Igneous Cores Research Articles

Exploration of the Sarfartôq carbonatite complex, southern West Greenland

Carbonatite was emplaced in two stages at c. 600 Ma in the border between the Archaean craton and the Nagssugtoqidian mobile belt. During and after emplacement hydrothermal activity gave rise to fenitisation and alteration with increased contents of Nb and REE adjacent to the igneous core.

Saponite and celadonite in layer 2 basalts, DSDP Leg 37

Detailed examination of igneous core recovered during Leg 37 of the DSDP, has revealed that basalt diagenesis, an alteration effect resulting from low temperature interaction with seawater, is pervasive to depths of greater than 600 meters in layer 2. Phyllosilicates are among the main components of this low grade alteration effect and occur exclusively as saponite, celadonite and interlayered mixtures of these two phases. Saponites fall into two chemical groups as (a) Mg rich-Fe poor types characteristic of oxidative diagenesis and (b) Fe rich-Mg poor types characteristic of non-oxidative diagenesis. Celadonite is entirely restricted in occurrence to the oxidative diagenetic environment (oxidation halos) and rarely, if ever, occurs as a pure phase. In this respect, X-ray and electron microprobe analyses suggest the presence of interlayered smectite in some cases and iron-oxide contamination in others.

Resource Potential and Plate-Margin Geology of Frontier Basins of North Pacific and Bering Sea: ABSTRACT

Few of Alaska's offshore frontier basins have been explored by drilling. However, regional geologic data and tectonic considerations can be used to assess the likelihood that commercial volumes of oil and gas are present in the basins. Basins of the northern margin of the Gulf of Alaska, and the contiguous Aleutian Ridge on the west, have formed along the Aleutian subduction zone, a tectonic terrane 3,600 km long that separates the Pacific and North American plates. The eastern gulf shelf is underlain by Cenozoic deposits that are as much as 10 km thick, but adequate reservoir beds are thought to be absent in Neogene and younger beds. However, the discovery that potential reservoir and source beds of early Tertiary age underlie the continental slope enhances the oil and gas prospects of the eastern gulf margin. Basins of the central gulf shelf (Kodiak Island area) contain upper Cenozoic beds that are as much as 5 to 6 km thick. These beds are broadly deformed and unconformably overlie more deformed rocks of Paleogen and Cretaceous age. Grabens (unusual for the gulf margin) filled with 6 to 8 km of Neogene and younger beds are present beneath the western gulf shelf (Sanak Island). Publicly available data imply that reservoir and End_Page 1279------------------------------ source beds adequate to form large hydrocarbon deposits are probably absent from the central and western gulf. Farther west the lower Tertiary igneous core of the Aleutian Ridge is overlain by broadly deformed Neogene and younger deposits. These beds are 2 to 4 km thick in summit basins, and probably much thicker below the Aleutian terrace along the ridge's southern flank. Although these basins include diatomaceous and turbidite sequences, the probable abundance of readily altered volcanic detritus cautions against optimistic expectations of large quantities of oil and gas along the Aleutian Ridge. Five extensive (25,000 sq km) basins filled with as much as 15 km of mostly Cenozoic beds are present beneath the Beringian shelf, and, therefore, north of the Aleutian subduction zone. Except near Siberia, the deposits in these basins are little deformed. Elongate St. George and Navarin basins, along the southern or outer edge of the shelf, have formed on a collapsed foldbelt of miogeoclinal rocks that include beds of Jurassic and Cretaceous age. Subsidence of the foldbelt occurred after subduction of oceanic crust ceased beneath the Beringian margin (60 to 70 m.y. ago) and shifted south to the Aleutian Ridge. In contrast, Norton basin, which underlies the inner or northern edge of the shelf, is floored by Paleozoic and older rocks of Brooks Range affinity that subsided in response t Cenozoic strike-slip faulting in western Alaska. A speculative reading of the geologic history of the Beringian basins implies that some of them could harbor commercial volumes of oil and gas. South of the Beringian margin, the abyssal floor (3 to 4 km) of the Bering Sea basin is underlain by 4 to 10 km of undeformed deposits chiefly of Cenozoic age. Drilling results, and the detection of deep-water bright spots (VAMPs), suggest that hydrocarbon deposits (of unknown volume) occur in the basin. Its basement of Lower (?) Cretaceous oceanic crust was presumably separated from the north Pacific by the formation of the Aleutian Ridge in latest Cretaceous or earliest Tertiary time. Since early Mesozoic time, the evolution of the structural framework of the north Pacific margin has been controlled by the subduction of more than 10,000 km of oceanic lithosphere. However, recognition that segments of the margin are underlain by deeply submerged miogeoclinal rocks of Mesozoic and early Tertiary age, and the results of DSDP drilling at Pacific margins, attest that the evolution of Alaskan and Bering Sea margins is not adequately described by models of accretionary tectonics or back-arc spreading. Little understood aspects of subduction and post-subduction tectonics that cause and control marginal uplift and subduction are thought to hold important clues to the economic potential of the frontier basins of the north Pacific and Bering Sea regions. End_of_Article - Last_Page 1280------------

Paleomagnetic results from igneous and sedimentary rocks from the Michigan Basin borehole

The results of thermal and alternating field demagnetization of 13 specimens from a fully oriented mafic igneous core taken from a depth of 5.32 km near the center of the Michigan basin give a pole position at 9°S, 159°E (k = 69, α95 = 5°). This pole is close to late Hadrynian and Cambrian pole positions from the North American craton. Furthermore, one specimen was cut from each of three vertical red bed core segments, unoriented in declination, from depths of 4.59, 4.09, and 4.08 km. Thermal demagnetization revealed a large secondary magnetization in these samples, which was fully eliminated at 480°C. Further thermal demagnetization revealed closely grouped characteristic inclinations that averaged 8°. By assuming the secondary magnetizations to be parallel to the present‐day geomagnetic field direction it was possible to obtain characteristic declinations as well. The pole from the red beds is located at 7°N, 174°E and agrees well with the pole obtained from the late Keweenawan sediments (Freda sandstone and Nonesuch shale) of the Upper Peninsula in Michigan. These results suggest that the red beds acquired their characteristic magnetization before the igneous basement rock. If the igneous rock is an extrusive body of Keweenawan age, its characteristic magnetization must be entirely secondary.

Cooling rates of group IVA iron meteorites

Kamacite Ni profiles in low-Ni and high-Ni IVA irons are distinctly different, and cannot be fit with the same α/(α + γ) boundary in the low-temperature Fe-Ni phase diagram. This is attributed to an expansion of the α field to higher Ni contents resulting from the substantially higher P contents of the high-Ni irons. New α/(α + γ) boundaries are derived for P contents of 0.03 and 0.16%. Cooling rates of six group IVA iron meteorites were estimated by a taenite central Ni concentration-taenite half-width method similar to that of Wood [1]. Narrow (<20 μm) taenite lamellae were used to minimize uncertainties resulting from differences in nucleation temperatures. The calculated cooling rates range between 13 and 25°C/Myr, with an average of 20°C/Myr. No correlation between cooling rate and bulk Ni content is observed, and the data appear to be consistent with a uniform cooling rate as expected from an igneous core origin. This result differs from previous studies reporting a wide range in cooling rates that were strongly correlated with bulk Ni contents. The differences mainly result from differences in the phase diagram and the selected diffusion coefficients. Cooling rates inferred from taenite Ni concentrations at the interface with kamacite are consistent with those based on taenite central Ni contents.

Shallow inclinations of remanent magnetism in Deep-Sea Drilling Project igneous cores: Geomagnetic field behavior or postemplacement effects?

Paleomagnetic results from abyssal igneous cores collected during the Deep-Sea Drilling Project (DSDP) are compared to reference data sets from lava sequences for the period between 13 and 2 m.y. ago in Iceland. Statistical modeling shows that the paleomagnetic data from Iceland can be modeled approximately by a virtual geomagnetic pole distribution which is 84% Fisherian and 16% random. Transforming this result into values of inclination which can be expected at other latitudes provides a measurement of the probability that negative inclinations can occur during times of normal polarity in the northern hemisphere. It appears that inclination of remanent magnetism from holes within 30° of the paleoequator cannot provide reliable measurements of polarity. Comparison of paleomagnetic inclinations in basalt from DSDP leg 37 with our model shows that factors other than the geomagnetic field behavior almost certainly were the cause of the measured inclinations. Postemplacement tectonic rotation around an axis parallel to the ridge crest is therefore inferred. Data from leg 46 contain too few measurements for a reliable comparison to be made.

Changes of Sea-Level in West Spitsbergen: A New Interpretation

achieve and maintain such extraordinarily long and steep radial slopes without suffering very rapid gravitational collapse. The igneous core of multiple, superimposed and radially directed lava flows would hardly be touched, only the superficial ash and calcareous sediments being affected by the slides in this way. The genetic types of the volcanoes involved may be worth future consideration. Goenoeng Api and in fact many of the islands examined so far, are of andesitic character, being well supplied with ash intercalations in the lava flows. On the other hand, the arcuate-margined atolls of the Central Pacific can hardly be included in the andesitic province and, since the slides may very well be confined to the superficial sediments, it does not appear that any satisfactory genetic relationship between volcanic province and slide incidence can yet be drawn. Evolution of atoll forms

Intrusive domes of the Claremont-Newport area, New Hampshire

Study of the Croydon and Unity domes sheds considerable light on the mode of intrusion of the Oliverian magma series in western New Hampshire. Each dome consists of an igneous core (late Devonian) surrounded by concentric belts of older (Ordovician?, Silurian, and early Devonian) metamorphosed sedimentary and volcanic rocks. The foliation of the plutonic cores is chiefly a primary flow structure as indicated by the parallel arrangement of inclusions, the parallelism between foliation and contacts, and the marked development of foliation near contacts. The bedding-plane schistosity of the surrounding metamorphic rocks is to be attributed to effects of the magma because (1) it is confined to the rocks immediately surrounding the igneous cores; (2) it parallels the contacts of the igneous cores; (3) it is relatively flat-dipping over large areas and thus rules out regional compression as a possible explanation. Several lines of evidence show that the numerous Oliverian domes are laccoliths: (1) The structure of each dome resembles that of a laccolith; (2) all the domes have roofs at the same horizon; (3) the intense folding and crumpling of the metamorphic rocks between domes indicate crowding of adjacent and isolated igneous cores; (4) laccolithic intrusion best explains the bedding-plane schistosity of the surrounding metamorphic rocks; (5) laccolithic intrusion best explains the foliation of the igneous cores. The eastern contact of the Mt. Clough pluton (New Hampshire magma series) is briefly considered, and the mode of intrusion of the body is explained.

New Interpretation of Some Laccolithic Mountains and Its Possible Bearing on Structural Traps for Oil and Gas

Each of the five Henry Mountains consists of nearly symmetrical dome, several miles in diameter and a few thousand feet high. Their flanks are wrinkled by radial anticlinal noses, each a mile or two long and a few thousand feet wide. At the center of each dome is a stock, and evidence is presented to show that physical injection of the stocks produced the domes. The anticlinal noses were produced by elongate laccoliths injected radially from the stocks. Igneous cores exposed at the centers of large domes at numerous localities in the United States and other countries have been interpreted generally as mushroom-shaped laccoliths and many of the domes have not been tested for oil and gas, perhaps because the arched strata are breached by erosion and the older strata were thought to pass undisturbed beneath the intrusion. Recent studies in the Henry Mountains, however, indicate that these domes may be produced by stocks as well as by laccoliths and it is suggested that oil or gas may be trapped on the flanks of such domes in oil-bearing regions in much the same way as oil and gas are trapped on the sides of salt domes.

Structural and Commercial Oil and Gas Possibilities of Central Valley Region, California

Relationship of the igneous core of the Marysville Buttes to the petroliferous Cretaceous rocks is presented. Lenticular sands, buttress sands, and structural terraces, or low anticlines, around a great bow in the sediments east of Kettleman Hills, afford favorable conditions for accumulation of oil and gas. Evidence of similar general conditions north of Wheeler Ridge is presented and discussed.

Devonian of the Santa Rita Mountains, Arizona

Introduction The Santa Rita Mountains, in southern Arizona, begin about 10 miles north of Nogales and extend northward about 40 miles, curving slightly to the east and terminating in Empire Mountain. Davidson Canyon partly separates Empire Mountain from the main range. The highest peak, Old Baldy, has an elevation of 9,432 feet, or about a mile above the surrounding country, and therefore is a conspicuous feature. General Geology The Santa Rita Mountains consist of thick masses of plutonic and volcanic rocks, which during late Mesozoic and early Tertiary time penetrated the strata. These igneous rocks constitute the highland, and the slopes and bordering plains are mantled by detritus, largely derived from the igneous core of the range. The pressure of the intrusive masses arched up large areas of the Cretaceous rocks, especially in the northern part of the range, and at some places the Paleozoic rocks were brought to the . . .

The thermal conductivity of wires and rods : T. Barratt and R. M. Winter. (Ann. d. Phys., No. 9, 1925.)

We present a new technique for directly dating, by conventional techniques, the rims of zircons. Several circumstances, such as a xenocrystic or inherited component in igneous zircon and metamorphic overgrowths on igneous cores, can result in grains with physically distinct age components. Pneumatic abrasion has been previously shown by Krogh to remove overgrowths and damaged areas of zircon, leaving more resistant and isotopically less disturbed parts available for analysis. A new abrader design, which is capable of very gently grinding only tips and interfacial edges of even needle-like grains, permits easy collection of abraded material for dating.Five examples demonstrate the utility of the “dust-collecting” technique, including two studies that compare conventional, ion microprobe and abrader data. Common Pb may be strongly concentrated in the outermost zones of many zircons and this Pb is not easily removed by leaching (even in weak HF). Thus, the benefit of removing only the outermost zones (and avoiding mixing of age components) is somewhat compromised by the much higher common Pb contents which result in less precise age determinations. A very brief abrasion to remove the high common Pb zones prior to collection of material for dating is selected.

Dust from cement works: A. W. Stockett (U. S. Bureau of Mines)

We present a new technique for directly dating, by conventional techniques, the rims of zircons. Several circumstances, such as a xenocrystic or inherited component in igneous zircon and metamorphic overgrowths on igneous cores, can result in grains with physically distinct age components. Pneumatic abrasion has been previously shown by Krogh to remove overgrowths and damaged areas of zircon, leaving more resistant and isotopically less disturbed parts available for analysis. A new abrader design, which is capable of very gently grinding only tips and interfacial edges of even needle-like grains, permits easy collection of abraded material for dating.Five examples demonstrate the utility of the “dust-collecting” technique, including two studies that compare conventional, ion microprobe and abrader data. Common Pb may be strongly concentrated in the outermost zones of many zircons and this Pb is not easily removed by leaching (even in weak HF). Thus, the benefit of removing only the outermost zones (and avoiding mixing of age components) is somewhat compromised by the much higher common Pb contents which result in less precise age determinations. A very brief abrasion to remove the high common Pb zones prior to collection of material for dating is selected.

On the connexion of Volcanic Action with Changes of Level

The purpose of this paper was to show that “volcanic action is not the cause, but the effect, of secular changes of level; and secular changes of level are due to the subsidence of the surface of the interior, as the interior contracts in cooling.” Change of level is a differential action, and consequently cannot be due to the cooling of a sphere by radiation into space. Volcanic action cannot be due to a spontaneous outburst of the expansive force of the earth’s internal heat; for this could not burst through a crust once formed by cooling. Changes of level and volcanic action were explained as follows:—The interior of the earth is constantly cooling, and as it cools must contract; but the cold surface-strata cannot contract with it; and as their weight keeps them in contact with the core, they are compelled to form ridges like those on the skin of an apple which shrinks in drying. When such a ridge rises into an arch, the hot matter below rises and fills the arch, forming the igneous core of a mountain-chain. Volcanoes are formed when in these foldings the surface is broken through, so as to liberate the expansive force of the internal heat. Darwin has shown, in his work on Volcanic Islands, that volcanoes are formed only in regions which are rising or have recently risen. The reason of this is, that an upward bend in a stratum is resisted only by the weight of the stratum itself, while a downward bend is resisted by the pressure on the strata below; so that an upward bend is more likely than a downward one to become great enough to produce fracture.