Geochemistry and Crustal Evolution of Volcano-sedimentary Successions and Orthogneisses in the São Gabriel Block, Southernmost Brazil — Relics of Neoproterozoic Magmatic Arcs

Precambrian metaplutonic rocks of the São Gabriel block in southernmost Brazil comprise juvenile Neoproterozoic calc-alkaline gneisses (Cambaí Complex). The connection with associated (ultra-)mafic metavolcanic and metasedimentary rocks (Palma Group) is not well established. The whole complex was deformed during the Brasiliano orogenic cycle. Both metasedimentary and metavolcanic rocks as well as metaplutonic rocks of the Cambaí Complex have been sampled for geochemical analyses in order to get constraints on the tectonic setting of these rocks and to establish a tectonic model for the São Gabriel block and its role during the assembly of West-Gondwana. The major element compositions of the igneous rocks (Palma Group and Cambaí Complex) indicate a subalkaline character; most orthogneisses have a calc-alkaline chemistry; many metavolcanic rocks of the Palma Group show signatures of low-K tholeiitic volcanic arc basalts. Trace element data, especially Ti, Zr, Y, Nb, of most igneous samples from both the lower Palma Group and the Cambaí Complex indicate origin at plate margins, i.e., in a subduction zone environment. This is corroborated by relative enrichment in LREE, low contents of Nb and other high field strength elements and enrichment in LILE like Rb, Ba, and Th. The data indicate the possible existence of two suites, an oceanic island arc and a continental arc or active continental margin. However, some ultramafic samples of the lower Palma Group in the western São Gabriel block indicate the existence of another volcanic suite with intra-plate character which possibly represents relics of oceanic island basalts (OIB). Trace element data indicate contributions from andesitic to mixed felsic and basic arc sources for the metasedimentary rocks. The patterns of chondrite- and N-MORB-normalized spider diagrams resemble the patterns of the igneous rocks, i.e., LILE and LREE enrichment and HFS depletion. The geochemical signatures of most igneous and metasedimentary samples and their low (⁸⁷Sr/⁸⁶Sr)_t ratios suggest only minor contribution of old continental crust.A geotectonic model for the São Gabriel block comprises east-ward subduction and following accretion of an intra-oceanic island arc to the eastern border of the Rio de la Plata Craton at ca. 880 Ma, and westward subduction beneath the newly formed active continental margin between ca. 750 and 700 Ma. The São Gabriel block represents relics of an early Brasiliano oceanic basin between the Rio de la Plata and Kalahari Cratons. This ocean to the east of the Rio de la Plata Craton might be traced to the north and could possibly be linked with Neoproterozoic juvenile oceanic crust in the western Brasília belt (Goiás magmatic arc).     

Kinetic study of the kaolinite-mullite reaction sequence. Part I: Kaolinite dehydroxylation

The decomposition reaction of kaolinite has been investigated as a function of the defectivity of the starting material and the temperature of reaction. Time resolved energy-dispersive powder diffraction patterns have been measured using synchrotron radiation, both under a constant heating rate (heating rates from 10 to 100° C/min) and in isothermal conditions (in the temperature range 500 to 700° C). The apparent activation energy of the dehydroxylation process is different for kaolinites exhibiting a different degree of stacking fault density. The results of the analysis of the kinetic data indicate that the starting reaction mechanism is controlled by diffusion in the kaolinite particle. The diffusion process is dependent on the defective nature of both kaolinite and metakaolinite. At high temperatures, and at higher heating rates, the reaction mechanism changes and the resistance in the boundary layer outside the crystallites becomes the rate-limiting factor, and nucleation begins within the reacting particle. During the final stage of the dehydroxylation process the reaction is limited by heat or mass transfer, and this might be interpreted by the limited diffusion between the unreacted kaolinite domains and the metakaolinite matrix.     

Structural and petrological evidence for the continuation of the Isfahan fault system across the Urumieh-Dokhtar zone of central Iran

The Isfahan fault system, a north-trending, dextral strike-slip fault across the Sanandaj-Sirjan zone, represents the boundary between the northwestern and the southeastern parts of the Sanandaj-Sirjan zone and it terminates in the north at the southern boundary of the Urumieh-Dokhtar zone. This paper focuses on the continuation of the Isfahan fault system across the Urumieh-Dokhtar zone north of Isfahan city. The Urumieh-Dokhtar magmatic assemblage belt is located along the active margin of the Iranian plate and the Arabian plate. The Karkas fault strikes nearly north-south, has a length of about 40 km, a normal component of movement, and it truncates the Urumieh-Dokhtar zone. Due to the location of this fault and the mechanism similar to the Isfahan fault system, the Karkas fault can be considered a continuation of the Isfahan fault system that has been displaced dextrally by the southwestern bordering faults of the Urumieh-Dokhtar zone. The unique juxtaposition association of the Silurian volcanic rocks in the Urumieh-Dokhtar zone, near the Karkas fault, provides an important evidence regarding the major role of this fault in the geological evolution of the region. The Silurian volcanic rocks outcrop in two districts of the study area and generally are composed of basalts. The alkaline basalt composition is determined from mineralogy and immobile elements geochemistry. The geotectonic setting diagrams classified the Silurian volcanic rocks as the within plate basalts. Thus, an intracontinental rifting under extensional tectonic regime can be inferred as the setting that controlled formation of these volcanic rocks. They were created by an alkaline to transitional magmas generated due to low partial melting at depth. The alkaline basalts were most likely derived from an asthenosphere-dominated mantle source due to extension and partial melting. The north trending extensional faults affected thinned overlying continental lithosphere in the Paleozoic era, facilitating magma penetration and eruption.     

First record of 1.2 Ga quartz dioritic magmatism in the Archaean Yilgarn Craton,Western Australia,and its significance

Farming of southern bluefin tuna in South Australia currently contributes to more than 30% of the value of the aquaculture production in Australia. This study investigated the natural sedimentary setting of the area designated for this important industry in coastal waters off Port Lincoln, and explored the links between the natural distribution of sediments and potential environmental effects and risks to the industry. Sediments were mostly composed of poorly sorted silts and fine sands, predominantly skeletal remains of carbonate-secreting organisms. The contribution of plankton to the organic matter remaining in the sediments was calculated to be in excess of 80% using concentration-dependent stable-isotope mixing models. An erosional area was identified south of Rabbit Island where sediments contained up to 50% siliciclastic material, grainsize distributions were better sorted and coarser, and organic carbon and total nitrogen contents were very low. In contrast, deeper waters north of Cape Donington were identified as a depocentre for fine sediments, which contained organic matter levels twice those elsewhere in the region despite the extremely high carbonate contents (>80%). The heavier stable isotopic signature of nitrogen suggested that this organic matter comprised a greater fraction of weathered components, probably advected to the area by suspended and bedload transport. This local variability of sediment characteristics in the farming zone suggests that the benthic assimilative capacity of farmed sites will depend on their location. Wastes from pens located south of Rabbit Island in particular are likely to be quickly winnowed out by wave and tidal action. These pens are also less likely to be affected by resuspension of fine sediments that might be associated with unusually severe storms.     

Improved Petrological Modal Analyses from X-ray Powder Diffraction Data by use of the Rietveld Method I. Selected Igneous, Volcanic, and Metamorphic Rocks

Mineral modes have been determined for specimens of eight rocktypes from CuK X-ray powder diffraction data using the Rietveldmethod. The samples include two granites, a granodiorite, adamellite,gabbro, basalt, trachyte, and two granulite-facies metamorphicrocks. Up to eight individual mineral components have been measuredin each sample (no glassy phases were observed), with a detectionlimit of {small tilde}1 wt.%, depending on the mineral assemblage.Marked variations in grain size (i.e., granite vs. trachyte)provide no difficulties for the X-ray method. The X-ray resultscompare very favourably with (1) optical modes determined forthe medium–coarse-grained samples by point counting, (2)normative calculations obtained using locally enhanced catanormand mesonorm software, and (3) corresponding Rietveld modesdetermined, for two samples, from neutron powder data. Wheredifferences occur, these are discussed in relation to the limitationsof each of the methods. The improved accuracy of the X-ray method is due primarily tothe incorporation of the full diffraction profile in the Rietveldanalysis calculations, and the elimination of preferred orientationby collecting the data from samples packed in glass capillaries(i.e., Debye–Scherrer mode). The good agreement of theX-ray and neutron modes shows that the usual problems encounteredwith microabsorption, extinction, and sampling are of littleconcern in these rocks. The results highlight one of the majoradvantages provided by Rietveld modal analysis over the moretraditional ‘reference intensity’ X-ray methods,namely, that the crystal chemistry (and thus the calibrationconstants) of the individual phases can be adjusted dynamicallyduring each individual analysis. This not only provides moreaccurate phase abundances, but also gives important supplementaryinformation about the mineralogy of the major components.     

Geology of the Victor Kimberlite, Attawapiskat, Northern Ontario, Canada: cross-cutting and nested craters

The pipe shapes, infill and emplacement processes of the Attawapiskat kimberlites, including Victor, contrast with most of the southern African kimberlite pipes. The Attawapiskat kimberlite pipes are formed by an overall two-stage process of (1) pipe excavation without the development of a diatreme (sensu stricto) and (2) subsequent pipe infilling. The Victor kimberlite comprises two adjacent but separate pipes, Victor South and Victor North. The pipes are infilled with two contrasting textural types of kimberlite: pyroclastic and hypabyssal-like kimberlite. Victor South and much of Victor North are composed of pyroclastic spinel carbonate kimberlites, the main features of which are similar: clast-supported, discrete macrocrystal and phenocrystal olivine grains, pyroclastic juvenile lapilli, mantle-derived xenocrysts and minor country rock xenoliths are set in serpentine and carbonate matrices. These partly bedded, juvenile lapilli-bearing olivine tuffs appear to have been formed by subaerial fire-fountaining airfall processes.

The Victor South pipe has a simple bowl-like shape that flares from just below the basal sandstone of the sediments that overlie the basement. The sandstone is a known aquifer, suggesting that the crater excavation process was possibly phreatomagmatic. In contrast, the pipe shape and internal geology of Victor North are more complex. The northwestern part of the pipe is dominated by dark competent rocks, which resemble fresh hypabyssal kimberlite, but have unusual textures and are closely associated with pyroclastic juvenile lapilli tuffs and country rock breccias±volcaniclastic kimberlite. Current evidence suggests that the hypabyssal-like kimberlite is, in fact, not intrusive and that the northwestern part of Victor North represents an early-formed crater infilled with contrasting extrusive kimberlites and associated breccias. The remaining, main part of Victor North consists of two macroscopically similar, but petrographically distinct, pyroclastic kimberlites that have contrasting macrodiamond sample grades. The juvenile lapilli of each pyroclastic kimberlite can be distinguished only microscopically. The nature and relative modal proportion of primary olivine phenocrysts in the juvenile lapilli are different, indicating that they derive from different magma pulses, or phases of kimberlite, and thus represent separate eruptions. The initial excavation of a crater cross-cutting the earlier northwestern crater was followed by emplacement of phase (i), a low-grade olivine phenocryst-rich pyroclastic kimberlite, and the subsequent eruption of phase (ii), a high-grade olivine phenocryst-poor pyroclastic kimberlite, as two separate vents nested within the original phase (i) crater. The second eruption was accompanied by the formation of an intermediate mixed zone with moderate grade. Thus, the final pyroclastic pipe infill of the main part of the Victor North pipe appears to consist of at least three geological/macrodiamond grade zones.

In conclusion, the Victor kimberlite was formed by several eruptive events resulting in adjacent and cross-cutting craters that were infilled with either pyroclastic kimberlite or hypabyssal-like kimberlite, which is now interpreted to be of probable extrusive origin. Within the pyroclastic kimberlites of Victor North, there are two nested vents, a feature seldom documented in kimberlites elsewhere. This study highlights the meaningful role of kimberlite petrography in the evaluation of diamond deposits and provides further insight into kimberlite emplacement and volcanism.     

A methodology for locating the original quarries used for constructing historical buildings: application to Málaga Cathedral, Spain

It is often necessary to locate the original quarry which supplied the stone for a particular historical building. This stones could be used for future restoration work and for testing in the laboratory (artificial aging tests, physical properties determination, control of the efficacy of conservation treatments, etc.). Generally, reviewing historical documentation gives information about the geographical setting of quarries and location of the stones in the monument, but this information needs to be proved by field and laboratory studies. The comparative study of stone from quarries and monuments should basically include the following: (1) mineralogical and petrographical studies; (2) the chemical analysis of major, minor and trace elements; (3)stable isotopes determinations; (4) physical properties of quarry materials and unweathered building stone (water absorption, ultrasound transmission velocity, porosity and porous system, density, bulk density, compressive strength, etc.). This methodology was applied to Málaga Cathedral stones represented in the main façade, towers, and the western zone of the terrace, which, according to historical literature, came from Almayate (Miocene–Pliocene limestones) and Cerro Coronado (Permotriassic sandstone) in Málaga. The conclusion of the comparative study carried out on quarries and building stones was consistent with the information available from the historical documentation.     

Petrology of Lamproites from Smoky Butte, Montana

Hyalo-armalcolite-phlogopite lamproites and sanidine-phlogopitelamproites occurring at Smoky Butte, Montana are rocks formedfrom rapidly quenched, high temperature, uncontaminated lamproiticmagma. Petrographic variations are attributable to differentcooling histories of several batches of compositionally identicalmagma. Compared with other occurrences of lamproite, the rocksare unusually rich in TiO₂ and are characterized by the presenceof abundant armalcolite and the most TiO₂-rich phlogopites yetfound in this paragenesis. Compositional data are given fortitanian phlogopite, olivine, diopside, titanian potassian richterite,armalcolite, sanidine, analcite, and glass. The mafic mineralsare Al-deficient and exhibit very little compositional variation.Original leucite has been pseudomorphed by sanidine or analcite.The latter mineral was probably formed at the same time thatthe glass lost K, and gained Na, during alteration by groundwater.All of the lamproites are strongly enriched in Ta, Hf, and thelight REE (La /Yb = 162–280), and have high MgO and Crcontents. Mineralogical, geochemical, and previously publishedisotopic data are combined in developing a petrogenetic modelwhich suggests that these lamproites were derived from an ancient(2.5 Ga) doubly metasomatized harzburgitic source, and thatthey represent relatively primitive lamproites which were intrudedat near-liquidus temperatures.     

Postsedimentation transformation of triassic terrigenous rocks in West Chukotka as an indicator of folding conditions

Postsedimentation alteration and structural assemblies of the Triassic sedimentary complexes of West Chukotka are discussed. Zoning of the alteration is based on examination of newly formed structural and mineral assemblages, the chemical composition, and the polytypes of clay minerals. Three zones of postsedimentation transformation of sandstones are distinguished: (1) the zone of chlorite, illite, and mixed-layer disordered chlorite-smectite; (2) the zone of illite and chlorite; and (3) the zone of phengite and ferroan chlorite. The grade of postsedimentation transformation and the composition of the newly formed micas are correlated with the cleavage type. The development of two-three types of cleavage leads to the highest degree of rock transformation. The assemblages of clay minerals and the crystal chemistry of the authigenic phengite show that the grade of postsedimentation transformation of the Triassic rocks attains the stage of greenschistfacies metamorphism in the zone of development of two cleavage types. Where the second cleavage is not documented or poorly developed, the rocks remain unmetamorphosed. Evidence is given that postsedimentation transformation of terrigenous rocks in the foldbelt is controlled largely by deformation.