Volatile contents and ferric-ferrous ratios of basalt,ferrobasalt, andesite and rhyodacite glasses from the Galapagos 95.5°W propagating rift

Volatiles and major elements in abyssal glasses ranging in composition from basalt, ferrobasalt, andesite to rhyodacite from the Galapagos Spreading Center (GSC) near 95°W were analyzed using electron microprobe and high temperature mass spectrometry. Total volatile content ranged from 0.32 wt.% to 2.74 wt.%. Volatile abundances of MORB glasses from the 95.5°W propagating rift are similar to those from the adjacent normal rift (avg. 0.34 wt.%) and lower than those of N-type MORB from the Mid-Atlantic Ridge (avg. 0.49 wt.%). Although both propagating and non-propagating rift glasses contain trace amounts of methane (<0.01 wt.%) and carbon monoxide (0.04 wt.%), significantly higher 100 $\text{Fe}{}_2\text{O}{}_3\text{FeO}\text{+ Fe}{}_2\text{O}{}_3$ ratios are observed for the primitive propagating rift glasses. Water contents of the most primitive GSC glasses are ~0.09 wt.% suggesting a water content for the mantle source of ~0.02 wt.% which indicates that source masses with very low water content can be involved in the generation of MORB.In fractionated ferrobasalt, andesite and rhyodacite glasses from the 95.5°W propagating rift, increasing abundances of H₂O, Cl and F indicate highly incompatible behavior, whereas CO₂ and reduced carbon species appear to decrease in abundance with increasing differentiation. Ferric-ferrous ratios increase from basalt to andesite and reduce to near zero in the rhyodacite. These values are not distinguishable from those previously reported for similar fractionated glasses from the Galapagos 85°W propagating rift, despite the apparent suppression of oxide precipitation in the 85°W suite.     

三叠纪西秦岭西北部洋俯冲的记录:来自镁安山岩/高镁安山岩的证据

三叠纪以来,西秦岭地区构造-岩浆活动极为强烈,中酸性侵入岩呈北西向弧状分布,前人做了大量卓有成效的工作,但对出露较少的火山岩鲜有研究。本次工作选取青海省同仁县麦秀和甘肃省夏河县甘加、德乌鲁3个地区的安山岩,从岩石学、岩石地球化学、年代学等进行分析。结果显示安山岩均属于钙碱性系列,具有富钾火山岩的特征。其中,麦秀安山岩中既有高镁安山岩,也有镁安山岩,而甘加与德乌鲁安山岩属于镁安山岩,为洋壳俯冲作用形成的两种典型火山岩类型。西秦岭地区安山岩形成时间东早西晚,由西向东安山岩基性程度降低,K_2O、Na_2O、∑REE等含量增高,MF、FL、DI、La/Sm等值变大,SI、Mg~#等值降低,并且呈明显的线性特征,构成同仁—合作镁安山岩(镁闪长岩)岩浆弧组合,和北部隆务峡蛇绿岩带一起,组成岩性分布极性,从而判断出西秦岭西北部在三叠纪存在隆务峡蛇绿岩代表的洋壳板块向南俯冲作用。     

Environmental characterization of a karst polje: an example from Izeh polje,southwest Iran

Groundwater contamination is a well-known phenomenon, which occurs on local and regional scales in Izeh polje. The aims of this paper are investigation of the impact of human activities on the polje ecosystem, determination of the vulnerability of ground water, and to solve environmental problems. Nitrate contamination of groundwater in the Izeh polje was predicted using a solute transport model. The nitrate concentration in groundwater in most parts of Izeh polje is greater than maximum concentration permissible for drinking water, i.e., 45 mg/l. The main source of nitrate in the eastern underground areas of Izeh city is the domestic sewage. Bacterial pollution of shallow ground water in Izeh polje is severe and widespread. About 45% of ground water samples in May and September 2001 have positive MPN coliforms. Infiltration of polluted surface waters and decrease of water table depth, have lead to bacterial pollution of 80% of ground water samples in January 2002. The northeast, south and southwest areas of Izeh polje have higher pollution potential rather than its middle parts. The aquifer vulnerability indices in the middle, eastern, and northern parts of the polje are moderately lower as a result of decreased sediment size of the aquifer. The pollution in the polje depends on the amount and presence of pollutants. If they do exist, the possibility of pollution is considerable due to the coarseness of materials and shallow depth of groundwater table.     

相山“流纹英安岩”单颗粒锆石U-Pb年龄及地质意义

相山火山-侵入杂岩主要由晚中生代打鼓顶组、鹅湖岭组火山岩和浅成-超浅成侵入岩所组成,主要岩石类型有酸性火山碎屑岩、碎斑熔岩、花岗斑岩和“流纹英安岩”等,“流纹英安岩”长期作溢流相火山岩处理。应用单颗粒锆石U-Pb法,对相山“流纹英安岩”进行了测年,获得其年龄为129.54±7.93Ma,该年龄小于相山火山-侵入杂岩的主体岩石——碎斑熔岩的年龄,结合其产状形态特征,认为“流纹英安岩”属火山期后浅成-超浅成侵入的流纹英安斑岩。结合前人测年资料,参照上侏罗统与下白垩统的145.5Ma界线年龄,提出相山火山-侵入杂岩的形成时代为早白垩世的新认识。     

Chromite-bearing spessartites from Kasuga-mura,Japan, and their bearing on possible mantle origin andesite

An unusual association of chromite and hornblende was found in the spessartites of andesite composition, occurring as a dike swarm associated with a Cretaceous granite batholith. The spessartites are largely porphyritic with phenocrysts of either hornblende or augite. One dike, comprising a finegrained spessartite, exhibits distinct chilled selvages of aphanitic facies. The chromites in the fine-grained and augite-spessartites are significantly higher in Cr/ (Cr+Al) than those occurring rarely as inclusions in the phenocrystic hornblendes in the hornblende spessartite, although both are similar in Mg/ (Mg+Fe), Fe₂O₃, and TiO₂. The phenocrystic hornblendes are titaniferous pargasite with high Mg/ (Mg+Fe), and differ in their higher octahedral Al from the groundmass hornblendes including those in the fine-grained spessartite. The crystallization sequence in the phenocrystic hornblende-bearing spessartites is Al-rich chromite, phenocrystic hornblende, and plagioclase without pyroxene, suggesting a high water content in the magma and the start of the crystallization at relatively high pressures. The finegrained spessartite from which the porphyritic spessartites have been derived by fractionation of dominant mafic minerals, has the high Mg-value and Cr content equivalent to those in primitive, undifferentiated basalts, although still andesitic in SiO₂ content. Chemically similar magnesian andesites, although uncommon, found in some orogenic calc-alkalic suites may represent a magma composition in equilibrium with mantle peridotite under the condition of high water pressures.     

Maestrichtian-Danian andesite series of the Eastern Sikhote Alin: Mineralogy,geochemistry, and petrogenetic aspects

In the stratigraphic sequence of volcanic rocks in the Eastern Sikhote Alin, Maestrichtian-Danian predominantly andesitic volcanics are characterized by a boundary position between the Late Cretaceous subduction, mostly acid volcanic rocks and Cenozoic post-subduction basaltoids. Data on these rocks are important for elucidating the genesis of andesitic magmas, constraining and specifying the geodynamic evolutionary stages in this territory, and revealing the conditions under which the parental melts of these rocks were derived and evolved. Results of detailed mineralogical and geochemical studies, including ICP-MS analysis for trace elements point to a hybrid character of the andesitic volcanic rocks and an important role of fractional crystallization and crustal contamination in their genesis. Although geological evidence (variations in the style of volcanism, the composition of its products, and the character of their distribution) testifies to a change in the geodynamic environment in the Eastern Sikhote Alin in the Maestrichtian-Danian, geochemically the volcanics of this age range are typical subduction-related rocks with anomalously low concentrations of Nb and high contents of K, Ba, Rb, Pb, and U. The volcanic piles contain no adakites, which are indicators of the geodynamic environment in which slab windows are formed. The inconsistency between geological and geochemical indicators of the geodynamic environment suggests certain genetic features of the transitional magmatic series. The parental magmas of the andesitic volcanics were derived from the suprasubduction mantle wedge, which had been metasomatically recycled in the course of the dehydration and melting of the subducted oceanic slab. The increasing extension provided the possibility for the parental basaltic magmas to enter upper crustal levels, where they could interact with the host rocks and form hybrid andesitic melts.     

The hydrous phase equilibria (to 3 kbar) of an andesite and basaltic andesite from western Mexico: constraints on water content and conditions of phenocryst growth

We have conducted high pressure (to 3 kbar), water saturated melting experiments on an andesite (62 wt% SiO₂) and a basaltic andesite (55 wt% SiO₂) from western Mexico. A close comparison between the experimental phase assemblages and their compositions, and the phenocryst assemblages of the lavas, is found in water saturated liquids, suggesting that the CO₂ content was minimal in the fluid phase. Thus the historic lavas from Volcan Colima (with phenocrysts of orthopyroxene, augite, plagioclase, and hornblende) were stored at a temperature between 950–975 °C, at a pressure between 700–1500 bars, and with a water content of 3.0–5.0 wt%. A hornblende andesite (spessartite) from Mascota, of nearly identical composition but with only amphibole phenocrysts, had a similar temperature but equilibrated at a minimum of 2000 bars pressure with a dissolved water content of at least 5.5 wt% in the liquid. Experiments on the basaltic andesite show that the most common natural phenocryst assemblages (olivine, ±augite, ±plagioclase) could have precipitated at temperatures from 1000–1150 °C, in liquids with a wide range of dissolved water content (∼2.0–6.0 wt%) and a corresponding pressure range. A lava of the same bulk composition with phenocrysts of hornblende, olivine, plagioclase, and augite is restricted to temperatures below 1000 °C and pressures below 2500 bars, corresponding to <5.5 wt% water in the residual liquid. Although there is some evidence for mixing in the andesites (sporadic olivine phenocrysts), the broad theme of the history of both lava types is that the phenocryst assemblages for both the andesitic magmas and basaltic andesitic magmas are generated from degassing and reequilibration on ascent of initially hydrous parents containing greater than 6 wt% water. Indeed andesitic magmas could be related to a basaltic andesite parent by hornblende-plagioclase fractionation under the same hydrous conditions. Received: 10 December 1996 / Accepted: 21 August 1997     

Mid-Pleistocene lavas from the Seguam volcanic center,central Aleutian arc: closed-system fractional crystallization of a basalt to rhyodacite eruptive suite

In contrast to adjacent volcanic centers of the modern central Aleutian arc, Seguam Island developed on strongly extended arc crust. K-Ar dates indicate that mid-Pleistocene, late-Pleistocene, and Holocene eruptive phases constitute Seguam. This study focuses on the petrology of the mid-Pleistocene, 1.07–07 Ma, Turf Point Formation (TPF) which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst-poor andesite to rhyodacite lavas. Increasing whole-rock FeO^*/MgO from basalt to dacite, the anhydrous Plag+Ol+Cpx±Opx±Mt phenocryst assemblage, groundmass pigeonite, and the reaction Ol+Liq=Opx preserved in the mafic lavas indicate a tholeiitic affinity. Thermometry and comparison to published phase equilibria suggests that most TPF basalts crystallized Plag+Ol+Cpx±Mt at 1160°C between about 3–5 kb (±1–2% H₂O), andesites crystallized Plag+Cpx+Opx±Mt at 1000°C between 3–4 kb with 3–5% H₂O, and dacites crystallized Plag +Cpx±Opx±Mt at 1000°C between 1–2 kb with 2–3% H₂O. All lavas crystallized at f _{o 2} close to the NNO buffer. Mineral compositions and textures indicate equilibrium crystallization of the evolved lavas; petrographic evidence of open-system mixing or assimilation is rare. MgO, CaO, Al₂O₃, Cr, Ni, and Sr abundances decrease and K₂O, Na₂O, Rb, Ba, Zr, and Pb increase with increasing SiO₂ (50–71%). LREE enrichment [(Ce/Yb)_n=1.7±0.2] characterizes most TPF lavas; total REE contents increase and Eu anomalies become more negative with increasing SiO₂. Relative to other Aleutian volcanic centers, TPF basalts and basaltic andesites have lower K₂O, Na₂O, TiO₂, Rb, Ba, Sr, Zr, Y, and LREE abundances. ⁸⁷Sr/⁸⁶Sr ratios (0.70361–0.70375) and ratios of ²⁰⁶Pb/²⁰⁴Pb (18.88–18.97), ²⁰⁷Pb/²⁰⁴Pb (15.58–15.62), ²⁰⁸Pb/²⁰⁴Pb (38.46–38.55) are the highest measured for any suite of lavas in the oceanic portion of the Aleutian arc. Conversely, Nd values (+5.8 to+6.7) are among the lowest from the Aleutians. Sr, Nd, and Pb ratios are virtually constant from basalt through rhyodacite, whereas detectable isotopic heterogenity is observed at most other Aleutian volcanic centers. Major and trace element, REE, and Sr, Nd, and Pb isotopic compositions are consistent with the basaltic andesitic, andesitic, dacitic, and rhyodacitic liquids evolving from TPF basaltic magma via closed-system fractional crystallization alone. Fractionation models suggest that removal of 80 wt% cumulate (61% Plag, 17% Cpx, 12% Opx, 7% Ol, and 3% Mt) can produce 20 wt% rhyodacitic residual liquid per unit mass of parental basaltic liquid. Petrologic and physical constraints favor segregation of small batches of basalt from a larger mid-crustal reservoir trapped below a low-density upper crustal lid. In these small magma batches, the degree of cooling, crystallization, and fractionation are functions of the initial mass of basaltic magma segregated, the thermal state of the upper crust, and the magnitude of extension. Tholeiitic magmas erupted at Seguam evolved by substantially different mechanisms than did calc-alkaline lavas erupted at the adjacent volcanic centers of Kanaga and Adak on unextended arc crust. These variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew. At Seguam, intra-arc extension promoted eruption of voluminous basalt and its differentiates, unmodified by interaction with lower crustal or upper mantle wallrocks.     

东天山觉罗塔格构造带晚三叠世镁安山岩的厘定及其构造意义

新疆东天山地区印支期岩浆活动的岩石成因、源区性质及相关动力学背景一直是区域地质问题争论的焦点。对东天山构造带阿奇山安山岩开展岩石学、地球化学及锆石U-Pb-Hf系统研究,结果表明安山岩具斑状结构和气孔状构造,斑晶矿物主要为单斜辉石和斜长石,基质以针状斜长石微晶为主。锆石U-Pb定年结果表明镁安山岩喷发于235 Ma(晚三叠世),并含有石炭纪年龄的继承锆石。锆石Hf同位素分析显示岩浆期锆石具有低的ε_Hf(t)值(-31.7~-7.6)和古老的亏损地幔模式年龄(T_DM=2.2~0.7 Ga)。岩石地球化学特征显示样品为高MgO含量(3.7%~4.7%)和K₂O/Na₂O比值(0.6~1.1)及低TFeO/MgO比值(1.6~1.8)的高钾钙碱性镁安山岩。岩石地球化学特征结合区域构造-岩浆-成矿作用揭示阿奇山镁安山岩为板内伸展背景下含金云母的富集岩石圈地幔在较浅深度(<80 km)部分熔融的产物,且母岩浆混有古老地壳物质。晚三叠世阿奇山镁安山岩的厘定反映了该时期东天山地区处于岩石圈伸展-减薄阶段。     

Compositional relations of coexisting orthopyroxene,pigeonite and augite in a tholeiitic andesite from Hakone Volcano

The compositions of five different coexisting pyroxenes hypersthene, pigeonite and augite in groundmass and bronzite and augite of phenocryst in a tholeiitic andesite from Hakone Volcano, Japan have been determined by the electron probe microanalyser. It is shown that there is a compositional gap of about 25 mole per cent CaSiO₃ between groundmass pigeonite and augite, compared with 35 per cent CaSiO₃ between phenocrystic augite and bronzite. Subcalcic augite or pigeonitic augite was not found. The groundmass augite, which occurs only as thin rims of pigeonite and hypersthene, is less calcic and more iron-rich than the phenocryst augite. It is also shown that the groundmass pigeonite is 3–4 mole per cent more CaSiO₃-rich than the coexisting groundmass hypersthene. The Fe/(Mg + Fe) ratios of these coexisting hypersthene and pigeonite are about 0.31 and 0.33, respectively. It is suggested from these results that a continuous solid solution does not exist between augite and pigeonite of the Fe/(Mg + Fe) ratio at least near 0.3 under the conditions of crystallization of groundmass of the tholeiitic andesite. It is suggested from the Mg-Fe partition and the textural relation that the groundmass augite crystallized from a liquid more iron-rich than that from which groundmass hypersthene and pigeonite crystallized.     

Magnesian andesite and basalt from Shodo-Shima Island,southwest Japan,and their bearing on the genesis of calc-alkaline andesites

Calc-alkaline andesites and olivine tholeiitic basalts are widely distributed on Shodo-Shima island, southwest Japan. The Fo content of olivine phenocrysts in the andesite is higher than in the basalt. The primary magma of the andesite, estimated on the basis of the olivine fractional crystallization model, is not basaltic but andesitic. The basalt contains both chromite and titanomagnetite as inclusions in olivine phenocrysts, while only chromite appears in the andesite. The Cr content of chromite in the andesite is higher than in the basalt. These facts again indicate that the andesite cannot be a fractionation product of the basalt, and that andesitic and basaltic primary magmas were generated independently.     

Rare Earth data and petrogenesis of andesite from the North Chilean Andes

Abundances of 9 Rare-Earth Elements are reported for 21 andesite and dacite samples from a small group of volcanoes in the North Chilean segment of the Andean plate margin. The data are correlated with major and trace element data (Roobol et al., in press) and are used to critically assess several models proposed to account for the origin of andesite. A three-stage model for andesite petrogenesis is proposed. This involves: (i) initiation of melting at the subduction zone and rise of magmas into the overlying mantle-wedge, which thereby becomes richer in garnet-pyroxene components and large-ion lithophile (LIL) elements, (ii) partial melting of this LIL-enriched garnet pyroxenite material to form andesitic magmas within the mantle wedge, (iii) minor crystal fractionation during rise through the crust.     

Tholeiitic andesite of high-pressure origin from the tweed shield volcano,northeastern New South Wales

A high-alumina tholeiitic andesite from the southern portion of the Tweed Shield Volcano in northeastern New South Wales contains abundant megacrysts of plagioclase (Ab₅₀An₄₆Or₄) and megacrysts of aluminian bronzite (Ca₄Mg₇₁Fe₂₅) and relatively Ca-poor aluminian augite (Ca₃₆Mg₄₁Fe₂₀). The pyroxenes commonly occur as inclusions in the plagioclase megacrysts. Electron probe microanalyses of the pyroxene megacrysts indicate that they differ in character and composition from the groundmass ferromagnesian phases, namely a more Al- and Na-poor augite (Ca₄₁Mg₄₂Fe₁₇) and olivine (Fa₅₃). The bulk composition of the plagioclase megacrysts is slightly more Ab-rich than that of the groundmass plagioclase, but differences in the two compositions are extended by microanalyses of groundmass plagioclases. Evaluation of the megacryst compositions in the light of experimental data and analogous occurrences in alkaline volcanics leads to the interpretation that the megacrysts represent cognate precipitates formed at pressures broadly equivalent to the crust-mantle boundary. More important, they provide strong evidence for the high pressure origin of tholeiitic andesites, customarily interpreted as the products of low pressure fractional crystallization of tholeiitic magma.     

Geochemistry characterization of groundwater in an agricultural area of Razan,Hamadan, Iran

This study was conducted to evaluate factors regulating groundwater quality in an area with agriculture as main use. Thirty groundwater samples have been collected from Razan area (Hamadan, Iran) for hydrochemical investigations to understand the sources of dissolved ions and assess the chemical quality of the groundwater. The chemical compositions of the groundwater are dominated by Na⁺, Ca²⁺, HCO₃ ⁻, Cl⁻ and SO₄ ²⁻, which have been derived largely from natural chemical weathering of carbonate, gypsum and anthropogenic activities of fertilizer’s source. The production of SO₄ ²⁻ has multiple origins, mainly from dissolution of sulphate minerals, oxidation of sulphide minerals and anthropogenic sources. The major anthropogenic components in the groundwater include Na⁺, Cl⁻, SO₄ ²⁻ and NO₃ ⁻, with Cl⁻ and NO₃ ⁻ being the main contributors to groundwater pollution in Razan area.     

Halogen signatures of biotites from the Maher-Abad porphyry copper deposit,Iran: characterization of volatiles in syn- to post-magmatic hydrothermal fluids

Copper and gold mineralization in the Maher-Abad area, eastern Iran, is closely related to multiple episodes of emplacement of a late Eocene granodiorite into a quartz-monzonitic stock and andesitic volcaniclastic rocks. Hypogene and supergene porphyry Cu–Au mineralization occurred within the porphyritic granodiorite and quartz-monzonite host rocks extensively altered into dominantly potassic, propylitic, phyllic, and argillic assemblages. Temperature and pressure estimates using the plagioclase–hornblende thermometer and Al-in-hornblende barometer indicate that the granodiorite intruded at 758 ± 10°C and 1.4 ± 0.2 kbar.

Biotites from the alteration zones have more variable Al^IV than those in the fresh granodiorite, but nearly all are close to the ideal phlogopite composition. Biotite compositions display an increase in Al₂O₃, FeO, TiO₂, and Cl, but a decrease in SiO₂ and F, from the porphyritic granodiorite and potassic to the transitional phyllic alteration zones. Biotite from the potassic zone (X _phl?=?0.63–0.67) possesses a moderate F content (0.53 to 0.82 wt.%) that is significantly higher than that in the phyllic zone (0.22 to 0.38 wt.%), exhibiting a positive correlation with X _Mg and negative correlation with Cl.

With a decrease in the temperature, log (fH₂O/fHF) and log (fH₂O/fHCl) values calculated for fluids equilibrated with biotite increase progressively from the granodiorite through the potassic to the phyllic zones, whereas log (fHF/fHCl) shifts towards more negative values. Fugacity ratio trends in the Maher-Abad porphyry copper deposit are quite similar to those of other porphyry copper systems. The decrease in halogen content of hydrothermal fluids towards outer parts of the deposits reflects an increase in the degree of mixing between magmatic fluid and meteoric water.     

新疆西天山智博铁矿岩浆-热液成矿作用—来自安山岩矿物学的证据

本文以智博铁矿区内的安山岩为研究对象,通过详细的野外地质调查,并利用电子显微镜和电子探针,对安山岩中的主要矿物进行了系统的岩相学观察和矿物学研究。研究表明,智博安山岩中斜长石主要为Na-高钠长石,具低TiO2,高Na2O和Al2O3的特点;辉石主要为普通辉石,具高TiO2,高Al2O3的特点;角闪石主要为镁角闪石和阳起石,具低TiO2和Al2O3,高MgO的特点;副矿物磁铁矿具高TiO2,低MgO和Al2O3的特点。辉石、角闪石矿物化学特征表明,智博铁矿安山岩的母岩浆属于壳幔混源的玄武质岩浆,构造环境为火山岛弧环境。智博安山岩中单斜辉石结晶温度为1225℃左右,结晶压力约0.795GPa,结晶深度约26km。智博铁矿后期的热液作用也参与了磁铁矿成矿,对智博铁矿的成矿有一定的贡献。     

On the origin of andesite in the northern Mariana Island Arc: Implications from Agrigan

Magmatic evolution on the active volcano of Agrigan in the northern Mariana Island Arc is interpreted as resulting in the production of calc-alkaline andesites by the fractional crystallization of high-alumina basalt. Basaltic products predominate, but the ratio of andesites to basalts increases with time up to an event of voluminous andesitic pyroclastic ejection accompanied by caldera-collapse; post-collapse lavas are entirely basaltic. Moderate iron-enrichment is demonstrated for the volcanic suite, with indications of a progressive, pre-caldera decrease in iron-enrichment; post-caldera lavas display a return to moderate Fe-enrichment. Overall, the lavas are enriched in the LIL elements (K, Rb, Ba, Sr) and depleted in Ti, Mg, Cr, and Ni. From the oldest to the youngest pre-caldera volcanic sequence, the LIL elements increase 3-6X while Ca and Mg decrease by 50% or more. Approximately constant K/ Rb (430±60) and ⁸⁷Sr/⁸⁶Sr (0.7032–0.7034) indicate consanguinity of the basalts and the andesites. Cumulate plutonic xenoliths, common in the lavas, are composed of mineral phases also encountered as phenocrysts. The following order of crystallization is indicated: olivine; anorthite-bytownite; clinopyroxene; orthopyroxene and titanomagnetite. Co-existing xenolithic olivines (Fo_74–83) and plagioclase (An_88–96) are typical of calc-alkaline island-arc assemblages and contrast with assemblages in the tholeiites from the Mariana Trough to the west. The relatively fayalitic composition and low abundances of Ni in olivines and Cr in clinopyroxenes indicate equilibrium with an already-fractionated liquid. These data, along with structural evidence, high Ca in the olivines, and comparison of the observed assemblages with experimental studies, suggests that these xenoliths formed as crystal cumulates at the floor of a shallow ( 7 km) crustal magma chamber.Major element modeling studies using the separation of observed xenocrystic and phenocrystic phases from assumed parental liquids reproduce the observed temporal and geochemical variations in the lavas. Trace element modeling parallels this evolution with the exception of Cr and Ni in the andesites. An extensive (16.3 km³) gabbroic body is required by this modeling to be present beneath Agrigan to produce the inferred volumes of the various lithologies preserved in the volcano's evolution. The sum of stratigraphic, geochemical, and isotopic evidence on Agrigan supports the derivation of calc-alkaline andesite by the removal of about 75% solids from a high-alumina basalt accompanied by a process of K and Rb enrichment, such as volatile-transfer. Considerations of ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and ³He/⁴He isotopic data indicate that the source region of these parental liquids lies in the mantle, not subducted crust. In the northern Marianas, the model of a shallow-crustal origin for andesite is preferred over one requiring andesite generation in the deeper mantle and/or subducted slab.     

大别造山带宿松球颗结构安山岩的地球化学特征

大别造山带南部宿松球颗结构安山岩的球颗结构指示此安山岩是喷发于水下环境。球颗由放射状斜长石（蚀变为钠长石和更长石）及少量位于斜长石间隙的单斜辉石、普通角闪石和蚀变矿物石英组成，基质由玻璃质、绿泥石、黝帘石、角闪石组成，SiO2=56.17%-58.31%,AI2O3=14.08% -14.67%,TiO2=0.91%-0.94%,NaO=4.19%-4.58%,K2O=0.93%-1.42%,MgO含量高达5.66%-6.98%,100Mg/(Mg Fe)=64.1-66.2。球颗结构安山岩和闪长斑岩、英安岩均为亚碱性的钙碱性系列岩石。球颗结构安山岩富集Si(1259-1588μg/g）、Ba(880-1134μg/g）、Th、Rb等大离子亲石元素。大离子亲石元素相对高场强元素富集，例如La/Nb=5.56-6.07。高场强元素的比值具有类似MORB特性，例如Zr/Nb=22.08-26.14,Hf/Ta=5.62-9.62.亏损Ta、Nb、P、Ti等高场强元素，自Ce元素向右，各元素的含量下降很快，表现出活动大陆边缘的与俯冲作用有关的火山岩的特征。轻稀土元素强烈富集，稀土分馏显著（La/Yb)N=32.84，没有Eu异常。具有岛孤安山岩的特性，但有异常高的(La-Yb)N=28.63-26.74,(La/Y)N=70.33-81.04。并且Y和Yb元素有显著的亏损，Y＜20μg/g，YN=2.74-2.86,YbN=2.18-2.35.从La-(La/Sm)图解看，本安山岩为部分熔融作用的产物。La/Nb的平均值为5．83，这在岛弧火山岩是很广泛的特点。Nd的εNd(0)为－18．71～－19．16，物源是大陆物质或被大陆物质所混染。Nd模式年龄为1．9Ga，可能反映了Nd模式年龄为1．7Ga的俯冲扬子板引起Nd模式年龄为2．5Ga的华北大陆岩石圈地幔楔物质部分熔融，产生这种玄武质岩浆。因而具有大陆基底的活动大陆边缘的岛弧火山岩的地球化学特征。     

Amphibole crystallization from a water-saturated andesite melt: Experimental data at P = 2 kbar

This paper reports the results of an experimental investigation of the influence of iron oxidation state in the water-saturated andesite melt of Shiveluch Volcano (Kamchatka) on amphibole composition. Experiments with durations of 2–4 h were carried out at a pressure of 2 kbar and temperatures close to the amphibole liquidus (950°C). The degree of iron oxidation (NNO and NNO + 1) in the initial glass was determined by Mössbauer spectroscopy. The analyses of experimental amphiboles were recalculated to the distribution of elements among crystallographic sites with the estimation of the degree of iron oxidation on the basis of the IMA-97 method (Leake et al., 1997). A negative correlation was established between Al^VI and the sum (Fe³⁺ + Ti). In the most oxidized amphiboles, (Fe³⁺ + Ti) reaches 1.8 atoms per formula unit (a.f.u.). The kinetics of growth affects the amphibole composition more significantly than pressure (according to the data of Schmidt, 1992). Quenching (supercooling by more than 100°C) produces amphibole crystals with Al^VI = 1.3 a.f.u. and Fe³⁺ = Ti = 0. The obtained results allowed us to interpret the conditions of amphibole growth in the andesites of Shiveluch Volcano.