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Silica-rich granites and rhyolites are components of igneous rock suites found in many tectonic environments, both continental and oceanic. Silica-rich magmas may arise by a range of processes including partial melting, magma mixing, melt extraction from a crystal mush, and fractional crystallization. These processes may result in rocks dominated by quartz and feldspars. Even though their mineralogies are similar, silica-rich rocks retain in their major and trace element geochemical compositions evidence of their petrogenesis. In this paper we examine silica-rich rocks from various tectonic settings, and from their geochemical compositions we identify six groups with distinct origins. Three groups form by differentiation: ferroan alkali-calcicmagmas arise by differentiation of tholeiite, magnesian calc-alkalic or calcicmagmas form by differentiation of high-Al basalt or andesite, and ferroan peralkalinemagmas derive from transitional or alkali basalt. Peraluminous leucogranitesform by partial melting of pelitic rocks, and ferroan calc-alkalicrocks by partial melting of tonalite or granodiorite. The final group, the trondhjemites, is derived from basaltic rocks. Trondhjemites include Archean trondhjemites, peraluminous trondhjemites, and oceanic plagiogranites, each with distinct geochemical signatures reflecting their different origins. Volcanic and plutonic silica-rich rocks rarely are exposed together in a single magmatic center. Therefore, in relating extrusive complements to intrusive silica-rich rocks and determining whether they are geochemically identical, comparing rocks formed from the same source rocks by the same process is important; this classification aids in that undertaking.

In the system CaO-MgO-FeO-Fe2O3-TiO2--SiO2, equilibria among titanite, pyroxene(s), olivine, spinel phase, ilmenite, and quartz constrain fO₂, T, aSiO₂, aTiO₂, aFe, and the compositional variables, μTiFe2+Fe3+-2, μCa(Mg,Fe)-1, and consequently μMg,Fe-1among the silicate and oxide phases. These equilibria can provide the framework for a better understanding of relevant titanite-bearing natural assemblages. In addition, such equilibria can constrain the variables that operate during crystallization of igneous and metamorphic rocks. Titanite-bearing assemblages with pyroxene(s), Fe-Mg-Ti ilmenite and spinel, olivine, and quartz are rare in metamorphic rocks but are common in igneous rocks where they are more commonly reported in plutonic than in volcanic rocks. Probably because, at high temperatures, titanite cannot coexist with the relative common in volcanic rocks sub-assemblage augite + orthopyroxene + olivine + spinel phase + ilmenite. Moreover, decreasing pressure and temperature conditions appear to favor titanite. Thus, it is not surprising that titanite is commonly observed in slowly cooled rocks, albeit, most typically in association with amphibole. We argue that the titanite + amphibole association is favored by high aH₂Oand, because aH₂Otypically increases during crystallization of a pluton, titanite + amphibole (and consequently titanite) is more common in plutonic than in volcanic rocks.

Proterozoic granitoids in the Laramie Range of southeastern Wyoming crop out on both sides of an inferred suture between the Archean Wyoming craton and Proterozoic terranes to the south. These rocks offer an unusual opportunity to test the various hypotheses for the geometry of the suture. No difference in major or trace element composition is observed across the suture, nor is there a large difference in isotopic composition. Nd and Sr isotopic data indicate that only a small proportion of Archean material has contributed to this granitoid suite, even in two samples that intrude Archean gneiss. The isotopic data therefore require a dominantly Proterozoic lower crust both beneath the Proterozoic rocks and beneath the margin of the Archean block. This observation is inconsistent with models in which the suture between the Archean and Proterozoic blocks dips to the south. This inconsistency can be reconciled by magmatic underplating or mechanical underthrusting of Proterozoic crust beneath the Archean block during terrane accretion or post-accretion strike-slip faulting.

There are three geothermometers based on reversed experimental data and applicable to granulites: the two-pyroxene, two-oxide, and garnet-clinopyroxene thermometers. All have apparent closure temperatures below those of the granulite facies. This casts significant doubt on the concept that "peak" temperatures are routinely obtained from ion-exchange thermometers, particularly those that are empirically calibrated. The best way to recover high temperatures in granulite terrains is through the reintegration of exsolution lamellae or the use of relict mineral assemblages; we call these features fossil thermometers. Because fossil thermometers can be destroyed by deformation, in many terrains it may be impossible to tell whether the temperature they record is a true maximum or whether it is a temperature locked in during cooling. Low closure temperatures also affect the oxygen fugacites obtained from two-oxide equilibria. Compilation of the available data indicate that a number of granulite terrains either crystallized on the graphite saturation surface or underwent re-equilibration until they intersected this surface. Subsequent oxyexsolution appears to have taken place on the graphite saturation surface by the reaction: $$6 Fe_{2}TiO_{4} + CO_{2} = 2 Fe_{3}O_{4} + 6 FeTiO_{3} + C$$, suggesting that a $$CO_{2}$$vapor phase was present during the cooling history of these granulites. Recognition of the low closure temperature of ion-exchange thermometers also calls into question the validity of P-T paths calculated from retrograde garnet zoning profiles. Because geobarometers are likely to lock in at higher temperature than do geothermometers, the P-T paths obtained from the zoned rims of granulite-grade garnets may be artifacts of the differential closure temperatures for both the barometer and the thermometer. Consideration of the garnet-plagioclase-aluminosilicate-quartz, garnet-rutile-aluminosilicate-ilmenite-quartz, and garnet-plagioclase-orthopyroxene-quartz barometers indicates that the apparent P-T path obtained due to differential closure temperatures will be indicative of isobaric cooling, regardless of the actual P-T path followed by the rock. This problem is illustrated with an example from the Kerala Khondalite Belt of southern India in which the garnet zoning profile records an apparent isobaric cooling path, although fossil thermobarometry strongly suggests that the rock underwent considerable decompression during cooling.

There are three populations of fluid inclusions in quartz from the Sybille Monzosyenite: early CO2, secondary CO2, and rare secondary brines. The oldest consist of low density CO2 (??0.70) inclusions that appear to be co-magmatic. The densities of these inclusions are consistent with the inferred crystallization conditions of the Sybille Monzosyenite, namely 3 kilobars and 950–1000° C. The other types of inclusions are secondary; they contain CO2 (??0.50) and secondary brine inclusions that form trains radiating out from a decrepitated inclusion. The sites of these decrepitated inclusions are now marked by irregularly shaped fluid inclusions and solid inclusions of salt and carbonate. Rather than fluid inclusions, feldspar contain abundant solid inclusions. These consist of magmatic minerals, hedenbergite, hornblende, ilmenite, apatite, and graphite, intimately associated with K, Na chlorides. We interpret these relations as follows: The Sybille Monzosyenite formed from a magma that contained immiscible droplets of a halide-rich melt along with a CO2 vapor phase. The salt was trapped along with the other obvious magmatic minerals during growth of the feldspars. CO2 may have also been included in the feldspars but it probably leaked later during exsolution of the feldspars and was not preserved. Both the saline melt and the CO2 vapor were trapped in the quartz. The melt inclusions in the quartz later decrepitated, perhaps due to progressive exsolution of fluids, to produce the secondary H2O and CO2 inclusions. These observations indicate that the Sybille Monzosyenite, which is a markedly anhydrous rock, was actually vapor-saturated. Rather than being H2O, however, the vapor was CO2-rich and possibly related to an immiscible chloride-rich melt.

The discovery that granulites contain CO2-rich fluid inclusions while amphibolites in the same terrane contain H2O-rich or mixed CO2–H2O inclusions1has led to a debate over the processes that produce granulite metamorphism. One theory2–4maintains that granulites form as the result of a massive influx of CO2from lower crustal or mantle sources. Such an influx could be responsible for the marked depletion many granulite terranes show in large-ion lithophile elements2,5. Others, however, maintain that granulite-facies metamorphism is a necessary complement of crustal melt ing6–9. Recently, Lamb and Valley10,11have argued that granulite metamorphism may be a product of any one of three processes: CO2-streaming, partial melting, or recrystallization of originally dry rocks. Here we suggest that these processes need not be independent and that they all may be related to the passage of melts through the crust.

Using graphical analysis of the system CaO-Al2O3-SiO2-H2O-CO2, this paper derives a topology relating the minerals calcite, laumontite, wairakite, prehnite, quartz, and zoisite. Simple thermodynamic reasoning allows this system to be applied to natural rocks and indicates that the first appearance of the assemblage epidote-chlorite-quartz (±albite) should mark the upper boundary of zeolite facies. This assemblage forms at the expense of laumontite+prehnite, laumontite+calcite, or laumontite+pumpellyite, with wairakite likely to replace laumontite as the stable zeolite at low pressures. In natural systems this proposed facies boundary is multivariant and, hence, it is likely to be strongly sensitive to compositional variables. For example, Na-bearing wairakite will be more stable than pure Ca-wairakite and increasing abundance of Fe3+ will tend to stabilize epidote+quartz at the expense of the zeolites. Because of this, monitoring the composition of minerals such as epidote, prehnite, or wairakite from lowvariance assemblages may provide a more-sensitive indicator of metamorphic grade than the presence or absence of any particular mineral assemblage.

During regional metamorphism of the Malenco serpentinized peridotite (Sondrio, northern Italy), the mineral assemblage pentlandite-awaruite-magnetite-native copper-antigorite-brucite-olivine-diopside is formed. The opaque assemblage indicates very reduced fluids with fO2 values 4 log units below QFM. Primary fluid inclusions were trapped in diopside overgrowth, contemporaneous with the opaque assemblage. These metamorphic fluids are saline aqueous solutions (about 10.4 mol% NaCl equivalent) and contain molecular H2 of approximately 1 mol%, as shown by micro-Raman analysis and microthermometry. The fluids are interpreted to have been formed during deserpentinization at the olivine-in isograd under strong reducing conditions.

The record of the first two billion years of Earth’s history (the Archean) is notoriously incomplete, yet crust of this age is present on every continent. Here we examine the Archean record of the Wyoming craton in the northern Rocky Mountains, USA, which is both well-exposed and readily accessible. We identify three stages of Archean continental crust formation that are also recorded in other cratons. The youngest stage is characterized by a variety of Neoarchean rock assemblages that are indistinguishable from those produced by modern plate-tectonic processes. The middle stage is typified by the trondhjemite-tonalite-granodiorite (TTG) association, which involved partial melting of older, mafic crust. This older mafic crust is not preserved but can be inferred from information in igneous and detrital zircon grains and isotopic compositions of younger rocks in Wyoming and other cratons. This sequence of crust formation characterizes all cratons, but the times of transition from one stage to the next vary from craton to craton. [ABSTRACT FROM AUTHOR]