Metamorfismo ercinico di bassa-pressione: evoluzione tettonico-metamorfica del complesso di mandatoriccio (massiccio della Sila - Calabria)

Low-pressure/high-temperature (LP/HT) metamorphic belts are characterised by rocks that experienced abnormal heat flow in shallow crustal levels (T > 600 °C; P < 4 kbar) resulting in anomalous geothermal gradients (60-150 °C/km). The abnormal amount of heat has been related to crustal underplating of mantle-derived basic magmas or to thermal perturbation linked to intrusion of large volumes of granitoids in the intermediate crust. In particular, in this latter context, magmatic or aqueous fluids are able to transport relevant amounts of heat by advection, thus favouring regional LP/HT metamorphism. However, the thermal perturbation consequent to heat released by cooling magmas is responsible also for contact metamorphic effects. A first problem is that time and space relationships between regional LP/HT metamorphism and contact metamorphism are usually unclear. A second problem is related to the high temperature conditions reached at different crustal levels. These, in some cases, can completely erase the previous metamorphic history. Notwithstanding this problem is very marked in lower crustal levels, petrologic and geochronologic studies usually concentrate in these attractive portions of the crust. However, only in the intermediate/upper-crustal levels of a LP/HT metamorphic belt the tectono-metamorphic events preceding the temperature peak, usually not preserved in the lower crustal portions, can be readily unravelled. The Hercynian Orogen of Western Europe is a well-documented example of a continental collision zone with widespread LP/HT metamorphism, intense crustal anatexis and granite magmatism. Owing to the exposure of a nearly continuous cross-section of the Hercynian continental crust, the Sila massif (northern Calabria) represents a favourable area to understand large-scale relationships between granitoids and LP/HT metamorphic rocks, and to discriminate regional LP/HT metamorphic events from contact metamorphic effects. Granulite-facies rocks of the lower crust and greenschist- to amphibolite-facies rocks of the intermediate-upper crust are separated by granitoids emplaced into the intermediate level during the late stages of the Hercynian orogeny. Up to now, advanced petrologic studies have been focused mostly in understanding P-T evolution of deeper crustal levels and magmatic bodies, whereas the metamorphic history of the shallower crustal levels is poorly constrained. The Hercynian upper crust exposed in Sila has been subdivided in two different metamorphic complexes by previous authors: the low- to very low-grade Bocchigliero complex and the greenschist- to amphibolite-facies Mandatoriccio complex. The latter contains favourable mineral assemblages in order to unravel the tectono-metamorphic evolution of the Hercynian upper crust. The Mandatoriccio complex consists mainly of metapelites, meta-arenites, acid metavolcanites and metabasites with rare intercalations of marbles and orthogneisses. Siliciclastic metasediments show a static porphyroblastic growth mainly of biotite, garnet, andalusite, staurolite and muscovite, whereas cordierite and fibrolite are less common. U-Pb ages and internal features of zircons suggest that the protoliths of the Mandatoriccio complex formed in a sedimentary basin filled by Cambrian to Silurian magmatic products as well as by siliciclastic sediments derived from older igneous and metamorphic rocks. In some localities, metamorphic rocks are injected by numerous aplite/pegmatite veins. Small granite bodies are also present and are always associated to spotted schists with large porphyroblasts. They occur along a NW-SE trending transcurrent cataclastic fault zone, which represents the tectonic contact between the Bocchigliero and the Mandatoriccio complexes. This cataclastic fault zone shows evidence of activity at least from middle-Miocene to Recent, indicating that brittle deformation post-dated the Hercynian orogeny. P-T pseudosections show that micaschists and paragneisses of the Mandatoriccio complex followed a clockwise P-T path characterised by four main prograde phases: thickening, peak-pressure condition, decompression and peak-temperature condition. During the thickening phase, garnet blastesis started up with spessartine-rich syntectonic core developed within micaschists and paragneisses. Coevally (340 ± 9.6 Ma), mafic sills and dykes injected the upper crustal volcaniclastic sedimentary sequence of the Mandatoriccio complex. After reaching the peak-pressure condition (≈4 kbar), the upper crust experienced a period of deformation quiescence marked by the static overgrowths of S2 by Almandine-rich-garnet rims and by porphyroblasts of biotite and staurolite. Probably, this metamorphic phase is related to isotherms relaxation after the thickening episode recorder by the Rb/Sr isotopic system (326 ± 6 Ma isochron age). The post-collisional period was mainly characterised by decompression with increasing temperature. This stage is documented by the andalusite+biotite coronas overgrown on staurolite porphyroblasts and represents a critical point of the metamorphic history, since metamorphic rocks begin to record a significant thermal perturbation. Peak-temperature conditions (≈620 °C) were reached at the end of this stage. They are well constrained by some reaction textures and mineral assemblages observed almost exclusively within paragneisses. The later appearance of fibrolitic sillimanite documents a small excursion of the P-T path across the And-Sil boundary due to the heating. Stephanian U-Pb ages of monazite crystals from the paragneiss, can be related to this heating phase. Similar monazite U-Pb ages from the micaschist combined with the lack of fibrolitic sillimanite suggest that, during the same thermal perturbation, micaschists recorded temperatures slightly lower than those reached by paragneisses. The metamorphic history ended with the crystallisation of cordierite mainly at the expense of andalusite. Consequently, the Ms+Bt+St+And+Sill+Crd mineral assemblage observed in the paragneisses is the result of a polyphasic evolution and is characterised by the metastable persistence of the staurolite in the stability fields of the cordierite. Geologic, geochronologic and petrographic data suggest that the thermal peak recorded by the intermediate/upper crust could be strictly connected with the emplacement of large amounts of granitoid magmas in the middle crust. Probably, the lithospheric extension in the relatively heated crust favoured ascent and emplacement of granitoids and further exhumation of metamorphic rocks. After a comparison among the tectono-metamorphic evolutions of the different Hercynian crustal levels exposed in Sila, it is concluded that the intermediate/upper crustal level offers the possibility to reconstruct a more detailed tectono-metamorphic history. The P-T paths proposed for the lower crustal levels probably underestimate the amount of the decompression. Apart from these considerations, the comparative analysis indicates that P-T paths at various crustal levels in the Sila cross section are well compatible with a unique geologic scenario, characterized by post-collisional extensional tectonics and magmas ascent.

A 300 million year-long history. The metamorphic evolution of the Northern Apennine Variscan basement

The Variscan basement of Northern Apennines (Northern Italy) is a polymetamorphic portion of continental crust. This thesis investigated the metamorphic history of this basement occurring in the Cerreto Pass, in the Pontremoli well, and in the Pisani Mountains. The study comprised fieldwork, petrography and microstructural analysis, determination of the bulk rock and mineral composition, thermodynamic modelling, conventional geothermobarometry, monazite chemical dating and Ar/Ar dating of muscovite. The reconstructed metamorphic evolution of the selected samples allowed to define a long-lasting metamorphic history straddling the Variscan and Alpine orogenesis. Some general petrological issues generally found in low- to medium-grade metapelites were also tackled: (i) With middle-grade micaschist it is possible to reconstruct a complete P-T-D path by combining microstructural analysis and thermodynamic modelling. Prekinematic white mica may preserve Mg-rich cores related to the pre-peak stage. Mn-poor garnet rim records the peak metamorphism. Na-rich mylonitic white mica, the XFe of chlorite and the late paragenesis may constrain the retrograde stage. (ii) Metapelites may contain coronitic microstructures of apatite + Th-silicate, allanite and epidote around unstable monazite grains. Chemistry and microstructure of Th-rich monazite relics surrounded by this coronitic microstructure may suggest that monazite mineral was inherited and underwent partial dissolution and fluid-aided replacement by REE-accessory minerals at 500-600°C and 5-7 kbar. (iii) Fish-shaped white mica is not always a (prekinematic) mica-fish. Observed at high-magnification BSE images it may consist of several white mica formed during a mylonitic stage. Hence, the asymmetric foliation boudin is a suitable microstructure to obtain geochronological information about the shearing stage. (iv) Thermodynamic modelling of a hematite-rich metasedimentary rock fails to reproduce the observed mineral compositions when the bulk Fe2O3 is neglected or determined through titration. The mismatch between observed and computed mineral compositions and assemblage is resolved by tuning the effective ferric iron content by P-XFe2O3 diagrams.

Geocronologia u-pb del Plutone della Valle del Cervo (Biella)

I complessi intrusivi alcalini, e i loro prodotti di alterazione, costituiscono una delle principali risorse di materie prime ad alto valore economico. Il magmatismo alcalino è di solito associato a fenomeni di rifting continentale che può essere legato o ad un’incipiente divergenza di placche litosferiche (Rift dell’Africa Orientale) o ad eventi distensivi post-collisionali. La caratterizzazione petrografica, geochimica e geocronologia di questi complessi sono prerequisiti essenziali per ogni progetto di prospezione mineraria. In Italia uno dei complessi alcalini più conosciuti è il Plutone Oligocenico della Valle del Cervo (Biella). La petrografia e la composizione degli elementi maggiori e in traccia dei diversi litotipi che lo compongono sono conosciuti nella letteratura geologica. Le datazioni radiometriche disponibili, tuttavia, sono limitate a dati Rb-Sr su separati di biotite e a più recenti dati U-Pb su zircone separato dal piccolo nucleo granitico del Plutone Oligocenico della Valle del Cervo. In entrambi i casi sono state ottenute età radiometriche di circa 30 Ma. Questo lavoro di tesi si basa sull’analisi degli isotopi U-Pb di zirconi separati dalla sienite e dal monzogranito a megacristalli di ortoclasio che costituiscono gran parte del Plutone Oligocenico della Valle del Cervo. Dopo una caratterizzazione petrografica e geochimica dei campioni scelti si è proceduto alla separazione degli zirconi tramite i metodi di routine che includono separazione gravimetrica, magnetica e liquidi pesanti. I rapporti isotopici U-Pb degli zirconi sono stati determinati presso l’Istituto di Geoscienze e Georisorse, Unità di Pavia, usando la tecnica di spettrometria di massa accoppiata ad una microsonda ad ablazione laser (LA ICP-MS). Le età U-Pb medie determinate per la sienite (33 Ma) e per il monzogranito a megacristalli di ortoclasio (29 Ma) sono da considerarsi età magmatiche relative alla cristallizzazione dei fusi che diedero origine a queste rocce.

Microtermometría, direcciones de flujo hidrotermal y metalogenia del yacimiento de tungsteno de Pasto Bueno (Ancash, Perú)

El yacimiento de Pasto Bueno se localiza en el extremo nordeste del Batolito de la Cordillera Blanca, comprende diversas vetas, generalmente subverticales, asociadas al stock cuarzomonzonitico de Consuzo, datado como Terciario Superior, que intruye a las pizarras de la fm. Chicama y cuarcitas de la fm. Chimu. Las principales vetas discurren con direccion N-S cortando al stock, aunque tambien existen sistemas NE-SW asi como NW-SE encajados sobre las rocas metamorficas. La mineralogia de mena reconocida comprende wolframita (hubnerita), tetraedrita/tenantita, esfalerita y galena, en una ganga de cuarzo, fluorita, sericita, pirita y carbonatos, ademas de molibdenita, calcopirita, bornita, arsenopirita, enargita (luzonita), stolzita, scheelita, zinnwaldita, topacio, tungstita y arsenico nativo. Estudios previos han caracterizado Pasto Bueno como un yacimiento con una gran componente de greisen, con una evolucion de las vetas desde un episodio temprano esteril de 400 oC, depositando la mineralizacion economica en torno a los 220-250 oC y con un evento postumo de 175-220 oC rico en CO2. La precipitacion de la wolframita se produjo a partir de un fluido netamente hidrotermal, sin embargo, dicha precipitacion estuvo controlada por el aporte al sistema de aguas externas meteoricas y/o metamorficas. El trabajo llevado a cabo ha consistido en la realizacion de un estudio microtermometrico de las 3 principales estructuras del distrito: Consuelo, Alonso-Fenix y Chabuca, para caracterizar la evolucion del fluido mineralizador desde el stock (veta Consuelo) hacia las rocas metasedimentarias de las fm. Chicama y Chimu (manto Alonso-Fenix y veta Chabuca). Para ello se realizo un muestreo sobre el evento principal de mineralizacion. Dichas muestras se sometieron a un estudio petrografico de lamina gruesa para seleccionar las muestras optimas para el posterior estudio microtermometrico. Previamente a la obtencion de las medidas de temperatura de fusion del hielo (criotermometria) y de homogenizacion del fluido; se realizo un estudio de petrografia de inclusiones fluidas para caracterizarlas y seleccionar las representativas. La interpretacion de los resultados ha permitido confirmar la existencia de un episodio previo de alta temperatura, superior a 282 oC y un evento mineralizador con temperaturas en torno a los 200-240 oC. Sin embargo, las salinidades obtenidas son mucho menores que las previamente publicadas, en torno al 5 % peso eq. NaCl, frente a 11-17 % peso eq. NaCl. Tambien se ha observado un fluido postumo rico en CO2, pero de temperatura superior, en torno a los 270 oC. Los gradientes isotermicos muestran dos focos para dichos fluidos hidrotermales: el primero asociado al stock en la veta Consuelo, y el segundo en la veta Chabuca, asociado a la zona de cabalgamiento de las pizarras de la fm. Chicama sobre las cuarcitas de la fm. Chimu. Este segundo foco puede corresponder con los aportes externos de aguas metamorficas. Para finalizar, se dan una serie de pautas para guiar las futuras exploraciones en el yacimiento. ABSTRACT The Pasto Bueno deposit is located at the northeastern end of the Cordillera Blanca Batholith. It comprises several veins, generally subvertical, associated with the quartz-monzonite stock of Consuzo, dated as Tertiary, which intrudes the Chicama fm. slates and the Chimu fm. quartzites. The main veins trend N-S cutting the stock, although there are also NE-SW and NWSE systems, hosted by the metamorphic rocks. The ore mineralogy comprises wolframite (hubnerite), tetrahedrite/tennantite, sphalerite and galena in a gangue of quartz, fluorite, sericite, pyrite and carbonates, and minor molybdenite, chalcopyrite, bornite, arsenopyrite, enargite (luzonite), stolzite, scheelite, zinnwaldite, topaz, tungstite and native arsenic. Previous studies have characterized Pasto Bueno as a deposit with a large component of greisen, with an evolution of the veins from an early barren 400 oC event , followed by economic mineralization of about 220-250 °C and a late event of 175 -220 oC rich in CO2. Wolframite precipitation occurred from a purely hydrothermal fluid; however, this precipitation was controlled by an external flux of meteoric and/or metamorphic waters. Microthermometric studies of the 3 main structures of the district (Consuelo, Alonso-Fenix and Chabuca veins) have been carried out to depict the evolution of the mineralizing fluid coming from the stock (Consuelo vein) into the metasedimentary rocks of the Chimu and Chicama fm. (Alonso-Fenix and Chabuca veins). The sampling was performed over the main event of mineralization. These samples were subject to a quick plate petrography study in order to select the optimal samples for further microthermometry studies. Before the freezing/heating measures, a fluid inclusion petrography study was done to characterize and select the representative F.I. Interpretation of results has confirmed the existence of a previous episode of higher temperature, over 282 °C, and a mineralizing event with temperatures of about 200-240 °C. However, obtained salinities, about 5 wt% NaCl equivalents, are much lower than those previously reported, about 11-17 wt% NaCl equivalents. A last fluid, rich in CO2, but of higher temperature, about 270 oC, has been characterized. Isothermal gradients show two foci for the hydrothermal fluids: the first one associated to the Consuzo stock as shown in the Consuelo vein, and the second one related to the thrust fault which places the Chicama fm. slates over the Chimu fm. quartzites in the Chabuca vein area. This second focus may correspond to an external input of metamorphic waters. Finally, some guidelines have been given to guide future explorations.

Maciços alcalinos do Itatiaia e de Passa Quatro (sudeste do Brasil): contribuição à Geologia e Petrologia

Neste trabalho são apresentados os resultados de estudo geológico-petrográfico da província alcalina Itatiaia-Passa Quatro, localizada na Serra da Mantiqueira, nos limites dos Estados de Minas Gerais, Rio de Janeiro e São Paulo. Esta região já foi mencionada como exemplo de uma das maiores ocorrências de rochas sieníticas, com área de aproximadamente 1.300 km2, conforme estimativa de Lamego 45. De acordo com os dados do mapa geológico elaborado durante as nossas pesquisas a área de rochas alcalinas é estimada em 330 km2, correspondendo a menos da metade da extensão do maciço alcalino de Poços de Caldas, Minas Gerais. Do total desta área, o maciço do Itatiaia compreende 220 km2 e o Passa Quatro 110 km2. Ambos os corpos de rochas alcalinas são de contorno elíptico, sendo que o do Itatiaia tem seu maior eixo, com 31 km, na direção NW-SE e o menor, com 12 km, na direção NE-SW. O maciço de Passa Quatro possui o maior eixo na direção NE-SW e o menor na direção NW-SE, respectivamente com 17 e com 8 km de extensão. O complexo alcalino do Itatiaia é formado de sienitos, foiaítos, quartzo-sienitos e brechas. A existência dos diferentes tipos petrográficos é mais uma consequência da distribuição dos minerais em proporções variáveis, bem como de modificações na textura, do que das diferenças mineralógicas. Principalmente na área do planalto, onde afloram os quartzo-sienitos, há uma transição gradual de rochas saturadas e supersaturadas. Assim é que os teores de quartzo aumentam gradativamente de 2% nos quartzo sienitos em contato com as brechas, a mais de 5% nos nordmarkitos, e atingem o máximo de 27,5% no granito alcalino que aflora na parte central do maciço. Os quartzo-sienitos que afloram mais ou menos na região central do maciço alcalino, devem representar a fase final da diferenciação magmática. A textura granofirítica, frequente nos quartzo-sienitos e no granito alcalino do Itatiaia, ) sugerem cristalização final em cúpula de sistema fechado. O contato das rochas sieníticas, na maior parte da área, é com gnaisses pré-cambrianos, com orientação predominante N-NE, mergulhando para o sul. A sudeste, o maciço do Itatiaia está em contato pouco nítido com sedimentos clásticos pertencentes à bacia terciária (?) de Resende e com talus mais recentes que possivelmente recobrem rochas do embasamento cristalino. As brechas magmáticas do Itatiaia mostram variações quanto à natureza, forma e dimensões dos fragmentos, quanto à relação quantitativa matriz-fragmentos e consequentemente quanto à cor. Além das brechas magmáticas, em algumas zonas ocorrem brechas monolitológicas de origem tectônica. Para a gênese das rochas alcalinas é admitida uma provável diferenciação a partir do magma basáltico, responsável pela grande atividade vulcânica do Mesozóico, que se estendeu sobre enorme área ao sul do continente. Por outro lado, é também ressaltada a importância do ambiente tectônico nestes eventos. Os halos pleocróicos evidenciados em cristais de biotita de alguns dos sienitos e foiaítos do Itatiaia, possivelmente se originaram em biotita primariamente ligada ao magma alcalino. Na província alcalina Itatiaia-Passa Quatro há ocorrências de bauxito, algumas das quais já em exploração como fonte de matéria prima para a produção de sulfato de alumínio.

Geochemical and Sr, Nd, Pb isotope investigation of the New Caledonia ophiolite

The New Caledonia ophiolite hosts one of the largest obducted mantle section in the world, hence providing a unique insight for the study of upper mantle processes. These mantle rocks belong to an “atypical” ophiolitic sequence, which is dominated by refractory harzburgites but it also includes minor spinel and plagioclase lherzolites. Upper crust is notably absent in the ophiolite, with the exception of some mafic-ultramafic cumulates cropping out in the southern part of the island. Although the New Caledonia ophiolite has been under investigation for decades, its ultra-depleted nature has made its characterization an analytical challenge, so that few trace element data are available, while isotopic data are completely missing. In this thesis a comprehensive geochemical study (major, trace element and Sr-Nd-Pb isotopes) of the peridotites and the associated intrusive mafic rocks from the New Caledonia ophiolite has been carried out. The peridotites are low-strain tectonites showing porphyroclastic textures. Spinel lherzolites are undepleted lithotypes, as attested by the presence of 7-8 vol% of Na2O and Al2O3-rich clinopyroxene (up to 0.5 wt% Na2O; 6.5 wt% Al2O3), Fo content of olivine (88.5-90.0 mol%) and low Cr# of spinel (13-17). Conversely, harzburgites display a refractory nature, proven by the remarkable absence of primary clinopyroxene, very high Fo content in olivine (90.9-92.9 mol%), high Mg# in orthopyroxene (89.8-94.2) and Cr# in spinel (39-71). REE contents show abyssal-type patterns for spinel lherzolites, while harzburgites display U-shaped patterns, typical of fore-arc settings. Spinel lherzolites REE compositions are consistent with relatively low degree (8-9%) of fractional melting of a DMM source, starting in the garnet stability field. Conversely, REE models for harzburgites indicate high melting degrees (20-25%) of a DMM mantle source under spinel faies conditions, consistent with hydrous melting in forearc setting. Plagioclase lherzolites exhibit melt impregnation microtextures, Cr- and TiO2-enriched spinels and REE, Ti, Y, Zr progressive increase with respect to spinel lherzolites. Impregnation models indicate that plagioclase lherzolites may derive from spinel lherzolites by entrapment of highly depleted MORB melts in the shallow oceanic lithosphere. Mafic intrusives are olivine gabbronorites with a very refractory composition, as attested by high Fo content of olivine (87.3-88.9 mol.%), very high Mg# of clinopyroxene (87.7-92.2) and extreme anorthitic content of plagioclase (An = 90-96 mol%). The high Mg#, low TiO2 concentrations in pyroxenes and the anorthitic composition of plagioclase point out an origin from ultra-depleted primitive magmas in a convergent setting. Geochemical trace element models show that the parental melts of gabbronorites are primitive magmas with striking depleted compositions, bearing only in part similarities with the primitive boninitic melts of Bonin Islands. The first Sr, Nd and Pb isotope data obtained for the New Caledonia ophiolite highlight the presence of DM mantle source variably modified by different processes. Nd-Sr-Pb isotopic ratios for the lherzolites (+6.98≤epsilon Ndi≤+10.97) indicate a DM source that suffered low-temperature hydrothermal reactions. Harzburgites are characterized by a wide variation of Sr, Nd and Pb isotopic values, extending from DM-type to EM2 compositions (-0.82≤ epsilon Ndi≤+17.55), suggesting that harzburgite source was strongly affected by subduction-related processes. Conversely, combined trace element and Sr-Nd-Pb isotopic data for gabbronorites indicate a derivation from a source with composition similar to Indian-type mantle, but affected by fluid input in subduction environment. These geochemical features point out an evolution in a pre-Eocenic marginal basin setting, possibly in the proximity of a transform fault, for the lherzolites. Conversely, the harzburgites acquired their main geochemical and isotopic fingerprint in subduction zone setting.

Caracterização petrográfica e litogeoquímica de granitoides ediacaranos no extremo NE da Província da Borborema (NE do Brasil) com ênfase nos plútons Gameleira, Pitombeira e Taipu

The studied area is situated in the northeastern extremity of the Rio Grande do Norte State, between the municipalities of Taipu and Poço Branco, and is geologically inserted into the São José do Campestre Crystalline Terrain within the Borborema Province, where the analysis of field relationships, petrographic and geochemical data allowed the distinction of three plutons named: Gameleira, Taipu and Pitombeira. The Gamaleira Pluton is composed of granodioritic rocks characterized by zoned plagioclase phenocrysts, with amphibole and biotite as the main mafic phases. Geochemically, these are metaluminous rocks of calc-alkaline nature and magnesian character. The Pitombeira Pluton encompasses two facies: (a) a coarse-grained to porphyritic monzo- to syenogranitic facies marked by K-feldspar phenocrysts; and (b) a quartz dioritic to tonalitic facies with partially zoned plagioclase laths showing chilled rims. Geochemically, rocks of the monzo- to syenogranitic facies are transitional between metaluminous and peraluminous, display a subalkaline nature (high K calc-alkaline) and a ferroan character, whereas rocks of the quartz dioritic to tonalitic facies are metaluminous, with shoshonitic affinity and ferroan character. Lastly, the Taipu Pluton is made of monzoto syenogranitic rocks with biotite as the chief mafic mineral. They are peraluminous rocks of subalkaline nature (high-K calc-alkaline) and ferroan character. Regarding the rare-earth elements (REE), it is possible to conclude that the three studied plutons display negative Eu anomalies and a relative enrichment of LREE over HREE, with LaN/YbN ratios between 9.39 to 16.20 (Gameleira Pluton), 17.99 to 31.39 (granitic facies of the Pitombeira Pluton), 14.15 to 21.81 (dioritic facies of the Pitombeira Pluton) and 15.17 to 175.41 (Taipu Pluton). Based on the combined investigation of geochemical data and discrimination tectonic diagrams, a late- to post-collisional tectonic environment is suggested for the plutons here studied

Petrologia e geocronologia U-Pb do plúton granítico Serra da Rajada, porção central do domínio Rio Piranhas - Seridó, Província Borborema, NE do Brasil

The ediacaran plutonic activity related to the Brasilian/Pan-African orogeny is one of the most important geological features in the Borborema Province, represented along its extension by numerous batholiths, stocks, and dikes.The object of this study, the Serra Rajada Granitic Pluton (SRGP), located in the central portion of the Piranhas-Seridó River Domain is an example of this activity. This pluton has been the subject of cartographic, petrographic, geochronological and lithogeochemical studies and its rocks were characterized by two facies. First, the granitic facies were described as monzogranites consisting of K-feldspar, plagioclase (oligoclase - An23-24%), quartz and biotite (main mafic) and opaque minerals such as titanite, allanite, apatite, and zircon as accessories. Alteration minerals are chlorite, white mica and carbonate. Second, the dioritic facies consist of rocks formed by quartz diorite containing plagioclase (dominant mineral phase), quartz and K-feldspar. Biotite and amphibole are the dominant mafic minerals; and titanite, opaque minerals, allanite, zircon and apatite are the accessories. However, previous geological mapping work in the region also identified the presence of other lithostratigraphic units. These were described as gneisses and migmatites with undifferentiated amphibolite lenses related to the Caicó Complex (Paleoproterozoic) and metasedimentary rocks of the Seridó Group (Neoproterozoic) composed of paragneiss with calc-silicate lenses, muscovite quartzite and biotite schist (respectively, the Jucurutu formations, Equador and Seridó), the host rocks for the SRGP rocks. Leucomicrogranite and pegmatite dikes have also been identified, both related to the end of the Ediacaran magmatism and colluvial- eluvial and alluvial deposits related to Neogene and Quaternary, respectively. Lithogeochemical data on the SRGP granite facies, highlighted quite evolved rocks (SiO2 69% to 75%), rich in alkalis (Na2O+K2O ≥ 8.0%), depleted of MgO (≤ 0.45%), CaO (≤ 1.42%) and TiO2 (≤ 0.36%) and moderate levels of Fe2O3t (2.16 to 3.53%). They display transitional nature between metaluminous and peraluminous (predominance of the latter) with sub-alkaline/monzonitic (High K calcium-alkali) affinity. Harker diagrams show negative correlations for Fe2O3t, MgO, and CaO, indicating mafic and plagioclase fractionation. REE spectrum shows enrichment of LREE relative to heavy REE (LaN/YbN = 23.70 to 10.13), with negative anomaly in the Eu (Eu/Eu* = 0.70 to 0.23), suggesting fractionation or accumulation in the feldspars source (plagioclase). Data integration allows to correlate the SRGP rocks with those described as Calcium-Alkaline Suite of equigranular High K. The crystallization conditions of the SRGP rocks were determined from the integration of petrographic and lithogeochemical data. These data indicated intermediate to high conditions of ƒO2 (mineral paragenesis titanite + magnetite + quartz), parent magma saturated in H2O (early biotite crystallization), tardi-magmatic processes of fluids rich in ƒCO2, H2O and O2 causing part of the mineral assembly to change (plagioclase carbonation and saussuritization, biotite chloritization and opaques Sphenitization). Thermobarometrical conditions were estimated based on geochemical parameters (Zr and P2O5) and CIPW normative minerals, with results showing the liquidus minimum temperature of about800°C and the solidus temperature of approximately 700°C. The final/minimum crystallization pressure are suggested to be between 3 and 5 Kbar. The presence of zoned minerals (plagioclase and allanite) associated with lithogeochemical data in bi-log diagrams for Rb vs. Ba and Rb vs. Sr suggest the role of fractional crystallization as the dominant process in the magmatic evolution of SRGP. U-Pb Geochronological and Sm-Nd isotope studies indicated, respectively, the crystallization age of biotite monzogranite as 557 ± 13 Ma, with TDM model age of 2.36 Ga, and εNd value of -20.10 to the crystallization age, allowing to infer paleoproterozoic crustal source for the magma.

Petrologia e geocronologia U-Pb do plúton granítico Serra da Rajada, porção central do domínio Rio Piranhas - Seridó, Província Borborema, NE do Brasil

The ediacaran plutonic activity related to the Brasilian/Pan-African orogeny is one of the most important geological features in the Borborema Province, represented along its extension by numerous batholiths, stocks, and dikes.The object of this study, the Serra Rajada Granitic Pluton (SRGP), located in the central portion of the Piranhas-Seridó River Domain is an example of this activity. This pluton has been the subject of cartographic, petrographic, geochronological and lithogeochemical studies and its rocks were characterized by two facies. First, the granitic facies were described as monzogranites consisting of K-feldspar, plagioclase (oligoclase - An23-24%), quartz and biotite (main mafic) and opaque minerals such as titanite, allanite, apatite, and zircon as accessories. Alteration minerals are chlorite, white mica and carbonate. Second, the dioritic facies consist of rocks formed by quartz diorite containing plagioclase (dominant mineral phase), quartz and K-feldspar. Biotite and amphibole are the dominant mafic minerals; and titanite, opaque minerals, allanite, zircon and apatite are the accessories. However, previous geological mapping work in the region also identified the presence of other lithostratigraphic units. These were described as gneisses and migmatites with undifferentiated amphibolite lenses related to the Caicó Complex (Paleoproterozoic) and metasedimentary rocks of the Seridó Group (Neoproterozoic) composed of paragneiss with calc-silicate lenses, muscovite quartzite and biotite schist (respectively, the Jucurutu formations, Equador and Seridó), the host rocks for the SRGP rocks. Leucomicrogranite and pegmatite dikes have also been identified, both related to the end of the Ediacaran magmatism and colluvial- eluvial and alluvial deposits related to Neogene and Quaternary, respectively. Lithogeochemical data on the SRGP granite facies, highlighted quite evolved rocks (SiO2 69% to 75%), rich in alkalis (Na2O+K2O ≥ 8.0%), depleted of MgO (≤ 0.45%), CaO (≤ 1.42%) and TiO2 (≤ 0.36%) and moderate levels of Fe2O3t (2.16 to 3.53%). They display transitional nature between metaluminous and peraluminous (predominance of the latter) with sub-alkaline/monzonitic (High K calcium-alkali) affinity. Harker diagrams show negative correlations for Fe2O3t, MgO, and CaO, indicating mafic and plagioclase fractionation. REE spectrum shows enrichment of LREE relative to heavy REE (LaN/YbN = 23.70 to 10.13), with negative anomaly in the Eu (Eu/Eu* = 0.70 to 0.23), suggesting fractionation or accumulation in the feldspars source (plagioclase). Data integration allows to correlate the SRGP rocks with those described as Calcium-Alkaline Suite of equigranular High K. The crystallization conditions of the SRGP rocks were determined from the integration of petrographic and lithogeochemical data. These data indicated intermediate to high conditions of ƒO2 (mineral paragenesis titanite + magnetite + quartz), parent magma saturated in H2O (early biotite crystallization), tardi-magmatic processes of fluids rich in ƒCO2, H2O and O2 causing part of the mineral assembly to change (plagioclase carbonation and saussuritization, biotite chloritization and opaques Sphenitization). Thermobarometrical conditions were estimated based on geochemical parameters (Zr and P2O5) and CIPW normative minerals, with results showing the liquidus minimum temperature of about800°C and the solidus temperature of approximately 700°C. The final/minimum crystallization pressure are suggested to be between 3 and 5 Kbar. The presence of zoned minerals (plagioclase and allanite) associated with lithogeochemical data in bi-log diagrams for Rb vs. Ba and Rb vs. Sr suggest the role of fractional crystallization as the dominant process in the magmatic evolution of SRGP. U-Pb Geochronological and Sm-Nd isotope studies indicated, respectively, the crystallization age of biotite monzogranite as 557 ± 13 Ma, with TDM model age of 2.36 Ga, and εNd value of -20.10 to the crystallization age, allowing to infer paleoproterozoic crustal source for the magma.

Petrologia e geoquímica do magmatismo ediacarano Serra do Caramuru, Rio Grande do Norte, NE do Brasil

The Serra do Caramuru and Tapuio stocks, located in the extreme NE of Rio Piranhas-Seridó Domain (RN), are representative of the Ediacaran-Cambrian magmatism, an important magmatic feature of the Brasilian / Panafrican orogeny of the Borborema Province. These bodies are lithologically similar, intrusive in paleoproterozoic gneiss embasement, being separated by a thin belt of mylonitic orthogneiss. The field relations show a magmatic stratigraphy initiated by dioritic facies that coexists with the porphyritic granitic and equigranular granitic I facies, and less frequently with equigranular granitic II facies. These rocks are crosscut by late granitic dykes and sheets with NE-SW / NNE-SSW orientation. The dioritic facies (diorite, quartz diorite, quartz monzodiorites, tonalite and granodiorite) is leucocratic to melanocratic, rich in biotite and hornblende. The granitic facies are hololeucocratic to leucocratic, and have biotite ± hornblende. Petrographic and geochemical (whole rock) data, especially from Serra do Caramuru pluton, suggest fractionation of zircon, apatite, clinopyroxene (in diorites), opaque minerals, titanite, biotite, hornblende, allanite, plagioclase, microcline and garnet (in dykes). The behavior of trace elements such as Zr, La and Yb indicates that the dioritic magma does not constitute the parental magma for the granitic facies. On the other hand, the granitic facies seems to be cogenetic to each other, displaying differentiation trends and very similar rare earth elements (REE) spectra [12.3≤(La/Yb)N≤190.8; Eu/Eu*=0.30-0.68]. Field relationships and REE patterns [6.96≤(La/Yb)N≤277.8; Eu/Eu*=0.18-0.58] demonstrate that the granitic dykes and sheets are not cogenetically related to the Serra do Caramuru magmatism. The dioritic facies is metaluminous (A/CNK = 0.88-0.74) and shoshonitic, whereas the granitic ones are metaluminous to peraluminous (A/CNK = 1.08-0.93) and high potassium calc-alkaline. Dykes and sheets are strictly peraluminous (A/CNK = 1.01-1.04). Binary diagrams relating compatible and incompatible trace elements and microtextures indicate the fractional crystallization as the dominant mechanism of magmatic evolution of the various facies. The Serra do Caramuru and Tapuio stocks have well preserved magmatic fabric, do not show metamorphic minerals and are structurally isotropic, showing crosscutting contact with the ductile fabric of the basement. These observations lead to interpretate a stage of relative tectonic stability, consistent with the orogenic relaxation period of the Brasiliano / Pan-African orogeny. Chemical plots involving oxides and trace elements indicate late to post-collisional emplacement. In this context, the assumed better mechanism to describe the stocks emplacement within an extensional T Riedel joint, with ENE-WSW extensional vector. The U-Pb zircon age of 553 ± 10 Ma allows correlating the Serra do Caramuru magmatism to the group of post-collisional bodies, equigranular high potassium calc-alkaline granites of the NE of Rio Piranhas-Seridó Domain.

Petrologia e geoquímica do magmatismo ediacarano Serra do Caramuru, Rio Grande do Norte, NE do Brasil

The Serra do Caramuru and Tapuio stocks, located in the extreme NE of Rio Piranhas-Seridó Domain (RN), are representative of the Ediacaran-Cambrian magmatism, an important magmatic feature of the Brasilian / Panafrican orogeny of the Borborema Province. These bodies are lithologically similar, intrusive in paleoproterozoic gneiss embasement, being separated by a thin belt of mylonitic orthogneiss. The field relations show a magmatic stratigraphy initiated by dioritic facies that coexists with the porphyritic granitic and equigranular granitic I facies, and less frequently with equigranular granitic II facies. These rocks are crosscut by late granitic dykes and sheets with NE-SW / NNE-SSW orientation. The dioritic facies (diorite, quartz diorite, quartz monzodiorites, tonalite and granodiorite) is leucocratic to melanocratic, rich in biotite and hornblende. The granitic facies are hololeucocratic to leucocratic, and have biotite ± hornblende. Petrographic and geochemical (whole rock) data, especially from Serra do Caramuru pluton, suggest fractionation of zircon, apatite, clinopyroxene (in diorites), opaque minerals, titanite, biotite, hornblende, allanite, plagioclase, microcline and garnet (in dykes). The behavior of trace elements such as Zr, La and Yb indicates that the dioritic magma does not constitute the parental magma for the granitic facies. On the other hand, the granitic facies seems to be cogenetic to each other, displaying differentiation trends and very similar rare earth elements (REE) spectra [12.3≤(La/Yb)N≤190.8; Eu/Eu*=0.30-0.68]. Field relationships and REE patterns [6.96≤(La/Yb)N≤277.8; Eu/Eu*=0.18-0.58] demonstrate that the granitic dykes and sheets are not cogenetically related to the Serra do Caramuru magmatism. The dioritic facies is metaluminous (A/CNK = 0.88-0.74) and shoshonitic, whereas the granitic ones are metaluminous to peraluminous (A/CNK = 1.08-0.93) and high potassium calc-alkaline. Dykes and sheets are strictly peraluminous (A/CNK = 1.01-1.04). Binary diagrams relating compatible and incompatible trace elements and microtextures indicate the fractional crystallization as the dominant mechanism of magmatic evolution of the various facies. The Serra do Caramuru and Tapuio stocks have well preserved magmatic fabric, do not show metamorphic minerals and are structurally isotropic, showing crosscutting contact with the ductile fabric of the basement. These observations lead to interpretate a stage of relative tectonic stability, consistent with the orogenic relaxation period of the Brasiliano / Pan-African orogeny. Chemical plots involving oxides and trace elements indicate late to post-collisional emplacement. In this context, the assumed better mechanism to describe the stocks emplacement within an extensional T Riedel joint, with ENE-WSW extensional vector. The U-Pb zircon age of 553 ± 10 Ma allows correlating the Serra do Caramuru magmatism to the group of post-collisional bodies, equigranular high potassium calc-alkaline granites of the NE of Rio Piranhas-Seridó Domain.

Pirometamorfismo provocado por intrusões básicas cenozoicas em rochas da Bacia Potiguar, RN: integração de dados geológicos e petrofísicos

The Potiguar Basin is located in the Brazilian Equatorial Margin and presents sedimentary rocks affected by Cenozoic basic igneous intrusions, known as Macau Magmatism. The most prominent effect related to these intrusions is the formation of buchites, pyrometamorphic rocks that occur at very high temperatures and very low pressures in the sanidinite metamorphic facies. Through literature review, field observations, petrographic and petrophysical data, accessing the database of previous studies and results from this research, it was possible to characterize and estimate the effects produced in the thermal aureole of some hypabyssal bodies in the basin. The most relevant features associated with the intrusions are: compactation, hydraulic fracturing, partial melting and recrystallization of country rocks. According to the observed mineral occurrences, temperature of 800 to 1200 °C and pressure below 0,5 kbar were estimated at the contacts of the igneous bodies. The thermal modeling of the São João plug indicates thermal effects extending up to 150 m away from the contact and cooling time of approximately 265,000 years. After the peak of temperature, followed a cooling phase registered by remobilization and precipitation of minerals at low-temperature in faults, fractures and geodes, interpreted as derived from reactions with sedimentary rocks and metasomatic / hydrothermal fluids with abundant carbonatization and silicification.