Field and petrological evidence for the development of an ensialic marginal basin related to the Hercynian orogeny in the Massif Central,France

Six lithologic units in tectonic contact with each other have been defined during mapping of the Devonian in the Beaujolais area of the northeastern Massif Central. Five main igneous suites have been recognized:

1. A transitional basaltic suite restricted to a single unit.
2. An acid volcanic-plutonic suite the members of which are related by fractional crystallization and magma mixing.
3. Low-TiO₂ volcanic rocks with calc-alkaline affinities.
4. A TiO₂-rich tholeiitic suite related to an ophiolitic complex.
5. A plutonic suite with close resemblances to Alaskantype intrusions.

The transitional metabasalts (1) form the oldest igneous suite and could represent either an intraplate magmatic forerunner of rifting or tectonic slices of weakly metamorphosed rocks representing a pre-Acadian event. The next three suites may be related to a short-lived ensialic marginal basin that developed between the Acadian and Bretonian orogenies. The basin is asymmetric, with the ophiolite of the central part flanked by an acid ridge on one side and a passive continental margin on the other. Quartz-keratophyres (2) and calc-alkalic basic volcanic rocks (3) were intercalated in varying proportions to form a bimodal volcanic pile before the rifting event that led to the formation of the ophiolites (4). The acid ridge (2) may be due to the reactivation of a continental basement. Cumulate rocks with Alaskan-type affinities occur as olistoliths, emplaced before the formation of the rift basin, supporting a comparison of the Hercynian belt with accreted magmatic arc terranes.     

Petrochemistry of the volcanic rocks of the Island of Principe,Gulf of Guinea

Principe is one of the volcanic centres comprising the Cameroun line in West Africa. The volcanic rocks can be divided into two stratigraphic units:

1. Younger lava series — basanite and nephelinite overlying.
2. Older lava series — transitional to mildly alkaline basalt and hawaiite.

These units lie on a basement of palagonite breccias of tholeiitic affinities. The basic lavas are intruded by plugs ranging in composition from tristanite to phonolite and are overlain by phonolite lavas. These rocks form two chemically and mineralogically distinct suites:

1. Phonolites which evolved by low pressure crystal fractionation of the younger lava series basanitic magma, and
2. Tristanite — trachyte — trachyphonolite suite which may have evolved by high pressure crystal fractionation of the older lava series magma.

     

Ein Beitrag zur Vulkanologie Süd-Perus

The Nevado Coropuna (6400 m/19 500 ft) is the largest and highest volcano of Peru and is situated 150 km NW of the town of Arequipa at a distance of 110 km from the Pacific coast. Results of a thorough petrographic study are presented including microprobe and radiometric measurements.

1. The constituent rocks building up the Coropuna volcano are lavas and rhyodacitic ash flows intercalated between older and younger lavas at the foot of the cone. The volcanic edifice rests on older ignimbrite sheets (14 m. y.) exposed only in the surrounding valleys.
2. The lavas are typically latite-andesites which contain some normative quartz in the groundmass. Plagioclase has 37–47% An. The depth of the phenocryst crystallization is calculated at 8–12 km based on the equilibrium between plagioclase, clinopyroxene and groundmass.
3. The Coropuna volcano has existed since the Late Miocene (5 m. y.). Approximately 2 m. y. ago a catastrophic explosion produced large rhyodacitic ignimbrite deposits around the foot of the mountain. Thereafter the effusion of lavas was dominant through Holocene times with the latest lavas becoming slightly more acidic (62% SiO₂).
4. 30–40 km to the W and SW of the Coropuna some outliers of the coastal batholites are exposed. Both their radiometric age (Cretaceous, 97 m. y.) and their chemical composition are in disagreement with the notion of these granodioritic to gabbroic rocks as the intrusive equivalents of the young volcanics.

     

Some Xenoliths from the alkalic rocks of Teneriffe,Canary Islands

1. Xenoliths of ultrabasic, ultramafic, gabbroic or syenitic type occur in Teneriffe: dunites and clino-pyroxenites in the old alkalic basalt formations of Teno and Anaga peninsulas; gabbroic xenoliths in the Pedro Gil region; nepheline-syenite xenoliths in the Las Canadas and Vilaflor regions where intermediate and phonolitic lavas are abundant; ultramafic, clino-pyroxenite and syenitic xenoliths in the Anaga peninsula where there are many intrusions of nepheline-syenite and phonolitic syenite. Several xenoliths show signs of cataclasis, recrystallisation or reaction of their minerals with the host liquids.
2. The ultrabasic, ultramafic and anorthoclase-rich xenoliths appear to be of cumulus origin, subtracted from basic to intermediate alkalic liquids. Major cumulus phases are: magnesium-rich olivine, sub-silicic, aluminous pyroxene, titanomagnetite, sub-silicic potassic kaersutite, and anorthoclase. It is suggested that the xenoliths formed at depths between 11 km and 30 km, largely under wet conditions that helped suppress formation of cumulus plagioclase.
3. The subtraction of kaersutite from liquids of intermediate composition is thought to be a means of producing the gap in silica content between the Teneriffe trachybasalts and the more siliceous trachyphonolites and phonolites. It is also suggested that the subtraction of kaersutite and anorthoclase would considerably deplete residual liquids in alumina whilst enriching then in soda and this might be the means of producing peralkaline liquids.
4. The presence of the xenoliths supports the geophysical data that indicated that Teneriffe has a sub-crustal structure of plutonic rocks. Correlation of the Teneriffe plutonic xenoliths with exposed plutonic basement rocks from other Canary Islands, which are believed to have similar sub-crustal structures, is considered necessary.

     

Crustal features of some main suture zones along the European Geotraverse

The European Geotraverse (EGT) crosses along a 4000 km profile from the North Cape to Tunisia the following main suture zones:

the Tornquist-Teisseyre zone between the Baltic Shield and the Variscan realm,

the transition zones between Rhenohercynian and Saxothuringian as well as between Saxothuringian and Moldanubian zones in the Variscan part of central Europe, and

the collision zone between the European continent and the Adriatic microplate.

Some structural aspects of these suture zones are described.     

Some examples of the Middle-Triassic Marine transgression in South-Western Mediterranean Europe

In Southwestern Mediterranean Europe (NE Spain, Sardinia and NW Apennines e. g. Monti Pisani and Punta Bianca) the Middle-Triassic transgression on a margin of an intracratonic basin appears to be controlled by the different morphology and tectonic activity of the margin itself. The analysis of some sequences leads to recognize three different kinds of margins:

1. margin with a narrow shelf sloping toward a very shallow restricted basin (Central and Southwestern Sardinia);
2. margin with a broad shelf gently sloping toward a shallow basin (NE Spain, Monti Pisani, NW Sardinia);
3. margin with a narrow shelf and steep slope marked by tectonic and volcanic activity (Punta Bianca).

     

The structural geology of the Dalradian rocks of Slieve Gamph,Cos. Mayo and Sligo,Western Ireland

The paper presents novel information on the Caledonian orogeny in Ireland. A series of Dalradian (Upper Precambrian-Lower Cambrian) metasedimentary rocks occur as an envelope to a granitic igneous complex at Slieve Gamph, Western Ireland. These metasedimentary rocks have been deformed at several distinct times and evidence is shown for the following sequence of events:

1. formation of major nappe structures and a tectonic slide. The axial-plane traces of the folds probably trended N. E.-S. W.
2. formation of upright, gently plunging folds with axial-plane traces of the folds trending N. E.-S. W. Emplacement of the components of the Slieve Gamph igneous complex.
3. formation of a conjugate set of folds:
   1. Open folds with N. N. E.-S. S. W. trending axial-planes which dip to the east,
   2. Open folds with E.W. trending axial-planes which dip to the north.
4. formation of kink-bands, open and conjugate folds with an axialplane trace trending N. W.-S. E.

Late phase of faulting. No isotopic dates are available for these structural events.     

Phase equilibria among pumpellyite,lawsonite, epidote and associated minerals in low grade metamorphic rocks

Phase relations of pumpellyite, epidote, lawsonite, CaCO₃, paragonite, actinolite, crossite and iron oxide are analysed on an Al-Ca-Fe³⁺ diagram in which all minerals are projected from quartz, albite or Jadeite, chlorite and fluid. Fe²⁺ and Mg are treated as a single component because variation in Fe²⁺/Mg has little effect on the stability of phases on the diagram. Comparison of assemblages in the Franciscan, Shuksan, Sanbagawa, New Caledonia, Southern Italian, and Otago metamorphic terranes reveals several reactions, useful for construction of a petrogenetic grid:

1. lawsonite+crossite + paragonite = epidote+chlorite + albite + quartz + H₂O
2. lawsonite + crossite = pumpellyite + epidote + chlorite + albite+ quartz + H₂O
3. crossite + pumpellyite + quartz = epidote + actinolite + albite + chlorite + H₂O
4. crossite + epidote + quartz = actinolite + hematite + albite + chlorite + H₂O
5. calcite + epidote + chlorite + quartz = pumpellyite + actinolite + H₂O + CO₂
6. pumpellyite + chlorite + quartz = epidote + actinolite + H₂O

     

Geological results of deep exploration in the Vienna basin

The exploration for hydrocarbons in the deepest tectonic »floor« underneath the Vienna basin with depths of 6.5–8.5 km, was undertaken between 1977 and 1985 and based on several important conditions:

-The assumption that an autochthonous sedimentary cover lies upon the Crystalline Basement (Bohemian massif) below the Neogene basin infill and the Alpine-Carpathian nappes.

-Expressed high zones exist within the Vienna basin with exploration targets at depths reachable by drilling.

-The significant accumulation of oil- and gasfields m shallower position over the area of interest.

As a result of the 4 deep wells drilled for the abovementioned targets more information has been acquired concerning the stratigraphy, facies distribution and depth positions of the autochthonous Jurassic, Upper Cretaceous and Tertiary Molasse along the Eastern flank of the Crystalline basement spur of the Bohemian Massif. The allochthonous, Waschberg- und Flyschzone, both Alpine-Carpathian units underneath the Vienna basin, have been penetrated by these wells for the first time and the overthrust of the Calcareous Alps over the Flysch nappes has been proven (well Aderklaa UT1). Additional information about Neogene sedimentation and faulting was obtained. Drilling results made it possible to get a more comprehensive picture of the 3 tectonic »floors« of the Vienna basin, m detail represented by the Zistersdorf and Aderklaa profiles. Thick basin marls of the Upper Jurassic represent a large source potential for hydrocarbons. The favourable reservoir layers, detected in the Mesozoic sections of the foreland area have not been encountered here till now. A high supply of free hydrocarbons within the deepest floor must be assumed on the basis of many oil and gas shows, a major gas kick in Zistersdorf ÜTla and a limited oil production from a fractured zone along a thrustline in the Maustrenk ÜTla well, both occurring in an overpressured environment.     

Comparative magma/xenolith relationships in some volcanic and plutonic rocks from the French Massif Central

A comparative study of xenoliths from lavas and granitic rocks of central France indicates that:

1. The evolution of the xenoliths is essentially the same in intermediate to acid lavas as in granites. The major factor in this evolution is the assimilation by the host magmas of the quartzofeldspathic components of the xenoliths, which then become Al, Fe, Mg-rich restites.
2. Liquid immiscibility between melted xenoliths and host magmas can only be postulated for acid xenoliths in alkali basalts.
3. The basic microgranular xenoliths in some granites do not show important compositional gaps with their host rocks, though they are regularly more basic. They can be considered as early segregations (cumulates).

     

Rare earth element geochemistry of the Scourian complex N.W. Scotland — Evidence for the granite-granulite link

Medium-to high-pressure granulite facies complexes represent samples of lower crustal material and are, therefore, important in the study of crustal processes. New rare earth element data for the Scourian granulite facies terrain of the Precambrian Lewisian complex of N.W. Scotland indicate that:

1. Overall, the Scourian complex has a light rare earth enriched pattern with a small but distinct positive Eu anomaly;
2. While some rare earth element trends for the complex as a whole (e.g., Σ REE vs SiO₂) are similar to those observed in upper crustal cogenetic sequences, others (e.g., Eu/Eu* vs SiO₂) are reversed;
3. Compared to average upper crust, the Scourian complex is depleted in REE (except Eu) by a factor of 2 to 3.

These new data, along with previously reported major and trace element data, isotopic abundances, and trace element modelling support the hypothesis that the Scourian terrain is the residuum left after genesis and removal of granitic melts.     

Geologische und geochemische Untersuchungen im Zentralgneis und in der Greiner Schiefer Serie (Zillertaler Alpen,Tirol)

Geologic, petrological and geochemical investigations have been carried out in the western part of the “Zillertaler Alpen”. Important results are:

1. The premetamorphic material of the Greiner series consists of conglomerates, breccias, arcosic-sandstones or greywackes, bituminous shales, volcanic lavas and tuffs.
2. The southern part of the “Zentralgneis” shows a differentiation trend from alkaline granite to quarzdiorite with predomination of granodiorite.
3. Chemical relationships of granodiorite to its restitic inclusions allow the supposition of a palingenetic origin of the granitic rocks.
4. At least two stages of metamorphism can be differentiated.
5. Parts of the Greiner series, covered by triassic metasediments, are supposed to be of Permian age. A lower age boundary can not yet be given.
6. Some vertical, northeast striking faults with throws of more than 1 or 2 kilometers produced southward verging drag folds.
7. Geodynamic aspects, revealed from regional metamorphism and tectonics, are discussed.

     

Comparison between “magnetic” and sedimentary fabric in graded and cross-laminated sand layers,Southern California

Paleocurrent directions can be reconstructed by determining optically the preferred orientation of longest grain axes of sediments; this can be estimated by measuring the anisotropy of magnetic susceptibility (Rees, 1965). In order to check the applicability of both methods, the orientation of large- and small-scale sedimentary structures (cross-bedding, sole markings, grain orientation) was compared with the “magnetic fabric” of some sand and silt layers of different ages and origins:

1. Rosario Formation (Late Cretaceous), La Jolla: thin siltstone layers with small-scale current-ripple cross-lamination;
2. Monterey and Topanga Formations (Miocene, Los Angeles Basin): graded and laminated sandstone beds (“turbidites”);
3. Holocene submarine La Jolla Fan: graded and cross-laminated sand layers (Rees, v.Rad &Shepard, 1968).

Most of the investigated sand layers have a “primary-style” magnetic fabric, characterized by well-grouped, horizontal maximum susceptibility axes, sometimes slightly imbricated upcurrent, minimum susceptibilities normal to the bedding, and intermediate values of the factor q (about 0.1–0.5), suggesting moderately strong current conditions during deposition. The orientation of maximum susceptibility axes correlates with the directions of sand transport, inferred from cross-lamination foresets and sole markings (a). Also conventional optical grain orientation measurements give very similar directions as maximum susceptibilities (b). In Recent deep-water sand layers (c) cross-lamination foresets and maximum susceptibility axes are parallel, and influenced by the local morphology (e. g. fan-valley axis). Since current-parallel orientation of the longest grain axes and maximum susceptibility axes is prevailing in deep-water sands, the magnetic method is a useful tool for palcocurrent analysis. Characteristic differences of the magnetic fabrics from different depositional environments may also give clues to the mechanisms of sediment transport.     

Les granites du Massif Central Français: étude comparée des leucogranites et granodiorites

Chemical and mineralogical compositions of granitic rocks of the French Massif Central enable us to classify them into two distinct groups: (i) leucogranites, (ii) granodiorites or quartz-monzonites, separated by a natural gap. The differences between these two groups are not only chemical and mineralogical, but are also reflected by:

conditions of crystallization as deduced from compositions and from thermal metamorphism of xenoliths.

nature of inclusions and, especially, the occurrence of basic fine-grained igneous xenoliths restricted to the granodiorite — quartz-monzonite group

relations between intrusive and autochtonous migmatitic masses;

levels of formation and of intrusion; the granodioritic and quartz-monzonitic magmas have originated at deeper horizons of the earth's crust than the leucogranitic one, but have risen higher.

In our opinion these differences are linked, and the specific characteristics of the granodiorites and quartz-monzonites are related to the presence of basic igneous inclusions. These inclusions are relics of deep basic intrusions in the earth's crust, which have overheated the surrounding acid rocks beyond the minimum temperature of melting. Thus granodioritic or quartz-monzonitic melts were produced and were able to rise to superficial levels. These inclusions are missing in leucogranites. The compositions of the latter and their poor mobility suggest a wet anatectic origin in minimal PT conditions.     

The role of water in the 1944 Vesuvius eruption

The P-T path of magma associated with the 1944 Vesuvius eruption has been outlined on the basis of probe mineralogy and the relationships between the crystallising phases. Equilibrium P-T values, obtained from the reactions:

1. CaMgSi₂O_6(liq) = CaMgSi₂O_6(cpx)
2. NaAlSi₃O_{8 (liq)} = NaAlSi₃O_{8 (plag)}
3. CaAl₂Si₂O_{8 (plag)}=CaAl₂SiO_6(cpx)+SiO_2(liq) have been established for three intracrustal crystallisation stages: I) 8.0 kbar and 1255 °C; II) 4.0 kbar and 1178 °C; III) 0.5 kbar and 1105 °C.

The H₂O content in the magma has been estimated from an experimental calibration of \(a_{^{CaMgSi_2 O_6 } }^{liq}\) as a function of \(X_{H_2 O}^{liq}\) at P _tot = 2 kbar. The estimated water contents of the magma for the three stages, I) 0.7%; II) 0.9%; III) 1.1%, are consistent with the pattern of activity of the 1944 Vesuvius eruption and with the relationship between the lavas. The shallow depth of H₂O-saturation of the magma, 0.24 kbar at 1100 °C, is consistent with the eruption sequence of lava flows followed by lava fountain activity.     

Looking at computer cartography

Computer Cartography has gone through many changes during its short life. This article tries to document some of the results of the development. The major conclusions are:

- whereas the application of computer mappins is fluurishing, conceptual development is slow;

- as a result the applications are simpler than they could be, given the development of computer science, and brute force reigns over elegance.

To exemplify these points, the main types of Geographic Information Systems are discussed with respect to their recent achievements: Catastral mapping, Thematic mapping, Topographic mapping, Resource Information Systems, and Digital Terrain Models.     

The structure of the guinean continental margin: Implications for the connection between the central and the south atlantic oceans

The results of a recent geological-geophysical survey, conducted off Guinea, are combined with previous data to establish a preliminary stratigraphy and provide a structural sketch of this portion of the West African continental margin. Three sectors are distinguished:

A northwestern portion of the margin comprises a wide and deeply submerged plateau — the Guinean Marginal Plateau underlain by a thick sedimentary sequence and facing westward toward the Gambia Abyssal Plain. Scismic stratigraphy and structures show clear analogies to the Jurassic margins of the central Atlantic. Including the presence of a Cretaceous paleoslope covered by Cenozoic deposits.

A southern area of the margin comprises a series of aligned (W-E trending), acoustic basement features extending along the slope and bounding the Guinean Plateau to the south. These features, basement ridges and volcanic piles are related to a fracture zone system also documented by magnetic anomalies and gravity data. The bordering deep Sierra Leone abyssal plain, also dissected by E-W-trending oceanic fracture zones, contains a sedimentary cover apparently not older than middle Cretaceous.

Between both sectors and between two NW-SE trending scarps lies an intermediate area. The seismic profiles show that here, the margin is dissected by faults creating a series of asymmetric horst and graben features progressively narrowing towards the S-E and covered by untectonized (but partly eroded) Upper Cretaceous to Cenozoic sediments.

The overall structure of the Guinean Margin is interpreted as the result of two major events. During a first phase the margin was created at the southern extremity of the central Jurassic Atlantic and developed like other comparable margins. During a s econd phase (beginning in Early Cretaceous times) the margin was progressively submitted to the opening of the equatorial South Atlantic. This process gave rise to the margin of the southern Guinean plateau (locally injected by volcanics) and generated the tectonic features of the intermediate zone. This protion may thus represent a part of the rifted Jurassic margin discordantly dissected by the oblique opening of the south Atlantic in the area. The oceanic crust of the central and south Atlantic were definitively connected only during Late Albian times as indicated by the end of the tectonic activity and the early Upper Cretaceous unconformity.     

Gehlenite stability in the system CaO-Al2O3-SiO2-H2O-CO2

P, T, \(X_{{\text{CO}}_{\text{2}} }\) relations of gehlenite, anorthite, grossularite, wollastonite, corundum and calcite have been determined experimentally at P _f =1 and 4 kb. Using synthetic starting minerals the following reactions have been demonstrated reversibly

1. 2 anorthite+3 calcite=gehlenite+grossularite+3 CO₂.
2. anorthite+corundum+3 calcite=2 gehlenite+3 CO₂.
3. 3anorthite+3 calcite=2 grossularite+corundum+3CO₂.
4. grossularite+2 corundum+3 calcite=3 gehlenite+3 CO₂.
5. anorthite+2 calcite=gehlenite+wollastonite+2CO₂.
6. anorthite+wollastonite+calcite=grossularite+CO₂.
7. grossularite+calcite=gehlenite+2 wollastonite+CO₂.

In the T, \(X_{{\text{CO}}_{\text{2}} }\) diagram at P _f =1 kb two isobaric invariant points have been located at 770±10°C, \(X_{{\text{CO}}_{\text{2}} }\) =0.27 and at 840±10°C, \(X_{{\text{CO}}_{\text{2}} }\) =0.55. Formation of gehlenite from low temperature assemblages according to (4) and (2) takes place at 1 kb and 715–855° C, \(X_{{\text{CO}}_{\text{2}} }\) =0.1–1.0. In agreement with experimental results the formation of gehlenite in natural metamorphic rocks is restricted to shallow, high temperature contact aureoles.     

Environmental impacts by input of substances in landscapes of the District (Bezirk) of Leipzig,Germany

Problems of landscape and resource protection resulting from the intensification of land-use can be mastered only by intersectoral planning and a land management considering (landscape-)ecological principles right from the beginning. In the district of Leipzig ecological studies in the '80ies have focussed on:

1. Determination of the regional pattern of atmospheric immissions;
2. Registration of heavy metals in soil and vegetation;
3. Soil compaction, soil erosion;
4. Study of stress indicators in the aeration zone and in the top-most aquifer in order to examine barrier effects in the percolation process.

First results are discussed.