Late Hercynian Research Articles

Un “fit” structural de l'Atlantique Nord: Arguments geologiques pour correler les marqueurs geophysiques reconnus sur les deux marges

Geophysical markers have been correlated between European and Canadian shelves; the credibility of this reconstruction is mainly based upon equal spacing between equivalent structures on both sides of the North Atlantic. Five major trends are recognized: the Grenville Front, the Dover—Hermitage fault, the South Armorican shear zone, the Chedabucto shear zone and the Ibero—Armorican Arc. The reconstruction of the Hercynian Front is only tentative. Such a pattern could be used to improve North Atlantic pre-opening fits with respect to latitude. Because Upper Paleozoic cross-cutting wrench faults are only responsible for limited offsets, the proposed reconstruction is not significantly different from a Late Hercynian fit.

Certain inferences from correlations of deep and near-surface structure of earth's crust in southern mountain frame of Siberia

In major constituents of the frame (fig. 1), Early Proterozoic to Late Hercynian and Pacific, thicknesses of basaltic, dioritic, and volcanogenic-sedimentary- granitic layers in eugeosynclinal and mesogeosynclinal structures show a dependence upon tectogenetic cyclism (fig. 2) and upon the age (figs. 3, 4). Such relationships are believed to be regional rather than global, particularly east of Lake Baykal. -- V.P. Sokoloff.

Tectonics of Arctic Region of Yakut ASSR: ABSTRACT

The Arctic sector of Yakut ASSR includes the ancient Siberian platform, the late Hercynian Taimyr fold belt, the Mesozoic Verkhoyansk-Chukotsk fold ranges, and shelf seas that represent the recent Laptev-East Siberian Seas paraliageosynclinal region. The Taimyr fold structures are cut off at the continental slope. Fold systems in the Verkhoyansk-Chukotsk region are divided by median masses, and are traced within the shelf seas. The Verkhoyansk-Kolyma system is divided by the Shelon, Kular, and Laptev buried massifs into 3 branches; one of these, in the northwest, abuts the Hercynides of Taimyr and the others extend northward toward the continental slope. The Novosibirsk-Chukotsk fold system, confined on the north by the Novosibirsk and De-Longa massifs, is traceable northwest to the continental slope; on the southeast it is subdivided into 2 branches. One branch terminates with Mesozoides of the Anyuy zone, and another extends toward Wrangel Island. Outliers of fold structures of Mesozoides are exposed on islands of the shelf seas. Parts of median massifs represent uplifts of the basement of the recent paraliageosynclinal region. Within the latter there are northern and southern trough systems. The northern eugeosynclinal(?) system includes foreshelf and marginal-shelf troughs and uplifts, separated by systems of deep faults which probably have large associated intrusions of mafic magmatic rocks. The southern system consists of the miogeosynclinal sublatitudinal troughs that are separated by the Lyakhov uplift. Upper Mesozoic and Cenozoic strata of the troughs may reach thicknesses of 10-12 km. The cut-off by shelf seas and the continental slope of the Mesozoides, Hercynides, and median massif structures; features associated with the building of the Arctic Ocean crust; the nearby presence of mid-ocean and other submarine ridges, which are nearly perpendicular to the continental margins and which commonly are extensions of the continental fold structures of different age; and the formation of a juvenile paraliageosynclinal region within marginal areas of the ocean are End_Page 2496------------------------------ indicative of the extreme youth of large parts of the Arctic Ocean and of the formation of the ocean by oceanization of continental crust as a result of large vertical subsidence movements. End_of_Article - Last_Page 2497------------

Intrusive complexes at sources of Aksaut, Bol'shoy Zelenchuk and Laba rivers (Main Caucasus Range area)

From studies carried out by the author (1954-1962), the metallogeny of the various groups of intrusive rocks of different ages, developed in the Laba-Teberda region of the Main Caucasus Range, emerges clearly. The oldest intrusions - serpentinized hyperbasites - appear totally unpromising metallogenically. Mid-Hercynian intrusions, a strongly differentiated complex, are exceedingly low in mineral content, perhaps because the parent magma was sterile originally. Granites of the late Hercynian are most numerous in the region. Only indication of the metal concentration in young Mesozoic intrusions is the small number of ore shows known to be associated with them in the region under consideration; most important of these are the garnet-magnetite skarns at the source of the Sancharo river, occurring at the contact between Paleozoic limestones and microdiorite dikes.—IGR Staff.

DISTRIBUTION OF URANIUM AND THORIUM IN THE GRANITES OF CENTRAL KAZAKHSTAN

A total of 65 radiochemical uranium and thorium analyses of igneous rocks from the Hercynian Kylcha and Kuu granitic intrusions of Central Kazakhstan is presented. Data on leaching studies and on the uranium contents of mineral separates of Kylcha rocks are given. For comparative purposes, 14 uranium and thorium analyses of Zhirov's for Hercynian Shet granitic rocks are included. Petrographical data include general descriptions of the rocks, modal analyses, and six whole rock chemical analyses. Geologically the Kylcha biotite- and biotite-hornblende granites are determined to be early Hercynian in age whereas the leucogranites of Kuu and Shet intrusions are late Hercynian. Average thorium and uranium contents, and Th/U ratios for the three intrusions are respectively: 31 ppm, 4.2 ppm, and 7.4 for Kylcha granites; 42 ppm, 11 ppm, and 3.8 for Kuu granites; and 46 ppm, 10 ppm, and 4.6 for Shet granites. The uranium and thorium data are treated statistically by the authors and they conclude that there is a real difference between the distributions of radioactive elements in these early and late Hercynian granitic intrusions.--Z. Peterman.

THE STRUCTURES OF THE ORE DISTRICTS, ORE FIELDS AND ORE DEPOSITS OF THE RUDNYY ALTAY

Polymetallic and pyritic-polymetallic ores of the Rudnyy Altay and its intersection with the Gornyy Altay are analyzed as to structure and classified as to ore districts, ore fields and ore deposits. In relating these categories to their containing structures, the author finds that ore districts are associated with long-developing tectonic zones, ore fields with second-order and thirdorder folds, chiefly in the marginal parts of anticlinoria, and ore deposits with shearing and tension fractures. Vein deposits are more common in fractured siliceous rocks. Lenticular and saddle-shaped deposits are commonly interbedded with pre-ore rocks. Concentrated ore complexes are particularly common in regions of block-folds or where synclinoria and anticlinoria intersect. Most of the Rudnyy Altay deposits are assigned to the late Hercynian.—A. Eustus.

Le gite de pyrrhotite de Lacourt (Ariege)

Abstract The Lacourt granite, SSE of Saint Girons in the Pyrenees of southwestern France, is a post-tectonic intrusive cutting Hercynian folds. The granite, characterized by a 1:2 ratio between microcline and plagioclase, and by incipiently-chloritized biotite, had contact-metamorphic effects on the country rocks. Granite and the contact-metamorphic aureole in turn are cut by several large, nearly vertical dikes of quartz, one of which is mineralized with pyrrhotite. The age relationships are not clear. The granite and quartz may be late Hercynian, or much younger, or may even represent several successive phases. Thick pyrrhotite emplacements are locally important.