Syenite Gneiss Research Articles

A model for lower continental crust

This proposed model is based on geological, geophysical and geochemical data. Previous models suggested for the lower continental crust consisted of basalt, gabbro, or charnockitic rocks; however, experimental and field petrological data indicate that the bulk of crustal rocks are metamorphic. A lower crust of heterogeneous metamorphic rocks also agrees with seismic reflection results which show numerous reflections from “layering”. Geothermal conditions favor a “dry” charnockitic or gabbroic lower crust rather than an amphibolitic lower crust because heat production data imply that wet amphibolitic rocks would have a higher heat production than their dry metamorphic equivalents. Relatively high velocities from field and laboratory measurements in such low-density rocks as granite, syenite, anorthosite and granulitic rocks in general imply that the composition of the lower crust is more felsic than gabbro. Variation in seismic velocity and depths from crustal refraction studies and numerous seismic reflections all indicate a highly heterogeneous lower crust. The lower crust, which has traditionally been described as gabbroic or mafic, may consist of such diverse rocks as granite gneiss, syenite gneiss, anorthosite, pyroxene granulite, and amphibolite, interlayered on a small scale, deformed, and intruded by granite and gabbro. Interlayering of these rocks explains the presence and character of seismic reflections. Abrupt changes in dip, tight folding, disruption of layers, intrusion, and changes in layer thickness explain the characteristic discontinuity of deep reflections. Igneous intrusions may be floored by metamorphic rocks. The lower crust consists of a complex series of igneous and metamorphic rock of approximate intermediate composition.

Petrology of Alkalic Gneiss in the Dahomeyan of Ghana

A layer of alkalic gneiss, 7 to 20 m thick, extends almost continuously for 60 km in the Precambrian Dahomeyan basement complex of southeastern Ghana. Leucocratic nepheline–syenitic gneiss predominates, whereas mafic nepheline syenitic gneiss and rare feldspar-poor nepheline gneiss occur locally. Silica-saturated leucocratic gneiss is intercalated with the undersaturated rocks throughout the layer, but oversaturated peralkaline types are rare. The alkalic rocks are chemically similar to phonolite, alkali trachyte, comendite, mugearite, and ijolite; the leucocratic rocks plot in the experimentally determined low-temperature region for salic magmatic liquids. The alkalic gneiss has been derived by the isochemical metamorphism of pre-existing alkaline igneous rocks; the parental alkalic rocks may have been of volcanic origin. Regionally associated with the alkalic rocks are mafic gneiss and amphibolite similar in chemical composition to basanite, alkali basalt, and hawaiite. Mafic nepheline gneiss, chemically similar to mugearite, forms a geochemical link between the mafic gneiss and amphibolite and the leucocratic alkaline gneiss. Petrochemical trends on variation diagrams and close field relations suggest that the parent igneous rocks of the alkalic gneiss were derived by fractionation from mafic magmas that were parental to the mafic gneiss and amphibolite. The alkalic gneiss, mafic gneiss, and amphibolite have undergone at least two stages of regional metamorphic recrystallization that were separated by a period of strong penetrative deformation. The latest recrystallization was in the epidote-amphibolite facies and probably was associated with the Pan-African thermotectonic event.

Thermal convection below the solidus in a mantled gneiss dome, Fungwi Reserve, Rhodesia

Previously unrecorded structures inside a mantled gneiss dome are described and interpreted as having important geological implications. The gneisses which make up this basement structure were domed on at least three scales near the thermal acme of the post-Umkondo 500 m.y. orogeny in the Zambezi orogenic belt. Groups of domes on one scale occur inside those of successively larger scales, forming a hierarchy of structures which are interpreted as being due mainly to gravitational, rather than lateral forces. The rocks involved have an essentially uniform density at the present and it is argued that a concentration of the regional orogenic thermal gradient would have induced unstable density gradients capable of driving thermal convection below the range of temperatures at which the rocks would have melted. For the domes (or convection cells) to have been preserved in the manner in which they are found the convection can only have been due to marginal mechanical instability and must have been stabilized after the first half-cycle of motion. Heat is conducted faster parallel to a gneissose foliation than normal to it and, in restricted ranges of thermal gradients, the mechanism of heat transfer, conduction or convection-with-conduction, may depend solely on the attitude of the foliation in relation to the thermal gradient. The hierarchy of structures is interpreted as a chain reaction of alternations of conduction and convection caused by the rotation of this thermal anisotropy as the structures formed. The genetic model suggested is tested and found to be geologically acceptable in termsthe thicknesses of the convecting layers, their effective viscosities, and the time necessary for each structural pattern to develop. It is argued that had the regional orogenic thermal gradient persisted or been exceeded the incipient convection preserved in Fungwi would have become cyclic. This would have resulted in the homogenization of the granite- syenitic gneiss to what would subsequently look like a normal magmatic rock in a very short time geologically. Other basement domes in the region are interpreted as recording a predictable sequence of normal gneiss domesto completely ‘igneous’ plutons. The production of homogeneous rocks from a gneissose basement by convective overturn may be anticipated as a reasonably common orogenic phenomenon inside local thickenings of mechanically uniform rock sequences. It is suggested that this simple mechanism is a link often assumed or alluded to by earlier writers who have described migma and magma domes in close proximity.

Gravity Study of a Hypersthene Syenite in the Laramie Anorthosite Complex, Wyoming

On the northwest border of the Laramie anorthosite complex, 236 gravity stations were located approximately 1 per square km over a hypersthene syenite body and country rock. The hypersthene syenite has a maximum width of about 6 km, extends about 18 km, and is bordered principally by anorthosite, hornblende syenite, and granite gneiss. Three 15 to 18-mgal anomalies with maximum gradients of about 4 mgal/km are centered over the hypersthene syenite. Bouguer anomaly values range from a mean of about —150 mgal in the anorthosite to a high of —128 mgal over the hypersthene syenite to Bouguer mean values of —150 mgal over granite gneisses to the west. Strikingly, isoanomaly lines locally crosscut geologic contacts. Computed minimum density contrasts of about δσ = .23 indicate that a subsurface mafic mass, possibly norite, is the principal disturbing mass and not the hypersthene syenite. Models indicate that this mafic mass is within .5 km of the surface in places and extends to a depth of about 5 km. The geometry of the hypersthene syenite is difficult to ascertain.

Geology and geochemistry of the Blue Mountain nepheline syenite

The Blue Mountain nepheline syenite sill was formed from a nepheline syenite magma that intruded metasediments and syenite gneiss about 1300 m.y. ago. The magma was fractionated during emplacement into several zones, distinguished by their chemistry, mineralogy, and texture. Pegmatites were formed from late-stage fluids. Mafic bodies in the sill represent slabs of metasediments which were stoped from the roof of the sill by the magma, and which were plasticized and strung out into narrow sheets parallel to the direction of flow of the magma. Strong contortion of some sheets was produced by non-laminar flow of the magma around the included material. A contact aureole was developed above and to a minor extent below the sill; alkalies, alumina, water, and carbon dioxide were added to the metasediments from the magma.The sill and surrounding rocks were folded into their present positions during the Grenville orogeny about 1000 m.y. ago. Country rocks were recrystallized, but the rocks of most of the sill were only slightly affected by the metamorphism accompanying the orogeny.

The petrology of a potassic syenite and its associated biotite pyroxenite at Shaki, Western Nigeria

A syenite gneiss associated with biotite pyroxenite and biotite-muscovite gneiss forms an elongated mass covering about 150 km2 in the basement complex around Shaki in Western Nigeria. It lies conformably in the biotite-museovite gneiss to which it is similar in texture. The biotite pyroxenite occurs as patches of varying sizes widely distributed throughout the syenite but is not found in any of the surrounding rocks.

PRECAMBRIAN CORUNDUM-BEARING ROCKS, MADISON RANGE, SOUTHWESTERN MONTANA

Corundum-bearing rocks are associated with metamorphosed mafic rocks in a metamorphic terrane mainly of metasedimentary rocks including gneiss, schist, amphibolite, and minor iron formation. Granulite(?) facies metamorphism of an aluminous sediment produced biotite syenite gneiss; next, perhaps late in this phase, unoriented corundum crystals surrounded by alkali feldspar halos formed by replacement of the gneiss with the addition of Al and K. The final step was local development of muscovite-sillimanite schist probably during almandine amphibolite facies metamorphism, Petrographic and minor element studies are presented in support of these conclusions.

Preferred Orientation of Nepheline in Deformed Nepheline Syenite Gneisses from Sørøy, Northern Norway

AbstractEvidence is presented which shows that nepheline develops a preferred c-axis orientation in response to deformative stresses. It is believed that this is the first record of a stress-induced preferred orientation of nepheline.

The Laurentian of the Ottawa District

Research Article| January 01, 1892 The Laurentian of the Ottawa District* R. W. Ells R. W. Ells Search for other works by this author on: GSW Google Scholar Author and Article Information R. W. Ells Publisher: Geological Society of America First Online: 02 Mar 2017 Online Issn: 1943-2674 Print Issn: 0016-7606 © 1892 Geological Society of America GSA Bulletin (1892) 4 (1): 349–360. https://doi.org/10.1130/GSAB-4-349 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. W. Ells; The Laurentian of the Ottawa District. GSA Bulletin 1892;; 4 (1): 349–360. doi: https://doi.org/10.1130/GSAB-4-349 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Views of earlier Writers.In discussing the structure of the Laurentian rocks, as developed in the valley of the Ottawa river, their characteristics, as given in the first report of the late Sir William Logan, in 1845–’46, on “The Laurentian of the Upper Ottawa,” may here be presented. After stating that a low anticlinal crosses that river between the mouth of the Mattawa and the foot of Lake Temiscamingue, he says:“The lowest rocks which this undulation brings to the surface are of a highly crystalline quality belonging to the order which in the nomenclature of Lyell is called metamorphic instead of primary, as possessing an aspect inducing the theoretic belief that they may be ancient sedimentary formations in an altered condition. Their general character is that of syenitic gneiss. Their general color is reddish, and it arises from the presence of reddish feldspar, which is the prevailing constituent . . . This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

1. Report of the Boulder Committee, with Remarks by the Convener

Between Crinan Bay on the north and the head of Loch Fyne on the south, there is a trough or hollow now occupied by the Crinan Canal. The highest point along this trough, is about 150 feet above the sea. A series of locks occur at this summit-level, to allow of the passage of vessels between the two sea-lochs.At the summit-level, the rocks form a sort of ridge across the valley, with smooth surfaces towards the north, and rough surfaces towards the south.On both sides of this rocky ridge, there are large boulders; on the north side, I counted between forty and fifty, on the south side, there are not more than two or three. The boulders are a syenitic gneiss; the rocks in situ, are shivery clay slate, nearly vertical, dipping steeply towards the south.

Note regarding the Occurrence of Jade in the Karakash Valley, on the Southern Borders of Turkestan

The portion of the Kuen-Lun range which extends from Shahidulla eastwards towards Koten, appears to consist entirely of gneiss, syenitic gneiss, and metamorphic rocks, the latter being quartzose, micaceous, and hornblendic schists. On the southern declivity of this range, which runs along the right bank of the Karakash river, are situated the old jade-mines, or rather quarries, formerly worked by the Chinese; they are about seven miles distant from the Khirgiz encampment of Belakchi, which is itself about twelve miles S.E. from Shahidulla. I had the pleasure of visiting the mines in company with Dr. Bellew and Captain Biddulph, a Yarkundee official being our guide. We found the principal jade locality to be about one mile and a half from the river, and at a height of about 500 feet above its level. Just in this portion of the range, a few short spurs spring from the higher hills, all of which, however, as is usual, are thickly covered with débris and sand, the result of disintegration of the original rock; the whole has an appearance as if an extensive slip of the mountain-side had occurred.

Geological Notes on the Route traversed by the Yarkund Embassy, from Shahidulla to Yarkund and Kashgar

In a former communication I had already occasion to notice that the rocks composing the Kuen-Lun range, near Shahidulla, chiefly consist of syenitic gneiss, often interbedded and alternating with various metamorphic and quartzose schists. Similar rocks continue the whole way down the Karakash river for about twenty-four miles. After this the road runs in a somewhat north-westerly direction, following a small stream leading to the Sanju (or Grimm) Pass. Here the rocks are chiefly true mica-schist, in places full of garnets. Near and on the pass itself chloritic and quartzose schists prevail, in which veins of pale green jade occur, numerous blocks containing this mineral having been observed near the top of the pass. All the strata are very highly inclined, often vertical—the slopes of the hills, and, in fact, of the entire range, being on that account rather precipitous, and the crests of the ridges themselves very narrow. To the north of the Sanju Pass, we meet again with metamorphic, mostly chloritic, schists, until we approach the camping-place Yám, where distinctly bedded sedimentary rocks cap the hills of both sides of the valley. These are dark, almost black, silky slates, resting unconformably on the schists, and are overlain by a grey, partly quartzitic sandstone, passing into conglomerate. The last rock contains particles of the black slates, and is therefore clearly of more recent date.