Igneous Mass Research Articles

Depth Control of Some Concordant Intrusions

Analysis of geologic data from 54 localities, mainly in the western United States, shows that concordant igneous masses intruded zones in nearly flat-lying sedimentary rocks where the thickness of cover was 3000 to 7500 feet. The depth of intrusion was apparently affected by a well-defined parting surface (bedding plane or unconformity), static load of the overburden (lithostatic pressure), and presence of a fluid barrier above the intrusion. Most bodies intruded along well-defined bedding planes in sedimentary rocks and flow surfaces in extrusive igneous rocks; some intruded along unconformities. The range in overburden pressure is about 3000 to 7500 psi for intrusions at depths of 3000 to 7500 feet. Magma pressure for lateral injection at the lower limit of the depth range, therefore, must have exceeded 7500 psi to lift 7500 feet of overburden. A fluid barrier, such as a shale, overlies almost all intrusions studied and tended to retard upward advance of magma. At depths less than 3000 feet, the barrier shales may be brittle and easily ruptured by the fluid pressure, permitting steam and magma to escape to the surface. Below 3000 feet the shales may become ductile.

Belts of major mineralization in western United States

A study of the literature on the centers of major mineralization in western United States reveals that in the preponderant number of those of the mesothermal and hypothermal environments the mineralizing fractures strike northeasterly; that in some of these centers the mineralization and the igneous masses with which it is genetically related are confined to a relatively narrow zone that trends northeasterly in general parallel to the strike pattern of the mineralizing fractures; and that the contained dikes usually parallel the zone, and often the larger igneous masses are elongated in that direction. Furthermore, the centers of the three main metallogenic epochs of the region--Precambrian, Nevadan-Laramide, and late Tertiary--are aligned in belts that also trend northeasterly, closely paralleling the trends in the centers.It is proposed that the belts of mineralization reflect zones of deep rupture in the earth9s crust all of which formed simultaneously in response to a common regional stress, and that there is a genetic relationship between the zones of rupture and the magma reservoirs from which the mineralizing solutions were derived.

Faults of South and Central Texas: ABSTRACT

The map entitled Faults of South and Central Texas shows the location, depth of trace, and approximate vertical displacement of the various normal faults located in the area covered by the investigation. The information on the faults was derived from: published sources (listed in text); various individuals including especially, William Pittman, Wilford Stapp, and Porter Montgomery; and studies in company files. The writer found that most reports on a particular area were broadly about the same, but might differ considerably in detail. Certainly, no two maps were identical; each geologist seems to have his particular style of fault interpretation. Additional faults undoubtedly will be discovered in the future and the interpretations presented will require alteration. Most of the faults in the region occur within the End_Page 2171------------------------------ major fault systems. A few of the faults are over structures of punch-type origin such as salt domes and igneous masses; radial fault patterns and grabens are commonly associated with these structures (for example, see northern McMullen Co.). Several faults are associated with folds such as the Pearsall and Chittim anticlines. Finally, there are many faults which appear to be structurally independent and which lie outside the fault systems. The largest known fault displacement in the region is less than 1,000 ft; most displacements do not exceed 200 ft. The total displacement of the down-to-the-coast faults is usually about the same as the total displacement of the opposing nearby up-to-the-coast faults. The individual faults and fault systems generally have strikes which are within 10 to 20 degrees of the strikes of the beds in which they occur. Surface and subsurface studies both show that the ends of controlled faults commonly turn in a downdip direction. The actual movements on the faults constituting the major fault systems appear to have occurred only in the downthrown blocks; these blocks are almost invariably lower than persistent regional stratigraphic surfaces. The upthrown blocks are usually even with these surfaces. End_of_Article - Last_Page 2172------------

Salt Temperature : a Fundamental Factor in Salt Dome Intrusion

IN a previous article1, I dealt with the emplacement of igneous masses (volcanic plugs, granite batholiths, diamond pipes, and serpentine bodies), salt domes, and such related bodies as mud volcanoes, ice piercements or pingos, and frost boils. In reviewing the subject for presentation at a recent symposium on salt domes in New Orleans, Louisiana2, it dawned on me that heat is a very important factor and may actually initiate salt intrusion.

The use of An and Sr data on plagioclase in a study of basic xenoliths in a gabbroic mass at Hamar, Somalia

SummaryIn this paper we give data for An and Sr in plagioclase to demonstrate the presence of numerous amphibolite xenoliths within parts of a basic igneous mass where, after initial reconnaissance mapping, none was suspected. The data also show that the concentration of Sr in plagioclase from the margin of some amphibolite xenoliths is appreciably higher than in plagioclase from either the cores of the xenoliths or the enclosing metagabbro even though the modes and An contents of the plagioclase are similar. An explanation is offered, which implies that some of the xenoliths were amphibolites rather than anhydrous equivalents prior to their enclosure in the basic magma.

Geomorphic evolution in Hebridean Scotland

Synopsis The view is challenged that the Highland plateau in Scotland originated during later Palaeozoic times and evolved as a complex ‘peneplain’. The view is also challenged that the Highland plateau originated as an emergent Cretaceous sea-floor. Although Cretaceous rocks were repeatedly deformed by Palaeogene earth-movement and igneous action, there is no sign of a ‘Hebridean rift’ to which the complexities of Hebridean geology may be relegated. Arran, of Hebridean igneous facies in an alien structural context, confirms other evidence in demonstrating that the evolution of regional relief is not to be separated into one mode in a conjectural rift, another in a mainland massif. The primary Hebridean landform appears to be the product of the pulsed emergence of little-warped wave-cut benches. The benches continue uninterruptedly from the Hebridean province into the Highlands, and prove an original geomorphic unity of the whole country. Since the benches bevel the Palaeogene igneous masses and are later than the mid-Tertiary folds and faults, the regional landscape is essentially Neogene in origin. So also appears to be the landscape of other parts of Britain.

X.—A Gravity Survey in Ayrshire and its Geological Interpretation

SynopsisBouguer anomaly maps covering most of Ayrshire at a density of about one station per sq. km., show a close relationship between anomalies and the distribution of the Upper Palæozoic rocks in the area south of the Inch-gotrick Fault, but are less clearly interpreted to the north, where thick dense igneous masses are present.In central and south Ayrshire the gravity field may be largely interpreted in terms of the known density-contrasts at the interfaces separating Upper and Lower Old Red Sandstone, and Lower Old Red Sandstone and Lower Palæozoic rocks. The major structure, the Mauchline Basin, is reflected clearly in the largest anomaly, and there is evidence of a culmination of its south-western limb near Kirkoswald. The important N.E.–S.W. faults also give rise to large anomalies, which may be connected with the known geology. It is inferred that they moved as normal faults in Carboniferous times, and that the adjacent synclines are essentially sags associated with the fault displacements. There is geophysical evidence that both the Southern Upland and Kerse Loch Faults existed in Middle O.R.S. (proto-Armorican) times. It is concluded that a hypothesis of N.–S. Armorican stress is not valid in south Ayrshire.In north Ayrshire, many of the anomalies are best explained by changes of thickness of the Millstone Grit lavas and of the Clyde Plateau lavas, and by the presence of thick dolerite intrusions. Additional evidence is needed, however, before final conclusions may be drawn.

A statistical approach to the exploration of copper in the Singhbhum shear zone, Bihar, India

Rank-correlation analysis suggests that the most favorable areas for copper deposits lie 2-4 kilometers south of the northern limit of slip cleavage (eastern sector) or 1-3 km south of the northern limit of the Singhbhum shear zone (western sector). No relation to distance from igneous rock masses is apparent.

Corona Structures in the Basic Igneous Masses of East Aberdeenshire

THE presence of corona structures in the east Aberdeen-shire intrusions is well known. Read1 described and figured such reaction relations from olivine-gabbros of the Huntly mass. Stewart2 noted in the Belhelvie mass common reaction rims surrounding the olivines of the troctolites, the inner being colourless orthopyroxene, the outer amphibole. Sadashivaiah3 described from ultrabasic rocks of the Bourtie area of the Insch mass “Very beautiful double coronas … are developed around the olivines at the contacts with feldspar; the narrow inner rim is usually of orthopyroxene and the broader outer rim of fibrous amphibole …”. Read, Sadashivaiah and Haq4 recorded from olivine-gabbros of the Insch mass “intergrowths of various kinds and reaction-associations are beautifully developed.… The discontinuous reaction series involved in the reaction association consists of the following members: olivine, hypersthene, augite, hornblende and biotite.” In my investigation of the southern end of the Huntly mass I have observed in troctolites double coronas of enstatite and hornblende, and in olivine-gabbros and olivine-norites double coronas of hypersthene and hornblende.

Notes, mainly geochemical, on the granite-diorite complex of the Insch igneous mass, with an addendum on the Aberdeenshire quartz-dolerites

New analytical data on the Insch granite and diorite are given and are discussed with reference to the origins of these rocks. It is concluded that the granite cannot have resulted from the further differentiation of the parent basic magma that gave the Insch syenites. Less definite conclusions are possible concerning the basic material that contaminated the granite to produce the dioritic rocks. The Permo-Carboniferous quartz-dolerite is found to have a trace-element pattern markedly different from those of the igneous rocks of the Caledonian Insch complex.

An Occurrence of Regionally Metamorphosed Pre-Cambrian Schists in South-West Pembrokeshire

AbstractRecent research work has revealed an area of regionally metamorphosed Pre-Cambrian schists (previously undescribed) in south-west Pembrokeshire. The name “Dutch Gin Schists” is proposed for this newly described group of rocks. The Johnston Plutonic Series (Dimetian) are seen to be intruded into these schists, and the nature of the intrusive contact is described. The structure and petrology of the schists, and the nature of diorites adjacent to the contact and that seen elsewhere in the igneous mass is compared, and a suggestion made regarding the age of the foliation. A comparison is made with the Pre-Cambrian metamorphosed rocks of Malvern, Shropshire, and Anglesey.

The structure of the Rogart igneous complex, Sutherland, Scotland

The Rogart igneous complex, Sutherland, is of Newer Granite age and consists of a zoned tonalite-granodiorite-granite body (the central granodiorite complex) emplaced in Moine metasediments which to the east and north are involved in a peripheral zone of migmatization (the migmatite complex). Although the migmatite complex is genetically related to the central complex, it is not a static contact-effect. Evidence is presented which suggests the following sequence of events. An ascending body of granodiorite magma was preceded by a 'front' of migmatization. When it reached a higher crustal level, migmatization ceased and continued forceful intrusion of the magma deformed the migmatites into a plastic envelope around the expanding magma body, which consolidated as tonalite and granodiorite. Lateral distension exceeded upward movement and a partly funnel-shaped structure resulted. The granite cross-cuts structures in the tonalite and granodiorite and was emplaced later under more brittle conditions. An analysis of the deformation of 'regional' linear structures around the complex confirms the forcefully intrusive nature of the igneous mass. The geometry of the deformation of the migmatites during the emplacement of the central complex is related to the stress field obtaining at this period as deduced from planar and linear structures in the tonalite.

Petrography and provenance of the Millstone Grit of Central Scotland

The mineralogy of the Millstone Grit sandstones is described. It is shown that the heavy mineral assemblage is quite different from those of the Upper Limestone Group and Coal Measures. Lateral variations in the accessory minerals are considered to be due to derivation from several sources rather than to sorting or to differential intrastratal solution. Vertical variations are attributed to changes in the relative importance of the different source areas. An examination of both light and heavy minerals suggests that the major source areas were composed largely of low-grade metamorphic rocks intruded by acid igneous masses and that several of the minor assemblages may have been derived by local erosion of the Upper Limestone Group.

Petrology and Structure of the Southern Wet Mountains, Colorado

The Precambrian core of the southern Wet Mountains consists in part of metasedimentary gneiss and schist concordantly foliated with granite gneiss and composite rocks—migmatite and lit-par-lit gneiss. These rocks developed by alteration during intrusion of the San Isabel Granite batholith and associated smaller plutons. Migmatite borders the batholith and grades outward into lit-par-lit gneiss as a result of more intensive reconstitution and metasomatism nearer the igneous mass. Foliation in the metamorphic and composite units tends to “wrap around” the plutons and is discordant to the foliation (planar flow structure) of these bodies. Lineation, which developed in the metamorphic and composite rocks during folding and neocrystallization of the sequence, plunges northward. Lineation (linear flow structure) of the granite plutons also generally plunges northward and supports the belief that the plutons are of common origin. Field relations and petrographic investigations, including heavy-mineral studies, indicate a genetic relation among the granite bodies; the smaller plutons are probably contaminated differentiates of the magma which formed the San Isabel batholith. Lead-alpha age dating of zircon from the San Isabel Granite indicates that this batholith represents part of the orogenic activity which occurred about 1500 m. y. ago. The southern Wet Mountains form a southeast-plunging anticline with the dominant fracture pattern striking N. 60° W., parallel to the range axis, and a minor trend N. 40° E. Complex structural relations reflect both Precambrian deformation and later superimposed fracturing. Late Paleozoic uplift (Ancestral Rocky Mountains) occurred along pre-existing lines of weakness; Laramide deformation and Tertiary block faulting, although influenced by these trends, formed additional fracture patterns. The Wet Mountains were uplifted to present heights from mid-Tertiary to Pleistocene. Probably the Wet Mountain massif acted as a resistant block to regional compression and yielded by upward movement, although uplift may be the result of solely vertical stresses. Movement occurred in stages along prominent high-angle boundary faults and by slight displacements along innumerable joints.

Differentiation in the Olivine-Gabbro of the Insch Mass, Aberdeenshire

Geological, petrographic and geochemical accounts of two of the basic units of the Insch Igneous Mass are provided. One, the Insch Differentiated Suite, consists of iron-enriched olivine-gabbro with up to 24% total iron-oxides, that passes upwards through syeno-gabbro into syenite, with over 7% potash. This suite is inter-preted as resulting from the seeping of low-temperature constituents towards the roof of the crystallising olivine-gabbro magma which, itself, may have been enriched in iron by the abstraction of refractory magnesium-rich phases from the regional gabbro magma. The second basic unit dealt with is the Bourtie Series of dunites, peridotites and troctolitic gabbros, all rich in magnesia, and interpreted as early cumulates. The possibility of the geological connection of the two units to make a wider differentiated series ranging from dunite to syenite is discussed. Whilst this interpretation is favoured, it is agreed that special geochemical and mineralogical investigations are necessary to test its validity. Thirty-seven new analyses are used in the petrogenetic discussions. A revised geological map of the whole Insch Igneous Mass is presented.

The structural problem of the younger gabbros of north-east Scotland

The evidence for and against deformation of the gabbroic masses is discussed. It is suggested that some of the so-called “older” igneous rocks, at Portsoy and along the western margin of the Huntly mass, are actually parts of the “younger” Huntly mass. The younger masses are later than the early fold structures of the Banffshire coast. Evidence from the primary structures of the igneous masses strongly favours deformation by the Boyndie-Buchan group of folds. Evidence against such folding, although suggestive, cannot be accepted without further work. It is likely that a solution will be found when the Insch and Huntly masses and the rocks of their thermal aureoles have been studied in detail.

The dislocated south-western margin of the Insch igneous mass, Aberdeenshire

The natural contacts of the great Insch igneous mass are roof, and the exposed igneous rocks may form only a small part of a much greater body. The southwestern portion of the contact is a mechanical one, however, along which considerable upward movement of the adjacent country-rocks relative to the igneous rocks has led to the removal of the thermal aureole, to intense dislocation-metamorphism and lenticularisation of the igneous and country-rocks, and to the emplacement of tectonic pods of serpentine and calc-silicate rocks coming from depth. Over great parts of the whole Insch mass the igneous rocks show comparable dislocation effects, with silicification and amphibolitisation in appropriate rocks, increasing towards the southwestern contact shear-zone. Such effects are concentrated within the igneous body itself along two intense shear-belts aligned with the main contact dislocation. All dislocations are later than all the igneous rocks belonging to the Insch intrusion.

Geotectonics of the Pacific with Reference to South-western Japan:II., Geotectonics of the Kii Peninsula in Connection with the Subjacent Igneous Mass, occupying the Greater Part of it

Geotectonics of the Pacific with Reference to South-western Japan:II., Geotectonics of the Kii Peninsula in Connection with the Subjacent Igneous Mass, occupying the Greater Part of it

The Jersey lead-zinc deposit, Salmo, B. C

The orebody lies in a dolomitized zone at the base of a limestone bed in a series of lower Cambrian sediments. The sediments are isoclinally folded and the ore is in the upper limb of the fold. The ore minerals are pale-colored iron-free sphalerite, and galena. Silver is virtually absent but cadmium is associated with the sphalerite. Pyrite and some pyrrhotite are present.The major control of ore deposition has been open and overturned folding. In places the folding is accompanied by brecciation and minor pre-mineral faulting. The ore occurs in the troughs and on the limbs of the folds. The sediments have been intruded by granite, which has formed a tactite or skarn zone beneath the ore. This zone is in places overfolded and high-grade ore occurs around the nose and within the overfold.The deposit is typical of a type of world-wide distribution normally far removed from any igneous masses. The adjacent granite is not necessarily the source of the ore solutions.Trace element analyses show considerable zinc in the overlying argillite and this is considered to be a possible source of the mineralization that has been concentrated in a favorable zone in the limestone by meteoric water.

The Granite—Diorite Complex of the Insch Igneous Mass, Aberdeenshire

AbstractA complex of granitic, syenitic, and dioritic rocks forms the western part of the Insch mass. All these rocks appear to be related, but they are later than, and have no direct connection with, the basic rocks forming the bulk of the Insch mass. The granitic and syenitic varieties are fairly uniform, but the dioritic types are variable rocks rich in xenoliths which are apparently derived from basic rocks. It is suggested that the diorites were formed by contamination of the magma producing the granitic and syenitic rocks.