Small Massifs Research Articles

L'évolution du socle précambrien dans la région de Qagssimiut, Groënland méridional

The rocks of the Qagssimiut area, South Greenland, belong, apart from a few late dolerite dykes, to the Precambrian shield. Their history was found to be very complex. An orogenic period affected formations mainly composed of quartzites and amphibolitic schists which now occupy but a very small percentage of that area. These rocks, having been submitted to at least two periods of deformation (during which a set of folds with a NW~SE trend, and a major, younger NE-SW set were developed), were subsequently intruded by massifs and plugs of ultrabasic material, which are very much more widely preserved. Widespread granitisation then occurred; in particular, the pre-existent rocks were (partly) transformed into granite, which is by far the principal component of the Qagssimiut area. The processes apparently took place in tranquil conditions. Basic intrusion also takes the form of sheets and dykes, concentrated mainly in the southern parts of the area. Some of these sheets are conspicuously veined and replaced by granite or aplite. The striking characteristics of this phenomenon, which involves static replacement and progressive consumption of basic material by acid, are described in some detail. The contrast between basic and acid material is remarkable, as is the alternation of phases of basic intrusion and granitisation, which occurred under conspicuously stable conditions. The emplacement of small massifs of homogeneous, pink or white microgranite preceded the Gardar period, characterised by the intrusion of dykes (lamprophyres, dolerites, trachytes, porphyries) and block-faulting. A long hiatus separated the end of the Gardar period from the next event, the (Tertiary?) intrusion of two generations of dolerite dykes. They are fresh and never faulted. Quaternary phenomena are mainly related to the activity of the ice-cap, whose traces are ubiquitous.

Sur le granite orbiculaire de Tisselliline (Hoggar, Sahara central)

Abstract Orbicular granite outcrops in a small area near the northern edge of the Tisselliline granitic massif in the Ahaggar mountains (Algeria) in the central Sahara. Petrographic and chemical study leads to a double diffusion theory of its origin. Whereas orbicular rocks are generally found near a migmatite front, this particular example indicates that an orbicular facies can also be associated with small granite massifs, thus suggesting the possibility that a phase analogous to migmitization may be involved in the genesis of certain intrusive massifs of this type.

Observations sur la chromite du mont Ahito (Togo)

Abstract The chromite mineralization of Ahito, one of a series of small serpentine massifs in French Togo, is shown to have been emplaced in discontinuous lenticular veins, under pneumatolytic-hydrothermal conditions, along north-south fractures paralleling the main structural trend of the massif.

Tectonics of Mexico

It has been attempted to present in this paper, for the first time, an idea of the tectonics of Mexico as a whole, based on the concepts developed mainly by Alpine geologists for the study of orogenic processes. With this object in view, the paper was divided in two parts; the first, in which was established the meaning of the concepts in order to fix the sense in which they will be used and to point out in a general way the orogenic processes to which the region of Mexico was subjected; the second part in which are described the major structural units, from a tectonic point of view, especially as to the character and orientation of their folds, since the character of the major units is mainly established from the character of the folds. There have been established in the foreland of the Mexican geosyncline, three shelves, a large massif, two small ones, and four basins. There have been postulated for the hinterland a great massif or continent, two large massifs, two medium and four small massifs, and three basins. The foredeep of the geosynclinal fold has been indicated in three regions, as established by the flysch sediments deposited therein. It is considered that the geosyncline of the Mexican Cordillera begins west of Ciudad Juarez extending southward to Torreon, where it turns eastward to Monterrey, and from there, southeastward to the Coatzacoalcos River, thence northeastward to the Chalchijapan River, and upon reaching the narrowest part of the Isthmus of Tehuantepec, it turns east-southeastward into Guatemala. It is established that the principal orogeny which gave rise to this folding started at the end of the Cretaceous in the northern part of Mexico and at the beginning of the Eocene in the rest of the country, and ended at the end of the Eocene excepting in the southeast where it lasted until the beginning of the Oligocene. Furthermore, there were embryonic and posthumous movements, the last of which is very important, in the Isthmus and the Macuspana and Pichucalco basins. End_Page 1319------------------------------ Fig. 1. TECTONIC MAP OF THE REPUBLIC OF MEXICO End_Page 1320------------------------------

Tectonics of Republic of Mexico: ABSTRACT

It has been attempted to give in this paper, for the first time, an idea of the tectonics of the Republic of Mexico as a whole, based on the concepts developed, mainly by alpine geologists, for the study of orogenic processes. With this object in view, the paper was divided in two parts; the first, in which was established the meaning of the concepts in order to fix the sense in which they will be used and to point out in a general way the orogenic processes to which the Republic of Mexico was subjected. The second part in which are described the major structural units, from a tectonic point of view, especially as to the character and orientation of their folds, since the character of the major units is mainly established from the character of the folds. There have been established in the foreland of the Mexican geosyncline, three shelves, a large massif, two small ones, and four basins. There have been postulated for the hinterland a great massif or continent, two large massifs, two medium, and four small massifs as well as three basins. The foredeep of the geosynclinal folds has been indicated in three regions, as established by the flysch sediments deposited therein. It is considered that the geosynclinal folds of the Mexican Cordillera begin west of Ciudad Juarez toward Torreon, where they turn toward Monterey, and from there, southeast down to the Coatzacoalcos River, then northwest up to the Chalchijapa River and on reaching the narrowest part of the Isthmus of Tehuantepec they turn east-southeast into Guatemala. It is established that the orogeny which gave rise to this folding started at the end of the Cretaceous in the northern part of the Republic and at the beginning of the Eocene in the rest of the country, and ended at the end of the Eocene except in the southeast where it lasted until the beginning End_Page 2312------------------------------ End_Page 2313------------------------------ of the Oligocene. Furthermore, there were previous and posthumous movements, the last of which is very important in the Isthmus and in the Macuspana and Pichucalco basins. End_of_Article - Last_Page 2314------------

Mother Lode and Sierra Nevada Batholith

A mineralized and silicified zone which dips more gently than the wall rock and is accompanied by reverse faulting or thrusting is denned as a mother-lode structure. The western border of the Sierra Nevada batholith is accompanied by a belt of such structures. A tripartite arrangement which coincides with three distinctly different portions of the batholith can be recognized in this belt: (I) a southern portion of scattered lodes within the boundaries of the widest portion of the batholith; (2) a medial continuous zone (the Mother Lode proper) west of the batholith; and (3) a northern extension of scattered, rich lodes which are closely related to the contacts of scattered and small massifs. The divisions of the batholith are believed to be accidental and due to a northward plunge of the mass; lower levels are exposed in the southern region and higher levels in the north. Gold production and mining values increase from south to north and seem to be highest where the shallowest levels of the batholith ar...