Information provided by previous workers shows a considerable diversification of lava types in the early Tertiary deposits of the Kettle River region of British Columbia. The aim of the present study is to establish, on the basis of mineralogical and chemical evidence, whether or not these lavas form a single magma series. A suitable spectrochemical technique was adapted for the determination of Sr, Ba, CaO, MgO, and FeO in rock samples and mineral extracts. A method based on the work of Shaw and Filby, using lanthanum as an internal standard for determination of Sr, Ba, and Ca, is extended to include the determination of MgO and FeO. Extension of the available chemical data was possible using a fused glass bead technique for refractive index determination of the lavas. In view of good correlation between refractive index and the most refractory constituents of the lavas CaO, MgO, and FeO, the refractive index serves as a good magmatic differentiation index. Examination of the refractive indices of three suites of samples, taken from widely separated points within the Kettle River region, shows that the lavas range from semi-basic to acid composition with intermediate types, andesites and trachyandesites, predominating. There is a marked paucity of basaltic rocks. The stratigraphic distribution of the lavas reveals no consistant trends toward either acid or basic compositions; neither is there any relation between stratigraphic position and lava types within the region concerned. In spite of close age and spatial association of the lavas, petrological evidence points to a three-fold division of these rocks (termed 'A', 'B' , and 'C' series). 'A' series is similar to the silica-enrichment 'calc-alkali' trend showing regular mineral and chemical progression from andesites to latite and rhyolite. 'B' series is characterized by a somewhat less well defined group of two-feldspar lavas, trachyandesites, and trachytes. An unusual undersaturated lava, termed 'analcite rhomb-porphyry' or 'shackanite' is thought to be genetically related to 'B' series but probably belongs to a poorly developed semi-basic alkali trend, 'C' series. A mechanism involving crystal fractionation of hornblende, or alternately, augite and plagioclase, is envisaged as the probable mode of origin for many of the rocks of 'A' series. On the other hand, Daly's view that the shackanite lavas were formed as a result of limestone assimilation is supported by field evidence as well as mineral and chemical data. The rocks of 'B' series, the trachyandesites and trachytes, are intermediate in composition to 'A' and 'C' series and have probably formed by process of crystal fractionation in consequence of normal cooling conditions and limestone assimilation. / Thesis / Master of Science (MSc)

No description available.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1982. / MICROFICHE COPY AVAILABLE IN ARCHIVES AND LINDGREN / Bibliography: leaves 393-408. / by Michael Frank Roden. / Ph.D.

The Belchertown Intrusive Complex is a ~164 km2 Devonian pluton that intruded Paleozoic metasedimentary and metavolcanic rocks in west-central Massachusetts. Intrusion of the pluton was synchronous with Acadian deformation (Ashwal, 1974). The complex is concentrically zoned, with a core of orthopyroxene-biotite monzodiorite, a middle zone of clinopyroxene-hornblende-biotite granodiorite, and an outermost zone of hornblende-biotite granodiorite. Zoning from a more to less hydrous mineral suite from the outside to the inside of the pluton led Ashwal (1974) to suggest that metamorphic hydration most strongly affected the outermost zones of the complex. Basaltic inclusions occur most commonly near the edges of the pluton. Many of these inclusions preserve textures suggestive of mafic-felsic magma interaction. The abundance of basaltic enclaves on the edges of the complex may suggest a bowl-shaped structure, with lower more mafic-dominated rocks exposed on the edges of the complex. Bulk rock major element analyses of granitoids, basalts and gabbros, and diorites from all zones of the complex show that intermediate samples, regardless of zone, plot on a mixing line between mafic and felsic end members, supporting a model in which mafic and felsic magmas may have mingled and mixed. Trace element analyses of intermediate-composition samples are similar across the entire complex, with enrichment in large ion lithophile elements and a pronounced Nb trough. In contrast, trace element concentrations in both the mafic rocks (basaltic enclaves and gabbroic inclusions), and in the granitoids, show considerable variation. The diversity of composition within both the mafic and felsic end-members suggests that either fractionation or differing degrees of partial melting of source rocks may account for these compositional ranges. Sanukitoids comprise a majority of the inner zone, and point to the mixing of parental melts at depth in a subduction tectonic regime as a likely model of petrogenesis for the complex.

The Striped Rock granite pluton is an epizonal A-type intrusion emplaced into Grenville age gneisses at 748 Ma. Isotopic, petrographic, and textural evidence indicate that the facies comprising this pluton are differentiates of a homogeneous magrna and not the result of multiple intrusions. Strontium isotopic data preclude the Striped Rock granite from being a partial melt of basement gneisses similar to those presently exposed in the Blue Ridge province. Isotopic data also suggest that it is unlikely that the Striped Rock granite formed by simple differentiation of a mantle derived melt. Of the many models proposed for the petrogenesis of A-type granites the models that are most consistent with the physical and chemical characteristics of Striped Rock granite are those employing interaction between a basaltic magma and an evolved crustal source. Of particular interest, here, is the Inodel proposed by Barker et al. (1975) for the formation of the Pikes Peak batholith. The environment of formation, intensive variables (as interpreted from petrographic studies), mineralogy, and chemistry of the fayalite free granite of Pikes Peak and the Striped Rock granite closely parallel one another, suggesting a similar process for their formation. The "reaction melting" process outlined by Barker et al. (1975) is consistent with both isotopic and limited trace element data from Striped Rock granite. The Striped Rock granite is part of the Late Precambdan Magmatic Province (LPMP). A detailed study of Striped Rock granite has allowed this pluton to be compared and contrasted with other LPMP rocks. This comparison has lead to the recognition of two distinctly different categories of late Precanlbrian felsic intrusives; (i) Crossnore-Lansing type (Crossnore and Lansing granites) and (ii) Striped Rock type (Striped Rock, Beech, and Stewartsville(?) granites). Crossnore-Lansing type granites are characterized by small volumes (as inferred from areal distribution, ~lkm²), high Sr initial ratio, sodic amphibole, and multiple zircon populations. Striped Rock type granites are characterized by relatively large volumes (>65km²), intermediate Sr initial ratio, a single zircon population, and hornblende, where amphibole is present. Recent radiometric age determinations support a temporal relationship between Striped Rock type granites. The age of formation for the Striped Rock granite, determined here, strongly supports the emerging model that late Precambrian magmatism associated with extensional tectonism occurred in at least two pulses. The earlier pulse of magmatism was dominated by felsic intrusives and lasted for 60 My between 760 Ma and 700 Ma. The Second pulse of magmatism was dominated by basaltic magmatism and occurred approximately at 570 Ma. This later magmatic event was probably related to the opening of the Iapetus ocean. / Master of Science

The Alnö Complex is a Late Precambrian alkaline and carbonatite intrusion (c. 30km2) into Early Proterozoic country rock that extends from the north east, to the north western shoulder of Alnö Island. Carbonatites are rare among volcanic provinces, with Oldoinyo Lengai of northern Tanzania being the only active carbonatite volcano in the world today. The high carbonate mineral volumes and rare earth element (REE) concentrations of carbonatites, in combination with the intrusive-extrusive nature of their suites contribute to the rarity of these rocks. Carbonatites, through their peculiar petrological and geochemical compositions, provide vital insights to the composition and condition of the Earth’s mantle. The genesis of the Alnö carbonatites and their relation to other lithological units at the complex is however, only partially understood. This stems from the epistemological division of carbonatites as having either a ‘magmatic’ or ‘reactive’ origin. This study focuses on sampled carbonatites from the Alnö Complex, employing an oxygen and carbon isotope approach on their native calcite, complemented with petrological and mineralogical methods in order to constrain petrogenesis. As a reference, oxygen and carbon isotope data of calcite from an earlier Alnö investigation as well as from an array of data from comparative alkaline complexes elsewhere are also discussed. The combined data and the derived findings support a scenario that is consistent with the ‘magmatic’ model wherein carbonatites have a primary mantle-derived origin, and prospectively stem from a parent magma akin to that of Oldoinyo Lengai, but have experienced a degree of silicate and sedimentary assimilation. The extraction of the Alnö carbonatites for their rare earth metals is a looming possibility due to the current volatility in the rare earth market. The risks and opportunities involved in this kind of natural resource extraction provide a context wherein sustainable development paradigms can be applied. The capacity of the Alnö environment to withstand the impact of development in the mining sector is discussed through a perspective of establishing a quarry, and quarry-related methods for rare earth extraction.

Thesis submitted for the degree MAGISTER SCIENTIAE to the Faculty of Science, Department of Geology, University of the Witwatersrand, Johannesburg / The Koperberg Suite intrusion at Jubilee, Namaqualand varies in composition from anorthosite, through mica diorite to pyroxene leucodiorite and pyroxene diorite. Detailed mapping and petrological studies of these rocks indicate that they were sequentially emplaced into the Concordia Granite country rocks, and that each of the rock types represent discrete magmatic events. The sequence of intrusion is from more acidic to more basic. Whole-rock geochemical analyses indicate that these rocks represent cumulates involving variable proportions of plagioclase, orthopyroxene, mica, quartz, oxides and immiscible sulphides. These cumulate phases intruded into the Concordia Granite at the time of peak of metamorphism and deformation in the Okiep Copper District, i.e, about 1030Ma ago, At the time of intrusion, the country rocks were under going partial melting under high grade (granulite facies) metamorphic conditions. and granitic anatects were present in the crust. Hybridisation of basic magma and granitic melts occurred within the shear zones along which the basic magmas ascended. The En content of orthopyroxene in the Koperberg Suite exceeds the An content of plagioclase. This is atypical of basic intrusions and is a consequence of this mixing. Mixing calculations based on the initial 87Sr/86Srratio (Ru) of the Jubilee samples at 1030Ma, imply high levels of assimilation (as much as 80% assimilation in the case of anorthositic rocks) between a granitic component, similar in composition to the Nababeep Gneiss and a mantle-derived basic magma. Sulphide mineralisation was initiated by the assimilation process, which caused the separation of immiscible sulphides from the hybridised magma. Subsequent oxidation of the original sulphide assemblage produced bornite, chalcopyrite and Ti-poor magnetite. / Andrew Chakane 2019

O maciço alcalino máfico-ultramáfico Ponte Nova (SP-MG) apresenta uma associação litológica eminentemente gabróide, gerada por sucessivos pulsos magmáticos, há aproximadamente 86 Ma. Constitui a única ocorrência de tendência alcalina do setor norte da província Serra do Mar com predomínio acentuado de rochas máficas e ultramáficas cumuláticas. Apresenta duas áreas de exposição: uma principal, maior (~5,5 km2), de forma elíptica e com grande variedade de litotipos, e uma menor (~1 km2), localizada a sul da primeira, estando ambas separadas por rochas do embasamento Pré-Cambriano. Na área principal, o pulso central é constituido de uma seqüência inferior, cumulática, caracterizada pela presença de cumulatos ultramáficos e melagábricos (e.g., olivina clinopiroxenitos e melagabros com olivina), e uma seqüência superior, com rochas gábricas e monzogábricas porfiríticas e equigranulares. Tais seqüências associadas a um mesmo pulso são confirmada pelas variações crípticas em minerais, pela composição geoquímica das rochas e pelas assinaturas isotópicas obtida. À região oeste e sul deste pulso central encontra-se, separada por falhas, uma seqüência inferior muito semelhante, cumulática porém com a seqüência superior caracterizada principalmente pela ocorrência de rochas bandadas e com maior concentração de nefelina em relação às rochas da área central. Estas ocorrências parecem estar relacionadas a um segundo pulso associado à esta área principal, conforme indicado pela evolução da seqüência superior, pelas assinaturas isotópicas e condições de fO2 calculadas e por variações crípticas encontradas em alguns minerais das rochas bandadas, como olivina. Já na área satélite a sul, predominam melamonzonitos com nefelina que, embora permitam algumas correlações com as rochas da seqüência superior do pulso central, o enriquecimento em diferentes traços bem como a assinatura isotópica apontam para um pulso magmático isolado. Esta área ainda apresenta litotipos mais evoluídos (e.g., monzonitos com nefelina) que, conforme as características isotópicas e também a distribuição dos traços, permite individualizá-lo como um pulso separado. Outros pulsos isolados e menores são caracterizados, predominando rochas melagábricas, variando entre olivina melamonzodioritos a melamonzodioritos com olivina no pulso satélite norte e rochas mais evoluídas, variando entre nefelina monzodioritos a monzodioritos com nefelina, no pulso satélite leste. Diferenciados félsicos finais ocorrem sob a forma de diques, vênulas e possivelmente bolsões, e variam de leucocráticos a mesocráticos, com rochas de composições monzoníticas a monzossieníticas, chegando a nefelina sienitos em alguns casos, e podem ser considerados representativas do líquido residual dos diferentes pulsos que sofreram migração para diferentes porções do maciço. Um corpo de brecha magmática ocorre confinado à região leste, posterior aos pulsos anteriormente descritos, com fragmentos líticos de todos os litotipos gabróides anteriormente descritos. Diques máficos (lamprófiros, tefritos e basanitos) e félsicos (tefrifonólitos a fonotefritos) intrudem as rochas do maciço, sendo estes representativos de diferentes fontes mantélicas e possivelmente ocorrendo em estágios distintos. As diferentes assinaturas isotópicas registradas para os diques, que abrangem o amplo espectro obtido para os diferentes pulsos do maciço, confirmam o caráter multi-intrusivo desta ocorrência. O caráter cumulático é bastante pronunciado nos principais pulsos do maciço Ponte Nova. Os altos índices de máficos (M), os baixos conteúdos de Na e K, o caráter ultrabásico e a composição de picrito e picrobasalto de parte das amostras evidenciam este caráter e apontam a cristalização fracionada como principal mecanismo atuante na evolução do maciço. A variação composicional das fases cumulus ao longo de todo maciço, especialmente relacionada aos índices envolvendo a razão Mg/(Mg+Fe2+) tanto na olivina quanto no clinopiroxênio, com a progressiva diminuição deste índice em direção às rochas da seqüência superior, indicam que mecanismos de fracionamento magmático dominam a variação vertical modal e geoquímica do maciço em seus principais pulsos. Variações crípticas obtidas também em fases intercumulus, compatíveis com o trend evolutivo dos minerais cumulus, favorecem a idéia de estas fases serem representativas principalmente de um líquido aprisionado (trapped liquid) no momento da acumulação, guardando portanto a composição do líquido em equilíbrio com o cumulato formado. Assim, infere-se que o processo de acumulação envolvido, com conseqüente aprisionamento de líquido, deve ter-se dado de maneira relativamente rápida. Tal consideração tende a indicar um processo gravitacional de acumulação para grande parte das rochas do maciço. Já os casos onde são encontradas estruturas bandadas, alternando-se bandas máficas e félsicas (associadas a regiões próximas ao contato com o embasamento), apontam para uma possível ação mais efetiva de correntes de convecção. Com relação aos parâmetros intensivos, as rochas do maciço Ponte Nova cristalizaram-se a uma profundidade relativamente rasa (entre 1 e 0,5 kbar), conforme indicado pela composição dos clinopiroxênios. A história de cristalização do maciço inicia-se algo acima de 1030ºC, que representa o início do equilíbrio olivinaclinopiroxênio, terminando em ±600º C, com o equilíbrio apatita-biotita (fases intersticiais finais). Conforme modelamentos geoquímicos evidenciam, os diques máficos junto ao Maciço Ponte Nova e os que são encaixados no embasamento adjacente a este, de composição principalmente basanito-tefrítica, podem ser considerados representativos do magma parental que levou à formação das rochas cumuláticas do maciço. Modelos de fusão indicam que os diques máficos que cortam o maciço e, conseqüentemente, o magma parental do Maciço Ponte Nova, podem ter como fonte mantélica tanto espinélio lherzolitos como granada lherzolitos. Em ambos os casos o manto deve estar previamente enriquecido em elementos traços. A este enriquecimento é atribuido como causa o metassomatismo mantélico. As assinaturas isotópicas encontradas para os diferentes litotipos do maciço Ponte Nova pressupõe uma fonte mantélica heterogênea, sendo representativas dos diferentes graus de enriquecimento do manto litosférico. As idades modelo (TDM) obtidas, que podem ser atribuídas aos períodos de enriquecimento metassomático do manto, são correlacionáveis com os eventos regionais de evolução crustal neoproterozóica, principalmente ligados a eventos de subducção. As evidências significativas das heterogeneidades mantélicas (tanto em escala regional quanto numa escala local) com assinaturas isotópicas tipicamente litosféricas, do enriquecimento geoquímico da fonte (indicando um metassomatismo mantélico e uma fonte rica em voláteis) e do claro controle tectônico dos pulsos alcalinos (associados à reativação das principais zonas de fraqueza regionais), tendenciam uma interpretação favorável a modelos relacionados principalmente a fenômenos litosféricos, se comparadas aos modelos envolvendo plumas mantélicas. / The Ponte Nova alkaline mafic-ultramafic massif (~85 Ma) is mainly composed of a gabbroic association, generated by successive magmatic pulses. It is the single alkaline massif of the northern sector of Serra do Mar Province with predominance of mafic and ultramafic cumulitic rocks. The Ponte Nova massif crops out in two areas: the larger one (~5.5 km2), with elliptical shape and a wide variety of lithotypes, and the smaller satellite area (~1 km2), located south of the main area. These are separated by outcrops of Precambrian basement. The central pulse of the larger area is composed by a lower sequence, cumulitic, characterized by the presence of ultramafic and melagabbroic cumulates (e.g., olivine clinopyroxenites and olivine-bearing melagabbros), and an upper sequence, with porphyritic and equigranular gabbroic and monzogabbroic rocks. Such sequences are associated with the same magmatic pulse, as indicated by cryptic variations in minerals, whole-rock geochemistry and isotopic signatures. At the western and southern adjoining regions of this central pulse, separated by faults, a similar cumulitic lower sequence crops out. However, the upper sequence of these regions is characterized by the occurrence of banded rocks with higher concentration of nepheline than in the central area. These occurrences seem to be related to a second magmatic pulse, as indicated by evolution of its upper sequence, by isotopic signatures, calculated fO2 conditions and cryptic variations in some minerals of the banded rocks, such as olivine. In the southern satellite area, nepheline-bearing melamonzonites are the predominant rocks. Although correlations with rocks of central pulse upper sequence can be established, the enrichment in several trace elements as well as its isotopic signatures point to an isolated magmatic pulse. This area also presents more evolved lithotypes (e.g., nepheline-bearing monzonites) that, as indicated by isotopic characteristics and the distribution of the trace elements, could be interpreted as a distinct pulse. There are other isolated and smaller pulses in the larger area. Melagabbroic rocks varying between olivine melamonzodiorites to olivine-bearing melamonzodiorites are found in a northern satellite pulse. More evolved rocks varying between nepheline monzodiorites and nepheline-bearing monzodiorites are found in an eastern satellite body. Late-stage felsic rocks occur as dykes, venules and patches, and vary from leucocratic to mesocratic rocks, monzonitic to monzosyenitic in composition (nepheline syenites in some cases). These rocks are possibly representative of residual liquids that had suffered migration for different portions of the massif. A magmatic breccia occurs in the eastern region of the main area, subsequent to the described pulses, with the previously described lithic fragments of all gabbroic lithotypes. Mafic (lamprophyres, tephrites, basanites) and felsic (tephriphonolites to phonotephrites) dykes intrude the massif rocks. These are representative of different mantle sources and possibly occur in distinct magmatic stages. The wide-range isotopic signatures of these dykes, that comprise the wide range obtained for the different pulses of the massif, confirm the multi-intrusive character of this occurrence. The cumulitic character is strongly characterized in the main pulses of the Ponte Nova massif. The high mafic index (M), the low Na and K contents, the ultrabasic character and the composition of picrite and picribasalt of part of the samples evidence this character and point to fractional crystallization as the main operating mechanism in the evolution of the massif. The compositional variation of the cumulus phases throughout all the massif, particularly in terms of Mg/ (Mg+Fe2+) ratios, either in olivine or clinopyroxene, with the gradual reduction of this index towards the upper sequence, indicates that magmatic fractionation dominates the modal and geochemical vertical variation of the massif in its main pulses. Cryptic variations obtained also in intercumulus phases, compatible with evolutive trend of cumulus minerals, suggest that these phases represent a trapped liquid at the moment of the accumulation, and the composition of liquid and cumulate were in equilibrium. Thus, it may be inferred that the process of accumulation must have been relatively fast, indicating a gravitational process of accumulation for most rocks of the massif. The banded structures near the contact with the basement, alternating mafic and felsic banding, suggest a more effective action of convection currents. The Ponte Nova massif crystallized at relatively low depth (between 1 and 0,5 kbar), as indicated by clinopyroxene compositions. The massif crystallization sequence begins above 1030ºC, representing the beginning of the olivine-clinopyroxene equilibrium, and did proceed until ±600oC, with the apatite-biotite equilibrium (final interstitial phases). The mafic dykes intruding the Ponte Nova massif and those in the adjacent basement, mainly of basanitictephritic composition, possibly represent the parental magma of the cumulitic rocks of the massif, as indicated by geochemical models. The Ponte Nova massif isotopic signatures of the different lithotypes indicate a heterogeneous mantle source, with variable degrees of lithospheric mantle enrichment. Model ages (TDM) can be attributed to periods of mantle metassomatic enrichment and are correlated with the regional events of Neoproterozoic crustal evolution, mainly related to subduction events. The significative evidences of mantle heterogeneities (both at regional and local scale) with typically lithospheric isotopic signatures, of geochemical source enrichment (indicative of mantle metassomatism and a volatile-rich source) and of clearly tectonic control of the alkaline pulses (associated to the reactivation of the main regional zones of weakness), led to a favorable interpretation of models mainly related to lithospheric phenomena, if compared with models involving mantle plumes.

O maciço alcalino máfico-ultramáfico Ponte Nova (SP-MG) apresenta uma associação litológica eminentemente gabróide, gerada por sucessivos pulsos magmáticos, há aproximadamente 86 Ma. Constitui a única ocorrência de tendência alcalina do setor norte da província Serra do Mar com predomínio acentuado de rochas máficas e ultramáficas cumuláticas. Apresenta duas áreas de exposição: uma principal, maior (~5,5 km2), de forma elíptica e com grande variedade de litotipos, e uma menor (~1 km2), localizada a sul da primeira, estando ambas separadas por rochas do embasamento Pré-Cambriano. Na área principal, o pulso central é constituido de uma seqüência inferior, cumulática, caracterizada pela presença de cumulatos ultramáficos e melagábricos (e.g., olivina clinopiroxenitos e melagabros com olivina), e uma seqüência superior, com rochas gábricas e monzogábricas porfiríticas e equigranulares. Tais seqüências associadas a um mesmo pulso são confirmada pelas variações crípticas em minerais, pela composição geoquímica das rochas e pelas assinaturas isotópicas obtida. À região oeste e sul deste pulso central encontra-se, separada por falhas, uma seqüência inferior muito semelhante, cumulática porém com a seqüência superior caracterizada principalmente pela ocorrência de rochas bandadas e com maior concentração de nefelina em relação às rochas da área central. Estas ocorrências parecem estar relacionadas a um segundo pulso associado à esta área principal, conforme indicado pela evolução da seqüência superior, pelas assinaturas isotópicas e condições de fO2 calculadas e por variações crípticas encontradas em alguns minerais das rochas bandadas, como olivina. Já na área satélite a sul, predominam melamonzonitos com nefelina que, embora permitam algumas correlações com as rochas da seqüência superior do pulso central, o enriquecimento em diferentes traços bem como a assinatura isotópica apontam para um pulso magmático isolado. Esta área ainda apresenta litotipos mais evoluídos (e.g., monzonitos com nefelina) que, conforme as características isotópicas e também a distribuição dos traços, permite individualizá-lo como um pulso separado. Outros pulsos isolados e menores são caracterizados, predominando rochas melagábricas, variando entre olivina melamonzodioritos a melamonzodioritos com olivina no pulso satélite norte e rochas mais evoluídas, variando entre nefelina monzodioritos a monzodioritos com nefelina, no pulso satélite leste. Diferenciados félsicos finais ocorrem sob a forma de diques, vênulas e possivelmente bolsões, e variam de leucocráticos a mesocráticos, com rochas de composições monzoníticas a monzossieníticas, chegando a nefelina sienitos em alguns casos, e podem ser considerados representativas do líquido residual dos diferentes pulsos que sofreram migração para diferentes porções do maciço. Um corpo de brecha magmática ocorre confinado à região leste, posterior aos pulsos anteriormente descritos, com fragmentos líticos de todos os litotipos gabróides anteriormente descritos. Diques máficos (lamprófiros, tefritos e basanitos) e félsicos (tefrifonólitos a fonotefritos) intrudem as rochas do maciço, sendo estes representativos de diferentes fontes mantélicas e possivelmente ocorrendo em estágios distintos. As diferentes assinaturas isotópicas registradas para os diques, que abrangem o amplo espectro obtido para os diferentes pulsos do maciço, confirmam o caráter multi-intrusivo desta ocorrência. O caráter cumulático é bastante pronunciado nos principais pulsos do maciço Ponte Nova. Os altos índices de máficos (M), os baixos conteúdos de Na e K, o caráter ultrabásico e a composição de picrito e picrobasalto de parte das amostras evidenciam este caráter e apontam a cristalização fracionada como principal mecanismo atuante na evolução do maciço. A variação composicional das fases cumulus ao longo de todo maciço, especialmente relacionada aos índices envolvendo a razão Mg/(Mg+Fe2+) tanto na olivina quanto no clinopiroxênio, com a progressiva diminuição deste índice em direção às rochas da seqüência superior, indicam que mecanismos de fracionamento magmático dominam a variação vertical modal e geoquímica do maciço em seus principais pulsos. Variações crípticas obtidas também em fases intercumulus, compatíveis com o trend evolutivo dos minerais cumulus, favorecem a idéia de estas fases serem representativas principalmente de um líquido aprisionado (trapped liquid) no momento da acumulação, guardando portanto a composição do líquido em equilíbrio com o cumulato formado. Assim, infere-se que o processo de acumulação envolvido, com conseqüente aprisionamento de líquido, deve ter-se dado de maneira relativamente rápida. Tal consideração tende a indicar um processo gravitacional de acumulação para grande parte das rochas do maciço. Já os casos onde são encontradas estruturas bandadas, alternando-se bandas máficas e félsicas (associadas a regiões próximas ao contato com o embasamento), apontam para uma possível ação mais efetiva de correntes de convecção. Com relação aos parâmetros intensivos, as rochas do maciço Ponte Nova cristalizaram-se a uma profundidade relativamente rasa (entre 1 e 0,5 kbar), conforme indicado pela composição dos clinopiroxênios. A história de cristalização do maciço inicia-se algo acima de 1030ºC, que representa o início do equilíbrio olivinaclinopiroxênio, terminando em ±600º C, com o equilíbrio apatita-biotita (fases intersticiais finais). Conforme modelamentos geoquímicos evidenciam, os diques máficos junto ao Maciço Ponte Nova e os que são encaixados no embasamento adjacente a este, de composição principalmente basanito-tefrítica, podem ser considerados representativos do magma parental que levou à formação das rochas cumuláticas do maciço. Modelos de fusão indicam que os diques máficos que cortam o maciço e, conseqüentemente, o magma parental do Maciço Ponte Nova, podem ter como fonte mantélica tanto espinélio lherzolitos como granada lherzolitos. Em ambos os casos o manto deve estar previamente enriquecido em elementos traços. A este enriquecimento é atribuido como causa o metassomatismo mantélico. As assinaturas isotópicas encontradas para os diferentes litotipos do maciço Ponte Nova pressupõe uma fonte mantélica heterogênea, sendo representativas dos diferentes graus de enriquecimento do manto litosférico. As idades modelo (TDM) obtidas, que podem ser atribuídas aos períodos de enriquecimento metassomático do manto, são correlacionáveis com os eventos regionais de evolução crustal neoproterozóica, principalmente ligados a eventos de subducção. As evidências significativas das heterogeneidades mantélicas (tanto em escala regional quanto numa escala local) com assinaturas isotópicas tipicamente litosféricas, do enriquecimento geoquímico da fonte (indicando um metassomatismo mantélico e uma fonte rica em voláteis) e do claro controle tectônico dos pulsos alcalinos (associados à reativação das principais zonas de fraqueza regionais), tendenciam uma interpretação favorável a modelos relacionados principalmente a fenômenos litosféricos, se comparadas aos modelos envolvendo plumas mantélicas. / The Ponte Nova alkaline mafic-ultramafic massif (~85 Ma) is mainly composed of a gabbroic association, generated by successive magmatic pulses. It is the single alkaline massif of the northern sector of Serra do Mar Province with predominance of mafic and ultramafic cumulitic rocks. The Ponte Nova massif crops out in two areas: the larger one (~5.5 km2), with elliptical shape and a wide variety of lithotypes, and the smaller satellite area (~1 km2), located south of the main area. These are separated by outcrops of Precambrian basement. The central pulse of the larger area is composed by a lower sequence, cumulitic, characterized by the presence of ultramafic and melagabbroic cumulates (e.g., olivine clinopyroxenites and olivine-bearing melagabbros), and an upper sequence, with porphyritic and equigranular gabbroic and monzogabbroic rocks. Such sequences are associated with the same magmatic pulse, as indicated by cryptic variations in minerals, whole-rock geochemistry and isotopic signatures. At the western and southern adjoining regions of this central pulse, separated by faults, a similar cumulitic lower sequence crops out. However, the upper sequence of these regions is characterized by the occurrence of banded rocks with higher concentration of nepheline than in the central area. These occurrences seem to be related to a second magmatic pulse, as indicated by evolution of its upper sequence, by isotopic signatures, calculated fO2 conditions and cryptic variations in some minerals of the banded rocks, such as olivine. In the southern satellite area, nepheline-bearing melamonzonites are the predominant rocks. Although correlations with rocks of central pulse upper sequence can be established, the enrichment in several trace elements as well as its isotopic signatures point to an isolated magmatic pulse. This area also presents more evolved lithotypes (e.g., nepheline-bearing monzonites) that, as indicated by isotopic characteristics and the distribution of the trace elements, could be interpreted as a distinct pulse. There are other isolated and smaller pulses in the larger area. Melagabbroic rocks varying between olivine melamonzodiorites to olivine-bearing melamonzodiorites are found in a northern satellite pulse. More evolved rocks varying between nepheline monzodiorites and nepheline-bearing monzodiorites are found in an eastern satellite body. Late-stage felsic rocks occur as dykes, venules and patches, and vary from leucocratic to mesocratic rocks, monzonitic to monzosyenitic in composition (nepheline syenites in some cases). These rocks are possibly representative of residual liquids that had suffered migration for different portions of the massif. A magmatic breccia occurs in the eastern region of the main area, subsequent to the described pulses, with the previously described lithic fragments of all gabbroic lithotypes. Mafic (lamprophyres, tephrites, basanites) and felsic (tephriphonolites to phonotephrites) dykes intrude the massif rocks. These are representative of different mantle sources and possibly occur in distinct magmatic stages. The wide-range isotopic signatures of these dykes, that comprise the wide range obtained for the different pulses of the massif, confirm the multi-intrusive character of this occurrence. The cumulitic character is strongly characterized in the main pulses of the Ponte Nova massif. The high mafic index (M), the low Na and K contents, the ultrabasic character and the composition of picrite and picribasalt of part of the samples evidence this character and point to fractional crystallization as the main operating mechanism in the evolution of the massif. The compositional variation of the cumulus phases throughout all the massif, particularly in terms of Mg/ (Mg+Fe2+) ratios, either in olivine or clinopyroxene, with the gradual reduction of this index towards the upper sequence, indicates that magmatic fractionation dominates the modal and geochemical vertical variation of the massif in its main pulses. Cryptic variations obtained also in intercumulus phases, compatible with evolutive trend of cumulus minerals, suggest that these phases represent a trapped liquid at the moment of the accumulation, and the composition of liquid and cumulate were in equilibrium. Thus, it may be inferred that the process of accumulation must have been relatively fast, indicating a gravitational process of accumulation for most rocks of the massif. The banded structures near the contact with the basement, alternating mafic and felsic banding, suggest a more effective action of convection currents. The Ponte Nova massif crystallized at relatively low depth (between 1 and 0,5 kbar), as indicated by clinopyroxene compositions. The massif crystallization sequence begins above 1030ºC, representing the beginning of the olivine-clinopyroxene equilibrium, and did proceed until ±600oC, with the apatite-biotite equilibrium (final interstitial phases). The mafic dykes intruding the Ponte Nova massif and those in the adjacent basement, mainly of basanitictephritic composition, possibly represent the parental magma of the cumulitic rocks of the massif, as indicated by geochemical models. The Ponte Nova massif isotopic signatures of the different lithotypes indicate a heterogeneous mantle source, with variable degrees of lithospheric mantle enrichment. Model ages (TDM) can be attributed to periods of mantle metassomatic enrichment and are correlated with the regional events of Neoproterozoic crustal evolution, mainly related to subduction events. The significative evidences of mantle heterogeneities (both at regional and local scale) with typically lithospheric isotopic signatures, of geochemical source enrichment (indicative of mantle metassomatism and a volatile-rich source) and of clearly tectonic control of the alkaline pulses (associated to the reactivation of the main regional zones of weakness), led to a favorable interpretation of models mainly related to lithospheric phenomena, if compared with models involving mantle plumes.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / O tema abordado nesta dissertação de mestrado é a petrogênese dos diabásios do Eocretáceo que integram o Enxame de Diques da Serra do Mar na região de Angra dos Reis (RJ). Estes diques intrudem rochas do Terreno Ocidental e do Arco Magmático Rio Negro, inserindo-se no contexto geodinâmico de fragmentação do Gondwana no Cretáceo Inferior. Essas intrusões são subverticais e orientam-se preferencialmente a N039E. Intrusões retilíneas são as mais observadas em campo. Os diabásios são compostos, essencialmente, por plagioclásio e clinopiroxênios (augita e pigeonita). Quartzo, minerais opacos, apatita e olivina ocorrem como minerais acessórios. A composição mineral secundária é marcada pela presença de biotita, uralita e saussurita. Os diabásios estudados compreendem uma série transicional com afinidade toleítica. Os estudos petrológicos revelam a existência de cinco suítes de alto-TiO2, tendo sido as mesmas discriminadas com base nas razões La/Yb de amostras com o mesmo teor de MgO. Destas, apenas a suíte 2 têm quantidade mínima de amostras para avaliação de processos evolutivos. O resultado da modelagem petrogenética aponta que o processo evolutivo mais provável para a geração dos diabásios da suíte 2 é cristalização fracionada sem mudança na assembleia fracionante. As amostras representativas dos líquidos parentais das cinco suítes apresentam razões La/Nb e La/Yb maiores que a unidade, típicas de derivação de fonte enriquecida, que, em várias províncias basálticas continentais, tem sido associada à fusão exclusiva do manto litosférico subcontinental ou a sua participação em processos de mistura de fontes. A discriminação de fontes mantélicas foi feita exclusivamente com base nos teores de elementos traços de amostras representativas de líquidos parentais das cinco suítes discriminadas na área de estudo. Os modelos indicam que todas as cinco suítes estudadas estão relacionadas a fontes harzburgíticas com granada residual. A suíte 5 da área de estudo não pode ser associada à mesma fonte mantélica geradora das suítes 1, 2, 3 e 4 que, por sua vez, se originaram por diferentes quantidades de fusão parcial (respectivamente, 44%, 35%, 31% e 25%) de uma mesma fonte. Cabe ressaltar que a ocorrência de duas suítes, com razões La/Yb muito distintas, porém geograficamente muito próximas, sugere que processos de heterogeneidade mantélica, tipicamente laterais, devem também ocorrer verticalmente. / The petrogenesis of Early Cretaceous dolerites within the Serra do Mar Dyke Swarm in the region of Angra dos Reis (RJ) is presented in this dissertation. The dykes are associated with the Gondwana breakup and intrude rocks of the so-called Occidental Terrane and the Rio Negro Magmatic Arc within the Late Proterozoic-Ordovician Ribeira Orogen. Tipically, the intrusions are subvertical bearing a N039E preferential trend with sharp, linear contacts with their country rocks. The dolerites are essentially composed of plagioclase and clinopyroxene (augite and pigeonita). Quartz, opaque minerals, apatite and olivine occur as accessory minerals. The secondary phases are biotite, eterog and eterogene. The dolerites constitute a transitional series with tholeiitic affinity. Whole-rock geochemical data reveal the existence of five high-TiO2 suites which were discriminated on the basis of La/Yb ratios for samples with similar MgO contents. Petrogenesis analysis were done for suite 2 only due to scarcity of samples in the other four suites. As such, petrogenetic interpretation and modeling points to fractional crystallization as the likely evolutionary process associated with suite 2 dolerites. Samples representing parental liquids of five the high-TiO2 suites show values of La/Nb and La/Yb ratios greater than unity which points to at least a contribution of the subcontinental lithospheric mantle (SCLM) for their genesis. Nevertheless, source mixing processes may have operated since the marked composition of the SCLM (as the Nb depletion, for instance) tends to obliterate most of the geochemical characteristics of other mantle sources. Petrogenetic eteroge indicates that the all five studied suites are related to harzbugitic sources with residual garnet pointing to a thick SCLM by the time of the Gondwana breakup. Suite 5 cannot be associated with the same mantle source that generated suites 1, 2, 3, and 4 which in turn can be related to different amounts of partial melting from a common mantle source (respectively, 44%, 35%, 31% and 25%). As all five suites are close related in space, processes involving vertical chemical eterogeneities within the SCLM as opposed to lateral ones should be considered when putting constraints on geodynamic models for the Gondwana breakup.