Geology Reports⌕ Search

SEARCH · Geology Reports

Results for “Journal of 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 37 records · Page 2Linked to original sources
USGS2020

Generation of calc-alkaline magmas during crystallization at high oxygen fugacity: An experimental and petrologic study of tephras from Buldir Volcano, western Aleutian Arc, Alaska, USA

Despite agreement that calc-alkaline volcanism occurs at subduction zones and is responsible for the genesis of continental landmasses, there is no consensus on the source of the Fe-depleted signature hallmark to calc-alkaline volcanism. In this study, we utilize mafic tephras collected from Buldir Volcano to address the genesis of strongly calc-alkaline volcanic rocks (those with a low Tholeiitic Index; ≤0·7) in a segment of the western Aleutian Arc to determine if the eruptions are plausibly part of a liquid line of descent, if they are mixtures of crustal melts and parental magmas, or if they are mixtures of melts of the mantle and the subducting slab. We conducted a series of H 2 O-saturated phase equilibrium experiments (1175–1000°C; 100 MPa) in a rapid-quench cold-seal (MHC) apparatus on the most primitive natural lava from Buldir (9·34 wt % MgO) at oxidizing conditions near the Re–ReO 2 buffer. We confirmed that all experiments equilibrated 0·3 ± 0·23 log units above the Re–ReO 2 buffer (ΔQFM ∼ +2·8) using X-ray Absorption Near Edge Structure (XANES) spectroscopy. Chromite is the liquidus phase, followed by olivine, then plagioclase, then clinopyroxene, and finally hornblende. Once clinopyroxene saturates, spinel composition shifts to magnetite. We compared our experimental results to the major element geochemistry and petrology of six tephras (51·9–54·8 wt % SiO 2 ) from Buldir collected during the 2015 field season of the GeoPRISMS shared platform. Tephras contain olivine + plagioclase + clinopyroxene + spinel ± hornblende; plagioclase comprises most of the crystalline volume, followed by either olivine or hornblende. Spinel is ubiquitous; with Cr-rich spinel inclusions in olivine and hornblende, and magnetite in the groundmass. Variations in phenocryst assemblages and compositions between samples can be attributed to differences in pre-eruptive temperatures, where hotter samples are devoid of hornblende, and contain Fo-rich olivine and plagioclase with lower An-contents, owing to the position of the mineral-in curves at fluid-saturated conditions. Experimental glasses match the depletion in FeO T observed in the tephra whole rock compositions. The continuous depletion in FeO T is attributable to saturation of spinel as a liquidus phase (initially as chromite) and continuous crystallization through the experimental series (changing to magnetite at colder temperatures). In contrast to the natural samples, the experiments show enrichment in TiO 2 with decreasing MgO, suggesting that differentiation did not occur at 100 MPa on Buldir. The TiO 2 depletion in volcanic rocks from Buldir can be accounted for if hornblende crystallization occurs close to the liquidus of a parental magma; a condition that is met at higher pressures and hydrous conditions. The emerging picture for Buldir Island is that (1) oxidizing conditions are required to drive the observed depletions in FeO T via crystallization of spinel, and (2) elevated H 2 O contents and high pressures are required to saturate hornblende close to the liquidus to reproduce the entire suite of major elements. Our study provides a mechanism to generate the calc-alkaline trends observed at Buldir without requiring mixing of slab and mantle melts. We conclude that calc-alkaline volcanic rocks with extremely low Tholeiitic Indices (0·7), like those from Buldir, cannot be generated in absence of high oxygen fugacity, even at high pressure and/or elevated water pressures.

Alaska↗
USGS2013

Hydrogen isotope investigation of amphibole and glass in dacite magmas erupted in 1980-1986 and 2005 at Mount St. Helens, Washington

In active, shallow, sub-volcanic magma conduits the extent of the dehydrogenation–oxidation reaction in amphibole phenocrysts is controlled by energetic processes that cause crystal lattice damage or conditions that increase hydrogen diffusivity in magmatic phases. Amphibole phenocrysts separated from dacitic volcanic rocks erupted from 1980 to 1986 and in 2005 at Mount St. Helens (MSH) were analyzed for δD, water content and Fe 3 +/Fe 2 +, and fragments of glassy groundmass were analyzed for δD and water content. Changes in amphibole δD values through time are evaluated within the context of carefully observed volcanic eruption behavior and published petrological and geochemical investigations. Driving forces for amphibole dehydrogenation include increase in magma oxygen fugacity, decrease in amphibole hydrogen fugacity, or both. The phenocryst amphibole (δD value c. –57‰ and 2 wt % H 2 O) in the white fallout pumice of the May 18, 1980 plinian eruptive phase is probably little modified during rapid magma ascent up an ∼7 km conduit. Younger volcanic rocks incorporate some shallowly degassed dacitic magma from earlier pulses, based on amphibole phenocryst populations that exhibit varying degrees of dehydrogenation. Pyroclastic rocks from explosive eruptions in June–October 1980 have elevated abundances of mottled amphibole phenocrysts (peaking in some pyroclastic rocks erupted on July 22, 1980), and extensive amphibole dehydrogenation is linked to crystal damage from vesiculation and pyroclastic fountain collapse that increased effective hydrogen diffusion in amphibole. Multiple amphibole δD populations in many 1980 pyroclastic rocks combined with their groundmass characteristics (e.g. mixed pumice textures) support models of shallow mixing prior to, or during, eruption as new, volatile-rich magma pulses blended with more oxidized, degassed magma. Amphibole dehydrogenation is quenched at the top surface of MSH dacite lava lobes, but the diversity in the δD amph populations in original fresh lava flow surfaces may occur from blending magma domains with different ascent histories in the sub-volcanic environment immediately before eruption. Multi-stage open-system magma degassing operated in each parcel of magma rising toward the surface, whereas the magma below ∼7 km was a relatively closed system, at least to the October 1986 eruption based on the large population of minimally dehydrogenated, rim-free amphibole in the lavas. Magma degassing and possibly H isotope exchange with low-δD fluids around the roof zone may have accompanied the ∼1·5 km upward migration of the 1980 magma body. The low-δDamph (c. –188 to –122‰) oxy-amphibole phenocrysts in lava spines extruded in May 2005 reflect dehydrogenation as ascending viscous magma degassed and crystallized, and fractures that admitted oxygen into the hot solidified lava spine interior facilitated additional iron oxidation.

Washington↗
USGS1996

Evidence for multiple mechanisms of crustal contamination of magma from compositionally zoned plutons and associated ultramafic intrusions of the Alaska Range

Models of continental crustal magmagenesis commonly invoke the interaction of mafic mantle-derived magma and continental crust to explain geochemical and petrologic characteristics of crustal volcanic and plutonic rocks. This interaction and the specific mechanisms of crustal contamination associated with it are poorly understood. An excellent opportunity to study the progressive effects of crustal contamination is offered by the composite plutons of the Alaska Range, a series of nine early Tertiary, multiply intruded, compositionally zoned (Peridotite to granite) plutons. Large initial Sr and Nd isotopic contrasts between the crustal country rock and likely parental magmas allow evaluation of the mechanisms and extents of crustal contamination that accompanied the crystallization of these ultra-mafic through granitic rocks. Three contamination processes are distinguished in these plutons. The most obvious of these is assimilation of crustal country rock concurrent with magmatic fractional crystallization (AFC), as indicated by a general trend toward crustal-like isotopic signatures with increasing differentiation. Second, many ultramafic and mafic rocks have late-stage phenocryst reaction and orthocumulate textures that suggest interaction with felsic melt. These rocks also have variable and enriched isotopic compositions that suggest that this felsic melt was isotopically enriched and probably derived from crustal country rock. Partial melt from the flysch country rock may have reacted with and contaminated these partly crystalline magmas following the precipitation and accumulation of the cumulus phenocrysts but before complete solidification of the magma. This suggests that in magmatic mush (especially of ultramafic composition) crystallizing in continental crust, a second distinct process of crustal contamination may be super imposed on AFC or magma mixing involving the main magma body. Finally, nearly all rocks, including mafic and ultramafic rocks, have ( 87 Sr/ 86 Sr) i that are too high, and ε(T) Nd that are too low, to represent the expected isotopic composition of typical depleted mantle. However, gabbro xenoliths with typical depicted-mantle isotopic compositions are found in the plutons. This situation requires either an additional enriched mantle component to provide the parental magma for these plutons, or some mechanism of crustal contamination of the parent magma that did not cause significant crystallization and differentiation of the magma to more felsic compositions. Thermodynamic modeling indicates that assimilation of alkali-and water-rich partial melt of the metapelite country rock by fractionating, near-liquidus basaltic magma could cause significant contamination while suppressing significant crystallization and differentiation.

Journal of Petrology↗
USGS1975

Chemical compositions of Kilauea east-rift lava, 1968–1971

The major element chemical compositions of lava from four eruptions on the east rift zone of Kilauea between August 1968 and October 1971 reflect three petrologic processes: Production of chemically distinct batches of magma in the mantle. Separation of olivine, augite, and plagioclase from liquid during flow in the rift-zone conduits. Mixing of different magmas during ascent to the surface. Chemically none of the four Kilauea east-rift eruptions matches the preceding summit eruption in Halemaumau that ended in July 1968. The Mauna Ulu eruption, May 1969 to October 1971 (the last of flie east-rift eruptions), can be divided into five olivine-controlled and chemically distinct variants. Three of these characterize the first seven months of the eruption and are closest in composition to the 1967–8 Halemaumau eruption. Variants 4 and 5 were erupted later and have compositions that are distinctly different from that of the 1967–8 eruption. Major differences are higher Al 2 O 3 (0·15–0·23 per cent), and lower K 2 O (0·07–0·10 per cent) and TiO 2 (0·12–0·23 per cent) in variants 4 and 5 at the same MgO content. Some lavas from eruptions in August and October 1968 and February 1969, have olivine-controlled magma compositions that are identical to mixtures of Mauna Ulu variants 1–3 and the 1967–8 composition. This observation fits an hypothesis advanced earlier by T. L. Wright and R. S. Fiske that magmas in the central magma chamber become mixed with magmas in the rift zone and can be identified as mixing components of rift eruption magmas before they appear as distinctive magmas in summit eruptions. Lavas representing mixing of olivine-controlled magma with differentiated magma were erupted in October 1968, February 1969, and in May and December 1969. The changes in amount of K 2 O and TiO 2 during the latter part of the 1969–71 Mauna Ulu eruption are the reverse of the overall secular change in composition of Kilauea summit lavas from pre-1750 through 1967–8. The K 2 O and TiO 2 contents of the latest overflows during the 1969–71 Mauna Ulu eruption (April 1971) are comparable to that of lava erupted at Kilauea summit prior to 1750. The changing chemistry of Kilauea magma is found to be of use as a ‘tracer’ in the complex Kilauea conduit system. Application of these data to older lava sequences is difficul because of the complexity of the processes controlling lava composition and the absence of detailed information about the time-space chemical variation during individual eruptions.

Hawaii↗
USGS1995

Petrology of Submarine Lavas from Kilauea's Puna Ridge, Hawaii

We have studied 30 quenched tholeiitic lava flows recovered by 20 dredge hauls and one submersible dive along Puna Ridge, the submarine part of the East Rift Zone of Kilauea Volcano, Hawaii Glass grains from numerous additional flows were recovered in turbidite sands cored in the Hawaiian Trough. These quenched lavas document variable primary magma compositions; olivine and multiphase crystallization and fractionation; degassing; wall-rock stoping and assimilation; mixing in the crustal reservoir and the rift zone; entrainment of olivine xenocrysts from a hot, ductile, olivine cumulate body; and disruption of gabbro wallrocks in the rift zone. Glass grains in turbidite sands contain up to 15⋅0wt% MgO, in contrast to < 7⋅0wt% MgO for the sampled glass rinds on lavas. The most forsteritic olivine phenocryst (F0 90·7 ) is in equilibrium with primary Kilauea liquid containing an average 16⋅5 wt% MgO, but ranging from 13⋅4 to 18⋅4%. Lavas and glass grains have more restricted P 2 O 5 /K 2 O and TiO 2 /K 2 O than glass inclusions in olivine, because more diverse liquids trapped as glass inclusions are mixed and homogenized before eruption. Variable trace element compositions in glass grains and whole rocks indicate that the primary liquids form by partial melting of mantle sources retaining clinopyroxene and garnet. Orthopyroxene xenocrysts formed at moderate pressures provide evidence for a sub-crustal staging zone. Chromite and olivine crystallize in the crustal magma reservoir as the liquid cools from an average 1346°C to ∼1170°C. Low viscosities of the primary liquids (0·4 Pas) facilitate olivine settling, and the crystallized olivine forms an olivine cumulate body at the base of the reservoir. Olivine is deformed as the hot ductile dunite body flows down and away from the summit. This flow drives instability of the Hilina landslide on Kilauea. Dikes intrude the dunite, and magma flowing through the dikes disaggregates and entrains olivine xenocrysts in Puna Ridge magmas. Primary liquids pond at or near the base of Kilauea's crustal reservoir because they are denser than more fractionated liquids that occupy the upper parts of the reservoir. The sulfur and water contents of glass rinds indicate that fractionated liquids near the top of the reservoir degas at low pressure, a process that increases their density and causes them to sink to levels where they mix with resident undegassed, near-primary liquid. The fractionated liquids near the top of the magma reservoir acquire excess Cl, owing to assimilation of hydrothermally altered roofrocks. Magma flowing into the rift zone encounters and mixes with low-temperature, multiphase-fractionated melt. The mixed magmas typically contain rare orthopyroxene, plagioclase as sodic as andesine, olivine as fayalitic as F0 75 and Fe-rich augite derived from the fractionated magma. Magma flowing through dikes also dislodged fragments of gabbroic wallrocks that occur as xenoliths. The interrelations in the Kilauean submarine lavas between host glass and glass inclusion compositions, volatile contents and mineral chemistry reveal an extraordinarily complex sequence of petrogenetic processes and events that are difficult or impossible to determine in subaerial Kilauea lavas because of crystallization, reequilibration and degassing during or after their eruption.

Hawaii↗
USGS2025

Melt generation sources and conditions in the wake of a migrating slab window: Geochemistry and petrology of the million-year history of primitive volcanism at Clear Lake volcanic field, California

Clear Lake volcanic field (CLVF) is the northernmost and youngest (~2.2 Ma to 8 ka) of the volcanic centers distributed along the San Andreas transform fault in western California. The initial phase of CLVF volcanism (interval one) occurred between ~2.2 and 1.3 Ma and extends ~35 km southeast of Clear Lake, forming a semi-continuous upland plateau capped by lava flows, with isolated volcanic remnants on the periphery. This volcanism is broadly characterized by geochemically primitive compositions that reflect three source compositions and conditions of melt generation. (1) Partial melting of upwelling asthenospheric mantle lherzolite at moderate pressures (1.2–1.4 GPa) and temperatures (1297–1329 °C) produced high-CaO (9.8–11.3 wt %) basalts with high Al 2 O 3 (16.8–17.6 wt %), Mg#s (66–70), MgO (8–10 wt %), Ni (103–262 μg/g), and Cr (284–609 μg/g). These high-CaO basalts contain olivine (Fo 87–91 ) phenocrysts with Cr-spinel inclusions ± subordinate plagioclase and crop out only in the southern part of the CLVF. (2) Partial melting of depleted sub-continental lithospheric mantle harzburgite at variable pressures (0.7–1.5 GPa) and temperatures (1097–1299 °C) produced a compositional continuum of med-K 2 O, calc-alkaline, high-MgO basalts through high-MgO andesites with high Mg#s (67–77), MgO (8–14 wt %) and high Ni and Cr abundances (154–439 and 340–1124 μg/g, respectively). Mineral assemblages are olivine (Fo 88–93 ) with Cr-spinel inclusions ± subordinate clinopyroxene, orthopyroxene and plagioclase. Small (<2.5 cm) mantle harzburgite xenoliths and mantle olivine xenocrysts are also found in several of these samples. These high-MgO basalts through andesites represent the largest volume of primitive compositions and have erupted predominantly along the main, fault-controlled northwest-southeast trending axis of volcanism with peripheral outcrops to the north, west, and east. (3) Partial melting of the Gorda eclogite slab edge produced adakitic silicic slab melts with strong depletion in the heavy rare earth elements (Yb = 0.6 μg/g). Subsequent reaction of those melts with depleted ultramafic rocks during ascent imprinted the adakitic dacites with high Mg#s (65–78) and elevated Ni (117–210 μg/g) and Cr (191–283 μg/g). Phenocrysts of orthopyroxene (En 87–94 ) with spinel inclusions (Cr# = 80–88) and extremely Ni-rich (9483 μg/g) olivine cores (Fo 84–93 ) record those reactions. Small-volume outcrops of the adakites on the eastern periphery of the CLVF track the passing slab edge. The trio of melting sources recorded by early CLVF magmatism reflect the tectonically complex environment and the hot (1097–1329 °C), shallow (0.7–1.5 GPa) melting conditions for these primitive compositions and provide estimates of the heat delivered to the crust. Over time, this flux led to maturation of the CLVF magmatic system toward the more voluminous and silicic volcanism that characterizes the balance of its subsequent volcanic history and maintains the present-day anomalously high heat flow in the region. The current interval (interval four) of volcanic activity at CLVF is characterized by low-volume, fault-controlled eruptions of basaltic andesite and andesite suggestive of mantle magma and heat delivery to the crust, similar to interval one. This analogous activity provides motivation for the current study and begs the question of whether the system is undergoing thermal priming for renewed silicic volcanism.

California↗
USGS2009

Petrogenesis of basaltic volcanic rocks from the Pribilof Islands, Alaska, by melting of metasomatically enriched depleted lithosphere, crystallization differentiation, and magma mixing

The Pribilof Islands, Alaska, are located in the Bering Sea in a continental intraplate setting. In this study we examine the petrology and geochemistry of volcanic rocks from St. Paul (0??54-0??003 Ma) and St. George (2??8-1??4 Ma) Islands, the two largest Pribilof Islands. Rocks from St. George can be divided into three groups: group 1 is a high-MgO, low-SiO. 2 suite composed primarily of basanites; group 2 is a high-MgO, high-SiO 2 suite consisting predominantly of alkali basalts; group 3 is an intermediate- to low-MgO suite that includes plagioclase-phyric subalkali basalts and hawaiites. Major and trace element geochemistry suggests that groups 1 and 2 formed by small-degree partial melting of amphibole-bearing to amphibole-free garnet peridotite. Group 1 rocks were the earliest melts produced from the most hydrous parts of the mantle, as they show the strongest geochemical signature of amphibole in their source. The suite of rocks from St. Paul ranges from 14??4 to 4??2 wt % MgO at relatively constant SiO 2 contents (43??1-47??3 wt %). The most primitive St. Paul rocks are modeled as mixtures between magmas with compositions similar to groups 1 and 2 from St. George Island, which subsequently fractionated olivine, clinopyroxene, and spinel to form more evolved rocks. Plagioclase-phyric group 3 rocks from St. George are modeled as mixtures between an evolved melt similar to the evolved magmas on St. Paul and a fractionated group 2 end-member from St. George. Mantle potential temperatures estimated for primitive basanites and alkali basalts are ???1400??C and are similar to those of mid-ocean ridge basalts (MORB). Similarly, 87 Sr/. 86 Sr and 143 Nd/. 144 Nd values for all rocks are MORB-like, in the range of 0??702704-0??703035 and 0??513026-0??513109, respectively. 208 Pb/. 204 Pb vs 206 Pb/. 204 Pb values lie near the MORB end-member but show a linear trend towards HIMU (high time-integrated 238 U/. 204 Pb). Despite isotopic similarities to MORB, many of the major and trace element characteristics are similar to those of ocean island basalts (OIB), including enrichment in alkalis and incompatible trace elements. These characteristics are interpreted to indicate that their mantle source experienced an ancient melt-removal event that is reflected in depleted radiogenic isotopic compositions and was then re-enriched by metasomatism that elevated incompatible trace element contents, but was too young to produce a time-integrated change in radiogenic isotopic ratios. Evidence suggests that the Pribilof Island basalts did not form in either a plume or a back-arc basin tectonic setting. Rather, they were produced by melting of metasomatically hydrated upper mantle peridotite at relatively low temperatures and were able to erupt at the surface through extensional or transtensional faults that served as conduits for the magmas. ?? The Author 2009. Published by Oxford University Press.

Journal of Petrology↗
USGS2016

Probing the volcanic-plutonic connection and the genesis of crystal-rich rhyolite in a deeply dissected supervolcano in the Nevada Great Basin: Source of the late Eocene Caetano Tuff

Late Cenozoic faulting and large-magnitude extension in the Great Basin of the western USA has created locally deep windows into the upper crust, permitting direct study of volcanic and plutonic rocks within individual calderas. The Caetano caldera in north–central Nevada, formed during the mid-Tertiary ignimbrite flare-up, offers one of the best exposed and most complete records of caldera magmatism. Integrating whole-rock geochemistry, mineral chemistry, isotope geochemistry and geochronology with field studies and geologic mapping, we define the petrologic evolution of the magmatic system that sourced the >1100 km 3 Caetano Tuff. The intra-caldera Caetano Tuff is up to ∼5 km thick, composed of crystal-rich (30–45 vol. %), high-silica rhyolite, overlain by a smaller volume of comparably crystal-rich, low-silica rhyolite. It defies classification as either a monotonous intermediate or crystal-poor zoned rhyolite, as commonly ascribed to ignimbrite eruptions. Crystallization modeling based on the observed mineralogy and major and trace element geochemistry demonstrates that the compositional zonation can be explained by liquid–cumulate evolution in the Caetano Tuff magma chamber, with the more evolved lower Caetano Tuff consisting of extracted liquids that continued to crystallize and mix in the upper part of the chamber following segregation from a cumulate-rich, and more heterogeneous, source mush. The latter is represented in the caldera stratigraphy by the less evolved upper Caetano Tuff. Whole-rock major, trace and rare earth element geochemistry, modal mineralogy and mineral chemistry, O, Sr, Nd and Pb isotope geochemistry, sanidine Ar–Ar geochronology, and zircon U–Pb geochronology and trace element geochemistry provide robust evidence that the voluminous caldera intrusions (Carico Lake pluton and Redrock Canyon porphyry) are genetically equivalent to the least evolved Caetano Tuff and formed from magma that remained in the lower chamber after ignimbrite eruption and caldera collapse. Thus, the Caetano Tuff contradicts models for the mutually exclusive origins of voluminous volcanic and plutonic magmas in the upper crust. Crystal-scale O isotope data indicate that the Caetano Tuff is one of the most 18 O-enriched rhyolites in the Great Basin (δ 18 O magma = 10·2 ± 0·2‰), supporting anatexis of local metasedimentary basement crust. Metapelite xenoliths in the Carico Lake pluton and ubiquitous xenocrystic zircons in the Caetano Tuff provide constraints for the anatexis process; these data point to shallow (<15 km) dehydration melting of a protolith similar to the Proterozoic McCoy Creek Group siliciclastic sediments in eastern Nevada, projected beneath Caetano in fault-stacked shelf sediments that were thickened during Mesozoic crustal shortening. Mean zircon U–Pb ages for different stratigraphic levels of the intra-caldera Caetano Tuff are 34·2–34·5 Ma, 0·2–0·5 Myr older than the caldera sanidine 40 Ar/ 39 Ar age of 34·00 ± 0·03 Ma, documenting protracted duration of assembly and homogenization of isotopically diverse upper crustal melts, followed by crystallization and zonation to generate the Caetano Tuff magma chamber. Sanidine rims in the least evolved Caetano Tuff and in the Carico Lake pluton and Redrock Canyon porphyry have sharply zoned Ba domains that point to crystal growth during magmatic recharge events. The recharge magma is inferred to have been compositionally similar to the Caetano Tuff magma, with increased Ba resulting from remelting of Ba-rich sanidine cumulates. Mush reactivation to generate the Caetano Tuff eruption was sufficiently rapid to preserve compositional gradients in the intracaldera ignimbrite, calling into question models that predict homogeneity as a prerequisite for remobilizing crystal-rich ignimbrite magmas.

Journal of Petrology↗
USGS1995

Sources of the Early Cretaceous plutons in the Turtle and West Riverside Mountains, California

Ages and initial isotopic ratios of Early Cretaceous (˜100 Ma) plutons of the Cordilleran Interior in the southern Turtle and West Riverside mountains distinguish them from Late Cretaceous plutons in surrounding ranges in the eastern Mojave Desert. Furthermore, the studied plutons have isotopic and geochemical characteristics more similar to plutons of Cretaceous age in the coastal batholiths (Peninsular Ranges and Sierra Nevada) than to most Mesozoic plutons in the Cordilleran Interior. The studied plutons are calcic, in contrast to the mostly cak-alkaline Mesozoic plutons of the eastern Mojave Desert. Distinctive isotopic signatures of the granitoids include lower initial 87 Sr/ 86 Sr of 0⋅705–0⋅710, δ 18 O of +6⋅3 to +7⋅7‰, 208 Pb/ 204 Pb of 38⋅3–39⋅5, and higher ε Nd of −3⋅86 to −9⋅60 than the Late Cretaceous plutons in the region. The distinctive characteristics of these Early Cretaceous plutons are probably both location and time specific and result from: (1) emplacement in a cold, untapped ‘Mojave-type’ Proterozoic upper crust, (2) a significant component of basaltic magmas partially melted from the asthenosphere or subcontinental lithosphere and (3) a magmatic component derived from Proterozoic, mafic, lower crust. They interacted less with their crustal hosts than did the later, more voluminous Late Cretaceous plutons.

California↗
USGS1997

Phengite-hosted LILE enrichment in eclogite and related rocks: Implications for fluid-mediated mass transfer in subduction zones and arc magma genesis

Geochemical differences between island arc basalts (LAB) and ocean-floor basalts (mid-ocean ridge basalts; MORB) suggest that the large-ion lithophile elements (LILE) K, Ba, Rb and Cs are probably mobilized in subduction zone fluids and melts. This study documents LILE enrichment of eclogite, amphibolite, and epidote ?? garnet blueschist tectonic blocks and related rocks from melanges of two subduction complexes. The samples are from six localities of the Franciscan Complex, California, and related terranes of Oregon and Baja California, and from the Samana Metamorphic Complex, Samana Peninsula, Dominican Republic. Most Franciscan blocks are MORB-like in their contents of rare earth elements (REE) and high field strength elements (HFSE); in contrast, most Samana blocks show an LAB signature of these elements. The whole-rock K2O contents of both groups range from 1 to 3 wt %; K, Ba, Rb, and Cs are all strongly intercorrelated. Many blocks display K/Ba similar to melasomatized transition zones and rinds at their outer margins. Some transition zones and rinds are enriched in LILE compared with host blocks; others are relatively depleted in these elements. Some LILE-rich blocks contain 'early' coarse-grained muscovite that is aligned in the foliation defined by coarse-grained omphacite or amphibole grains. Others display 'late' muscovite in veins and as a partial replacement of garnet; many contain both textural types. The muscovite is phengite that contains ???3??25-3??55 Si per 11 oxygens, and ???0??25-0??50 Mgper 11 oxygens. Lower-Si phengite has a significant paragonite component: Na per 11 oxygens ranges to ???0??12. Ba contents of phengite range to over 1 wt % (0??027 per 11 oxygens). Ba in phengite does not covary strongly with either Na or K. Ba contents of phengite increase from some blocks to their transition zones or rinds, or from blocks to their veins. Averaged KlBa ratios for phengite and host samples define an array which describes other subsamples of the block and other analyzed blocks. Phengite carries essentially all of the LILE in otherwise mafic eclogite, amphibolite, and garnet blueschist blocks that are enriched in these elements compared with MORE. It evidently tracks a distinctive type of LILE metasomatism that attends both high-T and retrograde subduction zone metamorphism. An obvious source for the LILE is a fluid in equilibrium with metasedimentary rocks. High-grade semipelitic schists from subduction complexes and subductable sediment display LILE values that resemble those seen in the most LILE-rich blocks. Modeling of Ba and Ti suggests that 1-40 wt % of phengite added to MORB can produce their observed LILE enrichment. Thus, the release of LILE from such rocks to fluids or melts in very high-T and -P parts of subduction zones probably depends critically on the stability and solubility relations of phengite, which is thought to be stable at pressures as high as 95-110 kbar at T= 750-1050??C.

Journal of Petrology↗
USGS1997

Comagmatic A-type granophyre and rhyolite from the Alid volcanic center, eritrea, northeast Africa

Granophyric blocks within late-Pleistocene pyroclastic flow ejecta from the Alid volcanic center, northeast Africa, are the rapidly crystallized, intrusive equivalent of pumice from the pyroclastic flow. Phenocryst compositions and geochemical characteristics of the pumice and granophyre are virtually identical. Silicate melt inclusions and other geochemical and geological constraints reveal those processes leading to development of the granophyric texture. Rhyolitic (A-type) magma with ∼2.6 wt % dissolved H 2 O and a temperature near 870°C was intruded to within 2–4 km of the surface, causing deformation and structural doming of shallow marine and subaerial strata. Eruptions of crystal-poor rhyolite from this shallow magma chamber caused degassing, which forced undercooling and consequent granophyric crystallization of some of the magma remaining in the intrusion. The most recent eruption from Alid excavated the crystallized granitic wall of the magma chamber, bringing the granophyric clasts to the surface.

Journal of Petrology↗
USGS1997

The role of diffusion-controlled oscillatory nucleation in the formation of line rock in pegmatite-aplite dikes

The George Ashley Block (GAB), located in the Pala Pegmatite District, San Diego County, California, is a composite pegmatite–aplite dike of 8 m thickness displaying striking mineralogical layering in the aplite portion of the dike, referred to as line rock. Rhythmic layering is characterized by garnet-rich bands alternating with albite–quartz–muscovite-rich bands. Cumulus textures are notably absent from the layered portion of the dike. Elongated quartz megacrysts are oriented perpendicular to the garnet-rich layers and poikilitically include garnet, albite, and muscovite. Calculated crystal-free magma viscosity with 3% H 2 O is 10 6.2 Pa s and the calculated settling velocity for garnet is 0.51 cm/year. Conductive cooling calculations based on emplacement of a 650°C dike into 150°C fractured gabbroic country rock at 1.5 kbar, and accounting for latent heat of crystallization, demonstrate that the line rock portion of the dike cools to 550°C in about 1 year. Crystal size distribution studies also suggest very rapid nucleation and crystallization. Diffusion-controlled gel crystallization experiments yield textures virtually identical to those observed in the layered aplite, including rhythmic banding, colloform layering, and band discontinuities. Thus, observed textures and calculated magmatic parameters suggest that mineralogical layering in the GAB results from an in situ diffusion-controlled process of oscillatory nucleation and crystallization. We propose that any event that promotes strong undercooling has the potential to initiate rapid heterogeneous nucleation and oscillatory crystal growth, leading to the development of a layer of excluded components in front of the crystallization front, and the formation of line rock.

Journal of Petrology↗
USGS1998

Magmagenesis at Soufriere volcano St Vincent, Lesser Antilles Arc

Soufriere volcano of St Vincent (<0.6 Ma) is composed of basalts and basaltic andesites, the most mafic of which ( mg -number 75) may be representative of the parental magmas of the calc-alkaline suites of the Lesser Antilles arc. Parental, possibly primary, magmas at Soufriere had MgO ∼12.5 wt % and were probably nepheline-normative. They last equilibrated with mantle at ∼17 kbar pressure, at temperatures of around 1130°C and f(O 2 ) exceeding FMQ (fayalite–magnetite–quartz) + 1. They fractionated, along several liquid lines of descent, through to basaltic andesites and rarer andesites over a range of crustal pressures (5–10 kbar) and temperatures (1000–1100°C), separating initially olivine + Cr-spinel + clinopyroxene + plagioclase ± titanomagnetite and then clinopyroxene + plagioclase + titanomagnetite + orthopyroxene assemblages. The total amount of crystallization was some 76 wt %. Amphibole was apparently not a fractionating phase. Sr and Nd isotopic and trace element systematics show no evidence for significant crustal assimilation. There is conflicting evidence as to the pre-eruptive water contents of Soufriere magmas; compositions of clinopyroxene phenocrysts and melt inclusions suggest H 2 O >3 wt %, whereas various projections onto phase diagrams are more consistent with relatively anhydrous magmas. Primary magmas at Soufriere were generated by around 15% melting of mid-ocean ridge basalt type mantle sources which had been modified by addition of fluids released from the slab containing contributions from subducted sediments and mafic crust.

Journal of Petrology↗
USGS1998

Petrochemistry of late miocene peraluminous silicic volcanic rocks from the Morococala field, Bolivia

Late Miocene peraluminous volcanic rocks of the Morococala field, Bolivia, define a layered stratigraphy of basal andalusite-, biotite- (± muscovite)-bearing rhyolite tuffs (AR), overlain by cordierite-, biotite-bearing rhyolite tuffs (CR), and capped by biotite-bearing quartz latite tuffs, lavas, and late domal flows (QL). Mineral and whole-rock compositions become more evolved from top to bottom, with differentiation reflected by decreasing Ca, Ba, Mg, Fe, and rare earth elements (REE) versus increasing F, Na/K, and aluminosity from QL to AR. Mineral, whole-rock, and glass inclusion compositions are consistent with derivation of all three rock types from a single stratified magma reservoir, but age and spatial relations between the three units make this unlikely. Genesis of the QL involved biotite-dehydration melting of an aluminous source at T > 750°C and P ≥ = 4–6 kbar. If not co-magmatic with QL, the other units were generated primarily by muscovite-dehydration melting at T = 730–750°C and P ≥ = 3.5–4.5 kbar for CR, and T ≤ = 750°C for AR with pre-eruptive residence at low pressure (1.5–3.0 kbar). Low hematite contents (X Hem ≤ = 0.06) of ilmenite grains in AR, CR, and early grains (as inclusions in plagioclase and sanidine cores) in QL indicate reduced conditions imposed by a graphite-bearing source. Compositional variability among texturally later oxides (ilmenite with X Hem = 0.06–0.50, primary magnetite), however, apparently records progressive increases in pre-eruptive f(O 2 ) in QL. Plagioclase-melt equilibria and electron microprobe analysis difference for quartz-hosted glass inclusions suggest pre-eruptive melt H 2 O contents ≥ = 5–7 wt % for the AR, ∼4–6 wt % for the CR, and ∼3–5 wt % for the QL.

Journal of Petrology↗
USGS1998

The petrogenesis of felsic calc-alkaline magmas from the southernmost Cascades, California: Origin by partial melting of basaltic lower crust

The majority of felsic rocks from composite centers in the southernmost Cascades have geochemical and Sr, Nd and Pb isotopic ratios that suggest derivation by partial melting of lower crust that is compositionally similar to calc-alkaline basalts observed in the region. Only a few felsic rocks have δ 18 O and Pb isotopic compositions that indicate interaction with the upper crust. Mineralogical and geochemical differences among the felsic magmas result primarily from melting under variable f (H 2 O) and temperature conditions. Partial melting under low f (H 2 O) and high temperature conditions leaves an amphibole-poor residuum, and produces magmas that have orthopyroxene as the most abundant ferromagnesian phenocryst, relatively low silica contents, and straight rare earth element patterns. Partial melting under higher f (H 2 O) and lower temperature conditions leaves an amphibole-rich residuum, and produces magmas that have amphibole ± biotite phenocrysts, relatively high silica contents, and pronounced middle rare earth element depletions. These conclusions are consistent with published thermal models that suggest that reasonable volumes of basaltic magma emplaced beneath large composite centers in the southernmost Cascades can serve as the heat source for melting of the lower crust. Melting of the lower crust under variable f (H 2 O) conditions is likely to result from differences in the H 2 O contents of these basaltic magmas.

Journal of Petrology↗
USGS1999

A complex magma mixing origin for rocks erupted in 1915, Lassen Peak, California

The eruption of Lassen Peak in May 1915 produced four volcanic rock types within 3 days, and in the following order: (1) hybrid black dacite lava containing (2) undercooled andesitic inclusions, (3) compositionally banded pumice with dark andesite and light dacite bands, and (4) unbanded light dacite. All types represent stages of a complex mixing process between basaltic andesite and dacite that was interrupted by the eruption. They contain disequilibrium phenocryst assemblages characterized by the coexistence of magnesian olivine and quartz and by reacted and unreacted phenocrysts derived from the dacite. The petrography and crystal chemistry of the phenocrysts and the variation in rock compositions indicate that basaltic andesite intruded dacite magma and partially hybridized with it. Phenocrysts from the dacite magma were reacted. Cooling, crystallization, and vesiculation of the hybrid andesite magma converted it to a layer of mafic foam. The decreased density of the andesite magma destabilized and disrupted the foam. Blobs of foam rose into and were further cooled by the overlying dacite magma, forming the andesitic inclusions. Disaggregation of andesitic inclusions in the host dacite produced the black dacite and light dacite magmas. Formation of foam was a dynamic process. Removal of foam propagated the foam layer downward into the hybrid andesite magma. Eventually the thermal and compositional contrasts between the hybrid andesite and black dacite magmas were reduced. Then, they mixed directly, forming the dark andesite magma. About 40–50% andesitic inclusions were disaggregated into the host dacite to produce the hybrid black dacite. Thus, disaggregation of inclusions into small fragments and individual crystals can be an efficient magma-mixing process. Disaggregation of undercooled inclusions carrying reacted host-magma phenocrysts produces coexisting reacted and unreacted phenocryst populations.

Journal of Petrology↗
USGS1999

The chemical and isotopic differentiation of an epizonal magma body: Organ Needle pluton, New Mexico

Major and trace element, and Nd and Sr isotopic compositions of whole rocks and mineral separates from the Oligocene, alkaline Organ Needle pluton (ONP), southern New Mexico, constrain models for the differentiation of the magma body parental to this compositionally zoned and layered epizonal intrusive body. The data reveal that the pluton is rimmed by lower ε Nd (∼ −5) and higher 87 Sr/ 86 Sr (∼0.7085) syenitic rocks than those in its interior (ε Nd ∼ −2, 87 Sr/ 86 Sr ∼0.7060) and that the bulk compositions of the marginal rocks become more felsic with decreasing structural depth. At the deepest exposed levels of the pluton, the ε Nd ∼ −5 lithology is a compositionally heterogeneous inequigranular syenite. Modal, compositional and isotopic data from separates of rare earth element (REE)-bearing major and accessory mineral phases (hornblende, titanite, apatite, zircon) demonstrate that this decoupling of trace and major elements in the inequigranular syenite results from accumulation of light REE (LREE)-bearing minerals that were evidently separated from silicic magmas as the latter rose along the sides of the magma chamber. Chemical and isotopic data for microgranular mafic enclaves, as well as for restite xenoliths of Precambrian granite wall rock, indicate that the isotopic distinction between the marginal and interior facies of the ONP probably reflects assimilation of the wall rock by ε Nd ∼ −2 mafic magmas near the base of the magma system. Fractional crystallization and crystal–liquid separation of the crustally contaminated magma at the base and along the margins of the chamber generated the highly silicic magmas that ultimately pooled at the chamber top.

Journal of Petrology↗
USGS1999

Magmatic interactions as recorded in plagioclase phenocrysts of Chaos Crags, Lassen Volcanic Center, California

The silicic lava domes of Chaos Crags in Lassen Volcanic National Park contain a suite of variably quenched, hybrid basaltic andesite magmatic inclusions. The inclusions represent thorough mixing between rhyodacite and basalt recharge liquids accompanied by some mechanical disaggregation of the inclusions resulting in crystals mixing into the rhyodacite host preserved by quenching on dome emplacement. 87Sr/86Sr ratios (~0.7037-0.7038) of the inclusions are distinctly lower than those of the host rhyodacite (~0.704-0.7041), which are used to fingerprint the origin of mineral components and to monitor the mixing and mingling process. Chemical, isotopic, and textural characteristics indicate that the inclusions are hybrid magmas formed from the mixing and undercooling of recharge basaltic magma with rhyodacitic magma. All the host magma phenocrysts (biotite, plagioclase, hornblende and quartz crystals) also occur in the inclusions, where they are rimmed by reaction products. Compositional and strontium isotopic data from cores of unresorbed plagioclase crystals in the host rhyodacite, partially resorbed plagioclase crystals enclosed within basaltic andesite inclusions, and partially resorbed plagioclase crystals in the rhyodacitic host are all similar. Rim 87Sr/86Sr ratios of the partially resorbed plagioclase crystals in both inclusions and host are lower and close to those of the whole-rock hybrid basaltic andesite values. This observation indicates that some crystals originally crystallized in the silicic host, were partially resorbed and subsequently overgrown in the hybrid basaltic andesite magma, and then some of these partially resorbed plagioclase crystals were recycled back into the host rhyodacite. Textural evidence, in the form of sieve zones and major dissolution boundaries of the resorbed plagioclase crystals, indicates immersion of crystals into a hotter, more calcic magma. The occurrence of partially resorbed plagioclase together with plagioclase microlites and olivine crystals reflects disaggregation of inclusions and mingling of this material into the silicic host. These processes are commonplace in some orogenic magma systems and may be elucidated by isotopic microsampling and analysis of the plagioclases crystallizing from them.

Journal of Petrology↗