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1.
The British Tertiary Volcanic Province (BTVP) comprises within-platecentral igneous complexes associated with plateau lavas andregional dyke swarms. Lundy is the southernmost complex of theBTVP and comprises granite ({small tilde}90%) emplaced intodeformed Devonian sedimentary rocks within the Hercynian Cornubiangranite province of southwest England. The complex is intrudedby a northwest-southeast trending dyke swarm. In common withother BTVP igneous complexes, Lundy is associated with positivegravity and magnetic anomalies which are interpreted in termsof the presena of an underlying basic intrusion at shallow depth,with a volume exceeding that of the overlying granite. The Lundy intrusion is a coarse-grained megacrystic granitecontaining up to 20% alkali feldspar megacrysts in a coarse-grainedgroundmass composed of alkali feldspar, quartz, lithium-bearingmuscovite, and biotite (lithian siderophyllite),with a range of aaxssory minerals. The main granite has a coarse-grained(locally miarolitic) pegmatitic facies and is intruded by thinsheets and veins of fine-grained aplite and microgranite. Themineralogy indicates crystallization of the Lundy granite froma highly fractionated H2O- and halogen-rich magma at a relativelyshallow crustal level. The main Lundy granite is a peraluminous leucogranite with Na2O=34%,K2O{small tilde}5%, low TiO2, MeO, CaO, Zr, and Sr, and highRb and Rb/Sr in comparison with many other peralurninous granites,including those from the Cornubian batholith and the BTVP. Anew Rb-Sr whole-rock isochron for the granite yields an ageof 58?7?1?6 Ma with an initial 87Sr/86Sr of 0?715?0?006. Ndvalues for the granite (0?9 to 1?9) plot betweencontemporaneous mantle (positive Nd and Cornubian granites (Nd=ca.11). The trace element data (Rb, Y, Nb) show affinities with syn-collisionand within-plate granites. As the Sr isotope data indicate amajor crustal component, and the Nd isotope data suggest bothmantle and crustal components, we propose that the Lundy graniteis derived from a parental magma comprising crustal components(derived from a similar source to that of the Cornubian granitebatholith) and a mantle-derived component (derived from a differentiateof contemporaneous basaltic magma This magma experienced fractionalcrystallization of plagioclase, alkai feldspar, Fe-Mg minerals,and REE-bearing accessory minerals before emplacement, and theLundy granite experienced further in situ fractional crystallization,associateded with crustal contamination by the Devonian shaleafter emplacement. 相似文献
2.
BRUGMANN G. E.; HANSKI E. J.; NALDRETT A. J.; SMOLKIN V. F. 《Journal of Petrology》2000,41(12):1721-1742
The Palaeoproterozoic NiCu sulphide deposits of the PechengaComplex, Kola Peninsula, occur in the lower parts of ferropicriticintrusions emplaced into the phyllitic and tuffaceous sedimentaryunit of the Pilgujärvi Zone. The intrusive rocks are comagmaticwith extrusive ferropicrites of the overlying volcanic formation.Massive lavas and chilled margins from layered flows and intrusionscontain <37 ng/g Pd and Pt and <0·022·0ng/g Ir, Os and Ru with low Pd/Ir ratios of 511. Theabundances of platinum group elements (PGE) correlate with eachother and with chalcophile elements such as Cu and Ni, and indicatea compatible behaviour during crystallization of the parentalmagma. Compared with the PGE-depleted central zones of differentiatedflows (spinifex and clinopyroxene cumulate zones) the olivinecumulate zones at the base contain elevated PGE abundances upto 10 ng/g Pd and Pt. A similar pattern is displayed in intrusivebodies, such as the Kammikivi sill and the Pilgujärvi intrusion.The olivine cumulates at the base of these bodies contain massiveand disseminated NiCu-sulphides with up to 2 µg/gPd and Pt, but the PGE concentrations in the overlying clinopyroxenitesand gabbroic rocks are in many cases below the detection limits.The metal distribution observed in samples closely representingliquid compositions suggests that the parental magma becamesulphide saturated during the emplacement and depleted in chalcophileand siderophile metals as a result of fractional segregationof sulphide liquids. Relative sulphide liquidsilicatemelt partition coefficients decrease in the order of Ir >Rh > Os > Ru > Pt = Pd > Cu. R-factors (silicate-sulphidemass ratio) are high and of the order of 104105, andthey indicate the segregation of only small amounts of sulphideliquid in the parental ferropicritic magma. In differentiatedflows and intrusions the sulphide liquids segregated and accumulatedat the base of these bodies, but because of a low silicatesulphidemass ratio the sulphide liquids had a low PGE tenor and Pt/Irand Cu/Ir ratios similar to the parental silicate melts. Duringcooling the sulphide liquid crystallized 4050% of monosulphidesolid solution (mss) and the residual sulphide liquid becameenriched in Cu, Pt and Pd and depleted in Ir, Os and Ru. TheCu-rich sulphide liquid locally assimilated components of thesurrounding S-rich sediments as suggested by the radiogenicOs isotopic composition of some sulphide ores ( 相似文献
3.
The Cretaceous Igneous Province of Madagascar: Geochemistry and Petrogenesis of Lavas and Dykes from the Central-Western Sector 总被引:1,自引:0,他引:1
The Cretaceous lava sequence and associated mafic dyke swarmin centralwestern Madagascar (Mailaka and Bemaraha areas)range in composition from picrite basalts to cordieriteorthopyroxene-bearingrhyodacites (MgO from 14 to 0·6 wt %). Petrographic andchemical data indicate the presence of both tholeiitic and transitionalmagma series, with variable degree of rare earth element enrichment[(La/Nd)n = 11·4 for tholeiites vs (La/Nd)n =0·651 for transitional rocks]. Initial (at 88Ma) 87Sr/86Sr and 相似文献
4.
Hybridization of a Shallow 'I-type' Granitoid Pluton and its Host Migmatite by Magma-Chamber Wall Collapse: the Tokuwa Pluton, Central Japan 总被引:1,自引:0,他引:1
The Miocene Tokuwa pluton of I-type granitoidaffinity was emplaced discordantly into a Cretaceous to Paleogeneaccretionary complex and induced a contact aureole in whichvarious thermally metamorphosed rocks were developed, includinghornfels, metatexite, diatexite and cordierite-bearing tonalite(Crd-tonalite) of S-type granite affinity. Thethermally metamorphosed rocks show low-pressure reaction texturesculminating in partial melting. Peak PT conditions of3 kbar at 780°C are estimated on the basis of the TWQ thermobarometerfor the garnet-bearing rocks. The rocks in the contact aureoleexhibit a gradual transition from hornfels, through metatexiteand diatexite to Crd-tonalite. The Sr-isotopic composition atthe time of Tokuwa pluton emplacement at 12 Ma decreases systematicallyfrom metatexite (0·71000·7112) throughdiatexite (0·70780·7094) to Crd-tonalite(0·70670·7068); this trend is interpretedin terms of mixing between the Tokuwa magma and the aureolemigmatites. The field relationships, geochemical data, and isotopicdata collectively suggest that the emplacement of the Tokuwapluton triggered partial melting of the surrounding metasedimentaryrocks. Subsequent hybridization of the Tokuwa magma with themetatexite in variable proportions produced the Crd-tonaliteand diatexite. The hybridization was caused by invasion of theTokuwa magma into the migmatite zone, accompanied by gravitationalcollapse of the previously crystallized wall of the magma chamber.The data presented demonstrate that even a relatively low-temperature,shallow, I-type granitoid pluton can induce contactanatexis and hybrid S-type granitoid formationat the intrusive contact. KEY WORDS: contact metamorphism; hybridization; magmahost-rock interaction; migmatite; S-type granitoid 相似文献
5.
Magmatic Evolution of the La Pacana Caldera System, Central Andes, Chile: Compositional Variation of Two Cogenetic, Large-Volume Felsic Ignimbrites 总被引:10,自引:3,他引:10
LINDSAY J. M.; SCHMITT A. K.; TRUMBULL R. B.; DE SILVA S. L.; SIEBEL W.; EMMERMANN R. 《Journal of Petrology》2001,42(3):459-486
La Pacana is one of the largest known calderas on Earth, andis the source of at least two major ignimbrite eruptions witha combined volume of some 2700 km3. These ignimbrites have stronglycontrasting compositions, raising the question of whether theyare genetically related. The Toconao ignimbrite is crystal poor,and contains rhyolitic (7677 wt % SiO2) tube pumices.The overlying Atana ignimbrite is a homogeneous tuff whose pumiceis dacitic (6670 wt % SiO2), dense (4060% vesicularity)and crystal rich (3040 % crystals). Phase equilibriaindicate that the Atana magma equilibrated at temperatures of770790°C with melt water contents of 3·14·4wt %. The pre-eruptive Toconao magma was cooler (730750°C)and its melt more water rich (6·36·8 wt% H2O). A pressure of 200 MPa is inferred from mineral barometryfor the Atana magma chamber. Isotope compositions are variablebut overlapping for both units (87Sr/86Sri 0·70940·7131;143Nd/144Nd 0·512220·51230) and are consistentwith a dominantly crustal origin. Glass analyses from Atanapumices are similar in composition to those in Toconao tubepumices, demonstrating that the Toconao magma could representa differentiated melt of the Atana magma. Fractional crystallizationmodelling suggests that the Toconao magma can be produced by30% crystallization of the observed Atana mineral phases. Toconaomelt characteristics and intensive parameters are consistentwith a volatile oversaturation-driven eruption. However, thelow H2O content, high viscosity and high crystal content ofthe Atana magma imply an external eruption trigger. KEY WORDS: Central Andes; crystal-rich dacite; eruption trigger; high-silica rhyolite; zoned magma chamber 相似文献
6.
The Younger Andesites and Dacites of Iztacc?huatl volcano, Mexico,constitute a medium-K calcalkaline rock suite (5866 wt.per cent SiO2) characterized by high Mg-numbers (100Mg/(Mg+0?85Fe2+=5566) and relatively high abundances of MgO (2?56?6wt. per cent), Ni(17158 p.p.m.), and Cr (42224p.p.m.). Chemical stratigraphy plots of eruptive sequences indicatethe existence of a plexus of long-lived dacite magma chambersperiodically replenished by influxes of basaltic magma ascendingfrom depth. Short-term geochemical evolution after batch influxwas dictated by magma mixing and eventual dilution of the basalticcomponent by quasi-steady state hornblende dacitemagma. The chemical data support textural and mineralogicalevidence for rapid homogenization of originally diverse magmasby convective blending of residual liquids accompanied by dynamicfractional crystallization (Nixon, 1988). Internally-consistent mixing calculations used to derive thecomposition of basaltic magma influx incorporate analyticaluncertainties and the observed range of salic end-member compositions.Mafic end-members are basalts to basaltic andesites (5254wt. per cent SiO2) with Mg-numbers (7376), MgO (911wt. per cent), Ni (250 p.p.m.), and Cr (340510 p.p.m.)concentrations, and liquidus olivine compositions (Fo9088),appropriate for unfractionated partial melts of mantle peridotite.The majority of model compositions are Ol-Hy-normative, similarto those of primitive basaltic lavas on the flanks of Iztacc?huatland in the Valley of Mexico. However, calculated magma batchesrange from weakly Qz-normative to strongly Ne-normative. Bothcalculated and analyzed basaltic compositions are distinguishedby highly variable abundances of alkalies and incompatible traceelements, notably Rb, Ba, Sr, P, Zr, and Y. Initial 87Sr/86Sr ratios for Iztacc?huatl lavas (0?70400?7046;n=24) are comparable to those for primitive basaltic rocks (0?70370?7045;?=4) and indicate that (1) mantle source regions are isotopicallyheterogeneous; and (2) contamination of iztacc?huatl magma chambersby radiogenic crustal rocks was not a significant factor inthe evolution of calc-alkaline andesites and dacites. The replenishment of Iztacc?huatl dacite reservoirs by Ne-normativemagmas late in the history of cone growth precludes exhaustionof mantle source regions by progressive partial melting. Thewaning stages of volcanic activity at Iztacc?huatl appear toreflect the inability of dense basaltic influxes to successfullypenetrate a large high-level chamber of low density hornblendedacite magma. 相似文献
7.
Chemistry of the Shiant Isles Main Sill, NW Scotland, and Wider Implications for the Petrogenesis of Mafic Sills 总被引:3,自引:0,他引:3
Major and trace element data for the Tertiary, Shiant IslesMain Sill, NW Scotland, are used to discuss its complex internaldifferentiation. Vertical sections through the sill exhibitsharp breaks in chemistry that coincide with changes in texture,grain size and mineralogy. These breaks are paired, top andbottom, and correspond to the boundaries of intrusive units,confirming a four-phase multiple-intrusion model based on fieldrelations, petrography, mineralogy and isotopes. Whole-rockchemistry is consistent with this model and necessitates onlyminor revisions to the intrusive and differentiation mechanismspreviously proposed. The rocks contain strongly zoned minerals(e.g. olivine Fo705, clinopyroxene Mg# = 755,plagioclase An755) indicating almost perfect fractionalcrystallization, but whole-rock compositions do not show suchextreme variations. Thus, while residual liquids became highlyevolved in situ, they mainly became trapped within the crystalnetwork and did not undergo wholesale inward migration. Someinward (mainly upward) concentration of residual liquids didoccur to form a sandwich horizon, but the morevolatile-rich, late-stage liquids that did not crystallize insitu appear to have migrated to higher levels in the sill toform pegmatitic horizons. Parental liquid compositions are modelledfor the intrusive units and it is concluded that the originalparent magma formed by partial melting of upper mantle thatwas more depleted in LREE than the sources of most ScottishTertiary basaltic rocks. Incompatible trace elements in thepicrodoleritecrinanite intrusive unit support isotopeevidence that its parent magma was contaminated by crustal material.Attempts to reconcile the chemical characteristics of the sillwith a recently proposed petrogenetic model based on a singleintrusion of magma differentiated by novel, but controversial,processes fail comprehensively. It is predicted that the complexpetrogenetic history of the Shiant Isles sill is not unusualand could become the model for other large (>50 m thick)sills. KEY WORDS: alkali basalt; differentiation; geochemistry; multiple intrusion; Shiant Isles; sill 相似文献
8.
U-Pb Age, Setting and Tectonic Significance of the Anorthosite-Mangerite-Charnockite-Granite Suite, Lofoten-Vesteralen, Norway 总被引:1,自引:0,他引:1
Mangerites, charnockites, anorthosites, gabbros and granitesoccur within a high-grade metamorphic complex in the LofotenVesterålenislands of northern Norway. UPb dating of zircon, titaniteand monazite indicates a three-stage magmatic history beginningat 18701860 Ma with the emplacement of the Lødingenand Hopen plutons, followed by a dominant stage at 18001790Ma that formed the bulk of the suite, and concluded by the emplacementof pegmatites, local rehydration and retrogression between 1790and 1770 Ma. On the scale of the Baltic Shield the 18701860Ma episode corresponds to contraction, amalgamation of arcs,and regional deformation. By contrast, the episode at 18001790Ma was characterized by major shifts in plate convergence, byintraplate deformation, and by a diversity of magmatic associationsincluding suites derived from the subcontinental mantle andwidespread granitoid rocks extracted from the continental crust.The diversity of concurrent magmatic events across the Svecofennianorogen, and the temporal coincidence with collisional eventsin coeval orogenic belts, suggests that the genesis of the suiteof magmatic rocks may have been related to tectonically drivenmechanisms of magma generation. KEY WORDS: anorthositemangeritecharnockitegranite; lithospheric processes; LofotenVesterålen; Svecofennian orogen; UPb geochronology 相似文献
9.
Evolution and Genesis of Magmas from Vico Volcano, Central Italy: Multiple Differentiation Pathways and Variable Parental Magmas 总被引:4,自引:0,他引:4
PERINI GIULIA; FRANCALANCI LORELLA; DAVIDSON JON P.; CONTICELLI SANDRO 《Journal of Petrology》2004,45(1):139-182
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/86Srinitial = 0·710370·71115)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% FeTi 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 1029%clinopyroxene + 1215% plagioclase + 4467% sanidine+ 24% phlogopite + 13% apatite + 710% FeTioxides. In contrast, leucite-bearing rocks of Period III (87Sr/86Srinitial= 0·708120·70948) were derived from a potassictrachybasalt by assimilationfractional crystallizationwith 2040% of solid removed and r = 0·40·5(where r is assimilation rate/crystallization rate) at differentpressures. Silica-saturated magmas of Period II (87Sr/86Srinitial= 0·710440·71052) 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 veinperidotite/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 相似文献
10.
Initial Nd, Pb, and Sr isotopic data from carbonatites and associatedintrusive silica-undersaturated rocks from the early Jurassic,Chilwa Island complex, located in southern Malawi, central Africa,suggest melt derivation from a Rb/Sr- and Nd/Sm-depleted butTh/Pb- and U/Pb-enriched mantle source. Initial 143Nd/144Nd(0.512650.51270) isotope ratios from the Chilwa Islandcarbonatites are relatively constant, but their initial 87Sr/86Sr(0.703190.70361) ratios are variable. The 18Osmow (9.5314.15%0)and 13CPDB (3.27 to 1.50%0) isotope ratios ofthe carbonates are enriched relative to the range of mantlevalues, and there is a negative correlation between 18O andSr isotope ratios. The variations in Sr, C, and O isotopic ratiosfrom the carbonatites suggest secondary processes, such as interactionwith meteoric groundwater during late-stage carbonatite activity.The initial 143Nd/144Nd (0.51246 0.51269) and initial 87Sr/86Sr(0.703440.70383) isotope ratios from the intrusive silicaterocks are more variable, and the Sr more radiogenic than thosefrom the carbonatites. Most of the Pb isotope data from Chilwa Island plot to the rightof the geochron and close to the oceanic regression line definedby MORBs and OIBs. Initial Pb isotopic ratios from both carbonatites(207Pb/204Pb 15.6315.71; 206Pb/204Pb 19.1319.78)and silicate rocks (207Pb/204Pb 15.6115.72; 206Pb/204Pb18.1820.12) show pronounced variations, and form twogroups in Pb-Pb plots. The isotopic variations shown by Nd, Pb, and Sr for the ChilwaIsland carbonatites and intrusive silicates suggest that thesemelts underwent different evolutionary histories. The chemicaldata, including isotopic ratios, from the carbonatites and olivinenephelinites are consistent with magmatic differentiation ofa carbonated-nephelinite magma. A model is proposed in whichdifferentiation of the carbonatite magma was accompanied byfenitization (metasomatic alteration) of the country rocks bycarbonatite-derived fluids, and subsequent alteration of thecarbonatite by hydrothermal activity. The chemical and isotopicdata from the non-nephelinitic intrusive silicate rocks reveala more complex evolutionary history, involving either selectivebinary mixing of lower-crustal granulites and a nephelinitemagma, or incremental batch melting of a depleted source andsubsequent crustal contamination. 相似文献
11.
Subduction Style Magmatism in a Non-subduction Setting: the Colville Igneous Complex, NE Washington State, USA 总被引:1,自引:0,他引:1
The Colville Igneous Complex is located within the Eocene MagmaticBelt of the North American Cordilleran interior. It straddlesthe USCanadian border in northeast Washington and southernBritish Columbia. The complex consists of three intrusive andtwo extrusive phases, the first extrusive phase being contemporaneouswith the latter two intrusive phases. As a consequence of sub-solidusre-equilibration in the plutonic rocks, this study concentrateson the two extrusive phases, the Sanpoil Volcanic Formationand the Klondike Mountain Formation. The Sanpoil Volcanic Formationconsists of andesites, dacites and rare trachyandesites (SiO2= 5570 wt %) exhibiting a slight decrease in total alkalis(Na2O + K2O) with increasing silica. The Klondike Mountain Formationconsists of basalts, basaltic andesites, andesites, dacitesand rhyolites (SiO2 = 5175 wt %) with total alkalis increasingwith increasing silica. The calc-alkaline affinity of the rocksof the Colville Igneous Complex, coupled with the presence ofa subduction signature of enriched large ion lithophileelements (LILE) and depleted high field strength elements (HFSE),has traditionally been attributed to petrogenesis in a subduction-relatedmagmatic arc, the Challis Arc. New trace and rareearth element and isotopic data (87Sr/86Sri, 相似文献
12.
Laser fusion 40Ar/39Ar ages of titanian pargasite from a microgranodiorite dyke swarm in the southern Bohemian Massif effectively date the early Permian (late Autunian) emplacement of dykes into a cool Moldanubian crust. This intrusion represents the youngest magmatic phase recorded in this part of the Moldanubian Zone. Strontium and neodymium isotopic ratios of microgranodiorites point to magma derivation from re-melting the lower crustal rocks with a possible component of upper mantle composition. Spatial and temporal association of the dykes with movements on a major N-S (NNE-SSW) tectonic discontinuity (Blanice-Kaplice-Rödl fault zone) suggests that their emplacement corresponds to the maximum age of fault movements associated with the E/W-oriented extension in this part of the Bohemian Massif. 相似文献
13.
14.
Blue Mountain is a central-type alkali ultrabasic-gabbro ringcomplex (1?1?5 km) introducing Upper Jurassic sediments, Marlborough,New Zealand. The ultrabasic-gabbroic rocks contain lenses ofkaersutite pegmatite and sodic syenite pegmatite and are intrudedby ring dykes of titanaugite-ilmenite gabbro and lamprophyre.The margin of the intrusion is defined by a ring dyke of alkaligabbro. The plutonic rocks are cut by a swarm of hornblende-biotite-richlamprophyre dykes. Thermal metamorphism has converted the sedimentsto a hornfels ranging in grade from the albite-epidote hornfelsfacies to the upper limit of the hornblende hornfels facies. The rocks are nepheline normative and consist of olivine (Fo82-74),endiopside (Ca45Mg48Fe7-Ca36Mg55Fe9), titanaugite (Ca40Mg50Fe10-Ca44Mg39Fe17),plagioclase (An73-18), and ilmenitetitaniferous magnetite, withvarious amounts of titaniferous hornblende and titanbiotite.There is a complete gradation between end-iopside and titanaugitewith the coupled substitution Ry+z+Si(Ti+4+Fe+3)+Al+3 and asympathetic increase in CaAl2SiO6 (0?2-10?2 percent) and CaTiAl2O6(2?1-8?1 per cent) with fractionation. Endiopside shows a small,progressive Mg enrichment along a trend subparallel to the CaMgSi2O6-Mg2Si2O6boundary, and titanaugite is enriched in Ca and Fe+2+Fe+3 withdifferentiation. Oscillatory zoning between endiopside and titanaugiteis common. Exsolved ilmenite needles occur in the most Fe-richtitanaugites. The amphiboles show the trend: titaniferous hornblende(1?05?7 per cent TiO2)kaersutite (6?4 per cent TiO2)Fe-richhastingsite (18?019?1 per cent FeO as total Fe). Biotiteis high in TiO2 (6?67?8 per cent). Ilmenite and titaniferousmagnetite (3?510?6 per cent TiO2) are typically homogeneousgrains; their composition can be expressed in terms of R+2RO3:R+2O:R2+3O4. The intrusion of igneous rocks was probably controlled by subterraneanring fracturing. Subsidence of the country rock within the ringfracture provided space for periodic injections of magma froma lower reservoir up the initial ring fracture to form the BlueMountain rocks at a higher level. Downward movement of the floorof the intrusion during crystallization caused inward slumpingof the cumulates which affected the textural, mineralogical,and chemical evolution of the rocks in different parts of theintrusion. The order of mineral fractionation is reflected by the chemicalvariation in the in situ ultrabasic-gabbroic rocks and the successiveintrusions of titanaugite-ilmenite gabbro and lamprophyre ringdykes, marginal alkali gabbro and lamprophyre dyke swarm. Aninitial decrease, then increase in SiO2; a steady decrease inMgO, CaO, Ni, and Cr: an initial increase, then decrease inFeO+Fe2O3, TiO2, MnO, and V; almost linear increase in Al2O3and late stage increase in alkalis and P2O3, implies fractionationof olivine and endiopside, followed by titanaugite and Fe-Tioxides, followed by plagioclase, hornblende, biotite, and apatite.Reversals in the composition of cumulus olivine and endiopsideand Solidification Index, indicate that the ultrabasic-gabbroicsequence is composed of four main injections of magma. The ultrabasic rocks crystallized under conditions of high PH2Oand fairly high, constant PO2; PH2 and PO2 increased duringthe formation of the gabbroic rocks until fracturing of thechamber roof occurred. The abundance of euhedral amphibole inthe latter injection phases suggests that amphibole accumulatedfrom a hydrous SiO2 undersaturated magma when an increase inPO2, stabilized its crystallization. Plutonic complexes similar to Blue Mountain are found withinand beneath the volcanic piles of many oceanic islands, e.g.Canaries, Reunion, and Tahiti, and those intruding thick sedimentarysequences, as at Blue Mountain, e.g. the pipe-like intrusionsof the Monteregian Hills, Quebec. 相似文献
15.
Low-O18 Igneous Rocks from the Intrusive Complexes of Skye, Mull, and Ardnamurchan, Western Scotland
Oxygen isotope analyses have been obtained on rocks and coexistingminerals from the Tertiary stocks, ring-dikes, and cone-sheetsin Ardnamurchan (18 samples), Skye (41 samples), and Mull (18samples); these include a few samples of the plateau basaltcountry rocks. Almost all of the rocks in the vicinity of thecentral ring complexes (within 2 mi. of an intrusive contact)are strongly depleted in O18 relative to normaligneous rocks from other areas. The rocks in Skye ( 150 sq.mi.) and Mull ( 150 sq. mi.) have suffered an overall O18 depletionof about 6 to 7 per mil, and those in Ardnamurchan ( 30 sq.mi.) about 3 to 6 per mil. These data indicate that very large hydrothermal convectionsystems involving heated low. O18 meteoric ground waters wereestablished in these areas at the time of igneous intrusion.The heated ground waters exchanged with the gabbros, granities,and basaltic lavas, locaclly lowering their O18/ O16 rationsby at least 12 per mil. Much, if not all, of this exchange occurredafter crystallization of a given igneous rock was essentiallycomplete, inasmuch as feldspars invariably have undergone muchmore depletion in O18 than has coexisting quartz or pyroxen.The meteoric-hydrothermal process and the presence of an aqueousgas phase may possibly be responsible for the widespread epidote-chloritealteration, the turbid or cloudy feldspars, the abundant feldspathicveins, the felsitic and granophyric textures, the miaroliticcavities, and much of the explosive volcanic activity and brecciationthat are found in these areas. 相似文献
16.
Guo Feng; Nakamuru Eizo; Fan Weiming; Kobayoshi Katsura; Li Chaowen 《Journal of Petrology》2007,48(4):661-692
Palaeocene (c. 5558 Ma) adakitic andesites from the Yanjiarea, NE China, are typically clinopyroxene-bearing sodic andesitescontaining 60· 962· 2% SiO2 and 4·024· 36% MgO, with high Mg-number [100 Mg/(Mg+ Fe) atomic ratio] from 65· 5 to 70· 1. Whole-rockgeochemical features include high Cr (128161 ppm) andNi (86117 ppm) concentrations, extremely high Sr (20132282ppm), low Y (1011 ppm) and heavy rare earth elements(HREE; e.g. Yb = 0· 791· 01 ppm), and mid-oceanridge basalt (MORB)-like SrNdPb isotopic compositions[e.g. 87Sr/ 86Sr(i) = 0· 702980· 70316,Nd(t) = +3· 8 to +6· 3 and 206Pb/ 204Pb = 17·98 18· 06], analogous to high-Mg adakites occurringin modern subduction zones. However, mineralogical evidencefrom clinopyroxene phenocrysts and microcrystalline plagioclaseclearly points to magma mixing during magma evolution. Iron-richclinopyroxene (augite) cores with low Sr, high Y and heavy REEcontents, slightly fractionated REE patterns and large negativeEu anomalies probably crystallized along with low-Ca plagioclasefrom a lower crustal felsic magma. In contrast, high Mg-numberclinopyroxene (diopside and endiopside) mantles and rims havehigher Sr and lower HREE and Y concentrations, highly fractionatedREE patterns (high La/Yb) and negligible Eu anomalies, similarto those found in adakites from subduction zones. The Yanjiadakitic andesites can be interpreted as a mixture between acrust-derived magma having low Mg-number and Sr, and high Yand HREE, and a mantle-derived high Mg-number adakite havinghigh Sr and low Y and HREE concentrations. During storage and/orascent, the mixed magma experienced further crustal contaminationto capture zircons, of a range of ages, from the wall rocks.The absence of coeval arc magmatism and an extensional tectonicregime in the Yanji area and surrounding regions suggest thatthese Palaeocene adakitic andesites were formed during post-subductionextension that followed the late Cretaceous IzanagiFarallonridge subduction. Generation of these adakitic andesites doesnot require contemporaneous subduction of a young, hot oceanicridge or delamination of eclogitic lower crust as suggestedby previous models. KEY WORDS: magma mixing; adakitic andesites; Palaeocene; NE China 相似文献
17.
Open-System, Sub-Volcanic Magmatic Evolution: Constraints on the Petrogenesis of the Mount Brome Alkaline Complex, Canada 总被引:1,自引:0,他引:1
The Mount Brome alkaline complex of southern Quebec (Canada)comprises gabbroic to silica-oversaturated and -undersaturatedfelsic rocks which have variable initial 87Sr/86Sr and 143Nd/144Ndratios: 0.703450.70431 and 0.512700.51258, respectively,in mafic lithologies (gabbro, nepheline diorite, and biotitemonzodiorite); 0.703530.70403 and 0.512700.51263in silica-undersaturated felsic units (pulaskite and nephelinesyenite); and 0.70510.7115 and 0.512620.51259in silica-oversaturated nordmarkite. Negatively correlated 87Sr/86Srvs. 143Nd/144Nd trends for the various rock types appear toconverge at the composition 87Sr/86Sr = 0.70345 and 143Nd/144Nd= 0.51270 (Nd = 4.3). This suggests that all rock types sharea common parental magma composition, either through derivationfrom a single batch of liquid, or (more probably) through twoepisodes of melting of the same depleted mantle source region.Delta 18O ranges from + 5.5 to + 8% and is broadly correlatedwith radiogenic isotopes and bulk composition. Isotopic, and major and trace element compositions suggest thatfractional crystallization (FC) of the parental magma at deeplevels produced evolved magmas, apparently without significantcrustal contamination [FC without assimilation (A)]. Duringascent and emplacement into the upper crust, these magmas thenevolved by simultaneous crystal fractionation and country-rockassimilation (AFC). Within both basic and felsic rocks thereis a clear relationship between silica saturation and degreeof contamination. Indeed, the oversaturated nordmarkites owetheir origin to assimilation of siliceous country rocks by acritically undersaturated magma. More generally, it seems likelythat this type of process is the normal mode of origin for coexistingquartz and nepheline syenites in many sub-volcanic alkalineigneous complexes. Additionally, such complexes would be likelyto develop by punctuated FC and AFC processes throughout theirpetrogenetic history. 相似文献
18.
Processes and Conditions During Contact Anatexis, Melt Escape and Restite Formation: the Huntly Gabbro Complex, NE Scotland 总被引:1,自引:1,他引:1
The Huntly Gabbro is one of a suite of large, Ordovician, syn-orogenic,mid-crustal, layered, mafic intrusions, emplaced into Proterozoicmetaclastic rocks of NE Scotland soon after the thermal peakof static, high-T, low-P regional metamorphism. This gabbroand its associated contact metamorphic rocks illustrate a varietyof processes operating during contact anatexis and subsequentmelt segregation and extraction. These processes may closelymirror those occurring at much larger scales in the deep crustduring high-grade regional metamorphism and the generation ofgranitic magmas. The emplacement of the Huntly mafic magma resultedin high-grade contact metamorphism and, locally, anatexis ofmetapelites, leading to the formation of migmatites. The migmatitesand country-rock schists were studied to establish the physicalconditions of metamorphism and anatexis, the nature of the meltingreactions, the compositions of the melts produced, and the extentto which melting was a closed- or open-system process. The country-rockschists immediately to the south of the Huntly Complex containmineral assemblages characteristic of the regional andalusitezone. Thermobarometry of an andalusite schist yields regionalmetamorphic conditions of 537 ± 42°C and 0·27± 0·12 GPa, consistent with previously publishedPT estimates. The contact metamorphic rocks include sillimanitehornfelses, metatexites and diatexites. The metatexites consistof cordieriteK-feldspar hornfels melanosomes and K-feldspar-richgarnetiferous leucosomes. The diatexites consist of schollenof fine-grained granoblastic hornfels and metatexite suspendedin igneous-textured matrix rocks composed of abundant sub/euhedralgarnet, cordierite, plagioclase and, locally, orthopyroxene,with minor interstitial biotite, K-feldspar and quartz. Thehornfels melanosomes and schollen retained their structuralintegrity during partial melting, but the matrix rocks did not.In the highest-grade diatexites, the assemblage Grt + Opx +Crd + Hc + Pl characterizes both the hornfels schollen and thesub/euhedral minerals of the matrix rocks. Application of phaseequilibria to Opx-bearing rocks yields estimated peak-metamorphicconditions of 900 ± 50°C, 0·45 ± 0·1GPa and aH2O < 0·3. The pressure estimate impliesan emplacement depth of 相似文献
19.
The Petrology and Geochemistry of Oto-Zan Composite Lava Flow on Shodo-Shima Island, SW Japan: Remelting of a Solidified High-Mg Andesite Magma 总被引:1,自引:0,他引:1
TATSUMI Y.; SUZUKI T.; KAWABATA H.; SATO K.; MIYAZAKI T.; CHANG Q.; TAKAHASHI T.; TANI K.; SHIBATA T.; YOSHIKAWA M. 《Journal of Petrology》2006,47(3):595-629
The Oto-Zan lava in the Setouchi volcanic belt is composed ofphenocryst-poor, sparsely plagioclase-phyric andesites (sanukitoids)and forms a composite lava flow. The phenocryst assemblagesand element abundances change but SrNdPb isotopiccompositions are constant throughout the lava flow. The sanukitoidat the base is a high-Mg andesite (HMA) and contains Mg- andNi-rich olivine and Cr-rich chromite, suggesting the emplacementof a mantle-derived hydrous (7 wt % H2O) HMA magma. However,Oto-Zan sanukitoids contain little H2O and are phenocryst-poor.The liquid lines of descent obtained for an Oto-Zan HMA at 0·3GPa in the presence of 0·72·1 wt % H2Osuggest that mixing of an HMA magma with a differentiated felsicmelt can reasonably explain the petrographical and chemicalcharacteristics of Oto-Zan sanukitoids. We propose a model wherebya hydrous HMA magma crystallizes extensively within the crust,resulting in the formation of an HMA pluton and causing liberationof H2O from the magma system. The HMA pluton, in which interstitialrhyolitic melts still remain, is then heated from the base byintrusion of a high-T basalt magma, forming an H2O-deficientHMA magma at the base of the pluton. During ascent, this secondaryHMA magma entrains the overlying interstitial rhyolitic melt,resulting in variable self-mixing and formation of a zoned magmareservoir, comprising more felsic magmas upwards. More effectiveupwelling of more mafic, and hence less viscous, magmas througha propagated vent finally results in the emplacement of thecomposite lava flow. KEY WORDS: high-Mg andesite; sanukitoid; composite lava; solidification; remelting 相似文献
20.
Magmatic Evolution of the Skye Igneous Centre, Western Scotland: Modelling of Assimilation, Recharge and Fractional Crystallization 总被引:3,自引:2,他引:1
The Skye igneous centre, forming part of the British Tertiarymagmatic province, developed over a 7 Myr period (6154Ma) and is characterized by a complex suite of lavas, hypabyssaland intrusive rocks of picritic to granitic composition. Theintrusion of magma from mantle to crust at 2x103km3/yr(6 Mt/yr) advected magmatic heat of roughly 0·2 GW averagedover the period of magmatism supporting an excessheat flux of about 130 mW/m2, or about twice the present-dayaverage continental heat flow. The volume of new crust generatedat Skye (15000 km3) spread over the present-day area of Skyecorresponds to 9 km of new crust. The geochemical evolutionof the Skye magmatic system is constrained using the Energy-ConstrainedRecharge, Assimilation, and Fractional Crystallization (EC-RAFC)model to understand variations in the Sr- and Pb-isotopic andSr trace-element composition of the exposed magmatic rocks withtime. The character (composition and specific enthalpy) of bothassimilant and recharge magma appears to change systematicallyup-section, suggesting that the magma reservoirs migrated toprogressively shallower levels as the system matured. The modelof the magma transport system that emerges is one in which magmabatches are stored initially at lower-crustal levels, wherethey undergo RAFC evolution. Residual magma from this stagethen migrates to shallower levels, where mid-crustal wall rockis assimilated; the recharge magma at this level is characterizedby an increasingly crustal signature. For some of the stratigraphicallyyoungest rocks, the data suggest that the magma reservoirs ascendedinto, and interacted with, upper-crustal Torridonian metasediments. KEY WORDS: assimilation; EC-RAFC model; geochemical modelling; magma recharge; Skye magmatism 相似文献