Silicic Igneous Rocks Research Articles

The nature and origin of lamprophyres: an overview

Summary Lamprophyres are ‘alkaline rocks’ because they carry high alkalis at a given percentage of SiO 2 , together with one or more of normative ne, lc or ac , modal foids, and Na-K-Ti-rich amphiboles or pyroxenes. They reach higher whole-rock H 2 O, CO 2 , Sr and Ba contents than other silicate igneous rocks. Contents of related element subsets in amphiboles (Ti, Ba), K-feldspars (Ba, Fe 3 ), phlogopites (Ti, Ba, Fe 3 ) and pyroxenes (Ti, Al, Fe 3 ) include among the highest values known in nature for these minerals. ‘Primitive’ minerals (diopside, forsterite) commonly coexist with ‘evolved’ minerals (albite + orthoclase, quartz). Four well-defined ‘branches’ of the lamprophyre ‘clan’ have distinctive compositions: calc-alkaline (shoshonitic) lamprophyres (minettes etc.), alone among lamprophyres, have mixed alkaline-calc-alkaline affinities; alkaline lamprophyres (camptonites etc.) are basanitic to nephelinitic and, alone among lamprophyres, usually have Na < K; ultramafic lamprophyres (alnöites etc.) are the most Si poor and Ca rich of silicate igneous rocks, and grade into carbonatites; lamproites (orendites etc.) are uniquely rich in K, Rb, Ba, Th, Mg, Cr and Ni at mainly ‘andesitic’ SiO 2 contents, and carry a suite of diagnostic minerals (wadeite etc.). Each branch comprises at least four rock types which resemble each other much more than rock types of other branches; however, some rock types can be grouped into slightly distinct ‘families’ within one branch (e.g. phlogopitic and madupitic lamproites). A case can be made for including kimberlites as a fifth branch of the lamprophyre clan. Synoptic plots and tables, based on some 5000 major, trace-element and mineral analyses, are presented, to aid identification and classification. Lamprophyres are far more common than generally stated, occurring worldwide in more tectonic settings than many other alkaline rocks and throughout the geological record. They may approximate intratelluric magma compositions. Nearly all represent primitive magmas, and many represent primary magmas. Some represent parental magmas to a wide range of hydrous alkaline intrusive suites: calc-alkaline lamprophyres to potassic pyroxenite-diorite-shonkinite-syenite-granite plutons (Cortlandt); alkaline lamprophyres to hornblendic gabbro-syenite plutons (Monteregian Hills); and ultramafic lamprophyres to ijolite-carbonatite complexes (Fen).

The role of silica and iron oxide mobility in the formation of gold-bearing fluvial sediments in the Proterozoic Mporokoso Basin, northern Zambia

AbstractGold-bearing sandstone and conglomerate near the base of the Proterozoic Mporokoso Group were deposited in a braided river system. The detrital sand grade material is mainly of quartz, sericite and haematite, and the pebbles are of vein-quartz, chert, silicic volcanics, quartzose metasediment and jaspilite. The basement rocks presently exposed in the basement are silicic igneous rocks and quartzose metasediments.The petrography of the fluvial sediments suggests that silica and, to a lesser extent, iron oxide were mobile both in the source-area and in the braided river system. Evidence for silica-mobility includes jaspilite pebbles with spherulites and glaebules of chalcedony, abundant vein-quartz pebbles, intra-basinal sandstone pebbles, and the silicification of volcanic pebbles. The detrital haematite in the fluvial sandstone forms pseudomorphs after magmatic magnetite. Authigenic iron oxide occurs in several forms which suggest that iron oxide was mobile in the source-area and in the fluvial sediments. Uranium is locally abundant in basement and sedimentary rocks, cassiterite is a common heavy mineral in the fluvial sediments, and fluorite has been found in the basement.These features may be explained by intense weathering which mobilized both silica and iron. The silica was concentrated near the surface to form silcretes in the basement and later in the overlying fluvial sediments. Hydrothermal convection cells driven by the granites may have carried silica, iron, tin, fluorine and uranium towards the surface before and during the erosion of the igneous basement.

Advances in Geology of the Porphyry Copper Deposits: Southwestern North America

This book summarizes recent advances in information on the porphyry copper deposits of southwestern North America. It is a sequel to a n earlier summary volume published by the same group in 1966. The new volume is indispensable for anyone interested in copper deposits and a valuable reference for those working in fields ranging from the petrology of silicic igneous rocks to the tectonic evolution of western North America. The volume is particularly significant because porphyry coppers are by far the most important class of deposits in terms of global copper reserves and, most importantly, because southwestern North America contains an unusually large number and variety of these deposits.

Abundance and distribution of uranium and thorium in the northern units of the Leinster granite, Ireland

SummaryAbundance data, determined by epithermal neutron activation analysis, are reported for radioelements in the northern units of the Leinster granite. Compared to similar data for silicic igneous rocks and British Caledonian granites, the Leinster granite shows normal U and K contents but is low in Th. Inter-element correlations and fission-track results suggest that most Th and up to 40% of the U is bound in resistate accessory phases (zircon, sphene, apatite), but that the bulk of U is associated with late-stage alteration products and is located at interstitial labile sites within the host-rock matrix. Mobilization of U located at such sites in the Leinster granite by hydrothermal leaching could lead to redeposition and enrichment in marginal sediments of Old Red Sandstone or younger age.

Fractography of highly crosslinked polymers

The fracture surfaces of highly crosslinked polymers sometimes include regions in which regular arrays of features are disposed along tracks running in the direction of crack propagation. Generally the features appear to be fibres but in some cases there is evidence that they are curls of rolled-up film. The features may remain attached to the tracks, be partially detached, or entirely missing. Such effects are illustrated by reproduction of results for a phenol-formaldehyde resin in Fig. 1 [1, 2]. Subsequently, similar effects were observed in other highly crosslinked polymers [3] such as polyesters [4], epoxy resins [5] and poly(ethylene glycol dimethacrylate) [6]. The purpose of the present letter is to suggest adoption of a mechanism proposed long ago by Preston to account for "stries" formation in silicate glasses, igneous rocks, and bitumen [7]. This mechanism dealt with the consequences of fracture in an ideal brittle material but, in order to account for the cylindrical shapes of the features actually observed in organic networks, it is necessary to invoke, additionally, the occurrence of localized plastic deformation. Preston's analysis made repeated use of the acceptable premise that crack propagation always proceeds in a direction at right angles to the principle tensile stress in the immediate neighbourhood of the crack tip. He then accounted for "stries" formation by analysis of the consequences of a sudden change in orientation of the principle tensile stress. This was illustrated by reference to a continuous crack front which may be imagined as moving upwards through a glass block (Fig. 2a). Suppose now that the front suddenly moves into an uncracked region in which the tensile stress is at a slightly different orientation. The entire crack

The distribution of uranium and thorium in granitic rocks of the basin and range province, Western United States

Some secondary uranium deposits are thought to have formed from uranium derived by the weathering of silicic igneous rocks such as granites, rhyolites, and tuffs. A regional geochemical survey was made to determine the distribution of uranium and thorium in granitic rocks of the Basin and Range province in order to evaluate the potential for secondary uranium occurrences in the area. The resulting geochemical maps of uranium, thorium, and the Th:U ratio may be useful in locating target areas for uranium exploration. The granites were sampled according to a five-level, nested, analysis-of-variance design, permitting estimates to be made of the variance due to differences between:(1) two-degree cells; (2) one-degree cells; (3) plutons; (4) samples; and (5) analyses. The cells are areas described in units of degrees of latitude and longitude. The results show that individual plutons tend to differ in uranium and thorium concentrations, but that each pluton tends to be relatively homogeneous. Only small amounts of variance occur at the two degree and the between-analyses levels. The three geochemical maps that were prepared are based on one-degree cell means. The reproducibility of the maps is U > Th ⪢ Th:U. These geochemical maps may be used in three methods of locating target areas for uranium exploration. The first method uses the concept that plutons containing the greatest amounts of uranium may supply the greatest amounts of uranium for the formation of secondary uranium occurrences. The second method is to examine areas with high thorium contents, because thorium and uranium are initially highly correlated but much uranium could be lost by weathering. The third method is to locate areas in which the plutons have particularly high Th:U ratios. Because uranium, but not thorium, is leached by chemical weathering, high Th:U ratios suggest a possible loss of uranium and possibly a greater potential for secondary uranium occurrences to be found in the area.

Evaluation of multispectral middle infrared aircraft images for lithologic mapping in the East Tintic Mountains, Utah

Six channels of multispectral middle infrared (8 to 14 micron) aircraft scanner data were acquired over the East Tintic mining district, Utah. This area has high relief and moderate vegetation and consists mainly of Tertiary silicic igneous rocks and Paleozoic quartzite and carbonate rocks that have been locally hydrothermally altered. These digital-image data were computer processed to create a color-composite image based on principal component transformations. Color differences in this image are related to the spectral differences in the surface material and allow discrimination of several rock types, depending primarily on their silica content. When combined with a visible and near infrared color-composite image from a previous flight, with limited field checking, it is possible to discriminate quartzite, carbonate rocks, quartz latitic and quartz monzonitic rocks, latitic and monzonitic rocks, silicified altered rocks, argillized altered rocks, and vegetation.

Rb–Sr ages of silicic igneous rocks and deformation, Burlington Peninsula, Newfoundland

The Burlington Peninsula, east of the Baie Verte Lineament, is underlain by the Eastern Division of the Fleur de Lys Supergroup, a sequence of metasediments and metavolcanic rocks that is intruded by plutonic masses of granitic to granodioritic composition. This sequence has been designated as Cambrian or older and the deformation and metamorphism in the area interpreted as pre-Ordovician. Recently, however, it has been shown that at least part of the succession is post Lower Ordovician and a mid or late Palaeozoic age has been proposed for the major deformation affecting the area.Rb–Sr ages obtained in the present study area are: (a) whole-rock isochrons—Dunamagon Granite, 413 ± 10 Ma; Cape Brulé Porphyry, 393 ± 25 Ma; Cape St. John Group Ignimbrites, 343 ± 15 Ma and 429 ± 50 Ma; Mic Mac Group Ignimbrite, 375 ± 15 Ma; (b) Biotite ages—Dunamagon Granite, 334, 356 and 358 Ma; (c) Mineral isochron—Burlington Granodiorite, 422 ± 40 Ma. Because of extensive trace element migration in at least some of these units, the interpretation of the ages is complex and does not allow a unique history of the area to be determined. However, the ages do support the proposed mid or late Palaeozoic age for the major deformation and indicate that pre-Ordovician orogeny did not prevail on the scale previously envisaged.

Low-Potash Lush's Bight Tholeiites: Ancient Oceanic Crust in Newfoundland?

The Lush's Bight Group of north-central Newfoundland consists mainly of regionally metamorphosed pillowed basalts. Mafic dikes and sills, comagmatic with the basalts, are abundant and dioritic stocks and silicic igneous rocks constitute a small portion of the group. Most rocks belong to a tholeiitic suite derived from low-K tholeiite that has a trace element content similar to oceanic tholeiite. A few rocks are slightly alkaline, implying that alkali basalt may have contributed to the volcanic pile. The thickness, chemical composition, and lithology of the group suggest it represents the upper (dominantly volcanic) part of layer two of oceanic crust. Alternatively, formation as a tholeiitic shield beneath a mid-ocean volcanic island is suggested by the alkaline affinities of some rocks and by a thick pyroclastic sequence that conformably overlies the group.The Lush's Bight Group contains many volcanogenic iron-copper sulfide occurrences.Mid-ocean ridges are recognized as magmatically dominated environments in which hydrothermal fluids of plutonic and volcanic origin might constitute ore fluids.

Correlations between O18/O16ratios and chemical compositions of tektites

The limits of oxygen isotope variation in tektites have been determined by analyzing 33 tektites that cover the widest possible spectrum of chemical compositions. δO18 values were obtained for 7 australites (8.9 to 11.0), 4 javaites (9.4 to 10.0), 3 indochinites (9.3 to 10.6), 5 philippinites (9.8 to 10.6), 8 moldavites (10.7 to 11.8), and 6 Ivory Coast tektites (13.0 to 13.6), as well as 2 samples of Darwin glass (14.4 to 14.9). Various tektite groupings based on chemical composition and geographic occurrence all show a systematic increase in O18 with decreasing SiO2 content, similar to that previously found for the bediasites. These systematic correlations must arise either by (1) vapor fractionation of tektite material during impact melting or (2) mixing of a SiO2-rich igneous component and a low-SiO2 component formed at a much lower temperature; perhaps the mixing occurred during weathering at the earth's surface or during hydrothermal alteration of silicic igneous rocks either on the earth or the moon.

Thorium, uranium and potassium concentrations in granites and gneisses of the Rum Jungle complex, Northern Territory, Australia

Rich uranium-copper mineralization occurs in metasediments adjacent to and mantling the older granites and gneisses of the Rum Jungle complex. The uranium and thorium content of these older rocks, measured by gamma-ray spectrometry, is more than twice the average for silicic igneous rocks in general. Younger granites of the area also have high concentrations, suggesting that this is a uranium-thorium-rich province. During low-grade metamorphism, mobilization of these elements in the older complex or in the sediments derived from it may have led to formation of the ore deposits.

The relation of tektites to lunar igneous activity

The theory that tektites are of lunar origin has frequently been criticized on the grounds that rocks of this chemical composition could not have been formed on the Moon. The purpose of this paper is to show that tektites may have been derived from silicic igneous rocks, specifically rhyolitic tuffs, forming the upper layers of the maria. The essentially igneous nature of tektites is indicated by their bulk composition, their restricted compositional range, and the high probability that they have not been derived from sedimentary rocks by random processes such as meteoritic impact. The differences between tektites and normal igneous rocks probably reflect their formation by extremely high temperature fusion of previously solid igneous rock. It is shown that if the Moon originally had a chondritic composition, the fusion curve of the lunar material should intersect the thermal gradients computed by G. J. F. MacDonald at the present time at 400 to 500 km depth, depending on the slope of the fusion curve, even if the Moon had originally been at 0°C. Additional heat sources such as the kinetic energy of accretion and capture-induced internal friction would probably have raised the temperature well above the “cold” Moon gradient. Magmas generated by partial fusion of the assumed chondritic material should be basaltic. It is proposed that the maria are the lunar equivalents of terrestrial lopoliths such as the Bushveld and Sudbury complexes, which are large basins filled mainly with basaltic rock overlain by tuffs, rhyolites, and granophyres. Tektites may have been derived from these silicic rocks by some process involving meteoritic impact.

SEISMIC REFLECTIONS FROM LAYERING WITHIN THE PRE‐CAMBRIAN BASEMENT COMPLEX, OKLAHOMA

Reflections from within the pre‐Cambrian basement complex were recorded in the vicinity of the Wichita Mountains in southwestern Oklahoma. The reflections, of good quality and persistence, depict a section in excess of 20,000 ft of igneous rocks that appears like a seismic section of sedimentary formations. A well in the area drilled 4,000 ft of this pre‐Cambrian section, encountering alternating layers of silicic and gabbroic igneous rocks exhibiting high contrast in density. Pre‐Cambrian outcrops of much of the Wichita Mountains, comprising comparable types of rocks, display sheet‐like, gently dipping layers, some of which persist for several miles. The seismic reflections are thus produced by the igneous layers of differential acoustic properties. An abrupt change of direction of dip occurring at about mid‐depth of the seismic section precludes the possibility that the seismic events are multiple reflections.

TRACE ELEMENTS IN PELITIC ROCKS

The range in composition of pelitic shales, schists, and gneisses has been investigated for a number of trace elements and has been compared with previous work on similar materials. An interesting distribution law has appeared (logarithmic), namely that the range in composition extends from zero to about twice the mean value for almost every element. The probability that a rock will fall outside this range is low. Fairly good positive correlation has been found between chromium, vanadium, and nickel, with cobalt participating to some extent. All these elements tend to be high or low together, but the most regular correlation is between nickel and chromium. In general, the content of the trace elements studied is what would be expected from erosion and transfer from the average igneous rock, with no loss or gain on the way. Copper is an exception with a clear loss, whereas lithium and strontium show an increase. Comparison with suites of silicic and basic igneous rocks emphasizes some differences between these and pelitic rocks, but also shows that in many cases the composition fields overlap extensively. Certain revisions in crustal abundance (average igneous rock) figures are suggested.

Pre-Cambrian Crystalline Rocks of North-Central Utah

Four classes of rocks are recognized, namely: metamorphic sedimentary rocks, metamorphic silicic igneous rocks, injection gneisses, and metamorphic mafic rocks. The sedimentary rocks are now metaquartzites, quartz schists, arkosites, conglomerate schists, and possibly metamorphosed graywackes. The pebbles in the quartz-chlorite conglomerate schist were definitely derived from a pre-existing siltstone, and the metaquartzites may possibly represent older sediments that have been re-worked. Other rocks such as the arkosites were probably derived directly from some near-by pre-existing igneous rock. After sediments of considerable thickness were deposited they were thoroughly injected by sills and dikes and also intruded by a few small stocks. The igneous material consists of granite, quartz monzonite, and pegmatite. The injection occurred during a prolonged orogeny which resulted in widespread development of foliation and various cataclastic structures. The recrystallization, cataclastic features, and mineral composition of the Farmington Canyon complex, as the ancient crystalline rocks are here designated, indicate that the metamorphism was of the low- to medium-grade dynamic type. This main period of metamorphism occurred in early or Middle pre-Cambrian time and was more intense than any deformation since. Later crustal disturbances affected very little the structures left in the rocks by this early metamorphism, except locally in fault zones.