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The common association of mid-crustal migmatites with an upper-level granite pluton could indicate that the migmatites are a feeder zone for the pluton. If magma from a deeper level pervasively intrudes a high temperature metamorphic complex, most of the intruded magma would not freeze because of the prevailing temperature. The interaction between the magma and country rocks, which could include partial melting and crystallisation of the magma passing through, would modify magma to a more granitic composition, as found in the higher-level pluton.The physical aspect of the magma transport through such a hot feeder zone is modelled by introducing a dimensionless melt transport (MT) number, which is the ratio of the rate of melt movement caused by the bulk flow of the entire mass (melt+solid) to that of porous media flow of melt only through the solid framework. The MT number is strongly dependent on the melt content of the melt-rich zone (MRZ), the diameter of the MRZ and typical particle size in the MRZ.The ∼300-Ma, diatexitic, Lauterbrunnen migmatites (LM) in the Aar massif, Swiss Alps, may be such a feeder zone for the nearby 303-Ma Gastern granite (GG). The chemical and field evidence indicates that the LM formed by an intrusion of intermediate composition magma, which interacted with country rocks to produce a magma of GG composi

A statistically rigorous approach based on the isocon method (Grant, 1986) to evaluate mass transport is presented. Chemical analyses of multiple samples of unaltered (parent) and altered rock are used to calculate the average oxide or element concentration and its standard deviation for each rock population. In a typical application, the uncertainties in element concentration associated with the lack of homogeneity in each population outweigh the analytical uncertainty. An efficient algorithm is presented to select the immobile elements by identifying the maximum number of elements that are, within their uncertainties, compatible with the same isocon. The actual isocon is constructed by forcing a line through the origin and optimizing its slope using a weighted least-squares procedure for the selected immobile elements. Thus, individual uncertainties for each immobile element are taken into account. As an illustration, the procedure is applied to a published set of data on potassic and sericitic alterations of quartz monzonite of the Bingham porphyry copper system. The surprising result indicates that the apparent increase of potassic alteration phases is a consequence of acid leaching of calcium and sodium. No mass transport of potassium is indicated. Silica, water, and sulfur were added during alteration. The best fit isocon requires a small mass increase during each alteration process which, however, is within the uncertainty obtained for the slope of the isocon. A software package is available from the authors in the form of a FORTRAN 77 source code.

Petrologic and oxygen isotope data indicate that water-rich fluids infiltrated metasedimentary rocks of the Waterville and Sangerville (formally Vassalboro) Formations, south-central Maine, during peak metamorphism, and depleted Sangerville rocks in alkalis but not equivalent Waterville rocks. Fluid inclusion data from two outcrops, ∼ 1 km apart, one of the Waterville and the other of the Sangerville Formations, suggest a cause for the geochemical difference between the two units. Postulated peak metamorphic inclusions, the texturally earliest of aqueous inclusions in the metasediments, approximate the water-rich compositions of peak fluids predicted by mineral-fluid equilibria, and have average salinity in the Sangerville Formation ∼ three times that of equivalent Waterville inclusions. The higher salinity in the Sangerville fluids could explain the greater alkali depletion in these rocks. Probable pre-peak or prograde inclusions are preserved in metasediments as the texturally earliest carbonic inclusions which contain CO2, CH4, N2±H2O, as determined by microthermometry and Raman spectrometry. They may have formed by breakdown of organic matter. Probable retrograde inclusions occur as texturally late aqucous inclusions in healed fractures with salinity ranges indistinguishable between the two formations. Synmetamorphic granitic dikes present in the two outcrops were ruled out as a source for fluids in metasediments because composition and density ranges of inclusions in dikes and metasediments are fundamentally different, and because there is no correlation between the abundance or composition of inclusions in a sample and proximity to dikes. Isochores for many of the inclusions in both metasediments and dikes are not consistent with the inferred P—T conditions of their trapping, but intersect at ∼ 300° to 400°C and 1 to 2 kbar. The intersections probably resulted because inclusion densities continued to equilibrate during uplift and cooling until quartz became rigid. The present densities are those at the last equilibration, not the time of trapping. In contrast, the clear distinctions in inclusion compositions between dikes and between dike and country rock show that the original compositional differences generally have been preserved.

Gneiss domes in the Maryland Piedmont near Baltimore contain a wide variety of metamorphic rock types, all metamorphosed to amphibolite grade and collectively named Baltimore Gneiss. Most were derived from interlayered sedimentary and volcanic rocks into which granitic plutons were emplaced in both Precambrian and Paleozoic time. Migmatitic features are common in the Baltimore Gneiss. Most seem to refl ect either original sedimentary layering or original layering that has been enhanced by later metamorphic differentiation, but some involved partial melting, perhaps during the Precambrian metamorphism. We fi nd no evidence for widespread melting in rocks at the present level of exposure during the Paleozoic. Limited fl uid-driven K-metasomatism may well have occurred locally, but we fi nd no evidence for metasomatism on a regional scale. Pressure-temperature conditions for the Baltimore Gneiss during the Paleozoic metamorphism are estimated to be 7–9 kb and 625–675 °C. For half a century, the anticlinal structures exposing basement gneiss at Baltimore have been cited as classic examples of mantled gneiss domes, but more recent fi eld studies and geophysical investigations suggest that they are the result of interference between late upright folds and early nappe structures formed by a combination of isoclinal folding along subhorizontal axial surfaces and thrust faults rooted in a regional decollement.

Monophase negative-crystal shaped CO2 inclusions occurring isolated, in small clusters, or in well-healed intragranular fractures are common in the leucosome quartz of the 1700m.y.-old migmatites from the east-central Colorado Front Range. They are, however, quite rare in the mafic selvage and paleosome (host rock) quartz. The mode of occurrence suggests that these are the earliest inclusions to form. In addition to the difference in abundance of the inclusions, there is a difference in CO2-density distribution between migmatitic zones. The temperatures of homogenization for the leucosome inclusions range and ∼ +l°C from −67° C to +20° C with two maxima (at ∼ −21° C) while those for the paleosome and selvage inclusions are −37° C to +20° C with a single maximum at ∼ + 5° C. These differences between the migmatitic zones which occur on the scale of a few centimeters suggest that the formation of these inclusions was related to the migmatization process. The densities corresponding to the Th maxima are appropriate for the P-T conditions for migmatization estimated from the mineral geobarometer/geothermometer. These inclusions must contain nearly pure CO2, as their final melting temperatures (−56.5° to −57.2° C) are very close to that of the triple point of CO2. Their composition also was confirmed by Raman spectroscopic analyses.

The migmatite problem has always been interrelated with the granite problem. The long and sometimes heated debates of the granite controversy between ‘magmatists’ and ‘transformationists’ were ended, for the time being, by Tuttle and Bowen (1958). Their study also convinced many geologists that migmatites were the product of simple partial melting and that the leucosomes in migmatites represented squeezed-out minimum melts. However, it quickly became clear that this assumption had many difficulties when applied to regional migmatites. The problems most often cited were: (1) the leucosomes do not have the minimum melt compositions (for example, Misch, 1968); (2) certain migmatitic structures, such as relict mineral orientations (for example, Hopson, 1964), metamorphic texture of and replacement textures in the leucosome (for example, King, 1965), cannot be reconciled with the idea of the leucosome as a melt. Winkler and Breitbart’s melting experiment study (1978) illustrates clearly the type of dilemma that crops up in migmatite research. They found ‘with surprise’ that at least six out of nine leucosomes of ‘in-situ migmatites’ probably were not completely liquid at the temperature reasonable for a high-grade metamorphism even at P H2O = P total = 5 kbar. It is very unlikely that such crystal mush could be filter-pressed out of the wall rocks, especially when the melt must contain plagioclase and quartz but leave mafic minerals in the selvages.

The fluid inclusions provide petrologists with the only direct method for studying a possible metamorphic fluid from a high P-T environment. Basic principles of a fluid inclusion study are as old as modern petrography (Sorby, 1858), but only in recent years have advances in the technology (heating-freezing stages), and in the understanding of fluid systems at high P-T conditions, permitted the studies of fluid inclusions in many rock types. For specific techniques and basic principles of data interpretation, readers are referred to the recent publications on the subjects (e.g. Weisbrod et al., 1976; Touret, 1977; Hollister and Crawford, 1981; Roedder, 1984).

The net compositions of neosome plus mafic selvage in two biotitic layered migmatites from the Baltimore Gneiss at Piney Creek, Maryland, closely approximate the paleosome compositions: the migmatization must have occurred in rocks closed to all except possibly volatile components. Anatexis or metamorphic differentiation is indicated as the migmatization mechanism. Parallel tie lines between the neosome, selvage, and paleosome in a modal biotite-microcline plot suggest a subsolidus metamorphic differentiation mechanism in which microcline replaces biotite in the neosome by a reaction such as biotite + 6H+ → microcline + 3 (Fe++, Mg++) + 4H2O; the Fe and Mg released by the reaction diffuse from the neosome to the selvage; biotite replaces microcline in the selvage by the reverse of the reaction. Anatexis (simple partial fusion) in the Piney Creek rocks is also indicated because (1) the closer a paleosome composition is to the granite minimum the more extensive is the migmatization in the rock; and (2) the neosomes are closer to the granite minimum than the paleosomes. It is postulated that the Piney Creek migmatites formed by metamorphic differentiation induced by anatexis. Anatexis decreases f H2o locally because much water must be dissolved in the first melt, thus initiating the reaction by which biotite breaks down to microcline. The f H2o gradient between the neosome and paleosome maintained by the presence of melt in the neosome drives the metamorphic differentiation and should also create an a sio 2 gradient leading to the observed quartz migration from the selvage to the neosome.