The largest accumulations of rhyolitic melt in the upper crust occur in voluminous silicic crystal mushes, which sometimes erupt as unzoned, crystal-rich ignimbrites, but are most frequently preserved as granodioritic batholiths. After approximately 40–50% crystallization, magmas of intermediate composition (andesite–dacite) typically contain high-SiO2 interstitial melt, similar to crystal-poor rhyolites commonly erupted in mature arc and continental settings. This paper analyzes the feasibility of system-wide extraction of this melt from the mush, a mechanism that can rationalize a number of observations in both the plutonic and volcanic record, such as: (1) abrupt compositional gaps in ignimbrites; (2) the presence of chemically highly evolved bodies at the roof of subvolcanic batholiths; (3) the observed range of ages (up to 200–300 ka) recorded by zircons in silicic magmas; (4) extensive zones of low P-wave velocity in the shallow crust under active silicic calderas. We argue that crystal–melt segregation occurs by a combination of several processes (hindered settling, micro-settling, compaction) once convection is hampered as the rheological locking point of the crystal–melt mixture ( 50 vol. % crystals) is attained. We constrain segregation rates by using hindered settling velocities and compaction rates as endmembers. Time scales estimated for the formation of >500 km of crystal-poor rhyolite range from 10 to 10 years, within the estimated residence times of mushes in the upper crust (>10 years, largely based on U/Th and U/Pb dating). This model provides an integrated picture of silicic magmatism, linking the evolution of plutonic and volcanic systems until storage in the upper crust, where granitoids become the leftovers from rhyolitic eruptions.

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On the Origin of Crystal-poor Rhyolites: Extracted from Batholithic Crystal Mushes

Do coeval mafic and felsic magmas in post-collisional to within-plate regimes necessarily imply two contrasting, mantle and crustal, sources? A review

Effects of repetitive emplacement of basaltic intrusions on thermal evolution and melt generation in the crust

Partial melting of mafic (amphibolitic) lower crust by periodic influx of basaltic magma

recharge mass and Teq, the mass of wallrock heated to Teq is Geochemical data for igneous rocks provide definitive evidence for computed and the geochemical trajectory of melt is determined as the occurrence of open-system processes in magma bodies, including magma temperature approaches Teq from its starting value, Tm°. Replenishment by intrusion of primitive magma, Assimilation of The effects of imperfect extraction of anatectic wallrock melt may anatectic wallrock melt and cumulate formation by Fractional be accounted for by introduction of an extraction efficiency factor. Crystallization. A general class of models (Energy ConservedMathematical details of a simpler model, EC-AFC (no reRAFC or EC-RAFC) can be constructed, which allow simulation plenishment) are provided. Energy-constrained models have the of geochemical paths for trace elements and isotopic ratios for magma advantage of linking thermal and chemical properties of magma undergoing simultaneous replenishment, assimilation and fractional chambers. Compared with ‘classical’ models that conserve mass crystallization during the approach to thermal equilibration. The and species only, they represent more complete assessments of general problem of EC-RAFC is formulated as a set of 3 + t the complex physicochemical dynamics governing the geochemical + i + s coupled nonlinear ordinary differential equations, where evolution of open-system magma bodies. Results of EC-AFC the number of trace elements, radiogenic and stable isotope ratios simulations demonstrate that geochemical trends can differ sigsimultaneously modeled are t, i and s, respectively. Partial melting nificantly from predictions based on ‘classical’ AFC even when of wallrock, modeled as fractional melting, is incorporated, as are recharge plays no role. Incorporation of energy conservation and sensible and latent heat effects. Temperature-dependent partition partial melting into geochemical models allows important coupled coefficients are used to describe trace element distributions. Solution effects to play their natural role. of the set of differential equations, with magma temperature (Tm ) as the independent variable, provides values for the average temperature of wallrock (Ta ), fraction of melt within the magma body (Mm ),

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Energy-constrained open-system magmatic processes I : General model and energy-constrained assimilation and fractional crystallization (EC-AFC) formulation

Emplacement of mantle-derived magma (magmatic accretion) is the total thermal budget of the continental lower crust, often presumed or inferred to be an important cause of regional produce regional granulite facies metamorphism, and granulite facies metamorphism and crustal anatexis. The juxgenerate Yand heavy rare earth element (HREE)taposition of mafic cumulates and regionally distributed granulite depleted granitoids (Ellis, 1987). Accretion of mafic facies rocks has led some to consider the Ivrea zone (northern Italy, magma and migration of partial melt will internally Southern Alps) as an important exposure that demonstrates this stratify, chemically differentiate, and deplete the concausal relationship. However, regional PTt paths indicated by tinental lower crust in large ion lithophile elements metamorphic reaction textures and PT conditions inferred from (LILE). Magmatic accretion has been invoked to provide geothermobarometry indicate that the emplacement of mafic plutonic the heat and mass necessary for sustained magmatism in rocks (Mafic Complex) at the Ivrea zone occurred during detectonic settings such as Phanerozoic extensional terranes compression from ambient pressures at the regional thermal maximum. (Lister et al., 1986; Gans, 1987; Fountain, 1989; Mareschal Field and petrographic observations, supported by PT estimates, & Bergantz, 1990; Jarchow et al., 1993) and magmatic indicate that regional retrograde decompression and emplacement of arcs (Hamilton, 1981; Kay & Kay, 1981; Bohlen & the upper parts of the Mafic Complex probably accompanied Lindsley, 1987; Hildreth & Moorbath, 1988). extension during the Late Carboniferous–Early Permian. A spatially However, as few exposed sections of lower continental restricted decompression-melting event accompanied final emcrust show contiguous mafic intrusions and regionally placement, depleting supracrustal rocks enclosed by an >2–3 km distributed granulite facies rocks, estimates of the extent aureole overlying the upper Mafic Complex by 20–30% granite of anatexis and metamorphism accompanying magmatic component. The upper Mafic Complex provided the thermal energy accretion have relied on numerical and analog simto reset mineral assemblages and locally overprint the regional ulations. These models yield disparate results depending prograde metamorphic zonation. The limited extent of the contact on whether heat transfer within the mafic intrusion aureole suggests that magmatic accretion may not inexorably cause and surrounding country rocks is primarily convective regional metamorphism and crustal anatexis. (Campbell & Turner, 1987; Huppert & Sparks, 1988) or conductive (Marsh, 1989; Bergantz & Dawes, 1994; Barboza & Bergantz, 1996). A comparison with field evidence is required to discriminate between the model

Metamorphism and Anatexis in the Mafic Complex Contact Aureole, Ivrea Zone, Northern Italy

One widely accepted paradigm for the development of continental lower crust is that regional granulite facies metamorphism is caused by intrusion of mafic magma beneath or into the crust (magmatic accretion). The amphibolite to granulite facies supracrustal section exposed in the Ivrea zone (southern Alps, northern Italy) is commonly cited as a classic example establishing this postulated genetic relationship. Our interpretation of the pattern of metamorphic isograds, compositional trends in high-grade metasedimentary rocks, and textural evidence in metapelite, however, indicates that final emplacement of the mafic plutonic rocks (the Mafic Complex) occurred subsequent to the regional thermal maximum. Field and petrographic relations suggest that a spatially restricted contact-melting event in crustal rocks accompanied the emplacement of the Mafic Complex. This inference is consistent with leucosome compositions in migmatites and a low-pressure, high-temperature metamorphic overprint recorded by mineral assemblages in wall rocks proximal to the intrusion. Therefore, evidence of anatexis and metamorphism of crustal rocks associated unequivocally with emplacement of the Mafic Complex is found only within an ~2-km-wide contact aureole overlying the intrusion. The narrow aureole associated with emplacement of the Mafic Complex demonstrates that, in some cases, emplacement of large volumes of mafic magma within the crust does not inexorably lead to regional-scale granulite facies metamorphism and large ion lithophile element depletion by melt loss.

Regional granulite facies metamorphism in the Ivrea zone: Is the Mafic Complex the smoking gun or a red herring?

Rheological transitions and the progress of melting of crustal rocks

Dehydration melting of crustal rocks may commonly occur in response to the intrusion of mafic magma in the mid- or lower crust. However, the relative importance of melt buoyancy, shear or dyking in melt generation and extraction under geologically relevant conditions is not well understood. A numerical model of the partial melting of a metapelite is presented and the model results are compared with the Ivrea-Verbano Zone in northern Italy. The numerical model uses the mixture theory approach to modelling simultaneous convection and phase change and includes special ramping and switching functions to accommodate the rheology of crystal-melt mixtures in accordance with the results of deformation experiments. The model explicitly includes both porous media flow and thermally and compositionally driven bulk convection of a restitecharged melt mass. A range of melt viscosity and critical melt fraction models is considered. General agreement was found between predicted positions of isopleths and those from the Ivrea-Verbano Zone. Maximum melt velocities in the region of porous flow are found to be 1 × 10−7 and 1 × 10−1m per year in the region of viscous flow. The results indicate that melt buoyancy alone may not be a sufficient agent for melt extraction and that extensive, vigorous convection of partially molten rocks above mafic bodies is unlikely, in accord with direct geological examples.

Dynamic model of dehydration melting motivated by a natural analogue: applications to the Ivrea–Verbano zone, northern Italy

The binary-eutectic model of phase change limits the application of continuum-mixture simulations to particular systems. The authors present a phase-change model that allows systems with more complex phase relations to be addressed. The model includes multiple invariant points, which yield a nonlinear production of melt as a function of temperature. A constitutive relation based on the hybrid model for viscosity and permeability is used to account for transport behavior as a function of the local melt fraction. Results are reported of two series of simulations, one utilizing the new two-invariant-point model and the other using a one-invariant-point model. The comparison demonstrates that the efficiency of convective heat transport can be increased by over 200% in the two-invariant-point case. The total kinetic energy for the two-invariant-point system is typically 2 orders of magnitude greater. This comparison highlights the important link between melt productivity and the constitutive relations and illustrates the necessity of accurately modeling the variation of melt fraction with temperature in partially molten systems.

Scott A. Barboza

Papers

Metamorphism and Anatexis in the Mafic Complex Contact Aureole, Ivrea Zone, Northern Italy

Regional granulite facies metamorphism in the Ivrea zone: Is the Mafic Complex the smoking gun or a red herring?

Rheological transitions and the progress of melting of crustal rocks

Dynamic model of dehydration melting motivated by a natural analogue: applications to the Ivrea–Verbano zone, northern Italy

Melt Productivity and Rheology: Complementary Influences on the Progress of Melting