Revista da Universidade de Aveiro. Geociências

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Joaquim Renato Ferreira de Araújo — 1987-01-01

Os responsáveis pela "Revista da Universidade de Aveiro - Geociências" quiseram dedicar este número ao Prof. João Manuel Cotelo Neiva. (...)

Constituição do núcleo da terra

Frederico Machado — 1987-01-01

Using for the Earth's interior the HB1 (of Haddon & Bullen), anda assuming for the core a chemical composition Feo, 9Nio, 10o. 6, it is shown that the actual conditions are adequate for generating the Earth's magnetic field. The assumed core material is certainly a semicondutor with a forbidden band of about 7.4 eV (and possibly some intermediate levels which facilitate the jumpsm of electrons between allowed bands).

A sedimentologia da "Formação arenitos e argila de Aveiro": cretácico superior, bacia ocidental portuguesa

C. A. Bernardes — 1987-01-01

In the upper Cretaceous of the north section of the Western Portuguese Basin, four facies associations have been defined (I, II, III, IV), belonging a tidal flat complex.The association I, was deposited in a subtidal environment, represented by a lagoon separated from the open marine water by a barrier island; it includes central and marginal lagoonal facies and washover deposits.The intertidal environment, association II, is formed by a mud flat subject to oscillations of the water level and periods of subaereal exposures. Three morphological areas have been identified: lower mud flat related to the margin lagoon, middle mud flar and higher mul flat covered by halophites.The association III, was deposited in tidal channels of high sinuosity; according to its characteristics, two types of channels with different positions in the tidal flat can be distinguished: tidal creeks of small dimensions and simple structure, normally associated with tidal bars end tidal inlets oh greater dimensions and complex structure.The association IV belongs to a supratidal environment in witch the main processes were: vegetation colonization and organic matter accumulation in lower level, in transition to the higher mud flat, strong oscillations of water level and long period subaereal exposures in the higher levels where the evaporitic conditions are prevalent.

Sistemática, caracterização e interesse dos defeitos estruturais da caulinite

C. S. F. Gomes — 1987-01-01

A defect systems for the kaolinite structural defects is proposed herein. Yhe defects included in such systems are caracterizes and some examples of the applied importance of the defects are reported.

Minerais da carga sólida em suspensão de rios e ribeiras que afluem à Ria de Aveiro

C. S. F. Gomes — 1987-01-01

This paper reports some experimental data of the preliminary studies carried out in samples of the suspended solids transported by rivers and other water streams into the Ria de Aveiro lagune.The composition of the soils eroded within the drainage basins determines the mineral association of suspended solids transported by the water streams.

Efeito das variações de cristalinidade dos minerais argilosos na respectiva quantificação por difracção de raios x

M. C. Carapito — 1987-01-01

Experimental data show as the crystallinity variations of clay minerals affect the quantification of these minerals where the crystallinity variations are quite commom.

Sondagens eléctricas verticais, serão mesmo verticais?

M. J. Senos Matias — 1987-01-01

Electrical soundings, sometimes called vertical electrical soundings (VES), can be used in areas of complex geology, where pronouced orientational variation of resistivity measurements can be encountered. However, even in the circinstances, it is usual to use bo the field and interpretation techniques developed for models in which resistivity measurements vary with depth only. The present paper discusses and illustrates with field and model data the difficulties inherent to such aproach when carrying out and interpreting resistivity soundings in areas of complex geology. Then, alternative techniques more suitable to use in these areas reviewed and discussed. Finally, economical aspects on the use of these alternative techniques are discussed.

Geochemical prospecting and panning method at the Coriscada zone (Beira Alta - Portugal). Comparative essay of methodologies

L. Rodrigues — 1987-01-01

The studie area (16Km2) is situated to the south of Meda (Beira Alta - Portugal). In the geological point of view two great units predominate in the area: a hercinian medium-grain and two-mica granite and schist-granite-migmatite complex. The complex components are, however, difficult to separate.The comparative study of the score factors mapping, the W and Sn geochemical anomalies mapping and the cassiterite, volframite and schellite panning maps leads to the conclusion that the panning method is suitable to obtain a prior delimitation of the W and Sn anomalous areas.Nevertheless, the panning procedure cannot dispense the geochemical prospecting, since the utilisation of multivariate statistical analysis of geochemical data shows more representative images of geological context and mineralizations occuring in the area.

Granitoids from Fornos de Algodres and Castro Daire areas (Northern Central Portugal): a comparison and some problems

M. Serrano Pinto — 1987-01-01

A comparison is made between the most representative granitoid units of the Castro Daire zoned pluton and some of the granotoid units that occur in the Fornos de Algodres area, the areas being close to each other. In each area one amphibole-bearing granodiorite and four granites have been selected.The granites of both areas show the same internal order of cristalization. Contrary to the granodiorites, that are metaluminous rocks, they plot in the aluminous domain of Debon & Le Fort's "characteristic minerals" diagram.The granodiorite also plot apart from the granites in Rb-Sr diagram. The zoned pluton granite units show compositional continuity in this diagram which is not the case of the rocks of the other area.Rb-Sr whole rock isochron dta show two of the Fornos de Algodres units to belong to a 370 +- 10 Ma age group and two other units to be ca, 255 Ma in age, whereas four units of the zoned pluton yielded ages ranging from 322 Ma up to 291 Ma. The age of 285 +- 5 Ma is the only one shared by rocks that outcrop in both areas and it relates to coarse grained porphyritic biotite granites. Granodiorites are the oldest dated rocks in both areas, but they differ in age from each by about 50 million years.Initial 87Sr/86Sr ratios are either low (< 0.704) or high (> 0.712) in the Fornos de Algodres rocks and intermediate (0.707 +- 0.002) in the case of zoned pluton rocks.Most of the granites of both areas show S-type granite features wheares the granodiorites are probably I-type granitoids with some crustal contamination.Some granites evidence moscovitization and albitication effects, as well as megacrysts, for which a metasomatic origin related to the intrusion of later granites cannot be clearly deduced from the data.The granites are not considered to be related to the granodiorites by fractional crystallization, but the granites of the zoned pluton could be mutually related by the process, contrary to the granites of the Fornos de Algodres area.

Geochronology of the Ossa-Morena granitoids (Iberian Massif) in the framework of the Ibero-Armorican arc

M. C. Serrano Pinto — 1987-01-01

A comparison is made between the Ossa-Morena Zone (ZOM) in the Iberian Massif and the northernmost domains of the Armorican Massif, mainly based upon granitoids geochronology.In the ZOM evidence is given for a VAriscan granitic plutonism, mainly calc-alkalic, ranging from Middle/Upper Devonian up to Lower Permian. Several age groups can be distinguished in the ZOM and in the neighbouring Central Iberian Zone (ZCI): 280-290 Ma, 300-310 Ma, 310-330 Ma and 340-380 Ma. In opposition to the ZCI, the ZOM is characterized by the occurence of pre-Variscan granotoids in the 400-600 Ma range; they are tentatively gathered in four age groups: 420-470 Ma, 470-500 Ma, 500-550 Ma and 550-600 Ma. Both in the Portuguese and Spanish sectors of the ZOM pre-Variscan alkaline rocks occur in the age group ca, 485 Ma. Alkaline rocks dated ca, 260 Ma have been reported only in the Spanish sector.In the Armorican Massif, Cadomian (650-550) and Late-Cadomian (550-500 Ma) granitoids occur largely and (almost) exclusively in the northern Damnomean-Mancelian Zone (ZDM); they become virtually absent in the neighbouring Central Armorican Zone (ZCA), thus matching the situation in the Iberian Massif when one goes from the ZDM to the ZCI. Several age groups identical to ZOM can be distinguished in ZDN: 420-480 Ma, 500-550 Ma and 550-600 Ma. Granitoids in the 430-480 Ma range are scarce both in the ZDM and in the ZOM.The Variscan age groups 380-340 Ma, 330-310 Ma and 305-280 Ma can be distinguished both in the ZDM (and its northeastern adge) and in ZDM/Domnean domain (and its southern edge).The analogies recognized here between the Iberian ZOM and the Armorican ZDM (Domnonean domain in particular) support some geodynamic models recently proposed that visualize a Cadomian ZOM-ZDM active margin (cordillera or island arc) and an associated (late) Cadomian ZCI-ZCA basin. In Hercynian tomes the northward advancing of the postulated Cantabrian block would have caused a strong reactivation of the ZCI-ZCA belt. Obviously, much more geochronological data are necessary (mainly in the Portuguese sector of ZOM) to clarify the picture.

Alguns aspectos da alteração de granitóides da área de Oliveira de Azemeis - Castro Daire

L. Serrano — 1987-01-01

Results are presented of the chemistry and mineralogy of 6 weathering profiles on granitic rocks from the Oliveira de Azemeis - Castro Daire areal. Chemical evolution versus apparent specific weight using the Reiche's weathering potential index and product index, the Parker index and Ruxton index have been tested.The mineralogical composition of the minus 4 μm fraction by X-ray diffraction analysis has been determined.