Role of cryptic amphibole crystallization in magma differentiation at Hudson volcano, Southern Volcanic Zone, Chile

Hudson volcano (Chile) is the southern most stratovolcano of the Andean Southern Volcanic Zone and has produced some of the largest Holocene eruptions in South America. There have been at least 12 recorded Holocene explosive events at Hudson, with the 6700 years BP, 3600 years BP, and 1991 eruptions the largest of these. Hudson volcano has consistently discharged magmas of similar trachyandesitic and trachydacitic composition, with comparable anhydrous phenocryst assemblages, and pre-eruptive temperatures and oxygen fugacities. Pre-eruptive storage conditions for the three largest Holocene events have been estimated using mineral geothermometry, melt inclusion volatile contents, and comparisons to analogous high pressure experiments. Throughout the Holocene, storage of the trachyandesitic magmas occurred at depths between 0.2 and 2.7 km at approximately ~972°C (±25) and log fO₂ −10.33–10.24 (±0.2) (one log unit above the NNO buffer), with between 1 and 3 wt% H₂O in the melt. Pre-eruptive storage of the trachydacitic magma occurred between 1.1 and 2.0 km, at ~942°C (±26) and log fO₂ −10.68 (±0.2), with ~2.5 wt% H₂O in the melt. The evolved trachyandesitic and trachydacitic magmas can be derived from a basaltic parent primarily via fractional crystallization. Entrapment pressures estimated from plagioclase-hosted melt inclusions suggest relatively shallow levels of crystallization. However, trace element data (e.g., Dy/Yb ratio trends) suggests amphibole played an important role in the differentiation of the Hudson magmas, and this fractionation is likely to have occurred at depths >6 km. The absence of a garnet signal in the Hudson trace element data, the potential staging point for differentiation of parental mafic magmas [i.e., ~20 km (e.g., Annen et al. in J Petrol 47(3):505–539, 2006)], and the inferred amphibolite facies [~24 km (e.g., Rudnick and Fountain in Rev Geophys 33:267–309, 1995)] combine to place some constraint on the lower limit of depth of differentiation (i.e., ~20–24 km). These constraints suggest that differentiation of mantle-derived magmas occurred at upper-mid to lower crustal levels and involved a hydrous mineral assemblage that included amphibole, and generated a basaltic to basaltic andesitic composition similar to the magma discharged during the first phase of the 1991 eruption. Continued fractionation at this depth resulted in the formation of the trachyandesitic and trachydacitic compositions. These more evolved magmas ascended and stalled in the shallow crust, as suggested by the pressures of entrapment obtained from the melt inclusions. The decrease in pressure that accompanied ascent, combined with the potential heating of the magma body through decompression-induced crystallization would cause the magma to cross out of the amphibole stability field. Further shallow crystallization involved an anhydrous mineral assemblage and may explain the lack of phenocrystic amphibole in the Hudson suite.     

Adakite-like Lavas from Antisana Volcano (Ecuador): Evidence for Slab Melt Metasomatism Beneath Andean Northern Volcanic Zone

Extensive sampling of the Antisana volcano in Ecuador (NorthernVolcanic Zone of the Andes) has revealed the presence of adakite-likerocks throughout the edifice, i.e. rocks with geochemical characteristicsclose, but not identical, to those of slab melts. Two main volcanicgroups have been distinguished, characterized by two distinctevolutionary trends. The AND group, mostly composed of andesites,shows the clearest adakitic characteristics such as high La/Yband Sr/Y ratios and low heavy rare earth element (HREE) contents.The CAK group, composed of high-K andesites and dacites, displaysless pronounced adakitic-like characteristics. Although themore basic rocks of each group are difficult to distinguishon many geochemical diagrams, a geochemical study shows thatthe evolution of the AND and CAK groups is dominated by differentpetrogenetic processes. The isotopic characteristics of theCAK rocks suggest that evolution of this group is dominatedby a limited assimilation–fractional crystallization processwithin the granitic continental basement of the cordillera.In the AND group, the abundances of incompatible elements, suchas Nb or HREE, suggest that the series was produced by a partialmelting process in a mantle rich in garnet, amphibole and/orclinopyroxene. Such a mantle source has been demonstrated (experimentallyand by exhumed mantle xenoliths) to be produced in subductionzones where slab melts react with and metasomatize the mantlewedge. In Ecuador, magmas erupted in the Western Cordillera(trenchward relative to Antisana volcano) are true adakites,suggesting that slab melts can be responsible for the metasomatismof the mantle wedge beneath the NVZ in Ecuador. If mantle convectioncan drag down this modified mantle beneath Antisana volcano,destabilization of metasomatic amphibole at appropriate pressuresin this modified garnetiferous mantle can adequately explainthe formation and the geochemical features of Antisana lavas. KEY WORDS: subduction; adakite; metasomatism; Ecuador; AFC; Sr and Nd isotopes     

Temporal Evolution of Magmatism in the Northern Volcanic Zone of the Andes: The Geology and Petrology of Cayambe Volcanic Complex (Ecuador)

In the Northern Volcanic Zone of the Andes, the Cayambe VolcanicComplex consists of: (1) a basal, mostly effusive volcano, theViejo Cayambe, whose lavas (andesites and subordinate dacitesand rhyolites) are typically calc-alkaline; and (2) a younger,essentially dacitic, composite edifice, the Nevado Cayambe,characterized by lavas with adakitic signatures and explosiveeruptive styles. The construction of Viejo Cayambe began >1·1Myr ago and ended at 1·0 Ma. The young and still activeNevado Cayambe grew after a period of quiescence of about 0·6Myr, from 0·4 Ma to Holocene. Its complex history isdivided into at least three large construction phases (Angurealcone, Main Summit cone and Secondary Summit cone) and compriseslarge pyroclastic events, debris avalanches, as well as periodsof dome activity. Geochemical data indicate that fractionalcrystallization and crustal assimilation processes have a limitedrole in the genesis of each suite. On the contrary, field observations,and mineralogical and geochemical data show the increasing importanceof magma mixing during the evolution of the volcanic complex.The adakitic signature of Nevado Cayambe magmas is related topartial melting of a basaltic source, which could be the lowercrust or the subducted slab. However, reliable geophysical andgeochemical evidence indicates that the source of adakitic componentis the subducted slab. Thus, the Viejo Cayambe magmas are inferredto come from a mantle wedge source metasomatized by slab-derivedmelts (adakites), whereas the Nevado Cayambe magmas indicatea greater involvement of adakitic melts in their petrogenesis.This temporal evolution can be related to the presence of thesubducted Carnegie Ridge, modifying the geothermal gradientalong the Wadati–Benioff zone and favouring slab partialmelting. KEY WORDS: adakites; ⁴⁰Ar/³⁹Ar dating; Cayambe volcano; Ecuador; mantle metasomatism; Andes     

Adakite-like porphyries from the southern Tibetan continental collision zones: evidence for slab melt metasomatism

We present new whole rock trace element and Pb-isotope data for a suite of Neogene adakitic rocks that formed during the post-collisional stage of the India-Asia collision in an east-west- trending array along the Yalu Tsangpo suture. Compared to classic ‘adakites’ that form along certain active convergent plate margins, the Tibetan adakitic rocks show even stronger enrichment in incompatible elements (i.e. Rb, Ba, Th, K and LREEs) and even larger variation in radiogenic (Pb, Sr, Nd) isotope ratios. Tibetan adakitic rocks have extraordinarily low HREE (Yb: 0.34–0.61 ppm) and Y (3.71–6.79 ppm), high Sr/Y (66–196), high Dy_n/Yb_n and La_n/Yb_n. They show strong evidence of binary mixing both in isotopic space (Sr-Nd, common Pb, thorogenic Pb) and trace element systematics. The majority of the adakitic rocks in south Tibet, including published and our new data, have variational Mg# (0.32–0.70), clear Nb (and HFSE) enrichment, the lowest initial ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios, and the highest ¹⁴⁴Nd/¹⁴³Nd ratios of all Neogene volcanic rocks in south Tibet. These results indicate an involvement of slab melts in petrogenesis. Major and trace element characteristics of the isotopically more enriched adakites are compatible with derivation from subducted sediment but not with assimilation of crustal material. Thus, the south Tibetan adakitic magmas are inferred to have been derived from an upper mantle source metasomatised by slab-derived melts. An interesting observation is that temporally coeval and spatially related lamproites could be genetically related to the adakitic rocks in representing partial melts of distinct mantle domains metasomatised by subducted sediment. Our favoured geodynamic interpretation is that along-strike variation in south Tibetan post-collisional magma compositions may be related to release of slab melts and fluids along the former subduction zone resulting in compositionally distinct mantle domains.     

Wet and Dry Basalt Magma Evolution at Torishima Volcano, Izu Bonin Arc, Japan: the Possible Role of Phengite in the Downgoing Slab

The arc-front volcanoes of Sumisu (31·5°N, 140°E)and Torishima (30·5°N, 140·3°E) in thecentral Izu–Bonin arc are similar in size and rise asrelatively isolated edifices from the seafloor. Together theyprovide valuable along-arc information about magma generationprocesses. The volcanoes have erupted low-K basalts originatingfrom both wet and dry parental basaltic magmas (low-Zr basaltsand high-Zr basalts, respectively). Based on models involvingfluid-immobile incompatible element ratios (La/Sm), the parentalbasalts appear to result from different degrees of partial meltingof the same source mantle (20% and 10% for wet and dry basaltmagmas, respectively). Assuming that the wet basalts containgreater abundances of slab-derived components than their drycounterparts, geochemical comparison of these two basalt typespermits the identification of the specific elements involvedin fluid transport from the subducting slab. Using an extensiveset of new geochemical data from Torishima, where the top ofthe downgoing slab is about 100 km deep, we find that Cs, Pb,and Sr are variably enriched in the low-Zr basalts, which cannotbe accounted for by fractional crystallization or by differencesin the degree of mantle melting. These elements are interpretedto be selectively concentrated in slab-derived metasomatic fluids.Variations in K, high field strength element and rare earthelement concentrations are readily explained by variations inthe degree of melting between the low- and high-Zr basalts;these elements are not contained in the slab-derived fluids.Rb and Ba exhibit variable behaviour in the low-Zr basalts,ranging from immobile, similar to K, to mildly enriched in somelow-Zr basalts. We suggest that the K-rich mica, phengite, playsan important role in determining the composition of fluids releasedfrom the downgoing slab. In arc-front settings, where slab depthis 100 km, phengite is stable, and the fluids released fromthe slab contain little K. In back-arc settings, however, wherethe slab is at 100–140 km depth, phengite is unstable,and K-rich fluids are released. We conclude that cross-arc variationsin the K content of arc basalts are probably related to differingcompositions of released fluids or melts rather than the widelyheld view that such variations are controlled by the degreeof partial melting. KEY WORDS: arc volcano; degrees of melting; mantle wedge; water; wet and dry basalts     

Water-saturated magmas in the Panama Canal region: a precursor to adakite-like magma generation?

Amphibole, while uncommon as a phenocryst in arc lavas, is increasingly recognized as a key constituent in the petrogenesis of arc magmas. Fractional crystallization of water-saturated arc magmas in the lower crust can yield substantial volumes of amphibole cumulates that, depending on the pressure of crystallization, may also contain garnet. Fractionation of this higher pressure assemblage has been invoked as a possible mechanism in the production of magmas that contain an adakitic signature. This study examines newly dated Late-Oligocene (25.37 ± 0.13 Ma) hypabyssal amphibole-rich andesites from Cerro Patacon in the Panama Canal region. These andesites contain nodules of amphibole cumulates that are ~4–6 cm in diameter and are almost entirely composed of 5–10-mm amphibole crystals (dominantly ferri-tschermakite). Geochemical variations, optical and chemical zoning of the Cerro Patacon amphiboles are consistent with their evolution in a crystal mush environment that had at least one recharge event prior to entrainment in the host andesite. Amphiboles hosted within the cumulate nodules differ from those hosted in the Cerro Patacon andesite and contain consistently higher values of Ti. We suggest these nodules represent the early stages of fractionation from a water-saturated magma. Cerro Patacon andesites have REE concentrations that plot at the most depleted end of Central American Arc magmas and exhibit a distinctive depletion in the middle REE. These geochemical and petrographic observations strongly support significant amphibole fractionation during formation of the Cerro Patacon andesite, consistent with the petrographic evidence. Fractionation of water-saturated magmas is a mechanism by which adakitic compositions may be produced, and the Cerro Patacon andesites do exhibit adakite-like geochemical characteristics (e.g., elevated Sr/Y; 28–34). However, the relatively elevated concentrations of Y and HREE indicate garnet was not stable in the fractionating assemblage during this early stage of arc development.     

Andesite and dacite genesis via contrasting processes: the geology and geochemistry of El Valle Volcano,Panama

The easternmost stratovolcano along the Central American arc is El Valle volcano, Panama. Several andesitic and dacitic lava flows, which range in age 5–10 Ma, are termed the old group. After a long period of quiescence (approximately 3.4 Ma), volcanic activity resumed approximately 1.55 Ma with the emplacement of dacitic domes and the deposition of dacitic pyroclastic flows 0.9–0.2 Ma. These are referred to as the young group. All of the samples analyzed are calc-alkaline andesites and dacites. The mineralogy of the two groups is distinct; two pyroxenes occur in the old-group rocks but are commonly absent in the young group. In contrast, amphibole has been found only in the young-group samples. Several disequilibrium features have been observed in the minerals (e.g., oscillatory zoning within clinopyroxenes). These disequilibrium textures appear to be more prevalent among the old- as compared with the young-group samples and are most likely the result of magma-mixing, assimilation, and/or polybaric crystallization. Mass-balance fractionation models for major and trace elements were successful in relating samples from the old group but failed to show a relationship among the young-group rocks or between the old- and young-group volcanics. We believe that the old-group volcanics were derived through differentiation processes from basaltic magmas generated within the mantlewedge. The young group, however, does not appear to be related to more primitive magmas by differentiation. The young-group samples cannot be related by fractionation including realistic amounts of amphibole. Distinctive geochemical features of the young group, including La/Yb ratios〉15, Yb〈1, Sr/Y〉150, and Y〈6, suggest that these rocks were derived from the partial melting of the subducted lithosphere. These characteristics can be explained by the partial melting of a source with residual garnet and amphibole. Dacitic material with the geochemical characteristics of subducted-lithosphere melting is generated apparently only where relatively hot crust is subducted, based on recent work. The young dacite-genesis at El Valle volcano is related to the subduction of relatively hot lithosphere.     

Phase Relationships of Hydrous Alkalic Magmas at High Pressures: Production of Nepheline Hawaiitic to Mugearitic Liquids by Amphibole-Dominated Fractional Crystallization Within the Lithospheric Mantle

Experimental melting studies were conducted on a nepheline mugearitecomposition to pressures of 31 kbar in the presence of 0–30%added water. A temperature maximum in the near-liquidus stabilityof amphibole (with olivine) was found for a water content of3·5 wt % at a pressure of 14 kbar. This is interpretedto have petrogenetic significance for the derivation of nephelinemugearite magmas from nepheline hawaiite by amphibole-dominatedfractional crystallization at depth within the lithosphericmantle. Synthetic liquids at progressively lower temperaturesrange to nepheline benmoreite compositions very similar to thoseof natural xenolith-bearing high-pressure lavas elsewhere, andsupport the hypothesis that continued fractional crystallizationcould lead to high-pressure phonolite liquids. Independent experimentaldata for a basanite composition modeled on a lava from the sameigneous province (the Newer Basalts of Victoria) permit theinference that primary asthenospheric basanite magmas undergopolybaric fractional crystallization during ascent, and mayevolve to liquids ranging from nepheline hawaiite to phonoliteupon encountering cooler lithospheric mantle at depths of 42–50km. Such a model is consistent with the presence in some evolvedalkalic lavas of both lithospheric peridotite xenoliths indicativeof similar depths and of megacryst suites that probably representdisrupted pegmatitic segregations precipitated from precursoralkalic magmas in conduit systems within lithospheric mantle. KEY WORDS: experiment; high pressure; alkalic magmas; amphibole; nepheline mugearite; basanite; lithosphere     

Petrology of a Late Archaean, Highly Potassic, Sanukitoid Pluton from the Baltic Shield: Insights into Late Archaean Mantle Metasomatism

The late Archaean Panozero pluton in Central Karelia (BalticShield) is a multi-phase high-Mg, high-K intrusion with sanukitoidaffinities, emplaced at 2·74 Ga. The magmatic historyof the intrusion may be subdivided into three cycles and includesmonzonitic and lamprophyric magmas. Compositional variationsare most extreme in the monzonite series and these are interpretedas the result of fractional crystallization. Estimates of thecomposition of the parental magmas to the monzonites and lamprophyresshow that they are enriched in light rare earth elements, Sr,Ba, Cr, Ni and P but have low contents of high field strengthelements. Radiogenic isotope data indicate a low U/Pb, highTh/U, high Rb/Sr, low Sm/Nd source. The magmatic rocks of thePanozero intrusion are also enriched in H₂O and CO₂; carbonisotope data are consistent with mantle values, indicating afluid-enriched mantle source. The similarity in trace elementcharacter of all the Panozero parental magmas indicates thatall the magmas were derived from a similar mantle source. Thepattern of trace element enrichment is consistent with a mantlesource enriched by fluids released from a subducting slab. Nd-isotopedata suggest that this enrichment took place at c. 2·8Ga, during the main episode of greenstone belt and tonalite–trondhjemite–granodioriteformation in Central Karelia. Sixty million years later, at2·74 Ga, the subcontinental mantle melted to form thePanozero magmas. Experimental studies suggest that the monzoniticmagmas originated by the melting of pargasite–phlogopitelherzolite in the subcontinental mantle lithosphere at 1–1·5GPa. The precise cause of the melting event at 2·74 Gais not known, although a model involving upwelling of asthenosphericmantle following slab break-off is consistent with the geochemicalevidence for the enrichment of the Karelian subcontinental mantlelithosphere by subduction fluids. KEY WORDS: Archaean; sanukitoid; monzonite; Karelia; mantle metasomatism     

Depths of Partial Crystallization of H2O-bearing MORB: Phase Equilibria Simulations of Basalts at the MAR near Ascension Island (7 11{degrees}S)

Phase equilibria simulations were performed on naturally quenchedbasaltic glasses to determine crystallization conditions priorto eruption of magmas at the Mid-Atlantic Ridge (MAR) east ofAscension Island (7–11°S). The results indicate thatmid-ocean ridge basalt (MORB) magmas beneath different segmentsof the MAR have crystallized over a wide range of pressures(100–900 MPa). However, each segment seems to have a specificcrystallization history. Nearly isobaric crystallization conditions(100–300 MPa) were obtained for the geochemically enrichedMORB magmas of the central segments, whereas normal (N)-MORBmagmas of the bounding segments are characterized by polybariccrystallization conditions (200–900 MPa). In addition,our results demonstrate close to anhydrous crystallization conditionsof N-MORBs, whereas geochemically enriched MORBs were successfullymodeled in the presence of 0·4–1 wt% H₂O in theparental melts. These estimates are in agreement with direct(Fourier transform IR) measurements of H₂O abundances in basalticglasses and melt inclusions for selected samples. Water contentsdetermined in the parental melts are in the range 0·04–0·09and 0·30–0·55 wt% H₂O for depleted and enrichedMORBs, respectively. Our results are in general agreement (within±200 MPa) with previous approaches used to evaluate pressureestimates in MORB. However, the determination of pre-eruptiveconditions of MORBs, including temperature and water contentin addition to pressure, requires the improvement of magma crystallizationmodels to simulate liquid lines of descent in the presence ofsmall amounts of water. KEY WORDS: MORB; Mid-Atlantic Ridge; depth of crystallization; water abundances; phase equilibria calculations; cotectic crystallization; pressure estimates; polybaric fractionation     

Origin of silicic volcanism in the Panamanian arc: evidence for a two-stage fractionation process at El Valle volcano

In the Central American Volcanic Arc, adakite-like volcanism has often been described as volumetrically insignificant. However, extensive silicic adakitic volcanism does occur in the Panamanian arc and provides an opportunity to evaluate the origin of this magma-type as well as to contrast its origin with other Central American silicic magmas. The Quaternary volcanic deposits of El Valle volcano are characterized by pronounced depletions in the heavy rare earth elements, low Y, high Sr, high Sr/Y, relatively high MgO, and low K₂O/Na₂O, when compared with other Quaternary Central American volcanics at similar SiO₂. These chemical features are also diagnostic of adakitic signatures. Our new ⁴⁰Ar/³⁹Ar ages of lava flows and ash flows that compose the volcanic edifice of El Valle volcano illustrate that the eruptive volume of adakitic-like volcanism is substantial during the Quaternary (~120 km³). Adakitic-like magmas dominate the stratigraphic record. Common to all models for the origin of an adakite geochemical signature is the involvement of garnet, as a residual or fractionating phase. The stability of garnet in hydrous magmas has been recently reevaluated with important consequences; garnet is a stable primary igneous phase at pressure and temperature conditions expected for magma differentiation at the roots of a mature island arc. Moreover, adakite-like volcanism erupted at El Valle volcano displays the middle rare earth element depletion observed in other Panamanian volcanic centers that has been attributed to significant amphibole fractionation. Extensive amphibole fractionation may have occurred in two stages. The first stage of fractionation, garnet + amphibole fractionation, occurs from hydrous basaltic–andesitic parental magmas that have ponded at the base of an overthickened crust. The second stage occurs at mid-lower crustal levels where abundant amphibole + plagioclase and minor sphene crystallized from water-rich magmas. These two stages combined may have resulted in an amphibole-rich cumulate layer. This amphibole layer is likely the source of the abundant amphibole-rich cumulate enclaves and blobs found in volcanic products across the Panamanian arc. Stalling of water-rich magmas during this two-stage fractionation process could drive the interstitial liquids to the evolved compositions typical of continental crust, while leaving behind amphibole-rich cumulate rocks that may eventually be returned to the asthenosphere. Differentiation of H₂O-rich magmas under the conditions appropriate for the roots of island arcs may therefore be a key process in developing a better understanding of the generation of continental crust in island arc environments.     

From basalt to dacite: origin and evolution of the calc-alkaline series of Salina, Aeolian Arc, Italy

The island of Salina comprises one of the most distinct calc-alkaline series of the Aeolian arc (Italy), in which calc-alkaline, high-K calc-alkaline, shoshonitic and leucite-shoshonitic magma series are developed. Detailed petrological, geochemical and isotopic (Sr, Nd, Pb, O) data are reported for a stratigraphically well-established sequence of lavas and pyroclastic rocks from the Middle Pleistocene volcanic cycle (430–127 ka) of Salina, which is characterized by an early period of basaltic volcanism (Corvo; Capo; Rivi; Fossa delle Felci, group 1) and a sequence of basaltic andesites, and andesites and dacites in the final stages of activity (Fossa delle Felci, groups 2–8). Major and trace element compositional trends, rare earth element (REE) abundances and mineralogy reveal the importance of crystal fractionation of plagioclase + clinopyroxene + olivine/ orthopyroxene ± titanomagnetite ± amphibole ± apatite in generating the more evolved magma types from parental basaltic magmas, and plagioclase accumulation in producing the high Al₂O₃ contents of some of the more evolved basalts. Sr isotope ratios range from 0.70410 to 0.70463 throughout the suite and show a well-defined negative correlation with ¹⁴³Nd/¹⁴⁴Nd (0.51275–0.51279). Pb isotope compositions are distinctly radiogenic with relatively large variations in ²⁰⁶Pb/²⁰⁴Pb (19.30–19.66), fairly constant ²⁰⁷Pb/²⁰⁴Pb (15.68–15.76) and minor variations in ²⁰⁸Pb/²⁰⁴Pb ratios (39.15–39.51). Whole-rock δ¹⁸O values range from +6.4 to +8.5‰ and correlate positively with Sr isotope ratios. Overall, the isotopic variations are correlated with the degree of differentiation of the rocks, indicating that only small degrees of crustal assimilation are overprinting the dominant evolution by crystal–liquid fractionation (AFC-type processes). The radiogenic and oxygen isotope composition of the Salina basalts suggests derivation from primary magmas from a depleted mantle source contaminated by slab-derived fluids and subducted sediments with an isotopic signature of typical upper continental crust. These magmas then evolved further to andesitic and dacitic compositions through the prevailing process of low-pressure fractional crystallization in a shallow magma reservoir, accompanied by minor assimilation of crustal lithologies similar to those of the Calabrian lower crust. Received: 29 November 1999 / Accepted: 16 April 2000     

Role of the subducted slab,mantle wedge and continental crust in the generation of adakites from the Andean Austral Volcanic Zone

 All six Holocene volcanic centers of the Andean Austral Volcanic Zone (AVZ; 49–54°S) have erupted exclusively adakitic andesites and dacites characterized by low Yb and Y concentrations and high Sr/Y ratios, suggesting a source with residual garnet, amphibole and pyroxene, but little or no olivine and plagioclase. Melting of mafic lower crust may be the source for adakites in some arcs, but such a source is inconsistent with the high Mg# of AVZ adakites. Also, the AVZ occurs in a region of relatively thin crust (<35 km) within which plagioclase rather than garnet is stable. The source for AVZ adakites is more likely to be subducted oceanic basalt, recrystallized to garnet-amphibolite or eclogite. Geothermal models indicate that partial melting of the subducted oceanic crust is probable below the Austral Andes due to the slow subduction rate (2 cm/year) and the young age (<24 Ma) of the subducted oceanic lithosphere. Geochemical models for AVZ adakites are also consistent with a large material contribution from subducted oceanic crust (35–90% slab-derived mass), including sediment (up to 4% sediment-derived mass, representing approximately 15% of all sediment subducted). Variable isotopic and trace-element ratios observed for AVZ adakites, which span the range reported for adakites world-wide, require multistage models involving melting of different proportions of subducted basalt and sediment, as well as an important material contribution from both the overlying mantle wedge (10–50% mass contribution) and continental crust (0–30% mass contribution). Andesites from Cook Island volcano, located in the southernmost AVZ (54°S) where subduction is more oblique, have MORB-like Sr, Nd, Pb and O isotopic composition and trace-element ratios. These can be modeled by small degrees (2–4%) of partial melting of eclogitic MORB, yielding a tonalitic parent (intermediate SiO₂, CaO/Na₂O>1), followed by limited interaction of this melt with the overlying mantle (≥90% MORB melt, ≤10% mantle), but only very little (≤1%) or no participation of either subducted sediment or crust. In contrast, models for the magmatic evolution of Burney (52°S), Reclus (51°S) and northernmost AVZ (49–50°S) andesites and dacites require melting of a mixture of MORB and subducted sediment, followed by interaction of this melt not only with the overlying mantle, but the crust as well. Crustal assimilation and fractional crystallization (AFC) processes and the mass contribution from the crust become more significant northwards in the AVZ as the angle of convergence becomes more orthogonal. Received: 1 March 1995 / Accepted: 13 September 1995     

Pre-eruptive Conditions of the Huerto Andesite (Fish Canyon System, San Juan Volcanic Field, Colorado): Influence of Volatiles (C-O-H-S) on Phase Equilibria and Mineral Composition

Crystallization experiments at 400 MPa, oxidized condition (logfO₂= NNO + 1, where NNO is nickel–nickel oxide buffer) andover a range of temperatures (850–950°C) and fluidcomposition (XH₂O_in = 0·3–1) have been carriedout to constrain the storage conditions of the sulphur-richmagma of the Huerto Andesite (an anhydrite, pyrrhotite, andS-rich apatite-bearing, post-Fish Canyon Tuff mafic lava). Theresults are used to evaluate the role of fluids released fromthe crystallization of magmas such as the Huerto Andesite onthe remobilization of the largely crystallized dacitic FishCanyon magma body. Experiments were performed using the naturalandesitic bulk composition with and without added sulphur. Thepresence of sulphur slightly affects the phase equilibria bychanging the phase proportions, stability fields of plagioclase,pyroxenes and ilmenite, and also affects the plagioclase composition.Phase equilibria and mineral composition data indicate thatthe magma may have contained 4·5 wt % water in the meltand that the pre-eruptive temperature was 875 ± 25°C.Assuming that the magma was in equilibrium with a fluid phase,the CO₂ concentration of the melt is estimated to be in therange 2000–4000 ppm (at 400 MPa). Before eruption, theandesite had an oxidation state very close to, or slightly within,the co-stability field of anhydrite–pyrrhotite at NNO+ 1·1. At these conditions, the sulphur content in themelt is 500 ppm. Assuming open-system degassing resulting fromcontinuing crystallization at depth, most of the CO₂ dissolvedin the andesitic melt should be released after the crystallizationof <10 vol. % of the magma, corresponding to a cooling from875 to 825–850°C. Thus, the fluids released owingto crystallization processes should be mainly composed of waterat temperatures below 825°C. KEY WORDS: experimental study; andesite; volatile; Fish Canyon Tuff; Huerto Andesite     

Hornblende- and Phlogopite-Bearing Gabbroic Xenoliths from Volcan San Pedro (36{degrees}S), Chilean Andes: Evidence for Melt and Fluid Migration and Reactions in Subduction-Related Plutons

Two groups of gabbroic xenoliths (I and II) containing largeproportions of late-crystallized hornblende (up to 50 vol. %)and Na-rich phlogopite (up to 15 vol. %), were brought to thesurface by a late Holocene eruption of Volcán San Pedro,the youngest edifice of the Tatara–San Pedro VolcanicComplex (36°S, Chilean Andes). Group I are inferred to befragments of partially solidified Holocene plutons because theycontain residual interstitial glass, whereas exsolution anddeformation textures in Group II indicate that they are fragmentsof pre-Quaternary plutonic basement. On the basis of texturalrelations plus the mineral and whole-rock compositions of bothgroups of xenoliths, we suggest that hornblende and phlogopitewith high mg-numbers and Cr contents have formed by reactionsbetween refractory cumulus minerals (olivine, Cr-spinel, pyroxenesor plagioclase) and evolved melts ± aqueous fluids thatmigrated through partly solidified crystalline frameworks. Thus,the hydrous minerals are not early-crystallized phases in thebasaltic magmas from which the cumulus minerals precipitated.The high proportions of hornblende in many subduction-relatedgabbroic plutons and xenolith suites compared with its paucityin basaltic or basaltic andesitic lavas may be partially explainedby multistage plutonic crystallization histories involving reactionand migration of evolved melt ± aqueous fluids that eithercould have originated within the cumulus pile of the mafic intrusionor were derived externally, from broadly contemporaneous felsicmagmas. KEY WORDS: Tatara–San Pedro; gabbroic xenolith; hornblende; phlogopite; melt migration     

The Liquid Line of Descent of Anhydrous, Mantle-Derived, Tholeiitic Liquids by Fractional and Equilibrium Crystallization--an Experimental Study at 1{middle dot}0 GPa

Two series of anhydrous experiments have been performed in anend-loaded piston cylinder apparatus on a primitive, mantle-derivedtholeiitic basalt at 1·0 GPa pressure and temperaturesin the range 1060–1330°C. The experimental data provideconstraints on phase equilibria, and solid and liquid compositionsalong the liquid line of descent of primary basaltic magmasdifferentiating in storage reservoirs located at the base ofthe continental crust. The first series are equilibrium crystallizationexperiments on a single basaltic bulk composition; the secondseries are fractionation experiments where near-perfect fractionalcrystallization was approached in a stepwise manner using 30°Ctemperature steps and starting compositions corresponding tothe liquid composition of the previous, higher-temperature glasscomposition. Liquids in the fractional crystallization experimentsevolve with progressive SiO₂ increase from basalts to dacites,whereas the liquids in the equilibrium crystallization experimentsremain basaltic and display only a moderate SiO₂ increase accompaniedby more pronounced Al₂O₃ enrichment. The principal phase equilibriacontrols responsible for these contrasting trends are suppressionof the peritectic olivine + liquid = opx reaction and earlierplagioclase saturation in the fractionation experiments comparedwith the equilibrium experiments. Both crystallization processeslead to the formation of large volumes of ultramafic cumulatesrelated to the suppression of plagioclase crystallization relativeto pyroxenes at high pressures. This is in contrast to low-pressurefractionation of tholeiitic liquids, where early plagioclasesaturation leads to the production of troctolites followed by(olivine-) gabbros at an early stage of differentiation. KEY WORDS: liquid line of descent; tholeiitic magmas; equilibrium crystallization; fractional crystallization     

Dolomite-bearing orogenic garnet peridotites witness fluid-mediated carbon recycling in a mantle wedge (Ulten Zone,Eastern Alps,Italy)

We document the presence of dolomite ± apatite in orogenic peridotites from the Ulten Zone (UZ, Italian Alps), the remnants of a Variscan mantle wedge tectonically coupled with eclogitized continental crust. These dolomite peridotites are associated with dominant carbonate-free amphibole peridotites, which formed in response to infiltration of aqueous subduction fluids lost by the associated crustal rocks during high-pressure (HP) metamorphism and retrogression. Dolomite-free and dolomite-bearing peridotites share the same metamorphic evolution, from garnet- (HP) to spinel-facies (low-pressure, LP) conditions. Dolomite and the texturally coexisting phases display equilibrium redistribution of rare earth elements and of incompatible trace elements during HP and LP metamorphism; clinopyroxene and amphiboles from carbonate-free and carbonate-bearing peridotites have quite similar compositions. These features indicate that the UZ mantle rocks equilibrated with the same metasomatic agents: aqueous CO₂-bearing fluids enriched in incompatible elements released by the crust. The P–T crystallization conditions of the dolomite peridotites (outside the field of carbonatite melt + amphibole peridotite coexistence), a lack of textures indicating quench of carbonic melts, a lack of increase in modal clinopyroxene by reaction with such melts and the observed amphibole increase at the expense of clinopyroxene, all suggest that dolomite formation was assisted by aqueous CO₂-bearing fluids. A comparison of the trace element compositions of carbonates and amphiboles from the UZ peridotites and from peridotites metasomatized by carbonatite and/or carbon-bearing silicate melts does not help to unambiguously discriminate between the different agents (fluids or melts). The few observed differences (lower trace element contents in the fluid-related dolomite) may ultimately depend on the solute content of the metasomatic agent (CO₂-bearing fluid versus carbonatite melt). This study provides strong evidence that C–O–H subduction fluids can produce ‘carbonatite-like’ assemblages in mantle rocks, thus being effective C carriers from the slab to the mantle wedge at relatively low P–T. If transported beyond the carbonate and amphibole solidus by further subduction, dolomite-bearing garnet + amphibole peridotites like the ones from Ulten can become sources of carbonatite and/or C-bearing silicate melts in the mantle wedge. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. In memory of Lauro Morten 1941–2006.     

Contrasting Fractionation Trends in Coexisting Continental Alkaline Magma Series; Cantal, Massif Central, France

Two coexisting series of strongly alkaline (basanite-tephritephonolite)and weakly alkaline (alkali basalt-trachyandesite-trachyte-rhyolite)lavas occur in the Cantal volcano (French Massif Central). Theparental magmas appear to be derived by variable degrees ofpartial melting of a common asthenospheric mantle source. Derivativetrachyandesites and feldspathoidbearing tephrites show depletionsand enrichments in trace elements which indicate that they havebeen generated by broadly similar fractionation processes, relatedto the removal of a mineral extract, from the parental alkalibasalts and basanites respectively, dominated by olivine, clinopyroxene,amphibole, apatite and titaniferous magnetite±plagioclase.In the most extreme differentiates (trachytes, rhyolites andphonolites) fractionation of zircon, sphene and alkali feldsparexerts a major control on the trace element characteristicsof the magmas. Sr-Nd-Pb isotopic data for the two magma series suggest theimportance of combined assimilation-fractional crystallizationprocesses (AFC) within the lower crust in their evolution. Modellingdemonstrates that the AFC process amplifies the original compositionaldifferences between the parent magmas. After 55% crystallizationin the strongly alkaline series and 65% in the weakly alkalineseries crustal contamination ceases, although fractional crystallizationcontinues beyond this point to produce the most evolved differentiates(phonolites and rhyolites). This may reflect progressive sealingof the lowercrustal magma reservoirs. The tendency of the magmasto follow over- or under-saturated evolutionary trends, producingrhyolitic and phonolitic residua respectively, appears to beestablished at the early stages of magmatic differentiation,reflecting inherent differences in the compositions of the parentalmagmas. KEY WORDS: alkaline magmas; Massif Central; Cantal; AFC; magmatic differentiation     

Geochemistry, Petrogenesis and Geodynamic Relationships of Miocene Calc-alkaline Volcanic Rocks in the Western Carpathian Arc, Eastern Central Europe

We report major and trace element abundances and Sr, Nd andPb isotopic data for Miocene (16·5–11 Ma) calc-alkalinevolcanic rocks from the western segment of the Carpathian arc.This volcanic suite consists mostly of andesites and dacites;basalts and basaltic andesites as well as rhyolites are rareand occur only at a late stage. Amphibole fractionation bothat high and low pressure played a significant role in magmaticdifferentiation, accompanied by high-pressure garnet fractionationduring the early stages. Sr–Nd–Pb isotopic dataindicate a major role for crustal materials in the petrogenesisof the magmas. The parental mafic magmas could have been generatedfrom an enriched mid-ocean ridge basalt (E-MORB)-type mantlesource, previously metasomatized by fluids derived from subductedsediment. Initially, the mafic magmas ponded beneath the thickcontinental crust and initiated melting in the lower crust.Mixing of mafic magmas with silicic melts from metasedimentarylower crust resulted in relatively Al-rich hybrid dacitic magmas,from which almandine could crystallize at high pressure. Theamount of crustal involvement in the petrogenesis of the magmasdecreased with time as the continental crust thinned. A strikingchange of mantle source occurred at about 13 Ma. The basalticmagmas generated during the later stages of the calc-alkalinemagmatism were derived from a more enriched mantle source, akinto FOZO. An upwelling mantle plume is unlikely to be presentin this area; therefore this mantle component probably residesin the heterogeneous upper mantle. Following the calc-alkalinemagmatism, alkaline mafic magmas erupted that were also generatedfrom an enriched asthenospheric source. We propose that bothtypes of magmatism were related in some way to lithosphericextension of the Pannonian Basin and that subduction playedonly an indirect role in generation of the calc-alkaline magmatism.The calc-alkaline magmas were formed during the peak phase ofextension by melting of metasomatized, enriched lithosphericmantle and were contaminated by various crustal materials, whereasthe alkaline mafic magmas were generated during the post-extensionalstage by low-degree melting of the shallow asthenosphere. Thewestern Carpathian volcanic areas provide an example of long-lastingmagmatism in which magma compositions changed continuously inresponse to changing geodynamic setting. KEY WORDS: Carpathian–Pannonian region; calc-alkaline magmatism; Sr, Nd and Pb isotopes; subduction; lithospheric extension     

The influence of amphibole fractionation on the evolution of calc-alkaline andesite and dacite tephra from the central Aleutians,Alaska

Petrologic studies of tephra from Kanaga, Adak, and Great Sitkin Islands indicate that amphibole fractionation and magma mixing are important processes controlling the composition of calc-alkaline andesite and dacite magmas in the central Aleutians. Amphibole is ubiquitous in tephra from Kanaga and Adak Islands, whereas it is present only in a basaltic-andesite pumice from Great Sitkin. Dacitic tephra from Great Sitkin do not contain amphibole. Hornblende dacite tephra contain HB+PLAG+OX±OPX±CPX phenocrysts with simple zoning patterns, suggesting that the dacites evolved in isolated magma chambers. Andesitic tephra from Adak contain two pyroxene and hornbelende populations, and reversely zoned plagioclase, indicating a more complex history involving mixing and fractional crystallization. Mass balance calculations suggest that the andesitic tephra may represent the complements of amphibole-bearing cumulate xenoliths, both formed during the evolution of high-Al basalts. The presence of amphibole in andesitic and dacitic tephra implies that Aleutian cale-alkaline magmas evolve in the mid to lower crust under hydrous (>4 wt.% H₂O) and oxidizing (Ni–NiO) conditions. Amphibole-bearing andesites and pyroxene-bearing dacites from Great Sitkin indicates fractionation at several levels within the arc crust. Despite its absence in many calc-alkaline andesite and dacite lavas, open system behavior involving amphibole fractionation can explain the trace element characteristies of lavas found on Adak Island. Neither open nor closed system fractionation involving a pyroxene-bearing assemblage is capable of explaining the trace element concentrations or ratios found in the Adak suite. We envision a scenario where amphibole was initially a liquidus phase in many calc-alkaline magmas, but was later replaced by pyroxenes as the magmas rose to shallow levels within the crust. The mineral assemblage in these evolved lavas reflects shallow level equilibration of the magma, whereas the trace element chemistry provides evidence for a earlier, amphibole-bearing, mineral assemblage.