Geology and petrology of the Salto do Céu Suite: tectonic and stratigraphic implications on the SW Amazonian Craton

The basic rocks of the Salto do Ceu Suite outcrop as sills and flows in the southwestern Amazon Craton, Rondonian-San Ignacio Province, with an age of about 1.44 Ga. Sills are 2 to 30 m thick being hosted by pelites of the Aguapei Group. The flows are up to 5 m thick and cover this unit. Typical textures of magma mingling are observed near the contact with granites of Rio Branco Suite. The Rio Branco Intrusive Suite is composed of basic and acid rocks, as well as hybrid rocks that indicate mixing processes between basic and acids magmas. U-Pb (TIMS zircon) results indicate ages around 1.4 Ga for both terms. The Salto do Ceu Suite rocks have tholeiites affinity classified as subalkaline and iron-rich tholeiitic basalts, with mg# values between 0.30 and 0.51. They can be separated into two groups, based on La N ; one is richer in ETR with La N greater than 100, while the other one has La N less than 100. Rocks of the Salto do Ceu Suite and Rio Branco Suite are interpreted as a bimodal suites showing magma mingling features such as those developed in continental intraplate settings, extensional regime associated to the Columbia/Nuna breakup.


INTRODUCTION
The mafic sills found in the region of Salto do Céu municipality, state of Mato Grosso, central Brazil, belonging to the Salto do Céu Suite (SCS) are interpreted as a record of the breakup of the Rodinia Supercontinent (Ruiz et al. 2010a, Lima et al. 2012) on the basis of K-Ar analyses that yields a cooling age around 0.9 Ga (Barros et al. 1982). Recent U-Pb (TIMS) analyses on baddeleyite show crystallisation ages around 1.44 Ga (Teixeira et al. 2016). This new geochronological result challenges the widely accepted geological history of the SW Amazonian Craton both in terms of this magmatic event of basic nature as well as the age of its country rock, that is, the Aguapeí Group. Distensive magmatic events, with ages similar to those obtained for the Salto do Céu Suite, have been associated with the evolution of the Columbia Supercontinent by many authors (Ernst et al. 2008, 2013, Rogers & Santosh 2002. The Rio Branco Intrusive Suite comprises basic (gabbro, diabase and basalt) and felsic (porphyritic and rapakivi granite) rocks. Hybrid rocks (monzosyenite) with rapakivi-like textures indicate commingling and mixing among basic and felsic magmas. U-Pb isotopic data indicate ages close to 1.4 Ga for both terms (Geraldes et al. 2004), situated in the Rondonian-San Ignacio geochronological province (Bettencourt et al. 2010).
Based on field work and new geochronological data available in the literature, as well as in unpublished petrographic and geochemical results of rocks from the SCS, the petrogenetic evolution of this mafic magmatism is discussed, as well as its relationship to felsic rocks of the Rio Branco Intrusive Suite (RBIS), and its tectonic implication to the evolution of the southwestern Amazonian Craton.

REGIONAL GEOLOGICAL SETTINGS
The south-southwest portion of the Amazonian Craton records a widespread occurrence of mafic sills and dyke swarms ( Fig. 1) which have been interpreted as evidence for the breakup of the Rodinia Supercontinent (Ruiz et al. 2010a, Sécolo et al. 2011, Lima 2011, Lima et al. 2012. Gabbros and diabases found in the regions of Salto do Céu and Rio Branco, southwest of the Brazilian state of Mato Grosso, were first reported by Oliva (1979), and later attributed to the Rio Branco Group by Barros et al. (1982). Leite et al. (1985) have attributed this exposure of mesocratic and melanocratic rocks of the Rio Branco Intrusive Suite to windows in the Vale da Promissao Formation cut by erosion. Therefore, these rocks are described as a differentiated stratiform igneous complex with typical bimodal character, which may be an indicative of anorogenic magmatism likely developed in rift settings.
According to Geraldes (2000) and Geraldes et al. (2001Geraldes et al. ( , 2004, the basal portion of the Rio Branco Intrusive Suite is composed of basic rocks, while its upper portion comprises a plutonic-volcanic association chiefly composed of acid to intermediate rocks. These authors presented a U-Pb age of 1471 ± 8 Ma for basic rocks, and an age of 1427 ± 10 Ma for acid rocks that are interpreted as an extensional intracratonic magmatism, a reflection of the Santa Helena magmatic arc (1.47-1.42 Ga).
The Santa Helena magmatic arc is characterized in a precocious phase (1480-1485 Ma) by calc-alkaline batholiths, peraluminous, tonalitic and monzogranitic (Santa Helena and Água Clara Intrusive Suites) and by minor bodies of Figure 1. Tectonic Map of the South/Southwestern Amazonian Craton highlighting the fissural mafic magmatism (dykes of the intrusive suites Rio Perdido, Nova Lacerda and Huanchaca; and sills of the Salto do Céu and Huanchaca intrusive suites), and felsic magmatism (plutons of the Guapé Intrusive Suite and granitic dykes of the Vila Bela Granite). Extracted and modified from Ruiz et al. 2010b. the Intrusive Suite Pindaituba (1465-1425 Ma), with large compositional variation (Ruiz 2005). Araújo et al. (2005) named as Salto do Céu Intrusive Suite the assemblage of intrusions occurring parallel to the bedding of pelites and psammites of the Vale da Promissão Formation (Aguapeí Group), and composed of mafic sills 1 to 5 m thick. Araújo-Ruiz et al. (2007) recognized two main plutonic series in the Rio Branco Intrusive Suite, one of basic nature with discontinuous distribution located in the intrusion borders, and another one of acid/intermediate nature that is composed of three petrographic facies. It was concluded that the occurrence of gabbroic rocks represent two separate magmatic events: the basic plutonic rocks (gabbros and diorites) belonging to the Rio Branco Intrusive Suite, and the shallow intrusions (diabases, microgabbros) belonging to the Salto do Céu Intrusive Suite.
Araújo (2008) Teixeira et al. (2016) obtained U-Pb baddeleyite ages of 1439 ± 4 Ma for the Salto do Céu sills, and of 1387 ± 17 Ma for the Nova Lacerda mafic dyke swarms. These ages reflect a single magmatic event for these units associated with the evolution of the Columbia Supercontinent.
Mesoproterozoic sedimentary and metasedimentary rocks occurring in the southwest of Mato Grosso, and in Eastern Bolivia were first reported in the LASA report (1968). These rocks were latter named Aguapeí Unit by Figueiredo & Olivatti (1974). Souza & Hildred (1980) used the term Aguapeí Group in order to group the formations Fortuna, Vale da Promissão and Morro Cristalina, which was then described as a marine transgressive-regressive platform cover sequence.
According to Saes (1999), the Fortuna Formation is composed of sandstones and orthoquartzitic conglomerates. These rocks with widespread cross-bedding indicates deposition in braided fluvial systems in the areas of Huanchaca and Sao Vicente Hills which eventually prograded into a shallow-marine platform influenced by tidal currents, fan deltas and tempestites in the region of Pontes and Lacerda, and Rio Branco. The Vale da Promissão Formation represents a progradational wedge of marine sediments thickening towards the SE. In the regions of Rio Branco, and Pontes and Lacerda, this unit consists of pelitic rocks intercalated with sandstones that were deposited by storms on shallow marine platform. The Morro Cristalina Formation is chiefly composed of quartz-arenites, and rarely composed of conglomerates whose main characteristics are the high level of maturity and the exclusive continental character (fluvial and eolian) of its deposits. Geraldes et al. (2014) obtained U-Pb (LA-ICP-MS) ages on detrital zircons for samples from the Aguapeí Group collected in three hills: Rio Branco, Ricardo Franco and Santa Bárbara. The results from the Rio Branco Hill, same region where the target rocks crop out, yield four main age peaks at 1544, 1655, 1812 and 2515 Ma. According to these authors, the first peak is likely related to the rocks of the Cachoeirinha orogeny, the second peak represents the rocks of the Lomas Manechis Complex (Bolivia) and, in their turn, the last two peaks may represent the older units not mapped yet. Table 1 shows geochronological data available for the acid to intermediate rocks and basic rocks, respectively, from the Rio Branco and Salto do Céu suites.

GEOLOGICAL AND PETROGRAPHIC CHARACTERIZATION
The basic rocks crop out in the study area as sills and lava flows, blocks or as large low-lying outcrops along drainages. The main outcrops are exposed on the road joining the town of Salto do Céu to the Progresso Village, on the upper reaches of the Bracinho Stream in the municipality of Rio Branco, and on the slopes of the homonymous hill (Fig. 2).
Locally, typical magma mingling features can be observed in the Rio Branco Suite outcrops. These features consist of mafic microgranular enclaves within the felsic rocks (Figs. 3A and 3B). The mafic microgranular enclaves show alkali feldspar xenocrysts with reabsorption features and recurrent transitional and diffuse boundaries, and centimeter size hybridization zones in the contact with the felsic rock.
Mafic microgranular enclaves are pointed by Barbarin and Didier (1992) and Barbarin (2005) as indicative of magma mingling processes. The hybridization zone, restricted to the enclaves margins, indicates the subordinated actuation of mixing processes, commonly observed in granitoid evolved by magma interaction, as suggested by Barbey et al. (2008).
Sills are 2 to 30 m thick, and show well-defined intrusive contacts with pelites of Vale da Promissão Formation (Fig. 3C) as well as tectonic contacts with the sandstones of Morro Cristalina Formation as a result of normal faulting. They are gently dipping between 10º and 15º WSW, and in some places their dip is 5º toward both SW and ENE. The shape-rounded outcrops display typical spheroidal exfoliation.
Lava flows are about 6 m thick and show vertical internal structures as well as flow-top structures, typical of thin basaltic lava flows (< 10 m) as highlighted by Aubele et al. (1988) and Cashman & Kauahikaua (1997). Their internal structure is marked by zoning consisting of an upper vesicular portion (~ 1.7 m), and another intermediate portion without vesicular texture (~ 4.0 m); no vesicular lower portion was recognized as usual in the basaltic lava flows.
The abundance and size of vesicles increase toward the top (Figs. 4A and 4B). Flow folds (Figs. 4C and 4D) deformed during lava flow motion result in surfaces of pahoehoe flows observed at the top of flows (30 to 40 cm below the top). The deformation caused by lava flow motion also led to fragmentation of ropy lava in the upper crusts resulting in its brecciated aspect (Figs. 4E and 4F), in places showing rapakivi texture (Fig. 4G) and formation of alkali feldspar phenocrysts partially resorbed (Fig. 4H). Internal structures and flow-top aspects are characteristics similar to those pointed out by Chitwood (1994), Hon et al. (1994) and Self et al. (1998) for pahoehoe inflated flows deposited in subaerial settings. Vesicles are mostly filled with fibrous to fibro-radiated material consisting of zeolites chlorite, fluorite and opaque minerals.
Both sills and lava flows are mesocratic to melanocratic, greenish-gray to black in colour, and equigranular varying from very fine-to medium-grained.
The sills consist of diabases and massif gabbros that under the microscope display ophitic, subophitic, intergranular, and coronitic textures (Figs. 5A, 5B and 5C). They are essentially composed of plagioclase and pyroxene, having as accessory mineral assemblage opaque minerals, acicular apatite crystals, and subhedral brown sphene crystals. Alteration paragenesis is represented by amphibole, biotite, epidote/clinozoisite, sericite, calcite, clay minerals, and chlorite. Rare grains of alkali feldspar and quartz exhibiting graphic intergrowth are found as late-forming minerals. The plagioclase, identified as labradorite occurs as subhedral to tabular euhedral crystals showing albite, pericline and Carlsbad twinning; in places, albite and Carlsbad twinning are combined. It is intensely argilized and saussuritized resulting in the formation of epidote/clinozoisite, sericite/muscovite and calcite.
The monoclinic pyroxene is identified as augite-titanoaugite and pigeonite being euhedral, prismatic, well-preserved, and mostly pink to dark pink in colour which indicates a Ti-rich composition, and exhibits sectorial twinning in places. Green amphibole occurs in reaction borders characterizing uralitization processes (Figs. 6A, 6B and 6C).   Occasionally, pyroxene shows complete pseudomorphism by amphibole, chlorite and biotite, but it may also occur as relict mineral.
The types of amphibole here observed are actinolite-tremolite and hornblende. Light green actinolite-tremolite occurs as acicular subhedral crystals, single crystals or fibrous aggregates that fills microvenules and microfractures following a fibrorradiated arrangement. They are mainly associated to the borders of pyroxene featuring coronitic texture, and may completely replace the pyroxene. Brown hornblende is observed as anhedral grains or pseudomorphs of pyroxene. It is usually altered to biotite and chlorite that may replace it completely.
Opaque minerals (Figs. 7A and 7B) are common in these rocks as well-developed, subhedral, skeletal and symplectite crystals, and may be partially replaced by biotite, chlorite, rutile and sphene.
Biotite is a common product from the alteration of amphibole, pyroxene or opaque minerals. These minerals are in places surrounded by a biotite fringe consisting of tiny fibrous crystals. Biotite seldom occur as well-developed blades with reddish brown to brownish pleochroism. Chlorite is observed in thin microscopic greenish blades displaying a fibrous to fibro-radiated habit. Chlorite also occurs associated with amphibole in pseudomorphs of pyroxene and opaques, or in a pervasive way as fracture filling in plagioclase.
The amygdales are round to ellipsoidal in shape, with diameter up to 3 mm, filled by fibrous to fibro-radiated material (  minerals. They may present reaction rims composed of a red-coloured mixture of iron oxides/hydroxides, biotite and rutile.
Labradorite/andesine occurs as euhedral to subhedral tabular phenocrysts and submillimetre-sized laths displaying albite, pericline and Carlsbad twinning. Alteration processes such as argilization, sericitization, and mostly saussuritization are observed. Some crystals delineate a pseudotrachytic flow texture and may show normal, oscillatory and reverse zoning recognized by its higher degree of saussuritization in the more calcium-rich portions of plagioclase.  Clinopyroxene is identified as augite and pigeonite, white to pink in colour, in places exhibiting zoning and twinning, partially to completely uralitized or pseudomorphized by a mixture of amphibole, chlorite and biotite; some lithotypes contain orthopyroxene recognized as colourless to beige enstatite, partially altered to tremolite-actinolite and chlorite.
The amphibole types are products of pyroxene transformation occurring as hornblende prismatic crystals and grains, dark-green to brown in colour, and show drop-like quartz texture as well as acicular, fibrous and fibro-radiated, and white to greenish in colour tremolite-actinolite is seen. Both of them alters to biotite, chlorite and opaques.
Opaque minerals occur mostly as primary minerals or result from alteration of mafic minerals. They are more developed crystals showing dendritic habit and symplectitic texture, in places partially altered to biotite, chlorite, rutile, and sphene. Biotite occurs as tiny brown to brownish blades, sparsely distributed in the rock or in association with amphibole. Rare biotite crystals are found well-preserved and often result from the alteration of opaque minerals.

LITHOCHEMICAL CARACTERIZATION
The lithochemical study of sills and lava flows from SCS was carried out on fourteen samples; also, sills and lava flows were studied together due to their chemical affinity. These samples were previously crushed and pulverized in the Sample Preparation Laboratory of the Geosciences College, Federal University of Mato Grosso, and forwarded to the Acme Analytical Laboratories (Vancouver, Canada) in order to measure major and minor elements, using ICP-ES, and trace-elements including rare earth elements using, ICP-MS. Analytical data are shown on Table 2. Data processing was performed using GCDkit software (version 3.0, Geochemical Data Toolkit for Windows; Janoušek et al. 2006).   (Jaques & Green 1979, Takahari & Kushiro 1983. Major and trace-elements variation diagrams using MgO as differentiation index (Fenner's diagrams; Figs. 11 and 12) define relative linear trends. A negative relationship is observed in the major-elements diagrams between MgO and contents of SiO 2 , TiO 2 , P 2 O 5 , Fe 2 O 3 and MnO, while CaO and Na 2 O are randomly distributed, which suggests they were largely mobilized during post-magmatic events. K 2 O yields a divergent pattern in which its content decreases and then increases again as the MgO content decreases.

MgO contents vary between
Trace-elements show that concentrations of Ba, Nb, Zr, La, Ce and Y decrease as MgO content increases.
The rocks from Salto do Céu Suite have tholeiitic affinity with typical FeO t enrichment relative to MgO taking into account nearly constant alkali values, as seen in the AFM diagram (Irvine & Baragar 1971;Fig. 13A), and are classified as sub-alkaline basalts and iron-rich tholeiitic basalts following the classification of Winchester & Floyd (1977;Fig. 13B) and Jensen (1976; Fig. 13C), respectively.
Pattern distribution of rare earth elements (REE) normalized to primitive mantle values of McDonough & Sun (1995) is illustrated in Figure 14A. Heavy REE (HREE) fractionation relative to light REE (LREE) is observed. There is a clear distinction between the two rock groups in which one of them is richer in REE, with La N greater than 100 and a discrete negative Eu anomaly (group 1), corresponding to the rocks with lower MgO contents; the second group does not show this pattern having La N less than 100 (group 2). There is no relation between the shape of occurrence (sills and flows) with the groups of high and low LaN. Figure 14B shows a primitive-mantle normalized multi-element spidergram according to values of McDonough & Sun (1995). Here rocks also show a separation into two groups, where the main difference is the negative Sr anomalies for the group with La N greater than 100, and positive Sr anomalies for the group with La N less than 100, which corresponds to the group with discrete Eu anomaly. For other elements, both groups follow a very similar pattern with negative Rb, K, and Nb anomalies, and positive Ti anomaly likely reflecting no fractionation of Fe-Ti oxides. The negative Nb anomaly, in its turn, may be attributable to crustal assimilation processes.

DISCUSSION AND FINAL CONSIDERATIONS
Based on data from semi-detailed geological mapping as well as petrographic and lithochemical characterization, here we conclude the basic rocks from the region of Salto do Céu (Mato Grosso) occurring as sills and lava flows belong to the same magmatic event that gave rise to the Salto do Céu Suite. Moreover, the U-Pb baddeleyite age of 1439 ± 4 Ma (Teixeira et al. 2015), and geochronological dataset available for rocks from the Rio Branco Intrusive Suite (1427 ± 10 Ma, 1423 ± 0.2 Ma and 1423 ± 6 Ma obtained by Geraldes et al. 2001, 2004, Araújo 2008; respectively) indicate that both suites are derived from a bimodal coeval magmatism.
The temporal relation of Salto do Céu (mafic rocks) and Rio Branco (felsic rocks) suites is corroborated by the presence of magma mingling diagnostic features, such as mafic microgranular enclaves with alkali feldspar xenocrysts, diffuse and transitional boundaries and hybridization zones, as previously described. Moreover, oscillatory or reverse zoning in plagioclase can result from the interaction of felsic and mafic magmas. Ginibre et al. (2002) described zoning patterns in plagioclase crystals of the Parinacota Volcano (Chile) and assumed that this features can be generated by a magma chamber recharge with mafic magma.  The TIMS U-Pb baddeleyite ages of 1110 ± 2 Ma and 1112 ± 2 Ma measured by Teixeira et al. (2015) for the mafic sills from Rincon del Tigre, respectively, in Bolivia and Huanchaca as well as that U-Pb baddeleyite age of 1439 ± 4 Ma obtained by Teixeira et al. (2016) for the basic magmatism of the Salto do Céu Suite, in Brazil, show that they are not related to a single igneous-tectonic event attributable to an older crustal extension episode that resulted in mantellic magma ascent, opposing previous proposals of Ruiz et al. (2010a), Lima et al. (2012), among others.
Geochronological data made available by Leite & Saes (2003), Santos et al. (2005) and Geraldes et al. (2014) point out that the sedimentary cover hosting sills and lava flows of the Salto do Céu Suite is not attributable to the defined Aguapeí Group of the type-area in Santa Barbara Hill (Souza & Hildred 1980).
The diagenesis of Fortuna Formation, basal unit of Aguapeí Group, occurred between 1165 ± 27 Ma and 1149 ± 7 Ma based on SHRIMP U-Pb dating of diagenetic xenotime from Santa Barbara Hill (Santos et al. 2005). A maximum deposition age of 1.3 Ga on detrital zircons is attributed to it according to Leite & Saes (2003; bottom of Aguapeí Group in São Vicente and Lavrinha hills) and Geraldes et al. (2014;Fortuna Formation in Ricardo Franco and Santa Barbara hills). The younger basement rocks of this group in the surrounding region are the post-kinematic granites of the Pensamiento Suite dated at 1290 Ma by U-Pb (LA-IPC-MS) (Jesus et al. 2010). Geraldes et al. (2014) report a maximum deposition age around 1540 Ma (U-Pb, LA-ICP-MS) for this group in the Rio Branco Hill, which supports the notion that it is a different sedimentary cover, likely another stratigraphic unit deposited in the Calymmian once the younger rocks of the underlying basement consist of post-kinematic granites of the Alvorada Suite dated around 1440 Ma by U-Pb (TIMS) on zircon (Geraldes et al. 2001, Ruiz 2005. The mafic-ultramafic suites named Figueira Branca and Indiavaí (1.42 a 1.41 Ga; Teixeira et al. 2011) are part of the Jauru Terrane and are chrono-correlated to the Salto do Céu and Rio Branco suites as well as may be derived from a single magmatic event.
The mafic sills and lava flows from Salto do Céu Suite and the rocks from Rio Branco Suite are here interpreted as an anorogenic bimodal suite formed in intraplate settings, in extensive tectonic regime, which may reflect an important tectonic milestone associated with the breakup of the Columbia (Nuna) Supercontinent.