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At least 199 records · Page 11Linked to original sources
USGS1973

Petrogenesis of the Superstition-Superior volcanic area as inferred from strontium- and oxygen-isotope studies

Apparent initial Sr 87 /Sr 86 ratios of five ash-flow tuffs (0.7063 to 0.7139) and several mafic to silicic lavas (0.7055 to 0.7131) indicate that the magmas were derived below the base of the Precambrian granitic crust (0.7231 to 1.0906). Liquidus compositions in the system Q-Or-Ab-H 2 O and oxygen-isotope geother-mometry suggest that the silicic magmas started to crystallize quartz, magnetite, and two feldspars in a water-undersaturated environment of high pressure (∼10 kb) and moderate temperature of at least 830°C. During or after ascent into the crust, the magmas underwent varying degrees of crystal-melt re-equilibration. Measured plagioclase-biotite O 18 fractionations (0.5 to 0.7) imply a temperature that is too high for the observed mineral assemblage, and the inference is that the two minerals did not crystallize in equilibrium. Prior to eruption, the upper part of the magma column assimilated crustal Sr such that the base of each ash flow is now enriched in Sr 87 . In some cases, this assimilation was too rapid to allow crystal-melt equilibration of Sr isotopes. δO 18 values for the magmas are within the range typical of similar magma types, indicating that no significant interaction took place between the melts and meteoric water; however, some rocks have exchanged oxygen isotopes with meteoric water at low temperatures after eruption. Some of the magma appears to have had a long residence in the crust at lower pressure (1 kb) and temperature (750°C), because two of the ash-flow tuffs and one lava are greatly enriched in Sr 87 and have largely re-equilibrated under the P-T conditions of a shallow magma chamber. Even these, however, have retained evidence for a multistage genesis.

Geological Society of America Bulletin↗
USGS1973

Interpretation of a high-grade Precambrian terrane in northern Idaho

A terrane of high-grade metamorphic rocks in northern Idaho and northeastern Washington is almost completely surrounded by low-grade rocks of the Precambrian Belt Supergroup. The high-grade terrane includes both Belt and pre-Belt rocks. Four events of folding and metamorphism occurred in the high-grade terrane. The first three events may have been associated with the Late Cretaceous emplacement of quartz monzonite of the Kaniksu batholith; the fourth may have been associated with a slightly later emplacement of granodiorite or with a Tertiary plutonic and volcanic episode. A much older event of plutonism in the high-grade terrane is recorded by zircon, which was dated by the Pb-U method at 1,500 m.y. from pre-Belt meta-igneous augen gneiss. Evidence of regional events intermediate in age between 1,500 and 100 m.y. has been found in the surrounding low-grade rocks but not in the high-grade terrane.

Idaho↗
USGS1973

Origin of andesitic and granitic magmas in the northern Sierra Nevada, California

The early magmas of the northern Sierra Nevada, calc-alkaline andesite of island-arc type and its derivatives, all low in potassium, were generated during the Devonian(?) period, possibly along an eastward-dipping sub-duction zone. These magmas could have been derived from mantle peridotite of the continental plate by introduction of water from the descending oceanic plate. Later, during the Permian(?) period, the magmas became basaltic, with potassium-rich silicic derivatives indicating anhydrous conditions and a deeper level of magma generation. Plutonism began in Jurassic time, at the end of a period of intense deformation and metamorphism. The earliest intrusive rocks are gabbro and diorite. At the end of the Jurassic period, large granitic plutons were emplaced. These grade from hornblende quartz diorite at the borders to monzotonalite at the centers. Trondhjemite occurs as the latest product of crystallization differentiation of plutonic magmas. Exchange of elements between plutonic and metamorphic rocks suggests that the plutonic magmas were composite. The partial melts of the downfolded volcanic and sedimentary rocks were modified by partial melts from the mantle and the subducted oceanic lithosphere below. Relative amounts of material contributed by each of the three sources of plutonic magma changed with time, and these changes, along with differentiation processes, were responsible for the diversity in composition of magmas.

California↗
USGS1973

Species diversity: Patterns in modern and Miocene foraminifera of the eastern margin of North America

Patterns of foraminiferal species diversity were examined along the eastern margin of North America by utilizing the number of species, S, the information function, H(S), and species equitability, E. The 350 modern samples we studied extended from the Arctic to the Gulf of Mexico at depths ranging from a few meters to more than 5,000 m. In addition, 29 samples from Miocene strata of the Atlantic Coastal Plain and continental shelf were studied. Modern species diversity as measured by S and H(S) generally increases as depth increases and latitude decreases. Some notable exceptions occur, however, which are difficult to explain. For example, species diversity in the Arctic depth interval of 0 to 100 m is as high or higher than that found immediately south of Nova Scotia, in the Gulf of Maine, on Browns and Georges Banks, and even off the Gulf of Mexico deltas. At the moderate depth interval of 100 to 1,000 m, however, the entire margin north of Browns and Georges Banks has lower diversities than that to the south. The highest diversity by far in this depth interval occurs in the northeastern Gulf of Mexico. At the depth interval greater than 1,000 m, the more southern areas studied generally have a higher species diversity than the more northern Cape Cod to Maryland area. An exception to this is the northwestern Gulf of Mexico; this area is also an exception in that species diversity is significantly lower in the deeper waters than in the shallower waters in the same area. The measure of species equitability, E, showed no clear pattern with depth or latitude. This may be so because no simple pattern of species proportions exists or because the sampling was inadequate to measure it. Samples from the Miocene strata show a striking resemblance in species diversity to modern samples at similar depths and latitudes. Our observations indicate that species diversity and equitability have not increased during the last 15 × 10 6 yrs. The fossil and modern data indicate that each environment has its own carrying capacity and that this capacity is reached rather quickly. Although time and environmental stability are undoubtedly important in determining species diversity, as presently defined they are inadequate to explain all observed patterns. Long-term observations in various environments will be required to determine the relative importance of variables that affect species diversity.

Geological Society of America Bulletin↗
USGS1973

Eclogites from southwestern Oregon

Eclogite, high-grade blueschist, and amphibolite blocks occur within the Mesozoic Otter Point Formation of southwestern Oregon and are inferred to have been tectonically emplaced by eastward-directed overthrusting involving Colebrooke Schist and serpentinite. Eclogite from southwestern Oregon is very similar in bulk chemistry and mineralogy to the well-studied eclogite of California. Calculations of phase equilibria at load pressures of 7 and 10 kb and T = 400°C to 550°C suggest that many of the hydrates found in eclogite could have been stable at very low H 2 O fugacities. The lack of lawsonite and the presence of almandine-grossular garnet set a maximum limit on H 2 O fugacity for a given P s -T. Chemically, Group C eclogite from Oregon and California characteristically is nepheline normative and is enriched in normative diopside relative to basaltic compositions. The present chemistry of this eclogite may be the result of metasomatism in an ultramafic environment with low a SiO 2 and high a Ca, but outside the stability field of serpentine. The generally high jadeite content of clinopyroxene from Group C eclogite compared with Group A and Group B eclogite is largely a function of bulk rock chemistry. Crystallization under low a SiO 2 conditions stabilizes jadeite in clinopyroxene at lower P s for a given T.

Oregon↗
USGS1973

Crandall conglomerate, an unusual stream deposit, and its relation to heart mountain faulting

The Crandall Conglomerate (Eocene) is a channel deposit, more than 350 ft (100 m) thick, believed to have formed as a result of preliminary movement of the Heart Mountain detachment fault in northwestern Wyoming. Initial movement of the Heart Mountain fault opened a deep rift in which the conglomerate was deposited. The rift was less than a mile (1.6 km) wide and was bordered by 2,000-ft (600 m) cliffs, mostly of Paleozoic limestone. Before the gravel was deposited, unconfined Cambrian shale below the rift was deformed into the Blacktail fold, a sharp anticline without apparent roots, while streams carried away the upwelling shale and cut a channel several hundred feet deep. The debris that accumulated in this channel is the Crandall Conglomerate. Deposition of the conglomerate was followed by Cathedral Cliffs volcanism, by movement on the Reef Creek detachment fault, and by the main movement on the Heart Mountain detachment fault. The main movement on this fault left the lower part of the conglomerate in place but carried the upper part with deposits of the upper plate roughly 15 mi (24 km) southeastward. Most of the deposits of the lower plate rest directly on the Blacktail fold. Of the 15 known deposits of Crandall Conglomerate, five are in place but have been overridden by the upper plate of the Heart Mountain fault, and ten have been transported as part of the upper plate. After this movement, volcanic rocks of the Wapiti Formation blanketed the region. © 1973 Geological Society of America.

Geological Society of America Bulletin↗
USGS1973

Structure and development of the continental margin of British Honduras

The continental margin of British Honduras is characterized by elongate ridges that are sub-parallel with the coast and that diverge slightly northward. Each ridge apparently is formed by aligned fault blocks composed of continental basement rock that rotated and subsided during rifting of the margin. The western part of the Cayman Ridge may be underlain by similar rock that extends southwestward into the margin. Little sediment has been trapped in the basins between the ridges, presumably as a result of long-term, low rates of sedimentation. A canyon system that is controlled by both fault-block ridges and cross faults transects the margin and opens into the Yucatan Basin. Deep-sea sediments are characterized by a succession of pelagic deposits above acoustic basement rocks; these deposits are covered by turbidites that have formed deep-sea fan deposits in some places. The change from pelagic to turbidite deposition may have occurred at the end of Cretaceous time when regional uplift provided a source for clastic materials. The principal channel for transport of sediment to the deep sea is believed to be a graben that extends northeastward from the southwest corner of the Gulf of Honduras. Development of the margin may have begun in Late Jurassic time when sphenochasmic rifting created the ancestral Gulf of Honduras. Near the end of Cretaceous time, a new set of motions began, with the Caribbean plate moving relatively eastward and slightly away from the North American plate. This new motion resulted in continued movement of basement blocks through Tertiary and Quaternary time. Although this margin is not young, it has a geomorphically youthful aspect in that the sedimentary sequences do not obscure the rift structures.

British Honduras, Caribbean Sea, Gulf of Honduras↗
USGS1973

Glauconites from New Jersey-Maryland coastal plain: Their K-Ar ages and application in stratigraphic studies

Glauconite samples from various stratigraphic levels in the northern Atlantic Coastal Plain were dated by the K-Ar method. Twenty-eight samples were collected from glauconite-bearing sands in four traverses across the outcrop belt of Upper Cretaceous–lower Tertiary formations from northern New Jersey to eastern Maryland, thus providing a framework on which to test the reliability of glauconite for dating in vertical sequences and within some formations along strike. Sample ages were determined paleontologically and radiogenically. The results show that most radiogenic determinations of glauconite followed a systematic pattern; the resultant ages were generally younger upward through the Stratigraphic section. In addition, consistent ages were obtained from many of the stratigraphic units along strike. A combination of paleontologically derived ages and the numerical radiogenic ages suggests that the stages of the Late Cretaceous are unequal rather than equal as some have proposed. For example, Campanian and Paleocene times are of significantly greater duration than the Maestrichtian. Several samples obtained across the Cretaceous-Tertiary boundary indicate that the age of the boundary is between 63.2 and 59.1 m.y., probably very near 61.1 m.y. Dates considered to be reliable were obtained from 25 of 28 samples. Two of the three discordant ages are ascribed to incomplete glauconitization of the pre-existing mineral phase and hence to mineral impurity. No adequate explanation for the third significantly younger age has been determined.

Delaware, Maryland, New Jersey, Pennsylvania↗
USGS1973

Granitic rocks of the White Mountains area, California-Nevada: Age and regional significance

Potassium-argon ages have been determined on 25 biotite and hornblende samples (four coexisting biotite-hornblende pairs were dated) from a number of granitic formations in the more than 500 sq mi of dominantly granitic outcrop in the White Mountains. These new data, together with earlier published radiometric ages, indicate a group of plutons about 70 to 85 m.y. old, a single body about 210 m.y. old, another that may be about 225 m.y. old, and several between 130 and 185 m.y. old. Evidence of intrusion is lacking in the intervals of about 90 to 130 m.y. and 185 to 200 m.y. The oldest age, about 226 m.y., may represent the chance preservation of a “primary” hornblende in a body in which the dark minerals are largely recrystallized and their radiometric ages reset. This age suggests that Triassic magmatism may be more widespread along the eastern margin of the Sierra Nevada batholith than had previously been considered. The radiometric age data from the White Mountains, together with similar data from northwestern and north-central Nevada and abundant data from the central Sierra Nevada, suggest essentially continuous, albeit irregular, magmatic activity from Triassic to the end of Cretaceous, except for periods of little or no activity in earliest Jurassic and earliest Cretaceous. Episodes of magmatic activity may, and probably do, characterize specific areas, but when sufficiently large blocks of the Sierra Nevada batholith are considered, the sum of the episodes approaches a continuum.

California, Nevada↗
USGS1973

Trans-Andean geophysical profile, southern Colombia

Negative Bouguer anomalies (−80 mgals) near the Pacific coast of southern Colombia define the position of the Tertiary Bolívar trough. Values increase eastward to a huge positive anomaly (+75 mgals) over Mesozoic “eugeosynclinal” rocks of the western Andes. This anomaly is part of the West Colombian gravity high, which extends from Panamá into western Ecuador and is caused by shallow mafic crust. Bouguer anomalies are strongly negative (−220 mgals) over pre-Mesozoic(?) metamorphic rocks, Mesozoic(?) granitic bodies, and Tertiary to Holocene volcanic rocks of the central Andes between Pasto and Ipiales. The steep gravity gradient between the West Colombian gravity high and the negative anomaly of the central Andes represents the transition between mafic crust to the west and continental crust to the east. This zone parallels the Romeral-Cauca megashear system. East of the Andes, Bouguer anomalies range from −50 to −120 mgals over a Mesozoic-Tertiary basin of the Putumayo district, indicating that the crust there is thinner or denser than it is beneath the central Andes. Models derived from gravity data suggest that the crust is about 45 km thick under the south-central Colombian Andes. If this is correct, the crust must thicken southward along the strike of the Andes, as thicknesses of 70 km have been reported in the Andes of southern Perú, Bolivia, and northern Chile by Lomnitz (1962) and James (1971a). Such differing crustal thicknesses may reflect different intensities of tectonic activity, greater crustal thickness indicating more intense or rapid growth of the volcano-plutonic arc or foreshortening of an existing crustal section.

Geological Society of America Bulletin↗
USGS1973

Regional gravity anomalies and crustal structure in northern Colombia

The central range of the Colombian Andes gives way northward to a series of Cenozoic fault-bordered basins and uplifts near the Caribbean Sea. Pre-Cenozoic structures exposed in the uplifts curve increasingly toward the east to become parallel to the continental margins along the south side of the Caribbean. Major Cenozoic faults, with large vertical and horizontal displacements, cut across older structures which include Permian-Triassic(?) and Late Cretaceous to early Tertiary metamorphic zones, Precambrian gneiss, and Jurassic batholiths. Gravity anomalies have large amplitudes in the Santa Marta area. Bouguer anomalies rise to +130 mgals over the crystalline rocks of the high Santa Marta massif. Over adjacent Cenozoic basins, they range down to −80 mgals over the Lower Magdalena basin and to −65 mgals over the Baja Guajira basin. Steep gravity gradients characterize the Santa Marta and Oca faults on the west and north sides of the massif, respectively. In the Guajira Peninsula region, Bouguer anomalies increase to +105 mgals over a serpentinite zone at Cabo de la Vela and to +55 mgals over Cretaceous volcanic rocks in the southern peninsula. Two smaller basins on the peninsula are characterized by negative Bouguer anomalies. Steep gradients characterize many of the major Cenozoic faults, and two concealed faults are postulated on this basis. Though useful for evaluating the relative vertical displacements, which may exceed 10 km along faults bounding the Santa Marta massif, the gravity data yield no definitive information on the large horizontal displacements postulated along some of the major faults of the area. The Bouguer anomalies do indicate, however, the extension of some of the Cenozoic basins into offshore areas. Strong positive Bouguer anomalies of the Santa Marta massif and its great relief, which exceeds 9 km relative to the floor of the adjacent Caribbean, indicate thin continental crust, lack of isostatic balance, and relatively recent uplift for the massif. After corrections are made for the gravitational effects of Tertiary sedimentary basins in the Guajira Peninsula, most of the peninsular region also has positive anomalies, suggesting a relatively thin continental crust and a lack of isostatic balance. A mechanism of overthrusting, in relatively recent time, of the continental margin over the adjacent Caribbean upper mantle and crust to the northwest can account for the observations.

Geological Society of America Bulletin↗
USGS1973

Fusion Relations in the System NaAlSi3O8-CaAl2Si2O8-KAlSi3O8-SiO2-H2O and Generation of Granitic Magmas in the Sierra Nevada Batholith

Chemical analyses of 167 typical specimens indicate that about 95 percent of the intrusive rocks of the central Sierra Nevada contain more than 79 percent normative Ab + An + Or + Qz. If the composition of the lower continental crust is similar to or slightly more felsic than andesite, as seems likely, the system NaAlSi 3 O 8 -CaAl 2 Si 2 O 8 -KAlSi 3 O 8 -SiO 2 -H 2 O provides an excellent chemical model for testing various schemes of fusion of the lower crust and crystallization of the resulting magmas. From consideration of this system in conjunction with field and petrographic data, we conclude that the intrusive rocks are best explained by repeated episodes of equilibrium fusion corresponding to magmatic sequences defined by field, petrologic, chemical, and geochronologic data. Fractional crystallization of the crystal-liquid mush generated by equilibrium fusion, coupled with periodic upward or lateral movement of the less crystallized central part of the magma, would produce the characteristic mafic to felsic sequence of intrusion; each mafic to felsic sequence corresponds to a separate equilibrium fusion event. In contrast, a close approach to fractional fusion of the lower crust is inadequate for obtaining most of the plutonic rocks, because rock compositions capable of being produced by this process do not match those observed. Normal amounts of conductive heat from the mantle and from radioactive decay in the crust may have been capable of causing fusion in the deepest parts of a thickened crust under the central part of the Sierra Nevada without the aid of a transient heat source from the mantle, but would have been inadequate where the crust was thin in the western Sierra Nevada. However, upward transport of andesitic and basaltic magmas generated along a Mesozoic subduction zone dipping beneath the Sierra Nevada would have provided sufficient additional heat to make fusion of the lower crust unavoidable. This implies that a major portion of the present batholith must have been derived from the lower crust.

California↗
USGS1973

Variations in Sr, Rb, K, Na, and Initial Sr87/Sr86 in Mesozoic Granitic Rocks and Intruded Wall Rocks in Central California

Initial Sr 87 /Sr 86 of granitic rocks which are exposed north of the Garlock fault in California, and which represent the entire 130-m.y. time span of emplacement during the Mesozoic, ranges mainly from 0.7031 to 0.7082, with one value of 0.7094. A systematic areal variation, independent of age, exists for initial Sr 87 /Sr 86 in these granitic rocks and is the same as the areal variation in initial Sr 87 /Sr 86 of superjacent upper Cenozoic basalts and andesites. Two values of initial Sr 87 /Sr 86 , 0.7040 and 0.7060, mark natural separations of granitic rock data on K-Rb, K-Sr, and Rb/Sr-Rb variation diagrams, and also, when contoured, seem to represent geographic markers of paleo-geographic, geochemical, and physiographic significance. Upper Precambrian sedimentary and metamorphic rocks in California crop out only in the region where initial Sr 87 /Sr 86 of granitic rocks is greater than 0.7060. A line of initial Sr 87 /Sr 86 = 0.7060 is approximately coincident with the boundary between Paleozoic eugeosynclinal and miogeosynclinal rocks. Granitic rocks intruded into Paleozoic miogeosynclinal rocks have initial Sr 87 /Sr 86 greater than 0.7060, whereas those intruded into eugeosynclinal Paleozoic rocks have initial Sr 87 /Sr 86 less than 0.7060. The line of initial Sr 87 /Sr 86 = 0.7040 is the eastern limit of principal exposures of ultramafic rocks, the western limit of Cretaceous granitic rocks, and is coincident with an abrupt change in “topographic expression” on the Bouguer gravity map of California. Correlation of the isotopic variations with these major crustal features suggests that there has been a sharp lateral contrast in crust-mantle chemistry across the region of study that has been fixed in position from the Precambrian to the present time. The chemical and isotopic variations observed are best explained if the parent magmas of the majority of granitic rocks investigated were derived in a region that was laterally variable in composition and in a zone of melting that intersected both upper mantle and lower crust. However, some igneous rocks, such as Jurassic volcanic rocks in wall rocks and roof pendants and some granitic rocks with high strontium concentrations and low Rb-Sr ratios, suggest that deeper sources are also involved in the total spectrum of igneous rocks in the region.

California↗
USGS1973

Variations in lead-isotopic compositions in Mesozoic granitic rocks of California: A preliminary investigation

Six alkali feldspar and two whole-rock samples of granitic rocks from the Sierra Nevada batholith and adjacent Klamath Mountains were analyzed for their lead-isotope compositions. The samples represented each of three 87 Sr/ 86 Sr groupings (< 0.704, 0.704 to 0.706, and > 0.706) for granitic rocks north of the Garlock fault in California. The isotopic compositions of lead in the samples from the Sierra Nevada batholith range from 18.73 to 19.37 for 206 Pb/ 204 Pb, 15.61 to 15.71 for 207 Pb/ 204 Pb, and 38.44 to 39.10 for 208 Pb/ 204 Pb. A crude parallel correspondence was found between lead and strontium isotopes, in that the specimens with the most radiogenic strontium also tend to have the most radiogenic lead similar to the previously studied Boulder batholith of Montana. A parallel correspondence is thought to imply characteristics of the source rocks for the plutons rather than consequences of partial melting or natural contamination. Lead-isotopic compositions for the Sierra Nevada batholith and the Boulder batholith differ, average values of 206 Pb/ 204 Pb being at least 18.8 for the Sierra Nevada batholith and about 18 for the Boulder batholith. In the Late Cretaceous part of the Sierra Nevada batholith, the secondary isochron “age” for the lead data in these rocks is about 2,900 m.y., far older than known Precambrian in California. Sources are proposed for these plutons from the lower continental crust and upper continental mantle or dominantly recycled continental materials, probably of intermediate composition and possibly carried down to the zone of melting by subduction. This source material may have been formed in Pre-cambrian times but did not undergo a Precambrian metamorphism greater than upper amphibolite facies which would have reduced the values of 238 U/ 204 Pb in the source rocks and resulted in Mesozoic leads like those found in the Boulder batholith and elsewhere in the Rocky Mountain region. A trondhjemite from the Klamath Mountains has a lead-isotope composition ( 206 Pb/ 204 Pb, 18.57; 207 Pb/ 204 Pb, 15.50; 208 Pb/ 204 Pb, 38.08) similar to that of oceanic volcanic rocks, particularly like those of island volcanics on oceanic ridges. Derivation of this trondhjemite from an oceanic mantle or recycled mantle material is indicated by this observation and supports the conclusion of Kistler and Peterman (1973) based on its alkali abundances and 87 Sr/ 86 Sr value.

California↗
USGS1973

Age and correlation of the Windermere Group in northeastern Washington

Greenstone of basaltic composition forms the middle part of the Windermere System in southern British Columbia and the correlative Windermere Group in northeastern Washington. The greenstone, together with the rest of the Windermere in this region, is highly sheared, altered, and metamorphosed, except for a small exposed mass of relatively unaffected rocks near the community of Chewelah, Washington. K-Ar ages on whole rocks and mineral separates from the Chewelah area indicate the greenstone was probably extruded between 827 and 918 m.y. ago.

Washington↗