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At least 991 records · Page 55Linked to original sources
USGS1991

Isotopic and chemical evidence concerning the genesis and contamination of basaltic and rhyolitic magma beneath the Yellowstone Plateau Volcanic Field

Since 2.2 Ma, the Yellowstone Plateau volcanic field has produced ∼6000 km 3 of rhyolite tuffs and lavas in >60 separate eruptions, as well as ∼ 100 km 3 of tholeiitic basalt from >50 vents peripheral to the silicic focus. Intermediate eruptive products are absent. Large calderas collapsed at 2·0, 1·3, and 0·6 Ma on eruption of ash-flow sheets representing at least 2500, 280, and 1000 km 3 of zoned magma. Early postcollapse rhyolites show large shifts in Nd, Sr, Pb, and O isotopic compositions caused by assimilation of roof rocks and hydrothermal brines during collapse and resurgence. Younger intracaldera rhyolite lavas record partial isotopic recovery toward precaldera ration. Thirteen extracaldera rhyolites show none of these effects and have sources independent of the subcaldera magma system. Contributions from the Archaean crust have extreme values and wide ranges of Nd-, Sr-, and Pb-isotope ratios, but Yellowstone rhyolites have moderate values and limited ranges. This requires their deep-crustal sources to have been pervasively hybridized (and the Archaean components diluted) by distributed intrusion of Cenozoic basalt, most of which was probably contemporaneous with the Pliocene and Qualernary volcanism. In hybrid sources yielding magmas parental to the subcaldera rhyolites, half or more of the Nd and Sr may have been contributed by such young basalt. Parents for the extracaldera rhyolites, generated beyond the leading edge of the northeast-propagating focus of basaltic intrusion and deep-crustal mobilization, contained smaller fractions of mantle-derived components.

Journal of Petrology↗
USGS1991

Geochemistry of lavas from Taal volcano, southwestern Luzon, Philippines: evidence for multiple magma supply systems and mantle source heterogeneity

Taal Volcano is a highly active center in the southwest Luzon volcanic field. Recent eruptions are confined to Volcano Island, a plexus of cinder cones and tuff rings surrounding a central crater, occupying (and post-dating) a massive volcano-tectonic depression resulting from multiple phases of collapse. Taal lava series can be distinguished from each other by differences in major and trace element trends and trace element ratios, indicating multiple magmatic systems associated with discrete centers in time and space. On Volcano Island, contemporaneous lava series range from typically calc-alkaline to iron-enriched. Major and trace element variation in these series can be modelled by fractionation of similar assemblages, with early fractionation of titano-magnetite in less iron-enriched series. However, phase compositional and petrographic evidence of mineral-liquid disequilibrium suggests that magma mixing played an important role in the evolution of these series. We interpret this to mean that mixing occurred along a line of descent defined by fractional crystallization, with mixing occurring over larger compositional intervals in the less iron-enriched series, consistent with numerical modelling. Between-series incompatible element ratio differences are not explicable through differential partial melting of a homogeneous source (also confirmed by variation in Pb isotopic ratios), suggesting a supply system from discrete melt zones in heterogeneous mantle.

Journal of Petrology↗
USGS1993

Emplacement and differentiation of the york haven diabase sheet, Pennsylvania

Many of the high-Ti quartz-normative tholeiitic intrusive sheets in the early Mesozoic rift basins of the Eastern USA exhibit lateral differentiation from mafic cumulate units, through diabase, to relatively evolved iron-rich rock types. We have investigated a representative example in detail, the York Haven sheet in the Gettysburg basin of south-central Pennsylvania. It ranges in thickness from 330 m to 675 m, and we have sampled it from base to top along four separate stratigraphic sections evenly spaced over the extent of the intrusion. The easternmost section (York Haven) is entirely basaltic bronzite cumulate (average 15 vol. % bronzite), whereas the westernmost (Reesers Summit) consists of diabase and low-MgO diabase with a middle to upper 'sandwich zone' of ferrogabbro. The intervening sections feature rock types transitional between the two end-member sequences. Chemically, the rock series shows a gradual east to west depletion of compatible elements (Mg, Ca, Ni, and Cr), and enrichment of incompatible elements [Ti, Fe, Na, K, P, Cu, Zr, Th, Ta, Hf, Sb, Cs, As, platinum group elements (PGEs), and rare earth elements (REEs)].We suggest two main processes for the trends observed in the York Haven sheet. First, flow differentiation during ascent and lateral injection of the parental magma produced a tongue of basaltic bronzite cumulate that thins from southeast to northwest and passes laterally into diabase, and, at the distal end of the intrusion, into low-MgO diabase. Then, in the latter stages of crystallization, densitydriven hydrothermal fluids transported incompatible elements westward, into structurally higher parts of the intrusion. Reaction of this residual aqueous fluid with partly crystallized low-MgO diabase produced a zone of ferrogabbro rich in hydrothermal replacement products (e.g., Cl-amphibole, biotite, ferrohypersthene, and skeletal ilmenite) and precipitates (e.g., quartz, fayalite, Cl-apatite, sulfides, and PGE minerals). © 1993 Oxford University Press.

Pennsylvania↗
USGS1994

Origin of phenocrysts and compositional diversity in pre-Mazama rhyodacite lavas, Crater Lake, Oregon

Phenocrysts in porphyritic volcanic rocks may originate in a variety of ways in addition to nucleation and growth in the matrix in which they are found. Porphyritic rhyodacite lavas that underlie the eastern half of Mount Mazama, the High Cascade andesite/dacite volcano that contains Crater Lake caldera, contain evidence that bears on the general problem of phenocryst origin. Phenocrysts in these lavas apparently formed by crystallization near the margins of a magma chamber and were admixed into convecting magma before eruption. About 20 km3 of pre-Mazama rhyodacite magma erupted during a relatively short period between ~400 and 500 ka; exposed pre-Mazama dacites are older and less voluminous. The rhyodacites formed as many as 40 lava domes and flows that can be assigned to three eruptive groups on the basis of composition and phenocryst content. -from Authors

Oregon↗
USGS1994

Evidence from xenoliths for a dynamic lower crust, eastern Mojave Desert, California

Garnet-rich xenoliths in a Tertiary dike in the eastern Mojave Desert, California, preserve information about the nature and history of the lower crust. These xenoliths record pressures of ∼ 10–12 kbar and temperatures of ∼ 750–800°C. Approximately 25% have mafic compositions and bear hornblende + plagioclase + clinopyroxene + quartz in addition to garnet. The remainder, all of which contain quartz, include quartzose, quartzofeldspathic, and aluminous (kyanite±sillimanite-bearing) varieties. Most xenoliths have identifiable protoliths—mafic from intermediate or mafic igneous rocks, quartzose from quartz-rich sedimentary rocks, aluminous from Al-rich graywackes or pelites, and quartzofeldspathic from feldspathic sediments and/or intermediate to felsic igneous rocks. However, many have unusual chemical compositions characterized by high FeO(t), FeO(t)/MgO, Al 2 O 3 , and Al 2 O 3 /CaO, which correspond to high garnet abundance. The mineralogy and major-and trace-element compositions are consistent with the interpretation that the xenoliths are the garnet-rich residues of high-pressure crustal melting, from which granitic melt was extracted. High 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd, together with highly discordant zircons from a single sample with Pb/Pb ages of ∼ 1.7 Ga, demonstrate that the crustal material represented by the xenoliths is at least as old as Early Proterozoic. This supracrustal-bearing lithologic assemblage may have been emplaced in the lower crust during either Proterozoic or Mesozoic orogenesis, but Sr and Nd model ages> 4 Ga require late Phanerozoic modification of parent/daughter ratios, presumably during the anatectic event. Pressures of equilibration indicate that peak metamorphism and melting occurred before the Mojave crust had thinned to its current thickness of <30 km. The compositions of the xenoliths suggest that the lower crust here is grossly similar to estimated world-wide lower-crustal compositions in terms of silica and mafic content; however, it is considerably more peraluminous, has a lower mg -number, and is distinctive in some trace element concentrations, reflecting its strong metasedimentary and restitic heritage.

Journal of Petrology↗
USGS1994

Multiple isotopic components in Quaternary volcanic rocks of the Cascade Arc near Crater lake, Oregon

Quaternary lavas and pyroclastic rocks of Mount Mazama, Crater lake caldera, and the surrounding area have variable Sr, Nd, and Pb isotopic compositions. High-alumina olivine tholeiites have 87Ar/86Ar ratios of 0.70346-0.70364; basaltic andesite, 0.70349-0.70372; shoshonitic basaltic andesite, 0.70374-0.70388; and andesite, 0.70324-0.70383. Dacites of Mount Mazama have 87Sr/ 86Sr ratios of 0.70348-0.70373. Most rhyodacites converge on 0.7037. Andesitic to mafic-cumulate scoriae of the climatic eruption, and enclaves in pre-climactic rhyodacites, cluster in two groups but show nearly the entire 87Sr/86Sr range of the data set, confirming previously suggested introduction of diverse parental magmas into the growing climactic chamber. Magma evolution is described.

Oregon↗
USGS1994

Rare earth element evidence for the petrogenesis of the banded series of the Stillwater Complex, Montana, and its anorthosites

A rare earth element (REE) study was made by isotope-dilution mass spectrometry of plagioclase separates from a variety of cumulates stratigraphically spanning the Banded series of the Stillwater Complex, Montana. Evaluation of parent liquid REE patterns, calculated on the basis of published plagioclase-liquid partition coefficients, shows that the range of REE ratios is too large to be attributable to fractionation of a single magma type. At least two different parental melts were present throughout the Banded series. This finding supports hypotheses of previous workers that the Stillwater Complex formed from two different parent magma types, designated the anorthositic- or A-type liquid and the ultramafic- or U-type liquid. On the basis of our data, one melt has a REE pattern with a distinctive shallow slope and is represented by samples from the thick, massive Anorthosite zones I and II (AN I and AN II) of the Middle Banded series. Although samples from AN I and AN II are separated by as much as 1400 m stratigraphically, they have remarkably similar calculated parent liquid characteristics, with (Ce/Sm)n = 1.7–1.9, (Nd/Sm)n = 1.3–1.4 and (Ce/Yb)n = 2.9–4.6 (where n denotes chondrite-normalized). These calculated liquids are probably close to representing A-type magma. In addition, plagioclase-bronzite cumulates from Norite zones I and II (N I and N II), although thought to be U-type cumulates, contain plagioclase that has A-type REE characteristics, implying that A-type magmas were injected into the magma chamber during formation of those zones. In contrast, calculated parent liquids of cumulus augite-bearing rocks have REE patterns that display distinctly steeper slopes than the A-type REE pattern. The extreme is the calculated parent liquid of a plagioclase-bronzite-augite cumulated with (Ce/Sm)n = 2.9, (Nd/Sm)n = 1.7, and (Ce/Yb)n = 10.1.

Journal of Petrology↗
USGS1995

The Grayback Pluton: Magmatism in a Jurassic back-arc environment, Klamath Mountains, Oregon

The Jurassic Grayback pluton was emplaced in a back-arc setting behind a contemporaneous oceanic arc. Th\alphae main stage of the pluton consists of an early, reversely zoned tonalite to gabbro that was intruded by synplutonic noritic and gabbroic magmas. Late-stage activity was characterized by intrusion of tonalitic and granitic dikes, many of which contain mafic enclaves and hybrid zones. Most mafic rocks in the pluton are calc-alkaline, with characteristic magnesian clinopyroxene, calcic cores in plagioclase, and elemental abundances similar to H 2 O-rich arc basalts. However, some mafic rocks contain relatively Fe-rich clinopyroxene, lack calcic cores in plagioclase, and are compositionally similar to evolved high-alumina tholeiite. Compositional variation in the main stage can be modeled in part by fractional crystallization and crusted assimilation during which parental calc-alkaline basalt evolved to granitic compositions. Cumulates related to this process are represented by modally variable melagabbro and pyroxenite. Mixing of basaltic and tonalitic magmas accounts for the compositions of most main-stage intermediate rocks, but mixing of basaltic and granitic magmas was uncommon until late in the pluton's history. Oxygen, Sr and Nd isotopic data indicate that virtually all main-stage magmas in the pluton contain a crustal component. Isotopic and trace element data further suggest that late-stage tonalitic dikes represent melts derived from older, metavolcanic arc crust Deep crustal contamination of main-stage rocks took place below the level of emplacement, probably in a magma-rich zone where basalts ponded and mixed with crustal melts. The Grayback pluton illustrates the diversity of Jurassic back-arc magmatism in the Klamath province and demonstrates that ancient magmatism with arc-like features need not be situated in an arc setting.

Oregon↗
USGS1995

The generation of oceanic rhyolites by crystal fractionation: the basalt-rhyolite association at Volcán Alcedo, Galápagos archipelago

Alcedo volcano is one of six shield volcanoes on Isabela Island in the western Galápagos Islands. Although Alcedo is dominantiy basaltic, it is unusual in that it also has erupted ∼1 km 3 of rhyolite. The rhyolitic phase marked a 10-fold decrease in the mass-eruption rate of the volcano, and the volcano has returned to erupting basalt. The basalts are tholeiitic and range from strongly to sparsely porphyritic. Olivine and plagiodase are the liquidus phases in the most primitive basalts. The MgO and Ni concentrations in the most primitive basalts indicate that they have undergone substantial differentiation since extraction from the mantle. The rhyolites contain the assemblage oligoclase-augite-titanomagnetite-fayalite-apatite and sparse xenoliths of quenched basalt and cumulate gabbros. Intermediate rocks are very rare, but some are apparently basaltrhyolite hybrids, and others resulted from differentiation of tholeiitic magma. Several modeling approaches and Sr-, Nd-, and O-isotopic data indicate that the rhyolites resulted from ∼ 90% fractionation (by weight) of plagiodase, augite, titanomagnetite, olivine, and apatite from the most primitive olivine tholeiite. The data are inconsistent with the rhyolites originating by crustal anatexis. The extreme Daly gap may be caused by the large increase in viscosity as the basaltic magma differentiates to intermediate and siliceous compositions; highly evolved magmas are eruptible only after they become saturated with volatiles by second boiling. The close association of the hybrid intermediate magmas and magmatic inclusions with the climactic plinian eruption indicates mixing between mafic and silicic magmas immediately before eruption. Rhyolite production was favored by the decrease in supply of basaltic magma as Alcedo was carried away from the focus of the Galápagos hotspot. A three-stage model for the magmatic evolution of a Galápagos volcano is proposed. In the first stage, the supply of basaltic magma is large. Basaltic magma continually intrudes the subcaldera magma chamber, buffering the magmas' compositional and thermal evolution. As the volcano is carried away from the basaltic source, the magma chamber is allowed to cool and differentiate, as exemplified by Alcedo's rhyolitic phase. Finally, the volcano receives even smaller influx of basalt, so a large magma chamber cannot be sustained, and the volcano shifts to isolated basaltic eruptions.

Galapagos Islands↗
USGS1963

Properties of synthetic triclinic KFeSi3O8, iron-microcline, with some observations on the iron-microcline⇌iron-sanidine transition

Triclinic KFeSi 3 O 8 , iron-microcline, has been synthesized from oxide mixes and by complete conversion of monoclinic KFeSi 3 O 8 , iron-sanidine. Iron-microcline is triclinic, C ī, a =8·68±0·01 Å, b =13·10±0·01, c =7·34±0·01, α =90° 45′±10′, β =116° 03′±10′, γ=86°14′±10′. The optical properties (Na light) are: α =1·585±0·002, β =1·596±0·002, γ=1·605±0·002, 2 V α =85° (calc.), X≈ b , Z ∧ c =20°±5°. A reversible phase transition between triclinic and monoclinic KFeSi 3 O 8 occurs at 704°±6° C at 2000 bars total pressure. Iron-microcline is the low-temperature polymorph; no intermediate polymorphs were observed in either hydrothermal or dry heating experiments.

Journal of Petrology↗
USGS1963

Degree of oxidation of adirondack iron oxide and iron-titanium oxide minerals in relation to petrogeny

Partial chemical analyses of the accessory iron oxide and iron-titanium oxide minerals from more than one hundred samples of Adirondack igneous, metamorphic, and metasomatic rocks portray the degree of oxidation of the minerals and afford a basis for a discussion of their relation to the accompanying silicate assemblage and petrogeny. Among the granitic igneous rocks, the green pyroxenic facies have the least oxidized iron-titanium oxide minerals and a pink potassium-rich microperthitic microcline facies the most oxidized. Granites and syenites may crystallize with either an iron-rich variety of hornblende (or pyroxene) plus ilmenite, or with an iron-magnesium variety of hornblende (or of pyroxene) plus magnetite plus ilmenite, depending on the degree of oxidation. The granite facies of the orthogneisses of the Diana Complex (Precambrian) have a more oxidized mineral assemblage than the syenitic and quartz syenitic facies. The anorthosites and gabbro anorthosites have relatively oxygen-rich oxide minerals. Orthogneisses in the granulite facies have a lower ratio of Fe 2 O 3 /FeO than similar gneisses in the amphibolite facies. At least part of this lower ratio is due to metamorphism at higher temperatures and pressures. A regional belt of granitic orthogneiss metamorphosed in the amphibolite facies has a magnetite-sphene instead of a magnetite-ilmenite assemblage. Metasomatism of biotite-quartz-plagioclase gneiss to sillimanitic quartz-microcline gneisses is accompanied by decrease in mafic silicates and a series of changes of minerals in intermediate stages such that, in general, biotite yields with increasing degrees of oxidation successively such assemblages as (1) biotite, garnet, magnetite, hemo-ilmenite, and ilmeno-hematite; (2) pale brown mica, sillimanite, magnetite, and ilmeno-hematite; and (3) rutilo-hematite, rutile, and meagre silicates including sillimanite, muscovite, and chlorite. Diabase altered to monzodiorite by potassium-bearing solutions has ilmeno-magnetite partly altered to ilmeno-maghemite.

Journal of Petrology↗
USGS1963

Glaucophane-bearing metamorphic rock types of the Cazadero area, California

A detailed field and laboratory study has been made of a well-exposed glaucophane schist sequence within the Jurassic and Cretaceous Franciscan Formation of northern California. Three types of glaucophane-bearing metamorphic rocks have been distinguished in and around the area of the detailed study. Each is characterized by distinctive textures and mineral assemblages that are interpreted to represent different grades of metamorphism within the glaucophane schist facies. From the combination of small- and large-scale mapping in the area described it is clear that coarsely schistose blocks, tens of feet in diameter, commonly rest directly upon and within less intensely metamorphosed terrain. In the Cazadero area these isolated blocks of coarsely crystalline rocks are concentrated in a band that is roughly concordant with some of the major faulting, and their metamorphic fabric shows no consistent relation to local or regional structures. It is tentatively suggested that these blocks have been transported upward tectonically and that they are not stratigraphically equivalent to the other types of glaucophane-bearing metamorphic rocks in the area. Chemical and petrographic evidence indicates that basalt and sediments have been converted to glaucophane-bearing rocks under conditions of metamorphism that were essentially isochemical, except for fugitive components and some minor elements.

California↗
USGS1963

The atomic ratios of natural ferruginous biotites with reference to 'the stability relations of the ferruginous biotite, annite'

The atomic ratios given by Eugster and Wones (1962) for analyses of high FeO-low MgO micas cited by Foster (1960) are in error. The recalculated atomic ratios herein presented show that only three of the thirteen have low (OH+F), and only one has characteristics that may possibly be due to loss of water during determination. None of the analyses exhibit characteristics suggestive of oxidation of iron and loss of H, as in progression towards oxyannite. The average recalculated octahedral occupancy is 2.69, not 2.91 as in the previous calculation. The differences between atomic ratios calculated on the basis ot determined H 2 O + and F and those calculated on the basis of the theoretical O 10 (OH) 2 content are not great enough to change the essential compositional character of these micas as interpreted by Foster (1960). However, they do affect significantly the calculated octahedral occupancy.

Journal of Petrology↗
USGSDate not supplied

Garnet types from the Cazadero area, California

This is the third in a series of papers on glaucophane schists from the Franciscan Formation near Cazadero, California. Previous papers describe three distinct types of glaucophane-bearing Franciscan metamorphic rocks near Cazadero. The purpose of this study is to investigate the garnets present in metamorphic types III (bedrock schists) and IV (tectonic blocks) as defined by Coleman & Lee (1963). Twenty-four garnet analyses are presented. Sixteen of these are from (aragonite-bearing) type III glaucophane schists, and eight are from type IV glaucophane schists. Type IV rocks include California eclogites. Type III rocks include metabasalt, metachert, metashale, meta-ironstone, and metacarbonate that were formed under high pressure and relatively low temperature. These rocks contain garnets that display a wide range of composition, but the dominant molecules represented are consistently almandine, spessartine, and grossular. Type IV rocks are mainly metabasalts that were probably formed under higher temperatures and pressures than type III rocks. There is a distinct difference between garnets from type III rocks and those from type IV (including eclogites); the lattercontain less spessartine and more pyrope, and the dominant molecules are almandine and grossular. The four analyses of garnets from California eclogites have an average pyrope content of about ten molecular per cent, and they extend the range of composition reported for eclogite garnets. Quantitative spectrographic determinations of minor elements are listed for each of the garnets described. The values determined for some of the minor elements have a wide range and a capricious distribution over a few feet of outcrop area. As a group, both the garnets from type III rocks and those from type IV are pyralspites with large contents (as much as 35 molecular per cent) of ugrandite. This unusual admixture of the pyralspite and ugrandite garnet series may have resulted in part from the conditions (high pressures and relatively low temperatures) under which the enclosing rocks were recrystallized.

California↗
USGS2000

Magma mixing, recharge and eruption histories recorded in plagioclase phenocrysts from El Chichón Volcano, Mexico

Consistent core-to-rim decreases of 87 Sr/ 86 Sr ratios and coincident increases in Sr concentrations in plagioclase phenocrysts of varying size (∼1 cm to 2 mm) are reported from samples of the 1982 and pre-1982 (∼200 ka) eruptions of El Chichón Volcano. Maximum 87 Sr/ 86 Sr ratios of ∼0·7054, significantly higher than the whole-rock isotopic ratios (∼0·7040–0·7045), are found in the cores of plagioclase phenocrysts, and minimum 87 Sr/ 86 Sr ratios of ∼0·7039 are found near some of the rims. Plagioclase phenocrysts commonly display abrupt fluctuations in An content (up to 25 mol %) that correspond to well-developed dissolution surfaces. The isotopic, textural and compositional characteristics suggest that these plagioclase phenocrysts grew in a system that was periodically recharged by higher-temperature magma with a lower 87 Sr/ 86 Sr ratio and a higher Sr concentration. Rim 87 Sr/ 86 Sr ratios in plagioclase phenocrysts of rocks from the 200 ka eruption indicate that, at that time, the magma had already attained the lowest recorded 87 Sr/ 86 Sr value of the system (∼0·7039). In contrast, cores from plagioclase phenocrysts of the 1982 eruption, inferred to have grown in the past few thousand years, have the highest recorded 87 Sr/ 86 Sr ratios of the system. Collectively, the Sr isotopic data (for plagioclase and whole rock), disequilibrium textural features of the phenocrysts, known eruption frequencies, and inferred crystal-residence times of the plagioclases are best interpreted in terms of an intermittent magma chamber model. Similar processes, including crustal contamination, magma mixing, periodic recharge by addition of more mafic magma to induce plagioclase disequilibrium (possibly triggering eruption) and subsequent re-equilibration, apparently were operative throughout the 200 ky history of the El Chichón magma system.

Chiapas↗
USGS2002

Ancient and modern subduction zone contributions to the mantle sources of lavas from the Lassen region of California inferred from Lu-Hf isotopic systematics

Hafnium isotopic compositions have been determined on a suite of calc-alkaline and high-alumina-olivine tholeiitic lavas from the Lassen region of California and are used, in conjunction with previously published mineralogical, geochemical, and isotopic data, to constrain their petrogenesis. Positive correlation between ε Hf values and geochemical indices of the modern subduction component indicates that the isotopic compositions of the calc-alkaline lavas record addition of radiogenic Hf from the subducted slab. However, the addition of the modern subduction component increases the ε Hf values of most calc-alkaline lavas by <0·5 units over estimates of non-subduction enriched peridotites of the mantle wedge. The Lu–Hf isotopic systematics of the Lassen lavas suggest that the calc-alkaline magmas have equilibrated with garnet at some point in their history, whereas the tholeiitic magmas have not. These observations require the two lava types to be derived from different sources. The isotopic variability of the Lassen lavas cannot be produced by mixing mantle sources inferred to be present in the eastern–central Pacific and western USA with a modern subduction component. Instead, the isotopic variability is consistent with mixing of a depleted mantle source, a more fertile mantle source enriched by an ancient subduction component, and a modern subduction component.

California↗
USGS2002

The Fish Canyon magma body, San Juan volcanic field, Colorado: Rejuvenation and eruption of an upper-crustal batholith

More than 5000 km 3 of nearly compositionally homogeneous crystal-rich dacite (∼68 wt % SiO 2 : ∼45% Pl + Kfs + Qtz + Hbl + Bt + Spn + Mag + Ilm + Ap + Zrn + Po) erupted from the Fish Canyon magma body during three phases: (1) the pre-caldera Pagosa Peak Dacite (an unusual poorly fragmented pyroclastic deposit, ∼200 km 3 ); (2) the syn-collapse Fish Canyon Tuff (one of the largest known ignimbrites, ∼5000 km 3 ); (3) the post-collapse Nutras Creek Dacite (a volumetrically minor lava). The late evolution of the Fish Canyon magma is characterized by rejuvenation of a near-solidus upper-crustal intrusive body (mainly crystal mush) of batholithic dimensions. The necessary thermal input was supplied by a shallow intrusion of more mafic magma represented at the surface by sparse andesitic enclaves in late-erupted Fish Canyon Tuff and by the post-caldera Huerto Andesite. The solidified margins of this intrusion are represented by holocrystalline xenoliths with Fish Canyon mineralogy and mineral chemistry and widely dispersed partially remelted polymineralic aggregates, but dehydration melting was not an important mechanism in the rejuvenation of the Fish Canyon magma. Underlying mafic magma may have evolved H 2 O–F–S–Cl-rich fluids that fluxed melting in the overlying crystal mush. Manifestations of the late up-temperature magma evolution are: (1) resorbed quartz, as well as feldspars displaying a wide spectrum of textures indicative of both resorption and growth, including Rapakivi textures and reverse growth zoning (An 27–28 to An 32–33 ) at the margins of many plagioclase phenocrysts; (2) high Sr, Ba, and Eu contents in the high-SiO 2 rhyolite matrix glass, which are inconsistent with extreme fractional crystallization of feldspar; (3) oscillatory and reverse growth zoning toward the margins of many euhedral hornblende phenocrysts (rimward increases from ∼5·5–6 to 7·7–8·5 wt % Al 2 O 3 ). Homogeneity in magma composition at the chamber-wide scale, contrasting with extreme textural and chemical complexities at the centimeter–millimeter scale, is consistent with a dynamic environment, wherein crystals with a variety of growth and resorption histories were juxtaposed shortly before eruption by convective currents.

Colorado↗
USGS2003

Petrogenesis of mesozoic, peraluminous granites in the Lamoille canyon area, Ruby mountains, Nevada, USA

Two groups of closely associated, peraluminous, two-mica granitic gneiss were identified in the area. The older, sparsely distributed unit is equigranular (EG) with initial ε Nd ∼ − 8·8 and initial 87 Sr/ 86 Sr ∼0·7098. Its age is uncertain. The younger unit is Late Cretaceous (∼80 Ma), pegmatitic, and sillimanite-bearing (KPG), with ε Nd from −15·8 to −17·3 and initial 87 Sr/ 86 Sr from 0·7157 to 0·7198. The concentrations of Fe, Mg, Na, Ca, Sr, V, Zr, Zn and Hf are higher, and K, Rb and Th are lower in the EG. Major- and trace-element models indicate that the KPG was derived by muscovite dehydration melting (<35 km depth) of Neoproterozoic metapelitic rocks that are widespread in the eastern Great Basin. The models are broadly consistent with anatexis of crust tectonically thickened during the Sevier orogeny; no mantle mass or heat contribution was necessary. As such, this unit represents one crustal end-member of regional Late Cretaceous peraluminous granites. The EG was produced by biotite dehydration melting at greater depths, with garnet stable in the residue. The source of the EG was probably Paleoproterozoic metagraywacke. Because EG magmatism probably pre-dated Late Cretaceous crustal thickening, it required heat input from the mantle or from mantle-derived magma.

Nevada↗