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G. R. Himmelberg

Publications and source records attributed to G. R. Himmelberg.

7 recordsLinked to original sources
USGS2012

The Kanuti ophiolite, Alaska

The Kanuti ophiolite is a mafic-ultramafic thrust sheet of probable Jurassic age, formerly considered to be the upper part of the Yukon-Koyukuk ophiolite belt (Angayucham terrane). It is here called the Kanuti ophiolite after the Kanuti River region on the southeastern flank of the Yukon-Koyukuk Basin. The thrust sheet crops out discontinuously for a distance of more than 900 km along the northern and southeastern margins of the basin. It is probably correlative with similar ophiolite thrust sheets to the north in the western Brooks Range and to the south in the Ruby geanticline. Technically, the ophiolite is considered to be the Kanuti thrust panel of the Angayucham-Totzitna terrane. The Kanuti consistently overlies another extensive thrust sheet, consisting mostly of pillow basalt and radiolarian chert of Devonian to Jurassic age (Narvak thrust panel). This sheet is thrust over a third sheet consisting of probable Devonian phyllite and metagraywacke, which is in turn thrust over older metamorphic rocks (Slate Creek thrust panel). The Kanuti ophiolite is a partial ophiolite that consists of a lower residual mantle suite and an upper magmatic suite, but dikes, extrusives, and sediments are absent. The residual mantle suite is composed of harzburgite and dunite with refractory mineral compositions. The harzburgite is attributed to partial melting and extraction of basaltic magma; residual dunite is attributed to partial melting or to reaction of orthopyroxene out of harzburgite in contact with ascending melt diapirs. The magmatic suite consists of layered ultramafic and gabbroic rocks, containing minerals having limited iron enrichment. The absence of large volumes of magmatic rocks intermediate in composition between cumulus ultramafics and evolved gabbros favors periodic introduction of magma, rather than closed system fractional crystallization. The ultramafic rocks of both the residual mantle and magmatic suites are tectonites, which have undergone high-temperature deformation involving isoclinal folding on all scales and related syntectonic recrystallization. The olivine fabric is consistent with the glide system {0kl} [100], which has been produced experimentally at 800°–1190°C at 20 kbar. Olivine Z axes and subparallel isoclinal fold axes have consistent, northeast trends throughout the Kanuti region (>100 km NE-SW) and may be close to the original upper mantle flow direction, despite later low-angle thrust faulting. The order of crystallization in the cumulus ultramafic rocks of the magmatic suite is olivine, clinopyroxene, plagioclase, and orthopyroxene. The high Mg numbers of clinopyroxene (0.85–0.93) coexisting with olivine suggest that the cumulus ultramafic rocks crystallized at relatively high pressures (>10 kbar). The effects of parental magma composition cannot be evaluated, but the small difference in Mg numbers of coexisting olivine and clinopyroxene in the cumulus ultramafic rocks and in residual harzburgite suggests that regardless of absolute pressures, the pressure difference between the melting that produced the basalt magma and the initial fractional crystallization of the magma is small. Because of the limited range in rock types in the ophiolite, the tectonic environment cannot be interpreted unambiguously. However, the structural and petrological data are best reconciled with an origin in a volcanic arc tectonic setting.

Journal of Geophysical Research Solid Earth↗
USGS1989

Evolution of the western part of the Coast plutonic–metamorphic complex, South-Eastern Alaska, USA: A summary

The western Cordillera of North America extends for over 6000 km from the tip of Baja California to the Alaska Range. It includes a wide variety of metamorphic and plutonic terrains, but none is more spectacular scenically or geologically than the Coast plutonic-metamorphic complex (Brew & Ford 1984) of western Canada and south-eastern Alaska. This report briefly describes the evolution of the western part of the complex, integrating information from the deformational, plutonic and metamorphic events. Most of the original studies are reported by the authors in U.S. Geological Survey Circular numbers 733, 751, 823-B, 868, 939, 945, 967 and 978, and are not cited specifically here. This summary does not contain either a comprehensive bibliography or a comparison of the metamorphic histories of south-eastern Alaska with the adjacent parts of British Columbia. The Coast plutonic-metamorphic complex is here divided into three major elements: the western metamorphic, the central granitic and the eastern metamorphic zones (Fig. 1). The western metamorphic belt is extremely long (900 km), and narrow (7–25 km). It consists of regional dynamothermally and regional thermally metamorphosed rocks with mineral assemblages ranging from prehnite-pumpellyite to upper amphibolite facies, scattered mesozonal to epizonal granitic bodies, and a few concentrically zoned mafic-ultramafic masses. The metamorphic grade and the amount of deformation increase from south-west to north-east, culminating at, or slightly to the north-east of, the ‘great tonalite sill’: a remarkable 700-km-long, 3- to 25-km-wide vertical to northeast-dipping belt of mostly syntectonic plutons of approximately the same age, composition and structural

Alaska↗
USGS1987

Petrogenesis of gabbronorite at Yakobi and northwest Chichagof Islands, Alaska

On Yakobi Island and at Mirror Harbor on the northwest coast of Chichagof Island, gabbronorite occurs as irregular bodies, as much as 5.5 km in maximum dimension, mostly within a 40 to 43 m.y. composite pluton consisting largely of tonalite. The gab-bronorites are the host rocks for a magmatic nickel-copper sulfide deposit consisting predominantly of pyrrhotite, pentlandite, and chalcopyrite. The gabbronorites characteristically have more orthopyroxene than augite and have a significant amount of hornblende. Rock types mapped as gabbronorite range from hornblende pyroxenite to hornblende-pyroxene gabbronorite to quartz-bearing norite and gabbronorite. The tonalite pluton is composed of hornblende diorite, biotite-hornblende diorite, hornblende quartz diorite, biotite-hornblende tonalite, and biotite granodiorite. Contacts between types of gabbronorite are generally gradational on a scale of centimetres to metres; contacts between gabbronorite and the tonalite pluton are gradational on a scale of metres to tens of metres. Rock textures, pyroxene-hornblende relations, and rock and mineral chemistry of the gabbronorites show systematic changes as the gabbronorites grade into the tonalites. The field, petrographic, and chemical data, including trace-element abundances, of the gabbronorites and tonalite pluton rocks can best be explained by either (1) crystallization of gabbronorite from a tholeiitic magma with subsequent assimilation by tonalite that was simultaneously undergoing fractional crystallization or (2) fractional crystallization of a quartz diorite parent magma yielding the range of gabbronorites and tonalite pluton rocks.

Geological Society of America Bulletin↗
USGS1976

Zoned Cr, Fe-spinel from the La Perouse layered gabbro, Fairweather Range, Alaska

Zoned spinel of unusual composition and morphology has been found in massive pyrrhotite-chalcopyrite-pent-landite ore from the La Perouse layered gabbro intrusion in the Fairweather Range, southeastern Alaska. The spinel grains show continuous zoning from cores with up to 53 wt.% Cr 2 O 3 to rims with less than 11 wt.% Cr 2 O 3 . Their composition is exceptional because they contain less than 0.32 wt.% MgO and less than 0.10 wt.% Al 2 O 3 and TiO 2 . Also notable are the concentrations of MnO and V 2 O 3 , which reach 4.73 and 4.50 wt.%, respectively, in the cores. The spinel is thought to have crystallized at low oxygen fugacity and at temperatures above 900°C, directly from a sulfide melt that separated by immiscibility from the gabbroic parental magma.

Earth and Planetary Science Letters↗
USGS1971

Structure and petrology of the alpine-type peridotite at Burro Mountain, California, U.S.A.

The alpine-type peridotite at Burro Mountain is a partially serpentinized harzburgite-dunite body approximately 2 km in diameter. It lies in a chaotic mélange derived from the Franciscan Formation (Upper Jurassic to Upper Cretaceous) of the southern Coast Ranges of California. The peridotite is bounded on the east by a vertical fault in the Nacimiento fault zone that brings sedimentary rocks of Taliaferro's (1943 b ) Asuncion Group (Upper Cretaceous) into contact with the peridotite. The peridotite appears to be one of a number of tectonic lenses, having a wide range in size, that make up the mélange . These lenses include metagraywacke, metachert, greenstone, amphibolite, and blueschist, as well as ultramafic rocks, and represent a wide range of pressure-temperature environments. The outer shell of the peridotite is a sheared serpentinite zone 10–15 m thick. The peridotite was tectonically emplaced at its present level as a cold solid mass and had little effect on the mineral assemblages of the Franciscan Formation. Local development of lawsonite and aragonite in shear zones may be related to the peridotite emplacement. Foliated harzburgite forms approximately 60 per cent of the peridotite. It is a lithologically uniform rock that has an olivine: orthopyroxene ratio of approximately 75:25. Accessory clinopyroxene and chromian spinel generally make up less than 5 per cent of the harzburgite. Dunite, composed of olivine, accessory chromian spinel (< 5 per cent), and trace amounts of pyroxene, makes up approximately 40 per cent of the peridotite and occurs as dikes, sills, and irregular bodies in the harzburgite. Olivine and pyroxene show small but significant compositional variations and chromian spinel shows a large range in the cation ratio Cr/(Cr+Al+ Fe 3+ ). The compositional variations in these minerals are related to original differences in bulk chemical composition. The following compositional ranges were determined for minerals in the harzburgite: olivine, Fo 91.1 −Fo 91.4 ; orthopyroxene, En 89.8 −En 91.1 ; clinopyroxene, Ca 47.0 Mg 50.0 Fe 3.0 −Ca 48.7 Mg 48.2 Fe 3.1 ; chromian spinel, Cr/(Cr+Al+Fe 3+ ) 0.37−0.55. The pyroxenes have a range in A1 2 O 3 content of 1.3−3.0 wt per cent. Olivine from dunite ranges from Fo 91 to Fo 92 7 and the chromian spinel has a range in the Cr/(Cr+Al+Fe 3+ ) ratio of 0.30−0.75. Although all the dunites are lithologically similar, three distinct types are recognized on the basis of composition of coexisting olivine and chromian spinel. Structural relations between the three types of dunite suggest three periods of emplacement (possibly overlapping) of dunite into harzburgite. The evidence indicates that the dunite, and probably also the harzburgite crystallized from an ultramafic magma, probably in the upper mantle. After the magmatic episode and crystallization, the peridotite was subjected to a deep-seated plastic deformation and recrystallization. The first phase of the deformation produced a pervasive, planar structural element (S 1 ) that crosscuts many harzburgite-dunite contacts. It is probable that some of the dunite sills were emplaced during this deformation. The foliation, S 1 , is defined by layers of different orthopyroxene content in harzburgite, and by discontinuous layers of chromian spinel in dunite. Flow or slip along S 1 produced slip folds in harzburgite—dunite contacts with axial planes parallel to S 1 . At a later stage, isoclinal folds developed in S 1 , and the present olivine microfabric was probably formed by recrystallization in the stress field that produced the isoclinal folding. In the olivine microfabric, X tends to be perpendicular to the axial planes (S 2 ) of the isoclinal folds and Y and Z tend to form double maxima in S 2 approximately 90° apart. Mg−Fe 2+ distribution between coexisting mineral pairs yields a calculated temperature of formation of approximately 1200 °C. Although this temperature is only a nominal value, it indicates that the mineral pairs equilibrated at a significantly high temperature. In view of the deformation and recrystallization, the calculated temperature possibly represents subsolidus re-equilibration of the minerals during this event. The deformation and recrystallization probably occurred shortly after crystallization while the peridotite was still at a high temperature. A later deep-seated deformation produced small scattered kink folds in S 1 that tend to disrupt the major olivine microfabric. The kink folding was accompanied or followed by the development of kink bands in olivine that reflect intragranular gliding on the system T = [ Okl ], t = [100]. The kink bands probably formed at a minimum temperature of 1000 °C. Following the deep-seated deformation, which probably took place in the mantle, the peridotite mass was tectonically detached and moved upward to its present level in the crust. Cleavages, joints, and faults provided channels for water to pervade the peridotite and allow alteration of the primary minerals.

California↗
USGS1969

Coexisting amphiboles from blueschist facies metamorphic rocks

Four pairs of associated calcic and sodic amphiboles from blueschist facies metamorphic rocks were analyzed with the electron microprobe and studied by single-crystal X-ray diffraction techniques. Except for ranges in the ratios Mg/(Mg+Fe) and Fe 3+ /(Fe 3+ +Al+Ti), the sodic amphiboles are similar in chemical composition. The amount of calcium in the M(4)-site ranges only from 0·18 to 0·21 ion per formula unit. The calcic amphiboles, in addition to a range in Mg/(Mg+Fe), vary in Na/(Na+Ca) ratio (0·29–0·48). Three of the calcic amphiboles contain less than 1·5 calcium ions per formula unit, indicating a significant solid solution of sodic amphibole components in the calcic amphibole phase. The a and b unit-cell parameters of the calcic amphiboles decrease with increased content of the sodic component.

Journal of Petrology↗