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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.

Natural occurrences of Cl-bearing amphibole indicate crystallization in the presence of a highly saline fluid. Amphibole with varying Cl contents (0.1 to 5 wt% Cl) found in lower oceanic crustal rocks demonstrates the activity of saline hydrothermal fluids at depth. Here we present an experimental study done in cold seal pressure vessels (CSPV) and internally heated pressure vessels (IHPV) to illustrate the process by which gabbro-hosted amphibole-rich parageneses evolve in the presence of a hydrothermal saline fluid. The starting material used was a natural gabbroic rock containing high-Ti magnesio-hastingsite with the addition of a moderately to highly saline fluid (NaCl-H2O) with XNaCl = 0.02, 0.07 and 0.24 (6, 20 and 50 wt% NaCl). We evaluated a range of conditions from hydrothermal (500–750 °C) to magmatic (900 °C) at pressures of 200 MPa and fO2 close to NNO. Fluid/rock mass ratios used were 0.2 and 1 in subsolidus (hydrothermal) experiments, and 0.07 in partial melting (magmatic) experiments. New amphibole was formed on rims of the starting high-Ti magnesio-hastingsite, with product compositions corresponding to magnesio-hastingsite, high-Si magnesio-hornblende, tschermakite, edenite, hastingsite and ferro-pargasite with varying Cl contents up to 0.47 wt% Cl. Our results from subsolidus experiments demonstrate a decrease in olivine, plagioclase and clinopyroxene in the starting rock, and formation of new amphibole, with decreased IVAl and Ti with respect to starting amphibole. Product amphibole does not display any correlation between Cl and Fe2+, IVAl and K, in contrast to natural highly Cl-rich amphibole, suggesting that these correlations hold true only at Cl contents higher than those attained in our experiments. In addition, we found that increasing NaCl in the fluid correlates with increased maximum Cl contents in amphibole. Compositional variations found in product amphibole highlight the heterogeneities in fluid infiltration and Cl activity that account for the complexity of hydrothermal fluid/rock interactions in deep oceanic geological systems.

The Xilinhot gabbro–diorite pluton in central Inner Mongolia comprises a microgabbro–hornblende gabbro–quartz diorite suite and diorite–microdiorite–quartz microdiorite suite, which intrude the Erdaojing complex in the northern orogenic belt of the southeastern Central Asian Orogenic Belt (CAOB). Zircons separated from hornblende gabbro and quartz diorite yielded sensitive high-resolution ion microprobe U–Pb ages of 326.8 ± 3.4 and 324.0 ± 1.9 Ma, respectively. There are two types of hornblende in the hornblende gabbro: isolated hornblende and hornblende clusters surrounding orthopyroxene. Only hornblende clusters occur in the quartz diorite. The amphibole is edenite in the diorite, whereas the microdiorite and quartz microdiorite contain hornblende. The isolated hornblende formed at higher P–T–H2Omelt conditions (3.3–5.0 kbar; 749–814 °C; 7.4–8.3 wt% H2O) than the hornblende clusters in the hornblende gabbro (0.9–2.2 kbar; 670–731 °C; 3.8–5.5 wt% H2O). Edenite in the diorite crystallized at higher P–T and lower H2Omelt conditions (1.8–4.8 kbar; 777–979 °C; 3.2–5.2 wt% H2O) than hornblende in the microdiorite (1.3–1.5 kbar; 697–711 °C; 5.0–5.6 wt% H2O). These thermodynamic parameters reveal a multi-level magmatic storage system. Hornblende gabbro, quartz diorite, diorite, microdiorite, and quartz microdiorite are generally characterized by flat middle to heavy rare earth element patterns and no Eu anomalies. Geochemical modeling of heavy rare earth element concentrations in hornblende gabbro suggest that the magma was generated by >17.5% partial melting of spinel lherzolite. Slab-derived fluid input can account for the enrichments in Sr, Th, and light rare earth elements, and depletions in Nb and Ta. The multi-level magmatic storage system and slab-derived fluid input suggest that the early Carboniferous magmatism was generated in an arc-related tectonic setting. The northern orogenic and Hegenshan ophiolite belts constitute an early Carboniferous fore-arc basin–arc–back-arc basin system.

Amphibole growth rates were experimentally determined at hydrous (3.3–4.2 wt% H2O), isobaric (0.8 GPa) conditions, variable temperature (1030 and 1080 °C) and dwell time (0.25, 3, 6, and 9 h), using as starting material a primitive alkaline basalt from Procida island (Campi Flegrei Volcanic District, south Italy). Amphibole growth rates decrease from 1.5·10−7 to 2.9·10−8 cm s−1 as the duration of the experiments increase from 0.25 to 9 h. Moreover, increasing both temperature and water content leads to similar growth rate increase at constant dwell time. The comparison between amphibole and clinopyroxene growth rates determined at the same experimental conditions reveals for amphibole a faster growth relatively to the coexisting clinopyroxene, regardless of the dwell time. Furthermore, the experimental time appears to be a critical parameter for the composition of synthetic amphiboles; specifically, edenite is the dominant composition in short experiments (≤3 h), particularly, at low temperature (1030 °C), whereas the magnesiohastingsitic amphibole becomes progressively more important shifting towards longer duration and higher temperature run conditions. The magnesiohastingsite, on the basis of the amphibole-liquid Fe-Mg exchange coefficient values, results to be the compositional term representative of amphibole-melt equilibrium at the investigated P-T-H2O conditions. Finally, experimental growth rates from this study have been used to investigate the crystallization time of natural amphiboles and clinopyroxenes from the Oligo-Miocene cumulates of north-western Sardinia (i.e. Capo Marargiu Volcanic District, Italy), yielding crystallization times of 1.46–3.12 yr.

Adrar Izzilatene in the Egere-Aleksod terrane of the LATEA metacraton (Hoggar, Algeria) exposes one of the best preserved examples of eclogite facies metamorphism in Hoggar. Three distinct stages of metamorphic development are recognised, namely, the pre-peak stage (M1), characterised by garnet, amphibole, epidote, quartz and rutile, the peak eclogite facies stage (M2), consisting of omphacite, garnet, edenite, epidote, quartz and rutile, and the retrograde stage (M3), where initial decompression resulted in the appearance of plagioclase, the development of pargasite + plagioclase kelyphites and finally the formation of anhydrous plagioclase + diopside coronas. Porphyroblastic omphacite has a jadeite content of up to X Jd = 0.36, which is the highest yet observed for eclogite facies rocks from the Tuareg Shield. Garnet growth zoning patterns are characterised by flat profiles in the cores ( X Alm = 0.55–0.60; X Prp = 0.12–0.16; X Grs = 0.26–0.30) before showing a decrease in almandine to X Alm = 0.45, coupled to an increase in pyrope to X Prp = 0.29 and decrease in grossular to X Grs = 0.26 at the rims. Calculated P–T–M H2O pseudosections show that the prograde M1 assemblage equilibrated at 13–14 kbar and 580 °C, before pressure and temperature increased to 19 kbar and 650–700 °C at fluid-saturated conditions during peak metamorphism. Retrogression involved near-isothermal decompression to 8–9 kbar and 700–750 °C at fluid-undersaturated conditions. Prograde-to-peak metamorphism of the Izzilatene eclogite could have involved either oceanic or continental subduction, followed by exhumation as the area was obducted towards the LATEA metacraton during the Pan-African orogeny and the assembly of Western Gondwana.

At Okataina volcano, a caldera-collapse event at 46 ka followed by frequent intra-caldera eruptions (every 1–5 ka) provides an opportunity to examine the temperature and pressure conditions from amphiboles (and other phases) through the life cycle of a rhyolite magma system. In each of 18 eruptions examined, the amphibole population is heterogeneous (e.g., Al 2 O 3 = 4.45–10.88 wt.%; and MgO = 10.81–16.35 wt.%), and lacks trends in space or time or with magma composition and volume. The crystals are a ‘cargo’ gathered rapidly from a range from T-P-X conditions prior to eruption, perhaps due to buoyancy instabilities in the system causing overturn and blending. Nearly all of the compositional variation can be accounted for by temperature-sensitive edenite and Ti-Tschermak exchange mechanisms, and ‘Plagioclase exchange’ that is influenced by co-crystallizing plagioclase composition. The pressure-sensitive Al-Tschermak exchange is insignificant. This highlights that the application of Al-in-amphibole barometers would produce erroneous conclusions if atomic exchange mechanisms were not assessed. Variations in temperature implied by atomic substitutions can be explained by petrographic evidence for frequent mafic intrusions. Amphibole temperature estimates based on empirical models are supported by equilibrium calculations involving plagioclase and melt, and indicate some amphiboles are relicts from a hotter thermal regime than that of the dominant plagioclase population and melt. Fast equilibrating melts and Fe–Ti oxides show a dramatic change to high temperature rhyodacitic compositions immediately following caldera-collapse (42–36 ka), before returning to typical low temperature high-SiO 2 rhyolite production over the last ~ 30 ka. Throughout this cycle, the amphiboles record a contiguous thermally zoned system (700–900 °C), buffered from eruption events, perhaps reflecting its large size or a lack of connectivity in the melt generation zones. Hence, the crystallization state of the system has little bearing on its potential for eruption.

The Late Neoproterozoic Greendale Complex, located within the Avalon terrane of Nova Scotia, is a suite of appinitic rocks ranging from ultramafic to felsic in composition that were intruded during regional ensialic arc magmatism and crystallized at shallow crustal levels under conditions of high pH 2 O. Amphibole is the dominant mafic mineral in ultramafic to mafic rocks and displays the extraordinary variability in texture and modal abundance that is characteristic of appinite suites. These features allow sensitivity of amphibole composition (major, trace and REE) to the evolution of water-rich magma to be investigated. All amphiboles in mafic and ultramafic rocks are calcic, with (Ca + Na) B ≥ 1.34 and Si IV between 6.1 and 7.3. They predominantly range in composition from tschermakite to magnesiohornblende and display a dominance of edenite (Na,K A + Al IV = Si IV ) substitution. Although each sample exhibits remarkably uniform Mg# over a wide range in Si of up to one formula unit, the mafic rock amphiboles are characterized by lower (0.5 to 0.7) Mg#, compared to the ultramafic rocks (0.7 and 0.9). REE profiles are bow-shaped, and are characterized by depletion in LREE (La/Sm ≈ 0.61), a slight depletion in HREE (Gd/Yb ≈ 1.55) as well as a negative Eu anomaly, which is attributed to co-precipitation of plagioclase. REE and trace element profiles of ultramafic amphiboles are divided into two groupings. Group A amphiboles occur in all specimens analyzed and their REE profiles are very similar to the whole-rock analyses of the mafic rocks and to those predicted from amphibole/melt partition coefficients. In contrast, Group B amphiboles display relative enrichment in light REEs (La/Sm ≈ 2.05), have lower ΣREE, and lack a negative Eu anomaly relative to Sm and Gd. Group B amphiboles are more enriched in Th and U and show a more pronounced depletion in Nb, Ti, Y and HREE. Group B amphiboles probably grew in a reaction relationship with olivine and pyroxene, and their contrasting profiles suggest that partition coefficients traditionally used for petrogenetic modeling may not be appropriate for such amphiboles. Groups A and B are virtually indistinguishable with respect to the major elements used for amphibole classification, suggesting that REE and selected trace element data, when combined with textural observations, may provide important additional insights into phase equilibria and conditions of crystallization.

The Rio Espinharas pluton, northeastern Brazil, belongs to the shoshonitic series and consists mainly of syenogranite, quartz-monzonite and porphyritic quartz-monzonite, but diorite, quartz-monzodiorite, quartz-syenite and microsyenogranite also occur containing microgranular enclaves, except for the diorite. Most variation diagrams of rocks, amphiboles, biotites and allanites show linear trends, but K, Zr, Sr and Ba of rocks display curved scattered trends. The rocks ranging from diorite to syenogranite define a pseudo-errorchron and have similar REE patterns. Syenogranite and microsyenogranite are derived from two distinct pulses of granite magma with initial 87Sr/86Sr ratio of 0.7083±0.0003 and 0.7104±0.0007, respectively. Modelling of major and trace elements shows that the syenogranite evolved by fractional crystallization of plagioclase, microcline, edenite, biotite and titanite, whereas quartz-monzonite, porphyritic quartz-monzonite, quartz-monzodiorite and quartz-syenite resulted from simple mixing between an upper mantle-derived dioritic magma and the upper crust-derived syenogranite magma. Dioritic enclaves are globules of a mafic magma from the upper mantle. http://www.sciencedirect.com/science/article/B6V6J-46X86NR-5/1/0e5ce1dcc0c8fe91ebd680e6f2ac523b

A series of granitoids from Proterozoic to Cenozoic age occurred in the western Kunlun orogenic belt, Xinjiang, northwestern China. Several intrusions such as the West Datong (Middle Caledonian age), North Kuda (Late Caledonian age) and Kuzigan, Karibasheng, Zankan (Himalayan age) plutons have shoshonitic affinity. Their rock assemblages include (quartz) monzodiorite–(quartz) monzonite–quartz syenite (Middle Caledonian) or monzonitic granite–granite (Late Caledonian) or biotite (monzonitic) granite–diopside granite–diopside syenite (Himalayan). Generally, biotite is iron–phlogopite, with some eastonite and high Mg/(Mg+Fe T ) and Fe 3+ /Fe 2+ ratio. Amphibole is mainly edenitic hornblende and magnesian hastingsitic hornblende, with some edenite and higher Mg/(Mg+Fe T ) and Fe 3+ /Fe 2+ ratio. The rocks show SiO 2 contents of 52.77–71.85% and high K 2 O+Na 2 O (mostly >8%, average 9.14%), K 2 O/Na 2 O (mostly >1, average 1.50) and Fe 2 O 3 /FeO (0.85–1.51, average 1.01) and low TiO 2 contents (0.15–1.12%, average 0.57%). Al 2 O 3 contents (13.01–19.20%) are high but variable. The granitoids are prominently enriched in LILE, LREE and volatiles such as F. However, the studied shoshonitic granitoids among the three intrusive periods also show differences in isotopic compositions and trace element concentrations, suggesting their different geneses: the origin of the West Datong pluton is probably related to the involvement of subducted oceanic crust sediments into the mantle source; the North Kuda and Himalayan plutons could have been generated by partial melting of subducted oceanic crust sediments or metasediments of thickened continental lower crust in the process of late-orogenic slab break-off or lithospheric thinning.

The Variscan metamorphic evolution of eclogite-amphibolites from the Savona Crystalline Massif is marked by the occurence of calcic amphiboles. Microtextural relations together with coupled electron microprobe and X-ray investigations on amphiboles pointed out a complex decompressional evolution. A prograde stage of the evolution in the eclogite facies (temperature conditions estimated as ca. 620°C) is recorded by compositional zoning and inclusions in garnet. The eclogitic event (pressure conditions 12kbar) produced assemblages of garnet, omphacite, Ca-amphibole, zoisite, quartz and rutile. Composition and zoning of matrix eclogitic amphiboles reveal that the subsequent evolution in the eclogite facies contemplates a late re-equilibration under lower conditions of temperature (ca. 540°C) and pressure. The eclogitic amphiboles are pargasitic to edenitic hornblendes, and are marked by rather high Na contents at the M4 site (up to 0.5 apfu). The forming of coarse-grained diopside-plagioclase symplectites and of amphibole-plagioclase coronas represent the early results of the destabilization of omphacites and garnets in the amphibolite facies. Pressure conditions of this stage have been estimated of ca. 10 kbar, while temperature conditions are poorly constrained. However, the increase in edenite (NaAlSi −1 ) at the outermost rims of the matrix eclogitic amphiboles and the very high values of edenite and tschermack (Al 2 Mg −1 Si −1 ) components in the coronitic amphiboles (subsilicic pargasites) suggest that temperature increase accompanied the early decompression to the amphibolite facies. Subsequent retrogression in the low-grade amphibolite facies resulted in the complete breakdown of omphacite, producing finer-grained symplectites. This stage is also recorded by the amphibole growth (magnesio-hornblendes to edenitic hornblendes with negligible Na amounts at the M4 site) at the expense of the clinopyroxene symplectites and as outward rims around the coronitic amphiboles. Frequently, amphibolites are completely recrystallized and do not retain any relic of high pressure assemblages. Temperature conditions for this late amphibolitic event have been estimated of ca. 510°C, while pressure conditions are inferred in the range 4–7 kbar. Such a complicated decompressional evolution is indicative of complex uplift tectonics after a subduction event.

Amphiboles approached edenite (NaCa 2 Mg 5 Si 7 AlO 22 (OH) 2 ), richterite (Na 2 CaMg 5 Si 8 O 22 (OH) 2 ), tremolite (□Ca 2 Mg 5 Si 8 O 22 (OH) 2 ) solid solutions were studied by conventional hydrothermal techniques employing the bulk compositions edenite, and edenite + additional quartz, all with excess H 2 O. For the stoichiometric edenite bulk composition + excess H 2 O, the equilibrium phase assemblage is diopside + Na-phlogopite + forsterite + fluid at, and just above the amphibole high-temperature limit at 850 ± 5°C, 500 bar, and 880 ± 5°C, 1000 bar. The breakdown temperature of sodic phlogopite is 855 ± 3°C at 500 bar, and 890 ± 5°C at 700 bar, producing nepheline + plagioclase (or melt), additional forsterite and fluid. Diopside and Na-phlogopite solid solution coexist over a broad P fluid - T region, even within the amphibole field, where they are associated with an edenite-richterite (-tremolite) solid solution of approximate composition Ed 35 Rc 50 Tr 15 . In the system edenite + 4 quartz + excess H 2 O, nearly pure tremolite and albite coexist stably between 670° and 830°C at 1000 bar and give way to the possibly metastable assemblage diopside + talc + albite below 670°C. In the presence of albite, tremolite reacts to produce diopside + quartz + enstatite + fluid above 830°C at 1000 bar. For the investigated silica-rich bulk composition, amphibole P fluid - T stability is divided by the albite melting curve into a tremolite + albite field, and a tremolite + aqueous melt field. Substantial equilibrium solid solution of tremolite towards edenite or richterite was not observed for silica-excess bulk compositions. Metastable edenite-rich amphiboles initially synthesized change to tremolite with increasing run length in the presence of free SiO 2 . Edenitic amphibole is stable only over a very limited temperature range in silica-undersaturated environments, thus accounting for its rarity in nature. Na-phlogopite solid solutions are also disfavored by high a SiO 2 ; even for nepheline-normative lithologies, a hypothesized rapid low-temperature conversion to vermiculite or smectite could partly explain the scarcity of sodic phlogopite in rocks.

A compositional gap between hypersolidus plagioclase and subsolidus plagioclase, most of which must have formed by devitrification, decreases in An % with decreasing cooling rate, probably reflecting a melt composition derived by in situ differentiation. Augitic pyroxenes, crystallized from a quickly cooling magma, tend to contain more Al, Ti, Na(+K) and Fe3+, and less Si and Mn. Compositional breaks between hypersolidus and subsolidus amphiboles with respect to the components of edenite and titanoamphibole, decrease in width with decreasing cooling rate. Subcalcic hornblende seems to grow from differentiated magmas in the course of moderate to rapid cooling.

An expression of 200 amphibole analyses from Deer et al. (1963) and Engel et al. (1964) in terms of x tremolite , x ferrotremolite , x edenite , x tschermakite , and x pargasite is generally accompanied by a large errorbetween the observed and computed chemical analysis. In addition it is common to have x j ar j , for many analyses which other authors have shown graphically as falling within the chemical composition space defined by these five molecules. Most major chemical variations within the amphibole group are expressed in terms of the 11 oxides SiO 2 , TiO 2 , Al 2 O 3 , Fe 2 O 3 , FeO, MnO, MgO, CaO, Na 2 O, K 2 O, H 2 O. Granting this assumption the total error, → , for a given chemical analysis b → is minimized when the amphibole composition space is represented by an 11×10 matrix A and x → is computed using some 10×10 matrix from A. It has not been possible thus far, using common amphibole ‘end’ members to define a representation A within which x j ≧0 for all j for a single amphibole analysis. It is possible, however, to generate a set of molecules which obey charge balance and certain structural constraints of the amphibole group and which define a composition space within which natural amphiboles lie. It is concluded that the natural amphiboles cannot be represented graphically.