O maciço alcalino máfi co-ultramáfi co Ponte Nova, localizado no setor norte da Província Serra do Mar, junto à porção oriental da Serra da Mantiqueira, na região Sudeste do Brasil, encontra-se encaixado em granitoides e gnaisses migmatíticos Pré-Cambrianos. O maciço apresenta duas áreas distintas de exposição, estando ambas separadas por rochas do embasamento: uma maior (~5,5 km 2), de forma elíptica e com grande variedade de litotipos; outra menor (~1 km 2), de forma irregular e petrografi camente menos complexa, e situada a sul da primeira. É predominantemente composto por uma associação gabroide alcalina, gerada por sucessivos pulsos magmáticos. As rochas mais abundantes são cumulatos máfi cos e ultramáfi cos (e.g., olivina clinopiroxenitos e melagabros com olivina) presentes nos níveis inferiores do maciço, ao lado de rochas gábricas e monzogábricas porfi ríticas, equigranulares e bandadas nos níveis superiores, todos representativos da evolução magmática dos principais pulsos. Um corpo de brecha magmática e uma suíte de diques (lamprófi ros a tefrifonólitos) interceptam todas as rochas intrusivas. Análises geocronológicas por K/Ar para diferentes rochas indicam uma idade média de 87,6 Ma (Cretáceo Superior), que é similar às obtidas para as demais ocorrências do setor norte da referida província. A incompatibilidade das idades torna difícil sustentar a hipótese de sua associação genética com a pluma mantélica de Trindade.

ABSTRACT:The question of whether the antecryst assemblage affects the bulk composition of lamprophyre dykes, and could be responsible for the compositional zonation between their centers and borders is addressed through a detailed study involving four monchiquite and camptonite dykes (basanites and tephrites) representative of the Arco de Ponta Grossa and Serra do Mar alkaline provinces. In them, antecrysts are interpreted as early-crystallized minerals that are no longer in equilibrium with their host-liquid, albeit still linked to the same magmatic system. They represent recycled crystals of earlier stages of the magmatic system at depth. The antecryst microtextures, such as zoned clinopyroxene megacrysts (augite cores and titanaugite rims) with partly corroded cores, olivine crystals with corroded rims surrounded by biotite coronas, chrome-spinel inclusions in clinopyroxene and olivine megacryst cores, and titanomagnetite crystals surrounded by biotite coronas, suggest chemical re-equilibrium with the matrix. The greatest antecryst cargo in these dykes is found in their centers. After subtracting the antecryst volume from the center analyses of each body, the calculated compositions are very similar to the border analyses. The mafic antecryst cargo of each occurrence proportionally leads to enrichment of MgO, FeO, TiO 2 , CaO, compatible trace elements (Cr, Ni and Co), and depletion of SiO 2 , K 2 O, Na 2 O, Al 2 O 3 and incompatible trace elements (Ba, Sr and REE). The whole-rock geochemical analyses of each dyke represent the combination of accumulated crystals and melt. The compositional zonation of the studied dykes is associated with the antecryst cargo rather than different magmatic pulses. KEYWORDS:Antecrysts; Alkaline lamprophyres; Petrology; Geochemistry. RESUMO: A questão de como a assembleia de antecristais influencia a composição química de diques de lamprófiro e se esta pode ser responsável pelo zoneamento composicional entre núcleos e bordas é abordada por meio de um estudo detalhado em quatro diques de monchiquito e camptonito, representativos das províncias alcalinas Arco de Ponta Grossa e Serra do Mar. Nestes diques, os antecristais são interpretados como minerais cristalizados precocemente que não se encontram mais em equi-

ABSTRACT:The Late Cretaceous Búzios Island alkaline massif intrudes Precambrian charnockites and consists dominantly of syenitic rocks that are cut by a large number of dikes, mostly NE-trending, and representing two distinct suites, a felsic one and a mafic-ultramafic one. Alkali feldspar is the most abundant mineral; other constituents are clinopyroxene, commonly replaced by amphibole/ biotite, and opaques. Accessory minerals include occasionally rare phases bearing Zr, Ti, Nb and Rare Earth Elements (REE). The felsic dikes may also have nepheline (sodalite). The mafic-ultramafic suite, in particular the lamprophyres, shows a primary mineral assemblage with olivine, clinopyroxene and amphibole in addition to a groundmass having glassy material and carbonates (ocelli). The Búzios rocks are chemically evolved, mostly of potassic affinity and mainly belong to the miaskitic series. Variation diagrams for major and trace elements show a bimodal distribution, suggesting an origin from different magmatic pulses. The rocks are interpreted as having been derived by fractional crystallization processes from a basanitic parental magma. The SiO 2 -undersaturated and SiO 2 -oversaturated associations present in the massif are apparently not linked to a single magmatic source, and in the petrogeny residual system two trends are evident: the first one towards the phonolitic minimum and the second one towards the rhyolitic minimum, possibly pointing to amphibole fractionation.KEYWORDS: Igneous petrology; Alkaline rocks; Geochemistry. RESUMO: O maciço alcalino da Ilha dos Búzios, do Cretáceo Superior, é intrusivo em charnockitos precambrianos e consiste predominantemente de rochas sieníticas cortadas por grande número de diques, orientados preferencialmente para NE, representando duas suítes distintas: félsica e máfica-ultramáfica. Feldspato alcalino é o mineral mais abundante; outros constituintes são clinopiroxênio, comumente substituído por anfibólio/biotita, e opacos. Minerais acessórios incluem ocasionalmente fases raras contendo Zr, Ti, Nb e elementos terras raras (ETR). Os diques félsicos podem possuir também nefelina (sodalita). A suíte máfica-ultramáfica, particularmente os lamprófiros, apresenta uma assembleia mineralógica primária com olivina, clinopiroxênio e anfibólio, além de massa fundamental com material vítreo e carbonatos (ocelli

Three series of alkaline dikes are characterized in a restricted area of the Mantiqueira Range: a weakly silica-undersaturated series (normative ne < 7), of potassic character, represented by basanite, tephrites, alkali basalts, basaltic trachyandesites, phonotephrites, tephriphonolites, and trachyandesites, an intermediate series (7 > normative ne > 13), of potassic character, consisting only of basanites and tephrites, and a strongly silica-undersaturated series (normative ne > 13), of sodic character, consisting of basanites, tephrites, phonotephrites, and phonolites. Ar-Ar data for biotite from a tephriphonolite dike yielded an average age of 85.4±0.4 Ma. All three series show similar evolution trends controlled mainly by early-fractionation of olivine, spinel and clinopyroxene and, secondly, fractionation of plagioclase and apatite. Different degrees of crustal assimilation are suggested for each series. Assimilation processes are responsible for the wide 87 Sr/ 86 Sr i (0.70447-0.71172) and 143 Nd/ 144 Nd i (0.51237-0.51244) ranges, for the increased K 2 O/Na 2 O, Ba/Sr and Rb/Sr ratios, and for the reduced Ce/Pb ratios. In general, a significant contribution from partially melted host rocks is associated to the weakly silica-undersaturated potassic series. A phlogopite-bearing carbonate-metasomatized heterogeneous peridotite source accounts for the main chemical variations in the least contaminated samples of each series. Geochemical signatures of the strongly sodic alkaline series indicate a greater contribution from carbonated pyroxene-rich veins in the mantle source.