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Hydrothermal veins and dykelets that cross-cut layered olivine gabbros deep in the plutonic section of the Samail Ophiolite, Sultanate of Oman, point towards the occurrence of hydrothermal circulation in the deep oceanic crust, and these features record interactions between rock and high temperature seawater-derived fluids or brines. Deep penetration of seawater-derived fluids down to 100 m above the Moho transition zone and the consequent interactions with the host rock lead to hydrothermal alteration from granulite facies grading down to greenschist facies conditions. Here we present a study of veins and dykelets formed by hydrothermal interaction cutting layered gabbro in the Wadi Wariyah, using petrographic, microanalytical, isotopic, and structural methods. We focus on amphiboles, which show a conspicuous compositional variation from high-Ti magnesiohastingsite and pargasite via magnesiohornblende and edenite, to Cl-rich ferropargasite and hastingsite (up to 1.5 a.p.f.u. Cl) and actinolite. These minerals record a wide range of formation conditions from magmatic to hydrothermal, and reveal a complex history of interactions between rock and hydrothermal fluid or brine in a lower oceanic crustal setting. Large variations in Cl content and cation configurations in amphibole suggest formation in equilibrium with fluids of different salinities at variable fluid/rock ratios. The presence of subsolidus amphibole extremely enriched in chlorine implies phase separation and brine/rock interactions. 87Sr/86Sr values of 0.7031 to 0.7039 and stable δ18O isotopic compositions of 4.1 to 5.6‰ of the different amphibole types suggest a rock-dominated environment, i.e. with low fluid/rock ratios. However, the slight departure from mean Oman isotope values may indicate there was some influence of seawater in the aforementioned fluid-rock interactions. Our study provides new petrological data for the subsolidus evolution of gabbro-hosted amphibole-rich veins in the presence of a seawater-derived fluid.

The North Qilian Orogen in NW China hosts several commercial quality serpentine jade deposits, although nephrite jade has not so far been reported from this region. Here, we investigate a green nephrite (tremolite) jade from Yushigou ophiolite suite in the North Qilian orogen. We present results from petrographic, scanning electron microscopy (SEM), X-ray diffraction, Electron microprobe analyses (EMPA) and zircon U-Pb geochronological studies to characterize the nephrite jade and its genetic setting. Our results suggest that the high-quality green nephrite jade is predominately composed of tremolite and actinolite, with transformation to actinolite with increasing of Fe and Cr, together with minor chlorite, calcite, magnetite, and titanite. In contrast, the serpentine jade consists mainly of antigorite with minor magnesite, talc, calcite, magnetite, chromite, and apatite. Zircon U-Pb dating yields a lower intercept age of 500 ± 12 Ma, representing the formation age of the Yushigou ophiolite. A few young zircon grains yield a weighted mean age of 227.9 ± 5.3 Ma, which is interpreted to represent the crystallization age of the tremolite, in response to the tectonics associated with late Paleo-Tethys to early Neo-Tethys. Accordingly, we propose that the host serpentinites in this region formed within a Late Cambrian supra-subduction zone through water-rock interaction, involving Ca-rich and COH-bearing fluid systems. The Ca- or COH-bearing fluids released from mantle-derived magma, and then migrated to higher levels during late metamorphic-metasomatic processes in the Mesozoic, finally generating the nephrite jade. Moreover, our study also provides insights into the tectonic processes associated with the formation of jade within subduction zone settings.

Extending approximately 1300 km and located in the Western Kunlun Mountains, the Hetian nephrite belt is the largest nephrite belt in the world and contains approximately 11 major deposits and more than 20 orebodies including the Alamas deposit. Hetian nephrite deposits can be classified as Mg-skarn deposits with Precambrian dolomitic marble host rock and green, green–white and white nephrite zones are distributed gradually in the zone of a granodiorite pluton. The green nephrite is mainly predominately composed of tremolite with generally minor to trace constituents of diopside, grossularitic garnet, actinolite and other minerals. Also green nephrite has higher content of TFe2O3, than green–white and white nephrites have. We subdivided the zircons from the green nephrites into four types, depending on their internal textures, mineral inclusions, and SHRIMP U–Pb ages. Type I zircons are round instead of idiomorphic in shape and lack obvious zoning. Type II and IV zircons have broad, clear oscillatory zoning and are hypidiomorphic or idiomorphic in shape; they contain inclusions of diopside, tremolite, chlorite and calcite. Most Type III zircons are narrow rims ( 0.1), similar REE and trace element patterns, a Ce anomaly (Ce/Ce* > 5), and ΣREE contents of 454 to 922 ppm and 102 to 3182 ppm with averages of 627 ppm and 855 ppm, respectively. The similar geochemical signatures, morphologies, and ages indicate that most zircons (or fragments of zircon) in the nephrite came from the granodiorite and some experience partially recrystallized during skarnization. This is consistent with the field observation that original granodiorite–dolomitic marble boundary is now represented within a nephrite sequence, with the green nephrite close to the granodiorite and the white/white–green nephrites adjoining the dolomitic marble. Typical skarn deposits experience prograde and retrograde metasomatism stages. According to the field observations and petrographic studies, both prograde metasomatism and the early retrograde altered stages are two main stages for the formation of Alamas nephrite deposits. The replacements of coarse-grained tremolite by fine-grained tremolite (nephrite) lead to the formation of nephrite. Based on petrographic studies, the main formation processes of the nephrite are 1) diopside ← dolomite; 2) tremolite (nephrite) ← diopside; and 3) chlorite ← tremolite (nephrite). Thus, the timing of the formation of nephrite is later than that of Mg-skarn.

Trace element concentrations, particularly rare earth elements and yttrium (REY) in feldspars and accessory minerals, have been determined in a suite of albitized igneous, metasedimentary and metasomatite rocks from the Moonta-Wallaroo district, Olympic Cu–Au Province, South Australia. Results show that changes in REY-fractionation trends and concentrations in feldspars and common accessories are associated with key textures in albite-bearing associations from different lithologies. In granitic rocks, pseudomorphic replacement of pre-existing feldspars is typified by porous albite with cleavage-oriented intergrowths of sericite and pore-attached hematite. These observations are comparable with albitization features of granitic terranes elsewhere. A mineral association (albite-sericite ± chlorite), similar to that from granitoids, is observed as pervasive spots in limestone, inferring prograde skarnoid reactions at low fluid/rock ratio in an impure carbonate. In metasedimentary and metasomatite rocks with comparable Na2O content (~ 5–6 wt.%), fine-grained granoblastic albite suggests growth under high fluid/rock ratios irrespective of lithology. In such cases, albite with the highest REY content (ΣREY ~ 200 ppm) accounts for the entire REY budget, e.g., in albite–biotite-schist with the lowest abundance of accessory minerals. Nanoscale investigation confirms this albite to be a REY carrier (elements incorporated within the crystal lattice); no pore-attached inclusions are observed. In contrast, albite with the lowest REY-concentration (~ 14 ppm) is encountered in the metasomatite. In such rocks, recording the highest ΣREY (~ 1000 ppm) in whole-rock, partitioning of REY is favoured among the abundant accessories (titanite, apatite) and calc-silicates (actinolite, clinozoisite) rather than albite. Comparable low-REY albite is also found in granitoid-derived albitite (Na2O ~ 5 wt.%), in which abundant accessories and discrete REY-minerals formed during albitization account for the high ΣREY content (~ 700 ppm) in whole rock. The role of coupled dissolution–reprecipitation reactions (CDRR) is critical for REY (re)distribution within albitized igneous rocks, where REY-release from early magmatic accessories and/or feldspars assists REY-enrichment into late albite. The presence of abundant nanopore-attached inclusions in plagioclase demonstrates the nanoscale nature of CDRR-driven albitization in granitoids, consistent with published experimental work on altered granites. Such porosity offers sites for REY entrapment seen within discrete REY-minerals in new-formed K-feldspar. Similarly, release and uptake of REY, concurrent with albitization, is seen in formation of coarser REY-minerals (xenotime, bastnasite, synchysite) during CDRR-driven replacement of accessory Fe–Ti-oxides by symplectites of chlorite and hematite. Based on the differences identified between the albitization pathways in igneous and metasedimentary rocks, we discuss how albitization proceeds via a series of complex fluid–mineral reactions, each involving the redistribution, accumulation and retention of REY. These reactions are critical for defining the endowment and deportment of REY in rocks that have undergone sodic alteration. Contrary to previous models, albitization appears controlled by pH rather than redox conditions. Despite regional differences in local geological environment and alteration style across the Olympic Cu–Au Province, albitization, the initiation of hydrothermal alteration, is a pre-requisite stage for REY-enrichment in Iron-Oxide–Copper–Gold (IOCG) systems. REY distribution patterns in feldspars may thus have value in mineral exploration as criteria enabling alteration associated with mineralization to be distinguished from the regional background. Strong albitization without superposition of later potassic alteration may not, however, be automatically linked to the formation of giant IOCG deposits. Albitization enhances rock permeability and in a strongly faulted structural environment without a suitable trap, hydrothermal fluids may be more readily lost from the system.

The Hillside Cu-(Au) deposit, Yorke Peninsula, South Australia, is a recently-discovered ore system within the 1.6. Ga World-class Olympic iron oxide-copper-gold (IOCG) Province. The deposit is characterized by a skarn-style alteration zone. Analyses of feldspar, calcite, skarn minerals (garnet, pyroxene, clinozoisite and actinolite) and accessories (titanite, apatite and allanite), and grain-scale element mapping by laser-ablation inductively-coupled plasma mass spectrometry are used to assess the distributions of rare earth element (REE), incompatible and ore-forming elements in host rocks, prograde and retrograde skarn.Garnet is a major repository of HREE, especially in prograde skarn, whereas LREE-enriched clinozoisite is the principal REE-host in retrograde skarn. REE distribution patterns define a pronounced partitioning of elements among the dominant coexisting minerals. Compositional variation between assemblages, and also within individual grains, defines an evolution from early feldspar-pyroxene skarn through main-stage calcic skarn to the ore-stage. A switch from a prograde, HREE-dominant signature to a LREE-enriched signature is observed in both retrograde and distal skarn. Zr-in-titanite geothermometry supports transition from magmatic to hydrothermal, skarn-forming processes at temperatures of ~. 660. °C; the initiation of ore-stage is about 100. °C lower.Understanding REE distributions in all minerals within a complex, multistage ore system assists the development of vectoring tools that use trace element chemistry in exploration for similar IOCG deposits beneath regolith cover across the Olympic Province. Titanite and apatite show particular promise because of their characteristically distinct REE patterns in magmatic and hydrothermal stages, trace element responses to redox changes, and their widespread abundance throughout different lithologies in the area. Refereed/Peer-reviewed

The Likamannikko quartz alkali feldspar syenite complex is located in eastern Finland, in the Suomussalmi district, within the Archean Karelian Craton. The intrusive complex covers an area of 2 km 2 and is cut by both Neoarchean leucogranite and Paleoproterozoic dolerite dykes. Three zircon fractions define a concordant U–Pb age of 2741 ± 2 Ma for the quartz alkali feldspar syenite. The syenitic complex also contains carbonatite patches and angular fragments of ultrabasic rock less than 50 cm across. However, the paucity of outcrop has prevented direct observation of the relationship between carbonatites and other rock types. The quartz alkali feldspar syenite is not alkaline sensu stricto , but has alkaline affinities, with major element compositions in the range SiO 2 = 62.6–70.3 wt.%, K 2 O = 1.09–5.73 wt.%, Na 2 O = 4.41–7.19 wt.% and MgO = 0.65–2.06 wt.%. The ultrabasic fragments are interpreted as cumulates containing clinopyroxene (generally altered to actinolite), apatite, allanite, epidote, and albite; these minerals occur in varying proportions, leading to considerable whole-rock compositional heterogeneity, with major elements in the range SiO 2 = 31.4–48.9 wt.%, CaO = 16.35–30.74 wt.% and MgO = 2.00–14.07 wt.%, as well as notable enrichment in P 2 O 5 (3.2–13.5 wt.%). The carbonatites are calcitecarbonatites displaying mantle-like stable isotope values with δ 18 O VSMOW from 5.2 to 6.9‰ and δ 13 C VPDB values from − 3.8 to − 5.5‰, also trace element characteristics are consistent with a magmatic origin. Based on Sm–Nd model ages, the carbonatites are approximately coeval with the host quartz alkali feldspar syenite and thus possibly among the oldest known carbonatites in the world. All three rock types are interpreted to have originated from metasomatized mantle and the intrusive complex consists of two or more silicate magma pulses. Positive e Nd values and their small scatter from − 0.2 to 2.0 at 2741 Ma suggest only limited assimilation of older crust or Paleoproterozoic disturbance of Sm–Nd systematics.

Microstructural and petrologic analyses of 7 gabbroic rocks sampled from the medial area of the Godzilla Megamullion (site KH07-02-D18), located along the Parece Vela Basin spreading ridge (Parece Vela Rift), Philippine Sea, reveal the development of a high-temperature ductile shear zone associated with hydrothermal metamorphism in the lower crust. The deformed gabbroic rocks are petrographically classified into mylonites and an ultramylonite, and are characterized by porphyroclastic textures consisting mainly of coarse plagioclase and clinopyroxene/amphibole porphyroclasts in a fine-grained matrix. Plagioclase crystallographic-preferred orientations vary from (010)[100] and (001)[100] patterns in the mylonites to a weak (001)[100] pattern in the some mylonites and ultramylonite, suggesting a change in the deformation mechanism from dislocation creep to grain-size-sensitive creep with increasing intensity of deformation. The chemical composition of matrix plagioclase is generally more sodic than that of porphyroclasts. Secondary amphibole is ubiquitous, consisting mainly of pargasite and magnesiohornblende (brown hornblende) and actinolite (green hornblende). The mineral assemblage is consistent with the hydrothermal metamorphic reaction: clinopyroxene + calcic plagioclase + fluid → amphibole + sodic plagioclase. Compared with deformed gabbroic rocks from the breakaway and termination areas of the Godzilla Megamullion, the samples record ductile shearing under high temperature conditions, possibly related to the development of the Godzilla Megamullion, although hydrothermal activity in the medial area appears to have been less intense than in both the breakaway and termination areas.

Thermodynamic reaction path models were applied to provide insights into the formation of rodingites at mafic/ultramafic boundaries. The models were set up to investigate the shifts in fluid–rock equilibria as fluids move from peridotite undergoing serpentinization into a gabbroic body. Phase relations were investigated in the direction of increasing extent of reaction of the serpentinization fluid with gabbro at 200 °C, 300 °C, and 400 °C. Phase assemblages typical of rodingite (grossular + diopside ± chlorite) are predicted to form at 200 °C and 300 °C, but only in areas where the fluid is essentially unaffected by reactions with gabbro, i.e., near the contact with ultramafic rock or adjacent to a fissure filled with serpentinization fluid. As the fluid becomes more affected by reactions with the gabbro, prehnite or epidote-ss replace garnet and tremolite replaces clinopyroxene. Once the fluid chemistry is completely reset by reactions with gabbro, the predicted assemblage is typical of greenschist facies: albitic plagioclase, actinolite, chlorite, and epidote-ss. These transitions predicted in the model are very similar to what is observed in natural rodingites from different settings. Our model results hence support the hypothesis that rodingites form during serpentinization and only in areas where fluids controlled by serpentinization reactions are present. Our calculations further indicate that the formation of mineral assemblages and spatial mineralogical variability within rodingites from the ocean floor is mainly driven by steep activity gradients in protons and aqueous silica in pore fluids. Mass transfer of Ca is likely by diffusion of the CaOH + species, which is predicted to show a very steep concentration gradient across a mafic–ultramafic boundary. Rodingites act as Ca traps because of the great stability of diopside and garnet at low SiO 2 activities. Our model calculations further predict the formation of diopsidite from gabbroic and clinopyroxenite precursor rocks at 200 °C and 300 °C, suggesting that very high temperatures on the order of 800 °C are not required in the formation of diopsidite veins.

Four generations of mineral assemblages have been identified in ultrahigh-pressure (UHP) eclogites from the Hong'an area, recording four metamorphic stages of the exhumation trajectory, i.e., garnet + omphacite + kyanite+ zoisite/epidote + rutile + coesite formed at the UHP stage, garnet + omphacite + paragonite + phengite + amphibole + epidote + quartz at the high-pressure (HP) eclogitic stage, amphibole + epidote + albite–oligoclase + paragonite + quartz at the epidote-amphibolite facies (EA) stage, and actinolite+ albite and pumpellyite + albite + K-feldspar + muscovite + chlorite + margarite + epidote + quartz at the greenschist facies (GS) stage. Garnet formed at the UHP stage was overgrown by atoll garnet at the HP stage. X-ray images, mineral compositions and compositional profiles indicate that the UHP garnet experienced a variable Fe–Mg modification in response to Fe–Mg exchange between garnet and omphacite prior to the overgrowth of atoll HP garnet. A volume diffusion zone was also observed between the UHP and HP garnets. The following P–T conditions were estimated for these mineral assemblages: 480 to 560 °C and 2.5 to 2.9 GPa with median values of 520 °C and 2.8 GPa at UHP stage, 575 to 685 °C and 1.6 to 1.9 GPa with the average results of 634 ± 37 °C and 1.8 ± 0.06 GPa at the HP stage, 500 to 640 °C and 0.4 to 1 GPa at the EA stage, and 160 to 320 °C and 0.2 to 0.8 GPa at the GS stage. The Hong’an UHP eclogites experienced a temperature increase of over 100 °C from the UHP to HP stages and continued to exhume from the HP to EA stages through a process of approximately isothermal depression, which was followed by a significantly cooling from the EA to GS stages. In the process of exhumation, UHP eclogites witnessed a series of events involving fluid influx, resulting in the formation of hydrous minerals at different stages. The nature of fluids changed from K-rich at the HP stage to K-poor at the EA stage and then K-rich again at the GS stage.

Albite-, quartz-, actinolite-, apatite-rich rocks with accessory titanite form a carapace that caps a small dome-like intrusion of Roxmere pluton, and provide evidence of the accumulation and release of magmatic fluids that may have contributed to regionally extensive Na–Ca alteration in the Proterozoic Cloncurry district, Australia. The Roxmere pluton was emplaced after the peak amphibolite facies metamorphism, and into psammitic metasedimentary rocks that are stratigraphically younger and overlie the calc-silicate-rich Mary Kathleen Group. Na–Ca hydrothermal assemblages closely spatially related to the Roxmere pluton suggest that Na–Ca alteration, in this case, is inconsistent with excepted up-temperature metamorphic fluid circulation models. The carapace rocks comprise two variants, aplite and pegmatite, both of which were affected by alteration and brecciation. Aplitic material predominates and the carapace has a brecciated top, and a distinct textural zonation, from an aplite-rich, pegmatite-poor base, to a relatively aplite-poor/pegmatite-rich top. This zonation is interpreted to represent the progressive accumulation of volatile phases toward the top of the carapace, which culminated in fluid overpressuring and brecciation of the granitoid stock. The convoluted and ptygmatic habit of the aplitic and pegmatitic material within the carapace is similar in nature to the comb quartz unidirectional solidification textures that are developed in the roof zones of some porphyry stocks. These complex textures are interpreted to have formed from a single phase of fluid saturation. The Roxmere pluton, or a deeper equivalent, has a Na-, Mg- and Ca-rich composition and a clinopyroxene-, amphibole-rich mineralogy that is consistent with the formation of the contained albite-, actinolite-, apatite-rich phases. However, the source of the rocks within the carapace is equivocal given that their contact relationships are obscured. The δ 18 O composition of minerals within the carapace has the following ranges: albite, 7.1–8.7‰; actinolite, 8.0–8.5‰; and quartz, 8.9–11.4‰. Albite and quartz δ 18 O equilibria suggest that these minerals in the pegmatitic and aplitic components formed at temperatures of ∼510°C to 540°C. The calculated δ 18 O composition of a fluid in equilibrium with these minerals (∼6.0‰ to 8.0‰), at temperatures between 450°C and 550°C, is consistent with a magmatic origin. These estimated temperatures and δ 18 O fluid composition are comparable to estimates for regional Na–Ca alteration. In contrast, δD analysis of actinolite (−136 to −150‰) within the carapace demonstrates that these fluids have a significantly lower δD signature compared to Na–Ca alteration assemblages associated with regional alteration (−70‰ to −90‰). The low values in the carapace could either be interpreted as being produced from meteoric fluids or from degassed magmatic fluids, although a meteoric fluid model is deemed improbable as meteoric fluids in the district have a documented δD composition of ca. 0.0‰. As a consequence, an open-system degassed magmatic–hydrothermal fluid origin is considered to be the most plausible explanation for the low isotopic signatures, and is also consistent with the complex textural relationships described within the carapace.

The retrograde chemical zonal structure of amphibole in hematite-bearing basic and quartz schists from the higher grade zone in the Saruta-gawa area of the Sanbagawa belt was studied to investigate the relationships between the prograde and retrograde P–T paths of the Sanbagawa metamorphism. This amphibole coexists with chlorite, epidote, muscovite, albite, quartz and hematite, and is composed of Al-rich core and Al-poor mantle. The core is fairly homogeneous and has a barroisitic composition. In the mantle part, [B] Na increases with decreasing [4] Al towards the margins, which have winchite–magnesioriebeckite compositions. The barroisite–winchite–magnesioriebeckite composite crystal is sometimes rimmed by actinolite and/or winchite with low [4] Al and [B] Na . The Al-rich core and Al-poor mantle are regarded as prograde and retrograde products, respectively. The retrograde mantle in the Saruta-gawa area: (1) is systematically richer in [B] Na [0.40–1.73 per formula unit (pfu; for O=23)] than that from the same grade zone in the Asemi-gawa area (0.19–0.78 pfu), about 8 km south of the studied area; (2) tends to be [B] Na -poorer (less than 1.73 pfu) than prograde sodic amphibole (up to 1.93 [B] Na pfu) produced in the peak temperature stage from the lower grade zone in the same and other areas; and (3) extends its compositional range towards higher [B] Na and lower [4] Al than prograde-formed amphibole from the same grade zone in the same area. These zonal characteristics imply that (1) the Saruta-gawa samples experienced retrograde metamorphism under higher P/T conditions than the Asemi-gawa samples, (2) the retrograde P–T path of the Saruta-gawa area passes on the lower pressure side of the metamorphic field gradient, and (3) the Saruta-gawa samples underwent retrograde metamorphism under higher P/T conditions than the prograde metamorphism. The higher P/T conditions of the retrograde metamorphism suggests an increasing dP/dT of the geotherm during exhumation. Retrograde P–T conditions during the formation of magnesioriebeckite can be roughly estimated at 7–8 kbar, 400–450°C based on semi-quantitative phase relations of actinolite–winchite–magnesioriebeckite–barroisite series associated with chlorite, epidote, muscovite, albite, quartz and hematite.

Laminated to massive rhodochrosite, hausmannite, and Mn-silicates from the Smith prospect and Manga-Chrome mine, Sierra Nevada, California were deposited as ocean floor sediments associated with chert and shale. The principal lithologies at Smith are chert, argillite, rhodochrosite-, hausmannite- and chlorite-rich layers, and relatively uncommon layers of jacobsite. The Manga-Chrome mine also contains layers rich in manganoan calcite and caryopilite. Tephroite, rhodonite, spessartine, and accessory alleghanyite and sonolite formed during metamorphism. Volcaniclastic components are present at Manga-Chrome as metavolcanic clasts and as Mn-poor, red, garnet- and hematite-rich layers. There is no evidence, such as relict lithologies, that Mn was introduced into Mn-poor lithologies such as chert, limestone or mudstone. Replacement of Mn-poor phases by Mn-rich phases is observed only in the groundmass of volcanic clasts that appear to have fallen into soft Mn-rich mud. Manganiferous samples from the Smith prospect and Manga-Chrome mine have high Mn Fe and low concentrations of Ni, Cu, Zn, Co, U, Th and the rare-earth elements that are similar to concentrations reported from other ancient Mn deposits found in chert-greenstone complexes and from manganiferous sediments and crusts that are forming near modern sea floor vents. The Sierra Nevada deposits formed as precipitates of Mn-rich sediments on the sea floor, probably from mixtures of circulating hydrothermal fluids and seawater. The composition of a metabasalt from the Smith prospect is consistent with those of island-arc tholeiites. Metavolcanic clasts from the Manga-Chrome mine are compositionally distinct from the Smith metabasalt and have alkaline to calc-alkaline affinities. A back-arc basin is considered to be the most likely paleoenvironment for the formation of the Mn-rich lenses at the Manga-Chrome mine and, by association, the Smith prospect. Layers of rhodochrosite, hausmannite and chert preserve the composition and some textures of the sedimentary protoliths at both Sierra Nevada deposits. Jacobsite-rich layers probably represent a Fe-rich protolith. Caryopilite and manganoan calcite represent additional protoliths at the Manga-Chrome mine. The metamorphic assemblage prehnite-chlorite-epidote-calcite in a metabasalt from the Smith prospect constrains regional metamorphic conditions to a maximum temperature of 325°C and a pressure of 2 kbar. Slightly higher temperatures are indicated by the presence of actinolite in another metabasalt. Compositions of Mn-rich minerals in Smith samples are consistent with these metamorphic conditions.

The asymmetricaly zoned Santa Olalla pluton consists of several facies; gabbro; a tonalitic group that includes both hornblende diorite and cordierite-bearing quartz monzonite; granitic dykes; and enclaves. Enclaves are an ubiquitous component and in places they occur in swarms. The tonalites and enclaves display extremely abundant disequilibrium microstructures: pyroxene replaced by actinolite; actinolite by hornblende; hornblende by biotite; biotite by hornblende; and both calcic plagioclase and biotite phenocrysts have been corroded in a disequilibrium melt phase. Most of these microstructures can develop during the evolution of a single magma, but the replacement of biotite suggests that crystal-melt disequilibrium through magma mixing is responsble. Hornblende and actinolite occur in clots enclosing pyroxene. The growth of actinolite was probably metastable at magmatic temperatures, and replacement of pyroxene was controlled by diffusion of Al. Subsequently, hornblende replaced actinolite, with a melt phase present. Plagioclase microstructures and compositions are consistent with batches of phenocrysts immersed in a disequilibrium melt that resulted from a bulk compositional change in the magma, with which they re-equilibrated. Bulk-rock geochemistry indicates that the series gabbro through tonalite to granitic dykes cannot be simply related by a single fractionation trend. Gabbros vary by pyroxene fractionation. The tonalites are a mixture of a cordierite-bearing S-type magma and a somewhat fractionated pyroxene gabbro: thus, it is H(Hybrid)-type plutonism, not I-type. The granitic dykes developed after mixing, by segregation of a residual melt from the hybrid tonalites. There is evidence of some degree of fractionation of the tonalite facies after mixing: the presence of aplites, granitic dykes, and divergence of incompatible trace elements from simple mixing ratios. No variation can be attributed to restite unmixing.

In high temperature geothermal areas ( T .200° C ) in Iceland the alteration minerals formed show a regular zoning with increasing temperature i.e. smectite, chlorite, epidote and actinolite. To test the effect of varying composition of the hydrothermal circulation fluid drillhole chips from two contrasting areas were analyzed in detail. The Reykjanes area is fed with sea water while Krafla is fed with meteoric water and has a Cl − concentration of 28 ppm. Both areas have tholeiitic basalts, lavas and hyaloclastites as original reservoir rocks. Although both systems have the same sequence of alteration minerals the actual composition of the minerals differs between systems reflecting the different chemical composition of the circulating waters. In Reykjanes, with its brine fluid, the composition of the original rock appears not to affect the composition of the alteration minerals while the reverse appears to be the case in Krafla.

Fluid inclusions in emeralds from the Habachtal, Central Tauern Window, have been studied by microthermometry. Results allow a detailed reconstruction of trapping history and evolution of the metamorphic fluids during the Middle Alpine Tauernkristallisation metamorphic event and some of the subsequent cooling period. Five different types of fluid inclusions, corresponding to at least five trapping periods, have been distinguished. In general, the earliest primary (type 1) inclusions, which occur as negative crystals or thin long tubes, are represented by low salinity ( < 10 wt. % NaCl equivalent) aqueous fluids with or without CO2 with up to XCO2 ≈ 0.04. Later primary type 2 inclusions are distinguished by different morphologies and distribution patterns. Lower salinity CO2-free brines and CO2-bearing denser inclusions with higher CO2 contents (up to XCO2 ≈ 0.11) are characteristic of this stage. The type 2 inclusions may also occur as pseudosecondary arrays. The effects of necking have been studied, and found to be considerable in the type 1 primary inclusions. This mechanism has occasionally resulted in the appearance of almost pure CO2 fluids. The possibility of fluid immiscibility has been examined, and rejected, for the apparent “coexistence” of primary brine and CO2-bearing inclusions. Instead, mixing of fluids which fluctuated between two different compositions is proposed. The fluctuation was probably due to the sequence of hydration reactions during the Tauernkristallisation. Maximum trapping pressures (3.6 kbar) obtained for stage 1 of the Tauernkristallisation are thought to represent a situation where sublithostatic fluid pressures exested in shear zones during the crystallisation period of many of the emerald cores and coexisting biotite and actinolite. Maximum fluid pressures of 7 kbar were obtained from the type 2 inclusions. This is similar to pressure estimates obtained from mineral equilibria. At least four phases of deformation are indicated by the trapping history. A pressure-temperature-time path for the Tauernkristallisation and the subsequent cooling/uplift period has been constructed for the Habachtal area, using the maximum pressure estimates obtained in this work together with previously existing data. In the cooling period, fluid pressures lower than the lithostatic load again prevailed. This difference, about 1–2 kbar, was probably due to late stage fracturing and/or the development of an open system. At least two more phases of minor deformation and three more stages of entrapment have been defined for this period. During this time, fluids gradually evolved towards more CO2-poor, and less saline compositions. The present work shows that the possibility of fluctuations in fluid pressures must be considered seriously when attempting to define the PT cooling path from fluid inclusions in metamorphic rocks, especially those in shear zones. Postulations of retrograde PT paths based on fluid inclusions alone may result in pressure estimates which are too low.

Variation of calcic amphibole chemistry with increasing grade of metamorphism in different metamorphic facies series is reviewed, and the nature of reactions generating hornblende in natural assemblages is discussed. Recent publications on the variation of calcic amphibole composition with metamorphic grade have provided a variety of conflicting evidence regarding the reality or otherwise of a miscibility gap within the calcic amphibole series. It is here concluded that, in most cases, composition gaps may be better ascribed to disequilibrium and kinetic factors, and to abrupt or discontinuous changes in the compositions of, or in the appearance or disappearance of, other phases involved in proposed reactions occurring at the actinolite-hornblende transition. The weight of current evidence does not support the existence of a miscibility gap.

From the observed textures and mineral associations, chemographic relations have been derived in isobaric T-X CO 2 space for the minerals actinolite, ankerite, calcite, chlorine, epidote, and hornblende in Scottish Dalradian metabasites from Central Perthshire. Mineralogical variations in the carbonate-bearing metabasites within the distances of 0.5-1 km or so, are attributed to varying partial pressures of CO 2 operating during metamorphism.

Sanbagawa metabasites metamorphosed at conditions near the upper limit of the pumpellytic-actinolite facies were examined in terms of phase equilibria in the five component system Al2O3Fe2O3CaOMgOFeO. The Fe3+ content of epidote measured as X Ep Fe (= Fe/(Fe + Al ) of epidote) in the assemblage epidote-chlorite-actinolite-pumpellyite decreases gradually towards the higher-grade, pumpellyite-free areas. The progressive change in XFeEp can be detected within one metabasite bed 200 meters thick near the upper limit of the pumpellyite-actinolite facies. The MgFe2+ substitution, as expressed by variation of Fe/(Fe + Mg) in chlorite (0.40–0.55) has little effect on the Fe3+ + Al) ratios of epidote and pumpellyite in the above-mentioned assemblage. The lowet XFeEp in the pumpellite-bearing assemblage is 0.15 and hence the upper limit of the pumpellyite-actinolite facies is defined by the appearance of an epidote-chlorite-actinolite assemblage with X Ep Fc = C .15

Calcic amphiboles from the metabasites of the medium-pressure type from Mineoka, central Japan, are chemically continuous from Al-poor actinolite to Al-rich hornblende even in a single grain. Calcic amphibole with the intermediate composition between actinolite and hornblende was confirmed to be a single phase by analytical electron microscope and X-ray diffraction studies. Calcic amphiboles from Mineoka metabasites, transitional between greenschist and epidote amphibolite facies, are characterized by the continuous deficiency of Ca, which is due to the increase of crossite and cummingtonite components from actinolite to hornblende. In metabasites of medium-pressure type, however, calcic amphiboles with low Ca deficiency have the compositional gap between actinolite and hornblende. It is most probable that the miscibility gap in calcic amphiboles of metabasites of medium-pressure type tends to be narrowed with increase of crossite and cummingtonite components.

Microcline phenocrysts in a monzonite gneiss, and hornblende and actinolite crystals in a series of amphibole schists, form strong lineations of elongate crystals perpendicular to axes of small folds. The crystallographic c axes of the amphibole and microcline crystals form strong maxima parallel to the A fabric axis of the rocks; the b axes of microcline form a strong maxima parallel to the C fabric axes and the b axes of the amphiboles form maxima parallel to the B fabric axis. If the folds and foliation planes are products of inelastic shear strain, these orientations correspond to the maximum elastic strain energies per unit stress for these crystals and agree with the prediction of preferred orientation by MacDonald (1960) . A predicted effect of positive and negative shear strain was not observed in these crystals. Preferred mineral orientations and recrystallization may be interpreted by use of elastic strain theory to give additional insight into the problems of rock deformation.

Based on detailed analyses of parageneses of greenstone and greenschist containing actinolite, it is concluded that the defining reactions of actinolite isograds are different in different metamorphic terranes, each representing particular physical conditions. In one district, a decomposition reaction of pumpellyite into epidote-actinolite is important in defining the actinolite isograd. In another area, a reaction of chlorite, calcite and quartz which also produces epidote-actinolite is essential in marking the actinolite isograd. In several other areas prehnite is decomposed into epidote-actinolite at the actinolite isograds. Replacement of relic augite by actinolite is a separate reaction which does not define an actinolite isograd.

A statistical study of about 550 chemical analyses of amphiboles in the hornblende-actinolite series makes it possible to recognize their paragenetic types. The similarities or differences in the above types have been demonstrated by applying Student's t-test and Fischer's test. Typical hornblendes of regional metamorphic rocks are stable within the range 900 to 550°C. Actinolite is characteristic of greenschist facies rocks that formed at temperatures below 550°C. The contents of Al IV and alkalies decrease with decreasing temperatures and the content of Al IV increases with significant increase of pressure. The Al content also increases (due to AlvI) during the transition from clinopyroxene bearing rocks to garnet bearing rocks. Correlations revealed some peculiarities in isomorphism which confirmed the scheme Fe +3 Al IV → MgSi, but failed to confirm the replacement of AM AlIv by MgSi. The role of a glaucophane component has been shown. Curves showing the relationship of Ng and f in the various paragenetic types of amphiboles have been derived using regression equations.