Geology Reports⌕ Search

SEARCH · Geology Reports

Results for “Petrology”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 811 records · Page 45Linked to original sources
USGS1961

Jadeite deposits of the clear creek area, New Idria district, San Benito county, California

Tectonic inclusions within the New Idria serpentine body contain jadeite in two distinct assemblages: (1) Lenslike inclusions containing a monomineralic green jadeite core surrounded by a calc-silicate rim. (2) Jadeite veins cross-cutting albite-crossite schist inclusions. In these veins jadeite coexists with low albite; green jadeite (Jd 75 Ac+Di+He 25) coexists with low albite within the host schist. Albite schists are related to the pre-metamorphic keratophyres and it can be shown that desilication of such rocks produces a bulk composition similar to that of jadeite. Metamor-phism of keratophyric tectonic inclusions within serpentine produced jadeite-albite schists along margins of the inclusions. Jadeite-albite veins formed from mobilized liquids rich in the jadeite molecule and deficient in water. Geological evidence such as would indicate extreme pressures or temperatures during jadeite formation is lacking; rather, a low silica and dry system allowed its formation at reduced temperatures and pressures.

California↗
USGS1961

Magmatic differentiation in the Uwekahuna Laccolith, Kilauea Caldera, Hawaii

Petrographic and chemicoal studies of a suite of rocks from the Uwekahuna laccolith of Kilauea Volcano show that the original mafic tholeiitic magma differentiated into tholeiitic picrite, tholeiitic olivine gabbro, and an aphanitic rock approaching quartz-basalt in composition. Mechanisms involved were an initial gravity settling of olivine and a final filter pressing of the residual liquid. The range of composition represented by the rocks of the laccolith is as great as that found among all hitherto analysed lavas of the volcano.

Hawaii↗
USGS1986

Aluminum enrichment in silicate melts by fractional crystallization: some mineralogic and petrographic constraints.

The degree of aluminum saturation of an igneous rock may be described by its Aluminum Saturation Index (ASI) defined as the molar ratio Al 2 O 3 (CaO + K 2 O + Na 2 O). One suggested origin for mildly peraluminous granites (ASI between 1 and about 1.1) is by fractional crystallization of subaluminous (ASI < 1) magmas; hornblende, having ASI < 0.5, could be a major driving force in such a fractionation process. The efficacy of the process depends not only on precipitation of hornblende and its effective removal from the reacting system, but on the composition and nature of other coprecipitating phases, weighted by their modal abundances in the reactive system. Precipitation of feldspar (ASI = 1), for instance, would retard or even prevent aluminum enrichment in the melt if the ASI of melt is < 1, but would enhance such evolution if the ASI of the melt is > 1. Discussion of the efficacy of any mineral must be made in the context of the total reacting system. For hornblende to effectively cause a melt to evolve into a peraluminous composition, it must be able to coexist with peraluminous magmas. Experimental phase equilibrium data show that at pressure > 5 kb hornblende can coexist with strongly peraluminous melts (ASI ˜ 1.5). Scantily phyric volcanic rocks show that hornblende can coexist with granitic magma having ASI ˜ 1.1 –1.2. The aggregate ASI of last-stage minerals of a typical granite is less than this value; therefore, even after hornblende has reacted out, the residual magma may be expected to continue to evolve toward more aluminous compositions.

Journal of Petrology↗
USGS1987

Geochemistry of high-silica peralkaline rhyolites, Naivasha, Kenya rift valley

The Recent (<15000 y) volcanic complex of southwest Naivasha, Kenya, consists of mildly peralkaline (comenditic) rhyolite domes, lava flows, air fall pumices, and lake sediments, with minor, peripheral, basalts and hawaiites. The comendites are either aphyric or sparsely porphyritic, few samples containing >5 per cent phenocrysts. Phenocryst minerals are quartz-sanidine-ferrohedenbergite-fayalite-titanomagnetite-ilmenite-riebeckite-arfvedsonite-aenigmatite-biotite-zircon. Ferrohedenbergite and zircon are restricted to less peralkaline, and amphibole, aenigmatite, and biotite to more peralkaline, rocks.The comendites show unusually strong enrichment in Cs, F, Hf, Nb, Rb, REE, Ta, Th, U, Y, Zn, and Zr, and extreme depletion in Mg, Ca, Ba, Co, and Sr. REE patterns are moderately LREE-enriched, with large, negative Eu anomalies. Values of LIL/HFS element ratios, such as Th/Ta and Rb/Zr, are unusually high for peralkaline rhyolites, and are consistent with a substantial crustal component in the comendites. Parameters such as LREE/HREE and Zr/Nb ratios indicate that the Naivasha rhyolites represent several pulses of closely related, but subtly different, magmas. Sanidine/glass partition coefficients for Ba, Pb, Rb, Sr, U, and the REE are presented for one specimen.Major and trace element modelling, and feldspar-rock relationships, show that closed system crystal fractionation cannot alone account for the overall compositional variations in the comendites. A model involving partial melting of variable crustal source rocks and migration of dissolved volatile-metal complexes may be appropriate at Naivasha. © 1987 Oxford University Press.

Journal of Petrology↗
USGS1962

Stability relations of the ferruginous biotite, annite

Annite , KFe 3 AISi 3 O 10 (OH) 2 a member of the iron biotites and the ferrous analogue of phlogopite, has been synthesized and its phase relations have been determined as functions of temperature, fugacity of oxygen (fo 2 ), and total pressure (P total ≈PH 2 O+PH 2 ). A method for controlling fo 2 at high total pressures is described, and data for the 'oxygen buffers' used are given. Buffers range from quartz+iron+fayalite assemblages (low fo 2 ) to magnetite-hematite assemblages (high fo 2 ). Optical properties and unit-cell dimensions of synthetic annites depend on the conditions of synthesis.By recalculating published analyses of natural iron-rich biotites it can be shown that one cannot assume a constant hydrogen content for such biotites. Oxidation may have occurred by drying at 115°C. Octahedral occupancy therefore cannot be calculated from such data.Phase relations of annite are presented in 2,070 and 1,035 bar sections. Depending on fo 2 -T values annite was found to decompose to one of the following assemblages: hematite+ sanidine, magnetite+sanidine, fayalite+leucite+kalsilite, iron+sanidine. All decompositions are dehydration and redox reactions and are sensitive to changes in fH 2 0 and fo 2 (or fH 2 0 and fH 2 ). At 2, 070 bars total pressure annite +magnetite+sanidine can coexist between 425°C and 825° C, depending upon the magnitude of fo 2 .In the presence of quartz the stability field of annite is more restricted. Phase equilibria in the system KAlSiO 4 -SiO 2 -Fe-O 2 -H 2 have been summarized schematically.Wherever possible, thermodynamic extrapolations are made to test the internal consistency of the data. Enthalpies of formation are calculated for both annite and phlogopite. Ranges of fo 2 values in nature as well as mechanisms for changes in fo 2 are investigated. It is useful to distinguish between assemblages which are internally buffered with respect to fo 2 changes and those which are not buffered. The applications of individual reactions involving annite to specific geologic problems are discussed with respect to igneous, metamorphic, and sedimentary rocks.

Journal of Petrology↗
USGS1962

Lower temperature terminations of the three-phase region plagioclase-alkali feldspar-liquid

Geological and experimental evidence indicate that the three-phase field, plagioclase-alkalifeldspar-liquid, may terminate in several different ways. The possible terminations have been developed from Schreinemakers' rules governing the disappearance of three-phase fields. In igncous rocks, these different terminations may arise from variations in the relative amounts of additional components in magmas, or from changes of total pressure, or from structural changes that effect the extent of solid solution in feldspar.The three-phase region originates from the intersection of the solidus and the feldspar solvus. The available evidence regarding this intersection is reviewed, as is the evidence for the existence and form of the boundary curve on the feldspar liquidus. The data are used to project a series of isobaric polythermal and isobaric isothermal diagrams for each possible termination . Subsequent discussions relate the theoretical arguments to the natural evidence, suggest the more probable geological environments of some of the terminations , and indicate characteristic features of each termination .It may be possible to identify the type of termination involved in the crystallization of some rocks. The necessary data are the amounts and compositions of both kinds of coexisting feldspars and of the feldspar components of the coexisting liquid at one or more stages of the crystallization process. Volcanic rocks are most suitable for such studies.The terminations explain many of the compositional relationships possible between feldspar crystals and silicate melt under geological and experimental conditions. These compositional relationships are fundamental to understanding the crystallization of igneous rocks and the formation of melts by partial fusion. Other geological applications include a simple explanation for some resorbed feldspars, the separation, correlation, and comparison of porphyritic rock units, interpretation of compositional changes of successive zones of zoned feldspars, and mantling of one feldspar by another. With all types of termination , extensive fractionation yields a liquid rich in Or and Ab that may crystallize to alkali feldspar or feldspars.

Journal of Petrology↗
USGS1990

Calc-alkaline, shoshonitic, and primitive tholeiitic lavas from monogenetic volcanoes near Crater Lake, Oregon

Quaternary monogenetic volcanism in the High Cascades of Oregon is manifested by cinder cones, lava fields, and small shields. Near Crater Lake caldera, monogenetic lava compositions include: low-K (as low as 0·09% K 2 O) high-alumina olivine tholeiite (HAOT); medium-K. calc-alkaline basalt, basaltic andesite, and andesite; and shoshonitic basaltic andesite (2·1% K 2 O, 1750 ppm Sr at 54% SiO 2 ). Tholeiites have MORB-like trace element abundances except for elevated Sr, Ba, and Th and low high field strength elements (HFSE), and they represent near-primary liquids. They are similar to HAOTs from the Cascades and adjacent Basin and Range, and to many primitive basalts from intraoceanic arcs. Calc-alkaline lavas show a well-developed arc signature of high large-ion lithophile elements (LILE) and low HFSE. Their Zr and Hf concentrations are at least partly decoupled from those of Nb and Ta; HREE are low relative to HAOT. Incompatible element abundances and ratios vary widely among basaltic andesites. Some calc-alkaline lavas vented near Mount Mazama contain abundant gabbroic microxcnoliths, and are basaltic andesitic magmas contaminated with olivine gabbro. A calc-alkaline basalt and a few basaltic andesites have MgO and compatible trace element contents that suggest only minor fractionation. There appears to be a compositional continuum between primitive tholeiitic and calc-alkaline lavas. Compositional variation within suites of comagmatic primitive lavas, both tholeiitic and calc-alkaline, mainly results from different degrees of partial melting. Sources of calc-alkaline primary magmas were enriched in LILE and LREE by a subduction component and contained residual garnet, whereas sources of HAOTs had lower LILE and LREE concentrations and contained residual clinopyroxene. High and variable LILE and LREE contents of calc-alkaline lavas reflect variations in fluid-transported subduction component added to the mantle wedge, degree of partial melting, and possibly also interaction with rocks or partial melts in the lower crust. Andesites were derived from calc-alkaline basaltic andesites by fractionation of plagioclase+augite+magnetite+apatite ± orthopyroxene or olivine, commonly accompanied by assimilation. Many andesites are mixtures of andesitic or dacitic magma and a basaltic or basaltic andesitic component, or are contaminated with gabbroic material. Mingled basalt, andesite, and dacite of Williams Crater formed by multi-component, multi-stage mixing of basaltic andesitic magma, gabbro, and dacitic magma. The wide range of compositions vented from monogenetic volcanoes near Crater Lake is a result of the thick crust coupled with mild tectonic extension superimposed on a subduction-related magmatic arc.

Oregon↗
USGS1990

Petrogenetic evolution of the torfajökull volcanic complex, Iceland I. relationship between the magma types

The Torfajökull volcano, Iceland's largest silicic centre, is situated close to the junction of the active, southerly propagating Eastern Rift Zone and the South Eastern Zone, an older crustal segment. This paper provides major, trace, and some Sr isotope data on postglacial (<10000 y) rocks, i.e., tholeiitic magmas of the Eastern Rift Zone and transitional basalts, icelandites, and rhyolites of the Torfajökull centre, and assesses the relationships between the magma types in terms of the development of the Icelandic crust.Tholeiitic magmas from the Eastern Rift Zone are LILE-enriched relative to MORB. They have undergone extensive olivine-plagioclase-clinopyroxene fractionation at low pressures. Compared with the tholeiites, Torfajökull transitional basalts show LILE/HFS enrichment and higher ( 87 Sr/ 86 Sr) 1 ratios. They define several magmatic lineages and have equilibrated over a wide range of pressures. Both basalt types were derived by very small degrees of partial melting of compositionally similar mantle sources, the main difference being that the tholeiites were generated in the spinel Iherzolite, and the transitional basalts in the garnet lherzolite, stability fields, a conclusion previously reached by Meyer et al. (1985). The mantle sources may have contained LILE-enriched streaks.Low-pressure differentiation of Torfajökull transitional basalt produced an iceiandite to sub-alkaline rhyolite sequence by crystal fractionation, the rhyolites representing >90% crystallization of parental basalts. The rhyolites were emplaced as nine separate lava fields, formed during 11 eruptive episodes. The compositional range within each field is limited, and, although similar, the ranges define several magmatic lineages. Continued fractionation of plagioclase-alkali feldspar-clinopyroxene-magnetite-apatite-zircon assemblages generated peralkaline rhyolites in certain post-glacial eruptions. Chemical variations in the deposits from the Hrafntinnusker peralkaline eruption were related predominantly to alkali feldspar fractionation, and the melts were erupted from a zoned magma chamber. All postglacial volcanic rocks at Torfajokull have been mantle derived and thus represent new additions to the Icelandic crust. © 1990 Oxford University Press.

Journal of Petrology↗
USGS1990

Petrogenetic evolution of the torfajökull volcanic complex, Iceland II. The role of magma mixing

In southern Iceland, tholeiitic basalt magmas propagating laterally from the active Eastern Rift Zone into the older cmstal segment of the South Eastern Zone have been injected into Torfajökull, a mature volcanic centre dominated by rhyolites. Eruptions of complex suites of mixed and hybrid rocks have been triggered, involving tholeiites of the rift zone and transitional basalts and rhyolites of the Torfajökull centre. Three-component hybrids are an unusual feature of the activity. The distribution of various magma mixing and hybrid types is related to the periodic injection of tholeiite into a magma chamber, or chambers, where rhyolite overlies parental transitional basalts.Pre-postglacial rhyolites (>10000 y) at Torfajokull are predominantly peralkaline, whereas later rhyolites are, with few exceptions, subalkaline. Furthermore, the injection of rift zone magmas, and the consequent abundance of rhyolite-basalt mixing, have been important features of magmatism at the centre only in postglacial times. Reduced repose times in the magma reservoirs have prevented the production of peralkaline rhyolites. These trends are interpreted in terms of the southerly migration of the Eastern Rift Zone. © 1990 Oxford University Press.

Journal of Petrology↗
USGS1990

Metamorphism in the Adirondacks: II. The role of fluids

Quantitative estimates of metamorphic fluid speciation, stable isotopic analyses, and studies of fluid inclusions all document the local complexity of fluids in the deep crustal rocks exposed in the Adirondack Mountains, NY. Estimates of the activity of H 2 O in the granulite facies are substantially lower than in the amphibolite facies gneisses. The onset of low water activities in semi-pelitic gneisses generally correlates with migmatitic textures in the uppermost amphibolite facies, suggesting that partial melts absorbed H 2 O at the peak of metamorphism. In granulite facies marbles and calc-silicates, conditions varied from extremely undersaturated in H 2 O-CO 2 fluid to fluid saturated, and α H2O and α CO2 show sharp gradients within single outcrops. Low values of f O2 and f H2O , or of f CO2 , and f H2O indicate fluid-absent conditions for some orthogneisses and marbles, which are inferred to have been ‘dry’ rocks before and during granulite facies recrystallization. Wollastonite is preserved from early contact metamorphism and serves as an index mineral for fluid-absent conditions in granulites where α H2O is low. Values of f O2 range from near the hematite + magnetite buffer in metamorphosed iron formation to substantially below the quartz + magnetite + fayalite buffer in some orthogneisses. The anorthosite suite is more oxidized than some associated granitic gneisses. Halogens (F and Cl) substitute extensively for OH in micas and amphiboles, extending their stability, although F 2 , Cl 2 , HCl, and HF are minor components in any fluid. Oxybiotite-type exchanges involving O for OH are also important, extending the stability of biotite. Stable isotopic ratios of O and C demonstrate that premetamorphic whole-rock compositions are commonly preserved whereas mineral compositions generally reflect equilibration at the peak of metamorphism. The Marcy Anorthosite Massif was intruded as a high δ 18 O magma. The combination of mineral equilibria, stable isotope data, and fluid inclusions is used to identify and to distinguish among pre-orogenic contact metamorphic/hydrothermal events, peak metamorphic events, and retrograde/postmetamorphic events. Polymetamorphism is documented at skarn zones adjacent to anorthosite, where large volumes of hydrothermal fluid were channeled during early, shallow contact metamorphism and where conditions were fluid poor during subsequent regional metamorphism. Peak metamorphic events are inferred to have been caused primarily by magmatic processes of intrusion and anatexis. Partial melting has caused low values of α H2O in many rocks, but in other cases low values of α H2O are recorded in orthogneisses derived from H 2 O-poor magmas. Isotopic studies show that maximum fluid/rock ratios were <0·l and possibly 0·0 for infiltrating fluids at the peak of metamorphism in many localities. No evidence of pervasive, regional infiltration by a fluid at the peak of metamorphism has been substantiated in the Adirondacks. Fluid inclusions containing high-density CO 2 or CO 2 + H 2 O represent conditions from after the peak of metamorphism and document isobaric cooling, in agreement with estimates from garnet zoning. Fine-scale retrograde veins are common and are associated with high-density CO 2 -rich fluid inclusions.

New York↗
USGS1990

Geochemistry and intrusive history of the Ashland pluton, Klamath Mountains, California and Oregon

The Ashland pluton is a calc-alkaline plutonic complex that intruded the western Paleozoic and Triassic belt of the Klamath Mountains in late Middle Jurassic time. The pluton comprises a series of compositionally distinct magma pulses. The oldest rocks are hornblende gabbro and two-pyroxene quartz gabbro with initial 87 Sr/ 86 Sr = 0˙7044, δ 18 O = 8˙7%, and REE patterns with chondrite normalized La/Lu = 7. These units were followed by a suite of tonalitic rocks (La N /Lu N = 7) and then by a suite of K 2 O- and P 2 O 5 rocks of quartz monzodioritic affinity (La N /Lu N = 13–21; La N /Sm N = 2˙4–3˙) The quartz monzodioritic rocks were then intruded by biotite granodiorite and granite with lower REE abundances but more fractionated LREE(La N /Lu N = 13–19; La N /Sm N = 4˙3–6 and they, in turn, were host to dikes and bosses of hornblende diorite. The latest intrusive activity consisted of aplitic and granitic dikes. Combined phase equilibria and mineral composition data, indicate emplacement conditions of approximately P total = 2˙3kb, P H2O between 1˙5 and 2˙2 kb, and f O2 between the nickel-nickel oxide and hematite-magnetite buffers. Successive pulses of magma display increasing SiO 2 together with increasing δ 18 O and decreasing initial 87 Sr/ 86 Sr. The isotopic data are consistent with either (1) combined fractional crystallization of andesitic magma and concurrent assimilation of crustal material characterized by low Sr 1 and high (δ 18 O or, more probably, (2) a series of partial melting events in which sources were successively less radiogenic but richer in 18 O Each intrusive stage displays evidence for some degree of crystal accumulation and/or fractional crystallization but neither process adequately accounts for their compositional differences. Consequently, each stage appears to represent a distinct partial melting or assimilation event. The P 2 O 5 -rich nature of the quartz monzodiorite suite suggests accumulation of apatite. However, the suite contains abundant mafic microgranitoid enclaves and most apatite in the suite is acicular. These observations suggest that magma mixing affected the compositional variation of the quartz monzodiorite suite. Mass balance calculations are consistent with a simple mixing process in which P 2 O 5 -rich alkalic basalt magma (represented by the mafic microgranitoid enclaves) was combined with a crystal-poor felsic magma (represented by the tonalite suite), yielding a quartz monzodioritic magma that then underwent differentiation by crystal fractionation and accumulation.

California, Oregon↗
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↗
USGS1993

Pressure increases, the formation of chromite seams, and the development of the ultramafic series in the Stillwater Complex, Montana

This paper explores the hypothesis that chromite seams in the Stillwater Complex formed in response to periodic increases in total pressure in the chamber. Total pressure increased because of the positive δ V of nucleation of CO 2 bubbles in the melt and their subsequent rise through the magma chamber, during which the bubbles increased in volume by a factor of 4–6. By analogy with the pressure changes in the summit chambers of Kilauea and Krafla volcanoes, the maximum variation was 0⋅2–0⋅25 kbar, or 5–10% of the total pressure in the Stillwater chamber. An evaluation of the likelihood of fountaining and mixing of a new, primitive liquid that entered the chamber with the somewhat more evolved liquid already in the chamber is based upon calculations using observed and inferred velocities and flow rates of basaltic magmas moving through volcanic fissures. The calculations indicate that hot, dense magma would have oozed, rather than fountained into the chamber, and early mixing of the new and residual magmas that could have resulted in chromite crystallizing alone did not take place.

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↗