Geology, Petrology and Magmatic Evolution of the Felsic Alkaline Rocks of the Vitória Island, São Paulo State, Brazil Geologia, Petrologia e Evolução Magmática das Rochas Alcalinas Félsicas da Ilha de Vitória, São Paulo, Brasil

This paper presents geological, petrographic, geochemical characteristics and radiometric ages for the felsic alkaline rocks of the Vitória Island Intrusive Complex, São Paulo State, Brazil, and discusses its geochemical evolutionary processes. This intrusive body is about 3 km in diameter and the emerged island areas correspond to its northeastern border. The main intrusion is constituted by nepheline syenite at the central zone, nepheline-bearing syenite at the outer zone, syenite at the border, and quartz syenite at the contact zone with alkali feldspar represented by orthoclase. Clinopyroxene vary from hedenbergite, soda-augite to aegirine-augite, and some crystals have resorption shape and amphibole reaction rim. They are intruded by phonolite, trachyte and lamprophyre dykes. Based on the intrusion relation, the dykes are subdivided into two generations: the first one encompass a radial system and the second a parallel system with NE-SW orientation and sinistral displacement. The alkaline rocks vary from silica undersaturated to oversaturated, show peralkaline to peraluminous compositions and belong to the potassic series. The K-Ar ages for amphibole of the nepheline-bearing syenite and syenite are 89.58 ± 3.18 Ma and 84.35 ± 3.92 Ma, respectively. The rocks are strongly fractionated with high differentiation index (DI) of 88.15 in average, indicating mafic minerals fractionation. K2O/ (K2O+Na2O) ratios are low, in average 0.51, in accordance to crystallization of pseudo leucite and potassic feldspar the last observed in thin sections.


Introduction
In the coastal region of southeast Brazil, Cretaceous to Early Cenozoic felsic alkaline intrusive bodies represent an expressive magmatic event called Serra do Mar Magmatic Province (Figure 1).
They are constituted mainly of nepheline syenite, syenite, phonolite and trachyte with local occurrences of subvolcanic breccia and lamprophyre dykes.
The alkaline intrusive bodies form three magmatic alignments: A) Poços de Caldas -Cabo Frio with ESE-NEW trend; B) Mendanha -Morro de São João with ENE-SWS trend; C) Monte de Trigo -Vitória Island with ENE-SWS trend.The former two alignments were proposed in a very close region and nowadays, only the Poços de Caldas -Cabo Frio Alignment (Almeida, 1976;Sadowski & Dias Neto, 1981;Thomáz-Filho & Rodrigues, 1999) has been cited in the literature.Herz (1977) and Thomáz-Filho & Rodrigues (1999) interpreted that the Poços de Caldas -Cabo Frio Alignment would continue to the Vitória-Trindade Chain, forming a single hot-spot track.However, the absolute motion of the South America Plate is NW to NNW (O'Connor & Roex, 1992;Steinberger, 2000), which is widely diferent from the alkaline rock trends, about E-W.Therefore, it is not considered as resulted from hotspot tracks (Skolotnev et al., 2010) although recent works present evidences of possible mantle plume contribution to their genesis (Santos, 2013(Santos, , 2016;;Bongiolo et al., 2015;Pires et al., 2016).Although both magmatic chains have parallel layout, they are separated from each other about 380 km (Motoki et al., 2012 a), and do not form a single continuous chain (Sichel et al., 2012).
In addition, felsic alkaline rocks, such as nepheline syenite, constitute the Poços de Caldas-Cabo Frio whereas the Vitória-Trindade is mainly composed by ultramaic rocks such as basanite and nephelinite, possibly inluenced by crustal thickness and chemical composition variation (continental and oceanic crusts environments).
The Intrusive Complex of the Vitória Island is situated at the easternmost part of Monte de Trigo -Vitória Island felsic alkaline rock alignment (Motoki, 1986).The rocks of this alignment have some diferences from those of the Poços de Caldas-Cabo Frio one.The intrusive ages of the former range from 80 to 90 Ma, which are slightly older than the latter (55 to 72 Ma, according to Sonoki & Garda, 1988;Motoki et al., 2010;2012 b).Vent-illing subvolcanic pyroclastic bodies are common in the latter (Motoki & Sichel, 2006;Motoki et al., 2008 b;c;Sichel et al., 2008) but it does not occur in the former.
The aforementioned intrusive bodies present gradual transition from silica-oversaturated rocks to  Santos; Camila Cardoso Nogueira; Werlem Holanda dos Santos; Mauro Cesar Geraldes; Susanna Sichel; & Thais Vargas undersaturated in the same intrusive body, hypothetically crossing over the thermal divide, which is a controversy subject.Bonin (1990) stated that during magma ascent and solidiication, water content from wall rocks may play an important role in the melt changing its composition, moving from undersaturated to over-saturated alkaline melts.Higher initial Sr isotopic ratio is also related to crustal contamination in Evisa Complex in Corsica where crossing over thermal divide is related to a set of magmatic changes beyond F-rich luids migration (Bonin, 1990).
According to literature, magma can cross over thermal divide under some conditions such as continental crust assimilation/contamination, water and luids content (F, Cl, CO 2 , for example).Motoki (1986) showed, for analyzed rocks from Vitória Island, high chlorine, carbonate and luor contents and high water content, compared to North Nyasa Alkaline Province of Malawi (Eby et al., 1998).Water-rich luids are less efective in REE luid-melt partition coeicient.Observing the REE content in analyzed rocks we can state that water played an important role in the evolution of undersaturated to oversaturated rock.This paper presents geological, petrographic, geochemical characteristics and radiometric ages of the Vitória Island felsic alkaline intrusive body, located at São Paulo state, Brazil.

Geologic Setting
The Archipelago of Vitória Island is situated at 23°45'S and 45°01'W, about 35 km to the south of Ubatuba city and 35 km to the east of Ilha Bela city, at the North Coast of the São Paulo State, Brazil.It consists of the main island and two small islets: Vitória Island, Pescador Island and Cabras Island (Figure 2).They are composed mainly by syenitic rocks (Figure 3A).The nepheline syenite and nepheline-bearing syenite occur in the southwestern portion of the body.The quartz-bearing syenite showing strongly deformed host rock enclaves is observed locally at the northeastern domain, in the border of the intrusion.The ring-like shape of the structure indicates that the main intrusive body has semi-circular form.Similar structure is observed also in Tanguá (Motoki et al., 2010) and Cabo Frio Island (Motoki et al., 2012 b) intrusive bodies of the Poços de Cal-das-Cabo Frio Alignment.The grain size is homogenous, about 4 mm, but at the contact zone it becomes smaller, about 3 mm.The nepheline-bearing rocks are grey in colour, but syenite and quartz syenite are dark blue changing into dark green due to oxidation.
The main syenitic body is cut by numerous dykes of phonolite and trachyte composition (Figure 2).According to the intrusive relation, they are divided into two generations: 1) The irst generation of dykes forms a radial system (Figure 3B); 2) The second-generation constitutes a parallel system (Figure 3C).The center of the radial dykes corresponds approximately to the main syenitic body, suggesting the intrusion WAS caused by hydraulic tensile fracturing (e.g.Hubbert & Willis, 1957;Nicolas & Jackson, 1982;Motoki & Sichel, 2008).The parallel dykes have NE-SW orientation.They show 'en-echelon' pattern and sinistral strike-slip displacement, reforcing a hydraulic shear fracturing mechanism (e.g.Phillips, 1974;Motoki et al., 2008c;2009).Some dykes of lamprophyre composition were also observed (Figure 3D).

Petrographic Characteristics
The syenitic rocks are composed mainly of alkali feldspar (~80-90% in mode), nepheline (<11%), clinopyroxene, amphibole, and biotite.Some rare samples contain small amount of quartz (<3%).The accessory minerals are magnetite, ilmenite, apatite and analcime.Orthoclase is xenomorphic characterised by interlocking perthite (Figure 4A).The nepheline-bearing rocks sometimes show antiperthite patches.The nepheline is xenomorphic and often altered into analcime and cancrinite (Figure 4B).The modal content is generally lower than 10 vol.%.Only in a small area in the inner portion of the main body nepheline syenite occurs.The outer domain of the body is constituted by nepheline-bearing syenite, so-called pulaskite.The border zone is made up of syenite without nepheline and quartz.At the contact zone, there is alkaline syenite with small amount of xenomorphic quartz, so-called nordmarkite.
The syenitic rocks are leucocratic with maic minerals content less than 10%.The clinopyroxene is green to light yellow in colour, with extinction angle of about 45°, suggesting an augite composition.Some crystals have resorption shape and amphibole reaction rim (Figure 4C).Similar characteristics are also observed in clinopyroxene from Cabo Frio  (Motoki, 1986;Woolley, 1987).VTR represent locations where rock samples were collected in the Vitória Island.Island Intrusive Body (Araújo, 1995;Motoki et al., 2012 b).The amphibole is brown to brownish/red suggesting that the composition must be close to barkevikite.Some idiomorphic crystals present short prismatic shape suggesting that they are pseudomorph after clinopyroxene.The rocks present biotite in brown to yellow-brownish colour besides idiomorphic magnetite and ilmenite crystals with ca.0.3 mm in size and some grains showing resorption.Euhedral apatite with size from 0.1 to 0.5 mm is scattered all over the matrix.
The ine-grained felsic rocks are characterized by well-developed trachytic texture showing microcrystalline groundmass and alkali feldspar microlites (Figure 4D).However, the central part of the dykes has a width of up to 1 m being composed of rocks with interstitial texture and alkali feldspar framework.The clinopyroxene phenocrysts are stron-gly green with extinction angle of ca.20° suggesting they are aegirine-augite.Phenocrysts of alkali feldspar, amphibole, and biotite are also observed.

Geochemical Classiication of the Alkaline Rocks
The whole-rock chemical analyses of the Vitória Island samples have been performed at the Geochemical Laboratory of the GeoSol™ S.A., Belo Horizonte, Minas Gerais State, Brazil, using ICP atomic emission (ICP-AES) and atomic absorption spectrometry (AAS) according to the methodology described in Hofman (1992).The results are presented in Table 1.
Figure 5 Santos; Camila Cardoso Nogueira; Werlem Holanda dos Santos; Mauro Cesar Geraldes; Susanna Sichel; & Thais Vargas Table 1 Major and trace elements for the alkaline rock of the Vitória Island Intrusive Complex, São Paulo State, Brazil.Pulaskite corresponds to nepheline-bearing syenite.The Magma Diferentiation Index (D.I.) corresponds to sum of the all of Norm felsic minerals (Thornton & Tuttle, 1960) (1986).The coarse-grained rocks fall mainly on the trachyte (syenite) ield and the inegrained ones correspond to the phonolite (nepheline syenite) ield.These rocks are less alkaline than ones presented in the Poços de Caldas-Cabo Frio Alignment.The lamprophyre plots in the tephrite ield.
In the Na 2 O vs. K 2 O diagram of Figure 6 almost all felsic rocks are classiied in the potassic series.The K 2 O+Na 2 O is approximately 13 wt.%,which is about 2 wt.% lower than those rocks from Poços de Caldas-Cabo Frio Alignment.
In the alkaline-alumina saturation diagram (Figure 7), the felsic rocks form a linear trend from peralkaline to peraluminous ields.Similar tendency is also observed for the felsic rocks from Poços de Caldas-Cabo Frio Alignment.Kenji Motoki; Anderson Costa dos Santos; Camila Cardoso Nogueira; Werlem Holanda dos Santos; Mauro Cesar Geraldes; Susanna Sichel; & Thais Vargas

Radiometric Dating
Four radiometric ages have been performed at the Geochronological Centre of São Paulo University, Brazil: 1) K-Ar in biotite from nepheline-bearing syenite; 2) K-Ar in amphibole from nepheline-bearing syenite; 3) K-Ar in amphibole from syenite; 4) Whole-rock K-Ar dating for phonolite.The quantitative analyses of potassium for K-Ar ages have been performed by frame photometry method and for 40 Ar quantiication, by mass spectrometer for gas, such as described in Motoki (1986) and results are shown in  (Dalrymple & Lamphere, 1969).Phonolite sample (VTR-4A) yielded an age of 80.32 ± 1.97 Ma. 87 Sr/ 86 Sr initial ratios have been determined by thermal ionization mass spectrometer (TIMS), according to procedures described in Heilbron et al., (2013) and the results are displayed on Table 3.The 87 Sr/ 86 Sr initial ratio of 0.7055 indicates mantle origin being comparable to that of EM1.It is similar to those of the Poços de Caldas-Cabo Frio Alignment and also of Tanguá, Cabo Frio Island, Itatiaia and Morro de São João intrusive complexes, which have Sr initial ratios of 0.7062, 0.7049, 0.7049, and 0.7047, respectively (Motoki et al., 2010;2012 b).

Fractional Crystallization
The SiO 2 content of the felsic rocks, with exception for syenitic aplite, ranges from 58.10 to 62.20 wt.%. Figure 5 shows that the silica variation could be related to fractional crystallization process, which increases water content while magma ascend to the surface.These felsic alkaline rocks are highly fractionated magmas with low MgO, CaO, and FeO*, with respective mean value of 0.62, 1.88, and 3.28 wt.%, and with high magma diferentiation index (D.I. -88.15, Thornton & Tuttle, 1960).
The main agent of fractional crystallization of this stage are the felsic minerals.Valença (1980) proposed that by means of leucite fractionation in nepheline syenite magma a sodic signature becomes potassic.Ulbrich (1984) conirmed this idea proposing that the felsic alkaline magma becomes peralkaline (agpaitic) from meta-aluminous (miaskitic).
Alkaline rocks of Vitória Island are characterized by relatively low K 2 O/(K 2 O+Na 2 O) wt.%, ranging from 0.39 to 0.57, with mean value of 0.51.These ratios point to advanced fractional crystallization, being comparable to that of Cabo Frio Island (Figure 8), with phonolite samples (VTR-4A and VTR-74A) being the highest fractionated rocks in Vitória alkaline complex.Motoki (1986) presents data for residual system of SiO 2 -NeAlSiO 4 -KAlSiO 4 , which expresses rock samples plotting close to 1 Kbar minimum suggesting that they represent fractionated liquids.Amphibole K-Ar ages of nepheline-bearing syenite (VTR-4) and syenite (VTR-74), are, respectively, 89.58 ± 3.18 Ma and 84.35 ± 3.92 Ma.The atmospheric 40 Ar contents are lower than that obtained in biotite grains, 18.5% and 26.56%, respectively; therefore, these ages seem to be more accurate.They are similar to the other ages of intrusive complexes of the Monte de Trigo-Vitória Island Alignment (Sonoki & Garda, 1988).

Conclusions
Field observations, petrographic descriptions, geochemical features, and radiometric ages lead to the following conclusions: Vitória Island felsic alkaline intrusive body, São Paulo state, Brazil, is about 3 km in diameter.The main intrusive body is constituted by nepheline syenite in its inner zone, nepheline-bearing syenite in the outer zone, syenite in the border, and quartz-bearing alkaline syenite along the contact zone.The emerged island areas correspond to the northeast border of the intrusive body.
The main body is intruded by phonolite, trachyte, and lamprophyre dykes.According to the ield relationship, they are subdivided into two generations.The irst-generation dykes form a radial system, indicating intrusion by hydraulic tensile fracturing.The second-generation constitutes a parallel system with NE-SW orientation and sinistral displacement, suggesting intrusion by hydraulic shear fracturing.
The alkali feldspar of the coarse-grained rocks is orthoclase and clinopyroxene composition varies from hedenbergite to aegirine-augite.Some crystals have resorption features and reaction rim of amphibole.
These alkaline rocks belong to the potassic series which compositions vary from silica-undersaturated to oversaturated, and from peralkaline to peraluminous.Geochemically, these rocks correspond to syenite and nepheline syenite.K-Ar ages in amphibole from the nepheline-bearing syenite and from syenite are 89.58 ± 3.18 Ma and 84.35 ± 3.92 Ma, respectively.
The alkaline rocks are highly fractionated with magma diferentiation index of ca.88.15.The K 2 O/ (K 2 O+Na 2 O) wt.% ratios of ca.0.51 indicates that felsic mineral fractionation played an important role in the magmatic evolution.Other characteristics such as low Ba and Sr contents, low V and Co, low Ba and relatively low Sr (data from this work and Motoki, 1986) represent, respectively, fractionation of feldspar, magnetite and clinopyroxene beyond amphibole due to low MREE content (Motoki, 1986).

Geology,
Figure2Geologic map of the Vitória Island Intrusive Complex(Motoki, 1986;Woolley, 1987).VTR represent locations where rock samples were collected in the Vitória Island.

Figure 6
Figure 6 Na 2 O vs. K 2 O diagram from Middlemost (1975) for alkaline rocks of Vitória Island.Data from Poços de Caldas-Cabo Frio Alignment are as on Figure 5.

Figure 7
Figure 7 Alkaline-alumina saturation diagram plot from Maniar & Piccoli (1989) for alkaline rocks from Vitória Island Intrusive Body.Data from Poços de Caldas-Cabo Frio as on Figure 5.

Figure 8
Figure 8 K 2 O/(K 2 O+Na 2 O) wt% ranges of alkaline rocks from Vitória Island Intrusive Complex, São Paulo State, Brazil, in comparison to those from Poços de Caldas-Cabo Frio Alignment.

Petrology and Magmatic Evolution of the Felsic Alkaline Rocks of the Vitória Island, São Paulo State, Brazil
shows the geochemical classiication of the rocks on SiO 2 vs. Na 2 O+K 2 O diagram based Akihisa Motoki U ; KenjiMotoki; Anderson Costa dos Santos; Camila Cardoso Nogueira; Werlem Holanda dos Santos; Mauro Cesar Geraldes; Susanna Sichel; & Thais Vargas

Table 2 .
Biotite of nepheline-bearing syenite, sample VTR-4, yielded an age of 80.75 ± 4.54 Ma, with high atmospheric 40 Ar, 40.91%.In such case, the apparent age tends to become younger

Table 2 K
-Ar dating data for the Vitória Island alkaline rocks, São Paulo State, Brazil.The pulaskite corresponds to nepheline--bearing syenite.

Table 3
Rb-Sr isotope data for minerals and whole-rock from syenite (VTR-108) of the Vitória Island Intrusive Complex, São Paulo state, Brazil.