40 Ar/ 39 Ar age, lithogeochemistry and petrographic studies of the Cretaceous Alkaline Marapicu Intrusion, Rio de Janeiro, Brazil

: The Marapicu Alkaline Massif is an intrusion into the Marapicu-Gericinó-Mendanha Igneous Complex that is part of the Cretaceous Poços de Caldas-Cabo Frio magmatic lineament located in the Southeastern region of Brazil. Nepheline syenites and phonolites are the most abundant rocks in the massif that also include syenites forming an alkaline series SiO 2 - undersatured. Chemically this series is predominantly metaluminous and to a lesser extent peralkaline. This series presents both potassic and sodic suites being the first one in greater content. The data show that both basic and intermediary rocks with parental composition sampled in this area have no genetic relationship with the other rocks of the body. Geochemistry data shows that evolution processes involved fractional crystallization with or without continental crust assimilation and also indicates that this alkaline magma was generating from an enriched mantle source. The 40 Ar/ 39 Ar age of hornblende (extracted of nepheline syenite) from Marapicu massif is 80.46 ± 0.58 Ma, which it is contrasting, with the idea of age decrease of the hotspot track from west to east on the Poços de Caldas-Cabo Frio magmatic lineament.


INTRODUCTION
The Marapicu-Gericinó-Mendanha Igneous Complex (Figure 1) is composed of two adjacent alkaline intrusions named as Marapicu and Mendanha; these bodies are intruded into the Central segment of the Neoproterozoic Ribeira Mobile Belt and make part of the Brazilian Southeast Alkaline Province (Almeida, 1983).The Marapicu-Gericinó-Mendanha Igneous Complex is detached related to other alkaline provinces of the South America Platform for including plutonic, sub-volcanic (mainly dykes) and volcanic (rarely seen in that provinces) rocks (Ulbrich & Gomes, 1981).The Marapicu Massif forms a smaller body with approximately circular shape having close to 10 km 2 in area and height over 400 m.The Gericinó-Mendanha is the biggest body localized east side of the Marapicu and has approximately shape the ellipse.Its higher accessible point is over than 850 m.In this body is localized the Nova Iguaçu Volcanic Complex which is composed for central agglomerate, bombs, tuffs and abundant lapilli (Klein & Vieira, 1980;Klein, 1993).In this study, we carry out new petrographical observations and, geochemical and 40 Ar/ 39 Ar analyses of samples from the Marapicu Massif.The aim of this study is to constrain the emplacement models of the Cretaceous-Tertiary magmatism, which have taken place in the South-American Plate, southern from Brazil.
Figure 1.Geologic map of Marapicu massif (smaller body in the left) and Gericinó-Mendanha massif (bigger body in the right) adapted of Mota (2008).

TECTONIC SETTING
Alkaline rocks occurrence in the southeast part of Brazil are observed in two great magmatic lineaments of Cretaceous-Tertiary age in the South-American Platform.The first presents a NW-SE direction and is called Poços de Caldas-Cabo Frio Lineament (Figure 2) and the second which have a NW-SW direction is called Southern Coast (Ulbrich & Gomes, 1981).These two lineaments make part of the Alkaline Province of Brazilian Southern (Almeida, 1986).
The magmatism dated from Mesozoic to Cenozoic periods is widely registered on the Brazilian territory as tholeitic lava flows, dykes, alkaline plugs and stocks (Almeida, 1976(Almeida, , 1986;;Almeida et al., 1996;Thomaz Filho & Rodrigues, 1999;Thomaz Filho et al., 2005).These magmatic manifestations are associated with two great events, which occurred in the South-American Platform, stabilized in the Cambrian-Ordovician and, can be sub-divided in the tholeitic magmatism that is associated to Gondwana break-up and consequently opening of South Atlantic Ocean.These magmatic events occurred approximately 130 to 120 Myr ago and are associated with the implantation of a Brazilian passive margin basin.The other event is the alkaline magmatism related to uplift phenomena such as the occurred in the Serra do Mar and sedimentary tertiary basin of Brazilian southern formation.
The alkaline rock bodies' lineament Poços de Caldas-Cabo Frio (Almeida, 1983(Almeida, , 1986(Almeida, , 1991;;Almeida et al., 1996;Freitas, 1947) form a located magmatic sequence represented by alkaline rocks in the form of stocks, plugs, dykes and rare lavas and pyroclastic flows.These rocks present ages between Upper Cretaceous to Eocene and setting in WNW-ESE direction into the Rio de Janeiro state, cutting by oblique way the preferential direction of tectonic structures from the Neoproterozoic Ribeira Mobile Belt.Almeida (1991) refers to that alkaline rocks being exclusively felsics and presented mainly for nepheline syenite, pulaskite, foiaite, phonolite, tinguaite and trachyte.

GEODYNAMIC MODELS
Three geodynamic models have been proposed for explaining the Upper Cretaceous alkaline magmatism in the meridional part of South-American Plate (Almeida, 1991;Fainstein & Summerhayes, 1982;Thomaz Filho et al., 2000;Thompson et al., 1998) as following: reactivation of deep faults; hotspot activity and; the combination of two previous models.
The reactivation of deep faults proposal is directly related to the sub-parallelism of both continental and oceanic lineaments observed in the Atlantic passive margin of the South-American Plate (for instance: Gorini & Bryan, 1976).
The plumes and hotspots related magmatism model (for instance: Gibson et al., 1995) suggest that the alkaline activity of Upper Cretaceous in the southern of Brasil may have been caused for the Trindade-Martin Vaz mantle plume at the base of continental lithosphere.
The third model attempted to associate the fault reactivation by the presence of a thermic anomaly (plume) in the Upper Cretaceous of the Brazilian southeast region (Fainstein & Summerhayes, 1982;Thomaz Filho et al., 2000).This model explains the magmatic activity related to the Poços de Caldas-Cabo Frio lineament as a response for the thermic anomaly also responsible for the formation of the Vitoria-Trindade magmatic lineament.

FIELD WORK
The methodology applied in this study starting with fieldwork when 28 outcrops were visited at the parts Southwest, North and Northeast of the area and 52 samples were collected.The greater part of the sampling includes nepheline syenites and phonolites, which are the most representatives in the area.Furthermore, we have collected one basanite/tefrite and one tefrite phonolitic.The nepheline syenites and syenites presents granulometry ranging from fine to coarse including pegmatitic types.The plutonic rocks occur intruded into the granites and gneiss of the Domínio Costeiro from the Ribeira Mobile Belt, the phonolites occur as hipoabissal bodies the way of the dikes with up to 3 m width.These dikes have two main trends: NW-SE e NNE-SSW.

40
Ar/ 39 Ar METHODOLOGY We selected a fresh sample from Marapicu for 40 Ar/ 39 Ar dating and separated unaltered, optically transparent, 250-500 µm size, hornblende.These minerals were separated using a Frantz magnetic separator, and then carefully hand-picked under a binocular microscope.The selected hornblende minerals were further leached in diluted HF for one minute and then thoroughly rinsed with distilled water in an ultrasonic cleaner.This step was carried out at the Geologic Laboratory of Sample Preparation from State University of Rio de Janeiro.
After the hornblende grain separation the 40 Ar/ 39 Ar dating was carried out at the Curtin University from Australia where the Samples were loaded into a large wells of 1.9 cm diameter and 0.3 cm depth aluminum disc.This well was bracketed by small wells that included Fish Canyon sanidine (FCs) used as a neutron fluence monitor for which an age of 28.294 ± 0.036 Ma (1σ) was adopted of Renne et al. (2011).The discs were Cd-shielded (to minimize undesirable nuclear interference reactions) and irradiated for 2 hours in the Hamilton McMaster University nuclear reactor (Canada) in position 5C.The mean J-values computed from standard grains within the small pits and determined as the average and standard deviation of J-values of the small wells for each irradiation disc is given along with the raw data in Table 1.Mass discrimination is given in Table 2 for each sample and was monitored using an automated air pipette and calculated relative to an air ratio of 298.56 ± 0.31 (Lee et al., 2006).The correction factors for interfering isotopes were ( 39 Ar/ 37 Ar) Ca = 7.30x10-4 (± 11%), ( 36 Ar/ 37 Ar) Ca = 2.82x10-4 (± 1%) and ( 40 Ar/ 39 Ar) K = 6.76x10-4 (± 32).
The 40 Ar/ 39 Ar analyses were performed at the Western Australian Argon Isotope Facility at Curtin University.The sample was step-heated in a double vacuum high frequency Pond Engineering© furnace.The gas was purified in a stainless steel extraction line using two AP10 and one GP50 SAES getters and a liquid nitrogen condensation trap.Ar isotopes were measured in static mode using a MAP 215-50 mass spectrometer (resolution of ~400; sensitivity of 4 x 10 -14 mol/V) with a Balzers SEV 217 electron multiplier using 9 to 10 cycles of peak-hopping.
The data acquisition was performed with the Argus program written by M.O.McWilliams and ran under a LabView environment.The raw data were processed using the ArArCALC software (Koppers, 2002) and the ages have been calculated using the decay constants recommended by Renne et al. (2011).Blanks were monitored every 3 to 4 steps and typical 40 Ar blanks range from 1 x 10 -16 to 2 x 10 -16 mol.Ar isotopic data corrected for blank, mass discrimination and radioactive decay are given in Table 3. Individual errors in Table 3     Our criteria for the determination of plateau are as follows: plateaus must include at least 70% of 39 Ar.The plateau should be distributed over a minimum of 3 consecutive steps agreeing at 95% confidence level and satisfying a probability of fit (P) of at least 0.05.Plateau ages are given at the 2σ level and are calculated using the mean of the entire plateau steps, each weighted by the inverse variance of their individual analytical error.Inverse isochrons include the maximum number of steps with a probability of fit ≥ 0.05.The 40 Ar/ 36 Ar intercept value is provided.All sources of uncertainties are included in the calculation.

PETROGRAPHY Nepheline syenite and syenite
The plutonic lithotypes include nepheline syenites and syenites showing very similar petrographic and hand sample characteristics, they presents holocrystalline and equigranular texture and grain size ranging from coarse (3 to 6 mm) to fine (± 1 m) (Figures 3A,3B and 3C).Its main mineralogy include: alkali feldspar, nepheline, plagioclase and hornblende.The accessory minerals include: biotite, scapolite, muscovite, carbonate, zircon, apatite, augite and opaque mineral (mainly magnetite).Fine-grained white clay mineral is common in all samples, formed by the decomposition of alkali feldspar.
The alkali feldspar grains with simple Carlsbad twins and plagioclase both occur in different shapes anhedral, subhedral and tabular euhedral, they have coarse (3 to 6 mm) to medium sizes (1 to 3 mm).Abundant alkali feldspar grains are microperthitic (Figures 3D and 3F) and show transformation into very fine ragged grains of sericite and ultrafine -grained clay with a cloudy appearance, a radial texture is present too (Figure 3D).The plagioclase with K-feldspar lamellae also shows an alteration grade, which occults almost totally its polysynthetic twinning.The nepheline occurs, in smaller proportion related to feldspars, with anhedric and subhedric coarse crystals and takes place in interstitial spaces.
The hornblende grains are strongly pleochroic from greenish-brown to dark brown, occur in anhedral, subhedral and euhedral shapes forming hexagonal grains.Some zoned grains present a variation of green colour being more dark in the rim and more light in the core, besides the simple twinning visible (Figure 3E).Other aspect is that the hornblende presents inclusions of apatite and biotite (more often), but opaque minerals, scapolite and carbonate are also observed.In their rim occur grains of biotite and opaque minerals, muscovite sometimes as alteration product is present too.
The biotite is observed forming anhedral and subhedral grains or as rips.It appears as inclusion in hornblende or in contact with their rims.The apatite is sub-milimetric, also disseminated and occurs commonly included in hornblende, as anhedral and euhedral grains with hexagonal or neddle habits.The scapolite occurs frequently as xenomorphic crystals but few present triangular habits.Carbonate, muscovite, augite and opaque mineral are found in subordinated amount.Opaque minerals are mainly magnetite forming few grains with anhedral habit.Zircon is rare and generally occurs < 1 mm in size.

Phonolites
The phonolites can be divided in two groups based on their phenocrysts content.The first group of phonolite is represented by a fine-grained, inequigranular porphyritic rock (Figure 4A) with phenocrysts of alkali-feldspar, nepheline and hornblende that comprise ~10% by volume of the rock (Figure 4C) in a hypocrystalline matrix.The groundmass of the rock has a grain size < 1 mm.It exhibits a strong trachytic texture, and is dominated by sanidine, hornblende and biotite.The alkali feldspar phenocrysts show Carlsbad twinning and large size (3 to 5 mm), are anhedral and subhedral with tabular habit.The nepheline phenocrysts are subhedral and euhedral with hexagonal habit occurring as medium sized crystals (2 to 3 mm).Hornblende phenocrysts are anhedral and euhedral and up 3 mm with hexagonal, octagonal and tabular habits.Some grains present compositional zoning and carlsbad twinning.Isotropic glass and opaques occur within interstitial areas.
The second type of phonolite (Figure 4B) is a rock inequigranular porphyritic with a fine-grained matrix and phenocrystals of sanidine and nepheline, which have a medium size (about 3 mm), also occur a few millimetric phenocrysts of biotite.The accessory minerals are biotite, scapolite, augite, opaque minerals and zircon.The sanidine integrates the matrix forming submilimetric crystals commonly subhedral and euhedral.It is common their presence as euhedral phenocrysts with tabular habit and carlsbad twinning.Many sanidine grains are elongate laths (Figure 3D).The nepheline occurs as subhedral and euhedral phenocrystals, with hexagonal habit.Microscopic examination reveals the trachytic texture of the fine matrix.

Whole rock Geochemistry
A subset of 27 samples was prepared for whole rock geochemical analyses at the Geologic Laboratory of Sample Preparation from State University of Rio de Janeiro.The hand sample were cut the way of chip being two pieces for each sample and them these chips were fragmented in a pestle.The fragments were washed with distilled water and alcohol so left to dry.These fragments were powdered in a tungsten ball mill; this machine is also a mixer ensuring the homogeneity of the sample.The rock powder for each sample is stored in 10 g container and then send to geochemical analyses.
All geochemical analyses were performed in the Activation Laboratories Ltd. from Canada by using relatively fresh samples.Whole rock major and trace element concentrations were measured using Fusion Mass Spectrometry (FUS-MS) and Fusion Inductively Coupled Plasma (FUS-ICP) techniques.The sum of the major elements and Loss on Ignition is between 98.19 and 100.55 wt%.The Loss on Ignition values are less than 5%.The iron analyses were carried out in the way of total Fe 2 O 3 as indicated in Appendix.
The Marapicu massif is constituted of an alkaline series, which have mainly nepheline syenites and phonolites as it is shown in the Figure 5.This series it is characterized for increasing of the alkalis (Na 2 O+K 2 O) content along with decreasing of the SiO 2 content, in this way, geochemically the more evaluated members of the series are the phonolites and the less evaluated members are the syenites.
The samples analyzed form an alkaline series with intermediary character and SiO 2 content ranging of 53.46 wt% to 61.87 wt%.Into the TAS diagram (SiO 2 versus Na 2 O+K 2 O) of Cox et al. (1979), the most part of the samples lie in the nepheline syenite field, and a smaller group lie in the syenite field.The sample MAR-28C have a basic composition with SiO 2 content of 47.5 wt% and high content of Fe 2 O 3 T (8.24 wt%), MgO (7.02 wt%) and CaO (8.51 wt%) which are mafic rock-forming mineral elements, besides, presents both high content of Ni (130 ppm) and Cr (370 ppm).This way the sample cited would be representative the parental magma of the alkaline serie in study but as will be showed there is no genetic relationship with the other samples.The sample MAR-24 also have a mafic composition with high content of Fe 2 O 3 T (9.01 wt%), MgO (2.5 wt%) and CaO (5.66) and have not genetic relationship with the other samples too.
The A/CNK versus A/NK (Al/2Ca+Na+K versus Al/ Na+K molar prop.) by Shand (1943) (Figure 6) was used to discriminate the alumina saturation classes.According to this diagram the most part of the samples (all of the plutonic nepheline syenites and syenites in addition to part of the phonolites) are metaluminous (Al 2 O 3 / Na 2 O+K 2 O > 1), which indicate that had excess of Ca after the alumina cumulate on feldspar, this is agreed with mineral assemblage formed for hornblende and biotite observed in the petrography.A lesser part of the samples (phonolites not included in the metaluminous group) are peralkalines (Al 2 O 3 /Na 2 O+K 2 O < 1), and indicating an excess of alkalis related to alumina and more alkalis that the necessary to make feldspars, as agreed with hornblende present in the mode.Furthermore the sample MAR-19A (nepheline syenite) presents peraluminous character (Al/(2Ca+Na+K)) indicating more alumina than necessary to make feldspar and corroborated by normative corundum (Table 4).The CIPW norm applied these samples show that it is a SiO 2 -undersatured series including a normative assemblage of Or+Ab±An+Ne±Di±Ol (Table 4).This way, metaluminous plutonic nepheline syenites and syenites are anorthite and diopside normatives.On the other hand, peralkaline volcanic phonolite presents sodium metasilicate (Ns) and lack of anorthite in the norm.Harker diagrams were confectioned for major elements K 2 O, Na 2 O, CaO, TiO 2 , P 2 O 5 , Fe 2 O 3 (T), MgO, Al 2 O 3 (Figure 7) and also for the following trace elements Y, Rb, U, Eu, Pb, Ba, Sr and Th (Figure 8).The Figure 7 shows that Al 2 O 3 (R 2 = 0.0744), and TiO 2 (R 2 = 0.0811) content roughly decrease with SiO 2 increase, Fe 2 O 3 presents the same behavior although with better linear correlation (R 2 = 0.1818).MgO (R 2 = 0.0089), CaO (R 2 = 0.0323) and P 2 O 5 (R 2 = 0.0159) roughly increase along with SiO 2 .In fact, the R 2 values (linear correlation) for these oxides indicate dispersion of the data.Still looking in Figure 7, Na 2 O content decrease with SiO 2 increase (R 2 = 0.4653), whereas K 2 O (R 2 = 0.2509) content increase along with SiO 2 .This way, only both Na 2 O and K 2 O presents robust linear correlation.The Figure 8 shows that Rb (R 2 = 0.5697), U (R 2 = 0.1604), Th (R 2 = 0.4104), Y (R 2 = 0.6370) and Pb (R 2 = 0.5426) content decrease with SiO 2 increase.All of these trace elements present robust data according to their R 2 values.On the other hand, Ba (R 2 = 0.1842), Sr (R 2 = 0.1368) and Eu (R 2 = 0.0542) content increase along with SiO 2 with poor linear correlation except for Ba.
Rare earth elements (REE) of the alkaline series were normalized to chondrite Boynton (1984) (Figure 9).This diagram showed that the REE pattern for the most part of the samples have in general a Eu negative anomaly, moreover, there is a bigger fractionating for the light REE (La, Ce, Pr, Nd, Pm, Sm) being more than one hundred times the condritic values related to heavy REE (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu).The samples MAR-28C and MAR-24 (basanite tephrite and phonolitic tephrite respectively) presents unexpectedly high REE content indicating lack of cogeneticity with the alkaline series studied.The high concentration of light REE suggests a source rich in these elements and the Eu negative anomaly suggest the fractionation of plagioclase in the least stages of the crystallization in addition to the fractionating of K-feldspar.The primitive mantle normalized spidergram by Sun & McDonough (1989) of the Figure 10 shows that the comportment of the elements of the Marapicu magma relate to their possibly mantle source.This diagram show a general pattern to which occurs a progressive depletion of Ba, Sr, P, and Ti, on the other hand, the remaining of the more incompatible REE elements are enriched.

RESULTS AND DISCUSSION
The 40 Ar/ 39 Ar dating of the Marapicu intrusion has been carried out by using hornblende grain extracted of the nepheline syenite sample as above mentioned.The hornblende sample yielded a plateau age of 80.46 ± 0.58 Ma (MSWD = 1.08;P = 0.37) including 100% of the 39 Ar released (Figure 11) that we interpret as the cooling age of the hornblende, which should be temporally very close to indistinguishable from its crystallization age.
The K/Ca ratio (derived from the 39 Ar/ 37 Ar ratio) shows the presence of a K-rich component at the lowtemperature steps (Figure 12), but show a relatively constant composition (~0.2) for the medium to high temperature steps, suggesting that the selected crystals were homogenous and did not contain any inclusions.
The data were plotted in an inverse isochron diagram (Figure 13) and gave an age of 80.27 ± 0.62 Ma statistically indistinguishable from the plateau age, and a 40 Ar/ 36 Ar ratio of 326 ± 36, indistinguishable from atmospheric composition.This suggest that no excess 40 Ar* is present in the sample.
The present data suggest that the Marapicu massif form an alkaline series mostly constituted of phonolites and nepheline syenites.This series has SiO 2 -undersatured character and it is observed SiO 2 decreasing along with alkalis (Na 2 O+K 2 O) increasing in the TAS diagram.Binary diagrams from Marapicu massif samples present linear correlations without large compositional variations, which it is indicative of fractional crystallization with or without assimilation.Both Al 2 O 3 and Na 2 O presents higher concentrations in the most evaluated members (nepheline syenites and phonolites) whereas MgO, CaO, TiO 2 and P 2 O 5 shows higher concentrations in syenites.Fe 2 O 3 contents have no significant concentration difference for analyzed lithotypes.The negative linear correlation of Na 2 O is related to the fractionation of the feldspar as a main mineral phase.The major elements presents poor linear correlation with silica (except for Na 2 O and K 2 O) according to their R 2 values which not permit to make reliable insights.
The variation diagrams presented in the Figure 8 shows the negative linear correlation for Rb, Pb, Th, Y and U whereas Ba, Sr and Eu presents data dispersion.U, Th, Pb e Rb are markedly more concentrated in the nepheline syenites and phonolites than in syenites, Y have not so clear behavior.For Sr, Ba and Eu the concentrations are less easily detachable among the lithotypes.The negative linear correlation observed for Rb is related to fractionating of K-feldspar, main modal phase in these rocks, decreasing of Y is coherent with fractionation of apatite which is an accessory phase present in the mode.
Light Rare earth elements are more fractionated than heavy rare earth elements indicating a mantle source enriched in these elements such as Enriched Mantle 1 (EM1), Enriched Mantle 2 (EM2) or Ocean Island Basalts (OIB).The multielement spidergram presented shows depletion for Ba, Sr, P and Ti whereas Th, U, Pb and Zr are enriched.Depletion for Ba is coherent with the fractionation of K-feldspar (also indicated by the Sr depletion), biotite and hornblende.Depletion for P is corroborating by apatite fractionation and Ti for titanite fractionation, all of these phases are presents in the modal assemblage.Both MAR-28C and MAR-24 samples have not genetic relationship with the studied series despite of their high REE content related to the other samples.
Both Sr and Nd isotopic data from literature (Morbidelli et al., 1995;Mota, 2008Mota, , 2012) corroborate a magma origin from an astenospheric mantle.Such isotopic studies also indicating preferentially enriched mantle sources as EM1 or EM2 and the Pb isotopic results suggest an array among Mid-Ocean Ridge Basalt (MORB), OIB and EM2, indicating a mixture of two mantle components.The Pb isotopic data from the Tristan da Cunha Island presents similarities with the Pb isotopic signatures from alkaline intrusions and it is different of the signature observed in the Trindade and Açores ocean islands.
Structural geology and geophysics studies (magnetometry and gravimetry) Mota (2008) indicate that alkaline intrusions have little geometric relationship with the structural features from the basement, where the contacts forming rounded boundaries moreover isotropic texture observed on the samples.On the other hand, the alkaline intrusion presents regional distribution suggesting agreement alignment with the regional structural features from Neoproterozoic Ribeira Orogen and in addition have close relationship with the Cenozoic sediments.This correlation may be genetically interpreted where the extensional stresses which forming the basins also would be that responsible for alkaline magma intrusions.

CONCLUSION
The 40 Ar/ 39 Ar age presented in this work is the older for Poços de Caldas-Cabo Frio magmatic lineament.This fact do not support the idea of age decreasing from west to east in this magmatic province (Herz, 1977;Thomaz Filho & Rodrigues, 1999) based on K-Ar ages.According to this model, the passage of the South American Plate over a hot spot would generate a perfectly age decreasing among the emplaced bodies.Indeed, considering the experimental errors associated with K-Ar ages (3 Ma), support this model.However, new methodologies applied to the alkaline rocks may bring different interpretations and contribute to the knowledge for these magmatic manifestations.This way, more accurate 40 Ar/ 39 Ar dating shows that Marapicu Massif with 80 Ma is older than Poços de Caldas and Itatiaia (K-Ar ages of 74.6 Ma and 73.1 Ma respectively) which would be older considering their geographic location.This data shows how complex is the passage of the hot spot through the continental crust.
The data presented in this work together with those already published lead for two probably models: the first model take account the existence of a mantle plume originated into the asthenosphere from enriched sources (probably of deep subduction of continental crust slabs in previous events).The hot spot ways was recorded on the continental crust, even though being less marked than oceanic crust, due to their greater thickness.Generally, the trajectory of the hot spots are responsible for kimberlitics and lamproitics body generation besides the alkaline complexes and carbonatite.It is in agreement with this model the petrologic, isotopic and geochemistry data, and the 40 Ar/ 39 Ar age.
On the other hand, both isotopic and geochemistry data from Trindade may not enable relate the alkaline rocks from continent with this island rocks.The secondly proposed model, in agreement with the recent 40 Ar/ 39 Ar data, suggest a crustal flexure.In this way, the sediments deposited into the continental shelf trigged a stress so that flexure the continental crust producing deep fractures allowing the entering of the mantellic magmas.Thus, future geochronology works of the alkaline rocks will provide fundamental information to the understanding questions about southeast Brazilian alkaline rocks genesis.

ACKNOWLEDGEMENT
The present research work has been performed under the financial support of Rio de Janeiro State University, Brazil.Part of the fieldwork instruments, office materials, and the resources of the informatics is supported by the FAPERJ (Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro, Carlos Chagas Filho).The chemical analyses were performed by Activation Laboratories Ltd., Ontario, Canada.The 40Ar/39Ar analyses were performed at the Western Australian Argon Isotope Facility at Curtin University.The authors are grateful to the abovementioned institutions.APPENDIx.Geochemistry data of the 27 analyzed samples from Marapicu massif, classification is according to Cox et al. (1979)

Figure 3 .
Figure 3. Characteristics of the facies syenitic: A, B and C) macroscopic aspect of the handle sample; D, E and F) photomicrographs in cross-polarized light.Legends: Af = alkali feldspar; hbl = hornblende.

Figure 4 .
Figure 4. Characteristics of the two phonolite groups, (A) macroscopic aspect of the phonolite of first group, (B) macroscopic aspect of phonolite of second type, (C) photomicrograph them with a strong trachytic texture in cross-polarized light, and (D) photomicrograph them, in cross-polarized light.Legends: Ne = nepheline; Sn = sanidine; Af = alkali feldspar; hbl = hornblende; Mtx = matrix.

Figure 5 .
Figure 5.Total alkalis versus silica chemistry classification diagram by Cox et al. (1979) for all Marapicu samples analized.Triangle represents plutonic nepheline syenites and syenites, filled squares represents phonolites, open square and asterisk represents basic rocks.

Figure 9 .
Figure 9. Rare earth elements plot by Boynton (1984) for syenites and phonolites of the Marapicu massif.Note general pattern with enrichment of light rare earth elements related to heavy rare earth elements and Eu negative anomaly.Triangle represents plutonic nepheline syenites and syenites, filled squares represents phonolites, open square and asterisk represents basic rocks.

Figure 10 .
Figure 10.Spidergram primitive mantle normalized by Sun & McDonough (1989).Note depletion in Ba, Sr, P and Ti and enrichment of Rb, Th, U, Nb and Pb.Triangle represents plutonic nepheline syenites and syenites, filled squares represents phonolites, open square and asterisk represents basic rocks.

Figure 12
Figure 12.K/Ca ratio showing a K-rich component at the low-temperature steps and relatively constant compositions for the medium to high temperature steps.

Figure 13 .
Figure 13.Inverse isochron with step heating data of hornblende providing the age of (80.27 ± 0.62 Ma).
are given at the 1σ level.

Table 2 .
Mass discrimination for each step.

Table 3 .
Ar isotopic data corrected for blank, mass discrimination and radioactive decay. .