Lack of low-grade metamorphism in the Triassic formations of the Argana basin, Morocco: an illite crystallinity re-evaluation

Analysis of mineral assemblages and illite crystallinity of the Arganan Triassic formations show that:

• 1. 

  i) mixed-layers are present across almost all of the section; and

• 2. 

  ii) values of illite crystallinity are quite dispersed and lie mostly in the diagenetic zone, with no gradient related to burial.

It is therefore concluded that, contrary to ideas proposed recently, there is no evidence for very low-grade metamorphism related to crustal thinning during the early rifting of the Central Atlantic. Fluctuations of illite crystallinity values are interpreted as due to complex interactions of detrital heritage and several factors governing illitization and improvement of illite crystallinity. The increase in temperature during burial constitutes only one of them.     

Geochemical framework for water quality studies in sub-Saharan Africa

The scope for using hydrogeochemical techniques in water quality studies in Africa is reviewed as a background to a set of thematic papers. Water quality problems are emerging as a key issue in Africa either:

• 1. 

  i) in view of the pressures of man-made pollution on finite resources; or

• 2. 

  ii) the existence of regions with naturally induced geological problems, for example fluoride endemic areas.

Such natural problems are the focus of this paper and the following topics were emphasised during a workshop in Sodere, Ethiopia: the need to determine natural baselines as a means of recognizing anthropogenic pollution; the need for high quality samples and field data, but relatively basic analytical data; the use of chloride to assist in recharge estimation and water-balance studies; an understanding of depth stratification of water quality as part of the design of well/borehole drilling programmes; the use of hydrogeochemistry in geothermal studies; the recognition of health and/or acceptability problems, especially for F, Fe, Mn, As, I and Al. Water quality standards for African countries need to be considered in the context of local geochemical environments and some of the WHO limits, especially for major ions, may be unattainable owing to naturally high total mineralization.     

Drowning discontinuities and stratigraphic correlation in platform carbonates. The late Barremian–early Aptian record of southeast France

In Provence and Languedoc, four drowning events were identified in platform carbonates of late Barremian–Bedoulian age. Their recognition is based on sedimentological and stratigraphical evidence, and their timing, referred to ammonite zones or subzones, is as follows:

• (1) 

  Late Barremian, at the G. sartousiana–Imerites giraudi transition, or merely the lowermost part of the I. giraudi zone,

• (2) 

  Middle Bedoulian, at the Deshayesitesweissi–Deshayesites deshayesi transition,

• (3) 

  Mid late Bedoulian in correspondence with the “Roloboceras hambrovi subzone”,

• (4) 

  Late Bedoulian at the Deshayesites grandis–Deshayesites furcata transition.

Corresponding events are also well expressed in basinal settings where they are marked by significant facies and faunal changes.These four successive drowning events distinguish four successive steps in platform development and demise. Step 1 was coeval with the onset of the Bedoulian palaeogeography and started after drowning event (1) with a drastic reduction of shallow platform settings with rudists, usually replaced by Palorbitolina facies. The ensuing recovery of rudist facies and, following drowning event (2), subsequent step 2 marked the developmental phase of the platform system, whereas steps 3 and 4, each prefaced by a drowning event, were associated with its demise. Step 1 represents the major spreading phase of the Urgonian type facies spectrum including bioclastics, coral and rudist facies groups. In Provence, step 1 was characterized by a bipolar (N-S) progradation, and aggradation was coeval with a maximum of subsidence. The termination of step 1 was marked by the emergence of the antecedent platform margin. Step 2, which followed the disappearance of rudist facies and the extreme spatial reduction of both coral and bioclastic facies, started with the flooding of the antecedent platform and the development of Palorbitolina and cherty limestones. Shallow water bioclastics and/or coral facies recovered rapidly on top of the pre-existing emerged areas and developed locally as bioclastic shoals. Step 2 documents a regional reorganisation of subsidence patterns.The infralittoral (high illuminated environments) “Urgonian facies” are therefore essentially present in the Lower Bedoulian, and circalittoral (relatively deep low illuminated environments) deposits dominate in the Upper Bedoulian. This pattern, typical for SE France and wide parts of the Helvetic shelf, departs from that of adjacent regions (e.g. SW France, Spain) where late Bedoulian platform carbonates have a significant record. The record thus shows that the demise of the Urgonian platform was a step-wise phenomenon which cannot be ascribed to a single event, i.e. the Goguel/Selli OAE1a main event.     

Paleomagnetism and the alpine tectonics of Eurasia V: The paleomagnetism of some mesozoic and cenozoic volcanic rocks from the lebanon

A sampling of Mesozoic and Tertiary basalts in Lebanon yielded the following information:

Age	D	I	α95	Pole position	dp	dm
Upper Jurassic	95	+21	10.6	114E 2N	5.9	11.2
				66W 2S
Lower Cretaceous	122	+2	9.0	105E 25S	4.5	9.0
				75W 25N
Upper Pliocene	2	+46	7.7	169E 88N	6.3	9.8
				11W 88S

Full-size table

These results confirm and amplify earlier work by Van Dongen et al., and can be interpreted as indicating a net anticlockwise rotation of Lebanon relative to the African tectonic plate amounting to about 70° during the Late Jurassic-Pliocene interval. This could have resulted from differential movement between the African and European plates as they made way for the growing Atlantic Ocean.     

Earthquake recurrence on the Calaveras fault east of San Jose, California

Occurrence of small (3 M_L < 4) earthquakes on two 10-km segments of the Calaveras fault between Calaveras and Anderson reservoirs follows a simple linear pattern of elastic strain accumulation and release. The centers of these independent patches of earthquake activity are 20 km apart. Each region is characterized by a constant rate of seismic slip as computed from earthquake magnitudes, and is assumed to be an isolated locked patch on a creeping fault surface. By calculating seismic slip rates and the amount of seismic slip since the time of the last significant (M 3) earthquake, it is possible to estimate the most likely date of the next (M - 3) event on each patch. The larger the last significant event, the longer the time until the next one. The recurrence time also appears to be increased according to the moment of smaller (2 < M_L < 3) events in the interim. The anticipated times of future larger events on each patch, on the basis of preliminary location data through May 1977 and estimates of interim activity, are tabulated below with standard errors. The occurrence time for the southern zone is based on eight recurrent events since 1969, the northern zone on only three. The 95% confidence limits can be estimated as twice the standard error of the projected least-squares line. Events of M 3 should not occur in the specified zones at times outside these limits. The central region between the two zones was the locus of two events (M = 3.6, 3.3) on July 3, 1977. These events occurred prior to a window based on the three point, post-1969 slip-time line for the central region.

Latitude	Longitude	Depth	Mag.	Target date	Standard error (days)
37°17′± 2′N	121°39′±2′W	5.0 ±2 km	3.0–4.0	7-22-77	22.3
37°26′± 2′N	121°47′±2′W	6.0 ± 2 km	3.0–4.0	9-02-77	8.0

Full-size table

     

Rare earth element diffusion in a natural pyrope single crystal at 2.8 GPa

Volume diffusion rates of Ce, Sm, Dy, and Yb have been measured in a natural pyrope-rich garnet single crystal (Py₇₁Alm₁₆Gr₁₃) at a pressure of 2.8 GPa and temperatures of 1,200-1,450 °C. Pieces of a single gem-quality pyrope megacryst were polished, coated with a thin layer of polycrystalline REE oxide, then annealed in a piston cylinder device for times between 2.6 and 90 h. Diffusion profiles in the annealed samples were measured by SIMS depth profiling. The dependence of diffusion rates on temperature can be described by the following Arrhenius equations (diffusion coefficients in m²/s): % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBam % XvP5wqSXMqHnxAJn0BKvguHDwzZbqegm0B1jxALjhiov2DaeHbuLwB % Lnhiov2DGi1BTfMBaebbfv3ySLgzGueE0jxyaibaieYlf9irVeeu0d % Xdh9vqqj-hEeeu0xXdbba9frFj0-OqFfea0dXdd9vqaq-JfrVkFHe9 % pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr0-vqpWqaaeaabiGaciaaca % qabeaadaabauaaaOqaauaabeqaeeaaaaqaaiGbcYgaSjabc+gaVjab % cEgaNnaaBaaaleaacqaIXaqmcqaIWaamaeqaaOGaemiraq0aaSbaaS % qaaiabbMfazjabbkgaIbqabaGccqGH9aqpcqGGOaakcqGHsislcqaI % 3aWncqGGUaGlcqaI3aWncqaIZaWmcqGHXcqScqaIWaamcqGGUaGlcq % aI5aqocqaI3aWncqGGPaqkcqGHsisldaqadaqaaiabiodaZiabisda % 0iabiodaZiabgglaXkabiodaZiabicdaWiaaysW7cqqGRbWAcqqGkb % GscaaMe8UaeeyBa0Maee4Ba8MaeeiBaW2aaWbaaSqabeaacqqGTaql % cqqGXaqmaaGccqGGVaWlcqaIYaGmcqGGUaGlcqaIZaWmcqaIWaamcq % aIZaWmcqWGsbGucqWGubavaiaawIcacaGLPaaaaeaacyGGSbaBcqGG % VbWBcqGGNbWzdaWgaaWcbaGaeGymaeJaeGimaadabeaakiabdseaen % aaBaaaleaacqqGebarcqqG5bqEaeqaaOGaeyypa0JaeiikaGIaeyOe % I0IaeGyoaKJaeiOla4IaeGimaaJaeGinaqJaeyySaeRaeGimaaJaei % Ola4IaeGyoaKJaeG4naCJaeiykaKIaeyOeI0YaaeWaaeaacqaIZaWm % cqaIWaamcqaIYaGmcqGHXcqScqaIZaWmcqaIWaamcaaMe8Uaee4AaS % MaeeOsaOKaaGjbVlabb2gaTjabb+gaVjabbYgaSnaaCaaaleqabaGa % eeyla0IaeeymaedaaOGaei4la8IaeGOmaiJaeiOla4IaeG4mamJaeG % imaaJaeG4mamJaemOuaiLaemivaqfacaGLOaGaayzkaaaabaGagiiB % aWMaei4Ba8Maei4zaC2aaSbaaSqaaiabigdaXiabicdaWaqabaGccq % WGebardaWgaaWcbaGaee4uamLaeeyBa0gabeaakiabg2da9iabcIca % OiabgkHiTiabiMda5iabc6caUiabikdaYiabigdaXiabgglaXkabic % daWiabc6caUiabiMda5iabiEda3iabcMcaPiabgkHiTmaabmaabaGa % eG4mamJaeGimaaJaeGimaaJaeyySaeRaeG4mamJaeGimaaJaaGjbVl % abbUgaRjabbQeakjaaysW7cqqGTbqBcqqGVbWBcqqGSbaBdaahaaWc % beqaaiabb2caTiabbgdaXaaakiabc+caViabikdaYiabc6caUiabio % daZiabicdaWiabiodaZiabdkfasjabdsfaubGaayjkaiaawMcaaaqa % aiGbcYgaSjabc+gaVjabcEgaNnaaBaaaleaacqaIXaqmcqaIWaamae % qaaOGaemiraq0aaSbaaSqaaiabboeadjabbwgaLbqabaGccqGH9aqp % cqGGOaakcqGHsislcqaI5aqocqGGUaGlcqaI3aWncqaI0aancqGHXc % qScqaIYaGmcqGGUaGlcqaI4aaocqaI0aancqGGPaqkcqGHsisldaqa % daqaaiabikdaYiabiIda4iabisda0iabgglaXkabiMda5iabigdaXi % aaysW7cqqGRbWAcqqGkbGscaaMe8UaeeyBa0Maee4Ba8MaeeiBaW2a % aWbaaSqabeaacqqGTaqlcqqGXaqmaaGccqGGVaWlcqaIYaGmcqGGUa % GlcqaIZaWmcqaIWaamcqaIZaWmcqWGsbGucqWGubavaiaawIcacaGL % Paaaaaaaaa!0C76!

Thermodynamic properties and stability of AlF-bearing titanite CaTiOSiO4–CaAlFSiO4

Calorimetric and experimental data on AlF-bearing titanite are presented that yield thermodynamic properties of CaAlFSiO₄, as well as activity-composition relations of binary titanite CaTiOSiO₄-CaAlFSiO₄. The heat capacity of synthetic CaAlFSiO₄ was measured with differential scanning calorimetry between 170 and 850 K: C_P=689.96-0.38647T+2911300T^-2-8356.1T^-0.5+0.00016179T² Based on low-temperature heat capacity calculations with lattice vibrational theory (Debye model), the calorimetric entropy of CaAlFSiO₄ can be expected to lie between 104.7 and 118.1 J mol^-1 K^-1. The temperature of the P2₁/a to A2/a phase change was determined calorimetrically for a titanite with X_Al=0.09 (T_transition=390 K). The decrease of the transition temperature at a rate of about 11 K per mol% CaAlFSiO₄ is in good agreement with previous TEM investigations. The displacement of the reaction anorthite + fluorite = CaAlFSiO₄ in the presence of CaTiOSiO₄ was studied with high P-T experiments. Titanite behaves as a non-ideal, symmetrical solid-solution. The thermodynamic properties of CaAlFSiO₄ consistent with a multi-site mixing model are: % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfKttLearuavTnhis1MBaeXatLxBI9gBam % XvP5wqSXMqHnxAJn0BKvguHDwzZbqegm0B1jxALjhiov2Daebbnrfi % fHhDYfgasaacH8srps0lbbf9q8WrFfeuY-Hhbbf9v8qqaqFr0xc9pk % 0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8frFve9 % Fve9Ff0dmeaabaqaciaacaGaaeqabaWaaeaaeaaakeaafaqaaiWaca % aabaGaeeyrauKaeeOBa4MaeeiDaqNaeeiAaGMaeeyyaeMaeeiBaWMa % eeiCaaNaeeyEaKNaeeiiaaIaee4Ba8MaeeOzayMaeeiiaaIaeeOzay % Maee4Ba8MaeeOCaiNaeeyBa0MaeeyyaeMaeeiDaqNaeeyAaKMaee4B % a8MaeeOBa4MaeeiiaaIaeeikaGIaeeyzauMaeeiBaWMaeeyzauMaee % yBa0MaeeyzauMaeeOBa4MaeeiDaqNaee4CamNaeeykaKIaeeiiaaIa % emizaq2aaSbaaSqaaiabdAgaMbqabaGccqWGibasdaahaaWcbeqaai % abicdaWaaaaOqaaiabg2da9iabgkHiTiabikdaYiabiEda3iabisda % 0iabicdaWiabc6caUiabiIda4iabgglaXkabiodaZiabc6caUiabic % daWiabbccaGiabbUgaRjabbQeakjabb2gaTjabb+gaVjabbYgaSnaa % CaaaleqabaGaeyOeI0IaeGymaedaaaGcbaGaee4uamLaeeiDaqNaee % yyaeMaeeOBa4MaeeizaqMaeeyyaeMaeeOCaiNaeeizaqMaeeiiaaIa % ee4CamNaeeiDaqNaeeyyaeMaeeiDaqNaeeyzauMaeeiiaaIaeeyzau % MaeeOBa4MaeeiDaqNaeeOCaiNaee4Ba8MaeeiCaaNaeeyEaKNaeeii % aaIaee4uam1aaWbaaSqabeaacqqGWaamaaaakeaacqqG9aqpcqqGXa % qmcqqGWaamcqqG0aancqqGUaGlcqqG5aqocqGHXcqScqqGXaqmcqqG % UaGlcqqGXaqmcqqGGaaicqqGkbGscqqGTbqBcqqGVbWBcqqGSbaBda % ahaaWcbeqaaiabgkHiTiabigdaXaaakiabbUealnaaCaaaleqabaGa % eyOeI0IaeGymaedaaaGcbaGaeeyta0KaeeyyaeMaeeOCaiNaee4zaC % MaeeyDauNaeeiBaWMaeeyzauMaee4CamNaeeiiaaIaeeiCaaNaeeyy % aeMaeeOCaiNaeeyyaeMaeeyBa0MaeeyzauMaeeiDaqNaeeyzauMaee % OCaiNaeeiiaaYaamWaaeaacqWGxbWvdaWgaaWcbaGaemisaG0aaWba % aWqabeaacqGHsislaaaaleqaaOGaeeivaqLaem4vaC1aaSbaaSqaai % abdohaZbqabaaakiaawUfacaGLDbaaaeaacqGH9aqpcqaIXaqmcqaI % ZaWmcqGGUaGlcqaI2aGncqGHXcqScqaIWaamcqGGUaGlcqaI0aanca % aMe8UaeeOsaOKaeeyBa0Maee4Ba8MaeeiBaW2aaWbaaSqabeaacqGH % sislcqaIXaqmaaaaaaaa!E403!

Seleniferous minerals of palladium and platinum from ouro preto-bearing mineralisation in Brazil

Foram determinadas as composições químicas e as idades Rb–Sr de mica branca, feldspato potássico e de rochas totais das mineralizações de esmeraldas de Capoeirana e Belmont, de pegmatitos sem esmeraldas e dos gnaisses Borrachudos, Monlevade e Guanhães da região de Nova Era–Itabira–Ferros (Minas Gerais, Brazil). Os gnaisses graníticos Borrachudos, os gnaisses bandados Monlevade, seus respectivos pegmatitos e veios/schlieren pegmatóides, e os gnaisses Guanhães, adquiriram suas texturas e composições mineralógicas atuais há cerca de 1.9 Ga no contexto do evento Transamazônico.As rochas regionais encaixantes típicas das ocorrências de esmeraldas são os gnaisses Monlevade que pertencem a uma sequência metavulcano-sedimentar de tipo greenstone belt. O evento principal de formação de esmeraldas em Belmont e Capoeirana foi uma reação metassomática dos pegmatitos anatéticos ricos em Be com rochas ultrabásicas ricas em Cr durante o evento Transamazônico em torno de 1.9 Ga. Em Capoeirana nesse contexto os pegmatitos com feldspato potássico ricos em Be foram transformados em rochas de plagioclasio–quartzo. As idades Rb–Sr de cerca de 480 Ma de minerais das mineralizações de esmeralda resultaram da reequilibração de biotitas e feldspatos Transamazônicos durante o evento Brasiliano.Os pegmatitos não-metamórficos e sem esmeralda da região estudada foram formados no evento Brasiliano há 477±14 Ma. O grau de diferenciação dos pegmatitos, estudado em diagramas indicadores específicos como por exemplo Cs vs. K/Rb de micas brancas e feldspatos potássicos, varia desde pegmatitos cerámicos até muscovita-pegmatitos, à pegmatitos de metais raros e até berilíferos. Alguns dos pegmatitos apresentam marcante diferenciação interna.

Characterization of Quaternary faults by electric resistivity tomography in the Andean Precordillera of Western Argentina

El río San Juan, situado en la Provincia de San Juan (Argentina) cruza la Precordillera y otras unidades geológicas incluyendo la Depresión de Ullum y la Zona de La Laja, entre las latitudes 31°S y 32°S. El curso del río tiene un cierto caracter antecedente como puede deducirse por sus dos trazas perpendiculares unidas por otra casi paralela a las alineaciones estructurales principales. En la zona de la Precordillera, el valle del río San Juan muestra numerosos abanicos aluviales, situados en las zonas de confluencia entre el río principal y sus tributarios. Las superficies de los abanicos aluviales cuaternarios estan cortadas por una serie de escalones que consideramos como terrazas aluviales generadas por episodios repetitivos de agradación y degradación. El sector estudiado incluye una zona con una importante actividad sísmica reciente(La Laja), otra sin una importante actividad sísmica reciente (Precordillera), y una zona subsidente (Ullum) donde se formó un gran lago natural hace unos 6500 años. El antiguo río San Juan fue capturado por el valle de la Quebrada de Ullum mediante una incisión del orden de 25 m, que implicó una nueva adecuación del gradiente del río mediante los efectos de la erosión remontante. El gradiente del río San Juan muestra algunas irregularidades que, aunque no se presenten relacionadas directamente con las estructuras principales, estan relacionadas con la propia dinámica fluvial que acentúa la diferenciación litológica. La anchura del valle del río principal, la geometria y el gradiente de cada tributario, junto a las litologias del basamento y a las dimensiones de cada area fuente local, son los factores principales que controlan los procesos de la generación de las terrazas aluviales. En la zona de La Laja, donde la terraza mas alta soporta un nivel de travertino, la datación de los depósitos travertínicos proporciona datos como para suponer una tasa de incisión del orden de 0,9–1 mm/año, asociada a la actuación periódica de la falla de La Laja.