Progress in the understanding of the volumes and viscosities of granitic and related pegmatitic melts generated by experimental studies are reviewed. The results of a series of investigations of the volumes and viscosities of melts derived from a haplogranitic base composition, HPG8, located near the 2 kbar water-saturated minimum melt composition in the albite—orthoclase—silica system are discussed. Melt volumes, obtained using a combination of dilatometric and calorimetric methods at 1 atm and relatively low temperatures yield an internally consistent set of partial molar volumes for 18 components in granitic melts. These partial molar volumes, combined with an estimate for water, allow the estimation of melt densities for granitic and related pegmatitic magmas.

Melt viscosities, obtained using a combination of high and low range viscometry techniques, provide a template for the estimation of melt viscosities in more complex natural systems. The parameterisation of the non-Arrhenian temperature-dependence of the viscosity of such melts is presented, together with some structural implications of the variation of melt viscosity with temperature and composition. Outstanding questions related to the PVT equation of state of granitic melts and to the mechanical response to shear stresses are discussed, with an outlook for the experimental solutions to those questions in the next few years.

The rheological and chemical behaviour of the lower crust during anatexis has been a major focus of geological investigations for many years. Modern studies of crustal evolution require significant knowledge, not only of the potential source regions for granites, but also of the transport paths and emplacement mechanisms operating during granite genesis. We have gained significant insights into the segregation and transport of granitoid melts from the results of experimental studies on rock behaviour during partial melting. Experiments performed on crustal rock cores under both hydrostatic conditions and during deformation have led, in part, to two conclusions. (1) The interfacial energy controlling melt distribution is anisotropic and, as a result, the textures deviate significantly from those predicted for ideal systems—planar solid-melt interfaces are developed in addition to triple junction melt pockets. The ideal dihedral angle model for melt distribution cannot be used as a constraint to predict melt migration in the lower crust. (2) The ‘critical melt fraction’ model, which requires viscous, granitic melt to remain in the source until melt fractions reach >25 vol%, is not a reliable model for melt segregation. The most recent experimental results on crustal rock cores which have helped advance our understanding of melt segregation processes have shown that melt segregation is controlled by several variables, including the depth of melting, the type of reaction and the volume change associated with that reaction. Larger scale processes such as tectonic environment determine the rate at which the lower crust heats and deforms, thus the tectonic setting controls the melt fraction at which segregation takes place, in addition to the pressure and temperature of the potential melting reactions. Melt migration therefore can occur at a variety of different melt fractions depending on the tectonic environment; these results have significant implications for the predicted geochemistry of the magmas themselves.

To form a granite pluton, the felsic melt produced by partial melting of the middle and lower continental crust must separate from its source and residuum. This can happen in three ways: (1) simple melt segregation, where only the melt fraction moves; (2) magma mobility, in which all the melt and residuum move together; and (3) magma mobility with melt segregation, in which the melt and residuum move together as a magma, but become separated during flow. The first mechanism applies to metatexite migmatites and the other two to diatexite migmatites, but the primary driving forces for each are deviatoric stresses related to regional-scale deformation. Neither of the first two mechanisms generates parental granite magmas. In the first mechanism segregation is so effective that the resulting magmas are too depleted in FeOT, MgO, Rb, Zr, Th and the REEs, and in the second no segregation occurs. Only the third mechanism produces magmas with compositions comparable with parental granites, and occurs at a large enough scale in the highest grade parts of migmatite terranes, to be considered representative of the segregation processes occurring in the source regions of granites.

Diapirism has been discredited as a transport mechanism for magmas partly because diapirs seem to be unable to bring magmas to shallow crustal levels (<10km) and partly because recent developments in the theory of dyke propagation have shown that sufficiently wide dykes are able to efficiently transport felsic magmas through the crust. However, it is still unclear how felsic dykes grow to widths that allow them to propagate faster than they close by magma freezing. Ultimately, it may be the ability of felsic dykes to grow within the source that controls which mechanism dominates ascent.

The ability of dykes to propagate, from the top of rising diapirs depends among other factors on the changing temperature gradient of the wall rocks. The steep gradient around rapidly rising diapirs in the low viscosity lower crust will cause dykes to freeze. As diapirs rise to colder stiffer crust and decelerate, heat diffuses further from the diapir, resulting in shallower temperature gradients that favour dyke propagation. The mechanism may thus swap, during ascent, from diapirism to dyking. Calculations of the thermal evolution of diapirs and their surroundings show that basaltic diapirs may never form because they would be drained by dykes at a very early stage; felsic diapirs may be unable to give rise to successful dykes, whereas diapirs of intermediate magmas may propagate dykes during ascent.

Until the last few years, diapirism reigned supreme among granitoid ascent mechanisms. Granitoid masses in a variety of material states, from pure melt through semi-molten crystal mushes to solid rock, were believed to have risen forcefully through the continental crust to their final emplacement levels in a way analogous to salt domes. The structural analogy between granite plutons and salt diapirs, which gained acceptance in the 1930s, has clearly been attractive despite the pessimistic outcomes of thermal models and, at best, ambiguous field evidence.

In contrast with traditional diapiric ascent, dyke transport of granitoid magmas has a number of important implications for the emplacement and geochemistry of granites that have yet to be fully explored. Rapid ascent rates of ≍ 10 2m/s predicted for granite melts in dykes (cf. m/a for diapirs) mean that felsic magmas can be transported through the continental crust in months rather than thousands (or even millions) of years, and that large plutons can in principle be filled in <104 a. Granitic melts are likely to rise adiabatically from their source regions, leading to the resorption of any entrained restitic material. Ascending melts in dykes close to their critical minimum widths may have little opportunity to assimilate significant amounts of country rock, and if source extraction is sufficiently rapid, most crustal contamination will be restricted to the site of emplacement. Rates of pluton and batholith inflation will be determined by the amount and rate of melt extraction at source.

The construction of large plutons and batholiths piecemeal from a number of magma pulses separated by periods of relative quiescence provides a means of reconciling rapid ascent rates with times for batholith construction based on average rates. Field and seismic evidence that shows batholiths as large, sheet-like structures with flat roofs and floors is consistent with a general model for plutons and batholiths as laccolith-type structures, fed from depth by dykes. The overall geometry of this type of structure helps ameliorate the space problem, which developed as a consequence of the unrealistic volumes of upwelling granite associated with the classical diapir model.

Buddington (1959) pointed out that the construction of large crustal magma chambers involves complex internal processes as well as multiple country rock material transfer processes (MTPs), which reflect large horizontal, vertical and temporal gradients in physical conditions. Thus, we have attempted to determine the relative importance of different magmatic and country rock MTPs at various crustal depths, and whether country rock MTPs largely transport material vertically or horizontally, rather than seeking a single model of magma ascent and emplacement.

Partially preserved roofs of nine plutons and in some cases roof–wall transitions with roof emplacement depths of 1·5–11 km were mapped. During emplacement, these roofs were not deformed in a ductile manner, detached or extended by faults, or significantly uplifted. Instead, sharp, irregular, discordant contacts are the rule with stoped blocks often preserved immediately below the roof, even at depths of 10 km. The upper portions of these magma chambers are varied, sometimes preserving the crests of more evolved magmas or local zones of volatile-rich phases and complex zones of dyking and magma mingling. Magmatic structures near roofs display a wide variety of patterns and generally formed after emplacement. Transitions from gently dipping roofs to steep walls are abrupt. At shallow crustal levels, steep wall contacts have sharp, discordant, stepped patterns with locally preserved stoped blocks indicating that the chamber grew sideways in part by stoping. Around deeper plutons, an abrupt transition (sometimes within hundreds of metres) occurs in the country rock from discordant, brittle roofs to moderately concordant, walls deformed in a ductile manner defining narrow structural aureoles. Brittle or ductile faults are not present at roof–wall joins.

Near steep wall contacts at shallow to mid-crustal depths (5–15 km), vertical and horizontal deflections of pre-emplacement markers (e.g. bedding, faults, dykes), and ductile strains in narrow aureoles (0·1–0·3 body radii) give a complete range of bulk strain values that account for 0–100% of the needed space, but average around 30%, or less, particularly for larger batholiths. A lack of far-field deflection of these same markers rules out significant horizontal displacement outside the aureoles and requires that any near-field lateral shortening is accommodated by vertical flow. Lateral variations from ductile (inner aureole) to brittle (outer aureole) MTPs are typically observed. Compositional zoning is widespread within these magma bodies and is thought to represent separately evolved pulses that travelled up the same magma plumbing system. Magmatic foliations and lineations commonly cross-cut contacts between pulses and reflect the strain caused either by the late flow of melt or regional deformation.

Country rocks near the few examined mid- to deep crustal walls (10–30 km) are extensively deformed, with both discordant and concordant contacts present; however, the distinction between regional and emplacement-related deformation is less clear than for shallower plutons. Internal sheeting is more common, although elliptical masses are present. Lateral compositional variations are as large as vertical variations at shallower depths and occur over shorter distances. Magmatic foliations and lineations often reflect regional deformation rather than emplacement processes.

The lack of evidence for horizontal displacement outside the narrow, shallow to mid-crustal aureoles and the lack of lateral or upwards displacement of pluton roofs indicate that during emplacement most country rock is transported downwards in the region now occupied by the magma body and its aureole. The internal sheeting and zoning indicate that during the downwards flow of country rock, multiple pulses of magma travelled up the same magma system. If these relationships are widespread in arcs, magma emplacement is the driving mechanism for a huge crustal-scale exchange process.

Most granitic batholiths contain plutons which are composed of low-variance mineral assemblages amenable to quantification of the P– conditions that characterise emplacement. Some mineral thermometers, such as those based on two feldspars or two Fe–Ti oxides, commonly undergo subsolidus re-equilibration. Others are more robust, including hornblende–plagioclase, hornblende–clinopyroxene, pyroxene–ilmenite, pyroxene–biotite, garnet–hornblende, muscovite-biotite and garnet–biotite. The quality of their calibration is variable and a major challenge resides in the large range of liquidus to solidus crystallisation temperatures that are incompletely preserved in mineral profiles. Further, the addition of components that affect Kd relations between non-ideal solutions remains inadequately understood. Estimation of solidus and near-solidus conditions derived from exchange thermometry often yield results >700°C and above that expected for crystallisation in the presence of an H2O-rich volatile phase. These results suggest that the assumption of crystallisation on an H2O-saturated solidus may not be an accurate characterisation of some granitic rocks.

Vapour undersaturation and volatile phase composition dramatically affect solidus temperatures. Equilibria including hypersthene–biotite–sanidine–quartz, fayalite–sanidine–biotite, and annite–sanidine–magnetite (ASM) allow estimation of Estimates by the latter assemblage, however, are highly dependent on . Oxygen fugacity varies widely (from two or more log units below the QFM buffer to a few log units below the HM buffer) and can have a strong affect on mafic phase composition. Ilmenite–magnetite, quartz–ulvospinel–ilmenite–fayalite (QUILF), annite–sanidine–magnetite, biotite–almandine–muscovite–magnetite (BAMM), and titanite–magnetite–quartz (TMQ) are equilibria providing a basis for the calculation of .

Granite barometry plays a critical part in constraining tectonic history. Metaluminous granites offer a range of barometers including ferrosilite–fayalite–quartz, garnet–plagioclase–hornblende–quartz and Al-in-hornblende. The latter barometer remains at the developmental stage, but has potential when the effects of temperature are considered. Likewise, peraluminous granites often contain mineral assemblages that enable pressure determinations, including garnet–biotite–muscovite–plagioclase and muscovite–biotite–alkali–feldspar–quartz. Limiting pressures can be obtained from the presence of magmatic epidote and, for low-Ca pegmatites or aplites, the presence of subsolvus versus hypersolvus alkali feldspars.

As with all barometers, the influence of temperature, , and choice of activity model are critical factors. Foremost is the fact that batholiths are not static features. Mineral compositions imperfectly record conditions acquired during ascent and over a range of temperature and pressure and great care must be taken in properly quantifying intensive parameters.

Recent water-undersaturated phase equilibrium data on the subsystems of the granite-H2O system have provided important new constraints on the topology of the cotectic surfaces and hence on the compositional evolution of felsic magmas. The effect of water on phase relations can be deduced from a comparison of anhydrous and H2O-saturated data or from data obtained in the presence of a CO2-bearing fluid. However, although new experimental evidence indicates that the silica enrichment of evolving H2O-undersaturated, H2O-unbuffered melts during the co-precipitation of quartz and feldspar is as previously thought for orthoclase-rich compositions, it suggests that such a trend is considerably less for Ab-rich compositions. For water-poor trachytic melts, the newly recognised strong destabilisation of the sanidine melt component relative to the anorthite melt component with increasing water content indicates that the co-precipitation of two feldspars will result in saturation of the melt with ternary alkali feldspar at an earlier stage (i.e. higher melt anorthite content) than previously thought. This, in turn, implies that the melt differentiation path will have a greater component of anorthite depletion during the equilibrium co-precipitation of ternary feldspars and that the melt will remain in the peritectic region of the two feldspar plus liquid surface over a greater interval of crystallisation, thereby enhancing the possibility that the resoption of plagioclase during the early stages of equilibrium with alkali feldspar may go to completion. Comparison of CO2-free and CO2-bearing haplogranitic phase equilibrium data suggests that CO2 may be playing an independent part in the modification of phase equilibria and may induce a significant destabilisation of the orthoclase melt component.

Petrological investigations of granite commonly reveal multiple periods of growth punctuated by resorption for many of the constituent minerals. Complementary to such textures are mineral compositional heterogeneity manifested by zoning or grain to grain variability. These features ultimately reflect changes in the intensive parameters or activities of components during melt solidification. Such complexities of granite crystallisation can be simultaneously modelled in a reaction space constructed from the set of linearly independent reactions describing the equilibria among all phases and components in the system of interest.

The topology of the linearly independent reactions that define the reaction space for garnetmuscovite-biotite granites yields the following insights: (1) there is no one unique reaction that produces or consumes aluminous minerals (e.g. garnet); (2) minerals can alternate as reactants or products in different reactions accounting for textures indicating multiple periods of crystallisation separated by resorption; (3) mineral compositions are regulated by the reaction(s) producing them and vary as the stoichiometry of the reaction(s) producing them varies; (4) resorption of early crystallising garnet is likely to reflect decreasing pressure, presumably during magma ascent; (5) late crystallisation of garnet, at the expense of biotite, reflects an increase in melt aluminosity and does not necessarily require high Mn activities for the melt and (6) increasing melt H2O, at H2Oundersaturated conditions, favours the formation of biotite–muscovite granite.

Application of the reaction space method to other granite types holds considerable promise for elucidating reactions that regulate mineral assemblages and compositions during crystallisation.

Granites within suites share compositional properties that reflect features of their source rocks. Variation within suites results dominantly from crystal fractionation, either of restite crystals entrained from the source, or by the fractional crystallisation of precipitated crystals. At least in the Lachlan Fold Belt, the processes of magma mixing, assimilation or hydrothermal alteration were insignificant in producing the major compositional variations within suites. Fractional crystallisation produced the complete variation in only one significant group of rocks of that area, the relatively high temperature Boggy Plain Supersuite. Modelling of Sr, Ba and Rb variations in the I-type Glenbog and Moruya suites and the S-type Bullenbalong Suite shows that variation within those suites cannot be the result of fractional crystallisation, but can be readily accounted for by restite fractionation. Direct evidence for the dominance of restite fractionation includes the close chemical equivalence of some plutonic and volcanic rocks, the presence of plagioclase cores that were not derived from a mingled mafic component, and the occurrence of older cores in many zircon crystals. In the Lachlan Fold Belt, granite suites typically evolved through a protracted phase of restite fractionation, with a brief episode of fractional crystallisation sometimes evident in the most felsic rocks. Evolution of the S-type Koetong Suite passed at about 69% SiO2 from a stage dominated by restite separation to one of fractional crystallisation. Other suites exist where felsic rocks evolved in the same way, but the more mafic rocks are absent. In terranes in which tonalitic rocks formed at high temperatures are more common, fractional crystallisation would be a more important process than was the case for the Lachlan Fold Belt.

The paradox of Lachlan Fold Belt (LFB) granitoids is that although contrasted chemical types (S- and I-types) imply melting of distinct crustal sources, the simple Nd–Sr–Pb–O isotopic arrays indicate a continuum, suggesting mixing of magmatic components. The paradox is resolved by the recognition that the previously inferred, isotopically primitive end-member is itself a crust-mantle mix, so that three general source components, mantle, lower crust and middle crust, comprise the granitoids. Based on Nd isotopic evidence, mantle-derived basaltic magmas melted and mixed with Neoproterozoic-Cambrian, arc-backarc-type material to produce primitive I-type, parental granitoid magmas in the lower–middle crust. Ordovician metasediment, locally underthrust to mid-crustal levels, was remobilised under the elevated geotherms and is most clearly recognised as diatexite in the Cooma complex, but it also exists as gneissic enclaves in S-type granites. The diatexite mixed with the hybrid I-type magmas to produce the parental S-type magmas. Unique parent magma compositions of individual granite suites reflect variations within any or all of the three major source components, or between the mixing proportions. For example, chemical tie-lines between Cooma diatexite and mafic I-type Jindabyne suite magma encompass almost all mafic S-type granites of the vast Bullenbalong supersuite, consistently in the proportion Jindabyne: Cooma, 30:70. The modelling shows that LFB S-type magmas are heavily contaminated I-type magmas, produced by large-scale mixing of hot I-type material with lower temperature diatexite in the middle crust. The model implies a genetic link between migmatite and pluton-scale, crustally derived (S-type) granites.

Given the chemical and isotopic contrasts of the crustally derived source components, and their typically unequal proportions in the magmas, it is not surprising that the LFB granitoids are so distinctive and have been categorised as S- and I-type. The sublinear chemical trends of the granitoid suites are considered to be secondary effects associated with crystal fractionation of unique parental magmas that were formed by three-component mixing. The model obviates the necessity for multiple underplating events and Proterozoic continental basement, in accordance with the observed tectonostratigraphy of the Lachlan Fold Belt.

The Cretaceous granitic rocks and associated regional metamorphic rocks in SW Japan were formed by a Cordilleran-type orogeny. Southwest Japan is regarded as a hypothetical cross-section of the upper to middle crust of the Eurasian continental margin in the Cretaceous, comprising (1) high-level granitoids (called San-yo type) and weakly to unmetamorphosed accretionary complexes that are exposed on the back-arc side and (2) low-level (Ryoke type) granitoids with high-grade metamorphites up to migmatitic gneisses on the forearc side. All these granitoids are of the ilmenite series, and predominantly I-type, with a subordinate amount of garnet- or muscovite-bearing varieties in the Ryoke zone, but none of these contains cordierite. These mineralogical variations are likely to depend more on their slightly peraluminous chemistry rather than the pressure differences during crystallisation.

In the eastern part of SW Japan, the granitoids of both levels give K–Ar biotite ages of approximately 65 Ma, whereas the magmatic age of high-level granitoids is approximately 70 Ma, 15 Ma younger than the nearly 85 Ma old lower level granitoids. This implies that the formation of the middle crust started approximately 15 Ma before that of the upper crust. The middle crust material was kept over 500°C for 15–20 Ma after solidification, then it cooled together with the upper crust to 300°C, 6–7 Ma after the formation of the upper crust. The coincidence of cooling history below 500°C of the upper and middle crust may reflect the regional uplift of the crust.

The low-level granitoids have higher 87Sr/86Sr initial ratios than those of high-level granitoids in the middle-western part (Chugoku district), but the relationship appears to be opposite in the eastern part. This may imply that the two plutonic series formed by separate magmatic pulses at an interval of c. 15 Ma, even though they are not independent, but rather part of a larger episode of crustal growth.

In Patagonia a Triassic-Early Jurassic Cordilleran interior magmatic belt preceded the widespread eruption of Middle Jurassic syn-extensional rhyolites. Two plutons (La Calandria and La Leona) represent the easternmost plutonic rocks of this belt, > 750 km east of the present oceanic trench. They define a high-K calc-alkaline monzonite series in contrast with the main Andinotype arc magmatism of the Pacific margin: they are enriched in large ion lithophile elements (K, Rb, Ba, Sr and Th), LREE and P2O5 and depleted in HREE and Y, with low FeO*/MgO ratio. The range of observed compositions (56-76% SiO2) resulted from high-level fractionation of plagioclase, hornbleńde, biotite, K-feldspar and accessories (sphene, apatite and zircon).

Initial 87Sr/86Sr ratios, average εNdt and mean depleted-mantle Nd model ages of the two plutons are 0·70487, -0·5 and 1050 Ma for La Calandria and 0·70509, -1·4 and 1125 Ma for La Leona, respectively. They are thus isotopically more primitive than the Middle Jurassic rhyolites, previously attributed to partial melting of Mesoproterozoic mafic lower crust. The preferred model for the origin of the monzonites is remelting of an amphibole- + garnet-bearing, plagioclase-poor, high-K mafic source (?underplating). This occurred in a distal sector of a dying oblique subduction regime, immediately preceding the extensional silicic volcanism.

The prospect of partial melting of the subducted oceanic crust to produce arc magmatism has been debated for over 30 years. Debate has centred on the physical conditions of slab melting and the lack of a definitive, unambiguous geochemical signature and petrogenetic process. Experimental partial melting data for basalt over a wide range of pressures (1–32 kbar) and temperatures (700–1150°C) have shown that melt compositions are primarily trondhjemite–tonalite–dacite (TTD). High-Al (> 15% Al2O3 at the 70% SiO2 level) TTD melts are produced by high-pressure (≥ 5 kbar) partial melting of basalt, leaving a restite assemblage of garnet + clinopyroxene ± hornblende. A specific Cenozoic high-Al TTD (adakite) contains lower Y, Yb and Sc and higher Sr, Sr/Y, La/Yb and.Zr/Sm relative to other TTD types and is interpreted to represent a slab melt under garnet amphibolite to eclogite conditions. High-Al TTD with an adakite-like geochemical character is prevalent in the Archean as the result of a higher geotherm that facilitated slab melting. Cenozoic adakite localities are commonly associated with the subduction of young (<25 Ma), hot oceanic crust, which may provide a slab geotherm (≍9–10°C km−1) conducive for slab dehydration melting. Viable alternative or supporting tectonic effects that may enhance slab melting include highly oblique convergence and resultant high shear stresses and incipient subduction into a pristine hot mantle wedge. The minimum P–T conditions for slab melting are interpreted to be 22–26 kbar (75–85 km depth) and 750–800°C. This P–T regime is framed by the hornblende dehydration, 10°C/km, and wet basalt melting curves and coincides with numerous potential slab dehydration reactions, such as tremolite, biotite + quartz, serpentine, talc, Mg-chloritoid, paragonite, clinohumite and talc + phengite. Involvement of overthickened (>50 km) lower continental crust either via direct partial melting or as a contaminant in typical mantle wedge-derived arc magmas has been presented as an alternative to slab melting. However, the intermediate to felsic volcanic and plutonic rocks that involve the lower crust are more highly potassic, enriched in large ion lithophile elements and elevated in Sr isotopic values relative to Cenozoic adakites. Slab-derived adakites, on the other hand, ascend into and react with the mantle wedge and become progressively enriched in MgO, Cr and Ni while retaining their slab melt geochemical signature. Our studies in northern Kamchatka, Russia provide an excellent case example for adakite-mantle interaction and a rare glimpse of trapped slab melt veinlets in Na-metasomatised mantle xenoliths.

Most natural systems display non-linear dynamic behaviour. This should be true for magma mingling and mixing processes, which may be chaotic. The equations that most nearly represent how a chaotic natural system behaves are insoluble, so modelling involves linearisation. The difference between the solution of the linearised and ‘true’ equation is assumed to be small because the discarded terms are assumed to be unimportant. This may be very misleading because the importance of such terms is both unknown and unknowable. Linearised equations are generally poor descriptors of nature and are incapable of either predicting or retrodicting the evolution of most natural systems. Viewed in two dimensions, the mixing of two or more visually contrasting fluids produces patterns by folding and stretching. This increases the interfacial area and reduces striation thickness. This provides visual analogues of the deterministic chaos within a dynamic magma system, in which an enclave magma is mingling and mixing with a host magma. Here, two initially adjacent enclave blobs may be driven arbitrarily and exponentially far apart, while undergoing independent (and possibly dissimilar) changes in their composition. Examples are given of the wildly different morphologies, chemical characteristics and Nd isotope systematics of microgranitoid enclaves within individual felsic magmas, and it is concluded that these contrasts represent different stages in the temporal evolution of a complex magma system driven by nonlinear dynamics. If this is true, there are major implications for the interpretation of the parts played by enclaves in the genesis and evolution of granitoid magmas.

Interaction processes between acid and basic magmas are widespread in the Sardinia–Corsica Batholith. The resulting hybrid magmas are extremely variable and can be broadly divided into: (i) microgranular mafic enclaves with geochemical characteristics of both magmatic liquids and cumulates; (ii) basic gabbroic complexes with internal parts mainly formed by cumulates and with interaction zones developing only in the marginal parts; and (iii) basic septa with the form of discrete, lenticular-like bodies often mechanically fragmented in the host rock. Different styles of interaction, ranging from mixing to mingling, have been related to variations in several physicochemical parameterś, such as: (i) the initial contrast in chemical composition, temperature and viscosity; (ii) the relative mass fractions and the physical state of interacting magmas; and (iii) the static versus dynamic environment of interaction.

A model is presented for the origin and history of interaction processes between basic and acid magmas based on the geochemical characteristics of hybrid magmas. Physico-chemical processes responsible for the formation of hybrid magmas can be attributed to: (i) fractional crystallisation of basic magma and contamination by acid magma; (ii) loss of the liquid phase from the evolving basic magma by filter pressing processes; (iii) mechanical mixing between basic and acid magmas; and (iv) liquid state isotopic diffusion during the attainment of thermal equilibrium.

Plutonic complexes with interlayered mafic and silicic rocks commonly contain layers (1–50 m thick) with a chilled gabbroic base that grades upwards to dioritic or silicic cumulates. Each chilled base records the infusion of new basaltic magma into the chamber. Some layers preserve a record of double-diffusive convection with hotter, denser mafic magma beneath silicic magma. Processes of hybridisation include mechanical mixing of crystals and selective exchange of H2O, alkalis and isotopes. These effects are convected away from the boundary into the interiors of both magmas. Fractional crystallisation aad replenishment of the mafic magma can also generate intermediate magma layers highly enriched in incompatible elements.

Basaltic infusions into silicic magma chambers can significantly affect the thermal and chemical character of resident granitic magmas in shallow level chambers. In one Maine pluton, they converted resident I-type granitic magma into A-type granite and, in another, they produced a low-K (trondhjemitic) magma layer beneath normal granitic magma. If comparable interactions occur at deeper crustal levels, selective thermal, chemical and isotopic exchange should probably be even more effective. Because the mafic magmas crystallise first and relatively rapidly, silicic magmas that rise away from deep composite chambers may show little direct evidence (e.g. enclaves) of their prior involvement with mafic magma.

Anhydrite has been identified as a phenocrystic phase in some silicic volcanic magmas, but it is not commonly described in plutonic rocks. Anhydrite-bearing magmas tend to form in arc environments and to contain hydrous, low-temperature, oxidised mineral assemblages. Phenocrystic anhydrite coexists with sulphur-enriched apatite and sometimes with pyrrhotite, in silicate melt that contains from 50 ppm to 1 wt% S, depending on temperature and conditions. Vapour coexisting with anhydrite- and water-saturated magma may contain from a few tenths of a mole per cent to a few mole per cent sulphur gases (SO2 and H2S), with the exact composition and gas speciation depending on temperature and oxygen fugacity. Samples of one anhydrite-bearing magma, the 1991 Pinatubo dacite, have been experimentally crystallised to determine whether the magma retains its characteristic sulphur-rich mineral phases during solidification. Results show that anhydrite and sulphur-rich apatite are retained throughout crystallisation and vapour phase evolution. This suggests that anhydrite-bearing intrusive equivalents of the Pinatubo dacite should be present in arc plutonic complexes.

Numerical models that account for fluid flow, magmatic and metamorphic fluid production, topography and thermal expansion of the fluid following emplacement of a granitic magma in the upper crust reveal controls on the distribution of magmatic fluids during the evolution of a hydrothermal system. Initially, fluid pressures are close to lithostatic in and near an intrusion, and internally generated magmatic and metamorphic fluids are expelled. Later, fluid pressures drop to hydrostatic values and meteoric fluids circulate throughout the system. High permeabilities and low rates of fluid production accelerate this transition. Fluid production in the magma and wallrocks is the dominant mechanism elevating fluid pressures to lithostatic values. For granitic intrusions, about three to five times as much magmatic fluid is produced as metamorphic fluid. Continuous fluid release from a granitic magma with a vertical dimensions of 10 km produces a dynamic permeability of up to several tens of microdarcies.

Near the surface, topography associated with a typical volcano acts to maintain a shallow meteoric flow system and drive fluids laterally. The exponential decay with depth of the influence of topography on fluid pressures results in a persistent zone of mixing at a depth of 1-2 km between these meteoric fluids and magmatic fluids despite variations in the strength of the magmatic hydrothermal system. However, in shallow systems where fluid release is episodic, dramatic changes in the region of mixing are still possible because fluid pressure is sensitive to variations in the rates of fluid production. At depth, high rates of metamorphic fluid production in the wallrocks and low permeabilities (< 1 μD) produce elevated fluid pressures, which hinder the lateral flow of magmatic fluids. Together, these patterns are consistent with the distribution and evolution of skarns and hydrothermal ore deposits around granitic magmas.