Space-time relation of post-collisional granitic magmatism in Santa Catarina, southern Brazil: U-Pb LA-MC-ICP-MS zircon geochronology of coeval mafic-felsic magmatism related to the Major Gercino Shear Zone

LA-MC-ICP-MS U-Pb zircon dating was performed on syntectonic, early post-collisional granitic and associated mafic rocks that are intrusive in the Brusque Metamorphic Complex and in the Florianopolis Batholith, major tectonic domains separated by the Neoproterozoic Major Gercino Shear Zone (MGSZ) in south Brazil. The inferred ages of magmatic crystallization are consistent with field relationships, and show that the syntectonic granites from both domains are similar, with ages around 630-620 Ma for high-K calc-alkaline metaluminous granites and ca. 610 Ma for slightly peraluminous granites. Although ca. 650 Ma inherited zircon components are identified in granites from both domains, important contrasts on the crustal architecture in each domain are revealed by the patterns of zircon inheritance, indicating different crustal sources for the granites in each domain. The granites from the southern domain (Floriandpolis Batholith) have essentially Neoproterozoic (650-700 Ma and 900-950 Ma) inheritance; with a single 2.0-2.2 Ga inherited age obtained in the peraluminous Mariscal Granite. In the northern Brusque Metamorphic Complex, the metaluminous Rio Pequeno Granite and associated mafic rocks have scarce inherited cores with ages around 1.65 Ga, whereas the slightly peraluminous Serra dos Macacos Granite has abundant Paleoproterozoic (1.8-2.2 Ga) and Archean (2.9-3.4 Ga) inherited zircons. Our results are consistent with the hypothesis that the MGSZ separates domains with distinct geologic evolution; however, the contemporaneity of 630-610 Ma granitic magmatism with similar structural and geochemical patterns on both sides of this major shear zone indicates that these domains were already part of a single continental mass at 630 Ma, reinforcing the post-collisional character of these granites. (C) 2012 Elsevier B.V. All rights reserved.

Comparative Studies of Oklahoma and Missouri Granites with Special Attention to the Heavy Minerals of the Oklahoma Granites

When examined petrographically the granites of Oklahoma show a marked similarity to the granites of South­eastern Missouri. The same heavy accessory mineral suites are present in the granites of both regions and include: fluorite, zircon, apatite, titanite and epidote. This similarity was further shown by the actual correlation of the heavy mineral suites by types, these types being, based on the heavy mineral distributions of the Missouri Granites.

Experimental dissolution of molybdenum-sulphides at low oxygen concentrations: A first-order approximation of late Archean atmospheric conditions

The abundance of atmospheric oxygen and its evolution through Earth's history is a highly debated topic. The earliest change of the Mo concentration and isotope composition of marine sediments are interpreted to be linked to the onset of the accumulation of free O2 in Earth's atmosphere. The O2 concentration needed to dissolve significant amounts of Mo in water is not yet quantified, however. We present laboratory experiments on pulverized and surface-cleaned molybdenite (MoS2) and a hydrothermal breccia enriched in Mo-bearing sulphides using a glove box setup. Duration of an experiment was 14 days, and first signs of oxidation and subsequent dissolution of Mo compounds start to occur above an atmospheric oxygen concentration of 72 ± 20 ppmv (i.e., 2.6 to 4.6 × 10−4 present atmospheric level (PAL)). This experimentally determined value coincides with published model calculations supporting atmospheric O2 concentrations between 1 × 10−5 to 3 × 10−4 PAL prior to the Great Oxidation Event and sets an upper limit to the molecular oxygen needed to trigger Mo accumulation and Mo isotope variations recorded in sediments. In combination with the published Mo isotope composition of the rock record, this result implies an atmospheric oxygen concentration prior to 2.76 Ga of below 72 ± 20 ppmv.

Exhumation rates in the Archean from pressure-ime paths: Examplefrom the Skjoldungen Orogen (SE Greenland)

The discussions on the orogenic evolution during Earth's history converge to the question of a different thermal structure in the Archean compared to the Phanerozoic and the applicability of the plate tectonic paradigm. However, geothermal structures are transient in orogens and are difficult to translate into large-scale tectonics and exhumation rates. Therefore, we propose depth–time data in the Archean Skjoldungen Orogen (SE Greenland, North Atlantic Craton) that allow for reconstruction of an exhumation rate independent of geothermal gradients. The resulting exhumation rate of ca. 0.4 km/Ma is similar to exhumation rates during erosion-controlled processes in modern orogens. These exhumation rates can only be established by erosion time constants similar to modern orogens. The occurrence of erosion-controlled exhumation is best explained by a stiff foreland promoting localized deformation in the orogen. Therefore, a switch from magmatic-dominated processes to localized deformation is proposed in the Skjoldungen Orogen area. This is supported by a change in magma composition and volume, from widespread granodiorite to localized alkaline intrusions. In addition, the involved metasedimentary rocks include detrital zircons of the only 50 Ma older foreland, which also correspond to erosion and tectonics as in modern orogens, i.e. flysh-type sediments. Relatively fast exhumation rates and the structural-magmatic evolution of the Neoarchean Skjoldungen Orogen thus indicate modern-style tectonic processes where stiff Mesoarchean continental crust forms a foreland to a collisional orogen instead of typical accretionary tectonics of weak island arc-like terranes in granite-greenstone terranes.

Mineral composition and 40Ar-39Ar data of samples from Garnet Point, Aurora Peak, Correll Nunatak, Penguin Point and Horn Bluffs, East Antarctica

George V Land (Antarctica) includes the boundary between Late Archean-Paleoproterozoic metamorphic terrains of the East Antarctic craton and the intrusive and metasedimentary rocks of the Early Paleozoic Ross-Delamerian Orogen. This therefore represents a key region for understanding the tectono-metamorphic evolution of the East Antarctic Craton and the Ross Orogen and for defining their structural relationship in East Antarctica, with potential implications for Gondwana reconstructions. In the East Antarctic Craton the outcrops closest to the Ross orogenic belt form the Mertz Shear Zone, a prominent ductile shear zone up to 5 km wide. Its deformation fabric includes a series of progressive, overprinting shear structures developed under different metamorphic conditions: from an early medium-P granulite-facies metamorphism, through amphibolite-facies to late greenschist-facies conditions. 40Ar-39Ar laserprobe data on biotite in mylonitic rocks from the Mertz Shear Zone indicate that the minimum age for ductile deformation under greenschist-facies conditions is 1502 ± 9 Ma and reveal no evidence of reactivation processes linked to the Ross Orogeny. 40Ar-39Ar laserprobe data on amphibole, although plagued by excess argon, suggest the presence of a ~1.7 Ga old phase of regional-scale retrogression under amphibolite-facies conditions. Results support the correlation between the East Antarctic Craton in the Mertz Glacier area and the Sleaford Complex of the Gawler Craton in southern Australia, and suggest that the Mertz Shear Zone may be considered a correlative of the Kalinjala Shear Zone. An erratic immature metasandstone collected east of Ninnis Glacier (~180 km east of the Mertz Glacier) and petrographically similar to metasedimentary rocks enclosed as xenoliths in Cambro-Ordovician granites cropping out along the western side of Ninnis Glacier, yielded detrital white-mica 40Ar-39Ar ages from ~530 to 640 Ma and a minimum age of 518 ± 5 Ma. This pattern compares remarkably well with those previously obtained for the Kanmantoo Group from the Adelaide Rift Complex of southern Australia, thereby suggesting that the segment of the Ross Orogen exposed east of the Mertz Glacier may represent a continuation of the eastern part of the Delamerian Orogen.

Chemical and isotopic compositions of Archean magmatic and metasedimentary rocks and minerals from the Podolia domain, Ukrainian Shield

Zircons from the oldest magmatic and metasedimentary rocks in the Podolia domain of the Ukrainian shield were studied and dated by the U-Pb method on a NORDSIM secondary-ion mass spectrometer. Age of zircon cores in enderbite gneisses sampled in the Kazachii Yar and Odessa quarries on the opposite banks of the Yuzhnyi Bug River reaches 3790 Ma. Cores of terrigenous zircons in quartzites from the Odessa quarry as well as in garnet gneisses from the Zaval'e graphite quarry have age within 3650-3750 Ma. Zircon rims record two metamorphic events around 2750-2850 Ma and 1900-2000 Ma. Extremely low U content in zircons of the second age group indicates conditions of the granulite facies metamorphism in Paleoproterozoic within the Podolia domain. Measured data on orthorocks (enderbite-gneiss) and metasedimentary rocks unambiguously suggest existence of the ancient Paleoarchean crust in the Podolia (Dniester-Bug) domain of the Ukrainian shield. They contribute in our knowledge of scales of formation and geochemical features of the primordial crust.

Caracterização geocronológica (U-Pb), geoquímica e isotópica (Sr, Nd, Hf) do complexo Rio Capivari no terreno Embu

O Complexo Rio Capivari (CRC) é constituído por ortognaisses migmatíticos de composições graníticas a tonalíticas e anfibolitos subordinados (magmas toleíticos) em lascas tectônicas no Terreno Embu. As composições dos gnaisses do CRC são predominantemente cálcio-alcalinas a álcali-cálcicas. Idades U-Pb em núcleos de zircão com zoneamento oscilatório indicam cristalização magmática dos protólitos em três períodos principais 2.4, 2.2-2.1 e 2.0 Ga. Idades metamórficas foram reconhecidas em bordas de zircão totalmente escuras nas imagens de catodoluminescência e variam entre 620-590 Ma. A suíte sideriana (2.4 Ga) apresenta caráter juvenil, como evidenciado pelos valores positivos de \'\'épsilon\'\'IND.Nd\' (+3.8) e \'\'épsilon\'\'IND.Hf\' (+0.3 a +4.8) e pela ausência de núcleos de zircão herdado, comumente encontrados em rochas que sofreram retrabalhamento crustal. A suíte de idades riacianas (2.2-2.1 Ga) apresenta idades modelos TDM arqueanas (2.6-3.3 Ga), valores negativos de \'\'épsilon\'\'IND.Nd\' (-12.0 a -4.0) e negativos a levemente positivos de \'\'épsilon\'\'IND.Hf\' (-7.8 a +0.5). Portanto, tais rochas derivam de retrabalhamento de reservatórios crustais antigos. A suíte de idade orosiriana (2.0 Ga) apresenta fontes mais antigas e retrabalhadas com valores altamente negativos de \'\'épsilon\'\'IND.Nd\' (-10.4) e \'\'épsilon\'\'IND.Hf\' (-1.2 a -13.6), sugerindo prolongada residência crustal com idades modelo \'T IND.DM\' e \'T IND.Hf\' >3.3 Ga. As assinaturas de elementos traços em rocha total e a química de zircão sugerem fontes máficas para o gnaisse sideriano. Reservatórios de crosta média, mas de profundidades variáveis, parecem ser a principal fonte dos gnaisses riacianos e orosirianos. Análises em diagramas tectônicos discriminantes baseados em elementos traços de rocha total com elevadas razões \'La/Yb IND.(N)\' (>10), Nb/Yb (>2) e Th/Yb (>1), somados aos valores de \'Y IND.2\'\'O IND.3\' (<3000 ppm), U/Yb (>0.5) e Nb/Yb (0.01-0.10) da química de zircão, sugerem que ambas as suítes de idades foram geradas em ambientes de arco magmático continental, mas com um gap de 200-300 Ma entre o gnaisse sideriano e os gnaisses riacianos sem dados ou informações geológicas. Perfis multielementos (elementos traços) comparativos entre representação de amostras típicas de arco continental associado à subducção de crosta oceânica (margem andina) e amostras de arcos de ilha (Ilhas Mariana) confirmam afinidade com ambiente de arco continental para o CRC, associado à subducção de placa oceânica, principalmente para o gnaisse sideriano. Apesar de pouco representativo, devido ao número de amostras (n=1), uma acresção juvenil em 2.4 Ga colabora para uma dinâmica contínua da evolução da crosta continental. O papel desempenhado pelo CRC na evolução geral do Terreno Embu permanece enigmático. Os dados isotópicos de \'\'épsilon\'\'IND.Nd(590)\' e \'ANTPOT.87 Sr\'/\'ANTPOT.88 Sr IND.(i)\' do CRC (-27.3 a -19.7 e 0.704 a 0.722, respectivamente) indicam evolução temporal não compatível com o requerido para as fontes dos granitos ediacaranos do Terreno Embu, que exigem a participação de reservatórios mais primitivos (\'\'épsilon\'\'IND.Nd(590)\' -13 a -7) e empobrecidos em Rb (\'ANTPOT.87 Sr\'/\'ANTPOT.88 Sr IND.(i)\' \'\'QUASE IGUAL A\' 0,710).

Geologia e petrologia da ilha de São Sebastião (Estado de São Paulo)

A ilha de São Sebastião consta principalmente de rochas alcalinas que formam um maciço de 300 km2 aproximadamente, constituindo o terceiro em área no Brasil. Apresenta-se em um "stock" alongado segundo NE-SW, encaixado em estruturas de gnais. As formações geológicas encontradas consistem em 1 - Granitos e Gnais (ARQUEANO), 2 - Eruptivas básicas (RÉTICO), 3 - Eruptivas alcalinas (JURÁSSICO) e 4 - Depósitos recentes (HOLOCENO). O método de estudo empregado foi o petrográfico e a coluna geológica estabelecida em base de dados petrográficos, tectônicos e fisiográficos. O arqueano é determinado por definição dos seus tipos petrográficos (1- gnais facoidal, 2- oligoclásio-gnais, 3- hornblenda-gnais, 4- biotita-gnais e 5- microlina-granito) idênticos aos concorrentes no considerado arqueano do Brasil meridional. O triássico (rético) é conferido às rochas básicas (diabásios e basaltos) pela sua semelhança tectônica e petrográfica com as congêneres que cortam de maneira semelhante o arqueano no continente. A "mise-en-place" das eruptivas alcalinas (1- Nordmarkito, 2- Biotita-pulaskito, 3- Pulaskito, 4- Nefelina-sienito, 5- Foiaito, 6- Essexito-foiaito, 7- Essexito e 8- Teralito) pode ser considerada jurássica devido suas relações com as eruptivas básicas referidas réticas, pois na praia do Bonete (foto 14) observa-se um dique de nordmarkito cortando outro de diabásio. As eruptivas quartzo-dioríticas (quartzo-microdiorito e quartzo-andesito) cortam as alcalinas no cume do Zabumba, indicando sua idade mais moderna que estas. Além deste fato, preenchem linhas de fraturas tectônicas recentes, como as falhas ao longo do canal de São Sebastião, indicando que a topografia deveria ser a mesma que a atual para permitir rios efusivos ao nível do canal ou que pelo menos toda a zona de extrusão estivesse, como hoje está, em superfície. Os depósitos aluviais marinhos e continentais são considerados recentes, (holocênicos) pelo favor da topografia onde se dispõe, ocupando o fundo os vales e os bordos do atual modelado costeiro, idade esta conferida em base fisiográfica. A tectônica que afetou a ilha de São Sebastião participa da que atuou em todo o litoral meridional brasileiro. Pode-se distinguir duas fases distintas: na primeira ocorreram as erupções básicas e as alcalinas subsidiárias e na segunda deram-se os falhamentos escalonados em blocos basculados para NW, com as fraturas de tensão preenchidas pelas eruptivas quartzo-dioríticas. Toda a atividade tectônica foi regulada pela direção NE-SW privilegiada da estrutura do arqueano, correspondente a antigos eixos dos dobramentos laurencianos e huronianos. A geomorfologia da ilha consta de uma antiga superfície de erosão rematada até a senilidade, - o peneplano cretáceo, hoje reduzida às cristas culminares do maciço alcalino e às satélites das estruturas gnáissicas, desnivelada pelo falhamento em blocos e ligeiramente adernada para NW devido ao basculamento. Ao lado desta topografia vestigial existe o modelado atual da ilha caracterizado por uma juventude do estágio evolutivo. Esta escultura foi inaugurada com os últimos levantamentos epirogênicos que ascenderam as eruptivas alcalinas plutônicas a mais de 1.300 m sobre o nível do mar. O modelado costeiro apresenta uma costa típica de submergência com esculturas em rias, no estágio da juventude. A presença de terraceamentos marinhos de abrasão, atualmente elevados cerca de 20 a 30 m, lembra as oscilações epirogênicas ou eustáticas do litoral.

Conversion of a digital camera into a non-contact colorimeter for use in stone cultural heritage: The application case to Spanish granites

In this study, a digital CMOS camera was calibrated for use as a non-contact colorimeter for measuring the color of granite artworks. The low chroma values of the granite, which yield similar stimulation of the three color channels of the camera, proved to be the most challenging aspect of the task. The appropriate parameters for converting the device-dependent RGB color space into a device-independent color space were established. For this purpose, the color of a large number of Munsell samples (corresponding to the previously defined color gamut of granite) was measured with a digital camera and with a spectrophotometer (reference instrument). The color data were then compared using the CIELAB color formulae. The best correlations between measurements were obtained when the camera works to 10-bits and the spectrophotometric measures in SCI mode. Finally, the calibrated instrument was used successfully to measure the color of six commercial varieties of Spanish granite.