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The Azufre volcano (21°47'S; 68°14'W) is emplaced above the western boundary of the Altiplano Puna Magma Body. The youngest lava of the Azufre volcano (50-331 ka) was selected for detailed studies because of the relevant petrological information that could emerge to understand its reservoir as a potential heat source of the neighboring Cerro Pabellón geothermal system. The studied lava corresponds to an andesite-dacite (61-63 SiO.sub.2 wt%) with phenocrysts of plagioclase, amphibole (Group 1), biotite, pyroxenes, quartz, and olivine. The lava also contains aphanitic enclaves (58-60 SiO.sub.2 wt%), whose groundmass have the same mineralogy of lava samples groundmass consisting of amphibole microphenocryts (Group 2) and microlites of plagioclase, pyroxenes, and Fe-Ti oxides. Disequilibrium textures are commonly observed in the studied samples such as partially resorbed plagioclase phenocrysts, amphibole breakdown, and reverse zoning in pyroxene and plagioclase phenocrysts. Thermobarometry calculations indicated pressures of ~ 2 kbar for Group 1 amphiboles, temperatures of 905-1097 °C for Mg-rich pyroxene phenocrysts and Ca-rich plagioclase (An [greater than or equal to] 66), and near-solidus temperatures of 712-788 °C for Group 1 amphibole-plagioclase pairs. Group 1 amphiboles also indicate crystallization from an evolved liquid (63-79 wt% of SiO.sub.2). Oxidation conditions of QFM + 0.9-2.5 log units were recorded in amphibole and Fe-Ti oxides. Rhyolite-MELTS models reproduce the composition of the high-temperature phases from a melt composition (andesitic enclave) at similar P-T-fO.sub.2. The arrival of new hot andesite magma into a crystal-rich shallow reservoir, thermally and compositionally zoned, would have triggered the studied eruption. Diffusion models in Fe-Ti oxides microlites indicated cooling temperatures of 742-866 °C during the sub-aerial emplacement of the lava., Author(s): Darío Salvador Hübner [sup.1] [sup.2], Miguel-Ángel Parada [sup.1] [sup.2], Eduardo Morgado [sup.3], Francisca Mallea-Lillo [sup.1] [sup.2] Author Affiliations: (1) grid.443909.3, 0000 0004 0385 4466, Department of Geology, Faculty of [...]

The Karoo large igneous province has been divided into rift zone and basin-related groups, with picrites from the Luenha river, Mozambique, representing an end-member of the latter. New O isotope, major and trace element data for olivine have been combined with MELTS crystallisation modelling to deconvolve compositional diversity associated with magma differentiation from source-derived heterogeneity. Three olivines populations have been discerned as follows: (1) the 'main trend', which records crystallisation from a variety of magma compositions; (2) the 'low Cu trend', which is inferred to represent xenocrysts or antecrysts; and (3) the 'high CaO' olivines, which record polybaric crystallisation of a primitive, little fractionated magma. The trace element variability in olivine phenocrysts relates partially to sampling of different parts of the same overall magma transport and storage systems, and partly to heterogeneity of parental magmas and their mantle sources. When the measured [delta].sup.18O.sub.olivine values have been converted into [delta].sup.18O.sub.melt values, the mean [delta].sup.18O.sub.melt values for the 'main trend' and 'low Cu' groups are indistinguishable from each other (5.7 ± 0.1â°, 2[sigma]); however, the mean [delta].sup.18O.sub.melt value of 6.1 ± 0.1â° for the 'high CaO' group is distinctly enriched. These data record source heterogeneity and suggest contributions from two mantle sources, one with elevated [delta].sup.18O, and another with more 'typical' mantle [delta].sup.18O. Combining these data with previously reported trace element and Nd and Sr isotope data support derivation from a mantle source similar to non-chondritic bulk silicate earth, but with minor contributions (1-2% for the enriched magmas) from a recycled sedimentary component. This points to the importance of a primitive mantle source for the basin-related successions in Karoo., Author(s): Kieran A. Iles [sup.1] [sup.2], Arto V. Luttinen [sup.2], Christoph Beier [sup.1], Jussi S. Heinonen [sup.1], Martin J. Whitehouse [sup.3], Sanni T. Turunen [sup.2] Author Affiliations: (1) grid.7737.4, 0000 [...]

The Earn Group of central Yukon records the transition from a passive to an active margin along western Laurentia in the Late Devonian. Fine-grained clastic rocks and chert of the lower Earn Group contain late Early to Middle Devonian fossils and were deposited in an offshelf environment. The upper Earn Group comprises a mixture of sandstone and conglomerate, fine-grained siliciclastic rocks, and widespread crystal lithic tuff. Zircons from this succession are precisely dated using chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) methods on igneous (ca. 363 Ma) and detrital (ca. 378-363 Ma) grains and the ages were confirmed by Frasnian to Famennian fossils. Abrupt, along-strike facies changes within the upper Earn Group of the Glenlyon-Tay River area occur across mapped faults that are inferred to have originated as syndepositional extension faults in the Late Devonian. Occurrences of ca. 363 Ma tuff horizons within all facies of the upper Earn Group provide a temporal correlation across the area. Diorite plutons intrude lower Paleozoic rocks in the area and have U-Pb zircon crystallization dates of ca. 364 Ma. The diorite has calc-alkaline composition consistent with arc magmatism or crustal contamination. The Late Devonian magmatism in the Earn Group is coincident with onset of arc magmatism in the allochthonous Yukon-Tanana terrane, and extension related to rifting and opening of the Slide Mountain ocean in a back-arc setting. Magmatic rocks in the Earn Group of central Yukon thus represent part of a remnant continental arc and back-arc stranded behind the Slide Mountain ocean in the Mississippian. Key words: Late Devonian, magmatism, Earn Group, extension, western Laurentia, biostratigraphy. Le Groupe d'Earn dans le centre du Yukon preserve des preuves d'une transition d'une marge passive a une marge active le long de la Laurentie occidentale au Devonien tardif. Des roches clastiques a grains fins et deschertsdu Groupe d'Earn inferieur renferment des fossiles d'age devonien precoce tardif a devonien moyen et ont ete deposes dans un milieu oceanique. Le Groupe d'Earn superieur comprend un melange de gres et de conglomerats, de roches silicoclastiques a grains fins et de tuf cristallolithique ubiquiste. Des zircons provenant de cette succession sont dates de maniere precise par des methodes de spectrometrie de masse d'ionisation thermique d'ionisation d'isotope d'abrasion chimique (CA-ID-TIMS) appliquees a des grains ignes (~363 Ma) et detritiques (~378-363 Ma), ces ages etant confirmes par des fossiles frasniens et famenniens. Des changements de facies abrupts paralleles a la direction des strates dans le Groupe d'Earn superieur de la region de Glenlyon-riviere Tay sont observes de part et d'autre de failles cartographiees interpretees comme etant, a leur origine au Devonien precoce, des failles d'extension synsedimentaires. La presence d'horizons de tuf de ~363 Ma dans tous lesfaciesduGrouped'Earn superieur fournit une correlation temporelle sur l'ensemble de la region. Des plutons de diorite recoupent des roches du Paleozo'ique inferieur dans la region et donnent des ages U-Pb de cristallisation des zircons de ~364 Ma. La diorite presente une composition calco-alcaline que pourraient expliquer un magmatisme d'arc ou une contamination crustale. Le magmatisme d'age devonien tardif dans le Groupe d'Earn coincide avec le debut du magmatisme d'arc dans le terrane allochtone de Yukon-Tanana et avec l'extension associee au rifting et a l'ouverture de l'ocean de Slide Mountain dans un milieu d'arriere-arc. Les roches magmatiques dans le Groupe d'Earn du centre du Yukon representent donc une partie des vestiges d'un arc continental et d'un arriere-arc echoues derriere l'ocean de Slide Mountain au Mississipien. [Traduit par la Redaction] Mots-cles: Devonien tardif, magmatisme, Groupe d'Earn, extension, Laurentie occidentale, biostratigraphie., Introduction The western edge of North America (or Laurentia in the Paleozoic) was a passive continental margin from early Cambrian to Middle Devonian (Bond et al. 1983; Bond and Kominz [...]

We explore crystal growth and magma recharge during the formation of intermediate lavas using bulk rock compositions and zoning patterns and textural variation in plagioclase feldspars from Hasandag volcano in south-central Turkey. Hasandag intermediate lavas formed primarily through fractionation of the observed mineral phases, and also show abundant evidence for magma mixing and thermochemical disequilibrium. Sparse basaltic andesites originated through mixing of observed mafic and felsic lavas rather than fractionation. Plagioclase phenocrysts from all lava types have uniform core (An.sub.~39) and variable rim (An.sub.39-64) compositions, suggesting plagioclase feldspars from a common magma reservoir are present in all erupted lavas. An-rich crystal rims are regularly enriched in Mg, Fe and Ti, indicating mafic recharge and magma mixing. Mixing timescales determined by diffusion modeling of Mg profiles across core-rim boundaries suggest recharge to eruption occurs on the order of up to ~ 45 days. Thermobarometric calculations constrain shallow crustal storage of dacite to 1.5-2 kbar (~ 5 km) and 800-890 °C, with deeper reservoirs for more mafic magmas (up to 35-40 km for basalts). Abundant disequilibrium textures (sieve-textured zones, oscillatory zoning, resorbed and patchy-zoned cores) indicate mobilization of a homogeneous crystal-rich dacitic reservoir by injection of mafic magma. We suggest that incomplete and dynamic physical mixing at shallow crustal levels results in distinct crystal morphologies, some of which record punctuated ascent and storage, while others are erupted rapidly after the influx of new magma., Author(s): Helen Gall [sup.1] [sup.3], Biltan Kürkçüoglu [sup.2], Jacob Cipar [sup.1], Katherine Crispin [sup.1] [sup.4], Tanya Furman [sup.1] Author Affiliations: (1) grid.29857.31, 0000 0001 2097 4281, Department of Geosciences, Pennsylvania [...]

The Margaree pluton extends for >40 km along the axis of the Ganderian Aspy terrane of northern Cape Breton Island, Nova Scotia. The pluton consists mainly of coarse-grained megacrystic syenogranite, intruded by small bodies of medium-grained equigranular syenogranite and microgranite porphyry, all locally displaying rapakivi texture. The three rock types have similar U-Pb (zircon) ages of 363 [+ or -] 1.6, 364.8 [+ or -] 1.6, and 365.5 [+ or -] 3.3 Ma, respectively, consistent with field and penological evidence that they are coeval and comagmatic. The rare earth elements display parallel trends characterized by enrichment in the light rare earth elements, flat heavy rare earth elements, moderate negative Eu anomalies, and, in some cases, positive Ce anomalies. The megacrystic and rapakivi textures are attributed to thermal perturbation in the magma chamber caused by the mixing of mafic and felsic magma, even though direct evidence of the mafic magma is mainly lacking at the current level of exposure. Magma evolution was controlled by fractionation of quartz, K-feldspar, and Na-rich plagioclase in molar proportions of 0.75:0.12:0.13. The chemical and isotopic (Sm-Nd) signature of the Margaree pluton is consistent with the melting of preexisting continental crust that was enriched in heat-producing elements, likely assisted by intrusion of mantle-derived mafic magma during Late Devonian regional extension. The proposed model involving magma mixing at shallow crustal levels in a cryptic silicic-mafic magma chamber during post-Acadian extension is consistent with models for other, better exposed occurrences of rapakivi granite in the northern Appalachian orogen. Key words: rapakivi, megacrysts, U-Pb zircon, Devonian, crystal fractionation, PER diagrams. Le pluton de Margaree s'etale sur plus de 40 km le long de l'axe du terrane ganderien d'Aspy du nord de l'île du Cap-Breton (Nouvelle-Ecosse). Le pluton est principalement constitue de syenogranite megacristique à grains grossiers, recoupe par de petits corps de syenogranite equigranulaire à grains moyens et de porphyre microgranitique, qui presentent tous une texture rapakivi. Les trois types de roche donnent des âges U-Pb sur zircons semblables de 363 [+ or -] 1,6, 364,8 [+ or -]1,6 et 365,5 [+ or -] 3,3 Ma, respectivement, ce qui concorde avec des observations petrologiques et de terrain indiquant qu'ils sont contemporains et comagmatiques. Les terres rares presentent des spectres paralleles caracterises par un enrichissement des terres rares legeres, des tronçons des terres rares lourdes plats, des anomalies negatives moderees d'Eu et, dans certains cas, des anomalies de Ce positives. Les textures megacristique et rapakivi sont attribuees à une perturbation thermique dans la chambre magmatique causee par le melange de magmas mafique et felsique, bien que des preuves directes de la presence d'un magma mafique soient generalement absentes au niveau d'exposition actuel. L'evolution du magma etait controlee par la cristallisation fractionnee du quartz, du feldspath potassique et du plagioclase riche en Na dans des proportions molaires de 0,75:0,12:0,13. Les signatures chimique et isotopique (Sm-Nd) du pluton de Margaree concordent avec la fusion de croûte continentale preexistante qui etait enrichie en elements thermogenes, vraisemblablement aidee par l'intrusion de magma mafique derive du manteau durant l'extension regionale au Devonien tardif. Le modele propose qui comprend le melange de magmas à des niveaux crustaux peu profonds dans une chambre magmatique siliceuse-mafique cryptique durant l'extension post-acadienne concorde avec les modeles proposes pour d'autres cas mieux exposes de granite rapakivi dans le nord de l'orogene des Appalaches. [Traduit par la Redaction] Mots-cles: rapakivi, megacristaux, U-Pb sur zircons, Devonien, cristallisation fractionnee, diagramme de Pearce., Introduction Rapakivi granite sensu stricto is characterized by the presence of plagioclase-mantled K-feldspar, independent of K-feldspar morphology or origin (Ramo and Haapala 1995). The debate over the formation of rapakivi [...]

The Machangqing Cu-Mo (Au) deposit is located in the central part of the Jinshajiang - Red River belt in the Sanjiang orogen, which lies across the Qiangtang terrane and western Yangtze craton, southwestern China. Zircon U-Pb dating constrains that the granite porphyry and porphyritic granite emplacements occurred at 35.92 [+ or -] 0.31 Ma and 34.92 [+ or -] 0.31 Ma, respectively. The Re-Os model ages of molybdenite are 34.94 [+ or -] 0.38 Ma. The new ages presented here, along with previously published data in the region, define a short duration of potassic magmatism and mineralization from 37 Ma to 34 Ma in the Jinshajiang - Red River belt. Zircon [Ce.sup.4+]/[Ce.sup.3+] values of the porphyritic granite and granite porphyry vary from 50.32 to 1579.20 (averaging 481.01) and 33.18 to 1511.80 (averaging 452.98), respectively, and the log(f[o.sub.2]) values vary from -6.66 to -23.86 and -9.88 to -25.18, respectively, which plot within the range of the fayalite-magnetite-quartz buffer curve to the magnetite-hematite buffer curve, indicating an oxidized magma source, which may have facilitated the Cu-Au enrichment. Zircons from granitoids show [epsilon]Hf(t) values ranging from -0.75 to +2.33 and crustal model ages between 0.9 and 1.1 Ga. The features of Lu-Hf isotopes and wide range of Mg#, Cr, and Ni contents imply that the magmas of the Machangqing granitoids were probably derived from partial melting of juvenile lower crust and mixed with some mantle melts. Combined with the features of the Machangqing granitoids, the following evolution process are concluded. During the Cenozoic, the India-Asia continental collision triggered upwelling of hot asthenosphere and underplating of the thickened juvenile lower crust, which caused the formation of mafic and felsic magmas. Those magmas ascended, mixed, crystallized, and formed Machangqing ore-bearing granitoids in an intracontinental extension setting. Key words: LA-ICP-MS zircon U-Pb dating, molybdenite Re-Os dating, oxidation magma, Machangqing Cu-Mo (Au) deposit, Jinshajiang - Red River belt, eastern Tibetan plateau. Le gisement de Cu-Mo (Au) de Machangqing est situe dans la partie centrale de la ceinture de failles de Jinshajiang - riviere Rouge de l'orogene de Sanjiang, qui enjambe le terrane de Qiangtang et le craton du Yangtze (sud-ouest de la Chine). La mineralisation à Cu-Mo (Au) est genetiquement associee à des stocks de granitoïdes comprenant un granite porphyrique et un porphyre granitique. La datation U-Pb sur zircons par LA-ICP-MS permet d'etablir que le porphyre granitique et le granite porphyrique ont ete mis en place à 35,92 [+ or -] 0,31 Ma et 34,92 [+ or -] 0,31 Ma, respectivement. L'âge modele Re-Os sur la molybdenite est de 34,94 [+ or -] 0,38 Ma, ce qui indiquerait un episode de magmatisme et de mineralisation paleogene à Machangqing. Les nouveaux âges presentes ici, combines à des âges dejà publies pour la region definissent un court intervalle de magmatisme potassique et de mineralisation de 37 Ma à 34 Ma dans la ceinture de Jinshajiang - riviere Rouge. Les valeurs de [Ce.sup.4+]/[Ce.sup.3+] des zircons du granite porphyrique et du porphyre granitique vont de 50,32 à 1579,20 (moyenne de 481,01) et de 33,18 à 1511,80 (moyenne de 452,98), respectivement, et les valeurs de log(f[o.sub.2]) vont de -6,66 à -23,86 et de -9,88 à -25,18, respectivement, du champ tampon redox fayalite-magnetite-quartz au champ tampon redox magnetite-hematite, indiquant une source de magma oxydee qui pourrait avoir facilite l'enrichissement en Cu-Au. Les zircons des granitoïdes presentent des valeurs de [epsilon]Hf(t) allant de -0,75 à +2,33 et des âges modeles crustaux entre 0,9 Ga et 1,1 Ga. Les caracteristiques des isotopes de Lu-Hf et les grandes fourchettes definies par les valeurs de Mg#, Cr et Ni indiqueraient que les magmas des granitoïdes de Machangqing sont probablement derives de la fusion partielle d'une croûte inferieure juvenile, melanges avec des produits de fusion du manteau. Ces donnees, combinees à l'histoire geologique, la geochimie et les donnees isotopiques à l'echelle regionale, permettent de conclure au processus d'evolution suivant: durant le Cenozoïque, la collision continentale de l'Inde et de l'Asie a declenche la remontee d'asthenosphere chaude et son plaquage sous la croûte inferieure juvenile epaissie, ce qui a entraîne la formation de magmas mafiques et felsiques. Ces magmas ont remonte, se sont melanges, ont cristallise et ont forme les granitoïdes mineralises de Machangqing dans un milieu d'extension intracontinentale dans l'est du plateau tibetain. [Traduit par la Redaction] Mots-cles: datation U-Pb sur zircons par LA-ICP-MS, datation par Re-Os sur molybdenite, magma oxydant, gisement de Cu-Mo (Au) de Machangqing, ceinture de Jinshajiang - riviere Rouge, est du plateau tibetain., Introduction Porphyry Cu-(Mo-Au) deposits are among the most important types of economic deposits that account for a significant proportion of global Cu production (e.g., Cooke et al. 2005; Sillitoe 2010). [...]

We present systematic U-Pb age data collected by laser ablation multi-collector inductively coupled plasma mass spectrometry, precise geochemical data, and Nd isotope data for igneous rocks from the southeastern Lesser Xing'an Range (SE LXR). The results indicate that the formation ages as follows: Maojiatun alkaline granite, 207.2 [+ or -] 0.84 Ma and 204.6 [+ or -] 0.93 Ma; Diorite porphyrite, 164.5 [+ or -] 0.97 Ma; and Tieli syenogranite, 186.7 [+ or -] 1.50 Ma. The alkaline granite has high silicon, potassium, alkali, and Fe[O.sub.T] contents; it is enriched in high field strength elements, Zr, Hf, Th, Rb, and U; is depleted in Ba, Sr, Nb, Ta, P, Ti, etc.; and has high ratios of 10000Ga/Al. It shows an [A.sub.2]-type granite affinity. The Tieli alkali-feldspar granite has high total alkali contents and is enriched in high field strength elements and rare earth elements and depleted in Sr, Ba, Ti, and P, and shows varying degrees of alkalinity. Rocks from SE LXR display similar [[epsilon].sub.Nd] (t) values with corresponding to Nd model ages of 1095 to 813 Ma. The igneous rocks from the SE LXR are proposed to be derived from melting of the Neoproterozoic lower crust and potential magma mixing with ancient crystalline basement. The formation of the Maojiatun alkaline granite occurred in response to a postorogenic event following the closure of the Paleo-Asian Ocean. However, the SE LXR exhibited an extensional back-arc tectonic setting in the Early Jurassic. The Middle Jurassic diorite porphyrite could be related to the temporary stagnation of the westward subduction of the Paleo-Pacific plate. Key words: alkaline rocks, Lesser Xing'an Range, A-type granite, lithospheric delamination. Nous presentons des donnees de datation U-Pb systematique obtenues par spectrometrie de masse avec plasma a couplage inductif et collection d'ions multiples jumelee a l'ablation au laser, des donnees geochimiques precises et des donnees sur les isotopes du Nd pour des roches ignees du sud-est de la chaine du Petit Xing'an (SE LXR). Les resultats indiquent les ages de formation suivants: granite alcalin de Maojiatun, 207,2 [+ or -] 0.84 Ma et 204,6 [+ or -] 0,93 Ma; porphyre dioritique, 164,5 [+ or -] 0,97 Ma; et syenogranite de Tieli, 186,7 [+ or -] 1,50 Ma. Le granite alcalin a des concentrations elevees en silice, potassium, alcalis et Fe[O.sub.T], et il est enrichi en elements a forte liaison atomique, Zr, Hf, Th, Rb, et U, appauvri en Ba, Sr, Nb, Ta, P, Ti, et ainsi de suite, et est caracterise par des rapports 10000Ga/Al eleves. Il montre une affinite aux granites de type [A.sub.2]. Le granite a feldspaths alcalins de Tieli a de fortes concentrations totales d'alcalis et est enrichi en elements a forte liaison atomique et en terres rares et appauvri en Sr, Ba, Ti et P, et il presente des degres d'alcalinite variables. Les roches de la SE LXR presentent des valeurs de [[epsilon].sub.Nd] (t) semblables correspondant a des ages Nd modeles de 1095 Ma a 813 Ma. Il est propose que les roches ignees de la SE LXR sont derivees de la fusion de la croute inferieure neoproterozoique et du possible melange du magma avec des roches du substrat cristallin ancien. La formation du granite alcalin de Maojiatun s'est produite en reponse a un episode post-orogenique qui a suivi la fermeture de l'ocean paleo-Asiatique. Le contexte tectonique de la SE LXR au Jurassique precoce etait toutefois celui d'un bassin d'extension d'arriere-arc. Le porphyre dioritique d'age jurassique moyen pourrait etre associe a la stagnation temporaire de la subduction vers l'ouest de la plaque paleo-Pacifique. [Traduit par la Redaction] Mots-cles: roches alcalines, chaine du Petit Xing'an, granite de type A, delamination lithospherique., Introduction The southeastern Lesser Xing'an Range (SE LXR) innortheastern China belongs to the northern portion of the Central Asian Orogenic Belt (Sengor and Natal'in 1996; Jahn et al. 2000a, 2000b; [...]

Late Mesozoic granitic rocks are widely distributed in the Sulu orogenic belt, but the source, tectonic affinity, and associated geodynamic setting that produced the respective magmas remain controversial. To provide insights into these issues, we present field-based petrological, whole-rock major and trace element and Sr-Nd isotope geochemical, zircon U-Pb dating, and Lu-Hf isotope studies on two types of granite porphyry dykes that are newly recognized from the central Sulu belt. U-Pb dating of magmatic zircons from both types yields consistent ages that vary between 124 [+ or -] 2 and 118 [+ or -] 2 Ma, constraining the timing of intrusion as Early Cretaceous. The granitic rocks have high-K calc-alkaline peraluminous compositions with low Mg# values and are characterized by fractionated rare earth element patterns with strong depletion in high field strength elements. Compared with type I of the granite porphyry dykes, type II exhibits higher Si[O.sub.2] but slightly lower [Na.sub.2]O and [K.sub.2]O abundances, contains higher Rb/Sr and [.sup.87]Sr/[.sup.86]Sr ratios, and shows more pronounced negative Eu, Sr, and Ba anomalies. Both types I and II have high initial [.sup.87]Sr/[.sup.86]Sr ratios of 0.709-0.711 and negative [[epsilon].sub.Nd](t) values of-19.8 to -18.4. The magmatic zircons possess negative [[epsilon].sub.Hf](t) values of-29.1 to -20.8, with mostly Neoarchean Hf model ages of 3001-2478 Ma. These features, together with the presence of Neoproterozoic inherited zircons, indicate that the two types of granite porphyries successively crystallized from a joint granite magma that derived from partial melting of the continental crust of the Yangtze Craton. Therefore, an interrelationship between the granite porphyry dykes and massive magmatic granitoids from adjacent regions in the Sulu belt may be documented, recording evidence of a joint ancient crustal reworking and recycling in a fossilized continental subduction zone during the Early Cretaceous. Key words: collisional orogen, paleo-Pacific plate subduction, granite porphyry, U-Pb zircon geochronology, Sulu orogenic belt. Des roches granitiques d'age mesozoique tardif sont largement repandues dans la ceinture orogenique de Sulu, mais la source, l'affinite tectonique et le contexte geodynamique associe qui ont produit leurs magmas respectifs demeurent controverses. Afin de jeter un nouvel eclairage sur ces questions, nous presentons les resultats d'analyses geochimiques des elements majeurs et en traces sur roche totale et des isotopes de Sr-Nd et de Lu-Hf, et de la datation U-Pb sur zircons pour deux types de dykes de granite porphyrique nouvellement decouverts dans la partie centrale de la ceinture de Sulu. La datation U-Pb sur zircons magmatiques des deux types de granite donne des ages coherents allant de 124 [+ or -] 2 a 118 [+ or -] 2 Ma, ce qui fixe le moment de leur intrusion au Cretace precoce. Les roches granitiques ont des compositions hyperalumineuses calco-alcalines riches en K avec de faibles Mg#, et elles sont caracterisees par des spectres de terres rares fractionnes montrant un appauvrissement marque des elements a forte liaison atomique. Comparativement au premier type de dykes de granite porphyrique, le deuxieme type presente de plus fortes teneurs en Si[O.sub.2], mais des concentrations de [Na.sub.2]O et [K.sub.2]O legerement plus faibles, des rapports Rb/Sr et [.sup.87]Sr/[.sup.86]Sr plus eleves et des anomalies negatives plus prononcees de Eu, Sr et Ba. Les deux types presentent des rapports initiaux de [.sup.87]Sr/[.sup.86]Sr eleves de 0,709-0,711 et des valeurs negatives de [[epsilon].sub.Nd](t) allant de -19,8 a -18,4. Les zircons magmatiques ont des [[epsilon].sub.Hf](t) negatifs de -29,1 a -20,8, et des ages modeles Hf principalement neoarcheens de 3001 Ma - 2478 Ma. Ces caracteristiques, combinees a la presence de zircons neoproterozoiques herites, indiquent que les deux types de porphyre granitique ont cristallise successivement d'un meme magma granitique derive de la fusion partielle de la croute continentale du craton du Yangzi. Une interrelation entre les dykes de granite porphyrique et des granitoides magmatiques massifs de regions attenantes dans la ceinture de Sulu pourrait ainsi etre documentee, qui temoigne de leur remaniement crustal et de leur recyclage communs dans une zone de subduction continentale fossilisee durant le Cretace precoce. [Traduit par la Redaction] Mots-cles: orogene de collision, subduction de la plaque paleo-Pacifique, porphyre granitique, geochronologie U-Pb sur zircons, ceinture orogenique de Sulu., Introduction Migration of granitic magmas from their source regions in the deep orogenic crust to sites of accumulation and emplacement at shallower crustal levels is an important process for mass [...]

A suite of six Devonian granites and one syenite were emplaced into the upper crust of northern Yukon between 364.8 [+ or -] 2.7 and 371.2 [+ or -] 1.4 Ma. The Bear Mountain syenite and related rhyolite porphyry in adjacent Alaska intruded at 52.3 [+ or -] 0.4 and 53.5 [+ or -] 0.2 Ma, respectively. A felsic volcaniclastic unit and quartz-phyric sill are newly documented adjacent to the Mount Sedgwick granite. The volcaniclastic unit may indicate the presence of a related volcanic edifice. The presence of xenocrystic zircon grains in most of the intrusions suggests initial emplacement of magmas began 10-20 Myr before final emplacement into the upper crust. A Famennian final intrusion age coincides with Late Devonian encroachment of Ellesmerian deformation into the region. Attendant crustal flexure, or evolving foreland structures, may have facilitated upward migration of the magmas. Geochemistry of the intrusions indicates that the Devonian magmatism was largely derived from partial melting of lower and middle crust, implying widespread mafic magmatic underplating in Middle to Late Devonian time. Only Dave Lord syenite retains evidence of an original mantle geochemical signature. Mantle underplating may have played a role in localizing extension, volcanism, and rifting that led to the Late Devonian opening of the Angayucham ocean basin. The Eocene Bear Mountain pluton is inferred to be a northerly example of widespread Cenozoic within-plate magmatism in Alaska. Une serie de six granites et d'une syenite d'ages devoniens ont ete mis en place dans la croûte superieure du nord du Yukon entre 364,8 [+ ou -] 2,7 et 371,2 [+ ou -] 1,4 Ma. La syenite de Bear Mountain et le porphyre rhyolitique associe en Alaska voisin se sont mis en place a 52,3 [+ ou -] 0,4 et 53,5 [+ ou -] 0,2 Ma, respectivement. Une unite volcanoclastique et un filon-couche a phenocristaux de quartz viennent d'être documentes a côte du granite de Mount Sedgwick. L'unite volcanoclastique pourrait indiquer la presence d'un edifice volcanique relie. La presence de xenocristaux de zircon dans la plupart des intrusions laisse croire que la mise en place initiale des magmas a debute quelque 10 a 20 millions d'annees avant leur emplacement final dans la croûte superieure. Un age d'intrusion finale famennien coïncide avec l'empietement au Devonien tardif de la deformation ellesmerienne dans la region. La flexion crustale associee, ou des structures en evolution dans l'avant-pays, pourraient avoir facilite la migration vers le haut des magmas. La geochimie des intrusions indique que le magmatisme devonien est en bonne partie derive de la fusion partielle de la croûte inferieure a intermediaire, ce qui suppose un sous-placage magmatique mafique repandu au Devonien intermediaire a tardif. Seule la syenite de Dave Lord conserve des indices d'une signature geochimie mantellique initiale. Le sous-placage mantellique pourrait avoir contribue a localiser l'extension, le volcanisme et le rifting qui ont mene a l'ouverture au Devonien tardif du bassin oceanique d'Angayucham. Le pluton eocene de Bear Mountain est interprete comme etant un exemple plus septentrional du magmatisme intraplaque cenozoïque repandu en Alaska. [Traduit par la Redaction], Introduction The tectonic evolution of the Arctic region has been a topic of debate and speculation for many decades (see reviews by Lawver and Scotese 1990; Lane 1997; Lane et [...]

Magmatic-hydrothermal gold-copper deposits in post-subduction settings represent essential targets for mineral exploration, but controls on their formation remain controversial. The early Cretaceous lode Au districts that formed during lithosphere destruction of the North China Craton provide an ideal opportunity to better understand the key tectono-magmatic factors responsible for the genesis of Au-rich deposits in post-subduction settings. Here, we present a LA-ICP-MS study of silicate melt inclusions and sulfide inclusions from ore-related mafic to intermediate rocks in the central Taihangshan Au district in the interior of the North China Craton to constrain the content and evolution of magmatic ore metals ± volatiles. The results, combined with numerical modeling, suggest that the ore-related magmas contained only a few ng/g Au, which is similar to the Au content of non-mineralization-related mafic to intermediate magmas worldwide. The low Au content of the lode Au-related magmas suggest that large volumes of magmas had to accumulate in the middle to lower crust through trans-lithospheric fault systems to produce the lode Au deposits. It is further suggested that the lode Au-related magmas were alkali-rich, hydrous, oxidized and relatively rich in sulfur and chlorine (mafic melt inclusions contain 0.14â0.24 wt% S and 0.1â0.2 wt% Cl). These properties are considered critical for the generation of auriferous ore fluids. By comparing the tectono-magmatic setting of the giant Jiaodong Au province (~ 4000 t Au) with the central Taihangshan district (~ 150 t Au), we propose that the much larger total Au tonnage of the Jiaodong district results from the accumulation of a much larger volume of ore-forming magmas at deep crustal levels, induced by a stronger degree of lithosphere modification. In addition, given that the composition of lode Au-related magmas is similar to that of porphyry Cu-Au-related magmas, the lack of giant, early Cretaceous porphyry Cu-Au deposits in the North China Craton suggests that strong extensional settings favor the formation of lode Au deposits instead of porphyry Cu-Au deposits. The present study, therefore, has general implications for the genesis of Au-rich deposits in strongly extensional settings., Author(s): Jia Chang [sup.1] [sup.2], Andreas Audétat [sup.2], Jian-Wei Li [sup.1] Author Affiliations: (1) grid.503241.1, 0000 0004 1760 9015, School of Earth Resources and State Key Laboratory of Geological Processes [...]

The 1.26 Ma Tshirege Member of the Bandelier Tuff is the second of two major (~ 400 km.sup.3, dense rock equivalent) compositionally zoned rhyolitic eruptions from the Valles caldera. Here we analyze 25 samples of a minor component of compositionally and texturally variable silicic dacite pumice (~ 67 to 72% SiO.sub.2) that is widely distributed through the unit. The dacite has a phenocryst assemblage dominated by feldspar and hornblende and is presumed to be a recharge magma. Quenching of dacite against cooler rhyolite, melting of rhyolitic crystal mush, and mixing between dacite and rhyolite contributed to textural complexity. The dacite can be broken into three petrographic pumice types resulting from different degrees of dacite-rhyolite interaction. The earliest stage in the history of the dacite discernable from mineral chemistry, thermobarometry and hygrometry is mid-crustal storage at temperatures close to 900 °C and water content ~ 5 wt%. Plagioclase zoning suggests that the dacite was subject to more mafic recharge at this stage. The dacite was then injected into rhyolitic crystal mush at temperatures between 700 and 800 °C and pressures ~ 0.3 GPa. Consequences of mixing with mushy rhyolite include the following: (1) cooling and partial crystallization of dacite; (2) growth of large, dendritic feldspars with ternary compositions; (3) ingestion of and melting of feldspar and quartz from the rhyolitic mush; (4) enrichment in fluorine due to melting of biotite in the mush; (5) enrichment in light REE contents in some samples due to melting of chevkinite-rich domain(s) in the mush; (6) second boiling of quenched dacite rendering it buoyant and distributing dacite 'enclaves' through the zoned rhyolite magma column. The dacite was likely injected into the rhyolite over a protracted period and eventually triggered the Tshirege eruption., Author(s): Joseph R. Boro [sup.1], John A. Wolff [sup.1], Owen K. Neill [sup.2] Author Affiliations: (1) grid.30064.31, 0000 0001 2157 6568, School of the Environment, Washington State University, , 99163, [...]

Crustal storage and transit play a critical role in the compositional evolution of arc magmas; however, the enigmatic nature of lower crustal magma storage and early differentiation limit our understanding of the connections between the physical processes of subduction zones and the architecture of the arc crust. We present new geochemical data and applications of existing barometric and chronometric tools to interrogate the mantle source compositions, crustal storage depths, and ascent timescales of a primitive, high-Mg, clinopyroxene-bearing cinder cone, the basaltic andesite of Box Canyon, located in the vicinity of Lassen Volcanic National Park, CA, in the southern Cascades. Petrographic examination in addition to bulk and in situ geochemical analyses (XRF, LA-ICP-MS, and EPMA) of tephra and lava-derived samples reveals co-crystallization of clinopyroxene and olivine as phenocrysts and glomerocrysts with ~ Mg# 80 and > Fo.sub.85, respectively. Phase equilibria experiments of analogous Cascade Arc magma compositions estimate crustal storage of the observed phase assemblage at pressures in the lower crust > 700 MPa. Reverse zonation in olivine and clinopyroxene phenocryst interiors from core-rim analytical profiles record a lower crustal mafic mixing event. Results from one-dimensional, multi-elemental olivine and clinopyroxene diffusion models fit to these interior mixing zones provide an assessment of available trace element (Ti, La, Yb, Ce, and Nd) diffusion chronometers in clinopyroxene by considering multiple element profiles within two phases that experienced the same pre-eruptive conditions. While multi-elemental and multi-phase diffusion timescales span two to three orders of magnitude, Ni in olivine profiles provide the most-robust estimate of 19.1 ± 8.6 years from mixing-to-eruption. Our results provide new constraints on arc crustal differentiation processes, indicate rapid crustal transit timescales in agreement with a growing database of diffusion-based ascent timescales of basaltic magmas, and demonstrate significant, systematic deviation of diffusion timescales calculated from natural zonation of rare earth elements in clinopyroxene and Ti in clinopyroxene and olivine., Author(s): A. E. Hollyday [sup.1] [sup.2], S. H. Leiter [sup.1] [sup.3], K. J. Walowski [sup.1] Author Affiliations: (1) grid.260002.6, 0000 0000 9743 9925, Department of Geology, Middlebury College, , Middlebury, [...]

A geochemical study was undertaken on basaltic lava flows intercalated with Oligocene to Miocene strata in the Sabzevar region, northern Iran, to examine their petrogenesis in a regional tectonic framework. The lavas are either aphyric or have phenocrysts and micro-phenocrysts of olivine and, to a lesser extent, clinopyroxene. Geochemically, the lavas are silica-undersaturated alkali basalts characterized by relatively high Mg# (~ 57-66) and Na.sub.2O/K.sub.2O (~ 2.0-6.7). They have distinctive trace element patterns characterized by strong rare-earth element fractionation, negative Nb-Ta and Zr-Hf anomalies and a positive Sr anomaly. Significant contamination by crustal materials either in the magma source or during ascent is ruled out on the basis of trace element compositions and Sr-Nd isotopic compositions (.sup.87Sr/.sup.86Sr = 0.7037-0.7048 and .sup.143Nd/.sup.144Nd = 0.5128-0.5130), both of which differ markedly from continental crustal rocks. Phenocryst assemblage, analysis of multiple saturation points in lherzolite systems, and covariations of La/Yb with MgO of the studied lavas are generally consistent with an origin involving high-pressure fractionation of peridotite-derived melts. Primary magma compositions calculated by reversed fractionation of clinopyroxene and olivine for the relatively primitive samples (> 9 wt.% MgO), however, do not plot on the lherzolite multiple saturation points. Also, high-pressure fractionation predicts increasing trends of silica undersaturation and alkalinitiy with differentiation, and such trends are not indicated by the geochemical data. We suggest that the mixed trends shown by the data might be related to melt generation from both peridotite and silica-deficient pyroxenite sources, superimposed by variable degrees of high-pressure fractionation. The role of pyroxenite in magma genesis is indicated not only by the positive Sr anomaly shown by the trace element patterns, but also first-row transition element systematics of the studied lavas. The silica-deficient pyroxenites contributing to melt generation might have been transformed from mafic-ultramafic cumulates in subducted, lower oceanic crust, or might have formed in the lower crust or mantle lithosphere under continents during earlier magmatic episodes., Author(s): Mojtaba Rostami-Hossouri [sup.1], Habibollah Ghasemi [sup.1], Kwan-Nang Pang [sup.2], J. Gregory Shellnutt [sup.3], Mehdi Rezaei-Kahkhaei [sup.1], Laicheng Miao [sup.4], Mohsen Mobasheri [sup.1], Yoshiyuki Iizuka [sup.2], Hao-Yang Lee [sup.2], Te-Hsien [...]

Supersaturation Nucleation and Growth of Plagioclase (SNGPlag) is a numerical model that predicts the nucleation and growth of plagioclase crystals in a decompressing magma as a function of time. The model is written in Matlab, but is available as a standalone compiled program. SNGPlag uses the MELTS webservice to determine equilibrium plagioclase mode, for a user-defined magma composition, as a function of pressure and temperature. User inputs include decompression path, the presence and size distributions of antecrysts and phenocrysts, and crystal shape. At each time step, the model evaluates the difference between the calculated crystallinity and equilibrium crystallinity for a given pressure and temperature to determine the degree of supersaturation, which then sets plagioclase nucleation and growth rates. Growth rates are used to grow the existing crystals whereas nucleation adds new crystals. SNGPlag produces results that can be compared to quantitative textures in natural volcanic rocks, including total crystallinity, microlite number density, microlite crystal size distribution, the characteristic size of microlite crystals, as well as a time series of crystallinity. Model results are consistent with the established crystallization theory. As expected, microlite crystallinity increases as decompression rate slows. Decompression path greatly affects microlite textures. For the same average decompression rate, single-step paths have higher crystallinities and microlite number densities than multi-step decompressions, which are in turn more crystalline than continuous paths. Pre-existing crystals damp microlite crystallization, as these crystals provide a substrate to accommodate crystal growth and thus reduce supersaturation. The size distribution and volume fraction of these pre-existing crystals determines the magnitude of the damping. SNGPlag predicts that melt composition and temperature also exert important controls. Higher temperatures and higher silica contents both reduce microlite crystallization. In comparison with the previous studies of decompression rate based on microlite crystallization experiments, SNGPlag generally predicts minimum decompression rates that are up to three-to-four times slower. The difference is likely because those studies applied single- or multi-step decompression experiments to simulate natural magma ascent, which may be better represented by continuous decompression pathways or series of continuous decompression intervals punctuated with pauses. Previous studies also fail to account for the effects of phenocrysts or antecrysts on microlite nucleation and growth., Author(s): Benjamin J. Andrews [sup.1], Kenneth S. Befus [sup.2] Author Affiliations: (1) 0000 0000 8716 3312, grid.1214.6, Global Volcanism Program, Smithsonian Institution, , 20560, Washington, DC, USA (2) 0000 0001 [...]

Volcaniclastic sequences drilled during IODP Expedition 330 on top of Burton Guyot preserve a unique record of rejuvenated magmatic activity along the Louisville Seamount trail. Geochemical analysis of clinopyroxenes in primary volcaniclastic deposits of this rejuvenated phase allows the reconstruction of magmatic evolution from the shield to post-erosional phases of a Louisville seamount, and to compare this evolution to that of Hawaiian volcanoes. Our results reveal the occurrence of three main types of clinopyroxenes in the rejuvenated volcaniclastic deposits at Burton Guyot, with a Na (and Al)-poor phenocrystic clinopyroxene and two types of Na-rich clinopyroxenes from disaggregated ultramafic xenoliths. The rejuvenated Na-poor phenocrysts have the same compositional range as clinopyroxenes associated with the shield stage of the volcano, indicating an overlap in shield and rejuvenated magma compositions. The dominant type of Na-rich clinopyroxene (Type 1) is very similar to clinopyroxenes in Hawaiian pyroxenitic xenoliths thought to represent high pressure cumulates. Their relatively low Mg/(Mg + Fe), Cr, and Sc contents, similar trace element abundances and high Al(vi):Al(iv) to Hawaiian cumulates indicates that they too are cumulates. This contrasts with lower Al(vi):Al(iv) of the Na-poor phenocrysts that crystallized between 6-7 kbars and 1150-1200 °C. Type 2 clinopyroxenes are Mg-rich, and have major and trace element compositions very similar to clinopyroxenes in Hawaiian peridotites. These clinopyroxenes are interpreted as fragments of mantle xenoliths. They show intermediate amounts of incompatible element depletion, between more enriched Hawaiian peridotites and strongly depleted abyssal peridotites. Some grains exhibit the effects of mantle metasomatism, having spoon-shaped, chondrite-normalized REE patterns like those of Hawaiian peridotite xenoliths. The occurrence of disaggregated pyroxenitic cumulates and metasomatized mantle xenoliths in rejuvenated magmas of both Burton Guyot and Hawaiian islands suggests that the plumbing system of these volcanic systems share significant similarities. However, consistently with previous geochemical studies of the Louisville seamounts, geochemical consistency of shield and rejuvenated clinopyroxenes at Burton Guyot show that this volcano experienced similar alkaline magmatism from shield to rejuvenated stages. This is an important difference with the evolution of Hawaiian volcanoes that includes a dominantly tholeiitic shield stages and alkaline post-shield and rejuvenated stages, which suggests that the model of Hawaiian island formation may not be fully applicable to Louisville seamounts., Author(s): Michael J. Dorais [sup.1], David M. Buchs [sup.2] Author Affiliations: (1) 0000 0004 1936 9115, grid.253294.b, Department of Geological Sciences, Brigham Young University, , 84602, Provo, UT, USA (2) [...]

Rapakivi and alkali feldspar phenocrysts from the Devonian Deer Isle Granite Complex, Maine were investigated to see if magma mixing or isothermal decompression was responsible for their formation. Pb-isotope and trace-element analyses of alkali feldspar ovoids and their plagioclase inclusions were obtained by LA-ICP-MS to determine the characteristics of the initial magma; these were compared with the plagioclase mantles. Rapakivi grains show a decrease in Pb-isotopic values and an increase in Ba, La, and Ce concentrations from the ovoids and inclusions to the innermost plagioclase mantles adjacent to the ovoids. These variations, along with CL imagery of quartz phenocrysts, indicate an open system with compositional changes in the magma chamber occurring during rapakivi feldspar growth. Repeated episodes of magma mixing/replenishment by Ba and LREE enriched magmas with lower Pb-isotopic ratios (along with hybridized variations with the host magma) created localized dissolution of alkali feldspar and quartz. Alkali feldspar phenocrysts proximal to these zones of thermal perturbation were first resorbed and then mantled by plagioclase that records the more primitive signature of the more mafic melt. Pb-isotopic values and Ba, La, and Ce concentrations trend back to the initial magma composition at the mantle rims as the effects of the mixing event dissipated. For grains that were somewhat more distal to these zones of mixing, resorption of the alkali feldspar crystals did not occur, yet the fresh supply of Ba resulted in Ba-rich alkali feldspar mantles over Ba-poor alkali feldspar cores. Other alkali feldspar crystals were too far from the site of mixing; hence, they lack any evidence of open system crystallization. As crystallization of the chamber continued along solidification fronts, batches of cooler crystal-rich magmas settled en masse to the floor. Disaggregation of these batches during settling, and subsequent accumulation on the reservoir floor, brought grains with disparate crystallization histories together. Filter pressing of the cumulate pile flushed highly evolved melts/fluids out from interstitial pores to the high silica, upper portions of the chamber. The crystallization, transportation, and juxtaposition of disparate types of feldspar phenocrysts suggest that a relatively large and active magma chamber, periodically recharged by localized batches of melt, must have existed. In this case, it is clear that rapakivi mantled feldspars are the result of magma mixing; such interpretations may apply to many other occurrences as well., Author(s): William Desmond O'Brien [sup.1] , Michael J. Dorais [sup.1] , Eric H. Christiansen [sup.1] , David Gibson [sup.2] Author Affiliations: (Aff1) 0000 0004 1936 9115, grid.253294.b, Department of Geological [...]

Volcanism during the Neogene in the Central Volcanic Zone (CVZ) of the Andes produced (1) stratovolcanoes, (2) rhyodacitic to rhyolitic ignimbrites which reach volumes of generally less than 300 km.sup.3 and (3) large-volume monotonous dacitic ignimbrites of up to several thousand cubic kilometres. We present models for the origin of these magma types using O and Sr isotopes to constrain crust/mantle proportions for the large-volume ignimbrites and explore the relationship to the evolution of the Andean crust. Oxygen isotope ratios were measured on phenocrysts in order to avoid the effects of secondary alteration. Our results show a complete overlap in the Sr-O isotope compositions of lavas from stratovolcanoes and low-volume rhyolitic ignimbrites as well as older (>9 Ma) large-volume dacitic ignimbrites. This suggests that the mass balance of crustal and mantle components are largely similar. By contrast, younger ( Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1152-5) contains supplementary material, which is available to authorized users., Author(s): Heye Freymuth[sup.1] [sup.2] , Melanie Brandmeier[sup.1] [sup.3] , Gerhard Worner[sup.1] Author Affiliations: (1) Geowissenschaftliches Zentrum, Universität Gottingen, Goldschmidtstr. 1, 37077, Gottingen, Germany (2) Bristol Isotope Group, School of Earth [...]

A thermodynamic model for estimating the saturation conditions of H.sub.2O-CO.sub.2 mixed fluids in multicomponent silicate liquids is described. The model extends the capabilities of rhyolite-MELTS (Gualda et al. in J Petrol 53:875-890, 2012a (See CR52)) and augments the water saturation model in MELTS (Ghiorso and Sack in Contrib Mineral Petrol 119:197-212, 1995 (See CR47)). The model is internally consistent with the fluid-phase thermodynamic model of Duan and Zhang (Geochim Cosmochim Acta 70:2311-2324, 2006 (See CR33)). It may be used independently of rhyolite-MELTS to estimate intensive variables and fluid saturation conditions from glass inclusions trapped in phenocrysts. The model is calibrated from published experimental data on water and carbon dioxide solubility, and mixed fluid saturation in silicate liquids. The model is constructed on the assumption that water dissolves to form a hydroxyl melt species, and that carbon dioxide both a molecular species and a carbonate ion, the latter complexed with calcium. Excess enthalpy interaction terms in part compensate for these simplistic assumptions regarding speciation. The model is restricted to natural composition liquids over the pressure range 0-3 GPa. One characteristic of the model is that fluid saturation isobars at pressures greater than ~100 MPa always display a maximum in melt CO.sub.2 at nonzero H.sub.2O melt concentrations, regardless of bulk composition. This feature is universal and can be attributed to the dominance of hydroxyl speciation at low water concentrations. The model is applied to four examples. The first involves estimation of pressures from H.sub.2O-CO.sub.2-bearing glass inclusions found in quartz phenocrysts of the Bishop Tuff. The second illustrates H.sub.2O and CO.sub.2 partitioning between melt and fluid during fluid-saturated equilibrium and fractional crystallization of MORB. The third example demonstrates that the position of the quartz-feldspar cotectic surface is insensitive to melt CO.sub.2 contents, which facilitates geobarometry using phase equilibria. The final example shows the effect of H.sub.2O and CO.sub.2 on the crystallization paths of a high-silica rhyolite composition representative of the late-erupted Bishop Tuff. Software that implements the model is available at ofm-research.org, and the model is incorporated into the latest version (1.1+) of rhyolite-MELTS. Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1141-8) contains supplementary material, which is available to authorized users., Author(s): Mark S. Ghiorso[sup.1] , Guilherme A. R. Gualda[sup.2] Author Affiliations: (1) OFM-Research, 98115, Seattle, WA, USA (2) Earth and Environmental Sciences, Vanderbilt University, 37235, Nashville, TN, USA Introduction and [...]

Mt. Merapi in Central Java is one of the most active stratovolcanoes on Earth and is underlain by a multistage plumbing system. Crystal size distribution analyses (CSD) were carried out on recent Merapi basaltic-andesites and co-eruptive magmatic and plutonic inclusions to characterise the crystallisation processes that operate during storage and ascent and to obtain information on respective time scales. The basaltic-andesites exhibit log-linear, kinked-upwards CSD curves for plagioclase and clinopyroxene that can be separated into two main textural populations. Large plagioclase phenocrysts (≥ 1.6 mm) make up one population, but correspond to crystals with variable geochemical composition and reflect a period of crystal growth at deep to mid-crustal levels. This population was subsequently influenced by crystal accumulation and the onset of crustal assimilation, including the incorporation of high-Ca skarn-derived xenocrysts. Textural re-equilibration is required for these crystals to form a single population in CSD. A second episode of crystal growth at shallower levels is represented by chemically homogenous plagioclase crystals Keywords Basaltic-andesite * Crystal size distribution * Plutonic inclusions * Volcanic plumbing system * Merapi volcano, Introduction Gunung (Mount) Merapi in Central Java, Indonesia, is one of the most active stratovolcanoes on Earth (Voight et al. 2000a) and is situated in a densely populated area that [...]

Li, Be, B and [δ.sup.7]Li SIMS analyses of plagioclase phenocrysts from the 1040-1941 Niki dacite lava (Nea Kameni, Santorini, Greece) exhibit varied processes. From their anorthite contents alone, the crystals may be segregated into four main types: type-N shows the normal decline in An during crystallisation ([An.sub.62-40]); type-O has only oscillatory zoning accompanied by resorption surfaces ([An.sub.58-39]); type-C is complex with high-An cores (subtype C1: [An.sub.64-58], subtype C2: [An.sub.88-73]) and normal rims ([An.sub.55-42]). Type-A plagioclase with high An content ([An.sub.92-82]) is found within mafic enclaves. On the basis of their Li concentrations, type-O crystals may be subdivided into subtype O1 with flat Li concentration profiles and subtype O2 with decreasing Li concentration from core to rim. The concentrations of Be and B of all four types show a negative correlation with anorthite content (An), but Li concentration profiles differ amongst the different plagioclase types. Types N and O1, and the cores of type-C, are equilibrated in Li concentration. Types O2 and A, and the mantles of type-C display an initial enrichment in Li, probably from volatile influx into the melt. Consistent with the propensity towards equilibrium with the melt, these crystals display dramatic rim-ward declines in Li concentration. All analysed plagioclase crystals, except for the xenocrystic type-A, have nearly the same Li, Be and B concentrations at their rims. These coincide with the composition of plagioclase microlites in the groundmass, thereby affording estimates of plagioclase-melt partitioning for the light elements: [K.sub.Li] = 0.19-0.28, [K.sub.]Be = 0.24-0.38 and [K.sub.B] = 0.007-0.009. [δ.sup.7]Li profiles in type-O2 and type-A phenocrysts manifest an unmistakable inverse relation to Li concentration, with variations of up to ~39 ‰, revealing preferential kinetic diffusion. This may have been driven by Li loss from the melt, most likely through degassing during decompression, perhaps in the course of magma ascent to subsequent eruption. Considering the rapid diffusion of Li in plagioclase, in situ phenocryst analyses may yield useful information about processes leading up to, or even causing, eruptions. Keywords Lithium * Beryllium * Boron * [δ.sup.7]Li * Diffusion * Plagioclase * Dacite * Nea Kameni * Santorini, Introduction The last two decades have seen the proliferation of research involving the geochemistry of the light elements Li, Be and B. Initially considered most useful in tracking materials during [...]

The gas and fluid transport in magmas via permeable flow through interconnected bubble networks controls the rate of outgassing from magmas ascending in volcanic conduits and the fluid transport in the mushy boundary layer of magma reservoirs. Hence, clarifying its mechanism and rate is crucial to understanding the explosivity of volcanic eruptions and the evolution and dynamics of a magma reservoir. Recent experimental studies have determined the gas permeabilities in crystal-free rhyolite and basalt. However, no experimental study has investigated the effect of the crystal contents on the permeable gas transport in magmas. In this study, we performed decompression experiments for hydrous rhyolitic melts having crystallinities of 30 and 50 vol% to examine the effect of crystals on the bubble microstructure and gas permeability during magma vesiculation. Size-controlled (100-meshed) corundum crystals were used as an analog of the phenocrysts in silicic magmas. Microstructural analyses using X-ray CT showed that bubbles coalesce and their connectivity increases with a decrease in the final pressure after the decompression, that is, an increase in the vesicularity. As long as the vesicularities of melt part in the crystal-free basis (melt vesicularity) were similar, no clear effect of the crystallinity on the degree of bubble coalescence and connectivity was observed at melt vesicularities < 68 vol%. The corundum showed a large contact angle with aqueous fluid as well as plagioclase and alkaline feldspar; this failed to induce the efficient heterogeneous nucleation and coalescence of bubbles on its surface. The gas permeabilities of all the run products were lower than the detection limits of the present analysis (the order of [10.sup.-16] [m.sup.2]) at melt vesicularities < 68 vol%. These results show that silicic magmas containing 30 and 50 vol% phenocrysts with a large contact angle have low gas permeabilities until the vesicularity becomes large (at least > 68 vol%). This result indicates that the permeable fluid transport through a deep volcanic conduit, which has been proposed on the basis of the observations of volcanic gases and natural products, is so slow that other processes, like shear deformation or magma convection, may be needed to explain the observations. Keywords Open-system degassing * Gas permeability * Crystalline silicic magma * Volcanic eruptions * Gas bubble * Crystal mush, Introduction The transport of gas and supercritical aqueous fluid (fluid, hereafter) in magmas is an important process for understanding volcanism on the terrestrial planet. The outgassing (gas transport to the [...]

The 3.119 ± 0.010 Ma Chefe Donsa phreatomagmatic deposits on the shoulder of the Ethiopian Rift mark the northern termination of the Silti-Debre Zeyit Fault Zone, a linear zone of focused extension within the modern Ethiopian Rift. These peralkaline pumice fragments and glass shards span a wide range of glass compositions but have a restricted phenocryst assemblage dominated by unzoned sanidine. Glass shards found within the ash occupy a far more limited compositional range (75-76 wt% Si[O.sub.2]) in comparison with the pumice (64-75 wt% Si[O.sub.2]), which is rarely mingled. Thermodynamic modeling shows that liquids broadly similar to the least evolved glass composition can be achieved with 50-60% fractionation of moderately crustally contaminated basalt. Inconsistencies between modeled solutions and the observed values of CaO and [P.sub.2][O.sub.5] highlight the important role of fluorine in stabilizing fluor-apatite and the limitations of current thermodynamic models largely resulting from the scarce experimental data available for the role of fluorine in igneous phase stability. On the basis of limited feldspar heterogeneity and crystal content of pumice at Chefe Donsa, and the difficulties of extracting small volumes of Si-rich melt in classical fractional crystallization models, we suggest a two-step polybaric process: (1) basaltic magma ponds at mid-upper-crustal depths and fractionates to form a crystal/magma mush. Once this mush has reached 50-60% crystallinity, the interstitial liquid may be extracted from the rigid crystal framework. The trachytic magma extracted at this step is equivalent to the most primitive pumice analyzed at Chefe Donsa. (2) The extracted trachytic liquid will rise and continue to crystallize, generating a second mush zone from which rhyolite liquids may be extracted. Some of the compositional range observed in the Chefe Donsa deposits may result from the fresh intrusion of trachyte magma, which may also provide an eruption trigger. This model may have wider application in understanding the origin of the Daly Gap in Ethiopian magmas--intermediate liquids may not be extracted from crystal-liquid mushes due to insufficient crystallization to yield a rigid framework. The wide range of glass compositions characteristic of the proximal Chefe Donsa deposits is not recorded in temporally equivalent tephra deposits located in regional depocenters. Our results show that glass shards, which represent the material most likely transported to distal depocenters, occupy a limited compositional range at high Si[O.sub.2] values and overlap some distal tephra deposits. These results suggest that distal tephra deposits may not faithfully record the potentially wide range in magma compositions present in a magmatic system just prior to eruption and that robust distal-proximal tephra correlations must include a careful analysis of the full range of materials in the proximal deposit. Keywords East African Rift * Ethiopia * Peralkaline * Tephra * MELTS, Introduction The magmatic sequences of the East African rift system have long been recognized as important probes of the tectonic processes active within this type location of continental rifting. In [...]

The water-saturated phase relations have been determined for a primitive magnesian andesite (57 wt% Si[O.sub.2], 9 wt% MgO) from the Mt. Shasta, CA region over the pressure range 200-800 MPa, temperature range of 915-1,070°C, and oxygen fugacities varying from the nickel-nickel oxide (NNO) buffer to three log units above NNO (NNO+3). The phase diagram of a primitive basaltic andesite (52 wt% Si[O.sub.2], 10.5 wt% MgO) also from the Mt. Shasta region (Grove et al. in Contrib Miner Petrol 145:515-533; 2003) has been supplemented with additional experimental data at 500 MPa. Hydrous phase relations for these compositions allow a comparison of the dramatic effects of dissolved [H.sub.2]O on the crystallization sequence. Liquidus mineral phase stability and appearance temperatures vary sensitively in response to variation in pressure and [H.sub.2]O content, and this information is used to calibrate magmatic barometers-hygrometers for primitive arc magmas. [H.sub.2]O-saturated experiments on both compositions reveal the strong dependence of amphibole stability on the partial pressure of [H.sub.2]O. A narrow stability field is identified where olivine and amphibole are coexisting phases in the primitive andesite composition above 500 MPa and at least until 800 MPa, between 975-1,025 °C. With increasing [H.sub.2]O pressure ([MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]), the temperature difference between the liquidus and amphibole appearance decreases, causing a change in chemical composition of the first amphibole to crystallize. An empirical calibration is proposed for an amphibole first appearance barometer-hygrometer that uses Mg# of the amphibole and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] This barometer gives a minimum [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] recorded by the first appearance of amphibole in primitive arc basaltic andesite and andesite. We apply this barometer to amphibole antecrysts erupted in mixed andesite and dacite lavas from the Mt. Shasta, CA stratocone. Both high [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] pressures (500-900 MPa) and high pre-eruptive magmatic [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] contents (10-14 wt% [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]) are indicated for the primitive end members of magma mixing that are preserved in the Shasta lavas. We also use these new experimental data to explore and evaluate the empirical hornblende barometer of Larocque and Canil (2010). Keywords Amphibole * Experimental petrology * Shasta * Water in magma * Subduction zone volcanism, Introduction This study examines the [H.sub.2]O-saturated phase relations of two primitive arc magmas; a basaltic andesite and primitive magnesian andesite from the Mt. Shasta region, CA. Each of these compositions [...]

Byline: Heather Katherine Handley (1,2), Colin G. Macpherson (1), Jon P. Davidson (1) Keywords: Crustal contamination; Fractional crystallisation; Geochemistry; Magma mixing; Sunda arc Abstract: The Gede Volcanic Complex (GVC) of the Sunda island arc (West Java, Indonesia) consists of multiple volcanic centres and eruptive groups with complex magmatic histories. We present new petrological, mineralogical, whole-rock major and trace element and Sr--O isotopic data to provide constraints on the relative importance of fractional crystallisation and magma mixing in petrogenesis, as well as on the role and nature of the arc crust. Banded juvenile scoria from Young and Old Gede provide unequivocal evidence for the (late-stage) interaction of distinct magmas at Gede volcano. However, the relatively small-degree compositional zoning observed in plagioclase phenocrysts of all eruptive groups (up to ~20 mol% An) may be attributed to physical changes in magma properties (e.g. P, T, and P[H.sub.2]O) rather than changes in melt composition. Major element and trace element variations within each eruptive series are inconsistent with magmatic evolution through simple mixing processes. Instead, mixing of variably fractionated magma batches is suggested to account for the significant scatter in some element variation diagrams. No correlation is observed between textural complexity and/or mineral disequilibrium and whole-rock geochemistry. REE data and geochemical modelling indicate that fractional crystallisation involving amphibole in the mid- to lower crust, and fractionation of plagioclase, clinopyroxene, Fe--Ti oxide +- olivine +- orthopyroxene provide strong control on the geochemical evolution of GVC rocks. Two-pyroxene geothermobarometry provides pre-eruption crystallisation temperatures of 891--1,046degC and pressures of 3.4--6.5 kbar, equivalent to ~13--24 km depth beneath the volcanoes (mid- to lower crust). Low, mantle-like clinopyroxene [delt[a].sup.18]O values of GVC lavas and poor correlation of Sr isotope ratios with indices of differentiation precludes significant assimilation of isotopically distinct crust during magmatic differentiation. Therefore, we suggest that the geochemical character of the moderately thick West Javan arc crust is relatively immature compared to typical continental crust. Trace element ratios and strontium isotopes show that the magmatic source composition of the older geographical units, Gegerbentang and Older Quaternary, is distinct from the other GVC groups. Author Affiliation: (1) Department of Earth Sciences, Durham University, Durham, DH1 3LE, UK (2) GEMOC Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW, 2109, Australia Article History: Registration Date: 22/10/2009 Received Date: 17/02/2009 Accepted Date: 22/10/2009 Online Date: 21/11/2009 Article note: Communicated by T.L. Grove. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-009-0460-z) contains supplementary material, which is available to authorized users.

Byline: Fidel Costa (1,2), Laurence A. Coogan (3), Sumit Chakraborty (2) Keywords: Plagioclase; Diffusion; MORB; Mixing Abstract: We have studied the chemical zoning of plagioclase phenocrysts from the slow-spreading Mid-Atlantic Ridge and the intermediate-spreading rate Costa Rica Rift to obtain the time scales of magmatic processes beneath these ridges. The anorthite content, Mg, and Sr in plagioclase phenocrysts from the Mid-Atlantic Ridge can be interpreted as recording initial crystallisation from a primitive magma (~11 wt% MgO) in an open system. This was followed by crystal accumulation in a mush zone and later entrainment of crystals into the erupted magma. The initial magma crystallised plagioclase more anorthitic than those in equilibrium with any erupted basalt. Evidence that the crystals accumulated in a mush zone comes from both: (1) plagioclase rims that were in equilibrium with a Sr-poor melt requiring extreme differentiation and (2) different crystals found in the same thin section having different histories. Diffusion modelling shows that crystal residence times in the mush were 11 wt%). Partial equilibration in some crystals can be modelled as starting Author Affiliation: (1) Institut de Ciencies de la Terra 'Jaume Almera', CSIC, 08028, Barcelona, Spain (2) Institut fuer Geologie, Mineralogie und Geophysik, Ruhr-Universitat Bochum, 44780, Bochum, Germany (3) School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, V8W 3P6, Canada Article History: Registration Date: 29/07/2009 Received Date: 19/02/2009 Accepted Date: 29/07/2009 Online Date: 14/08/2009 Article note: Communicated by T.L. Grove. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-009-0432-3) contains supplementary material, which is available to authorized users.

Byline: Lorenzo Fedele (1), Alberto Zanetti (2), Vincenzo Morra (1), Michele Lustrino (3,4), Leone Melluso (1), Riccardo Vannucci (2,5) Keywords: Campi Flegrei; Partition coefficient; Potassic; Clinopyroxene; Elastic strain theory; LA-ICP-MS Abstract: Trace element partition coefficients between clinopyroxenes and associated glassy matrix (.sup.Cpx/L D) have been determined for 13 REE, HFSE.sup.4+,5+, U, Th, Sr, Pb, Sc and V from combined LA-ICP-MS/EMP analyses in selected trachytes and trachyphonolites from Campi Flegrei. Composition of clinopyroxene and glass is pretty homogeneous in the trachyphonolites, pointing to an overall attainment of the equilibrium conditions. In trachytes, conversely, phases show some compositional heterogeneity (due to the presence of clinopyroxene xenocrysts) that requested a more careful petrographic and geochemical inspection of the samples to assess the equilibrium clinopyroxene composition. In the trachyte clinopyroxenes, REE are compatible from Nd to Lu (.sup.Cpx/L D up to 2.9), like Y, Ti, Sc and V. The .sup.Cpx/L D for Eu is lower than those of the adjacent REE, highlighting Eu.sup.2+ contribution. High D values are also shown by U, Th, Pb, Zr, Hf, Nb and Ta relatively to basaltic and andesitic systems, whereas the D .sub.Sr is roughly similar to that found for less evolved magmas. Trachyphonolites are characterized by an overall decrease of the .sup.Cpx/L D for highly-charged cations (with the exception of V), and by a slight increase of D .sub.Sr. REE are still compatible from Nd to Lu (.sup.Cpx/L D up to 2.1), like Ti, Y, Sc and V. This variation is also predicted for REE and Y by models based on the elastic strain theory, being consistent with the slightly lower polymerization degree estimated for the trachyphonolites. However, the observed .sup.Cpx/L D .sub.(REE,Y) are matched by the modelled ones only considering very low T (a$?825degC), which are believed unlikely. This mismatch cannot be attributed to effects induced by the water-rich composition of the trachyte--trachyphonolite suite, since they would lower the observed .sup.Cpx/L D .sub.(REE,Y). Moreover, the anomalous inflections of measured .sup.Cpx/L D for HREE suggests some crystal-chemical control, such as the entrance of these elements in a site distinct from M2. It is concluded that the large .sup.Cpx/L D determined for trachytes and trachyphonolites are likely induced by hitherto unconstrained changes of the Z.sup.3+ activities related to the composition of melt and/or solid. All these considerations strongly highlight the importance of a direct characterization of trace element partitioning in natural samples from magmatic systems poorly characterized by experimental studies. Author Affiliation: (1) Dipartimento di Scienze della Terra, Universita di Napoli Federico II, Via Mezzocannone 8, 80134, Naples, Italy (2) CNR, Istituto di Geoscienze e Georisorse (IGG), Sezione di Pavia, Via Ferrata 1, 27100, Pavia, Italy (3) Dipartimento di Scienze della Terra, Universita degli Studi di Roma La Sapienza, Piazzale Aldo Moro 5, 00185, Rome, Italy (4) CNR-Istituto di Geologia Ambientale e Geoingegneria (IGAG), Piazzale Aldo Moro 5, 00185, Rome, Italy (5) Dipartimento di Scienze della Terra, Universita di Pavia, Via Ferrata 1, 27100, Pavia, Italy Article History: Registration Date: 22/01/2009 Received Date: 03/09/2008 Accepted Date: 22/01/2009 Online Date: 14/02/2009 Article note: Communicated by T. L. Grove. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-009-0386-5) contains supplementary material, which is available to authorized users.

Byline: Jose F. Molina (1), Jane H. Scarrow (1), Pilar G. Montero (1), Fernando Bea (1) Abstract: Central Iberian Variscan granite batholiths and anatectic complexes are punctuated by coeval stocks of hydrous, high-K calc-alkaline, ultrabasic to intermediate rock series. Despite their overall calc-alkaline affinity, the mafic--ultramafic members contain high-Ti amphibole oikocrysts rimmed by lower-Ti amphibole +- cummingtonite and high-Ti amphibole replacing early phlogopite. To understand the factors controlling the saturation of high-Ti amphibole in the parental magmas, clinopyroxene-melt, phlogopite-melt and amphibole-melt relationships are reviewed. This analysis reveals that for melts with intermediate compositions, the affinity of TiO.sub.2 for amphibole rises in alkalic magmas. Accordingly, mildly alkalic trachytoid to subalkaline medium- to high-K andesite and dacite compositions are estimated for interstitial high-Ti amphibole-saturated melts. Amphibole Ce/Pb ratios reveal a mantle--crust hybrid nature for interstitial melts with subalkaline trachytoid compositions. The hydrous character of the Variscan basic magmas favoured an overall magmatic evolutionary trend with a low rate of variation of [Na.sub.2]O with respect to silica during amphibole crystallization. Author Affiliation: (1) Department of Mineralogy and Petrology, Campus Fuentenueva, University of Granada, 18002, Granada, Spain Article History: Registration Date: 29/12/2008 Received Date: 12/03/2008 Accepted Date: 29/12/2008 Online Date: 08/02/2009 Article note: Communicated by M. W. Schmidt. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-008-0371-4) contains supplementary material, which is available to authorized users.

Byline: Anna Pietranik (1), Jurgen Koepke (2) Keywords: Magma mingling; Magma mixing; Subsolidus interactions; Diorite; Granodiorite; Plagioclase; Sr; Bohemian Massif Abstract: Dioritic and granodioritic rocks coexist in the GAsiniec Intrusion in SW Poland showing typical relationships in many mafic--felsic mingling zones worldwide, such as dioritic syn-putonic dykes and microgranular enclaves within granodioritic host. Plagioclase zonation from granodioritic rocks suggests late stage mixing probably with dioritic magma, whereas no magma mixing is recorded in plagioclase from dioritic rocks. The diorites seem to show effects of interaction with evolved, leucocratic melts derived from granodiorite, not with the granodioritic melt itself. We conclude that the diorites' compositions were modified after their emplacement within the granodioritic host, when the diorites were essentially solidified and injection of evolved melt from granodiorite did not involve marked modification of plagioclase composition. Compositional zoning patterns of plagioclase in diorites can be modeled by closed system fractional crystallization interrupted by resorption induced probably by decompression. Granodioritic plagioclase seems to be affected by the same resorption event. Plagioclase that crystallized in dioritic magma before the resorption does not record interaction between dioritic and granodioritic magmas, suggesting that both magmas evolved separately. Author Affiliation: (1) Institute of Geological Sciences, University of WrocAaw, pl. Borna 9, 50-205, WrocAaw, Poland (2) Institute of Mineralogy, University of Hannover, Callinstrasse 3, 30167, Hannover, Germany Article History: Registration Date: 12/12/2008 Received Date: 11/07/2007 Accepted Date: 12/12/2008 Online Date: 10/01/2009 Article note: Communicated by B. Collins. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-008-0368-z) contains supplementary material, which is available to authorized users.

Byline: Shiho Hirotani (1), Masao Ban (2), Mitsuhiro Nakagawa (3) Keywords: Calc-alkaline mixed rocks; Cr content; Assimilation and fractional crystallization; Partial remelting; Low-K basalts; Shirataka volcano Abstract: Eruptive products of the Shirataka volcano (0.9--0.7 Ma) in NE Japan are calc-alkaline andesite--dacite, and are divisible into six petrologic groups (G1--G6). Shirataka rocks possess mafic inclusions--basalt--basaltic andesite, except for G3 and G4. All rocks show mixing and mingling of the mafic and silicic end-members, with trends defined by hosts and inclusions divided into high-Cr and low-Cr types both types coexist in G1, G2, and G5. Estimated mafic end-members are high-Cr (1120--1170degC, 48--51% SiO.sub.2, olv +- cpx +- plg) and low-Cr type magmas (49--52% SiO.sub.2, cpx +- plg) except for the Sr isotopic composition. In contrast, the silicic end-members of both types have similar petrologic features (790--840degC, 64--70% SiO.sub.2, hbl +- qtz +- px + plg). High-Cr type mafic and corresponding silicic end-members have lower 87.sup.Sr/ [.sup.86]Sr ratios than the low-Cr ones in each group. The trace element model calculations suggest that the low-Cr type mafic end-member magma is produced through ca. 20% fractional crystallization (olv +- cpx +- plg) from the high-Cr type one with assimilation of granitoids (r = 0.02--0.05). The silicic magmas are producible through Author Affiliation: (1) Division of Interactive Symbiosphere Sciences, Graduate School of Science and Engineering, Yamagata University, 1-4-12, Kojirakawa-machi, Yamagata, 990-8560, Japan (2) Department of Earth and Environmental Sciences, Faculty of Science, Yamagata University, 1-4-12, Kojirakawa-machi, Yamagata, 990-8560, Japan (3) Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, N10 W8, Kita-ku, Sapporo, 060-0810, Japan Article History: Registration Date: 30/10/2008 Received Date: 25/03/2008 Accepted Date: 30/10/2008 Online Date: 20/12/2008 Article note: Communicated by T.L. Grove. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-008-0360-7) contains supplementary material, which is available to authorized users.

Byline: Madeleine C. S. Humphreys (1), Thomas Christopher (2), Vicky Hards (2,3) Keywords: Magma mixing; Mafic inclusions; Microlites; Hybridisation; Disaggregation; Soufriere Hills volcano; Montserrat Abstract: The Soufriere Hills volcano on Montserrat has for the past 12 years been erupting andesite with basaltic to basaltic--andesite inclusions. The andesite contains a wide variety of phenocryst textures and strongly zoned microlites. Analysis of minor elements in both phenocrysts and microlites allows us to put detailed constraints on their origins. Compositions of clinopyroxene, from overgrowth rims on quartz and orthopyroxene and coarse-grained breakdown rims on hornblende, are identical to those from the mafic inclusions, indicating that these rims form during interaction with mafic magma. In contrast, resorbed quartz and reversely zoned orthopyroxenes form during heating. Microlites of plagioclase and orthopyroxene are chemically distinct from the phenocrysts, being enriched in Fe and Mg, and Al and Ca respectively. However, microlites of plagioclase, orthopyroxene and clinopyroxene are indistinguishable from the compositions of these phases in the mafic inclusions. We infer that the inclusions disaggregated under conditions of high shear stress during ascent in the conduit, transferring mafic material into the andesite groundmass. The mafic component of the system is therefore greater than previously thought. The presence of mafic-derived microlites in the andesite groundmass also means that care must be taken when using this as a starting material for phase equilibrium experiments. Author Affiliation: (1) Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, UK (2) Montserrat Volcano Observatory, Flemmings, Montserrat, West Indies (3) British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK Article History: Registration Date: 20/10/2008 Received Date: 18/07/2008 Accepted Date: 20/10/2008 Online Date: 07/11/2008 Article note: Communicated by J. Blundy.

Byline: Brian R. Jicha (1), Garret L. Hart (1,2), Clark M. Johnson (1), Wes Hildreth (3), Brian L. Beard (1), Steven B. Shirey (4), John W. Valley (1) Keywords: Mount Adams; Cascade arc; Isotopes; Arc magmatism; Stratovolcano Abstract: Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P).sub.n and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. [delta] 18.sup.O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal interaction in this juvenile arc, in stark contrast to Os isotopes, which are highly sensitive to interaction with young, mafic material in the lower crust. Author Affiliation: (1) Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton Street, Madison, WI, 53706, USA (2) School of Earth and Environmental Sciences, Washington State University, 1228 Webster, Pullman, WA, 99163-2812, USA (3) US Geological Survey, 345 Middlefield Road MS910, Menlo Park, CA, 94025, USA (4) Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, N.W, Washington DC, 20015, USA Article History: Registration Date: 12/07/2008 Received Date: 14/02/2008 Accepted Date: 11/07/2008 Online Date: 06/08/2008 Article note: Communicated by T.L. Grove.

Byline: Don Francis (1), William Minarik (1) Keywords: Clinopyroxene; Trace element partitioning; Aluminum; Ankaramite; Carmacks; Phenocryst Abstract: As technical advances have dramatically increased our ability to analyze trace elements, the need for more reliable data on the compositional dependence of trace element partitioning between minerals and melt has become increasingly important. The late-Cretaceous Carmacks Group of south central Yukon comprises a succession of primitive high-Mg ankaramitic lavas characterized by shoshonitic chemical affinities and containing large complexly zoned clinopyroxene phenocrysts. The compositional zonation of the clinopyroxene phenocrysts is characterized by relatively Fe-rich (Mg.sup. = Mg/(Mg + Fe) = 0.85), but mottled, cores surrounded by mantles of cyclically-zoned clinopyroxene whose Mg.sup. varies repeatedly between 0.9 and 0.80. These cyclically zoned clinopyroxene mantles appear to record the repeated influx and mixing of batches of primitive with more evolved magma in a deep sub-crustal ([proportional to]1.2 GPa) magma chamber(s). Laser ablation ICP-MS was used to analyze the trace element variation in these zoned clinopyroxenes. The results indicate more than a threefold variation in the absolute concentrations of Th, Zr, rare earth elements (REE), and Y within individual clinopyroxene phenocrysts, with no apparent change in the degree of REE or high field strength element (HFSE) fractionation. The variation in absolute abundances of trace elements correlates closely with the major element composition of the clinopyroxene, with the most enriched clinopyroxene having the lowest Mg.sup. and highest Al contents. The problem is that the amount of crystal fractionation required to explain the major element variation ([proportional to]20%) in these clinopyroxene phenocrysts cannot explain the increase in the abundance of the incompatible trace elements, which would require more than 70% crystal fractionation, if constant partition coefficients are assumed. The anomalous increase in incompatible trace elements appears to reflect an increase in their partition coefficients with increasing Al.sup.IV in the clinopyroxene with an increase in [Al.sub.2]O.sub.3 from 1.5 to 4.0 wt.% during [proportional to]20% crystal fractionation over a temperature decrease of [proportional to]100degC being associated with more that a threefold increase in the partition coefficients of Th, Zr, REE, and Y. The magnitude of these increases may indicate that the substitution of these trace elements into clinopyroxene is better modeled in some natural systems by a local charge balance model, rather than the distributed charge model that better replicates the results of annealed experiments. These findings indicate that the effect of Al on the partition coefficients of incompatible trace elements in clinopyroxene may be under appreciated in natural magmatic systems and that the application of experimentally determined clinopyroxene partition coefficients to natural systems must be done with caution. Author Affiliation: (1) Earth and Planetary Sciences, McGill University, GEOTOP UQAM-McGill, Montreal, QC, Canada, H3A 2A7 Article History: Registration Date: 03/03/2008 Received Date: 16/05/2007 Accepted Date: 03/03/2008 Online Date: 28/03/2008 Article note: Communicated by J.Blundy.

Dome lavas from the 2004 eruption of Mount St. Helens show elevated Li contents in plagioclase phenocrysts at the onset of dome growth in October 2004. These cannot be explained by variations in plagioclase-melt partitioning, but require elevated Li contents in coexisting melt, a fact confirmed by measurements of Li contents as high as 207 [micro]g/g in coexisting melt inclusions. Similar Li enrichment has been observed in material erupted prior to and during the climactic May 1980 eruption, and is likewise best explained via pre-eruptive transfer of an exsolved alkali-rich vapor phase derived from deeper within the magma transport system. Unlike 1980, however, high Li samples from 2004 show no evidence of excess ([sup.210]Pb)/([sup.226]Ra), implying that measurable Li enrichments may occur despite significant differences in the timing and/or extent of magmatic degassing. Diffusion modeling shows that Li enrichment occurred within ~1 yr before eruption, and that magma remained Li enriched until immediately before eruption and cooling. This short flux time and the very high Li contents in ash produced by phreatomagmatic activity prior to the onset of dome extrusion suggest that vapor transfer and accumulation were associated with initiation of the current eruption. Overall, observation of a high Li signature in both 1980 and 2004 dacites indicates that Li enrichment may be a relatively common phenomenon, and may prove useful for petrologic monitoring of Mount St. Helens and other silicic volcanoes. Lithium diffusion is also sufficiently rapid to constrain vapor transfer on similar time scales to short-lived radionuclides. Keywords: Mount St. Helens, diffusion, plagioclase, lithium, vapor transport.

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.sub.n]/[Yb.sub.n] and [La.sub.n]/[Yb.sub.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 [sup.87]Sr/[sup.86]Sr and [sup.206]Pb/[sup.204]Pb ratios, and the highest [sup.144]Nd/[sup.143]Nd 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 alongstrike 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.

Major- and trace-element compositions of zircons and whole rocks from 14 barren and seven ore-bearing calc-alkaline intrusions from the Chuquicamata-El Abra porphyry copper belt of northern Chile have been measured in situ by excimer laser ablation (ELA) ICP-MS. These data permit the Ce(IV)/Ce(III) ratio within zircon to be calculated using a lattice-strain model for mineral-melt partitioning of Ce(IV) and Ce(III). Zircon Ce(IV)/Ce(III) and [Eu.sub.N]/[Eu.sub.N.sup.*] ratios, and by inference magmatic oxidation states, generally increase from older, mafic to younger, felsic units. Within this sequence, porphyry copper mineralization is directly associated only with intrusions with zircon Ce(IV)/Ce(III) > 300 and [Eu.sub.N]/[Eu.sub.N.sup.*] > 0.4. Such trends can be understood in terms of interdependent relations between oxygen fugacity, sulfur speciation and solubility, and chalcophile element partitioning in silicate magmas. Because zircon occurs in most calc-alkaline intrusions and is resistant to subsolidus alteration, zircon Ce(IV)/Ce(III) ratios provide a useful tool for evaluating the economic potential of such rocks for magmatic-hydrothermal Cu [+ or -] Au mineralization. The approach is general and may provide a means to infer relative oxidation state in a wide range of intermediate to felsic igneous rocks.

An extensive electron microprobe survey of amphibole compositions in the Fish Canyon magma (2146 analyses), more than 80% of which are from high-resolution ( Hornblende phenocrysts are variable for most major elements (e.g., 5-9 wt% [Al.sub.2][O.sub.3] and 44-50 wt% Si[O.sub.2]). This compositional range is controlled by two major temperature-sensitive coupled substitutions. Approximately 50% of the total Al variation (~0.8 atoms per formula unit = apfu) is due to the edenite exchange [[sup.T]Si + [sup.A][??] = [sup.T]Al + [sup.A](Na + K)] and another 25-30% is the consequence of a Ti-Tschermak exchange ([sup.T]Si + [sup.M1-M3]Mn = [sup.T]Al + [sup.M1-M3]Ti). In contrast, the pressure-sensitive Al-Tschermak substitution ([sup.T]Si + [sup.M1-M3]Mg = [sup.T]Al + [sup.M1-M3]Al) did not play a significant role, as [sup.M1-M3]Al does not correlate with [sup.T]Al and is always In order to constrain the ranges of absolute P and T over which these hornblendes crystallized and to assess the sensitivity of the recent thermo-barometric algorithms of BH1990, HB1994TA (requiring silica saturation), HB1994TB (not requiring silica saturation) and AS1995, we have calculated pressures and temperatures for two selected populations of analyses wherein [Al.sub.2][O.sub.3] contents are within analytical error (5.95 to 6.05 wt% [Al.sub.2][O.sub.3], N = 78 and 7.7 to 7.8 wt% [Al.sub.2][O.sub.3], N = 40). The barometric formulation of AS1995 gives a mean pressure of 2.24 [+ or-] 0.05 for the high-Al population at 760[degrees]C, which is indistinguishable from the 2.4 [+ or -] 0.5 kbar estimate of Johnson and Rutherford (1989a). A high sensitivity to temperature at low P is suggested by the geologically implausibly shallow depths calculated for the low-Al population ( Nine out of 14 traverses across large phenocrysts from the Fish Canyon magma display rimward increases in [sup.T]Al, [sup.A](Na + K), and [sup.M1-M3]Ti, compensated by decreases in [sup.T]Si, and [sup.M1-M3]Mn. Using the HB1994TB algorithm, the low-Al population, typical of near-core compositions, gives a mean temperature of ~715[degrees]C, which is ~35-45[degrees]C above the water-saturated granite solidus at 2-2.5 kbar. The high-Al population, representing the average rim composition, gives a value around 760[degrees]C, which is indistinguishable from independent T determinations using coexisting Fe-Ti oxides and Qtz-Mag oxygen isotope thermometry. These profiles suggest that Fish Canyon hornblendes crystallized during near-isobaric reheating over a temperature range of ~40[degrees]C, which is consistent with our model of rejuvenation and remobilization of a pre-existing near-solidus crystal mush of batholithic dimensions via shallow intrusion of more mafic magma (Bachmann et al. 2002). Crystallization of hornblende from a high-Si[O.sub.2], low-MgO melt during reheating requires an open system, in which both heat and mass, in particular volatiles, are transferred from the underlying mafic magma.

Locke's porphyry argument at 2.8.19 of the Essay has not been properly appreciated. On my reconstruction, Locke argues from the premise that porphyry undergoes a mere Cambridge change of color in different lighting conditions to the conclusion that porphyry's colors do not belong to it as it is in itself. I argue that his argument is not quite sound, but it would be if Locke chose a different stone, alexandrite. Examining his argument teaches us something about the relation between explanatory qualities and real alterations and something about the ways that colors inhere in bodies.

3He/4He ratios in olivine phenocrysts have been measured in 58 Ma basaltic lavas from the Hold with Hope region of northeast Greenland. These lavas constitute a part of the North Atlantic volcanic province. The samples exhibit a wide range of 3He/4He ratios between 1.7[R.sub.a] and 21[R.sub.a] ([R.sub.a] = the atmospheric 3He/4He ratio = 1.386 x [10.sup.-6]), which is comparable to the previously recorded range for Quaternary Icelandic basalts. These He isotope variations may partly result from crustal assimilation, but 3He/4He ratios higher than those of mid-ocean-ridge basalts support early participation of plume-head material at the onset of volcanic activity in the North Atlantic volcanic province. As with the Deccan-Reunion plume and the Ethiopia-Afar plume, this long-term contribution of 3He-rich material is fully consistent with models that ascribe a deep (i.e., deeper than the convective mantle that drives plate tectonics) origin for plume-source material, but contradicts those that postulate a shallow-mantle origin for continental flood basalts and large igneous provinces.

Detailed chemical surveys of phlogopite/biotite in diverse tectonic lavas provide a database to understand degassing-related variations in the ferric-ferrous ratios, O/H2O contents, and delta-D values. Cation/anion-based normalization techniques accurately unravel mineral formulae of most of the cations/anions, but seem unable to provide realistic estimates of Fe3+/Fe(super T) ratios. Analysis of the phenocrysts in arc-related lavas of western Mexico and eastern California reveals the highest ferric-ferrous ratios pointing to high magmatic oxygen fugacities.

Textural, chemical and Sr isotopic studies of feldspars from the subsolvus Shap granite, northern England, demonstrate that a number of magma mixing events have dominated the evolution of this pluton. K-feldspar megacrysts are phenocrysts formed in the magma chamber. They contain a number of Ba-rich zones that developed during periods of slight dissolution and regrowth linked to the hybridization of the granite by the intrusion of basic magmas. Diorite enclaves represent the relicts of these magmas and these also contain K-feldspar megacrysts, which show evidence of major dissolution. They are xenocrysts picked up from the host granite and incorporated in the basic magma. Increasing [H.sub.2]O contents during fractional crystallization caused a late switch from growth of megacrysts to finer-grained K-feldspars in the matrix. The chemically and isotopically zoned K-feldspar megacrysts preserve an exceptional record of the evolution of the magma, and the zones also had a significant influence on the development of exsolution microtextures during cooling. Keywords: K-feldspar, granites, 87Sr/86Sr, barium, hybridization.

Laser probe 40Ar/39Ar dating of quartz phenocrysts with rhyolite glass inclusions from the Bishop Tuff air-fall and ignimbrite deposits reveals that the Long Valley magma system existed as a long-lived silicic magma chamber throughout most of the Pleistocene. Sanidine phenocryst and matrix glass analyses show that the Bishop Tuff eruption occurred at 759 [+ or -] 1 to 761 [+ or -] 1 ka. Initial and radiogenic Ar isotope ratios indicate isotopic equilibrium between the sanidine phenocrysts and their host melt at the time of eruption. The quartz phenocrysts, in contrast, the most abundant phenocryst phase of the Bishop rhyolite, crystallized, trapped their glass inclusions, and became a closed system with respect to Ar at 1.89 [+ or -] 0.03 to 2.3 [+ or -] 0.3 Ma. Consequently, the Bishop rhyolite magma already resided in the Long Valley basement and had formed most of its quartz phenocrysts [approximately] 1.1 m.y. before its principal eruption, providing important constraints on the longevity of large silicic magma chambers.

Fresh volcanic glasses in the form of melt inclusions within quartz phenocrysts are commonly present in Paleozoic K-bentonites from New York State, Iowa, Kentucky, Pennsylvania, Newfoundland, and Quebec. Because these glasses are compositionally distinct from one layer to another, their geochemistry can be used to define chronostratigraphic horizons. In New York State, the K-bentonites occur within flat-lying, calcareous black shales of the Middle Ordovician Utica Formation. This glass has survived (1) because it has been sealed within a host crystal that is stable under most diagenetic conditions, and (2) because of the modest burial depths of the Ordovician strata in this region. To demonstrate the potential of these volcanic glasses for stratigraphic correlations, isochronous surfaces have been established among three localities separated by 35 km. The immaculate preservation of these Ordovician glasses bodes well for the general application of this approach to younger, and perhaps even older, strata where geologic conditions have favored the survival of glass inclusions.

Low-density, fluid phase within growing phenocrysts revealed the presence of rare, copper-rich bubbles in some melt inclusions in the magmas which erupted from the valley of Ten Thousand Smokes at Alaska in 1912. Cryptic fractionation of phenocryst phases and division into a co-existing fluid phase influenced the behavior of volatile substances such as sulfur, copper and chloride ions. Sulfides also regulated the distribution of copper in the magma.