Cone Sheets Research Articles

Structure of the coastal dyke swarm and associated plutonic intrusions of East Greenland

The coastal dyke swarm and associated flexure, plutonic intrusions and volcanics are the products of a short episode of rifting between normal and thinned continental crust during initial opening of the Atlantic Ocean between Greenland and the Rockall Plateau 56–52 m.y. ago. They constitute a continental rift zone which provides deeply eroded onshore examples of phenomena which probably lie buried beneath the sea along major rifted continental margins. The dyke swarm occurs in a series of zones arranged en echelon, similar to dyke and fissure swarms in Iceland. Most dykes were intruded vertically before flexuring rather than as a fan during flexuring as postulated by Wager and Deer [1]. Layered gabbro plutons and basic cone sheets were emplaced during early stages of flexuring. Magma was tapped westwards along the upper limb of the developing flexure to form the Skaergaard and Kap Edvard Holm intrusions, but intrusions such as Imilik and Kap Gustav Holm in the steep limb show more complex histories of synplutonic tilting, slumping and deformation. Most flexuring occurred after consolidation of the gabbros and was followed by the intrusion of linear and radial swarms of intermediate dykes and ring dykes associated with the emplacement of syenite and granite plutons by cauldron subsidence.

Plinian-type eruptions in the medicine lake highland, california, and the nature of the underlying magma

Contemporaneous Plinian eruptions of rhyolite pumice from Glass Mountain and Little Glass Mountain during the last 1100 years B.P. were followed by extrusion of lava flows. 1.2 km of material was erupted and 10% by volume is tephra. All of the tephra deposits consist of very poorly sorted coarse ash and lapilli that are mostly pumice pyroclasts. Eruptive sequences, chemical composition and petrographic character of the rhyolites at Little Glass Mountain and Glass Mountain suggest that they came from the same magma body. The 1:9 ratio of tephra to lavas is typical of small silicic magma chambers. Eruption from a small chamber, 4–6 km deep, at vents 15 km apart is possible if magma rose along cone sheets with dips of 45–60°. The caldera rim and arcuate lines of vents near it may represent the surface expression of several concentric cone sheets. Pumice pyroclasts erupted at Glass Mountain and Little Glass Mountain may have formed in the following manner: (1) vesicle growth and coalescence beginning at 1–2 km depths; (2) elongation of the vesicles by flow within the cone sheets; (3) disruption of the vesiculated magma when it reached the surface by an expansion wave passing down through it; and (4) eruption of comminution products as pumice pyroclasts. Plinian activity at Little Glass Mountain and Glass Mountain continued until the volatile-rich top of the magma chamber had been depleted.

Geology of the Urad and Henderson molybdenite deposits, Clear Creek County, Colorado, with a section on a comparison of these deposits with those at Climax, Colorado

The Urad and Henderson stockwork molybdenite orebodies at Red Mountain, Clear Creek County, Colorado, are related to a rhyolitic subvolcanic center of Oligocene age referred to as the Red Mountain Complex. The two orebodies are well separated in space and are specifically related to different intrusive rocks of the complex. The Urad orebody was deposited first and at shallow depth. The Henderson formed deeper within the complex and is much the larger of the two. The Oligocene rocks intrude a batholith of Precambrian Silver Plume Granite near the western edge of the Colorado mineral belt and are within a mile of the north-north-east-trending Berthoud Pass fault. This fault is the major structural element of the area and has been identified by Tweto and Sims (1963) as a Precambrian fault reactivated in Tertiary time.The part of the Red Mountain Complex crops out over an irregular area about 2,400 feet long and 800 to 1,200 feet wide. Dikes related to the complex surround it and are present as much as 1 mile from the igneous center. Known rocks of the complex represent more than 15 stages and substages of igneous activity. All rocks are chemically and mineralogically similar and consist of essential quartz, K-feldspar, albite, and minor biotite. Clastic and fragmental textures are common features of the rocks exposed at surface, porphyries predominate at intermediate depths, and a granitic rock is present at the greatest depths reached in exploratory drilling. The major rock types of the complex exposed at the surface are columns that either grade into, or are cut off by, an underlying composite stock, the apex of which lies about 2,500 feet below the peak of Red Mountain. The stock, although much larger than the shallower columns, appears to be a steep-walled intrusive to explored depths. The space now occupied by the stock is thought to have been a magma chamber throughout the time the rocks exposed at the surface were emplaced; the magma chamber was their immediate source. The sequence of rocks exposed at surface is as follows: Tungsten Slide and East Knob units (relative ages unknown), Square quartz porphyry, rhyolite porphyry radial dikes, and the Red Mountain porphyry unit. Each unit consists of several members. Rock types which constitute the buried stock are, from oldest to youngest, the Urad porphyry, the Primos porphyry, and the Henderson granite. The Square quartz porphyry is the youngest known igneous rock predating the Urad orebody and is considered to have been the source intrusive for the Urad mineralized zone of which the Urad orebody is a part. The Urad mineralized zone is envisioned as a cylindrical body of mineralized rock. The orebody was localized within part of this zone by a series of moderately dipping arcuate fractures referred to as the main fissure system. The fissures are interpreted as cone sheet fractures caused by the emplacement of the Square quartz porphyry. About half of the orebody was contained in rocks of the Red Mountain Complex; the rest was in the Silver Plume Granite. The Urad mine, now worked out, produced about 14 million tons of ore. Intrusion of the Red Mountain porphyry, the principal rock of the complex at surface, destroyed much of the Square quartz porphyry, a large part of the Urad mineralized zone, and part of the Urad orebody. The magma from which the Red Mountain porphyry crystallized probably vented; a potential orebody may have been lost to the atmosphere during this event.The Henderson orebody is an umbrella-shaped stockwork entirely within the intrusive complex and contains more than 300 million tons of ore. The difference in the shape of the Urad mineralized zone and the Henderson orebody is thought to reflect the difference in the depth of formation. The Henderson deposit is genetically related to one or more phases of the Primos porphyry and to the Henderson granite and is a compound orebody. A master reservoir is postulated at depth beneath Red Mountain to explain the repetition of igneous and hydrothermal events and to provide the quantity of metal necessary for the formation of the ores. We believe that this magma derived its ore metal from crustal rocks anomalously rich in molybdenum during its rise toward the surface. The deposits at Red Mountain are remarkably similar in many respects to those at Climax, Colorado, but there are some notable differences. The abundance of Precambrian metamorphic rocks, possibly available for reaction with, or incorporation by, Oligocene magma at Climax, and the scarcity of these rocks at Red Mountain may account for the large amount of tungsten in the Climax deposit compared to that in the Urad and Henderson orebodies.

Multidimensional wave radiation from a source with Gaussian time variation and Gaussian-approximated distribution about a spherical sheet

This paper concerns the wave field of a source with the title-indicated space–time function which, additionally, possesses an arbitrary directional variation. The multivariate solution obtained comprises an estimated error plus peak-induced spherical harmonics that are hyperconically confined, i.e., bounded by diverging and converging spherical fronts. Such fronts are not necessarily singular. Compliance with the radiation principle ensues, through contour integration, from Cauchy initial conditions. For an odd number of spatial dimensions, an inner zone created after a focusing phenomenon exhibits an analogy with a Petrowsky’s lacuna. Naturally, the wave field varies with direction, but only because its source does so. Spherically as well as axially symmetric cases constitute major corollaries. Asymptotic developments, evolving ultimately into steady limits, are also deducible. An indirect application is illustrated for magnetoacoustic flow parallel to a magnetic field; on induction by a cylindrical Gaussian-approximated current distribution, weak effects appear everywhere during the steady state and are superposed upon strong stationary wave effects bounded by cone sheets which project either (i) downstream for a supersonic–super-Alfvénic flow, or (ii) upstream for a restricted subsonic–sub-Alfvénic flow. Finally, the main results are directly applied to elastic wave propagation from a two-component Gaussian body force concentrated about a spherical base; a spherically symmetric radial component generates a strong irrotational wave field normally involving an instantaneous point singularity; an axisymmetric azimuthal component generates a strong solenoidal wave field.

Application of age and Sr isotope data to the petrogenesis of the Marangudzi ring complex, Rhodesia

The Marangudzi ring complex, Rhodesia, consists essentially of a gabbro mass intruded by ring dykes of quartz syenite and cone sheets of nepheline syenite. Five intrusive units (gabbro, two quartz syenite and two nepheline syenite units) have been studied using Rb-Sr and K-Ar methods. A total of 24 whole rock samples define a Rb-Sr isochron which gives an age of 186 ± 3m.y. and an initial ( 87Sr/ 86Sr) 0 ratio of 0.70769 ± 0.00006 (±2sigma; based on λ = 1.42 × 10 −11yr −1) . K-Ar and Rb-Sr analyses on biotite and hastingsite separates are consistent with this age assignment. Whole rock Rb-Sr isochrons for the different units treated individually agree with the above age and initial Sr ratio within analytical uncertainties. This is believed to indicate that the different rock types are comagmatic forming by fractional crystallization of a parental, mantle-derived, K 2O-rich basaltic magma, having an initial Sr ratio of 0.7077, without appreciable assimilation of the Precambrian country rock. The entire differentiation, emplacement and crystallization processes took place over a rather short time span.

Discussion of the stress control of the formation of inclined sheets

D r W. J. P hillips writes: In his recent paper Dr Walker (1975a) adopted Bradley's (1965) concept of isopiestic surfaces and derived theoretical isopiestic surfaces of lithostatic pressure for a model in which "a body of low density (2.3 g cm −3 ) acid magma is emplaced in rocks of appreciably higher density" (2.6 to 2.85 g cm −3 ), but is underlain by basic intrusive rocks of density, 3.0 g cm −3 ( Walker 1975a Fig. 3 a ). Walker's hypothesis for the formation of cone sheets is based on the assumption that isopiestic surfaces of lithostatic pressure derived from this model may be superimposed on another set of theoretical isopiestic surfaces (Fig. 3 b ) which is derived for "the hydrostatic pressure of a basaltic magma of density 2.65 g cm −3 ” if it occupied the space of the acid diapir and basic intrusive rocks of the first model. Clearly the superposition of the two sets of isopiestic surfaces, Fig. 3 c, is incorrect because the space can be occupied by only one of the two alternative components of the model and therefore the supposed isopiestic surfaces of the other component would be absent. This restriction invalidates the concept of excess hydrostatic pressure and consequently also the supposed new concept of the evolution of the British Tertiary intrusive centres. However it is of interest to consider a number of other points which give cause for concern. In general, basaltic magma does not penetrate the pore spaces of the envelope rocks and therefore the isopiestic surfaces derived for the hydrostatic pressure. in

DISCUSSION OF PREVIOUS TWO PAPERS

DR M. J. L e B as offered his congratulations to Dr Walker on his lucid presentation of the role of density during magmatic ascent. However, density effects alone cannot explain the majority of continental basalts which have been extruded through less dense materials. That density control can be important is shown at Carlingford and Skye, where layered gabbros intrude as sheets above sialic crust and at the base of the Tertiary lava pile: Bailey’s mechanism for ring dyke formation is almost impossible here. Dr Walker likened the inclined sheets of Iceland to cone sheets, but the former are planar and dip to the riftzone which gives a different stress field from that of the conical sheets in the Hebrides. It seems that cone sheets raise their central mass of country rock as a result of vertical forces, and that this mechanism is well exemplified by highlevel carbonatite complexes. How did he reconcile the fact that his model appears to depend on acid preceding basic magma, contrary to the relations at most Hebridean centres? Had he considered the mechanical strength of the crust through which magma penetrates: the nature and extent of joints and fractures in strata along which magma can find weaknesses? Finally, did he distinguish between the different effects, apart from viscosity, of ‘wet’ and ‘dry’ magmas, which can either soften the envelope of an intrusive boss followed by deformation and folding, or give brittle-type fracture of the country rocks permitting sheet intrusion? DR R. R. S kelhorn : It was very

A new concept of the evolution of the British Tertiary intrusive centres

An early event in the history of most or all of the intrusive centres in the British Tertiary province was probably the uprise to a high level of an acid diapir. This fact, combined with the idea that the relative densities of magmas and crust play a major role in the localisation of igneous intrusions, forms the basis for a new interpretation of the mechanism of formation of the intrusions and enables a model to be developed for the evolution of each centre. Firstly it is postulated that the emplacement of cone sheets is governed by a tendency of rising magma to move in the direction of maximum excess hydrostatic pressure, P He (this being the amount by which the hydrostatic pressure in the magma exceeds the lithostatic pressure). It is suggested that the perturbation of surfaces of equal P He near a high level diapir of low-density acid magma can cause uprising magma to be diverted to form cone sheets instead of dykes. Secondly by a similar mechanism some batches of basaltic magma are diverted into a magma trap situated at the base of each diapir and there they gradually build up the great cylinder of basic intrusions the existence of which has been inferred from gravity anomalies. This cylinder marks the upward course of the diapir, and heat supplied by this basic magma aided its uprise. A late stage in the development of an intrusive centre is the elimination of the diapir because of the escape of successive batches of acid magma from it in a near-surface environment. Subsidence then takes over as a dominant process and ring dykes are emplaced. Intrusions of bell-jar or “curved flange” type also occur and a mechanism involving downsagging of the basic cylinder is invoked for their emplacement.

The Lochnagar granitic ring complex, Aberdeenshire

Synopsis The Lochnagar granitic ring complex consists of nine concentric annular and crescentic masses of adamellite and granodiorite. The successive members of the complex were emplaced in order of increasing potassium and silica content and decreasing size and grain size from the margin towards the centre of the complex. The early members of the complex, the granodiorites and the first adamellite, are foliated and crowded with inclusions of the contiguous wall rocks. They are partly circumscribed by an adamellite cone sheet and appear to have been forcibly intruded into the complex. The later finer-grained adamellites appear to have been emplaced into the central part of the complex by cauldron-subsidence; the last two phases, the smallest, finest-grained and most felsic adamellites, being emplaced into the centre of the complex under flat roofs of earlier adamellite.

The dynamic emplacement of cone sheets

Explanations of the formation of radial dykes, ring dykes and cone sheets are reviewed and the explanations of the formation of cone sheets are found to be unsatisfactory. Ring dykes develop on shear fractures formed in the overlying rocks when the magma subsides as explained by E.M. Anderson. Radial dykes and cone sheets form as a result of the upward pressure of the magma, and the problem is to explain the development of the two very different stress distributions which gave rise to the two kinds of intrusion. The radial dykes are simple hydraulic tension fractures formed periodically during the upwelling of the magma. It is suggested that the cone sheets occupy shear fractures formed as the result of dynamic stresses arising from the rapid expansion of a magma undergoing retrograde boiling.

Hebridean tholeiitic magmas: A geochemical study of the Ardnamurchan cone sheets

The Tertiary complex of doleritic cone sheets in Ardnamurchan has been sampled and a geochemical study made on 98 rock specimens by X-ray fluorescence analysis. A range in composition from 48 to 66% (wt.) SiO2 is associated with, for example, an increase in Rb from 2 to 126 ppm, and a decrease in Ni from 86 to < 1 ppm. R-mode factor analysis indicates element groupings that are most easily related to a history of crystal fractionation beneath the central intrusive complex. No chemical basis can be found that would add support to either the two-centre or the spiral-fracture hypotheses relating to the disposition of the cone sheets. Plots of oxide and normative data show the Hebridean cone-sheet complex to have a tholeiitic character, which is compared with the Hawaiian tholeiitic and Hebridean alkalic series. A transitional nature for the two Hebridean series, compared with the two Hawaiian series, is probably related to derivation from an olivine basalt parent rather than from an olivine tholeiite as in Hawaii.

Geologic Interpretation of the Study of Lunar Rocks

The hypothesis on an internal origin of lunar rocks of gabbroic type is supported by experimental petrologic data obtained by the author. The temperature interval of their crystallization from a dry silicate melt is given.Lunar gabbroic rocks of the Tranquillity Base are compared with paleobasalts of north-eastern Bohemia, eucrites and associated rocks of the British Isles and meteoritic eucrites of poikilophitic texture.The metasomatic origin of lunar anorthosites displaying a mosaic texture in deeper parts of the volcanic apparatus is postulated.An analogy is thought to exist between the Tertiary volcanic areas of Mull, Ardnamurchan and Rum in Scotland and the Ptolemaeus, Alphonsus and Arzachel regions of the Moon in the overall tectonic structure and the development of tectono-volcanic cirques. By analogy the possibility of the presence of more basic rocks in the rim and on the floor of the Moon subsidence calderas and of more acidic rocks of anorthositic to trachyandesitic chemistry in the central peak is presumed.The mode of origin of double-craters of Cyrillus-Theophilus type is suggested to be similar to the origin of two consecutive volcanic rings of Mull.The ring- and radial complexes of wrinkle ridges of Lamont type are compared with cone sheets and radial dykes of the central type tectono-volcanic complex of Mull.

Feldspathoidal Rocks of the Red Hill Igneous Complex, New Hampshire

The Red Hill intrusive complex in the towns of Moultonboro and Sandwich, New Hampshire is a small elliptical stock of syenite, feldspathoidal syenite, and granite. It is divided into three distinct intrusive groups which are, from oldest to youngest, the Older Quartzose Group consisting of slightly oversaturated syenites occupying the crescent-shaped northwestern part of the intrusive, the Feldspathoidal Group consisting of undersaturated nepheline- and sodalite-bearing syenites which form a funnel-shaped intrusive body and a swarm of ring dikes or cone sheets, and the Younger Quartzose Group consisting of oversaturated syenites and granite occurring as crosscutting plugs. The compositions and textures of the Feldspathoidal Group rocks are believed to be the result of the intrusion of magma intermittently tapped from a subjacent body of fractionally crystallizing slightly undersaturated, chloride-rich syenitic magma. All the Feldspathoidal Group rocks cooled together under closed-system conditions, local equilibrium being maintained down to about 250° C. These rocks solidified at a depth of less than 6 or 7 km.

Lake Lappajarvi, Central Finland: a Possible Meteorite Impact Structure

SVENSSON1 has drawn attention to a most interesting cryptic structure—one more in the growing list of such cryptoexplosion structures. I would, however, question how much weight should be attached to the recognition of shock lamellae in quartz grains as a criterion of meteorite impact involvement. I have previously endorsed Bucher's scepticism regarding numerous so-called astro-blemes2, and since that publication Snyder and Gerdemann3 have put forward what seems to me to be an overwhelming case for the endogenous origin of the numerous cryptoexplosion structures east of the Appalachians. Currie and Shafiqullah4 have recovered potassic trachyte and alnöite from the Brent Crater, Ontario. Both these accounts confirm my conclusions, and the Brent discovery supports the idea that some sort of highly explosive alkalic vulcanism produced these structures. It is apparent that shatter cones and coesite, both found in association with cryptic structures of this group, are not restricted to meteoritic cryptoexplosion structures. One may well question whether shock lamellation in quartz has any more validity: surely it has been reported from the Brent and Holleford craters, which are clearly of endogenous origin ? (A fortuitous association of alnöite with an astro-bleme is extremely unlikely, for alnöites are very rare igneous rocks associated, in most cases, with localized, highly explosive diatreme activity, involving carbonatites, and such endogenous eruptivity produced the Pretoria Salt Pan which appears just like many of the cryptoexplosion craters attributed to meteorite impact explosion.) Geologists who, like myself, have worked extensively on carbonatites5 must realize that the supposed restriction on the energy involvement involved in volcanic explosions (accepted without question by many authorities because it is repeated so often in the literature) is without foundation. The sort of phase change reaction that must be involved in the production of the immense carbonatite explosion breccia fillings of some carbonatite vents, and of the associated cone sheets—a reaction producing a colossal explosion at a point focus at depth—does not seem to be limited in any way by the strength of the overlying thickness of crustal rocks. Such an explosion, involving adiabatic expansion, could, physicists assure me, involve a virtually unlimited amount of energy, and so we are left with the possibility of two mechanisms, one internal and one of extraterrestrial origin, capable of producing these cryptoexplosion structures. No one would argue that there is much conflicting evidence: and this conflict applies to the Rieskessel, New Quebec Crater, Manacouagan Lake, Clearwater Lakes, Bosumtwi Crater, and even to Wolf Creek Crater. One thing, however, is clear from the literature: there are at present no valid criteria that afford strong support for either theory, except the actual association of meteoritic material; and even where that is present, as at Wolf Creek6,7, one can reasonably harbour just an iota of doubt7.

Cone sheet swarm, resurgence of tejeda caldera, and the early geologic history of gran Canaria

The southwestern half of Gran Canaria probably represents the lower part of a large Tertiary volcano. The following stages in its development are recognized: alkali olivine basalt shield volcano; caldera collapse and emission of large quantities of alkali trachytic to soda rhyolitic ash flows; building of a central volcano inside the caldera, and high level intrusion and resurgent doming of the central volcano’s substructure by thick trachytic sills, a central syenite stock, and numerous trachytic cone sheets. The syenite stock and the younger cone sheet system inside the caldera suggest that two stages of doming followed caldera collapse.

The Tertiary Geology of Rhum, Inner Hebrides

The Tertiary Igneous rocks of Rhum are emplaced in Lewisian gneiss and Torridonian sediments, the latter overlain by a small area of Trias sandstone. In Tertiary times two phases of intrusive activity were separated by the formation of the Main Ring Fault. The earlier phase began with the intrusion of small masses of gabbro, followed by explosion-breccias and tuffisites, felsites and granophyres. The early igneous activity was dominantly acid in character. The members of the early phase, together with the underlying Lewisian and lower members of the Torridonian, were uplifted within the Main Ring Fault. Renewed intrusive activity involved the formation of small ultrabasic masses, and the emplacement of the main body of layered ultrabasic rocks in an essentially solid condition within a ring fracture lubricated by basaltic magma. Basaltic dykes, in a radial system, and cone sheets were intruded at various stages in the development of the complex. Small areas of marble caught in the ring fault system of eastern Rhum may represent highly metamorphosed Jurassic limestones. After the formation of the Main Ring Fault, and probably after the emplacement of the main body of layered ultrabasic rocks there was a period when erosion cut down to almost the present level. During this time basaltic and mugearitic lavas flowed over the irregular land surface, which was mantled in places with coarse conglomerates derived from the earlier rocks. From the gravity survey of Rhum it appears that the Tertiary complex is underlain by a large body of dense basic or ultrabasic rock. This is thought to have been the prime-mover in the Tertiary history of the area, causing central uplift and ring faulting. In addition it probably generated by rheomorphism the acid rocks of the complex: abundant evidence of rheomorphism is found in the rocks at the present level of erosion.

Structural and economic aspect of the Vatukoula caldera, Fiji

The Vatukoula caldera is semi-elliptical in shape with the long axis trending north-easterly and occupies about 14 square miles of an undulating topographical basin located near the central north coast of Viti Levu, the largest island of the Fiji Group. The caldera formed when Tertiary basalts collapsed after prolonged explosion from a central vent area. The ensuing subsidence, which appears to have been cyclic, was accompanied by the deposition of andesitic volcanic material to form 5,000 to 7,000 feet of rhythmic tuffs, breccias and agglomerates partly under lacustrine conditions. The peripheral basalts were shattered during the stages of collapse forming a ring fault zone around the caldera. The depositional and subsidence stages were followed by an intrusive augite andesitic one from which extensive cone sheets formed in the caldera rocks. Radial and tangential dykes formed around the caldera in the peripheral basalts. After a time interval, the comparatively shallow central depression of the caldera received biotite andesitic pyroclastics and flows. Biotite andesite dykes followed a similar structural pattern to the augite andesitic ones. Finally, plug like bodies of porphyrite and monzonite intruded into the highly fractured zones, particularly the ring fault zone in the peripheral basalts. An important younger structural development with economic significance was the formation of a north-westerly shear system across the caldera. Flatly dipping structures formed in the peripheral basalts from the resettling of major blocks around the caldera. After the monzonite intrusions, epithermal mineralisers were liberated with economic amounts of gold in the form of telluride and auriferous pyrite. The mineralisers favoured the north-westerly shear system and, in the peripheral basalts, the accompanying flatly dipping structures. Thermal spring activity appears to mark the last phase of volcanicity.

XIII.—Granites and Associated Rocks of the Eastern Part of the Western Redhills Complex, Isle of Skye

SynopsisThe country lying between the head of Loch Ainort and the northern end of the Blaven Range in central Skye forms part of a Tertiary igneous complex. Country rocks in the area are Jurassic sediments. The earliest igneous rocks are basalt lavas which show the effects of thermal metamorphism by a later intrusion of eucrite and gabbro. This latter, which is part of the Blaven mass, has features of a layered intrusion such as igneous lamination, repetition of layers rich in certain minerals and comparable textures. It may be correlated with zones of the Cuillin layered intrusion. The eucrites are cut by a cone sheet swarm. Adjacent to the eucrite is a vent breccia which contains eucrite blocks, is not cut by cone sheets and is, therefore, of later age than the eucrite. The major part of the area under discussion is formed from four granitic ring dykes and a hybrid suite with ring dyke form, all of which belong to the Western Redhills intrusive complex; in addition, part of a fifth granitic ring dyke, which probably belongs to the Eastern Redhills intrusive complex is included in the area. The granites are mainly fayalite-hedenbergite types with a variety of phenocrysts of felspar and features characteristic of high level intrusions such as granophyric texture and varying optical and chemical properties of the felspar indicating higher temperature modifications. Three new chemical analyses of the granites and one of ferrohedenbergitic pyroxene are presented. The granites may be regarded as partial remelts of the underlying basement gneiss and a simple investigation of thermal requirements indicates that the heat necessary for the partial melting could be provided by a mass of basic rock presumed from geophysical evidence to underlie the complex. Various acid and basic minor intrusions in the area are also described.

The anisotropy of magnetic susceptibility of some igneous and metamorphic rocks

The anisotropy of magnetic susceptibility of some basic igneous and metamorphic rocks has been found to be due to the preferred orientation of the long axes of grains of magnetite. The degree of anisotropy is in a few samples as great as 40 per cent but usually is less than 10 per cent. The variation in anisotropy is believed to be due to variation in the degree of preferred orientation, and in susceptibility due to grain size and chemical composition. Half of the susceptibility ellipsoids determined were found to approach an oblate shape while only a quarter of the remainder approached prolate shapes. The magnetic ellipsoids of a number of Tertiary lava flows, banded gabbros, dikes, cone sheets, a ring dike, and Precambrian basic gneiss have been determined. The principal susceptibility directions of most of the lavas were widely dispersed but in some flows were tightly grouped—the minima round the pole of the flow plane, the maxima normal to the flow direction in the flow plane, and the intermediates in the flow direction. Similar groupings were found with the banded gabbros but not with the other intrusives. Basic gneisses show a very marked concentration of maxima in the direction of the visible lineations and of minima normal to the observed foliation. The applicability of anisotropy measurements in structural petrology is discussed.

The Mt. Barney Central Complex, S.E. Queensland

AbstractThe general geology of the Mt. Barney Central Complex is described. This intrusive complex shows close similarity in type to the classic Tertiary ring complexes of Western Scotland and Northern Ireland. In this discussion of the complex, the similarities shown by ring dykes, cone sheets, a forcefully intruded granophyre boss, and a closed ring fault are indicated.