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At least 1,387 records · Page 77Linked to original sources
USGS1962

Hornblendes formed during progressive metamorphism of amphibolites, northwest Adirondack Mountains, New York

Hornblendes in amphibolite interlayers in the paragneiss of the northwest Adirondack Mountains undergo systematic changes in color, composition, and density during progressive metamorphism from almandine- amphibolite to hornblende -granulite facies. In contrast, indices of refraction of the hornblendes remain about constant. In the almandine- amphibolite facies the amphibolite layers have the bulk composition of a saturated basalt and consist of bluish-green hornblende , andesine, and quartz. As these layers are traced into the hornblende -granulite facies, their composition undergoes a progressive change to that of an olivine basalt with brownish-green hornblende , clinopyroxene and orthopyroxene, and calcic andesine as major constituents. Compositional changes in the hornblendes with increasing grade of metamorphism include increases in Ti, Na, K, Cr, V, and Sc. Decreases occur in the amounts of Mn, Zn, OH + F + Cl, and in the ratios Fe 2 O 3 /FeO and Fe/Mg. Density of the hornblendes increases from 3.260 to 3.278 with the increasing grade of metamorphism . These changes in the hornblendes with increasing T and P, although well denned, are less pronounced than those measured in biotites and garnets of the enclosing paragneiss. Large variations in the physical and chemical properties of hornblendes in metamafic rocks reconstituted above the epidote- amphibolite facies appear to be induced principally by critical changes in the bulk composition of the total rock, and not by the regional gradients in T, P, or by changes in kind, or composition, of the coexisting minerals.

New York↗
USGS1965

Isotopic ages of minerals from granitic rocks of the central Sierra Nevada and Inyo Mountains, California

Potassium-argon ages of biotite and hornblende from specimens of 17 granitic plutons in the central Sierra Nevada and the western Inyo Mountains, California, range from 69 to 183 m. y. The Mount Givens, Lamarck. and Round Valley Peak Granodiorites and related younger and more felsic quartz monzonites represent a pulse of magma emplaced in the general time interval of 80-90 million years ago, during Cretaceous time. Mineral ages of granitic rocks that flank these plutons on both the east and the west have been reduced during the emplacement of the Cretaceous intrusive rocks and are minimum ages for the time of crystallization. The ages of hornblende from the Tinemaha Granodiorite (150 to 180 m. y.) may approach crystallization dates. In conjunction with ages for other intrusive rocks in the Sierra Nevada and adjacent desert ranges they strongly suggest a magmatic episode during the Early Jurassic.

California↗
USGS1965

Stratigraphy and heavy minerals of the bays formation, Bays Mountain synclinorium, northeast Tennessee

The Bays Mountain synclinorium is in the Valley and Ridge province in northeast Tennessee , southwest of Kingsport and west of Greeneville. The more clastic part of the Bays formation lies in the east section of the synclinorium . The thickness of the Bays decreases from about 870 feet on the east to about 600 feet on the west. Presumably, the red beds and primary features of the Bays formation formed under deltaic conditions. A Camarocladia Zone near its base indicates a time-transgression of the Bays . The Bays is older at the base to the east than at its base to the west. The heavy minerals of the Bays were compared with those of the Cambrian and Precambrian(?) sediments, volcanic rocks, and accessory minerals of the crystalline complex which now comprise the Blue Ridge. The minerals identified include nearly all those in the Cambrian and Precambrian(?) sediments as well as additional minerals not found in these older sediments but that are in the crystalline complex. Such minerals indicate that at least a part of the crystalline complex was above sea level during the Middle Ordovician.

Tennessee↗
USGS1965

Structure, metamorphism, and plutonism in the south-central Klamath Mountains, California

In the south-central Klamath Mountains 50 miles of the the north-trending central metamorphic belt and adjacent parts of the eastern Paleozoic and western Paleozoic and Triassic belts have been mapped and studied in detail. Within the central metamorphic belt a sequence of three lithologically distinctive metamorphic units has been recognized (from bottom to top): (1) siliceous metasedimentary rocks and greenstones of the Stuart Fork Formation; (2) the Salmon Hornblende Schist; and (3) siliceous, calcareous, and amphibolitic rocks, predominantly metasedimentary, of the Grouse Ridge Formation. The age of these metamorphic rocks is uncertain; they are known only to predate intrusion of Late Jurassic (Nevadan) granitic rocks. Ultramafic rocks, mainly alpine-type peridotites, were emplaced before the granitic rocks and occur primarily in a single large sheetlike body which separates the central metamorphic belt from the eastern Paleozoic belt. Granitic plutons, including quartz diorites, trondhjemites, granodiorites, diorites, and gabbros, in decreasing order of abundance, range in size from less than 1 to about 80 square miles in area. Two orogenic phases in the central metamorphic belt have been distinguished by structural and textural features. A late deformation uniformly affected the metamorphic terrane and the ultramafic rocks but predated granitic rocks. It was accompanied by some metamorphism in the lower to middle greenchist facies and produced upright folds that trend south and plunge gently. An earlier phase affected the various rock units differentially; it produced widespread recumbent folding and upper greenschist- to amphibolite-facies metamorphism in Salmon and Grouse Ridge rocks, but involved the underlying Stuart Fork Formation less severely, producing at least local recumbent folding and lower greenschist-facies metamorphism. The preferred interpretation of this upward increase in structural complexity and metamorphic grade is that the Salmon-Grouse Ridge sequence is a thrust sheet which overrode the Stuart Fork rocks concurrently with emplacement of the ultramafic rocks during the culmination of early recumbent folding and metamorphism. Thrusting was then followed by upright folding during the waning stages of regional metamorphism. The first deformational phase, and possibly the second, occurred during late Paleozoic time as indicated by recent isotopic ages of Salmon Hornblende Schist.

California↗
USGS1969

Carbon isotopes in pelites of the Precambrian Uncompahgre Formation, Needle Mountains, Colorado

Carbon isotopic ratios and weight percentages of carbon were measured in 15 samples of slate, phyllite, and schist of the approximately 1500- to 1600-m.y.-old Uncompahgre Formation of the Needle Mountains, southwestern Colorado. Rocks with less than 1 percent total carbon, all of which is reduced, have δC 13 values of −23 to −28 per mil, whereas those with 1 to 6.4 percent carbon have δC 13 from −29 to −31 per mil. In general, the slates and phyllites contain more carbon and isotopically lighter carbon than do the schists of higher metamorphic rank. Increasing loss of C 12 -enriched methane with increasing intensity of metamorphism is suggested to account for these differences.

Colroado↗
USGS1970

Faulting in the Burro Mountain area, California Coast Ranges, and its relation to the Nacimiento fault

The northwest-striking Nacimiento fault, in the southern Coast Ranges of California, has generally been regarded as the boundary between two major structural blocks: the Nacimiento block to the southwest, in which the basement rocks belong to the Franciscan Formation (Upper Jurassic to Upper Cretaceous), and the Salinian block to the northeast, in which the basement rocks are granitic and high-grade metamorphic. It has been found, however, that in the Burro Mountain area of the southern Santa Lucia Range, the “Nacimiento” fault of Jennings (1959) is nearly vertical and is within the Nacimiento block. In this area, the Franciscan Formation crops out northeast of the “Nacimiento” fault through windows in an older, low-angle thrust fault that brings the Asuncion Group of Taliaferro (1943) (Upper Cretaceous) over the Franciscan Formation. The fault boundary between the Nacimiento and the Salinian blocks must therefore lie farther to the northeast, where it may be buried beneath the Asuncion Group and younger strata. This conclusion is supported by Hanna's recent aeromagnetic work (1969).

California↗
USGS1970

Thermal infrared investigations, Arbuckle Mountains, Oklahoma

Thermal-infrared images obtained on flights over the Tishomingo anticline and South Flank areas near Mill Creek in the Arbuckle Mountains, Oklahoma, were used to study the possibility of identifying some common rock types from their diagnostic reflection and emission characteristics, and to evaluate the usefulness of infrared images in structural geologic investigations. The areas flown are underlain by folded and faulted Paleozoic dolomite, limestone, sandstone, shale, and Precambrian granite. Images were obtained at 6:00 a.m., 11:00 a.m., and 2:00 p.m. The predawn (6:00 a.m.) image is the most useful in distinguishing rock types. Of particular interest is a thermal contrast of dolomite (warm) and limestone (cool), sufficient to distinguish those rock types and to reveal facies changes between them. Theoretical considerations indicate that this thermal contrast arises from a combination of albedo and thermal-inertia characteristics distinctive of dolomites and limestones in many areas. The daytime images display much stratigraphic and structural detail. Small-scale bedding detail is enhanced in the morning images of low-relief areas, and contrasts of alternating formations that form hogbacks and valleys are enhanced in the afternoon images of higher relief areas. The difference in features displayed in morning and afternoon images appears to be a function of the insolation on sunward and shadowed slopes of differing scale. Fault or fracture zones are best displayed in the predawn image; they appear cooler than surrounding ground, because of greater water content and concomitant evaporation. The abundance and throughgoing nature of lineaments (which coincide for the most part with joint systems) are more obvious in the infrared images than in aerial photographs. Lineaments striking northwest are preferentially enhanced in the morning images, and lineaments striking northeast are preferentially shown in the afternoon images. This enhancement cannot be ascribed to the effects of topography, insolation, or wind; it may relate to a combination of ground-water and vegetation effects.

Oklahoma↗
USGS1971

Relations of folded dikes and Precambrian polyphase deformation, Gardner Lake area, Beartooth Mountains, Wyoming

Two cross-cutting mafic dikes in the headwall of Gardner Lake in the eastern Beartooth Mountains, Wyoming, have structural relations with Archean migmatite and gneiss that suggest intrusion between deformational phases recognized in the eastern part of the range. Fabric data show that the older dike, an orthoamphibolite, was emplaced subsequent to the F 1 deformational event, but prior to the main episode of metamorphism, metasomatism, and folding, F 2 . The younger dike, a metanorite, was intruded after F 2 , but is folded by west-trending, open F 3 folds. The style and general trend of these F 3 folds are consistent with those observed elsewhere in the eastern part of the range. The data available and absolute age relations in other parts of the range indicate that F 2 occurred 2750 m.y. ago and F 3 1600 to 1800 m.y. ago.

Wyoming↗
USGS1971

Paleozoic metasediments in the northern Ruby Mountains, Nevada

New evidence indicates that high-grade regionally metamorphosed marble and quartzite in the northern Ruby Mountains are Paleozoic strata. Correlation is based on a match of the lithologic sequence to strata of Cambrian to Devonian age in nearby areas; particularly diagnostic is a brown dolomite at the base of the Cambrian carbonate sequence. The metamorphic complex contrasts with unmetamorphosed terranes exposed in nearby areas, and apparently formed in the Mesozoic as part of a metamorphic infrastructure.

Nevada↗
USGS1971

Thrust and strike-slip faulting in the Plomosa Mountains, southwestern Arizona

Thrust and strike-slip faulting are recognized in the Plomosa Mountains, southwestern Arizona. The distribution of rock types and the geometry of the thrust faults necessitate that the upper plate moved from east to west. The amount of displacement is not known, but is considered to be large. Apparent separations along the strike-slip faults are in a right-lateral sense and are greater than 19,000 ft. Rhyodacite flows, dated by K-Ar methods at 19 to 20 m.y., unconformably overlie the thrust faults and are cut by the strike-slip faults.

Arizona↗
USGS1973

Radiometric ages of intrusive rocks in the Little Belt Mountains, Montana

Radiometric ages indicate that most, if not all, of the major intrusions in the Little Belt Mountains, central Montana, were emplaced during the Eocene epoch, between 48 and 54 m.y. ago. In the Hughesville area, igneous activity continued, or was episodic until 42 m.y. ago. As a result of the continued igneous activity, radiometric ages in the Hughesville area can be interpreted either as primary ages or as reset ages.

Montana↗
USGS1973

Lead isotope systematics and uranium depletion in the Granite Mountains, Wyoming

Isotopic composition and concentration of lead in whole rock and microcline and concentration of uranium and thorium in whole-rock samples of granite from the Granite Mountains, Wyoming, have been determined. The lead isotopic composition in the whole rocks was found to be highly radiogenic with a range in Pb 206 /Pb 204 of 19.58 to 42.27; the corresponding range in microclines is 15.39 to 22.44. A Pb 206 /Pb 204 versus Pb 207 /Pb 204 plot of the whole-rock data yields an apparent isochron age of 2,790 ± 80 m.y. as the time of crystallization of the granite. Chemically determined values of U 238 /Pb 204 in the whole rocks lie between 3.3 and 18.4 and are too low to account for the amount of radiogenic lead observed. A material balance of lead, thorium, and uranium components indicates that an average of approximately 75 percent of the amount of uranium required to produce the radiogenic lead was removed from the rocks, whereas, on the average, there was no apparent loss of thorium. Loss of uranium from the granite is demonstrated to extend at least to a depth of 165 ft in a drill core. The average uranium loss from the samples analyzed represents about 20 g uranium per 1,000 kg of rock that apparently was removed during the Cenozoic and that probably constitutes the major source of uranium now in ore deposits in central Wyoming basins. The lead isotopic composition of the microclines indicates that lead was mobilized within the granite and was isolated in the feldspar during a thermal event about 1,640 + 120 m.y. ago. However, there is no evidence that the whole rocks themselves became open systems at that time. Whole-rock and microcline isochrons intersect at Pb 206 /Pb 204 and Pb 207 /Pb 204 of 13.77 and 14.86, respectively, indicating a characteristic U 238 /Pb 204 of 8.96 in the source region of the granite magma.

Wyoming↗
USGS1974

Garnet-clinopyroxenite from the Red Mountain pluton, Alaska

Several lens-shaped and irregular masses of garnet-clinopyroxenite occur along the north margin of the Red Mountain pluton, near Seldovia, Alaska. The pluton is composed of intercalated dunite, clinopyroxenite, and banded chromite layers. The chromite layers appear to dip toward the center of the mass, but they have been deformed, and in some places display isoclinal overturned folds. The garnet-clinopyroxenite bodies are compositionally layered and appear to have been deformed along with the host peridotite. The garnet-clinopyroxenite bodies have whole-rock compositions analogous to alkali-poor anorthite gabbro or mafic anorthosite. Garnet compositions are dominated by the pyrope, grossularite, and almandine components, and the clinopyroxene bodies are rich in Ca-Tschermaks molecule (CaAl 2 SiO 6 ). Although crystallization from the liquidus is a possibility, textures, structures, and the chemical composition of the constituent mineral phases indicate that the garnet-clinopyroxenite has equilibrated in the solid state at pressures of 15 to 25 kb and temperatures of 1100°C to 1300°C. Such P-T parameters suggest an upper-mantle origin for the garnet-clinopyroxenite and the enclosing pluton.

Alaska↗
USGS1976

Trace-element variations at Summer Coon volcano, San Juan Mountains, Colorado, and the origin of continental-interior andesite

The Oligocene Summer Coon center, an eroded continental-interior volcano of the eastern San Juan Mountains, Colorado, was the source of magmas ranging in composition from basaltic andesite to rhyolite. Previous Pb and Sr isotope studies indicate derivation of the magmas from an isotopically homogeneous source. This study presents new data for rare-earth elements (REE), U, Th, Ba, Sr, Rb, and Ni from 10 samples of the Summer Coon sequence. Alkali elements are high in all rocks; as SiO 2 increases, Ba increases from 900 to 2,000 ppm, Rb increases from 35 to 90 ppm, Sr decreases from 900 to 350 ppm, K/Rb decreases slightly, Ba/Sr increases, U increases from 0.5 to 2.5 ppm, and Th increases from 2 to 7 ppm. Chondrite-normalized REE patterns are strongly fractionated in comparison with oceanic-arc andesite-dacite sequences. La is 80 to 120 times chondritic abundance, but Yb and Lu are less than 10 times chondritic abundance. Small negative Eu anomalies characterize the rhyolites. Nickel in the andesites is 40 to 70 ppm. The origin of the andesite is interpreted in terms of nonmodal partial melting of a trace-element—enriched garnet-bearing source, possibly subducted crust that has converted to eclogite. Rhyodacite and rhyolite are interpreted as low-pressure crystal-fractionation products of silicic andesite, in which crystallizing phases are hornblende rich in REE and plagioclase.

Colorado↗
USGS1983

The ophiolitic North Fork terrane in the Salmon River region, central Klamath Mountains, California

The North Fork terrane is an assemblage of ophiolitic and other oceanic volcanic and sedimentary rocks that has been internally imbricated and folded. The ophiolitic rocks form a north-trending belt through the central part of the region and consist of a disrupted sequence of homogeneous gabbro, diabase, massive to pillowed basalt, and interleaved tectonitic harzburgite. U-Pb zircon age data on a plagiogranite pod from the gabbroic unit indicate that at least this part of the igneous sequence is late Paleozoic in age. The ophiolitic belt is flanked on either side by mafic volcanic and volcaniclastic rocks, limestone, bedded chert, and argillite. Most of the chert is Triassic, including much of Late Triassic age, but chert with uncertain stratigraphic relations at one locality is Permian. The strata flanking the east side of the ophiolitic belt face eastward, and depositional contacts between units are for the most part preserved. The strata on the west side of the ophiolitic belt are more highly disrupted than those on the east side, contain chert-argillite melange, and have unproven stratigraphic relation to either the ophiolitic rocks or the eastern strata. Rocks of the North Fork terrane do not show widespread evidence of penetrative deformation at elevated temperatures, except an early tectonitic fabric in the harzburgite. Slip-fiber foliation in serpentinite, phacoidal foliation in chert and mafic rocks, scaly foliation in argillite, and mesoscopic folds in bedded chert are consistent with an interpretation of large-scale anti-formal folding of the terrane about a north-south hinge found along the ophiolitic belt, but other structural interpretations are tenable. The age of folding of North Fork rocks is constrained by the involvement of Triassic and younger cherts and crosscutting Late Jurassic plutons. Deformation in the North Fork terrane must have spanned a short period of time because the terrane is bounded structurally above and below by Middle or Late Jurassic thrust faults. The North Fork terrane appears to contain no arc volcanic rocks or arc-derived detritus, suggesting that it neither constituted the base for an arc nor was in a basinal setting adjacent to an arc sediment source. Details of the progressive accretion and evolutionary relationship of the North Fork to other terranes of the Klamath Mountains are not yet clear.

California↗
USGS1994

Late Mesozoic and possible early Tertiary accretion in western Washington State: The Helena-Haystack mélange and the Darrington-Devils Mountain fault zone

The Helena-Haystack mélange (HH mélange) and coincident Darrington-Devils Mountain fault zone (DDMFZ) in northwestern Washington separate two terranes, the Northwest Cascade System (NWCS) and the western and eastern mélange belts (WEMB). The two terranes of Paleozoic and Mesozoic rocks superficially resemble each other but record considerable differences in structural and metamorphic history. The HH mélange is a serpentinite-matrix mélange containing blocks of adjacent terranes but also exotic blocks of schistose metavolcanic rocks and Jurassic tonalite and associated amphibolite. The HH mélange must have formed between early Cretaceous and late middle Eocene time, because it contains tectonic clasts of early Cretaceous Shuksan Greenschist and is overlain by late middle Eocene sedimentary and volcanic rocks. Less certain constraints on its age are a tectonic clast of metarhyolite that yields 90 Ma metamorphic ages and the presumption that the mélange was emplaced before the outboard Olympic terrane arrived at about 50 Ma. The apparent continuity of the HH mélange and the Decatur terrane of the San Juan Islands suggests that the mélange is the strongly tectonized equivalent of the Fidalgo ophiolite. The out-crop pattern suggests that the HH mélange overlies rocks of the NWCS and it may have formed when the WEMB terranes were thrust over rocks of the NWCS. Much of the exposed belt of the HH mélange is overlain by late middle Eocene feldspathic sandstone and volcanic rocks of the Barlow Pass Volcanics of Vance (1957a), which are cut by numerous faults of the DDMFZ paralleling the mélange. The Barlow Pass Volcanics appear to overlie the Straight Creek fault without large offset, but a displaced exotic block of amphibolite with attached early or early middle Eocene(?) sandstone in the mélange suggests that strike-slip movement along the DDMFZ was synchronous with movement on the Straight Creek fault, and stretched cobbles in the conglomerates of the Barlow Pass Volcanics suggest post-Straight Creek movement. The possible continuation of the DDMFZ to the northwest as the San Juan and the West Coast faults on Vancouver Island suggests That the structure has had a major role in the emplacement of all the westernmost terranes in the Pacific Northwest. This major suture is strongly bowed to the northeast opposite the great oroclinal bend of the Olympic terrane, suggesting that the emplacement of that terrane may have deformed a once straighter strike-slip zone.

Washington↗
USGS1999

Importance of mechanical disaggregation in chemical weathering in a cold alpine environment, San Juan Mountains, Colorado

Weathering of welded tuff near the summit of Snowshoe Mountain (3660 m) in southwestern Colorado was studied by analyzing infiltrating waters in the soil and associated solid phases. Infiltrating waters exhibit anomalously high potassium to silica ratios resulting from dissolution of a potassium-rich glass that occurs as a trace phase in the rock. In laboratory experiments using rock from the field site, initial dissolution generated potassium-rich solutions similar to those observed in the field. The anomalous potassium release decreased over time (about 1 month), after which the dominant cation was calcium, with a much lower potassium to silica ratio.

Colorado↗
USGS1977

Significance of Mesozoic radiolarians from the pre-Nevadan rocks of the southern Klamath Mountains, California

Ribbon cherts and siliceous tuffs of the North Fork and Rattlesnake Creek terranes of the Klamath Mountains yield Mesozoic radiolarians. Rocks of the North Fork terrane were previously considered to be of Paleozoic age and those of the Rattlesnake Creek to be of Paleozoic and Triassic age, on the basis of fossiliferous limestone bodies that are now considered to be exotic blocks. In both terranes, however, red cherts that are closely associated with ophiolitic rocks contain Late Triassic radiolarians; overlying cherts and siliceous tuffs contain Early or Middle Jurassic radiolarians. The Jurassic radiolarian fauna from the North Fork terrane is similar to a fauna contained in Franciscan chert near Santa Barbara in southern California. The change in age assignment of the dominant rocks of these terranes, based on this new radiolarian data, indicates that the suture between the North Fork terrane and the Devonian rocks of the central metamorphic belt on the east probably formed during Middle or Late Jurassic time.

California↗