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Brazilian Journal of Geology

Print version ISSN 2317-4889On-line version ISSN 2317-4692

Braz. J. Geol. vol.48 no.2 São Paulo Apr./June 2018  Epub Feb 22, 2018

http://dx.doi.org/10.1590/2317-4889201820170091 

ARTICLE

Geology, geochemistry and petrology of basalts from Paraná Continental Magmatic Province in the Araguari, Uberlândia, Uberaba and Sacramento regions, Minas Gerais state, Brazil

Lucia Castanheira de Moraes 1  

Hildor José Seer 1  

Leila Soares Marques 2  

1Centro Federal de Educação Tecnológica de Minas Gerais - Araxá (MG), Brazil. E-mails: 2013luciam@gmail.com; hildorster@gmail.com

2Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo - São Paulo (SP), Brazil. E-mail: leila.marques@usp.br

Abstract:

This study covers the region between the cities of Sacramento and Araguari/Uberlândia (Minas Gerais State, Brazil), where basalt flows from the Paraná Continental Magmatic Province outcrop. The investigated rocks present tholeiitic signature, with high titanium content, and are classified as Pitanga magma-type. The preserved basalt thickness is between 10 and 200 meters and individual flows do not exceed 15 meters thick. Flows were identified as sheet lobes, smaller and thinner flows units - stacked laterally and vertically forming compound lavas -, or frontal, centimetric lobes. The basalt flows show decimetric to metric intercalations of clastic sedimentary rock, with depositional characteristics that can vary from aeolian to lacustrine, and are important markers on prevailing environmental conditions. The plagioclases are dominantly labradorite and pyroxene is augite, whereas olivine can be hyalosiderite or hortonolite/ferrohortonolite. The behavior of the major, minor and trace elements is compatible with the presence of at least two parental magmas, which were subjected to fractional crystallization mainly of plagioclase, clinopyroxene, ilmenite and magnetite. There is a chemistry distinction between basalts from Sacramento to those from Araguari/Uberlândia region, the former one showing more evolved than the last one. The high (La/Lu)N values are indicative of partial melting of a garnet peridotite, while the Rare Earth Elements (REE) values are indicative of fractional crystallization.

Keywords: Paraná-Etendeka Province; Lithostratigraphy; Petrography; Lithochemistry; Mineral chemistry

INTRODUCTION

Continental flood basalt provinces are huge eruptive events characterized by their phenomenal volumes of tholeiitic basalts erupted in a very short time range and in a continental environment from fissure systems in the Earth’s crust (Hooper 2000). One of these, the Paraná-Etendeka Province (PEP), associated with the opening of the South Atlantic Ocean (Renne et al. 1992, Peate et al. 1992, Turner et al. 1994), occupies an estimated area of 1,300,000 km2 distributed across Brazil, Argentina, Uruguay, Paraguay and Namibia (Fig. 1).

Figure 1: (A) Geological illustration of the Paraná Continental Magmatic Province (PCMP) with its magmatic rocks distribution overlapped by the Bauru Basin. Urubici and Ribeira type magma occurrences are not compatible with this scale. In detail, sketch of the Paraná-Etendeka Province (PEP). Modified from Janasi et al. (2011) and Waichel (2006); (B) geologic map of the Triângulo Mineiro region, modified from Pinto and Silva (2014); 1, 2, 3 and 4 are geologic sections represented in Figure 2. Dashed line limits the studied area; (C) location of analyzed samples and cities of the Triângulo Mineiro region mentioned in the text.  

The Brazilian portion of the PEP volcanic rocks (Serra Geral Group; Rossetti et al. 2017) was formed in the final phase of the Paraná Basin. This was an intracratonic and polyhistoric basin, which was part of the Gondwana Continent, dominated in its interior, at that time, by the arid climate (Scherer 2002, Jerram & Stollhofen 2002). These volcanic rocks are mainly of basaltic composition and overlap the aeolian deposits of the Botucatu Formation, and, in some places, they are intercalated. Sills, mainly located in the northern area, as well as the mafic dyke swarms of Ponta Grossa, Serra do Mar and Florianópolis, are associated to the volcanism (Ernesto et al. 2002), and together the extrusive and intrusive rocks compose the Paraná Continental Magmatic Province (PCMP). The intrusive rocks are predominantly basic in composition (micro-gabros), and all of them show similar geochemical characteristics to the associated volcanic rocks (Bellieni et al. 1984, Piccirillo et al. 1990, Ernesto et al. 1999).

The PCMP is dominated by tholeiitic basalts and andesibasalts (> 90%), although significant quantities of two compositionally distinct groups of silicic volcanics - Palmas (dacites and rhyolites with low-Ti content) and Chapecó types (dacites with higher-Ti content) - are found along the southern and eastern parts of the province (Mantovani et al. 1985, Bellieni et al. 1986, Piccirillo & Melfi 1988, Peate 1997). Peate et al. (1992) subdivided mafic magmatism into six types (Fig. 1A): Urubici, Pitanga, Paranapanema (high TiO2) and Gramado, Esmeralda and Ribeira (low TiO2). The northern PCMP is dominated by Pitanga and Paranapanema basalts, whereas the Gramado and Esmeralda tholeiites occur in southern PCMP. The Ribeira and Urubici flows are scarce and occur in the northern and southern PCMP, respectively. Recent studies have shown that some Urubici flows are present in the border region between Minas Gerais and São Paulo states (Machado 2005), as well as dikes crosscutting rocks of the São Francisco Craton, Minas Gerais (Seer et al. 2011, Marques et al. 2016). The age of the main phase of the volcanic activity is concentrated at 134 to 132 Ma, according with data obtained by Renne et al. (1992), Thiede and Vasconcelos (2010), and Janasi et al. (2011).

The regional location of the study area is shown in Figure 1B. It is also possible to see the current limits of the PCMP and the stratigraphic column for the Triângulo Mineiro region, Minas Gerais state, in the northeast of the province. Most of the basalts of the PCMP is covered by fluvio-lacustrine sedimentary Upper Cretaceous rocks belonging to the Bauru Basin. The analyzed samples, as well as some by Rocha-Júnior et al. (2013) used for comparison (KS samples), are located in Figure 1C. Much of the information available today from the northern portion of the PCMP is restricted to lithochemical data, without any knowledge of the lithostratigraphic point of view, with the exception of the works by Araujo (1982), Ferreira (1985), Machado (2005), Pacheco et al. (2017) and Moraes and Seer (2017). The Triângulo Mineiro region is limited by the Alto Paranaíba Arch, a regional topographic high present at least from the Lower Cretaceous and tectonically reactivated during the Upper Cretaceous and Cenozoic. The Alto Paranaíba Arch (Fig. 1A) composes the northeast border of the PCMP and hosts a series of dykes with NW direction and alkaline-carbonatite bodies of Upper Cretaceous age. The NW lineaments, inherited from the Neoproterozoic structure of the region, control the directions of the dykes. According to preliminary petrographic and geochemical data, dykes are potential feeders of the PCMP volcanism in the area. The Alto Paranaíba Arch was the source area of sediments of both, the Botucatu Formation and the Bauru Basin (Hasui 1967, Barcelos 1989, Batezelli 2003), besides an important watershed in the Triângulo Mineiro. It also seems to have behaved as a geographic barrier controlling the climate and the advance of the dunes and lavas of the PCMP.

GEOLOGICAL CONTEXT

The map of isopacs for the PCMP volcanic rocks in the Triângulo Mineiro region (Batezelli 2003, p. 147) shows a thickness over 1,000 meters in the west (Iturama in Fig. 1C) that decreases eastwards, being less than 100 meters in the Sacramento region. This information can be confirmed in Figure 2, in which both the paleotopography and the lithology distribution of the Botucatu Formation and PCMP volcanic rocks are represented.

Source: modifed from Moraes and Seer, 2017.

Figure 2: Paleorelief of the NE limit of the Paraná Continental Magmatic Province (PCMP) represented in regional geologic cross-sections (location on Figure 1B). The relation between basalts from PCMP and aeolian (+), alluvial (o) and lacustrine (*) dominant sediments of the Botucatu Formation are made explicit. 

Figure 2 shows an irregular paleotopography, higher in the direction of the Alto Paranaíba Arch, to the northeast. The thickness of sandstones and basalts and the relationships between them are variable. The occupation of the paleorelief begins with the sedimentation in paleovalleys by aeolian sandstones of the Botucatu Formation, whose thicknesses vary from less than two to 110 meters. In the Sacramento region, sections 1 and 2 show dunes over the basement and several aeolian sandstone lenses interspersed in basalt (Figs. 3A and 3B). Between Sacramento and Uberaba, section 3 shows no basal dunes, but many aeolian sandstone lenses. In the Uberlândia/Araguari region (section 4), just two small dunes occur to the north of Araguari city. On the other hand, lenses of alluvial fan and lacustrine deposits are present (Figs. 3C and 3D).

Figure 3: Sedimentary rocks of the Botucatu Formation and their field relations. (A) Aeolian sandstone lenses interspersed with basalt flows; (B) aeolian sandstone occupying voids in the upper crust blocks of basalt flow; (C) alluvial fan matrix supported conglomerate and sandstone from the Botucatu Formation overlaid by lava flow in irregular contact. The light green layer at the base of the lava flow represents rapid quenching of the lava, already altered; (D) lacustrine deposit (white dashed line) interspersed with lava flow.  

Alluvial fans occur as conglomerate levels dominantly matrix supported and can reach thicknesses of about 1 meter in recurrent channeled shapes. Lacustrine deposits are limited in area and thickness - with minimum length and depth of 50 and 2 meters, respectively -, and cover alluvial fans or interspersed basalt flows. Ostracod fossils were found in three of the four lacustrine deposits. Fossilized conifer woods were described in Botucatu Formation, near Uberlândia city (Pires et al. 2011).

The lavas also fill topographical depressions and free spaces between dunes, and the preserved thickness is between 10 and 200 meters. The contact between sedimentary and basaltic rocks is concordant. Locally it is possible to see interaction between lavas and sand forming peperites/pseudopeperites, degassing pipes in sandstone and grooves produced by basalt flows on still unconsolidated surface (Figs. 4A, 4B, 4C and 4D). The intertrapp sandstone can vary from a few centimeters to a few meters - and may occupy the voids between the upper crust blocks of a pahoehoe flow -, indicating the intermittent nature of the two processes. Nevertheless, these recurring intercalations indicate that aeolian processes and volcanism occurred simultaneously.

Figure 4: Sediment/lava field relationships: (A) heavily weathered peperite, with rounded fragments of oxidized-aphanitic basalt that has clay-sandy infilled vesicles in the mid of a clay-sandy matrix. Fragment edges show a reaction rim; (B) brecciated rubble surface filled by aeolian sand or cemented by carbonate/zeolite/chalcedony. The field relationship allow distinguishing both A and B structures; (C) blister of degassing in fractured sandstone due to lava flow action; (D) grooves and basalt remnants due to the passage of small lava lobes in still unconsolidated sandstone surface.  

Taking into account the quality and extent of the outcrops and the scale studied, it was not possible to determine the actual size of the lava flows. The outcrops visited do not exceed 15 meters thick. Lobes are found in sheet, structured in lava core and fractured crust facies (Fig. 5A), as well as smaller and thinner flows units, < 2 meters thick, stacked laterally and vertically forming compound lavas (Fig. 5B). Small frontal lobes, centimetric, similar to toes, were also identified in loose blocks or as grooves on the upper crust pahoehoe and on still unconsolidated sandstone surface (Figs. 5C and 5D).

Figure 5: Characteristics of basaltic flows: (A) sheet flows with many lobes. Note short but well developed colonnade at the base flows and entablature, which is dominant and extremely fractured; (B) compound lavas formed by thinner, smaller lava units stacked up; (C) small basalt toes, rusty and vesicular, flowing through the void spaces of a brecciated rubbly surface. Well-formed crystals of analcime can be seen in both; (D) grooves of small lava lobes on the top of brecciated pahoehoe flow. Note the breccia crust was still unconsolidated when the basaltic lobes flowed over it; (E) fan radiated columns from a core infilled with brecciated and hydrothermalized material.  

Sheet flows are exposed in basalt quarries. Colonnade usually occurs in the first third from the base of the flow, < 2 m thick, and eventually is also exposed near the top. Entablature of aphanitic basalt dominates lava flows, being narrower (< 30 cm wide), irregular and twisted. Fan radiate columns can be present (Fig. 5E) and exhibit a nucleus infilled with brecciated and hydrothermally altered material. The brecciated rubble top of the flow has abrupt contacts with the underlying entablature unit via decimetric chill-zone. In general, this level is rich in vesicles/amygdales that can be flattened or spherical. They vary between 0.5 and 1 cm in diameter and are filled by silica, calcite, zeolites, or celadonite, which is predominant. At the entablature rubbly-top transition, gas blisters (5 to 25 cm) that can be flattened, infilled with chalcedony and quartz, ± calcite crystals occur. The rubbly surface of the flow hardly exceeds 1 meter in length and consists of clasts of basalt within a fragmental basalt matrix-supported breccia. Alternatively, the clasts may be highly vesicular scoria and the spaces between the fragments may be filled by aeolian sand or cemented by carbonate.

Noteworthy is the presence of two sets of pillow lavas (5.7 and 7.4 meters average thickness, respectively) that interact with fluvio-lacustrine sediments in area situated between the cities of Uberlândia and Araguari, along the Central Atlantic Railway (FCA), in a location known as Fundão (Moraes & Seer 2017). This interaction gives rise to peperites as interpillow material, which may also be formed by hyaloclastite and fragments of lava, intrinsically mixed to the sediments in a complex way. The basal set of pillows, sized between 30 to over 150 centimeters, with hollow or filled center, is well preserved, and the pillows have chilled rim. The upper set is heavily weathered, and pillows are wrapped in fragments of sandstone, sandy mudrock and mudrock, but show compact arrangement - unlike the basal set, that may show high or very low packaging.

In general, the basalt is fine-grained to aphanitic, with a dark-gray to a greenish-dark gray color, sometimes reddish due to weathering, and consists of microphenocrysts of plagioclase and pyroxene and, less commonly, olivine. The mesostasis is dominated by crystals of plagioclase, pyroxene, magnetite, maghemite and ilmenite. There is a difference between textures of entablature and colonnade basalts; in the former one, volcanic glass is conspicuous, microlites are abundant, opaque crystals are predominantly skeletal, and swallowtail-like textures are common in plagioclase (Fig. 6A). On the other hand, colonnade basalts have phenocrysts with euhedral tendency, sometimes zoned, and coarser mesostasis in predominantly intergranular texture, sub- to euhedral opaque minerals and interstitial and localized glass (Fig. 6B). Where present, microvesicules/microamygdales (< 2 mm) are spherical to irregular, dominantly filled by celadonite, but also by silica, zeolites and calcite (Fig. 6C). The borders of basalt flows tend to be vitreous, with dispersed euhedral crystals of plagioclase and clinopyroxene and amoeboid vesicles (Fig. 6D).

Figure 6: Representative features in thin sections. (A) Plagioclase and olivine (altered to iddingsite) phyric basalt with acicular plagioclase crystals and seeds of clinopyroxene in the middle of indistinguishable oxides and glassy matrix. Parallel polarizers; (B) magnetite and augite intergranular to plagioclase crystals and sub-ophitic texture in fine-grained basalt with subordinated glass. Parallel polarizers; (C) fine-grained basalt with spherical vesicle filled with celadonite and silica. Crossed polarizers; (D) amoeboid amygdule, filled with celadonite and silica, set in a vitreous mesostasis and sparse pyroxene and plagioclase crystals, ≤ 0.5 mm, which may be zoned. Parallel polarizers.  

MATERIALS AND METHODS

The petrographic studies were performed on 28 polished thin sections at the Centro de Pesquisas Professor Manoel Teixeira da Costa of the Geoscience Institute of the Universidade Federal de Minas Gerais (UFMG). The minerals were analyzed by electronic microprobe in the laboratories of the Universidade Federal de Goiás (UFG) (153 analyses) and the Universidade Estadual Paulista “Júlio de Mesquita Filho” (UNESP) (252 analyses), of which 113 in pyroxene, 184 in plagioclase, 88 in oxides, 9 in olivine and 11 in alteration minerals. The polished thin sections were carbon film coated, and analytical work was performed at 15 kV and 20 nA. The data were processed using the Gabbrosoft Project worksheets (http://www.gabbrosoft.org), the WinPyrox (Yavuz 2013) and the Igpet software.

Whole-rock chemical analysis of four samples for major, minor, trace and rare earth elements were performed at Geosol laboratories, while 17 at UNESP ones. The percent differences for major and minor elements of sample HL004C (analyzed at GEOSOL and at UNESP) are lower than 4.8%, indicating that the results are compatible, without analytical bias. Samples were crushed, split and pulverized in mechanical agate mortar. At UNESP, they were analyzed by X-ray fluorescence (XRF) after Li2B4O7/LiBO2 fusion and LOI determined by gravimetric analysis (Nardy et al. 1997). At GEOSOL, major elements were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES); minor and trace elements by inductively coupled plasma mass spectrometry (ICP-MS). The relative accuracy for major and minor elements in both laboratories is 1%, whereas for trace elements it is within ± 3%. Major oxide contents were recalculated to 100% on an anhydrous basis. The data were processed and plotted using the softwares Igpet and Petrograph (Petrelli et al. 2005).

MINERAL CHEMISTRY

Modal mineralogy is on average 51% plagioclase, 36% pyroxene, 7% oxides and 4% olivine, which, however, may reach 6% or be absent. This result is consistent with the norms calculated for the chemically analyzed samples, where five samples have normative olivine and 17 do not. In the latter one, normative quartz is present. Normative apatite and ilmenite occurs in all samples.

The Ab-Or-An diagram (Fig. 7) shows the result of the analysis of 100 microphenocrysts and 41 microlites of plagioclase (Tab. 1). Some plagioclases are characterized by compositional zoning (Figs. 8A and 8B) and, when it occurs, their core and edge were analyzed separately. Among the micro-phenocrysts analyzed, sample HL002 has the most calcic crystal (An75Ab24Or1), while the HL003b (An51Ab46Or3), the least one. In mesostasis, the variations are between An63Ab35Or2 (HL002) and An47Ab49Or4 (HL003b), in a coherent and expected way. Although normal zonation is present (for example in HL004c sample), reverse zoning occurs too, and in some of the analyzed samples the nuclei of micro-phenocrysts are less calcic than those ones of the mesostasis. Fig. 9 shows edge-to-edge profiles for two zoned micro-phenocrysts. Profile A (FU463) shows composition An68 at the edges and An47 at the core. Profile B (FU687b) shows rims with composition around An48, depletion in calcium to An46.5 followed by enrichment, reaching the core with An50. Although the variation is small, it draws attention to a complex magmatic trajectory.

Table 1: Representative analysis and formulae of plagioclase.  

Sample FU17 P C FU17 P R FU17 MP C HL001 P C HL001 P R HL001 MP C HL004c P C HL004c P R HL004c MP C LH05 P C LH05 P R LH05 M PC
Component
SiO2 50.580 53.330 52.330 52.400 51.150 52.320 50.420 52.970 54.220 53.430 51.860 51.530
TiO2 0.110 0.170 0.130 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Al2O3 30.080 27.840 28.880 30.220 30.650 29.950 31.320 29.310 28.430 29.140 30.340 30.440
FeOt 0.850 0.920 0.930 0.730 0.870 0.790 0.800 0.750 0.970 0.750 0.730 0.860
CaO 13.380 11.390 11.920 12.830 13.600 12.730 14.370 12.060 11.240 11.860 13.140 12.910
Na2O 3.140 4.070 3.690 3.910 3.480 3.950 3.090 4.200 4.790 4.360 3.760 3.840
K2O 0.280 0.500 0.430 0.290 0.240 0.280 0.240 0.380 0.460 0.370 0.280 0.290
Total 98.420 98.220 98.310 100.380 99.990 100.020 100.240 99.670 100.110 99.910 100.110 99.870
Cations based on 32 oxygen
Si 9.369 9.843 9.663 9.470 9.337 9.519 9.198 9.653 9.831 9.705 9.436 9.405
Ti 0.015 0.024 0.018 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Al 6.565 6.057 6.285 6.482 6.593 6.420 6.733 6.294 6.075 6.238 6.506 6.548
Fe(ii) 0.132 0.143 0.143 0.111 0.133 0.121 0.122 0.114 0.147 0.114 0.111 0.131
Ca 2.655 2.252 2.358 2.501 2.660 2.481 2.808 2.354 2.183 2.308 2.561 2.524
Na 1.127 1.456 1.321 1.380 1.232 1.395 1.093 1.483 1.683 1.536 1.326 1.360
K 0.066 0.118 0.101 0.066 0.056 0.066 0.057 0.085 0.107 0.087 0.066 0.067
Cat sum 19.929 19.892 19.888 20.011 20.011 20.001 20.011 19.984 20.026 19.990 20.007 20.034
End-member (%)
An 69.000 58.870 62.390 63.370 67.370 62.950 70.950 60.010 54.950 58.710 64.780 63.880
Ab 29.290 38.050 34.940 34.950 31.210 35.380 27.620 37.820 42.370 39.090 33.550 34.430
Or 1.710 3.080 2.680 1.680 1.420 1.670 1.430 2.170 2.680 2.200 1.670 1.690

P: phenocryst; MP: micro-phenocryst; C: core; R: rim.

Figure 7: Compositional variation of plagioclase of the basalt flows (Pitanga magma type) studied according Or-Ab-An diagram of Deer et al. (2003).  

Figure 8: Microscopic aspects representative of basalt flows. (A) Micro-phenocryst of concentrically zoned labradorite in a mesostasis rich in plagioclase laths and granules of clinopyroxene, magnetite and olivine already transformed into iddingsite. Part of the black material is volcanic glass. Crossed polarizers; (B) back-scattered image showing phenocryst of plagioclase with irregular zoning and inclusions of glass and oxides in sample HL001; (C) augites in micro-phenocrysts, showing inclusions of plagioclase, oxides and glass, and immersed in a mesostasis of clinopyroxene, plagioclase, oxides and glass. Parallel polarizers; (D) back-scattered image of augite in micro-phenocryst with distinct marginal zoning and delicate “patchy zoning” within the crystal.  

Figure 9: Rim to rim profile in two plagioclase phenocrysts that show very distinct crystallization history. Sample FU 463 shows gradual Ca-enrichment from the center to the edge of the crystal, while the FU687b sample has a more complex history, with a Ca-enriched center and border and a depleted intermediate portion. 

The composition of 37 micro-phenocrysts and 30 microlites of pyroxenes is shown in Fig. 10A, the Wo-En-Fs ternary diagram (Morimoto 1988), and Table 2 shows some representative analyses and formulae of them. With the exception of two crystals from LH04 sample that fall in the pigeonite field, all the other pyroxenes analyzed are augites. Fig. 10B shows the compositional variation profiles obtained in zoned micro-phenocrysts. A normal variation, with enrichment in Fs towards the rim, is prevailing, except for the micro-phenocrysts present in samples HL004B1, HL004B2, LH04-2 and LH04-3. In these ones, some micro-phenocrysts are fairly homogeneous or show significant inverted zoning. In general, there is a negligible variation of the chemical composition from the core (Wo35-41En 39-47Fs15-24) to the rim (Wo36-39En38-47Fe14-23) of the zoned micro-phenocrysts, and a trend of differentiation is not identified.

Table 2: Some analysis and formulae of representative pyroxenes.  

Sample FU17 P C FU17 P R FU17 MP C HL001 P C HL001 P R HL004c P C HL004c P C HL004c MP C HL004c MP C LH04 P C LH04 P R LH04 MP C
Component
SiO2 49.430 49.820 47.140 50.120 46.990 49.690 50.540 49.520 50.280 49.520 50.130 50.110
TiO2 1.260 1.150 2.150 1.160 2.640 1.460 1.070 1.440 1.110 1.180 0.960 1.140
Al2O3 2.870 2.090 4.000 2.790 3.980 3.280 2.810 3.100 1.880 3.410 2.810 2.110
Cr2O3 0.260 0.030 0.080 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
FeOt 9.480 12.410 12.260 9.710 14.010 10.550 10.230 12.280 13.450 11.440 11.630 12.430
MnO 0.230 0.230 0.250 0.230 0.300 0.260 0.240 0.270 0.370 0.270 0.310 0.300
MgO 15.190 14.480 13.500 15.870 12.960 15.000 16.150 14.470 14.70 14.570 14.870 14.460
CaO 19.270 17.660 18.230 18.860 17.890 19.030 18.110 17.600 17.600 18.340 18.030 18.070
Na2O 0.270 0.200 0.300 0.260 0.300 0.240 0.240 0.230 0.200 0.360 0.260 0.230
K2O 0.010 0.030 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Total 98.260 98.100 97.900 99.000 99.070 99.510 99.390 98.910 99.590 99.090 99.000 98.850
Cations based on 6 oxygen
Si 1.880 0.033 1.824 1.887 1.810 1.870 1.894 1.882 1.908 1.876 1.899 1.910
Ti 0.036 0.094 0.063 0.033 0.077 0.041 0.030 0.041 0.032 0.034 0.027 0.033
Al 0.129 0.001 0.182 0.124 0.181 0.146 0.124 0.139 0.084 0.152 0.126 0.095
Cr 0.008 0.398 0.002 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Fe(ii) 0.301 0.007 0.397 0.306 0.451 0.332 0.320 0.390 0.427 0.362 0.368 0.396
Mn 0.007 0.828 0.008 0.007 0.010 0.008 0.008 0.009 0.012 0.009 0.010 0.010
Mg 0.861 0.726 0.778 0.891 0.745 0.842 0.902 0.820 0.832 0.823 0.840 0.821
Ca 0.785 0.015 0.756 0.761 0.739 0.768 0.727 0.717 0.716 0.744 0.732 0.738
Na 0.020 0.001 0.022 0.019 0.022 0.018 0.018 0.017 0.015 0.026 0.019 0.017
K 0.001 0.033 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Cat sum 4.027 4.016 4.032 4.028 4.033 4.024 4.023 4.016 4.025 4.027 4.020 4.019
End-member (%)
Wo 39.800 36.790 38.590 38.380 37.640 39.060 36.850 36.730 35.820 37.940 37.200 37.260
En 43.650 41.980 39.760 44.960 37.950 42.830 45.730 42.010 41.620 41.930 42.700 41.490
Fs 15.550 20.480 20.520 15.700 23.310 17.210 16.530 20.370 21.810 18.790 19.140 20.400

P: phenocryst; MP: micro-phenocryst; C: core; R: rim.

Figure 10: Pyroxene analysis. (A) Compositional variation of pyroxenes according to the ternary diagram (Wo-En-Fs) of Morimoto (1988); (C) compositional variation of pyroxenes according to core-rim profile; cpx crystals HL004B 1, HL004B 2, LH04-2 and LH04-3 show inverted zoning, unlike the others, including the two other crystals analyzed for these same samples (HL004B 3 and LH04-1). 

Some subhedral micro-phenocrysts of clinopyroxenes in subophitic texture are shown in Fig. 8C, while in the back-scattered image of Fig. 8D a zoned augite micro-phenocryst with about 0.4 mm in length with subtle zoning can be seen. It partially involves plagioclase crystals, in subophitic texture, and is immersed in mesostasis rich in plagioclase, pyroxene, ilmenite and magnetite microlites, besides glass in the process of devitrification.

Oxides occur as subhedral to euhedral crystals, tabular or skeletal (Figs. 11A and 11B). The ternary FeO vs. TiO2 vs. Fe2O3 diagram (Akimoto & Katsura 1959) shows the major solid solution series magnetite-ulvöspinel, hematite-ilmenite and ferropseudobrookite-pseudobrookite. The data of the analyzed oxides (Tab. 3 and Fig. 12) plot on the field of ilmenite. The subordinate presence of titanomaghemite (Katsura & Kushiro 1961) suggests weathering or low-T oxidation of titanomagnetite or magnetite. The presence of magnetite was attested by magnet in hand samples.

Table 3: Analysis and formulae of representative ilmenite.  

Sample FU17 MP C FU17 MP C FU17 MP C HL012a MP C HL012a MP C HL004c MP C HL004c MP C HL004c MP C HL004b MP C LH05 MP C
Component
SiO2 0.010 0.040 0.000 0.020 0.000 0.040 0.050 0.050 0.030 0.290
TiO2 47.970 47.450 47.860 49.330 48.930 48.280 48.330 47.620 48.780 49.750
Al2O3 0.310 0.290 0.280 0.140 0.190 0.240 0.270 0.280 0.150 0.160
FeOt 48.120 48.370 48.340 46.930 47.340 47.810 47.910 47.580 46.970 45.960
MnO 0.440 0.390 0.370 0.460 0.420 0.450 0.440 0.390 0.460 0.480
MgO 1.830 1.530 1.600 1.280 1.310 1.290 1.440 1.350 0.910 1.590
CaO 0.170 0.140 0.070 0.030 0.080 0.120 0.120 0.150 0.000 0.120
Total 98.840 98.170 98.520 98.190 98.270 98.230 98.560 97.420 97.300 98.350
Cations based on 4 oxygen
Si 0.000 0.002 0.000 0.001 0.000 0.002 0.003 0.003 0.002 0.015
Ti 1.862 1.859 1.867 1.918 1.905 1.886 1.880 1.876 1.919 1.918
Al 0.019 0.018 0.017 0.008 0.011 0.015 0.018 0.017 0.009 0.010
Fe(ii) 2.076 2.107 2.096 2.029 2.049 2.076 2.071 2.084 2.054 1.970
Mn 0.019 0.017 0.016 0.020 0.019 0.020 0.019 0.017 0.020 0.021
Mg 0.141 0.119 0.124 0.099 0.101 0.100 0.111 0.106 0.071 0.122
Ca 0.009 0.008 0.004 0.002 0.005 0.006 0.007 0.009 0.000 0.007
Cat sum 4.127 4.130 4.124 4.077 4.089 4.105 4.109 4.112 4.075 4.062

MP: micro-phenocryst; C: core.

Figure 11: (A) Crystals of ilmenite between clinopyroxene and plagioclase showing intergranular, intersertal and subophitic textures. Some small plagioclase laths show swallowtail termination. Parallel polarizers; (B) back-scattered image showing some subhedral crystals of ilmenite and abundant skeletal microlites in the mesostasis in process of devitrification; (C) small euhedral crystals of olivine, replaced by iddingsite, together with laths of plagioclase, granular clinopyroxene and oxides. Parallel polarizers; (D) back-scattered image showing some micro-phenocrysts of well-preserved olivine, plagioclase and clinopyroxene, in a mesostasis rich in clinopyroxene and plagioclase apart from oxides and glass.  

Figure 12: Ternary FeO vs. TiO2 vs. Fe2O3 diagram (Akimoto and Katsura, 1959) showing the main solid solutions of Fe-Ti oxides. Discussion in text. 

When present, olivine shows alteration to iddingsite, so that preserved crystals are very rare (Figs. 11C and 11D). Nine olivine crystals of three basaltic samples were analyzed (Tab. 4, Fig. 13). None of them has significant zonation. FU017 sample shows olivine with a more magnesian composition (Fo59Fa41-hyalosiderite), while in samples FU250 and LH05 olivine is markedly ferriferous (between Fo35Fa65 and Fo28Fa70 - hortonolite/ferrohortonolite) according to Deer et al. (2003). Despite this difference in composition, no significant textural distinctions were observed. However, the olivines of sample FU017 are subhedral and slightly rounded, suggesting corrosion, while other ones have more inclusions and almost absence of faces. As for the size, all are about 75 µm in the longest length. The few data available do not allow further consideration.

Table 4: Representative analysis and formulae of olivine.  

Sample FU 17 P C FU 17 P C FU 250 MP C FU 250 MP C
Component
SiO2 34.840 34.890 31.570 31.730
TiO2 0.070 0.080 0.260 0.120
Al2O3 0.040 0.050 0.020 0.000
FeOt 33.960 34.010 54.020 53.410
CaO 0.330 0.390 0.380 0.350
MnO 0.450 0.480 0.920 0.830
MgO 28.270 27.960 12.230 12.690
NiO 0.060 0.070 0.010 0.000
Total 98.020 97.930 99.420 99.147
Cations based on 4 oxygen
Si 0.990 0.993 0.988 0.992
Ti 0.002 0.002 0.006 0.003
Al 0.001 0.002 0.001 0.000
Fe(ii) 0.807 0.809 1.414 1.396
Mn 0.011 0.012 0.024 0.022
Mg 1.197 1.186 0.571 0.592
Ni 0.001 0.002 0.000 0.000
Ca 0.010 0.012 0.013 0.012
Cat sum 3.019 3.018 3.017 3.017
End-member (%)
Fo 59.430 59.107 28.407 29.434

P: phenocryst; MP: micro-phenocryst; C: core.

Source: modified from Sarmento et al. (2017).

Figure 13: Compositional variation of olivine from samples FU017 (full triangle), FU250 and LH05 (full circle). 

The presence of glass in the basalts mesostasis is very common, as interstitial and localized form or as significant proportions. In either case, devitrification processes are common, and the presence of crystallites is conspicuous (Figs. 14A and 14B).

Figure 14: (A) Typical example of vitreous mesostasis in the devitrification process; crystallites of clinopyroxene and plagioclase in plumose texture are abundant and associated with skeletal oxide crystals; (B) back-scattered image showing mesostasis with crystalites of plagioclase, pyroxene and opaque minerals.  

ROCK CHEMISTRY

The 22 analyzed samples were divided according to their areal distribution (Tab. 5), being six of them from the Sacramento region, to the southeast, one from the Uberaba area, one from the Perdizes region and 14 from the area between Uberlândia and Araguari cities, to the northwest (see Fig. 1 for location). Table 5 also shows the CIPW Norm for these rocks.

Table 5: Whole-rock analysis of 22 basalt samples and CIPW Norm data. 

Sample HL001 HL003A HL003D HL004C HL004E HL004J* HL011B* HL011C HL013A* HL013B HLA1
Long (-) 47.6235 48.2108 48.2108 48.2136 48.2136 48.2136 48.3241 48.3241 48.3240 48.3240 48.2240
Lat (-) 19.2819 18.7259 18.7259 12.8183 12.8183 12.8183 18.8754 18.8754 18.8863 18.8863 18.7289
Major elements (wt. %)
SiO2 49.7600 47.6100 49.5800 49.7500 47.7100 49.9300 50.5300 49.8000 50.9000 49.7300 47.5900
TiO2 3.2100 3.3700 3.2400 3.5400 3.5600 3.6100 3.5800 3.5600 3.7100 3.5200 3.5700
Al2O3 12.6200 11.9800 12.6600 12.5000 12.6700 12.5300 12.5900 12.4800 12.7500 12.3600 12.5000
Fe2O3(t) 14.2900 18.4800 14.2700 14.6200 14.7700 14.8400 14.3300 14.7800 14.5200 14.7400 15.3100
CaO 9.7700 8.8600 9.8400 9.2900 9.6600 9.4000 9.8500 9.3600 9.6200 9.2900 9.0000
MgO 5.4300 4.6100 5.3300 4.8200 4.9300 4.7200 4.8200 4.7700 5.0300 4.8000 4.8000
Na2O 2.4700 2.7800 2.6600 2.8700 2.3500 2.8800 2.8900 2.8200 2.8600 2.8300 2.6400
K2O 0.9900 0.7700 0.8000 0.8400 1.4900 0.8500 0.8200 0.9300 0.8900 0.9000 1.3000
P2O5 0.3600 0.4200 0.3700 0.4400 0.4300 0.4300 0.4700 0.4500 0.4300 0.4400 0.4500
MnO 0.1900 0.2400 0.1800 0.2000 0.1800 0.2000 0.2200 0.1900 0.2200 0.1800 0.2100
LOI 0.8800 1.0100 1.0400 1.1100 2.2400 1.1500 0.9900 0.9500 0.9000 1.2200 2.6300
Total 99.9700 100.1300 99.9700 99.9800 99.9900 100.5400 100.0900 100.0900 101.8300 100.0100 100.0000
CIPW Norm (%)
qz 0.0000 0.0000 1.3900 1.8200 0.0000 1.2100 2.1200 1.7400 2.1300 1.8100 0.0000
or 9.7600 4.8000 4.9000 5.2000 9.3500 5.2000 5.0000 5.7500 5.3500 5.6000 8.2000
ab 23.1000 26.1000 24.8500 26.9500 22.3500 26.8500 26.7000 26.3500 26.2500 26.5000 25.2000
an 21.2000 18.7800 21.0500 19.5000 20.6800 18.8800 19.5300 19.4300 19.7300 19.2000 19.4800
di 21.3200 19.4800 21.6800 22.8800 21.6000 23.2000 22.1200 20.6800 20.9200 20.7600 19.8400
en 10.8400 18.1600 18.4600 15.3800 16.1600 16.4000 16.3800 17.8000 17.4200 17.9400 16.3800
ol 6.2700 3.8700 0.0000 0.0000 1.5300 0.0000 0.0000 0.0000 0.0000 0.0000 2.4000
mt 2.0300 2.6400 2.0300 2.7800 2.1300 2.1000 2.0100 2.1000 2.0300 2.1000 2.2200
il 4.6600 4.9200 4.7000 5.1400 5.2600 5.2200 5.1400 5.1800 5.2800 5.1400 5.2800
ap 0.8000 0.9300 0.8300 0.9500 0.9600 0.9400 1.0100 0.9900 0.9300 0.9600 1.0100
total 99.9800 99.6800 99.8900 100.6000 100.0200 100.0000 100.0100 100.0200 100.0400 100.0100 100.0100
Trace elements (ppm)
Zn 104.0000 107.0000 103.0000 108.0000 105.0000 117.0000 112.0000 107.0000 113.0000 101.0000 116.0000
Cu 168.0000 163.0000 180.0000 164.0000 171.0000 185.0000 188.0000 161.0000 186.0000 162.0000 99.0000
Cr 134.0000 57.0000 121.0000 69.0000 59.0000 - - 68.0000 57.0000 57.0000 12.0000
Ni 66.0000 53.0000 66.0000 60.0000 51.0000 59.0000 64.0000 53.0000 60.0000 50.0000 22.0000
Ba 340.0000 402.0000 347.0000 422.0000 412.0000 490.0000 396.0000 426.0000 394.0000 435.0000 498.0000
Co 37.0000 38.0000 39.0000 41.0000 38.0000 39.2000 39.6000 41.0000 38.5000 39.0000 38.0000
Cs - - - 0.3200 - 0.3300 0.2500 - 0.2600 - -
Ga 20.0000 20.0000 20.0000 22.5000 20.0000 22.4000 22.6000 20.0000 22.8000 21.0000 19.0000
Hf - - - 6.2400 - 6.0800 5.8400 - 5.8700 - -
Nb 20.0000 20.0000 19.0000 22.3600 19.0000 20.9100 21.7100 20.0000 20.1200 20.1200 22.0000
Rb 23.0000 20.0000 20.0000 20.1000 28.0000 23.0000 22.9000 21.0000 21.7000 20.0000 18.0000
Sn - - - 1.5000 - 1.1000 0.9000 - 0.9000 - -
Sr 375.0000 380.0000 388.0000 381.1000 403.0000 480.0000 491.0000 377.0000 488.0000 377.0000 405.0000
Th - - - 4.8000 - 3.8000 3.2000 - 3.1000 - -
U - - - 0.7100 - 0.6900 0.7800 - 0.6400 - -
Sample HL001 HL003A HL003D HL004C HL004E HL004J* HL011B* HL011C HL013A* HL013B HLA1
Long (-) 47.6235 48.2108 48.2108 48.2136 48.2136 48.2136 48.3241 48.3241 48.3240 48.3240 48.2240
Lat (-) 19.2819 18.7259 18.7259 12.8183 12.8183 12.8183 18.8754 18.8754 18.8863 18.8863 18.7289
Major elements (wt. %)
Zr 175.0000 197.0000 176.0000 198.0000 191.0000 229.0000 242.0000 196.0000 246.0000 198.000 230.000
Y 27.0000 29.0000 26.0000 30.9700 29.0000 29.9100 32.0100 29.0000 30.6100 29.000 31.0000
La 12.0000 25.0000 25.0000 32.5000 29.0000 30.3000 40.3000 24.0000 33.9000 31.000 30.0000
Ce 74.0000 73.0000 66.0000 81.0000 73.0000 67.5000 74.3000 67.0000 68.8000 73.000 83.0000
Pr - - - 8.7900 - 8.6600 9.2900 - 8.7000 - -
Nd - - - 37.4000 - 37.2000 39.6000 - 37.9000 - -
Sm - - - 8.1000 - 8.2000 8.5000 - 8.2000 - -
Eu - - - 2.5300 - 2.5300 2.6100 - 2.5100 - -
Gd - - - 8.2200 - 8.0400 8.4500 - 8.1200 - -
Tb - - - 1.2000 - 1.1900 1.2500 - 1.1800 - -
Dy - - - 6.6400 - 6.5600 6.8200 - 6.6000 - -
Ho - - - 1.2600 - 1.2500 1.2900 - 1.2500 - -
Er - - - 3.4600 - 3.3300 3.5500 - 3.3800 - -
Tm - - - 0.4800 - 0.4600 0.4600 - 0.4400 - -
Yb - - - 2.8000 - 2.7000 2.8000 - 2.7000 - -
Lu - - - 0.4100 - 0.3900 0.4200 - 0.3900 - -
Sample HLA7 FU17* FS065 FS066 FS129 FS229 FS236 FS252 LH03 LH04 LH05
Long (-) 48.2264 47.9590 47.4383 47.4529 47.4402 47.3380 47.3674 47.4322 48.2191 48.2192 48.2193
Lat (-) 18.7118 19.9550 19.6112 19.6050 19.9639 19.7701 19.7733 19.8493 18.7262 18.7261 18.7260
Major elements (wt. %)
SiO2 49.7100 50.1200 50.5300 50.6800 49.9300 51.4200 50.8900 50.4800 46.7300 50.7000 49.3200
TiO2 3.2300 3.5400 3.8800 3.9000 3.9700 3.6400 3.6300 3.9100 3.5900 3.4600 3.2500
Al2O3 12.6700 12.5400 11.8700 11.9400 12.6300 12.1600 12.2100 11.8500 12.6100 12.1100 12.5700
Fe2O3(t) 14.4200 14.6800 15.8500 14.9400 14.5000 15.0100 15.2800 15.7300 15.6800 15.0200 14.7700
CaO 9.8000 9.4000 8.0400 8.2200 8.1400 8.0700 8.1800 8.0300 9.2500 8.3300 9.7900
MgO 5.4900 4.4600 3.9200 4.1200 4.3500 4.0300 4.1800 3.9400 5.0200 3.9300 5.4700
Na2O 2.5700 2.6900 2.6300 2.8300 3.0000 2.6800 2.7800 2.6300 2.4900 3.0100 2.5300
K2O 0.9400 1.0400 1.6400 1.4900 1.4900 1.5200 1.4900 1.5300 1.0600 1.1700 1.1700
P2O5 0.3600 0.4400 0.6600 0.7900 0.6500 0.5100 0.5100 0.6600 0.4600 0.4500 0.3700
MnO 0.1800 0.2100 0.2200 0.2000 0.2000 0.2000 0.2000 0.2200 0.2200 0.1900 0.1900
LOI 0.6400 0.1500 0.9500 0.9200 1.1300 0.7400 0.6600 0.9800 2.9000 1.5800 0.8200
Total 100.0100 99.2700 100.1900 100.0300 99.9900 99.9800 100.0100 99.9600 100.0100 99.9500 99.9800
CIPW Norm (%)
qz 0.5200 2.2600 4.0200 2.0600 1.2300 4.6700 3.3400 1.9200 0.0000 3.1900 0.9000
or 5.8000 6.4000 9.7000 9.2500 9.200 9.4000 9.1500 9.5500 6.6500 7.3000 5.5500
ab 23.9000 25.1000 22.1000 26.6000 27.8200 25.2500 26.0000 24.8500 23.8500 28.4500 23.6000
an 20.9500 19.8000 16.4000 16.1500 17.3000 17.3500 17.1800 16.8300 21.4000 16.9000 21.0800
di 23.5200 20.5200 17.3300 21.2400 19.9200 16.8000 17.3200 18.2600 19.4000 18.8800 21.4000
en 18.3400 17.7800 19.1000 16.8800 16.1600 17.9800 18.4200 18.6400 17.2000 17.1000 19.8400
Sample HLA7 FU17* FS065 FS066 FS129 FS229 FS236 FS252 LH03 LH04 LH05
Long (-) 48.2264 47.9590 47.4383 47.4529 47.4402 47.3380 47.3674 47.4322 48.2191 48.2192 48.2193
Lat (-) 18.7118 19.9550 19.6112 19.6050 19.9639 19.7701 19.7733 19.8493 18.7262 18.7261 18.7260
ol 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 2.8500 0.0000 0.0000
mt 2.0400 2.0700 2.9100 0.0500 2.0600 2.1500 2.1800 2.2500 2.2800 2.1500 2.0900
il 4.6200 5.1200 6.4300 5.6800 5.7800 5.3000 5.2200 5.7200 5.3400 5.0800 4.7200
ap 0.8000 0.9600 2.0000 1.7500 1.4200 1.1200 1.1200 1.4400 1.0400 0.9600 0.8300
Total 100.4900 100.0100 99.9900 99.6600 100.8900 100.0200 99.9300 99.4600 100.0100 100.0100 100.0100
Trace elements (ppm)
Zn 98.0000 118.0000 139.0000 125.0000 125.0000 120.0000 109.0000 121.0000 114.0000 105.0000 98.0000
Cu 179.0000 188.0000 136.0000 60.0000 85.0000 63.0000 58.0000 138.0000 88.0000 119.0000 176.0000
Cr 126.0000 18.0000 6.0000 30.0000 17.0000 15.0000 10.0000 16.0000 9.0000 116.0000
Ni 66.0000 62.0000 20.0000 21.0000 43.0000 21.0000 20.0000 23.0000 20.0000 22.0000 64.0000
Ba 383.0000 458.0000 517.0000 528.0000 583.0000 500.0000 501.0000 497.0000 458.0000 539.0000 368.0000
Co 42.0000 40.3000 38.00000 38.0000 39.0000 38.0000 35.0000 34.0000 42.0000 38.0000 38.0000
Cs - 0.2700 - - - - - - - - -
Ga 20.0000 22.4000 20.0000 20.0000 21.0000 21.0000 21.0000 21.0000 20.0000 21.0000 19.0000
Hf - 5.7800 - - - - - - - - -
Nb 19.0000 19.6400 27.00000 25.0000 26.0000 22.0000 21.0000 28.0000 22.0000 21.0000 17.0000
Rb 21.0000 18.5000 32.00000 29.0000 30.0000 28.0000 26.0000 31.0000 14.0000 21.0000 21.0000
Sn - 0.9000 - - - - - - - - -
Sr 376.0000 475.0000 394.0000 471.0000 504.0000 378.0000 382.0000 386.0000 392.0000 393.0000 377.0000
Th - 2.9000 - - - - - - - - -
U - 0.6400 - - - - - - - - -
V 423.0000 481.0000 397.00000 319.0000 417.0000 407.0000 390.0000 400.0000 464.0000 418.0000 421.0000
W - 1.4000 - - - - - - - - -
Zr 175.00000 243.0000 243.0000 230.0000 234.0000 225.0000 225.0000 244.0000 228.0000 228.0000 171.0000
Y 27.0000 30.3500 33.0000 32.0000 30.0000 31.0000 32.0000 32.0000 30.0000 31.0000 27.0000
La 22.0000 32.2000 23.0000 35.0000 22.0000 23.0000 23.0000 27.0000 33.0000 31.0000 18.0000
Ce 65.0000 67.9000 93.0000 98.0000 94.0000 95.0000 83.0000 114.0000 94.0000 91.0000 74.0000
Pr - 8.7300 - - - - - - - - -
Nd - 37.2000 - - - - - - - - -
Sm - 8.3000 - - - - - - - - -
Eu - 2.4900 - - - - - - - - -
Gd - 7.9800 - - - - - - - - -
Tb - 1.1600 - - - - - - - - -
Dy - 6.6700 - - - - - - - - -
Ho - 1.2600 - - - - - - - - -
Er - 3.3000 - - - - - - - - -
Tm - 0.4500 - - - - - - - - -
Yb - 2.7000 - - - - - - - - -
Lu - 0.4000 - - - - - - - - -

Source: Fujimori, 1990; HL and LH: Uberlândia/Araguari region; FU: Uberaba region; FS: Sacramento region; qz: quartz; or: orthoclase; ab: albite; an: anorthite; di: diopside; en: enstatite; ol: olivine; mt: magnetite; il: ilmenite; ap: apatite; *analyzed by Geosol. Trace and rare earth elements from HL004C were determined by Geosol. See text for details.

The flows are classified as basalts, in the total alkali-versus-silica (TAS) diagram (Le Bas et al. 1986). The SiO2 concentrations range from 48.10 to 51.81 wt% and total alkalis between 3.43 and 4.49 wt% (Fig. 15), and all samples plot in the field of subalkaline volcanic series (Irvine & Baragar 1971). The rocks have a tholeiitic character, with strong enrichment of iron in relation to alkalis and magnesium (Fe2O3t/MgO between 2.6 and 4.0) and high-TiO2 contents, belonging to the HTi Basalt Group of the PCMP (Bellieni et al. 1984, Peate et al. 1992).

Figure 15: Classification and chemical nomenclature of the studied samples. Total alkali-versus-silica (TAS) diagram (alkalis vs. SiO2), according to Le Bas et al. (1986).  

The main characteristic shown by the variation diagrams of major, minor and trace elements against MgO (Figs. 16 and 17), considered as a magmatic differentiation index, is the separation of two specific fields: one for the more evolved rocks from the Sacramento region, in the southeast of the surveyed area; and the other one for those less evolved rocks from the Uberlândia/Araguari region, at the northwest. The only representative sample of the Uberaba region falls in the intermediate area, between the two groups. Samples from the pillow lavas related basaltic flows behave independently by plotting in between the groups mentioned; four of them overlap the basalts of the Uberlândia/Araguari region, while the other one has lower MgO, SiO2, Al2O3, CaO and Na2O and overlaps the Sacramento Group. Analyzed together, a negative correlation trend is observed for SiO2, Na2O, K2O, P2O5 and (subtle) for TiO2 and positive for CaO and Al2O3. There is a strong correspondence with Rocha-Júnior et al. (2013) data, even though those present lower values for Na2O and higher for Al2O3.

Empty squares: 18 samples from the Triângulo Mineiro and northeast of São Paulo (Rocha Junior et al., 2013); empty diamonds: Sacramento; full square: Uberaba; empty triangles: Uberlândia/Araguari and crosses: basalt flows related to pillow lavas).

Figure 16: Diagrams for major and minor elements vs. MgO. 

Empty squares: 18 samples from the Triângulo Mineiro and northeast of São Paulo (Rocha Junior et al., 2013); empty diamonds: Sacramento; fullsquare: Uberaba; empty triangles: Uberlândia/Araguari and crosses: basalt flows related to pillow lavas).

Figure 17: Diagrams for trace elements vs. MgO.  

According to the parameters proposed by Peate et al. (1992), the studied basalts belong to the Pitanga magma type. They show Fe2O3 between 14.27 and 18.48 wt% with average of 15.04 wt% and Sr variation between 375 and 504 ppm with average 412 ppm. The TiO2 content varies between 3.21 to 3.97 wt% with average 3.7 wt%, similar to Urubici magma-type, but in the Sr vs. TiO2 diagram the samples plot on the Pitanga field (Tab. 6, Fig. 18).

Table 6: Classification of the basaltic rocks from the studied area according to Peate et al. (1992) parameters.  

Sample TiO 2 Sr ppm Ti/Y Zr/Y Magma type
HL001 3.24 375 740 6.48 Pitanga
HL003A 3.41 380 703 6.79 Pitanga
HL003D 3.28 388 753 6.77 Pitanga
HL004C 3.58 381 764 7.07 Pitanga
HL004E 3.65 403 689 6.59 Pitanga
HL004J 3.61 480 722 7.66 Pitanga
HL0011B 3.58 377 668 7.56 Pitanga
HL0011C 3.60 377 741 6.76 Pitanga
HL0013A 3.71 488 725 8.04 Pitanga
HL0013B 3.57 377 738 6.83 Pitanga
HLA1 3.67 405 645 7.42 Pitanga
HLA7 3.25 376 741 6,48 Pitanga
FU17 3,54 475 698 8.01 Pitanga
FS 065 3.88 394 703 7.36 Pitanga
FS 066 3.90 471 731 7.19 Pitanga
FS 129 3.97 504 793 7.8 Pitanga
FS 229 3.64 378 703 7.26 Pitanga
FS 236 3.64 382 681 7.03 Pitanga
FS 252 3.91 386 732 7.60 Pitanga
LH03 3.59 392 716 7.60 Pitanga
LH04 3.46 393 647 7.13 Pitanga
LH05 3.25 377 722 6.33 Pitanga

Empty squares: 18 samples from the Triângulo Mineiro and northeast of São Paulo (Rocha Junior et al., 2013); empty diamonds: Sacramento; full square: Uberaba; empty triangles: Uberlândia/Araguari and crosses: basalt flows related to pillow lavas).

Figure 18: Sr vs. TiO2 diagram for high titanium magma-types based in Peate et al. (1992) and Machado (2005).  

The two groups of samples identified by major elements are also distinguished by the trace elements (Fig. 17). Negative correlation is observed for incompatible elements Nb, Ba, Ce, Rb and (subtle) Y and Zr. Ni shows defined positive correlation, whereas for Sr there is no clear correlation. Figure 19 reinforces the presence of two groups of basalts and the evidence of two parental magmas in this region. With regard to data from Rocha-Júnior et al. (2013), the correspondence observed for the major and minor elements is observed for the trace elements, although it is not so clear for some samples. In addition, all those samples are more Y-enriched than these under analysis.

Figure 19: Diagrams variation in some major, minor and trace elements suggestive of different parental magmas. 

Only five samples - from the Uberaba and Uberlândia/Araguari regions - were analyzed for Rare Earth Elements (REE). The REE were normalized to the chondrites (Sun & McDonough 1989) (Fig. 20) and resemble the data of Rocha-Júnior et al. (2013). Heavy REE’s are strongly depleted compared to light REE’s in the Pitanga magma type. (La/Lu)N varies between 8.04 to 9.93, while the data from Pitanga magma by Rocha-Júnior et al. (2013) are in the 6.1 to 10.3 range. Eu shows a very slightly negative anomaly with Eu/Eu* values of 0.92 to 0.94.

Figure 20: Chondrite-normalized rare Earth elements (REE) (black field) compared with Rocha Junior et al. (2013) data (gray field = Pitanga Magma; empty field = Paranapanema Magma). 

The multi-elemental distribution patterns normalized to primitive mantle show negative Nb anomalies, also observed for all high- and low-Ti tholeiites of the PCMP. The (Rb/Ba)PM ratios < 1 (varying from X to Y) is an indicative that the investigated basalt flows were not affected by low-pressure crustal contamination processes (e.g., Peate 1997, Marques et al. 1999, Marques et al. 2017).

DISCUSSION

In the Lower Cretaceous, the north and northeast edge of the Paraná Basin was delimited by topographically elevated terrains, known as Alto Paranaíba Arch, whose palaeorelief was highly irregular. These peculiarities determined the design and characteristics of the sedimentary and volcanic deposits. In such peridesertic region, occasional aqueous flows generated alluvial fan deposits and/or fluvio-lacustrine systems and, in addition, allowed the occurrence of pillow lavas. These characteristics - together with the broad domain of entablature on colonnades, very fine textured basalts, mesostasis with significant amount of glass and fossil records in contemporaneous sediments - are indicative of a more humid environment than the dominant in the rest of the PCMP. On the other hand, the predominance of intertrapped aeolian sandstone lenses in thin basalt flows suggests irregular topography and peripheral location.

Under microscopy, thin sections of entablature show abundant microlites and frequent plagioclase with swallowtail-like texture, opaque minerals with skeletal texture and volcanic glass in mesostasis. These characteristics are less frequent in thin-sections of colonnade samples, indicating a less quick cooling rate for the last one.

An unusual feature is the presence of inverted or alternating zoning from the core to the edge in some plagioclase and pyroxene crystals. According to Vernon (2004, p. 139-141), even though it has been produced in the laboratory, reverse discontinuous zoning is uncommon in igneous rocks. However, it could be produced by a rapidly induced temperature variation (caused by, for example, rapid movement of magma in a chamber, volcanic eruption, rapid release of volatiles in a water-saturated magma, or magma mixing). This author also discusses patchy zoning which, as stated by Vance (1965 apudVernon 2004), is due to initial crystallization of relatively calcic plagioclase in a water-undersaturated magma at depth, that suffer a rapid decrease in confining pressure, causing resorption. A new crystallization of more sodic plagioclase begins to fill cavities in the nuclei - forming pseudoinclusions - and enveloping it with a composition different from that of the previous zones. Not rarely, small inclusions (e.g., of glass or pyroxene) occur in the more sodium-rich patches, generated from trapped melt in the corroded core. Normal and reverse oscillatory zoning may also occur in clinopyroxene - generally oscillating between augite and subcalcic augite, - and is explained as the response to local diffusion limitations, as in the case of plagioclase. It should also be recorded that two pigeonitic cores were identified in pyroxene phenocrysts.

Still with respect to plagioclases, the overlaps between the compositions of groundmass and phenocrysts, illustrated in the ternary classification diagram Ab-An-Or (Fig. 7), suggest the simultaneous crystallization of both phenocrysts edges and groundmass (Rao et al. 2012).

Olivines, with hyalosiderite (Fo59), hortonolite and ferrohortonolite (Fo28) composition, were identified in samples from Uberaba and Uberlândia/Araguari regions. The current level of knowledge does not permit discussing the relationships/meaning between the basaltic flows with these distinct olivines. However, the data are in agreement with ones presented by Machado (2009) for magmas of the Pitanga type, in the northwest of the Province. This kind of record is present in olivine phenocrysts of lava flows from Linga area of the Eastern Deccan Volcanic Province. According to Ganguly et al. (2012), while the high iron content denotes a magma with much-evolved composition, the broad compositional spectrum of phenocrysts corroborates a changing crystallization condition in the predominant magmatic environment. More detailed studies are needed to test this hypothesis for Pitanga magma type.

These rocks are classified as basalts and have a tholeiitic character, with strong enrichment of iron in relation to alkalis and magnesium. Although the TiO2 content shows 3.7 wt% in average, similar to Urubici magma type, the samples plot on the Pitanga field in the Sr vs. TiO2 diagram. Two specific fields are clear in major oxides / trace elements vs. MgO diagrams: one for less evolved rocks (MgO > 4.65 wt%) from the Uberlândia/Araguari region; and other one for those more evolved rocks from the Sacramento region (MgO < 4.35 wt%), with the single sample of the Uberaba region in the intermediate area, between the two groups. Samples of the basalt flows related to pillow lavas plot scattered among the groups mentioned. Sample HL04, however, shows an enrichment in SiO2, Na2O and Rb, and impoverishment in CaO and MgO, which suggests alteration by water interaction. In general, there is a correspondence with Rocha-Júnior et al. (2013) data, although these exhibit lower values for Na2O and higher for Al2O3, suggesting the possibility of some analytical bias.

Heavy REE are depleted compared to light rare-earth elements (LREE) with (La/Lu)N between 8.04 to 9.93, and, although some enrichment in LREE is expected as a result of fractional crystallization, the high (La/Lu)N values are indicative of partial melting of a garnet peridotite, constraining the mantle source to depths greater than 80 km (Wilson 2007). Taking into account that (Rb/Ba)PM ratios are < 1 in all analyzed basalts, suggesting that the magmas did not suffer crustal contamination during their ascension to surface, the Nb negative anomaly observed in the multi-elemental trace element patterns is a mantle source feature. According to Rocha-Júnior et al. (2012, 2013) the sources of PCMP basalts were affected by Neoproterozoic subductions (generating the depletion in Nb) related with the Gondwana Assembly.

The trends observed for major, minor and trace elements in the variation diagrams (Figs. 16 and 17) are broadly compatible with fractional crystallization of clinopyroxene, plagioclase and Ti-magnetite. The slightly negative Eu anomalies (Eu/Eu*= 0.92 - 0.94) in the REE distribution patterns reinforce plagioclase fractionation. Besides that, the geochemical data point to evolution involving at least two parental magmas, as evidenced by their distinct Ba/Zr, Nb/Zr and Nb/Y ratios (Fig. 19). One of them would generate the basalts of Sacramento, whereas the other one originated the Araguari/Uberlândia, Uberaba and Perdizes tholeiite group. It is noteworthy that the last group presents some variation in major, minor and trace elements, such as TiO2, P2O5, Rb/Sr, Zr/Nb, which is also suggestive of different parental magmas, but additional highly incompatible trace element data are necessary to corroborate this possibility.

CONCLUDING REMARKS

The northeast edge of the South American portion of the PEP is formed by heterogeneous lava packages with distinct morphology and chemistry. The rocks are basalts, with a tholeiitic signature, belong to the Serra Geral Group and are classified as Pitanga magma-type, of the high titanium group of basalts. The intertrapped sedimentary beds, aeolian or lacustrine in character, are important markers on prevailing environmental conditions, as well as quiescence periods in the volcanic activity.

Some zoned plagioclase and pyroxene crystals show inverted or alternating zoning from the core to the edge, which could indicate rapidly temperature variation. Still with respect to plagioclases, the ternary classification diagram Ab-An-Or shows overlaps between groundmass and phenocrysts compositions, suggesting simultaneous crystallization of them.

A pronounced chemical distinction can be seen between the rocks from the Uberlândia/Araguari region and those from the Sacramento region, which are more evolved than the first one. Attention is drawn to the independent behavior of rocks associated with pillow lavas, suggesting a possible alteration by water interaction. The origin of these magmas may be to the partial melting of a garnet peridotite mantle source, and the negative anomaly of Nb may be an inheritance of the subduction processes that affected the region during the Neoproterozoic.

The data presented reinforce the need for a more detailed lithostratigraphic control in order to advance the understanding of the processes that acted in the origin of this region of PCMP.

ACKNOWLEDGMENTS

This work was supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) - Project 2012/06082-6. Professor Dr. Evandro F. de Lima gave us several hours of work on the UFG electronic microprobe, and Professor Dr. José Affonso Brod opened the doors of this laboratory generously, so we are grateful to both. The authors are really thankful to the reviewers’ contribution for the paper, especially to Dr. Valdecir A. Janasi.

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Received: August 07, 2017; Accepted: November 29, 2017

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