (magmatic curve for H₂O saturated granitoids). The regions with light orange and light yellow are P-T of plutons studied by Sial et al. (1999), the light blue region temperature and pressure of crystallization of the Tamboril and Olho d’Água plutons, and the region in pink are near- liquidus temperature and pressure of these plutons.

These calculated near liquidus temperature are similar to the range of those estimated using the method of apatite saturation of Green & Watson (1982), between 800°C and 900°C as illustrated in the Fig. 3a, as well as using the relationship TiO₂ and silica contents (Green & Pearson 1986) that indicates T< 900°C (Fig. 3b).

DISCUSSION

Source rock

Silica contents of the studied rocks range from ~63.7 to 67.9 wt% in the Tamboril pluton and from ~62.1 to 65.1 wt% in the Olho d’Água pluton (Siqueira et al. 2018). Samples from the two plutons form almost continuous, although distint groups, in silica vs. oxide diagrams, as illustrated for the correlation between silica and P₂O₅ and TiO₂ (Figs 3a, 3b).

Granitic/granodioritic magmas are usually considered to have formed from evolution of basaltic magmas or by crustal anatexis (e.g. Jagoutz 2010, Brown 1994, 2013, and references therein). Neverthless, experiments show that granitic/granodioritic magmas can be produced from dehydration of amphibolites (e.g. Beard & Lofgren 1991, Rushmer 1991, Rapp & Watson 1995, Douce 1999) or from basaltic compositions (e.g. Beard & Lofgren 1991, Sisson et al. 2005). Several lines of evidence suggest that the studied rocks derived from magmas formed from melting of mafic rocks. For instance, major element compositions of the studied plutons are similar to those of experimentally-obtained melts from amphibolites studied by Douce (1999) (Fig. 3c). Besides, the widespread presence of amphibole-rich mafic clots, as well as diorite enclaves in the studied plutons are suggestive of a source derivation. One of the type of amphibole-rich clots presents an external layer of biotite + amphibole surrounding actinolite with magnesio-hornblende margins (Fig. 2d). Usually this type of ARC show angular shape (Fig. 2c) suggesting that they were captured as solid by the tonalite magma. The mafic enclaves are commonly found in the rocks of both plutons. Usually thet have sharp contacts with the host rocks, although they tend to have elliptical to rounded shapes. They are mainly fine-grained dioritic rocks, including diorite and pyroxene diorite with igneous textures, and having condensing edges with rich fine dark minerals.

Similar features are observed in many other plutons of this type in the Cachoeirinha-Salgueiro Terrane (e.g. Sial & Ferreira 2015) in the mafic clots (chiefly calcic amphibole, plus clinopyroxene, biotite, euhedral titanite) are interpreted to be fragments, possibly restites, from partial fusion of amphibolitic source (Sial & Ferreira 2015). The mafic fragments could have been transported upward by granodiorite/tonalite magmas from a metabasaltic source to crystallize at shallower depth (Sial 1993, Sial et al. 1998, 2008, Ferreira et al. 2003, 2011). This way, we infer a similar source for the magmas that originated the studied plutons, and a similar mechanism of fragment transportation.

Low whole-rock magnetic susceptibility (≈ 0.3 × 10–³ SI) reported by Siqueira et al. (2018) for the studied stocks is compatible with an S–type origin for the magmas (Chappell & White 1974), but within the range for ilmenite-series of granitoids (Ishihara 1977, 1998, Takahashi et al. 1980). Nevertheless, Sial et al. (1999) identified magmatic epidote-bearing granites in the Cachoeirinha–Salgueiro Terrane that have low magnetic susceptibility (<0.5 × 10–³ SI) , and interpreted them to be I-type rocks. They attributed this low magnetic susceptibility for I-type granite as due to incorporation of Fe⁺³ into epidote rather than into iron oxide minerals. In fact, Schmidt & Thompson (1996) noted that magnetite is more abundant in epidote-free granites than in epidote-bearing granites (2.1–18 kbar and 550–850 °C).

Magmatic crystallization

Inclusion-free euhedral clinopyroxene grains are interpreted here as crystallized from the host tonalite magma, whereas inclusion-rich subhedral grains in diorite enclaves are interpreted as residues from the magma source. Thompson & Ellis (1994) proposed that during melting, epidote + amphibole + quartz can produce liquid + clinopyroxene at T >800 °C and P between 10 and 25 kbar. Long et al. (2005) proposed that during dehydration-melting of an amphibolitic source rock, amphibole-rich clots + clinopyroxene could produce melt + residual clinopyroxene. Dehydration melting reactions can produce clinopyroxene at the expense of biotite and amphibole (e.g., Chappell et al. 2012).

Titanite in the Olho d’Água pluton tends to be enriched in Al and F, which suggests crystallization at moderate to high pressure (Erdmann et al. 2019; Evans & Patrick 1987, Franz & Spear 1985, Tropper et al. 2002). The Mg/(Mg+Fe) ratios (0.45–0.70) in amphibole are consistent with the range proposed by Mason (1985) for calc–alkalic granitoids, while Fe/(Fe+Mg) ratios indicate moderately high oxygen fugacity at crystallization of this phase.

Textural criteria to recognized primary epidote includes euhedral to subhedral grains with allanite core and oscillatory zoning (e.g. Sial 1990) similar to the ones shown in the studied plutons (Fig. 2g, 2h). Chemical criteria to distinguish magmatic epidote on the other hand is based on either pistacite content of epidote (Ps = molar [Fe³⁺/(Fe³⁺+Al)]100)>25%, (Tulloch 1979), or on TiO₂ contents, as suggested by Evans and Vance (1987), for whom magmatic epidote typically has < 0.2 wt% TiO₂, whereas secondary epidote that is a replacement of biotite has > 0.6 wt% TiO₂. Epidote compositional variation from the Tamboril (Ps₁₇–₂₀) and Olho d’Água (Ps₁₈–₂₆) plutons are slightly below the values proposed by Tulloch (1979) for typically magmatic epidote (Ps₂₅ to Ps₂₉), and would suggest that these epidote formed from alteration of plagioclase. However, these pistacite contents are close to the range of pistacite reported as typical for mEp (Ps₂₀–₂₅) by Sial (1990), Sial et al. (1998), Brasilino et al. (2011) and Sial & Ferreira (2015), for Ediacaran granitoids in the Cachoeirinha–Salgueiro Terrane. Despite the slightly lower pistacite contents, Sial et al. (2008) interpreted euhedral to subhedral epidote crystals rimmed by biotite, with zoned allanite core as typical of magmatic epidote in the Cachoeirinha–Salgueiro Terrane (Sial et al. 2008, Ferreira et al. 2011, Sial & Ferreira 2015).

In samples from the two studied plutons in which clinopyroxene occur together with epidote, pyroxene is less abundant than epidote, as also observed in the Pedra Branca pluton, a similar calc-alkalic mEp granite (Sial & Ferreira 2015). These authors interpret the relation between clinopyroxene and epidote as consistent with Schmidt & Thompson (1996) experiments on water–saturated tonalitic magmas, which shows that the appearance of clinopyroxene occurs at the expense of epidote according to the reaction clinopyroxene + liquid = epidote + hornblende + H₂O, and so epidote increases at the expense of clinopyroxene.

Granitoid magmas have been classified into magnetite series (high ƒO₂) and ilmenite series (low ƒO₂) (Ishihara 1977). The boundary separating these series is between the HM and FMQ buffers (Wones 1989). Oxygen fugacity relates to magma source, a critical controlling parameter of magmatic processes. Although it is difficult, on the basis of an evolved final product (granite) to infer the oxygen fugacity of primary magma, some T and P conclusions may be used for mineral chemistry and mineral rock assemblage (Enami et al. 1993). Several line of evidence indicate that the Tamboril and Olho D´Água magma was oxidized. Among them are the presence of quartz, amphibole-rich clots, primary biotite, primary epidote (that carries Fe³⁺) and euhedral titanite, that together imply a relatively oxidized, silica-saturated host magma. Besides the pistacite contents in the epidote of the two plutons in this study and Fe# contents of biotite are consistent with crystallization between FMQ and HM buffers (Sial & Ferreira 2015). On the other hand, biotite from the Tamboril stock exhibits slightly higher Fe#, corresponding to biotite crystallization under slightly lower oxygen fugacity, than biotite from the Olho D´Água stock. The difference in the ƒO₂ conditions between Tamboril and Olho d’Água magmas may be associated with different magmatic pulses extracted from the same source or with the evolution of two distinct magmas...

Estimated temperature for the Tamboril (587-641^oC) and Olho d’Água (637-679^oC) stocks, both using Holland & Blundy (1994) are lower than those expected for crystallization temperature of granitic magmas (650° to 700 °C, at 4-10 kbar; Luth et al. 1964). These rather low temperature could be explained by aluminum reequilibration between coexisting amphibole and plagioclase during sub-solidus cooling (e.g. Ferreira et al. 2011). Another possibility is that mineral chemistry in both stocks have changed during magma intrusion into the phyllites of Cachoeirinha-Salgueiro Terrane. Many plagioclase grains in both stocks show alteration suggesting interaction with fluids.

Estimated pressure of crystallization using the Al^t-in-hornlende geobarometer of Anderson & Smith (1995) for the Tamboril stock ranges from 6.2 to 7 kbar while that from the Olho d’Água stock varies from 5.1 to 6.6 kbar. These values are higher than those calculated using Mutch et al. (2016) barometer (4.9 to 5.2 kbar for the Tamboril pluton, and from 3.8 to 5.3 kbar for the Olho d’Água pluton). The higher pressure estimated using Anderson & Smith (1995) barometer is compatible with the presence of magmatic epidote in the studied rocks, which is indicative of high pressure of crystallization, as suggested by Schmidt & Thompson (1996) who demonstrated experimentally that epidote and plagioclase can coexist around 10 kbar in tonalitic magmas.

Oscillatory textural and compositional zoning are observed in plagioclase grains of the studied plutons (Fig. 2a, 2b). Oscillations in chemical composition are apparently incompatible with slow, near-equilibrium growth (e.g. Shore & Fowler 1995). Many authors interpret this type of zoning as representing changes in temperature during feldspar growth possibily due to magma convection or to eventual reheating of the chamber by replenishment by more mafic magma (e.g. Mariano & Sial 1988, Shcherbakov et al. 2011). This evidence together with presence of biotite rows in plagioclase megacrysts (Fig. 2a) suggest that plagioclase crystallization occurred in a convective magma chamber probably due injection of mafic magma pulses.

Altogether, our data suggest that the studied plutons resulted from crystallization from different magmatic pulses, formed from fractional melting of a single source. These two magma pulses underwent subsequent crystallization, in a convective magmatic chamber, at rather high pressure.

CONCLUSIONS

Our data indicate that the rocks that compose the Tamboril and Olho D´Água stocks are calc-alkalic, metaluminous tonalite to granodiorite, that carry magmatic epidote and diopside, probably formed from crystallization of a magma derived from partial melting of amphibolitic source. The two plutons represent crystallization of two different magma pulses, in a convective magma chamber at intermediate to high oxygen fugacity conditions. Crystallization occurred under rather high pressure, as suggested by presence of magmatic epidote and Al-in-hornblende geobarometer. These two plutons are similar to many other magmatic epidote-bearing granitoids intrusive in the low-grade metapelites, and formed in magmatic-arc setting.

ACKNOWLEDGMENTS

We thank Prof. Nilson Botelho (University of Brasilia) for permitting us to use the electron microprobe laboratory of that University. ANS acknowledges grants from FACEPE (APQ 1073-1.07/15) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, 407171/2018-5) and VPF, a grant from CNPq (471034/2012-6) which helped defraying costs of this study. We are grateful to two anonymous reviewers whose comments and suggestions greatly contributed to improve our original text. This is the NEG-LABISE contribution n. 292.

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SUPPLEMENTARY MATERIAL

Table SI. Microprobe analyses of plagioclase from Tamboril (TAM) and Olho d’Água (OA) plutons.

Table SII. Microprobe analyses of amphibole from Tamboril (TAM) and Olho d’Água (OA) plutons. Tindle and Webb (1994) methods calculated to Fe partition (Fe2+ ˗ Fe3+) and H2O contents.

Table SIII. Microprobe analyses of biotite from Tamboril (TAM) and Olho d’Água (OA) plutons. Tindle & Webb (1990) approach was used to calculate Li2O and H2O contents.

Table SIV. Microprobe analyses of epidote from Tamboril (TAM) and Olho d’Água (OA) plutons.

Table SV. Microprobe analyses of titanite from the Tamboril (TAM) and Olho d’Água (OA) plutons.

Table SVI. Microprobe analyses of clinopyroxene from Tamboril (TAM) and Olho d’Água (OA) plutons.

Publication Dates

• Publication in this collection
  01 Nov 2021
• Date of issue
  2021

History

• Received
  9 Apr 2021
• Accepted
  9 July 2021