Pyrometallurgical slags from three Cu-Co smelters (Nkana, Mufulira, Chambishi) in the Copperbelt Province, Zambia, were studied from mineralogical and chemical points of view. The slags were enriched in metals and metalloids, mainly Cu (up to 35 wt.%), Co (up to 2.4 wt.%) and As (up to 3650 ppm). The following primary phases were observed in slags: Ca-Fe silicates (clinopyroxene, olivine) and leucite, oxides (spinel-series phases), ubiquitous silicate glass and sulphide/metallic droplets of various sizes. The presence of glass and skeletal/dendritic crystal shapes indicated rapid cooling of the slag melt. Copper and cobalt were found in low concentrations in the majority of silicates (olivine, clinopyroxene) and oxides, substituting for Fe in their structures (up to 7.15 wt.% CoO in olivine, 4.11 wt.% CuO in spinel). Similarly, up to 0.91 wt.% CoO and 6.90 wt.% CuO were observed in the interstitial glass. Nevertheless, the main carriers of these metals in the slags studied were Cu sulphides (digenite, chalcocite, bornite, chalcopyrite), Co-Fe sulphides (cobaltpentlandite), Co-bearing intermetallic phases ((Fe,Co)2As) and alloys. Weathering features corresponding to the presence of secondary metal-bearing phases, such as malachite (Cu2(CO3)(OH)2), brochantite (Cu4SO4(OH)6) and sphaerocobaltite (CoCO3), were observed on the slag surfaces. They indicate that the slags studied are reactive on contact with water/atmosphere and that their environmental stability and release of potentially harmful metals and metalloids must be evaluated further.

The Cerro Prieto geothermal system provides a unique opportunity for the detailed study of calc-silicate mineral transitions between the diagenetic clay-carbonate and greenschist facies within the terrigenous sediments of the Colorado River delta. In this system, progressive devolatization reactions within carbonate-cemented, quartzofeldspathic sediment have produced a distinct hydrothermal mineral zonation at temperatures between 200–370°C and fluid pressures below 0.3 kbar. Descriptive and compositional data are presented for these minerals which include wairakite, epidote, prehnite, actinolite, clinopyroxene, garnet, sphene, biotite, microcline, and calcite. Partitioning of octahedral Fe, Mg, and Al between coexisting authigenic silicates is comparable with data from higher temperature metamorphic rocks and demonstrates an approach to local equilibrium within this system. Calculated fugacities of oxygen at temperatures above 300°C are (with rare exception) more reducing than that defined by the quartz-fayalite-magnetite buffer, a result consistent with the scarcity of hematite and grandite and the ubiquitous presence of organic material in Cerro Prieto sandstones.

The Shiant Isles Main Sill, of Tertiary age, is a classic example of a composite, differentiated alkaline basic sill. The first unit to be intruded was a 2 m thick olivine teschenite which was emplaced with phenocrysts of olivine (mg > 83) [mg ≡ Mg#] and, perhaps, plagioclase. This was intruded by a 24 m thick picrite sill consisting of a mush of melt and suspended olivine phenocrysts (mg > 83) with a D-shaped modal profile. The 140 m thick picrodolerite-crinanite unit was formed by a magma carrying ∼ 10% olivine (mg > 83) as the main phenocryst phase, together with some calcic plagioclase phenocrysts, being emplaced into the top of the picrite unit before the host rock was completely solidified. The olivine phenocrysts settled towards the bottom to form the picrodolerites. In-situ differentiation processes occurred under conditions of almost perfect fractional crystallization, during which very strongly zoned ophitic crystals of olivine (fayalitic rims) and clinopyroxene (hedenbergitic rims), and zoned laths of plagioclase (anorthoclase rims), formed. The last unit consists of ∼ 2 m of granular olivine picrodolerite which was intruded into the upper crinanites, again before the host rock was fully solid.

The mineral zoning patterns are interpreted using published cation diffusion coefficient data, and used to show that the picrite unit might have cooled to the blocking temperatures for Mg and Fe diffusion in < 5 years, and that even the relatively thick crinanite unit cooled very fast, so preserving the zoned Fe-Mg olivine and pyroxene compositions. The compositions of coexisting ilmenites and spinels define a redox trend which initially lies close to fayalite-magnetite-quartz buffer conditions, but later becomes more reducing and approaches magnetite-wustite buffer conditions. The final stages of development occurred during sub-solidus deuteric processes and involved formation of analcime and zeolites, as well as localized sulphide mineralization.

One metamorphic and four magmatic aegirines, together with two end-member aegirines synthesized at atmospheric pressure and different temperatures, were investigated by single-crystal X-ray diffraction. The limited compositional differences allow the polyhedral volumes to be almost constant in all the aegirines investigated( VM1 ≈ 11.0 Å3; VM2 ≈ 26.3 Å3; VT ≈ 2.21 Å3). However, differences in polyhedral distortions are responsible for the cell-volume variations, reflected mainly in the change of a and β cell parameters. Cell volume is only partly related to the composition of these aegirines: with increasing formation temperature, an increase in the unit-cell volume of ~1.2 Å3 is observed, while a significant contraction of the cell volume occurs during high-pressure formation. As the difference in cell volume between the two synthetic aegirines is ascribed to the different conditions of synthesis temperature, the same interpretation could be adopted for the differences observed in natural aegirines.

An inclusion of corundum (ruby) was found in a clinopyroxene xenocryst in alkali basalt from the late-Cenozoic Chanthaburi-Trat volcanics of eastern Thailand. The clinopyroxene is fairly sodic, highly aluminous and magnesian (0.12–0.14 Na, 0.31–0.33 AlIV and 0.36–0.40 AlVI per 6(O), and Mg/(Mg+Fe2+) > 0.9)) and is chemically similar to clinopyroxene inclusions in rubies from nearby alluvial gem deposits, suggesting a common origin for the two types of occurrence. Sapphirine (Mg/(Mg+Fe2+) = 0.91–0.94) and garnet (py56–67alm11–18grs18–23) also occur as inclusions in alluvial rubies. Thermodynamic calculation of the equilibrium 2 di + 2 crn = 2 cats + en constrains the temperatures of clinopyroxene + corundum crystallization to between 800 and 1150 ± 100°C. Use of other equilibria as stability limits places the pressures of crystallization between 10 and 25 kbar, implying depths of between 35 and 88 km. The most Fe-rich clinopyroxene crystallized at a pressure in the lower part of the range. The pyropic garnet inclusions in corundum crystallized at pressures of >18 kbar (i.e. at depths > ~63 km).

The xenocrystic clinopyroxene could have coexisted in equilibrium with garnet of similar composition to the observed inclusions at the deduced temperatures of crystallization. The rubies probably crystallized in rocks of mafic composition, i.e. garnet-clinopyroxenites or garnet-pyriclasites, within the upper mantle.

The clinopyroxenes kanoite and spodumene were studied by single-crystal X-ray diffraction in a diamond anvil cell (DAC) at room temperature. At the displacive phase transitions between the C2/c and P21/c polymorphs both phases coexist within the same crystal. In this two-phase region the pressure in the DAC remains constant. The experimental results demonstrate that pressure buffering at first-order phase transitions can occur in the DAC.

Concentrations of trace elements in coexisting garnet, clinopyroxene and completely miscible carbonate-silicate melt (formed at 7 GPa from the Chagatai silicocarbonatite rock known to be diamondiferous) were determined using LA-ICP-MS. The partition coefficients for Li, Rb, Cs, Ba, Th, U, Ta, Nb, La, Ce, Pb, Pr, Sr, Nd, Zr, Hf, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, Lu, Sc and Zn were determined. The new experimental data for trace-element partitioning between garnet, clinopyroxene and carbonate-silicate melt have been compared with published data for partitioning between garnet, clinopyroxene and carbonatite melt, and garnet, clinopyroxene and silicate melt. The results show that the trace-element partitioning is not significantly altered by changes in melt composition, with HREE always concentrated in the garnet. Carbonate-silicate melt, as a diamond-forming medium, and carbonatite or silicate melt equilibrated with mantle silicate minerals, behave similarly in respect of trace-element distribution.

The crystal structure of a clinopyroxene with composition (Na0.75Ca0.25)(Cr0.75Mg0.25)Si2O6 was refined at 100, 150, 200, 250 and 298 K. The work was performed in the context of an investigation on the low-temperature behaviour of A+M3+Si2O6 (with A dominant in Na and M = transition elements) pyroxenes in order to provide new insights concerning the phase transition and anomalies recently found in the low-temperature behaviour of NaTiSi2O6 and NaGaSi2O6 compounds. The refinements were done in the C2/c space group (wR2 between 0.048 and 0.068), and no change of symmetry was observed down to 100 K. Highly-anisotropic axial thermal expansion occurs with the scheme αb ≥ αb > αc.

The M2, M1 and T polyhedra expand with αM2 > αM1 αT as generally observed in pyroxenes. A discontinuity in the M1 polyhedral volume is observed between 200 and 250 K, similar to the one observed in NaGaSi2O6 between 190 and 235 K.

The atomic displacement parameters are scaled according to the following pattern: UM2 > UO2 > UO3 ≥ U01 > UT ≥ UM1 Comparison with previous data along the CaMgSi2O-NaCrSi2O6 join suggests significant positional disorder for the O1 oxygen, due to repulsion of the 2p orbitals of O1 and the non-bondine 3d electrons of Cr.

Detailed chemical analyses of different portions of laboratory-grown sector-zoned clinopyroxene been provided by Skulski et al. (1994). These data have been plotted here in PC-IR diagrams results indicate equilibrium crystallization and protosite control of element selection.

Available experimental data on chemical composition and crystal structure of K-bearing clinopyroxenes are compiled together with the results of atomistic simulations and thermodynamic calculations of mineral equilibria. It is shown that the limited solubility of K2O in clinopyroxene from crustal rocks cannot be ascribed to the strong non-ideality of mixing between diopside (CaMgSi2O6) and K-jadeite (KAlSi2O6) components. The more likely reason is the instability of the potassic endmember with respect to other K-bearing phases. As the volume effects of typical K-jadeite-forming reactions are negative, the incorporation of K in the clinopyroxene structure becomes less difficult at higher pressure. Atomistic simulations predict that the thermodynamic mixing properties of diopside-K-jadeite solid-solutions at high temperature approach those of a regular mixture with a relatively small positive excess enthalpy. The standard enthalpy of formation (ΔfH° = —2932.7 kJ/mol), the standard volume (V° = 6.479 J mol–1 bar–1) and the isothermal bulk modulus (K0 = 145 GPa) of K-jadeite were calculated from first principles, and the standard entropy (S° = 141.24 J mol–1 K–1) and thermal-expansion coefficient (α = 3.3 x 1CP–5 K–1) of the K-jadeite endmember were estimated using quasi-harmonic lattice-dynamic calculations based on a force-field model. The estimated thermodynamic data are used to compute compositions of K-bearing clinopyroxenes in diverse mineral assemblages within a wide P-T interval. The review substantiates the conclusion that clinopyroxene can serve as an effective container for K at upper-mantle conditions.

Titanian aegirine in a pegmatoid differentiate from leucitite sensu stricto (i.e. without modal plagioclase) was found to contain significant K (up to 0.07K atoms per formula unit; 0.11 wt.% K2O) by electron probe microanalysis. A transmission electron microscope study showed that this pyroxene contained a lamellar amorphous phase and other more irregular, amorphous domains where K and Al are concentrated. The microstructures suggest that the aegirine-amorphous phase system did not achieve textural equilibrium. Also, the chemistry of this phase is variable, particularly in Ca and Mg, suggesting lack of chemical equilibrium within the liquid from which the phase derived. The average composition of the latter approaches that of the coexisting K-richterite and is interpreted as trapped residual liquid, i.e. glass, from pyroxene crystallization which failed to develop a crystalline amphibole. In view of the extensive stability of richterite coexisting with pyroxene to high pressure, the reported occurrence of ‘K-bearing pyroxene’ may not be considered as a homogeneous K-bearing phase and a predictable potassium carrier in the petrogenesis of K-rich rocks like leucitites and lamproites.

Ortho- and clinopyroxenes within partially-hydrated harzburgites from Elba and Val di Cecina (Italy) show lamellar exsolution textures and variable replacement by biopyriboles, talc-chlorite-serpentine mixed layers and serpentine. Chemical and geothermometric data suggest that the pyroxenes crystallized at 1240–1051°C, followed by subsolidus exsolution at slightly lower T (1145–1025°C for clinopyroxene lamellae + orthopyroxene matrix pairs and a 1033–982°C range for orthopyroxene lamellae + clinopyroxene matrix pairs).

Investigation by transmission electron microscopy of exsolved enstatite and augite reveals a multistage hydration process. The first stage (highest T, probably in the amphibole stability field) leads to the formation of biopyribole lamellae within exsolved augite, leaving the enstatite matrix unaffected. The second stage (~500–300°C) corresponds to the topotactic replacement of enstatite by layer silicates (talc + chlorite + serpentine, with (001)layer silicates parallel to (100)enstatite). Enstatite is also replaced by randomly oriented, poorly crystalline serpentine. The last hydration stage (<300°C) corresponds to the disappearence of augite and recrystallization of serpentine, leading to completely hydrated bastites with random lizardite lamellae, polygonal serpentine and minor chrysotile.

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.

Two C2/c pyroxene suites from sodic alkaline rocks (Basanite-Phonolite: B-Ph and Alkali basalt-Trachyphonolite: AB-T) belonging to the McMurdo Volcanic Group (Mt. Melbourne province and Mt. Erebus, Antarctica) were investigated by single-crystal X-ray diffraction combined with electron probe microanalysis which together provide accurate site occupancy and site configuration parameters. Variations in site volumes distinguish the clinopyroxenes belonging to the more alkaline B-Ph suite from those of the AB-T suite. The former have higher VM2 and, for similar cell volume, lower VM1 and higher VT. In these C2/c pyroxenes, the bond lengths of M1 depend on R3+ content, necessary to balance AlIV in the T site. M1 geometric variations are similar for both B-Ph and AB-T suites. However, the M2 site is crucial for variations in polyhedral configurations. The increase in AlIV affects the shortest M2-O bond lengths in different ways depending on (Ca+Na) contents in the M2 site. Thus, the clinopyroxenes were distinguished in two different groups not related to the two suites. The first group is characterized by (Ca+Na) higher than 0.90 atoms per formula unit but shows a good positive correlation between the shortest M2-O bond lengths and AlIV content, at quite constant (Ca+Na). The second group has (Ca+Na) less than 0.90 atoms per formula unit but shows a poor correlation between the shortest M2-O bond lengths and AlIV content. In general, shortening of the longest M2-O bond lengths is associated with increase in AlIV. The cell and M1 volumes suggest that the clinopyroxenes, including the larger and sometimes resorbed macrocrysts, crystallized at a pressure lower than 5 kbar, fitting the petrography and evolved character of the rocks in question.

The Island of Linosa is a small part of the large submarine volcanic complex which is locatedat the SW edge of the Linosa Graben, Sicily Channel. The island was formed between 1.06±0.10 and 0.53±0.07 Ma, through three main stages of activity: Paleo-Linosa, Arena Bianca and Monte Bandiera. Major and trace element data show that the compositional variability of the three activity stages is limited, with most of the rocks showing basaltic to hawaiitic composition. Evolved benmoreites and trachytes are foundas lithics in some pyroclastic units of Paleolinosa. The mafic rocks of the three stages show porphyritic texture, with phenocryst assemblages characterized by olivine, clinopyroxene and plagioclase. The volume ratio of olivine vs. clinopyroxene decreases from early to late stages of activity in mafic rocks with comparable major element composition. Clinopyroxene phenocrysts from mafic rocks of the three stages have poorly variable composition, clustering in the augite field. Phenocrysts from the first activity stage (Paleo-Linosa), show a slight increase in TiO2, Al2O3 and CaO, and a decrease of Fe2O3 (total) with the increasing SiO2 content of the host rocks. Crystals from the second and the third stage (Arena Bianca and Monte Bandiera) display a slightly more restricted range of FeOtot, frequently with very high MgO, Al2O3 and TiO2 contents. Crystal chemical investigation of clinopyroxenes from rocks of the three stages with comparable degrees of evolution, revealed significant variation of structural parameters, in particular VM1 and Vcell. These show a consistent decrease, passing from clinopyroxenes of the early stage to crystals extracted from the mafic lavas of stages 2 and 3. Given the similar compositional ranges of the host rocks, structural variations of clinopyroxenes are interpreted to reveal modifications of crystallization pressure, which increased, passing from Paleo-Linosa to the Arena Bianca and Monte Bandiera stages. Given this information, the observed crystal-chemical variations provide information on the depth of magma reservoirs and on the evolution of the plumbing system of Linosa volcano.

Clinopyroxene phenocrysts and groundmass crystals are relict phases in altered basalt and basaltic andesite lavas, and arenites of the Cudgegong Volcanics and Toolamanang Volcanics, Cudgegong-Mudgee district, New South Wales. Petrography, field relationships and clinopyroxene compositions indicate that basaltic blocks in the latter unit are reworked from the Cudgegong Volcanics. Clinopyroxene phenocrysts show a restricted compositional range and minor Feenrichment from core to rim, features considered indicative of a calc-alkaline parent magma. It is proposed that the Cudgegong Volcanics crystallized under hydrous conditions, at least in the later stages, with rising fO2 resulting in a Fe-Ti oxide crystallizing as a primary phase. The clinopyroxenes are considered to have crystallized at moderate (5–6 kbar) and falling pressures and at minimum temperatures in the range 900 to 1000°C. Coupled substitutions affecting the “other” components in the clinopyroxene structural formula indicate that the ivAl-viFe3+, ivAl-viAl and ivAl-viTi4+ couples are important. The Sofala Volcanics, south of the study area, and the Cudgegong Volcanics are similar in age, petrography and stratigraphic position, and contain relict clinopyroxenes which are chemically similar. This suggests that the units are laterally equivalent and adds further evidence to the proposal that an oceanic island arc system was active in central western New South Wales during the Late Ordovician.

Overgrowths of whiskers of hedenbergite (Ca(Fe2+,Mg)Si2O6) on the hydrous pyroxenoid babingtonite (Ca2Fe2+Fe3+[Si5O14(OH)]) have been observed at Arvigo in Switzerland and Kreimbach/Kaulbach in Germany, and we have studied them with transmission electron microscopy in order to understand their structural relationships and formation. The boundaries between babingtonite and hedenbergite are sharply defined, and the two minerals are in direct contact with no additional phases present. The relationships of babingtonite (Bab) and hedenbergite (Hd) were determined as Bab[100]//Hd[112] in the Arvigo specimen and Bab[$\bar 1$00]//Hd[1$\bar 1$2] in the Kreimbach/Kaulbach specimen. Diffraction derived from Bab(031) and Hd(02$\bar 1$) in the Arvigo samples and Bab(031) and Hd(021) in the Kreimbach/Kaulbach samples were observed in identical positions. The reciprocity between the babingtonite and hedenbergite structures is governed by the direction of the SiO4-tetrahedral chains, and the related configuration of octahedra. Thus, hedenbergite is apparently an epitaxial phase grown on a base of {010} plates of babingtonite. The defined orientation relationship is also consistent with that shown in topotaxial intergrowths of other clinopyroxenes and pyroxenoids. The topotaxial intergrowths may result from diffusion-controlled solid-state reactions, whereas rapid whisker growth is characteristic of supersaturated solutions or a vapour medium. The epitaxial growth of hedenbergite whiskers on babingtonite with an abrupt but coherent change of structure at the interface represents an ideal example where the similar chemical compositions of host and guest contribute strongly to the close structural relationship.

The Proterozoic Sushina Hill Complex is the only agpaitic complex, reported from India and is characterized by a eudialyte-rinkite-bearing nepheline syenite. The complex is considered a ‘metamorphosed agpaitic complex'. This study describes the mineral assemblages formed during successive stages of evolution from magmatic to hydrothermal stages and low-temperature subsolidus re-equilibration assemblage. The primary-late magmatic assemblage is characterized by albite, orthoclase, unaltered nepheline, zoned diopside-hedenbergite, rinkite, late magmatic eudialyte and magnesio-arfvedsonite formed at ∼700°C with maximum aSiO2 of 0.60. In contrast, a deuteric assemblage (400-348°C) is represented by aegirine-jadeite-rich clinopyroxene, post-magmatic eudialyte, sodalite, analcime and the decomposition assemblages formed after eudialyte with decreasing aSiO2 (0.52-0.48). A further low-temperature subsolidus assemblage (≤250°C) represented by late-forming natrolite could be either related to regressive stages of metamorphism or a continuum of the subsolidus processes. Considering the P/T range of the greenschist - lower-amphibolite facies of metamorphism it is evident that the incorporation of a jadeite component within pyroxene is related to a subsolidus process between ∼400°C and 348°C in a silica deficient environment. We emphasize that the deuteric fluid itself acted as an agent of metamorphism and the decomposition assemblage formed after eudialyte is retained even after metamorphism due to the convergence of subsolidus and metamorphic domains. The formation of jadeite-rich aegirine is not considered to result from metamorphism. Overall it is near-impossible to discern any bona fide metamorphic textures or mineral assemblages in these syenites which appear to preserve a relict mineralogy regardless of their occurrence in country rocks which have experienced greenschist - amphibolite facies metamorphism. The Sushina complex is very similar in this respect to the Norra Kärr complex (Sweden).

Bulk rock geochemistry and major- and trace-element compositions of clinopyroxene have been determined for three suites of peridotitic mantle xenoliths from the North Atlantic Craton (NAC) in northern Scotland, to establish the magmatic and metasomatic history of subcontinental lithospheric mantle (SCLM) below this region. Spinel lherzolites from the southernmost locality (Streap Com'laidh) have non-NAC mantle compositions, while the two northern xenolith suites (Loch Roag and Rinibar) are derived from the thinned NAC marginal keel. Clinopyroxene compositions have characteristic trace-element signatures which show both 'primary' and 'metasomatic' origins. We use Zr and Hf abundances to identify ancient cryptic refertilization in 'primary' clinopyroxenes. We suggest that Loch Roag and Rinibar peridotite xenoliths represent an ancient Archaean-Palaeoproterozoic SCLM with original depleted cratonic signatures which were overprinted by metasomatism around the time of intrusion of the Scourie Dyke Swarm (∼2.4 Ga). This SCLM keel was preserved during Caledonian orogenesis, although some addition of material and/or metasomatism probably also occurred, as recorded by Rinibar xenoliths. Rinibar and Streap xenoliths were entrained in Permo-Carboniferous magmas and thus were isolated from the SCLM ∼200 Ma before Loch Roag xenoliths (in an Eocene dyke). Crucially, despite their geographical location, lithospheric mantle peridotite samples from Loch Roag show no evidence of recent melting or refertilization during the Palaeogene opening of the Atlantic.

A volcano-plutonic complex in the Susa area, southwest Japan, consists of the Yamashima andesites, the Koyama gabbros and syn-plutonic porphyrite dykes derived from a common basaltic andesite magma. The complex is closely associated with middle Miocene turbidite deposits. The Yamashima andesites are composed mainly of basaltic andesite feeder dykes, massive submarine lavas with hyaloclastites, and their reworked deposits. The lavas and deposits immediately overlie turbidite deposits, indicating submarine volcanic activity. The Koyama gabbros formed hornfels by contact metamorphism of the surrounding turbidites and andesites. Highly purified clinopyroxene and plagioclase mineral separates from the Yamashima andesites were dated by a K–Ar method using an ultra-low blank K analysis procedure. Ages obtained from duplicate analyses are 16.5±1.5, 15.2±1.4, 15.8±1.7, and 16.5±2.0 Ma for clinopyroxene, and 14.2±0.8, 15.2±0.9, and 15.6±0.9 Ma for plagioclase. The clinopyroxene and plagioclase data define a mineral isochron age of 14.7±0.9 (1σ) Ma with an initial 40Ar/36Ar ratio of 297.3±2.4 (1σ), suggesting that clinopyroxene has no excess argon and can be reliably dated by K–Ar. Most of the groundmass ages are considerably younger (12.1–14.6) than the isochron age, perhaps due to argon loss during alteration. The gabbros give ages of 14.2±0.3 and 14.1±0.3 Ma for biotite, and 13.7±0.3 and 13.7±0.7 Ma for green hornblende. The porphyrite dyke yields an age of 12.5±0.3 Ma for the groundmass, and the pelitic hornfels gives a biotite age of 14.8±0.3 Ma. Our new K–Ar ages, together with previous studies, show that a series of geological events took place in the Susa area between 16 and 13 Ma. Conglomerates and sandstones were deposited in the beginning of marine transgression. Subsequent abrupt deepening led to deposition of a thick black shale unit, turbidite deposits and large-scale submarine channel-fill deposits. Coeval igneous activity formed the volcano-plutonic complex. The magmato-tectonic event was synchronous with the opening of the Japan Sea and the associated clockwise rotation of the southwest Japan arc sliver, recording a unique tectonic setting.