Geology topics

du Bray

Publications and source records attributed to du Bray.

7 recordsLinked to original sources

USGS2012

Gravity model and structural implications of the Goddard Pendant, Sierra Nevada, California

A subsurface model for the Goddard pendant is constructed from a residual gravity high of about 7 mGal over the pendant. The model, which is the simplest and most geologically reasonable possibility, shows a metamorphic block that tapers with depth and extends about 3.5 km below the surface. The structures in the Goddard pendant are similar in style and orientation to those in other Sierra Nevada pendants, indicating that the country rock was neither deformed nor rotated during pluton emplacement. Consequently, emplacement must have been a passive rather than a forceful process. The pendant itself represents a piece of country rock trapped between plutons which are dome shaped in cross section.

Journal of Geophysical Research Solid Earth↗

USGS1999

Recrystallization and anatexis along the plutonic-volcanic contact of the Turkey Creek caldera, Arizona

Unusual geologic and geochemical relations are preserved along the contact between intracaldera tuff and a resurgent intrusion within the 26.9 Ma Turkey Creek caldera of southeast Arizona. Thick intracaldera tuff is weakly argillically altered throughout, except in zones within several hundred meters of its contact with the resurgent intrusion, where the groundmass of the tuff has been variably converted to granophyre and unaltered sanidine phenocrysts are present. Dikes of similarly granophyric material originate at the tuff-resurgent intrusion contact and intrude overlying intracaldera megabreccia and tuff. Field relations indicate that the resurgent intrusion is a laccolith and that it caused local partial melting of adjacent intracaldera tuff. Geochemical and petrographic relations indicate that small volumes of partially melted intracaldera tuff assimilated and mixed with dacite of the resurgent intrusion along their contact, resulting in rocks that have petrographic and compositional characteristics transitional between those of tuff and dacite. Some of this variably contaminated, second-generation magma coalesced, was mobilized, and was intruded into overlying intracaldera rocks.

Arizona↗

USGS1996

The Eocene Big Timber stock, south-central Montana: Development of extensive compositional variation in an arc-related intrusion by side-wall crystallization and cumulate glomerocryst remixing

The Eocene Big Timber stock in the Crazy Mountains of south-central Montana is an elliptical, 8 by 13 km, compositionally and texturally diverse composite intrusion with a well-developed radial dike swarm. A sharp intrusive contact separates its two phases: the core of the intrusion is fine-grained quartz monzodiorite, and the volumetrically dominant remainder is composed of medium-grained diorite and gabbro. Differentiation-related major oxide variation within the stock is extensive and spatially nonsystematic. However, abundances of most trace elements were not strongly influenced by differentiation; late zircon and apatite fractionation caused moderate heavy and slight light rare earth element abundance depletions, respectively. Mineral compositions and assemblages indicate crystallization between ≈950 and 700 °C at a pressure of ≈0.8 kbar (3 km). Mixing models indicate that fractionation of varying amounts of plagioclase, orthopyroxene, clinopyroxene, magnesio-hastingsite, hornblende, biotite, titanite, apatite, and magnetite (the stock's principal constituents, with quartz and potassium feldspar) and remixing of these minerals and residual liquids controlled compositional evolution in the reservoir. Crystals apparently nucleated at the reservoir wall while residual silicate liquid was displaced inward and remixed. Some crystals were plucked from the solidification front, as indicated by glomerocrysts present throughout the stock, and also remixed with residual liquid. Solidification of the reservoir represented by the stock involved heat loss to enclosing wall rock, side-wall crystallization, and subsequent, variably effective, crystal-liquid remixing. This process is an important variant of conventionally invoked models pertaining to solidification of intrusions and explains extensive, relatively nonsystematic compositional variation. The genesis of compositional evolution in other intrusions characterized by extensive, spatially nonsystematic variation may result from the important process documented herein. Compositional and geologic relationships are consistent with magma genesis related to subduction and magmatic-arc processes inboard from the western edge of the early Cenozoic North American plate. Arc magmatism in south-central Montana during Eocene time is consistent with models pertaining to early Cenozoic southward sweep and westward retreat of magmatism. Magmatism represented by the Big Timber stock provides significant new support for steepening subduction, westward retreat of the subduction hinge line, and development of an asthenospheric mantle wedge that fueled renewed magmatism beneath the western edge of the North American continent following early Cenozoic shallow subduction.

Geological Society of America Bulletin↗

USGS1994

Compositions of micas in peraluminous granitoids of the eastern Arabian shield - Implications for petrogenesis and tectonic setting of highly evolved, rare-metal enriched granites

Compositions and pleochroism of micas in fourteen peraluminous alkali-feldspar granites in the eastern part of the Late Proterozoic Arabian Shield are unlike those of micas (principally biotite) in most calc-alkaline granitoid rocks. Compositions of these micas are distinguished by elevated abundances of Li2O, F, and numerous cations and by low MgO abundances. These micas, constituents of highly evolved rare-metal enriched granitoids, represent an iron-lithium substitution series that ranges from lithium-poor siderophyllite to lithium-rich ferroan lepidolite. The eastern Arabian Shield also hosts six epizonal granitoids that contain colorless micas. Compositions of these micas, mostly muscovite, and their host granitoids are distinct from those of the iron-lithium micas and their host granitoids. Compositions of the analyzed micas have a number of petrogenetic implications. The twenty granitoids containing these micas form three compositional groups that reflect genesis in particular tectonic regimes; mica compositions define the same three groups. The presence of magmatic muscovite in six of these shallowly crystallized granitoids conflicts with experimental data indicating muscovite stability at pressures greater than 3 kbar. Muscovite in the Arabian granitoids probably results from its non-ideal composition; the presence of muscovite cannot be used as a pressure indicator. Finally, mineral/matrix partition coefficients are significantly greater than 1.0 for a number of cations, the rare-earth elements in particular, in many of the analyzed iron-lithium micas. Involvement of these types of micas in partial melting or fractionation processes can have a major influence on silicate liquid compositions. ?? 1994 Springer-Verlag.

Contributions to Mineralogy and Petrology↗

USGS1990

Temperature, size, and depth of the magma reservoir for the Taylor Creek Rhyolite, New Mexico

The 55 km3 mid-Tertiary Taylor Creek Rhyolite in southwestern New Mexico consists of 20 lava domes and flows. This rhyolite is metaluminous to weakly peraluminous. Compositional zonation in feldspar phenocrysts is very minor and nonsystematic. The compositions of each feldspar species vary little throughout the suite of analyzed samples. This chemical homogeneity of phenocrysts reflects similar whole-rock homogeneity and suggests that the lavas were tapped from a single large reservoir of magma. Ages of sanidine phenocrysts determined using 40Ar/39Ar indicate that the Taylor Creek Rhyolite lavas were emplaced during a period of less than 0.42 my. and possibly less than 0.13 m.y., which is consistent with the single-reservoir scenario. Two-feldspar geothermometry suggests that Taylor Creek Rhyolite phenocrysts crystallized at about 775??C, at an assumed pressure of 2 kbar. Fe-Ti-oxide geothermometry suggests phenocryst growth at about 800??C. Experimental studies suggest that quartz and potassium-feldspar crystals that grow from H2O-undersaturated granitic magmas should exhibit resorption texture, a texture ubiquitous to Taylor Creek Rhyolite quartz and sanidine phenocrysts. We tentatively conclude that the Taylor Creek Rhyolite magma was H2O undersaturated and subliquidus at an unspecified pressure greater than 0.5 kbar during phenocryst growth and that Taylor Creek Rhyolite pyroclastic deposits formed because volatile saturation developed during the ascent of magma to sites of eruption. -from Authors

American Mineralogist↗

USGS1988

Garnet compositions and their use as indicators of peraluminous granitoid petrogenesis - southeastern Arabian Shield

Garnet, an uncommon accessory mineral in igneous rocks, occurs in seven small peraluminous granitoid plutons in the southeastern Arabian Shield; textural equilibrium between garnet and other host granitoid minerals indicates that the garnets crystallized from their host magmas. Compositions of the garnets form three groups that reflect host-granitoid compositions, which in turn reflect source compositions and tectonic regimes in which the host magmas were generated. Garnets from the seven plutons have almandine-rich cores and spessartine-rich rims. This reverse zoning depicts host magma compositional evolution; i.e. rimward spessartine enrichment resulted from progressive, host-magma manganese enrichment. The garnets are heavy rare-earth element enriched; (Lu/La)N ranges from 13 to 355 and one of the garnets contains spectacularly elevated abundances of Y, Ta, Th, U, Zn, Zr, Hf, Sn, and Nb. Involvement of garnets with these trace element characteristics in magma genesis or evolution can have dramatic effects on trace element signatures of the resulting magmas. Other researchers suggest that Mn-enriched magmas are most conducive to garnet nucleation. Although the garnetiferous granitoids discussed here are slightly Mn enriched, other genetically similar peraluminous Arabian granitoids lack garnet; Mn enrichment alone does not guarantee garnet nucleation. The presence of excess alumina in the magma may be a prerequisite for garnet nucleation. ?? 1988 Springer-Verlag.

Contributions to Mineralogy and Petrology↗

USGS1986

Jabal Silsilah tin prospect, Najd region, Kingdom of Saudi Arabia

A significant Sn deposit at Jabal Silsilah (26°07′N, 42°41′E), a prominent ring complex in the NE Arabian Shield, is associated with greisen in flat-topped cupolas of a highly evolved, zinnwaldite- and topaz-bearing alkali-feldspar granite. This granite is peraluminous, enriched in incompatible trace elements and characterized by a chondrite-normalized REE profile showing a large negative Eu anomaly, but otherwise flat. It is overlain by a carapace of aplite and pegmatite, beneath which accumulated residual fluids caused greisenization and deposition of disseminated cassiterite. Quartz—wolframite veins cut both granite and carapace. The other components of the Silsilah complex, from oldest to youngest, are: alkaline dacite, peralkaline granite and hypabyssal rocks. Their occurrence in a single ring structure and geochemical affinities indicate that they comprise a differentiation series. Fractionation of sodic plagioclase, apatite and Fe and Ti oxides resulted in parental alkaline dacite magma becoming peralkaline and granitic. Subsequent fractionation of alkali feldspar, soda pyribole and zircon resulted in a peraluminous melt enriched in incompatible trace elements.

Journal of African Earth Sciences↗