Rocks Of Intermediate Composition Research Articles

Aeromagnetics, geology and ore environments, I. Magnetite in igneous, sedimentary and metamorphic rocks: An overview

Aeromagnetic (AM) surveys continue to play a prominent role in mineral exploration. The function of AM surveys is to map the distribution of magnetic minerals in the bedrock. Rock magnetic properties originate from the opaque minerals, of which the most abundant is magnetite. Interpretation is the drawing of inferences about the geology and ore potential of a given region from AM survey data; but it is hampered by a lack of clearly stated principles relating magnetic minerals to geology and to ore environments. This article, the first of a series, is a culling of materials from the geological literature which relate to the production and annihilation of magnetite in various geological environments. It begins with a description of the FeTi oxide minerals (the primary opaque oxide minerals in all rocks) and the factors which determine their induced and remanent magnetizations. The role of deuteric oxidation is of particular importance: rocks which have had different cooling histories will also have different magnetic properties, even if they have similar chemical compositions. The magnetic properties of rocks are determined by the partitioning of iron between the oxides of iron and titanium on the one hand, and (Fe, Mg) silicates on the other. The partitioning is controlled primarily by oxygen fugacity and temperature, and only secondarily by bulk chemistry. Biotite plays a special role in determining the amount of magnetite that can form during magmatic emplacement and also during subsequent metamorphism. Primary basaltic magmas differentiate along two different trends, depending upon whether oxygen levels are maintained externally, or whether they are allowed to drop. Magmas which follow the first trend produce primary Fe;Ti oxides and silica-rich end members. These give the calc-alkaline series, which are relatively deficient in magnetite. Thr second differentiation trend leads to primary Fe-silicates and FeO-rich end members which are silica-deficient. Rocks which follow this trend are relatively magnetic. The two trends correspond with quite different tectonic environments. The highest magnetic susceptibility values in either series should be found in rocks of intermediate composition approximating that of dacite or diorite, unless there has been alteration. During progressive regional metamorphism secondary magnetite can be produced at all stages, but mostly it develops under rather low-grade conditions by the conversion of hematite, or under high-grade conditions from the breakdown of hydrous (Fe, Mg) silicates such as biotite and hornblende into simpler orthosilicates. The magnetite-producing capacity of a rock during metamorphism is determined by attributes which are largely inherited from the premetamorphic state, of which the most important are total iron content, oxidation state, and degree of silica saturation. Other influencing factors include magnesium, aluminum and carbon content. Magnetite is destroyed during granitization and by low-temperature alteration. In interpreting AM survey data, close attention should be paid to all of the local factors that might have an influence upon magnetite, including mineralogy, chemical composition and metamorphic grade. Magnetite in metamorphic terrains often reflects the premetamorphic lithostratigraphy; hence magnetic interpretation maps should not be constrained to resemble maps of the existing geology.

Magma mixing in a main stage of formation of Montagne Pelée: the Saint Vincent-type scoria flow sequence (Martinique, F.W.I.)

Contrasting with the first stage of activity at Montagne Pelée, which built an andesitic volcano chiefly formed of brecciated lava flows and pyroclastic breccias, the second main stage (> 40,000 to 20,000 years B.P.) is characterized by the prevalence of scoria flows of more mafic composition. Many cross-sections of the deposits of stage 2 show an upward magmatic evolution from early acid andesites (plinian air-fall and pumice flow deposits) to late basaltic andesites (scoria flow deposits). Heterogeneous “banded” rocks, complex mineralogical associations and mineralogical disequilibrium features in the whole series are explained by mechanical mixing of contrasting magmas. Reversed zoning characterized plagioclase phenocrysts of both the acid and basic terms and also pyroxene phenocrysts of the mafic terms. Grossly uniform composition of residual glass in most macroscopically homogeneous rocks implies that chemical exchanges between liquids and local hybridization occurred. Plutonic rocks with cumulate textures are found in upper units of the sequence. Their phenocrysts (anorthite, salitic clinopyroxene, pargasitic hornblende and Ti-magnetite) are in equilibrium with a subalkaline basaltic interstitial liquid. The same phases are commonly present as xenocrysts in the basaltic andesites. It is inferred that basaltic andesites of stage 2 were firstly derived from a subalkaline basalt by crystal fractionation. Mixing of the basaltic andesites (52% SiO2) with acid andesites (62% SiO2) occurred in late stage of evolution of the series, producing heterogeneous rocks of intermediate composition. Successive eruption of acid up to mafic magmas, dominant amount of mafic rocks in the sequence and short duration of the magma mixing event shown by narrow rims of the reversely zoned phenocrysts, suggest that mixing was produced by intrusion of mafic magma into a differentiated magma body. We propose that hydrostatic overpressure induced by a high rate of intrusion triggered the eruption. Thus, magma mixing would be due to a large volume of mafic magma passing through the resident magma. Rapid admixture of the two batches increased gas exsolution, resulting in a highly explosive eruption. Magma mixing can thus explain the explosive dynamics of the basaltic andesites eruptions, which surprisingly contrast with the relatively effusive style of the preceding andesitic stage.

The Dunbar Gneiss-granitoid dome: Implications for early Proterozoic tectonic evolution of northern Wisconsin

The Dunbar dome in northeastern Wisconsin is a critical structural feature in the early Proterozoic Penokean orogen. It provides exposures of gneisses (Dunbar Gneiss) that structurally underlie the voluminous metavolcanic rocks of northeastern Wisconsin, and exposures of abundant granitoid rocks ranging from tonalite to granite. The granitoid rocks cut both the gneisses in the core and the supracrustal (cover) metavolcanic rocks and were emplaced essentially along the core-cover boundary. The Dunbar Gneiss is calc-alkaline and was derived from volcanic and intrusive rocks of intermediate composition. The various intrusive rocks have calcic, calc-alkaline, and alkali to alkali-calcic compositions, and they progress with time to more SiO 2 and K 2 -rich compositions. U-Pb zircon ages indicate that accumulation of the layered rocks in the core and cover, deformation and metamorphism, and intrusion of the granitoid rocks spanned a relatively short time, ∼1865–1835 Ma. We interpret the dome as being a large-scale, fold-interference structure resulting from polydeformation modified by diapirism. Northeast-oriented folds (F 3 ) and a related mylonitic foliation (S 3 ), nearly confined to the dome, are superposed on northwest-oriented folds (F 2 ) that developed during regional deformation. In the core-cover boundary, these structures are obliterated by a zone of intense deformation—a mylonitic foliation and a steeply plunging stretching lineation—as much as 500 m wide, which we interpret as resulting from diapirism. Metamorphic zoning is concentric: amphibolite facies in inner parts of the mantle and greenschist facies in the outer part of the mantle. The Wisconsin magmatic terrane, as represented by the rocks in the Dunbar dome, differs from the epicratonic, early Proterozoic sedimentary-volcanic sequence (Marquette Range Supergroup) in Michigan, to the north, in stratigraphy, structure, and volume and composition of igneous rocks. Whereas the basalts in northern Michigan are compositionally similar to continental rift basalts, the volcanic rocks in the Dunbar dome have over-all island-arc compositional affinities. The over-all calc-alkaline compositions of the intrusive rocks are similar to those in magmatic arcs formed at convergent plate-margin settings. Accordingly, we interpret the Wisconsin magmatic terrane as an oceanic-arc complex that was sutured to the North American continent during development of the Penokean orogen. Similar interpretations based on broad regional observations have been proposed previously.

Nivation landforms in the western Great Basin and their paleoclimatic significance

More than 10,000 nivation landforms occur in the higher mountain ranges of the western Great Basin. They range from small, subtle hollows with head scarps a few meters high and a few tens of meters long to broad, clearly defined terraces as much as 220 m wide bounded by bold, steeply sloping head scarps as much as 30 m high and 1600 m long. Distribution of these nivation hollows is strongly influenced by elevation, slope orientation, local relief, and substrate lithology. About 95% occur between 2200 and 3000 m elevation, and nearly 80% are situated on north-northwest-to east-northeast-facing slopes. They occur mainly in areas of moderately sloping terrain and moderate local relief, and they are preferentially developed on relatively incompetent substrates including terrigenous sedimentary deposits, volcanic and metavolcanic rocks of intermediate composition, and deeply weathered granitoid rocks. Nearly all of these nivation hollows are relict. They are most abundant near areas of late Pleistocene glaciation but rarely occur within such areas. Most are veneered with colluvium and are well vegetated, and many hollows in the Mono Basin area are veneered with volcanic ash at least 700 yr old. Distribution of nivation hollows suggests that (1) the full-glacial nivation threshold altitude (NTA) rose from north to south at 190 m per degree of latitude, subparallel to, and approximately 740 m lower than, the full-glacial equilibrium-line altitude (ELA) and about 1370 m lower than the estimated modern ELA; (2) the difference between the full-glacial and modern ELAs indicates an approximate 7°C full-glacial mean-annual-temperature depression throughout the Great Basin; and (3) the full-glacial mean annual temperature at the NTA is estimated to have been approximately 0° to 1°C, assuming little change in accumulation-season precipitation.

Os principais tipos de rochas vulcânicas da bacia do Paraná no planalto de Santa Catarina

The volcanic rocks of the Plateau of Santa Catarina, Southern Brazil, are composed, predominantly, by lava flows of basaltic to basaltic-andesite composition. Rocks of intermediate composition occur in the centro-west region and correspond to ranophyric trahyandesite. Acid rocks are represented by granophyric hemicristalline dacites in the South-east region and by granophyric riolites in the centro-west region, immediate to Paraná State.Magmatic activity in the Upper Mantle and in the Lower Crust, during Cretaceous, indued the diversity of petrographic types.The density of rocks provide different values and, by this reason, it is a good parameter to established the distinction among themselves.

Geology and major element geochemistry of anorthositic rocks associated with paleozoic hypabyssal ring complexes, air massif, Niger, West Africa

Four Paleozoic subvolcanic ring complexes located in the Air Massif, Republic of Niger, contain a bimodal basic/acidic series of rocks. The alkaline acidic series ranges in composition from syenite to granite while the basic series contains predominately anorthositic material with compositions ranging from iron-titanium oxide ores to anorthosite. Within each complex, field relations suggest that the basic series was intruded prior to the acidic series. The major minerals of the basic series are: (1) plagioclase, An 70-An 40; (2) olivine, Fo 65-FO 45; (3) clinopyroxene, Wo 45En 38Fs 17 (avg.); and (4) titaniferous magnetite and ilmenite. Major element geochemistry suggests that the acidic and basic series were not comagmatic, but rather were derived from independent sources. Rare rocks of intermediate composition are believed to be hybrid in nature resulting from the mixing of acidic liquids with previously intruded basic material. The recombined bulk composition of the basic series indicates parental magmas of highly aluminous basaltic compositions (anorthositic gabbro) which appear to have been transitional to alkaline in nature. In the two smaller complexes the basic series appears to have formed by flow differentiation, while in the two large complexes there is evidence that gravity differentiation formed the various rock types of the basic series.

FeMn crusts from the southern Tyrrhenian Sea

Dark crusts and concretions on volcanic and calcareous rocks dredged from seamounts west of the Aeolian Islands, southern Tyrrhenian Sea, consists mainly of amorphous FeMn hydroxides, with some goethite, todorokite, birnessite and clastic minerals. Chemically they consist of predominant Fe (up to 45%) and Mn (up to 12%), with significant amounts of Zn, Ni, Co and Cu. That the REE distribution patterns are similar to those of shallow Pacific derivation — from hydrothermal waters cognate or cycled through volcanic rocks of intermediate composition — is tentatively suggested.

U-Pb zircon and Rb-Sr whole-rock and mineral ages of Proterozoic intrusives on mapsheet Lannavaara, north-eastern Sweden

Abstract Radiometric data for igneous rocks, intruding early Proterozoic supracrustals, from two separate areas in north-eastern Sweden are presented. U-Pb analyses on zircons from plutonic rocks of intermediate composition around Lake Paittasjarvi yield an age of 1880 ± 28 Ma. A whole-rock isochron age of 1915± 145 Ma and a plagioclase-hornblendemica age of 1550± 40 Ma are calculated for the same rocks with the Rb-Sr method. Correlated granitoids from the Juoloyanjarvet area, which in part have suffered replacement of plagioclase by scapolite and are intruded by gabbros and younger granites, give a whole-rock Rb-Sr reference line of 1705 Ma and mineral ages of 1535 and 1495 ± 30 Ma. The whole-rock age probably lacks direct geological significance but the mineral ages obtained from both of the above areas may reflect influences from younger plutonism on the dated rocks. One of the intruding gabbros has a Rb-Sr mineral age of 1617 ± 107 Ma. This figure is considered a minimum age of the rock. The impact of...

Effects of Texture and Composition on Mechanical Behavior of Experimentally Deformed Carbonate Rocks

Dry, low-porosity carbonate rocks with a wide range of texture and dolomite content were experimentally deformed at room temperature, strain rate of 10-4 sec-1, and at confining pressures of 0, 50, and 100 MPa (1 MPa = 10 bars = 145 psi) to evaluate the effects of texture and composition on ultimate strength and ductility. Of all the factors considered, weighted mean grain size and microcrystalline carbonate (micrite) content have the highest linear correlation coefficients with ultimate strength. Grain size is, therefore, the dominant intrinsic rock property that affects ultimate strength in low-porosity carbonate rocks. Systematic increases in total dolomite content (determined by X-ray diffraction) do not correlate significantly with ultimate stre gth when all of the variously textured rocks are compared. However, ultimate strength does increase with increasing dolomite content in rocks of similar texture. The approximate ultimate strengths of carbonate rocks with intermediate dolomite and/or micrite content can be predicted by the best-fit plane to the ultimate strengths of pure end members (Yule marble, Solenhofen limestone, Hasmark dolomite, Blair dolomite). For experiments at 100 MPa confining pressure, this plane is defined by the equation: 0.90 D + 2.07 M + 269 = ^sgru, where D is dolomite content in percent, M is microcrystalline carbonate content in percent, and ^sgru is ultimate strength in MPa. The equation of the plane defined by ultimate strengths of rocks of intermediate composition and texture is: 1.07 D + 2.29 M + 258 = ^sgru. The mean difference between the strengths predicted by these two planes is only 9 MPa. Most precursive microfractures in nonmicritic elements are intragranular. Although the density of intragranular microfractures increases with increasing confining pressure, grain boundary cracks are suppressed by confining pressure. Very few microfractures occur in the micrite itself. At a given confining pressure, highly micritic rocks exhibit much less abundant precursive microfractures than coarser grained rocks, and hence they are stronger.

Geology of the El Abra porphyry copper deposit, Chile

The El Abra porphyry copper deposit, located 42 km north of Chuquicamata, was mined on a small scale in the past, and several studies of the deposit were carried out prior to 1972. From 1972 to 1975, the Corporacion Nacional del Cobre de Chile completed an evaluation program known as the El Abra Project.In common with several other major porphyry copper deposits in the region, El Abra is related to a regional structure known as the West Fissure and is emplaced in Tertiary intrusive rocks of intermediate composition, which in turn form part of an intrusive and volcanic complex, the oldest members of which are Jurassic in age. The orebody is controlled by a west-northwest structural trend which continues to the east where it is related to other minor hydrothermal occurrences.The host rocks for the mineralization range from diorite to syenite. Late in the magmatic cycle, these rocks were altered, first by a period of K-feldspar-quartz addition and later by a period of strong biotization. The intrusive complex, called the El Abra diorite, is intruded by several dacitic necks which produced a remobilization of potassic alteration, developing biotite breccias on their tops. Age relations are obscured by early alteration stages, but it can be concluded that El Abra primary mineralization was almost synchronic with the Chuquicamata orebody. Primary mineralization related to potassic alterations exhibit high copper/iron ratios, with increasing iron in time. The dacitic porphyries have their own zoning pattern.Hydrothermal alteration and mineralization is found inside the deposit as bands controlled by structures and as discontinuous occurrences in fringe zones. There, quartz veins with sericitic alteration in host rocks show a pyrite-rich assemblage, but only the roots of the phyllic zone are now exposed. Argillic and supergene alterations coexist with phyllic alteration in the same structures, being hydrothermal events obscured by this superimposition. The lack of pyrite and the basicity of host rocks had an unfavorable effect on supergene leaching and enrichment. However, an important oxidation blanket which has an area of one square kilometer covers primary sulfides. The dominant oxide mineral is chrysocolla.Alteration and mineralization at El Abra has been interpreted as a deep exposure of a porphyry copper due to erosion of upper zones. A multiple intrusion-alteration-mineralization model is proposed, which is essentially the classical model repeated under slightly different conditions during time. The deep exposure of the orebody in a porphyry copper column in this deposit approaches a magmatic environment with the geometric patterns deformed by premineral structural control and by the complexity of the intrusion-alteration history.

A Discussion on volcanism and the structure of the Earth - Cenozoic volcanism and plate-tectonic evolution of the Western United States. I. Early and middle cenozoic

Variations in Cenozoic volcanism in the Western United States correlate rather closely with changes in tectonic setting: intermediate-composition rocks and their associated differentiates were erupted through orogenic or fairly stable crust, whereas basaltic or bimodal basalt-rhyolite suites were erupted later— concurrently with crustal extension and normal faulting. Lower and middle Cenozoic continental lavas, erupted onto postorogenic terranes, are predominantly intermediate types (andesite to rhyodacite), commonly with closely associated more silicic ashflow sheets. Compositional zonations in individual ash-flow sheets, from rhyolite upward into quartz latite, record magmatic differentiation in underlying batholithic source chambers. The intermediate lavas probably represent the greater part of these batholiths and the ash-flow tuffs their differentiated tops. Continental volcanic activity of this type was most voluminous in the northwestern United States in Eocene time, but shifted southward in the Oligocene; contemporaneous sea-floor basalts occur in the Oregon-Washington coast ranges. Largely intermediate-composition calc-alkalic igneous suites, that become more alkalic toward the continental interior, are characteristic of most of the North and South American cordilleran belt. Similar volcanic associations are forming now around most of the Pacific margin where continental plates override oceanic crust along active subduction systems, marked by Benioff seismic zones and oceanic trenches. A similar subduction mechanism probably operated in the Western United States until late Cenozoic time. Analogy with chemical variations across active island arcs suggest that early and middle Cenozoic subduction occurred along two subparallel imbricate zones that dipped about 20° eastward. The western zone emerged at the continental margin, but the eastern zone was entirely beneath the continental plate, partly coupled to the western zone below the low-velocity layer. Predominantly intermediate-composition volcanism persisted throughout the Western United States until the initial intersection of North America with the East Pacific rise started the progressive destruction of the subduction system

A comparative study of the petrochemistry of the tholeiitic rocks of India

A comparative study of the petro-chemistry of the tholeiitic rocks (Cuddapah, Gwalior, Rajmahal and the Deccan traps) is made employing various crystallization parameters such as the differentiation index (D. I.) and the solidification index (S. I.). The D. I. values of the upper Deccan traps are much higher than those of the lower traps, rocks of intermediate composition being rare. The S. I. values of the Deccan traps allow them to be classified into three categories i.e., (1) the lower trap with higher S. I. values, (2) the differentiated upper division traps, (3) the unusual rock types of Western India.

Volcanic and intrusive structures in the Kila-Warji ring-complex, northern Nigeria

The Kila-Warji massif is one of a large number of ring-complexes of Jurassic age which cut the basement rocks of northern Nigeria. It is the only large member of this Nigerian Younger Granite province in which rocks of intermediate composition are dominant over acidic rocks. Structurally the complex presents several features of interest. The southern part——Warji—is a cauldron subsidence structure of volcanic rocks invaded by partly concordant intrusions. In the north the Kila ring-complex consists of a cluster of almost superimposed intrusive ring-structures. The complex lies in the extreme north of the Nigerian Younger Granite province (Fig. 1) and forms a group of low hills rising from an almost level plain developed on basement rocks. Farther north a few small complexes are exposed near the southern fringes of the Plio-Pleistocene sediments of the Chad basin, on the northern flanks of which Younger Granites reappear as large massifs in the Zinder-Gouré area of Niger ( Black 1963 ) and at Matsena in Bornu Province (Turner, in press). The Kila-Warji complex, composed mainly of acid and intermediate volcanics, syenites and peralkaline granites, has closer affinities to these northern complexes than to the main part of the Nigerian province, where biotite-granites are the most abundant rock type. Previous work on the area includes a preliminary survey by Raeburn (Falconer & Raeburn 1923) and an account of the petrography of some of the rocks by Beer (1952) . Field work for the present study was carried out from October 1965 to January 1966. This paper

Some problems of volcanism in Iceland

Until two decades or so ago the petrography of Iceland seemed to present a rather simple picture. About 99 per cent of the rocks were regarded as basaltic, 1 per cent as rhyolitic and rocks of intermediate composition as nearly non-existent. Recent research has changed this picture. Central volcanoes producing acid and intermediate lava and tephra have played a considerable role both in the Tertiary and Quaternary. Contrary to the lava massproducing basalt volcanoes, fed by a deep-seated and rather uniform basalt magma source, the differentiated central volcanoes are normally fed by separated magma chambers at smaller depth, where differentiation takes place between the eruptions, resulting in intermediate and acid products which bridge the “immiscibility gap” between the basalts and the rhyolites. Eight to ten per cent of the postglacial lavas and tephras and about 25 per cent of the historical ones are acid or intermediate. Opinions still differ as to whether the existence of these rocks can be explained solely as the result of gravitational differentiation of parent basalt magma or has to be explained partly by a remelting of a continental layer beneath the plateau-basalts. There are, however, some rather strong geological evidences in favour of the existence of such a layer and they seem at the moment not to be contradicted by geophysical research. The Surtsey eruption has demonstrated the close relation between the craterrows, shieldvolcanoes and tablemountains and convincingly shown how greatly external circumstances may influence volcanic activity and the shape of a volcano. The eruption focused interest on the neovolcanic zone of Iceland as a supra marine part of the Mid-Atlantic rift-ridge zone, where volcanic activity has taken place under similar conditions as along its submarine parts, viz. under water (glacial meltwater) when the Pleistocene ice-sheets blanketed the volcanic activity. Similar morphological types of volcanoes can therefore be expected in both cases, viz. steepsided mounds and long steepsided ridges.

Replacement magnetite on Vancouver Island, British Columbia

Magnetite. . . generally occurs at the conjunction of limestone, volcanic rocks, both of Upper Triassic age, and younger intrusive rocks of intermediate composition. Orebodies tend to be localized in small areas of intense deformation. The bulk of the iron was probably derived from the thick Karmutsen Group of volcanics underlying the limestone. Considerable volumes of the Karmutsen must have been incorporated by the ascending magmas, and excess iron was probably expelled with the residual magmatic fluids. Replacement was probably accomplished largely by individual ions and molecules diffusing. . . out along grain boundaries and cleavage cracks to points of a Hack on calcite and silicate crystals.

The Cader Idris granophyre and its associated rocks

Summary The Cader Idris granophyre and its surroundings have been remapped, using vertical aerial photographs on the scale of 18 inches to the mile, and the information recompiled on the scale of six inches to the mile. A petrographic study has been made of the rocks present, and a selection of these analysed for major elements. The results suggest that the granophyre was emplaced as magma but caused some mobilization and metasomatism of country rocks near its contacts, as well as in xenoliths and rafts of country rock trapped and surrounded by granophyre in a discordant vent-like portion of the intrusion on Mynydd Moel. It is believed that the associated quartz-diabase sill and certain rocks of intermediate composition occurring within the Upper Acid Group originated at depth by such processes of thermal mobilization and metasomatism. It is shown that the granophyre lifted its roof more than 1500 feet and that it is physically connected with the uppermost rhyolites of the Upper Acid Group. This fixes the age of the granophyre and suggests that its intrusion was associated with earth-movements of pre-Bala age.

Correlation of some igneous rocks of New Mexico by the fusion method

Thirty‐six samples of igneous rocks and ten of metamorphic rocks, for which complete chemical analyses have been obtained, were powdered and fused into artificial glasses in the carbon arc of a spectrograph. Curves showing the relation between the indices of refraction of such glasses and the composition of the rock samples may be used to estimate the approximate composition and may aid in the correlation of igneous rocks. By this method, the SiO2 content of an unknown rock sample may be estimated to within three per cent of accuracy, whereas MgO, CaO, and the combined Fe2O3 + FeO may be estimated to within two per cent of accuracy. The method is not so satisfactorily applied to rhyolite and quartz latite, because these rocks do not readily form homogeneous glasses in the carbon arc.Aside from the effect of changing proportions of SiO2, the indices of refraction of the artificial glasses are determined, in decreasing order, by the oxides Fe3O2, TiO2, FeO, MgO, CaO, Na20, and K20.From comparative studies of variation diagrams, it can be shown why the method is particularly effective for volcanic rocks of intermediate composition, but may not be applied satisfactorily to altered, metamorphic, or other igneous rocks.