Osmium Isotopic Evidence for Crust-Mantle Interaction in the Genesis of Continental Intraplate Basalts from the Newer Volcanics Province, Southeastern Australia

Basalts from the Quaternary Newer Volcanics Province in southeasternAustralia have a large diversity in their chemical and Nd, Srand Os isotopic compositions. Plains series olivine tholeiitesand Cones series nepheline hawaiites have distinctive isotopiccompositions and are clearly not related to each other by asimple genetic process. The Cones series nepheline hawaiiteshave trace element abundances and Nd (     

Processes in High-Mg, High-T Magmas: Evidence from Olivine, Chromite and Glass in Palaeogene Picrites from West Greenland

Uncontaminated volcanic rocks from the 60 Ma Vaigat Formation,West Greenland, contain 6·5–30 wt % MgO, averaging15·5 wt % MgO. Olivine (mg-number 77·4–93·3)forms diverse assemblages of zoned phenocrysts and xenocrystsshowing evidence for equilibrium and fractional crystallization,oxidation, partial to complete re-equilibration, as well asmagma mixing. The olivine crystals contain glass inclusionsand have high contents of Ca and Cr, indicating that all olivineswith up to mg-number 93·0 crystallized from melts. Associatedchromites (mg-number 45·4–77·2) are essentiallyunzoned and in equilibrium with the olivines. Matrix glassesfrom pillow breccias have 6·7–8·8 wt % MgOand quenched close to 1200°C with oxidation states one log-unitabove the NNO (nickel–nickel oxide) buffer. Compositionaldifferences between the glasses from different volcanic membersare inherited from the primary melts. The magmas erupted ascrystal-charged melts, and liquids with more than     

The Transition to Potassic Alkaline Volcanism in Island Arcs: The Ringgit--Beser Complex, East Java, Indonesia

The origin of potassic lavas with within-plate characteristicsin island are settings is unclear. The volcanic complex of Ringgit—Beser,situated in eastern Java, has erupted lavas of both normal islandare calc-alkaline type and atypical potassic lavas, includingsome highly magnesian lavas. The occurrence of these primitivelavas gives an unusual insight into the source characteristicsof the potassic lavas. The lavas from Ringgit—Beser have a wide range of K₂O(1.1–6.4 wt. %) and MgO contents (18.0–1.6 wt.%).The most magnesian lavas have high Ni and Cr contents. The calc-alkalinelavas have incompatible trace element patterns typical of islandare lavas with enrichments in large ion lithophile elements(LILE) and light rare earth elements (LREE) relative to highfield strength elements (HFSE) and heavy REE (HREE). The potassiclavas may be divided into two series on the basis of Ba andNb contents, with the enriched potassic (EK) series having higherBa and Nb contents for a given MgO content than the potassic(K) series. The EK and K series lavas have some incompatibletrace element ratios similar to within-plate lavas (e.g., highCe/Pb, low LILE/HFSE ratios, and low B/Be). However, both theEK series and K series lavas have negative Ti and Zr anomalies,and the EK series lavas have high Ba/La similar to are lavas.There is little distinction in Sr and Nd isotopes between theK and EK series, but the calc-alkaline lavas have lower ₈₇Sr/₈₆Srand higher ₁₄₃Nd/₁₄₄Nd ratios than the potassic lavas. The EKseries lavas have lower ₂₀₆Pb/₂₀₄Pb and higher ₂₀₈Pb/₂₀₄Pb thanthe K series lavas, but similar ₂₀₇Pb/₂₀₄Pb ratios. The K serieslavas define an almost horizontal trend in ₂₀₇Pb–₂₀₆Pbspace. The Pb isotopic ratios indicate that the EK series lavasare derived from a single mantle source, whereas the K seriesoriginate from a mixture of two mantle components. Calc-alkalinelavas have Pb isotope ratios similar to other calc-alkalineand tholeiitic lavas from Java, and plot on a mixing line betweenIndian Ocean mid-ocean ridge basalt (MORB) and Indian Oceansediment. Incompatible trace element and Pb isotope data for the calc-alkalinelavas indicate that these lavas have a similar source to othercalc-alkaline lavas erupted in Java, namely melts of the IndianOcean MORB mantle fluxed by fluids from the subducted slab.The potassic lavas originate from enriched mantle sources withinthe wedge which have not been affected by recent subductionprocesses. The EK series lavas are derived from a metasomatizedzone which has EMI-type characteristics. The K series lavasare derived from mixing of melts from Christmas Island-type(EMII) mantle and the metasomatized zone. The metasomatizedzone is probably situated at the base of the lithosphere andthe Indian Ocean MORB and Christmas Island-type mantle componentsare situated in the asthenosphere of the wedge. Isotopic datafor Ringgit—Beser lavas confirm that the mantle wedgeof the Sunda arc is extremely heterogeneous (Foden & Varne,1980; Varne, 1985; Wheller et al., 1987). The similarity in geochemistry between Indonesian potassic lavasand those erupted in continental settings indicates that themagma source is essentially the same, namely a metasomatizedphlogopite-rich layer generated by melts of recycled subductedlithosphere. The lack of negative Ti anomalies in the continentalpotassic lavas is ascribed to lower oxidation states in themantle in continental settings.     

Rates and Processes of Potassic Magma Evolution beneath Sangeang Api Volcano, East Sunda Arc, Indonesia

U-series isotopes can provide unique insights into the physicalprocesses of magma evolution by constraining the time scalesover which they operate. This, however, requires rock suitesthat provide a clear and complete record of the liquid lineof descent. Sangeang Api volcano, in the east Sunda arc, providessuch an opportunity because it erupts potassic lavas (SiO₂     

Liquid Lines of Descent of Island Arc Magmas and Genesis of Rhyolites: Evidence from the Shirahama Group, Japan

The Mio-Pliocene Shirahama Group, Izu Peninsula, Central Japan,a well-exposed submarine volcanic arc complex of lava flows,pyroclastic rocks and associated shallow intrusives, is characterizedby a tholeiitic series (basalt to dacite) and a calc-alkalineseries (andesite to dacite). Chemical variations in the tholeiiticseries and calc-alkaline series are consistent with crystalfractionation from basalt and magnesian andesite (boninite),respectively. Crystal–liquid phase relations of thesemagmas have been investigated by study of sample suites fromthese two series. Compositions of liquids in equilibrium withphenocrysts were determined by microprobe grid analyses, inwhich 49 points were averaged in 03 mm 03 mm groundmassareas. The liquid compositions, coupled with the phenocrystmineralogy of the same samples, define the liquid lines of descentof these volcanic arc magmas. Major findings include the following:(1) Crystallization of the tholeiitic series magma is consistentwith early stage crystallization in the simple system Fo–Di–Silica–H₂O,with olivine having a reaction relation to augite and the tholeiiticliquid. (2) The later stage products of the tholeiitic seriesmagma are, however, crystal-poor (<10%) dacites with no maficminerals, suggesting that tholeiitic liquids, hypersthene andaugite were no longer on the cotectic (3) A characteristic ofthe calc-alkaline series magmas is the development of rhyoliticliquids. Hypersthene, augite, plagioclase and Fe–Ti oxideoccur in most calc-alkaline rocks studied, and hornblende andquartz can be found in about half of these. However, their differentiationpaths show that the cotectic relation between quartz and liquidended at a later stage, resulting in the resorption of quartzphenocrysts and ultimately in the formation of quartz-free magmas.(4) The late-stage liquids of both the tholeiitic and calc-alkalineseries have deviated from their cotectics, which cannot be explainedby fractional crystallization alone. The addition of H₂O froman outside system is probably required to explain the differentiationpaths. (5) The formation of chilled margins, the in situ crystallizationof a magma chamber in the solidification zone, and/or the migrationof groundwater into the magma chamber are thought to be likelyprocesses affecting magmas during their migration and intrusioninto the crust. An extreme effect of H₂O addition would be tolower the liquidus temperatures of all precipitating silicatephases far below their restorable range before eruption, resultingin the production of aphyric magmas. Even when a temperaturedecrease in the magma chamber causes a liquid to intersect theliquidus of a pre-existing phase, the addition of H₂O shiftsthe cotectic toward SiO₂, resulting in quartz being the lastphase to crystallize. The resorption of quartz is interpretedto be the result of a liquidus boundary shift caused by theaddition of H₂O. The genesis of aphyric rhyolites is thereforeinferred to result from fractional crystallization followingaddition of H₂0. KEY WORDS: Shirahama Group; Japan; island arc; rhyolite; magma series     

Glass-rich, Cordierite-Biotite Rhyodacite, Valle Ninahuisa, Puno, SE Peru: Petrological Evidence for Hybridization of 'Lachlan S-type' and Potassic Mafic Magmas

The Revancha dyke, a 24·1 Ma,     

Evolution and Genesis of Magmas from Vico Volcano, Central Italy: Multiple Differentiation Pathways and Variable Parental Magmas

Vico volcano has erupted potassic and ultrapotassic magmas,ranging from silica-saturated to silica-undersaturated types,in three distinct volcanic periods over the past 0·5Myr. During Period I magma compositions changed from latiteto trachyte and rhyolite, with minor phono-tephrite; duringPeriods II and III the erupted magmas were primarly phono-tephriteto tephri-phonolite and phonolite; however, magmatic episodesinvolving leucite-free eruptives with latitic, trachytic andolivine latitic compositions also occurred. In Period II, leucite-bearingmagmas (⁸⁷Sr/⁸⁶Sr_initial = 0·71037–0·71115)were derived from a primitive tephrite parental magma. Modellingof phonolites with different modal plagioclase and Sr contentsindicates that low-Sr phonolitic lavas differentiated from tephri-phonoliteby fractional crystallization of 7% olivine + 27% clinopyroxene+ 54% plagioclase + 10% Fe–Ti oxides + 4% apatite at lowpressure, whereas high-Sr phonolitic lavas were generated byfractional crystallization at higher pressure. More differentiatedphonolites were generated from the parental magma of the high-Srphonolitic tephra by fractional crystallization of 10–29%clinopyroxene + 12–15% plagioclase + 44–67% sanidine+ 2–4% phlogopite + 1–3% apatite + 7–10% Fe–Tioxides. In contrast, leucite-bearing rocks of Period III (⁸⁷Sr/⁸⁶Sr_initial= 0·70812–0·70948) were derived from a potassictrachybasalt by assimilation–fractional crystallizationwith 20–40% of solid removed and r = 0·4–0·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (⁸⁷Sr/⁸⁶Sr_initial= 0·71044–0·71052) appear to have been generatedfrom an olivine latite similar to some of the youngest eruptedproducts. A primitive tephrite, a potassic trachybasalt andan olivine latite are inferred to be the parental magmas atVico. These magmas were generated by partial melting of a veinedlithospheric mantle sources with different vein–peridotite/wall-rockproportions, amount of residual apatite and distinct isolationtimes for the veins. KEY WORDS: isotope and trace element geochemistry; polybaric differentiation; veined mantle; potassic and ultrapotassic rocks; Vico volcano; central Italy     

五大连池等地钾质碱性玄武岩、幔源包体的微量元素特征及成因信息

黑龙江省五大连池、科洛、二克山火山岩的成因,是一个重要而又复杂的问题.作者根据岩相学、地球化学、模式计算等一系列工作,认为这组钾质玄武岩是含金云母的尖晶石二辉橄榄岩低度局部熔融的产物。岩浆在上升过程中,经历了结晶分异和同化混染,从而形成一套具成因联系的新生代大陆板内钾质碱性玄武岩。本文试从微量元素方面对此结论提供证据,并显示钾质熔岩及幔源包体的微量元素特征,由此说明地幔交代作用在钾质系列岩浆形成过程中的重要性。     

Petrogenesis of Tertiary Mafic Alkaline Magmas in the Hocheifel, Germany

Primitive nephelinites and basanites from the Tertiary Hocheifelarea of Germany (part of the Central European Volcanic Province;CEVP) have high Mg-number (>0·64), high Cr and Nicontents and strong light rare earth element enrichment butsystematic depletion in Rb, K and Ba relative to trace elementsof similar compatibility in anhydrous mantle. Alkali basaltsand more differentiated magmatic rocks have lower Mg-numberand lower abundances of Ni and Cr, and have undergone fractionationof mainly olivine, clinopyroxene, Fe–Ti oxide, amphiboleand plagioclase. Some nephelinites and basanites approach theSr–Nd–Pb isotope compositions inferred for the EAR(European Asthenospheric Reservoir) component. The Nd–Sr–Pbisotope composition of the differentiated rocks indicates thatassimilation of lower crustal material has modified the compositionof the primary mantle-derived magmas. Rare earth element meltingmodels can explain the petrogenesis of the most primitive maficmagmatic rocks in terms of mixing of melt fractions from anamphibole-bearing garnet peridotite source with melt fractionsfrom an amphibole-bearing spinel peridotite source, both sourcescontaining residual amphibole. It is inferred that amphibolewas precipitated in the asthenospheric mantle beneath the Hocheifel,close to the garnet peridotite–spinel peridotite boundary,by metasomatic fluids or melts from a rising mantle diapir orplume. Melt generation with amphibole present suggests relativelylow mantle potential temperatures (<1200°C); thus themantle plume is not thermally anomalous. A comparison of recentlypublished Ar/Ar ages for Hocheifel basanites with the geochemicaland isotopic composition of samples from this study collectedat the same sample sites indicates that eruption of earlierlavas with an EM signature was followed by the eruption of laterlavas derived from a source with EAR or HIMU characteristics,suggesting a contribution from the advancing plume. Thus, theHocheifel area represents an analogue for magmatism during continentalrift initiation, during which interaction of a mantle plumewith the overlying lithosphere may have led to the generationof partial melts from both the lower lithosphere and the asthenosphere. KEY WORDS: alkali basalts; continental volcanism; crustal contamination; partial melting; Eifel, Germany     

辽河盆地欧利坨子富钾质火山岩特征与成因探讨

欧利坨子位于辽河盆地东部凹陷中段,是新生代古近纪火山活动非常强烈的地区,主要岩性为玄武岩、粗面岩和火山碎屑岩。对该区富钾质火山岩(粗面岩)的喷发特征、喷发年代、地球化学和成因进行了研究。火山口位于该区的中部,属于裂隙式喷发,弧形裂隙的两端喷发强度大,并在晚期形成了破火山口。粗面岩的KAr年代学和地球化学表明,它们形成于沙三时期(38.0～43.0Ma),具有富碱(K2O Na2O=10.22%～12.47%)、富钾(K2O/Na2O=0.69～1.88)、高铝(Al2O3=17.82%～19.44%)、低钛(TiO2=0.19%～0.67%)的特点,属于典型的钾玄岩系。粗面岩的分布受火山裂隙控制,同时也受区域的断裂活动影响。在该区富钾质岩浆喷发前后,华北东部中、新生代构造体制存在由EW向挤压构造向NNE向以伸展为主的构造格局、由板缘向板内构造环境的重大转折,郯庐断裂由左旋走滑变为右旋走滑,辽河盆地在沙三期进入大幅度张裂期。欧利坨子富钾质火山岩的形成是裂谷期岩石圈伸展作用的产物,是玄武质岩浆经过大比例分离结晶作用的产物,可能与晚中生代太平洋板块俯冲残片的部分熔融有密切关系。     

Supra-Subduction Zone Environment and Economic Potential of the Nidar Ophiolite of Indus Suture Zone, Eastern Ladakh, the Himalaya

SUPRA-SUBDUCTION ZONE ENVIRONMENT AND ECONOMIC POTENTIAL OF THE NIDAR OPHIOLITE OF INDUS SUTURE ZONE, EASTERN LADAKH,THE HIMALAYA     

Mantle Processes and Sources of Neogene Slab Window Magmas from Southern Patagonia, Argentina

Neogene plateau lavas in Patagonia, southern Argentina, eastof the volcanic gap between the Southern and Austral VolcanicZones at 46·5° and 49·5°S are linked withasthenospheric slab window processes associated with the collisionof a Chile Ridge segment with the Chile Trench at 12 Ma. Thestrong ocean-island basalt (OIB)-like geochemical signatures(La/Ta <20; Ba/La <20; ⁸⁷Sr/⁸⁶Sr = 0·7035–0·7046;¹⁴³Nd/¹⁴⁴Nd = 0·51290–0·51261; ²⁰⁶Pb/²⁰⁴Pb= 18·3–18·8; ²⁰⁷Pb/²⁰⁴Pb = 15·57–15·65;²⁰⁸Pb/²⁰⁴Pb = 38·4–38·7) of these Patagonianslab window lavas contrast with the mid-ocean ridge basalt (MORB)-like,depleted mantle signatures of slab window lavas elsewhere inthe Cordillera (e.g. Antarctic Peninsula; Baja California).The Patagonian lavas can be divided into a voluminous     

Petrology and Genesis of Hercynian Alkaline Orthogneisses from Provence, France

Major and trace element variations in an Ordovician alkalineorthogneiss (ortholeptynites) series of the Maures massif, France,suggest that the pre-metamorphic rocks were alkaline and peralkalinerhyolites derived by fractional crystallization from trachy-rhyoliticmagma. Differentiation proceeded under low oxygen fugacity conditions,with the production of high-silica comendites. By comparisonwith unaltered alkaline and peralkaline rhyolites, it is possibleto prove that the parental magma was not similar to the metatholeiitesthat are associated with the orthogneisses in the field. Norwere the metarhyolites derived from the alkaline to transitionalmetabasalts of the same age which occur elsewhere in the Mauresmassif. The orthogneisses have geochemical features typicalof alkaline and peralkaline rhyolites occurring with basaltsin continental, anorogenic, bimodal associations. This hypothesisis also supported by zircon morphology and mineral chemistry.The Maures massif was a within-plate continental volcanic province,whose crust underwent doming and thinning, but not break up,during lower Palaeozoic times.     

Density,Viscosity and Velocity (Ascent Rate) of Alkaline Magmas

Three distinct alkaline magmas, represented by shonkinite, lamprophyre and alkali basalt dykes, characterize a significant magmatic expression of rift-related mantle-derived igneous activity in the Mesoproterozoic Prakasam Alkaline Province, SE India. In the present study we have estimated emplacement velocities (ascent rates) for these three varied alkaline magmas and compared with other silicate magmas to explore composition control on the ascent rates. The alkaline dykes have variable widths and lengths with none of the dykes wider than 1 m. The shonkinites are fine- to medium-grained rocks with clinopyroxene, phologopite, amphibole, K-feldspar perthite and nepheline as essential minerals. They exhibit equigranular hypidiomorphic to foliated textures. Lamprophyres and alkali basalts characteristically show porphyritic textures. Olivine, clinopyroxene, amphibole and biotite are distinct phenocrysts in lamprophyres whereas olivine, clinopyroxene and plagioclase form the phenocrystic mineralogy in the alkali basalts. The calculated densities [2.54–2.71 g/cc for shonkinite; 2.61–2.78 g/cc for lamprophyre; 2.66–2.74 g/cc for alkali basalt] and viscosities [3.11–3.39 Pa s for shonkinite; 3.01–3.28 Pa s for lamprophyre; 2.72–3.09 Pa s for alkali basalt] are utilized to compute velocities (ascent rates) of the three alkaline magmas. Since the lamprophyres and alkali basalts are crystal-laden, we have also calculated effective viscosities to infer crystal control on the velocities. Twenty percent of crystals in the magma increase the viscosity by 2.7 times consequently decrease ascent rate by 2.7 times compared to the crystal-free magmas. The computed ascent rates range from 0.11–2.13 m/sec, 0.23–2.77 m/sec and 1.16–2.89 m/sec for shonkinite, lamprophyre and alkali basalt magmas respectively. Ascent rates increase with the width of the dykes and density difference, and decrease with magma viscosity and proportion of crystals. If a constant width of 1 m is assumed in the magma-filled dyke propagation model, then the sequence of emplacement velocities in the decreasing order is alkaline magmas (4.68–15.31 m/sec) > ultramafic-mafic magmas (3.81–4.30 m/sec) > intermediate-felsic magmas (1.76–2.56 m/sec). We propose that SiO₂ content in the terrestrial magmas can be modeled as a semi-quantitative “geospeedometer” of the magma ascent rates.     

Genesis of the Gold Deposit in the Indus-Yarlung Tsangpo Suture Zone, Southern Tibet: Evidence from Geological and Geochemical Data

The Nianzha gold deposit,located in the central section of the Indus-Yarlung Tsangpo suture(IYS) zone in southern Tibet,is a large gold deposit(Au reserves of 25 tons with average grade of 3.08 g/t) controlled by a E-W striking fault that developed during the main stage of Indo-Asian collision(~65-41 Ma).The main orebody is 1760 m long and 5.15 m thick,and occurs in a fracture zone bordered by Cretaceous diorite in the hanging wall to the north and the Renbu tectonic melange in the footwall to the south.High-grade mineralization occurs in a fracture zone between diorite and ultramafic rock in the Renbu tectonic melange.The wall-rock alteration is characterized by silicification in the fracture zone,serpentinization and the formation of talc and magnesite in the ultramafic unit,and chloritization and the formation of epidote and calcite in diorite.Quartz veins associated with Au mineralization can be divided into three stages.Fluid inclusion data indicate that the deposit formed from H_2O-NaCl-organic gas fluids that homogenize at temperatures of 203℃-347℃ and have salinities of 0.35wt%-17.17wt%NaCl equivalent.The quartz veins yield δ~(18)O_(fluid) values of 0.15‰-10.45‰,low δD_(V-SMOW)values(-173‰ to-96‰),and the δ~(13)C values of-17.6‰ to-4.7‰,indicating the ore-forming fluids were a mix of metamorphic and sedimentary orogenic fluids with the addition of some meteoric and mantle-derived fluids.The pyrite within the diorite has δ~(34)S_(V-CDT) values of-2.9‰-1.9‰(average-1.1‰),~(206)Pb/~(204)Pb values of 18.47-18.64,~(207)Pb/~(204)Pb values of 15.64-15.74,and ~(208)Pb/~(204)Pb values of 38.71-39.27,all of which are indicative of the derivation of S and other ore-forming elements from deep in the mantle.The presence of the Nianzha,Bangbu,and Mayum gold deposits within the IYS zone indicates that this area is highly prospective for large orogenic gold deposits.We identified three types of mineralization within the IYS,namely Bangbu-type accretionary,Mayum-type microcontinent,and Nianzha-type ophiolite-associated orogenic Au deposits.The three types formed at different depths in an accretionary orogenic tectonic setting.The Bangbu type was formed at the deepest level and the Nianzha type at the shallowest.     

Underplating of Mesozoic Mantle-derived Magmas in Tongling, Anhui Province: Evidence from Megacrysts and Xenoliths

Lithological observations and mineralogical analyses on pyroxene and hornblende megacrysts and pyroxene and hornblende cumulates in xenoliths in the Mesozoic plutons of the Tongling region, Anhui Province, provide evidence for the magmatic underplating of mantle-derived alkali-olivine basalt at circa 140 Ma. The pyroxene and hornblende megacrysts and cumulates were formed through the AFC process at depths ranging from 27 to 35 km.     

The Genesis of Intermediate and Silicic Magmas in Deep Crustal Hot Zones

A model for the generation of intermediate and silicic igneousrocks is presented, based on experimental data and numericalmodelling. The model is directed at subduction-related magmatism,but has general applicability to magmas generated in other platetectonic settings, including continental rift zones. In themodel mantle-derived hydrous basalts emplaced as a successionof sills into the lower crust generate a deep crustal hot zone.Numerical modelling of the hot zone shows that melts are generatedfrom two distinct sources; partial crystallization of basaltsills to produce residual H₂O-rich melts; and partial meltingof pre-existing crustal rocks. Incubation times between theinjection of the first sill and generation of residual meltsfrom basalt crystallization are controlled by the initial geotherm,the magma input rate and the emplacement depth. After this incubationperiod, the melt fraction and composition of residual meltsare controlled by the temperature of the crust into which thebasalt is intruded. Heat and H₂O transfer from the crystallizingbasalt promote partial melting of the surrounding crust, whichcan include meta-sedimentary and meta-igneous basement rocksand earlier basalt intrusions. Mixing of residual and crustalpartial melts leads to diversity in isotope and trace elementchemistry. Hot zone melts are H₂O-rich. Consequently, they havelow viscosity and density, and can readily detach from theirsource and ascend rapidly. In the case of adiabatic ascent themagma attains a super-liquidus state, because of the relativeslopes of the adiabat and the liquidus. This leads to resorptionof any entrained crystals or country rock xenoliths. Crystallizationbegins only when the ascending magma intersects its H₂O-saturatedliquidus at shallow depths. Decompression and degassing arethe driving forces behind crystallization, which takes placeat shallow depth on timescales of decades or less. Degassingand crystallization at shallow depth lead to large increasesin viscosity and stalling of the magma to form volcano-feedingmagma chambers and shallow plutons. It is proposed that chemicaldiversity in arc magmas is largely acquired in the lower crust,whereas textural diversity is related to shallow-level crystallization. KEY WORDS: magma genesis; deep hot zone; residual melt; partial melt; adiabatic ascent     

The Petrology and Geochemistry of Vesteris Seamount, Greenland Basin--an Intraplate Alkaline Volcano of Non-Plume Origin

Vesteris Seamount is a solitary alkaline volcano in the GreenlandBasin some 280 km NW of Jan Mayen. Topographic and geophysicalstudies have shown no sign of an associated plume trace. Evidencefrom ash layers in sediment cores around the volcano and datingof dredged samples show that it has been active in Quaternarytimes. The lavas from Vesteris studied here consist of basanites,tephrites, mugearite, and alkali basalts. Crystal fractionationmodels are consistent with the generation of the tephrites andmugearite from a basanitic parent. Extensive kaersutite fractionationis required late in the fractionation sequence to produce theextreme mugearite composition. Na-Al-Fe-rich green cores tomany clinopyroxene phenocrysts at Vesteris suggest a fractionationhistory beginning at high pressure in the mantle. Differencesbetween Vesteris and Jan Mayen in the ratios of highly incompatibletrace elements such as Ce/Pb and Rb/Cs, which will not normallybe fractionated from one another during mantle melting, suggestthat the two are not derived from the same source. Relativelyunradiogenic Sr isotope ratios (compared with Bulk Earth), andhighly incompatible trace element patterns similar to thosefor St. Helena, suggest that Vesteris magmas are derived froma depleted, asthenospheric source. We propose that the Vesterisbasanites are very low degree partial melts ({small tilde}1%)of this source, most probably those which give rise to the seismiclow-velocity zone (LVZ). Such small-degree melts may preferentiallytap small-scale heterogeneities in the asthenosphere. Vesterislies at the intersection of two major structural trends in theGreenland Basin—(1) a zone of major reorientation of spreadingdirection on the Mohns Ridge north of Jan Mayen and (2) theextension of the Kolbeinsey Ridge axis. We propose that a combinationof the extensional stress fields related to these two lineamentsproduces sufficient dilation of the lithosphere at Vesteristo allow magmas from the LVZ to reach the surface.     

碱质A型花岗岩的判别、成因与构造环境

碱质A型花岗岩（简称AAG）是A型花岗岩的亚类之一。近年来,随着研究的深入,对AAG有了新的认识。本文总结了AAG独特的岩相学、地球化学特征及产出构造环境,讨论了其判别标志和判别方法,指出对AAG的成因和地球动力学机制的认识目前还存在明显分歧,亟待进一步的深入研究。     

Composition and Deep Sources of Early Devonian Potassic Rocks of the Sakmar Zone,Southern Urals

The isotope–geochemical composition of the Early Devonian magmatic rocks of the Chanchar potassium basaltoid volcano–plutonic Complex in the Sakmar Zone (Southern Urals) has been studied for the first time. The character of the noncoherent element distribution and their ratios suggest that the melts parental to the rocks of extrusive, subvolcanic and intrusive facies were formed from a single source of the primary magma. The low concentrations of HFSE relative to the MORB composition, relatively low εSr, and high εNd suggest the formation of primary melts from the moderately depleted mantle source. The enrichment of rocks with LILE indicates a fluid mantle addition introduced to the melts during evolution of the primary magma.