Wall-rock Assimilation Research Articles

Compositional evolution of magma from Parícutin Volcano, Mexico: The tephra record

The birth of Parícutin Volcano, Mexico, in 1943 provides an unprecedented opportunity to document the development of a monogenetic cinder cone and its associated lava flows and tephra blanket. Three ‘type’ sections provide a complete tephra record for the eruption, which is placed in a temporal framework by comparing both bulk tephra and olivine phenocryst compositions to dated samples of lava and tephra. Our data support the hypothesis of Luhr (2001) that the first four months of activity were fed by a magma batch (Phase 1) that was distinct from the magma that supplied the subsequent eight years of activity. We further suggest that the earliest erupted (vanguard) magma records evidence of temporary residence at shallow levels prior to eruption, suggesting early development of a dike and sill complex beneath the vent. Depletion of this early batch led to diminished eruptive activity in June and July of 1943, while arrival of the second magma batch (Phase 2) reinvigorated activity in late July. Phase 2 fed explosive activity from mid-1943 through 1946, although most of the tephra was deposited by the end of 1945. Phase 3 of the eruption began in mid-1947 with rapid evolution of magma compositions from basaltic andesite to andesite and dominance of lava effusion. The combined physical and chemical characteristics of the erupted material present a new interpretation of the physical conditions that led to compositional evolution of the magma. We believe that syn-eruptive assimilation of wall rock in a shallow complex of dikes and sills is more likely than pre-eruptive assimilation within a large magma chamber, as previously assumed. We further suggest that waning rates of magma supply from the deep feeder system allowed evolved, shallowly stored magma to enter the conduit in 1947, thus triggering the rapid observed change in the erupted magma composition. This physical model predicts that assimilation should be observable in other monogenetic eruptions, particularly those with low pressure melt inclusions and with eruption durations of months to years.

Platinum-group elements geochemistry in podiform chromitites and associated peridotites of the Mawat ophiolite, northeastern Iraq

Small lensoidal bodies of massive and disseminated chromitites have been examined in association with ultramafic rocks of the Mawat ophiolite complex (MOC), Iraqi Zagros Thrust Zone, northeastern Iraq. The chromitites are surrounded by dunite envelopes of variable thickness, exhibiting transitional boundaries to harzburgitic host rocks. The primary chromite composition exhibits high Cr varieties; the average Cr# of chromite is 0.73, and have <0.2% TiO 2 content, which may reflect the crystallization of chromite from boninitic magma. Partial melting in the upper mantle and assimilation of wall rocks by primitive melt might have played an essential role in the chromitite formation in the studied area. Unusually high platinum-group elements contents of up to 1094 ppb have been reported for the first time in these chromitites pods. Chondrite normalized patterns of platinum-group elements are typical for ophiolitic chromitite, with enrichment of the IPGE, and depletion in PPGE indicating a high degree of partial melting of the mantle source. The PGE diversity between different chromitite pods in the MOC may be attributed to the differences in chemistry of the magma involving in the chromitite formation. The mineral chemistry data and PGE geochemistry, indicates that the Mawat ophiolite was generated from an arc-related magma above a supra-subduction zone setting.

Lava shields and fissure eruptions of the Western Volcanic Zone, Iceland: Evidence for magma chambers and crustal interaction

Volcanic eruptions in Iceland occur either from fissures or central vents (lava shields). Within the post-glacial Western Volcanic Zone, the Thjófahraun fissure-fed lava field and Lambahraun lava shield were both erupted ~ 4000 yrs B.P. with eruptive centers separated by only ~ 25 km. Thjófahraun erupted ~ 1 km 3 of pāhoehoe and 'a'ā lava from a 9-km long fissure, whereas the Lambahraun lava shield erupted > 7 km 3 of low effusion-rate pāhoehoe. Thjófahraun lavas contain higher K, Rb, Y and Zr, and lower CaO than Lambahraun lavas at the same MgO, with variations broadly consistent with evolution by low-pressure crystal fractionation. Lambahraun spans a larger range of MgO, which generally decreases over time during the eruption. Lambahraun samples with high Al 2O 3 and low TiO 2 and FeO likely reflect up to 15% plagioclase accumulation. In addition, all samples from Lambahraun exhibit increasing CaO and Nb/Zr with decreasing MgO and overall incompatible-element enrichments greater than predicted by crystal fractionation alone. Although the increase in Nb/Zr and other incompatible elements could be explained by gradually more incompatible-element enriched parental magma being supplied to the magmatic system during the course of the Lambahraun eruption, this process requires very small-scale trace element heterogeneities in the mantle that are apparently decoupled from isotopic variations and a systematic relationship between parental magma composition and extent of differentiation. Alternatively, correlations among incompatible element concentration, increasing differentiation and time during the eruption can be related by concurrent wallrock assimilation and crystallization during melt migration through the crust. Geochemical modeling of assimilation of wallrock clinopyroxene concurrent with crystallization of olivine (± plagioclase) effectively reproduces the observed chemical variations of Lambahraun samples. Similar chemical characteristics exist in several other Western Volcanic Zone lava shields but not in fissure eruptions. Magmas that fed fissure eruptions may also have been modified by interaction with the crust prior to aggregation in crustal magma chambers, but the geochemical signature of this process is obscured by magma mixing. In contrast, Icelandic lava shields that preserve deeper level processes may not have developed shallow magma chambers; rather they probably represent slow effusion from magma systems that are continually being recharged and reacting with the crust during the course of their eruptions.

Scale of pluton/wall rock interaction near May Lake, Yosemite National Park, CA, USA

Interaction of magma with wall rock is an important process in igneous petrology, but the mechanisms by which interactions occur are poorly known. The western outer granodiorite of the Cretaceous Tuolumne Intrusive Suite of Yosemite National Park, California, intruded a variety of metasedimentary and igneous wall rocks at 93.1 Ma. The May Lake metamorphic screen is a metasedimentary remnant whose contact zone exhibits a variety of interaction phenomena including xenolith incorporation, disaggregation, and partial melting. The chemical contrast of these metasedimentary rocks with the invading pluton provides an excellent measure of pluton/wall rock interactions. Wall rock xenoliths (mostly pelitic quartzite) are predominantly located in an elongate horizon surrounded by a hybridized fine-grained granodiorite. Initial Sr and Nd isotopic ratios of the hybridized granodiorite indicate significant local incorporation of crustal material. Major- and trace-element geochemical data indicate that contamination of the granodiorite occurred via selective assimilation of both high-K and low-K, high-silica partial melts derived from pelitic quartzite. Although the hybridized granodiorite shows significant amounts of contamination, adjacent to xenoliths the proportion of contamination is undetectable more than a meter away. These results indicate that the chemical and isotopic variability of the Tuolumne Intrusive Suite is not caused by magma contamination via in situ wall rock assimilation.

Magmatic processes of Unzen volcano revealed by excess argon distribution in zero-age plagioclase phenocrysts

Excess argon ( 40Ar) is one of the most important problems in K–Ar geochronometry, because it renders incorrect the simple assumption that all initial argon in rock samples is derived from the atmosphere. Using the laser-microprobe analytical method we investigate and reveal the source and trapping site of excess argon in zero-age plagioclase phenocrysts extracted from the lavas erupted during 1990–1995 activity of the Unzen volcano, Kyushu, Japan. By combining the results of laser-microprobe analysis and conventional stepwise heating of separated plagioclase phenocrysts, we reveal that the majority of the excess argon is retained in melt inclusions in the dusty zone of the phenocrysts. We propose a model for the isotopic evolution of argon in the phenocryst-bearing magma of the Unzen volcano by taking into consideration the noble gas data from Unzen volcanic gases and the chemical composition of the melt inclusions in the phenocrysts. In our model, plagioclase phenocrysts crystallize from, or are captured by a low-temperature ( T) rhyodacitic magma. This magma has a comparatively high 40Ar/ 36Ar ratio for an arc magma due to assimilation of old Cretaceous granitic wall rock. Mantle-derived argon is replenished in the rhyodacitic magma over the life of the volcano when the magma chamber is recharged at intervals by high-T juvenile magma. The plagioclase phenocrysts crystallize rapidly, trapping excess argon ( 40Ar/ 36Ar > 970) during successive dissolution/crystallization events caused by intrusion of the high-T magma or water-rich fluids. The phenocryst-bearing mixed magma ascends at a relatively rapid rate following a final injection of the high-T magma, and interacts with shallow groundwater causing the 40Ar/ 36Ar ratio of the magma to decrease to the atmospheric ratio. Contamination of the phenocrysts by atmospheric argon in the surrounding magma is enhanced by rapid “short-circuit” diffusion in which dislocations and/or micro-cracks in the plagioclase crystals are used for argon transport. The remnant magma has been continuously degassing for a decade since the recent (1990–1995) eruptive activity, without significant change to 3He/ 4He and 40Ar/ 36Ar ratios.

Terrestrial Oxygen Isotope Variations and Their Implications for Planetary Lithospheres

Research Article| January 01, 2008 Terrestrial Oxygen Isotope Variations and Their Implications for Planetary Lithospheres Robert E. Criss Robert E. Criss Washington University St. Louis, Missouri 63130, U.S.A., criss@wustl.edu Search for other works by this author on: GSW Google Scholar Author and Article Information Robert E. Criss Washington University St. Louis, Missouri 63130, U.S.A., criss@wustl.edu Publisher: Mineralogical Society of America First Online: 03 Mar 2017 © The Mineralogical Society Of America Reviews in Mineralogy and Geochemistry (2008) 68 (1): 511–526. https://doi.org/10.2138/rmg.2008.68.18 Article history First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert E. Criss; Terrestrial Oxygen Isotope Variations and Their Implications for Planetary Lithospheres. Reviews in Mineralogy and Geochemistry 2008;; 68 (1): 511–526. doi: https://doi.org/10.2138/rmg.2008.68.18 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search Abstract Oxygen isotopes provide key data on the formation and evolution of the Solar System, particularly for Earth and other rocky bodies that are principally constituted of oxygen-rich phases. The dynamic interactions between Earth’s lithosphere, hydrosphere, biosphere and atmosphere are elucidated by a wealth of oxygen isotope data, that in turn provide a template for predicting the types of variations that occur on other planets. These isotopic variations are basically controlled by two master effects. First, isotopic fractionations among coexisting phases tend to be greatest at low temperatures, and are also large when they involve interactions between solids or liquids and gaseous phases. Second, material balance effects require the 18O contents of the most abundant phases to be close to the planetary average, and restrict extreme isotopic enrichments or depletions to minor phases. These dual controls of fractionation and material balance work in concert with practically any other dynamic process to foster the upward enrichment and increased heterogeneity of 18O contents on Earth, and, by implication, in planetary lithospheres in general. Upward 18O enrichment in planetary lithospheres is promoted by the tendency for the least dense silicate and oxide minerals to concentrate 18O relative to more dense minerals, and also by convective transfer, fractional crystallization, hydrothermal alteration, and other processes. Earth’s continents and the uppermost parts of its ocean floor consequently have higher 18O contents than average upper mantle material, and normal sediments, carbon dioxide and oxygen gas are all very rich in 18O. Heterogeneity is evident in the large ranges of δ18O values observed in sediments (at least +8 to +38‰) and terrestrial igneous rocks (at least −10.5 to +16‰), and in the even larger ranges in natural waters and atmospheric gases, all representing materials that constitute, or interacted with, oxygen reservoirs near Earth’s surface. Key identified processes include weathering, precipitation from water, phase changes, exchange with infiltrating hydrothermal fluids, and assimilation of wallrocks having disparate compositions. All such processes that promote the upward enrichment and heterogeneity of 18O complicate the deduction of the bulk planetary contents of this isotope. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

About this title ‐ Dynamics of Crustal Magma Transfer, Storage and Differentiation

Magmas are subject to a series of processes that lead to their differentiation during transfer through, and storage within, the Earth's crust. The depths and mechanisms of differentiation, the crustal contribution to magma generation through wall-rock assimilation, the rates and timescales of magma generation, transfer and storage, and how these link to the thermal state of the crust are subject to vivid debate and controversy. This volume presents a collection of research articles that provide a balanced overview of the diverse approaches available to elucidate these topics, and includes both theoretical models and case studies. By integrating petrological, geochemical and geophysical approaches, it offers new insights to the subject of magmatic processes operating within the Earth's crust, and reveals important links between subsurface processes and volcanism.

Introduction to Minerals, Inclusions and Volcanic Processes

Minerals are intrinsically resistant to the processes that homogenize silicate liquids—their compositions thus yield an archive of volcanic and magmatic processes that are invisible at the whole rock scale. Minerals and their inclusions record diverse magma compositions, the depths and temperatures of magma storage, the nature of open system processes, and the rates at which magmas ascend. The potential for understanding volcanic systems through minerals and their inclusions has long been recognized (Sorby 1858). Sorby’s (1863) study of James Hall’s reversal experiments helped resolve the “basalt controversy” in favor of a volcanic origin, while Zirkel’s (1863) discovery of quartz within a volcanic rock helped tip the balance in favor of a magmatic origin for granite (Young 2003). Studies of phenocrysts have also long illustrated the importance of wall rock assimilation and magma mixing (e.g., Fenner 1926; Finch and Anderson 1930; Larson et al. 1938), and the potential for geothermometry (Barth 1934). Darwin’s (1844) mineralogical field-studies in the Galapagos archipelago, followed by King’s (1878) studies at Hawaii, also inaugurated the establishment of fractional crystallization as an important evolutionary process (Becker 1897; Bowen 1915). Recent advances in micro-analytical techniques open a new realm of detail, building upon a long history of mineralogical research; this volume summarizes some of this progress. Our summary focuses on volcanologic and magmatic processes, but the methods reviewed here extend well beyond terrestrial applications. Samples from the Stardust return mission, for example, show that olivine, plagioclase and pyroxene pervade the solar system (Brownlee et al. 2006)—while the topics covered here surely apply to all terrestrial-like planetary bodies, relevance may extend to a cosmic scale. Our more modest hope is that this volume will aid the study of disparate fields of terrestrial igneous systems, and perhaps provide a catalyst for new collaborations and integrated studies. …

ENDOGENOUS AND EXOGENOUS PLUTONS: THE INFLUENCE OF EMPLACEMENT STYLE ON CONTAMINATION OF GRANITIC MAGMA

Whether granitic magmas can detach and digest fragments of wallrock or mingle with other magmas depends in large part on the style of magma emplacement. Many composite plutons grow by relatively small pulses of magma, some of which may reach their solidus before injection of the next pulse. Composite “nested” plutons may grow as new pulses of magma invade and displace their predecessors, or “sheeted” plutons may grow as new magma follows contacts between wallrock and older intrusions. Contrary to a common assumption, the innermost parts of a composite intrusion are not necessarily the last to be emplaced or the most fractionated. Emplacement kinematics of some plutons and lava domes are similar. Lava domes grow in two ways. Endogenous domes inflate as new lava ascends and ponds within a cooler lava crust, and exogenous domes accrete as new lava ascends, breaches the lava crust, and spreads over the exterior. Exogenous growth of sheeted plutons offers less opportunity for magma mingling and mixing, but allows more wallrock assimilation (contamination) than endogenous growth of nested plutons.

Lunar domes in the Doppelmayer region: Spectrophotometry, morphometry, rheology, and eruption conditions

In this study we examine a lunar volcanic region near Doppelmayer, composed of two domical structures previously not studied in detail and a well-known lunar pyroclastic deposit. Dome 1 is situated at selenographic coordinates 41 . 92 ∘ W and 30 . 08 ∘ S , dome 2 at 43 . 42 ∘ W and 30 . 66 ∘ S . We perform a spectrophotometric study of representative locations of the volcanic region based on Clementine UVVIS data. Relying on ground-based high-resolution CCD imagery, we furthermore examine the morphometric characteristics of the two domes, making use of a combined photoclinometry and shape from shading technique. Based on a rheologic model, we examine the physical conditions under which the domes were formed. Dome 1 is spectrally atypically red for mare domes and shows a very weak mafic absorption, implying a low TiO 2 and FeO content. The overall spectral signature corresponds to that of a mixture between mare and highland soils. Dome 1 was formed of lava with a relatively high viscosity value of ∼ 10 7 Pa s , situated between the ranges of values typically observed for mare domes and for the Gruithuisen and Mairan highland domes, respectively, erupting at moderate rates over a long period of time. Dome 1 is larger, steeper, and more voluminous than typical mare domes but has a much lower flank slope than the Gruithuisen and Mairan highland domes. It is an exemplar of a rare type of unusually steep and voluminous mare domes, similar to the well-known mare domes Hortensius 5 and 6 and Herodotus ω . We discuss the relevance of vertical (assimilation of crustal material) vs. lateral (distribution of material across mare-highland boundaries by random impacts) mixing mechanisms as being responsible for the observed spectral appearance of dome 1. The thermal conditions in the lunar interior did not favour the assimilation of crustal wallrock into the ascending magma. Due to the fact that dome 1 is located right on the boundary between hummocky terrain and a mare pond, lateral mixing of mare and highland soils is a much more natural explanation for the observed spectral signature. For dome 2, we find that it is a typical effusive mare dome, given its spectral and morphometric properties and inferred rheologic parameters. An estimation of the dimensions of the feeder dikes of the two domes reveals that the dike which formed dome 1 was five times as broad as the one which formed dome 2, while the dike lengths only differ by about one third. The dike dimensions suggest that their source regions were located below the lunar crust.

Plume–lithosphere interaction beneath Mt. Cameroon volcano, West Africa: Constraints from 238U– 230Th– 226Ra and Sr–Nd–Pb isotope systematics

Precise measurements of 238U– 230Th– 226Ra disequilibria in lavas erupted within the last 100 yr on Mt. Cameroon are presented, together with major and trace elements, and Sr–Nd–Pb isotope ratios, to unravel the source and processes of basaltic magmatism at intraplate tectonic settings. All samples possess 238U– 230Th– 226Ra disequilibria with 230Th (18–24%) and 226Ra (9–21%) excesses, and there exists a positive correlation in a ( 226Ra/ 230Th)–( 230Th/ 238U) diagram. The extent of 238U– 230Th– 226Ra disequilibria is markedly different in lavas of individual eruption ages, although the ( 230Th/ 232Th) ratio is constant irrespective of eruption age. When U-series results are combined with Pb isotope ratios, negative correlations are observed in the ( 230Th/ 238U)–( 206Pb/ 204Pb) and ( 226Ra/ 230Th)–( 206Pb/ 204Pb) diagrams. Shallow magma chamber processes like magma mixing, fractional crystallization and wall rock assimilation do not account for the correlations. Crustal contamination is not the cause of the observed isotopic variations because continental crust is considered to have extremely different Pb isotope compositions and U/Th ratios. Melting of a chemically heterogeneous mantle might explain the Mt. Cameroon data, but dynamic melting under conditions of high D U and D U/ D Th, long magma ascent time, or disequilibrium mineral/melt partitioning, is required. The most plausible scenario to produce the geochemical characteristics of Mt. Cameroon samples is the interaction of melt derived from the asthenospheric mantle with overlying sub-continental lithospheric mantle which has elevated U/Pb (>0.75) and Pb isotope ratios ( 206Pb/ 204Pb > 20.47) due to late Mesozoic metasomatism.

Tracing magma mixing in granite genesis: in situ U–Pb dating and Hf-isotope analysis of zircons

In situ zircon U–Pb and Hf-isotopic data have been determined for mafic microgranular enclaves and host granitoids from the Early Cretaceous Gudaoling batholith in the Liaodong Peninsula, NE China, in order to constrain the sources and petrogenesis of granites. The zircon U–Pb age of the enclaves (120 ± 1 Ma) is identical to that of the host monzogranite (120 ± 1 Ma), establishing that the mafic and felsic magmas were coeval. The Hf isotopic composition of the enclaves [e Hf(t) = +4.5 to −6.2] is distinct from the host monzogranite [e Hf(t) = −15.1 to −25.4], indicating that both depleted mantle and crustal sources contributed to their origin. The depleted mantle component was not previously revealed by geochemical and Nd and Sr isotopic studies, showing that zircon Hf isotopic data can be a powerful geochemical tracer with the potential to provide unique petrogenetic information. Some wall-rock contamination is indicated by inherited zircons with considerably older U–Pb ages and low initial Hf isotopic compositions. Hafnium isotopic variations in Early Cretaceous zircons rule-out simple crystal–liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, mixing of mantle-derived mafic magmas with crustal-derived felsic magmas, coupled with assimilation of wall rocks, is compatible with the data.

Is stoping a volumetrically significant pluton emplacement process?

Magmatic stoping is widely called upon to explain discordant pluton contacts, but several lines of evidence indicate that stoping is not volumetrically significant in the emplacement of most plutons. First, xenoliths rarely make up more than a few percent of most plutons; this sparseness, even on well-exposed pluton floors, argues that few xenoliths make it into plutons. Second, piles of rock fragments have sufficiently high porosities (∼50%) that a significant accumulation of stoped blocks on the floor of a magma chamber would trap much, if not all, of the available magma, producing pluton-scale magmatic breccias that are not observed. Third, although settling calculations show that large (meter-scale or larger) xenoliths could sink rapidly in magma, natural fragmentation processes are likely to produce a fractal distribution of particle sizes, with small particles far more abundant than large ones. The absence of small xenoliths at pluton contacts, where magma cools most rapidly, thus argues against stoping. Fourth, although contamination of magmas by anatectic liquids derived from wall rocks and xenoliths is common, geochemical data from many plutons clearly rule out significant bulk assimilation of local wall rocks. Finally, there is growing evidence that at least some large plutons never were large molten bodies of magma but grew incrementally. Stoping cannot be volumetrically important in incrementally emplaced plutons, because blocks can only fall through the volume that is molten at any given time. Where xenoliths are abundant in plutons, they may commonly represent isolation of wall-rock bodies between successive intrusive increments rather than wall-rock fragments that were engulfed by magma. However, even xenoliths that were engulfed by magma did not necessarily sink from the roof of a magma chamber. Although stoping undoubtedly occurs during the emplacement of some plutons, it is unlikely to be a volumetrically significant process.

A geochemical approach to model periodically replenished magma chambers: Does oscillatory supply account for the magmatic evolution of EPR 17–19°S?

We propose a new approach to model the geochemical evolution of continuously replenished and tapped steady-state magma chambers. We use a sinusoidal function to model cyclic magma supply. The temporal evolution of a reservoir is described using differential equations, in which the amount of refilling magma does not depend on the size of the chamber. These equations can be used to calculate incompatible trace element concentrations and magma quantities. We examine the geochemical consequences of episodic injections, noises and wall-rock assimilation. We also explore possible variations in crystallization rate. To show its potential, the theoretical treatment has been applied to the EPR 17–19°S, a site with a strong magma budget which has been the subject of several geological/geophysical studies. The practical application requires geological parameters to be constrained, as well as the extreme values of the lava concentration range. A first step specifies the incompatible trace element composition of the replenishing melt, which corresponds in the EPR case to a magnesian liquid (MgO = 9.5 wt%). It is then possible to determine other parameters such as cycle period (∼750 years), magma residence time (∼300 years), and reservoir size (from 4.1 to 8.6 km 3 per 20 km segment). Lastly, variations in crystallization rate do not significantly alter the results.

Amphibole fractionation—A constraint on the depth of arc magma evolution?

We have attempted to constrain the depth(s) at which the main differentiation processes in arc lavas occur, using two main approaches; REE characteristics. The REE patterns of volcanic rock suites produced by crystal-liquid differentiation processes are sensitive to garnet or amphibole fractionation at deep or intermediate levels, respectively. Not surprisingly La/Yb increases with differentiation (SiO2) in most of the volcanic suites we have studied. La/Yb increases are, however, not very great and cannot accommodate significant garnet fractionation. Perhaps more surprising, Dy/ Yb decreases with SiO2, which is consistent with amphibole fractionation. Garnet fractionation would overprint this effect. Gabbro (ol-cpx-plag) fractionation has limited effect on both La/Yb and Dy/Yb. Thus, it appears that a significant amount of differentiation occurs at intermediate crustal depths where amphibole is stable. Most of the suites studied do not contain modal amphibole. If it is responsible for the REE characteristics observed, amphibole fractionation is cryptic—consistent with the common occurrence of the mineral in cumulate inclusions if not in lavas. Ra-Th isotope systematics. The timescale of magma differentiation informs on the depth where it occurs: schematically, the deeper, the slower. Decreases in (Ra/Th) with increasing differentiation at arc volcanoes such as Mt St. Helens indicate timescales of differentiation of the order of a few thousand years. Moreover, modeling shows that if wall rock assimilation occurs, this timescale is significantly reduced, to times as low as a few hundred years. Thus if assimilation and concomitant crystallisation is important, it must happen quickly—which seems unlikely in the deep crust where high temperatures ought to make crystallisation rates slow. Interestingly amphibole fractionation may itself be responsible for some of the decrease in (Ra/Th) observed with differentiation, which would also tend to reduce inferred timescales.

Petrogenesis and tectonic setting of the peralkaline Pine Canyon caldera, Trans-Pecos Texas, USA

The Pine Canyon caldera is a small (6–7 km diameter) ash-flow caldera that erupted peralkaline quartz trachyte, rhyolite, and high-silica rhyolite lavas and ash-flow tuffs about 33–32 Ma. The Pine Canyon caldera is located in Big Bend National Park, Texas, USA, in the southern part of the Trans-Pecos Magmatic Province (TPMP). The eruptive products of the Pine Canyon caldera are assigned to the South Rim Formation, which represents the silicic end member of a bimodal suite (with a “Daly Gap” between 57 and 62 wt.% SiO 2); the mafic end member consists primarily of alkali basalt to mugearite lavas of the 34–30 Ma Bee Mountain Basalt. Approximately 60–70% crystallization of plagioclase, clinopyroxene, olivine, magnetite, and apatite from alkali basalt coupled with assimilation of shale wall rock ( M a/ M c = 0.3–0.4) produced the quartz trachyte magma. Variation within the quartz trachyte–rhyolite suite was the result of ∼70% fractional crystallization of an assemblage dominated by alkali feldspar with subordinate clinopyroxene, fayalite, ilmenite, and apatite. High-silica rhyolite is not cogenetic with the quartz trachyte–rhyolite suite, and can be best explained as the result of ∼ 5% partial melting of a mafic granulite in the deep crust under the fluxing influence of fluorine. Variation within the high-silica rhyolite is most likely due to fractional crystallization of alkali feldspar, quartz, magnetite, biotite, and monazite. Lavas and tuffs of the South Rim Formation form A-type rhyolite suites, and are broadly similar to rock series described in anorogenic settings both in terms of petrology and petrogenesis. The Pine Canyon caldera is interpreted to have developed in a post-orogenic tectonic setting, or an early stage of continental rifting, and represents the earliest evidence for continental extension in the TPMP.

Layered mafic sill complex beneath the eastern Snake River Plain: Evidence from cyclic geochemical variations in basalt

The eastern Snake River Plain in southern Idaho, western United States, is characterized by 1–2 km of Pleistocene to late Pliocene basalt overlying rhyolite caldera complexes. Cyclic variations in the chemical composition of basalts from 1136 m of scientific drill core show that the parent magmas of these lavas evolved by crystal fractionation at shallow to intermediate crustal depths, punctuated by episodic recharge with more primitive compositions and assimilation of adjacent wall rock. We have identified 10 upward fractionation cycles and four reversed cycles; assimilation of sialic crust was limited and mainly affects the oldest basalts, which directly overlie rhyolites. We infer that the crystal fractionation and/or recharge cycles took place in a series of sill-like intrusions at intermediate crustal depths that now form a layered mafic intrusion that underlies the eastern Snake River Plain at depth. This layered sill complex is represented by the ∼10-km-thick “basaltic sill” that has been imaged seismically at ∼12–22 km depth. The association of this mid-crustal sill complex with geochemical fractionation cycles in basalt supports the concept that exposed layered mafic intrusions may be linked to overlying basalt provinces that have since been removed by erosion.

A 2.5 ka History of Dacitic Magmatism at Nevado de Toluca, Mexico: Petrological, 40Ar/39Ar Dating, and Experimental Constraints on Petrogenesis

After 11 5 ka of quiescence (24 5–13 ka), the Nevado de Toluca volcano started a 2500 year period of activity. This period was characterized by a dome destruction event at 13 ka, a small Plinian event at 12 1 ka, and a large Plinian eruption at 10 5 ka. About 10 km of magma was erupted that was homogeneous in composition (63 3–65 7 SiO2 wt % whole-rock) and in mineralogy. Pumice consists of plagioclase (An30–59) > orthopyroxene (En56–59) > hornblende Fe–Ti oxides þ rare apatite (in opx) þ biotite, set in a rhyolitic matrix (72–76 SiO2 wt %). Ar/Ar analysis of single biotite crystals yielded ages (0 81–4 7 Ma), that do not correspond to eruption ages. The biotite represents partially assimilated xenocrysts, which could have resided in the magma for only a short period of time. Mineral chemical data, coupled with hydrothermal experiments, indicate that prior to eruption the dacitic magma stagnated at a depth of 4 5–6 km below the summit at water pressures of 160–210 MPa and a temperature of 824 12 C on the basis of Fe–Ti oxide thermometry, and under watersaturated conditions. To stabilize a homogeneous magma body of >10 km at 824 C in the upper crust, we propose that reheating of the dacitic reservoir by hotter magma batches was able to maintain the equilibrium between the temperature of the magma and the assimilation of wall-rock over a period of 2500 years. Based on similarities among the juvenile products, we suggest that the three eruptions were fed from the same magma body.

Petrogenesis of A-type Granitoids from the Alto Moxoto and Alto Pajeu Terranes of the Borborema Province, ne Brazil: Constraints from Geochemistry and Isotopic Composition

A-type granitoids (∼512 Ma) either intruded into Paleoproterozoic gneiss of the Alto Moxoto Terrane (Prata Complex and Serra da Engabelada Pluton) or into Early Neoproterozoic metavolcanic metasedimentary sequence of the Alto Pajeu Terrane (Serrote Santo Antonio Pluton), constitute a small proportion of Brasiliano (= Pan-African) granitoids in the Central Tectonic Domain of the Borborema Province, northeastern Brazil. The Prata Complex consists of syenogranites, monzogranites, mafic enclaves of diorites and norites. The felsic and mafic members are not genetically related through fractionation. Mingling and mixing were extensive processes within the Prata Complex. The granites evolution appears to have involved fractionation of alkali feldspar, biotite, apatite and sphene without significant wall-rock assimilation. The Serra da Engabelada and Serrote Santo Antonio plutons consist of biotite syenogranites, with rare mafic enclaves. The studied granitoids are dominantly metaluminous, characterized by Fe-rich biotite and Fe-hornblende. High total alkalis, Y, Nb and REE and low CaO, MgO and Sr abundances and high FeO/(FeO+MgO) ratios characterize these granitoids. Chondrite-normalized REE patterns show enriched LREE, moderate to strong negative Eu anomalies and more or less flat heavy REE. The studied granitoids and diabase from dykes and enclaves show negative eNd (512Ma) values (-14 to -10), high incompatible elements such as LILE, HFSE and REE, suggesting important contribution of Paleoproterozoic crust. The origin of the granites is thought to have involved partial melting of granodioritic or tonalitic lower crust. Such isotopic signature of the diabase from the dykes also reflects a Paleoproterozoic enriched lithospheric mantle in the area. The intrusion of the studied granitoids contemporary with sub-volcanic bimodal magmatism and deposition of many Cambrian "pull-apart" basins in the north and central tectonic Domain of the Borborema Province, suggest intrusion during post-tectonic relaxation of the Brasiliano orogeny following the assembly of West Gondwana.

Mineral-chemistry and stable-isotope constraints on the magmatism, hydrothermal alteration, and related PGE?(base-metal sulphide) mineralisation of the Mesoarchaean Baula-Nuasahi Complex, India

The Baula-Nuasahi Complex, on the southern flank of the Singhbhum Archaean nucleus in north-eastern India, exposes a series of Mesoarchaean igneous suites. These are (1) a gabbro–anorthosite unit, which is petrographically homogeneous, although mineral-chemistry data hint at a subtle eastward differentiation; (2) a peridotite unit (with three chromitite layers) together with (3) a pyroxenite unit which display cumulate textures, modal layering, and (for the peridotite unit) differentiation trends in both mineralogy and mineral chemistry; and (4) the Bangur gabbro (~3.1 Ga), which defines an oblong intrusion, crosscutting the older igneous suites in the southern part of the complex, with a curvilinear NW-trending apophysis, 2 km long and up to 40 m wide. Magmatic breccia comprising ultramafic and chromitite wall-rock clasts in a gabbro matrix is exposed at the contact of the main Bangur gabbro body and also forms the entire Bangur gabbro apophysis. Concentrations of platinum-group minerals (PGMs) are found where the breccia contains abundant chromitite clasts, and two types of platinum-group-element (PGE) mineralisation are recognised. Type 1 (Pt 1.1–14.2, Pd 0.1–2.1 ppm, with an average Pt/Pd=8–9) is a “contact-type” mineralisation which occurs in the breccia at the contact between the Bangur intrusion and its ultramafic host. The PGMs—Pt alloys (isoferroplatinum) and sulphides (braggite, malanite)—are enclosed by pyroxene and plagioclase, reflecting a magmatic origin. Significant wall-rock assimilation by the magma (giving rise to the Bangur gabbro) is indicated by changes in pyroxene composition and by the presence of relicts of chromite (from the host) now altered to secondary “ferritchromite” in the contact zone. Type 2 PGE mineralisation (Pt 0.3–1.6, Pd 1.8–6.0 ppm, with Pt/Pd~0.5–3.0) is restricted to the breccia apophysis of the Bangur gabbro where it occurs in the breccia matrix, associated with an intense hydrothermal alteration which does not exist in the contact zone. PGMs (PGE arsenides, tellurides, bismuthides and antimonides) and, where present, base-metal sulphides (BMSs) form intergrowths with hydrous silicates, reflecting a hydrothermal origin. Oxygen isotope geothermometry documents the main stages of hydrothermal alteration within a decreasing temperature range between 700–1,000 and 500–600 °C, and oxygen, hydrogen and sulphur isotopes show that the hydrothermal fluids were derived from the magma rather than an external source. Pervasive hydrothermal alteration in the breccia apophysis likely represents upward channelling of late-magmatic fluids along a narrow, near-vertical, subplanar conduit which led away from the main magma chamber. We suggest that Type 2 mineralisation was produced by late-magmatic hydrothermal remobilisation and reconcentration of Type 1 PGE mineralisation, and that the composition of the hydrothermal fluids controlled whether BMSs were enriched along with the PGMs.