Temporal variation of mineralogy and petrology in cognate gabbroic enclaves at Arenal volcano,Costa Rica

Gabbroic enclaves ejected during the current eruption phase (A-1) and during the latest prehistoric eruption phase (A-2) of Arenal Volcano show systematic variations in texture, mineralogy and composition as a function of host rock chemistry and timing of eruption. The most differentiated enclaves occur in the more differentiated A-2 lavas. Enclaves in the A-1 volcanics are consistently less evolved. Within the current A-1 eruption, the most mafic enclaves are amphibole-bearing rocks that were erupted during the first 2–3 years of activity (1968–1970). These enclaves occur in the most differentiated A-1 volcanics and are not in equilibrium with their host rocks. They crystallized from a hydrous melt that was slightly more mafic than anything erupted during the current cycle. We interpret the enclaves as sidewall crystallization products of a melt, possibly a high-alumina basalt, that was immediately parental to the A-1 lavas. Enclaves that occur in A-1 rocks erupted after 1970 and all of the A-2 enclaves are amphibole-free and less mafic than the early A-1 enclaves. Their chemistry suggests that they formed during the early to intermediate crystallization of their host lavas. None of the enclaves contain minerals that might have equilibrated with a primary, mantle-derived melt. Geothermometry is consistent with geochemistry, with amphibole-bearing A-1 enclaves yielding the highest pyroxene temperatures (ave. 1090° C) and A-2 enclaves the lowest (ave. 1030° C). Geobarometry suggests mid- to upper crustal depths for the crystallization of all enclaves. The enclaves are cognate and reflect pre-eruptive crystallization of Arenal magmas. They record evolution from a hydrous, basaltic magma to the drier basaltic andesites that characterize the current eruption. Volatiles appear to have been lost due to depressurization during the slow ascent of the magmas through the upper levels of the crust following the initial explosive eruption. Volatile loss and depressurization resulted in the destabilization and the progressive resorption of amphibole. The A-2 lavas may represent the long-term fractionation products of basaltic andesite magmas similar in composition to the A-1 lavas. Anorthitic plagioclase, commonly thought of as a phase stabilized by high Ca/Na and high water pressure, continued to crystallize in a system with relatively low Ca/Na and which had dehydrated and/or depressurized to the point at which amphibole was no longer stable. This suggests that compositional characteristics other than high Ca/Na or high water content may have stabilized the anorthite in the basaltic and basaltic andesite melts at Arenal. We speculate that the high-alumina content of the Arenal magmas may be the stabilizing factor.     

Timescales of magma differentiation from basalt to andesite beneath Hekla Volcano, Iceland: Constraints from U-series disequilibria in lavas from the last quarter-millennium flows

Measurements of ²³⁸U-²³⁰Th-²²⁶Ra disequilibria, Sr-Nd-Pb-Hf isotopes and major-trace elements have been conducted for lavas erupted in the last quarter-millennium at Hekla volcano, Iceland. The volcanic rocks range from basalt to dacite. Most of the lavas (excluding dacitic samples) display limited compositional variations in radiogenic Sr-Nd-Pb-Hf isotopes (⁸⁷Sr/⁸⁶Sr = 0.70319-0.70322; ¹⁴³Nd/¹⁴⁴Nd = 0.51302-0.51305; ²⁰⁶Pb/²⁰⁴Pb = 19.04-19.06; ²⁰⁷Pb/²⁰⁴Pb = 15.53-15.54; ²⁰⁸Pb/²⁰⁴Pb = 38.61-38.65; ¹⁷⁶Hf/¹⁷⁷Hf = 0.28311-0.28312). All the samples possess (²³⁰Th/²³⁸U) disequilibrium with ²³⁰Th excesses, and they show systematic variations in (²³⁰Th/²³²Th) and (²³⁸U/²³²Th) ratios. The highest ²²⁶Ra excesses occur in the basalt and most differentiated andesite lavas, while some basaltic-andesite lavas have (²²⁶Ra/²³⁰Th) ratio that are close to equilibrium. The ²³⁸U-²³⁰Th-²²⁶Ra disequilibria variations cannot be produced by simple closed-system fractional crystallization with radioactive decay of ²³⁰Th and ²²⁶Ra in a magma chamber. A closed-system fractional crystallization model and assimilation and fractional crystallization (AFC) model indicate that the least differentiated basaltic andesites were derived from basalt by fractional crystallization with a differentiation age of ∼24 ± 11 kyr, whereas the andesites were formed by assimilation of crustal material and fractionation of the basaltic-andesites within 2 kyr. Apatite is inferred to play a key role in fractionating the parent-daughter nuclides in ²³⁰Th-²³⁸U and ²²⁶Ra-²³⁰Th to make the observed variations. Our proposed model is that several batches of basaltic-andesite magmas that formed by fractional crystallization of a basaltic melt from a deeper reservoir, were periodically injected into the shallow crust to form individual magma pockets, and subsequently modifying the original magma compositions via simultaneous assimilation and fractional crystallization. The assimilant is the dacitic melt, which formed by partial melting of the crust.     

底侵玄武质下地壳的熔融:来自安徽沙溪adakite质富钠石英闪长玢岩的证据

沙溪石英闪长玢岩具有高Al2O3,Sr/Y,La/Yb，低Y，Yb,Sr正异常，Eu弱负异常－正异常，表现出与adakite岩类似的地球化学特征，与庐枞火山盆地双庙组粗面玄武岩的微量元素特征和Nd-Sr同位素组成类似，研究表明：（1）沙溪侵入岩不是由俯冲的洋壳熔融形成，而是由底侵的玄武质下地壳熔融形成，该玄武质下地壳的物质来源与双庙组玄武岩的来源相似，都为富集地幔；（2）燕山晚期，长江中下游地区可能存在玄武质岩浆的底侵作用，地壳发生垂向增生，沙溪地区地壳厚度曾大于40km，但白垩纪至现在，沙溪地区地壳明显减薄；（3）沙溪铜（金）矿床不是与大洋板片俯冲有关的斑岩铜（金）矿床，而可能是与底侵的玄武质下地壳熔融有关的斑岩铜（金）矿床。     

Sr-Nd-Pb isotopes from the Radicofani Volcano, Central Italy: constraints on heterogeneities in a veined mantle responsible for the shift from ultrapotassic shoshonite to basaltic andesite magmas in a post-collisional setting

The Radicofani Volcano is characterised by few lava flows, a cinder cone and a denudated neck, and is part of the Tuscan Magmatic Province, the northernmost volcanic region of the Italian peninsula. In spite of the short time span of activity, a large time-dependant chemical and isotopic variability is observed. Most of the rocks of the Radicofani volcano are ultrapotassic shoshonites associated to younger basaltic andesites, found at the bottom of the neck. K₂O contents are positively correlated with trace element and isotopic variations. Shoshonitic and high-K calc-alkaline rocks of the Radicofani volcano are significantly different from shoshonites occurring in association with leucite-bearing ultrapotassic rocks in the southernmost portion of the Roman Magmatic Province. The studied rocks are characterised by high, but variable, levels of incompatible trace elements with a subduction-related signature, with troughs at Ba, Ta, Nb, and Ti, and peaks at Cs, K, Th, U, and Pb. Initial values of ⁸⁷Sr/⁸⁶Sr range from 0.71333 to 0.71588, ¹⁴³Nd/¹⁴⁴Nd ranges from 0.512050 to 0.512183, while the lead isotope ratios vary between 18.672 and 18.716 for ²⁰⁶Pb/²⁰⁴Pb, 15.665 to 15.696 for ²⁰⁷Pb/²⁰⁴Pb, and 39.981 to 39.081 for ²⁰⁸Pb/²⁰⁴Pb. Ultrapotassic shoshonites show the highest incompatible trace element contents coupled with the highest ⁸⁷Sr/⁸⁶Sr and the lowest ¹⁴³Nd/¹⁴⁴Nd. On the basis of geochemical and isotopic signatures it is argued that magmas were generated in a modified lithospheric peridotitic source containing metasomatic veins generated by K-rich melts from recycled sediments within the mantle via subduction. A further metasomatic event generated by slab-derived fluids pervasively enriched the peridotitic source. Partial melting of the veins produced leucite-free ultrapotassic magmas (i.e. lamproite), and was triggered by rising of the isotherms after the orogenic front migrated eastward in the Italian Peninsula. Further rise of the isotherms induced larger degrees of partial melting inducing melting of the surrounding wall peridotite. The variation of the degree of partial melting of such a heterogeneous peridotitic source produced a wide spectrum of magma compositions, which mimic a mixing line between two components: ultrapotassic magma from partial melting of the metasomatic vein and a basaltic andesitic magma from partial melting of the surrounding peridotite.     

Petrogenesis of Ultramafic Xenoliths from the 1800 Kaupulehu Flow, Hualalai Volcano, Hawaii

The 1800 Kaupulehu flow on Hualalai Volcano, Hawaii, containsabundant xcnoliths of dunitc, wehrlite, and olivine clinopyroxenitewith minor gabbro, troctolite, anorthosite, and wcbstcrite.The petrography and mineral compositions of 41 dunite, wehrlite,and olivine clinopyroxenite xenoliths have been studied, andclinopyroxene separates from eight of these have been analyzedfor Ba, K, Rb, Sr, rare earth elements, ⁸⁷Sr/⁸⁶Sr, and ¹⁴³Nd/¹⁴⁴Nd.Temperatures of equilibration obtained by olivine-spinel andpyroxene geothermometry range from 1000 to 1200 C. Mineralogicaldata combined with published fluid inclusion data indicate depthsof origin in the range of 8–30 km. The rarity of orthopyroxene, the presence of Fe-rich olivine(Fo⁸¹–⁸⁹) and clinopyroxene (Fs⁵–¹²), and the occurrenceof high TiO² in spinel (0.9–2.8 wt.%) and clinopyroxene(035–1.33 wt%) all indicate that the xenoliths are cumulates,not residues from partial fusion. The separated clinopyrox-eneshave ⁸⁷Sr/⁸⁶Sr (0-70348.0-70367) and ¹⁴³Nd/¹⁴⁴Nd (0.51293–0.51299)values that are different from Sr and Nd isotope ratios of Pacificabyssal basalts (>0.7032 and >0-5130, respectively). Also,clinopyroxenes and spinels in the xenoliths have generally higherTiO₂ contents (>O.35 and >0.91 wt.%, respectively) thantheir counterparts in abyssal cumulates (<0.40 and <0.70wt%,respectively). These differences indicate that the xenolithsare not a normal component of oceanic crust. Because the xenoliths and alkalic to transitional Hualalai lavashave similar values for Cr/(Cr + Al) and Cr/(Cr + Al + Fe³⁺)of spinels, ⁸⁷Sr/⁸⁶Sr of clinopyroxenes, and whole-rock ³He/⁴He,we conclude that the xenoliths are cumulates from such magmas.Multiple parental magmas for the xenoliths are indicated byslightly heterogeneous ⁸⁷Sr/⁸⁶Sr of clinopyroxene separates.Depths of formation of the xenoliths are estimated to be {smalltilde}8–30 km. Extensive crystallization of olivine in the absence of pyroxenesand plagioclase is a characteristic and prominent feature ofHawaiian tholeiitic magmatism. Dunite xenoliths crystallizedfrom alkalic magmas have previously been reported from MaunaKea Volcano (Atwill & Garcia, 1985) and Loihi Seamount (Clague,1988). Our finding of an alkalic signature for dunite xenolithsfrom a third Hawaiian volcano, Hualalai, shows that early olivinecrystallization should be considered a characteristic not justof Hawaiian tholeiitic magmatism but also of Hawaiian alkalicmagmatism.     

Petrogenesis of Dunite Xenoliths from Koolau Volcano, Oahu, Hawaii: Implications for Hawaiian Volcanism

Ultramafic xenoliths from Koolau Volcano on the island of Oahu,Hawaii, are divided into spinel lherzolite, pyroxenite, anddunite suites. On the basis of a study of the petrography andmineral compositions of 43 spinel lherzolites, 12 pyroxenites,and 20 dunites, the following characteristics of the dunitesin relation to the other nodule types and to Hawaiian lavasemerge. (1) The forstente content of olivines in the Koolaudunites (Fo_82.6-Fo_{89 7} ) overlap those of Hawaiian tholeiiticand alkalic lavas and are generally lower than those in abyssallherzolites and dunites and in Koolau spinel lherzolites. (2)Most of the dunites contain no orthopyroxene, all except twocontain chrome spinel, and a few contain interstitial plagioclaseand clinopyroxene. (3) Chrome spinels from the Koolau dunitesare distinctly higher in Cr/(Cr+Al), lower in Mg/(Mg+ Fe²⁺)and higher in TiO₂ than those from abyssal basalts and peridotites.Chrome spinels in the dunites correspond closely in compositionto chrome spinels in Hawaiian tholeiitic and alkalic lavas.(4) The abundance of dunite relative to other nodule types decreasesoutward from the central part of the volcano. The dunites areinterpreted not as residues of partial fusion of the mantlebut as crystal accumulations stored at shallow depths beneaththe central part of Koolau Volcano and derived from picriticmagmas parental to the shield-building tholeiitic lavas.     

Metasomatism and Partial Melting in Upper-Mantle Peridotite Xenoliths from the Lashaine Volcano, Northern Tanzania

A group of chrome-spinel peridotite upper-mantle xenoliths fromthe Lashaine volcano, northern Tanzania, differs from otherxenoliths at this locality in containing glassy melt pockets.Modal, mineral chemical and isotopic evidence indicates that,before the melting that was coincident with the xenolith entrainmentand eruption in the Pleistocene, the sub-Tanzanian mantle lithospherehad a complex history. A major element depletion at     

Geochemistry of Rock-Forming Minerals in Mantle Xenoliths from Basalts of Sverre Volcano,Spitsbergen Archipelago

The paper reports the results of SIMS and SEM-EDS study of rock-forming minerals from melt pockets in the central part of a spinel peridotite xenolith taken from Quaternary alkaline basalts of Sverre Volcano in the northwestern part of West Spitsbergen Island. Olivine and clinopyroxene are analyzed to trace changes related to the metasomatic interaction between spinel lherzolite and a carbonate melt with formation of corresponding secondary minerals and silicate glass. It is established that the metasomatic interaction of the carbonate melt with minerals of host spinel lherzolite is accompanied by partial recrystallization of olivine and clinopyroxene, or crystallization of the second generation of these minerals. Percolating carbonate melt caused significant changes in the major, trace, and rare-earth element composition of the considered minerals, thus placing constraints on the use of the composition of these minerals for calculation of P–T parameters, estimating equilibrium, and modeling petrological processes in mantle.     

Petrology and Mineral Chemistry of Plutonic Igneous Xenoliths from the Carbonatite Volcano, Oldoinyo Lengai, Tanzania

Magmatic plutonic rocks in the Oldoinyo Lengai pyroclasticsare jacupirangite, pyroxenite, ijolite, nepheline syenite andwollastonitite. Mainly cumulates, they are combinations of nepheline,clinopyroxene, Ti-andradite, spinel (sensu lato), apatite, perovskite,titanite, wollastonite, sulphides, mica, glass and K-rich feldspar,most of which are strongly zoned. Low analytical sums for glasses,vesiculation of intergranular glasses, and the generally explosivenature of the volcanicity point to significant concentrationof dissolved volatiles in the parent magma; the absence of hydrousphases suggests that the dominant volatile is CO₂. Cumulatetextures, widely variable modes, veining and variation in specimenconsolidation and metasomatism all indicate derivation froma structurally complicated and multiply injected sub-volcaniccomplex. Complex zoning of phases and mineral disequilibrium is attributedto convective percolation of fluids through permeable cumulates,possibly complicated by magma replenishment during crystallizationof individual magma batches. Olivine, mica and pyroxene megacrystsin some ijolites indicate polybaric crystallization; mixingof potassic and sodic magmas may be the cause of these megacryst-bearingijolites, but the main parent is highly evolved, of carbonatedijolitic (nephelinitic) composition and with Nd and Sr isotopecharacteristics slightly more depleted than Bulk Earth. KEY WORDS: xenoliths; ijolite; jacupirangite; nepheline syenite ^*Corresponding author. Telephone 031 650 4837. Fax: 031 668 3184. e-mail: jbdawson{at}glg.ed.ac.uk     

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.     

Fluid venting activity on the Costa Rica margin: new results from authigenic carbonates

Carbonate precipitates on mounds and along tectonic scarps off the Costa Rica margin are manifestations of subduction-induced dewatering. The long-term dewatering history is recorded in mineralogical, petrological and isotope signals of carbonates recovered from these sites. The carbonates are strongly depleted in ¹³C (–11 to –53 PDB) and enriched in ¹⁸O (+4 to +8 PDB). Thermogenic methane and biogenic methane were identified as sources of the carbon. Chemoherm carbonates and seepage-associated carbonates formed in a focused flow regime have lighter ¹³C values, while others formed in a more diffusive flow regime have slightly enriched C isotope values. Three fluid components were inferred based on the calculation of equilibrium ¹⁸O: clay dehydration water, gas hydrate water and seawater. Calculated equilibrium ¹⁸O values of carbonates from different down-core depths as well as from different precipitation stages show that the ¹⁸O of the precipitating fluid is progressively depleted with time. Dolostones showing a methane-C source and a well constrained O-isotope signature are thought to have formed at depth in the sediment and subsequently became exhumed. Glauconitic sandstones cemented by methane-derived carbonate provide evidence that fluid and solid material have been expelled by the mud volcano.     

Clast shape and textural associations in peperite as a guide to hydromagmatic interactions: Upper Permian basaltic and basaltic andesite examples from Kiama,Australia

Upper Permian shallow marine siltstone and sandstone units of the Broughton Formation are intercalated with basaltic and basaltic andesite sheets at Kiama, New South Wales. Parts of the two sheets examined in this study display peperite texture. The lower example (Blow Hole Latite Member) can be divided into two units with peperitic contacts suggesting their intrusion into wet unconsolidated sediments of the overlying Kiama Sandstone Member. The Bumbo Latite Member overlies the Kiama Sandstone Member and has been interpreted by previous workers as a lava. Well‐developed columnar joints cut the interior of the sheets. Along contacts with sedimentary facies and peperitic dykes which penetrate the sheets, columnar joints merge into a several metre‐wide zone of blocky jointing, pseudo‐pillows and hyaloclastite. In peperitic facies, sandstone or siltstone fills joints and fractures that define pseudo‐pillows, polyhedral joint blocks and columns (closely packed fabric) or sediment matrix‐rich breccia contains fragments and apophyses of basalt and basaltic andesite (dispersed fabric). Along some contacts, peperite with dispersed fabric passes through a zone of closely packed peperite into coherent facies. Alternatively, closely packed peperite passes directly into coherent facies. Examples of peperite with more than one clast type (globular, blocky, platy), and involving sedimentary matrix of constant grain‐size, are common. In some examples, globular surfaces formed during an early, low‐viscosity phase of magma emplacement into wet sediment. Planar and curviplanar fractures cut some globular surfaces suggesting that these formed slightly later as the magma became more viscous (cooler) and/or vapour films at the magma‐sediment interface broke down. However, the complexities of peperite, in respect to clast types, abundances and distribution, as well as grainsize and structures in the sedimentary component, suggest that a spectrum of fragmentation and mixing processes were involved in fragmenting the sheets. Many peperitic domains include poorly and strongly vesicular parts, resulting in apparent polymictic breccias. Vesiculation of the sheets is interpreted to have occurred in two phases: an early degassing of primary magmatic volatiles and a later, scoria‐forming event, both of which progressed as the magma mixed with unconsolidated sediment. During the later phase, magma incorporated limited amounts of steam from the wet sediment and a vesicular front propagated out into the magmatic component. Confining pressures were insufficient to prevent vesiculation of the magma or to suppress fluidisation of the host sediment along magma‐sediment contacts, but large enough to inhibit large‐scale steam explosivity. Displacement of sediment along contacts may have reduced confining pressures sufficiently to promote vaporisation of pore water, and induce local vesiculation of the magma.     

Geochemistry and Evolution of Subcontinental Lithospheric Mantle in Central Europe: Evidence from Peridotite Xenoliths of the Kozakov Volcano, Czech Republic

Neogene basanite lavas of Kozákov volcano, located alongthe Lusatian fault in the northeastern Czech Republic, containabundant anhydrous spinel lherzolite xenoliths that providean exceptionally continuous sampling of the upper two-thirdsof central European lithospheric mantle. The xenoliths yielda range of two-pyroxene equilibration temperatures from 680°Cto 1070°C, and are estimated to originate from depths of32–70 km, based on a tectonothermal model for basalticunderplating associated with Neogene rifting. The sub-Kozákovmantle is layered, consisting of an equigranular upper layer(32–43 km), a protogranular intermediate layer that containsspinel–pyroxene symplectites after garnet (43–67km), and an equigranular lower layer (67–70 km). Negativecorrelations of wt % TiO₂, Al₂O₃, and CaO with MgO and clinopyroxenemode with Cr-number in the lherzolites record the effects ofpartial fusion and melt extraction; Y and Yb contents of clinopyroxeneand the Cr-number in spinel indicate 5 to 15% partial melting.Subsequent metasomatism of a depleted lherzolite protolith,probably by a silicate melt, produced enrichments in the largeion lithophile elements, light rare earth elements and highfield strength elements, and positive anomalies in primitivemantle normalized trace element patterns for P, Zr, and Hf.Although there are slight geochemical discontinuities at theboundaries between the three textural layers of mantle, theretends to be an overall decrease in the degree of depletion withdepth, accompanied by a decrease in the magnitude of metasomatism.Clinopyroxene separates from the intermediate protogranularlayer and the lower equigranular layer yield ¹⁴³Nd/¹⁴⁴Nd valuesof 0·51287–0·51307 (_Nd = +4·6 to+8·4) and ⁸⁷Sr/⁸⁶Sr values of 0·70328–0·70339.Such values are intermediate with respect to the Nd–Srisotopic array defined by anhydrous spinel peridotite xenolithsfrom central Europe and are similar to those associated withthe present-day low-velocity anomaly in the upper mantle beneathEurope. The geochemical characteristics of the central Europeanlithospheric mantle reflect a complex evolution related to Devonianto Early Carboniferous plate convergence, accretion, and crustalthickening, Late Carboniferous to Permian extension and gravitationalcollapse, and Neogene rifting, lithospheric thinning, and magmatism. KEY WORDS: xenoliths; lithospheric mantle; REE–LILE–HFSE; Sr–Nd isotopes; Bohemian Massif     

Modal mineralogy and chemical composition of the Uralide lower crust determined from physical properties data

Modeling of rock physical properties of the Uralide lower crust is carried out using the Physprops Excel worksheet, which calculates its P-wave (Vp), S-wave (Vs), Poisson's ratio (σ), and density (ρ). The mineral modal abundance and composition of mafic granulite, mafic garnet granulite, amphibolite, and gabbronorite is modeled, and the composition of each is determined by converting the aggregate mineralogy into wt.% oxides. The modeled wt.% oxide composition of mafic (+ garnet) granulite fit well with those estimated for post-Archean terrains, and all are within 1σ of the average of all lower crustal xenoliths. The modeled gabbronorite also shows a reasonable fit with these estimates. The modeled amphibolite compositions show variations in the wt.% SiO₂, FeO and MgO with respect to the estimates of mafic granulite and amphibolite. The differences in the composition across the Uralide lower crust are largely due to the presence or absence of garnet in the mafic granulite, which may be a result of higher temperatures having been reached in the interior part of the orogen.     

Chemical and isotopic composition of the lower crust beneath the Meguma Lithotectonic Zone,Nova Scotia: evidence from granulite facies xenoliths

Granulite-facies xenoliths from an Upper Devonian lamprophyre dyke near Tangier, Nova Scotia, provide new information about the lower crust in the Meguma Zone. Two mineralogically and chemically distinet groups of xenoliths occur. Both groups contain quartz+feldspar+biotite+Fe–Ti oxides+sulfides. In addition, group A contains garnet+graphite±[aluminosilicates+spinel±sapphirine (hight Al₂O₃ subgroups A1 and A2)] or [clinopyroxene+sphene+apatite (high CaO sub-group A3)]. Group B has highly variable proportions of orthopyroxene (B1), clinopyroxene (B2), and amphibole (B3). Trace-element contents of the highly aluminous xenoliths compare closely with average to upper crustal model compositions and are similar in many aspects to other undepleted granulite-facies rocks. LowP-T sedimentary assemblages of quartz-chlorite-clay minerals-calcite-albite or paragonite can account for the compositions of group A xenoliths. Within group B, a high-MgO (MgO>13 wt%) subgroup with high transition-metal contents, and low-MgO (MgO<9 wt%) sub-groups with higher LIL (large-ion-lithophile) element contents exist. Although the rare-earth and high-field-strength elements indicate a tholeiitic or low-K calc-alkaline chemistry, the LIL elements are as high as those from high-K calc-alkaline volcanic are rocks. Isotopically, group A ranges from Nd_t=-2.56 to-0.80 and⁸⁷Sr/⁸⁶Sr _t =0.7046 to 0.7182 fort=370 Ma. For group B these values are +1.45 to +5.33 and 0.7028 to 0.7048, respectively. Model ages (T_CHUR) are correspondingly low and are tightly constrained (544±52 Ma). These young ages contrast with the middle Proterozoic Nd model ages of the overlying Meguma Zone low-grade flysch. This time-inverted stratigraphy appears to be the product of a tectonic break between a younger autochthonous Tangier lower crust (Avalon), and an older allochthonous Meguma Group upper crust.     

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     

Magmatic processes that generate chemically distinct silicic magmas in NW Costa Rica and the evolution of juvenile continental crust in oceanic arcs

Northwestern Costa Rica is built upon an oceanic plateau that has developed chemical and geophysical characteristics of the upper continental crust. A major factor in converting the oceanic plateau to continental crust is the production, evolution, and emplacement of silicic magmas. In Costa Rica, the Caribbean Large Igneous Province (CLIP) forms the overriding plate in the subduction of the Cocos Plate—a process that has occurred for at least the last 25 my. Igneous rocks in Costa Rica older than about 8 Ma have chemical compositions typical of ocean island basalts and intra-oceanic arcs. In contrast, younger igneous deposits contain abundant silicic rocks, which are significantly enriched in SiO₂, alkalis, and light rare-earth elements and are geochemically similar to the average upper continental crust. Geophysical evidence (high Vp seismic velocities) also indicates a relatively thick (~40 km), addition of evolved igneous rocks to the CLIP. The silicic deposits of NW Costa Rica occur in two major compositional groups: a high-Ti and a low-Ti group with no overlap between the two. The major and trace element characteristics of these groups are consistent with these magmas being derived from liquids that were extracted from crystal mushes—either produced by crystallization or by partial melting of plutons near their solidi. In relative terms, the high-Ti silicic liquids were extracted from a hot, dry crystal mush with low oxygen fugacity, where plagioclase and pyroxene were the dominant phases crystallizing, along with lesser amounts of hornblende. In contrast, the low-Ti silicic liquids were extracted from a cool, wet crystal mush with high oxygen fugacity, where plagioclase and amphibole were the dominant phases crystallizing. The hot-dry-reducing magmas dominate the older sequence, but the youngest sequence contains only magmas from the cold-wet-oxidized group. Silicic volcanic deposits from other oceanic arcs (e.g., Izu-Bonin, Marianas) have chemical characteristics distinctly different from continental crust, whereas the NW Costa Rican silicic deposits have chemical characteristics nearly identical to the upper continental crust. The transition in NW Costa Rica from mafic oceanic arc and intra-oceanic magma to felsic, upper continental crust-type magma is governed by a combination of several important factors that may be absent in other arc settings: (1) thermal maturation of the thick Caribbean plateau, (2) regional or local crustal extension, and (3) establishment of an upper crustal reservoir.     

Generation of andesite through partial melting of basaltic metasomatites in the mantle wedge:Insight from quantitative study of Andean andesites

Continental crust in average exhibits a similar composition in both major and trace elements to andesites along ac-tive continental margins. For this reason, an...     

Modeling a tsunami from the Nicoya,Costa Rica,seismic gap and its potential impact in Puntarenas

Although subduction zones around the world are known to be the source of earthquakes and/or tsunamis, not all segments of these plate boundaries generate destructive earthquakes and catastrophic tsunamis. Costa Rica, in Central America, has subduction zones on both the Pacific and the Caribbean coasts and, even though large earthquakes (Mw = 7.4–7.8) occur in these convergent margins, they do not produce destructive tsunamis. The reason for this is that the seismogenic zones of the segments of the subduction zones that produce large earthquakes in Costa Rica are located beneath land (Nicoya peninsula, Osa peninsula and south of Limón) and not off shore as in most subduction zones around the world. To illustrate this particularity of Costa Rican subduction zones, we show in this work the case for the largest rupture area in Costa Rica (under the Nicoya peninsula), capable of producing Mw ～ 7.8 earthquakes, but the tsunamis it triggers are small and present little potential for damage even to the largest port city in Costa Rica.The Nicoya seismic gap, in NW Costa Rica, has passed its ～50-year interseismic period and therefore a large earthquake will have to occur there in the near future. The last large earthquake, in 1950 generated a tsunami which slightly affected the southwest coast of the Nicoya Peninsula. We present here a simulation to study the possible consequences that a tsunami generated by the next Nicoya earthquake could have for the city of Puntarenas. Puntarenas has a population of approximately eleven thousand people and is located on a 7.5 km long sand bar with a maximum height of 2 m above the mean sea level. This condition makes Puntarenas vulnerable to tsunamis.