Ultrabasic Intrusions Research Articles

The case against a lower palaeozoic geosyncline in Wales

AbstractThe author contends that we are in danger of spoiling further a term which has been, in its time, of great service in geology, if we describe as geosynclinal the Lower Palaeozoic rocks in Wales.Their facies and distribution yield no evidence that they were deposited in an elongated trough. At the present day they occupy an almost equidimensional area which is made up of a series of basins of deposition modified by tectonics of the platform type. The “mountains” of Wales (ca. 3000 ft.) are irregularly‐distributed monadnocks on a dissected plateau, and owe their elevation to resistant lavas, fragmental volcanic rocks and basic intrusions. There is no mountain chain of either Caledonian or later age. The rocks are predominantly conglomerates, sandstones, siltstones, mudstones and shales (slates): limestones are thin, scarce and scattered. Ultrabasic intrusives are very rare; and granites insignificant and of doubtful age. No metamorphic rocks (apart from slates) are known younger than Pre‐Cambrian. Shelf and basin facies have been identified at a few horizons and in certain areas, but much work remains to be done in this field. The statement that they are symmetrically distributed, and hence the styling of the geosyncline as the prototype of a new variety, the symmetrical geosyncline, are not warranted.No deep borings from which information about thicknesses could be obtained have been sunk. Estimates from field evidence show that no great thickness of sediments occurs at any particular locality. Breaks in the succession are numerous and widespread. The method of summating “restored” maximum thicknesses of formations and citing the aggregate is of doubtful value in view of the well‐known tendency for the compensatory thickening and thinning of formations as they are traced laterally. Local occurrences of high dips, steep folding and up‐thrusting are not regarded as evidence of orogenesis. It is therefore concluded that the Welsh region consists of a series of gently‐folded basins, in which the facies, thicknesses, trends and structures vary, situated on the south of the Scoto‐Scandinavian geosyncline.

Jadeite-rocks and glaucophane-schists from Angel Island, San Francisco Bay, California

The rocks of Angel Island comprise a series of Franciscan (Jurassic) arkosic graywackes with subordinate amounts of conglomerate and radiolarian chert. Almost all the graywackes have been metamorphosed to jadeite-bearing metagraywackes independently of proximity with basic or ultrabasic intrusions. A diabase sill, originally intrusive into the sedimentary series, has also been metamorphosed and its feldspar converted to chlorite, lawsonite, and jadeite. Glaucophane occurs in the altered diabase and is particularly abundant at its margins. Towards the diabase the jadeite-bearing metagraywackes become richer in glaucophane and pass into jadeite-free schistose glaucophane-rocks. Chemical analyses show that the change is accompanied by some desilication and an addition of Fe, Mg, Ca, Ti, and possibly Na. This metasomatism, which may have preceded, or coincided with, the metamorphism, was probably hydrothermal in nature and localized along igneous contacts and shear-planes. Similar, but more extensive developments of glaucophane-schist, also derived from graywacke, occur in association with a dike of serpentinized pyroxenite. Hydrothermal processes directly related to the serpentinization may have played a significant part in the chemical alteration of the graywacke. Movements along serpentinite contacts complicates the relationships, and many large masses of glaucophane-schist and jadeite-bearing metagraywacke occur as tectonic inclusions within the serpentinite. Plagioclase is absent from, and considered unstable in, all the metamorphic assemblages. While jadeite appears to be stable in the arkosic metagraywackes deficient in Fe, Mg, and Ca; its place is taken by acmitic-jadeite and eventually glaucophane in the glaucophane-schists.

The Pelitic Hornfelses of the Cashel-Lough Wheelaun Intrusion, County Galway, Eire

The pelitic envelope of a basic and ultrabasic intrusion, together with the pelitic xenoliths, have been studied by field and microscopic examination, and 14 chemical analyses of the hornfelsed pelites are given. Eighteen chemical analyses of unhornfelsed pelites from the region are available for comparison. There was every gradation from biotite-acid andesine-quartz-fibrolite-garnet "regional" schist through cordierite-prismatic sillimanite-labradorite-magnetite-biotite hornfels to spinel-magnetite-cordierite $\pm$ corundum $\pm$ orthopyroxene hornfels as desilication and dealkalization occurred. The $SiO_2$ content, 46.2 per cent, and $Na_2O + K_2O$ content, 6.2 per cent, in the average unhornfelsed nearby pelite fell in the xenoliths to less than 10 per cent $SiO_2$ and less than 0.25 per cent alkali, while the $FeO + Fe_2O_3$, 12.0 per cent, and MgO, 3.4 per cent, rose to over 40 per cent and 9 per cent, respectively. It is suggested that the composition of the material removed from the xenoliths to the ultrabasic magma had a composition resembling, but not identical with, that of Daly's average subalkaline granite, which result supports Bowen's theory of the interaction of basic magma and petlite.

Les roches ultrabasiques de la region de Tamatave; mode de gisement, origine et mineralisation

Abstract The Ambodiriana series (migmatites, mica schists, and so-called amphibole pyroxenites) of the Tamatave region, Madagascar, is cut by ultrabasic intrusions, some of which carry chromite. These ultrabasic rocks have been moderately metamorphosed since emplacement. The chromite ore occurs in veinlets that dip in the same direction as the surrounding schists. Their chromite content ranges from 60 to 86 percent, but the Cr/Fe ratio is too low for metallurgical purposes.

III.—Summary of Recent Research on the Pre-Tertiary Geology of the Northern Highlands

I. The Lewisian Gneiss The eclogites and their retrograde representatives, garnet-amphibolites, which occur in the Glenelg area were studied in detail by A. R. Alderman (1936) who showed that these rocks occur in bands which course N.N.E.-S.S.W. with the paragneisses of the locality. The eclogitic rocks have the composition of basalts and represent minor intrusions, and their association with the paragneisses is fortuitous according to Alderman. The retrograde metamorphism and the dispersion of these basic rocks among the banded gneisses of the region are due to the post-eclogite permeation by pegmatitic material. From the same general area Tilley (1936) described still more basic granulites which are definitely members of the paragneiss series. The rocks are granular aggregates of fayalite, grunerite, garnet, magnetite, hedenbergite in varying proportions, the variety in which fayalite is a dominant constituent being called eulysite. In composition they correspond to ferruginous sediments such as greenalite chert and siliceous ironstones. Two less specialised papers on areas of the Lewisian include (1) description of the Harris paragneisses and metagabbro-anorthosite suite by C. F. Davidson (1943), and (2) description of the complex lithology of the gneiss in the Torridon area together with discussion of the metamorphic history of the Lewisian of the mainland by J. Sutton and J. Watson (1951). Davidson showed that the paragneisses represent a series of interbanded shallow water sediments of siliceous, argillaceous and calcareous composition. These with associated minor basic and ultrabasic intrusions and larger masses of gabbro and anorthosite suffered deep regional metamorphism with the This 250-word extract was created in the absence of an abstract

神居古潭結晶片岩中の特殊鉱物について

The Kamuikotan metamorphic complex develops as irregular long zone between the Mesozoic and Cenozoic formations along the western side of the Hidaka mountain ridge in Central Hokkaido. It runs about 200 km in a direction of nearly north to south, from areas of boundary of Teshio and Kitami Provinces to Hidaka Province.In general the normal Kamuikotan complex is made up the products due to slight regional metamorphism, in which the chief petrological members are black siliceous schists and next comes an abundance of green schists, small amonnts of quartzite, phyllite, mylonite and crysrtalline limestone are intercalated in the complex. From the geological and petrographical points of view, it may reasonably be assumed that the Kamuikotan complex can be readily correlated with the Franciscan formation in California.Comparatively large lenses or belts of the ultra-basic rocks run parallel with the trend of the Kamuikotan complex, being restricted mainly to the tectonic lines is the zone of the complex. It is noticeable that various interesting crystalline schists locally develop between the normal schistose rocks of the Kamuikotan complex and the ultra-basic intrusives, which have attracted special attention because of the presence of some special minerals, especially soda-bearing silicates. These special rocks may be a product brought about by contact metamorphic action due to ultra-basic intrusion and occurrence of the special minerals in them originated from pyrometasomatism, primarily owing to the action of some especially soda-rich hydrothermal solution drived from the ultra-basic intrusions.The subject matter of this paper is an outline of these some special minerals in the pyro-metasomatized rocks, with special reference to the general properties of the following members ; (A) glaucophane and crossite, (B) riebeckite and crocidolite, (C) aegirine-augite, (D) stilpnomelane, (E) piedmontite, (F) garnet, (G) lawsonite, and (H) pumpellyite.

The basic and ultrabasic rocks of the Sanbagawa metamorphic region in the Tenryu river basin

In the Sanbagawa metamorphic region of the Tenryu river basin, the basic and ultrabasic intrusion can be divided into the following three stages. 1 st stage: The synkinematic intrusion of sills of “Topfstein” and epidote-amphibolite. The later was derived from a basic intrusive rock. Accompanying the intrusion of these rock, the crystalline schist were subjeoted to metasomatisms which were formed of the albitization and the chloritization. 2 nd stage: The intrusion of saussurite gabbro, periodtite, metadiabase and amphibolite. The latest was derived from basic and ultrabasic rocks. In the last of this stage, the banded rock, is composed of the peridotite and the saussurite gabbro, intruded into the space (Mikabu Zone) between the crystalline schist and the non-metamorphic palaeozoic rock. 3 rd stage: The intrusion of the serpentin was accompanied by the fault.

Tectonic Development of Cordilleran Region: ABSTRACT

The Cordilleran region of Western United States is a segment of the Cordilleran geosyncline. Its western and eastern borders are the Pacific Coast Ranges and the Wasatch or Teton line. The Cordilleran geosyncline differs from its cratonic neighboring regions, the Colorado Plateau and the Rocky Mountains, by (1) containing extreme thicknesses of sedimentary and volcanic rocks and by (2) having experienced true orogeny. Like its counterpart, the Appalachian geosyncline, it has dual facies expression. An inner (cratonward or miogeosynclinal) belt has thick sediments lithologically similar to cratonic rocks; an outer (seaward or eugeosynclinal) belt has very thick graywackes, volcanics, and other lithotopes. It is convenient to describe the tectonic development of the geosyncline in four stages, each of which exhibited a distinctive tectonic pattern. Stage I, comprising only the Cambrian period, was essentially simple failure of the continent margin by subsidence behind a progressively inward migrating hingeline. Sediment source was cratonic. Stage II was a complex interplay of orogeny, volcanism, and deposition with long duration. It End_Page 790------------------------------ opened in Ordovician time with development of a midgeosyncline hinge (the Manhattan line) which introduced the dual facies aspect. Dark, poorly sorted clastics in the outer belt were derived from tectonic island chains which rose within the belt itself (autocannibalism) while normal carbonates and clean sandstones accumulated in the inner belt. Folding and volcanism commenced in the Klamath Mountain area. Silurian and Devonian tectonic patterns resembled Ordovician. During the Carboniferous, intensified tectonic activity was expressed as folding and thrusting on the approximate site of the old Manhattan hingeline to produce an uplift (Manhattan geanticline of Eardley) which shed conglomerates into both belts. Folding, volcanism, and deposition of clastics continued in northern California. Further intensification of tectonic action occurred in the Permian, Triassic, and lower Jurassic periods. Profound subsidence of the outer belt accompanied deposition of autochthonous volcanics of regional extent and extreme thickness. A composite maximum of 70,000 feet (12 miles), centering in the California-Nevada area, dominated the entire outer belt. In addition, (1) strong tangential stresses produced local dynamometamorphism in the Sierra and Mohave areas of California in Permian time, (2) basic plutons were emplaced in central Oregon in Triassic time, and (3) ultrabasic intrusions of lower Jurassic age were injected in the Klamath area. These and earlier tectonic events indicate existence of a so-called Klamath tectogene in Washington, Oregon, and California. During the Mesozoi , midgeosyncline lands crowded the inner belt eastward, finally causing extinction. Stage III was climactic orogeny involving not only folding and thrusting but regional dynamometamorphism and large-scale batholithic intrusion. Beginning in upper Jurassic time, batholithic emplacement in the Klamath tectogene reached proportions unequalled in North America since the Precambrian. Repeated impulses of folding and thrusting marched eastward during late Jurassic and Cretaceous time to culminate as thrusts of major magnitude (Lewis, Bannock, Strawberry, Muddy) at the Teton-Wasatch line. Apparent post-orogenic relaxation of Stage IV caused collapse of the orogenic highland in the Great Basin. Extensive block faulting plus actual subsidence produced a Tertiary-Recent depositional basin (comparable with the Triassic of the Appalachian belt) which to-day exhibits internal sediment supply, and which in future may acquire a depositional history of some magnitude. Eventual broad, regional, epeirogenic uplift of the Cordilleran belt with associated erosional planation is predicted. End_of_Article - Last_Page 791------------

Biotitite in the Basement Complex of Southern Nyasaland

AbstractRecent work in the Middle Shire Valley has revealed a swarm of biotitite bodies emplaced in gneisses and granulites of the Basement Complex. The rocks of the Basement Complex are described briefly and referred to the Amphibolite and Granulite Facies of regional rnetarnorphism. The field relations, distribution, and petrology of biotitite bodies show that they are a series of metamorphosed ultrabasic intrusions. An occurrence is described in which hydrothermal agencies have altered biotitite to vermiculite rock.

METAMORPHIC AND IGNEOUS ROCKS OF THE MERRIMAC AREA, PLUMAS NATIONAL FOREST, CALIFORNIA

The pre-granitic rocks of an area in the northern Sierra Nevada consist of metamorphosed sedimentary and volcanic series ranging in age from Carboniferous to Jurassic. Synkinematic ultrabasic intrusives, now serpentines, cut these rocks concordantly and discordantly. Magmatic series ranging from basalt to dacite and soda-rhyolite occur together with the normal basalt-rhyolite series among the meta-volcanics. The younger intrusives (Sierra Nevada series), ranging from gabbros to granodiorites and granites, show great chemical similarity to the meta-volcanic series. Furthermore, soda-rich members are common among the pre-granitic intrusives and younger dike rocks. The pre-granitic rocks were folded and metamorphosed to green schist and epidote-amphibolite facies prior to emplacement of granodiorite and granite batholiths. The later contact metamorphism affected the areas next to the contacts of the batholitic intrusions, causing crystallization of such minerals as garnet, diopside, epidote, and andalusite. The plutonic rocks obtained the space needed partly by pushing the country rocks aside and partly by stoping and assimilation.

Franciscan-Knoxville Problem

Franciscan-Knoxville Problem

Serpentinization of Vermont Ultrabasics

INTRODUCTION THE QUESTION Ultrabasic intrusives have been problem rocks for many years, but, of late, interest has centered around the later changes that have taken place in these rock masses. The chief item is the autometamorphic or exometamorphic origin of the serpentine derived from them; in other words, the question is, did the solutions producing change come from outside the masses themselves or were they indigenous to the original magmas? CONDITIONS OF INVESTIGATION The Vermont ultrabasics are a series, exposed under conditions peculiarly favorable for study.1 Some of the best exposures are in verde antique marble quarries, where structural detail of the stone shows to advantage on the walls. Mapping of practically all similar intrusives, in a belt extending from the asbestos district of Quebec southward into Massachusetts, has been done on an extremely detailed scale, and surface exploration was supplemented by diamond drilling on many of the serpentine bodies. . . .

Genesis of the Anthophyllite Deposits near Kamiah, Idaho

Anthophyllite as mass-fiber asbestos forms a number of lenticular bodies near Kamiah, Idaho, where it occurs as a replacement of ultra-basic intrusives, particularly dunite. In these bodies the anthophyllite assumes its own characteristic crystal form in replacing the olivine, and occurs as fibers or bundles of fibers arranged in radial groups which on cross-fracture yield rosettes. Associated with the anthophyllite is a minor amount of antigorite, talc, carbonate, and pyrite, typical of mineral assemblages that have been formed by hydrothermal processes. It is believed that the anthophyllite and its associated minerals are the products of hydrothermal metamorphism related to igneous emanations from the Idaho batholith that underlies the deposits at slight depth.