A petrological and geochemical study has been made of the late Precambrian St. David's granophyre (SDG) and the Cwm Bach rhyolites (CBR). The SDG is trondhjemitic, peraluminous, and chemically zoned with lower Na/K ratios in its interior portions. Although the SDG contains only minor amounts of K, the Rb content is relatively high and ratios of immobile versus mobile elements indicate that K has been removed. The CBR are flow-banded and autobrecciated subaerial rhyolite flows which are more K-rich and less altered than the SDG, lying close to the ternary minimum composition of the system Q-Or-Ab-(H2O). K mobility is considered to result from the reaction K-feldspar→ K-mica which released excess K ions, imparting peraluminous compositions to the rocks. On the basis of this reaction the original K content of the rocks has been estimated, the values for which are more compatible with immobile element abundances and with normal igneous trends. Similar abundances of the relatively immobile elements Ti, Zr, Sr, Y and Nb, together with K-corrected K/Rb ratios, suggest that the SDG and CBR may be comagmatic volcanic-arc-collision-type granite and rhyolite.

The mineralogy of the highly peralkaline Junguni nepheline syenite intrusion of the Chilwa alkaline province has been investigated. The rocks comprise alkali feldspar, very abundant nepheline, locally exceptionally abundant sodalite, sodic pyroxenes, scarce biotite, rare amphibole, and an extensive range of accessory minerals. Electron microprobe analyses indicate that the pyroxenes define an evolutionary trend from salite through aegirine-augite to aegirine, which is unusual in its broadness and ill-definition. This is explained by a series of overlapping trends produced by fluctuating Fe3+/Fe2+ ratios caused by variations in alkali content of the magma, probably produced by periodic alkali loss by surface de-gassing. Much of the sodalite occurs as a primary liquidus phase, but the paragenesis of certain ramifying sodalite veins is more problematical. It is possible that such ramifying masses are the product of coupled migration of alkalis and volatiles by diffusion in a gravitation field under pressure gradients generated by eruptive events, or by the formation of immiscible Na- and Cl-rich liquids. Both early calcic and late mangan-fluor eckermannitic/arfvedsonitic amphiboles occur. The micas vary from biotites with Mg: Fe ratios < 0·5 to almost pure annites; they are fluor-micas and characterized by high Mn contents. Analyses of niobian rutile, mangan ilmenite, ferroan pyrophanite, manganese-rich eucolite, låvenite, mangan-titan låtvenite, rosenbuschite, wöhlerite, pyrochlore, eudialyte and what is believed to be only the second occurrence of kupletskite are given. Many of these minerals are rich in Zr, Nb, Na and Mn and thus typical of the assemblages found in extreme agpaitic complexes such as Ilimaussaq and Lovozero.

Metamorphic rocks of undoubted crustal origin have been described in recent years, principally from Mediterranean collision zones that have been subjected to PT conditions along very low geothermal gradients (∼ 7°C/km) and have reached pressures up to 30 kbar. MgAl-rich metapelites develop particularly diagnostic high-pressure minerals and mineral assemblages that have been and are being studied experimentally in model systems involving the components K2O, MgO, Al2O3, TiO2, SiO2, P2O5, and H2O up to pressures of 50 kbar and temperatures of 1000°C.

In the present review the following synthetic phases and phase assemblages are discussed, emphasizing their water-pressure-temperature stability fields (approximated in parentheses here), their reaction relationships, and their known or potential occurrences in metamorphic rocks. Sudoite (0 to ∼ 12 kbar, 150? to 380°C) occurs in very low-grade metapelites. Mg-carpholite (∼ 7 to ∼ 45 kbar, ∼ 200 to 600°C) is found in subducted metabauxites, metapelites, and related quartz veins. Mg-chloritoid (18 to 45 kbar?; 400 to 760°C) has not been found in nature as pure or nearly pure end-member; it requires silica-deficient environments. Yoderite, known in nature only from a single talc-kyanite schist occurrence, has only a small stability field (9 to 18 kbar?, 700 to 870°C?), cannot coexist with quartz, but may be stabilized by Fe3+. Pyrope (∼ 15 to at least 50 kbar, ∼ 700°C to melting), with or without relic coesite inclusions, occurs spectacularly in quartzites. Mg-staurolite (∼ 14 to some 90 kbar?, 700 to 1000°C), recently discovered as inclusions in pyrope, requires silica-deficiency. MgMgAl-pumpellyite is a new synthetic phase in which Mg totally replaces Ca of normal pumpellyite; because of its very high-pressure, low-temperature stability (∼ 37 to at least 55 kbar, < 400 to 780°C) it may not form within our globe. Ellenbergerite, the new high-pressure mineral forming inclusions in pyrope, apparently exhibits a rather composition-dependent stability with Ti-ellenbergerite, requiring higher pressures (> 20 kbar) than P-bearing, Ti-free members; a pure hydrous Mg-phosphate with ellenbergerite structure was synthesized at 10 kbar. Phengites, the widespread MgSi-substituted muscovites, require increasingly high water pressures (up to ∼ 20 kbar) for higher degrees of substitution, but the Al-celadonite end-member is not stable under any conditions; the compositions of phengites coexisting with limiting assemblages such as phlogopite, K-feldspar, and an SiO2 phase are useful geobarometers. The common assemblage Mg-chlorite + Al2SiO5 (mainly kyanite) has an extensive stability field ranging from near zero to 31 kbar at temperatures varying from some 320 to ∼ 760°C depending on pressure. The whiteschist assemblage talc + kyanite (6 to ∼ 45 kbar, 550 to 810°C) plays an important role in collision zone metamorphism as it forms from the greenschist assemblage chlorite + quartz at low grades but is also known to break down into pyrope + coesite at the highest grade observed thus far. The assemblage talc-phengite (11 to at least 35 kbar, 300? to 820°C depending on pressure), on the other hand, is well known from subducted metapelites. At pressures of 15–20 kbar and temperatures of 400–650°C a very K,Mg-rich, siliceous fluid forms as a consequence of the mutual reaction of the minerals K-feldspar and phlogopite (biotite) which are very common in crustal rocks including granites. Such fluids are bound to cause metasomatism in neighbouring mantle rocks which, upon subsequent increase of temperature, produce post-collisional ultrapotassic, lamproitic melts.

Six hundred and forty four gamma-ray spectrometer assays of U and Th obtained within the late Caledonian Galway Granite western Ireland are presented. The data cover the range of granodiorites, adamellites and leucogranites (Murvey Granite) present in the batholith. There is an overall increase in U and Th abundances with petrological evolution. The broad scatter of values that characterizes the Murvey Granite reflects the geographically separate occurrences of this leucogranite. Rubidium and strontium data imply varying degrees of fractionation among these separate Murvey Granite occurrences. Y and HREE are notably enriched in the Murvey granite at Costelloe which also contains the highest Th levels amongst the leucogranites. Thorite, uraninite, monazite and Y-zircon are present in this leucogranite and are responsible for the observed enrichments in U, Th, Y and HREE.

The Permian magmatic province of the Oslo rift, south-east Norway, includes large volumes of felsic and silicic rocks. Based on their geochemical character, these rocks may be divided into two main groups. The Larvik larvikites (monzonites) are highly enriched in large ion lithophile elements (LILE) (e.g. 10–32 ppm Th. 8–15 ppm Ta), and have an initial 87Sr/86Sr of 0.70391 ± 5. The syenites and granites have moderate to high concentrations of LILE (e.g. 7–88 ppm Th, 4–25 ppm Ta), and initial 87Sr/86Sr ratios between 0.705 and 0.707. The Larvik larvikites and extrusive equivalents (rhomb porphyry lavas) have similar initial Sr isotope ratios to uncontaminated basalts and gabbros in the rift, and are believed to have a mantle origin. The higher initial 87Sr/86Sr ratios in the silicic than in the felsic rocks reflect a crustal component representing the intermediate or low crust. After intrusion into the upper crust, the major and traceelement concentrations of the silicic magmas were modified through fractional crystallization dominated by removal of alkali feldspar, and transport of elements with a fluid phase. The silicic magmas appear not to have interacted significantly with the side rock at this stage.

The rock association of minette with silicic lavas and intrusions (granites, syenites, dacites) is a common geologic feature in both collisional and extensional tectonic settings. Considerable doubt exists as to whether a genetic link exists between these mafic and silicic rocks. We describe a Miocene sill from NW Colorado which is a clear example of a mixed magma consisting of originally-liquid inclusions of minette in a silicic trachydacite host. Chemical and Sr, Nd and Pb isotopic data are consistent with derivation of the silicic host magma of the sill dominantly by fractional crystallization of the minette magma. Correlations between the elemental compositions of the rock types and their Sr and Nd isotopic ratios imply minor assimilation of continental crust with relatively low values of both 87Sr/86Sr and 143Nd/144Nd, concomitantly with fractional crystallization. The parental minette magma was probably derived by partial melting of subcontinental lithospheric mantle. While the sill was emplaced in a rift-like tectonic setting, the chemical and isotopic composition of the lithosphere-derived minette magmas (and hence the silicic fractionates) was largely independent of this setting, but dependent upon the composition and age of the lithospheric mantle and crust.

The southern part of the Cretaceous Kohistan island arc is occupied by an extensive belt dominantly comprised of amphibolites. These include banded amphibolites of partly meta-volcanic parentage, and non-banded amphibolites derived from intrusive rock. In addition to being relict, banding has also been produced by shear deformation, metamorphic/metasomatic segregation and, possibly, by lit-par-lit injection of plagiogranitic material. Non-banded amphibolites also occur as retrograde products of noritic granulites forming the lopolithic Chilas complex. The chemistry of 37 rocks has been compared with those of known tectonic environments. The amphibolites have chemical characteristics similar to volcanic rocks found in island arcs and most of the analyses apparently support affinity with the calc-alkaline series. The amphibolites consist essentially of hornblende, plagioclase and/or epidote. Garnet and clinopyroxene have developed locally in rocks of appropriate bulk composition. Metamorphism may have taken place during the mid-Cretaceous under conditions of 550 to 680°C and 4.5 to 6.5 kbar PH2O. The metamorphic grade appears to increase from the centre of the southern belt toward the Chilas complex to the north and Indus-Zangbo suture (IZS) to the south. In the vicinity of the IZS, garnet-clinopyroxene ± amphibole assemblage developed locally in response to high P-T.

The Middle Zone (MZ) of the Insch intrusion lies in a geographically intermediate position between the Lower Zone (LZ) and Upper Zone (UZ) cumulate sequences, but is more complicated than either in comprising two intricately associated, but texturally distinct, components, the MZ cumulates and the fine-grained granular gabbros (FGG). In addition, there is a minor group of porphyritic granular gabbros (PGG), containing abundant plagioclase phenocrysts. A further variety of gabbroic rock, namely quartz-biotite norite (QBN) occupies a similarly intermediate position in the Boganclogh extension of the main Insch mass. These principal components (MZ cumulates, FGG, and QBN) show considerable mineralogical overlap with each other and with the lower part of the UZ succession (UZa). Unlike the UZ rocks, they are essentially olivine-free (apart from the most basic granular gabbros) and consist mainly of plagioclase, orthopyroxene, and Ca-rich clinopyroxene, with accessory Fe-Tioxides and minor interstitial biotite and/or hornblende. Mineral compositions are in the range An70–55 (plagioclase), En71–44 (opx), and Ca46Mg42Fe12 to Ca45Mg30Fe25 (cpx), but in detail each rock group can be characterized mineralogically as well as texturally. It is concluded that the MZ cumulates and FGG (including PGG) are essentially complementary, formed in different locations, and under slightly different conditions, but in the same magma chamber. The complex relationships between them, and the apparently haphazard geographical variations in mineral compositions, may be the combined results of the magmatic events, possibly including the movement of large xenolithic fragments, and later block faulting. The substantial compositional overlap of the more evolved MZ cumulates by UZa is explained in terms of replenishment by magma of slightly more primitive (and potentially olivine-bearing) composition after the deposition of the MZ. The Boganclogh QBN is believed to represent a more hydrous fraction of the Insch MZ magma.

The Kærven syenite complex, which reflects the hitherto earliest recorded stages in the Tertiary of East Greenland, outcrops in the middle reaches of the Kangerdlugssuaq Fjord as a peripheral intrusion to the Kangerdlugssuaq intrusion. The rocks of the Kærven complex range from syenite through alkali feldspar quartz-syenite to alkali feldspar granite. The general sequence of crystallization of the Kærven magmas was: alkali feldspar ± olivine(Fa96−99) ± plagioclase(An41−11), clinopyroxene (augite, ferrosalite, ferrohedenbergite), quartz and amphibole. Whole-rock major and trace-element data show coherent geochemical trends which suggest comagmatism. The data reveal that the Kærven rocks are distinct from the rocks from the adjacent Kangerdlugssuaq intrusion (e.g. higher TiO2, FeOT in low-SiO2 samples, lower Na2O, approx. constant Zr/Nb). The mineral chemistry supports this conclusion, as the Kærven samples typically have calcic amphiboles and clinopyroxenes with a very limited Na-enrichment in contrast to the sodic trends of the Kangerdlugssuaq intrusion. Normative feldspar compositions plot near to the Ab-Or cotectic in the Q-Ab-Or system and a maximum pressure of crystallization of 3–5 kbar with moderate to low PH2O is indicated.

Trace elements preferently incorporated in plagioclase and alkali feldspar, i.e. Sr, Ba and Rb, show systematics which are not compatible with an evolution of the rock suite by crystal fractionation of these phases, though possibly alkali feldspar may be partially accumulated in a few very evolved rocks. Numerical calculations do not suggest a magmatic evolution by fractional crystallization of the observed phases. The variation of Sr, Ba and Rb as well as of the incompatible elements Nb, Zr and Th support a derivation of the rock suite mainly by mixing two components, a syenitic and a granitic end-member. It is concluded that magma mixing was the most significant process in the formation of the Kærven rock suite accompanied by some crystal fractionation. Evidence for crustal contamination is detected in a few samples from the outer part of the intrusion but has not affected the main suite of rocks.

The surfaces of quartz crystals that were partially dissolved in superheated, H2O-saturated rhyolite melt are covered with hemispherical embayments; each embayment is judged to have formed where a gas bubble in the melt approached the crystal. ‘Flux-line attack’ and ‘upward-drilling’ of the refractory lining of glass tanks are analogous processes. As a bubble nears a dissolving solid it enters a compositional boundary layer in the melt, resulting in melt of variable surface tension surrounding the bubble. This unstable situation results in small-scale convection of the melt about the bubble (Marangoni convection) which can cause locally enhanced dissolution rate of the solid. It is suggested that this mechanism could cause round embayments to form in quartz phenocrysts in acid volcanic and sub-volcanic rocks. Criteria by which embayed phenocrysts formed by dissolution can be distinguished from those formed by unstable growth are reviewed briefly.

A study of xenocrystic material included in 57 Ma igneous rocks that outcrop in the Taourirt area of north-east Morocco has been made. The mineralogy of the host rocks is essentially clinopyroxene, rare olivine, titanomagnetite and either kaersutite or biotite. Feldspars (plagioclase and alkali feldspar) and nepheline are usually altered. Accessories may include haüyne, perovskite and ferriannite-rich annite. The complex xenocrystic assemblage includes various types of clinopyroxene, amphiboles, micas of differing compositions and various types of spinel. These inclusions did not dissolve in the melt and, as a consequence, the residual (alkali-rich) mineralogical assemblage is not modified. Their abundance is such that they may not significantly affect the major element compositions, but they may strongly influence the levels of certain trace elements such as Cr and Ni.

The first occurrence of orbicular rocks in Turkey is reported. They are gabbros in a dyke in the Baskil island-arc magmatic suite, of the southern branch of the Alpine-Himalayan chain. The orbicules have lithologically varied cores, regular shells of troctolitic composition, with a radial arrangement of olivine and plagioclase crystals, and a matrix of vari-textured gabbro. Rock and mineral analyses indicate that magmatic crystallization began near the inner margin of the troctolitic shells. Metamorphic hydration of minerals in cores, shells and matrix followed directly after the later stages of magmatic crystallization, as part of a continuous process, and thus a magmatic-metamorphic origin for the orbicules is proposed. Alternative hypotheses for the development of the orbicules are discussed: one involving the development of a protocrystalline magma shell, the other rapid crystallization during upward migration of xenolith-bearing magma.

Roxbyite (Cu1.74–1.82S), a new mineral, was isolated from drill core from the Olympic Dam deposit, from copper flotation concentrates from the Olympic Dam pilot plant, and from reaction residues obtained by treating certain concentrates with sulphuric acid. Powder X-ray diffraction data and the composition of the mineral are similar to results reported previously for the unnamed mineral of composition Cu1.83S (with a trace of iron) from the El Teniente deposit in Chile, and to copper sulphides of composition CuxS, 1.75 ⩽ x ⩽ 1.86, prepared electrochemically by several workers. A Weissenberg single-crystal study shows roxbyite to be monoclinic with a = 53.79, b = 30.90, c = 13.36Å, β = 90.0° and space group alternatives C2/m, Cm or C2. Some iron in the structure may stabilize the mineral.

Volatile species analysis of fluid inclusions in metamorphic quartz veins from the Llanbedr Formation, North Wales, show nitrogen to be an important component. Microthermometric and Laser Raman analysis indicates that the N2 is resident in a generation of very N2-rich inclusions and demonstrates the presence of both N2-rich and aqueous fluids during metamorphism of the Llanbedr Formation. N2-rich fluids do not appear to have been present in adjacent lithologies. Isotopic analysis of N2 in fluid inclusions and in the slates indicates that the N2 in the slates (thought to be present as substituting for K+ in muscovite and for Na+ in ) was probably originally derived from organic matter in the sediment and subsequently released to the fluid phase during metamorphism.

Mineral-fluid and fluid-phase equilibrium calculations show that the mineral assemblage in the slates could be in equilibrium with either N2-rich or aqueous fluid depending on redox conditions. The N2-rich and aqueous fluids in the veins could, therefore, have been trapped at different times under different conditions, though their coexistence as immiscible fluids is a possibility.

Rock deformation and metamorphism can interact at the mechanistic level in the following ways: (a) facilitation of cataclasis through the release of high-pressure fluid during dehydration and decarbonation reactions; (b) facilitation of intracrystalline plasticity through the stresses induced during solid-state volume changes; (c) enhanced deformability through the transient existence of fine-grained reaction products; (d) modification of chemical potential gradients driving diffusion if a reaction can occur along the diffusion path; (f) changes in the resistance to intracrystalline plasticity through the effect of reaction-induced changes, pore fluid pressure and chemistry on point defect chemistry of the solid phases.

Examples of experimental studies of each of these types of interaction are described. Special experimental problems arise through: (i) the effects of solid phase and pore space volume changes, and their effects on pore fluid pressure and measured strain; (ii) the effects of such microstructural changes on the determination of flow law parameters; and (iii) in many instances the need for very long duration deformation experiments if reaction kinetics are sluggish.

There is an outstanding need for experimental studies of the effects of non-hydrostatic stress on the conditions for the onset of metamorphic reactions and phase transformations, as a basis for understanding some classes of deformation/metamorphism interaction. However, it is emphasized that the threefold classification of rock deformation mechanisms into cataclastic, crystal-plastic, and diffusive mass transfer processes, established from the study of deformation of monomineralic rocks, forms an essential framework for the understanding of deformation/metamorphism inter-relationships.

Mafic dykes occur in close association with, and both cut and are cut by, the Eskdale granite in the south-western Lake District. The dykes range compositionally from magnesian basalt to andesite and are divided into two groups: (1) high-Fe-Ti rocks of tholeiitic affinity forming most of the dykes and (2) a lower-Fe-Ti group, comparable in composition to the lavas of the Borrowdale Volcanic Group. The dykes extend the range of tholeiitic magmatism in the Lakes into late Ordovician, and possibly Silurian times, and indicate that published plate tectonic models partly based on the distribution of magma types are perhaps over-simplified. The Eskdale dykes form one end of a spectrum of Lake District compositions from tholeiitic to calc-alkaline. All the magma types may have shared a common mantle source, their final composition reflecting residence times in the crust or LIL-enriched mantle.

New major and trace element chemical analyses of trioctahedral micas of the Cornubian batholith emphasize their enrichment in a ‘trace-alkali element’ association (Li, Rb, Cs, F, Nb, Mn) and their deficiency in a ‘femic element’ association (Zr, Ce, Th as well as Mg and Ti) compared with micas from many other granite suites, although there are similarities with some Hercynian granites and with rare metal pegmatites. The new data demonstrate a continuous series from siderophyllite through zinnwaldite to lepidolite, principally as a result of Li-R2+ substitution as indicated by Foster (1960), although a more complete replacement is (Li, Al) = (Fe2+, Fe3+, Ti). It is suggested that the ranges of these micas are based upon the Li content in the unit cell formula and the ratio of Li to R2+, in effect, a compromise between the ranges proposed by Foster (1960) and Rieder (1970). Microprobe analyses lack Li2O, but can be plotted on FeO-SiO2 and FeO-Al2O3-SiO2 diagrams (wt. or atom %) in order to locate compositions within the trioctahedral LiFe micas and to distinguish between lepidolite and muscovite.

An examination of 55 new mica analyses shows that hornfels biotites are richer in Mg and that the Cornubian Type A granite (as classifed by Exley and Stone, 1982) micas are richer in Ti and Fe compared with those of Type B granites. Micas from microgranite dykes appear to be poorer in femic elements and richer in trace alkalis and F compared with their Type B granite hosts, consistent with their differentiation from the latter. Mica chemistry is consistent with the magmatic evolution of the A-B-microgranite sequence in the biotite granites, the transformation of Types B to D upon emplacement of E, the derivation of Types E from B by extreme differentiation or metasomatic transformation and mobilization, and the in situ differentiation of Types G from E.

Ion microprobe analysis ofperovskite from kimberlites and alnöites permits the accurate determination of La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er and Yb at the part per million level. Other rare earth elements (REE) are subject to interferences in the mass spectrum caused by matrix-derived molecular ions and cannot easily be determined with comparable precision. Chondrite-normalized plots of the ion probe REE data are smooth curves, confirming the superiority of this technique over electron microprobe methods at the levels of REE abundance found in these perovskites. The perovskites analysed contain between 2.8 and 7.1 wt. % REE oxides and are highly enriched in the light REE, having La/Yb ratios of 577–3229. These La/Yb ratios are not representative of the parental magmas but result from REE fractionation during crystallization of the perovskite. Parental magma La/Yb ratios are estimated to be of the order of 120–650. The lower La/Yb ratios (80–200) found for whole-rock kimberlites are considered to result from contamination by relatively heavy REE-enriched crustal material.

The Visoke complex is one of the main Quaternary volcanic centres of the Virunga Range, located north of lake Kivu. Mineralogical (microprobe) data are given for two representative leucitite lavas; one sample contains a complex coarse-grained xenolith (phlogopite, diopside, leucite, titanomagnetite, perovskite and apatite) and megacrysts of pyroxene, phlogopite and olivine scattered in fine-grained leucite-rich host lava. Compared with the typical leucite-dominated, low-pressure phenocryst assemblage of the two samples studied, the chemical trends of ferromagnesian crystals suggest an earlier igneous event (high-pressure phenomenon) strongly related to the leucite-bearing magma suite.

The work on liquid immiscibility in carbonate-silicate systems of Freestone and Hamilton (1980) has been extended to include alkali-poor and alkali-free compositions. Immiscibility is shown to occur on the joins albite-calcite and anorthite-calcite at 5 kbar. These results make it possible to interpret ocellar structure between calcite-rich spheroids in lamproite or kimberlite host rock as products of liquid immiscibility. The common sequence of rock types found in carbonatite complexes of melilitite-ijolite-urtite-phonolite is interpreted as being the result of both fractional crystallization and liquid fractionation, the corresponding carbonatite composition changing from nearly pure CaCO3 (±MgCO3) progressively to natrocarbonate. A carbonate melt cooling in isolation will suffer crystal fractionation, the residual liquid producing the rarer ferrocarbonatites, etc., whilst the crystal accumulate of calcite (dolomite) plus other phases such as magnetite, apatite, baryte, pyrochlore, etc., are the raw material for the coarse-grained intrusive carbonatites commonly found in ring complexes.